Function of well-established mesoporous layers of recrystallized ZSM-22 zeolites in the catalytic performance of n-alkane isomerization

Abstract

ZSM-22 zeolite was desilicated and recrystallized to obtain a hierarchically porous material with well-established MCM-41 mesoporous structure on the outer surface. By choosing three different linear n-alkanes (n-heptane, n-dodecane, and n-hexadecane) as reactants, the impact of the MCM-41 mesoporous shell on isomerization performance was studied systematically. Compared with conventional and desilicated catalysts, the Pt-loaded recrystallized ZSM-22 catalyst exhibited lower cracking activity and rather higher isomerization selectivity. Detailed product analysis and kinetic study revealed that the isomerization activity was mainly determined by the interaction of hydrocarbon molecules in the confined reaction space with the channels of ZSM-22 zeolite. The Brønsted acid sites in the MCM-41 mesoporous shell in the recrystallized material also worked as active sites for the isomerization at high conversion leading to the significantly enhanced selectivity of multi-branched isomers due to the open mesoporous structure providing space for the formation of multi-branched isomers. Furthermore, with the diffusion restriction of the extra mesoporous shell, the residence time of reactants was prolonged, which promoted secondary isomerization at microporous pore openings. The Pt-loaded ZSM-22/MCM-41 hierarchically porous material was very attractive due to the high yield of multi-branched isomers, and the preferable hexadecane isomerization performance revealed its potential application in Fischer–Tropsch synthesis dewaxing processes.