N-Heptane catalytic cracking on hierarchical ZSM-5 zeolite: The effect of mesopores

Abstract

To investigate the effect of pore structure on catalytic cracking of n-heptane, hierarchical ZSM-5 zeolites with unchanged acidity were prepared by hydrothermal synthesis with organosilane ((3-aminopropyl) triethoxysilane, APTES) and tetrapropylammonium hydroxide (TPAOH) as double templates. Chemical and textural properties of samples were characterized by X-ray diffraction (XRD), N2 adsorption–desorption, scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), inductively coupled plasma atomic emission spectroscopy (ICP-OES) and NH3 temperature programmed desorption (NH3-TPD). The results revealed that the dual functions of APTES and increasing amount of TPAOH favored for forming higher intercrystalline mesopore volume in the hierarchical ZSM-5 zeolites while maintained the micropores. Additionally, the crystal size distribution of the hierarchical samples ranging from 250 to 330nm did not influence the diffusion rate of light olefins. The improved selectivity of both ethylene and propylene was then ascribed to the decreased diffusion resistance caused by the presence of secondary porosity, comparing with traditional ZSM-5. We also proposed a related model accordingly.