Fabrication and characterization of hierarchical ZSM-5 zeolites by using organosilanes as additives

Two synthesis strategies were utilized to fabricate hierarchical ZSM-5 zeolites by using 3-aminopropyltrimethoxysilane (APTMS), N-[3-(trimethoxysilyl)propyl] ethylenediamine (TMPED) and (N,N-diethyl-3-aminopropyl)trimethoxysilane (DATMS) as additives. The first synthesis strategy comprises three steps: preparation of the zeolite precursors, anchoring of the organosilanes onto the zeolite precursors, and formation of the hierarchical ZSM-5 zeolites. The second synthesis strategy is similar to the above strategy, except that the organosilanes are directly added into the original reaction solution. The hierarchical ZSM-5 zeolites exhibit an ellipsoidal shape with a long-axis length of ∼750 nm and short-axis length of ∼550 nm, which include lots of nanocrystals with a size of ∼25 nm. The aggregation of the nanocrystals leads to forming mesopores with a size of 2.8 nm and supermicropores with a size of 1.4 nm or 1.8 nm. However, the ZSM-5 zeolites prepared without using the organosilanes as additives exhibit an irregular shape with a broad particle size distribution. Benzene alkylation with ethene was carried out with ZSM-5 zeolites in a fixed bed reactor. The results show that the hierarchical ZSM-5 zeolites exhibit the higher C2H4 conversion and ethylbenzene selectivity than the conventional ZSM-5 zeolites.