Geometrical Constraints Shape-selective Catalysis

The active sites within the zeolite can be either the intrinsic acid sites or others introduced by ion exchange, etc. The function added most often is hydrogenation-dehydrogenation, which then allows bifunctional catalysis, or alternatively hydrogenation/dehydrogenation alone if the acid site is neutralized with base. 

Erionite and the related zeolite T exclude branched chain paraffins, so that n-paraffins are selectively cracked or hydrocracked over these catalysts. However, even n-paraffins have diffusities several orders of magnitude lower than in large pore zeolites. Earlier work on hydrocracking over erionite catalysts gave unexpected product distributions, with maxima at C4 and C12 and essentially no Cg product. Gorring measured diffusion coefficients for migration of n-paraffins in KT zeolite. An unexpected phenomenon, termed the window 

The pore size of mordenites depends both on the source of mordenite and on the extent and nature of cation exchange. Various shape-selective catalyses have been observed with mordenites of different types. Transition-state selectivity occurs as well as reactant and product selectivity, especially in the alkylation and transalkylation of alkyl benzenes. High selectivity for 1,2,4-trimethylbenzene has been found in the disproportation of toluene over partially-exchanged H-mordenites. Karge found the selective formation of monoalkyl benzenes in the ethylation of benzene over various cation-containing mordenites and attributed the results to transition-state selectivity. 

The ZSM-5 family of zeolites show further interesting shape-selective effects. Both normal and methyl-substituted paraffins have access to interior sites, so both hexane and 3-methylpentane are cracked by ZSM-5, but steric constraints cause hexane to be cracked faster than 3-methylpentane. Further shape selectivity was found between 3-methylpentane and 2,3-dimethylbutane. No window effect with paraffin chain length was found with ZSM-5. In the conversion of methanol to hydrocarbons over ZSM-5 catalysts, the distribution 94,152,195 of aromatic products ends at Cio- The distribution of tetramethylbenzenes is not far from equilibrium, but has excess 1,2,4,5-tetramethylbenzene. Measurements of diffusion coefficients of alkyl benzenes show rapid decrease, by orders of magnitude, as ring substitution increases. 

Zeolite Nu-1, of similar pore size to ZSM-5, also shows a similar distribution of aromatic products from methanol. The de-activated ZSM-5 catalysts have modified selectivities. The selective formation of p-xylene in various aromatic reactions could be due to further constraints on transport in the zeolite or to modified active sites. 

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