摘要:

J. Chem. SOC., Faraday Trans. I , 1986, 82, 1935-1943 A Comparative Study of the Protonation of myo-Inositol Hexakis(phosphate) Helene Bieth and Bernard Spiess* Laboratoire de Chimie Analytique, Facultt! de Pharmacie, 74 Route du Rhin, 6 7048 Stras b ourg, France Numerous studies of the protonation of myo-inositol hexakis(ph0sphate) (IP,) have lead to divergent results. Our values of the constants determined at 25 "C in 0.1 mol dm-3 Et4NC10, or Bu4NBr media are sometimes several orders of magnitude higher than those previously published. The reason for the discrepancy between the results is found from an examination of the theoretical titration curves computed with the protonation constants mea- sured in the presence of various support electrolytes at different ionic strengths. It is reasonable to suppose that complexation equilibria, involving alkali-metal cations, are superimposed on the protonation equilibria.Introduction 1,2,3,4,5,6-Hexakis(dihydrogen phosphate) myo-inositol, more commonly called myo- inositol hexakis (phosphoric acid) or phytic acid and its salts (designated IP,) are the major representatives of inositol ph0sphates.l A great variety of seeds used in alimentation, both cereals and legumes, contain important quantities of phytic acid. The ability of phytic acid to complex with metals is well known and it is generally believed that the presence of such complexes influences the nutritional properties of food. The occurrence and nutritional significance of the phytates, together with their interactions with food, have been the subject of several reviews21 Numerous other studies4-, have shown the ability of IP, to bind to proteins, particularly in the case of haemoglobins, which results in an important decrease in the latter's oxygen affinity.' The precise character and extent of binding of IP, with both metallic cations and proteins govern its functional and nutritional role as well as its biochemical activity.In addition to these fundamental interests, a quantitative study of the complexing properties of such a compound may shed new light on the problems of the bioavailability of metallic cations and of their interaction in food systems. Such a study first requires the knowledge of the protonation equilibrium constants, since the cation competes with the proton in complexation. There has long been much interest in this molecule.Neuberg* first proposed a hydrated tripyrophosphate structure for phytic acid, and later Andersong suggested a symmetrical hexaorthophosphate arrangement. Convincing evidence points towards the latter struc- ture following recent n.m.r., potentiometric and crystallographic studies1° of the hydrated dodecasodium salt. The same unanimity does not exist concerning the exact conformation of IP, in solution, which may be either equatorial (l-ax/s-eq) or axial (5-ax/l-eq). In the solid state IP, is unexpectedly found to have the 5-axll-eq conformation. In solution the results seem greatly dependent on the nature of the supporting ekctrolyte cation, the ionic strength and the pH. Barre et a2.,l1 on the basis of potentiometric measurements, envisage for the fully protonated species direct hydrogen bonds across the ring and therefore suggest the 5-ax/ 1 -eq conformation.However, n.m.r. studies by Costello et a1.12 19351936 Comparative Study of Protonation of myo-Inositol Hexakis(ph0sphate) indicate that the 1 -ax/5-eq conformation may exist in concentrated sodium hydroxide solutions. Other workers think that the conformation varies with the pH. Isbrandt and Oerte1,13 through the use of 13C n.m.r., 31P n.m.r. and Raman spectroscopy propose a l-ax/5-eq IP, conformer below pH 9.4 and a Sax/ 1-eq conformer above this pH. Emsley and Niazi14 also discuss the structure of IP, in solution in terms of conformation. They suggest an equatorial structure above pH 12. From pH 11 to pH 5 an axial conformation exists which reverts back to the equatorial one at the latter pH, owing to the formation of strong hydrogen bonds between phosphate groups.Below pH 2 the molecule seems to be preferentially axially orientated. Most previous s t ~ d i e s ~ ~ - ~ ~ ~ 15-17 give the ionization constants, which have been determined in various support electrolyte media and ionic strengths. It is surprising to see the discrepancy between the results from one author to another, although the experimental methods are well adapted and the measurements rigorously achieved. Our results determined by potentiometry at 25 "C in 0.1 mol dm-3 aqueous Et,NClO, or Bu,NBr solutions confirm this aspect of the problem. Experiment a1 Materials Hydrated Na,,IP, (Sigma Chemical Co.lot 83 F-0515 and B.D.H. lot 938038 D) were used without further purification. Recent l4 have shown that the commercial product has a satisfactory purity. A 4 x mol dm-3 solution of the sodium salt was converted into the acid by ion exchange using an Amberlite IRN 77(H) resin. Three successive passages were carried out to obtain a sodium concentration (analysed by an atomic absorption spectrometer, Varian model 275) below 4 x mol dm-3. The higher the initial Nal,IP, concentration, the lower was the pH of the solution after exchange, which is not favourable to the elimination of the last traces of Na+. A 4 x mol dmM3 Nal,IP, concentration seems satisfactory to obtain quantitative exchange. All the dilutions were made with deionized twice distilled water, and the solutions were stored in polypropylene vessels.We have shown (fig. 1) that solutions left in glass vessels slowly increase in Na+ content, whereas those in polypropylene flasks do not. The ionic strength was held constant at 0.1 mol dm-3 by addition of recrystallized tetraethylammonium perchlorate (Et,NClO,) or tetra-n-butylammonium bromide (Bu,NBr). Tetramethylammonium or tetra-n-butylammonium hydroxide solutions prepared in carbonate-free water were used to perform the titration. The concentration of phytic acid was determined potentiometrically by titration with one of the previously mentioned bases in the presence of NaClO, at molar concentration. The difference between the two inflexion points corresponds to the neutralization of six acidic protons of the phytic acid.Potentiometric Measurements The titrations were carried out in 50 cm3 solutions at 25f0.01 "C under a nitrogen atmosphere in a glass cell previously silanized with a 3% in CC1,-Rhodorsil solution. The pH measurements were made with a Tacussel combined glass electrode ref. TBC12HA, connected to a Tacussel Isis 20.000 pH meter. The standard liquid in the reference-electrodecompartment was replaced by a 0.0 1 mol dm-3 Et,NCl-0.09 mol dm--3 Et,NClO, solution saturated with AgCl. Haeringer and Schwingl* have shown that the variation of the junction potential with -log [H+] follows an exponential law. One can express the value of -log [H+lreal by the following relation : where a and b are constants and b [H+] represents the junction potential.H.Bieth and B. Spiess 1937 0.3 0.2 m 0.1 I I 7 14 21 time/days Fig. 1. Na+ content plotted against time for a 4 x mol dm-3 HJP, solution in a glass (a) or polypropylene (H) vessel. The standardization is taken at ph* 2 with a mol dm-3 HClO, and 9 x mol dm-3 Et,NClO, solution. Under these conditions -log [H+Ireal = -log [H+],,,, = 2 and b [H+],,,, = --a. The ph reading of a mol dm-3 HC10,- 9.9 x mol dm-3 Et,NClO, solution allows the determination of the coefficients a and b which we found to be, respectively, 0.124 and - 12.4. Results The general protonation equilibrium of 1P,(Lf2-) is given by (1) y H+ + L12- H, L(12-Y)- which can be characterized by the following overall thermodynamic stability constant, (H, L(12--Y)-) P," Bt" = (H-t), (L12-) ' (2) If the ionic strength remains constant, the activity factors of the different species are constant.One can define an apparent overall stability constant, which is In eqn (2) and (3) the parenthesis and the square brackets correspond to the activities and the concentrations, respectively. * Here ph is defined as the cologarithm of the concentration of the H+ species: ph = -log [H+].1938 Comparative Study of Protonation of myo-Inositol Hexakis(ph0sphate) Table 1. Logarithm of the protonation constants of myo-inositol hexakis(phosphate) at 25 "C, ionic strength 1 = 0.1 mol dm-3 (Et, NClO, or Bu,NBr). Et,NC10, Bu,NBr (0.1 mol dm-3); (0.1 mol dm-3) 3 experiments; 1 experiment; log Ku+aa N = 303 N = 101 y = 1-3 > 12.00 y = 4 1 1.47 & 0.28 y = 5 8.12 & 0.04 y = 6 6.5 1 & 0.02 y = 7 4.00 & 0.05 y = 8 2.68 0.01 y = 9 1.80 & 0.13 y = 1&12 < 1.50 > 12.00 1 1 S O & 0.06 7.97k0.01 6.41 & 0.02 3.93 & 0.03 2.73 0.03 2.00 & 0.04 < 1.50 a Values of a of the first set were calculated from the values obtained in each experiment. The values of a of the second set were given by the program MINIQUAD.N is the number of experi- mental points. It is also possible to envisage the step-by-step protonation of the acid in the general equation : We report here six stepwise protonation constants KY ( y = 4-9) of the twelve theoretical constants which exist for IP,. The logarithm of the constants log KY corresponds to the usual pK, i.e. the cologarithm of the apparent dissociation constants. Our potentiometric data corrected for the junction potential (see Experimental) have been refined by the computer program SCOGS1' and/or MINI QUAD.^^ SCOGS calculates the formation constants py of various species (up to 20) by minimizing the sum of the squares of the differences between the experimental and calculated titre (volume of base added during the titration in cm3) using estimates of unknown constants and the experimental values of ph.MINIQUAD, a more general program, treats the same type of data by minimization of the calculated and observed &values, & being the analytical concentration of the constituent i. and log pz have converged they are held constant and log p3 and log p4 are refined, and so on until convergence of the last constants. Secondly, the entire curve is treated and the constants refined simultan- eously.The values given by SCOGS and MINIQUAD are in good agreement, but SCOGS did not converge satisfactory in all cases. The results presented here were obtained by Table 1 shows two sets of logarithms of the protonation constants of IP,. The first were determined with 0.1 mol dm-3 Et,NClO, as the support electrolyte, the last were obtained in a 0.1 mol dm-3 Bu,NBr medium. All the constants are consistent. On the other hand, the constants are in agreement whatever acid is used, either from Sigma or from B.D.H. The first four constants, like the first protonation constant of orthophosphoric acid (log Kl = 12), are very high and correspond to very low acidic functions. The following four constants are in a large range of log KY, from log K5 = 8.12 to log K, = 2.68.Finally, H+ + J-J Y-1 L(13-Y)- + HyL(12-Y)-. (4) The constants are first refined in pairs. When log MINIQUAD.H. Bieth and B. Spiess 1939 0.9 0.8 0.7 0.6 2 0.5 0.4 0.3 0.2 0.1 0” XA 1 Y n H5L / Fig. 2. Distribution curves of the protonated species plotted against log [H+]. For clarity the charges are omitted. the last four constants are low, indicating extremely acidic functions in the same order as that of the second protonation of glycerol-2-phosphate (log K2 = 2.12).21 Fig. 2 shows the distribution of the protonated species in the ph range studied. These curves illustrate the percentage of each species at different ph values. For ph 2 to ph 10, three species are often in equilibrium and only LH!- and LH8,- exist in solutions > 90%.These results differ appreciably from those of other authors, and are sometimes several orders of magnitude higher than those previously measured. Unfortunately none of the results from the numerous studies of IP, protonation constants are in agreement. The major reason invoked to explain this dispersion of the constants is the influence of the nature of the cations present in solution, which in the case of alkali-metal ions should stabilize certain conformations. In an attempt to interpret these differences between the ensemble of results published to date, we have simulated with the program HALTAFALL~~ the theoretical titration curves resulting from the protonation constants listed in table 2, determined by different authors.This program allows the calculation, for heterogeneous as well as homogeneous systems, of the concentrations of the various species, given both the analytical concen- trations of the components and the formation constants. Our concentration conditions were used to construct the curves, thus permitting a comparison between these theoretical curves and our experimental data. The simulated titration curves and our experimental ones are reported in fig. 3. The differences between the curves illustrate the differences between the constants, which cannot be attributed either to experimental or interpretation errors. On the other hand, one can observe that the shift of each theoretical curve with our experimental curve is dependent on the concentration of support electrolyte containing alkali-metal cations.The most divergent curve is that corresponding to one set of values of Hoff-Jorgensen,15 who was working in a 1 .O mol dm-3 NaCl medium, then the shift decreases with the curve of Perrin et aZ.16 (values obtained in 0.5 mol dm-3 NaCl), the curve of Evans et a1.l’ andI940 Comparative Study of Protonation of myo-Inositol Hexakis(ph0sphate) Table 2. Logarithms of the protonation constants of myo-inositol heaxakis(phosphate) previously determined compared with oursa curve 1 2 3 4 5 6 7 ref. 15 16 15 17 11 12 our work 1% KY y = 12 y = 11 y = 10 y = 9 y = 8 y = 7 y = 6 y = 5 y = 4 y = 3 y = 2 y = l meth. medium I .oo I .oo I .oo I .oo 1.75 2.68 4.59 5.71 7.34 8.19 8.78 8.95 pot. NaCl 1 mol dm-3 I SO I S O I S O I SO 1 S O 2.87 4.74 5.90 7.61 8.88 8.90 9.05 pot.NaCl 0.5 mol dm-3 I S O 1 S O I S O I S O 2.20 3.24 5.15 6.39 7.96 8.82 9.61 9.83 pot. NaCl 0.2 rnol dm-3 1.92 1.92 1.92 2.38 2.38 3.16 5.20 6.25 7.98 9.19 9.53 9.53 pot. KC1 0.2 mol dm-3 25 11.84 11.84 11.84 11.84 11.84 11.84 6.30 6.30 9.70 9.70 9.70 9.70 pot. Na+ 1.10 1.50 1.50 1.70 2.10 2.10 5.70 6.85 7.60 10.0 10.0 12.0 RMN31P TBA 0-2.5 mol dm-3 28 < 1.50 < 1.50 < 1.50 1.80 2.68 4.00 6.51 8.12 1 1.47 > 12 > 12 > 12 pot. TEA 0.1 mol dm-3 25 a The values in italic were not given but were estimated for the computation (fig. 3). mole ratio Fig. 3. Simulated titration curves (1-6) from the values of the constants previously determined in various media (table 2) and our experimental curve (7). The last one was obtained by titration of a 2 x mol dm-3 IP, solution with a 1 x 10-1 mol dm-3 base.Hoff-J~rgensenl~ established, respectively, in 0.2 mol dm-3 KCl and 0.2 mol dm-3 NaCl, and the curve of Barre et a1.l1 (values obtained without support electrolyte but with NaOH as titrating solution). The curve relative to the values of Costello et aZ.12 should be superimposable on ours, since the medium theoretically does not contain alkali-metal cations. Fig. 3 shows that this is not the case. It is therefore possible that Na+ was still present in appreciable concentration after passage through the ion-exchange resin, or that sodium was released from the glass vessel in the presence of IP,. These types of curves are typical of titration curves where complexation occurs.In thisH. Bieth and B. Spiess 1941 0 - 1 2 3 4 5 6 7 8 9 10 11 12 1 3 1 4 mole ratio Fig, 4. Titration curves performed in (a) 0.1 mol dm-3 Et,NC10, and (b) 1 mol dm-3 NaC10, solutions and (c) the simulated curve from Hoff-Jorgensen obtained in 1 mol dm-3 NaC1. case a metallic cation competes with the proton for the coordination site. The shift between the curves obtained, either with or without metallic cations, is especially high, so that the complex is more stable or, for a given complex, that the concentration of the metal increases. Fig. 4 shows the theoretical titration curve obtained in a 1 .O mol dm-3 curves carried out in 0.1 mol dm-3 The last curve is very close to that NaCl medium together with our experimental Et,NC10, and 1.0 mol dm-3 NaC10, solutions.obtained by Hoff-J0rgen~en.l~ If we admit the hypothesis of an 'association proton (e.g. a metal, Mn+), to equilibrium ( equilibrium of the type of IP, with a species other than the ) is added a general complexation xMn+ + yH+ + ZL + M, H, Lkns+y)+. ( 5 ) This can be characterized by the following apparent overall stability constant: Equilibrium ( 5 ) must then be taken into account when determining the constants 8,. Glonek et aZ.23 were among the first to show the influence of the counter-cation on the chemical shifts of the middle phosphate groups, using 31P n.m.r. studies on tri- and tetra-metaphosphates and long-chain polyphosphate solutions. The tetra-n-butyl- ammonium and propylammonium salts exhibit the highest chemical shifts, indicating low interactions with the phosphate groups.In contrast, the alkali metals, particularly Na+ cations, have a very strong influence. The lower chemical shifts of the alkali-metal salts have been interpreted in terms of complex formation between these ions and the condensed phosphates. Tetra-n-butylammonium salts should be used when a minimal cation effect is required. For IP,, the comparison between our results and those of the previous studies shows that Bu,N+ or Et,N+ media lead to the same protonation constants, but this is not the case if the solutions contain Na+ or K+. It is thus likely that IP, exhibits the same affinity for various monovalent cations and that complexation reactions occur in solution. Other considerations also confirm such reactions. After the empirical HSAB rule of P e a r ~ o n , ~ ~ ~ 25 hard metal ions coordinate1942 Comparative Study of Protonation of myo-Inositol Hexakis(ph0sphate) preferentially with hard ligands and soft metal ions coordinate preferentially with soft ligands.Small cations with low polarisabilities, such as alkali-metal ions have been called ‘hard acids’ and form their most stable complexes with electronegative atoms such 0, N and F which belong to the hard-base class. The bonding in these complexes is largely ionic. IP,, which contains 18 donor oxygen atoms, appears to be particularly favourable to the formation of complexes with alkali-metal ions. On the other hand, all ligands known for their complexing properties with alkali-metal cations exhibit a macrocyclic structure. Examples thus far obtained include both natural antibiotics (such as valinomycine261 27) and synthetic molecules (such as Pedersen’s polyethers28 or Lehn’s crypt and^^^).In the case of IP, it also seems possible to define one or two pseudo-cavities, limited by the phosphate branches of the cycle, according to the type of conformation considered with equatorial or axial prevalence. The crystal structure of Na,,IP, also exhibits an unusual feature with regard to the type of coordination between sodium and oxygen. Octahedral, decahedral and hexahedral coordination have been displayed, showing bonds bridging the sodium atoms and the two phosphoryl oxygens or a phosphoryl oxygen and an ester oxygen. According to this mode of bonding, it was found that the sodium atoms should aid the stabilization of otherwise sterically undesirable conformations. Having only the log K values, we cannot discuss the conformation of IP, in the pH range studied.Nevertheless, if the very high values of the four first constants are taken into account, it seems possible to confirm that, at least for the LH:- species, the Sax/ 1 -eq conformer will predominate by setting up very strong hydrogen bonds. In this case two protons should play the same role as that of a sodium atom in the crystalline structure, i.e. the stabilization of an unfavourable conformation. However, the 1 -ax/5-eq form could reasonably be envisaged for the entire deprotonated species, since the minimization of steric interactions in the equatorial configuration is not opposed by hydrogen-bonding forces.Studies of the complexation of alkali-metal cations by myo-inositol hexakis(phosphoric acid) are currently underway in our laboratory. They will add to the more general framework of the complexation properties of inositol phosphates. In this series inositol 1,4,5-triphosphate is the objective of many current studies in other laboratories. Indeed this molecule, resulting from the hydrolysis of phosphoinositides, may have a second messenger function towards hormones and neurotransmitters, and initiates a signal cascade which includes the mobilization of calcium. References 1 D. J. Cosgrove, Inositol Phosphates, (Elsevier, Amsterdam, 1980). 2 M. Cheryan, Crit. Rev. Food Sci. Nutr., 1980, 13, 297. 3 J. A. Maga, J. Agric. Food Chem., 1982, 30, 1.4 R. S. Franco, K. Wagner, M. Weiner and 0. J. Martelo, Am. J . Hematol., 1984, 17, 393. 5 R. S. Franco and M. Weiner, U S . Patent 4, 478, 824. 6 K. Ruckpaul, S. Greschner, G. R. Jaenig, H. Lackeit, H. Grill and 0. Ristau, East German Patent DD 7 R. Benesch, R. E. Benesch and C. I. Yu, Proc. Nut1 Acad. Sci. USA, 1968, 59, 526. 8 C. Neuberg, Biochem. Z . , 1908, 9, 557. 9 R. J. Anderson, J. Biol. Chem., 1914, 17, 141. 210, 804. 10 G. E. Blank, J. Pletcher and M. Sax, Acta Crystallogr., Sect. B, 1985, 31, 2584. 11 R. Barre, J. E. Courtois and G. Wormser, Bull. SOC. Chim. Biol., 1954, 36, 455. 12 A. J. R. Costello, T. Glonek and T. C. Myers, Carbohydr. Res., 1976, 46, 159. 13 L. R. Isbrandt and R. P. Oertel, J . Am. Chem. SOC., 1980, 102, 3144. 14 J. Emsley and S. Niazi, Phosphorus Sulfur, 1981, 10,401. 15 Hoff-Jorgensen, Kgl. Dansk. Vid. Selsk., 1944, 7 , 21. 16 D. D. Perrin, personal communication in ref. (1). 17 W. J. Evans, E. J. McCountney and R. I. Shrager, J. Am. Oil Chem. SOC., 1982, 59, 189. 18 M. Haeringer and J. P. Schwing, Bull. SOC. Chim. Fr., 1967, 708. 19 I. G. Sayce, Talanta, 1968, 15, 1397.H. Bieth and B. Spiess 1943 20 A. Sabatini, A. Vacca and P. Gans, Talanta, 1974, 21, 53. 21 C. J. Christensen and R. M. Izatt, in Handbook of Biochemistry, ed. H. A. Sober, (The Chemical Rubber 22 N. Imgri, W. Kakolowicz, L. G. Sillen and B. Warnquist, Talanta, 1967, 14, 1261. 23 T. Glonek, R. A. Kleps, E. J. Griffith and T. C. Myers, Phosphorus, 1975,5, 157. 24 R. G. Pearson, J. Am. Chem. Soc., 1963,85, 3573. 25 R. G. Pearson, J. Am. Chem. Soc., 1968,45, 581. 26 C. Moore and B. C. Pressman, Biochem. Biophys. Res. Commun., 1964,15, 562. 27 M. Pinkerton, L. K. Steinrauf and P. Dawkins, Biochem. Biophys. Res. Commun., 1969, 35, 512. 28 C. J. Pedersen, J. Am. Chem. Soc., 1967,89,7017. 29 J. M. Lehn and J. P. Sauvage, J. Am. Chem. SOC., 1985,97, 6700. Co. Cleveland, Ohio, 1968), table 5.49. Paper 5/1362; Received 5th August, 1985

