Intracrystalline self-diffusion of benzene, toluene and xylene isomers in zeolites NaX

Intracrystalline self-diffusion of benzene, toluene and xylene isomers in zeolites N a - X A. Germanus, J. K~irger and H. Pfeifer

Sektion Physik der Karl-Marx-Universitiit Leipzi~ 7010 Leipzi~ Linndstrafle 5, German Democratic Republic and N.N. Samulevi~ and S.P. Zdanov

Grebenscicov Institute, Academy of Sciences of the USSR, 199164 Leningrad, Nab, Makarova 2, USSR The n.m.r, pulsed field gradient technique is applied to measure the intracrystalline self-diffusion coefficients of benzene, toluene and ortho; meta- and para-xylene in N a - X type zeolites with ratios Si/AI=I.2 and 1.8 in dependence on the sorbate concentration and the temperature. The diffusion behaviour observed can be explained by the specific interaction between the ~-electron system of the molecules with the cations. While at higher pore filling factors the mutual interaction of the molecules leads to a significant decrease in their translational mobility at lower concentrations the steric hindrance during the molecular jumps through the windows between adjacent cavities becomes dominant. In this concentration range the values for the self-diffusion coefficients of benzene, toluene and the xylenes are related to each other as 1:0.5:0.2. There is no perceptible difference between the self-diffusion coefficients of the different xylene isomers. Keywords: Nuclear magnetic relaxation; pulsed field gradient technique; zeolite NaX; aromatics; molecular diffusion

INTRODUCTION The activity of a porous catalyst depends on the density and intrinsic activity of its active sites as well as on the transport characteristics of the reactants ~,~. One of the major applications of zeolites in catalysis for which molecular migration was found to be decisive for overall reaction is the aromatic alkylation 3-9. Irrespective of their practical importance there is a remarkable lack of consistency in the relevant diffusion data. For Z S M - 5 , the diffusion coefficients of benzene and of the xylenes as calculated from conventional uptake experiments are found to scatter over an interval of at least three orders of magnitude 5,1°-~3. In the large-pore X type zeolites intracrystalline transport must be. assumed to be relatively rapid which is in accordance with previous n.m.r, studies j4,~5. Hence uptake 16 as well as c h r o m a t o g r a p h i c z measurements of intracrystalline diffusion are most likely masked by secondary processes such as adsorption heat release or intercrystalline diffusion ts-2°. It has been demonstrated in Ref 21 that unambiguous and direct information about the intracrystalline molecular mobility may be obtained by the n.m.r, pulsed field gradient technique 21-24. In the present paper this method has been used to study systematically the self-diffusion of benzene, toluene and xylene isomers adsorbed in X type zeolites.

and 8 #m, respectivelyz5.26. The zeolite material has been activated under shallow-bed conditions: keeping the pressure below 10 Pa, over a period of 48 h the temperature was monotonously elevated from room temperature up to 670 K. At this temperature the samples have been kept in contact with a mercury diffusion pump until the pressure decreased to values less than 0.01 Pa. After cooling to room temperature the adsorption was accomplished through the gas phase by freezing the adsorbate out of a vessel of known volume on the activated zeolite material. T h e amount adsorbed was checked gravimetrically. Afterwards, the loaded zeolite material was transferred under vacuum into 8 mm glass tubes, which were closed hermetically. T h e self-diffusion measurements were conducted using a home-built n.m.r, pulse spectrometer F E G R I S 80 at a proton resonance frequency of 60 M H z . As previously described 21.23, the self-diffusion coefficients were determined by studying the influence of the pulsed field gradients on the intensity of the spin-echo. T h e observation times--typically of the order of 5 m s - - w e r e chosen to be short enough to permit the unequivocal determination of the intracrystalline mobility ~1,~3,24. Even under the most unfavourable conditions (low sorbate concentrations and small self-diffusion coefficients) the mean error of the data presented did not exceed 50%. T h e relative error of self-diffusion coefficients obtained under identical experimental conditions is less than 10%.

EXPERIMENTAL Molecular self-diffusion has been studied in two different samples ( N a - X (1.2) and N a - X (1.8)) of laboratory-synthesized N a - X zeolites with Si/A1 ratios of 1.2 and 1.8 and mean crystallite diameters of 50 #m 0 1 4 4 - 2 4 4 9 / 8 5 / 0 2 0 0 9 1 - 0 5 $03.00 © 1985Butterworth&Co. (Publishers)Ltd.

RESULTS AND DISCUSSION

Figure 1 shows the concentration dependence of the selfdiffusion coefficients of benzene, toluene and the xylene isomers in N a - X (1.2).

ZEOLITES, 1985, Vo15, March 91

Molecular diffusion in N a - X : H. Pfeifer et al.

