摘要:

Comment on Rate of Polymorphic Transformation Between Phases I1 and 111 of Hexachloroethane BY HORIA METIU Department of Chemistry, University of California, Santa Barbara, California 93 106, U.S.A. JOHN ROSS* Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39, U.S.A. AND Received 27th February, 1978 We comment on an article by Koga and Miura in this journal by providing clarifications concerning the use of kinetic equations for phase transitions and of thermodynamic potentials for metastable conditions. The article by Koga and Miura' prompts us to make some clarifications concerning the thermodynamic meaning and the proper use of the kinetic equations describing the rate of phenomena related to phase transitions, such as nucleation,2 spinodal decomposition and c~ndensation.~ The observations made in the present note are not new? and their purpose is to complement the presentation in ref.(4) and hopefully thereby prevent confusion. The macroscopic theory of the kinetics of phase transitions is based on the requirement that the relaxation of the nonequilibrium system proceeds in a way that causes the decrease of some generalized thermodynamic potential. When implemented mathematically this requirement yields two equations : and We use here the example of a binary system in which the " order parameter " is the concentration C(Y, t ) of the component 1 at site Y and time t. F[T,p, C(Y, t)] is the generalized thermodynamic potential. The first equation is valid for cases (diffusion- like) in which the order parameter is conserved : the change in the amount of the component 1 at a given site is equal to the net flux through a surface surrounding the site.The second equation is valid for cases (chemical reaction-like) in which such conservation is not required. One example is provided by phase transitions in some biopolymers :' the order parameter is the concentration of hydrogen bonds and these bonds are formed at the site, by a " chemical reaction ", not transported from the neighbouring regions. The difference between the two equations is substantial and we feel that they are not interchangeable. Koga and Miura's use of eqn (2) instead of (1) makes, in our t The comments made in this Note are available but dispersed in the literature. 2750H.METIU AND J. ROSS 275 1 opinion, any conclusion about agreement between theory and experiment question- able. To see why this is so let us apply the two equations to a familiar case, such as relaxation of a concentration inhomogeneity in a system which is near equilibrium. We use for the generalized potential the customary * form F = 1 dr{flT, P, c(r, t)l+K(Vc)2). (3) Herefis the “ homogeneous part ” of the generalized potential and K(VC)~ takes into account the existence of the concentration gradients. Inserting eqn (3) into (1) and (2) yields ac ( r , tyat = rv2 - -rm4c C) . . and ?f ac ac ( r , tyat = ri - - riKv2c. (4) If we are interested in concentration relaxation near equilibrium we may assume that c(r, t ) - co is small (co is the homogeneous, equilibrium concentration) and hence write Inserting this in eqn (4) we obtain This is the customary diffusion equation (the second term at the right hand side can be neglected if the system is not very close to the consolution critical point, with the diffusion coefficient D = ra2f/dc& Comparing with the known thermodynamic definition of the diffusion coefficient we infer that df/dco is proportional to (,ul/ml) - (p2/m2), where p i and mi are the chemical potential and the mass of the component i and c is the concentration of component 1.Eqn (5), on the other hand, becomes Choosing the generalized potential so that af/aco = 0 at equilibrium [this amounts to subtracting a constant from af/aco and does not modify eqn (7)], we see that eqn (8) describes a system in which the component I undergoes a first order chemical reaction and diffuses.The “ reaction rate ” term is first order only because we consider the case when c(r, t ) - co is small. The time evolution of the concentration given by the two equations is very different and we hope that the application of eqn (1) and (2) to this familiar example illustrates the problems encountered using eqn (2) for cases in which eqn (1) is required. The second topic we want to comment upon concerns the properties and the meaning of the generalized potential. Our views have been strongly influenced by those of Langer.5 The customary thermodynamic treatment of irreversible processes uses the so- called local equilibrium assumption,’ which stipulates that one can coapute the value of a thermodynamic potential for a non-equilibrium state by inserting the non- equilibrium values of the state parameters [e.g., T, p and c(r, t ) ] into the functional2752 RATE OF POLYMORPHIC TRANSFORMATIONS form of the thermodynamic potential at equilibrium.Eqn (6) is one possible way of carrying out this procedure for states near equilibrium. In general terms this amounts to an analytic continuation of the equilibrium potential to non-equilibrium states. In the case of phase transition this procedure has difficulties. The thermo- dynamic potential function ends abruptly lo at the coexistence line of the phase diagram. The coexistence point is a singularity of the potential and it “ blocks ” lo the analytic continuation to values of c corresponding to metastable or unstable systems.The difficulty can, however, be circumvented in the following manner. Consider for example the Helmholtz free energy Here p = 1 /kT, V is the volume andfis the free energy density. We can compute a free energy f corresponding to the unstable homogeneous system of concentration c, by including in the computation of the trace only those states that correspond to the specified concentration c. By imposing this constraint we define a free energyf(c) that must be higher than the one corresponding to the equilibrium state. We emphasize that this method is not as artificial as it may at first appear. True equilibrium states are such only because we impose constraints, as €or example requiring that all the molecules are confined in a box of volume V.If we remove one wall of the box the true equilibrium state becomes unstable. The free energy of this unstable state (without the wall) is defined by the present procedure to be equal to the free energy of the stable equilibrium system obtained when the wall is present. While this procedure explains the molecular (statistical) meaning of the non- equilibrium thermodynamic potential (at concentrations for which a true equilibrium thermodynamic potential does not exist) it is not yet possible to implement it to obtain expressions for f. In order to establish the properties off (here we return to the general problem andfis again the homogeneous part of the generalized potential, not necessarily the free energy) we use the kinetic equation [e.g., eqn (4)] and require that f has features which describe the observed behaviour of the metastable states.Such a requirement is quite reasonable, in view of the fact that metastability is the result of kinetic constraints and that purely thermodynamic arguments predict that a metastable state does not exist. The observed qualitative behaviour of a metastable system is reproduced if we ask thatfhas two minima, one at the concentration of the metastable state c1 and the other at the concentration of the equilibrium state co (the absolute minimum). Between them we must have a maximum, at the concentra- tion c2, and as a consequence the two states, of concentration c1 and co, are separated by a barrier. This construction gives the kinetic equations the desired properties. The homogeneous equilibrium state is a steady state of the kinetic equation (af/aco = 0 implies aco/dt = 0).A small inhomogeneity imposed on the equilibrium state relaxes by diffusion (for conserved variables). The homogeneous metastable state is also a steady state (8f/acl = 0 implies &,/at = 0). A small inhomogeneity in this state relaxes by diffusion and with the diffusion coefficient Ta2f/dcf (which could be different from that of the equilibrium state, which is r a’fj’ac;). Thus a meta- stable state in which only small inhomogeneities are possible is stable forever. Inhomogeneities that are large enough to overcome the barrier between the metastable and the stable states destroy the metastable state (nucleation). The height of the barrier, which is the factor preventing nucleation, must therefore decrease with the degree of supercooling. When the spinodal line is crossed, the barrier disappears, the diffusion coeffcient becomes negative and the phase is unstable.This qualitative picture has been explored quantitatively in detail. 2-4H. METIU AND J . ROSS 2753 To obtain the equation used and criticized by Koga and Miura we have chosen the form of if/& to be given by a(c-cO)(c-c1)(c-c2), which is the simplest form that has the three extrema discussed above. This allows us to ohtain analytically a solution for the kinetic equation. Koga and Miura have objected to the form used by us, since they interpreted it to mean that the chemical potentials of the stable and metastable states are equal, a conclusion which is unacceptable.First, we note that the use of thermodynamk arguments in matters concerning metastability is not safe. Rigorous use of equilibrium thermodynamics would only tell us that metastable states cannot exist. It is necessary to use kinetic arguments, like those presented above, since metastability is a kinetic phenomenon. Second, it is possible to use mean field thermodynamic arguments to show that the type of choice made by us is consistent with a macroscopic, hence '' thermodynamic 'I analysis. The use of a mean field theory allows an analytic continuation of the thermodynamic potential "A Vi VB V 2 V C V D V o v FIG. 1.-Plot of derivative of Helmholtz free energy, a$/av, against v = V - Vc, where Vis the specific volume and V, the specific volume at the critical point.inside the metastable and unstable region since the mean field approximation does not display the singularity at the coexistence line that the exact potential does. If we denote by i,b the Helmholtz free energy per gram and by Y the specific volume, then we have l1 a@/av = -Azv-Bv3/3 +$(T) where v = Y - V,, z = T-T,, A and B are constants and $(T) is a function of temperature which we do not need to specify. A plot of a$(v)/av against v is shown in fig. 1. The points A and D correspond to the coexisting phases at the chosen temperature. Any specific volume between v A and vB, or vc and vD corresponds to a metastable phase ; those between v, and vc are unstable. Such a system can be prepared by starting at point 1, keeping the temperature constant and lowering the pressure in the direction indicated by the arrow.Consider now the Gibbs free energy g(v) = $(v)+plv, where p1 is the externally imposed pressure. We have ag/av = a$(v)/av+p, and we see from the graph that ag/dv = 0 for v = vl, v = v2 and v = vo. This shows that g has the properties required by kinetic arguments. If we put the metastable and the stable phases in contact, as required in a condensation problem (the boundary between the phases changes due to condensation), we see that ag/av = 0, for v 1 and Consider now a system at temperature 2, pressure p1 and volume vl. Here vo corresponds to the true equilibrium state.2754 RATE OF POLYMORPHIC TRANSFORMATIONS vo, is an expression of the mechanical equilibrium condition which requires that the two phases be at the same externally imposed pressure.It does not mean, as interpreted by Koga and Miura, that the two chemical potentials are forced to be equal. In fact, using dp = (V, +v)dP = - (V, + v) at,b/dv dv, one can show that the chemical potentials are equal only for the coexisting phases (pA = pD) which satisfy the Maxwell construction, and that pE # pF. When other slow variables are introduced, and this must often be done, the order parameter becomes very complex;12 the simple situations considered above are only illustrative. H. M. thanks the Research Corporation, and the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this work. J. R. thanks the National Science Foundation and the Air Force Office of Scientific Research for partial support of this work.Y. Koga and R. hi. Miura, J.C.S. Faraday I, 1978, 74, 191 3. J. S. Langer and L. A. Turski, Phys. Rev. A, 1973, 8, 3230. J. W. Cahn, Trans. Metal. SOC. A.I.M.E., 1968, 242, 166; J. S. Langer, Ann. Phys. ( N . Y.), 1973, 78, 421 ; J. S. Langer, in Fluctuations, Instabilities and Phase Transitions, ed. T. Riste (Plenum Press, N.Y., 1975). H. Metiu, K. Kitahara and J. Ross, J. Chem. Phys., 1976, 64, 292. J. S. Langer, Physica, 1974,73, 61. See, for example, H. Metiu, K. Kitahara and J. Ross, J. Chem. Phys., 1976, 65, 393. J. D. van der Waals, 2. phys. Chem. (Leigzig), 1894,6,657 ; V. L. Ginzburg and L. D. Landau, Zhur. Eksp. Teor. Fiz., 1950,20, 1064; J . W. Cahn and J. E. Hilliard, J. Chem. Phys., 1958, 28,258 ; 1959, 31, 688. L. D. Landau and E. M. Lifshitz, Fluid Mechanics (Pergamon Press, New York, 197% p. 221. l o M. E. Fisher, Physics, 1967, 3, 255 ; J. S . Langer, Ann. Phys. (N. Y.), 1967, 41, 108. l 1 L. D. Landau and E. M. Lifshitz, Statistical Physics (Addison-Wesley, Reading, Mass., 1969). l2 P. C. Hohenberg and B. I. Halperin, Rev. Mod. Phys., 1977, 49,435 ; K. Kawasaki, in Phase Transitions and Critical Phenomena, ed. C . Domb and M. S. Green (Academic Press, N.Y., 1976), vol. 5. ' E. Neumann, Angew. Chem. (Int. Edn.), 1972, 12, 356. (PAPER 8/356)

ISSN:0300-9599    

DOI:10.1039/F19787402750

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Analysis of Pressure Changes for Simultaneous First-order Decomposition Reactions in a Gas-kinetic System with Dead-space BY PETER J. ROBINSON* Department of Chemistry, Manchester Polytechnic, Manchester M 1 5GD GRAHAM G. SKELHORNE AND Lankro Chemicals, Manchester M30 OBH Received 2iid March, 1918 Methods are described for treating the problem of unreactive dead-space when extracting kinetic parameters from total pressure measurements on simultaneous first-order decomposition reactions in a constant-volume gas kinetic system (A - B --f C'+D'). The proposed methods are verified by applying them to computer-gene~~eCd+'Pexperimental" pressure against time data for systems with a wide range of parameter combinations. In the course of kinetic studies on the thermal decomposition of a mixture of cis- and trans-4-chloropent-2-ene, it was found necessary to investigate the mathe- matical treatment of a pair of first-order reactions in a constant-volume gas apparatus with unheated dead-space.The results are of general interest and are reported here. The case of a single first-order process A + B + C has already been discussed.2 The final pressure p m is equal to 2"p0, where p o is the initial pressure and a is the " dead- space factor," eqn (1) where V,, T, = volume and temperature of dead-space, Vh, Th = volume and temperature of reaction zone. The pressure of reactant at any time (pA) is approxi- mated only poorly by (2p"-p), where p is the total pressure at time t , and plots of In (2p"-p) against time are seriously curved. However, plots of In (p" - p ) or In (2"~"-p) against time are close to linearity, and the first-order rate constant k can be obtained from the initial slope, which is (2"- 1) k/a.In the present work we consider the decomposition of a mixture of two reactants, A and B, by parallel first-order reactions (2) and (3) a = (1 + VcTh/ V,T,)-' (1) kA kB A -+ C+D B --P C'+D'. (3) In the absence of dead-space, the combined reactant pressure is given by eqn (4), in which the symbols have the obvious meaning. Provided that kA and kB are not too similar in value, and that the shorter lived reactant is not present in great excess, the rate constant for the longer-lived reactant can be obtained from a PA +PB = p" - p = 2p" - p = p i eXp ( - kAt) +pi eXp (- kBt) (4) 27552756 GAS-KINETIC SYSTEM WITH DEAD-SPACE plot of In ( p a - p ) or In (2p0 -p) against time, which at sufficiently long times becomes linear.For example, fig. 1 (upper curve) illustrates the case with p s = 4 p i and kA z 10kB ; the slope of the linear portion is equal to -kB, and the extrapolated intercept at t = 0 is In p i . For times in the initial curved section, p B can be calculated as p i exp (- kBt), and p A can then be calculated from eqn (5) A plot of In p A against time then gives - kA from the slope and p i from the intercept (fig. 1, lower curve). The precise combinations of rate constants and concentrations for which this treatment is practicable depend strongly on the precision of the experi- mental results and the time resolution available. Useful data can typically be abstracted for cases where kA/kB > 2 when p i / p g i2l 0.1, or where kA/kB > 10 when 2p" - p - p i exp (- kBt).( 5 ) PA = 2p"-p-p, = n f 4 I a ' I 1 40 80 time/s FIG. 1 .-Method of analysis of pressure-time data (no dead-space). If we now consider the same reactions in a system with dead-space, derivations similar to those given previously show that the rate of pressure change beconies eqn (6) and the rates of change of partiaI pressures are given by eqn (7) and (8). In eqn (7), and similarly in eqn (8), the second term on the r.h.s. accounts for the movement of reaction mixture out of the reaction vessel resulting from the pressure increasc dp/dt = a(kA PA + kB P B ) (6) (7) (8) dpA/dt = -kAPA-(PA/P)(l -a)(kAPA+ ICBPB) dpB/dt = -kBpB- (PB/P)(l -a)(kAPA+ kBPB)* In the absence of dead space, a = 1 and the previous equations are recovered.In the presence of dead-space, it will be expected by analogy with previous work that p" # 2p", and that plots of In (2p"-p) against time will be curved and give erroneous rate constants.P . J . ROBINSON A N D G . G . SKELHORNE 2757 SEMI-EMP IRICAL TREATMENT in our previous work,' it was thought that eqn (6)-(8) could not be solved analytically. We therefore proposed, evaluated and used a semi-empirical treatment, which, by analogy with the case of a single reactant,2 was expected to provide a reasonably accurate correction for dead-space effects : (a) plot In (2"~" -p) against t ; (b) from the slope SB of the linear part, determine kB by eqn (9); (c) from the intercept I B of the linear part extrapolated back, determine p i by eqn (10); ( d ) plot In (2"pO-p-exp ( I B + S B t ) ) against time; and from the slope SA determine k A by eqn (1 1); (e) from the intercept IA, determine p i by eqn (12).(9) k B = - SB(2"- l)/a p i = exp (IA)/(2"- 1). A N A LY TI C A L TREAT MEN T During the preparation of the present paper, it was discovered that eqn (6)-(8) can be integrated by suitable substitutions to give eqn (13) for the total pressure, eqn (14) for pA and similarly for pB. For a system with dead-space, the 1.h.s. of eqn (13) can be used in place of the 1.h.s. of eqn (4), and the various parameters thus determined directly without any further correction for dead-space (pi +pi) - Por(PiPo)i/~ - 11 = p i exp (- kAt) + pg exp (- kBt) (13) eXp (- kAt).(14) 0 (1 - l / o ) PA = P i ( P / P TESTS OF PROPOSED METHODS The semi-empirical method was used satisfactorily for the analysis of extensive experimental data in our previous work.' Such data do not, however, fulfil the requirements for a comprehensive test of the mathematical accuracy of the proposed treatments, since for this purpose the reactions must be 100 % clean, and very precise a priuri data are required for all the parameters, including mixture composition, dead-space and the rate-constants. Test data were therefore generated by computer " experiments " before the analytical solution was discovered. The coupled differ- ential eqn (6)-(8) were integrated by the Runge-Kutta-Merson technique embodied in an interactive software package known as CMS (Conversational Mode Simulation) on the PDP-10 computer in the U.M.I.S.T.Control Systems Centre. The correct operation of the integration routine was originally checked by test cases with a = 1.000, for which the analytical solution was known. By appropriate adjustment of the integration error control parameter, agreement could be obtained to better than 1 % of the initial partial pressure of each reactant, even at 80-90 % reaction for that component (e.g. table 1). It was later possible to show that the numerical integration agreed very well with the analytical solution for a # 1 (e.g. table 2). Computer data were generated for various cases with a # 1 and analysed as above by both the semi-empirical and the analytical methods, to extract the initial pressures and rate constants for the two components.The analyses were performed by least- mean-squares cafculation on a Hewlett-Packard 98 10A " programmable calculator ", which automatically plotted the required graphs and calculated slopes and intercepts from points selected after plotting by the operator. The plots were essentially linear2758 GAS-KINETIC SYSTEM WITH DEAD-SPACE n the limiting regions corresponding to those in fig. 1 and enabled the straight lines to be fitted in a reasonably objective manner. The results of the calculations are shown in table 3, which indicates the generally good agreement obtained between the results of the analyses and the original input data, over a wide range of parameter TABLE l.-COMPARISON OF ANALYTICAL SOLUTION AND NUMERICAL INTEGRATION FOR a = 1 (RATE CONSTANTS : kA = 3 .6 6 ~ S-', kB = 3 . 6 0 ~ S-') time/s 0 200 400 750 1000 analytical 10.4 4.99 2.41 0.67 0.27 numerical 10.4 4.99 2.41 0.68 0.28 timell02 s 0 60 180 310 440 analytical 89.6 72.2 47.0 29.4 18.4 pB/mmHg{ numerical 89.6 72.2 47.3 29.9 19.1 TABLE 2.-cOMPARISON OF ANALYTICAL SOLUTION AND NUMERICAL INTEGRATION FOR a = 0.90 (kA = 1o-l s-', k~ = low3 s-l, pX = 10 mmHg, pb = 90 mmHg) time/s 0 25 1000 2000 analytical 100.0 110.2 158.6 176.3 100.0 110.2 158.6 176.3 total TABLE 3 .-ACCURACY OF DATA EXTRACTED BY ANALYSIS OF COMPUTER-GENERATED PRESSURE AGAINST TIME CURVES pg/rnmHg pg/mmHg 10 kA/S-l lo3 kB1S-I input a 10.0 90.0 1 .oo 1 .oo semi-emp.b 10.5 89.5 0.94 0.99 analyt .b 10.0 90.0 0.99 1 .oo input analyt.input analyt. input analyt. a = 0.95 semi-emp. { 20.0 20.5 19.9 40.0 40.2 40.0 10.0 10.6 9.8 80.0 79.6 80.1 60.0 59.7 60.0 90.0 89.3 90.2 1 .oo 0.97 1 .oo 1 .oo 0.99 1 .oo 1 .oo 0.99 1.02 1 .oo 0.99 1 .oo 1 .oo 0.99 1 .oo 10.0 9.8 10.0 input 90.0 10.0 1 .oo 10.0 a = 0.95 semi-emp. 89.7 10.2 0.99 10.0 anal yt . 89.7 10.3 1 .oo 10.2 input 10.0 90.0 1.00 1 .oo 10.9 89.0 0.91 0.99 { r analyt. 10.1 90.0 1 .oo 1 .oo a = 0.90 semi-emp. a " input " = data fed into numerical integration SE " true values " ; " semi-emp " and " analyt " = results of analysis = " observed values ". combinations. Thus either method provides a fair treatment of the dead-space problem; without correction, the plots for kB are distinctly curved at all times (e.g. (fig. 2), and meaningful parameter extraction can be virtually impossible.The analytical method is more accurate than the semi-empirical method ; indeed the former should be exact if the simulated experimental results are correct and ifP. J . ROBINSON AND G . G. SKELHORNE 2759 sufficient precision is used in the data. Thus the residual errors (a few percent) can be attributed mainly to “ experimental scatter,” introduced by limiting the computer- generated pressure data to a physically realistic accuracy of four significant figures (e.g. to 0.1 mmHg in 100-200 mmHg).* Thus when data for two competing reactions are extracted from a single pressure against time curve, the scatter and errors will be significantly greater than for a single clean reaction, unless the original data are of unusually high precision. 5 n 4 a I (Je s - a 3 Lc n Q4 I O 2 3 M 1 f 0 0 0 2000 3000 4000 timeis FIG. Z.-Plots, with and without correction for dead-space, for a = 0.95, kB/kA = 0.10 and PSIPA = 5.0. SINGLE REACTANT The present analytical solution suggests a useful treatment for first-order decom- position of a single reactant into two molecules by puttingp; = 0 in eqn (13). The more general case in which one molecule of reactant decomposes to give q molecules of product can be obtained from the integral of eqn (7) of ref. (2), which gives eqn (15). This should provide a precisely linear plot of slope k and could be a useful alternative to the plot of In (qapo-p) against t, as previously recommended We thank the S.R.C. for a studentship (G. G. S.), and the referees for their constructive comments. P. J. Robinson, G. G. Skelhorne and M. J. Waller, J.C.S. Perkin 11, 1978, 349. P. J. Robinson, Trans. Furaday SOC., 1965, 61, 1655. * mmHg = 133.3 Pa. (PAPER 8/389)

ISSN:0300-9599    

DOI:10.1039/F19787402755

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Electron Transfer Reactions Involving Cblorophylls a and b and Carotenoids BY JOSEPH LAFFERTY AND T. GEORGE TRUSCOTT" Chemistry Department, Paisley College, Paisley PA1 2BE EDWARD J. LAND AND Paterson Laboratories, Christie Hospital and Holt Radium Institute, Manchester M20 9BX Received 3rd March, 1978 Rates of positive and negative charge transfer from the radical cations and anions respectively of several carotenoids of varying double bond chain lengths to chlorophyll a and b are reported. The role of carotenoids in photosynthesis has been the subject of discussion for many years. It is probable that carotenoids have at least two functions, namely that of accessory pigments in which light energy is absorbed and transferred to chlorophyll and that of a protective species in which the potentially damaging effects of singlet oxygen, produced from the chlorophyll triplet, are quenched.It is thought that the primary process of photosynthesis involves electron ejection from chlorophyll a and various oxidation/reduction reactions. Thus the properties of the cations of carotenoids and chlorophylls (chi*+) are of importance. that all-trans-p-carotene and lycopene undergo charge transfer reactions of the type Our recent observations C*++chla -+ C+chla-+ (1) C*-+chla + C+chla-- (2) where C-+ and C-- are the radical cation and anion, respectively, of P-carotene and lycopene. imply that other roles of P-carotene in photosynthesis may be related to such charge transfer reactions. We now report further observations of electron transfer reactions involving both chlorophyll a.and chlorophyll b together with six polyenes varying from 7,7'-dihydro- /?-carotene (8 conjugated double bonds) to decapreno-P-carotene (1 5 conjugated double bonds). EXPERIMENTAL Chlorophyll a and b were extracted from spinach by a procedure similar to that described by Zscheile and Comar ;2 the purity was satisfactory as checked by the ratio of the blue/red absorption maxima. All polyenes were donated by Hoffmann-La Roche and were used without further purification. The solvent, hexane, was B.D.H. (Special for Spectroscopy) and was used as supplied. The pulse radiolysis equipment has also been described previ~usly.~ RESULTS In our earlier publication,' we discussed the rates of positive charge transfer from p-carotene and lycopene radical cations to chlorophyll a, and negative charge transfer 2760J .LAFFERTY, T. G . TRUSCOTT AND E . J . LAND 276 1 from the radial anions of the same pdyenes ta chlorophyll a. We now report the rates of + ve and - ve charge transfer from the radical cations and anions, respectively of several other polyenes of varying double bond chain lengths to chlorophyll a and chlorophyll b. The rate constants obtained, assuming monmeric chlorophylls, are listed in table 1 (chlorophyll a) and table 2 (chlorophyll b). As before the carotenoid radical ions were generated by pulse radiolysis of - mol dm-3 solutions of the corresponding carotenoids in hexane solution.4* The transfer rate constants were then obtained by monitoring the increased carotenoid radical ion decay rates resulting from addition of various concentrations el chlorophyll a or b.TABLE SE SECOND ORDER RATE CONSTANTS (k) FOR THE REACTION BETWEEN CAROTENOID RADICAL IONS AND CHLOROPHYU a number of monitoring conjugated wavelength/nm k/dm3 mol-1 s-1 carotenoid double bonds cation amon reaction (1) reaction (2) 7,7’-dihydro$-caro t ene 8 830 785 5.4 8.0 sep t apreno-/I-car o tene 9 915 785 10.7 7.0 1 5,l Y-ci.+fl-carotene 11 1040 880 11.8 8.7 all-trans-j?-caro tene 11 1040 880 6.0 8.5 all-tram-1 ycopene 11 1070 950 1.7 7.0 decapreno-#I-carotene 15 1250 1130 4.7 5.4 TABLE 2.-sECOND ORDER RATE CONSTANTS (k) FOR THE REACTION BETWEEN CAROTENOID RADICAL IONS AND CHLOROPHYLL b carotenoid number of monitoring conjugated wavelength/nm 10-10 k/dm3 mol-1 s-1 double bonds cation amon reaction (1) reaction (2) 7 ,? ’-di hy dro-b-car0 t ene 8 830 785 1 .o 2.5 septapreno-#I-carotene 9 915 785 0.6 8.0 1 5,I 5‘-cis-fl-carotene 11 1040 880 CO.01 1.45 all-trans-p-caro tene 11 1040 880 <0.01 1.75 decapren 0-p-car o t ene 15 1250 1130 ca.01 1 .o Solutions of chIorophy11 a and b alone in hexane and various other solvents including benzene, ether, acetonitrile and dimethyl sulphoxide were also pulsed in order to examine the absorptions of the chlorophyll a and b radical cations and anions.The transient absorptions thus obtained from chla solutions, although generally consistent with the spectra of c h h - and chlas+ found by Seki et oZ.,~ were too weak, in comparison with the intense absorptions of the earotenoid ions, to allow studies of the growth of chlaq- or chlae+ on pulse radiolysis of carotenoid+chlorophyll a mixtures.The transient absorptions from chlorophyll b solutions, were likewise very weak. For a single pair of carotenoids @carotene and septapreno-P-carotene) positive charge transfer from one carotenoid to another was investigated. Addition of lob5 mol dm-3 P-carotene to 2.5 x mol dm-3 septapreno-P-carotene resulted in an increase in the decay of the septapreno-/?-carotene cation absorption at 915 nm and a matching growth of 8-carotene cation absorption at 1050 nm.4 The rate of + ve charge transfer from the septapreno-P-carotene cation to p-carotene was thus found to be 5.2 x lo9 dm3 mol-l s-l. DISCUSSION Our earlier work involving the reaction of chlorophyll a with the p-carotene and lycopene radical cation [reaction (l)] and radical anion [reaction (2)] showed2762 ELECTRON TRANSFER I N CHLOROPHYLLS AND CAROTENOIDS that both processes occur at rates close to the diffusion limit for both carotenoids (k = 2-8 x lo9 dm3 mol-' s-l).Our new results show the same behaviour for all the carotenoids studied in that all such radical ions were quenched by chlorophyll a at similar rates (k = 2-12 x lo9 dm3 mol-' s-'). However, a marked difference was observed with chlorophyll b ; in this case the radical anions of all carotenoids were quenched while only the radical cation of the shorter polyenes (septapreno-P-carotene and 7,7'-dihydro-P-carotene) reacted with chlorophyll b. The important aspect is that we could detect no reaction between P-carotene.+ and chlorophyll b.Since septapreno-P-carotene*+ was quenched by chlorophyll b while /?-carotene*+ was not, this implied that septapreno-p-carotene*+ can gain an electron more readily than P-carotene-+. In an attempt to confirm this we studied the reaction between these two carotenoids and, in agreement with their reaction rates with chlorophyll b, we found that the process septapreno-P-carotene*+ + p-carotene -+ septapreno-/?-carotene + p-carotene-+ occurred at close to the diffusion limited rate. Recently, Beddard et aL7 have reported the quenching of chlorophyll a fluores- cence by P-carotene. Since the corresponding singlet energy levels preclude singlet- singlet energy transfer and P-carotene has a low ionisation potential, they interpret their observations in terms of an electron transfer process : chlorophyll a (S,) + P-carotene (So) -+ chlorophyll a*- +,&carotene-+. Thus, following our observation of the reaction p-carotenea+ + chlorophyll a. -+ /?-carotene + chlorophyll am+ Beddard et nl.speculate that the overall result of such electron transfer processes is to produce a charge separated pair, chla*+. . . chla.0-, which may be involved in the reaction centre of photosystem I1 (PSII). It is well known that the reaction centres in both PSI and PSII involve chlorophyll a and not chlorophyll b. Thus it is interesting that our results on the failure of p- carotene-+ to react with chlorophyll b preclude the formation of a chlb-+ . . . chlb*- charge separated pair and thus may offer an explanation for chlorophyll b not being involved in the primary process of PSII.Note added in proof.- One could also speculate that, after the eletron transfer processes described above have occurred, a " sandwich " type system chla*-. . . p-carotene . . chla*+ is produced, the carotene molecule preventing immediate charge recombination. Similar " sandwich " type systems can be envisaged involving an associated chlorophyll a radical cation.8 We thank the Cancer Research Campaign, the Medical Research Council and the S.R.C. for support. J. L. acknowledges a S.R.C. research studentship. J. LafTerty, E. J. Land and T. G. Truscott, J.C.S. Chem. Comm., 1976, 70. * F. P. Zscheile and C. L. Comar, Botan. Gas., 1941, 102,463. J. P. Keene, J. Sci. Instr., 1964, 41, 493. E. A. Dawe and E. J. Land, J.C.S. Faraday I, 1975, 71,2162. J . Lafferty, A. C. Roach, R. S. Sinclair, T. G. Truscott and E. J. Land, J.C.S. Faradayl, 1977, 73, 41 6. H. Seki, S. Arai, T. Shida and M. Imamura, J. Amer. Chem. Soc., 1973, 95, 3404. G. S. Beddard, R. S. Davidson and K. R. Trethewey, Nature, 1977, 267, 373. D. Holten and M. W. Windsor, Ann. Rev. Biophys. Bioeng., 1978,7,189. (PAPER 8/398)

