Synthesis and evaluation of seed-directed hierarchical ZSM-5 catalytic supports: Inductive influence of various seeds and aluminosilicate gels on the physicochemical properties and catalytic dehydrogenative behavior

Abstract

TPABr-silicalite-1 and TPAOH-silicalite-1 seeding suspensions were employed as structure directing agents through a seed-directed synthesis strategy to prepare a series of organotemplate-free hierarchical ZSM-5 zeolites with three different Si/Al molar ratios of 30, 60 and 120. No secondary meso-generating agent was implemented to construct wide supplementary pores. The physicochemical properties of the products were characterized by XRD, FESEM, FT-IR, EDX, AAS, TGA and N2 adsorption–desorption techniques. All zeolite samples with moderate hierarchy factors possessed a bi/tri-model porosity based on the governing nucleation mechanism and growth of zeolite crystals upon the surface of the silicalite-1 seeds. The prepared hybrid zeolites with intrinsic micropores and auxiliary meso/macro pores were served as the catalytic supports in propane dehydrogenation reaction to ascertain the influence of the seeding type, Si/Al ratio and consequent structural and particulate properties on the catalytic performance. The best catalytic result was acquired for Pt–Sn-based ZSM-5 catalyst, with Si/Al = 60, synthesized with less expensive TPABr-silicalite-1 seeds. Considerable activity (34.5% conversion) and high selectivity towards light olefins (90% for both ethylene and propylene); propylene in particular (84.5%) was observed after 4 h time on stream. This result is attributed to the high BET surface area and meso transport pores of the sample collaborated with high acidity to progressively impact on the reactivity and selectivity.