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and xylene 28 respectively. The decrease of the sorbate concentration with increasing temperature (cf. legend to Figure 2) is not considered. Taking into account this effect would lead to an even steeper decay in the high-concentration region

The steep decrease with increasing sorbate concentration for high pore filling factors may be attributed to the mutual hindrance of the molecules. The diminution of the molecular free volume leads to reduced mean j u m p lengths and hence to smaller selfdiffusion coefficients29. As might be anticipated on the basis of this model, there is no significant difference between the different aromatic compounds. In contrast to the saturated hydrocarbons, where the self-diffusion coefficients decrease over the whole concentration range 29, for small and medium concentrations of aromatic compounds there is only a minor influence of the amount adsorbed. In this concentration range, selfdiffusion is evidently determined by the interaction of the aromatic compounds with the adsorbent rather than by mutual encountera. O n the basis of calorimetrica° and nuclear magnetic relaxation 24 measurements it could be shown (see below) that the specific interaction between the a'-electrons of the unsaturated hydrocarbons and the sodium cations leads to the formation of surface complexes and hence it seems reasonable that for sufficiently small concentrations the dissociation of these compexes becomes the rate determining step in diffusion. Since the mean j u m p length of the free molecules depends on the steric interaction between the zeolite framework and the molecules, the observed dependence of the selfdiffusion coefficients on the nature of the molecules is in agreement with this model. Comparing the free apertures of the windows in the intracrystalline pore

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system ( = 0.75 nm) with the gas kinetic diameters of the molecules (in the van-der-Waals approximation one obtains about 0.46 nm, 0.49 nm and 0.53 nm for benzene, toluene and xylene, respectively3') the observed sequence D(benzene) > D(toluene) > D(xylene) may be clearly anticipated. It seems noteworthy to point out that within the limits of accuracy of our measurements there is no difference in the translational mobility of the xylene isomers, neither in the medium nor in the high concentration range. This finding is in contrast to the result for zeolites Z S M - 5 where significant differences in the mobility of the different xylene isomers have been observed 5,j°. In this latter case, however, the channel diameters are very close to the critical diameters of the xylene molecules. Figures 2a-d show the temperature dependence of the self-diffusion coefficients. According to the proposed model of molecular diffusion the activation energy measured at low concentrations should be comparable to the interaction energy between the a'-electron system of the molecules and the sodium cations. In agreement with this supposition, from the slope of the Arrhenius plots in Figures 2a-d one obtains for all molecules in N a - X (1.2) nearly the same activation energy between about 20 kJ mol-I at medium pore filling factors and about 25 kJ mo1-1 at low pore filling factors. Comparing our self-diffusion data for benzene and toluene with those determined in Ref 16 by conventional uptake experiments, a remarkable coincidence in the relative values of the self-diffusion coefficients and in the values of the activation energies is stated. However, there is a difference of about 3 orders of magnitude in the absolute values of the self-diffusion coefficients which in subsequent papers 19,37 could be shown to be most likely due to the finite heat dissipation rate during the uptake process. Recent uptake studies exhibit better 32 and even nearly complete agreement 33 between the n.m.r, and uptake data. Up to medium pore filling factors over the whole temperature interval considered, the concentration of molecules in the gas phase can be neglected in comparison with the amount adsorbed, so that the measurements are carried out under quasi-isosteric conditions. In the vicinity of saturation, however, the amount of molecules in the gas phase attains such values, that with increasing temperature the sorbate concentration effectively decreases. T h e values of selfdiffusion coefficients obtained under such conditions are denoted in Figures 2a-d by an asterisk. It is obvious therefore that the activation energy derived from the usual Arrhenius representation is only an apparent one, being as well determined by the concentration dependence of the self-diffusion coefficient and the temperature dependence of the actual sorbate concentration. T h e temperature dependence of the longitudinal proton relaxation time T~ of benzene adsorbed in a commercial zeolite N a - Y (Si/AI=2.6; Fe ~÷ impurities in the lattice corresponding to about 500 ppm Fe~O3) shows a sharp m i n i m u m s4.35 which must be described by thermal motion with a single correlation time. Due to the dominating magnetic dipolar interaction of the protons with the paramagnetic impurities (Fe 3÷) of the zeolite lattice for commercial zeolites (see Ref. 24), the correlation time is given by the mean time r i between

Molecular diffusion in Na-X: H. Pfeifer et al.

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" Actual sorbate concentration decreases with increasing temperature, since the amount of molecules in the gas phase cannot be neglected any more with respect to the amount adsorbed. Since the self-diffusion coefficient in this region strongly depends on the sorbate concentration the activation energy which would result from the respective Arrhenius plots is an apparent one only, which is as well determined by the temperature dependence of the actual sorbate concentration.

