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Structure and dynamics of hydrogen bonding guests in urea inclusion compounds Marina Brustolon,' Anna L. Maniero," Alessandro Marcomini" and Ulderico Segre* "Universita di Padova, Dipartimento di Chimica Fisica, Via Loredan 2, 35131 Padova, Italy bUniversita di Modena, Dipartimento di Chimica, Via Campi 183, 41100 Modena, Italy The radical obtained by y-irradiation of the 2-nonadecanone/urea (2-NDOU) and nonadecanoic acid/urea (NDAU) inclusion compounds have been studied by EPR spectroscopy. The spectra have first-order, fast-motion line shapes with anisotropic linewidths. For both compounds the spectra show the presence of two similar species. They originate from the two possible arrangements of the molecules inside the host channels, i.e. head-to-head or head-to-tail.The relative abundance of the two conformations is obtained from the intensities of their EPR signals and is accounted for in terms of the balance between guest-to- guest and guest-to-host hydrogen bonding. The transverse relaxation rate constants for the different hyperfine components have been obtained by computer simulation of the spectra. The relaxation originates from the librational motion of the p methylene group and from the hindered rotation of the radical inside the host channel. Evidence of pretransitional effects is shown by the spin relaxation rates above the order-disorder transition in NDAU. The urea inclusion compounds (UIC) are built up by urea molecules packed in an extended hydrogen-bonded arrange- ment which contains non-intersecting channe1s.l Inside these channels it is possible to accommodate molecules whose sizes are compatible with the cavity dimensions.Common examples are given by the normal alkanes or their derivatives, as ketones, alcohols, esters and fatty acids. At room temperature the symmetry of the urea host structure is described by the space group P6,22. The channels have a hexagonal symmetry axis and their walls are arranged in spirals.2 The UICs decompose at temperatures from 378 K (n= 10) to 409 K (n=30).3It should be noted that the long- chain UICs are stable above the melting point of pure urea, T, =405.8 K. On lowering the temperature the UICs undergo a phase transition to an orthorhombic phase. The longer the chain of the included molecule, the higher the transition temperature T,.For the alkane CnH2n+2 UIC, T, ranges from 110K (n=10) to 160K (n=20) and to 220K (i1=40).~ The latter transition has been modelled as an order-disorder phase transition induced by translation-rotation coupling between the orientational order of the included molecules and the transverse acoustic phonons of the urea host latti~e.~ Fatty acids are known to be included as dimers inside the urea channels because of their ability to form hydrogen bonds between themselves.6 As a consequence, their transition to the orthorhombic phase occurs at a temperature much higher than that of the corresponding alkane. As an example, T,= 167 K [by differential thermal analysis (DTA)] for the octadecane UIC and T,=239 K for the octadecanoic acid UIC.7 Moreover, the mobility of the fatty acid guests does not change sharply at the transition, but it is found to occur in a temperature range of some tens of In the UICs the host matrix and the guest molecules form two inter-penetrating ordered structures." It has been ques- tioned whether the repetition length of the guest structure along the channels cg is related to the pitch of the host helix ch.Different behaviours have been revealed according to the nature of the guest. In the case of the alkanes cg and ch are in general incommensurate, while for undecan-5-one it has been found that 2cg =3ch.11 This difference should be related to the modes of interaction of the latter guest compounds.The ketone molecules interact via long-range dipolar forces and can undergo hydrogen bonding with the urea molecules of the host matrix. Recently, the presence of an extended hydrogen-bonded array connecting hosts and guests has been reported for undecane-2,lO-dione UIC.I2 The role of hydrogen bonding is basic in accounting for the properties of host-guest assemblies as the UICs. It is therefore worth gaining a deeper understanding of the structure and the dynamics of the hydrogen-bonding guests in the urea channels. Magnetic resonance techniques are useful tools in studying both structure and dynamics in the solid state. Studies of single crystals give more detailed information with respect to dis- ordered samples. UIC crystals are often quite tiny, which makes the use of EPR advantageous because of its higher sensitivity than NMR.13 In this paper we report on our studies of hydrogen-bonding guests in urea by means of continuous wave (CW) EPR spectroscopy.In a forthcoming paper we will present the results obtained by pulsed EPR techniques. Several species of radicals have been obtained by irradiation of the UICs with hydrocarbon derivatives such as ketones, esters, ethers and carboxylic acids.14 In all these cases it is found that the only stable species is the one obtained by removal of one of the protons in the a position to the C=O group. Therefore, the main feature of the EPR spectrum is given by a hyperfine (hf) pattern of eight lines due to the coupling of the electron spin with one a and two p protons.It is found also that the lineshape is affected strongly by the temperature T and by the angle x between the magnetic field B, and the c6 crystal axis.15 As a consequence, at some particular values of x and T some lines can overlap. In this case the values of the hf splitting must be obtained by means of numerical spectral simulation. The small hf couplings with the y and 6 protons are observable only in the case of narrow lines, and their values can be measured accurately only by the ENDOR technique.16 Generally, in the high-temperature phase, the EPR lineshape can be simulated as a first-order spectrum, i.e. as a sum of a finite number of lines with different widths. The EPR spectra offer a set of data, the hf splittings and the linewidths, which can be exploited to obtain insight into the structural and dynamical properties of the guest radicals.In our previous works on the nonadecan-10-one UIC (10- NDOU),'5*'6 we have shown that the carbon chain largely deviates from the planar all-trans configuration, and that two kinds of motions affect the spin-relaxation properties of the radicals, the internal motions of the methylene chain and the uniaxial molecular rotation inside the host channels. In this paper we extend our investigation to non-symmetrical guest molecules composed of the same number of carbon atoms, it. nonadecan-2-one and nonadecanoic acid. Unsymmetrical guest J. Mater. Chem., 1996, 6(lo), 1723-1729 1723 molecules can assume two different arrangements inside the channels, head-to-head and head-to-tail (see Fig 1) It will be shown that both structures are present and that the dynamics of the guest radicals are remarkably different in the two cases Experimental We have studied by EPR spectroscopy the long chain radicals obtained from nonadecan-2-one and nonadecanoic acid in urea (2-NDOU and NDAU, respectively) Single crystals of 2-NDOU and NDAU were grown from a 1 1 methanol-acetone (spectrophotometric grade) solution of the urea and the guest in a 60 1 molar ratio The crystals were obtained either by slow evaporation at room temperature or by slow cooling (0 01 "C min-' from 50 to 20 "C) l7 The paramagnetic probes were obtained by y-irradiation with a dose of 2 Mrad at room temperature, giving rise to the stable nradicals I and I1 0 88It I CH~-C-C-C-(CB~)~&HJII HaHP' II The EPR spectra were obtained using an X-band CW Bruker ER 200 D spectrometer interfaced with a Bruker data system ESP 1600 and equipped with a Bruker variable-temperature unit The spectra were recorded in the range 290-170K and by varying the orientation x of the magnetic field with respect to the long prism axis of the hexagonal crystals coincident with the urea channel axis and which will be henceforth indicated as the 2 axis Results EPR spectra of irradiated UIC The first-order EPR transitions of an electronic spin doublet state are uniquely specified by the set of the spin quantum numbers {M}=MI, M2 M, of the nuclei which are coupled via the hf interaction to the electronic spin The transitions are located at any of the resonant values of the magnetic field Bres {M} =Bo+ 1ajMJ (1) J=1 fl where a, are the hf coupling constants (hfcc) When the I--I PESr B 0 <OH Fig.1 Possible arrangements of unsymmetrical guests within the host channels top, head-to-head, bottom, head-to-tail 1724 J Muter Chem, 1996, 6(10), 1723-1729 dynamics of the paramagnetic molecule are rapid, the Redfield- Freed theory applies," l9 and the EPR