Role of hierarchical micro-mesoporous structure of ZSM-5 derived from an embedded nanocarbon cluster synthesis approach in isomerization of alkenes, catalytic cracking and hydrocracking of alkanes

Abstract

Hierarchical micro-mesoporous ZSM-5 zeolites synthesized via an embedded nanocarbon cluster approach were studied for isomerization of n-butene, catalytic cracking of n-hexane, catalytic cracking and hydrocracking of n-hexadecane. The introduction of mesoporosity into ZSM-5 zeolite framework was achieved by using carbon/SiO2 composite obtained from a carbonaceous gas deposition on silica gel as a mesoporogen. The synthesized hierarchical ZSM-5 zeolites with different Si/Al ratios were characterized by means of XRD, ICP-OES, N2 adsorption-desorption isotherm measurement, TEM, 27Al MAS NMR, NH3-TPD, FTIR of pyridine adsorption and constraint index (CI) test. Compared with the conventional H-ZSM-5, the hierarchical H-ZSM-5 exhibits an enhancement in the catalytic activity in terms of higher reactant conversion and product yield for isomerization of n-butene and catalytic cracking of n-hexadecane, as the mesopores provide larger pores as well as improved accessible active sites. Moreover, Pt loaded on hierarchical H-ZSM-5 exhibit a remarkably enhanced catalytic activity in hydrocracking of n-hexadecane compared to Pt loaded on conventional H-ZSM-5 due to a smaller and higher dispersion of Pt nanoparticles. For n-hexane cracking, the presence of mesopores does not improve n-hexane conversion on the fresh catalyst because this reaction proceeds via monomolecular pathway and the corresponding reaction species are smaller than 10-ring pore windows of ZSM-5 zeolite. However, the hierarchical structure improves the catalytic stability in n-hexane cracking due to the presence of mesopores, which can shorten the diffusion path length, suppressing the consecutive reactions and, thus reducing the coke formation.