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Molecular diffusion in N a - X : H. Pfeifer et al.

two successive translational j u m p s 36. T h e values of rj for0 =0.2 and 0 =0.8 have been found to be 13 ns and 50 ns at room temperature, respectively. T h e activation energy, Ej does not depend on 0 within the limits of accuracy and amounts to (14.2 + 1.3) kJ mol -l ~4,36 Since in the case of N a - Y only sodium ions at sites SII are accessible to benzene molecules in the large cavities of the zeolite 37 the b e n z e n e - N a ÷ complexes are formed at sites SII, and Tj-I is the rate constant for the dissociation of these complexes. As a consequence of localized adsorption, Eyring's theory is applicable and the increase of r, with increasing values of 0 can be explained z4. For a (urther check of this model, the longitudinal proton relaxation time T I of toluene adsorbed in the same zeolite was measured. Although benzene and toluene differ in their intermolecular interactions in the liquid, as can be seen from their quite different melting points ( + 5 . 5 ° C and - 9 5 ° C , respectively), their molecular motion in the adsorbed state should be similar due to the d o m i n a t i n g interaction of the 7r-electron system of the carbon ring and the sodium ions. This indeed has been observed. T h e r e is no difference in Tl between benzene and toluene from about - 2 0 ° C to +180°C 24. If benzene is adsorbed in a commercial zeolite N a - X (Si/AI=I.8; about 500 p p m Fe203) the m i n i m u m of Tl shifts to much lower temperatures and is slightly flattened 14. Therefore, the mobility of benzene molecules adsorbed in N a - X is much higher (at room temperature r j = 10 ns in comparison to r j = 5 0 ns for N a - Y ) , and a reduced j u m p length seems probable. This is due to the additional non localizable sodium ions (denoted by S 3, cf. Ref. 37) by which the n u m b e r of adsorption centres (where b e n z e n e - N a ÷ complexes are formed) is enhanced and the height of potential walls between neighbouring centres (S 3 / S I I ) is lowered. A motion of the whole b e n z e n e - N a + S 3 complex as suggested in Ref. 24 seems to be less probable since Z3Na nuclear magnetic resonance measurements give no indication for mobile sodium ions 3s. T h e absolute value for the rms j u m p length < / 2 > ~ of the molecules can be determined on the basis of the relation:

= 6Drj

which is valid for diffusion by activated jumps. Using the above mentioned result r j = 10 ns which holds at room temperature for about four benzene molecules per large cavity in a zeolite N a - X (Si/AI=I.8), and the extrapolated value of D = 2 " 10-12m2s -l from Figure 2a, it follows: < F > '~ = 3.5" 10-1°m T h e controlling action of N a ÷ $3 is demonstrated by the resul09 that the value of rj is constant for Si/A1 _> 2.43 where no N a + S 3 exist, and decreases with decreasing ratio Si/A1 of the zeolite (increasing n u m b e r of N a ÷ $3). O u r model stating that in zeolites where no N a * S 3 exist, the benzene molecules are bonded to the sodium ions localized at sites SII, has been checked by an elimination of these sodium ions through an ion exchange with La 3÷ ions, because trivalent ions prefer sites SI, SI l outside of the large cavities, where they are not accessible to hydrocarbons 37. In agreement with our model, the

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m i n i m u m of the longitudinal proton relaxation time TI of benzene adsorbed in N a L a - Y shifts to a m u c h lower temperature (which corresponds to a drastic decrease of the value of the correlation time) with respect to N a - Y 14. Unfortunately, due to substantial differences in the genesis of the X and Y type zeolites 2s,26 until now we have no proper zeolite Y under our disposal which consists of crystallites large enough to permit an u n a m b i g u o u s m e a s u r e m e n t of intracrystaHine selfdiffusion coefficients. However, the expected tendency on going from X to Y type zeolites may already be observed by comparing the results of the same sorbate concentration for the zeolites N a - X (1.2) and N a - X (1.8) (cf. Figures 2a-d), since for the latter zeolite which approaches N a - Y , the intracrystalline self-diffusion coefficients are always smaller. Moreover, with comparable samples the activation energies for selfdiffusion of the aromatic c o m p o u n d s in zeolite N a - X (1.8) are larger than those in N a - X (1.2). This is in complete agreement with the above indicated tendency that the height of the potential walls between neighbouring adsorption centres increases with decreasing n u m b e r of (non-localizable) sodium ions. T h e effect of the mutual interaction of the molecules in the proposed model requires further investigations which deserves special attention in view of the decreasing differences between the self-diffusion coefficients in N a - X (1.2) and N a - X (1.8), and of the obvious increase of the activation energy with decreasing sorbate concentration, as stated in the present experiments. T h e b e n z e n e - N a ÷ complex was studied also theoretically using the semiempirical q u a n t u m chemical C N D O / 2 method 4°. T h e most favourable configuration found is that where the sodium ion lies on the sixfold axis of the aromatic ring (0.31 nm) in agreement with proton magnetic resonance m e a s u r e m e n t s of the second m o m e n t 4~ and recent infrared studies on the interaction of zeolites with benzene molecules 42. ACKNOWLEDGMENTS T h e authors are indebted to D. M. Ruthven for the stimulation to this work and for his continuous interest in its development, to M. Billow and J. Caro for helpful discussions, and to W. Heink for his engagement in the improvement of the n.m.r, pulse spectrometer F E G R I S 80.

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