transitions are predicted to have a Lorentzian shape The linewidth of a transition is the reciprocal of the transverse spin relaxation time T2and is written as a polynomial expansion in the quantum numbers (MI 1 -=A+ 1BkMk-t CCkM2-t 1Ekk MkMk (2)T, k k k<k where the sums are over all the coupled nuclei The coefficients are related to the correlation functions of the spin interactions which are modulated by the molecular motions For nuclei with 1=1/2, the M2 terms cannot be separated from the constant term A in eqn (2), which therefore reduces to 1 -=A'+ 1BkMk-k 1Ekk MkMk (3)T2 k k<k When the number of nuclear spins is n= 3 the EPR spectrum consists of eight lines whose widths are expressed in terms of seven coefficients by using eqn (3) Therefore, it is statistically more significant to obtain from the spectrum, by a fitting procedure, the width coefficients A', Bk and Ekk instead of the widths of all the individual lines The above theory cannot be applied when the frequencies of the molecular motions are slow with respect to the amplitude of the fluctuations of the magnetic interactions The EPR lineshape is no longer given by a simple superposition of Lorentzian lines and the proper slow-motion theory should be used2' In the low-temperature orthorhombic phase it is expected that the molecular rotation inside the urea channels is in the slow-motion regime Our study, therefore, was restric- ted to the hexagonal phase The EPR spectra of the included radicals were simulated by a least-squares fitting procedure l5 21 The adjustable param- eters are the centre of the spectrum, the hf splittings and the linewidth coefficients for the a, p and p' protons By this procedure, one obtains from the EPR spectrum a set of data regarding the structure of the radicals and their dynamical properties The results are given in the following sections Hyperfine couplings The EPR spectrum of 2-NDOU at T=290K and with B, parallel to the hexagonal C6 axis (x=O") is shown in Fig 2 together with its computer simulation The overall pattern is given by a triplet of doublets as a consequence of the accidental degeneracy of the principal hf couplings The splittings due to the methyl protons are also partially resolved However, some I II I I 3400 3440 3480 3520 3560 B1G Fig.2 Expenmental and simulated EPR spectrum of 2-NDOU at T=290K and x=O" spectral features (such as the line at very low field) cannot be accounted for by a unique set of hf interactions.We therefore hypothesised that the spectrum is due to the superposition of two different species, A and B. Their contributions may be disentangled by the following procedure. First, the lineshape of species A is computed by simulating the main features of the spectrum. Then, the computed contribution of A is sub- tracted from the spectrum to obtain the ‘experimental’ spec- trum of species B. The latter is simulated in its turn and the computed contribution of B is subtracted from the original spectrum to give a clearer ‘experimental’ lineshape for the species A. This procedure is iterated until reaching convergence. In Fig. 3 we display the computed contributions of species A and B to the total lineshape.The simulation in Fig. 2 is given by 88% A plus 12% B. The EPR spectrum of NDAU oriented with x =0 O at room temperature is displayed in Fig. 4. The overall shape of the spectrum is quite similar to that of 2-NDOU(B). In this case the presence of a second species is also apparent, which gives a minor contribution to the spectrum. However, the individual spectra cannot be resolved because of the very low intensity of the signal due to the second species. We have observed the EPR spectra of both 2-NDOU and NDAU at different orientations of the magnetic field. In the case of the ketone the contributions of the two species have been separated. The angular variations of the hf couplings constants are reported in Fig.5, together with those of 10-NDOU which were measured previously.I6 The splitting which exhibits the larger angular variation can be attributed to the coupling with the a proton, while the others originate from coupling to the p and p’protons. The rotational motion of the I 1 I O-NDOU(B) I 1 I 3-rvO 3440 3480 3520 3560 BIG Fig. 3 Computer-simulated spectra of 2-NDOU(A) and 2-NDOU(B), together with the ‘experimental’ spectra obtained by the subtraction procedure described in the text 3280 3320 3360 3400 3440 BIG Fig. 4 Experimental and simulated EPR spectra of NDAU at T= 290 K and x=O” 2-NDOU(A) 2-NDOU(B)l---l 30 10 -Q I I I 1 NDAU 10 -0306090 0 30 60 90 Xldegrees Fig.5 Angular dependence of the hf splittings for 2-NDOU, 10-NDOU and NDAU at T=290 K (0,a proton; 0, P proton; +, p’ proton) radical along its long axis is fast enough to average the anisotropic magnetic interactions which, therefore, are axially symmetrical along the Z axis.The principal values of the averaged tensors Aiand g are reported in Table 1. Fig. 5 also shows the computed angular dependence of the hf coupling Table 1 Principal components of the averaged hf tensors (in G) and g tensor at T= 290 K 10-NDOU 28.8 13.9 27.0 25.9 20.8 19.7 2.0037 2.0043 2-NDOU(A) 28.8 15.0 27.7 26.5 18.9 18.0 2.0037 2.0041 2-NDOU(B) 27.1 15.2 29.6 29.3 27.5 27.8 2.0043 2.0035 NDAU 29.3 15.4 35.5 35.4 28.9 29.0 2.0042 2.0029 J. Muter. Chem., 1996,6(lo), 1723-1729 1725 constants according to the theoretical expression 22 U’(X)=X,,~cos2 x+X12 sin2x (4) The temperature dependence of the principal value 2, in the range 200-290 K is displayed in Fig 6 for species A and B of 2-NDOU and for NDAU and 10-NDOU In the case of the ketone radicals the a proton Al,is nearly temperature independent, while both the pproton components are strongly affected by the temperature variation A similar trend was found for the parallel principal values of the hf coupling tensors On the other hand, in the case of NDAU, the components of the pproton hf tensors display a modest variation, while those of the a proton hf tensors display a modest variation, while those of the a proton increase slightly on lowering the temperature The latter behaviour is evidenced in Fig 3,where the temperature variation of the isotropic hfcc a= TrA of the a proton of NDAU is displayed with a finer T grain and an enlarged scale The hfcc value varies significantly between 210 and 240K, while it is quite constant outside this range The following features are worth noting in Fig 5 and 6 11111 40 r-l2-NDOU(A) 2-N DOU(6) -*o*** e-+++++++ -m i3 d :0 0 ~0000~ ++1 IIIIIQ10*40 11111’ 30 -*-e* **** 10-NDOU -20 +++++++++ + ~~00000000 lIllA10 I 200 240 280 200 240 280 TIK Fig.6 Temperature dependence of the 2, hf coupling principal values for 2-NDOU, 10-NDOU and NDAU measured at x=90° (0,a proton, 0, p proton, +,p proton) 2 20.5 200 250 300 TIK Fig.7 Temperature variation of the %-proton hyperfine coupling constant of NDAU 1726 J Muter Chern, 1996, 6(10), 1723-1729 (I) The hf couplings of the fl protons are inequivalent in all the four cases, but A@)and A@’)are more similar for NDAU and 2-NDOU(B) (ii) The anisotropy of the hf couplings of the pprotons is greater for 2-NDOU(A) and 10-NDOU (in) On the whole, the temperature and angular dependences of the hf splittings of 2-NDOU(A) are similar to those of 10-NDOU, while the variations of the hf splittings of 2-NDOU( B) look like those of NDAU Linewidth coefficients The spin-relaxation behaviour of radical probes in the solid state gives relevant information about the different types of motions affecting its magnetic interactions l3 In our previous study on 10-NDOU” we have shown that, in the fast-motion regime, the linewidth coefficient B, is especially affected by the molecular rotation around the long axis, which modulates the anisotropic hyperfine interaction of the a proton The value of the E,,, linewidth coefficient, instead, is related to the oscil- lation of the methylene group about its equilibrium position, because of the conformational dependence of the isotropic coupling constant of the pprotons 22 If the effects of these dynamical processes are considered to be uncoupled, it is possible to obtain simple expressions for the linewidth coefficients The value of the B, coefficient is maximum when the magnetic field is oriented at x=90° with respect to the channel axis The reorientation of the probe modulates the anisotropic interactions A, and g about their average values A, and El and the linewidth coefficient is given by 1 B, =4(P€3B0/fi2kxx-gyy)(4t xx -A, yy)G (5)0 where z, is the rotational correlation time In contrast, the linewidth coefficient E,, is expected to be angular independent The oscillation of the methylene groups modulates the isotropic coupling constants a, and a, about their averages a, and afl and the following equation is obtained E,, =2<(a,-<)(q3 --aa))tc (6) where T~ is the