Isotherms of p-xylene showed a sharp step rise from ca. 4 to 8 molecules uc- '. A clear phase boundary could be outlined. Adsorbed benzene behaved as a ...
Adsorption of aromatic compounds in large MFI zeolite crystals Chung-Kung Lee7 and Anthony S. T. Chiang Department of Chemical Engineering, National Central University, Chung-Li, Taiwan 32054, Republic of China
Very large (180 x 40 x 40 pm3) and well defined crystals have been used in a gravimetric system to investigate the adsorption of aromatic compounds in MFI (silicalite) zeolite. The isotherms and isosteric heats of adsorption (Q,,) were reported for future comparison with various adsorption models. Three types of phase transition were found. Isotherms of p-xylene showed a sharp step rise from ca. 4 to 8 molecules uc- '. A clear phase boundary could be outlined. Adsorbed benzene behaved as a dual-phase system in the range between 4.6 and 6 molecules uc-'. There were more phase transitions above 6 molecules uc-' but it was not possible to outline the phase boundary. Molar entropy changes of 210 and 180 J mol-' K-' were found for the observed p-xylene-MFI and benzene-MFI phase transitions. For ethylbenzene and toluene, a dual-phase region was also observed but the transition was less pronounced. isosteric heats, calculated from isotherms, showed a complex variation with loading and a strong dependence on temperature. It was also found that the adsorption kinetics are strongly influenced by the previous adsorption history. For freshly calcined samples the uptake rate was relatively fast. However, re-calcination after the adsorption of p-xylene created a diffusion barrier inside the crystalline material and subsequent adsorption of other aromatic compounds became much slower and displayed two-step kinetics.
1. Introduction MFI type zeolite is one of the most versatile and valuable zeolites in modern hydrocarbon processing. A wide range of applications, including catalytic and adsorptive processes, has been proposed based on the size and configuration of its pore structure. The adsorption of aromatic compounds in ZSM-5, and its aluminium-deficient structural analogue, silicalite, has been intensively studied in recent years. Various experimental methods have been used, including isotherms,'-' sorption isochore,' calorimetry measurements,' ' gas chromatograph~,'~,'~ and neutron25 powder diffraction and solid-state NMR.26--28Adsorption isotherms and calorimetric measurements provided data on the thermodynamics of adsorbate-host interactions. Diffraction data furnished information concerning the site location of adsorbate molecules and the changes in the lattice structure induced by both temperature and the presence of organic adsorbates. The changes in host structure may also be observed by solid-state NMR analysis. Collectively, these experimental results indicated some unusual adsorption characteristics for aromatic-MFI systems, which have been associated with the peculiar framework structure and the strong interactions induced by a tightfit situation. At least three locations in the MFI framework have been suggested as the adsorption site for aromatic compounds; the zigzag channel, the straight channel and the intersection between them. Site heterogeneity in MFI (silicalite) was first reported in the calorimetric studies of Thamm. '' The adsorption heat exhibited a strange jump at an intermediate loading for systems of benzene, ethylbenzene, toluene and p-xylene. This could only be explained in terms of a phase transition or a change in the state of the admolecules. Phase transition was first confirmed by experimental isotherms for the p-xyleneZSM-5 systema2 Talu et al." further outlined the phase boundaries of benzene-silicalite and p-xylene-silicalite systems with isotherms measured at different temperatures. 3914
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t Present address : Van-Nung Inst. of Technology, Chung-Li, Taiwan 32054, Republic of China.