ISSN:0300-9599    

DOI:10.1039/F19787402760

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Luminescence and Other Spectroscopic Studies of the Reaction of Pyridine and Oxygen with Thermally Activated SrO BY SALVATORE COLUCCIA,~ JEAN F. HEMIDY $ AND ANTHONY J. TENCH* Chemistry Division, A.E.R.E., Hanvell, Oxfordshire OX1 1 ORA Received 16th March, 1978 The reaction of pyridine and O2 on SrQ powder has been studied using several different spectro- scopic techniques to characterise the intermediate and the final products. Reflectance spectroscopy has been used to demonstrate the formation of the 4,4'-bipyridyl negative ion when pyridine is adsorbed on SrO. Subsequent adsorption of oxygen leads to the formation of the 0; ion by a one-to-one electron transfer process from the organic ion and photoluminescence studies confirm that the 4,4'-bipyridyl molecule is also formed on the surface when the electron transfer occurs.During recent years there has been considerable interest in electron transfer at oxide surfaces because of its close relation with various aspects of catalysis. Under normal conditions, there is little evidence that the alkaline earth oxides, when prepared in vacuu, will transfer electrons to oxygen without preirradiation. However, Iizuka 1 and Che et aL3 have reported formation of 0 2 on MgO and CaO when oxygen has been adsorbed on a surface which has previously been pretreated with pyridine (Py) or bipyridyls (Bpy) and recently it has been shown that, under similar conditions, 0; can be formed on Sr0.4 Up to now attention has been devoted mainly to the e.s.r. signal of the oxygen species. In this work we have extended the study of the reaction on SrO using optical absorption and luminescence techniques to characterise the intermediate organic species both in the anionic (before contact with 0,) and neutral (after contact with 0,) forms.Using e.s.r. we have shown that there is a one-to-one electron transfer from the organic anion to the adsorbed oxygen. EXPERIMENTAL Strontium oxide was prepared by thermal decomposition of strontium carbonate (Hopkins and Williams G.P.R. or Johnson Matthey Spec-pure) to give a polycrystalline sample with a specific surface area of N 10 m2 g-1.5 During the decomposition a zeolite trap at 77K was used to remove water vapour and carbon dioxide. The samples were sealed off after evacuation at a temperature of 1200 K to a pressure of < 1.5 x Pa Torr).The samples were prepared in a silica bulb with a break seal ; a side arm and a rectangular silica cell allowed the same sample to be studied by electron spin resonance, reflectance spectroscopy and photoluminescence. Pyridine (B.D.H.) was purified by the freeze-pump-thaw technique and was adsorbed on the sample under its room temperature vapour pressure. Spec-pure oxygen was used without further purification and adsorption was carried out under a pressure of 0.75 kPa (5 Torr). 4-4' Bpy (B.D.H.) and 2-2' Bpy (Hopkins and Williams) were used without further purification and adsorption on the sample was carried out by exposure to the sublimation pressure at 300 K ; the adsorption process was considered complete when the whole sample presented a homogeneous blue colour. t On leave from University of Turin.§ On leave from University of Caen. 27632764 E.s.r. spectra were recorded at 300 and 77 K using a Varian V4502 spectrometer with 100 kHz field modulation. Spin concentrations were calculated by direct double integration using an on-line computer system 6* and g values were measured relative to Cr3+ in MgO (g = 1.9797). The optical reflectance spectra were recorded on a Cary 14 spectrophotometer using magnesium carbonate as a reference.8 The photoluminescence spectra (at 300 and 77 K) were obtained using a 250 W Xenon lamp as an excitation source and the excitation wavelengths were selected using a double monochromator. An Ortec photon counting system was used to compare the photon pulse rates from two photomultipliers and the spectra were corrected for variations in excitation intensity with wavelength and time.A Corning 3-74 filter (cut-off at -400 pm) was used before the emission monochromator to eliminate scattered exciting light. REACTION OF PYRIDINE AND O2 WITH SrO RESULTS PHOTOLUMINESCENCE SPECTRA OF THE SYSTEM SrO+Py+O, The SrO samples prepared in vacuo were white and their luminescent spectra at 300 K showed a broad emission centred at about 475 nm [fig. l(u)J and a correspond- ing excitation spectrum with two maxima at 280 and 315 nm [fig. I@)]. The samples turned blue-violet on adsorption of pyridine and only very weak photoluminescence nm FIG. 1 .-Photoluminescence spectra of the SrO + Py + O2 system at 300 K : (Q) emission spectrum of SrO, Zx 1 ; (b) excitation spectrum of SrO, I x 1 ; (c) emission spectrum after adsorption of Py, Zx 250 ; ( d ) excitation spectrum after adsorption of Py, Zx 250.spectra were detected at 300 and 77 K with maxima for the emission at 475 nm [fig. l(c)J and for the excitation spectra at 280 and 315 nrn, [fig. l(d)]. Outgassing the excess Py at 300 K did not modify the spectra nor the colour of the sample. Admission of O2 at 300 K gave a colour change from blue-violet to yellow-pink and the emission spectrum excited at 280 nm showed a new maximum at 550 nm and [compare fig. 2 ( 4 and (b)] a slight increase of the intensity in the 400-450 nm region. The emission after the addition of oxygen [fig. 2(c)] measured at 77 K was much more intense than at 300 K and showed a structure of three main peaks at 429, 457 and 476 nm withS .COLUCCIA, J . F . HEMIDY AND A . J . TENCH 2765 shoulders at 450,490 and 515 nm ; the corresponding excitation spectrum [fig. 2(d)] had a maximum at 275 nm. Outgassing the excess of O2 at 300 K did not modify the spectra significantly. All the spectra of fig. 1 and 2 refer to the same sample and have been run in succession without moving the cell. The relative intensities of the spectra can be compared with reasonable confidence although the absolute intensities are not known. I------- nm nm FIG. 2.-Photoluminescence spectra of the SrO + Py+ O2 system. The sample is the same as for fig. 1 : (a) emission spectrum excited at 280 nm at 300 K after adsorption of Py, I x 250 ; (b) emission spectrum excited at 280 nm at 300 K after adsorption of O2 on preadsorbed Py, Zx 250 ; (c) as (b), but at 77 M, I x 12.5 ; (d) excitation spectrum of (c) Zx 12.5.nm FIG. 3.-Photoluminescence spectra of the SrO + Bpy + O2 system at 77 K : (a) emission spectra after adsorption of O2 on preadsorbed 4,4‘-Bpy; (b) excitation spectrum of (a); (c) emission spectrum after adsorption of O2 on preadsorbed 2,2’-Bpy ; (d) excitation spectrum of (c).2766 REACTION OF PYRIDINE AND Q2 WITH SrO PHOTOLUMINESCENCE SPECTRA OF THE SYSTEM SrOf BIPYR1DYLSfo2 Adsorption of either 2,2‘-Bpy or 4,4’-Bpy on the surface of SrO turned the samples blue and quenched the original emission. The blue colour was destroyed by exposure to O2 (5 Torr) and the samples turned yellow-pink. The photoluminescent spectra at 77 K were recorded after contact with 0, (fig. 3), for comparison with the equivalent spectra obtained when Py reacts with 0, on SrO.The emission spectrum excited at 276 nm when preadsorbed 4,4’-Bpy was contacted with O2 [fig. 3(a)] showed peaks at 440, 470 and 490 nm and shoulders at 459 and 51 5 nm, while the corresponding excitation spectrum has a maximum at 276nm. The emission spectra excited at 280 nm when preadsorbed 2,2’-Bpy was contacted with 0, on SrO [fig. 3(c)] showed a sharp peak at 490nm and a broad complex peak at 520nm with a shoulder at 560 nm. In addition it had a peak at 418 nm, whose real maximum could be at shorter wavelengths ; it is likely that we observed only the tail of this peak because of the filter. The excitation spectrum showed a maximum at 280 nin [fig 3jd)J.m Y .- El ,.-- aoo 600 400 nm FIG. 4.--Kubelka-Munk calculation on the reflectance spectra of SrO : (a) adsorption of pyridine ; (b) adsorption of pyridine followed by adsorption of 02. The photoluminescent spectra of the bipyridyls as mol dm-3 solutions in C2H50H have been run at 77 K for comparison with the earlier data. Under these conditions the 2,2‘-Bpy has an excitation maximum at 280 nm and an emission spectrum with bands at 427, 456 and 476 nm together with shoulders at 450, 490 and 515 nm. The 4,4’-Bpy has an excitation maximum at 275 nm and an emission spectrum with bands at 432 and 456 nm together with shoulders at 475 and 490 nm. REFLECTANCE SPECTRA OF THE SYSTEM SrO+Py+O, admission of O2 [fig. 4@)] have been transformed using the Kubelka-Munk function The reflectance spectra of SrO contacted with Py both before [fig.4(a)] and afterS . COLUCCIA, J . F . HEMIDY AND A . J . TENCH 2767 to give an absorbance scale. The spectrum before oxygen admission showed maxima at 254, 395 and 595 nm. After oxygen admission the two bands at 395 and 595 nm disappeared, the intensity of the band at shortest wavelength increased and the maxi- mum moved to 260 nm ; in addition a new band at 475 nm was evident. E . S . R . SPECTRA OF THE SYSTEM SrO+Py+O, The e.s.r. spectrum of SrO after adsorption of pyridine was a single isotropic line at g = 2.003 with a line-width which varied from 9-14 G depending on the sample. After adsorption of oxygen, the e.s.r. line disappeared and was replaced by a new spectrum (fig.4). The line shape of this new spectrum is characteristic of a species in an orthorhombic crystal field and at 77 K the principal g values were g, = 2.002, g, = 2.007 and g3 = 2.100. FIG. 5.43.s.r. spectrum at 77 K of a SrO sample treated with pyridine and then exposed to contact with 0 2 . Measurements of the spin concentration before and after the adsorption of oxygen indicated that the two paramagnetic species were present at essentially identical concentrations ("7 x 1017 spins g-I). From this evidence it is likely that an electron transfer reaction has occurred between the pyridine-type radicals and the oxygen molecules. DISCUSSION ADSORBED SPECIES The absorption and subsequent emission of light from SrO powders is very similar to that observed for high surface area MgO where it has been interpreted as arising from lattice ions on the surface in positions of unusually low coordination.A more extensive investigation has confirmed that this interpretation applies to SrO.ll Adsorption of Py does not significantly change the shape of the emission and excitation spectra of SrO but the intensities are much decreased. The remaining emission probably originates from a very small fraction of unreacted surface sites, but there is also the possibility that it originates from new surface species formed upon adsorption of Py. The emission spectrum of the Py sample is very weak and structureless even at 77 K. After admission of O2 on preadsorbed Py, the emission spectrum at 300 K is still very weak, but more complex. In contrast to the behaviour of the Py sample, lowering the temperature to 77 K affects both the intensity and the shape of the emission spectrum, increasing the intensity in the 450 nm region by 20 times.The very well defined structure, when compared with the data for bipyridyls2768 REACTION OF PYRIDINE AND O2 WITH SrO in frozen solutions and also adsorbed on the surface (fig. 3), strongly suggests that this emission is due to bipyridyl species formed on the surface. The emission spectra of 2,2’- and 4,4‘-Bpy in solution agree well with those reported by Gondo l2 for the free bipyridyls in different solvents and by other authors both for the free l 3 and the chelated 1 3 9 l4 2,2’-Bpy molecule. The clear similarities of the spectrum of fig.2 with the spectra of the free bipyridyls indicate that the product of the interaction of Py with O2 and SrO surface is a bipyridyl species. However, as the emission spectra of the solutions are very similar it would be difficult, on this basis alone, to decide which of the two bipyridyls is formed on the surface. But the emission spectra of the two bipyridyls are quite different when O2 is admitted to the molecules adsorbed on the surface of SrO, (fig. 3). The spectrum of 4,4‘- bipyridyl is very similar to the Py + 0, + SrO spectrum, both in the overall shape and in the spacings of the vibronic structure; in addition the excitation maximum gives better agreement with the maximum found for adsorbed 4,4’-Bpy (275 nm) than for 2,2’-Bpy (280 nm). This evidence leads us to conclude that the product of the inter- action of Py with 0, on SrO is 4,4’-Bpy.A full discussion of the emission spectra of the adsorbed bipyridyls is beyond the scope of this paper, but the difference in the spectra of the two molecules when adsorbed needs some comment. From fig. 3 it is clear that the spectrum of 4,4’-Bpy adsorbed on the surface is very similar to the spectrum of the frozen molecule; only a slight red shift and some variation in the relative intensities of the peaks is evident. In contrast, the spectrum of the adsorbed 2,2’-Bpy is quite different from that of the frozen molecules. This difference could be due to the different way in which the two molecules are likely t o interact with the surface sites. 2,2’-Bpy should be able to form a chelate species with both the nitrogen atoms of the two aromatic nuclei interacting with the coordinatively unsaturated surface cations, whilst the 4,4’-Bpy molecule can probably interact only with one nitrogen atom.The much stronger interaction of 2,2’-bipyridyl accounts for the large variation in the vibronic freedom of the molecule with respect to the free molecules, as has been observed in the i.r. spectra of complexe~.~’ The formation of 4,4’-Bpy by reduction of Py with alkali metals and subsequent oxidation has already been described 16-19 as well as the occurrence of the same reaction on the surfaces of alkaline earth oxides,2 but this is the first direct observation of the final product (neutral 4,4’-Bpy) on the surface. The formation of (4,4’-Bpy}- intermediate product has been suggested ; l 7 this negative species is characterised by two absorption bands at 380 and 580 nm.’ ‘ 9 For pyridine adsorbed on SrO we have found very similar values of 395 and 595 nm (fig.4) and this together with the initial strong e.s.r. signal indicates that the bipyridyl anion radical is formed on the surface of SrO. The e.s.r. spectrum does not show any hyperfine splitting and this can be explained by the large number of lines expected from so many nuclei with spin. The he-width variation from one sample to another probably indicates local concentration effects. In addition to the two bands of the anionic species, the reflectance spectrum of the Py+SrO samples show a weaker absorption of 254 nm that could well be due to an excess of Py still present on the surface after the short outgassing at 295 K.The two bands assigned to (4,4‘-Bpy}- disappear when O2 is admitted to the sample showing that an electron transfer process is taking pIace; the band at short wavelengths (260 nm) shows an enhanced intensity and this could be explained by the presence of the newly formed species on the surface, such as neutral 4,4’-Bpy and 0; which absorb in this region.2o* 21 After the adsorption of oxygen a reflectance peak at 475 nm is observed which is weak compared to the intensities of the 395 and 595 nm bands. It does not have the same thermal stability as the e.s.r. signal attributed toS . COLUCCIA, J . F. HEMIDY AND A . J . TENCH 2769 0 2 and may be associated with the formation of small amounts of polypyridyl species since similar bands have been observed in pyridine reduced by alkali metals.’ The e.s.r.spectra show that on adsorption of oxygen the organic radical species disappears and a new signal develops (fig. 5) with a g tensor very similar to that reported in literature for 0 2 22 and in particular for the alkaline earth oxides + Py + 0, systems, where I7O has been used to confirm the presence of OF, 3 p indicating that an electron transfer process takes place between the organic radical and O2 to form 0 2 adsorbed on the surface. 0 2 has been characterised in alkali halides 2o and in sodalite 21 systems by its emission spectra. In all these systems at 77 K a series of bands is observed centred at N 550 nm and extending from 400 to 650 nm, with a spacing be-tween the main peaks of - 1000 cm-l ; excitation maxima are quoted in the range 250 2o and 300 nm.21 The multiple band structure has been interpreted in terms of coupling with the vibration levels of the molecular ion.21 It is possible that there is some contribution from 0 2 species to the emission spectra seen in the O2 +Py+ SrO system, in particular the spectra at 300 and 77 K of fig.2. The spectrum at 77 K is clearly dominated by the emission in the 400-500 nm region and this has been assigned to 4,4’-Bpy ; no fine strlncture is observed at longest wavelengths where bands from the 0; spectrum would be expected. In contrast, the main feature of the spectrum at 300 K is a broad and complex emission peak centred at 550 nm and excited at 280 nm, which develops after admission of 02.A very similar spectrum at 300 K has been described for 0; in bromosodalite.21 It is possible that any contribution due to 0; adsorbed on SrO in the spectrum at 77 K [fig. 2(c)J, is hidden by the strongly temperature dependent emission of 4,4’-Bpy co-adsorbed on the surface. In addition energy transfer between different adsorbed species could strongly decrease the emission efficiency of OF. SURFACE SITES AND REACTION MECHANISM In the previous section it has been shown that adsorption of Py on SrO leads to the formation of the 4,4’-Bpy anion which can transfer an electron when 0, is adsorbed to give neutral 4,4’-Bpy and OF. The oxidation reaction to give the neutral molecule is straightforward. The formation of {2,2’-Bpy*}- and (4,4’-Bpy*}- when the bipyridyls are directly absorbed on the surface of SrO is indicated by the appearance of a blue colour which has been shown to occur on MgO as well using e.s.r.This demonstrates that the formation of the negative radical anions is not necessarily linked to a dehydrogenation stage and that electron donor sites exist on the surface. The intrinsic emission of the SrO is quenched when the Bpy and pyridine are adsorbed to form the blue anions, suggesting that the surface sites responsible for photoluminescence are involved in the electron transfer process. These sites have been described as ions on the surface in situations of unusually low coordina- tion.lo9 11* 23 We suggest that 02- ions on the surface in sites of low coordination have higher reactivity which is strongly enhanced by the lowering of the Madelung constant and are likely to be the electron donor sites on the surface.It must be stressed that the spin concentration quoted before shows that about 0.1 radical species are formed per 100A2, indicating that only the least stable 02- ions are able to take part in the electron transfer. Both the negative ions and the neutral molecules of Bpy are likely to be adsorbed on positive charged centres, i.e. on the cation Sr*+.2770 J. F. Hemidy acknowledges financial support from a European Exchange Fellow- ship of the Royal Society and provision of facilities by A.E.R.E., Harwell. S. Coluccia and A. J. Tench acknowledge financial support by NATO. The authors thank A. M. Deane for advice on experimental techniques and Prof.F. S. Stone for discussions. REACTION OF PYRIDINE AND O2 WITH SrO ' T. lizuka, Chem. Letters (Jupan), 1973, 891. T. Iizuka and K. Tanabe, Bull. Chem. Sue. Jupan, 1975,48,2527. M . Che, A. J. Tench, S. Coluccia and A. Zecchina, J.C.S. Faraduy I, 1976, 72, 1553. M. Che, S. Coluccia and A. Zecchina, J.C.S. Faraduy I, 1978, 74, 1324. R. L. Nelson, A. J. Tench and B. J. Harmsworth, Truns. Faraduy SOC., 1967, 63, 1427. A. M. Deane, C. Kenward and A. J. Tench, A.E.R.E. Report 7020. ' J. F. J. Kibblewhite and A. J. Tench, J.C.S. Faraduy I, 1974, '90, 72. * R. L. Nelson, J. W. Hale and B. J. Harmsworth, Trans. Furuduy Suc., 1971, 67,1164. G. Kortum, Reflexions Spektroscopie (Springer, Berlin, 1969). vol. 1, p. 171 ; A. J. Tench and G. T. Pott, Chem. Phys. Letters, 1974, 26, 590. lo S. Coluccia, A. M. Deane and A. J. Tench, Proc. Sixth Int. Congr. Cutulysis (London, 1976), ' ' S. Coluccia, A. M. Deane and A. J. Tench, J.C.S. Faraaizy I, in press. l 2 Y. Gondo and Y. Kaude, Bull. Chem. Soc. Japan, 1965,38,1187. l3 M . D. De Armond and J. E. Hillis, J. Chem. Phys., 1971,54,2247. l4 D. H. W. Carsteurs and G. A. Crosby, J. Mol. Spectr., 1970,34, 113. l5 J. S. Struckle and J. L. Walter, Spectrochim. Acta A, 1971, 27, 223. l6 R. L. Ward, J . Amer. Chem. Soc., 1961, 83, 3623. l7 A. Carrington and J. Dos. Santos Viega, Mol. Phys., 1962, 5,21. l 8 (a) J. W. Dodd, F. J. Hopton and N. S. Hush,Proc. Chem. SOC., 1962,61; (6) C. D. Schmulback, l9 V. Kalyanaraman, C. N. R. Rao and M. V. George, J. Chem. SOC. B, 1971, 2406. 2o J. Rolfe, F. R. Lipsett and W. J. King, Phys. Reu., 1961, 123, 447. 21 S. K. Deb and J. B. Gallivan, J. Luminescence, 1972,5, 348. 2 2 J. H. Lunsford, Catalysis Rev,., 1973, 8, 135. 23 A. Zecchina and F. S. Stone, J.C.S. Faraduy I, 1976,72,2364. C. C. Hinckley and D. Wasmund, J. Amer. Chern. Sac., 1968, 90,668. (PAPER 8/485)