ISSN:0300-9599    

DOI:10.1039/F19868201935

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1986,82, 1945-1953 Re20,/A1,03 B203 Metathesis Catalysts Xu Xiaoding," Cornelis Boelhouwer, Jeanet I. Benecke, Douwe Vonk and Johannes C. Mol University of Amsterdam, Institute of Chemical Technology, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands Low-loading Re,O,/Al,O, - B,O,-MR, (M = Sn or Pb, R = alkyl) catalysts show high activity and selectivity at room temperature (293 +2 K) for the metathesis of functionalized alkenes; this is attributed to their strong Bronsted acidity. The metathesis of alkenes, especially functionalized alkenes, is a reaction with many important potential appli~ations.l-~ At first only homogeneous catalyst systems, e.g. WCl,/SnMe,, were reported for the metathesis of functionalized alkenes,6 but later a heterogeneous catalyst, viz.Re,O,/Al,O, with a suitable co-cataly~t,~ was developed for this reaction. Catalyst systems for heterogeneous metathesis are especially attractive owing to their obvious advantages over their homogeneous counterparts, e.g. easy separation of reaction products, suitability for continuous operation and the pos- sibility of catalyst regeneration. Recently we reported that the catalyst systems M,O, - Re,0,/Al,03 (M,O, = V,O,, WO, or MOO,) and Re,O,/SiO, .Al,O,, with a tetra-alkyl tin or tetra-alkyl lead compound as a co-catalyst, show a high activity and selectivity for the metathesis of functionalized alkenes,sy which is attributed to their pronounced acidity (especially Bronsted a~idity).~? lo Here we report a new metathesis catalyst system, viz.Re,0,/A1,0,~B,03. The metathesis of methyl oleate [(a-methyl octadec-9-enoate1, a typical functionalized alkene, was used as the model reaction: 2Me[CH2],CH=CH[CH,],COOMe + Me[CH,],CH=CH[CH,],Me + Me00C[CH,],CH=CH[CH2]7COOMe. (1) Various factors influencing the activity of the catalysts, e.g. the loadings of Re,O, and B203, the catalyst preparation and the choice of co-catalyst, were studied. The role of Bronsted acidity in the reaction has been studied by poisoning experiments with hexamethyldisilazane (HMDS), a poison for Bronsted acid sites.ll Differential thermal gravimetry (d.t.g.) and temperature programmed reduction (t.p.r.) were also used. Experiment a1 The catalysts were prepared by impregnation of A1,0;B2O, (AB) carriers with an aqueous solution of ammonium per-rhenate (Drijfhout), followed by vacuum evaporation and drying at 383 K.The alumina-boria carriers used were prepared by impregnation of y-Al,O, (Ketjen CK 300, 250-500 pm) with an aqueous solution of orthoboric acid (H,BO,, Brocades). The amount of orthoboric acid u7as chosen in such a way that AB carriers of different wt % B203 (5, 10, 15 and 20 wt "/o ) were obtained after dehydration. The wt% of B,O, and Re,O, were calculated from the amounts of the corresponding compounds used for the impregnation. For comparative purposes a few catalysts were prepared with the same chemical composition, viz. 3 wt % Re,O, supported on 15 wt % B,O, alumina-boria, by either coimpregnation or stepwise impregnation, but with a reversed order of component deposition (first Re20, and subsequently B,O,).For 19451946 50 40 - 30 E 2 '30 20 E .r( h 10 0 Re,07/Al,03 - B,03 Metathesis Catalysts .f 0 1 2 tlh Fig. 1. Conversion of methyl oleate as a function of reaction time for a series of 3 wt % Re,O,/Al,O, - B,O, catalysts containing different wt % B,O,. The carriers were calcined at 773 K in oxygen. The numbers on the curves are the weight percentages of boria of the carriers. simplicity, in this article all the Al,03 B,03 supported catalysts are designated by x wt % Re,O,/AB(y-z), where x stands for the weight percentage of Re,07, y stands for the weight percentage of B,03 of the carrier and z stands for the calcination temperature in K of the carrier before impregnation with NH,ReO,. Activity measurements were carried out in a glass batch reactor at room temperature (293 2 K) as described earlier.** SnEt, (Aldrich Europe) was normally used as a co-catalyst.In a typical reaction 100 mg of catalyst activated at 823 K, 1 cm3 of a solution of SnEt, in hexane (7.2 pmol), and 250 mm3 (737 pmol) of methyl oleate were introduced in that order into the reactor. The conversion was calculated as (2xalkene)/ (2 x alkene + monoester) in the liquid phase. In poisoning experiments with HMDS (Merck) a catalyst sample (100 mg) was activated as described above. After activation a certain amount of HMDS was injected into the reactor by means of a hypodermic syringe at 423 K under Ar. After 1.5 h the vessel was degassed at 423 K to remove the ammonia produced and unreacted HMDS. Then the catalyst was cooled to room temperature and activity measurements were started as described above.D.t.g. measurements were carried out with commercial equipment (Setaram) in an oxygen stream at a heating rate of 15 K min-l. T.p.r. experiments were carried out in a t.p.r. set-up in a flowing H,-Ar mixture (67% H,) with a flow rate of 20 cm3 min-l (s.t.p.) and a heating rate of 10 K min-l, as described earlier.12Xu Xiaoding et al. 1947 50 40 n 30 E .4 h 2 8 20 10 0 Fig. 2. Conversion of methyl oleate for various catalysts with the same chemical composition, uiz. 3 wt% Re,O, catalysts on (a) AB(l5-688), (b)--(d) AB(l5-773), (e) AB((15-423) and cf) 15 wt % A,O, * B,O,, prepared by stepwise impregnation with the reverse sequence of component deposition. Catalysts (b) and (c) were activated at 743 and 693 K, respectively, while the other catalysts were activated at 823 K. Other conditions were kept the same in all the experiments.Results Fig. 1 shows the conversion of methyl oleate as a function of reaction time for various catalysts consisting of 3 wt % Re207 supported on AB(5-773), AB( 10-773), AB( 15-773), AB(20-773) and on y-alumina, respectively. The AB-supported catalysts are more active than the alumina-supported catalyst, except for the case of AB(20-773). The catalysts on AB(5-773) and on AB(1O-773) give the best results: equilibrium conversion (ca. 50%) is reached within 2 h. Fig. 2 shows the conversion of methyl oleate as a function of reaction time for various catalysts with the same chemical composition, 3 wt% Re207 on alumina-boria (1 5 wt% B203), but prepared and treated in a different way.In addition to the chemical composition, the method of preparation, the activation temperature and the calcination temperature of the carrier all affect the activity of the catalysts. Preparation by loading boria first and subsequently Re207 gives better results than preparation by co-impreg- nation (not shown) or by stepwise impregnation with a reverse sequence of component deposition [fig. 2(f)]. For the three catalysts prepared by the same stepwise im- pregnation method and supported on a carrier with 15 wt% B203, but calcined at different temperatures, the activity decreases in the order AB( 15-688) (a) > AB( 15-773) ( d ) > AB( 15-423) (e). Calcination of the carrier at higher temperatures seems to promote1948 5 0 40 - 30 E 2 c ." h W 20 10 0 Re,0,/Al,03 - B203 Metathesis Catalysts 6 3 9 18 1 0.25 0 1 2 tlh Fig.3. Conversion of methyl oleate as a function of reaction time for AB( 10-688)-supported Re,O, catalysts with different rhenium oxide loadings and an 18 wt% Re,O,/Al,O, catalyst, using 500 mm3 methyl oleate. The numbers after the curves are the wt% Re,O, of the catalysts. and V, AB-supported catalysts; A, alumina-supported catalyst. a higher activity. The activity of 3 wt % Re,O,/AB( 15-773) catalysts, activated at different temperatures, decreases in the following order of activation temperature : 743 K (b) > 693 K (c) > 823 K (d), the optimal temperature of activation thus being ca. 750 K. Fig. 3 shows the conversion of AB( 10-688)-supported catalysts with different wt% Re,O, as a function of reaction time.For comparison, the results of an 18 wt% Re,O,/Al,O, catalyst measured under the same conditions are also presented. In these experiments a double amount of methyl oleate (500mm3) was used. The influence of boria is clear; it appears that at longer reaction times (> 1.5 h) a 3 wt % Re,O,/AB( 10-688) catalyst gives a higher conversion than the 18 wt% Re20,/A1203 catalyst, and that even a 0.25 wt% Re,O,/AB(lO-688) catalyst shows substantial activity. Fig. 4 shows the conversion of methyl oleate divided by the wt% Re,O, as a function of reaction time for the catalysts in fig. 3. All the alumina-boria-supported catalysts are more effective than the 18 wt% Re,O,/Al,O, catalyst.The lower the Re,O, loading, the higher the efficiency of the catalyst. Fig. 5 presents the conversion of methyl oleate as a function of reaction time for a 3 wt% Re,O,/AB( 10-773) catalyst treated with various amounts of HMDS (given in mm3 per 100 mg catalyst in the curves). In all the metathesis experiments the AB-supported catalysts showed a high selectivity (ca. 99%). Fig. 6 presents the results of d.t.g. measurements of some of the catalysts, i.e.Xu Xiaoding et al. 40 c 1949 g 2 0 2 8 .r( 4 10 0 I / 0 . 2 5 t L 0 1 2 tlh Fig. 4. Conversion/wt% Re,O, for the metathesis of methyl oleate as a function of reaction time for the catalysts used in fig. 3. 3 wt% Re,O, on AB(10-773), AB(15-773), and AB(20-773), and of an AB(10-773) carrier. For all the 3 wt% Re,O,/AB catalysts three peaks were observed in d.t.g., while the AB carrier showed only two peaks.The third peak shifted towards lower temperatures with increasing boria loading. T.p.r. results of the same samples (fig. 7) showed that the peak due to the reduction of rhenium oxide becomes smaller with increasing boria loading, while the peak positions are about the same. Note that SnEt, was used as a co-catalyst in all the above-mentioned experi- ments. Experiments with other co-catalysts have shown that the promoting effect of the co-catalyst decreases in the following order: SnEt, 2 SnBu, > PbBu, > PbEt, > SnMe,. Also, other substrates (e.g. propene) are easily transformed with AB-supported catalysts. Discussion Boria is an acidic oxide.13 Deposition on alumina results in strong acid sites with a Hammet index Ho < -S.2.14-17 Owing to its strong acid character, alumina-boria catalyses many reactions, e.g.polymerization, isomerization and cracking.15-,l Izumi et aZ.17 measured the acidity of a number of alumina-boria catalysts and they found that the acidity is mainly of Bronsted character. That deposition of rhenium oxide on such a strongly acidic carrier results in a highly active metathesis catalyst gives support to the suggestion that acidity, especially Bronsted acidity, is intimately involved in the metathesis r e a c t i ~ n . ~ ~ lo This view is further supported by the results of HMDS poisoning experiments (fig. 5), from which it appears that the activity of a given catalyst decreases with increasing amount of HMDS used in the experiments.HMDS is a poison for1950 5 0 4 0 - 30 E 3 z G .A 00 20 10 0 Re20,/A120, * B20, Metathesis Catalysts Fig. 5. Conversion of methyl oleate for a 3 wt % Re,O,/AB( 10-773) catalyst (100 mg) as a function of reaction time. The catalyst was treated with different amounts of hexamethyldisilazane. The numbers after the curves are the volumes of HMDS used in mm3. Bronsted acid sites.ll Its reaction with surface Bronsted acid sites (-BH) can be described as follows. 2 -BH + HN(SiMe,), NH, + 2 -B-SiMe,. (2) The different activities in fig. 1 are due to different boria loadings, which also affect the acidity of alumina-boria.17 In this respect, maximum acidity and thus maximum activity could be expected at a boria loading of 15 wt% .17 On the other hand, however, boria, when fused, dissolves many metal oxides to give borate g1a~ses.l~ We believe that the formation of a solid solution or of metal borates can be the reason for the lower activity of Re20, supported on AB( 15-773) and on AB(20-773) and that catalysts with lower boria loadings could therefore be expected to be more active.Moreover, the stabilizing effect of alumina on rhenium oxide will become weaker with an increasing boria loading, and this also decreases the activity. That Re20, on AB(10-773) and on AB(5-773) show about the same activity might be due to the compensating effects of boria loading on activity. The lower acidity of the catalyst on AB(5-773) is compensated by a smaller decrease of available rhenium species on its surface.From fig. 2 it appears that the catalyst based on AB(15-688) shows a higher activity than that based on AB( 15-423). This can be explained by the influence of the calcination temperature on the formation of acid sites; it has been reported that acid sites are only developed at temperatures above 573 K.17 The different activity of the catalysts basedXu Xiaoding et al. 1951 I I 200 600 1000 TIK Fig. 6. D.t.g. results of 3 wt% Re,O, catalysts on (a) AB(20-773), (b) AB( 15-773) and (c) AB( 10-773) and of an AB( 10-773) carrier (d). The experiments were carried out in an oxygen stream at a heating rate of 15 K min-l. on AB( 15-773) and on AB( 15-688) might be due to a possible change in surface structure connected with the melting point of boria (733 K22). On the other hand, differences in activity might be caused by other complex effects occurring not only during calcination but also during activation.Additionally, the amount of available rhenium species on the surface of the catalysts might also be influenced by the formation of solid solutions with boria. The d.t.g. peaks in fig. 6 can be assigned as follows. The peaks at ca. 420 K and at ca. 560 K can be ascribed, respectively, to the desorption of physisorbed water plus the transition of orthoboric acid to metaboric acid (420 K peak) and to further dehydration of metaboric acid to boria (560 K peak).17 The reactions are as follows: H3BO3 e HBO, + H,O 2HB0, + B,03 + H20. (3) (4) The peak at higher temperature is obviously caused by the loss of rhenium species, since it is not observed with the AB carrier [curve ( d ) in fig.61. For Re,O,/Al,O, or Re,O,/SiO, - A1203 a similar peak was observed in d.t.g. at similar temperature^.^^ For AB-supported catalysts this peak shifts to lower temperatures with increasing boria loadings. The interaction of boria with alumina can develop acid sites,l5*l7 and this coverage of alumina surface might weaken the interaction of rhenium species with1952 Re207/A1203 * B,O, Metathesis Catalysts t 300 400 500 600 7 00 TIK Fig. 7. T.p.r. patterns of the samples used for fig. 6 (heating rate 10 K min-l). alumina, which is unfavourable for the retention of rhenium species. The fact that in fig. 7 the t.p.r. peak due to rhenium reduction decreases with increasing boria loadings indicates also that the amount of rhenium species available on the catalyst surface becomes less, leading to a lower activity for catalysts with higher boria loadings.The position of the t.p.r. peak responsible for rhenium reduction hardly shifts with boria loading, indicating the similar nature of the rhenium species on these catalysts. The much lower activity of catalysts prepared by a reversed sequence of component deposition might be caused by the possible coverage of the active rhenium sites by the added boria [fig. 2(f)]. The activities of alumina-boria-supported catalysts are comparable with those of silica-alumina-supported catalyst^.^ It is known that boron behaves similarly in several ways to si1ic0n.l~ It has been reported that alumina-boria contains more, but weaker acid sites than does silica-a1~mina.l~~ l6 However, not only may the strength of acid sites play a role, but also the unfavourable effect of the possible formation of a solid solution already mentioned.The fact that the 9 wt% Re207/AB(10-688) catalyst shown in fig. 3 has a lower activity than the corresponding 6 and 3 wt % Re207 catalysts might be caused by the loss of more Re,07 owing to a higher loading and by the coverage of acid sites by the excess of Re207.23 The trend shown in fig. 4, viz. the decreasing efficiency of AB-supported catalysts with increasing Re207 loading, was also observed with silica-alumina-supported catalyst^.^. This can be explained by the relatively small number of acid sites and available rhenium species on the surface.Xu Xiaoding et al.1953 Conclusions Alumina-boria-supported low-loading rhenium oxide catalysts show high activity in metathesis; this is due to their high Bronsted acidity. The activity is influenced by the loadings of rhenium oxide and boria, the co-catalyst used, the method of preparation of the catalysts and the activation procedures. Xu Xiaoding, on leave of absence from the Department of Chemistry, Fudan University, Shanghai, is the recipient of a fellowship on the basis of an exchange programme between The Netherlands and The People’s Republic of China. We thank Mr E. F. G. Woerlee for the d.t.g. measurements and helpful discussions. References 1 R. L. Banks, J. Mol. Catal., 1980, 8, 269. 2 K. J. Ivin, Olejin Metathesis (Academic Press, London, 1983).3 J. C. Mol, J. Mol. Catal., 1982, 15, 35. 4 J. C. Mol, Chemtech., 1983, 13, 250. 5 C. Boelhouwer and J. C. Mol, J. Am. Oil Chem. SOC., 1984, 61,425. 6 P. B. van Dam, M. C. Mittelmeijer and C. Boelhouwer, J. Chem. SOC., Chem. Commun., 1972, 1221. 7 E. Verkuijlen, F. Kapteijn, J. C. Mol and C. Boelhouwer, J. Chem. SOC., Chem. Commun., 1977, 198. 8 Xu Xiaoding, P. Imhoff, G. C. N. van Aardweg and J. C. Mol, J. Chem. Soc., Chem. Commun., 1985, 9 Xu Xiaoding and J. C. Mol, J. Chem. SOC., Chem. Commun., 1985,631. 273. 10 D. T. Laverty, J. J. Rooney and A. Stewart, J. Catal., 1976, 45, 110. 1 1 A. J. van Roosmalen and J. C. Mol, J. Catal., 1982, 82, 17. 12 Xu Xiaoding, A. Andreini and J. C. Mol, J. Mol. Catal., 1985, 28, 133. 13 F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry (John Wiley, New York, 1976), pp. 14 H. A. Benesi, J. Am. Chem. SOC., 1956,78, 5490. 15 K. Tanabe, Solid Acids and Bases (Academic Press and Kodanshd, Japan, 1970), p. 59. 16 H. A. Benesi and B. H. C. Winquist, Adv. Catal., 1978, 27, 97. 17 Y. Izumi and T. Shiba, Bull. Chem. SOC. Jpn, 1964, 37, 1797. 18 V. N. Voro’ev, D. R. Agzamkhodzhaeva, V. P. Mikitaand M. F. Abidova, Kinet. Katal., 1984,25,190. 19 M. Sato, T. Aonuma and T. Shiba, Proc. 3rd Znt. Congr. Catal. (North-Holland, Amsterdam, 1964), 20 V. A. Dzisko, Proc. 3rd Int. Congr. Catal. (North-Holland, Amsterdam, 1964), vol. 1, pp. 422-432. 21 A. Ozaki and Kimura, J. Catal., 1964, 3, 395. 22 Handbook of Chemistry and Physics, ed. R. C. Weast (Chemical Rubber Company, Cleveland, Ohio, 23 Xu Xiaoding, unpublished results. 289-325. vol. 1, pp. 396-407. 49th edn, 1969), p. B-183. Paper 511363; Received 5th August, 1985

