Hierarchical ZSM-5 Zeolite Fabricated with Loosely Nanocrystallite Aggregates without Secondary Template

Hierarchical ZSM-5 with loose nanocrystallites was synthesized by a traditional hydrothermal procedure. Factors controlling the formation of hierarchical zeolite were investigated in detail. Despite having comparable acid properties, the increased external surfaces offer the hierarchical ZSM-5n an excellent catalytic performance. After reaction for 414 h, the hierarchical ZSM-5n catalyst still keeps a 88.5% conversion of isopropylbenzene, while the ZSM-5 mu and ZSM-5m catalysts only show 37.5% (25.8 h), 28.2% (11 h) conversions, respectively; the initial conversion of n-octane over ZSM-5n is 94.3 wt %, obviously higher than 82.8 wt % on ZSM-5 mu, the half-life period related to the catalytic activity on ZSM-5n and ZSM-5 mu are 271 h, and 22 h, respectively. The prolonged catalysis life for hierarchical ZSM-5n during the catalytic cracking of n-octane is ascribed to the shortened diffusion path length and an "excretory system", carbon nanotubes (CNTs) self-generating during the cracking processing. The shortened microporous channels attribute to weakening adsorption energy barrier and promoting the coke precursors rapid diffusing toward the pore windows. Moreover, the autogenetic CNTs help the carbon precursors escaping from the pore windows and keep them far away from the zeolite crystals in the catalysts. That avoids the carbon deposit in the micropores as well as in the pore windows and on the external surface of the catalyst.