ISSN:0300-9599    

DOI:10.1039/F19787402763

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Aqueous Solutions Containing Amino Acids and Peptides Part 5.-Gibbs Free Energy of Interaction of Glycine with some Alkali Metal Chlorides at 298.15 K BY BARRY P. KELLEY? AND TERENCE H. LILLEY* Chemistry Department, The University, Sheffield S3 7HF Received 17th March, 1978 Cells with transference have been used to investigate the free energy of interaction of glycine with LiCI, NaCl and CsCl in aqueous soIutions at 298.15 K. The experimental data were analysed to give the Lewis-Randall free energy coefficients which represent pairwise interactions between the salt ions and the amino acid. The Lewis-Randall coefficients were transformed to the McMillan-Mayer scale and these were then deconvoluted in an approximate manner to give the contributions arising from excluded volume (hard-sphere), electrostatic (Kirkwood) and solvent reorganisation effects. The excluded volume and solvent reorganisation contributions are found to be approximately equal in magnitude but opposite in sign, so that the frequently used Kirkwood electrostatic model represents the nett interactions well.This is part of a continuing series of investigations on the thermodynamic behaviour of amino acids and peptides in aqueous solutions. Our principal aim is to obtain information on pairwise intermolecular interactions between solute mole- cules in such systems and in this paper we present results for the Gibbs free energy of interaction of lithium, sodium and caesium chlorides with glycine in water at 298.15 K, from cells with transference. EXPERIMENTAL The cells used were similar in design to those used by Covington and Prue.2 E.m.f.measurements were made using a Fenlow digital voltmeter with a resolution of 1 pV. The digital voltmeter was frequently checked and calibrated against a certified Weston standard cell. Silver-silver chloride electrodes were of the thermal electrolytic type and were pre- pared using previously described procedure^.^ Pairs of electrodes were selected which had bias potentials <3OpV. The procedure used to remove uncertainties arising from bias potentials was similar to that described ear lie^.^ All measurements were made on cells immersed in a water bath thermostatted at (298.15k0.01) K. The water used in the preparation of solutions was obtained by passage of laboratory distilled water through a mixed bed ion exchange column.UltraR grade sodium chloride was used without further purification after drying at 470 K for 48 h. The purification methods for lithium chloride and caesium chloride have been described elsewhere.’ AnalaR grade glycine was recrystallised twice from methanol+ water mixtures and dried at 320 K under vacuum. The apparatus used in the present investigation has been checked using aqueous sodium chloride solutions and used to obtain activity coefficients of LiCI6 and CsC17 in water. METHOD AND RESULTS Cells with transference have been used fairly extensively in the past to obtain information on either ionic transport numbers or activity coefficients.2* 4 9 * Their i Present address : Chloride Technical Ltd, Swinton, Manchester M27 2HB.27712772 AQUEOUS SOLUTIONS OF AMINO ACIDS use may be extended solutions containing added non-electrolyte. to the determination of activity coefficients of electrolytes in The cells used in the present investigation may be represented by : Ag I AgCl I MCl(m) 11 MCl(m), glycine(m,) I AgCl I Ag I I1 in which the aqueous phase I, containing alkali metal chloride (MCl) at molality m, is separated from aqueous phase 11, containing MCI at the same molality and glycine at molality mi, by a liquid junction. The e.m.f. (E) of such a cell is given by '9 lo f 11 l-11 E = -2k' J t d In (my)-RT J ti d In (miyi) I I where k' = RT/F, t is the cationic transference number, ti is a term allowing for the transference of the non-electrolyte across the liquid junction and y and yi are respec- tively the electrolyte mean ionic activity coefficient and the activity coefficient of the non-electrolyte.At present there is no experimental information available on the magnitude of ti but theoretical predictions based on an incompressible ion- dielectric continuum model indicate that the contribution to the e.m.f. from the second integral on the right hand side of eqn (1) is negligible compared with our experi- mental precision. It is worth pointing out however that it is possible to use Feakins' procedure l2 to measure the Washburn number (which is closely related to ti) and we have recently initiated a collaborative series of experiments with Prof. Feakins to investigate the theoretical predictions. For the present we shall use the information available and ignore the second term on the right hand side of eqn (1).Using this approximation eqn (1) may be rearranged to give In ( y / y o ) = -(E/2k'to)+(l/2k't:) At dE J o in which y and yo are respectively the mean ionic activity coefficients of the salt in the presence and absence of glycine and a term At is defined as where t and to are the cationic transference numbers in the presence and absence of the non-electrolyte. No experimental information is available on the rnolality dependence of At but unpublished observations indicate that the effect of added amino acid 011 the cationic transference number can be represented by where a is a constant characteristic of a given electrolyte-non-electrolyte system. If we now make the usual assumption that the logarithm of the activity coefficient of the salt may be expressed as a power series in mt and mi, i.e.( 5 ) then since the first term on the right hand side of eqn (2) is dominant, from eqn (2), (4) and (5) we have, At = t-to (3) At = ami (4) In ( y / y o ) = Ami + Bmtmi + Cmf + Dm mi + . . . At dE = ami dE rr -2k'toami d In ( y / y o ) = -2k'to~mi(A+Bmt+2Cmi+ Dm) dmi. (6) (7) Combination of this with eqn (2) and ( 5 ) yields, after slight rearrangement, in which F = Aa/2, G = &/2, H = Da/2 and .I = 2Ca/3. - E = 2k'mi { to ( A + Bm* -k Dm) + mi (Ct, -+ F -I- Gm3 % Hm + Jmi) 1B . P . KELLEY AND T . H . LILLEY 2773 Before this expression can be used in the analysis of experimental results, it is necessary to have values of the transference numbers of the cations at each molality of salt used in the experiments.The transference numbers of lithium l3 and sodium l4 ions in chloride solutions were obtained from published experimental data. The transference number of caesium ion in caesium chloride solutions has not been determined experimentally and it was necessary to estimate this. According to one formulation of the interionic attraction theory the cationic transference number in a solution of a 1 : 1 electrolyte is given by and the limiting (infinite dilution) transference number is where the symbols have their previously l 5 defined meanings. If t $ is 0.5 exactly then it is predicted that to does not vary with salt concentration. For potassium chloride in water at 25 "C, t $ = 0.4905 and the agreement between experimental values and those calculated from eqn (8) is excellent.The limiting transference number of the caesium ion in caesium chloride solutions is 0.5030 l6 and to predict its concentration dependence we have assumed an ion size parameter of 0.35 nm, although the concentration dependence is so small that any physically realistic value would be suitable. The transference numbers were then calculated from eqn (8) using appropriate density data to perform the conversion from the molar to the molal scale. The experimental results obtained are given in table 1 and the results for each system were fitted to eqn (7) by a least squares program. This program was such that if the 95 % confidence limit of a coefficient was greater than its numerical value, then the program reanalysed the data with this coefficient excluded.This procedure continued until all remaining coefficients had 95 % confidence limits less than their numerical values. It was found in the present investigation that the only coefficients required to fit the data were A , B, C and sometimes D. Table 2 represents the coefficients and their 95 % confidence limits. TABLE 1 .-EXPERIMENTAL E.M.F. VALUES FOR THE SYSTEM ALKALI METAL CHLORIDE+ GLYCINE AT 298.15 K LiCl + glyciiie mlmd kg-1 rn,/mol kg- 1 t 0 EImV AElmV 0.010 024 0.009 7445 0.022 478 0.022 837 0.039 998 0.039 965 0.062 408 0.089 848 0.099 788 0.301 55 0.490 63 0.302 98 0.502 39 0.100 12 0.302 69 0.492 47 0.100 26 0.504 23 0.100 46 0.296 47 0.452 80 0.099 994 0.488 51 0.300 44 0.506 11 0.3290 0.3291 0.3255 0.3255 0.3225 0.3225 0.3198 0.3175 0.331 0.928 1.427 0.945 1.514 0.302 0.850 1.327 0.289 1.354 0.273 0.754 1.128 0.246 1.098 0.657 1.022 - 0.01 1 O.OO0 0.022 - 0.01 1 - 0.03 1 - 0.005 0.007 0.007 0.008 0.005 om1 0.01 6 0.003 0.003 O.OO0 - 0.01 3 - 0.0032774 AQUEOUS SOLUTIONS OF AMINO ACIDS m/mol kg-I 0.010 127 0.023 942 0.040 117 0,062 187 0.090 594 TABLE 1 .-cont.mijmol kg-' fo 0.100 00 0.207 15 0.293 46 0.395 26 0.501 92 0.101 42 0.201 23 0.302 84 0.398 24 0.510 57 0.105 12 0.205 41 0.294 47 0.401 82 0.504 42 0,099 54 0.198 79 0.297 38 0.406 60 0.505 04 0.100 39 0.198 95 0.304 67 0.402 27 0.511 98 0.010 080 0.101 27 0.305 44 0.506 64 0.022 824 0.302 62 0.512 82 0.022 908 0.102 03 0.206 04 0.414 77 0.040 668 0.100 40 0.303 22 0.506 02 0.062 475 0.112 46 0.307 03 0.527 98 0.090 923 0.104 40 0.511 25 NaCl + glycine 0.391 8 0.3898 0.3883 0.3869 0.3857 CsCl + glycine 0.503 1 0,503 1 0.503 1 0.5032 0.5032 0.5033 E/mV 0.401 0.802 1.086 1.470 1.803 0.363 0.699 1.043 1.309 1.621 0.346 0.674 0.928 1.21 1 1.441 0.299 0.583 0.855 1.135 1.350 0.280 0.548 0.812 1.050 1.287 0.552 1.400 2.183 1.251 1.958 0.439 0.882 1.656 0.410 1.113 1.763 0.401 1.03 I 1.681 0.310 1.462 AE/mV - 0.003 0.001 0.027 - 0.01 1 -0.003 0.000 0.001 - 0.020 0.000 0.000 0.001 -0.017 -0.012 - 0.003 0.025 0.007 0.008 0.001 - 0.009 0.001 0.01 1 0.009 0.010 - 0.003 -0.010 - 0.064 - 0.004 - 0.004 0.007 0.023 0.006 - 0.009 - 0.005 - 0.01 1 0.017 - 0.003 0.008 0.012 - 0.029 0.041 0.001 TABLE 2.-cOEFFICIENTS OF EQN (7) FOR ALKALI METAL CHLORIDE+ GLYCINE SYSTEMS system A/kg mol-1 B/kg* mol-3 C/kgz mol-2 Dlkgz mol-2 LiCl+ glycine - 0.2191 0.006 0.254+ 0.01 6 0.0391 0.01 2 - KCl+ glycine* - 0.202+0.003 0.216f 0.009 0.034+ 0.004 - NaCl+glycine -0.2481 0.007 0.520+0.070 0.049* 0.007 -0.652kO.173 CsCl+ glycine - 0.232f0.017 0.45410.160 0.047+0.015 -0.43240.397 * Obtained from the data of Roberts and Kirkw~od.~B .P. KELLEY AND T. H . LILLEY 2775 Included in this table are the results we obtained from a reanalysis of the KClf glycine ~ystem,~ using only those experimental points for which the molality of the glycine was <0.5 mol kg-l and the molality of the salt was <0.1 mol kg-l. The final column of table 1 gives the difference (AE) between the experimental e.m.f. values and those calculated using the appropriate least squares parameters.COMPARISON WITH OTHER WORK It can be seen from table 2 that the A coeficients, which represent painvise inter- actions between the ions and the glycine, are all negative, indicating an attractive interaction between the electrolytes and glycine. This is what one would intuitively expect and is confirmed by other studies mentioned below. The sodium chloride + glycine system has been previously studied by a number of workers. Joseph l7 obtained activity coefficients for sodium chloride in glycine solutions, from cells without transference, utilising sodium amalgam and silver-silver chloride electrodes. Most of the work described was performed at 274.6 K and reanalysis of his data using our procedure gave a value for A of - 0.30 +O.10 kg mol-l. The rather large error arises because the e.m.f. values were only measured to a pre- cision of 1 mV. The few measurements at 298.15 I< reported by Joseph were obtained on solutions of such high molalities to preclude comparison with the present work. Scatchard and Prentiss * investigated the sodium chloride + glycine system using the freezing temperature depression technique and the value they quote of -0.327 kg mol-1 for the pairwise interaction term agrees favourably with the value obtained by Joseph. More recently Phang and Steele obtained activity coefficients of sodium chloride in mixtures with glycine by the use of sodium responsive glass and silver-silver chloride electrodes in cells without transference. The experimental results, as analysed by them, for the more dilute solutions investigated gave a value for A of -0.221 kg mol-1 at 298.15 K.No error limits were given for the various coefficients and it would have been interesting to reanalyse their data using our procedure but this was not possible, since the experimental e.m.f. data were not presented. There must be some uncertainties in the presented coefficients since, when an extended form of eqn ( 5 ) was used in order to allow the activity coefficients in the more concentrated solutions investigated to be included in the data analysis, the value obtained for the leading term was -0.203 kg mol-l. The result obtained at 0 "C of -0.252 kg mol-1 is at variance with that obtained from precise freezing temperature measurements. The sodium chloride + glycine system has also been investigated over a wide molality range by Schrier and Robinson,20 using the isopiestic vapour pressure technique.A polynomial expansion in solute molalities, [analogous to eqn (5)] containing ten coefficients, was necessary to represent the osmotic coefficient data. The value obtained for the A coefficient was -0.146 kg mol-l, which is in marked contrast to the value of -0.248 kg mol-l obtained from the present work. The large discrepancy almost certainly arises because of the relatively high molalities used in the isopiestic studies. It seems certain that the present result is the more nearly correct, since it was obtained from experiments in a molality range in which the A term was dominant, whereas in the molality range covered by the isopiestic experi- ments, very large contributions to the osmotic coefficient from triplet and higher order solute-solute interactions would be present.The discrepancy between results obtained from isopiestic measurements and those obtained from cells with transference is also apparent on comparing results obtained using the two methods for the KCl+glycine system. The value obtained for the leading term from cells is -0.202 kg mol-' and this contrasts with the value of 1-882776 AQUEOUS SOLUTIONS OF AMINO ACIDS - 0.130 kg mol-1 obtained from an isopiestic investigation.21 The KCl + glycine system has also been studied using a precise freezing temperature method and the value obtained for the A coefficient at the freezing temperature was - 0.21 8 & 0.008 kg mol-? The freezing temperature method was also used for the LiCl + glycine and CsCl + glycine systems and the respective values obtainedforthe A term were -0.158 kO.010 and -0.127+_0.006 kg mol-l.Mixtures of glycine with LiCl, NaCl and KCl were investigated many years ago 22 using solubility measurements but since these were performed in necessarily rather concentrated solutions, it is not possible to get a meaningful comparison between these and the present work. DISCUSSION The leading term in eqn (9, as stated above, arises from pairwise interactions between the ions and the non-electrolyte. In terms of our earlier nomenclature 5 * 23 we may identify A with ( g M i + g X i ) . It is apparent from the information given in table 2 that : (a) the ( g M i + g x i ) values are all negative indicating an attractive interaction between the electrolyte and the non-electrolyte and (b) these values change little with increasing cation size.TABLE 3.-cONTRIBUTIONS TO EQN (1 2) L(B&+ B&)/cm3 mol-1 experimental hard sphere Kirkwood solvent-reorganisation LiCI - 336+ 12 382 - 348 - 340 NaCl - 394f 14 386 - 360 - 420 KCI -292+ 16 432 - 350 - 373 CsCl - 340k 34 483 - 340 - 482 When trying to relate the experimental information to solute-solute inter- molecular potentials it is necessary to transpose the painvise interaction coefficients from the Lewis-Randall to the McMillan-Mayer (MM) These are related by : 5 9 2 3 The transposition from one scale to the other was performed using appropriate volumetric data " 9 2 8 and the values obtained for the MM coefficients are given in L(BGi+B&) = 2(~,i+g,i)F/,OM~~ + V L , + ~ V : - ~ R T K .(10) table 3. These second virial coefficients in eqn (10) are of the form BLi = 4n (1 -exp (- WMi/kT))r2 dr s," for spherically symmetric species. The intermolecular potential interaction of an ion and a non-electrolyte like glycine may be function for the considered to be composed of three contributions. These are : (a) short-range repulsive interactions, (b) long-range interactions arising from electrostatic ion-dipole effects, and (c) a solvent-reorganisation contribution. In order to proceed further, approximations must be made about these contribu- tions to WMi and Wxi. The electrostatic part of the intermolecular potential function is assumed to be given by the treatment of Kirkwood 29 and the short-range repulsive effects may be approximated by " hard-body " ' 9 23* 27 interactions between theB.P . KELLEY AND T. H. LILLEY 2717 ions and the non-electrolyte. The difference between the real intermolecular potential function and the contributions from these two calculable components is ascribed to solvent reorganisation effects or alternatively co-sphere-co-sphere or Gurney effects. 30 It should be recognised that the solvent reorganisation contributions must also include contributions arising from deficiencies in the approximations used to obtain the electrostatic and repulsive components. We may thus write where the subscripts refer to hard-sphere (HS), Kirkwood or electrostatic (K) and solvent reorganisation (SR) respectively.The first two terms on the r.h.s. of eqn (12) were estimated as b e f ~ r e , ~ by treating the ions as spheres of known radius and assuming that glycine could be represented as a sphere with a point dipole at its centre. The dipole moment of the glycine was taken to be 13.5 Debye.20 The values obtained for the three contributions to the virial coefficients derived from the experimental results are included in table 3. The most striking feature is that the hard sphere (excluded volume) term and the solvent reorganisation term are, for each system investigated, approximately equal but of opposite sign. This leads to a close correspondence between the experimental quality and the electrostatic contribution. This can only be fortuitous but it does explain the apparent success of earlier treatments 9* 18-21* 31 for the free energy of interaction of amino acids with salts.It should be pointed out that the same happy coincidence does not apply for the enthalpic and entropic contributions to ion-amino acid interactions.l* 3 2 9 3 3 (Bzi + BzJ = (B&i + B&)HS + (B&i + Bgi), + (BLi + B,*i)sR (12) We thank the S.R.C. for the award of a studentship to one of us (B. P. K.) and R. H. Wood for discussions. Addendum. Preliminary measurements from Prof. Feakins laboratory 34 indicate that the neglect of the second term on the r.h.s. of eqn. (1) is justified for the systems investigated. B. P. Kelley and T. H. Lilley, J. Chem. Thermodynamics, 1978,10,703. A. K. Covington and J. E.Prue, J. Chem. SOC., 1955, 3701. C. C. Briggs, Thesis (University of Sheffield, 1973) ; C. C. Briggs and T. H. Lilley, J. Chem. Thermodynamics, 1974, 6, 599. A. S. Brown and D. A. MacInnes, J. Amer. Chem. SOC., 1935,57, 1356. T. H. Lilley and R. P. Scott, J.C.S. Faraday I, 1976,72, 197. B. P. Kelley and T. H. Lilley, J. Chem. Thermodynamics, 1977, 9, 99. (a) T. Shedlovsky and D. A. MacInnes, J. Amer. Chem. Soc., 1936,58,1970 ; (b) T. Shedlovsky and D. A. MacInnes, J. Amer. Chem. SOC., 1937,59,503 ; (c) W. J. Hornibrook, G. J. Janz and A. R. Gordon, J. Amer. Chem. SOC., 1942,64,513; (d)R. E. Verrall, J. Solution Chem., 1975, 4, 319 ; (e) J. C. Ku, Thesis (University of Pittsburgh, 1971). R. M. Roberts and J. G. Kirkwood, J. Amer. Chem. SOC., 1941,63,1373.lo G. Scatchard, J. Amer. Chem. Soc., 1953,75,2883. A. M. Squires, Thesis (Cornell University, 1947). l 2 (a) D. Feakins, J. Chem. SOC., 1961, 5308 ; (6) D. Feakins and J. P. Lorrimer, Chem. Comm., 1971,646 ; (c) D. Feakins and J. P. Lorimer, J.C.S. Faraday I, 1974,70,1888 ; (d) D. Feakins, K. H. Khoo, J. P. Lorrimer and P. J. Voice, J.C.S. Chem. Comm., 1972, 1336; (e) D. Feakins, K. H. Khoo, J. P. Lorrimer, D. A. O'Shaugnessy and P. J. Voice, J.C.S. Faraday I, 1976,72, 2661. ' B. P. Kelley, Thesis (University of Sheffield, 1977). l 3 L. G. Longsworth, J. Amer. Chem. SOC., 1932,54,2741. l4 (a) L. G. Longsworth, J. Amer. Chem. SOC., 1935, 57, 1185 ; (6) R. W. Allgood, D. J. LeRoy l 5 R. H. Stokes, J. Amer. Chem. SOC., 1954, 76, 1988. and A. R. Gordon, J. Chem.Phys., 1940,8,418 ; 1942, 10,124.2778 AQUEOUS SOLUTIONS OF AMINO ACIDS l6 R. A. Robinson and R. H. Stokes, Electrolyte Solutions (Butterworth, London, 2nd edn, 1970), p. 463. N. R. Joseph, J. Biol. Chem., 1935,111,489. l8 G. Scatchard and S. S . Prentiss, J. Amer. Chem. SOC., 1934,56,2314. l9 S. Phang and B. J. Steele, J. Chem. Thermodynamics, 1974, 6, 537. 2o E. E. Schrier and R. A. Robinson, J. Biol. Chem., 1971,246,2870. 21 V. E. Bower and R. A. Robinson, J. Res. Nat. Bur. Standards, 1965,69A, 131. 22 References to this work are given by E. J. Cohn, in Proteins, Amino-acids andPeptides, ed. E. J. Cohn and J. T. Edsall (Reinhold, New York, 1943), chap. 10. 23 T. H. Lilley and R. P. Scott, J.C.S. Faraday I, 1976, 72, 184. 24 W. G. McMillan and J. E. Mayer, J. Chem. Phys., 1945,13,276. 25 H. L. Friedman, J. Solution Chem., 1972, 1, 387, 413, 419. 26 R. H. Wood, T. H. LiHey and P. T. Thompson, J.C.S. Faraday I, 1978,74,1301. 27 J. J. Kozak, W. S. Knight and W. Kauzmann, J. Chem. Phys., 1968,48,675. 28 F. J. Millero, in Water and Aqueous Solutions: Structure, Thermodynamics and Transport Processes, ed. R. A. Horne (Wiley-Interscience, New York, 1972), chap. 13. 29 J. G. Kirkwood, Chem. Rev., 1939,24,233 ; Proteins, Amino-acids and Peptides, ed. E. J. Cohn and J. T. Edsall (Reinhold, New York, 1943), chap. 12. 30 P. S. Ramanathan and H. L. Friedman, J. Chem. Phys., 1971,54,1086. 31 C. C. Briggs, T. H. Lilley, J. Rutherford and S. Woodhead, J. Solution Chem., 1974,3, 649. 32 J. W. Larson and D. G. Morrison, J. Phys. Chem. 1976,80, 1449. 33 B. P. Kelley, T. H. Lilley, E. M. Moses and I. R. Tasker, to be published. 34 D. Feakins, personal communication. (PAPER 8/502)

ISSN:0300-9599    

DOI:10.1039/F19787402771

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Aqueous Solutions Containing Amino Acids and Peptides Part 8.-Gibbs Free Energy of Interaction of some a,u-Amino Acids with Sodium Chloride at 298.15 K BY BARRY P. KELLEY 7 AND TERENCE H. LILLEY * Chemistry Department, The University, Sheffield S3 7HF Received 5th April, 1978 Cells with transference have been used to obtain information on the free energy of interaction between sodium chloride and the amino acids jl-alanine, y-aminobutyric acid and c-aminocaproic acid. The results obtained are compared with those obtained for some a-amino acids. It is shown that, whereas with the a-acids there is little change in the pairwise interaction parameter as the hydrocarbon side chain is extended, for the a,w-acids the interaction with the ions of the salt becomes increasingly attractive as the homologous series is ascended.In the last few years we have been involved with some thermodynamic investigations on aqueous solutions containing amino acids and salts and have recently presented a study of the free energy of interaction of sodium chloride with some a-amino acids.' The present work is complementary to this in that we now present results to illustrate the effect of the separation of the charged amino and carboxylate groups on the extent of interaction of sodium chloride with some apamino acids. EXPERIMENTAL The general experimental arrangement used was the same as that previously described.2 The purification of water and sodium chloride was as before.2 The racemic amino acids were of the best quality commercially available and were recrystallised from methanol+ water (fl-alanine) or ethanol+ water (7-amino-n-butyric and s-amino-n-caproic acids) before drying at 323 K.RESULTS The cells used may be represented schematically as Ag I AgCl I NaCl(m) 11 NaCl(m), amino acid (mi) 1 AgCl I Ag in which an aqueous phase I, containing sodium chloride at molality my is separated from another aqueous phase 11, containing sodium chloride at the same molality and amino acid at molality mi, by a liquid junction. As discussed if transport of the non-electrolyte is negligible, the e.m.f. (E) of such a cell is given by I I1 E = -2k' tdln(my) 1:' where k' = RT/F, t is the transference number of the cation and y is the mean ionic activity coefficient of the electrolyte. Eqn (1) may be re-expressed as - E = 2k'm,(t,(A+Bm*+ Dm)+mi(Cto+F+~m~+Hm+Jm,)) (2) t Present address : Chloride Technical Ltd, Swinton, Manchester M27 2HB.27792780 AQUEOUS SOLUTIONS OF AMINO ACIDS in which A , By C and D are coefficients in an activity coefficient series expansion in molalities, and F, G, H and J are composite terms which include contributions from A , B, C and D and a term representing the dependence of the cationic transference number on the molality of the non-electrolyte. In eqn (2) and (3), yo and to are the activity coefficient and the transference number in aqueous solution I and y is the activity coefficient in solution 11. The experimental results obtained are given in table 1. The results for each system were fitted by a least squares procedure to eqn (2) and the resulting co- efficients, with their 95 % confidence limits, are given in table 2.Not all of the In (yIyo) = ( A + Bma + Cmi + Dm)m,, (3) TABLE 1 .-EXPERIMENTAL E.M.P. VALUES FOR THE SYSTEMS SODIUM CHLORIDE+ AMINO ACID AT 298.15 K rn/mol kg-* 0.010 034 0.022 650 0.041 951 0.062 730 0.062 535 0.089 212 0.010 125 0.022 443 0.039 841 0.062 292 0.090 295 0.089 695 rnJmo1 kg-1 t0 E/mV p-alanine + NaCl 0.101 41 0.304 84 0.490 39 0.103 11 0.298 10 0.497 72 0.104 04 0.310 22 0.529 53 0.102 67 0.198 47 0.422 62 0.302 45 0.505 17 0.105 60 0.305 99 0.511 98 0.391 8 0.478 1.306 2.009 0.3899 0.449 1.156 1.821 0.3881 0.385 1.076 1.712 0.3869 0.374 0.630 1.300 0.3869 0.968 1.490 0.3858 0.317 0.864 1.370 y-aminobutyric acid+ NaCl 0.097 364 0.316 24 0.514 61 0.099 007 0.307 60 0.499 05 0,099 925 0.301 20 0.515 52 0.100 10 0.304 44 0.497 70 0.099 319 0.500 12 0.212 99 0.302 02 0.408 27 0.3918 0.590 1.661 2.490 0.3899 0.585 1.460 2.167 0,3883 0.459 1.255 1.981 0.3869 0.400 1.144 1.660 0.3857 0.328 1.508 0.3858 0.699 0.996 1.255 AEIrnV -0.019 0.005 - 0.001 - 0.025 0.005 0.007 0.000 0.002 0.000 - 0.025 0.023 - 0.01 3 - 0.006 0.004 0.01 1 0.018 - 0.010 - 0.01 2 0.03 1 - 0.009 - 0.067 0.006 - 0.003 -0.001 0.014 -0.014 0.001 - 0.019 0.032 0.01 8 -0.018 0.01 5 - 0.01 8 0.012B .P . KELLEY AND T. H . LILLEY 278 1 TABLE 1 .-contd. 8-amino-n-caproic acid+ NaCl m/mol kg- mi/mol kg - to EImV AEImV 0.010 087 0.100 32 0.3918 0.297 08 0.498 82 0.022 385 0.099 514 0.3899 0.293 07 0.508 30 0.040 253 0.099 873 0.3883 0.301 66 0.512 82 0.062 891 0.099 582 0.3869 0.302 38 0.509 86 0.089 563 0.104 73 0.3858 0.301 80 0.509 29 0.819 2.01 5 2.952 0.700 1.685 2.541 0.579 1 SO2 2.223 0.514 1.276 1.872 0.455 1.114 1.622 - 0.041 0.01 6 - 0.012 - 0.037 0.041 0.014 - 0.01 1 0.005 - 0.030 - 0.029 0.01 8 0.003 - 0.01 5 0.005 0.001 TABLE 2.-cOEFFICIENTS OF EQN (2) FOR NaCl+CC,O-AMINO ACID SYSTEMS A/kg mol-1 B/kgq rnol-9 C/kg2 mol-2 D/kgz mol-2 F/kg2 mol-2 NaCl+ p-alanine - 0.274 f0.011 0.470 fO.1 14 0.055 fO.0 1 1 - 0.3 18 k0.285 - NaCl+ y-amino-n- - 0.390 fO.040 0.883 f0.303 4.509 f 3.544 - 0.636f0.560 - 1.71 5 & 1.376 NaCI+ 6-amino-n- - 0.545 rt0.041 1.499 f0.3 12 6.952 5 3.90 1 - 1.447 f 0.594 - 2.633 f 1.5 1 6 butyric acid caproic acid coefficients in eqn (2) were required to represent the experimental results. Included in table 1 are the differences (AE) between the experimental e.m.f.values and those calculated using the least squares parameters. The leading term in the activity coefficient expansion [ A in eqn (2)] represents pairwise interactions between the sodium and chloride ions and the amino acid and is equivalent to the term (gNa+, +gel-, i) in our earlier termin~logy.~ We will revert to our earlier notation when discussing this pairwise interaction term since to do so stresses its molecular origin. TABLE 3 .-COMPARISON OF THE PRESENT RESULTS FOR THE PAIRWISE INTERACTION PARAMETER WITH THOSE OBTAINED FROM ISOPIESTIC STUDIES --(SNa+.f+gC1-.i)lkgmol-l amino acid (i) present work isopiestic glycine 0.248+ 0.007 0.146 p-alanine 0.274+ 0.01 1 0.196 y-aminobutyric acid 0.390~0.040 0.246 &-aminocaproic acid 0.545-+_0.041 0.245 In table 3 we compare the present values of interaction terms with those obtained from an earlier isopiestic vapour pressure investigation.The agreement between the two sets of information is poor. There is some uncertainty in the values obtained from the present investigation because of possible non-electrolyte transport contri- butions to the measured e.m.f. values although theoretical predictions indicate that these should be rather small and within our experimental errors. Our experience with the isopiestic technique leads us to believe that it is not a very satisfactory method to use to obtain painvise interaction coefficients in those situations when the2782 AQUEOUS SOLUTIONS OF AMINO ACIDS solute-solute higher order interactions are relatively strong since, because of the fairly high molalities necessarily used in isopiestic investigations, the pairwise effects may be masked by many-body interactions. This comment should not be taken to imply that the data obtained earlier are inaccurate or that isopiestic investigations are not useful for the determination of osmotic and thence activity coefficients.We simply wish to point out that in systems such as those investigated here, isopiestic measurements have limited application in the determination of pairwise interaction coefficients. DISCUSSION The interaction between sodium chloride and the a,o-amino acids and the cc-amino acids l s 2 is qualitatively the same, in that for the molality range investigated, attractive interactions occur between each of the acids and the salt.The general features \ I I I I I I I I I 0 0.0 2 0.04 0.06 0.0 8 rnlmol kg-' FIG. 1.-Trace activity coefficients of amino acids in sodium chloride solutions at 298.15 K. The numbers on the curves correspond to glycine (l), a-alanine (2), a-aminobutyric acid (3), norvaline (4), norleucine (5), /3-alanine (6), y-aminobutyric acid (7) and E-aminocaproic acid (8). Curve 1 was constructed from data presented in ref. (2) and curves 2-5 were constructed using data from ref. (1). associated with ascending the two homologous series are, however, different. This is illustrated in fig. 1 where we have used the parameters of eqn (2), for each system, to calculate the trace activity coefficient of the amino acid as a function of the saltB.P. KELLEY AND T. H. LILLEY 2783 molality. This trace activity coefficient (which corresponds to the situation when amino acid-amino acid interactions make no contributions) is given by It is apparent from fig. 1 that, not only are the values of the trace activity coefficients of the a-amino acids smaller than those for the corresponding a,w-acids, but also the trends within the two series are different in that the a-acid-salt interaction increases in a repulsive sense as the hydrocarbon chain is extended whereas the converse occurs in the a,cu series. This is illustrated in a different way in fig. 2 where we have plotted In y: = { 2A + (4B/3)m* + Dm)m. (4) 0.1 0.2 .-I -. 0.3 8 & M A 1 - .. I + 1 0.4 d I 0.5 0.6 I 2 3 4 5 number of C atoms in hydrocarbon chain FIG.2.-Pairwise interaction coefficient for (a) a- and (b) a,o-amino acids as a function of the number of carbon atoms in the hydrocarbon chains. the pairwise interaction coefficients for the two series. The values obtained for the a-acids appear to be constant after a-alanine whereas the values for the a,o-acids become increasingly more negative as this series is ascended. A qualitative explana- tion of these features would be that, since the interaction between the ions of the salt and the amino acids would intuitively be expected to occur primarily at the ionic head groups, then for the a-acids the side chain would not be “noticed” by the salt ions after some relatively short extension of the hydrocarbon chain, whereas for the a,w-acids, the interaction between a particular ion and the amino acid charged group of opposite charge would become more pronounced as the hydrocarbon chain is extended.2784 AQUEOUS SOLUTIONS OF AMINO ACIDS We attempted to quantify this by transposing our results from the Lewis-Randall (LR) to the McMillan-Mayer (MM) scales,’-11 since the latter is more closely related to the potential of mean force between solute molecules.The coefficients on the two scales are related by (5) where the terms have their previously defined meanings. The transposition from the LR to the MM scale was performed using appropriate volumetric data 2 * 4 9 l2 and the MM coefficients are given in table 4. L(B&+,i+B&-,i) = 2(9,,+,, +gcl-,i)v:M;l + v~,cl+2v:-2RTK TABLE 4.-MM PARAMETERS FOR THE NaCl+Q,U)-AMINO ACID SYSTEMS amino acid(i) L(B;.+, i+B;l-,)/cm3 mol-1 experimental excluded volume glycine - 394+ 14 388 p-alanine - 41 5 22 464 y-aminobu t yric acid - 612+ 80 529 8-aminocaproic acid - 866+ 82 657 chemical 782 879 1141 1523 0.6 0.8 r( * I 2 3 1.0 8 g 1.2 +, a \ E a I + * z ld 3 I 1.4 I.6 I 2 3 4 5 number of C atoms in hydrocarbon chain FIG. 3.-“ Chemical ” contribution to the MM second virial coefficients for (a) a- and (b) a,w-amino acids as a function of the number of carbon atoms in the hydrocarbon chains. These MM coefficients may be considered to consist of two principal contribu- tions lo which may be designated as an “ excluded volume ” term and a “ chemical ” term. The former contribution arises from the fact that when two molecules interact,B .P. KELLEY AND T . H . LILLEY 2785 the closed electronic shells of the molecules induce a repulsion at short intermolecular separations. We have estimated the excluded volume terms using the same procedure as that used earlier,l by assuming the amino acids .could be represented as hard ellipsoids and the ions may be represented as hard spheres. (The precise shape assumed for the amino acids has only a small effect on the value obtained for the excluded volume term. This is consistent with earlier lo* l3 conclusions). The difference between the experimental MM term and the calculated excluded volume term we call the " chemical " term. Table 4 includes the values obtained for this term. In fig. 3 we compare the " chemical " contributions obtained for the upacids with those obtained for the a-acids.The gross feature of this figure is that whereas the interaction between the a-amino acids and the salt ions remain roughly constant, i.e. independent of the hydrocarbon chain length, the corresponding interactions with the up-acids increases markedly as the hydrocarbon chain is extended. This is broadly the feature observed and discussed above from a consideration of the LR coefficients. We investigated the possibility of quantifying the electrostatic contributions to the " chemical " term by using the procedures developed by Kirkwood.14* l5 The most suitable of the models investigated by him which could be applied to the present systems would be that in which the amino acids are approximated by prolate ellipsoids with opposite charges at the foci.We spent some time investigating models such as these and were rather disappointed to find that relatively small changes in the geometry and charge disposition of the amino acids lead to rather marked changes in the electrostatic contributions to the interaction coefficients. We concluded that whilst the general features observed experimentally for the free energy coefficients are reproduced, any application of such an approach in a quantitative sense is not worthwhile, particularly in view of the fact that the electrostatic approach fails badly for the corresponding enthalpy measurements. 6-1 We acknowledge financial support from the S.R.C. to B. P. K. B. P. Kelley and T. H. Lilley, Canad. J. Chem., accepted for publication. B. P. Kelley and T. H. Lilley, J.C.S. Faraday I, 1978,742771. R. M. Roberts and J. G. Kirkwood, J. Amer. Chem. SOC., 1942,64,513. T. H. Lilley and R. P. Scott, J.C.S. Faraduy I, 1976,72, 184. E. E. Schrier and R. A. Robinson, J. Biol. Chem., 1971,246,2870. A. M. Squires, Thesis (University of Cornell, 1947). W. G. McMillan and J. E. Mayer, J. Chem. Phys., 1945,13,276. H. L. Friedman, J. Solution Chem., 1972, 1, 387. R. H. Wood, T. H. Lilley and P. T. Thompson, J.C.S. Faraday I, 1978,74, 1301. lo J. J. Kozak, W. S. Knight and W. Kaumann, J. Chem. Phys., 1968,48,675. l 1 J. E. Garrod and T. M. Herrington, J. Phys. Chem., 1969, 73, 1877. l2 J. J. Kozak, Thesis (Princeton, 1965). l 3 A. Isahara, J. Chem. Phys., 1950, 18, 1446; 1951, 19, 397. l4 J. G. Kirkwood, Chem. Rev., 1939,24,233. l5 J. G. Kirkwood, inproteins, Amino-acidsandpeptides, ed. E. J. Cohn and J. T. Edsall (Reinhold, l6 B. P. Kelley and T. H. Lilley, J. Chem. Thermodynamics, 1978, 10, 703. l7 J. W. Larson and D. G. Morrison, J. Phys. Chem., 1976,80,1449. l9 J . W. Larson, W. J. Plymale and A. F. Joseph, J. Phys. Chem., 1977, 81,2074. New York, 1943), chap. 12. B. P. Kelley, T. H. Lilley, E. M. Moses and I. R. Taker, to be published. (PAPER 8/610)