Furthermore, X-ray d i f f r a ~ t i o n ~ and ~ , ~ ~ solid-state NM R26 -2 8 studies on aromatic-MFI systems clearly demonstrated the presence of several types of admolecule-zeolite complexes. In some cases, the location of the admolecule was also identified. Using 'H NMR, Portsmouth and cow o r k e r proved ~ ~ ~ ~that ~ ~the adsorbed benzene and p-xylene molecules in silicalite were indeed present in different environments before and after the phase transition. However, it is known that phase transition is strongly scale dependent. The larger the experimental system, the sharper the phase transition observed. The typical zeolite sizes used in previous aromatic-MFI adsorption studies are in the range of a few pm. For these small crystals, the phase transition may have been smoothed out by edge effects. There is also a higher chance of lattice imperfection, and a larger effect of the external surface for smaller crystals. In this paper, we will present adsorption data measured with very large (180 x 40 x 40 pm3) and well defined crystals. The phase-transition phenomenon becomes very clear. Three types of polymorphic modifications of the MFI framework have been identified by a single-crystal diffraction study on the p-xylene-ZSM-5 system by van Koningsveld et ~ 2 The . structures ~ ~ of the host may be monoclinic (with P 2 , / n . 1 - 1 symmetry) or orthorhombic (either Pnma or P2,2,2,) depending on the temperature and the amount of p-xylene adsorbed. They have assigned the name MONO, ORTHO and PARA to these structures, respectively. Besides the difference in symmetry, there is also a slight variation in the lattice constants between these polymorphic structures. n p-xylene-MFI (the short hand for MFI zeolite loaded with n p-xylene molecules uc-') is in the ORTHO form if n < 4 and in the PARA form if n = 8. On the other hand, Sacerdote et a[.2 1-2 3 have found that ZSM-5 loaded with benzene, toluene or ethylbenzene is in the MONO form if n < 4. The step jump observed in the experimental isotherms has been ascribed to the transformation between these polymorphic phases. While the structure complexity of aromatic-MFI systems has been gradually uncovered with these microscopic techniques, more is yet to be learned about the adsorption thermodynamics. The goal is to describe, or eventually to predict, J . Chem. SOC.,Faraday Trans., 1996,92(18), 3445-3451
3445
the macroscopic adsorption isotherms. To accomplish this The isotherms, as well as the uptake rates, were measured goal, various thermodynamic models, based either on atomwith a Cahn-1000 microbalance in a constant volume system. ca. 100 mg of the crystals were used. Equilibrium was assumed istic sir nu la ti on^^^-^^ or on simplified phenomenological vers i o n ~ , ~have ~ - ~ been ~ proposed. Atomistic simulation when the sample weight changed by less than 0.01 mg for at provides a wealth of detailed molecular-level information and least 2 h. A complete adsorption isotherm was constructed by may be used to predict macroscopic thermodynamic properincreasing the pressure in a step-wise manner. In theory, the ties. However, such simulations are quite often computadesorption branch should be generated by a sequential tionally intensive. The situation is even more so for tight-fit decrease in the pressure. However, the desorption rate was systems such as aromatic compounds in MFI ~ e o l i t e . ~To " ~ ~ extremely slow, especially near and below the phase-transition circumvent this difficulty, Snurr et proposed recently a point. The sample weight hardly changed even under vacuum hierarchical atomistic/lattice strategy, based on the fact that for more than 10 h. Effective desorption could only be aromatic molecules are adsorbed in localized adsorption sites. achieved by increasing the temperature. Accordingly, only the Detailed atomistic simulations were used to obtain the adsorption branches of the isotherms were reported. required parameters for the more simple lattice gas model. The temperature of the system was controlled to within 1"C However, to account for the free energy change upon the by recirculating refrigerant from a thermal stat. The pressure transformation of framework structure, an additional empiriof the system was measured by two MKS absolute pressure cal parameter still had to be included. transducers within the 0-10 and 1-lo00 Torr ranges, respecPhenomenological models only deal with macroscopic tively. These pressure transducers have four digits of properties. They are useful in correlating the observed isoresolution. All adsorbates used were GR grade from Merck therms and adsorption heat curves. Earlier phenomenological co. models included the cell theory proposed by Stach et a1.,41 Originally, the O-ring in the system gave rise to various problems. Even with Viton O-rings, the aromatic compounds where an isotherm was obtained by summing probabilities of were absorbed slowly by the O-rings. This created a system occupancy on different sites. Pan and M e r ~ m a n nproposed ~~ another heterogeneous model