correlation time of the conformational motion On the grounds of the previous analysis, we have focused our attention on this pair of linewidth coefficients In the case of 2-NDOU, the A species only was considered In fact, the B species has a rather low intensity and the values of its linewidth coefficients obtained by the non-linear fitting procedure suffer from a large incertitude Therefore, it seems that the linewidth coefficients of the A species only are reliable to obtain infor- mation on the dynamical processes of the nonadecan-2-one UIC We recall that, as discussed previously, the above eqn (5) and (6) are valid as long as the motions are fast with respect to the interaction anisotropies The temperature dependences of B, and E,, for the A species of the 2-NDOU radical measured with the magnetic field perpendicular to the 2 axis are displayed in Fig 8 The data for NDAU are shown in Fig 9 The linewidth coefficient values increase on lowering the temperature, but in general they do not follow a simple trend If one assumes that the temperature dependence of a motional rate is due to an activated process, l/z =(l/zo) exp (-d/kT) (7) then a linewidth coefficient w(T)(w =B,,E,, should be fitted to the expression w(T)=woexp (d/kT) (8) In the case of the ketone radical, the E,, coefficient fits to a single activated process in the whole temperature range studied, while the B, coefficient increases up to a limiting value In the case of the carboxylic acid, instead, it is manifest that two 1 Q5 0.1 I I I I I 3 3.5 4 4.5 5 103 WT Fig.8 Temperature dependence of the linewidth coefficients B, and Epp, of2-NDOU I. I J I Qh 0.1 0.01 3.4 3.0 4.2 4.6 103 KIT Fig.9 Temperature dependence of the linewidth coefficients B, and ED,, of NDAU Table 2 Activation parameters for the linewidth coefficients of the UIC radicals 10-NDOU" 9.4 x lo4 7.5 -4.4 x 103 13 2-NDOU(A) 2.1 x lo3 18 -4.9 x lo2 19 NDAUb 1.3 x 10-5 47 -6.2~ 50 NDAU' 1.3 x 104 8.7 "Ref. 15. bT=216-231 K. 'T=238-290 K. different motional regimes are present. However, in the high- temperature region the linewidth contributions due to the motions are so small that it is not possible to obtain a reliable estimate of the E,,, coefficient. Both the coefficients can be fitted to eqn. (8) in the low-temperature region. We note that this region corresponds to the temperature interval in which the aproton hfcc shows the large variation displayed in Fig.7. The activation parameters wo and A corresponding to the straight lines in Fig. 8 and 9 are reported in Table 2. Discussion Structure By comparing the results obtained for the p protons in the different cases one notes two types of behaviour. In the cases of 10-NDOU and 2-NDOU(A),the hf coupling constants have very different values, they have a marked temperature depen- dence, and the dipolar part of the hf tensor shows an anisotropy of about 2G by rotating the crystal in the XZ plane. In the second case, 2-NDOU(B) and NDAU, the two hf coupling constants have more similar values, about 30G, they have a slight temperature dependence and they are nearly isotropic.These trends can be explained by the following consider- ations. The isotropic hf coupling of the p protons depends on the dihedral angle 8, between the z orbital of the unpaired electron and the CH, bond direction according to the relation,22 ui=acos2 el (i= i,2) (9) where a typical value for the proportionality constant is ca. 43 G. In the case of a planar conformation for the carbon chain, the values of the dihedral angles are 81.2 = 30 O and the hf coupling constants are equal. The non-equivalence of the p protons indicates that the local average conformation is distorted and the two angles are given by =y 300 (10) In the cases of NDAU and 2-NDOU(B) the distortion is small and displays a small temperature dependence. The value of the distortion angle can be obtained from the ratio ul/u2and it is given by y z3 O.On the other hand, in the cases of 10-NDOU and 2-NDOU(A) the values of a, and u2 cannot be obtained by using eqn. (lo), which takes into account a static distortion. As the large temperature variations of a, and u2 suggests, they are average values due to restricted rotation of the methylene group around the C,-C, b~nd.'~,'~It can be assumed that the dihedral angles vary between the two extrema 8, +a. The size of the average distortion angle y can be obtained by plotting a, us. u2 in the temperature range explored. In fact, it has been shown that the following relation hold^:'^,'^ a1 =ma, + 21 Gi? c 1-m1 (11) where cos(60"+27) f(y) = cos(60"-27) The values of the distortion angles have been obtained by a least-squares fit and they are y =25 O and y =42 O for 10-NDOU and 2-NDOU(A), respectively.The values of the fluctuation amplitudes at room temperature are a=4O0 for the former and a =50 O for the latter. From the above discussion we may conclude that the two species detected in the EPR spectra of 2-NDOU and NDAU are related to the two allowed arrangements of the guest molecules inside the channels (see Fig. 1). Species B of 2-NDOU and nearly all the molecules of NDAU are arranged head-to-head. In this case, the molecular pair forms a tight unit because of intra-pair hydrogen bonding. The conformation of the chain in the proximity of the molecular head, as tested by the p proton hf coupling, is quite rigid and nearly planar.In the case of the ketone this arrangement can give rise to a weak hydrogen bond between the CH3 protons of one molecule and the C=O group of the next one; on the other hand, the opposite arrangement allows the carbonyl to form stronger hydrogen bonds with the NH, groups of the urea molecules. In the NDAU case the head-to-head arrangement allows the formation of strong hydrogen bonds between the two facing COOH groups, and therefore this arrangement prevails neatly. Species A of 2-NDOU (and presumably the barely detectable second species of NDAU) are disposed in the head-to-tail arrangement. In this case, the molecular chain undergoes large oscillatory motions. If the oscillation was symmetrical about the planar zigzag conformation, the two hf coupling constants should be equal.The difference between their values indicates that the chirality of the host channel induces a chiral distortion J. Muter. Chem., 1996, 6(lo), 1723-1729 1727 in the guest molecule. It is well known that the presence of a chiral carbon atom in a radical induces the j3 protons to be inequivalent because of the different energies associated with two specular conformation^.^^ Therefore we can conclude that the formation of hydrogen bonds between the urea molecules and the guest molecule promotes a helix arrangement of the guest chain. Molecular dynamics calculations are in progress to check these conclusions.24 The temperature dependence of the proton hf couplings of the included radicals is a sensitive detector of the hexagonal to orthorhombic phase transition.In the case of the ketone radicals, the p proton hf coupling constants are discontinuous at ca. 160 K for 10-NDOU and at ca. 185 K for 2-NDOU. This variation should be related to the changes occurring in the host matrix structure, which affects the amplitude of the distortion of the CH2 group. Therefore, the above temperatures should correspond to the phase transitions for the two UICs. On the other hand, in the case of NDAU the p proton hf couplings have a small, continuous variation with temperature, while the a proton hf coupling has a significant variation in the range 240-210 K, with a maximum slope at 213+3 K, as displayed in Fig.7. We suggest that the last value corresponds to the transition temperature for NDAU. The higher value of T, for the acid UIC with respect to the ketone UICs is a confirmation of the dimerization of the guest molecules in the former case. Also the difference between the two ketone UIC transition temperatures should be related to the possibility for 2-NDOU of arranging head-to-head inside the urea channels, originating a loose dimer via hydrogen bonding. Since this disposition is adopted by a fraction of the guest molecules in the case of 2-NDOU, and by nearly all the molecules for NDAU, the transition temperature for 2-NDOU is intermedi- ate between those of 10-NDOU and NDAU. The value of the isotropic hfcc a, gives insight into the conformation of the CH, with respect to the carbonyl group.In fact, the Fermi contact coupling of the a proton is pro- portional to unpaired spin density on the carbon atom pa,22 If CH, and CO lie on the same plane, the spin density is partially delocalized over the carbonyl and pa is reduced. Therefore, the lower the value of a,, the greater the deviation of the CH,CO group from planarity. For NDAU, this deviation increases in the range 230-210 K and the carboxylic groups are expected to be more