Entropy driven preference for alkene adsorption at the pore mouth as the origin of pore-mouth catalysis for alkane hydroisomerization in 1D zeolites

Abstract

The hydroisomerization of n-paraffins to mono-methyl branched isomers on bifunctional metal acid-zeolite catalysts has been commonly considered in terms of the heuristic pore-mouth catalysis model developed to explain the highly selective formation of the monobranched isomer with the methyl group at the C2 position. This work presents a theoretical support of the pore-mouth model on the basis of semi-quantitative estimates of the entropy change upon adsorption at the opening of the zeolite channel and inside the pore, and the DFT calculated enthalpy for 3-heptene adsorption on the ZSM-23 zeolite. A key prediction is the entropy-driven preference for alkene (assumed to be readily produced by metal particle on the zeolite surface) to adsorb and isomerize only at the mouth of the zeolite pore being trapped by the Brønsted acid site via the alkene double bond located near the end of the molecule. This effect explains the origin of the pore-mouth catalysis and positional selectivity of the skeletal isomerization.