for the step-isotherm observed pressure drift as well as problems in the later evacuation in p-xylene-silicalite systems. A Langmuir isotherm was process. These problems were finally solved by replacing most parts of the system with stainless-steel vessels and all-metal adopted to model one type of adsorption site. A twovalves. The only O-rings left in the system were those on the dimensional lattice model with nearest neighbouring interbalance mechanism. action was employed for the second type of site. A similar compound model has also been proposed by Talu et ~ 1 . ' ~ Karsli et al.44 reported that the sorption history could influTo generalize the analysis of adsorption in a network of ence the sorption kinetics of aromatic compounds in silicalite. heterogeneous sites, we43 have proposed a lattice gas model It is, therefore, worthwhile to note clearly the order of our with three types of lattice sites and various nearest-neighbour sorption experiments here. We started our study with p-xylene interactions. This model was able to produce an isotherm with as adsorbate at 0 "C and followed with successively increasing a step jump as well as a qualitatively correct adsorption heat temperature. After the completion of each adsorption isocurve for the benzene-silicalite system. However, there were at therm, the zeolites were re-activated by increasing the temleast five energy parameters in the model, to be determined perature in steps to 100, 200 and 390°C under a vacuum from experimental data. Few literature isotherms were better than Torr. The last step lasted for 16 h. When the detailed enough for the regression of all these parameters. p-xylene isotherms were measured, the zeolite samples were All the phenomenological models mentioned above were taken out from the balance and recalcined. They were held for based on a fixed host structure, despite the polymorphic varia2 h at 200,300,400 and 500 "C and, finally, at 600 "C for 24 h. tion observed from structure analysis. Pan and M e r ~ m a n n ~ ~ The recalcined samples were then used in the study of ethylargued that when a molecule started to occupy the zigzag benzene (0, - 10"C), toluene (0, - 10 "C), m-xylene, o-xylene, channels, a large deformation of the host structure occurred. benzene (0, 30, 0, 10, 20°C) and again toluene (10, 30°C) and This deformation reduced the repulsive force between the ethylbenzene (10°C) in that order. The same activation proadsorbate and the host, and could be interpreted as a longcedure was used between the collection of successive isotherms. range interaction between the adsorbed molecules. They called it a 'crystal lattice mediated interaction'. Effectively, an attracThe initial adsorption of p-xylene was reasonably fast. When the loading was less than 3 molecules UC-', the system tive interaction was assumed between the admolecules on 'neighbouring' zigzag sites, when they were physically reached equilibrium within 10 min upon a step increase in the separated by the admolecules located at the channel intersecpressure. As the loading increased, the adsorption rate tions. reduced. A ca. 4 molecules uc- ', it took more than 4 h for the For either the comparison with theoretical prediction, or system to attain equilibrium. After we recalcined the zeolites, the adsorption rate in the for the regression of model parameters, good experimental isofollowing studies became much slower. Usually more than therms over a wide temperature range are indispensable. We report here a set of isotherms measured on very large and well 24 h were needed to obtain a single equilibrium point at low defined crystals of silicalite. The pressure steps have been kept loadings. As the loading increased, the adsorption rate as small as possible to give a clearly defined phase boundary. increased somewhat, but at least 10 h of equilibration time These data will be useful for a better understanding of the were still needed for each data point. For o-xylene and mxylene, no noticeable adsorption occurred, even after a few thermodynamics involved in the adsorption of aromatic compounds in MFI zeolites. days.
2. Experimental The silicalite crystals were indirectly obtained from Professor K. K. Unger of Johannes Gutenberg University, Germany. These crystals were uniform in size (180 x 40 x 40 pm3) and had an Si/Al ratio larger than 1OOO. An electron micrograph indicated that they were all perfect single crystals. Prior to adsorption experiments, the silicalite crystals were calcined in air at 600 "C for 24 h. 3446
J . Chem. SOC., Faraday Trans., 1996, Vol. 92
3. Results 3.1 Isotherms and heats of adsorption
Fig. 1-4 show the measured isotherms for the four adsorbates. Except for toluene, all the adsorbates reached a definite saturation capacity before a relative pressure of 0.1. The saturation capacities of benzene and p-xylene were both
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pressure/Pa Fig. 3 Adsorption isotherms for toluene in silicalite. The curves from left to right are for - 10,0, 10 and 30 "C, respectively.