ISSN:0300-9599    

DOI:10.1039/F19787402779

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Modified Zeolites Part 1 .-Dealuminated Mordenites and their Silanation BY RICHARD M. BARRER" AND JEAN-CHRISTIAN TROMBEt Physical Chemistry Laboratories, Chemistry Department, Imperial College, London SW7 2AY Received 23rd March, 1978 A series of partially dealuminated H-mordenites having different ratios Si01/A1203 has been prepared from Na-mordenite by various treatments with acid. From these a series of dealuminated Na-mordenites has been made by treating the corresponding H-mordenites with dilute NaOH. HO The loss of water, including that from the groups [ +A1 S i t ] and the " nests " of hydroxyls, H404, has been investigated as a function of temperature for the dealuminated forms. This showed at 360°C the existence of some OH groups not originating from [ +Al S i r ] .Such groups were attributed to -OH originating from nests. The silanation of the dealuminated H- and Na- mordenites was investigated quantitatively. In the mordenites free of zeolitic water the ratios H2 (evolved)/SiH4 (chemisorbed) were always between 1.5 and 2 and each ratio usually remained reasonably constant over the course of a run. This suggested a rather constant ratio of primary chemisorption to secondary reaction during many runs. The ratio increased above 2 only if zeolitic water was also present. No evidence was found for a significant amount of the reaction \ HO \ -OH HO- [-OH HO-] +SiH4 -+ in which SiH4 interacts with intact " nests ", H404, to regenerate defect-free framework. The Elovich equation appeared able to represent the kinetics of chemisorption of SiH4.Since Keough and Sand reported the stability of mordenite in acids and Barrer and Makki completely dealuminated clinoptilolite with mineral acid without destroying the essential crystal structure, considerable further work on acid and dealuminated zeolites has been undertaken. 3-7 Dealumination with acids necessarily involves also the formation of the H-zeolite, the two reactions being formally represented as 0 M+ 0 H30+ + A1 S i r +H30+ + +A1 S i e +M+ (1) \ / 0 \ / 0 and 0 *.*' \ H' H I 0 0- + 4HzO + A13+. (2) / le H 3 0 + -0-Al-0-+ 3H30f -+ -0 / H H I 0 t Present address : Institut National Polytechnique de Toulouse, Laboratoire de Physico-Chimie des Solides et des Hautes Temperatures, 38 rue des 36 Ponts, Toulouse, France. 2786R . M.BARRER AND J.-C. TROMBE 2187 Both reactions (1) and (2) may be followed by other reactions. On outgassing, and perhaps also slowly at room temperature,* one may have HO \ H30+ 3 A1 S i e + [+A1 Sie]+H,O. \ / (3) 0 The silanol OH groups have been found to react with SiH4 to give silanated zeolites 9 y through such processes as SiOH + H-SiH3 + + Si-0-SiH3 + H2 3 Si-O-SiH3 + HO-Si 4 + Z- Si-O-SiH2-O-Si f + H2 + Si-0-SiH2-0-Si f + HO-Si 4 -+ + Si-0-SiH-0-Si 4 + H2. (4) ( 5 ) (6) I 0-Si 4 Reactions ( 5 ) and (6) imply either mobility of protons from more distant -OH groups or else pairs or groups of three -OH groups close enough together to react with the same SiH4 molecule. The stability of the nests, H404, formed in reaction (2) may be low, due to loss of water on heating.Thus Thackur and Weller ’ reported no nests stable above 100°C in various dealuminated H-mordenites. However the reaction of a dealuminated H-mordenite with SiH4 required a number of -OH groups in excess of those formed in reaction (3),9 so that some hydroxyls originating from H404 nests appeared to have survived the prior outgassing at 360°C. The reaction of intact nests with SiH4 would, if complete, replace the A1 removed by dealumination by Si, and would yield crystalline silicas having the topology of the parent zeolite : 1 0 I I H404 + SiH4 -+ -0-Si-0- + 4H2. (7) 0 I Silanation of dealuminated H-zeolites offers the opportunity to study the hydroxyls in nests, and provides means of changing permanently the sorption, molecule sieving and catalytic behaviour of dealuminated crystals. It was therefore of interest to investigate silanation of such crystals to complement studies already made on silanation of H-mordenite and H-Y 9* lo and intracrystalline reactions of silane with zeolitic water.ll EXPERIMENTAL Na-mordenite and H-mordenite were supplied (as Na-Zeolon and H-Zeolon) by the Norton Co.Dealuminated H-mordenites were prepared from the Na-mordenite by refluxing 10 g samples in 150 cm3 AnalaR hydrochloric acid of different normalities and for different times (table 1). After the products had been washed free of acid they were dried at 50°C. Parts of the products were then treated with 0.1 mol dmV3 NaOH solution at room temperature for 24 h to generate dealuminated Na-mordenites* TABLE 1 .-PREPARATION OF DEALUMINATED MORDENITES conc.HCI/ reflux time designat ions rnol dm-3 /h 0.1 rnol d m - 3 NaOH 0.1 mol dm-3 NaOH 0.1 mol dm-3 NaOH -+ Na-M1 -+ Na-M2 >11 5 M3 -+ Na-M3 2 3 M1 6 4 Mz 6 28 M42788 MODIFIED ZEOLITES HO \ 8 Na+ [+A1 Si<]+NaOH + +A1 S i r +H20. (8) \ / 0 Analyses were made by standard methods : Si02 and A1203 were determined gravi- metrically and Na by absorption spectrophotometry. The total water was found by ignition (zeolitic and hydroxyl), and the weight loss was also measured as a function of temperature both by thermogravimetry, and more accurately on a silica spring balance by heating in vacuo over 24 h at each of a series of fixed temperatures. The monosilanol -OH content OH \ (associated with groups [ +A1 Sic]) was obtained for dealuminated H-mordenites both from the A1 content and by chemisorption of NH3 l2 in a silica spring gravimetric unit.The sample, outgassed at 360"C, was exposed to NH3 at 25°C for 24 h and the physically sorbed NH3 was evacuated until the weight of the sample was constant: HO 0 NH,+ Si<. (9) \ [+A1 Sir]+NH3-+ >A1 \ / 0 The apparatus used for silanation has been described previo~sly.~ At any time, t, the amounts of H2 evolved, of SiH4 chemisorbed and of SiH4 physisorbed were determined. Before all runs (dehydration, silanation, sorption) the zeolites were stored for at least one day over saturated Ca(N03)2 at -56 % RH. For silanation the zeolites were outgassed for not less than a day, nearly always at the temperature selected for the reaction with SiH4. In all tables or figures relating to silanation the amounts of SiH4 chemisorbed or H2 evolved are expressed in mmol/g-l of zeolite stored over saturated Ca(N03)2.RESULTS AND DISCUSSION ANALYSES AND THERMAL BEHAVIOUR Crystallinity to X-rays of all the dealuminated mordenites was good, and there was only a small contraction of the unit cell on extensive removal of Al. The molar ratios SiO2/Al,O3 ranged from 9.52 in parent Na-mordenite to 61-62 in M4 (table 2). The weight losses under vacuum at 360°C (LUV) and the loss on ignition (LOI) are given in columns 2 and 3 of the table. At 360°C under vacuum virtually all zeolitic water is removed [cf. fig. 1 (a) for Na-mordenite] so that LOI-LUV represents hydroxyl water retained at 360°C either in nests or as monosilanols or both.As expected, LOI-LUV for Na-mordenite is extremely small. The H-mordenite (H-Zeolon) with SiO2/Al2O3 = 1 1.84 is already partially dealuminated if derived Ca(N03), SATURATED SOLUTION TABLE 2.-ANALYTICAL RESULTS ON MORDENITES AIR DRIED AT 50°C AND STORED OVER A mordenite sample % wt loss vacuum Na-mordenite H-mordenite Na-M 1 MI Na-Mz Mz Na-M3 Na*-M3 M3 M4 at on 36OOC ignition (LUW (LO11 mmol g-1 of OH from Al analysis silanols nests asmono- in 12.27 12.60 7.22 1.22 11.22 l.18 9S2 - - 0.37 12.53 14.87 8.26 0.0 12.37 1.05 11.84 2.09 2.05 2.6 1.81 4.71 12.14 0.917 13.24 1.83 2.87 3.93 2.07 5.25 - 12.47 13.37 7.43 0.935 11.98 0.905 13.24 - 2.83 1.0 - 13.62 16.49 9.16 0.053 12.55 0.69 18.19 1.39 5.05 3.19 1.57 4.98 10.99 13.02 7.23 0.83 12.78 0.49 26.02 - 6.81 2.26 - 14.47 16.82 9.34 - 13.01 0.50 26.02 1.00 6.93 2.61 1.20 4.84 - 14.22 17.76 9.87 11.46 13.62 7.57 0.935 12.28 0.675 18.19 - 4.92 2.40 - - - 0.66 26.02 9.33 11.56 6.42 - 14.30 0.232 61.62 0.46 10.16 2.4s 0.79 3.87R. M.BARRER AND J.-C. TROMBE 2789 from a Na-mordenite with Si02/A1203 of -9.5. On this assumption reactions 3 and 2 give ideally 2.09 mmol of OH per g as monosilanols and 2.05 mmol g-' in H404 nests. The value calculated from LOI-LUV is 2.6 mmol of OH per g still present after outgassing at 360°C. This, and all the other values recorded in column 11 of table 2, lie between the extremes of -OH ideally present as monosilanols [reaction (3)] TABLE 3.-ESTIMATED WATER LOST FROM H404 NESTS AT 360°C fraction lost for monosilanol content mordenite derived from sample A1 content NH3 chemisorption M1 0.27 0.35 Mz 0.64 0.68 M3 0.77 0.80 M4 0.80 0.83 H-mordenite 0.75 0.61 13 13 1 ternperaturel'c FIG. 1.-Dehydration under high vacuum of mordenites : (a) Na-mordenite ; (b) curve 1, Na-MI ; curve 2, Na-M2; curve 3, Na-M3; (c) curve 1, Mz, curve 2, H-mordenite, curve 3, M4.and the sum of this -OH content plus that ideally present in nests [reaction (2)], as calculated in columns 9 and 10. Thus even if at 360°C no -OH present as monosilanols had been lost it is still necessary to assume a second source of --OH groups, some of which remain at 360°C. This source is ascribed to -OH from H404 nests. From the figures in columns 9, 10 and 11 , and assuming no loss of water from2790 MODIFIED ZEOLITES monosilanols, the fractions evolved up to 360°C of the total water which the nests could produce by complete dehydroxylation are those given in table 3, column 2.In the mordenites M1 to M4 the acidic -OH associated with groups HO [+A1 Si 43, i.e. the monosilanol content, determined by chemisorption of NH3 at 25°C (table 2, column 12) exceeds, but follows the trend of, values based upon the A1 content (column 9). For the H-mordenite, on the other hand, chemisorption of NH3 gave a 13 % lower value than that based on the A1 content. If the monosilanol content at 360°C is taken as that found by chemisorption of NH3 then the fractions of total water in H404 nests lost by 360°C are those given in table 3, column 3. There is reasonable consistency between the values in columns 2 and 3, with fractions lost tending to increase with increasing dealumination among the mordenites MI to M4.When samples of M, were outgassed at 200 and 360°C respectively the acidic silanol contents found by chemisorption of NH3 were 1.5, and 1.5, mmol g-l of zeolite. This supports the view that up to 360°C there is minimal loss of water from acidic silanols. Further information was obtained by measuring weight losses of the parent Na- and dealuminated mordenites which had been air-dried at 50°C and then stored over saturated Ca(NO,),, and finally outgassed at each of a series of fixed temperatures for at least 24 h (fig. 1). With all the dealuminated mordenites (which includes the H-mordenite of table 2) an upward inflexion appeared above 200°C [fig. l(b) and (c)], which was not observed with Na-mordenite, and therefore is not due to zeolitic water.The inflexion was observed also with each of the three dealuminated Na-mordenites of table 1, in which the acidic silanols present as \ HO \ 0 [+A1 S i r ] had been converted according to reaction (8) to [aAl-0-Sie]. Since these latter three zeolites should contain no acidic silanols, and also because for M, the ammonia chemisorption indicated no loss of acidic -OH up to 360"C, the inflexions above 200°C are attributed to an accelerated loss of water from non- acidic -OH in the original nests formed in reaction (2). The nests are evidently considerably less stable to heating than are the acidic silanols. Water can be evolved from them in two stages : The first stage could occur more easily than the second, especially if distortion and resultant steric factors arising from the first stage reorient the remaining pair of hydroxyls.It is probable that some dehydroxylation occurred below 200°C. In parent Na-mordenite with SiO2/A1,O3 = 9.S5 the curve of water loss against temperature [fig. l(a)] showed that 97.2 % of the zeolitic water had been removed at 200"C, assuming that all had been lost at 360°C. Table 4 gives the unit cell compositions of mordenites M1 to M4 and of H- mordenite, all air-dried at 50°C and then stored over saturated Ca(NO,),. In evaluating the zeolitic water contents it was assumed that at 50°C there had been no irreversible loss of water from H404 nests. If this were not the case the zeolitic water contents would increase by the amounts of the loss of water from nests at 50°C.Accordingly the zeolitic water contents are minimum values. The amounts of zeolitic water and of NH3 physisorbed at 25°C and 50 Torr (table 4) both decrease inR . M . BARRER AND J.-C. TROMBE 279 1 the sequence M1 to M4. Evidently selectivity towards NH, and retentivity for water decrease with increasing dealumination. DEAL U M I N A T ED Na-M 0 R D E N I T E S When the dealuminated H-mordenites MI, M2 and M3 were soaked in 0.1 mol dm-3 NaOH and washed to give Na-MI, Na-M, and Na-M,, the Na-contents exceeded those of the framework Al, especially for Na-M, and Na-M, (table 2). However further washing of Na-M, reduced the Na-content to give Na*-M, (table 2). The extra Na could thus represent free base imbibed by the crystals in excess of amounts required for reaction (8).SI LA NATION OF DE ALUMIN ATED H-MORDENI TES The kinetics of silanation at different temperatures are shown in fig. 2(a) for Mz outgassed at the silanation temperature. Reaction begins rapidly but soon becomes slow. The ratio R = H2 (produced)/SiH, (chemisorbed) was always considerably above unity (table 5) so that, as reported earliery9. lo reactions other than reaction (4) are involved. At 280°C a light brown colour was observed on the glass wall of the sample holder. This may be caused by formation of polymers (SiHx),.13 At 200°C or below no discolouration could be seen. TABLE 4.-uNIT CELL COMPOSITIONS OF MORDENlTES AIR DRIED AT 50°C AND STORED OVER A Ca(NO& SATURATED SOLUTION calculated LO1 due to water from cell OHas OH in HzO mordenite weight cell composition monosilanols nests zeottic Na-mordenite 3505 Na8 .3 ,[&.3 ,Si3 9. ci709 6124. 5H20 12.60 H-mordenite 3197 H6.70[A16.7~Si39.67(oH)6.52089.4,]19.8H20 1.88 1.84 11.15 M1 3266 H5.g9[A1s.99Si39.67(0H)g.36086.6,]24.54H20 1.65 2-58 13-52 M2 3 118 H4.36[A14.3 LSi3g.67(OH)i 5.88080.12]1 8S1H20 1.26 4.58 10.68 M3 3048 H3.05[A13.0sSi3g.67(OH)2i.i2074.88]16.39H20 0.90 6.24 9.68 For all runs in fig. 2(a), even though reaction was incomplete when the runs were ended, the H2 evolved (-2 mmol g-1 at 65,100 and 200°C) exceeded the acidic silanol content obtained by chemisorption of NH3 (1.58 mmol g-l). Other -OH groups are therefore involved which arise from nests or from residual zeolitic water.However, the outgassing at 200°C removed virtually all zeolitic water [fig. l(a)] so that residual water is not expected to influence the results for MI to M4 because these are increas- ingly less retentive of molecular water, as shown in a previous section. Accordingly it is considered that some -OH groups from partially dehydroxylated H404 nests are taking part in the chemisorption of SiH4. However, at the end of each run the number of SIH, niolecules chemisorbed was less than the maximum number of sites theoretically available. This maximum is the number of acidic silanol OH groups plus the number of nests, which for M2 is 1.39 + 1.26 = 2.65 mmol of sites per g. At 200°C fig. 2(a) shows only 1.2 mmol SiH4 chemisorbed per g. Likewise the H2 evolved when the run ended (-2 mmol g-l) was less than the -OH content remaining at 360°C (3.1 mmol g-I) as given in table 2, column 1 1.Reaction was still proceeding very slowly when the run was ended [fig. 2(a) and (b) and fig. 41, but evidently a considerable number of sites of hydroxyls do not react or react only very slowly with SiH,. The results in table 6 were obtained by silanation of M2 at 200°C but with different pretreatments. In the first run unreacted SiH4 was evacuated after 162.5 h M4 2766 Hi.2,[Al1.2gSi39.6,(oH)28.1~067.8~]3.0sHz0 0.42 9.16 2.002792 MODIFIED ZEOLITES TABLE 5.-REACTION OF SiH4 WITH MORDENITE Mz AT DIFFERENT TEMPERATURES time of 65°C lOO~C0 200"c 280°C treatmentlh R % Si R A si R % Si R % Si 0.5 1 2 3.5 4 19.5 21 26.5 41.5 44.5 56 65 79 92.5 101 125 1.76 1.69 1.89 l.S3 1.93 1.92 l.S7 2.09 1.g7 1.94 2.43 1.98 1.95 1.93 1.89 1.87 1.85 1.g3 2.60 1.91 2.72 r( bo I i!! 0 .5 l-++-l- 10 dtlh+ FIG.2.-(u) Silanation of mordenite M2 : 1.73 2.24 1.71 2-73 1.g4 1.70 2.60 1.73 2.S1 l.69 2.g1 1.72 3.OS 2.07 1.70 2.75 2.10 l.67 3.23 1.74 3.39 2.41 2.48 1.77 3.38 2.62 1.69 3.32 1.74 3.6, 1.82 3.88 H2 evolved SiH4 chemisorbed 65°C X + 100°C 0 0 200°C A A 280°C 0 .. (b) Silanation of various dealuminated mordenites at 200°C : H2 evolved SiH4 chemisorbed H-mordenite + X M1 0 0 M2 n A M3 0 M4 v v.R . M. BARRER AND J . - C . TROMBE 2793 and subsequent release of hydrogen was then measured. In 100 h over 0.22 mmol H2 per g were formed through slow secondary reactions. Finally oxidation by addition of excess water at 280°C brought the ratio H2 (evolved)/SiH, (chemisorbed) very near to the theoretical maximum of 4.The kinetics of chemisorption at 200°C were followed in greater detail in a special gravimetric unit [fig. 3(a)]. Fig. 3(b) indicates that the kinetics are reasonably represented by the Elovich l4 equation for t $ to : where Qt is the silane chemisorbed at time t and k and to are constants. This equation can be interpreted in terms of a model in which the energy of activation for chemisorption varies linearly with TABLE 6.-SILANATION OF MORDENITE Mt AT 200°C outgassing at 200°C. addition of 0.85 % time outgassing at 200°C outgassing at 360°C H 2 0 by wt at 200°C treatment/h silanation ato200"C silanation at 220°C silanation at 200°C R h Si R A Si R % Si 1 1.75 2.46 1.49 1.91 4 1.51 2.04 2.28 1.70 2 1.75 2.6, 22 1.75 2.92 23 1.52 2.36 24 2.18 2.Z5 44.5 1.57 2.44 56 1.75 3.12 66.5 1.58 2.53 92.5 1-76 3.20 101 1.6, 2.60 121 1.77 3.23 162.5 1.77 3.31 SiH4 pumped off, 200°C oxidation by water at 280°C 1st dose 2nd dose 100 h at 1.96 19 h 2.50 23 h 3.94 Fig.2(b) compares silanation and hydrogen evolution for M, to M4 and H- mordenite, all outgassed and silanated at 200°C. Table 7 records ratios R=H2 (evolved)/SiH, (chemisorbed). Particularly for M, to M4 the % Si added decreased with increasing dealumination. This could mean that reaction of SiH4 with the HO \ acidic -OH of the groups [ 3 A1 -OH originating from the nests. S i t ] occurs more readily than reaction with S I LAN AT I 0 N 0 F D E A L U MI N A TED Na-M OR D E N I T E S As noted earlier acidic -OH should not occur in the dealuminated Na-mordenites. Accordingly chemisorption of SiH4 should take place primarily at -OH groups2794 MODIFIED ZEOLITES originating from the nests, provided zeolitic water is absent.Therefore the zeolites Na-M,, Na-M2, Na-M3 and Na*-M, were outgassed and silanated at 2oo"c, as in table 7. In addition Na-M, was outgassed and silanated at 100°C. The H2 evolved and SiH4 chemisorbed are shown as functions of time in fig. 4 and the ratios R and % Si added in table 8. When tables 7 and 8 are compared it is seen for MI to M3 HO \ that removal of the acidic -OH of the groups [ s A l Si 41 has substantially FIG. 3.-Silanation of mordenite Mz at 200°C: kinetic analysis. (a) The rate curve; (b) test of Elovich equation. TABLE 7.-sILANATION OF DEALUMINATED H-MORDENITES AT 200°C AFTER OUTGASSING AT time of treatmentlh 1 4 20 22 26.5 43.5 56 70 72.5 90.5 92.5 114 121 137 139 158 1 62 182 H-mordzni te R XSi 2.03 1.69 2.00 2.04 1.98 2.41 1.99 2.4, 1-96 2.60 1.99 2.72 1-96 2-86 MI R %Si 1.73 2.95 1-75 3-06 1-78 3-26 1.79 3.38 1.81 3.45 1.84 3.58 1.86 3.60 1.75 2.92 1.75 3.12 1.76 3.20 1.77 3.23 MI R %Si 1.70 0.8, l.67 1.15 1.67 1.48 1.69 l.65 1.71 1.72 1.73 l.83 1.76 1.85R . M.BARRER AND J.-C. TROMBE 2795 reduced the initial rates of chemisorption of SiH4. This supports the view given in the previous section that -OH groups originating from nests are less reactive to SiH4 HO S i r ] groups. Removal of some Na from Na-M3 to give than those in [ +A1 Na*-M, (table 2) did not change the kinetics of chemisorption appreciably (table 8, columns 5 and 6).When the runs were terminated the ratios of the numbers of SiH4 molecules chemisorbed to the ideal numbers of nests (from table 4, column 3) were as follows : Na-MI, 1.3; Na-M,, 0.g8; Na-M,, 0.5,. Reaction in Na-M1 must therefore \2796 MODIFIED ZEOLITES As noted earlier these contain excess Na (table 2) which may be present as NaOH. This aspect of silanation requires further study. RATIOS H2 (FORMED)/SiH4 (CHEMISORBED) When tables 5-8 are considered together the following properties of R = H2 (produced)/SiH, (chemisorbed) are seen : (i) R lies in the range 1.5 to 2.0 for all the mordenites outgassed at 200 or 360°C. (ii) R does not depend strongly upon temperature of silanation, although at 65 and 100°C R is larger than at 200 and 280°C.(iii) During the course of any run R usually stays nearly constant. (iv) If zeolitic water is present R increases above 2, in agreement with an earlier study 11 (table 8, column 2 ; table 6, column 6). TABLE 8.-sILANATION OF DEALUMINATED Na-MORDENITE time of treatment/h 1 2 4 4.5 20 22 25.5 26.5 42.5 44.5 48 50 52 66 68 70.5 89 91 112 114 1 20 139 186 outgassing and silanation at 100°C. Na-M2 % Si 1 .26 1.49 1.31 1.67 1.67 1.79 1.87 outgassing and silanation at 200°C Na-l'$ R /,Si 1.75 1.47 1.70 1.74 1.73 lA4 1.70 2.36 l.69 2.55 1.69 2.84 1.70 3.13 1.72 3.21 1.71 3.33 Na-Mi R /,Si l.S7 1.84 1.39 1.81 1.56 l.S7 1& 1 . 8 ~ 2.26 1.86 2.43 1.g7 2.53 1-91 2.58 Na-v3 R Si 1.75 1.49 1.79 1.74 l.81 2.13 1.84 2.36 1.86 2.42 1.86 2.53 1.87 2.73 Na*+ R /,Si 1.83 1.51 l J O l.81 1.85 2.22 1.85 2.44 l.87 2.s3 l.89 2.63 Of the mordenites outgassed and silanated at 200°C the largest value of R was 2.0, for H-mordenite, the form richest in acidic hydroxyls.The smallest value of R was obtained with mordenite M, outgassed at 360°C and silanated at 200°C. In this latter system when reaction was terminated 0.g3 mmol of SiH4 had been chemisorbed per g of zeolite and 1.53 mmol of H2 had been evolved. The zeolite outgassed at this temperature had 1 .39 mmol g-l of acidic hydroxyls and 3.1 mmol total hydroxyl g-l (table 2). The H2 evolved exceeds the total acidic OH content but represents only about half the total hydroxyl. Because for all the zeolite samples nearly free of zeolitic water R lies between 1.5 and 2.0 the primary chemisorption reaction (4) cannot occur alone.The constant values of R during the runs also suggest that during a particular run the primary and secondary reactions occur in approximately constant proportions. Because R is in the range 1.5 to 2.0 a high proportion, if any, of reaction (7) is unlikely.R . M . BARRER AND J.-C. TROMBE A possible process additional to reactions (2)-(6), which gives R = 2, is 2797 ] +2H2. -0- ]+SiH4 + [ -O-SiH2-O- It involves the hydroxyl pairs generated by the first stage of reaction (10). CONCLUSION The existence has been demonstrated in dealuminated H-mordenites and Na- mordenites of -OH groups which can react with SiH4 but which do not arise from residual zeolitic water or as monosilanols in the groups [ + A1 Sir]. These have been attributed to hydroxyls originating from the H404 nests formed in the initial dealumination by acids.The analytical evidence shows that some -OH from this source still remains in dealuminated mordenites outgassed at 360°C. The loss of water from nests is necessarily a two-step process, and the second stage is likely to be less easy than the first. However, water loss from the nests has been found to occur considerably more easily than water loss from acidic -OH in the groups [+A1 Sir]. No evidence has been obtained that Si from SiH4 can replace to any marked extent the four H-atoms of the nest to recreate a defect-free framework, at least up to 200°C. Thus in zeolites free of molecular water the ratios H2 (evolved)/ SiH4 (chemisorbed) did not exceed 2 instead of reaching 4, even for Na-M1, Na-M2 and Na-M3 in which acidic -OH should be absent, but where nests are initially present. HO \ HO \ A. H. Keough and L. B. Sand, J. Amer. Chem. SOC., 1961,83,3536. R. M. Barrer and M. B. Makki, C d . J. Chem., 1964,42,1481. R. M. Barrer and D. L. Peterson, Proc. Roy. SOC. A, 1964,280,466. R. M. Barrer and B. Coughlan, in Molecular Sieves (SOC. Chem. Ind., London, 1968), p. 141. W. L. Kranich, Y. H. Ma, L. €3. Sand, A. H. Weiss and I. Zwiebel, in Molecular Sieve Zeolites in Adv. Chem. Ser. (Amer. Chem. SOC., 1971), no. 101, p. 502. G. T. Kerr, J. Phys. Chem., 1968,72,2594. ' D. K. Thakur and S. L. Weller, in Molecular Sieves, Adv. Chem. Ser. (Amer. Chem. SOC., 1973), no. 121, p. 596. R. M. Barrer and J. Klinowski, J.C.S. Furahy I, 1975,71,690. R. M. Barrer, R. G. Jenkins and G. Peeters, in Molecular Sieves 11, Amer. Chem. SOC. Symp. Ser., 1977, 40, 258. lo R. M. Barrer, E. F. Vansant and G. Peeters, J.C.S. Faraday I, 1978, 74, 1871. I f R. M. Barrer and J . 4 . Trombe, in preparation. I2 B. K. G. Theng, E. F. Vansant and J. B. Uytterhoeven, Trms. Furadby Soc., 1968, 64, 3370. I3 E. G. Rochow, Comprehensive InorQanic Chemistry (Pergamon Press, Oxford, 1973), vol. I, l4 S. Yu. Elovich and G. M. Zhabrova, Zhur. fiz. Khim., 1939,13, 1761. Is B. M. W. Trapnell, Chemisorption (Butterworth, London, 1955), p. 104 et seq. chap. 15, p. 1364. (PAPER 8/556)