ISSN:0300-9599    

DOI:10.1039/F19868201945

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J . Chem. Soc., Furuduy Trans. I , 1986,232, 1955-1963 Molar Gibbs (Free) Energies of Transfer of Silver@), Copper(1) and Pstassium(1) Gerhard Gritzner Institut f u r Chemische Technologie Anorganischer Stofle, Johannes-Kepler- Universitat Linz, A-4040 LinzlAuhof, Austria Molar Gibbs energies of Ag+ for the transfer from acetonitrile into formamide, N-methylformamide, N,N-dimethylformamide, N,N-diethyl- formamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methyl-2- pyrrolidone, tetramethylurea, dimethyl sulphoxide, hexamethylphosphoric triamide, acetone, propylene carbonate, benzonitrile, N,N-dimethylthio- formamide, N-methylthio-2-pyrrolidone and hexamethylthiophosphoric tri- amide have been calculated from the half-wave potentials of bis(bipheny1)chromium tetraphenylborate uerms the silver, 0.0 I mol dm-3 silver ion electrodes in the respective solvents as well as from polarographic half-wave potentials of Ag+ us. bis(bipheny1)chrornium (I/o) in benzonitrile, N,N-dimethylthioformamide, N-methylthio-2-pyrrolidone and hexamethy I- thiophosphoric triamide.Molar Gibbs energies of transfer for K+ have also been calculated from polarographic half-wave potentials for the solvents mentioned, except for formamide and for hexamethylphosphoric triamide. Molar Gibbs energies of transfer for Cu+ from acetonitrile into benzonitrile, N,M-dimethylthioformamide, N-methylthio-Zpyrrolidone and hexamethyl- thiophosphoric triamide have been obtained from polarographic half-wave potentials employing the bis(bipheny1)chromium assumption. The data have been compared with molar Gibbs energies of transfer derived via the tetraphenylarsonium tetraphenylborate assumption and the assumption of negligible liquid junction potentials.A comparison of Gibbs energies of transfer for Ag+ with the data for K+ showed the difference in interactions between ‘hard’ and ‘soft’ solvents. Gibbs energies of transfer for single ions are, as all other single-ion properties, quantities outside of the realm of strict thermodynamics. They are derived from thermodynamically exact measurements for salts uiu an assumption on which a division of the values for salts into the contributions from cations and anions is based. Among the so called ‘experimental’ procedures to derive Gibbs energies of transfer for single ions, the tetraphenylarsoniurn tetraphenylborate assumption,l the assumption of negligible liquid junction potentials2$ and the bis(bipheny1)chromium assumption4 have been more widely used than any other to obtain transfer data for cations.The tetraphenylarsonium tetraphenylborate assumption has generally been employed in connection with solubility studies, the assumption of negligible liquid junction potentials was used for potentiometric measurements, and the bis(bipheny1)chromium assumption served to obtain Gibbs energies of transfer from polarographic and cyclovoltammetric measurements. Gibbs energies of transfer from the gas phase as the reference state5 into solvents are dominated by the very large contribution caused by the transfer from the gaseous state into any condensed system, which tends to hide the subtle differences in Gibbs energies of cations between two solvents. It has therefore become common in solution chemistry to select one solvent as a reference solvent, in which Cibbs energies of all ions are assumed to be zero.By selecting a reference solvent one obtains Gibbs energies of transfer that better 19551956 Molar Gibbs (Free) Energies of Transfer reflect the relatively small changes in Gibbs energies upon the transfer from one solvent to another. In this study acetonitrife has been the preferred reference solvent, although, for comparison of the data based on the bis(bipheny1)chromium assumption with Gibbs energies of transfer derived from the tetraphenylarsonium tetraphenylborate assumption and the assumption of negligible liquid junction potentials, N,N-dimethylformamide has also been used.Gibbs energies of transfer of K+ and Cl- have been proposed as a secondary standard to quantify donor and acceptor properties of solvents.6 A comparison of the values for a typically ‘ hard’7 or ‘a-type’s cation, namely K+, with data for ‘soft’ or ‘b-type’ cations (Cu+, Ag+) will be made in this study to investigate the differences in interactions between solvents and ‘ hard ’ and ‘ soft’ cations. Experiment a1 The apparatus and the methods employed in polarography and cyclic voltammetry, and the chemicals used have been described in detailsg Gibbs energies of transfer (AG,“) have been calculated from the equation AG,“ = nF(E,- ERs) where n is the number of electrons, F is the Faraday constant, E, is the respective potential (e.g, the half-wave potential) in a given solvent and ERs is the respective potential in the reference solvent.Tetrakis(N-methylthio-Zpyrrolidone)copper(~) trifluoromethane sulphonate [Cu(NMTP),CF,SO,] (m.w. 637.37) was made by reacting anhydrous Cu(CF,SO,), with a tenfold molar excess of N-methylthio-2-pyrrolidone. This led to a yellow syrup from which pale yellow crystals could be obtained by adding a large amount of diethylether. Analysis: % found (% calculated): C , 37.23 (37.45); H, 5.12 (5.39); N, 8.17 (8.32); S, 23.16 (23.81); Cu, 9.23 (9.44). Results The data for the Gibbs energies of transfer for Cu+ and K+ have been calculated from polarographic half-wave potentials of reversible or quasireversible electrode processes measured uersus bis(biphenyl)chromium(I/o) as a reference redox system.The Gibbs energies of transfer for Ag+ have been calculated from the differences of the potentials of a silver, 0.01 mol drn+ silver ion electrode measured versus the half-wave potential of bis(biphenyl)chromium(r) in the respective supporting electrolytes and solvents. The reduction of silver@) cannot, as a rule, be observed in polarography, since the reduction occurs in most solvents at potentials more positive than the dissolution of mercury. Strong interactions of the solvents acetonitrile, benzonitrile, N,N-dimethylthioform- amide, N-methylthio-2-pyrrolidone and hexamethylthiophosphoric triamide with the silver ion, however, shifted the polarographic reduction of Ag+ to quite negative potentials, so that half-wave potentials could be measured.For these solvents Gibbs energies of transfer can also be calculated from the difference of the half-wave potentials in the respective solvent and acetonitrile. The agreement between the data obtained by these two procedures can be considered excellent. The half-wave potentials of Ag+, Cu+ and K+ as well as the half-wave potentials of bis(biphenyl)chromium(I) versus the silver, 0.0 1 mol dm-, silver ion electrode in the respective solvents have generally been taken from literature4, 9-18 and some additional data were measured during this study. The polarographic half-wave potential of Cut in N-methylthio-2-pyrrolidone (NMTP) was re-evaluated using CU(NMTP)~CF,SO, and found to be -0.1 1, V versus bis(biphe- nyl)chromium(I/o) as a reference redox system.The half-wave potential of Ag+ in this solvent was found to be 0.32, V and the potential of a silver, 0.01 mol dm-, silverG. Gritzner 1957 Table 1. Molar Gibbs energies of transfer (AG:) in kJ mol-l for silver(I), copper(1) and potassium@) at 25 "Ca no. 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 - 17 AG,"(K+) AG,"- AGWg+) abbrevi- solvent ation BCrb BCrC TATB NLPd (Cu') BCr TATB NLPd formamide N-methy 1 formamide N,N-dime t h y lformamide N,N-die t h ylformamide N,N-dime thy lace tamide N, N-dieth ylacetamide N-me t h y l-2-p yrrolidone te trame thylurea dime thy1 sulphoxide hexamethylphosphoric acetone propylene carbonate benzoni trile acetonitrile triamide FA 16.4 NMF 8.7 DMF 7.9 DEF 10.8 (10) (9) (1 1) (12) (10) DEA -0.3 (1 1) NMP 0.2 DMA -0.5 TMU 0.6 (14) DMSO -7.0 HMP -13.5 (16) AC 27.5 (14) PC 46.7 (14) BN 7.9 AN 0 (4) - - 14.2 - 12.1 - (19) (3) - -10.7 - - (9) - - 14.3 - 17.6 - 18.8 - -11.9 - - - -17.5 - -22.6 (11) (19) (3) (12) (1 0) (3) (1 1) - -14.7 - - - -15.0 -21.8 -25.9 (14) (15) (19) (3) - - 19.0 - -26.4 (1 6) (3) - -5.5 - - (17) - 1.3 -2.1 -7.1 (15) (19) (3) (4) (4) (19) (3) - -17.1 -20.1 -21.7 7.2 8.9 - - 0 0 0 0 hexamethylthiophosphoric HMTP - 56.5 - 55.5 - - -20.8 - - - triamide (16) (16) (16) N,N- dimethylthioformamide DMTF -74.2 -73.2 - -82.8 -47.9 19.6 - 17.2 (3) (18) (3) N-methylthio-2-pyrrolidone NMTP - 82.0 - 80.1 - - -52.1 18.5 - - a Reference solvent, acetonitrile.Literature references containing data from which Gibbs energies of transfer have been calculated are given in parentheses.BCr, Bis(bipheny1)chromium assumption. TATB, Tetraphenylarsonium tetraphenylborate assumption. NLP, assumption of negligible liquid junction potentials. Data from the potentials of the Ag, 0.01 rnol dmda silver ion electrode in the respective solvent. Data taken from table 1 of ref (3). Data from the half-wave potentials of Ag+- trifluoromethane sulphonate electrode was 0.18, V; both potential values have been recorded versus bis(biphenyl)chromium(I/o). The potential of the silver, 0.01 mol dm-3 silver trifluoromethane sulphonate electrode was found to be 0.95, V in dimethyl sulphoxide, 1.1 1, V in benzonitrile, 1.03, V in N-methyl-2-pyrrolidone, 0.26, V in N,N-dimethylthioformamide and 0.18, V in N-methylthio-2-pyrr olidone.The values for the Gibbs energies of transfer for Ag+, Cu+ and K+ from acetonitrile based on the bis(bipheny1)chromium assumption are listed in table 1, together with data1958 Molar Gibbs (Free) Energies of Transfer Table 2. Linear Correlations between Gibbs energies of transfer (AG;) at 25 "C in kJ m01-l for Ag+, Cu+ and K+ obtained from the bis(bipheny1)chromium assumption BCr, the tetraphenylarsonium tetraphenylborate assumption (TATB) and the assumption of negligible liquid junction potentials (NLP)a ~~ standard standard error error correlation X Y solventsb slope intercept slope estimate coefficient Ag+(BCr) Ag+(TATB) 1,3,7,9, 12 14 Ag+(BCr) Ag+(NLP) 1,3,5,7,9,10 12, 14, 16 Ag+(TATB) Ag+(NLP) 1,3,7,9, 12 14 K+(BCr) K+(TATB) 3,7,9,12,14 K+(BCr) K+(NLP) 3,5,7,9,10, 12, 14, 16 K+(TATB) K+(NLP) 1,3,7,9,12, 14 K+(BCr) Ag+(BCr) 2-12 Kf(BCr) Ag+(BCr) 13,14,16,17 Ag+(BCr) Cu+(BCr) 13-1 7 0.894 -4.81 0.087 3.76 0.982 1.01 - 5.29 0.038 3.47 0.995 0.999 1.05 0.178 0.013 0.522 0.131 0.093 2.32 3.35 0.98 1 0.980 1.15 1.11 - 1.95 -4.56 0.999 - 1.65 0.140 2.92 0.963 2.72 0.621 - 4.65 42.4 15.5 2.71 0.19 1.96 0.09 1 3.67 7.68 29.8 0.978 0.859 0.969 ~~~~ a Reference solvent, acetonitrile.Numbering of solvents as given in table 1. Table 3. Molar Gibbs energies of transfer (AG:) at 25 "C in kJ mol-l for silver(1) and potassium(I)a AGXAg+) AG,"(K+) solvent BCr TATB NLP BCr TATB NLP formamide " 6 1.7 N,N-dimeth ylformamide 0 0 N,N-dime t h y lace tamide - - 8., N-methyl-2-p yrrolidone -7., -13., dimethylsulphoxide -14., -16., hexamethylphosphoric triamide - 21 .4 - propylene carbonate 38., 33., acetonitrile - 7., -4.6 hexamethylthiophosphoric - 64.4 - triamide - N,N-dimethylthioformstmide - 82., 2.6 -7q - 18., - 26, 36.,, - 4., - 6O., 0 - 6., 6.7 0 - 3., - 7., - 2., - 7., 17., 18., 7.6 0 - 2., 0 - 5., - lo., 20., 15., 0 - 3., - O., - 2., -4., 15., 14., - - 87.0 33., 36., 33., a Reference solvent, N,N-dimethylformamide. BCr, Bis(bipheny1)chromium assumption.TATB, Tetraphenylarsoniurn tetraphenylborate assumption. NLP, Assumption of negligible liquid junction potentials. obtained from the tetraphenylarsonium tetraphenylborate assumption and the assump- tion of negligible liquid junction potentials. Gibbs energies of transfer for Ag+ as well as for K+ obtained by different assumptions have been plotted versus each other.A linear correlation best reffects the dependence of such data. Parameters obtained from linear regression analysis such as the slope, the intercept, the standard error of the slope, the standard error of estimate and the correlation coefficient for all the combinations tried are given in table 2.G. Gritzner 1959 Table 4. Linear correlations between Gibbs energies of transfer (AG,") at 25 "C in kJ mol-1 for Ag+ and K+ obtained from the bis(bipheny1)chromium assumption (BCr), the tetraphenylarsonium tetraphenylborate assumption (TATB) and the assumption of negligible liquid junction potentials (NLP)a standard standard error error correlation X Y solventsb slope intercept slope estimate coefficient Agf(BCr) Ag+(TATB) 1, 3, 7, 9, 0.894 -2.35 0.087 3.76 0.982 Ag+(BCr) Ag+(NLP) 1, 3, 5, 7, 9, 0.992 -1.41 0.037 3.89 0.994 12, 14 10, 12, 14, 15, 16 12, 14 12, 14, 16 12, 14, 16 10, 12, 14, Ag+(TATB) Ag+(NLP) 1, 3, 7, 9, 1.03 - 1.31 0.083 3.28 0.987 K+(BCr) K+(TATB) 3,5,7,9,10, 1.15 - 1.13 0.079 2.85 0.986 K+(BCr) K+(NLP) 3,5,7,9,10, 1.08 -0.728 0.091 3.28 0.980 K+(TATB) K+(NLP) 1, 3, 5, 7, 9, 0.924 -0.521 0.074 3.13 0.978 16 K+(BCr) Ag+(BCr) 3, 5, 7, 9, 2.82 -4.20 0.246 4.10 0.985 10,12 Reference solvent N,N-dimethylformamide.Numbers for the solvents are given in table 1 For the reasons given in the discussion Gibbs energies of transfer for Ag+ and K+ have also been calculated from N,N-dimethylformamide as a reference solvent (table 3). Data for linear correlations are summarized in table 4.Discussion Since Gibbs energies of transfer of single ions are extrathermodynamic properties it is advisable to consider data obtained from assumptions other than the one employed prior to any discussions based on such data. Thus for the Gibbs energies of transfer for Ag+, Cu+ and K+ values derived from the tetraphenylarsonium tetraphenylborate assumption and the assumption of negligible liquid junction potentials will be viewed together with those from the bis(bipheny1)chromium assumption. Gibbs energies of transfer from acetonitrile into various solvents yielded good agreement between the data from the tetraphenylarsonium tetraphenylborate assumption and the assumption of negligible liquid junction potentials. The data obtained via the bis(bipheny1)chromium assumption are generally more positive by 3-1 0 kJ mol-l than the data from the other two assumptions. Linear-regression analysis between the data from the bis(bipheny1)chromium assumption and the data from either the tetraphenyl- arsonium tetraphenylborate assumption or the assumption of negligible liquid junction potentials with acetonitrile as a reference solvent yielded slopes of about one and intercepts of ca.-5 kJ mol-l, with correlation coefficients better than 0.98 (fig. 1). Changing to N,N-dimethylformamide as a reference solvent instead of acetonitrile again yielded slopes close to unity; the values for the intercepts however are now only 1-2 kJ mol-l. The much better and acceptable agreement of the Gibbs energies of transfer derived from the mentioned assumptions from N,N-dimethylformamide indicates that the discrepancies between the bis(bipheny1)chromium assumption on one side and 65 FAR 11960 Molar Gibbs (Free) Energies of Transfer 40 - 20 - " c3 I 2 0: g - h 2 - ;= 4 -20- c - 6 -40- 0 0 - O r - u Q - -60 - -80 - - -80 -60 -40 -20 0 20 40 60 AG; [Ag+(BCr)]/kJ mol-' Fig.1. Correlation between Gibbs energies of transfer for Ag+ obtained from the bis(bipheny1)- chromium (BCr) assumption and the assumption of negligible liquid junction potentials (NLP). Reference solvent, acetonitrile. Abbreviations of the solvents are listed in table 1. the tetraphenylarsonium tetraphenylborate assumption and the assumption of negligible liquid junctions on the other are due to the different values for Ag+ in acetonitrile.Similar differences can be observed for the Gibbs energies of transfer for K+ from the bis(bipheny1)chromium assumption and the assumption of negligible liquid junction potentials from acetonitrile as reference solvent. The data for K+ based on the bis(bipheny1)chromium assumption on the other hand agree well with the Gibbs energies of KS derived from the tetraphenylarsonium tetraphenylborate assumption. The corre- lation coefficient is even better than the one for the values obtained from the tetraphe- nylarsonium tetraphenylborate assumption and the assumption of negligible liquid junction potentials. The data for K+ showed even better agreement between the three assumptions when N,N-dimethylformamide was employed as a reference solvent instead of acetonitrile.It is unfortunate that the data for acetonitrile differ by ca. 5 kJ mol-1 for Ag+, depending on the experimental techniques used. Changing to N,N- dimethylformamide as a reference solvent would result in good agreement of the data in nearly all the solvents for which values for different assumptions are available. However, such a change would prevent the calculation of Gibbs energies of transfer for Cu+, since monovalent copper is not stable in N,N-dimethylformamide. One could, of course, also continue to use acetonitrile as a reference solvent, arbitrarily choose one assumption and adjust the other two to this assumption by adding or subtracting from the values for the Gibbs energies of transfer a constant value, such as the value of the intercept.Such a procedure would then result in a very good agreement between the data. However, without knowing whether this discrepancy is due to errors in the experiment or due to the assumptions themselves, any selection of one assumption as the correct one would be arbitrary. Generally it can be said that Gibbs energies of transfer derived from any of these three assumptions do not differ by more than 8 kJ mol-l, in most cases the agreement is much better, since the standard error of estimate is ca. 4 kJ mol-1 for N,N-dimethylformamideG. Gritzner 1961 BN 0 DMTF t 0 -"I NMTP 0 t l 1 1 I I ' I I I ' ' I I I ' I ' I ' I -40 -20 0 20 40 60 AG,"[K+(BCr)]/kJ mol-' Fig. 2. Correlation between Gibbs energies of transfer for K+ and Ag+ obtained from the bis(bipheny1)chromium assumption.Reference solvent, acetonitrile. Abbreviations of the solvents are listed in table 1. as reference solvent. This is presumably as good an agreement as one can expect for Gibbs energies of transfer obtained via different experimental techniques and assumptions. The following considerations about ion-solvent interactions will therefore be based only on data obtained via the bis(bipheny1)chromium assumption, since there are considerably more data available for this assumption than for the other two assumptions. However, the conclusions drawn would be the same if data from any of the other two assumptions were used. A plot of Gibbs energies of transfer for Ag+ uersus the Gibbs energies of transfer for K+ is given in fig. 2. A linear correlation between these data exists only for 'hard' solvents.Thus the data points for the 'borderline ' solvents16 acetonitrile and benzonitrile and ' soft ' solvents N,N-dimethylthioformamide and N-methylthio-2-pyrrolidone, which are capable of ' soft-soft ' interactions with Ag+, deviate considerably from the line. Although there are only four data points for ' borderline' and ' soft' solvents, a line drawn through these points indicates that the increase in interactions of 'soft' solvents with 'soft' cations is accompanied by a decrease in the interaction of 'soft' solvents with ' hard ' cations. The Gibbs energies of transfer for Cu+ have also been correlated with the data for Ag+. Four points, namely the data for benzonitrile, acetonitrile, N,N-dimethylthio- formamide and N-methylthio-2-pyrrolidone, are on one line.The data for hexamethyl- 65-21962 - I 4 z 3 -20- 2 - n +8 3 V -40- u U Y 0.- -60 Molar Gibbs (Free) Energies of Transfer I- 1 . - 1 1 1 1 1 1 1 1 l 1 1 1 1 1 1 1 I l I t I t I I 1 thiophosphoric triamide deviate and are thus responsible for the increase in the standard error of estimation (fig. 3). From the data and the preceding discussion it becomes obvious that for solvents Gibbs energies of transfer for the 'hard' cation Kf and the ' soft ' cation Ag+ are linearly dependent. The slope of 2.72 obtained from the linear regression analysis shows that the interactions of the solvents with Ag+ lead to a more pronounced effect (larger change in the Gibbs energies of transfer) than for K+. This is surprising, since the ionic radii for IS+ (133 pm) and for Ag+ (126 pm) are very similar and one would expect a slope closer to unity.' Borderline' solvents and ' soft' solvents interact differently with ' soft ' cations than with 'hard' cations. This difference in interactions is not reflected in solvent parameters such as the dielectric constant, the dipole or quadrupole moments or even the donor numbers of solvents. It therefore becomes necessary to propose parameters to quantify the donor properties of ' soft' and ' borderline' solvents towards ' soft' cations ; Gibbs energies of transfer of Ag+ can be such a parameter. Based on this quantity the solvents can be arranged in the following order of increasing softness : benzonitrile, acetonitrile, hexame t h yl thiop hosp horic t riamide , N,N-dime t h yl t hio formamide and N-me t h yl t hi o -2 - pyrrolidone.Financial support from the Fonds zur Forderung der wissenschaftlichen Forschung (Austria) is gratefully appreciated. Thanks are due to Mr G. Slacik for help in developing computer programs. References 1 B. G. Cox and A. J. Parker, J. Am. Chem. SOC., 1973,95,402. 2 R. Alexander, A. J. Parker, J. H. Sharp and W. E. Waghorne, J. Am. Chem. SOC., 1972,94, 1148. 3 D. A. Owensby, A. J. Parker and J. W. Diggle, J. Am. Chem. SOC., 1974,96, 2682.G. Gritzner 4 G. Gritzner, Inorg. Chim. Acta, 1977, 24, 5. 5 M. Born, Z. Phys., 1920, 1, 45. 6 A. J. Parker, U. Mayer, R. Schmid and V. Gutmann, J . Org. Chem., 1978, 43, 1843. 7 R. Pearson, J. Am. Chem. SOC., 1963, 85, 3533. 8 S. Ahrland, J. Chatt and N. R. Davies, Q. Rev. Chem. SOC., 1958, 12, 265. 9 G. Gritzner, J. Electroanal. Chem., 1983, 144, 259. 10 G. Gsaller and G. Gritzner, Z. Phys. Chem., 1983, 138, 137. 11 J. Bock and G. Gritzner, 2. Phys. Chem., N.F., 1982, 130, 181. 12 S. Sperker and G. Gritzner, to be published. 13 G. Gritzner, P. Rechberger and V. Gutmann, J . Electroanal. Chem., 1977, 75, 739. 14 P. Rechberger, Z. Phys. Chem., N.F., 1980, 122, 15. 15 V. Gutmann and R. Schmid, Monatsh. Chem., 1969, 100, 21 13. 16 G. Kraml and G. Gritzner, J . Chem. SOC., Faraday Trans. I , 1985, 81, 2875. 17 0. Duschek, V. Gutmann and P. Rechberger, Monutsh. Chem., 1974, 105, 62. 18 V. Gutmann, K. Danksagmiiller and 0. Duschek, Z . Phys. Chem., N.F., 1974,92, 199. 19 B. G. Cox, G. R. Hedwig, A. J. Parker and D. W. Watts, Aust. J . Chem., 1974, 27, 477. 1963 Paper 5 / 1364; Received 5th August, 1985