8.04 molecules uc- while that of ethylbenzene was 6 molecules uc-l. The adsorption capacity of toluene should be larger than 7.5 molecules uc- ', but the isotherm did not level out, even after a relative pressure of 0.2. These saturation capacities were in good agreement with the earlier reported values.4.' Steps on the isotherm were observed for all aromatic compounds studied. Benzene had its first plateau at ca. 4.6 molecules uc- l, and a second step around 6 molecules uc- l. Such a two-step isotherm, to our knowledge, had not been reported previously in the literature. p-Xylene, toluene and ethylbenzene had only one plateau at 4 molecules uc-'. The isosteric heats of adsorption (Qst) for the four aromatic adsorbates in silicalite were determined directly from the isotherms. Some comparisons with the literature data and discussions were made in what follows.
3.1.1 p-Xylene. For p-xylene-ZSM-5, an S-shaped isotherm has long been known.2 Sometimes the two plateaus were connected by a hysteresis ~ O O P . Note ~ . ~ ~that this characteristic appeared, at different degrees of sharpness, in all published data, even though the samples used vary considerably. A survey of the literature indicated that the sharp phase transition and hysteresis were more pronounced for high-silica samples with larger crystal size. Because large and well defined crystals have been employed in our study, the phase change became very distinct. In Fig. 5 the Q,, curves for p-xylene in MFI zeolite obtained by isotherms in the range 303-313 K and 313-323 K are presented. Also plotted were the differential heat of adsorption determined by Thamm,' the isochoric differential enthalpy determined by PopeI4 and the Qst obtained in the range 313323 K by Richards and Rees." The heat curves obtained by
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Thamm” and Pope14 were nearly invariant over the entire coverage, despite noticeable changes in the isotherm. They There was a slight increase in packing density for the argued that there was no unusual adsorbate-adsorbent or ORTHO phase over the temperature range studied. The presadsorbate-adsorbate interactions, and the change of slope in sure P(T) at which the phase change occurred is re-plotted in the isotherm must have arised from other reasons. Fig. 7. The slope in this figure gave the molar change of However, the isosteric heat curves obtained by us and by entropy, -210 & 10 J mol-’ K - I , during the phase transition Richards and Rees displayed a more complex variation with from the ORTHO to the PARA form. coverage. For our 313-323 K curve, two minima, at ca. 2 and 4 molecules uc- were found. The first minimum was also 3.1.2 Benzene. In Fig. 8 the Q,, of benzene in MFI zeolite observed by Richards and Rees. The existence of a second is given, together with the Thamm” and Pope14 values. As minimum has been suggested by the activation energy of may be seen, benzene is adsorbed in MFI zeolite with a desorption from TPD analysis’* (not shown in figure). On the complex change in adsorption heat. The common feature in other hand, our heat curve in the 303-313 K range showed a these heat curves is that a maximum of unusual height is very sharp maximum at 4 molecules uc- This was similar to always found at high loading. This indicates that adsorbed the variation in QSt obtained by Lohse and Fahlke’ for p benzene molecules interact strongly with each other in MFI xylene on silicalite (not shown in figure). zeolite, leading to a decrease in the entropy of the system. The large difference between the isosteric heat curve Another similarity between our heat curve and that of obtained in different temperature ranges also implied that the Thamm’’ could be noticed. There were two maxima as well Q,, for this system was strongly temperature dependent. This may be a further reason for the dissimilarity of literature heat curves. 103 A detailed phase structure of the n (p-xy1ene)-MFI system has been given by the diffraction and NMR s t ~ d i e s . * ~ , ~ ~ + ~ ~ - ~ * Collectively, they have found that an adsorbate-free MFI zeolite possesses monoclinic symmetry. A polyphased region existed for 0 c n c 2. Presumably, the stable 2(p-xylene)-MFI 102 had its admolecules at alternating channel intersections. cp However, due to the limit of our pressure gauge, we were not % 2 able to observe the phase change for n < 2. 3 0 v) When more p-xylene molecules were admitted, methylL methyl interactions took place through the straight channels. Q A solid solution existed for 2 c n c 4. At n = 4, a polymer 10’ chain of sorbed p-xylene was formed along the straight channel. The ORTHO form of the 4(pxylene)-MFI complex had all its channel interactions occupied. For 4 n < 8, there was another polyphased region. The PARA form of 8 ( p xy1ene)-MFI complex had all its zigzag channels and the 100 channel intersections occupied. Both PARA and ORTHO phases existed over a substantial domain, perhaps an entire crystal, as evident by the sharpness of the NMR spectra.27It 5.60 5.65 5.70 5.75 5.80 has been pointed out by Snurr et aL3’ that local deformation In T of the framework, and thus a lattice-mediated interaction as Fig. 7 Relations between the pressure and temperature at phase proposed by Pan and Mer~mann,~’ may not be very likely. transition. The slope gives the molar entropy change for the tranA P-V plot was generated from the isotherm data and is sition. The solid lines are least-square error fit. The dotted lines are given in Fig. 6. A tentative phase boundary is also outlined. 9 5 % confidence range.