ISSN:0300-9599    

DOI:10.1039/F19787402786

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Modified Zeolites Part 2.-Sorption by Dealuminated, Silanated Mordenites BY RICHARD M. BARRER* AND JEAN-CHRISTIAN TROMBE~ Physical Chemistry Laboratories, Chemistry Department, Imperial College, London SW7 2AY Received 23rd March, 1978 A comparison has been made of sorption isotherms and kinetics of 02, N2 and Ar in each of a series of partially dealuminated mordenites before and after different extents of silanation. Before silanation partially dealuminated mordenites sorbed each gas freely but after silanation some very selective sorbents were obtained which at 77 K imbibed 0 2 much more copiously than either N2 or Ar. This selectivity was further improved when acidic H was replaced by Na in the dealuminated H-mordenites before silanation. In many of the sorbents a rapid initial uptake was followed by a slow diffusion at rates in the sequence O2 > N2 > Ar, which is the inverse order of their van der Waals cross-sectional diameters.The results provide evidence that after silanation the intracrystalline channels are not uniformly accessible to a given sorbate. A number of partially dealuminated H- and Na-mordenites were prepared (Part 1) in which the ratios Si02/A1,03 were in the range 11 .& to 61.6,. These zeolites were then variously silanated, to provide a range of crystals in which the mordenite channels had different contents of chemisorbed fragments. It had previously been shown that silanation of outgassed H-mordenite sorbents greatly altered their behaviour. * Also, while reactions of SiH4 with zeolitic water inside zeolites Na-Y and Na- mordenite produced little change in uptake of O2 and N, in the three-dimensional channel system of Na-Y, these reactions greatly and selectively altered sorption of the gases in the one-dimensional channel system of Na-m~rdenite.~ It was therefore of interest to investigate and compare the sorption behaviour of the partially de- aluminated and silanated mordenites inter se and with the modified mordenites examined previously, EXPERIMENTAL The sorbents employed are listed in table 1.The numbers in brackets are the percentages by weight of Si added as a result of chemisorption of SiH4 (Part l).4 Also Na*-M3 is Na-M3 after further extraction with distilled water, which reduced the Na content from 0.83 to 0.66 mmol g-l of zeolite (Part 1, table 2). The Na-M forms were each produced from the corresponding partly dealuminated H-form, M, by treatment with dilute NaOH, prior to silanation (Part 1).The sorbates were 02, N2 and Ar chosen for their increasing equilibrium distance cross- sectional diameters in the order given. Uptakes are all expressed as cm3 of sorbate g1 of zeolite outgassed at 360°C. Volumetric and gravimetric procedures were used for measuring uptakes and kinetics. In the kinetic runs the pressures were allowed to decline as the gases were sorbed. For isotherms, up to 30min was normally allowed per point. Because slow processes were sometimes present desorption points could lie a little above 7 Present address : Institut National Polytechnique de Toulouse, Laboratorie de Physico-Chimie des Solides et des Hautes Temperatures, 38 rue des 36 Ponts, Toulouse, France.2798R . M. BARRER AND J.-C. TROMBE 2799 adsorption points [e.g. fig. l(u), curve 4 ; fig. 2(6), curves 3 and 41. However in these situations a mean curve has been drawn. RESULTS AND DISCUSSION DEALUMINATED, SILANATED H-MORDENITES Some isotherms at 77K of 02, N2 and Ar in certain of the H-mordenites of table 1 are shown in fig. 1 and 2, and rates of sorption in fig. 3. Table 2 gives apparent sorption capacities at 50 Torr. Before silanation the H-mordenites gave capacities independent of equilibration times allowed ; after silanation gradual drifts were, as noted in the previous section, to be considered even though sorption and desorption points were usually close to a common curve.Accordingly for silanated sorbents TABLE 1 .-ZEOLITES USED* SiOz!A1203 in SiOz/.A1203 in unsilanated unsilanated zeolite form zeolite form M2 Na-M2 Na-M2 (3.33) H-mordenite (2.46) H-mordeni t e H-mordenite (2.5 6 ) H-mordenite (2.S6) M1 M1 (3.60) Na-M1 Na-M, (1.26) Na-M1 (1.g3) Na-M1 (2.5,) M3 M3 (3.02) . 13.24 Na-M3 Na-M3 (2.73) Na*-M3 (2.6,) .26.02 * " M " denotes " dealuminated H-mordenite " as in Part 1 .4 " Na-M " denotes these mordenites treated with 0.1 mol dm-3 NaOH (Part 1). PlmmHg FIG. 1.-Some isotherms at 77 K for (a) 02, (b) Ar and (c) N2 in H-mordenites and silanated H- mordenites. Curves 1, H-mordenite ; curves 2, H-mordenite (2.46 % Si) ; curves 3, H-mordenite (2.56 % Si) ; curves 4, H-mordenite (2.g6 % Si). + and open symbols denote adsorption points and x and filled symbols desorption points.Scale of ordinate the same in (a), (b) and (c).2800 MODIFIED ZEOLITES I 4 I 150c 1 100 150 20 0 50R . M. BARRER AND J . - C . TROMBE 2801 capacities are apparent ones appropriate for our conditions. Slow diffusion following initial rapid sorption is seen for example in fig. 3(a), and for Ar in M4 in fig. 3(b). The slow process sometimes gave linear plots of amount sorbed, Qt, against $ (t = time ; the ,/? law of diffusion). The line slope should then be proportional to D3/a, where D is the diffusivity and a is the mean crystal diameter in the powder. If, as expected, a is the same for the H-mordenites having 2.56 and 2A6 % by weight of Si added through chemisorption of SiH4 then for these samples the ratio of the two diffusivities for N2 [from curves 5 and 7 of fig.3(a)] is -6.8. Thus D appears to decline sharply as the extent of prior silanation at 200°C increases. The amount of initial rapid sorption is also a function of the extent of prior silanation [e.g. curves 5 and 7 of fig. 3(a)] and of molecular dimensions [e.g. curves 3 and 7, or 2 and 4, of fig. 3(a)]. The equilibrium separation cross-sectional diameters of 02, N2 and Ar are respectively 2.8, 3.0 and 3 . S 3 k 5 Effects of molecular dimensions upon isotherms can be seen by comparing curves 2 in fig. l(a), (6) and (c). At 77 K silanated H-mordenite can be very selective for 02. In the case of the 02, TABLE 2.-uPTAKES AT 50Torr AND 77K IN DEALUMINATED H-MORDENITES OUTGASSED AT 360°C, BEFORE AND AFTER SILANATION capacity of sorption at 77 K in cm3 g-1 dry, at 50 Torr weight loss under vacuum at 36OOC % N2 0 2 % Si added before after silanation before after before after material by wt silanation first run second run silanation silanation silanation silanation H-mordenite 2.S6 12.53 5.65 112 10 134 83 2.56 12.53 6.30 10.8 112 19 134 95 2.46' 12.53 8.86 9.9 112 17 134 97 M1 3.60 14.22 6.86 10.5 118 27 142 76 M2 3.31 13.62 9.41 10.1 113 40 143 92 M3 3.0, 14.47 7.16 10.2 110 55 148 92 M4 1.85 9.33 6.20 7.6 107 83 135 115 * In this case the silanation was performed at 150 "C, instead of 200 "C, for all the other runs.N2 pair selectivity is best for the most heavily silanated H-mordenite with SO2/ A1,03 = 11 -84 (table 2), and for comparable degrees of silanation tends to diminish as the extent of dealumination is increased [cf.H-mordenite (2.86) with M3 (3.0,) in table 21. For pairs of bigger molecules of graded and differing dimensions good selectivities might reappear in M3 (3.02), the larger molecule being more fully excluded. In M2 prior to silanation isotherms for 0, and N, at 77 K were measured after outgassing first at 200 and then at 360°C. Sorption was virtually unaltered for each gas [fig. 2(b), curve 1 and curve 21 so that any residual zeolitic water remaining at 200°C is insufficient to change the isotherms. Silanation of the H-mordenite of table 1 to give at 200°C 2S6 % of added Si, and at 150°C 2.46 % of extra Si, gave sorbents very much alike in uptakes of both O2 and N2 (table 2). Thus within this range, for almost equal extents of silanation, the modification was not sensitive to the silanation temperature.FIG. 2.-Isotherms at 77K in dealuminated, and in dealuminated and silanated H-mordenites Symbols filled and unfilled and + and x have the same sirncance as in fig, 1. (a) Curve 1, O2 in MI ; curve 2, N2 in Mi ; curve 3, O2 in MI (3.60 % Si); curve 4, N2 in MI (3.60 % Si). (b) Curve 1, O2 in M2 outgassed at 360 (0 and a), and at 200°C (V and V) ; curve 2, NZ in Ma outgassed at 360 (+ and x ), and at 200°C (0 and +) ; curve 3, O2 in M2 (3.31 % Si) ; curve 4, Nz in M2 (3.31 % Si). Outgassing before both runs was at 360°C. (c) Curve 1, O2 in M4 ; curve 2, Ar in Mq ; curve 3, O2 in M4 (1.85 % Si) ; curve 4, N2 in M4 ; curve 5, Nz in M e (1.85 Si) curve 6, Ar in Mq (1.85 % Si).Outgassing before runs was at 360°C.2802 MODIFIED ZEOLITES '""t- 1 1 5 10 15 qT/min+ FIG. 3.-Rates of sorption measured at 77 K in (a) silanated H-mordenite and (b) silanated de- aluminated H-mordenite Ma. Prior outgassing was at 360°C. (a) Curve 1, O2 in H-mordenite (2.56 % si, silanated at 200°C; at t = 116 min, p = 35.5 Torr) ; curve 2, O2 in H-mordenite (2.46 % Si, silanated at 100°C ; at t = 427 min, p = 21.5 Torr) ; curve 4,02 in H-mordenite (U6 % Si, silanated at 200°C; at f = 100min p = 9.0 Tom); curve 4, N2 in H-mordenite (2.46 % Si; at t = 121 min, p = 65.1 Tom) ; curve 5, N2 in H-mordenite (2.56 % Si ; at t = 180 min, p = 57.4 Tom) ; curve 6, Ar in H-mordenite (2.46 % Si ; at t = 100 mh,g = 53 Torr) ; curve 7, N2 in H-mordenite (2.86 % Si; at t = 196 min, p = 63.6 Tom).(b) Sorption in M4 (I& % Si). Curve 1, 0 2 (at t = 25 min, p = 6.8 Torr) ; curve 2, N2 (at t = 64 min, p = 8.7 Torr) ; curve 3, Ar (at t = 152 min, p = 46 Torr).R . M. BARRER A N D J . - C . TROMBE 2803 Table 2 records weight losses at 360°C under vacuum before and after silanation. With the exception of M2 (3.31) in the first runs (column 4) the samples were not oxidised after silanation but were evacuated at the silanation temperature, cooled in uacuo and only then contacted with air, prior to evacuation at 360°C. Weight losses in the first run following silanation are considerably less than the losses before silanation. After these first runs the silanated zeolites were stored for one day over saturated Ca(N03)2, and the weight losses obtained as before by outgassing at 360°C.The losses in these second runs were increased but remained significantly less than TABLE 3.-uPTAKES OF 0 2 AND Nz AT 77 K AND 50 TOIT IN DEALUMINATED Na-MORDENITES BEFORE AND AFTER SILANATION sorption at 77 K and p = 50 Torr in cm3 at s.t.p./g N2 0 2 % Si added before after before after material by wt silanation silanation silanation silanation N a - m o r d e n i t e 93 1 1 5 Na-M1 1 . 2 6 1 0 8 75 125 1 1 9 1.93 1 0 8 38 1 2 5 105 2 . 5 8 1 0 8 8 1 2 5 80 Na-Mz 3.33 1 0 9 2 1 1 2 9 88 Na-M3 2.73 1 0 0 1 8 121 90 Na*-M3 2.63 1 0 3 46 1 2 5 1 0 0 the losses from the unsilanated forms. The weight losses are largely due to evolution of zeolitic water, but in the first runs of column 4 of table 2 some of this zeolitic water may be trapped during heating by converting chemisorbed -SiH, and -SHY- to -Si(OH), and -Si(OH),,- within the zeolite and so may not all escape.Increased polarity due to these new -OH groups should also increase the selectivity for water. DEAL U MI N A T E D A N D S I LAN ATE D Na-M 0 R D E N I T E S Isotherms at 77 K for 02, Nz and Ar are shown in fig. 4 with Na-M1, Na-M1 (1.2& Na-MI (1.g3) and Na-Ml (2.5,) sorbents. As with the H-mordenites of the previous section the uptakes at 50 Torr depend strongly upon the extent of silanation and upon the dimensions of the sorbed molecules. The effect of molecular dimen- sions can be seen, for example, by comparing curves 2 in fig. 4(a), (b) and (c) (4 (4 -4----.-;t---+- - 106 bo 4 h x u M 3 50 100 50 100 50 100 1% 200 Plmm H g FIG.4.--Isotherms at 77K for 02, Ar and Nz in Na-M1 and Na-MI variously silanated and outgassed at 360°C. Symbols + and x , and filled and unfilled, have the same significance as in fig. 1. (a) O2 ; (b) Ar ; (c) Nz. Curves 1 refer to Na-Ml ; 2 to Na-M1 (1.26 % Si); 3 to Na-M1 (1 .g3 % Si) ; and 4 to Na-Ml (2& % Si). Silanation was effected at 200°C. Scale of ordinate is the same in (a), (b) and (c).2804 MODIFIED ZEOLITES ++-- lo 4 Lrr x l/T/min+ FIG. 5 . 4 0 ) Rates of sorption of 02, Ar and N2 in Na-Mi and variously silanated Na-M1 at 77 and at 195 K. Outgassing was at 360°C in (a) and (6); curve 1, O2 at 77 K in Na-Mi (1.26 % Si, p= 16.2 Torr at t = 36 min) ; curve 2, Oz at 77 K in Na-Mi (1.93 % Si,p = 37.9 Torr at t = 114 min) ; curve 3, NZ at 77 K in Na-Mi (1.26 % Si, p = 24.0 Torr at t = 121 rnin) ; curve 4, O2 at 77 K in Na-MI (2.58 % Si, p = 54 Torr at t = 640 min) ; curve 5, N2 at 77 K in Na-MI (1.93 % Si, p = 54.1 Torr at t = 121 min) ; curve 6, Ar at 77 K in Na-Mi (1.26 % Si,p = 64 Torr at t = 79 mm) ; curve 7, NZ at 195 K in Na-MI (2& % Si, p = 54.3 Torr at t = 165 min); curve 8, O2 at 195 K in Na-MI (2.58 % Si, p = 48 Torr at t = 121 min); curve 9, N2 at 77 K in Na-Mi (2.58 % Si, p = 49.1 Torr at t = 64 min) ; curve 10, Ar at 77 K in Na-M1 (2.58 % Si, p = 49.5 Torr at t = 60 min) ; curve 11, Ar at 195 K in Na-Ml (2.58 % Si, p = 52.9 Tom at t = 214 min) ; (b) rates of sorption at 77 K of Oz and Nz in Na-M3 (2.73) and in Na*-M3 (2.63) : curve 1, O2 in Na*-M3 (2.63 % Si, p = 57.6 Torr at t = 169 min) ; curve 2, O2 in Na-M3 (2.73 % Si, p = 23 Tom at t = 240 min); curve 3, NZ in Na*-M3 (2.63 % Si, p = 59.5 Torr at t = 196 min) ; curve 4, Nz in Na-M3 (2.73 % Si, p = 103 Torr at t = 570 min).R .M. BARRER AND J . - C . TROMBE 2805 respectively, the uptakes in Na-MI (l.26) being in the order 0, > N, > Ar, the inverse order of the cross-sectional diameters. Table 3 gives uptakes of 0, and N2 measured at 50Torr and 77K in parent Na-mordenite and in various modified mordenites, and table 4 gives these uptakes for O,, N2 and Ar at several temperatures. These tables show Na-M, (2.58) to be particularly effective in differentiating O2 from N2 and from Ar. As the extent of dealumination increased the selectivity for 0, over N, diminished, although it remains good for Na-M, (3.3,) and Na-M3 (2.7,) (table 3).When Na-M, (2.7,) was given extra washing which removed some Na to give Na*-M3 (2.6,) (table 1) the uptake of N, at 77 K and 50Torr increased 2S6-fold but that of 0, increased by only 11 %. Thus Na+ in the crystals, as well as dealumination and silanation are involved in differentiating between O2 and N,. When one compares the dealuminated and silanated H-mordenites of the previous section with analogous Na-mordenites it is seen that the Na-forms differentiate more strongly [e.g. M2 (3.3,) in table 2 and Na-M2 (3.3,) in table 31. TABLE UPTAKES OF Oz, N2 AND Ar AT SEVERAL TEMPERATURES IN Na-M1 (2.5,) IN cm3 AT S.T.P. PER g OF OUTGASSED ZEOLITE temperature /K sorption at p = 50f5 Tom NZ 0 2 Ar 77 8 80 5 195 30 13 8 273 0.7 0.7 0.3 Rates of sorption at 77 and 195 K are compared in fig.5(a) for O,, N2 and Ar in Na-M, with 1.26, 1.93 and 2S8 % Si added by silanation, and in fig. 5(b) for 0, and N2 at 77 K in Na-M, (2.7,) and Na*-M, (2.6,). After an initial rapid sorption, the extent of which, for a given gas and temperature, is seen from fig. 5(a) to decrease as the Si added increases, there may be a slow further uptake the rate of which is also a function of the degree of silanation. This is illustrated in fig. 5(a) by curves 1, 2 and 4 for 0, and 3, 5 and 9 for N,, at 77 K in Na-M, (l.26), Na-M, (1.9,) and Na-M, (2.58), respectively. A role of Na+ in such effects is seen from fig. 5(b) for 0, and N, for 77 K in Na-M, (2.73) and Na*-M, (2.6,).Removal of some Na+ has increased the extent of the initial rapid uptake and has changed the rate of the slow process. The slow process in the uptake of N, obeys the ,/Flaw [curves 3 and 4 of fig. 5(b)] and from the slopes of the plots of uptake against & as described in the previous section, one obtains -19 for the ratio of diffusivity, D, in Na*-M, (2.6,) to D in Na-M, (2.7,). Further evidence of slow, activated diffusion processes was obtained by deter- mining uptakes of 02, N, and Ar at -50 Torr at each of several temperatures in Na-M, (2.58). The isobars, as shown by the figures in table 4, exhibit maxima for N, and Ar at different intermediate temperatures because, although uptakes at 50 Torr at first increase as the temperature falls, eventually activated diffusion becomes so slow that these increased equilibrium uptakes cannot be realised on the time scale of the experiments, and so the amounts sorbed must pass through maxima.For 0, the maximum is at a temperature below 77 K and so is not observed in our experiments. At 195 K true equilibrium was established for N, and 0, and table 4 shows that N, is then selectively sorbed. This behaviour is attributed to the molecular quadrupole of N2 which interacts exothermally with intracrystalline electrostatic field gradients. The resultant contribution to the energy and free energy of sorption can be considerable.62806 MODIFIED ZEOLITES CONCLUDING REMARKS The kinetic measurements suggest that the intracrystalline channels in the silanated crystals are not uniformly accessible to the sorbates.The partial blocking associated with silanation may not have occurred equally among all crystallites in the powder, or even throughout one crystal. Nevertheless, it has been established that silanation of H-mordenites, dealuminated H-mordenites and dealuminated Na-mordenites can strongly and selectively influence both amounts and rates of uptake of 02, N2 and Ar, the effects observed at 77 K following the sequence of the equilibrium cross- sectional diameters. For H- and Na-mordenites, each dealuminated and silanated to the same extent, the Na-form differentiates more sharply than the H-form between O2 and N2 or between O2 and Ar. Before silanation and at 77 K the gases penetrate all the dealuminated mordenites to give true equilibria.At 50 Torr the uptakes in cm3 at s.t.p. per g of zeolite are given below : 0 2 Ar N2 OZ/NZ Oz/Ar Na-mordenite 115 - 93 1.24 - H-mordenite 134 121 112 1.20 1.11 Ml 142 - 119 1.19 - M2 143 - 113 1.27 - M3 149 - 111 1.34 - M4 134 124 107 1.25 1.08 Na-Ml 125 114 - 1.10 Na-M2 129 - 109 1.18 - Na-M3 121 - 100 1.21 - Na*-M3 125 - 103 1.21 - The ratios of the numbers of molecules sorbed, 02/N2 and OJAr as given in the last two columns of the above tabulation, vary only a little for most of the sorbents, suggesting that the intracrystalline pore volumes are approximately equally accessible in each, and that sorptions are nearly in accordance with Gurwitsch's rule? For this rule to be strictly valid the above ratios would be constant and in the inverse ratio of the molecular volumes of the sorbed fluids at 77 K. The experiences reported in this paper for mordenites are expected to be repeated in modified ways for other silanated zeolites having one dimensional channel systems and stable H-forms, examples being offretite and zeolites L and Q. We wish to acknowledge grants from the Royal Society and from the C.N.R.S. which made possible the participation of J.-C. T. in this work. R. M. Barrer, R. G. Jenkins and G. Peeters, in MoZecuIur Sieves I1 (Amer. Chem. SOC. Symp. Ser., No. 40, 1977), p. 258. R. M. Barrer, E. F. Vansant and G. Peeters, J.C.S. Faraday I, 1978, 74, 1871. R. M. Barrer and J.-C. Trombe, J.C.S. Faraday I, in press. R. M. Barrer and J.-C. Trombe, J.C.S. Furuday I, 1978, 74, 2786. L. Pauling, The Nature of the Chemical Bond (Cornell, 1940), p. 187 et seq. R. M. Barrer, J. Colloid Interface Sci., 1966, 21, 415. ' L. G. Gurwitsch, 2. phys. Chem., 1914,87, 323. (PAPER 8/557)