ISSN:0300-9599    

DOI:10.1039/F19868201955

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I, 1986,82, 1965-1972 Inhibition of the Thin-film Oxidation of n-Dodecane by p-Methoxyphenol Adiele D. Ekechukwu and Robert F. Simmons The University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 1QD The inhibition by p-methoxyphenol (PMP) of the oxidation of a thin film of n-dodecane has been studied over the temperature range 180-230 "C by following the consumption of gaseous oxygen, the disappearance of the inhibitor and the formation of peroxides. At the start of the reaction there is an induction period which increases linearly with the initial concentration of inhibitor. During the induction period [PMP] falls to zero and the removal of PMP follows a zero-order rate law. Thus PMP must react preferentially with the chain centres as they are formed in the initiation reaction.Once reaction starts, however, the consumption of oxygen is effectively independent of the initial concentration of PMP. It is shown that PMP must react with alkyl radicals and not alkylperoxy radicals in the primary inhibition reaction. This reaction involves the abstraction of the phenolic hydrogen to form phenoxy radicals, and these are sufficiently stable to either dimerse or react with alkyl or hydroperoxy radicals to give chain termination. The resistance of a lubricating oil to oxidation is increased by the addition of oxidation inhibitors, and these antioxidants can be divided into two main gr0ups.l The preventative antioxidants act by reducing the rate of chain initiation, while the chain-breaking antioxidants react preferentially with chain centres in the oxidation reaction.Phenolic compounds fall into the second category, and their effect is usually discussed in terms of the inhibition mechanism given in scheme 1. R02 + AOH f R02H + A 0 (1) RO, + A 0 -+ termination (2) A 0 +A0 -+ termination (3) AO+RH =AOH+R. Scheme 1. (4) The kinetic evidence for this mechanism, however, is limited and mainly depends on studies at low temperatures in which species such as benzoyl peroxide,2 a,a'-azo- bis-is~butyronitrile~ and 2,2,3,3-tetraphenylbutane4~ have been used to initiate the oxidation. For example, Mahoney** has studied the inhibiting effect of phenols on the oxidation of 9,lO-dihydroanthracene initiated by 2,2,3,3-tetraphenylbutane7 and this work shows that two distinct types of kinetic behaviour can be obtained.With sterically hindered phenol^,^ such as 2,6-di-t-butyl-4-methylphenol, the kinetics of the inhibited oxidation are relatively simple and can be explained if reactions (- 1) and (4) are slow compared with the rates of reactions (2) and (3). In contrast, with non-hindered phenols such as p-methoxyphenol (PMP) the reaction is kinetically complex and it is believed that both the alkylperoxy and phenoxy radicals are important chain centres in the reaction5 19651966 Inhibition of n-Dodecane Oxidation by p-Methoxyphenol All the above work was carried out at temperatures well below those encountered in a reciprocating engine, and the aim of the present work was therefore to examine the behaviour of a model system under conditions which were more representative of those in the engine.To this end, the effect of p-methoxyphenol on the oxidation of a thin film of n-dodecane has been studied in the temperature range 180-230°C. Under these conditions relatively simple kinetic behaviour is obtained, even though the inhibitor is a non-hindered phenol. There is an induction period at the beginning of the reaction during which the concentration of inhibitor decreases linearly with time to zero, and once the reaction starts, the rate of oxygen consumption is effectively independent of the initial concentration of the phenol. This is in marked contrast to the behaviour observed at lower temperatures, where the inhibitor reduces the rate of consumption of oxygen from the beginning of the reaction, and the mechanistic explanation of the present results also accounts for this difference in behaviour.Experiment a1 Methods In the present work, the required amounts of a solution of known concentration of p-methoxyphenol in n-dodecane and oxygen were sealed in a cylindrical Pyrex reaction tube and heated for a known time in a thermostatically controlled tube furnace. During the reaction, the Pyrex tube was rotated horizontally within the furnace (by means of rollers driven by an electric motor)6 to ensure that a thin film of liquid was maintained over the inner wall of the reaction tube. After quenching, the amount of oxygen consumed was determined, and in some experiments the liquid phase was analysed for PMP or the total peroxide content.The reaction tubes (26 cm x 2 cm 0.d.) were made from thick-walled Pyrex glass and had a narrow neck (0.6 cm 0.d.) which was drawn down to facilitate sealing. Before use, the tubes were cleaned with boiling fuming nitric acid, rinsed with distilled water and then treated with 10 cm3 of warm 40% hydrofluoric acid before being thoroughly rinsed with distilled water and dried overnight in an oven. The required volume (normally 2 cm3) of the dodecane solution was injected into the reaction tube by means of a gas-tight syringe (5cm3) and the solution degassed by successively cooling the tube in liquid nitrogen and then evacuating it. When the degassing was complete, the tube was allowed to warm up to room temperature, oxygen was admitted to a known pressure and the tube was sealed.Since the volume of the tube had been determined previously and the pressure of oxygen measured at room temperature, the absolute amount of oxygen in the reaction tube was also known. The furnace was a conventional tube furnace (47 cm x 6 cm i.d.) and the temperature variation along the central portion which housed the reaction tube did not vary by more than 00.5 "C. The furnace temperature was controlled to k0.2 "C by a Sirect proportional temperature controller using a platinum resistance thermometer as sensing element, and the actual temperature was measured using a copper-constantan thermo- couple junction positioned half-way along the length of the reaction tube. It took ca. 5 min for the furnace temperature to regain its steady value after the reaction tube had been introduced, and this warming-up period accounts for a major part of the induction time in the absence of inhibitor.After the required time, reaction was quenched by rapid cooling and the residual oxygen determined. The non-condensible gas at - 196 "C was transferred to a gas burette to determine the total amount, and the oxygen content of this gas was determined by gas chromatography using a 10 ft column of silica gel (30/60 mesh) with hydrogen as carrier gas and a katharometer detector. In some experiments a quantitative analysis was also made for PMP, and this was achieved using a capillary column with a wall coatingA . D. Ekechukwu and R. I;. Simmons 1967 10 30 50 70 tlmin Fig. 1. Effect of p-methoxyphenol (PMP) on the oxidation of n-dodecane.Temperature, 190 "C; initial pressure of oxygen, 600 Tort-; [PMP]/mol dm-3: @, 0; A, 5 x lov4; V, 1 x 0, 1.5 x A, 2 x and V, 3 x of free fatty acid (FFAP) as the stationary phase. In this case, a temperature programme was used (4 "C min-l, with a final temperature of 200 "C) with nitrogen as the carrier gas and a flame ionisation detector. The total peroxide content of the liquid phase was also determined in some experiments using the iron(I1) thiocyanate m e t h ~ d . ~ Since the liquid phase was not miscible with the thiocyanate reagent, 1.5 cm3 of the liquid products were extracted with 0.5 cm3 samples of the reagent until no more coloration was obtained (normally three times) and the resulting extract diluted volumetrically to a known volume.The optical density of this solution was then measured at 450nm using a Beckmann-Acta M, U.V. spectr opho tome ter . Materials Gas chromatographic analysis of the n-dodecane (B.D.H. Chemicals, > 99 % purity) showed a small quantity of impurity with a slightly shorter residence time, but since the extent of reaction was limited by the relatively small amount of oxygen no attempt was made to purify the n-dodecane any further. p-Methoxyphenol (B.D.H. Chemicals, > 99% purity) and gaseous oxygen (B.O.C.) were also used without further purification. Results The inhibition of the oxidation of a thin film of n-dodecane by known amounts of p-methoxyphenol has been studied over the temperature range 180-230 "C and some typical results are given in fig.1. These show that there is initially an induction period, during which the consumption of oxygen is negligible, and fig. 2 shows that this induction period increases linearly with increasing initial concentration of inhibitor. Once reaction starts, however, the rate of consumption of oxygen is rapid and is effectively independent of the initial concentration of PMP, although there is a slight reduction in the rate with the higher concentrations of PMP used at higher temperatures. This type of behaviour is analogous to that obtained for the inhibition by benzoquinone of the photochemical polymerisation of vinyl acetate8 and the thermal polymerisation of ~tyrene.~1968 Inhibition of n-Dodecane Oxidation by p-Methoxyphenol c I I I 1 \ 2 0.0001 0.0002 U 20 0.02 0.04 [PMPl/mol d r t ~ - ~ Fig.2. Variation in induction time with concentration of p-methoxyphenol [PMP]. Temperature : 0, 180; A, 190; A, 210 and 0, 230 "C. Each data point in fig. 1 comes from a separate kinetic experiment, and some indication of the reproducibility comes from the scatter of the data points about the mean lines. In addition, examination of three sets of data for the same experimental conditions, but obtained over intervals of a few months, suggests that the reproducibility in the oxygen consumption was normally _+ 1 % for oxygen consumptions ranging from 20 to 90%. While straight lines have been drawn through the data points in fig. 1 , which implies a zero-order reaction, a close examination of all the results suggests a steep sigmoid autocatalytic curve more accurately represents the behaviour, since the rate of consump- tion of oxygen varies with its initial pressure in the reaction tube.In contast, the induction period is independent of the initial oxygen pressure (over a sixfold variation) and thus most of the determinations have been made with an initial pressure of 608 rnrnHg* at room temperature. The consumption of PMP during the induction period has been followed by gas chromatography and the results are given in fig. 3. This shows that its concentration falls linearly with time to approximately zero at the end of the induction period, although there is a period of ca. 5 min at the start of the reaction during which it remains constant; this presumably reflects the heating-up period after the reaction tube was placed in the furnace.This linear relationship implies a zero-order dependence on the inhibitor concentration, so that the reaction removing the inhibitor is not the rate-controlling process during the induction period. Presumably the rate of removal of PMP (R = - d[PMP]/dt) is proportional to the rate of chain initiation and, if the concentrations of reactants are constant, the variation of * 1 mmHg z 133.3 Pa.A . D . Ekechukwu and R. I;. Simmons 1969 4 m E TJ a E 20 40 t/min Fig. 3. Disappearance of p-methoxyphenol (PMP) in the inhibited oxidation of n-dodecane. Temperature, 210 "C; initial pressure of oxygen, 600 Torr. [PMP]/mol dm-3: A, 2 x and a, 4 x 10-2. 20 20 4 0 6 0 t/min Fig. 4. Variation in the total amount of peroxide (expressed as hydrogen peroxide) during the oxidation of n-dodecane.Temperature, 190 "C; initial pressure of oxygen, 600 Torr. 0, A, V, Hydrogen peroxide; 0, A, T7, % oxygenconsumed; [PMP]/mol dm-3: @, 0, zero; A, A, 1 x and V, T7,3 x In R with reciprocal temperature gives the activation energy for the initiation reaction. The points for the three lower temperatures fall on a straight line with an associated activation energy of 297 kJ mol-l, and the slight decrease in the slope of the line when the temperature is above 210 "C can be attributed to an increasing contribution from the decomposition of peroxides as the temperature increases. Lastly, the total peroxide content of some reaction mixtures has been determined, but since the iron(r1) thiocyanate method does not enable a distinction to be made between hydrogen peroxides and alkyl peroxides, the results in fig.4 are expressed as equivalent amounts of hydrogen peroxide. There is a steady build-up in peroxide to a maximum1970 Inhibition of n-Dodecane Oxidation by p-Methoxyphenol value by the end of the induction period, even though there is no detectable consumption of oxygen. Fig. 4(a) shows that in the absence of PMP the maximum amount of 146 pg remains approximately constant between 8 and 15 min, during which the oxygen consumption increases from 0.1 to 80% , and then falls again. Fig. 4(b) shows that very similar behaviour is obtained when PMP is present, although the peroxide builds up much more slowly. For example, at 5 min reaction time, the amount of peroxide formed is reduced from 42.5 pg in the absence of PMP to 17 and 8 pg when the initial concentrations of PMP are 1 x mol dm-3, respectively.In contrast, the maximum amounts of 134 and 110 pg are not significantly different to that in the absence of PMP. and 3 x Discussion The interpretation of the above results is complicated by the fact that a two-phase system is involved in which the hydrocarbon is in the liquid phase and the oxygen is in the gas phase. The possibility that any significant reaction occurs in the gas phase can be eliminated, however, since it has been shownlO that under the same reaction conditions there is no gas-phase reaction between n-hexane and oxygen even after 30 min (compared with complete reaction of the oxygen in 15 min when the hydrocarbon is n-dodecane and no PMP is present).Thus the reaction must be occurring in the liquid phase. Similarly, increasing the amount of n-dodecane used in the reaction or the surface : volume ratio of the reaction tube by a factor of two did not affect the rate of the uninhibited reaction.1° Phenolic compounds are known to inhibit free-radical chain reactions by the preferential removal of chain centres,ll and the same basic mechanism must be operative in the present case. The zero-order dependence on PMP shows that it is not involved in the rate-determining step for its removal, and hence the initiator must react preferentially with the chain centres formed in the initiation reaction. Provided this reaction is fast compared with the normal chain-propagation reaction, this leads to no detectable consumption of oxygen until all the PMP has been consumed.The initiation reaction in liquid-phase oxidation of hydrocarbons is normally taken to be reaction ( 5 ) (scheme 2), but removal of HO, radicals by reaction with PMP can be eliminated since this produces hydrogen peroxide and the results show there is a marked reduction in the rate of its formation during the induction period when PMP is present. The possibility can also be rejected that the alkyl radicals react exclusively by reaction (7) to give alkylperoxy radicals and that these are removed by reaction with the inhibitor. Such a mechanism is known to be operative at lower temperatures,** but under these conditions the kinetic characteristics of the inhibition are different; there is a reduction in the rate of consumption of oxygen (not a complete inhibition), and the results have been explained in terms of a competition between reactions (1) and (8).It is inconceivable that increasing the temperature will lead to the exclusive reaction of alkylperoxy radicals with PMP, as required to explain the present results, and thus reaction (1) cannot be important under the present experimental conditions. It must be concluded, therefore, that the inhibition arises through the removal of alkyl radicals through reaction (6). RH+O,=HO,+R ( 5 ) R+ AOH e RH + AO (6) R + 0, e RO, (7) RO,+ RH e ROOH+ R (8) RO, + AOH + ROOH + AO (1) AO+AOSOAAO (9) Scheme 2.A . D. Ekechukwu and R. F. Simmons 1971 At first sight, this difference in kinetic behaviour is very unexpected, and a comparison of the present experimental conditions with those used earlier,? for the studies at the lower temperatures reveals only one possible explanation.Similar concentrations of inhibitor have been used in both types of study, so that no change in reaction path is expected from this source, and it seems most improbable that the use of an initiator to start the reaction should affect the kinetic characteristics of the inhibition. At the lower temperatures the alkyl radicals must react exclusively with dissolved oxygen, otherwise the kinetic characteristics of the inhibition cannot be controlled by a competition between reaction (1) and (8). At the higher temperatures used in the present study, however, the solubility of oxygen in the n-dodecane will be significantly lower (by inference from data at lower temperature for the solubility of oxygen in lower hydrocarbons), and under such circumstances reaction (6) can predominate over reaction (7), so that the kinetic characteristics of the inhibition are completely different.Thus the normal explanation of the antioxidant behaviour of phenols in lubricating oils in terms of reaction with alkylperoxy radicals is incorrect if the oil is hot. As only very small quantities of inhibitor have been used, no attempt has been made to identify the ultimate product(s) of the inhibition process and the above mechanism is only schematic in character. Undoubtedly the first step is the abstraction of the phenolic hydrogen, since this is a characteristic reaction of phenols,l and as the resulting phenoxy radical is resonance-stabilised it should be a relatively stable species.Thus the phenoxy radicals can either dimerse through reaction (9) (scheme 3) to give a quinone or react with other chain centres (R and HO,) to give chain termination, e.g. reaction (12). While it has not been possible to carry out a stationary-state analysis of this reaction scheme (there are too many quadratic terms in the stationary state equations) it explains qualitatively all the results. One particularly striking feature of the present results is the marked reduction in the rate of formation of hydrogen peroxide during the induction period when inhibitor is present, since it implies that the normal route for the formation of peroxides is affected by the presence of the PMP.In the absence of inhibitor, HO, radicals are converted into hydrogen peroxide by reactions (10) and (1 1). Both processes are important under the present experimental conditions, since a comparison of the initial rate of formation of hydrogen peroxide in the absence of inhibitor and the rate of initiation deduced from the state of consumption of PMP shows that ca. 70% of the HO, radicals are removed by reaction (1 1) and the remainder by reaction (1 0). HO, + HO, = H,O, + 0, HO,+RH = H,O,+R (1 1) HO, + AO = H0,AO Scheme 3. When inhibitor is present, however, reaction (12) must also be a major removal process for HO, radicals, otherwise the marked reduction in hydrogen peroxide formation cannot be explained.While an alkylhydroperoxide is formed in reaction (12) at an equivalent rate, the method used for the determination of the peroxides is much less sensitive to alkyl peroxides than to hydrogen peroxide and cannot distinguish between them. Thus the substitution of an alkyl peroxide for hydrogen peroxide appears as a reduction in the rate of formation, since all the results have been expressed as hydrogen peroxide. Abstraction of a secondary hydrogen atom by reaction (5) from a gaseous hydrocarbon is endothermic by 199 kJ mol-l, which is substantially smaller than the apparent activation energy of 297 kJ mol-l, but the interpretation of this activation energy is complicated. If equilibrium is maintained between gaseous and dissolved oxygen, the rate of initiation can be written as k,[RH]p,,/k,, where k , is the Henry’s law constant for1972 Inhibition of n-Dodecane Oxidation by p-Methoxyphenol the system, but this is not in accord with the experimental observations, as the induction period for given conditions is independent of po,.A similar argument eliminates chain initiation by the collision of a gaseous oxygen molecule with a molecule of n-dodecane in the liquid surface. It must be concluded, therefore, that if chain initiation occurs by reaction (5), dissolved and gaseous oxygen are not in equilibrium or that chain initiation occurs by some other process such as the heterogeneous decomposition of n-dodecane on the Pyrex surface. Once the inhibitor has been consumed, the consumption of oxygen proceeds in the same manner as in the uninhibited oxidation, but the reason for the very fast acceleration in rate at the end of the induction period is not obvious.This cannot be a consequence of the initial heating-up period, since the same transition occurs after an induction time of 60 min as after 8 min in the absence of inhibitor. Similarly, autocatalysis by decomposition of hydrogen peroxide or alkyl peroxides seems unlikely, as significant quantities of peroxide can be present during the induction period without affecting the rate at which PMP is removed from the system. One possibility is that reactions such as RO, + RO, e 2RO + 0, become important when the concentration of alkylperoxy radicals reaches a critical level, since any subsequent decomposition of the alkoxy radicals is effectively chain branching as it increases the number of chain centres.There is some qualitative support for this suggestion; alkoxy radicals must be present in the system since all the n-dodecanols are formed in the oxidation.6 Once reaction starts, the rate of removal of oxygen is probably controlled initially by the concentration of radical species in the dodecane surface. As this concentration increases, the probability of an oxygen molecule being assimulated on collision with the liquid surface also increases, but as the reaction proceeds this will be partly counter- balanced by a decreased collision frequency arising from the fall in the partial pressure of oxygen. The alternative possibility that the dodecane surface is saturated with adsorbed oxygen molecules and that these are replenished as soon as they are consumed can be rejected. This would require the rate of consumption of oxygen to be independent of its initial pressure, whereas this is not the case. In conclusion, therefore, the inhibition of the liquid-phase oxidation of n-dodecane by p-methoxyphenol occurs as a result of the inhibitor reacting with alkyl radicals as they are formed in the primary initiation process. This reaction involves the abstraction of the phenolic hydrogen to form phenoxy radicals, which are sufficiently stable to either dimerise or react with alkyl or hydroperoxy radicals. This last process is responsible for the reduced rate of formation of hydrogen peroxide during the induction period. Finally, after all the inhibitor has been consumed, the oxidation proceeds as in the absence of inhibitor. References 1 K. U. Ingold, Adv. Chem. Ser., 1968, 75, 296. 2 J. L. Bolland and P. ten Have, Trans. Faraday SOC., 1947, 43, 201. 3 J. A. Howard and K. U. Ingold, Can. J. Chem., 1964, 42, 2324. 4 L. R. Mahoney, J. Am. Chem. SOC., 1966,88, 3035. 5 L. R. Mahoney, J. Am. Chem. Soc., 1967,89, 1895. 6 A. D. Ekechukwu, Ph.D. Thesis (University of Manchester, 1982). 7 Sir A. C. Egerton, A. J. Everett, G. J. Minkoff, S. Rudrakanchana and K. C. Salooja, Anal. Chim. 8 G. M. Burnett and H. W. Melville, Proc. R. SOC. London, Ser. A, 1947, 189, 456. 9 S. G. Foord, J. Chem. SOC., 1940, 48. Acta, 1954, 10, 422. 10 N. J. Royster and R. F. Simmons, unpublished results. 1 1 K. U. Ingold, Chem. Rev., 1961, 61, 563. Paper 511374; Received 6th August, 1985