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as two minima at high adsorption loading (>4 molecules uc-’), the first occurring at around 4.6 molecules uc-l and the second at 6 molecules uc- which corresponded individually to the first and the second plateaus in the isotherm. In other words, there was a change of the state in the adsorbed phase at these loadings. A very complex phase behaviour has been reported on the n (benzene)-MFI system by the time-resolved X-ray powder diffraction study of Sacerdote et There is a dual-phase region limited by the 6 (benzene)-MFI (orthorhombic) and 4 (benzene)-MFI (monoclinic) compositions. There are additional dual-phase regions for 6 < n < 7 and 7 < n < 7.2. From n = 7.2 to 8, there is a single-phase solid solution. The adsorbed benzene molecules are located at the intersections when n = 4 and at both the intersections and the straight channels if n = 8. The locations of benzene molecules at other loading are not yet clear. The two-phase region in 4 < n < 6 was very clear from our data. A tentative phase boundary is sketched in Fig. 9. This phase boundary suggested a critical temperature of ca. 350 K, which is roughly the same temperature at which the thermalinduced phase transition of the MFI framework26 occurs. This figure also confirmed that there were other polyphased domains when n > 6. The second or third polyphased domain was not as clear as the first. The molar change of entropy as calculated from the slope in Fig. 7 was - 180 f 15 J mol-’ K - for the transition of 4 (benzene)-MFI to 6 (benzene)MFI.
Fig. 9 Phase diagram for benzene in silicalite. The solid lines outline a tentative two-phase region. There are other two-phase regions to the left and right of this phase boundary.
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expect the adsorbed ethylbenzene molecules to interact with each other at a loading close to 4 molecules uc-l. This was exactly what we observed from our heat curve. Furthermore, both our data and those of Thamm suggested that the framework is completely packed at around 6 (ethylbenzene)-MFI. The large variation of adsorption from 4(ethylbenzene)-MFI to 6 (ethylbenzene)-MFI also suggested that there is an excessive rearrangement of the admolecules. From analysis of X-ray powder diffraction profiles, it was found that both n (toluene)-MFI and n (ethylbenzene)-MFI complexes are monoclinic when n < 4.23The admolecules are located in the channel intersections with little interaction between each other.23 Above a loading of 4 molecules uc-’, the molecules start to occupy the zigzag channels and the framework changes to the ORTHO form. When there are two mono-substituted benzene derivatives residing on the intersection and the zigzag positions, respectively, there may be a
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3.1.3 Toluene and ethylbenzene. The heat of adsorption curve for toluene obtained by isotherms in the range 273-283 K is shown, together with the curves of Thamm” and Pope,14 in Fig. 10. Contrary to the result of calorimetric and isochoric measurements, the curve showed two maxima and one minimum, corresponding to 4, 7 and 6 molecules uc-’, respectively. Moreover, the adsorption heats obtained from the isotherms were larger than those from calorimetric and isochoric experiments. Since these curves corresponded to different temperatures, this implies that the adsorption of toluene in MFI zeolite is also strongly dependent on the temperature. The heat of adsorption curve for ethylbenzene obtained from isotherms in the 273-283 K range is displayed, together with the curve of Thamm,” in Fig. 11. These two curves were completely different. The length of ethylbenzene should be comparable to the length of the channel sections. One would
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double position disorder.23 The extra degree of freedom may have smoothed out the phase transition. Since ethylbenzene has the same molecular length and kinetic diameter as p-xylene, an explanation must be given for its lower saturation capacity. Algebraically, only half of the zigzag channel can be occupied at a loading of 6 molecules uc-l. We believe that the other half of the zigzag channels may have been blocked by the intruding ethyl group from the admolecules at the nearby channel intersection. If the admolecules at the intersection have an equal probability to orient the ethyl group in the four directions (two toward the straight channel, two toward the zigzag channel), only half of the zigzag channels will be free. Thus only six molecules of ethylbenzene can be packed into each unit cell.
