Synthesis, Characterization And Catalytic Activity Of Hierarchical ZSM-5Zeolite

Hierarchical zeolites possess good performance of diffusion, high thermal/hydrothermal stability and carbon deposition resistance, playing an important role in heavy oil cracking, macromolecular catalysis and the fine chemical field. The design, synthesis and catalytic application of hierarchical zeolites have become the hotspot. The synthesis method of hierarchical porous zeolites is roughly divided into two categories:the post-processing method, including desiliconization and dealuminated method and the direct synthesis method, including hard template method, soft template method, and the in-situ assembly of zeolite nanocrystals into mesoporous aggregate using small organic molecule as single template without secondary template method. However, two different templates for micropore and mesopore worked in a competitive manner rather than a cooperative manner, which easily results in the formation of physical mixtures of amorphous mesoporous material and bulky zeolite. To reduce the competition, double function template agent is a good choice. Although different morphologies of hierarchical zeolites can be prepared with them, it is difficult to large-scale application combined with the high cost and complexity in the fabrication of templates. A weak structure-directing agent of 1,6-diaminohexane (HDA) was used herein for the preparation of zeolite ZSM-5, simple and easy surfactant cetyl trimethyl ammonium bromide (CTAB) was used as the mesopogeneous template, by controlling synthesis conditions, to minimize the mismatch of nanocrystals and mesophase. Our works include two parts:In the first part, hierarchical ZSM-5 aggregates were fabricated using CTAB as mesoporogen together with 1,6-diaminohexane as structure-directing agent. The highly crystallized hierarchical ZSM-5 aggregates with large specific surface area (426.4 m2/g), pore volume (0.46 cm3/g) and regular mesopore size distribution were obtained. And they showed better catalytic activity in LDPE pyrolysis reactions. The influence of the silicon sources, silica alumina ratio, CTAB and its dosage on the physicochemical properties of hierarchical ZSM-5 zeolites was investigated. We also explored the mechanism of the synthesis of hierarchical ZSM-5 zeolite and the role of CTAB in the process of synthesis.In the second part, hierarchical ZSM-5 zeolites were fabricated using CTAB as mesoporogen together with Sililite-I colloidal seeds, SiO2/Al2O3 is between 30 to 600 while external specific surface area is between 54.5 to 113.3 m2/g. Moreover, the one with the SiO2/Al2O3=100 gave the highest conversion of 47% in the hydrodeoxygenation of guaiacol with one hour reaction time. The acidity of hierarchical ZSM-5 zeolites was a critical factor to affect the catalytic activities. Finally we optimized the best preparation condition of high silica alumina ratio of hierarchical ZSM-5 zeolites.