Fine-tuning hierarchical ZSM-5 zeolite by controlled aggregation of protozeolitic units functionalized with tertiary amine-containing organosilane

Abstract

Hierarchical ZSM-5 zeolite has been synthesized by hydrothermal crystallization of protozeolitic nanounits previously functionalized with two types of commercially available organosilanes. The first one (N,N-diethyl-3-aminopropyl-trimethoxysilane, DEAMP) is a monopodal organosilane that contains a tertiary amine group, whereas the second one (1,2-bis(trimethoxysilyl)decane, BTMDC) includes a long aliphatic chain and two tri-alkoxysilyl groups (dipodal organosilane). The properties of the samples obtained have been compared with two nanocrystalline ZSM-5 materials as references. While both organosilanes are effective in generation of a secondary porosity, enhancing strongly the textural properties of ZSM-5 zeolite, DEAMP affords for more uniform nanounits in the aggregates with narrower mesopore size distribution compared to BTMDC. On the other hand, varying the concentration of DEAMP during the silanization step (in the range of 2.5-10 mol%, referred to the silica content of the synthesis gel) it has been possible to synthesize hierarchical ZSM-5 samples showing progressively smaller size of the nanounits in the aggregates. This fact has led to important changes in both the textural and acid properties of the zeolite samples. In particular, a decrease in the Brunsted/Lewis acid sites ratio has been observed with increasing the concentration of DEAMP introduced in the synthesis gel. These materials have been tested as catalysts for low-density polyethylene (LDPE) cracking, showing enhanced catalytic activity in comparison with the two reference nanocrystalline ZSM-5 samples. A maximum of the LDPE conversion has been obtained with the h-ZSM-5 zeolite prepared with 5 mol% of silanization agent as this material exhibits an optimum combination of acidic features and accessibility.