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Fig. 12 Change in uptake kinetics from Fickian diffusion to a twostep mechanism in different samples. The upper part shows uptake curves for benzene (273 I(, 1.33 molecules uc-l), toluene (283 K, 1.66 molecules uc-') and ethylbenzene (283 K, 0.945 molecules uc-') in recalcined samples. The lower part shows the uptake curve for p xylene (283 K, 3.6 molecules uc-') in a fresh sample. MJM,,, is the ratio of molecules at time t to time co.
~ t r a t e dAlthough .~~ our sample came from a different source, it is very likely that coke may have formed during the decomposition of p-xylene and created a diffusion barrier along the interface between differently grown crystal sections. Therefore, the initial fast uptake may correspond to the adsorption of the outer crystal section, while the slower kinetics corresponds to the diffusion across the barrier into the inner section.
3.2 Adsorption rates Comparison of the uptake curves before and after recalcination is given in Fig. 12. Initially, when p-xylene was adsorbed in a freshly calcined sample, the uptake curves followed closely a Fickian-type diffusion. However, when the zeolite samples were removed from the balance and recalcined, the uptake characteristics changed dramatically. Not only was the adsorption rate reduced substantially, but also the uptake curve clearly exhibited two-step kinetics. This type of uptake curve has been observed el~ewhere.~'For the uptake curves that followed the Fickian diffusion mechanism, the diffusion time constant decreased monotonically with the amount of loading as shown in Fig. 13. The crystals we used are longer in one direction than the other directions. There is also a question about whether the diffusivity in MFI zeolite is isotropic. Therefore, we prefer to leave the result as an uptake time constant instead of an explicitly calculated diffusivity. One would not expect the uptake rate of p-xylene to be too much different from that of benzene or toluene, based on their similar molecular size. The changed uptake pattern can only be attributed to a change in zeolite properties due to the recalcination. This is the first time that a switch from normal diffusion to two-step kinetics has been directly related to the calcination procedure. The calcination temperature could not have induced the property change. The sample was calcined to the same temperature before the p-xylene experiments. It is calcination after p-xylene adsorption that induced this change. The formation of diffusion barriers inside a large single crystal of MFI zeolite during the removal of templates has recently been demon3450
J . Chem. Soc., Faraday Trans., 1996, V d . 92
4. Discussion A modified lattice model has been used by Rudzinski4' recently to fit simultaneously our isotherms and the adsorption heat curves of Thamm.I5 Owing to the number of parameters involved, and the complexity of the model solution, the fitting
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3 4 5 6 7 8 9 molecules per unit cell Fig. 13 Intracrystalline diffusion time constant for p-xylene at 283 K in fresh sample. (D/r2) is calculated by fitting the uptake curves to diffusion into a sphere of radius, r.