distorted in the low-temperature orthorhombic phase than in the hexagonal phase. Dynamics The activation energy values measured for the linewidth coefficients B, and E,,, have been reported in Table2. They are related to the energy barrier A, hindering the molecular rotation, and d, hindering the intramolecular conformational motion.By comparing the values for the two ketone UICs it is worth noting that the values of A, are quite comparable, while A, differs by more than a factor of two. The two radicals have equal lengths and similar interactions with the host matrix. Therefore, the difference should be related to the position of the radical centre (which is adjacent to the carbonyl group) within the molecule. The CH, group of 2-NDOU(A) can explore a larger set of orientations because of the higher probability of gauche defects at the end of the chain. In fact, there is much evidence that the motions of long alkyl chains are quite different in the middle of the chain and at its extremes.In particular, NMR,25 Raman26 and molecular dynami~s~~,~~ results have shown that in n-alkane UICs the gauche defects may be localized only at the ends of :he chains. The molecular rotation is therefore driven by the same conformational interconversion, and the energy barriers hindering the two motions are therefore very 1728 J. Muter. Chew., 1996, 6(lo), 1723-1729 similar. Further evidence of a larger freedom of motions for 2- NDOU is obtained by electron spin-echo experiments which show a dramatic reduction in the electron spin phase memory time in this case with respect to 10-NDOU.29 The values of the correlation times for the rotational and conformational motions can be obtained from the linewidth coefficients B, and E,,,, respectively, when the anisotropies of the relevant magnetic interactions are known.As long as the rotational motion is concerned, we have no direct knowledge of the x and y components of the A, and g tensors, which must be inserted into eqn. (5). However, we can obtain a reliable estimate of their values. It is known that the x, y, z components of the a proton hf tensor in a CH fragment have the relative values 1: 2 : 3.22 Therefore, from the values of A,,= A,, and Al = (Axx+ A,,)/2 in Table 1 we obtain A,, M 10 G and A,, ~20G. In an analogous manner, from the value g, = (gXx+g,,)/2in Table 1 and by assuming that component along the IT orbital is equal to the free electron value, g,,=2.0023, we have g,, M 2.006.In conclusion, the anisotropies A,, -A,, and gxx-gyy for 2-NDOU are very similar to those of 10- NDOU we reported previously,15 as is to be expected because of the similarity of the paramagnetic probes. Therefore, the ratio between the rotational correlation times for the two ketones is nearly equal to the ratio between the values of the linewid th coefficients Z, ( 1 0-NDOU) B, ( 10-NDOU)N q(2-NDOU) -Ba(2-NDOU) At T=240 K we obtained z, (10-NDOU)=0.85 ns,” and we have, therefore, q(2-NDOU) = 3.5 ns. This value corresponds to a rotational frequency which is of the order of magnitude of the interaction anisotropies that are averaged by the rotational motion, i.e. (A,, -Ayy)/h and ,uBBO(gxx-g,,)/h. Therefore, the Redfield-Freed theory cannot be applied at temperatures below 240 K, as has been discussed previously, and the anomalous behaviour of the B, coefficient of 2-NDOU(A) is explained.The dynamics of NDAU are more complex. In this case the guest molecules in the channel are oriented with their functional groups facing each other, as discussed above. The energy barrier hindering the molecular rotation, A,, is about equal to that of 10-NDOU, in the high-temperature region. At T M 230 K, i.e. about 20 K above the hexagonal to orthorhom- bic phase transition occurring at T,= 213 K, the variation of the rotational rate becomes steeper, and it can be described as an activated process in the temperature range down to T,. The energy barrier for the conformational motion also is very large and quite similar to that for the molecular rotation.In the same temperature range the hfcc a, increases by about 5%. The slowing of the motions in this range is related to the distortion of the dimeric unit formed by the carboxylic groups, which is induced by the phase transition to the low-temperature ordered phase. A similar trend has been found previously by monitoring the Raman spectra of the stearic acid UIC.8 The transition temperature for this compound is ca. 234 K and the bandwidth of the CH2 scissoring mode displays a very large variation in the range 220-250 K, while it varies slowly outside this range. Similar results have been recovered for icosan-1-01 UIC and icosanoic acid UIC.’ We conclude, therefore, that the onset of orientational order at the hexagonal to orthorhom- bic phase transition occurs together with the pretransitional effects which are monitored by the progressive distortion of the carboxylic groups.Conclusion The EPR spectra of the radicals produced by y-irradiation of unsymmetrical molecules included into the urea channels show that head-to-head and head-to-tail arrangements are possible. While the former is nearly totally dominant in the case of NDAU, both situations are present in the case of 2-NDOU. The EPR signal of head-to-tail arranged 2-NDOU is far more intense than that of the head-to-head case. This result should be compared with those obtained by solid-state NMR on 6 7 8 F. Laves, N. Nicolaides and K.C. Peng, Z. KristaIlogr., 1965, 121,258. Y. Chatani, H. Anraku and Y. Taki, Mol. Cryst. Liq. Cryst., 1978, 48,219. H. L. Casal, H. G. Camerun, E. C. Kelusky and A. P. Tulloch, J. Chem. Phys., 1984,81,4322. shorter alkanone guests (decan-2-one7 undecan-2-one and dodecan-2-one) in UIC.30This NMR study suggests that the guests are arranged randomly within the channels. Two poss- ible explanations can be put forward: (a)the degree of ordering of parallel or antiparallel dispositions is a function of chain length, so that parallel ordering is preferred for longer guest 9 10 11 12 H. L. Casal, J. Phys. Chem., 1985,89,4799. K. D. M. Harris, S. P. Smart and M. D. Hollingsworth, J. Chem. SOC., Faraday Trans., 1991,87,3423. M. D. Hollingsworth and C. R. Goss, Mol. Cryst.Liq. Cryst., 1992, 219,43. M. E. Brown and M. D. Hollingsworth, Nature (London), 1995, 376,323. ketones; (b) the radical yield for irradiated ketone/urea inclusion compounds is lower when the ketone molecules are disposed head-to-head. These two hypothetical mechanisms obviously are not alternative, but they could be both operative in determining the relative spectral intensities of parallel and 13 14 15 16 M. Brustolon and U. Segre, J. Chim. Phys., 1994,91,1820. 0.H. Griffith, J. Chem. Phys., 1964,41,1093; 1965,42,2644. U. Segre, M. Brustolon, A. L. Maniero and F. Bonon, J. Phys. Chem., 1993,97,2904. F. Bonon, M. Brustolon, A. L. Maniero and U. Segre, Appl. Magn. Reson., 1992,3, 779. antiparallel arrangements. More experiments, and comparison between the results obtained by means of different techniques on the same inclusion compound will be necessary to clarify this important point.17 18 19 20 M. S. Greenfield, R. L. Vold and R. R. Vold, J. Chem. Phys., 1985, 83, 1440, A. G. Redfield, Adv. Magn. Reson., 1966, 1, 1. J. H. Freed and G. K. Fraenkel, J. Chem. Phys., 1963,39,326. J. H. Freed, in Spin Labeling: Theory and Applications, ed. L. J. Berliner, Academic Press, 1976,vol. 1. This work has been supported by CSSMRE, C.N.R., Padova, and by MURST, Rome. Dr. P. G. Fuochi of FRAE, C.N.R., Bologna (Italy) is acknowledged for irradiating the compounds. We are grateful to Professor M. D. Hollingsworth for informing us of his results before publication. 21 22 23 24 25 M. Barzaghi and M. Simonetta, J. Magn. Reson., 1983,51, 175. N. M. Atherton, Electron Spin Resonance, Ellis Horwood, Chichester, 1973. P. Smith, W. H. Donovan, C. E. Mader, L. M. Dominguez and W. T. Koscielniak, Magn. Reson. Chem., 1995,33, 395. P. G. De Benedetti, C. Menziani and U. Segre, work in progress. F. Imashiro, D. Kuwahara, T. Nakai and T. Terao, J. Chem. Phys., 1989,90,3356. References 26 A. El Baghdadi and F. Guillaume, J. Raman Spectrosc., 1995, 26, 155. 1 K. D. M. Harris, J. Solid State Chem., 1993,106, 83. 2 A. E. Smith, Acta Crystallogr., 1952,5,224. 27 K. J. Lee, W. L. Mattice and R. G. Snyder, J. Chem. Phys., 1992, 96,9138. 3 J. Ahmad, A. J. Freeston and A. Hussein, J. Inclusion Phenom. Mol. Recognit. Chem., 1994,18,115. 4 K. Eukao, T. Horiuchi, S. Taki and K. Matsushige, Mol. Cryst. 28 29 30 A. R. George and K. D. M. Harris, J.Muter. Chem., 1994,4,1731. M. Brustolon, unpublished results. M. D. Hollingsworth, personal communication. Liq. Cryst. B, 1990,180,405. 5 R. M. Lynden-Bell, Mol. Phys., 1993,79, 313. Paper 5/05021I; Received 28th July, 1995 J. Muter. Chern., 1996, 6(lo), 1723-1729 1729