ISSN:0300-9599    

DOI:10.1039/F19787402798

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Magnetic and Optical Studies of Chromium Oxides Part 3.-Calcination of Coprecipitated Chromium and Aluminium Hydroxide Gels BY ALAN ELLIsoN*t AND KENNETH s. w. SING$. School of Science, Hull College of Higher Education, Kingston upon Hull HU6 7RT Received 16th December, 1977 Magnetic and optical studies were performed on coprecipitated chromium and aluminium hydroxide gels after calcination in air at temperatures between 50 and 1150°C. The results show that magnetic diIution was not achieved and that surface clusters of chromium oxide were formed. The magnetism, e.s.r. and optical spectra of oxidised samples are discussed in terms of a mixed- valency phase of partially-oxidised chromium containing Cr3+ and Cr6+ species. This new model ofy-phase chromium is shown to be consistent and an acceptable part of the extensive mixed-valency chemistry of chromium.This work continues a systematic study of the magnetic and spectral properties of chromium-alumina catalysts. It was shown in Parts 1 and 2 that the state of dispersion of chromium on the alumina support influences the stability of Cr6+, the occurrence and stability of y-phase chromium and the production of an " oxidised- phase " of chromium. There is some evidence that a well-dispersed state of chromium can be obtained using coprecipitation techniques and it is to be expected that on calcination of coprecipitated gels the chromium ion has the most favourable chance of occupying cation sites of well-defined symmetry in the host lattice. This paper is concerned with a study of the thermal decomposition of chromium and ahminium hydroxide gels coprecipitated from aqueous solution.EXPERIMENTAL The coprecipitated gels were prepared by allowing aqueous solutions of chromium (III) nitrate, aluminium nitrate and ammonium hydroxide to react under controlled conditions of pH, concentration and flow-rate in an apparatus similar to that of Harris and Sing,3 but modified to include a constant-head device. Three precipitates were prepared, containing 1.3, 6.1 and 12.3 wt. % Cr, respectively. The methods of calcination, analysis and measurement of magnetic susceptibility, e.s.r. and diffuse reflectance spectra are described in Part 1.' Samples are designated according to their chromium content (wt. %) and calcination temperature; thus, sample CAC 1.3 (50) contains 1.3 wt.% Cr and was calcined at 50°C for 5 h. RESULTS The mass susceptibility of Cry xz, measured at room temperature for samples CAC 1.3, CAC 6.1 and CAC 12.3 initially dried at 50°C, is plotted against the t Present address : School of Science, Faculty of Combined Studies, Hull College of Higher Education, Kingston upon Hull. $ Present address : School of Chemistry, Brunel University, Uxbridge, Middlesex. 1-89 28072808 MAGNETIC PROPERTIES OF COPRECIPITATED CHROMIUM HYDROXIDE GELS I l l l l l ~ ~ l l ' l 200 600 600 800 1000 1200 T/"C FIG. 1.---YCm' at room temperature plotted against calcination temperature T for the samples x CAC 1.3 (50) ; 0, CAC 6.1 (50) ; A, CAC 12.3 (50). temperature of calcination in fig. 1. Between 50 and 200°C the observed decrease in xz is accompanied by a change in colour from blue or green to yellow or brown depending upon chromium content.The mass susceptibilities of samples CAC (200), CAC (400) and CAC (600) are substantially magnetic-field dependent, see table 1. A field-independent set of measurements is included for comparison and it should be noted that the original gels dried at 50°C had field-independent susceptibilities. 3500 3000 2500A . ELLISON AND K. S. W. SING 2809 to O L 500 3500 3000 2500 (b) H/kG gain 1000 n - - 1 1000 500 1 I 3 500 3000 (4 H/kG FIG. 2.-Qualitative e.s.r. spectra at 77 K for the samples (low field, gain 1000, high field, gain 40). (a) CAC 1.3 (200); (b) CAC 1.3 (400); (c) CAC 1.3 (600). Qualitative e.s.r. spectra, obtained at 77 K are shown in fig.2 for samples CAC 1.3 (200), (400) and (600). Resonances of markedly different intensities are shown, recorded at different signal gains. The predominant resonance in each case is a strong narrow and symmetric signal at gain 40, peak to peak width (p.p.w.) 50 G, centred at ~3400 G. This is identified as the y-resonance of supported chromium." At a gain of 1000, &resonance is detected for the 200 and 400°C samples, but not for the 600°C sample. The fine structure is well resolved showing distinct lines at 11002810 MAGNETIC PROPERTIESOF COPRECIPITATED CHROMIUM HYDROXIDE GELS and 1700 G. The e.s.r. spectrum at 77 K for sample CAC 6.1 (400), consists of an intense y-resonance; P-resonance is also detected but is either of low intensity or, at this temperature, has been dipole-dipole broadened almost beyond detection.Fig. 3 shows molecular field plots, (xA)-'/m3 mol-1 against T/K, for samples CAC 1.3 (200), (400) and (600). These plots are curves convex to the temperature axis, giving extrapolated intercepts on this axis of between +90 and + 100 K. TABLE 1 .-MAGNETIC FIELD DEPENDENCE OF MAGNETIC SUSCEPTIBILITY 10: XF/rn3 kg-1 sample ( n ) (b) C) CAC 1.3 (50) 13.75 13.78 13.70 CAC 1.3 (200) 5.14 4.98 4.80 CAC 1.3 (400) 5.52 5.03 4.74 CAC 1.3 (600) 5.98 5.71 5.54 Field strength (a) 2.1 (b) 4.0 and (c) 6.5 kG. Quantitative e.s.r. spectra, for those samples showing only y-resonance at 3400 G, were determined over the temperature range 77 to 573 K. The resonance intensity I, calculated as the first moment of area of the y-resonance for samples CAC 1.3 (200), (400), (600) and CAC 6.1 (400) is shown in table 2 as a function of temperature.Examples of the spectra obtained are shown in fig. 4. The widths of the y-resonances 600 5 00 2 Y A00 53 L 300 d \ 4 8 ?3 200 100 1 1 I 0 100 200 300 TlK FIG. 3 . - 1 / ~ ~ plotted against temperature of measurement, T, (XA is the atomic susceptibility of Cr) for the samples (a) CAC 1.3 (200), x ; (b) CAC 1.3 (400), 0 ; (c) CAC 1.3 (W), A. remain constant within 1 G, over the whole temperature range and are independent of Cr content. Molecular field plots derived from these data are plotted in fig. 5 as I-1 against T. The plots show pronounced curvature, convex to the temperature axis, giving extrapolated intercepts close to those obtained from the xE-' against T plots for the same samples, see fig.3.281 1 A . ELLISON A N D K . S . W . SING TABLE 2.-E.S.R. SPECTRAL PARAMETERS FOR y-RESONANCE (T is the measurement temperature ; I is the relative number of spins ; p.p.w. is the peak to peak width) sample CAC 1 . 3 (200) CAC 1 3 (400) CAC 1.3 (600) CAC 6.1 (400) key T/K gain p.p.w./G 10-171 gain p.p.w:/G 10-17 Z gain p.p.w./G lO-17Z gain p.p.W./G 104’1 fig. 4 77 125 313 160 133 160 153 160 173 193 160 223 233 160 273 298 160 323 348 373 398 423 448 473 498 523 548 573 60 29.5 100 60 42.1 250 75 29.1 100 70 83.1 60 16.3 100 60 23.2 320 75 16.7 125 70 44.1 60 14.1 60 12.5 320 75 8.7 125 70 34.9 60 10.2 320 75 7.1 125 70 28.7 60 8.8 320 75 5.5 125 70 25.2 320 75 5.2 125 70 23.0 60 7.2 100 60 10.0 320 75 6.6 100 60 16.5 100 60 12.9 500 75 500 75 500 75 500 75 500 75 630 75 630 75 630 75 630 75 630 75 630 75 5.7 5.3 5.1 3.9 2.9 2.5 2.5 2.9 3.7 4.8 5.5 F~o.4.-Quantitative e.s.r. spectra of y-resonance for sample CAC 1.3 (600). Key for spectra (a) to ( I ) is given in table 2.2812 MAGNETIC PROPERTIES OF COPRECIPITATED CHROMIUM HYDROXIDE GELS 30 25 2 0 rl I 0 + 0 P( CI 1 E 1 5 - 5 1 0 L 5 - 35 - - - I 30 25 10 5 1 I I J 0 100 200 300 T/K FIG. 5.-Quantitative e.s.r. spectra ; I-' plotted against temperaturelof measurement, T. ( I is the relative e.s.r. intensity ofy-phase Cr). x , CAC 1.3 (200) ; 0, CAC 1.3 (400) ; A, CAC 1.3 (600)A. ELLISON AND K. S . W. SING 281 3 Values of 1 for the y-resonance of CAC 1.3 (600) are plotted against T in fig.6. The intensity at low temperatures is seen to increase with decreasing temperature at a much greater rate than expected for a paramagnet obeying the Curie or Curie- Weiss laws. The data was collected from two separate series of measurements, from 77 to 298 K and from 298 to 573 K causing the obvious discontinuity at 298 K. However, the increase in intensity with increasing temperature after x 470 K is anomalous and indicative of a transition temperature. DISCUSSION Between the calcination temperatures 50 and 200°C there is a decrease in ~2, accompanied by a change in sample colour, due to oxidation of Cr3+ ions. xE decreases to give a minimum centred at ~ 4 0 0 ° C for each sample, independent of Cr content, behaviour similar to that observed for CrC1,-Al,O, samples.2 The breadth of the minima and the temperature at which xg begins to increase, uiz.at 600°C for CAC 1.3 and at 400°C for CAC 6.1 and CAC 12.3, depends upon Cr content. This refiats the manner in which the stability of oxidised chromium depends upon the degree of dispersion of chromium on the support.l* Chromium at low concentration is more stable to oxidation than chromium at higher concen- tration. Indeed fig. 1 shows that even at 400°C some proportion of Cr3+ remains unoxidised. Samples CAC 6.1 and CAC 12.3 have identical Xz-minimum values; in the case of CAC 1.3 the position and value of the Xz-minimum is uncertain due to the marked field-dependent susceptibilities observed. Nevertheless the xE values all lie within a very narrow range. After calcination at 1120°C all of the chromium has been reduced to the Cr3+ state. The xE values are larger than the corresponding values in related systems suggesting that a more dispersed form of chromium has been achieved.This idea is confirmed on observing that the low-field derivative spectra in fig. 2 for CAC 1.3 are much more well defined and narrower than similar &resonances reported else- where.2 This width is usually interpreted in terms of a range of low-symmetry, zero-field terms, D and E, in the spin Hamiltonian. However an appropriate operator representing dipole-dipole broadening would provide an acceptable alter- native interpretation which is certainly more realistic for the planar clusters of chromium discussed in Part 2.2 On this model the 8-spectra of fig.2 suggest weaker dipole-dipole coupling due to greater dispersion of chromium in the support. The nature of the chromium responsible for the XC,'-minirna is worthy of some discussion. It has been shown that supported Cr3+, in both impregnated and coprecipitated conditions, is incompletely oxidised within a small temperature range, independently of concentration. This is not the behaviour of unsupported Cr3+. Cathers and Wendlandt have shown that all isomers of hydrated CrC1, decompose on heating to give Cr203, with loss of HCl and HzO, without passing through oxida- tion states higher than 3+. Similarly a-Cr,O,, does not undergo oxidation on heating in this temperature region. In addition, over the same temperature range, it is observed that supported Cr6+ is reduced and although the extent of reduction depends upon chromium content, e.s.r.and susceptibility measurements show that considerable reduction of low chromium content samples has occurred at 200"C.1 It is significant that not only do the xg-rninima occur in this region of coincident oxidation and reduction but also that the predominant paramagnetic phase, from e.s.r. evidence, is y-phase chromium independent of loading and of type of supported2814 MAGNETIC PROPERTIES OF COPRECIPITATED CHROMIUM HYDROXIDE GELS system. In addition susceptibilities are magnetic field-dependent and the values of Xz-minima show very little variation with chromium content. The chromium oxidation states whose identities have been firmly established in this region, by optical spectra and e.s.r.techniques, are Cr3+ and Cr6+. y-phase chromium, the predominant paramagnetic phase, has been interpreted as magnetically- isolated Crs+ ions which are not coupled by exchange interactions.6 If one accepts this model, for the sake of argument, the situation arises that in the calcination temperature range 200-600°C and at the Xz-minima, one must consider that there are three states of chromium present, viz., Cr3+, Cr6+ and Cr5+. x:, iteff and 6 depend markedly upon the relative amounts of the different chromium species and upon their individual degrees of dispersion. In fact we have observed that, in this calcination temperature range, xE remains remarkably constant even though the rates and magnitudes of oxidation and reduction and the values of 6 and of peff of each chromium species are very different and, moreover, depend upon chromium content.Further, the proportion of y-phase chromium produced on calcination of Cr3+ or Cr6+ depends markedly upon the surface area of the chromium, the nature of the support, the chromium content and the calcination temperature, resulting in modification of petf, 8 and ~2.'. x-' / .- / I /A . ELLISON AND K. S . W. SING 2815 of the mobile carrier spin during a " hop " from one lattice site to the next. The mobile carrier contributes to the binding energy of the system provided that the spins on neighbouring sites are parallel. The electric and magnetic properties of such compounds depend strongly upon their composition, x. Thus when x = 0 or 1 the oxides are good insulators and antiferromagnetic, while for a range of intermediate compositions the electric conductivity is several orders of magnitude greater and the materials are ferromagnetic. The theoretical temperature dependence of the magnetic susceptibility is shown in fig.7 and exactly describes the curved x-' against T plots reported here. Provided that the upper transition temperature is antiferromagnetic, a discon- tinuity in slope is observed at TN, yielding a negative, high-temperature Weiss constant 8. Low-temperature extrapolation yields a positive-temperature intercept, T,.' * The oxidised phase of chromium, 02--Cr3+-02--Cr6+-02- possesses magnetic properties described by the Zener model; moreover the existence of collective Zener electrons is shown by the colours, spectra and p-type semiconductivity of this phase.At the extremities of the series (Cr:' Cr;Lx)Oi- are Cri+03 (x = 0, n-type semi- conductor, antiferromagnetic) and Cr6+03 (x = 1, insulator, temperature independent paramagnetic). The stable Zener phase, when x > 0, occurs at the surface of chromium clusters where the antisotropic ferromagnetic coupling responsible for stability is maximised. It is important to realise that the existence of mixed-valency compounds is well established and in particular is characteristic of the chemistry of chromium. For example, the reviews of Allen and Hush l4 and of Robin and Day cite totals of 150 and 820 references, respectively. It is not possible to give here a complete review of the incidence of mixed-valency species in chromium chemistry.Neverthe- less this review does reveal that Crrrr-Crvr, Crrrr-Crrr, Crrrr-Crrv and Crv-Crvl mixed-valency compounds occur as stable stoichiometric and non-stoichiometric compounds. Most importantly Wilhelmi and co-authors 16* have shown that compounds of the type MCr308 and M2Cr309 (M = alkali metal ion)16 and intermediate oxides of chromium, including Cr205, Cr308, CrsOIz and Cr,OI5 l7 are black crystalline substances containing CrrIr-CrVr mixed-valency species alone. It is maintained that the magnetic and optical properties of the oxidised-phase of chromium are more completely explained on the basis of mixed-valency species of the type CrIrr-Crvr without recourse to the requirement of CrV ions stabilised at specific sites in the support l a t t i ~ e .~ Indeed the observed data are typical of those expected for a Zener double-exchange system. ~2 for samples at susceptibility minima is often magnetic field dependent. Molecular field plots (xi1 against T) show pronounced curvature towards the tempera- ture axis with intercepts of w -k90 IS. This unusual behaviour cannot be explained on uncoupled systems of Cr6+ or Cr3+ ions. Cr6+ ions have a temperature- independent susceptibiIity which would produce high-temperature deviation from linearity not observed here. Octahedral Cr3+ has a weak field 4A2, ground state with minimal orbital angular momentum contribution to peff or 6 through spin-orbit coupling. The amorphous Cr3+ phases in the temperature range 77 to 298 K are paramagnetic producing P-phase e.s.r., giving negative 6 values but no curvature in these plots.Ferrimagnetic structures could produce such curves but require well- ordered, three-dimensional interpenetrating-ferromagnetic sub-lattices. In, for example, C4" symmetry Cr5+ ions with a split 2T2 ground state can produce low- temperature deviation from Curie-Weiss behaviour but the deviation is away from the temperature axis, not as observed here. The Zener theory, however, does predict curved molecular field plots.2816 MAGNETIC PROPERTIES OF COPRECIPITATED CHROMIUM HYDROXIDE GELS Visible-u.v. reflectance spectra show Cr3+ and Cr6+ bands with, in addition, enhanced absorbance over the whole wavelength range, 200-1000 nm. Most signifi- cantly, these brown-black samples absorb radiation at wavelengths where the postulated individual species (Cr3+, Cr6+, Cr5+) are transparent.These observations are consistent with the existence of Zener mobile-electrons in a class 111 mixed- valency system. The predominant e.s.r. signal is y-resonance. Both 6- and P-resonances are often not detected even though the presence of Cr3+ species is confirmed from reflectance spectra. This suggests that the normal resonances of Cr3+ electron-spins have been obliterated through strong exchange forces. attaining a maximum intensity for alumina samples near 2 wt. % Cr where the proportion of small and thin, platelike chromium clusters is the greatest. Indeed, y-resonance appears at the expense of /?-resonance whilst &resonance is still often retained. y-resonance achieves greatest intensity for silica supported chromium, the most active catalysts in ethylene polymerisation.It has been asserted that &phase chromium has never been observed in silica systems, although after achieving a greater dispersion of chromium by thermal decomposition of mixed oxalato pre- cipitates, a small but unstable &phase signal has been reported.lg Cr5+ ions, often said to be responsible for y-resonance, need to occupy high symmetry support-lattice sites to acquire the necessary stabilisation energy. However, not only is a dispersed phase of CrS+ (the product of &phase) unlikely but also there is much evidence that chromium is insoluble in silica even at the liquidus temperature.' It would appear that although Cr3+ species occur in the oxidised chromium their normal magnetic properties are obliterated. Instead the oxidised phase itself exhibits different behaviour which is not characteristic of Cr5+ ions isolated in the support lattice.Moreover molecular field plots for y-resonance, I-' against T, in all of the supported systems studied show pronounced curvature with positive temperature intercepts, behaviour consistent with direct exchange interactions between ions of different oxidation state. For alumina-supported chromium the width of the y-resonance, from 77 to 593 K, is completely temperature-independent. The usual argument invoked is that spin- lattice relaxation is slow. However, if this is the case, and if as is observed the resonance does not show signs of saturation then another predominating relaxation mechanism must be efficiently operative.Thus Adrian 2 o maintains that spin-lattice interactions can become so inefficient that they are ineffective as a relaxation process. It has been estimated 21 that the contribution to the line width made by spin-lattice interactions of Cr3+ in A1203 is only 1 at room temperature. Spin-spin mechanisms must therefore provide the necessary route for the efficient release of magnetic energy during resonance absorption. This again implies that the origin of y-resonance lies in a magnetically-exchange-coupled system of spins rather than in magnetically-isolated species. Where temperature-dependent spectra are observed, in SO2 samplesY6 they are anomalous showing line-splitting at high temperatures and positive-temperature deviation from the Curie-Weiss law.This behaviour is not satisfactorily explained by these authors. In conclusion, a surface mobile-electron Zener phase creates the ideal environment in which catalytic activity is maximised, the current model for the polymerisation active sites requiring both ease of oxidation and reduction and a quality of coordinative unsaturat io n. y-phase resonance arises from the surface of clustered chromiumA. ELLISON AND K. S. W. SING A. Ellison, J. 0. V. Oubridge and K. S . W. Sing, Trans. Faraday Suc., 1970, 66, 1004. A. Ellison and K. S. W. Sing, J.C.S. Faraday I 1978. 74, 2017. M. R. Harris and K. S. W. Sing, J. Appl. Chem., 1957, 7, 397. B. E. O’Reilly and D. S. MacIver, J. Phys. Chem., 1962, 66, 276.R. E. Cathers and W. W. Wendlandt, J. Inorg. Nuclear Chem., 1365,27, 1015. L. L. Van Reijen and P. Cossee, Disc. Faraday SOC., 1966, 41, 277. M. P. McDaniel and R. L. Burwell Jnr., J. Catalysis, 1975, 36,404. (a) F. S. Baker, J. D. Carruthers, R. E. Day, K. S. W. Sing and L. J. Stryker, Disc. Furaday SOC., 1971, 52, 173; (b) 3. D. Carruthers, IS. S. W. Sing and J. Fennerty, Nature, 1967, 213, 66; (c) J. D. Carruthers, J. Fennerty and K. S. W. Sing, 6th Int. Symp. Reactivity of Solids, 1968, ed. J. W. Mitchell, R. C. de Vries, R. W. Roberts and P. Cannon (J. Wiley and Sons, 1969), lo (a) R. L. Burwell Jnr., G. L. Haller, K. C. Taylor and J. F. Read, Adv. Catalysis, 1969, 20, 1 ; l1 C. Zener, Phys. Rev., 1951,82,403. l2 P. W. Anderson and H. Hasegawa, Phys. Rea, 1955, 100, 675.l3 P. G. de Gennes, Phys. Rev., 1960, 118,141. l4 G. C. Allen and N. S. Hush, Progr. Inorg. Chem., 1967, 8, 357. l5 M. P. Robin and P. Day, Ado. Inorg. Chem. Radio Chem., 1967,10, 247. l6 (a) K. A. Wilhelmi, Chem. Comm., 1966,437 ; (b) K. A. Wilhelmi, 0. Jonsson and E. Lagervall, l7 K. A. Wilhelmi, Actu Chem. Scand., 1965,19,165. l9 D. E. O’Reilly, F. E. Santiago and R. G. Squires, J. Phys. Chem., 1969, 73, 3172. ‘O F. J. Adrian, J. ColloidInterface Sci., 1968, 26, 317. 21 J. S. Thorp and H. P. Buckley, J. Material Sci., 1974, 9, 1499. ’’ Yu. Ermakov and V . Zakharov, Adu. Catalysis, 1975, 24, 173. 2817 ’ A. Ellison, review to be published. p. 127-135. (b) M. P. McDaniel and R. L. Burwell Jm., J. Catalysis, 1975, 36, 394. Acta Chem. Scand., 1969, 23, 1074.C. P. Poole Jnr. and D. S. MacIver, Adv. Catalysis, 1967, 17,223. (PAPER 7/2201) Magnetic and Optical Studies of Chromium Oxides Part 3.-Calcination of Coprecipitated Chromium and Aluminium Hydroxide Gels BY ALAN ELLIsoN*t AND KENNETH s. w. SING$. School of Science, Hull College of Higher Education, Kingston upon Hull HU6 7RT Received 16th December, 1977 Magnetic and optical studies were performed on coprecipitated chromium and aluminium hydroxide gels after calcination in air at temperatures between 50 and 1150°C. The results show that magnetic diIution was not achieved and that surface clusters of chromium oxide were formed. The magnetism, e.s.r. and optical spectra of oxidised samples are discussed in terms of a mixed- valency phase of partially-oxidised chromium containing Cr3+ and Cr6+ species.This new model ofy-phase chromium is shown to be consistent and an acceptable part of the extensive mixed-valency chemistry of chromium. This work continues a systematic study of the magnetic and spectral properties of chromium-alumina catalysts. It was shown in Parts 1 and 2 that the state of dispersion of chromium on the alumina support influences the stability of Cr6+, the occurrence and stability of y-phase chromium and the production of an " oxidised- phase " of chromium. There is some evidence that a well-dispersed state of chromium can be obtained using coprecipitation techniques and it is to be expected that on calcination of coprecipitated gels the chromium ion has the most favourable chance of occupying cation sites of well-defined symmetry in the host lattice.This paper is concerned with a study of the thermal decomposition of chromium and ahminium hydroxide gels coprecipitated from aqueous solution. EXPERIMENTAL The coprecipitated gels were prepared by allowing aqueous solutions of chromium (III) nitrate, aluminium nitrate and ammonium hydroxide to react under controlled conditions of pH, concentration and flow-rate in an apparatus similar to that of Harris and Sing,3 but modified to include a constant-head device. Three precipitates were prepared, containing 1.3, 6.1 and 12.3 wt. % Cr, respectively. The methods of calcination, analysis and measurement of magnetic susceptibility, e.s.r. and diffuse reflectance spectra are described in Part 1.' Samples are designated according to their chromium content (wt. %) and calcination temperature; thus, sample CAC 1.3 (50) contains 1.3 wt.% Cr and was calcined at 50°C for 5 h. RESULTS The mass susceptibility of Cry xz, measured at room temperature for samples CAC 1.3, CAC 6.1 and CAC 12.3 initially dried at 50°C, is plotted against the t Present address : School of Science, Faculty of Combined Studies, Hull College of Higher Education, Kingston upon Hull. $ Present address : School of Chemistry, Brunel University, Uxbridge, Middlesex. 1-89 28072808 MAGNETIC PROPERTIES OF COPRECIPITATED CHROMIUM HYDROXIDE GELS I l l l l l ~ ~ l l ' l 200 600 600 800 1000 1200 T/"C FIG. 1.---YCm' at room temperature plotted against calcination temperature T for the samples x CAC 1.3 (50) ; 0, CAC 6.1 (50) ; A, CAC 12.3 (50).temperature of calcination in fig. 1. Between 50 and 200°C the observed decrease in xz is accompanied by a change in colour from blue or green to yellow or brown depending upon chromium content. The mass susceptibilities of samples CAC (200), CAC (400) and CAC (600) are substantially magnetic-field dependent, see table 1. A field-independent set of measurements is included for comparison and it should be noted that the original gels dried at 50°C had field-independent susceptibilities. 3500 3000 2500A . ELLISON AND K. S. W. SING 2809 to O L 500 3500 3000 2500 (b) H/kG gain 1000 n - - 1 1000 500 1 I 3 500 3000 (4 H/kG FIG. 2.-Qualitative e.s.r. spectra at 77 K for the samples (low field, gain 1000, high field, gain 40).(a) CAC 1.3 (200); (b) CAC 1.3 (400); (c) CAC 1.3 (600). Qualitative e.s.r. spectra, obtained at 77 K are shown in fig. 2 for samples CAC 1.3 (200), (400) and (600). Resonances of markedly different intensities are shown, recorded at different signal gains. The predominant resonance in each case is a strong narrow and symmetric signal at gain 40, peak to peak width (p.p.w.) 50 G, centred at ~3400 G. This is identified as the y-resonance of supported chromium." At a gain of 1000, &resonance is detected for the 200 and 400°C samples, but not for the 600°C sample. The fine structure is well resolved showing distinct lines at 11002810 MAGNETIC PROPERTIESOF COPRECIPITATED CHROMIUM HYDROXIDE GELS and 1700 G. The e.s.r. spectrum at 77 K for sample CAC 6.1 (400), consists of an intense y-resonance; P-resonance is also detected but is either of low intensity or, at this temperature, has been dipole-dipole broadened almost beyond detection.Fig. 3 shows molecular field plots, (xA)-'/m3 mol-1 against T/K, for samples CAC 1.3 (200), (400) and (600). These plots are curves convex to the temperature axis, giving extrapolated intercepts on this axis of between +90 and + 100 K. TABLE 1 .-MAGNETIC FIELD DEPENDENCE OF MAGNETIC SUSCEPTIBILITY 10: XF/rn3 kg-1 sample ( n ) (b) C) CAC 1.3 (50) 13.75 13.78 13.70 CAC 1.3 (200) 5.14 4.98 4.80 CAC 1.3 (400) 5.52 5.03 4.74 CAC 1.3 (600) 5.98 5.71 5.54 Field strength (a) 2.1 (b) 4.0 and (c) 6.5 kG. Quantitative e.s.r. spectra, for those samples showing only y-resonance at 3400 G, were determined over the temperature range 77 to 573 K.The resonance intensity I, calculated as the first moment of area of the y-resonance for samples CAC 1.3 (200), (400), (600) and CAC 6.1 (400) is shown in table 2 as a function of temperature. Examples of the spectra obtained are shown in fig. 4. The widths of the y-resonances 600 5 00 2 Y A00 53 L 300 d \ 4 8 ?3 200 100 1 1 I 0 100 200 300 TlK FIG. 3 . - 1 / ~ ~ plotted against temperature of measurement, T, (XA is the atomic susceptibility of Cr) for the samples (a) CAC 1.3 (200), x ; (b) CAC 1.3 (400), 0 ; (c) CAC 1.3 (W), A. remain constant within 1 G, over the whole temperature range and are independent of Cr content. Molecular field plots derived from these data are plotted in fig. 5 as I-1 against T. The plots show pronounced curvature, convex to the temperature axis, giving extrapolated intercepts close to those obtained from the xE-' against T plots for the same samples, see fig.3.281 1 A . ELLISON A N D K . S . W . SING TABLE 2.-E.S.R. SPECTRAL PARAMETERS FOR y-RESONANCE (T is the measurement temperature ; I is the relative number of spins ; p.p.w. is the peak to peak width) sample CAC 1 . 3 (200) CAC 1 3 (400) CAC 1.3 (600) CAC 6.1 (400) key T/K gain p.p.w./G 10-171 gain p.p.w:/G 10-17 Z gain p.p.w./G lO-17Z gain p.p.W./G 104’1 fig. 4 77 125 313 160 133 160 153 160 173 193 160 223 233 160 273 298 160 323 348 373 398 423 448 473 498 523 548 573 60 29.5 100 60 42.1 250 75 29.1 100 70 83.1 60 16.3 100 60 23.2 320 75 16.7 125 70 44.1 60 14.1 60 12.5 320 75 8.7 125 70 34.9 60 10.2 320 75 7.1 125 70 28.7 60 8.8 320 75 5.5 125 70 25.2 320 75 5.2 125 70 23.0 60 7.2 100 60 10.0 320 75 6.6 100 60 16.5 100 60 12.9 500 75 500 75 500 75 500 75 500 75 630 75 630 75 630 75 630 75 630 75 630 75 5.7 5.3 5.1 3.9 2.9 2.5 2.5 2.9 3.7 4.8 5.5 F~o.4.-Quantitative e.s.r. spectra of y-resonance for sample CAC 1.3 (600). Key for spectra (a) to ( I ) is given in table 2.2812 MAGNETIC PROPERTIES OF COPRECIPITATED CHROMIUM HYDROXIDE GELS 30 25 2 0 rl I 0 + 0 P( CI 1 E 1 5 - 5 1 0 L 5 - 35 - - - I 30 25 10 5 1 I I J 0 100 200 300 T/K FIG. 5.-Quantitative e.s.r. spectra ; I-' plotted against temperaturelof measurement, T. ( I is the relative e.s.r. intensity ofy-phase Cr). x , CAC 1.3 (200) ; 0, CAC 1.3 (400) ; A, CAC 1.3 (600)A.ELLISON AND K. S . W. SING 281 3 Values of 1 for the y-resonance of CAC 1.3 (600) are plotted against T in fig. 6. The intensity at low temperatures is seen to increase with decreasing temperature at a much greater rate than expected for a paramagnet obeying the Curie or Curie- Weiss laws. The data was collected from two separate series of measurements, from 77 to 298 K and from 298 to 573 K causing the obvious discontinuity at 298 K. However, the increase in intensity with increasing temperature after x 470 K is anomalous and indicative of a transition temperature. DISCUSSION Between the calcination temperatures 50 and 200°C there is a decrease in ~2, accompanied by a change in sample colour, due to oxidation of Cr3+ ions. xE decreases to give a minimum centred at ~ 4 0 0 ° C for each sample, independent of Cr content, behaviour similar to that observed for CrC1,-Al,O, samples.2 The breadth of the minima and the temperature at which xg begins to increase, uiz.at 600°C for CAC 1.3 and at 400°C for CAC 6.1 and CAC 12.3, depends upon Cr content. This refiats the manner in which the stability of oxidised chromium depends upon the degree of dispersion of chromium on the support.l* Chromium at low concentration is more stable to oxidation than chromium at higher concen- tration. Indeed fig. 1 shows that even at 400°C some proportion of Cr3+ remains unoxidised. Samples CAC 6.1 and CAC 12.3 have identical Xz-minimum values; in the case of CAC 1.3 the position and value of the Xz-minimum is uncertain due to the marked field-dependent susceptibilities observed. Nevertheless the xE values all lie within a very narrow range.After calcination at 1120°C all of the chromium has been reduced to the Cr3+ state. The xE values are larger than the corresponding values in related systems suggesting that a more dispersed form of chromium has been achieved. This idea is confirmed on observing that the low-field derivative spectra in fig. 2 for CAC 1.3 are much more well defined and narrower than similar &resonances reported else- where.2 This width is usually interpreted in terms of a range of low-symmetry, zero-field terms, D and E, in the spin Hamiltonian. However an appropriate operator representing dipole-dipole broadening would provide an acceptable alter- native interpretation which is certainly more realistic for the planar clusters of chromium discussed in Part 2.2 On this model the 8-spectra of fig.2 suggest weaker dipole-dipole coupling due to greater dispersion of chromium in the support. The nature of the chromium responsible for the XC,'-minirna is worthy of some discussion. It has been shown that supported Cr3+, in both impregnated and coprecipitated conditions, is incompletely oxidised within a small temperature range, independently of concentration. This is not the behaviour of unsupported Cr3+. Cathers and Wendlandt have shown that all isomers of hydrated CrC1, decompose on heating to give Cr203, with loss of HCl and HzO, without passing through oxida- tion states higher than 3+. Similarly a-Cr,O,, does not undergo oxidation on heating in this temperature region.In addition, over the same temperature range, it is observed that supported Cr6+ is reduced and although the extent of reduction depends upon chromium content, e.s.r. and susceptibility measurements show that considerable reduction of low chromium content samples has occurred at 200"C.1 It is significant that not only do the xg-rninima occur in this region of coincident oxidation and reduction but also that the predominant paramagnetic phase, from e.s.r. evidence, is y-phase chromium independent of loading and of type of supported2814 MAGNETIC PROPERTIES OF COPRECIPITATED CHROMIUM HYDROXIDE GELS system. In addition susceptibilities are magnetic field-dependent and the values of Xz-minima show very little variation with chromium content.The chromium oxidation states whose identities have been firmly established in this region, by optical spectra and e.s.r. techniques, are Cr3+ and Cr6+. y-phase chromium, the predominant paramagnetic phase, has been interpreted as magnetically- isolated Crs+ ions which are not coupled by exchange interactions.6 If one accepts this model, for the sake of argument, the situation arises that in the calcination temperature range 200-600°C and at the Xz-minima, one must consider that there are three states of chromium present, viz., Cr3+, Cr6+ and Cr5+. x:, iteff and 6 depend markedly upon the relative amounts of the different chromium species and upon their individual degrees of dispersion. In fact we have observed that, in this calcination temperature range, xE remains remarkably constant even though the rates and magnitudes of oxidation and reduction and the values of 6 and of peff of each chromium species are very different and, moreover, depend upon chromium content. Further, the proportion of y-phase chromium produced on calcination of Cr3+ or Cr6+ depends markedly upon the surface area of the chromium, the nature of the support, the chromium content and the calcination temperature, resulting in modification of petf, 8 and ~2.'.x-' / .- / I /A . ELLISON AND K. S . W. SING 2815 of the mobile carrier spin during a " hop " from one lattice site to the next. The mobile carrier contributes to the binding energy of the system provided that the spins on neighbouring sites are parallel. The electric and magnetic properties of such compounds depend strongly upon their composition, x.Thus when x = 0 or 1 the oxides are good insulators and antiferromagnetic, while for a range of intermediate compositions the electric conductivity is several orders of magnitude greater and the materials are ferromagnetic. The theoretical temperature dependence of the magnetic susceptibility is shown in fig. 7 and exactly describes the curved x-' against T plots reported here. Provided that the upper transition temperature is antiferromagnetic, a discon- tinuity in slope is observed at TN, yielding a negative, high-temperature Weiss constant 8. Low-temperature extrapolation yields a positive-temperature intercept, T,.' * The oxidised phase of chromium, 02--Cr3+-02--Cr6+-02- possesses magnetic properties described by the Zener model; moreover the existence of collective Zener electrons is shown by the colours, spectra and p-type semiconductivity of this phase.At the extremities of the series (Cr:' Cr;Lx)Oi- are Cri+03 (x = 0, n-type semi- conductor, antiferromagnetic) and Cr6+03 (x = 1, insulator, temperature independent paramagnetic). The stable Zener phase, when x > 0, occurs at the surface of chromium clusters where the antisotropic ferromagnetic coupling responsible for stability is maximised. It is important to realise that the existence of mixed-valency compounds is well established and in particular is characteristic of the chemistry of chromium. For example, the reviews of Allen and Hush l4 and of Robin and Day cite totals of 150 and 820 references, respectively.It is not possible to give here a complete review of the incidence of mixed-valency species in chromium chemistry. Neverthe- less this review does reveal that Crrrr-Crvr, Crrrr-Crrr, Crrrr-Crrv and Crv-Crvl mixed-valency compounds occur as stable stoichiometric and non-stoichiometric compounds. Most importantly Wilhelmi and co-authors 16* have shown that compounds of the type MCr308 and M2Cr309 (M = alkali metal ion)16 and intermediate oxides of chromium, including Cr205, Cr308, CrsOIz and Cr,OI5 l7 are black crystalline substances containing CrrIr-CrVr mixed-valency species alone. It is maintained that the magnetic and optical properties of the oxidised-phase of chromium are more completely explained on the basis of mixed-valency species of the type CrIrr-Crvr without recourse to the requirement of CrV ions stabilised at specific sites in the support l a t t i ~ e .~ Indeed the observed data are typical of those expected for a Zener double-exchange system. ~2 for samples at susceptibility minima is often magnetic field dependent. Molecular field plots (xi1 against T) show pronounced curvature towards the tempera- ture axis with intercepts of w -k90 IS. This unusual behaviour cannot be explained on uncoupled systems of Cr6+ or Cr3+ ions. Cr6+ ions have a temperature- independent susceptibiIity which would produce high-temperature deviation from linearity not observed here. Octahedral Cr3+ has a weak field 4A2, ground state with minimal orbital angular momentum contribution to peff or 6 through spin-orbit coupling. The amorphous Cr3+ phases in the temperature range 77 to 298 K are paramagnetic producing P-phase e.s.r., giving negative 6 values but no curvature in these plots.Ferrimagnetic structures could produce such curves but require well- ordered, three-dimensional interpenetrating-ferromagnetic sub-lattices. In, for example, C4" symmetry Cr5+ ions with a split 2T2 ground state can produce low- temperature deviation from Curie-Weiss behaviour but the deviation is away from the temperature axis, not as observed here. The Zener theory, however, does predict curved molecular field plots.2816 MAGNETIC PROPERTIES OF COPRECIPITATED CHROMIUM HYDROXIDE GELS Visible-u.v. reflectance spectra show Cr3+ and Cr6+ bands with, in addition, enhanced absorbance over the whole wavelength range, 200-1000 nm.Most signifi- cantly, these brown-black samples absorb radiation at wavelengths where the postulated individual species (Cr3+, Cr6+, Cr5+) are transparent. These observations are consistent with the existence of Zener mobile-electrons in a class 111 mixed- valency system. The predominant e.s.r. signal is y-resonance. Both 6- and P-resonances are often not detected even though the presence of Cr3+ species is confirmed from reflectance spectra. This suggests that the normal resonances of Cr3+ electron-spins have been obliterated through strong exchange forces. attaining a maximum intensity for alumina samples near 2 wt. % Cr where the proportion of small and thin, platelike chromium clusters is the greatest. Indeed, y-resonance appears at the expense of /?-resonance whilst &resonance is still often retained.y-resonance achieves greatest intensity for silica supported chromium, the most active catalysts in ethylene polymerisation. It has been asserted that &phase chromium has never been observed in silica systems, although after achieving a greater dispersion of chromium by thermal decomposition of mixed oxalato pre- cipitates, a small but unstable &phase signal has been reported.lg Cr5+ ions, often said to be responsible for y-resonance, need to occupy high symmetry support-lattice sites to acquire the necessary stabilisation energy. However, not only is a dispersed phase of CrS+ (the product of &phase) unlikely but also there is much evidence that chromium is insoluble in silica even at the liquidus temperature.' It would appear that although Cr3+ species occur in the oxidised chromium their normal magnetic properties are obliterated.Instead the oxidised phase itself exhibits different behaviour which is not characteristic of Cr5+ ions isolated in the support lattice. Moreover molecular field plots for y-resonance, I-' against T, in all of the supported systems studied show pronounced curvature with positive temperature intercepts, behaviour consistent with direct exchange interactions between ions of different oxidation state. For alumina-supported chromium the width of the y-resonance, from 77 to 593 K, is completely temperature-independent. The usual argument invoked is that spin- lattice relaxation is slow.However, if this is the case, and if as is observed the resonance does not show signs of saturation then another predominating relaxation mechanism must be efficiently operative. Thus Adrian 2 o maintains that spin-lattice interactions can become so inefficient that they are ineffective as a relaxation process. It has been estimated 21 that the contribution to the line width made by spin-lattice interactions of Cr3+ in A1203 is only 1 at room temperature. Spin-spin mechanisms must therefore provide the necessary route for the efficient release of magnetic energy during resonance absorption. This again implies that the origin of y-resonance lies in a magnetically-exchange-coupled system of spins rather than in magnetically-isolated species. Where temperature-dependent spectra are observed, in SO2 samplesY6 they are anomalous showing line-splitting at high temperatures and positive-temperature deviation from the Curie-Weiss law.This behaviour is not satisfactorily explained by these authors. In conclusion, a surface mobile-electron Zener phase creates the ideal environment in which catalytic activity is maximised, the current model for the polymerisation active sites requiring both ease of oxidation and reduction and a quality of coordinative unsaturat io n. y-phase resonance arises from the surface of clustered chromiumA. ELLISON AND K. S. W. SING A. Ellison, J. 0. V. Oubridge and K. S . W. Sing, Trans. Faraday Suc., 1970, 66, 1004. A. Ellison and K. S. W. Sing, J.C.S. Faraday I 1978. 74, 2017. M. R. Harris and K. S. W. Sing, J. Appl. Chem., 1957, 7, 397. B. E. O’Reilly and D. S. MacIver, J. Phys. Chem., 1962, 66, 276. R. E. Cathers and W. W. Wendlandt, J. Inorg. Nuclear Chem., 1365,27, 1015. L. L. Van Reijen and P. Cossee, Disc. Faraday SOC., 1966, 41, 277. M. P. McDaniel and R. L. Burwell Jnr., J. Catalysis, 1975, 36,404. (a) F. S. Baker, J. D. Carruthers, R. E. Day, K. S. W. Sing and L. J. Stryker, Disc. Furaday SOC., 1971, 52, 173; (b) 3. D. Carruthers, IS. S. W. Sing and J. Fennerty, Nature, 1967, 213, 66; (c) J. D. Carruthers, J. Fennerty and K. S. W. Sing, 6th Int. Symp. Reactivity of Solids, 1968, ed. J. W. Mitchell, R. C. de Vries, R. W. Roberts and P. Cannon (J. Wiley and Sons, 1969), lo (a) R. L. Burwell Jnr., G. L. Haller, K. C. Taylor and J. F. Read, Adv. Catalysis, 1969, 20, 1 ; l1 C. Zener, Phys. Rev., 1951,82,403. l2 P. W. Anderson and H. Hasegawa, Phys. Rea, 1955, 100, 675. l3 P. G. de Gennes, Phys. Rev., 1960, 118,141. l4 G. C. Allen and N. S. Hush, Progr. Inorg. Chem., 1967, 8, 357. l5 M. P. Robin and P. Day, Ado. Inorg. Chem. Radio Chem., 1967,10, 247. l6 (a) K. A. Wilhelmi, Chem. Comm., 1966,437 ; (b) K. A. Wilhelmi, 0. Jonsson and E. Lagervall, l7 K. A. Wilhelmi, Actu Chem. Scand., 1965,19,165. l9 D. E. O’Reilly, F. E. Santiago and R. G. Squires, J. Phys. Chem., 1969, 73, 3172. ‘O F. J. Adrian, J. ColloidInterface Sci., 1968, 26, 317. 21 J. S. Thorp and H. P. Buckley, J. Material Sci., 1974, 9, 1499. ’’ Yu. Ermakov and V . Zakharov, Adu. Catalysis, 1975, 24, 173. 2817 ’ A. Ellison, review to be published. p. 127-135. (b) M. P. McDaniel and R. L. Burwell Jm., J. Catalysis, 1975, 36, 394. Acta Chem. Scand., 1969, 23, 1074. C. P. Poole Jnr. and D. S. MacIver, Adv. Catalysis, 1967, 17,223. (PAPER 7/2201)