ISSN:0300-9599    

DOI:10.1039/F19868201965

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1986,82, 1973-1978 Rates and Activation Parameters of Alkaline Hydrolysis of the 2-Carbomethoxypropionate Ion in Aqueous Mixtures of Dimethyl Sulphoxide Prasanta Kumar Biswas and Mihir Nath Das* Department of Chemistry, Jadavpur University, Calcutta 700 032, India The rates of alkaline hydrolysis of the 2-carbomethoxypropionate ion (R-) have been measured at 15, 25 and 35 "C in water and aqueous dimethyl sulphoxide (DMSO) mixtures of six compositions with mole fractions of DMSO varying from 0.05 to 0.40. The activation parameters have been calculated for the reaction in all these media. The thermodynamic quantities for the transfer of the reacting ion R- have been obtained from the solubilities of the silver salt (AgR) in the media at different temperatures, using the reported transfer properties of Ag+.Utilising these values for the R- ion as well as the reported values for the OH- ion (the other reactant), together with the activation parameters for the reaction, the thermodynamic quantities for the transfer of the transition state have been calculated. Attempts have been made to correlate the kinetic results with thermodynamic parameters, and to compare the results with those for neutral ethyl acetate. Interest in the kinetics of the alkaline hydrolysis of neutral esters and allied compounds in aqueous dimethyl sulphoxide (DMSO) mixtures was stimulated by the observation that the rate of the reaction frequently increased gradually as the mole fraction of DMSO (x,) in the mixed medium increased.l? Kinetics of the alkaline hydrolysis of positively charged esters such as betaine ethyl ester and acetyl choline has been studied in 60 mol% aqueous DMSO as well as 60 mol% aqueous ethanol in order to investigate proximate charge The present paper reports the results of the kinetics of alkaline hydrolysis of a negatively charged ester, the 2-carbomethoxypropionate ion (R-), in aqueous DMSO mixtures of varying compositions at different temperatures, and attempts have been made to correlate the activation parameters with the solvent properties and thermodynamic transfer parameters of the reactants.The reaction is -OCOCH,CH,CO,CH, + OH- = -OCOCH,CH,CO; + CH,OH. Experiment a1 2-Carbomethoxypropionic acid (the half-ester of succinic acid) was prepared from succinic anhydride and methanol and purified by a standard m e t h ~ d .~ DMSO was purified by the procedure recommended by Coetzee and Ritchie.6 NaOH and NaCl used here were of analytical reagent quality (B.D.H.). Doubly distilled C0,-free water was used for preparing solutions. Freshly prepared solutions of the ester and alkali were used. Rates were measured at 15,25 and 35 "C, using each reactant at an initial concentration of 0.01 mol dm-, in the reaction mixture. Aqueous mixtures of DMSO at mole fractions (x,) of 0.05,0.10,0.20,0.30,0.35 and 0.40 were used as reaction media. In each medium the ionic strengths were varied from 0.02 to 0.12 mol dm-, by adding calculated amounts of standard NaCl solutions. Rates were measured by titration of aliquots with standard alkali.The second-order rate constants (k), obtained graphically, were reproducible within +2%. The rate constant at zero ionic strength for water (k,) and each mixed 19731974 Alkaline Hydrolysis of the 2-Carbomethoxypropionate Ion in DMSO Table 1. Variation of rate constants (k,/dm3 mol-l min-l) and of k,/kw with the composition of the solvent and with temperature ko kS/kW mole fraction DMSO (x,) 15 "C 25 "C 35 "C 15 "C 25 "C 35 "C - - 0 1.87 3.58 7.08 - 0.05 2.16 4.33 8.32 1.15 1.20 1.18 0.10 2.29 4.83 8.70 1.22 1.35 1.23 0.20 2.87 5.60 11.1 1.53 1.56 1.57 0.30 3.82 7.28 13.5 2.04 2.03 1.91 0.35 4.27 7.62 14.8 2.28 2.13 2.09 0.40 4.67 9.08 17.8 2.50 2.54 2.51 Table 2. Variation of activation parameters with solvent composition (molar scale) at 25 "C mole fraction DMSO (x,) AH#/kJ mol-1 AS#/J K-l mol-1 TAS+/kJ moll1 AG#/kJ mol-1 0 48.9 - 104.5 -31.1 80.0 0.05 48.3 - 104.8 -31.3 79.6 0.10 47.5 - 106.7 -31.8 79.3 0.20 46.9 - 107.5 - 32.0 78.9 0.30 46.1 - 107.8 - 32.1 78.3 0.35 45.4 - 109.9 - 33.0 78.1 0.40 47.8 - 100.4 - 29.9 77.7 medium (ks) were obtained by extrapolation of the linear plots of log k against the square root of the ionic strength.The results are recorded in table 1. Activation parameters, AG# , AH# and AS# , corresponding to zero ionic strength, were calculated for 25 "C by the usual procedure, and are given in table 2. Solubilities of the silver salt of the half-ester (AgR) were determined in each medium at 15, 25 and 35 "C by titrating aliquots of saturated solutions with standard KCNS solution, using iron(1Ir) alum as the indicator. The Ag salt (AgR) was obtained by precipitation on adding AgNO, to a solution of the sodium salt (NaR), followed by filtration and thorough washing with cold water.The solubility data are recorded in table 3. Results and Discussion Table 1 shows that the rate constant (at zero ionic strength) increases monotonically with increasing mole fraction (x,) of DMSO in the mixed medium. These results are, however, in sharp contrast with those obtained for the same reaction, as well as the alkaline hydrolysis of several negatively charged esters in aqueous mixtures of hydroxylic solvent~.~-l~ The rate constants in these cases decrease with increasing proportion of the non-aqueous solvent and consequent decrease in the dielectric constant, as expected from electrostatic considerations for a reaction between two like charges.Thus non-electrostatic factors, which must be favourable for the present reaction, more than compensate for the unfavourable electrostatic factor, which is rather small in view of the relatively small change in dielectric constant (78.5-67.7 at 25").16 A detailed analysis of medium effects on the kinetic parameters of the reaction requires a knowledge of the relevant thermodynamic transfer parameters, the transfer freeP. K . Biswas and M. N . Das 1975 Table 3. Variation of solubilities ( S / lo3 mol dmP3) with temperature; transfer free energies (AG:r/kJ mol-l) and transfer enthalpies ( A g r / k J mol-l) at 25 "C of AgR (molar scale) S mole fraction DMSO (xJ 15 "C 25 "C 35 "C A G r AKr - - 0 4.04 4.19 4.34 0.05 3.04 3.14 3.25 1.4 -0.2 0.10 2.60 2.70 2.79 2.2 -0.3 0.20 2.40 2.47 2.55 2.6 -0.3 0.30 2.25 2.32 2.40 2.9 - 0.4 0.35 2.10 2.17 2.25 3.3 -0.5 0.40 1.95 2.02 2.10 3.6 - 0.5 energies (AG;,.), enthalpies (A%') and entropies (AKr) for the reacting species from water to the respective mixed media.The AG;,. values for the OH- ion have been taken from the literature,17 and are based on the extrathermodynamic TATB assumption that the transfer free energies of tetraphenyl arsonium (Ph,As+) and tetraphenyl boride (Ph,B-) ions are equal. The corresponding A%,. values for the OH- ion1* are, however, based on the TPTB assumption that the transfer enthalpies of tetraphenylphosphonium (Ph,P+) and Ph,B- ions are equal. The two different thermodynamic assumptions have been found to yield reasonably concordant values in several cases.19-21 The thermodynamic quantities for transfer of 2-carbomethoxypropionate ion (R-) have been determined from solubilities of the silver salt (AgR) of 2-carbomethoxypropionic acid. The values of AGtr for AgR (table 3) were obtained from the equation where Ksp denotes the solubility product of AgR in water or in any mixed medium (S). The values of AGFr for the reacting ester ion (R-) were obtained by subtracting from the values of AG;,.(AgR) the respective values of the single-ion transfer free energies of Ag+ for the various media, the values being based on the TATB assumption.22 The enthalpies of transfer of AgR (table 3) were obtained from the temperature coefficients of the solubility products.The values of A%,. for the R- ion were then calculated by subtracting the corresponding values for the Agf ion, which are based on the TPTB ass~mption,~~ and hence A%,. values were obtained. All these transfer quantities, together with those for the OH- ion, are recorded in tables 4-6. The medium effect on any activation parameter (&jAX#) is given by ,SAX# = AX' (S) -AX' ( H 2 0 ) (2) where AX+ denotes AG#, AH# or AS# in water and solvent (S) as reported in table 2. The values of mdAG#, ,dA.H# and ,dAS+ are shown in tables 4, 5 and 6, respectively. The corresponding standard transfer properties for the transition state (AX,#,) are given by AX,#, = ,6AX# + AXr(OH-) + AXr(R-) (3) where A X r denotes AG;,., A K r or A%,.for the respective reacting species as recorded in tables 4, 5 and 6, respectively. The standard transfer properties thus calculated for the activated complex (AX,#,) are also shown in tables 4-6. Because of the intrinsic uncertainties involved, exact quantitative significance may not1976 Alkaline Hydrolysis of the 2-Carbomethoxypropionate Ion in DMSO Table 4. Standard free energies of transfer, AGfr/kJ mol-l, of Ag+, reactants and the transition state at 25 "C (molar scale) mole fraction DMSO (x2) AG:r(Ag+)a AG:r( R-) A GFr( OH-)b ,dAG +t AG2 0.05 - 1.9 3.3 4.8 - 0.4 7.7 0.10 - 4.0 6.2 9.8 - 0.7 15.3 0.20 - 8.0 10.6 20.1 - 1.1 29.6 0.30 - 13.6 16.5 30.1 - 1.7 44.9 0.35 - 16.5 19.8 35.1 - 1.9 53.0 0.40 - 19.2 22.9 40.2 - 2.3 60.8 a Ref. (22). Ref. (17). Table 5.Standard enthalpies of transfer, AKJkJ rnolp1, of Ag+, reactants and the transition state at 25 "C (molar scale) 0.05 -0.8 - 1.0 - 1.7 - 0.6 - 3.3 0.10 - 1.7 1.4 - 3.3 - 1.4 - 3.3 0.20 - 14.6 14.2 15.5 - 2.0 27.7 0.30 - 26.4 26.0 32.6 - 2.8 55.8 0.35 - 29.7 29.2 39.7 - 3.5 65.4 0.40 - 32.6 32.1 45.2 -1.1 76.2 a Ref. (18) and (23). Ref. (18). Table 6. Standard entropies of transfer, Agr/J K-' mol-l, of the reactants and the transition state at 25 "C (molar scale) 0.05 - 22.8 - 14.7 -0.3 - 37.5 0.10 - 43.9 - 16.1 - 2.2 - 62.2 0.20 - 15.4 - 12.1 - 3.0 - 6.3 0.30 8.4 31.8 - 3.3 36.9 0.35 15.4 31.5 - 5.4 41.5 0.40 16.8 30.9 4.1 51.8 be attached to the transfer parameters estimated for the reacting ions and the transition state ; however, certain general conclusions can definitely be drawn, at least qualitatively, from the observed trend.The rate enhancement of a reaction on changing the medium may occur as a result of destabilization of the reacting species and/or relative stabilization of the transition state. Anions are generally, although not necessarily, destabilized by DMS0.17y22-24 The effect on the OH- ion is indeed pronounced, as reflected in the large positive values of AGYJOH-). Table 4 shows that AGFr for the ester ion (R-) also has large positive values, indicating increasing destabilization with increasing x,. Thus both the reacting species are expected to become more and more reactive on adding DMSO to the medium. The free energies of transfer, AGZ, of the transition state are, however, found to have larger positive values than even those for the OH- ion.Thus the transition state is drastically destabilized in mixed media, much more so than either of the reacting species. However, the combined destabilization of the two reacting ions taken together beingP. K. Biswas and M. N . Das 1977 larger than that for the transition state, the overall effect leads to enhanced reaction rates in the mixed media. unlike those of a charged ester such as the 2-carbomethoxypropionate ion, are relatively small for water-DMSO mixtures at x, < 0.4. Hence the reactivity of a neutral ester such as ethyl acetate should not be much affected on going from water to DMSO mixtures, at least up to x, w 0.4. The higher rates for the alkaline hydrolysis of neutral esters can thus be ascribed to the high reactivity of the OH- ion in aqueous DMSO, and also to the fact that the singly charged transition state is much less destabilized in these media than the doubly charged transition state involved in the alkaline hydrolysis of an ester anion, both the electrostatic and solvation effects being more unfavourable for the formation of the activated complex in the latter case.For the alkaline hydrolysis of a neutral ester such as ethyl acetate,25 both AH# and AS# show minimum values around x2 w 0.065 and thereafter increase. AG# values, however, decrease monotonically with increasing x,, as in the present case. It is surprising that, although the two reactions belong to different charge types, no pronounced difference in AH# or AS# is observable. Reaction rates are, however, faster for the neutral ester in water as well as the corresponding mixed media. For the alkaline hydrolysis of the ester anion the rate increases by a factor of ca.2.5 on changing the medium from water to a DMSO mixture of mole fraction 0.40, and for the hydrolysis of ethyl acetate the rate also changes by a similar factor. This observation that the rate enhancement is more or less the same on adding DMSO for a neutral and a negatively charged ester constitutes a novel aspect of the present results, despite the fact that solubility studies (combined with an extrathermodynamic assumption) suggest that the anionic substrate is considerably destabilized relative to the neutral ester. Thus it may be concluded that the solvation of the carboxylate group is not substantially altered during reaction, i.e.it is independent of the reaction site. This is probably not surprising, as it should have very little tendency to approach the negatively charged hydroxide ion and is sufficiently far away not to have a large direct electrostatic effect. Transfer free energies of neutral We thank the Indian University Grants Commission, New Delhi for the award of a teaching fellowship to P. K. B. and the authorities of the R.B.C. College, Naihati, West Bengal, for granting leave of absence. References 1 M. J. Blandamer and J. Burges, Chem. SOC. Rev., 1975,4, 55. 2 M. J. Blandamer, in Advances in Physical Organic Chemistry, ed. V. Gold and D. Bethel (Academic 3 P. Haberfield and J. Pessin, J. Am. Chem. SOC., 1983, 105, 526.4 P. Haberfield and D. Foster, J . Org. Chem., 1983, 48, 4554. 5 Organic Synthesis, ed. J. Cason (John Wiley, New York, 1945), vol. 25, p. 19. 6 Solute-Solvent Interactions, ed. J. F. Coetzee and C. D. Ritchie (Marcel Dekker, New York, 1969), 7 A. K. Ray and M. N. Das, J . Chem. SOC. A, 1970, 1576. 8 A. K. Ray and M. N. Das, J. Chem. SOC. A , 1971, 1831. 9 Von V. Holba and J. Benko, 2. Phys. Chem. (Leipzig), 1977, 258, 1088. 10 J. Benko and V. Holba, Collect. Czech. Chem. Commun., 1978, 43, 193. 11 V. Holba, J. Benko and K. Okalova, Collect. Czech. Chem. Commun., 1978, 43, 1581. 12 J. Benko and V. Holba, Collect. Czech. Chem. Commun., 1980,45, 1485. 13 V. Holba and J. Benko, Collect. Czech. Chem. Commun., 1980,45, 2873. 14 B. C . Bag and M. N. Das, Ind. J . Chem., 1982,21A, 1035. 15 B. C. Bag and M. N. Das, J. Znd. Chem. SOC., 1983,60, 1118. 16 N. M. Galiyarova and I. M. Shakhparov, Vestn. Mosk. Gorud, Univ., Dep. Vinitino, 1976, 3613. 17 A. K. Das and K. K. Kundu, J . Solution Chem., 1979, 8, 259. Press, New York, 1977), vol. 14, pp. 203-352. p. 607.1978 18 R. Fuchs, C. P. Hagen and R. F. Rodewald, J . Phys. Chem., 1974, 78, 1509. 19 B. G. Cox and A. J. Parker, J . Am. Chem. Soc., 1973, 95, 402. 20 P. J. LaBrocca, R. Phillip, S. S. Goldberg and 0. Popovych, J . Chem. Eng. Dafu, 1979, 24, 171 1. 21 M. H. Abraham, A. Hill, H. C. Ling, R. A. Schulze and R. A. C. Watt. J. Chem. Soc., Faraday Trans. 22 K. K. Kundu and A. J. Parker. J . Solution Chem., 1981, 12, 847. 23 B. G. Cox and W. E. Waghorne, Chem. Soc. Rel;., 1980, 9, 381. 24 C. F. Wells, J . Chem. Soc., Faraday Trans. 1, 1981. 77, 1515. 25 E. Tommila and M. L. Murto, Acta. Chem. Scand., 1963, 17, 1947. Alkaline Hydrolysis of the 2-Carbomethoxypropionate Ion in DMSO 1: 1984, 80, 489. Paper 5/1412; Receiued 12th August, 1985

ISSN:0300-9599    

DOI:10.1039/F19868201973

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1986,82, 1979-1983 The Effect of Aqueous-phase Solubility on Free-radical Exit from Latex Particles Mary Adams, Donald H. Napper* and Robert G. Gilbert Departments of Physical and Theoretical Chemistry, The University of Sydney, N.S. W. 2006, Australia David F. Sangster Division of Chemical Physics, CSIRO Lucas Heights Research Laboratories, Private Bag 7, Sutherland, N.S. W. 2232, Australia The technique of y-radiolysis relaxation is used to study the rate coefficient for exit (desorption) of free radicals from styrene latex particles at 30 "C in the presence of methanol (which serves to increase the solubility of monomer in the aqueous phase). The observed exit rate coefficient is directly propor- tional to the solubility of monomer in the aqueous phase, until the latter becomes so high that the transfer-dominated limit is reached.These results are in quantitative agreement with the Nomura transfer/diffusion theory for exit. --_______________ ~ - ~ - ~ ~ ~ _ _ _ _ _~_________ The importance of exit (or desorption) of free radicals from growing polymer latex particles in emulsion polymerizations is now well established. For example, exit has been found to play a vital role, even for relatively water-insoluble monomers such as ~tyrene.l-~ Free-radical desorption in such cases is considered to occur by a three-step chain- transfer/diffusion mechanism :5 first, a propagating polymer chain transfers its free-radical activity to a monomer molecule (or to a chain-transfer agent) which may then diffuse to the surface of the particle; provided this species has not undergone significant propagation, it may then desorb from the particle.The free radical may then complete the exit process by diffusing away from the particle into the bulk aqueous phase. Any or all of these sequential steps in exit may in principle be rate-determining. A quantitative theory for this transfer/diffusion mechanism has been developed by Nomura and H a ~ a d a . ~ They showed that if all three of the foregoing events (transfer, intraparticle diffusion and aqueous-phase diffusion) are rate-determining, then the exit rate coefficient k is given by k = (3zDw/r,2) (ktr/kp)/(q + 6Dw/D,) (1) where z is the degree of polymerization of the exiting free radical, rs is the radius of the swollen latex particle, D, and Dw are the diffusion coefficients for the exiting species in the particle and in the aqueous phase, respectively, and k,, and k,, are the rate coefficients for free-radical transfer to monomer and for propagation, respectively. The parameter q is the partition coefficient of the exiting species between the (organic) latex particle and the aqueous phase.To a good approximation it may be assumed that q = C,/Cw, where C, and Cw are the solubilities of the monomer in the polymer and aqueous phases, respectively. The transfer/diffusion theory can be simplified in certain limits. For a sparingly soluble monomer such as styrene, q % 6Dw/0,, so that eqn (1) becomes 19791980 Free-radical Exit from Latex Particles Another limit of interest occurs when transfer is significantly slower than either diffusion step, in which case eqn (1) is replaced completely by k N k,, CM.(3) Finally, in the limit where diffusion within the particles is rate-determining (i.e. 6D,/D, % q), eqn (1) reduces to k = z(k,,/k,) DP/2r:. (4) The expected value of k will of course be given by the rate-determining step (or steps), i.e. the smallest of eqn (2)-(4). The dependence of k upon rs and on k,, has been investigated previously for styrene,l? with results that are in conformity with the predictions of eqn (2). It has also been shown6 that eqn (2) accurately predicts the measured2 temperature dependence of k for styrene. A further prediction of eqn (1)-(3) that can be experimentally tested is that the exit rate coefficient should increase as the solubility of the exiting species in the aqueous phase increases.The testing of this prediction can be accomplished by adding a substance to the aqueous phase which increases the water solubility of the hydrophobic exiting species. Thus for a styrene system the solubility of the monomer and thus the exiting species (presumably C,H,CHCH, and/or C6H,c=CH,) can be increased by the addition of methanol to the aqueous phase. Moreover, since methanol is a non-solvent for polystyrene, such an addition should not markedly alter the kinetic events occurring inside the latex particle. In this paper we report the effect of the addition of methanol on the kinetics of seeded styrene emulsion polymerization. By studying a seeded system, using y-radiolysis and following the kinetics after removal from the radiation source, one can obtain the exit rate coefficient directly.2 Experimental The exit rate coefficient was determined from the kinetics of a y-radiolytically initiated seeded system following removal from the source, as described in detail elsewhere.2 All experiments were carried out in Interval I11 (i.e.in the absence of monomer droplets) with an initial monomer concentration inside the latex particles of 4.8 mol dm-3. The mean unswollen radius of the polystyrene seed particles (as determined by calibrated electron microscopy) was 44 nm, with a standard deviation of 2 nm. All experiments were conducted at 30 "C. The value of k was deduced from the relaxation kinetics by fitting to the experimental data solutions of the generalized Smith-Ewart equations6? (incorporating propagation, free-radical entry and exit, and aqueous-phase re-entry and heterotermination of the exited free radicals).The data-reduction method used both the steady-state polymerization rate and the time required to reach this steady state. In evaluating k from the experimental relaxation data, it was assumed that all exited free radicals undergo subsequent re-entry into the particles (i.e. the ' fate ~arameter'~ a = + l), an assumption which has been amply j~stified.~ Results and Discussion Fractional conversion of monomer to polymer us. time curves for the system under study (both in the radiation source and following removal from it) are similar to those obtained previously in other styrene systems using this technique,lP4 and so will not be presented here.We note in passing that the steady-state polymerization rate in the radiation field was not changed significantly by the presence of methanol in the aqueous phase. This presumably arises because the average number of free radicals per particle in the steady state (z,~) for the type of system under study' is close to its limiting value o f t (i.e. it has been shown1? that for styrene systems such as the present, A,, exhibits a plateauM. Adams, D . H . Napper, R. G. Gilbert and D . F. Sangster methanol/water (% v/v) 1981 ~ 10 2 Fig. 1. Exit rate coefficient (k) as a function of volume fraction of methanol for styrene seeded emulsion polymerization at 30 "C. Points: experiment; line 1 : calculated from eqn (2); line 2: transfer-limiting value, from eqn (3); insert: experimental values for solubility of styrene in water at 30 "C as a function of volume fraction of methanol.value of 0.5 where it is independent of the entry and exit rate coefficients over a large range of values of these quantities). Thus the polymerization rate will be insensitive to changes in either entry or exit rate parameters. The variation of the measured exit rate coefficient with increasing aqueous-phase concentration of methanol is shown in fig. 1. It is apparent that, as predicted by eqn (2), k increases significantly on addition of methanol. Moreover, it can be seen that k apparently approaches a limiting maximum value as the methanol concentration increases. This is expected from the discussion above leading to eqn (3), which suggests that the plateau value of k should correspond to the transfer-dominated limit. Fig.1 shows also the values of k computed from eqn (2), using established values for the various rate parameters, uiz. k,, = 1.8 x dm3 mol-1 s-l (obtained using the rate parameters of Hui and Hamielec,8 a value which appears to be reliable6), k, = 126 dm3 mol-1 s - l y 2 9 and C , = 3 x mol dm-3.9 The value of C, ( ~ 4 . 8 dm3 mol-l) was calculated from the amount of monomer added to the system. A value of zD, = 8 x 1W6 cm2 s-I was chosen to ensure agreement between the prediction of eqn (2) and the experimental value of k in the absence of methanol; if one assumes z = 1, then this yields a value of D, which is consistent with measured values for similar substances.Included in this calculation is the experimentally obtained dependence of the aqueous-phase solubility of styrene on the amount of added methanol (see inset to fig. 1); this was measured by determining the first sign of enduring opalescence on the addition of increasing amounts of styrene to water-methanol mixtures. The increase in the exit rate coefficient upon addition of methanol is quantified in eqn (2). Qualitatively, one sees that this increase arises as follows. In the absence of methanol, diffusion of the exited free radical from the latex particle is rate-limiting. This step is slow because the low solubility of the exited free radical (presumably comparable in magnitude to that of styrene) ensures that the chemical potential gradient providing the driving force for diffusion away from the particles into the bulk phase is relatively small.This driving force is increased by the addition of methanol.1982 Free-radical Exit from Latex Particles Table 1. Values of respective rate coefficients for various limiting events in transfer/diffusion mechanism for exit step event eqn no CH,OH 2 0 x v/v CH,OH 1 chain transfer to monomer 3 8 8 2 diffusion to surface of particle 4 500 500 3 diffusion away from particle 2 4 > 10 It is apparent from fig. 1 that the observed increase in k with the amount of added methanol is in good agreement with the predictions of the theory as given by eqn (2),5 at least until the observed limiting value is reached. The experimental limiting value of k occurred when the methanol : water volume ratio exceeded ca.15%. This limiting value (ca. 7.8 x s-l) is almost double that (4.0 x s-l) observed in the absence of methanol. The limit for k [as quantified in eqn (3)] can be understood in terms of a change in the rate-determining step in the transfer/diffusion mechanism. Shown in table 1 are values for the various rate coefficients both in the presence and absence of methanol ( 2 20% v/v), calculated according to eqn (2) and (3). Transfer alone becomes rate-determining when the amount of methanol is close to that at which the limiting value is reached experimentally. Moreover, the calculated limiting value as found from eqn (3), k,, C, = 8.6 x s-l, is in reasonable accord with that observed experimentally if the uncertainty in the value of k,, is taken into account.We thus conclude that it is possible to increase the exit rate coefficient for styrene emulsion polymerization by increasing the solubility of the exiting species in the aqueous phase. The extent to which this is possible, however, is limited by chain transfer to monomer, which eventually becomes the rate-determining step. The experimental results are found to be in good quantitative agreement with the transfer/diffusion theory of Nomura and H a ~ a d a , ~ as given in eqn (1) and (3). This result, which is obtained by systematically changing the monomer solubility, provides important new evidence for the correctness of this theory, in accord with results from systematically changing the particle size,l transfer rate coefficient4 and temperature6 using other experimental methods.We thank the Australian Institute for Nuclear Science and Engineering for their continued support, and the University of Sydney Electron Microscope Unit for their provision of facilities. References 1 B. S. Hawkett, D. H. Napper and R. G. Gilbert, J. Chem. Soc., Faraday Trans. I , 1980,76, 1323. 2 S. W. Lansdowne, R. G. Gilbert, D. H. Napper and D. F. Sangster, J. Chem. SOC., Faraday Trans. I , 1980, 76, 1344. 3 B. C. Y. Whang, G. Lichti, R. G. Gilbert, D. H. Napper and D. F. Sangster, J. Polym. Sci., Polym. Lett. Ed., 1980, 18, 71 1 ; G. Lichti, D. F. Sangster, B. C. Y. Whang, D. H. Napper and R. G. Gilbert, J. Chem. SOC., Faraday Trans. 1, 1984, 80, 291 1. 4 G. Lichti, D. F. Sangster, B. C. Y. Whang, D. H. Napper and R. G. Gilbert, J. Chem. Sac., Faraday Trans. 1, 1982, 78, 2129. 5 M. Nomuraand M. Harada, J. Appl. Polym. Sci., 1981,26,17; M. Nomura, in Emulsion Polymerization, ed. I. Piirma (Academic Press, New York, 1982).M . Adams, D . H . Napper, R . G. Gilbert and D. F. Sangster 1983 6 R. G. Gilbert and D. H. Napper, J . Macromol. Sci. Rev., Macromol. Chem. Phys., 1983, C23, 127. 7 B. C. Y. Whang, D. H. Napper, G. Lichti, M. J. Ballard and R. G. Gilbert, J . Chem. SOC., Faraday 8 A. W. Hui and A. E. Hamielec, J . Appl. Polym. Sci., 1972, 16, 749. 9 W. H. Lane, Ind. Eng. Chem , Anal. Ed., 1946, 18, 295. Trans. I , 1982, 78, 1 11 7. Paper 5/1419; Received 14th August, 1985