ISSN:0959-9428    

DOI:10.1039/JM9960601723

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Magnesium- and iron-doped chromium fluoride/ hydroxyfluoride: synthesis, characterization and catalytic activity B. Adamczyk, A. Hess and E. Kemnitz" Humboldt- Universitat zu Berlin, Institut fur Chemie, Hessische Sty. 1-2, D-10115 Berlin, Germany The calcination of a-CrF, -3H20 results in the formation of a chromium hydroxyfluoride with the pyrochlore structure. The stepwise replacement of chromium by iron and magnesium leads to considerable alterations in the structure and the surface properties of the calcination products, accompanied by significant changes in the catalytic activity. The dismutation of dichlorodifluoromethane and the dehydrochlorination of l,l,l-trichloroethane act as a probe reactions for Lewis acid sites. The syntheses of the catalysts were carried out by coprecipitation of mixed metal fluoride trihydrates and subsequent calcination procedures.The stepwise replacement with iron leads to a rebuilding of the lattice from the cubic pyrochlore structure of CrF3 -,(OH), into the pseudo-hexagonal tungsten bronze (HTB) structure of P-FeF,. The maximum catalytic activity towards CH,CCl, dehydrochlorination was obtained for the 65% iron sample, which is accompanied by a maximum BET surface area and a maximal number of Lewis acid sites. CrF, -,(OH), exhibits a dramatic loss of catalytic activity as well as BET surface area. Possible explanations are given by comparing the pseudo-HTB structure with the cubic pyrochlore structure with regard to the accessibility of the Lewis acid metal cations. Upon substitution of chromium by magnesium we obtained a maximum Lewis acidity for samples with 65-92% magnesium leading to a corresponding maximum in catalytic activity.The Br~nsted acidity of both systems is predominantly weak. Bulk and surface hydroxy groups are distinguished. Various fluorinated aliphatics are produced by heterogeneously catalysed fluorination of the appropriate chloro compounds with HF [eqn. (l)]. Oxides, oxyfluorides and fluorides of Al, Fe and Cr have been reported as suitable fluorination catalysts. They also catalyse isomerization reactions [eqn. (2)] and dismutation reactions [eqn. (3)]. R-C1+ HF-+R-F +HCl (1) CHF, -CHF, +CF3 -CH2F 2 CC12F2 +CCl,F +CClF, (3) Previous studies have shown that catalyst activation is neces- sary when starting from an oxide During the activation period a chemical reaction between the solid surface and the haloalkane and/or HF takes place. In the case of y-Al,03, phases are formed which are similar to P-AlF, offering a particular catalytic For this reason the catalytic behaviour of p-metal fluorides was examined.First, it was found that active P-AlF, and P-CrF, cannot be synthesized via the same route. A further important factor is that the irreversible phase transformations into the stable a-phases must be avoided since the a-fluorides are catalytically less active or inactive. As is widely reported for oxides, partial substitution of the metal cations affects the catalytic In the case of partially replaced fluorides (Al/Mg and Al/Cr) the following results were obtained.A fluoride with a Al/Cr molar ratio of 1 gave the highest conversion with regard to CCl,F, dismu-tation, which can obviously be attributed to the significant enlargement of the specific surface area and the improved accessibility of the Lewis acid sites. Alterations in the strength of the sites were not observed. In contrast, the Mg-doped aluminium fluorides offered a decrease in Lewis acid strength with increasing Mg content, although the specific surface area had increased. Consequently, the catalytic activity was diminished with increasing Mg con- tent. MgF, was catalytically inactive. There is no uniform route to the synthesis of the p-metal fluorides. While P-AlF, can be obtained by thermal degradation of a-AlF3.3H,O under a self-produced atmosphere,8 P-CrF, is not formed via this route.Instead, a chromium hydroxyfluoride having a pyrochlore structure was formed under these conditions. This product is catalytically inactive, whereas P-CrF, synthesized by thermal decomposition of (NH4)3CrF6 exhibits catalytic activity with regard to CCl,F, dismutation. The different catalytic behaviour was recently discussed on the basis of chemical and structural differences between the hexagonal tungsten bronze (HTB) p-metal fluorides and the pyrochlore hydroxyfluorides.' The present work deals with the systems Fe/Cr and Mg/Cr. Experimenta1 Preparation of catalysts A defined mass of Fe(NO,), 9H20 [or Mg( NO,), * 6H20] and/or Cr(N03), 9H20 was dissolved in ethanol.The mixture was added dropwise to a stirred 40 mass% hydrofluoric acid solution. The precipitate (a-M,M,F, 3H20) was separated, washed with small amounts of water and ethanol and dried in air. Then, the sample powder was covered by an aluminium foil to allow a self-produced atmosphere and was subsequently heated under an argon flow (2 K min-', up to 300 "C for mixed Fe/Cr and 410°C for Mg/Cr compounds). The appro- priate final temperature was maintained for 2 h. All samples were characterized by X-ray powder diffraction (XRD 7 Seiffert-FPM, Cu-Ka radiation). The determination of the fluoride contents was carried out according to ref. 10. The standard deviation was <3%. For analysis of the metal contents, 10-20 mg of each sample was melted with an excess of KNO, (500 mg) in a Pt beaker and dissolved in concentrated H3PO4 (Fe samples) or 50 mass% H,S04 (Mg samples), respectively. The concentration was determined by ICP AES (50 mg l-l, Unicam 701).The relative experimental error did not exceed 2%. The determination of the specific surface area was performed using an ASAP 2000 (Micrometrics) instrument based on the nitrogen BET method (max. exptl. error & l?h). FTIR photoacoustic spectroscopy of chemisorbed pyridine This method was used for determining the nature of the acid sites (Lewis/Br~nsted) by adsorption of pyridine on the catalyst J. Muter. Chem., 1996, 6(lo), 1731-1735 1731 surface and subsequent detection of the characteristic IR bands.This method is sensitive to both Lewis and Brarnsted acid sites, although the sensitivity decreases rapidly with decreasing surface areas. The whole procedure was carried out under the same conditions as described previously." Determination of catalytic activity Dismutation. The dismutation of CC12F2 [eqn. (3)] proceeds on Lewis acid site^."^,^ The products undergo further dismu- tation reactions. In detail, a constant mass of 0.6 g of the catalyst was used in a flow reactor (nickel tube). A residence time of 2 s was set up by adjusting the appropriate gas flow. First, the sample was calcined under a nitrogen flow at 400 "C for 1 h and then subsequently treated with a CC12Fz flow at 390°C. The composition of the gas phase at the exit of the reactor was determined by gas chromatography (column: Poraplot u; i.d.0.53 mm; length 25 m). The conversion of the starting substance CCl,F, is given with an absolute experimen- tal error which did not exceed +0.5% conversion. Dehydrochlorinationof l,l,l-trichloroethane. Since P-FeF, is unstable at the dismutation temperature (phase transformation into the a-modification) it was necessary to introduce an additional probe reaction that is sensitive to Lewis acid sites. The dehydrochlorination of l,l,l-trichloroethane [eqn. (4)] is reported to depend on Lewis acid centresl29l3 and has the advantage of running at lower temperatures. C1,C-CH3 +C12C=CH2 +HCl (4) Ballinger and co-w~rkers~~'~~ employed this method in order to investigate the dehydrochlorination activity of y-alumina