ISSN:0300-9599    

DOI:10.1039/F19787402807

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Photolysis of Periodate and Periodic Acid in Aqueous Solution BY ULRIK K. KLANING" Department of Chemistry, Aarhus University, 140 Langelandsgade, DK-8000 Aarhus C, Denmark KNUD SEHESTED AND Accelerator Department, Risar National Laboratory, DK-4000 Roskilde, Denmark Received 30th January, 1978 The photochemistry of periodate and periodic acid in aqueous solution was studied (i) by quantum yield measurements at low light intensity (ii) by flash photolysis, and (iii) by photolysis of glassy samples at 77 K. The photochemical studies were supplemented with pulse radiolysis studies of aqueous periodate solutions and with kinetic studies using stopped-flow technique. In strongly alkaline solution the photodecomposition of periodate proceeds via formation of 0- and I v I . At pH < 12 an additional primary process is the formation of IV and H202.In neutral solution 03P is formed in a small yield. The energetics of the reaction of O'D with HzO with formation of H20z is discussed. It is suggested that oxygen atoms are formed only from 10; and not from other IVII species. Mechanisms for the secondary processes involving IvIII and IVI are given. IVIII and its relatively stable complex with IVII both form IV. IVII and 02. Depending on pH and concentration, IVI either disproportionates to IV and P I , reacts with IVII with formation of IV and P I 1 1 or dissociates into 0-(OH) and IV. Investigations of photodecomposition of the oxyanions XO,, n = 1-3 of chlorine, bromine and iodine in aqueous solution indicate that the primary reactions are one or more of reactions (1)-(4) ?l v xo, -+ xo,*- 1 + 0- xo,- -+ XO~--~ + 0 3 ~ xs, + X0,Y-I + UID xo,; -+ xo,z+o2 hv EL V k v where the formation of 0 3 P and of OID depends on the energy of the exciting light.lm4 A recent investigation of the photochemistry of perbromate, Br04 in aqueous solution has shown that the photochemistry of Br0: resembles that of other oxyanions of halogens, the primary reactions being reactions (3) and (4).5 Periodates are exceptional in the sense that several periodate species (in the following denoted col- lectively by IV") can exist in aqueous solution.We have studied the photochemistry of periodate in aqueous solution emphasizing the possible photochemical indi- vidualities that may arise from the different structures of the various IV1I species.The following IV"' species can exist in aqueous solution : H510s, H410g, 104, H310%-, H,I,O$C and H,IO2-. The equilibria among these IVI1 species are shown in (5)-(9) 281 8U. K. KLANING AND K . SEHESTED 2819 T = 298 K HSI06 = H410; +H+ H410; = H310%- +H+ pK1 = 3.3 pK2 = 6.7 (5) H410; = 10:+2H20 K, = 40 (7) (8) HJOg- = H210%-+H+ pK3 = 12.2'. (9) 2H310;- = H21204; +2H20 K, = 141 mol-1 dm3 ' Previous investigations have shown that irradiation of IVI1 in near neutral solution with ultraviolet light gives oxygen and iodate.8 It has been suggested that the photolysis proceeds via a free-radical chain mechanism, in which hydroxyl radicals are formed in a primary process. Irradiation of IV" dissolved in aqueous phosphate- borate glasses at liquid N2 temperature does not lead to decomposition of the Iv" species," a result which is not inconsistent with a free-radical chain mechanism at room temperature.In the present investigation we have studied the photochemistry of the various 1'" species at room temperature (21 & 1 ) O C at low light intensity under conditions of steady state and at high light intensity in flash photolysis. We have also studied the photochemistry of IV1I dissolved in various aqueous glasses at 77 K. We have supplemented the photochemical studies with pulse radiolysis studies of aqueous IV" solutions and with kinetic studies using the stopped-flow technique. EXPERIMENTAL MATERIALS All solutions were prepared from water doubly distilled in a Heraeus Bi 4 all-quartz still. N2 and Ar were AGA Special gases. NaOH, HzS04 and HC104 were Merck Suprapur.NaI04 and KI03 were Merck p.a. KBr04 and aqueous HBr04 was supplied by E. H. Appelman, Argonne National Laboratory, U.S.A. Carrier-free NaIl in aqueous NaOH was obtained from Institutt for Atomenergi, Kjeller, Norway. U02(C00)2 was prepared from AnalaR uranyl nitrate and oxalic acid (Merck p.a.) and recrystallized from water. Aqueous H202 was prepared from Merck unstabilized perhydrol and standardized against KMn04. Na3H2106 isotopically tagged with was prepared by oxidation of NaI containing Il3l with sodium hypochlorite in alkaline solution. To 5 x mol NaI containing 10 mC 1131 in 1 cm3 dilute aqueous NaOH solution was added 2 cm3 solution containing 1.5 x lo-" rnol NaOH and 4x rnol C12.The solution was heated to boiling for 2min. The precipitate of Na3H2106 formed was washed 4 times with water at 0°C. After each washing the precipitate was separated from the supernatant by centrifugation. ANALYSIS I-, 13, PHI [I-] and pH were measured on a Radiometer PHM 64 Research pH meter, fitted with a F 1032 I Selectrode and a K 701 reference electrode for [I-] measurements and fitted with the electrode set G K 2301C for pH measurements. [I;] was measured spectrophotometrically at 350 nm taking &I,- as 25 700 dm3 mo1-1 crn-l. 10,- 10, formed by photolysis of aqueous IvIr was measured by an isotope dilution technique. To aliquots (5 cm3) of solutions prepared from In1 isotopically tagged with 1131 was added subsequently : NaOH until pH > 13, 2 cm3 0.5 mol dm-3 BaCI2 and 2 cm3 0.2 mol dm-3 NaIO,. The precipitates of Ba(I03)2 were washed three times with a solution containing 0.05 mol dm-3 BaCI, and 0.1 rnol dm-3 HC104.The precipitate and supernatant were separated by centrifugation. The activities C and Co (in counts s-l) of Ba(103)2 precipitated2820 PHOTOLYSIS OF PERIODATE AND PERIODIC ACID from aliquots of irradiated and non-irradiated solutions were measured on a Harshaw gamma scintillation spectrometer. The total activity C, in the aliquot of the solution was determined by reducing all Ivrl to iodate with ethylene glycol prior to the precipitation of IOF. The extent of photolysis ~ 1 0 , - is given by a10,- = [IO~]/([IO;]+ [Iv1']) = (C- C O ) ~ (Ct- Co). It was found necessary to make the solution alkaline before the precipitation of Ba(103)2.The separation of IVT1 and 10, was poor in neutral and acid solution. IW' The decrease in [Ivr1] during photolysis was measured spectrophotometrically using a Beckman DU or a Cary 14 spectrophotometer. Denoting the absorbance before and after irradiation by go and E, respectively, and the extinction coefficients of Ivrl and 10; by EIVII and ~10,- , the extent of photolysis ap11 is given by ~ I V I I = (1 -e/so)/(l - ~ 0 ; /&PI). 0 3 mol dnr3 acetic acid as stabilizer was measured spectrophotometrically from the decrease in absorbance observed after flushing the solution with Ar, taking E O ~ at 253.7 nm equal to 3300 dm3 mol-l cm-l 11 [O,] was also measured iodometrically. 40cm3 solution was transferred with an all- glass syringe from the reaction cell to a gas-washing flask.Ar was passed through the solution and subsequently through two other gas-washing flasks each containing 20 cm3 0.1 mol dm-3 aqueous KI solution. [I;] formed in the two KI-containing gas-washing flasks was measured spectrophotometrically. Under the conditions employed 95 % of the ozone was absorbed in the first flask. [03] formed on photolysis of IvJJ solutions containing 10-3-5 x H202 In acid solution H202 reacts very slowly with IV1I. H202 was determined after removal of 03. In irradiated solutions containing 10-3-5 x lov3 mol dm-3 acetic acid and 2x rnol dmP3 [Iw1], H202 was identified and measured polarographically on a Metrom E 505 polarography stand attached to a E 506 Polarecord using the method of phase-selective AC polarography with a dropping Hg-electrode with controlled drop times and drop-synchronized current integration.To 25 cm3 of the irradiated solution was added 2 cm3 saturated KNOB solution. After flushing with N2 an a.c. polarogram was recorded between - 0.8 and - 1.3 V against s.c.e. H202 in known concentrations was then added and a polarogram was recorded after each addition of H202. The procedure was repeated with the non-irradiated solution. [H202] in the irradiated solution was determined by linear interpolation. In alkaline solution H202 reacts fast with IVI1 with formation of 10;. Analysis for 10: after the solution is made alkaline indicates that [H202] = [IO;]-A~vlr], where AIIvrr] is the decrease in [Ivr1] on irradiation of an acid solution of P I , in which H202 is stable.0-, OH saturated with O2 at 1 atm pressure [OF] was measured spectrophotometrically on the flash photolysis apparatus at 430 nm just after the flash irradiation, taking goo,- equal to 1900 dm3 mol-1 cm-l.13 [O-]+[OH] were found from eqn (111, which corrects for the reactions (12) and (13) (1 1) (12) IwI+OH+ IvIII (1 3) OH f H++O- (14) At 12.3 < pH < 14 0- was detected as 0; l2 by the observation of 0; in solutions 0 - + 0 2 + 0,. (10) i0-1 + [OH] = [o,]{ki 0[021~14 f [IvI11(ki 2Ki4 -k ki 3[Hfl)) /(kl 0[021K14) IVII+ 0- -+ IVITIu. K. KLANING AND K. SEHESTED 2821. where klo = 2 . 5 ~ lo9 dm3 mol-'~--~ ; l4 klz = 3 x lo7 dm3 mol-' s-l (this work), kls = 2 x lo8 dm3 mol-l s-l (this work), and in solutions containing 5 x 1 0 - ~ < [Cog-] < 0.1 mol dm-3 [O-]+ [OH] were taken equal to [COT], which was measured spectrophotometrically at 600 nm just after the flash irradiation, taking 8~0,- at this wavelength equal to 1860 mol-1 dm3 crn-l.l6 The decay of COT is of second order; the rate of decay increases with the ionic strength and is for [IV1I] ,< mol dm-3 independent of [Iw1] We find k16 = 5 x lo6 dm3 mol-I s-l at zero ionic strength in fair agreement with the value determined in pulse radiolysis 6 x lo6 dm3 mol-I At 0 < pH < 7, OH was detected by the formation of the hydroxyl adduct of benzene = 1.3 x mol dm3.IJ At 10 < pH < 13, OH+O- was detected by the formation of C o g CO$-+OH = CO$+OH-.(1 5 ) 2CO5-3 P. (1 6 ) formed on flashing a solution of IVI1 containing benzene.The concentration of the hydroxyl adduct was measured spectrophotometrically, extrapolating the logarithm of the absorbance at 312 nm to 20ps after the flash, and taking the extinction coefficient equal to 2100 dm3 mol-I CM-~.~' Due to the fact that benzene absorbs light from the flash, the concentration of hydroxyl adduct decreases with increasing benzene concentration. This effect was corrected for by extrapolating the absorbance at 312 nm to zero benzene concentration. APPARATUS FLASH PHOTOLYSIS The flash photolysis apparatus, reaction cell and the experimental procedure have previously been de~cribed.~~ The sensitivity at long wavelengths of absorbance measure- ments was enhanced by interchanging the original RCAlP28 photomultiplier with a RCA484-0 photomultiplier.In some experiments the energy E of the flash was attenuated by wrapping brass nets around the flash lamp. The transmittance of the brass nets were 0.43 at 250 nm. In other experiments E was varied by varying the voltage V and/or the capacity C of the condenser battery, It was found for 800 < E < 3200 J that the shape of the light pulse emitted at 300nm depends solely on E. With increasing E, the decay of the light pulse becomes slower, whereas the rate of growth of the light pulse is largely independent of E. The decay of the light pulse is exponential with a half life proportional to E*. At E = 3200 J the half life is -lops. LASER FLASH PHOTOLYSIS The laser flash photolysis experiments were made at the Department of Physical Chemistry, The Hebrew University, Jerusalem, Israel.The instrument used was a frequency quadrupled neodymium laser flash photolysis system constructed by Ch. R. Goldschmidt.' To get a high energy per pulse at 265 nm the laser was adjusted to give multimode pulses. The intensity of the 3 x s pulse was determined by measuring the absorbance at 415 nm of naphthalene triplet formed on pulsing a solution of naphthalene in cyclohexane. The absorbance of the triplet formed was - 0.5. Taking the quantum yield for triplet formation equal to 0.75 and the extinction coefficient at 415 nm equal to 25 000 dm3 mol-I cm-', a light intensity I in the irradiated volume of the 1 cm reaction cell equal to -2 x einstein dm-3 pulse-' results.20 At this intensity and with a limit of detection of transient absorbance of 0.005, a transient species is observable provided the extinction coefficient is greater than -300/@, where 0 is the quantum yield for formation of the species.PULSE R AD IOLY SIS The pulse radiolysis experiments were carried out at the Riso HRC-linac, using an optical detection system similar to that described previously,21 but modified by moving the detection2822 PHOTOLYSIS OF PERIODATE AND PERIODIC ACID system outside the radiation field. The essential features of the set up are a linear accelerator delivering 0.2-4 ps single pulses of 10 MeV electrons with a peak current of 800 mA, a Varian VIX 150 U.V. lamp, a Zeiss MM12 double quartz prism monochromator, an EM1 95584 photomultiplier and a Tektronix 555 double beam oscilloscope.The irradiation cell is cylindrical with two Suprasil windows and a double lightpass of 5.1 cm. The electron pulse current, recorded in every experiment by monitoring the current induced in a coil surrounding the electron beam, was used for relative dosimetry. The absolute dose was measured with the hexacyanoferrate (11) dosimeter,22 using G(e&+ OH) = 5.3 and &420nm 1000 moP1 cm-l. The dose used in the experiments was varied from 1 to 20 h a d in a 1 ps-pulse and up to 80 h a d in a 4 ps-pulse. The solutions were prepared in 100 cm3 syringes and deaerated by bubbling either N20 or Ar through the solution for 15 min. The pH of the solutions was adjusted with sodium hydroxide and was measured on a Radiometer digital pH meter, PHM52. All the chemicals were of p.a.quality and used without further purification. The water was triply distilled and all glassware was prebaked at 450OC. STEADY STATE PHOTOLYSIS The light source used in the steady state experiments was a flat spiral-shaped 100 W low pressure Hg lamp made of Spectrosil fitted with an optical filter, which consisted of 1 atm chlorine gas in a 4 cm long cylindrical Spectrosil cell. The filter limited the emission in ultraviolet to the Hg 253.7 nm resonance line. The 5 cm long cylindrical reaction celi with a volume of 66cm3 was placed in contact with the chlorine filter, which in turn was placed a few m i from the Hg lamp. During irradiation the solution in the reaction cell was stirred. The light intensity was determined by means of uranyl a~tinometer.~~ The actinometer solution was irradiated for 15-45 rnin in the reaction cell.The light intensity at 100 % absorption was 1.43 x einstein dm-3 s-l. The extent a of the photolysis of IV1I was <40 %. Quantum yields @ were determined in solutions containing 10-4-2x mol dm-3 IVI1. The light absorption at 253.7 nm in the solution was >85 %. The decrease in light intensity during the irradiation was so small (< 5 %) that the light intensity could be treated as a constant. 10; does not photodecompose to a measurable extent under the conditions of the present experiments. The quantum yield @VII for the photo- decomposition of IVI1 in alkaline solution is therefore equal to the quantum yield for formation of 10;. @p is given by eqn (1 8) (1 8) where [IVIr], is [IvJ1] before irradiation and t is the time. In acid solution, where H202 and O3 can be stabilized, the corresponding quantum yields ( D H ~ O ~ , (Do3, and (DIo,- are given by = @)IVII[X]/~[I~~I]~. @)IVII is given by eqn (18), and X stands for H202, O3 or 10,.When [H202] was not determined separately, ( D ) N ~ o ~ was taken equal @o; -QVIT. @IvII = [lvII]O[(l -EIO,- /cIvII)a- €10, /EIV~I In (1 - a)]/(tI) LOW TEMPERATURE PHOTOLYSIS The light sources in the low temperature photolysis experiment were a Philips 93106E Zn lamp and the lamp used in the steady state photolysis experiments fitted with a 0.2 cm Schott filter UG5 to cut off the visible light. The irradiation of the IV1l containing glasses was made in Spectrosil tubes of 0.2-0.3 em internal diameter submerged in liquid NZ in a clear Spectrosil dewar.The glasses consisted of 15 mol dm-3 aqueous LiCl, 10 mol drr3 aqueous NaC104, and 1 mol dmW3 HCI04+ 10 mol dm-3 aqueous NaCI04. E.s.r. spectra were recorded at the X-band on a Varian E-15 spectrometer. After heating to morn temperature 0.5-1 cm3 samples of the irradiated glass were analysed spectrophotometrically for IVI1 and by isotope dilution for 10;. STOPPED -FLOW hl E A S UR E Ivl E N TS Kinetic measurements of the reaction of H2Q2 with IVL1 in alkaline solution wcre carried The changes in absorbance at 270 nm The output from the out on an Aminco-Marrow stopped-flow apparatus. were monitored with a modified Beckman DU spectrophotometer.u. K. KLANING AND K . SEHESTED 2823 photometer was recorded on a Tectronix 5103 N storage oscilloscope fitted with a C-59 oscilloscope camera.All measurements were made at ambient temperature (21 & 1)"C. COMPUTATIONS The numerical integration of differential equations was performed using a second order predictor-corrector method combined with a fourth order Runge-Kutta method. RESULTS AND DISCUSSION FLASH PHOTOLYSIS AND PULSE RADIOLYSIS Besides 10; and unstable iodine-containing intermediates, the photo-products formed on flash irradiation of IvI1 solutions are: OH and H,O, in 1 mol dm-3 HC104 ; OH, O3 and H202 in 10-3-5 x acetic acid saturated with 1 atm O2 ; OH and H202 at 10.5 < pH < 11.5 (see below) and 0- in 0.2 mol dm-3 NaQH. Table 1 lists yields, Y, of the products and Y(IV1') of reduction of IV". Y is independent of the presence of 10, at pH < 11.However, on addition of 10: at pH > 12, Y(IV") decreases, whereas KO, increases. TRANSIENT ABSORBANCE AT 320 < 1 < 800nm The transient changes in absorbance observed in pulse radiolysis and in flash photolysis of IV" solutions depend on pM. In 1 mol d n r 3 HC104 no transient change in absorbance is observed. At 5 5 pH < 14, a transient increase in absor- bance is observed at 310 < 3, < 800 nm consisting of two overlapping bands 24 one at 360nm and another at 510-540nm depending on [Iv"] (see fig. 1). At [Iv"] - mol dm-3 the band maximum is at 540 nm ; at [Iv"] - lo-' mol dm-3 it is at 510nm. In the following we shall designate the band by its average position at 525 nm. At pH 5-7 the band at 360 nm decays in a pseudo first order process with a rate constant proportional to [Iv1'] which is too fast for observation on the present flash photolysis apparatus and which leaves a small slowly decaying absorbance belonging to the band centred at 525 nm.At pH > 9 the band at 360 nm is observed in flash photolysis decaying in a first order process at a rate which is independent of pH and [Iv"]. In pulse radiolysis at high doses (-20 krad) the decay at 360 nm starts off as a second order reaction and terminates as a first order reaction with a rate constant which equals the constant found in flash photolysis experiments and in pulse radiolysis of low dose (-2 krad). mol dm-3 butan-2-01 or of 10-3-10-1 rnol dm-3 CO,"-, substances which react fast with 0- and OH, the band at 525 nm disappears, whereas the band at 360 nm remains with unchanged decay kinetics.The species responsible for the absorbance at 360 nm are, however, not formed in the same process in flash photolysis and in pulse radiolysis. The 360 nm band does not appear on pulse radiolysis of N,O-containing IV" solutions, whereas no effect of adding N20 can be observed in flash photolysis. Also the absorbance at 360 nm, OD36o, is in pulse radiolysis proportional to the dose P of the electron pulse, whereas we find in flash photolysis d2(OD360)/dE2 < 0, indicating that the species absorbing at 360 nm is not formed in a primary process. At pH < 12.7 the band at 525 nin displays complex decay kinetics. At low IV" concentrations ([Iv1'] < 5 x msl dm-3) and high P or at high E the observed decay is a second order process.At higher IV" concentrations ( 5 x < [IVIr] < 5 x mol dnr3) and low P or E, the decay of the 525 nni band takes place in two steps. Finally at high Iv" concentrations and at low values o f P or of E, the decay is again of second order. However, the rate is much smaller than the one which is measured at low IV" concentrations and high values of P and E, fig. 1. The On addition of 10-3-5 xcd TABLE EXTENT OF PHOTODECOMPOSITION OF Y(Ivl') AND YIELDS, Y, OF PHOTOPRODUCTS ON FLASHING 45 Cm3 1"" SOLUTIONS WITH A 3240 J FLASH AT 21 & 1°C 4 0 [IVII] /lO-3 mol dm-3 0.2 0.9 1 .o 0.05 0.2 1 .o 10.0 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 0.0s [IO;] added /lO-3 rnol dm-3 pH 0 0 0 0 0 3-5 0 10.5 0 10.5 0 10.5 0 10.5 0 12 0 12.3 0 12.7 0 13.3 0.25 10.5 0.25 13.3 0.05 13.3 0 13.3 Y ( P ' ) 0.7 /lo+ rnol dm-3 - 21 0.7 1.7 - 1.2 4.4 0.9 1 .o Y(O-+OH) /lO-5 mol dm-3 -0.1 0.8 2.9 0.4-0.5 0.78 1.2 1.5 1.2 1.2 1.2 1.3 - 0.5 yIo - /lo- m d dm-3 1.46 7.9 1.26 3.1 4.6 33 - 5 4.1 3.3 2.1 1.3 4.6 1.5 1.3 0.4 0.76 0 0 13 0.5 0.6 0 1.4 0 0 - - 0 U Y 0 t3 Y c3U.K. KLANING AND K. SBHESTED 2825 rate of decrease in absorbance varies with pH. The rate of decrease in absorbance in the fast step increases with decreasing pH. The rate of decrease in absorbance in the slow step varies with pH attaining its smallest value at pH - 12. At pH > 12.7 the kinetics of decay in absorbance at 525 nm changes. The two steps of decay observed at 7 < pH < 12.7 merge to a single second order step, the rate constant of which decreases with increasing [Iv"].0.01 lo 1 I I I 1 300 4G0 500 600 700 nm FIG. 1 .-Spectra of the transient absorbance OD ; x , after electron pulse irradiation (dose 1.6 had) of a 5 x mol dm-3 IVII solution at pH 11.3 saturated with Ar, and 0, after flash irradiation (energy 3240 J) of a mol dm-3 I m I solution at pH 13.3 saturated with Ar normalized to the same OD at 550 nm. Inset : Oscillograms from which transient absorbances after flash irradiation are calculated, (a) at 360 nm, horizontal 1 ms/div, vertical 1 V/div ; 100 % light corresponds to 6.8 V ; (b) at 525 nm, horizontal 10 msldiv, vertical 2 V/div ; 100 % light corresponds to 12.8 V. (c) at 525 nm ; pH = 12.3 ; horizontal upper trace 5 ms/div, lower trace 20 ms/div ; vertical 1 V/div, 100 % light corresponds to 7.0 V.At pH > 9 we observe in pulse radiolysis an increase in absorbance at 525 nm on adding N20. This effect is not observed in flash photolysis or in pulse radiolysis of solutions in which pH < 9. ASSIGNMENTS IV"' The above observations lead to following asbignments : the transient absorbance at 525 nm is assigned to the hydroxyl adducts of IV1I species, IV1I1 and the relatively stable dinuclear species, (IvllIvlll), formed in reactions (12), (1 3) and (1 9) (Ivlll)z + OH -+ (IvllIvlll). (19)cd X 0 0 F TABLE 2.-kACTIONS INVOLVING IV1" SPECIES, 0- AND OH. EIVIJI = 1000 dm3 mOl-' Cm-' AT 540 nIll; E ~ I I V I I I ) =: 1200 dm3 I1101-l cm-' AT 510 nin (21 3- 1)"C. 2 12 IVII+O- + IVIII 13.3 lo-2 H2IOZ- 3x 10' 0 10.5 2~ 10-4 H310;- 2 .4 ~ lo8 cd 13 p + O H -+ P I 1 13 IV"+OH -+ IVI" -7 10-4-1 0-3 10, 4.5 x lo8 m z 1.5 x lo8 0 20 2IV'I' 3 IV+ IWI+ 0 2 -7 10-4-1 0-3 10, 20 21VIII -+ I V S P I + 0 2 10.5 < 2 x 10-4 H3@- 5 x 10' U * 4 20 2 F ' -+ IV+ P I + 0 2 13.3 < 2 x 10-4 H,IO: - 3.5 x 107 21 IVII + IVIII -+ (IVIIIVIIT) 10.5 10-3 HsIOZ-, H2120:; 4~ 105 m - 21 (IVIIIVIII) -+ p I + IVIII 10.5 10-3 H3IO;-, H2120:; 1 . 5 ~ 102 c + Z tl 24 2(Iv111v111) 3 Iv+ 3IV1I+ O2 7-12 10-3-10-2 IO,, HJOi-, H2120:; 3 x lo6 5 x 10-5-10-2 H2IOZ- 2 . 4 ~ lo6 cd 24 2(IV"IvT") 3 Iv+ 3IV1I+ 0 2 13.3 28 IVI+ IVII + IVIII+ IV 5-7 5 x 10-4-10-3 10, 1.3 x lo8 m z 0 - 6 x lo9 25 a 20H -+ H202 <11 - tl d - 9 x lo8 26 a 20- + H202 > 12 I a Ref. (15). b Estimated uncertainties 10 %-50 %.C s-'. 9 c U rate constant b reaction no. reaction PH [IvlI]/mol d i r 3 IVII species /mol-1 dm3 s-1 v1 H20 H UU . K . KLANING AND K . SEHESTED 2827 The kinetics of decay of the 525 nm transient absorbance is assigned to reactions such as 2 IV"' + IV + IV" + 0 2 (20) (21), (-21) (22) (23) (24) IV" +IVIII 2 - (Iv"Ivrll) IV"' + (1V"IV"') + I V + 2IV" + o2 2(IV1IIv1I1) 3 Iv + 3IV" + 02. IVIII + (IVII)~ + IVII + (IVIIIVIII) Table 2 lists the values of the extinction coefficients E ~ ~ ~ ~ I at 540 nm and E(IVIIIVIII) at 510 nm determined by pulse radiolysis and by flash photolysis at pH 13.3. Table 2 also shows which of the IV1' species 104, H,IOg-, H2120Lf;, H2102- dominates at the various listed pH values. The value for EIVII is found from &pII = ~ ~ 5 4 0 / ~ [ I v " ' ] and qIWr lvI1l) from qIvII IvIII) = 0 D5 lo/Z[(IvrrIvrrl)].At high concentration of IVIr, [(IvrlIvrrl)] was taken to equal [OH]+[O-1. At low [Iv"] it is necessary to correct for the reactions and 20H + H202 (25) 20- + H202. (26) (27) Eqn (27) is derived by integration of equations for d[IV"']/dt derived from H20 In this case [Iv"'] is calculated from eqn (27) where for pH > 13, x = k12[IV1r]/2k26 and y = [o-]/x, k26 = 9 x 10' mol-' dm3 s-l . 