6 V. R. Choudhary, K. R. Srinivasan, J. Catal., 1986,102, 328. is not trivial. However, the physical meaning of the param7 H. Stach, U. Lohse, H. Thamm and W. Schirmer, Zeolites, 1986, eters is yet to be clarified. 6, 74. The hierarchical atomistic/lattice approach of Snurr et ~ 1 . 8 ~ D. ~B. Shah, D. T. Hayhurst, G. Evanina and C. J. Guo, AZChE may be the preferred choice for a more general description of J., 1988,34, 1713. adsorption in aromatic-MFI systems. Interestingly, Snurr et 9 K. Beschmann, G. T. Kokotailo and L. Riekert, in Characterization of Porous Solids, ed. K. K. Unger, J. Rouguerol, K. s. W. al. assumed rigid ORTHO and PARA structures for the study Sing and H. Kral, Elsevier, Amsterdam, 1988, p. 355. of the benzene-silicalite system, despite the fact that M O N O and ORTHO structures were observed by X-ray d i f f r a ~ t i o n . ~ ~10 R. E. Richards and L. V. C. Rees, Zeolites, 1988,8,35. 1 1 C. J. Guo, 0.Talu and D. T. Hayhurst, AZChE J., 1989,35,573. They were, nevertheless, able to model the literature isotherm 12 0. Talu, C. J. Guo and D. T. Hayhurst, J. Phys. Chem., 1989, 93, successfully. The introduction of a fitting parameter for the 7294. free energy change upon phase transition might have helped, 13 C. G. Pope, J. Phys. Chem., 1984,88,6312. 14 C. G. Pope, J. Phys. Chem., 1986,90,835. but this rendered the model non-predictive. 15 H. Thamm, J . Phys. Chem., 1987,91,8. To calculate the free energy change, both the conventional 16 H. Thamm, Zeolites, 1987,7, 341. rigid framework and rigid admolecules approaches must be 17 H. Stach, H. Thamm, J. Janchen, K. Fiedler and W. Schirmer, in abandoned, as pointed out by Cheetham and Bull.31 To Proceedings of the 6th International Conference on Zeolites, ed. D. explore all possible framework symmetry of MFI, at least 24 H. Olson and A. Bisio, Butterworths, London, 1984, p. 225. framework T atoms must be included in the calculation, in 18 H. Lechert and W. Schweitzer, ref. 17, p. 210. 19 V. R. Choudhary and K. R. Srinivasan, Chem. Eng. Sci., 1987,42, addition to the admolecules. Such a calculation may require 382. more computation power than one could afford. 20 B. F. Mentzen, in Zeolites as Catalysts, Sorbents and Detergent A further point may be noted. Since the reduction from the Builders, ed. H. G. Karge and J. Weitkamp, Elsevier, Amsterdam, full atomistic model to the lattice gas model necessarily 1989, p. 477. involves some approximations, it may not be necessary to 21 M. Sacerdote, F. Bosselet and B. F. Mentzen, Mater. Res. Bull., evaluate the configurational integral as Snurr et al. did. 1990,25, 593. 22 M. Sacerdote, F. Bosselet and B. F. Mentzen, C . R. Acad. Sci. Instead, both the preferred adsorption location and orienta(Paris), 1991, 312(II), 1513. tion of admolecules may be identified simply by molecular 23 B. F. Mentzen, Mater. Res. Bull., 1992, 27, 831. mechanics. Once the relative positions of admolecules, as well 24 H. van Koningsveld, F. Tuinstra, H. van Bekkum and J. C. as framework atoms, under minimum potential-energy condiJansen, Acta Crystallogr., Sect. B, 1989,45,423. tions are determined for different loading, the free energy may 25 M. Sacerdote and B. F. Mentzen, Mater. Res. Bull., 1993, 28, 767; be evaluated directly. Next, a lattice model may be conJ. C. Taylor, Zeolites, 1987, 7, 31 1. 26 C. A. Fyfe, H. Strobl, G. T. Kokotailo, G. J. Kennedy and G. E. structed based on the free energies found. The approximations Barlow, J. Am. Chem. SOC., 1988,110,3373. involved here may still be acceptable if the assumption of 27 C. A. Fyfe, Y. Feng, H. Grondey and G. T. Kokotailo, J. Chem. localized adsorption is correct.
5. Conclusions The adsorption isotherms of several important aromatic compounds in large crystals of silicalite have been measured. A multiple-step change was observed for the benzene-MFI system. The isosteric heats calculated from these isotherms were compared with literature data. All results indicated a very pronounced variation in sorption heat with loading. The isotherms measured might serve to verify different adsorption models. It is, however, clear from the sudden change in the adsorption curves that the assumption of a rigid zeolite framework should be discarded. A strong dependence of the uptake rate on the sample preparation history has been found. The diffusion of p-xylene in a freshly prepared sample can be described by Fickian law with decreasing diffusivity upon increasing loading. The calcination of zeolite after p-xylene adsorption created some diffusion barrier inside the crystal. Subsequent uptakes of aromatic compounds became slower and exhibited two-step kinetics. The authors thank the National Science Council, Taiwan, Republic of China, for its continuous financial support through grant NSC80-0402-E008-05. References 1
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Paper 6/009061; Received 7th February, 1996
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