above 130°C.Of all the samples studied only P-CrF, as a standard material [formed by thermal decomposition of (NH,),CrF,] exhibits remarkable activity for dehydrochlori- nation reactions below 130 "C. Therefore, for better compari- sons all measurements were carried out above 130°C for all samples. In detail, 200-300mg of the catalyst (pore diameter 0.2-0.5 mm) were fed into a flow reactor and calcined at 200 "C under dry nitrogen for 2 h. The residence time (t, =1.7 s) was adjusted by controlling the flow rate using a mass flow controller (MKS) according to the height of catalyst packing. Then, a constant stream of nitrogen was allowed to flow through a thermostatted bottle with l,l,l-trichloroethane.All tubings were also thermostatted to avoid any condensation. The reactor temperature was 130°C. The conversion was followed by on-line GC (Shimadzu GC14a; column: Porapack Q, 2m) equipped with a thermostatted gas sampling valve. The experimental error did not exceed +5%. Double-bond isomerisation of but-l-ene. This probe reaction depends on the presence of Brarnsted acid Under the selected conditions (low temperature) (E/Z)-but-2-ene is the only product. The apparatus is similar as described above. But-l-ene (1 ml min-l) was mixed with the adjusted nitrogen flow. The reaction was carried out at 100 "C. After 4 min on- stream a gaseous sample was injected into the GC (MGC 4000; column: Poraplot U; 30m).Results Fe/Cr samples X-Ray diffraction and wet analysis. With pure a-FeF, 3H20, the HTB P-FeF, is obtained after calcination at 300°C. Temperatures >400 "C lead to the irreversible formation of a-FeF,. In contrast, if a-CrF, 3H20is calcined under compar- able conditions, a pyrochlore chromium hydroxyfluoride phase is formed that is stable even above 400°C. Owing to the similarity of the ionic radii [Fe 0.65 A;Cr 0.62 A (ref. 17)] the formation of solid solutions should be expected. In fact, depending on the Fe concentration there are two ranges (see Fig. 1): a concentration range from 100 to 65% Fe with the HTB structure and a second range from 41 to 0% Fe with the pyrochlore structure. As can be concluded from Table 1 the HTB range consists of metal trifluorides, whereas the pyrochlore range comprises hydroxyfluorides.It can be seen from Fig. 1 that Fe and Cr replace each other within the appropriate structures although the shift of the lattice constant is of the same order of magnitude as the experimental error. Nevertheless, particularly in the case of the pyrochlore phases a shift towards higher angles (lower d values, lower lattice constants) is observed with increasing Cr content. Besides, there is a certain amount of X-ray amorphous species present, particularly at ca. 50% Fe. The higher the Cr content, the higher the tendency to hydrolysis as described by eqn. (5). a-CrF, ~3H20-,Cr(OH),F,-x+xHF+(3-x)H20 (5) With increasing insertion of Fe into the chromium hydroxyfluo- ride lattice, the amount of bulk OH groups diminishes.Surface acidity. Fig. 2 shows the FTIR photoacoustic spectra of pyridine chemisorbed on the solid surfaces. The band at approximately 1450 cm-' can be assigned to coordinatively bonded pyridine (Lewis acid sites), whereas the band at about 1493 cm-' represents both Lewis and Brarnsted acid sites." If Fig. 1 XRD patterns (offset) of a-CrF, .3H20 calcination products with stepwise replacement by Fe. Note the change from the pyrochlore structure of CrF,-,(OH), into the HTB J3-FeF, structure with increasing Fe content. Table 1 Atomic compositions and BET specific surface areas specific surface area/ sample compositiona m2 g-' Cr 100/1 CrFO 39(OH)261 9 Fe 16 CrO 84Fe0 16F2 5 (OH)O 5 11 CrO 77Fe0 23F1 84(OH)116 13.5 CrO 59Fe0 41F2 95(OH)005 17 2 CrO 35Fe0 6SF3 38 9 Fe 90 Cro 1Feo 9F3 34.5 Fe 100 FeF, 24 Cr lOO/II CrFO 41(OH)259 9.5 Mg 26 CrO 74Mg026F1 42(OH)132 15 2 Mg 46 CrO 54Mg0 46F1 86(OH)068 17 5 Mg 56 CrO 44Mg0 56F2 ,,(OH), 32 31.1 Mg 65 CrO 35Mg065F2 06(OH)0 27 49.4 Mg 86 CrO 1dMg08GF1 95(OH)0 19 77.8 Mg 92 CrO OsMgO 9zF1 97(OH)0 11 78 Mg 100 MgF2 23 2 ~~ ~ ~ ~ a Atomic composition determined by ICP AES and by See1 lo 1732 J.Muter. Chern., 1996, 6(lo), 1731-1735 Cr l00fl Fc I6 Fe23 1493 1452 wavenumber/cm -' Fig. 2 FTIR photoacoustic spectra (offset) of pyridine chemisorbed on Fe-replaced a-CrF, *3H,O calcination products (background correction, 80 mg catalyst mass, 30 p1 pyridine adsorption at 150 "C, flow system) Brsnsted acid sites are absent then the intensity of the 1493 cm-' vibration is about one third of that of the 1450 cm-' band.Changes in the strength of the sites would cause a wavenumber shift (compare Mg/Cr system, sample Mg100). Here, this is not observed. As can be seen from Fig. 2, the intensity of the 1450 cm-I band reaches its maximum at the Fe65 sample and it decreases with further increases of the Cr content. This graduation is also observed for the specific surface areas (Table 1). Furthermore, one can state that the comparatively low BET areas correspond to the pyrochlore samples (small number of Lewis acid sites), whereas the HTB structure enables improved access to the Lewis acid sites.An enhanced number of Lewis acid sites with increasing Cr content have been observed for the HTB phases. This can be explained by a steady loss of crystallinity, accompanied by an increase of BET surface area, which improves the accessibility of the Lewis acid sites (metal cations). The strength of the sites is unaffected. The Brsnsted acidity in the Fe/Cr system is altogether very weak; a further graduation shows that the acidities of the samples Fe41 and Fe65 slightly exceed those of the others, but they remain weak. Hence, it can be concluded that the content of bulk OH groups does not correlate with the obtained Brsnsted acidity. Therefore, we assume that the large number of OH functions in the pyrochlore chromium hydroxyfluoride CrlOO is a fixed part of the bulk and does not act as a Brsnsted acid.Catalytic activity. If P-FeF, is present in the system then a maximum temperature of about 350 "C must not be exceeded (formation of the cr-phase). Therefore, the dismutation could not be applied as a probe reaction for Lewis acid sites. Hence, the dehydrochlorination of l,l,l-trichloroethane was used, which operates at lower temperature (see Experimental). Fig. 3 illustrates the obtained absolute and specific conversions (related to the surface area). There are considerable differences in the conversion depending on the crystal structure. The graduation corresponds with the appropriate number of Lewis acid sites indicated by the intensity of the 1450cm-' band in Fig.2. HTB phases offer a higher activity for Lewis acid-catalysed 2 absoluteconversion Y BET nonnalircd conversion 80 Pyrochlon phases Fe (mol%) FeF, Fig. 3 Conversion of l,l,l-trichloroethane us. Fe content fc)r various Fe-replaced a-CrF, *3H20 calcination products (catalyst mass = 300 mg, residence time =1.7s, 130"C) reactions than the pyrochlore phases. According to Fig. 2 the Lewis acidity of the pyrochlore samples is weak. Thr, double- bond isomerization of but-1-ene reveals very low coilversions (<3%). This is in good agreement with the results obtained from pyridine adsorption results (Fig. 2) which found very weak Brsnsted acidities only. Mg/Cr samples X-Ray diffraction and wet analysis.The thermal decompo- sition of the Mg/Cr precipitates results predominantly in the formation of amorphous products and MgF,, whereas the intensities of the latter diminish with increasing Cr content (Fig. 4). Samples Mg26 and Cr100/II exhibit very small reflec- tions which can be assigned to the pyrochlore chromium hydroxyfluoride. The higher calcination temperature possibly leads to the reduced crystallinity compared to thr, sample Cr100/I in the Fe/Cr system. The difference in the ionic radii [Mg 0.72 A; C c 0.62 A (ref. 17)] would be large enough to cause a detectablt shift of the lattice constant. As can be seen in Fig. 4 there is no shift of reflections with increasing dopant concentration f>x either MgF, or chromium hydroxyfluoride.A mutual substitution within the lattices does not take place. Instead, the appropriate dopant causes decreases in crystallinity of the host lattices. According to Table 1, one can state that the OH'F ratio increases with rising Cr content. -_ \ 0 .