Hierarchical ZSM-5 zeolites synthesized by silanization of protozeolitic units: mediating the mesoporosity contribution by changing the organosilane type

Abstract

Hierarchical ZSM-5 zeolites were prepared by crystallization of silanized protozeolitic units employing silylated polypropylene oxide diamine polymers as organosilanes. The influence of the (Sipol/ Sigel) molar ratio was investigated within 0 - 0.15 range. High synthesis yields (~90%) of well-crystallized hierarchical zeolites exhibiting a high proportion of secondary porosity (additional to the zeolitic micropores) was reached for (Sipol/Sigel) molar ratios lower than 0.08. The usage of the silylated polymer resulted in hierarchical ZSM-5 with larger mesopores (4-20 nm) in higher share than the hierarchical ZSM-5 prepared with a smaller organosilane (phenyl-aminopropyl-trimethoxysilane, PHAPTMS). However, it also contained meaningfully lower amount of acid sites and with less acid strength. The best catalytic performance in the cracking of low density polyethylene (LDPE) was showed by the material prepared from a (Sipol/Sigel) ratio of 0.03. Noteworthy, in addition to the gasoline range fraction (C6-C12), light C1-C5 olefins are the main reaction products, which are interesting feedstock for the petrochemical industry. Its catalytic performance is similar to the hierarchical ZSM-5 prepared using the smaller organosilane (PHAPTMS), which is indicative that the enhanced accessibility to the acid sites due to the presence of larger mesopores (4-20 nm) makes up for the lower amount and strength of its acid sites. Thereby, it is possible to enhance the mesoporosity by using bulkier organosilane (silylated polymers) but at the expense of losing acid properties.