ISSN:0300-9599    

DOI:10.1039/F19868201979

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Furuduy Trans. I, 1986,82, 1985-1998 Hydrogenolysis of Alkanes Part 3.-Hydrogenolysis of n-Hexane and Methylcyclopentane over variously treated Ru/TiO, Catalysts Robert Burch* Chemistry Department, Reading University, Whiteknights, Reading RG6 2AD Geoffrey C . Bond and Raj R. Rajaram Chemistry Department, Brunel University, Uxbridge UB8 3PH The hydrogenolysis and isomerisation of n-hexane and methylcyclopentane have been studied on 0.5, 1.0 and 5.0% Ru/TiO, catalysts pretreated in air at 623 K and in hydrogen at 758 or 893 K. Catalysts reduced without a prior oxidation treatment have activities for the n-hexane reaction which are intermediate between those of oxidised catalysts and of catalysts which, aftzr oxidation, are reduced a second time at 758 or 893 K.It is suggested that residual chloride on the catalysts after the first reduction diminishes the activity but prevents the creation of the strong metal-support interaction (SMSI) state, which is believed to be inactive for hydrogenolysis reactions. Re-oxidation eliminates most of the chloride and disperses the Ru. Reduction at 433 K leads to a very active catalyst. Subsequent high-temperature reduction in the absence of chloride produces a catalyst with a very low activity. A catalyst prepared from a chloride-free salt has intermediate properties. However, this catalyst still does not fully enter the SMSI state in the absence of a preoxidation treatment. The product distribution in the n-hexane hydrogenolysis reaction is almost invariant, irrespective of which catalyst or pretreatment is used.Catalysts after high-temperature reduction are selective for the isomerisation of n-hexane to 2- and 3-methylpentane. After the initial reduction, the activity of the 5% Ru catalyst for the hydrogenolysis of methylcyclopentane is almost identical to that of the oxidised catalyst. However, the selectivity in the ring-opening reaction to 2- and 3-methylpentane is very sensitive to the method of preparation. After a second reduction the activity decreases sharply but the selectivity is identical to that of the catalyst after the first reduction. The nature of the catalysts after each pretreatment and the effect of changes in the catalysts on the catalytic properties are discussed. Ti0,-supported metal catalysts have been studied extensively since it was reported1 that very dramatic, but reversible, changes in the chemisorption properties of the metal were obtained by reduction at elevated temperatures. Most subsequent work has been concerned with attempts to understand the nature of these so-called strong metal-support interactions (SMSI).,? the SMSI state as originally definedl probably does not exist.However, the term is now established in the vocabulary of catalysis and is taken to refer to the state of the catalyst after reduction at high temperature. It is believed that the main effect of the high-temperature reduction (HTR) is to cause a migration of species symbolised as TiO, from the support on to the surface of the metal particles. The presence of such species can in principle cause both physical blocking and localised electronic effects, and can account for the changes in the chemisorption capacity of the metals.Complementary catalytic investigations of Ti0,-supported metals have been rep~rted.~ 19851986 Hydrogenolysis of Alkanes Hydrogenolysis reactions are much more affected by HTR than, for example, hydro- genation or dehydrogenation reactions. Tests at high pressures have shown4 that after HTR Ru/TiO, and Ir/TiO, catalysts are selective for the skeletal rearrangement of n-heptane. Haller and coworkers5 have reported recently that Ir/TiO, and Rh/TiO, catalysts prepared by ion exchange become more selective for the isomerisation of n-butane after HTR. Recent work by Bond and Xu Yide6f on the hydrogenolysis of n-butane over Ru/TiO, catalysts pretreated in various ways has shown that variations in the pretreatment can lead to the formation of catalysts having very different properties.Catalysts beginning as RuClJTiO, given only an initial reduction had much higher activities than the same catalysts given an intermediate reoxidation followed by a second reduction. The distribution of hydrogenolysis products also depended on the method of pretreatment. Similarly, Burch and Garla* have demonstrated that the activity of Pt/Al,O, catalysts for the hydrogenolysis of n-pentane changes in different ways with the temperature of reduction depending on whether the initial state of the Pt is the oxide or the metal. As an extension of this earlier work on Ru catalysts, and in order to investigate more fully whether Ru/TiO, catalysts after HTR develop high selectivity for isomerisation and aromatisation reactions, we have studied the reactions of n-hexane and of methylcyclo- pentane over various Ru/TiO, catalysts pretreated using the method described previously. Experiment a1 Catalysts were prepared by impregnation to incipient wetness of Degussa P-25 titania with solutions of RuC1, (Johnson Matthey plc) to give catalysts containing 0.5 and 5.0% Ru by weight.The impregnated materials were dried in air at 373 K. A 1 % Ru/P25 catalyst, prepared using an aqueous solution of Ru nitrosyl nitrate, was acquired from the Coal Research Establishment, Stoke Orchard. Hydrogen chemisorption was measured in a conventional high-vacuum system equipped with a Texas Instruments precision transducer and capable of evacuation to better than 1.3 x lop3 Pa.Samples were reduced in situ in flowing hydrogen at 758 K for 16 H, after which the sample was flushed with N,, sealed and evacuated. The amount of hydrogen chemisorbed was measured at room temperature and at pressures up to 6 kPa, in each case allowing 15-20 min for equilibration. After measuring the hydrogen chemisorption isotherm, the system was flushed with nitrogen and the sample oxidised at 623 K for 1 h in flowing air. The sample was then flushed with nitrogen and reduced at 433 K for 1 h in flowing hydrogen. After further flushing with nitrogen the sample was sealed, evacuated and the hydrogen chemisorption repeated. The sample was further reduced at 758 K for another 16 h before the hydrogen chemisorption was repeated. The catalytic activities and selectivities of the various samples were determined after each in situ reduction pretreatment in a flow microreactor operating at atmospheric pressure.Hydrogen was obtained from cylinders (B.O.C., 99.995 % ) and used without further purification. The flow was maintained by a Negretti and Zambra controller. The hydrocarbon partial pressure was obtained by passing the hydrogen through two bubblers, the second of which was maintained at 273 K. The standard test conditions were: 6480 cm3 H, gcat-l h-l; 0.016 mol feedstock gcat-l h-l; H,/hydrocarbon ratio = 17; temperatures betwen 363 and 763 K. Products of the reactions were separated by gas chromatography using a 6 m 20% silicone fluid on Chromosorb P column operating at 353 K, and analysed with a Perkin-Elmer flame ionisation detector gas chromatograph linked to an Infotronics integrator.Product yields are expressed as selectivities S,, defined as the number of moles, ci, of product i formed from each mole of reactant. Thus Si = nc, i n ic,. Standard procedures were used to prepare samples for transmission electron microscopy. I 1R. Burch, G. C. Bond and R. R. Rajaram 1987 Table 1. Hydrogen chemisorption data for 5% Ru/TiO, catalysts treated in various ways 5D 27.3 43.5 0.1 1 8.3 5.2 3.0 f 0.5 5E 123.7 125.9 0.49 1.9 1.9 1.5 k0.5 5F 16.4 19.9 0.07 13.3 11.0 5.0 Ifs 1 .O a Po refers to zero pressure; P, refers to a pressure of 6 kPa. For convenience samples are referred to using the following code:7 R758(16), xD; 0623(1), xE; R758(16), xF; R893(16), xG; 0623(1), xH; and R893(16), XI, where x is the wt% Ru in the catalyst, and successive treatments (e.g.D, E, F) were performed on the same sample. Thus, for example, 0.5F represents a sample of the 0.5% Ru/TiO, catalyst reduced at 758 K for 16 h, oxidised at 623 K for 1 h and reduced at 433 K for 1 h, and finally reduced at 758 K for 16 h. The letters G, H and I denote samples for which the HTR was effected at 893 K. For further details of the pretreatment schedules, see ref. (7). Results Hydrogen Chemisorption The hydrogen isotherms for the 5% Ru catalyst were relatively well defined with a clear break at low pressures. However, after the first HTR (sample 5D) the linear region of the isotherm was not flat.Table 1 shows that the quantity of H, adsorbed at zero pressure, obtained by extrapolation of the linear part, is much less than that at 6 kPa. This is similar to the results reported in Part 2,7 where it was suggested that the shape of the first isotherm was affected by C1- ions retained by the catalyst. The sample reduced at 433 K after reoxidation (5E) gives an almost flat plateau; cf. VH at po and p s . Reoxidation is believed to remove any residual C1- ions. A second HTR gives an isotherm of intermediate form and a low H/Ru ratio. Table 1 shows that after reoxidation the H/Ru ratio increased markedly and the average particle size agrees well with the value obtained from TEM. Moreover, the TEM results show that the dispersion of the original, reduced catalyst is increased by the oxidation treatment and then decreases again after a second high-temperature reduction.The redispersion during oxidation is consistent with the model presented previ~usly,~ in which it was proposed that the prerequisite for spreading of the Ru oxide was removal of the residual chloride. After HTR the quantity of H, adsorbed at zero pressure is ca. 35% of that required to be consistent with the TEM results. We believe that after the first HTR the suppression of H, adsorption is mainly due to C1- contamination (hence the unusual shape of the isotherm), and that after the second HTR the suppression is due to the creation of the so-called SMSI state. The fact that some H, adsorption persists may be due to the large Ru particle size (5 nm). It is known that large particles succumb to the SMSI state less readily than small particle^.^ Catalytic Results The 5% Ru catalyst gave exclusively hydrogenolysis products after reduction at 758 K, but after reduction at 893 K some isomerisation products were also observed.The 0.5 :(, Ru catalyst gave both hydrogenolysis and isomerisation products under most experimental conditions. In discussing the catalytic results we shall consider the effect of pretreatment first on the rate of hydrogenolysis and then on the rate of isomerisation, taking each temperature of reduction in turn.1988 Hydrogenolysis of Alkanes 2.2 2.4 2.6 lo3 KIT Fig. 1. Plot of ln(rate) us. reciprocal temperature for the hydrogenolysis of n-hexane over a 5% Ru/TiO, catalyst treated in various ways.Letters on lines indicate the pretreatment used, see text for details. 2.2 2 4 2.6 lo3 KIT Fig. 2. 2.2 2 4 2.6 lo3 KIT Fig. 3. Fig. 2. Plot of ln(rate) us. reciprocal temperature for the hydrogenolysis of n-hexane over a 0.5% Ru/TiO, catalyst treated in various ways. Letters on lines indicate the pretreatment used, see text for details. Fig. 3. Plot of ln(rate) us. reciprocal temperature for the hydrogenolysis of n-hexane over a 1 % Ru/TiO, catalyst treated in various ways. Letters on lines indicate the pretreatment used, see text for details. Effect of Pretreatments on the Activity for the Hydrogenolysis of n-Hexane Fig. 1-3 show Arrhenius plots for the hydrogenolysis of n-hexane over three catalysts pretreated in different ways. The rates are expressed as mmol (gRu)-l h-l.The range of temperatures was chosen to keep the conversion below ca. 10%. Reductions at 758 K The activities of all three catalysts change dramatically as the pretreatment conditions are varied (see fig. 1-3 and table 2). After the initial reduction at 758 K the catalysts have a low activity. Oxidation followed by reduction at 433 K results in an increase in activity.R. Burch, G . C. Bond and R . R. Rajararn 1989 Table 2. Arrhenius parameters and rates of reaction for the hydrogenolysis of n-hexane and of methylcyclopentane over various Ru/TiO, catalysts catalyst E/kJ mol-l In A R&3 5D 5E 5F 5G 5H 51 O.5D O.5E O.5F 0.5G 0.5H 0.51 1D 1E I F 5D 5E 5F reactant: n-hexane 120.6 f 2.61 38.98 1 13.1 f 2.03 38.28 110.6f 1.75 34.12 174.4 f 1.45 52.26 124.4f 3.07 42.05 125.7 f 2.15 36.29 90.8 0.84 28.22 102.9 f 2.33 34.77 113.5 f 1.34 33.99 112.1 f 3.20 36.32 13 1.2 f 2.60 42.87 120.6f 1.85 37.13 116.8 f 1.81 35.88 112.8k1.45 37.19 106.5 f 2.01 3 1.78 46.5f2.51 16.88 60.8 f 1.71 21.31 95.5 & 3.17 28.69 reactant: MCP (240)b 29.9 45.4 (956) (1 800) 4.00 19.5 11.7 (484) 158 62 1 38.5 32.9 (351) 9.12 (52.6) (83.2) (8.69) a Rate of reaction at 433 K, units are mmol g& h-l.Values in brackets obtained by extrapolation using the Arrhenius parameters. A second reduction at 758 K lowers the activity by about a factor of 30 for the 5% catalyst, 50 for the 0.5% catalyst and 40 for the 1 % catalyst. In all cases the final activity is lower than after the initial reduction. Part of these variations can be attributed to changes in the dispersion of the Ru (see table 1).Thus, for the 5 % Ru catalyst the specific activities are calculated to be 6.12 x 1 O-, (5D), 5.48 x lo-, (5E) and 1.20 x lo-, (5F) molecule s-l (gRu)-l. From the amount of H, chemisorbed after HTR (see table 1) we can estimate that ca. 60% of the Ru surface is inaccessible to H, because of contamination by C1- (5D) or TiO, species (5F). Because catalysts 5D, 5E and 5F have very similar specific activities for hydrogenolysis it seems probable that the contaminants are not dispersed homogeneously over all the Ru particles. It is more likely that 60% of the Ru surface is totally contaminated and that the remainder is essentially free from contaminants. It is difficult to distinguish between the catalytic effects of residual C1 and of TiO,.It is possible that while there are C1 ions on, or at the peripheries of Ru particles, the formation of the SMSI state is inhibited, perhaps because H, spillover is restricted, or alternatively because C1- ions block the migration of TiO, species. The presence of Cl- ions may not be the only reason why Ru/TiO, catalysts do not readily enter the SMSI state. A similar pattern of activity is observed for the catalyst prepared from the Ru nitrosyl complex (cf. in particular the 0.5% Ru and the 1 % Ru catalysts after equivalent pretreatments, table 2). The reason why this catalyst is not converted fully into the SMSI state after the first reduction at 758 K may be because it contains large Ru particles, and a preliminary oxidation is needed to disperse the metal oxide irrespective of its origin.1990 Hydrogenolysis of Alkanes Reductions at 893 K When the 5% Ru catalyst is reduced at 893 K the activity is lower than even after the second reduction at 758 K; reoxidation restores all the original activity (cf.5E and 5H) but a further reduction produces a catalyst which only has a measurable activity at much higher temperatures (fig. 1 and table 2). This pattern of activity is as expected, since the higher temperature of the first reduction is sufficient to remove most of the residual C1- and to create the SMSI state. Presumably the residual activity is due to a small proportion of larger Ru particles still not converted into the SMSI state. Arrhenius Parameters The various pretreatments lead to differences in the Arrhenius parameters (see table 2).In the previous study of the hydrogenolysis of n-butane it was observed that increasing the temperature of the first reduction caused an increase in the activation energy. This is now also observed for the hydrogenolysis of n-hexane (cf. 5D/5G and 0.5D/0.5G). However, for n-butane it was observed that the oxidation produced a decrease in the activation energy and the second reduction restored the original value. In the case of the n-hexane reaction, oxidation decreases the activation energy for the 5 % Ru catalyst but there is no change after the second reduction. With the 0.5% Ru catalyst, oxidation produces an increase in the activation energy. Subsequent reduction at 758 K produces an increase but reduction at 893 K produces a decrease in the activation energy.Fig. 4 shows a compensation-effect plot of the Arrhenius parameters. The results fall remarkably neatly into three groupings according to whether the catalyst can be considered to be free of contaminants (top line, catalysts 5E, 5H, 5D, 0.5E, 0.5H and IE), in a contaminated state (bottom line, catalysts 5G, 51, 0.5F and 1F) or in an intermediate state (middle line, catalysts OSD, lD, 0.51 and 5F). An almost identical compensation-effect plot was previously obtained for the hydrogenolysis of n-butane over similar catalysts [see fig. 2 of ref. (7)]. From these results it would appear that the nature of the active site changes with the method of pretreatment. Comparison of catalysts having the same pre-exponential factors shows that as the catalyst becomes contaminated the activation energy for the hydrogenolysis reaction increases by ca.15 kJ mol-l. Isomerisation Selectivity in the n-Hexane Reaction Ru catalysts normally have such high hydrogenolysis activities that no isomerisation is detected. Table 3 confirms that this is the case for catalysts 5D, 5E and 5F. However, when the reduction temperature is increased to 893 K some isomerisation is observed (5G, table 3). Reoxidation virtually eliminates the isomerisation selectivity, but after a second reduction there is a further increase in the selectivity to C, products. Some isomerisation is detected with the 0.5% Ru catalyst even after the intermediate oxidation (0.5E and 0.5H, see table 3).It was suggested previously7 that this may be because the mild oxidation is insufficient to reverse totally the SMSI state for catalysts having a low metal loading. A second HTR leads to an isomerisation selectivity of over 30%. Table 3 also includes results for the 2-methylpentane/3-methylpentane (2MP/3MP) ratio obtained in the isomerisation of n-hexane. The fact that the pretreatment affects the 5 % and 0.5 % Ru catalysts differently is undoubtedly related to differences in particle size. In catalysts where the particle size is relatively large (5% Ru) HTR leads to an increase in the 2MP/3MP ratio. When the particle size is relatively small (0.5% Ru) the reverse is observed.R . Burch, G. C. Bond and R . R . Rajaram 1991 I I I I 100 