15 eqn (12) and (26) and eqn (13) and (25). At pH < 11, x = k13[IVr1]/2k25 and y = [OH]/x, k25 = 6 x lo9 mol-l dm3 s-l.15 The rate constants k12, k13, k20, k z r , k-21 and k,, are given in table 2. k20, k21, k-21 and k24 were determined by trial and error from the numerically integrated rate equation corresponding to reactions (20), (21) and (24) neglecting reactions (22) and (23) and simulating corresponding values of and time t.Fig. 2 shows OD525 plotted against t and a graph determined by the simulation procedure. k12 and kI3 were determined in pulse radiolysis of Ivxl solutions saturated with 1 atm N20. The observation that the decay of the transient absorbance at 525 nm is of second order at pH > 12.7 is ascribed to an increase in the rate constants of reactions (21) and (- 21) such that equilibrium between the species IV1I, IV"' and (IvllIvlrl) is maintained during the decay. Fig. 3 shows the second order constant for the decay at pH 13.3 of the transient absorbance at 525 nm, 2kapp, plotted against [Iv"] and a graph of eqn(24a) which may be derived by assuming equilibrium between IVrr, Ivrrl and [Ivr1'] = x In (y+ 1) (IV"IV1' '1 2kapp = (2k20 + 2k24[IV1r]2k~1/k~ 21)/(1+ [IV"]k21/k-2 (244 where the values of k20 and are taken from table 2 and k21/k-21 is taken equal to 2000 mol-1 dm3.~ 1 ~ 1 ~ 1 and E ( I V I I I V I I I ) at 525 nm are taken equal to 1000 mo1-I dm3 cm-l. IV' The absorbance at 360 nm is assigned to a species containing iodine in the oxida- tion state six, Ivl, which in pulse radiolysis stems from the electron adduct of IV" and in flash photolysis from a species formed in a primary reaction analogous to reaction (1).2828 PHOTOLYSIS OF PBRIODATE A N D PERIODIC ACID The pseudo first order decay observed in electron pulse irradiated IV" solutions at pH 5-7 is assigned to reaction (28) k2,q= 1.3 x lo8 mol-l dm3 s-l, fig.4. This assignment is based on the fact that addition of N20 has no effect on the yield of IV1". This means that Ivl and OH both react with IV" forming the hydroxyl adduct IV"' since in N20 saturated solutions e&+N,O 4 OH+N2. (28) IWI + IV' + IV + p i ' N2O 0.1 5b n 1 I t I 1 1 2 3 4 s FIG. 2.-Transient absorbance OD at 525 nm after flashing (energy 3240 J) a mol dm-3 IVII solution containing 2 x lW3 mol dm-3 NaOH plotted against time. Continuous curve is calculated by numerical integration of rate equations corresponding to reactions (20), (21), (-21) and (24). Inset : Oscillograms (a) and (b) from which absorbance data ( x ) are calculated. Horizontal (a) 20 msl div, (b) 5 ms/div vertical ; (a) and (b) 1 V/div 100 % light corresponds to 6.8 V.10-3[I~I]/mo1 dm-3 F'Io. 3.4tcond order rate constant 2kapp for the decay at pH 13.3 of transient absorbance at 525 nrn plotted against [IVII]. Continuous curve is calculated from eqn (244.U. K. KLANING AND K. SBHESTED 2829 The following observations indicate that the rate at which various Ivl species attain the most stable configuration by reaction with the solvent may be comparable to the rate at which the Ivl species disproportionate. An Ivl species, Irl, is formed in pulse radiolysis as the 0- adduct of 10; Iv+O- 3 IF; k29 = 3 x lo9 mol-1 dm3 s-1.25 (29) It has an absorption band at 360 nm [ E ~ ~ ~ ( I ~ I ) = 26001 25 decaying in a second order process at a rate which strongly depends on pH. We assign the decay to reaction (30) (at pH = 13.3, k30 = 7 x lo7 mol-1 dm3 s - ' ) .~ ~ 21r 3 IV + IV" (30) 'Ot 91- WL 71 + t - t I .I- 1 2 3 2 10-5 s FIG. 4.-First order plots of change in absorbance (OD- OD,) at 360 nm after electron pulse irradiation (dose 2.2 krad). a, [IVII] = 5 x lW3 mol dm-3 ; 0, [In11 = loA3 mol drr3 ; and $, [IvII] = 5 x mol dm-3 (pH = 5-7). ODm absorbance of IVIII after IVI has reacted. Inset : Oscillograms from which the plots are made (a) [IVII] = 5 x mol dm-3 ; horizontal 0.2 ps/div, vertical 0.01 V/div, 100 % light corresponds to 0.239 V; (b) BVII] = rnol d r r 3 ; horizontal 0.5 s/div, vertical 0.01 V/div, 100 % light corresponds to 0.243 V ; (c) [IVII] = 5 x mol d~n-~; horizontal 2 ,us/div, vertical 0.01 V/div, 100 % light corresponds to 0.225 V. Another Iv* species, If", is observed in flash photolysis at 360 nm, fig.I. Its decay is a first order process with a rate constant equal to 3.3 x lo3 s-l. A third Ivl species, IJI, formed as the electron adduct of IV1I, has an absorption band also centred at 360nm, with an extinction coefficient ranging, however, from 3400 to 4000 dm3 mol-' cm-l dependent on [Iv1']. At high dose the decay of Iyl is of second order The rate constant has the value kS1 = 4.5 x lo8 mol-1 dm3 s-1 independent of pH. At small dose the decay of I:1 is of first order process with a rate constant equal to The similarity of spectra and in decay kinetics suggests that reaction of I:* with 2111 3 IV+IV". (31) 4 x 103 s-1.2830 PHOTOLYSIS OF PERIODATE AND PERIODIC ACID water forming Iyl competes with reaction (31).We assign the decay of I:' to the reaction Further, we assume that I," is identical with If"'. Accordingly, we denote in the following IT' and 1:' by Ivl. Ivr observed in flash photolysis at 360 nm is not formed in a primary process. At pH = 13.3 we find for the yield Y,vI that d2YFI/dE2 < 0 and YIVI c Y(o-+oH). These observations may be rationalized by assuming that I:', formed in the primary reaction (1) either disproportionates or undergoes hydrolysis 1 1 + IV+0-. (32) 2Iy 3 IV+IV" and (33) (34) k34/(=33 q0- +OH)) FIG. 5.- XVI / Y(o- +OH) plotted against k34/(2k33 Y(o- +OH)). mol df~1-~, 0 [IVII] = rnol dm-3, * [IVII] = mol dm-3. 9, 0 and * [NaOW = 0.2 mol dnr3 ; A [NaOH] = 0.5 mol dm-3. k34/2k~3 = mol dm-3. Continuous curve is calculated from eqn (35).A and @ [IVII] = 2 x By assuming that 0-(OH) is formed in reaction (1) only and by neglecting the sub- sequent decay of Ivl in which reaction 0-(OH) also is formed [reaction (32)] we obtain eqn (35) from which equation we may estimate a lower limit for k34/k33. Fig. 5 shows that, when k34/2k33 is taken as mol dm-3, the corresponding values of YIV, Y(0- + OH) and k34/(2k33 Y(o- +OH)) fit the graph of eqn (35). The growing-inof the absorbance at 360 nm is too fast to be observed on the present apparatus, indicating that k34 > lo4 s-l and thereby implying that k33 must have a value close to the upper limit for a rate constant of a bimolecular reaction in aqueous solution. Direct observation of 1;' was attempted using the laser flash irradiation with a flash duration of 3 x s.However, no transient absorbance was detected at 300 < A < 850 nm. Taking the quantum yield for (0-+OH) formation to be 0.1, this means that ~~g~ < 3000 dm3 mol-l cm-l at 300 < A < 850 nm. In table 3 are summarized the reactions of Ivl mentioned above. -%/Y(O-+OH) = k34/[2k33y(O-+OH)I In [1+2k33 Y(O-+OH)/k341 (35)TABLE 3.-REACTIONS INVOLVING Ivl SPECIES (21 1)"c reaction PH [Ivlrllmol dm-3 1V" species rate constant b/mol-l dm3 s-1 remarks IV'I+IV' j IV+IV"I 5-7 5 x 10-4-5 x 10, 1.3 x lo8 pulse radiolysis 2 krad, 10, + 0- + 1:' > 12 0 3 x lo9 pulse radiolysis of 10; a 2 y + 1'+ IV" 13.3 0 7 x lo7 $" 2600 mol-l a 21y -+ I'+I~' 11.5-13.3 5x 10-4-5x H310g-, H2120i; 4.5 x lo8 &ifo 3400-4000 mol-1 1 PS dm3 cm-l s H2IO;- dm3 cm-1 pulse radiolysis 20 had, 1 P S reaction no.28 29 30 31 32 33 34 4 x lo3 C 3.3 x lo3 C pulse radiolysis 1.6 krad, flash photolysis 1 P Ivl -+ IV+O-(OH) 11.5-13.3 5 x 10-4-5x H,IOg-, Hz1204, H2IO;- k34/k33 > 2~ flash photolysis i 21x1 + IV+ IV" 11.5-13.3 5 x 10-4-5 x H310i -, H2130Lf; 1;' + I"' 11.5-13.3 5 x 10-4-5x H3IOg-, H2120f; k34 > lo4 H210, H2O H2IO;- a Ref. (25). b Estimated uncertainties 10 %-50 %. C s-' . dmol dm-3. c P 7f r *: z u Z 0 * z U h, 00 w c2832 PHOTOLYSIS OF PERIODATE A N D PERIODIC ACID The assignment of reaction (32) to the observed first order decay of Ivl is based on the observation that the concentration of 0 3 formed in reaction (10) on flashing an O2 containing IV" solution increases after the flash in a first order process, which matches the decay of Ivr. The assignment is further substantiated by the following observations.The transient absorbance at 360 nm observed on flashing a IV" solution, increases at pH 2 12 on addition of 1 0 3 to the solution. Furthermore, we observe a change h ""%\ 0 1 2 3 10-3 s FIG. 6.-Transient absorbance OD at 360 nm after flashing (energy 3240 J) rnol dm-3 IVII solutions containing 0.2 mol dm-3 NaOH and 10; in concentrations 0, 5 x mol dm-3 ; *, 2.5 x rnol dm-3 ; V , 6.25 x lW5 mol dm-3 and A, nil. Continuous curves are calculated from eqn (36). Inset : Oscillograms (a) and (b) from which absorbance data and A are calculated. Horizontal 500 ps/div, vertical (a) 1 V/div, (b) 0.5 Vldiv. 100 % light corresponds to in (a) 5.6 and in (b) 3.15 V.Oscillograms c1 and c2 show transient absorption at 525nm after flashing, in c1 [IO;] is nil, in c2 [IO;] = 2.5 x rnol dm-3 ; horizontal 20 ms/div, vertical 1 V/div, 100 % light in c1 and c2 corresponds to 6.8 V. in the kinetics of the decay of the transient at 360 nm. No effect on addition of 103' is observed at pH < 11, where 0- is protonated to OH which reacts slowly with 10; 2 5 or in the presence of substances which react with 0-, such as Cog- or butan- 2-01. We assign the increased absorbance to reaction (29), the rate constant of which is 30-40 times greater than the rate constant of the reaction of 0- with IV1I [reaction (12)]. However, due to reaction (32) the addition of 103 inhibits the photolysis of IV1I only to a relatively small extent since 0- formed in eqn (32) sub- sequently reacts with Iv" forming IV1*' [reaction (12)] (table 1) (cf.tables 2 and 3).U. K . KLANING AND K . SEHBSTED 2833 Fig. 6 shows agreement between the calculated and measured corresponding values of absorbance OD (at 360 nm) and time after the flash irradiation of a mol dm-3 IV1I solution containing 0.2 mol dm-3 NaOH and 103 in varying concentrations. OD as a function of t, OD(t), is calculated in the following way. Equations for d[Ivl]/dt and d[IV"'1/dt may be derived from eqn (12), (29), (30) and (32). By neglecting the decay of IV1I1 and applying the condition of steady state for [0-1, these equations may be integrated to give expressions for [I"] and [Ivr1'] as a function of the time t (cf. tables 2 and 3). As E ~ I and EPII are known, OD(t) may be calculated from these expressions [eqn (36)l oD(t) = [IV1]o(eFIZexp (-At)/[(2k,,[IV1~,/A)C1 -exp (-At))+ I]+ In [(2k30[Iv1]0/4(I -exp ( - A t ) ) + I])+ [IvIII]O&IvIII I (36) where Z is the opticaI length of the flash cell, EIVIII is taken equal to 350 mol-l dm3 cm-l at 360 nm, and [Iv1l0 and [Ivrlqo are concentrations at t = 0.A = k1zk32[1V1r]/ (k39[IV1 +k12[IV"I). I FIG. 7.-First order plots of change in absorbance OD at 270 nm ; 0 and * after flash irradiation (energy 3240 J) of IvIIsolutions, 0, [IVIIl = 4.5 x mol dm-3 ; 0, after mixing equal vdumes of 4 x Iwl and of 4 x rnol ~ I r n - ~ HzOz in stopped- flow apparatus (PH = 10.5). Inset: Oscillograms from which the plots are made. (a) and (b) flash irradiations, (c) stapped-flow measurement.(a) [IrVII] = 1.85 x lW4 rnol dm-3 ; horizontal 2 s/div ; vertical 1 V/div. (b) [ I 9 = 4.5 x mol dm-3 ; horizontal 5 s/div ; vertical 1 V/div. (c) [JW] = 2 x mol dm-3 ; [HzOz] - 2 x moI dm-3 ; horizontal 1 s/div ; vertical 0.1 V/div. mol dm-3 ; *, [IVII] = 1.85 x rnol Owing to the limited time resolution of the flash apparatus, [Ivqo and were calculated in the following way where ODo is the absorbance at 360 nm just after the flash. [Iv1] s t t = 2 x s is taken to be equal to OD(t=2xlo-4J(~P Z)-[Ivlll]o, and [Iv1], is then found by extrapolating to t = 0 using eqn(36). Thus we have in the calculation of [Iv'lO neglected the formation of IV1I1 in the first 2 x [IV"']0 = ODok12[IV"]/(ErVI Z)(k,,[IV] + k,,[IV1i]), s.2834 PHOTOLYSIS OF PERIODATE AND PERIODIC ACID TRANSIENT ABSORBANCE AT 270 < 3, < 310 STOPPED-FLOW EXPERIMENTS On flash irradiation of a 5 x 10-5-2 x mol dm-3 IV1I solution at pH - 11 with a 3240 J fiash, a decrease in absorbance occurring in two steps is observed.The decrease in absorbance in the initial fast step was roughly equal to the decrease in absorbance in the subsequent slow step. In the slow step the change in absorbance decreases exponentially with time with a rate constant which is proportional to [Iv"] and independent of pH at 10 5 pH 5 11. The fast step is assigned to a decrease in [Iv"] mainly taking place in the primary reaction. The slow step is assigned to reaction (37) of IV" with H202 formed in the fast step in a primary reaction We base the latter assignment on stopped-flow kinetic measurements of the reaction of IV" with H20z at 10.4 < pH < 11.1, [Iv"] N 2 x mol dm-3 and [H20,] 2: 2 x mol dm-3.Fig. 7 shows first order plots of the absorbance changes measured in the stopped- flow experiment and in the flash photolysis experiments. From the measured pseudo first order constants and the initial concentration [Ivlr]O we find, within the accuracy of the experiments, the same value for the second order constant in the stopped-flow measurement (2.6 x lo3 mol-l dm3 s-l) and in the flash photolysis experiment (2.7 x lo3 mol-1 dm3 s-l). The yields of H2Q2, Y H ~ o ~ , are shown in table 1. IV1r+H,02 -+ IO,+H,O++O,. (37) STEADY STATE A N D L O W TEMPERATURE PHOTOLYSIS It has been shown that complete reduction of periodate to iodate may be carried out, when aqueous 1'" solutions are irradiated with U.V.light, without significant photolysis of iodate due to the very small quantum yield of photolysis of iodate at wavelength 2 253.7 nm.26 In the present investigation the extent of the photolysis of IV" at 253.7 nm did not exceed 40 % and, accordingly, we did not detect I-, which is a photoproduct of iodate. @IO,-, ( 9 ~ ~ 0 ~ AND @03. IRRADIATION WITH LIGHT OF WAVELENGTH 253.7 nm, LIGHT INTENSITY TABLE 4.-QUANTUM YIELDS @ FOR REDUCTION OF IvI1, &I, AND FOR THE PHOTOPRODUCTS, I = 1.43 x einstein drn-3 s-l, [IVII] = 2x mol d ~ n - ~ (21 & 1)"C PH 1"" species qv11 OHzOz %OF % 3 0 ~ ~ 1 0 ~ 0.07 0.07 0.14 0 3-5 10, 0.28 0.20 0.56 0.e 3-5 10, 0.26 0.18 0.48 0 . 0 4 5 O - 0.17 - c 0.08 - a Saturated with 1 atm.02. * 02-free. 10.5 H3IOz- 0.15 13.3 HZIO2- 0.06 Table 4 lists the quantum yields for reduction of Ivrr, QDIVII, and the quantum yields for production of 103, H202 and O3 at varying pH. Also shown in table 4 are the 1''" species which dominate at the actual pH values. The above measurements do not exclude the possibility of an additional primary process (38) h v IV1I + Iv+20- or (OH) (38) the analogue of which plays an important role in the photolysis of Br02.5 In caseU. K . KLANING A N D K . SEHESTED 2835 of Br0; the formation of 0- was detected by e.s.r. measurements in aqueous NaOH glasses containing Br04 after irradiation with 253.7 nm light at 77 K. Irradiation at 77 K of IV" in aqueous glasses containing borate and phosphate or phosphoric acid alone does not lead to decomposition of IVr1.l0 In the present investigation glasses were prepared from 15 mol dm-3 aqueous LiC1, of aqueous 10 mol dm-3 NaC10, and of aqueous 10 mol dm-3 NaClO, containing 1 mol dm-3 HClO,.Irradiation for 2 h, with light of wavelength 253.7 nm at 77 K, of glasses containing mol dm-3 IVIr leads to 20-30 % decomposition in the LiCl and the NaClO, glass and 10-15 % decomposition of IV" in the NaClO, glass containing 1 mol dm-3 HClO,. No e.s.r. signals were detected by irradiation with light of wavelength 253.7 and 213.9 nm. KBrO, and HBrO, irradiated under the same conditions gave e.s.r. signals whereas no signals were detected from glasses containing the photoproduct H202 in the concentration 2 x lo-, mol dm-2.These measure- ments suggest that reaction (38) does not take place at 77 K. Formation of H202 and of OH radicals in a primary process is usually visualized as a process subsequent to formation of OID.ll This model originates in the com- parison of the photolysis of O3 in gas phase, where OID is formed at short wavelengths, with the photolysis of O3 in aqueous solution, where at the same wavelength H202 is forrned.ll The reactions leading to H2O2 are 20H 7 01D+H20 .+ (20H),,,, -+ H202 (39) (40) where two OH radicals formed within the solvent cage may combine to form Hz02 or break out of the solvent cage, depending on the excess kinetic energy liberated.27 However, the above findings suggest that O'D is formed only from the species 102 and not from other Ivrr species.The photolysis of halogen oxyanions usually results in the formation of 0 3 P when the irradiation is carried out of a wavelength longer than that required to produce H202. The only known exception is BrOz which does not absorb light at the wavelength which corresponds in energy to the threshold of formation of 03P.5 In the photolysis of aqueous IV1I, however, we observe 0 3 P formation only from solutions in which the predominant IV" species is 104 (table 1 and 4) despite the fact that the energy required to form O'D from 104 is much less than from other Ivrr species. The long wavelength edges for processes (41) and (42) h V 10, + 10, + 0% (41) BrO, 3 BrO, +OID (42) h v calculated from the enthalpies of formation of OID and of crystalline KI04, KBr0, and KBr03 are 301 and 329 nm, respectively.These values are expected to be close to the actual long wavelength edges, since the configuration of the initially formed Iv and BrV cannot be very different from the equilibrium configurations of 10: and Br0;. The long wavelength edges for reactions (43)-(45) h v h v h v HSI06 -P I0;+2H2O+O1D+H+ (43) H,IO; .+ 10:+2H20+01D (44) H3102- + I0~+HzO+OH-+O1D (45) calculated from the enthalpy of reaction (41), the standard enthalpy change in2836 PHOTOLYSIS OF PERIODATE A N D PERIODIC ACID reactions (5), (6) and (7) 6* ' and the enthalpy of ionization of water, are 270 nm for reactions (43) and (44) and 234nm for reaction (45). For reactions (43)-(45) the actual long wavelength edges may be much smaller than the values given above and are calculated from equilibrium data, since the configuration of the initially formed Iv can be different from the equilibrium configuration of 103.On the basis of these considerations, as well as the observation that H202 is the main photoproduct from photolysis of H5106 and H,IO%-, we suggest that H202 formed in the photolysis of H5106 and H,IO;- is formed directly in a primary process whereas oxygen atoms, whether in the state 3P or ID, are formed in the photolysis of 104 only.* These ideas may also explain the fact that OH radicals are formed from Br04 and not from IV" by irradiation at 77 K. The 1'" species present in the glasses at 77 K are not known. However, since the enthalpy of reaction (7) is positive '* ' the relative concentration of the species 104 will probably be negligible at 77 K.SUMMARY The primary photochemical reactions may be summarized as follows : (I) pH 13.3 (11) pH 11 (111) pH 11 (IV) pH 3-8 H,IOg- Ix'+O- 1:' + OH 7 L HJOZ- Iv + H202 1;1+ OH 7 4 H2IzOfo (Iv + H202) ? (Igl + OH) ? The secondary reactions in (I)-(IV) are summarized below (see tables 2 and 3). suggested that 10; is hydrated in aqueous solution Accordingly, the direct formation of H20z may be visualized as * Note added in proof: M. Anbar and S . Guttmann (J. Amer. Chem. Soc., 1961, 83, 781) have IOsfH2O = HzIOZ. hv h v hv H510s + H3104+H202 HJO; +- H2IO,+ H202 H3IOg- + HIO:-+ €3202.u. K . KLANING AND K . SEHESTED 2837 (1) The secondary reactions subsequent to (I) depend on the light intensity. Under steady state conditions, where intermediate-intermediate reactions may be neglected, the secondary reactions in I of 0- are reactions (12), (29) and (32), followed by some or all of reactions (20)-(24).The secondary reactions of 1;' are (34), (29) and (32). The mechanism is compatible with the observation that the quantum yield for reduction of IVI1, OIvII, is independent of the concentration of added 103, since a reaction of 0- with 10, does not lead to a formation of IV". In flash photolysis, however, reaction (33) is an important reaction of 1:' in addition to reaction (34), (29) and (32). This is in agreement with the finding that Y(IV") is close to Yo- at high flash energies (table 1). In reaction (30), Iv" is partly reformed. Therefore, Y(IV1I) decreases on addition of 103 to the solution.0- formed in reaction (1) and (32) reacts in eqn (29) with the added 10; with formation of Ivl, which again forms IVx1 in reaction (30) (table 1). However, we find that addition of 10.: causes a small increase in yIos, see table 1. KO, is determined as the yield of isotopically tagged I*O; formed from isotopically tagged I*v11. We suggest that the small increase observed on addition of " cold " 10; is due to the isotopic exchange reaction I*vl+Iv + I*v + IV1. (11) AND (111) The secondary reactions of OH with IV1I and (Iv11)2 are reactions (13) and (19) followed by some or all of reactions (20)-(24). The reaction of H202 is eqn (37). 1;' reacts in eqn (34) and (29), and further in (33) after flash photolysis. At pH 11, 0- is protonated to OH, which reacts slowly with 10; 25 and we observe no effect of adding 10; (table 1).YH202 was not measured at [Iv"] > 2 x 10-4mol dm-3; however, since KO, / Yo, is largely independent of [Iv"], we assume that the primary processes of H21204; are similar. (IV) The secondary reaction of Ivl is eqn (28). The secondary reaction of OH with IV1I is (13) followed by some or all of reactions (20)-(24). This means that we have the following relations among the yields Y and among the quantum yields 4 @ 10,- = @H202 + @ IV" + QO, (47) The values given for Y and O in tables 1 and 4 agree with reactions (46) and (47) within the estimated error. No secondary reactions are observed. The authors thank Henrik Loft Nielsen of the Physical Institute, this University, for help in the preparation of isotopically tagged periodate and in carrying out gamma spectrometric measurements. We thank Jarrgen Jakobsen of the Biochemical Institute, this University, for making the stopped-flow measurements, Shmouel R. Goldschmidt of the Department of Physical Chemistry, Hebrew University, Jerusalem, Israel, for making the laser flash photolysis measurements? James Norris of the Argonne National Laboratory? and Jsrgen Byberg of this Institute, for making the e.s.r measurements. The computer program employed to determine rate constants was2838 PHOTOLYSIS OF PERIODATE A N D PERIODIC ACID devised by Nis Bjerre. Evan H. Appelman of the Argonne National Laboratory, U.S.A., is thanked for supplying KBrO, and HBrO,. G. V. Buxton and M. S. Subhani, J.C.S. Furuduy I, 1972,68,958. ' G. V. Buxton and M. S. Subhani, J.C.S. Furudzy I, 1972, 68,970. F. Barat, L. Gilles, B. Hichel and B. Lesigne, J. Phys. Chem., 1971, 75, 2177. A. Treinin, Israel J. Chem., 1970,8, 103. U.K. Klaning, K. J. Olsen and E. H. Appelman, J.C.S. Furuduy I, 1975, 71,473. C. E. Crouthamel, A. M. Hayes and D. S. Martin, J. Amer. Chem. Soc., 1951, 83, 82. F. S. Head and H. A. Standing, J. Chem. SOC., 1952, 1457. M. C. R. Symons, J. Chem. Soc., 1955,2794. lo U. K. Klaning and M. C. R. Symons, J. Chem. Soc., 1960,977. l1 H. Taube, Trans. Faraduy SOC., 1957,53,656. l2 G. Czapski and L. M. Dorfmann, J. Phys. Chem., 1964,68,1169. l3 F. S. Dainton and P. Fowles, Proc. Roy. SOC. A, 1965,287,295. l4 G. B. Adams, J. W. Boag and B. D. Michael, Proc. Roy. Soc. A, 1966,289,321. l5 J. Rabani and M. S . Matheson, J. Amer. Chem. Soc., 1964, 86, 3175. l6 J. L. Weeks and J. Rabani, J. Phys. Chem., 1966,70,2100. l7 S . Gordon, K. H. Schmidt and E. J. Hart, J. Phys. Chem., 1977, 81, 104. l8 U. K. Klaning, J.C.S. Furuduy I, 1977, 73,434. l9 U. Lachish, A. Shafferman and G. Stein, J. Chem. Phys., 1976, 64,4205. 2o Ch. R. Goldschmidt, personal communication. 21 H. C. Christensen, G. Nielsson, P. Pagsberg and S. 0. Nielsen, Rev. Sci. Znstr., 1969, 40, 786. 22 R. Rabani and M. S. Matheson, J. Phys. Chem., 1966,70,761. 23 W . A. Noyes and P. A. Leighton, The Photochemistry of Gases (Reinhold, New York, 1941), 24 F. Barat, L. Gilles, B. Hichel and B. Lesigne, Chem. Comrn., 1971, 847. 2 5 F. Barat, L. Gilles, B. Hichel and B. Lesigne, J. Phys. Chem., 1972,76, 302. 26 L. Farkas and F. S. Klein, J. Chem. Phys., 1948,16,886. 27 E. Rabinowitch, Trans. Furuday SOC., 1937, 33, 1225. ' G. J. Buist, W. C. P. Hipperson and J. D. Lewis, J. Chern. Sac. (A), 1969, 307. p. 82. (PAPER 8 1162)

ISSN:0300-9599    

DOI:10.1039/F19787402818

出版商:RSC    

年代:1978

数据来源: RSC