Bimolecular cracking

As part of the same study selectivity data were provided at 10-100 kPa partial pressures of n-butane at 0-17% conversion over HZSM-5 [90]. With increase in pressure and conversion secondary reactions started to occur. These results are also summarized in Table 13.6. The lowered selectivity to hydrogen, methane and ethane was attributed to increasingly less favorable conditions for monomolecular cracking. The dramatic increase in selectivity to propane which was absent at zero conversion, along with decrease in propylene was considered as signature for bimolecular cracking. More specifically, it was suggested that hydride transfer... 

Formation of products in paraffin cracking reactions over acidic zeolites can proceed via both unimolecular and bimolecular pathways [4], Based on the analysis of the kinetic rate equations it was suggested that the intrinsic acidity shows better correlation with the intrinsic rate constant (kinl) of the unimolecular hexane cracking than with the apparent rate constant (kapp= k K, where K is the constant of adsorption equilibrium). In... 

Notwithstanding some obviously contradictory results in the literature, the data summarized above can be summarized as follows the general trend is that high aluminum contents are beneficial for the alkylation performance. This inference is supported by results from cracking experiments with zeolites having various Si/Al ratios. The bimolecular hydride transfer step is favored in materials with low Si/Al ratios (54,145,146). Thus, zeolites with low Si/Al ratios should exhibit better time-on-stream behavior than those with high Si/Al ratios. 

MCM-22, with a larger pore volume than ZSM-5, revealed behavior intermediate between what was observed for large- and medium-pore zeolites (126). Unverricht et al. (141) also examined MCM-22 at 353 and 393 K, it was found to produce mainly cracked products and dimethylhexanes and to deactivate rapidly. MCM-36 gained considerable interest that is evidenced by the patent literature (171-174). MCM-36 is a pillared zeolite based on the structure of MCM-22. Ideally, it should contain mesopores between layers of MCM-22 crystallites. This structure was found to be much more active and stable than MCM-22 (175). Alkane cracking experiments with zeolites having various pore dimensions evidenced the preference of monomolecular over sterically more demanding bimolecular pathways, such as hydride transfer, in small- and medium-pore zeolites (146). 

This bimolecular mechanism also applies to cycloalkanes which can be activated by intermolecular hydride transfer to small carbenium ions to form cyclohexyl cations prior to cracking. Alternately, the cyclohexyl cations can deprotonate and form cyclohexene. With two similar intermolecular hydride transfers an aromatic can also form [46]. 

Table 13.8 Monomolecular and bimolecular kinetics of alkane cracking [92],...

Krannila, H., Haag, W.O., and Gates, B.C. (1992) Monomolecular and bimolecular mechanisms of paraffin cracking n-butane cracking catalyzed by HZSM-5./. Catal, 135, 115-124. 

Historically, the earliest C8 aromatic isomerization catalysts tended to use amorphous supports with a halogen such as chloride or fluoride. Due to water sensitivity and corrosion issues, these were replaced by large-pore zeolites such as mordenite. The larger pore size was more favorable toward bimolecular transalkylation, whereas the chlorided alumina support tended to promote cracking. In both... 

The conditions where the bimolecular reaction path predominates are low temperature and high olefin concentration. Although both mono- and bimolecular limiting conditions can be experimentally realized to a good approximation, experiments are often carried out under conditions were both mechanisms contribute to product formation and the kinetics is complex. For example, kinetic evaluation of hexane cracking at 370°C and 150 torr hexane pressure shows that initially the reaction is slow and then accelerates (Fig. 4). 

It is noted that the microporous effect was greater in the disproportionation of 1,2,4-TrMB than in the cracking of cumene. As shown in the previous paper [14], the disproportionation of 1,2,4-TrMB at 200°C proceeds via a bimolecular transition state and obeys the second order kinetics. In contrast, the cracking of cumene is the first order kinetics with respect to cumene concentration. Thus, it seems that the microporous effect is exerted more significantly in the second order reaction (disproportionation) than in the first order reaction (cracking) if pore structure plays an important role in localizing concentration of reactant molecules. 

Over-cracking of PCC gasoline with either ZSM-5 or REHY results, in both cases, in a preferential loss of heavier olefin components. The major differences between the two zeolites is the increased C3/C4 ratio with ZSM-5 which has been assigned to pore size effects, and enhanced bimolecular hydrogen transfer reactions with REHY, resulting in a higher paraffin/olefin ratio. 

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