130 160 ElkJ mol-' Fig. 4.Compensation effect plot of the Arrhenius parameters for the variously pretreated Ru/TiO, catalysts. The letter beside each point indicates the pretreatment used, see text for details. A, 5% Ru; .,0.5% Ru; 0 , 1 % Ru. Table 3. Effect of pretreatment on selectivity for isomerisation of n-hexane over Ru/TiO, catalysts catalyst L / K S? (%) 2MP/3MP 5D 5E 5F 5G 5H 51 OSD 0.5E O.5F 0.5F/Sc 423 423 423 423 423 423 453 41 3 413 753 0 0 0 2.77 0.30 6.81 1.72 1.53 30.9 66.1 - 2.05 1.30 1.65 1.40 1.47 1.10 2.04 a Selectivity for the formation of isomers. Ratio of 2- methylpentane to 3-methylpentane. Catalyst deactivated with thiophene after reduction treatment F. Treatment with Thiophene The problem of converting all the 0.5% Ru catalyst into the SMSI state even after a second reduction at 758 or 893 K led us to investigate the effect of using a sulphiding treatment to moderate the activity of any Ru still in an uncontaminated state. Table 4 shows that after pulsing thiophene over catalyst 0.5F the activity is greatly reduced and no products for the reaction are detected until the test temperature is raised to 733 K.66 FAR 11992 Hydrogenolysis of Alkanes Table 4. Effect of thiophene treatment on the activity of a Ru/TiO, catalyst for the hydrogeno- lysis of n-hexane catalyst TeSt/K rate/mmol g& h-l 0.5F 403 1.22 413 2.07 423 5.80 0.5F/S 733 1.40 (12.1)a 753 4.14 (22.2) 763 7.12 (23.4) a Values in parentheses are rates of isomerisation + aromatisation. However, after this treatment the selectivity for the isomerisation of n-hexane at 753 K is over 66% (see table 3).The sulphiding treatment poisons any Ru remaining in the normal state after reduction at 758 K. The residual activity, and selectivity towards isomerisation, is thought to be due to Ru in the SMSI state. Product Distribution in n-Hexane Hydrogenolysis The temperature dependence of the product distribution varied from one catalyst to another, in some cases being almost temperature independent, in others showing a strong predominance of methane at higher temperatures. For the 5,0.5 and 1 .O% Ru catalysts, respectively, fig. 5, 6 and 7 show the product distributions obtained at two different temperatures. In each case the highest test temperature and a temperature 30 K lower than this are used for comparison. It is difficult to discern any simple patterns in these product distributions.There is no strong preference for breaking terminal C-C bonds in n-hexane. Carter et al.l0 have studied the hydrogenolysis of n-heptane over Ru; they also observe a fairly even spread of products. With the 5% Ru catalyst methane is the major product in all cases at the higher test temperatures, and there seems to be a pattern in so far as the selectivity to methane increases as the pretreatment is changed from initial reduction, to reoxidation, to second reduction. This is seen for both the 758 and 893 K reduction treatments. However, the variability in the temperature dependence of the product distribution is seen by comparing the results in fig. 5(a) and (b). At the lower temperatures the preference for methane formation is much reduced and there is no longer a smooth trend in methane selectivity as the pretreatment is changed at 758 K.The trend in methane selectivity is retained, however, for the treatments at 893 K. These hydrogenolysis patterns differ from those found previously for n-b~tane.~ There the methane selectivity was higher after oxidation, but decreased again after a second reduction. It was concluded that oxidised catalysts contained a different type of hydrogenolysis site. Possibly the most significant observation in the n-hexane reaction is that, in contrast to the n-butane reaction, there is so little difference in the product distribution between catalysts in the normal and in the SMSI state. Schepers et aZ.ll have suggested for Rh/TiO, catalysts that in the SMSI state the surface of the metal is covered by a blocking layer of TiO, which acts in a similar fashion to a self-poisoning carbonaceous deposit.This would account for the absence of any significant effect of pretreatment on the product distribution from the n-hexane reaction. With the 0.5% Ru catalyst the selectivity patterns are even less clearly defined (fig. 6). There is a slight preference for the formation of methane at the higher temperatures, butR. Burch, G. C. Bond and R . R. Rajaram 1993 1.0 0.5 D E F G H I Fig. 5. Selectivity patterns obtained in the hydrogenolysis of n-hexane over 5 % Ru/TiO, catalysts treated in various ways. Each set of data refer, left to right, to methane, ethane, propane, butane and pentane.Letters indicate the pretreatment used. Test temperatures used: (a) D, 413; E, 403; F, 413; G, 423; H, 393; I, 463; (b) D, 383; E, 373; F, 383; G, 393; H, 363; I, 433. D E F G H I Fig. 6. Selectivity patterns obtained in the hydrogenolysis of n-hexane over 0.5 % Ru/TiO, catalysts treated in various ways. Each set of data refer, left to right, to methane, ethane, propane, butane and pentane. Letters indicate the pretreatment used. Test temperatures used: (a) D, 463; E, 413; F, 463; G, 433; H, 413; I, 443; (b) D, 433; E, 383; F, 433; G, 403; H, 383; I, 413. 66-21994 Hydrogenolysis of Alkanes I 1.51 D E F Fig. 7. Selectivity patterns obtained in the hydrogenolysis of n-hexane over 1 % Ru/TiO, catalysts treated in various ways. Each set of data refer, left to right, to methane, ethane, propane, butane, pentane. Letters indicate the pretreatment used.Test temperatures used: (a) D, 443; E, 413; F, 463; (b) D, 413; E, 383; F, 433. compared with the 5 % Ru catalyst there is a much more even spread of products at both low and high temperatures. The 1% Ru catalyst (prepared from the nitrosyl complex) (fig. 7) compares more closely with the 5% Ru than the 0.5% Ru catalyst. Thus at the higher temperatures the major product is methane and the selectivity to methane increases as the pretreatment proceeds from reduction, through oxidation, to a second reduction. Activity and Selectivity in the Methylcyclopentane Reaction Fig. 8 summarises the kinetic results for the hydrogenolysis of methylcyclopentane using the 5% Ru catalyst reduced at 758 K.After the initial HTR the activity is comparable to that obtained after reoxidation and reduction at 433 K. Further HTR at 758 K produces a low-activity catalyst. It is significant that the pretreatment affects the activity of the catalysts for the n-hexane and methylcyclopentane reactions differently, especially after the initial reduction. Whereas the n-hexane results indicate that the Ru is in a modified state after the initial reduction (SMSI or C1 contaminated), the activity for the conversion of methylcyclo- pentane is very similar to that obtained for Ru in the normal state (i.e. catalyst 5E). A clear distinction between the n-hexane and methylcyclopentane reactions has been found also with Ti0,-supported Pt and Rh ~ata1ysts.l~ However, in those cases it is the methylcyclopentane reaction which is most affected by TiO, contamination.This is particularly so with Pt, where after HTR the activity for the methylcyclopentane reaction is essentially zero. It is clear that Ru metal in a contaminated state is capable of catalysing a ring-opening reaction much more effectively than the hydrogenolysis of straight-chain C-C bonds. The activation energies and pre-exponential factors for the methylcyclopentane reaction change very markedly with the method of pretreatment (see table 2). After the initial reduction the activation energy is very small, but this increases after the oxidation treatment and further increases after the second reduction. A compensation effect clearlyR . Burch, G. C. Bond and R.R . Rajaram 1995 lo3 KIT Fig. 8. Plot of In (rate) us. reciprocal temperature for the hydrogenolysis of methylcyclopentane over a 5% Ru/Ti02 catalyst treated in various ways. Letters on lines indicate the pretreatment used, see text for details. Table 5. Activity and product distribution in the methylcyclopentane reaction over variously treated Ru/TiO, catalysts selectivity c, c2 c3 c4 c5 2MP 3MP 2MP/3MP catalyst: 5D; ratea: 12.5 0.06 0.0 0.0 0.006 0.05 0.647 0.274 2.36 catalyst: 5E; ratea: 13.9 0.11 0.006 0.006 0.0 17 0.08 0.509 0.380 1.34 catalyst: 5F; ratea: 0.65 0.0 0.0 0.0 0.0 0.0 0.710 0.290 2.45 a Units are mmol g& h-l; test temperature was 393 K. operates since the In A values increase in line with the activation energies. The Arrhenius parameters obtained after the first two treatments fit very well with the extrapolated upper line in fig.4, suggesting that the active centres for MCP and n-hexane hydrogenolysis are similar. The electron-withdrawing properties of the residual Cl- ions on the Ru particles in sample 5D may provide a particularly favourable environment for the MCP reaction, and thus account for the low value of the activation energy. This is, however, compensated by a lower value of In A because of the smaller number of active centres available. Table 5 compares the selectivity results at 393 K for the three catalysts used in the methylcyclopentane work. There are only minor changes in selectivity with temperature and with conversion. The. results show that the selectivity for the formation of methane is 20 times lower for catalyst 5D with methylcyclopentane than with n-hexane (see fig.5). Reoxidation raises the selectivity for the formation of methane, but this is still an order of magnitude lower than that found with n-hexane. A second reduction totally1996 Hydrogenolysis of Alkanes suppresses the activity for the formation of products containing less than six carbon atoms. The 5% Ru catalyst converts n-hexane into cracked products, whereas the same catalyst converts methylcyclopentane mainly to 2- and 3-methylpentanes. Kramer and Zuegg12 have proposed that over Pt catalysts the ring-opening of methylcyclopentane to give n-hexane may occur at the metal-support interface. If the SMSI state arises because of a migration of TiO, onto the surface of the metal particles this should increase the interfacial area and lead to an increase in the selectivity to n-hexane.With our Ru catalysts no n-hexane was detected in any of the tests. There is, however, a definite change in the selectivity for the formation of 2- and 3-methylpentanes. After reduction at 758 K the 2MP/3MP ratio is ca. 2.4, whereas after reoxidation at 633 K and reduction at 433 K the 2MP/3MP ratio is only 1.34. Clearly the two types of catalyst bind the methylcyclopentane in different ways. Discussion Before discussing our results it will be helpful to describe the state of our catalysts after the various pretreatments. We assign the ‘state’ of the catalyst on the basis of rates of reaction. High activity is obtained for normal catalysts; low activity is obtained for SMSI catalysts; intermediate activity is obtained either when there is a mixture of SMSI (inactive) and normal (active) particles or when particles are partially coated with TiO,.We restrict the use of the term ‘partial SMSI state’ to particles which are partially contaminated with TiO,. All Ru/TiO, catalysts after oxidation and low-temperature reduction are in the normal state. These are characterised by high hydrogenolysis activity, high dispersion, low isomerisation selectivity, and a low 2MP/3MP ratio in the methylcyclopentane reaction. Reduction treatments lead to different forms of catalyst depending on the metal loading, average particle size, temperature of reduction, and whether the reduction has been preceded by oxidation. In general we believe that a second reduction at 758 K leads to complete deactivation of small particles by TiO,, partial coverage of medium-size particles and little or no coverage of large particles.A second reduction at 893 K will begin to convert large particles into the SMSI state. An initial reduction at 758 K produces catalysts having intermediate activity and here we suggest that C1 contamination prevents the creation of the SMSI state. Raising the initial reduction temperature to 893 K eliminates most of the C1 and the SMSI state is more readily formed. Because of the possible coexistence of three or four types of particle each having a different specific activity, rate measurements alone cannot be used to describe a catalyst. It is necessary also to take account of product distributions. We have shown that Ru/TiO, catalysts are difficult to convert into the SMSI state and that is true irrespective of the metal loading or of the type of salt used to prepare the catalyst.The influence of Cl, identified previ~usly,~ on the properties of reduced Ru has been confirmed in the hydrogenolysis of n-hexane, but similar effects have also been obtained for a C1-free catalyst. It seems that the resistance of Ru to entering the SMSI state has a more general origin, possibly linked to the morphology and particle size distribution of the Ru compound deposited during the catalyst preparation. The difficulty experienced in converting Ru into the SMSI state fits very well with other results13 obtained with Rh/Ti02 and Pt/TiO, catalysts.The ease of formation of the SMSI state is found to increase in the order Ru < Rh -c Pt, possibly reflecting a trend in the M-TiO, bond strength. The intermediate oxidation (catalysts E and H above) seems to result in the formation of a very well dispersed Ru oxide. TEM shows that after a subsequent low-temperature reduction the average particle size (1.5 nm) is only half that obtained after the initial reduction. It is probable that the main role of this intermediate oxidation is not only to remove C1 impurities, although this is undoubtedly important, but also to redisperseR. Burch, G . C. Bond and R. R. Rajaram Table 6. Product distributions in the n-hexane and n-butane reactions over a 5% Ru/TiO, catalyst pretreated in various ways selectivity catalyst T,,,,/K C, C , C, C, C , 5D 403 1.06 0.34 0.34 0.34 0.38 5E 403 1.32 0.28 0.27 0.32 0.41 5F 403 1.39 0.31 0.29 0.30 0.39 5Da 433 0.93 0.98 0.37 5E 403 1.05 0.53 0.63 5F 433 0.97 0.87 0.43 1997 a Data for the n-butane reaction are taken from ref.(7). the Ru. This would account for the fact that intermediate oxidation is necessary before the nitrosyl-derived catalyst converts into the SMSI form. However, the special importance of C1 can be judged from the fact that after identical reduction pretreatments the nitrosyl-derived catalyst is more fully converted into the SMSI state than either of the C1-derived catalysts. The use of TiO, as a support highlights the effect of changing the precursors or pretreatments but similar patterns of behaviour may also occur with more conventional oxide supports and with metals other than Ru.The similarities in the trends in activity between the present results for n-hexane and the earlier work on n-butane hydrogenolysis have been discussed already. There are differences, however, particularly with regard to the product distributions. Table 6 compares the product distributions for the 5 % Ru catalysts for the n-hexane and n-butane reactions. In the butane reaction normal Ru shows a preference for cleavage of the terminal C-C bond, whereas SMSI Ru develops a greater tendency for breaking the internal C-C bond. A change in product distribution for the n-butane reaction has also been reported for Ti0,-supported Rh and Ir catalysts in going from the normal to the SMSI ~ t a t e .~ However, for these metals exactly the opposite trend is observed, i.e. the selectivity to ethane decreases as the SMSI state develops. In the n-hexane reaction there is essentially no difference in the product distribution for normal and SMSI Ru (compare in particular 5E and 5F). (Note especially that there is no enhanced selectivity for internal bond cleavage.) The different sensitivities of n-butane and n-hexane to contamination of Ru by TiO, is observed over the same catalysts, so particle size effects can be discounted. The results indicate that the active centres for the two reactions are different. Two explanations are possible: (a) the hydrogenolysis of n-hexane occurs at a small active centre (1 or 2 Ru atoms) and n-butane reacts at a larger active centre (3 or 4 Ru atoms); (b) both TiO, and n-hexane contaminate the Ru in identical ways so that the steady-state catalyst is similar in both cases, e.g.if the HTR treatment results in a partial blockage of the surface of the Ru by TiO, to give the equivalent of a catalyst self-poisoned by carbonaceous deposits. The contrast between the reaction of methylcyclopentane and n-hexane is significant. After an initial HTR the 5 % Ru catalyst has a normal activity for the methylcyclopentane reaction, whereas the activity for the n-hexane reaction is suppressed. However, the change in the splitting pattern for methylcyclopentane (to give 2- or 3-methylpentane) with the type of pretreatment indicates that a different mechanism operates on catalysts after HTR. Normal Ru preferentially splits the C-C bond opposite the CH, group in methylcyclopentane, whereas SMSI Ru preferentially attacks the other secondary-Hydrogenolysis of Alkanes secondary C-C bonds. It appears that methylcyclopentane but not n-hexane can react on Ru particles heavily contaminated by TiO, species. The active centre may be at the metal-TiO, interface. We are grateful to the S.E.R.C. for the award of a Postdoctoral Fellowship to R.R.R. References 1 S. J. Tauster, S. C. Fung and R. L. Garten, J. Am. Chem. Soc., 1983, 100, 170. 2 Metal-Support and Metal-Additive Eflects in Catalysis, ed. B. Imelik et al. (Studies in Surface Science 3 G. C. Bond and R. Burch, in Specialist Periodical Report: Catalysis, ed. G. C. Bond and G. Webb 4 S. J. Tauster, L. L. Murrell and S. C. Fung, US Patent, 1976, 1576848. 5 D. E. Resasco and G. L. Haller, J. Phys. Chem., 1984, 88,4552. 6 G. C. Bond and Xu Yide, J. Chem. Soc., Chem. Commun., 1983, 1248. 7 G. C. Bond and Xu Yide, J. Chem. SOC., Furaday Trans. 1, 1984, 80, 3103. 8 R. Burch and L. C. Garla, J. Catal., 1982, 73, 20. 9 E. I. KO, S. Winston and C. Woo, J. Chem. Soc., Chem. Commun., 1982, 740. and Catalysis 11) (Elsevier, Amsterdam, 1982). (Royal Society of Chemistry, London, 1983), vol. 6, p. 27. 10 J. L. Carter, J. A. Cusumano and J. H. Sinfelt, J. Catal., 1971, 20, 223. 11 F. J. Schepers, J. G. van Senden, E. H. van Broekhoven and V. Ponec, J. Catal., to be published. 12 R. Kramer and H. Zuegg, J. Catal., 1983,80, 446. 13 J. B. F. Anderson and R. Burch, Appl. Catal., to be published. Paper 511489; Received 30th August, 1985