-3-$ a P A A - /c Mg 86 Mg 65 Mg 56 VIC Cg!-.-d Mg46 -Mg26 /I.,.).,.,.,.,,,.,. 5 10 IS 20 15 30 35 40 45 {O . 5'5 Cr ioom.-$0 28ldegrees Fig. 4 XRD patterns (offset) of rx-CrF, *3H,O calcination products with stepwise replacement by Mg. Note the change from the pyrochlore structure of CrF,-,(OH), (weak reflections) into the MgF, structure with increasing Mg content. J. Mater. Chem., 1996, 6(lo), 1731-1735 1733 Surface acidity. Fig. 5 shows the FTIR photoacoustic spectra of pyridine chemisorbed on the solid surfaces.Starting from Cr100/II, the number of Lewis acid sites increases with increas- ing Mg content, reaches its maximum at sample Mg86 and decreases to a comparatively low value at Mg100. This is accompanied by the formation of a distinct shoulder at lower wavenumbers (Mg92) at the position where MglOO exhibits the corresponding band. This can be explained by an attenu- ation of Lewis acid strength due to the lower positive charge of the Mg2+cations. The maximum number of Lewis acid sites is in agreement with the maximum of BET surface area (Table 1). It is remark- able that only 8% Cr in an MgF, environment (Mg92) leads to a dramatic change in the acidic properties (Fig.5). The Brernsted acidity behaves as follows. Samples Mg92, Mg86 and Mg65 exhibit a weak band at approximately 1540 cm-' that can be assigned to hydrogen-bonded pyridine (weak Brernsted sites). As can be concluded from the low intensity (Fig. 5), a catalytic activity based on Brmsted acid sites is not expected. Catalytic activity. Fig. 6 presents the CCl,F, conversion (Lewis acid probe reaction) as a function of the Mg content. The graduation corresponds to the number of Lewis acid sites derived from Fig. 5 with the exception of MgF,. In that case, the conversion is almost zero although there are a certain number of Lewis acid sites. The reason is that MgF, possesses significantly weaker Lewis acid sites, illustrated by the shift towards lower wavenumbers in Fig, 6.Obviously, their strength is no longer sufficient to catalyse the dismutation. As can be concluded from the surface-normalized conver- sions (Fig. 6) the Mg dopant causes an enlargement of the surface area whereas the specific conversion is not increased with increasing Mg content. Regarding the Brernsted probe reaction, the following results were obtained. The conversion of but-1-ene was generally very low, but samples Mg92, Mg86 and Mg65 were slightly more active (2-6%). This can be confirmed by the results of pyridine complexes, where very weak Brernsted acidity was found at these samples. Obviously, the bulk OH content (Table 1) does not correlate with the Brernsted acidity, otherwise it would increase with increasing Cr content.LPy+BPy LPy Mg 100 Mg 92 Mg86 Mg 65 Mg 56 Mg 46 Mg 26 Cr 100/11 *1.1.1.1.1.1.1 1540 1520 1500 1480 1460 1440 1420 Fig. 5 FTIR photoacoustic spectra (offset) of pyridine chemisorbed on Mg-replaced a-CrF, 3H20 calcination products (background correction, 80 mg catalyst mass, 30 pl pyridine adsorption at 150"C, flow system) 1734 J. Muter. Chem., 1996, 6(lo), 1731-1735 VI 70 1 absolute conversion=BETnormalizedconversion 4 9 60 50 h8 v C040.-r9 30 C8 20 10 0 Mg (mol%) Fig. 6 Conversion of CC12F2 us. Mg content for various Mg-replaced a-CrF, -3H20 calcination products (catalyst mass = 600 mg, residence time =2 s, 390 "C) Conclusions In the case of the Fe/Cr system significant differences in the catalytic activity can be attributed to the following factors.First, two different structures were obtained and secondly, different compositions were found. Metal trifluorides were formed by the HTB phases, whereas metal hydroxyfluorides predominated in the case of the pyrochlore structures. The latter exhibit a weak Lewis acidity, the HTB phases offer improved Lewis acidities which correspond to enhanced cata- lytic activities with regard to Lewis probe reactions. The catalytically less-active pyrochlore phases consist of M( F,OH), octahedra. Their special linkage results in the formation of hexagonal channels which pass along all six plane diagonals of the cubic unit cell.9 A cleavage of this low-density structure should cause a large number of Lewis sites to be attainable.According to our results this is not the case. We believe that the hydroxy groups which are present in all pyrochlore phases hinder the free access to the metal cations. The presence of hydroxy groups within the pyrochlore lattice enables the formation of hydrogen bridge bonds on the surface which is accompanied by a partial shielding of the Lewis acid sites. The OH groups do not act as Brmsted acid sites. HTB metal trifluorides which also contain hexagonal channels obvi- ously enable an enhanced access to the metal cations due to the absence of OH. Generally, the availability of the sites is improved if the crystallinity is lowered, e.g. when a certain amount of a dopant is introduced.This can also be monitored by increasing BET surface areas. Concerning the Mg/Cr system the catalytic activity was influenced mainly by the strength of the Lewis acid sites. This can be illustrated, e.g., by the samples MglOO (MgF,) and Mg92. MgF, possesses Lewis acid sites that are not strong enough to catalyse the Lewis probe reaction. In contrast, the introduction of 8 mol% Cr results in a significant enhancement of the Lewis acid strength, and the catalyst is now active. With further increases in the Cr content the specific conversion also rises until the pyrochlore lattice has been formed. The latter is again inactive for the reasons described earlier. Brernsted acidity, unsurprisingly, does not play an important role in systems containing Mg.We are grateful to the Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie for financial support. References 10 F. Seel, Angew. Chem., 1964,76,532. 11 A. Hess and E. Kemnitz, J. Catal., 1994,149,449. A. Hess and E. Kemnitz, J. Catal., 1994,148,270. 12 J. Thomson, G. Webb and J. M. Winfield, J. Chem. SOC., Chem. E. Kemnitz and A. Hess, J.Prakt. Chem., 1992,334, 591. Commun., 1991,323. L. Kolditz and G. Kauschka, 2.Anorg. Allg. Chem., 1977,434,41. 13 J. Thomson, G. Webb and J. M. Winfield, J. Mol. Catal., 1991, L. Kolditz, U. Calov, G. Kauschka and W. Schmidt, 2. Anorg. 68,347. 5 6 7 Allg. Chem., 1977,434, 55. L. E. Manzer, Eur. Pat., 331991,1989,to E. I. Du Pont de Nemours and Co. L. E. Manzer, US Pat., 4766259,1988,to E. I. Du Pont de Nemours and Co. S. Hirayama, PCT Int. Appl., WO 8910341, 1989, to Showa 14 15 16 17 T. H. Ballinger and J. T. Yates, Jr., J. Phys. Chem., 1992,%, 1417. T. H. Ballinger, R. S. Smith, S. D. Colson and J. T. Yates, Jr., Langmuir, 1992,8,2473. S. E. Tung and E. McIninch, J. Catal., 1964,3,229. A. F. Wells, Structural Inorganic Chemistry, Clarendon Press, Oxford, 1993. Denko K. K. 8 9 D. H. Menz and U. Bentrup, Z. Anorg. Allg. Chem., 1989,576,186. E. Kemnitz, A. Hess, G. Rother and S. Troyanov, J. Catal., 1996, Paper 6/03296F; Received 13th May, 1996 159, 332. J. Mater. Chern., 1996,6(lo), 1731-1735 1735

