Evidences for pore mouth and key-lock catalysis in hydroisomerization of long n-alkanes over 10-ring tubular pore bifunctional zeolites

The evidence for the pore mouth catalysis model for n-alkane methylbranchin g on Pt/H-ZSM-22 hydroisomerization catalyst is reviewed. It is based on ad sorption equilibria at catalytic temperatures determined using tracer and p erturbation chromatography, reaction product distributions obtained with nC (8)-nC(24) n-alkanes and rival model screening for catalytic conversions. I n the Henry regime, methylbranched isomers have lower adsorption entropy an d enthalpy compared to n-alkanes explained by the enhanced rotational and t ranslational freedom of methyl and methylene groups positioned outside the pore interacting with the external surface. Adsorption isotherms for isoalk anes are in agreement with dual site adsorption in pore mouths and on exter nal surfaces, respectively. The hydroisomerization can be modeled with a bi functional reaction scheme and adsorption on the external crystal surfaces and pore mouths. The selectivity for 2-methylbranching reflects the optimum van der Waals interaction of the n-alkane with the zeolite pore and methyl branching in that part of the chain that is located outside the first 10-ri ng of the zeolite pore to facilitate desorption. Very long n-alkanes (C-12( +)) exhibit key-lock adsorptions and penetrate simultaneously with their tw o ends into two different pores. Key-lock physisorption leads to branching at more central C atom positions. (C) 2001 Elsevier Science B.V. All rights reserved.