ISSN:0300-9599    

DOI:10.1039/F19868201985

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

CORRIGENDUM A Nuclear Magnetic Resonance Study of the Sodium Cryptate formed by 4,7,13,18-Tetraoxa-l,lO-diazabicyclo[8.5.5]eicosane (C211) in Various Solvents Stephen F. Lincoln Department of Physical and Inorganic Chemistry, University of Adelaide, South Australia 5001, Australia Ian M. Brereton and Thomas M. Spotswood Department of Organic Chemistry, University of Adelaide, South Australia 5001, Australia J . Chem. SOC., Faraday Trans. 1, 1985, 81, 1623-1630 In table 2 the units of k,(298.2 K) should be dm3 mol-1 s-l.

ISSN:0300-9599    

DOI:10.1039/F19868201999

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

Reviews of Books Surface and Colloid Science. Vol. 13. Ed. by E. Matijevie and R. J. Good. (Plenum Press, New York, 1984.) Pp. xiii+297. Price $45. This book deals primarily with experimental methods used in Surface and Colloid Science. A large variety of topics has been selected : electrochemistry at oil-water interfaces (A. Watanabe) ; kinetic theory of flotation of small particles (B. Derjaguin, S. Dukhin and N. Ruylov); specifically impermeable precipitate membranes (C. van Oss) ; dynamic surface tension and capillary waves (J. Mann); digital computer oriented numerical analysis in surface chemistry (J. Mann) ; advances in experimental techniques for mercury intrusion porosimetry (D. Winslow); and the contact angle of mercury on the internal surfaces of porous media (R.Good). The first chapter on electrochemistry at oil-water interfaces contains a broad survey of basic surface electrochemistry (surface potentials, electrocapillarity, adsorption, ion binding). Interesting results about electrocapillary emulsification and coalescence in the emulsions formed in this way are presented. Membranes electrical properties are also discussed. The second chapter is a very comprehensive survey of flotation aspects: conditions for the disjoining pressure and the 4‘ potential at the particle surface, action of surfactants, account of long range and short range hydrodynamic interactions, collisions, particle sizes, etc. The chapter on precipitate membranes deals with the specific impermeability of these membranes to the ions who formed the precipitate. This process is reversible and ‘deconditioning’ conditions are discussed. Precipitate lines can be formed in a similar way by double diffusion in gels.This is the basis of several accurate methods to characterize diffusion of biopolymers (antigen-antibodies for instance) in gels. The methods are described in detail. The precipitate membranes can be selective to small ions (Ca2+) and their curious analogies with real biological membranes are pointed out. The chapter on capillary waves contains a very broad survey on liquid surface theory: dynamic surface tension, surface viscoelasticity. The experimental methods to study the waves are then described in great detail: generated waves where the frequency range is typically 1-1000 Hz and thermally excited waves where the frequency range is larger 1-100 KHz.Many useful suggestions are given to make the best use of these techniques on the light of the most recent advances in the field. The following chapter contains computer-centered numerical analysis results that the author found useful in analysing the results of capillary ripple experiments. Chap. 6 describes recent advances in experimental techniques for mercury intrusion porosimetry, a very useful tool to investigate pore size distribution and other microstructural parameters in porous media. In the last chapter the role of the contact angle of mercury on the interior surface of porous solid is discussed in connection with porosimetry methods. In conclusion, all the topics discussed in the book are interesting and well documented. They are written for specialists or scientists willing to become specialists in these fields, rather than for a broad spectrum of scientists active in interfacial and colloid phenomena.This applies particularly to the chapters on capillary waves and mercury porosimetry. D. Langevin Received 25th October, 1985 Adsorption and the Gibbs Surface Excess. By D. K. Chattoraj and K. S. Birdi. (Plenum Press, New York, 1984). Pp. xiii+471. Price $59.50. The aim of this book is ‘to point out those specific areas of surface phenomena where the application of the Gibbs surface excess concept is essential or not yet fully appreciated’. Surface excess concentrations are used to characterize interfacial systems whenever the density or composition at the interface differs relatively little from that in the bulk phase from which adsorption occurs, or when a density or composition variable approaches its bulk value slowly, as in the case of polymer adsorption, or adsorption phenomena near a critical point of a bulk phase.200 12002 Reviews of Books Surface excess quantities depend on arbitrary conventions and, as R. B. Griffiths puts it, their chameleon-like behaviour when a convention is changed can prove to be a major conceptual hurdle for the beginning student, especially as there is nothing quite like it in bulk thermodynamics. Hence it is worth exploring this topic in more detail. Although the book has its merits, it fails to provide a basic understanding of the connections between the surface excess quantities and those physically significant quantities which do not depend on an arbitrary convention.It gives lengthy derivations and discussions of the Gibbs adsorption equation in terms of relative surface excess concentrations, but the treatment of more subtle questions is mostly rather superficial. For example, in a section on surface equations of state (p. 173) the quantities n; are introduced as ‘moles of components in the interfacial region as imagined by Gibbs’; but a few lines later: ‘n; is the moles of solvent present in the interfacial region whose value is positive (and not zero as imagined by Gibbs for relative excess)’. In the context of spread monolayers (p. 191), surface activity coefficients are defined on a surface excess concentration basis (!).By the way, the use of the symbol Ani for the (absolute) amount of a component in the inhomogeneous boundary layer, and the fact that concentration ratios are often written as a ratio of the amounts of two components, is sometimes confusing. The main part of the book is devoted to applications of the Gibbs adsorption equation to a variety of adsorption phenomena at the air-liquid, liquid-liquid and solid-liquid interface, and to related phenomena in biological systems. A large amount of experimental results is reviewed in separate chapters on adsorption from single and multicomponent solutions at air-liquid interfaces, adsorbed and spread monolayers, wettability and contact angles, adsorption at solid-liquid interfaces, adsorption of water vapour by biopolymers, and binding phenomena in biological systems. Some of these chapters, like that on adsorption from multicomponent electrolytes or those on biopolymers, will be most useful in view of the wealth of experimental material covered and the large number of references.As might be expected, the book emphasizes the authors’ own contributions. References to either of them appear nearly one in every two pages of the text. The resulting selection of examples is not always convincing; e.g. for clarification of basic concepts of adsorption at solid-liquid interfaces, the reader is first confronted with the adsorption of alcohols and glycols from aqueous solutions by gelatin powder. More well defined systems (studied by other authors) are reviewed later in the text. In conclusion, this book does not achieve its main goal, to lead a student through the in-depth understanding of the thermodynamics of interfaces. It can, however, serve as a useful source of information on selected topics of interfacial chemistry, in which part of the original literature is dispersed in journals not generally available.G. H. Findenegg Received 25th October, 1985 Alternating Copolymers (Speciality Polymers). Ed. by J. M. G. Cowie. (Plenum Press, New York, 1985.) Pp. xii+281. Price $47.50. Copolymerization reactions and copolymers constitute a very extensive field of work which is of considerable importance both scientifically and industrially. Within it the class of highly and strictly alternating copolymers (in which the molecular chains consist of a regular -ABAB- sequence of the two monomer units, irrespective of the exact composition of the reacting system in which they are formed) is a limiting case of much interest.The study of this type of system has made many advances in recent years, and the appearance of a book which seeks to draw together the various aspects is to be welcomed. The six contributions to the volume include a general survey, a review of the radical-initiated alternating copolymerizations, and a detailed account of the enhancement of the alternating tendency by Lewis acids (an effect which has greatly extended the range of systems available for study). There are also chapters reviewing the copolymerization of zwitterions, of olefins with diolefins and the physical properties of alternating copolymers.This last is timely because, although attention has largely focussed on the mechanisms which cause strict alternation, a small but increasing number of the copolymers is proving to be of technical interest. The book does not set out to be comprehensive or exhaustively detailed and a few minor gaps are apparent. There is rather little discussion of alkene-SO, copolymerizations, although their study contributed significantly to early work on the mechanism of alternation (and to recognition of the propagation4epropagationReviews of Books 2003 equilibrium) and is still a developing area. Nevertheless it is a well balanced, informative and up-to-date account of the subject, which should prove very useful to polymer scientists and technologists. K.E. Weale Received 25th October, 1985 Auger Electron Spectroscopy. By M. Thompson, M. D. Baker, A. Christie and J. F. Tyson. (Wiley-Interscience, Chichester, 1985.) Pp. vii + 394. Price &95. Auger electron spectroscopy (AES) is perhaps the most widely used surface analytical technique. Aside from importance in its own right in innumerable industrial and university laboratories, the technique finds frequent application as a means of monitoring surface composition and cleanliness in multitechnique surface science experiments. AES has also played an important part in the development of the broader field of gas-phase electron spectroscopy in the past two decades. The present text reflects something of this diversity. It aims to provide a comprehensive account of AES, but is directed particularly at the analytical chemistry community.Following introductions to the Auger mechanism of core hole decay and experimental aspects of the technique, there follow three major chapters dealing in turn with applications to gas-phase species, simple solids and technological samples of the sort likely to be encountered in metallurgical or materials-science laboratories. In the final chapter AES is compared critically with other methods of surface analysis. The text is generally well presented with few obvious typographical or other errors of presentation, although it was puzzling to find in the section on angle-resolved gas-phase Auger spectroscopy on p. 85 a diagram of an instrument designed specifically for angle-resolved U.V. photoelectron spectroscopy. I was also annoyed to find that the invaluable Varian chart of Auger electron energies was bound into my copy of the text at page 368 and not as intended at page 387 (publisher please take note).The book is the first to attempt a detailed coverage of all aspects of the technique, and despite its almost prohibitive price will be regarded as indispensible in many surface analytical laboratories. It can also be recommended to graduate students, where it will help broaden the perspectives of those involved in research on solid state and surface problems into the field of molecular spectroscopy. However, I feel that the book is weakened by devoting too much space to gas-phase AES at the expense of exciting and fast developing areas of application to solids.This criticism holds from both the perspective of the analytical chemist and the academic solid-state scientist. The chapter on gas-phase AES is by far the longest in the book and approaches its subject matter in the manner to be expected in a specialised review article. Assignments of spectra in terms of one electron configurations are discussed in painstaking detail and great importance is attached to questions of precedence in the publication of gas-phase spectra. By contrast there is little general discussion of Auger intensities in terms of localisation tendencies of molecular orbitals, a topic that would have provided an interesting starting point for discussion of solid-state core-valence spectra. In the discussion of the AES of ammonia it appears that one of the authors is using the book as a vehicle to air an old grudge against those who failed to recognise that his was the first spectrum of this molecule: not the sort of thing one expects to find in a textbook.However, the major criticism is that the length of the chapter on gas-phase AES is out of proportion to its importance either as an analytical technique or as a fundamental tool for the study of molecular electronic structure. The balance of the material in the chapters on solid-state and technological applications is better, although the discussion of the basic physics of the Auger process in solids is somewhat sketchy and topics such as hole-hole correlation in CVV spectra, plasmon excitation etc. are given fairly short shrift. These topics are discussed in an excellent review article by J.C. Fuggle [in Electron Spectroscopy: Theory, Techniques and Applications, ed. C. R. Brundle and A. D. Baker (Academic Press, London, 1981), vol. 4, p. 861 which is surprisingly missing from an otherwise useful bibliography of review articles at the end of the book. I was particularly disappointed that the authors believe that there is no need for AES ‘as a valence band spectroscopy to provide experimental evidence against which to test bulk bandstructure calculations’. This ignores important applications in the study of CVV and CCV spectra of alloys and other binary or ternary materials where Auger spectra give vivid meaning to the concept of the local density of states. This topic is perhaps of greater interest to the academic solid-state scientist than to the technological analyst, but nonetheless represents one of the areas where AES can furnish unique2004 Reviews of Books information.Viewed simply as a book on analytical applications, the text gives very much less emphasis to spatially resolved AES than would be expected. There is a brief discussion of submicron electron guns in the instrumental chapter, but I searched in vain for elemental Auger maps illustrating what can now be achieved in fields such as microelectronics. ceramic grain boundary analysis etc. All in all then this is a useful, but at times unbalanced text. For the most part, references are to papers published before 1982. AES continues to develop and I find it hard to believe that this book will establish itself as the definitive account of the technique.R. G. Egdell Received 23rd October, 1985 Handbook of Tritium NMR Spectroscopy and Applications. By E. A. Evans, D. C. Warrell, J. A. Elvidge and J. R. Jones. (Wiley-Interscience, Chichester, 1985.) Pp. xiv + 249. Price &37. 3H is unique in magnetic resonance in having the highest known gyromagnetic ratio of any nucleus (4.54 x lo7 Hz T-l); in a magnetic field in which lH frequencies are found near 90 MHz, 3H is close to 96 MHz. The advantages in sensitivity and chemical-shift range conferred by this high value are counteracted by other physical properties: 3H is a p-emitter of maximum energy 0.0186 MeV, decaying to 3He with a half-life of 12.43 years. Nevertheless, high-resolution 3H spectra were obtained as early as 1964 on a 10 Ci sample and since then, with gradually increasing sensitivity, from samples with levels now as low as 5 mCi per labelled position (0.017% isotopic abundance), with the possibility of recording trace levels in sight.The technique has thus become an important method of studying the distribution and stereochemistry of 3H labelling in molecules. Like 2H, it has the great advantage that the vast compilations of ‘H chemical-shift data in the literature can be applied directly to spectral prediction and assignment with the advantage over the lighter isotope of a much wider chemical-shift range and higher sensitivity. A monograph on the subject was clearly overdue and has been well provided by the authors of this excellent text; they themselves have made an extensive contribution to the subject over a period of over fifteen years, the experience of which has furnished the reader with a detailed and authoritative review, concerned largely with the three main methods (synthesis, exchange and biochemical) of preparing 3H labelled compounds, the labelling patterns observed and applications in organic analysis and biosynthesis.There are comprehensive tables giving the labelling patterns in nucleic acids, aromatic hydrocarbons, amino acids, carbohydrates, steroids and alkaloids, together with numerous 3H spectra, most of them fortunately of a considerably better quality than that on the front cover. The emphasis is placed on organic and biochemical mechanisms, but there is some discussion of catalytic studies and the isotopic shifts associated with hydrogen bonding, albeit mostly at a qualitative level.In most other respects, the monograph provides a most useful handbook to a subject which is surely due to experience a remarkable development with the wider use of FT ‘ supercon’ systems and cross-polarization methods. J. A. S. Smith Received 25th October. 1985 Dynamic Light Scattering. Application of Photon Correlation Spectroscopy. Ed. by R. Pecora. (Plenum Press, New York, 1985.) Pp. xiv+420. Price $59.50. The techniques of dynamic light scattering are, today, so well established that it comes as a slight surprise to be reminded in the introduction to this volume that the experimental and theoretical basis of most of the current work is just 21 years old.In the early sixties the laser was developed as a suitable source for optical work, at the same time as photomultiplier systems became widely used. In 1964 the editor of this volume published the results of his doctoral thesis at Columbia University, which cast the theory of dynamic light scattering in terms of correlation functions and indicated the way to observe the small energy changes characteristic of macromolecular diffusion in solution. Working also at Columbia, Cummins and coworkers demonstrated the experimental application of these ideas, closely followed by Ford and Benedek at M.I.T. It seems eminently suitable then that the effective ‘coming of age’ of such a widely applicable technique should be marked by the current volume. Published in the mid seventies, the books of Chu and of Berne and Pecora cover the general principles of the experimental and the theoretical treatments,Reviews of Books 2005 respectively, and together with the lecture notes from a NATO Advanced Study Institute, edited by Cummins and Pike, form the basic textbooks.The diversification of applications which has occurred since these volumes appeared means that an edited volume of contributions from experts in the various fields is probably more appropriate than a single author attempting to cover everything. While this does lead to some inevitable variation in style, the editing and production has been carefully carried out to give a unified look to the volume. Topics that have developed much in recent years and which merit whole chapters here are the studies of semidilute polymer solutions (C.C. Han and D. W. Schaefer), bulk polymers (G. D. Patterson), micelles and micro- membranes (N. A. Mazer) and polymer gels (T. Tanaka). An important area of theoretical development has been the treatment of systems of interacting particles (P. N. Pusey and R. J. S. Tough). As well as these new areas there is a comprehensive chapter on light-scattering apparatus by N. C. Ford Jr. The editor himself reviews the basic principle of the relatively neglected field of dynamic depolarised light scattering and discusses some promising applications, among others, to the study of intramolecular macromolecular motion (K. Zero and R. A. Pecora). To complete the ‘set’ there are reviews of applications of dynamic light scattering techniques to biological systems (V.A. Bloomfield, with over 400 references!) and to critical phenomena (B. Chu). While this volume will not replace for the serious student the standard texts on the bookshelf (and it is not intended to do so) it will, I believe, prove a very useful addition to any collection of reference works on dynamic light scattering and act as an introduction for research workers interested in finding out just what the technique is able to offer in their own area of study. J. S. Higgins Received 4th November, 1985 Stereochemistry of Heterogeneous Metal Catalysis. Ed. by (J. Czombos et al.) M. Bartok. (Wiley Interscience, Chichester, 1985.) Pp. xiv + 632. Price E85. Heterogeneous catalysis, apart from a few notable exceptions such as sulphur dioxide oxidation or synthesis of ammonia, is largely concerned with transformations of organic compounds on the surfaces of inorganic solids.Yet the subject in the West, and especially in the U.K., is now mainly the domain of physical chemists. That is a very ironic situation in the second half of the twentieth century, which has seen such rapid growth of the industrial importance of catalysis. This situation is even more strange because it is diametrically opposed to the early example shown by the great Eastern European pioneers, whose organic and inorganic skills led to the development of oil reforming and petrochemical industries by the end of the Second World War. One has only to think of Ipatieff or Pines to recognise the type whose guiding light is educated intuition based on a vast knowledge of factual information and a great deal of practical experience, rather than on physical concepts.The present book by M. Bartok and his associates from the Jozsef Attila University of Szeged is therefore very welcome because in dealing in depth with the stereochemistry of organic conversions on metal surfaces it highlights the continuity and importance of the organic trend in the subject, which has always been a major feature of the work of the Eastern European scientists. It is also a very timely publication because, as the authors themselves point out in the preface, it is the first book on the topic, even though there is already a vast literature in this particular area, and stereochemistry in catalysis of all kinds including that of enzymes is becoming increasingly important.The authors have attempted to fill this large gap, at least as far as metal catalysts are concerned, and their efforts have been rewarded by a considerable measure of success. They have written in all some 600 pages of highly condensed material divided into twelve chapters whose titles follow the sequence of most standard organic texts, i.e. alkanes, cycloalkanes, alkenes, alkynes, aromatics, alcohols, carbonyl compounds and so on. The penultimate chapter is about enantioselective hydrogenation, which is a major growth area, and the final one is mainly concerned with some recent developments in the field of hydrogenolysis, including that of Si-H and Si-C bonds. The standard and style of writing vary from one topic to another, which is to be expected considering the number of co-authors and greater research familiarity with some areas, such as that of chap.IX. However, my main criticism is that in a book of this type, which is meant to be comprehensive, every speculative mechanism ever proposed is included, whereas one would haveReviews of Books liked to have seen a more critical, selective and subjective approach. On the other hand, this volume is certainly very thorough in referring to most of the work up until the end of 1982. There are almost 4000 references to original papers and reviews supplemented by extensive author and subject indices. This above all makes it an invaluable reference text for all workers in the field of catalysis.The inclusion of numerous and extremely well drawn diagrams of organic structures is a most attractive feature. All of this has certainly added to the price, which at E85 is quite expensive, but the book is still good value for money even though it may be beyond the reach of the purses of most individuals. At the very least a copy should be available to each group which has a serious interest in catalysis, because its educative value for those with a physical approach can hardly be overestimated. J. J. Rooney Received 5th November, 1985 The Principles and Practice of Electron Microscopy. By Ian M. Watt. (Cambridge University Press, Cambridge, 1985.) Pp. 303. Price E40, $49.50. There have been several books published in the last few years on the subject of electron microscopy.This book differs from most, if not all of them in its approach to the subject. The author has been for many years, in his own words, a ‘general purpose microscopist providing a service in electron microscopy to customers from a range of disciplines ’. In this book, he gives his personal view of the subject to the sort of people who might use a microscopy service, or who encounter the results of electron microscopy, but have never carried out any microscopy themselves. This is not an academic textbook on the subject, and is not aimed at a student of electron microscopy. I envisage the typical reader to be a scientist or engineer of almost any discipline, who is working in industry, and who is not familiar with electron-beam instruments. The book explains in simple terms how the various electron-beam techniques work and what can be achieved.The coverage is very broad and, as the author is relying heavily on his own experience, this inevitably leads to questions as to whether the balance is correct. Chap. 1 discusses, qualitatively, the principles of light and electron optics and introduces the ideas of resolution, defects in images, and the interaction of electron beams with specimens. Chap. 2 deals with the family of electron beam instruments, how they work, and what they can achieve. This is a long chapter, which is rather curiously presented. Rather than confine himself to the modern ‘state of the art’ instruments, a mix of modern and old techniques and instruments is discussed, but there is no attempt at a proper historical survey.The balance seems to be too far towards the older equipment, making this section seem a little out of date. The third chapter discusses the preparation of specimens, and the interpretation of micrographs. The section on the preparation of specimens is rather uneven, there being excessive detail on some techniques, and virtual neglect of others. For example, two pages and two figures are devoted to the coating of SEM specimens by evaporation, a technique which has now been largely replaced by sputter coating. The section on interpretation of micrographs is mainly devoted to scanning electron micrographs, and to the secondary electron signal, although other signals are briefly mentioned. A chapter on specialised techniques follows. This includes the transmission electron microscopy of crystalline specimens, in which the microscopy of extraction replicas is discussed, and a substantial section on stereographic methods.The author returns to the subject of specimen preparation again to discuss preparation of biological specimens and crystalline specimens. There is some detailed discussion of instrumental defects, such as focussing and astigmatism, and a section on several other techniques for microscopy, including ion-beam methods. In the final chapter, the author presents a series of case studies, illustrating the application of microscopy to such diverse subjects as textile science, mineralogy, forensic science and pathology. This chapter is well worth the attention of anyone interested in electron microscopy.The book concludes with appendices on vacuum pumps, vacuum deposition of films, chemical analysis by X-rays and electron sources. Again, the choice of material is unusual. Three of the subjects are very specialised for a book of this sort, and the other, X-ray microanalysis, is so important that it really should have formed a major part of the book. Although the book is not particularly well balanced, it is clearly written, profusely illustrated,Reviews of Books 2007 and contains a lot of information, so that even an experienced microscopist will find something new in it. It should be comprehensible to scientists of any discipline, and will give them an idea of what electron microscopes are all about. F. J. Humphreys Received 21st November, 1985 ACS Symposium Series 282.Reactive Oligomers. Ed. by F. W. Harris and H. J. Spinelli. (American Chemical Society, Washington, 1985.) Pp. ix+261. Price $71.95. The content of this book is based on a symposium sponsored by the Divisions of Polymer Chemistry and Polymeric Materials Science and Engineering at the 187th Meeting of the ACS in St Louis in April 1984. As indicated by the title, the emphasis is on the synthesis of well defined oligomers terminated with reactive functional groups, which, on curing, produce high-molecular- weight resins and composites. Many of these find application in the electronic, adhesive and high-solid coating industries. The current interest in these high-temperature resistant, high-performance resins and plastics is reflected in contributions from U.S.state research centres, such as NASA and the U.S. Naval Research Laboratories, as well as the industrial research laboratories of Ford Motor Co., General Electric Co., and IBM. As is usual with the ACS Symposium Series there is a good balance of review material and original papers. The first article by P. M. Hergenrother sets the scene with a useful review of the various types of high-temperature resistance polymers. This is followed by papers on acetylene-terminated thermosetting resins based on Bisphenol-A and arylether sulphones, and polyaromatic oligomers based on poly(triethyny1benzene). A paper by Perced and Auman reviews general methods for the synthesis of polyaromatics containing either terminal or pendant styrene groups. Other papers deal with ethynyl end-capped polyimide oligomers, polyimide oligomers from substituted norbornenyl imides and the dynamic mechanical properties of thermosetting polymers from bis-phthalonitrile.N-Cyanourea-terminated resins and urethane oligomers of interest in high solid coating formu- lations are the subject of papers from the research laboratories of W. R. Grace and Company and Ford Motor Company. There is an interesting paper on a semi-continuous cationic polymerization of isobutene in the presence of a polyfunctional initiator - transfer agent, e.g. 1,4-di- or 1,3,5-tri-(2-chloro-2-propyl)benzenes to produce end-reactive or three-arm star telechelics. The syntheses of other telechelic oligomers by amination and carbonation of polymeric organolithium compounds are also described.Other papers describe the syntheses of reactive oligomers by free-radical ring-opening reactions ; the syntheses and characterisation of difunctional siloxane oligomers and poly( 1,6phenyleneoxides) (2 articles). A contribution from the BF Goodrich Company deals with the characterisation (by acetylation, molecular-weight determination and spectroscopic techniques) of hydroxy-terminated epichlorohydrin liquid polymers. There are also articles on the effect of structure on the thermal and U.V. stability of styrylpyridine based polyesters and polycarbonates, glycol bis(allylphtha1ate) cross-linking agents for diallylphthalate resins and the use of triglyceride oils, such as castor oil, in combination with polystyrene to form an interpenetrating polymer network.The majority of the contributions deal with the methods of synthesis of monomers and oligomers and their thermal properties and will be of interest mainly to the industrial and synthetic polymer specialist. However, this book does provide for the non-specialist a useful introduction to the latest developments in an area of polymer chemistry which is growing rapidly in importance. The articles have been reproduced from camera-ready typescript so that they are up-to-date and relatively free from error. One minor criticism is the excessive use of technical jargon and acronyms in many of the contributions. This may make it easier for the authors and it may help to reduce the publishing costs, but it does nothing for the reader. Nevertheless, to my knowledge this is the first book to deal with the important subject of reactive oligomer synthesis and, as such, it is a must for most good scientific libraries and for anyone actively involved in the preparation of composite resins, electronic coatings, high-solid coatings and thermosetting resins.B. L. Booth Received 28th October, 1985Reviews of Books Surface and Interfacial Aspects of Biomedical Polymers Vol. 1. Surface Chemistry and Physics. Ed. by J. D. Andrade. (Plenum Press, New York, 1985.) Pp. xvif470. Price $69.50. As stated by the editor in the preface to this book, many of the assumptions of classical surface chemistry are invalid when applied to polymer surfaces in contact with aqueous solutions. Since such surface properties are fundamental to the successful use of synthetic polymers in medical devices of many kinds, a treatise offered as being capable of providing a fundamental basis for study of the interactions of polymers with biological fluids is seeking to satisfy an exceedingly important and demanding role.The first chapter, written by the editor, is a useful summary of the pertinent iiterature, both journals and books, covering the last decade, together with details of societies and professional organisations related to polymer surfaces. Dynamic motions of polymers in the bulk and in the surface, the consequences of those motions and techniques for their study, are reviewed in the following chapter. Polymer systems illustrating particular aspects of surface behaviour, such as hydrophobicity, morphology and systemic hydration are reviewed in detail in the succeeding chapter.The role of polymerizable lipids in producing model membranes, monolayers and vesicles is investigated in detail in chap. 4. The next three chapters review techniques, previously mentioned in chap. 2, that may be used in studying polymeric surfaces. Chap. 5 is a very detailed review of X-ray photoelectron and Auger spectroscopy with applications to polymer surfaces. Chap. 6 previews surface infrared spectroscopy, with particular reference to total internal reflection techniques, and chap. 7 examines the problems arising in the use of contact-angle measurements on polymer surfaces, particularly those thought of as being ‘ soft ’. Chap. 8 is a straightforward review of electrostatic double-layer theory as applied to colloidal systems and, disappointingly, provides very little insight into applications involving polymer surfaces.Chap. 9, on the other hand, is an interesting and stimulating discussion of ‘polar’ con- tributions to surface energies from a viewpoint that such contributions should be more properly considered as acid-base interactions between opposing acidic or basic groups on the polymer and solvent. Variation of composition between bulk and surface in block copolymers and the effects upon surface properties from casting polymer films against differing surfaces is considered in chap. 1 1. Chap. 12 reviews the technique of Raman spectroscopy as applied to surfaces and logically should have followed the earlier chapters on experimental techniques.The book concludes with the editor’s view on expectations for the future course of studies of polymer surfaces. The book is, in general, satisfactorily free of printing errors, which makes for more comfortable reading, but the arrangement of chapters appears to follow no logical pattern - earlier chapters on dynamics and model polymer systems might have been better followed by the chapters on copolymers, acid-base properties and interfacial tension theories, with experimental techniques forming one clear section within the text, rather than being distributed throughout the book. If one poses the question ‘how well does the treatise satisfy the editor’s ambition for it to be a fundamental basis for the study of polymer surfaces with biomedical applications? ’ the answer for me is, in some respects, very well; the initial overview will be useful to newcomers to the field as will the sections dealing with polymer dynamics and model polymer systems, both bulk and monolayer. I am less convinced that detailed discussions of the principles of electric double-layer theory, ESCA, surface i.r.spectroscopy and Raman spectroscopy, with relatively few illustrations of suitable applications, have a useful contribution to make here. The review of difficulties that can arise in contact-angle studies, such as deformation and penetration should be valuable and the chapter on acid-base interactions is a very attractive contribution. In conclusion, I feel that certain chapters do fulfil the editor’s objective, but others provide no particular valuable contribution; overall the text will be very useful, but it falls short of being the talisman that the editor hoped to produce. D.Eagland Received 25th November, 1985Reviews of Books 2009 Specialist Periodical Reports. Spectroscopic Properties of Inorganic and Organometallic Compounds. Vol. 17. Senior Reporters G. Davidson and E. A. V. Ebsworth. (Royal Society of Chemistry, London, 1985.) Pp. 395. Price E95, $138. This volume covers within eight separate chapters, six different branches of spectroscopy including n.m.r., n.q.r., Mossbauer, vibrational and rotational spectroscopy plus electron diffraction. This is a disparate set of techniques to be found between one set of covers for which a relatively high price has to be paid. However, the fact that the volumes have reached number 17 suggests that there is a market for this particular selection of techniques. The diligence and fortitude of the reporters who have contributed to the volume is certainly admirable. For example, the first chapter on nuclear magnetic resonance spectroscopy by B. E. Mann contains a list of 2856 separate references, squeezed into 154 pages, almost 50% of the book. Unfortunately, this high density of information does not lead to an article which can be read easily or readily enjoyed. Indeed, in places the article is readable only in the same manner as one would use the London telephone directory. For example, some pages comprise lists of chemical formulae occupying the upper third of the page with the references filling the lower part of the same page. Many papers warrant only one sentence in description. The shortest chapter in the volume, on Rotational Spectroscopy by S. Craddock, has a more measured style with a mere 105 references to cover, excluding those in three pages of table. But even here a story line is absent. Overall therefore, this volume leaves the impression of an intimidating collection of facts. Analysis or digestion of the data are, by and large, absent. It seems to this reviewer that this defeats the essential purpose of such a volume. Today one can obtain from Chemical Abstracts Selects, for approximately the same price as this volume, a list every two weeks of all the abstracts of papers current in say n.m.r. spectroscopy. Such a list is more informative than the present volume. The use of computer-aided abstracts have now surely overtaken volumes such as this. Perhaps it should not be the purpose of suck a volume to aim for con;prehensive cover or to provide lists; rather a reader looks to a knowledgeable reporter to select the areas of excitement, progress and novelty within a subject and to exercise judgement in reporting this to the wider chemistry community. A. J. Thomson Received 7th Nooember, 1985

ISSN:0300-9599    

DOI:10.1039/F19868202001

出版商:RSC    

年代:1986

数据来源: RSC