Zeolite diffusional limitations

The activity of the Pt-exchanged catalyst for n-C f, transformation increases when the crystallites size increases, which was totally unexpected. External diffusional limitations cannot be invoked since the size of the grains of catalyst is the same. Moreover, this would lead to the opposite result. Other experiments showed that the activity of zeolite-... 

Crystallite Size Effects upon AP Catalyst Selectivity. Previous studies have shown that with the pellet sizes investigated, gross particle size does not affect activity or selectivity. If there are diffusional limitations, they must be intracrystalline and therefore a function of the crystallite size of the zeolite component. 

Tphe rate-limiting processes in catalytic reaction over zeolites remain A largely undefined, mainly because of the lack of information on counterdiffusion rates at reaction conditions. Thomas and Barmby (7), Chen et al. (2, 3), and Nace (4) speculate on possible diffusional limitations in catalytic cracking over zeolites, and Katzer (5) has shown that intracrystalline diffusional limitations do not exist in liquid-phase benzene alkylation with propene. Tan and Fuller (6) propose internal mass transfer limitations and rapid fouling in benzene alkylation with cyclohexene over Y zeolite, based on the occurrence of a maximum in the reaction rate at about 100 min in flow reaction studies. Venuto et al (7, 8, 9) report similar rate maxima for vapor- and liquid-phase alkylation of benzene and dehydro-... 

Three obvious models which could describe the observed reaction rate are (a) concentration equilibrium between all parts of the intracrystalline pore structure and the exterior gas phase (reaction rate limiting), (b) equilibrium between the gas phase and the surface of the zeolite crystallites but diffusional limitations within the intracrystalline pore structure, and (c) concentration uniformity within the intracrystalline pore structure but a large difference from equilibrium at the interface between the zeolite crystal (pore mouth) and the gas phase (product desorption limitation). Combinations of the above may occur, and all models must include catalyst deactivation. 

Chiche et a/.[56] have studied the oligomerization of butene over a series of zeolite (HBeta and HZSM-5), amorphous silica alumina and mesoporous MTS-type aluminosilicates with different pores. The authors found that MTS catalyst converts selectively butenes into a mixture of branched dimers at 423 K and 1.5-2 MPa. Under the same reaction conditions, acid zeolites and amorphous silica alumina are practically inactive due to rapid deactivation caused by the accumulation of hydrocarbon residue on the catalyst surface blocking pores and active sites. The catalytic behaviour observed for the MTS catalyst was attributed to the low density of sites on their surface along with the absence of diffusional limitations due to an open porosity. This would result in a low concentration of reactive species on the surface with short residence times, and favour deprotonation and desorption of the octyl cations, thus preventing secondary reaction of the olefinic products. 

Zeolite Beta has also been studied for isobutane/butene alkylation (65, 66), but it was less selective to the desired TMP than USY, suggesting some diffusional limitations for these highly branched products at the relatively low reaction temperatures used. In fact, an increase of activity was observed when decreasing the crystal size of the Beta zeolite (66). As for USY zeolites, the activity, selectivity and deactivation rate of Beta zeolite were influenced by the presence of EFAL species (67). Medium pore zeolites, such as ZSM-5 and ZSM-11 were also found active for alkylation, but at temperatures above 100°C (68, 69). Moreover, the product obtained on ZSM-5 and ZSM-11 contained more light compounds (C5-C7), and the Os fraction was almost free of trimethylpentanes, indicating serious pore restrictions for the formation of the desired alkylation products. 

These values allow to exclude external diffusional limitations, for which the pseudo-activation energies are in the 10-20 kJ /mol range. However, internal diffusion limitations caimot be excluded, as usual in the case of zeolites, particularly because of the high operating temperatures. So the above values should be regarded as apparent activation energies. 

In order to clarify this deviation some additional experiments with zeolite NaY were completed. Thus, the CO2 adsorption isotherm at 273K was repeated in the same sample after evacuation at 373 K in vacuum (to remove the physisorbed CO2). The isotherm obtained is very similar to the first one and no hysteresis is observed. These results indicate that CO2 is not significantly chemisorbed on the zeolite NaY at the experimental conditions used. Additionally, CO2 adsorption was performed at a higher temperature (i.e., 298 K) in order to discard any diffusional limitations of this adsorptive. The characteristic curves for the isotherm at 273 and 298 K are shown in Figure 7. The shape of the characteristic curves at the two temperatures is similar. Both curves exhibit a downward deviation at high adsorption potentials and the slope at lower potentials as well as the ordinate at the origin are very close (see dashed line). These experiments confirm that the deviation observed in the CO2 characteristic curve of zeolite NaY is due to neither CO2 chemisorption nor to diffusional problems as it was expected because the CO2 can enter into smaller pores like in the case of zeolite NaA (Table 2). Therefore, this deviation at low relative pressures must be related with the surface chemistry of the zeolite. 

