Dual Roles of Steam in the Dry Gel Synthesis of Mesoporous ZSM-5

Abstract

Mesoporous HZSM-5 was synthesized with a SiO2/Al2O3 ratio of 100 using a steam-assisted crystallization method. The resulting material was characterized by N2 physisorption, powder X-ray diffraction, focused ion beam-scanning electron microscopy, energy dispersive X-ray analysis, X-ray photoelectron spectrometer (XPS) depth profiling, NH3-temperature programmed desorption (TPD), pyridine-TPD, and collidine-TPD. Crystallization time was varied to determine the effect on material physical properties, total acidity, accessible acid sites, and aluminum distribution within the zeolite particles. The Barrett, Joyner, and Halenda mesopore volumes, Brunauer, Emmett, and Teller surface areas, and total pore volumes decreased with longer crystallization times, while the mesopore sizes increased with time. At prolonged crystallization time, the material underwent dealumination via steam etching followed by reforming of material, thus blocking accessibility to the mesopores. The total acidity was optimized at 18 h of crystallization time. In addition to total acidity, the ratio of external and mesoporous acid sites to total acid sites decreased with increased crystallization time. Although a relatively uniform distribution of Al was observed across the center of the 18 h particles, XPS depth profiling showed an enrichment of aluminum on the outer shell of all HZSM-5 particles (“aluminum zoning”) with this steam-assisted crystallization method. Thus, crystallization time should be strongly monitored when using this dry-gel synthesis method with soft templates.