ISSN:0959-9428    

DOI:10.1039/JM9960601731

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

MATERIALS CHEMISTRY COMMUNICATIONS Mesostructured lamellar chromium oxide S. Ayyappan, N. Ulagappan and C. N. R. Rao* CSIR Centre of Excellence in Chemistry and Materials Research Centre, Indian Institute of Science, Bangalore 560 012, India and Jawaharlal Nehru Centre For Advanced Scientijic Research, Jakkur, Bangalore 560 064, India Mesostructured l?mellar chromium oxide with an interlayer separation of 29 A has been prepared by employing a cationic surfactant. The introduction of phosphat? groups in the oxide increases the interlayer separation to 32 A. Mesoporous silica is known to occur with hexagonal, cubic and lamellar structures, although much of the work in the last three years has been on hexagonal phases.' There have been some efforts, however, to synthesize and characterize other types of mesoporous phases, in particular lamellar ones.Thus, Huo et aL2 have shown that lamellar metal oxides can be prepared by employing sodium alkyl sulfates (C,H,,+,OSO,Na; n=12, 14, 16, 18) and alkyl phosphates such as C1,H2,0P0,H2 as the templates, while Ciesla et aL3 have synthesized mesostructured lamellar molybdenum oxide using alkylammonium chlorides (C,H2,+ ,NMe,Cl; n = 12, 14, 16, 18) as the templates. Oliver et aL4 have obtained unusual lamellar aluminophosphates exhibiting surface patterns mim- icking diatom and radiolarion microskeletons by using decyl- ammonium dihydrogenphosphate. Sayari et aL5 have recently prepared lamellar aluminophosphates using neutral dodecyl- amine as the surfactant.In this communication, we report the successful synthesis of lamellar mesophases of chromium oxide by using cetyltrimethylammonium bromide (CTAB) as the surfac tant. In order to prepare mesostructured chromium oxide, we started with the molar composition CrO, :0.27CTAB: 14H20: 6C2H,0H, based on our experience with silica mesophases. In a typical synthesis, a solution of CTAB in aqueous ethanol was added to an aqueous solution of Cr03 and stirred for 30 min. To this solution ammonia was added to bring the pH to 6. The precipitate obtained was aged for 2 days, filtered, washed several times with water and dried at 373 K in a hot air-oven. Thermogravimetry showed the molar ratio of the oxide to the template to be 1: 1 and the adduct contained only the elements of chromium oxide along with the surfactant and water.The IR spectrum showed bands characteristic of the CH2 groups in the surfactant, as well as other vibration modes. The as-synthesized compound was subjected to X-ray diffraction. The X-ray diffraction pattern shown in Fig. l(a) clearly demonstrates the lamellar nature of the product with d-spacings $ue to 001 and other 001 reflections. The dool value of 29.43 A in the mesostructured chromium oxide found hFre is larger than that of the lamellar mesophase of MOO,(24.4 A)., In order to confirm the lamellar nature of the chromium oxide mesophase, we recorded high-resolution transmission electron microscope images using a JEOL JEM3010 microscope oper- ated at 300 kV. A typical image, as presented in Fig.2, reveals an interlayer spacing of ca. 30 A. Removal of the template by both thermal and solution routes destroys the lamellar meso- structure. The diffuse reflectance spectrum of the mesophase was recorded to establish the nature of the chromium species. The spectrum [Fig. 3(a)] of the as-synthesized sample shows a prominant band at 380 nm due to Cr6+; the shoulder around 450nm could be due to Cr3+ specie^.^.^ The EPR spectrum shows a very weak signal at g= 1.94 due to Cr3+. In order to investigate whether the introduction of phosphate 0 0 5 10 15 20 2Bldegrees (Cu-Ka) Fig. 1 X-Ray diffraction patterns of mesostructured lamellar (a)chro-mium oxide and (b)chromium oxide with some phosphate Fig.2 TEM image of mesostructured lamellar chromium oxide J. Muter. Chern., 1996,6( lo), 1737-1738 1737 wavelengthhm Fig. 3 Diffuse reflectance spectra of mesostructured (a) chromium oxide and (b)chromium oxide with some phosphate groups affects the lamellar structure in any way, we prepared mesophases starting with the molar composition Na20 CrO, 0 5P20, 0 2CTAB 150H20 In a typical syn- thesis, phosphoric acid was added to CrO, in an aqueous solution, to which the CTAB solution was then added After stirring for 30 min, the material was transferred to an air-tight polypropylene bottle and heated at 363 K for 2 days The product obtained was filtered, washed several times with water and dried at 373 K in a hot air-oven Analysis showed the phosphorus content to be 4 mol% with respect to chromium Thermogravimetry showed the molar ratio of the ‘chromium phosphate’ to the template to be 3 1 The X-ray diffraction pattern of the phosphate derivative [Fig l(b)], gives a doe, value of 32 13 A, which is higher than the value found for Cr03 alone E1ec:ron microscopy images show an interlayer separation of 32 A Thermal as well as solution treatments to remove the template resulted in the collapse of the mesostruc- ture in this case also The diffuse reflectance spectrum [Fig 3(b)] is similar to that of the chromium oxide-based mesophase discussed earlier S A thanks the CSIR, India, for a fellowship References 1 J S Beck and J C Vartuli, Curr Opin Solid State Muter SCI,1996, 1,76 2 Q Huo, D I Margolese, U Ciesla, D G Demuth, P Feng, T E Gier, P Sieger, A Firouzi, B F Chmelka, F Schuth and G D Stucky, Chem Muter, 1994,6, 1176 3 U Ciesla, D Demuth, R Leon, P Petroff, G D Stucky, K Unger and F Schuth, J Chem SOC,Chem Commun , 1994,1387 4 S Oliver, A Kuperman, N Coombs, A Lough and G A Ozin, Nature (London), 1995,378,47 5 A Sayari,V R Karra, J S Reddy and1 L Moudrakovski, Chem Commun ,1996,411 6 J S T Mambrim, E J S Vichi, H 0 Pastore, C U Davanzo, H Vargas, E Silva and 0 Nakamura, J Chem SOC, Chem Commun, 1991,922 7 N Ulagappan and C N R Rao, Chem Commun, 1996,1047 Communication 6/04395J, Received 24th June, 1996 1738 J Muter Chem, 1996, 6(10), 1737-1738

ISSN:0959-9428    

DOI:10.1039/JM9960601737

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

CORRIGENDUM Phasrnidic phases in macrocyclic liquid crystals Rachel P. Tuffin, Kenneth J. Toyne and John W. Goodby School of Chemistry, University of Hull, Hull, UK HU6 7RX J. Muter. Chem., 1996,6, 1271 The authors of this paper should read: Rachel P. Tuffin, Greg J. Cross, Kenneth J. Toyne and John W. Goodby J. Muter. Chem., 1996,6( lo), 1739 1739

ISSN:0959-9428    

DOI:10.1039/JM9960601739

出版商:RSC    

年代:1996

数据来源: RSC