Enhanced cracking of bulky hydrocarbons over hierarchical ZSM-5 materials: a comparative study

Abstract

A comparative study of hierarchical ZSM-5 materials including nanosized ZSM-5 (Nano-ZSM-5), nanosized ZSM-5/SBA-15 composite (Com-ZSM-5) and mesoporous ZSM-5 (Meso-ZSM-5) for the enhanced cracking of bulky hydrocarbons is reported. The studied materials were thoroughly characterized by XRD, SEM, TEM, N2-sorption, AAS, ICP-AES, NH3-TPD, FTIR of adsorbed pyridine before being tested in the cracking of 1,3,5 tri-isopropyl-benzene (TIPB) as a representative of bulky hydrocarbons. It was found that all synthesized hierarchical ZSM-5 materials exhibit the preservation of intrinsic, strong Bronsted acidity of ZSM-5 along with the enlarged external/mesoporous surface. The catalytic test results show that these hierarchical ZSM-5 materials indeed promote successive cracking reactions, leading to the enhanced TIPB conversion and selectivity to the deep cracking products, i.e. cumene and benzene compared to those of bulk, commercial ZSM-5. However, the development of mesoporosity by reducing the crystal size of ZSM-5 appears limited (Smeso = 134 m2 g−1). As a result, Nano-ZSM-5 affords only the moderate TIPB conversion and selectivity to deep cracking products (ca. 70% and ca. 20% respectively). Remarkably, the introduction of either intercrystalline mesopores by dispersion of Nano-ZSM-5 in the mesoporous SBA-15 analog matrix (Com-ZSM-5) or intracrystalline mesopores by the alkaline-acid treatments (Meso-ZSM-5) significantly improves the external/mesoporous surface (Smeso = 233–297 m2 g−1), giving rise to both the high TIPB conversion and selectivity to the deep cracking products (ca. 90% and ca. 25% respectively).