The decay of cracking catalysts is studied as a function of the coke content. The results suggest a deactivation by site fouling and zeolite pore plugging under diffusional limitation. 

In a zeolite catalyst diffusional limitations may occur at either the particle scale or the crystal scale. In the latter case the basic analysis remains the same, but since the rate constant is defined with respect to the concentration of reactant in the vapor phase while the intracrystalline diffusivity is defined with respect to the adsorbed phase concentration, the Thiele modulus must be redefined to introduce the dimensionless adsorption equUibriimi constant (K) ... 

Calculation shows that these are roughly equivalent, and if the inequality is not taken too literally, it is not really much different from the original Weisz-Prater criterion. However, for certain situations such as strong product inhibition, this is not the case—see Ex. 3.6.C-2. Finally, Brown [106] has considered macro-micro pore systems. For typical types of catalyst structure and diffusivities, the conclusion was that normally there will be no diffusional limitations in the micropores if there is none in the macroporcs. Thus, use of the standard criteria for the macropores should be sufficient to detect any pore diffusion problems however, the assumptions and calculations were probably not valid for zeolite molecular sieves, and so this case still needs special consideration. 

The ZSM-5 zeolite gives the acidity while it is stable to deactivation by coke. This is due to its channel dimensions which do not allow the formation of aromatic compounds with more than eleven carbon atoms, nor the formation of poliaromatics which can lead to coke deposits. Moreover, the tridirectional channel structure of ZSM-5 makes less critical the diffusional limitations created by coke deposits, than on other unidirectional zeolites such as mordenite, omega, ZSM-12, of etite, ferrierite, etc. (135-137). 

Figure 4.43. (a) The rate of a monomolecular zeolite-catalyzed reaction as a function of pressure (no diffusional limitation), (b) The rate of the monomolecular reaction that is diflfusional limited (no single-file diffusion), (c) The rate of the monomolecular reaction that is diffusional limited (single-file diffusion). 

Ej-Jennane et al. [156] compared H-ZSM-5, H-MOR, and H-Y zeolites in the disproportionation of toluene at long TOS. They found that the catalytic activity was related to the munber of acid sites for the H-ZSM-5 structure which did not show much deactivation, that there was a strong diffusional limitation with H-MOR due to pore blocking, and that the reactions over H-Y were limited by site poisoning. These results show that measurements after a long stabilization period are very hard to compare to acidity values which are measured on the fresh catalysts. If no initial activity is taken for comparison, different modes of deactivation over various catalysts will make it nearly impossible to evaluate the acidity of zeoHtes with different structures by means of this test reaction. 

The results reported indicate that cyclohexanol appears to be a good test molecule for probing acidity, due to the relatively low deactivation and the possibility of tracing acid sites with different strength. Unfortunately, the molecule seems to be too large to enter the pores of small and maybe even medium pore size zeolites and, therefore, diffusional limitations might affect the kinetic rates measured. 

In the pseudohomogeneous models described in the first section of this paper and by de Lasa et alo [6] the diffusional limitations in the macropore structure of the pellet was neglected. In fact, the rate expression proposed by Liederman et al. [ ] was obtained in a fluid bed where the dimensions of the zeolite particles were in the range of 50 ymo... 

MN acetylation is most likely related to the high polarity and bulkiness of the products with limitations in the reaction rate by product desorption. Dealumination would have a positive effect on the acetylation rate because of the decrease in the zeolite hydrophilicity and of the increase in the rate of diffusion of the bulky products owing to elimination of extra-framework A1 species. Curiously, in anisole acetylation, the Si/Al ratio of the HBEA zeolite had practically no effect on the reaction rate. However it is worth noting that most of the tested samples had Si/Al ratios between 11 and 30. Like for 2-MN acetylation,[28,32] the performance of HBEA zeolites in anisole acetylation depends on their crystallite size.[17] This was shown by comparing the activities of samples with large size (0.1-0.4 pm) and of a nanosize sample (0.01-0.02 pm) prepared within the pores of a carbon black matrix. The superior performance of the nanosize sample was ascribed to a decrease in diffusional constraints limiting the desorption of the bulky and polar p-methoxyacetophenone product from the BEA micropores. 

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