Synthesis And Catalytic Performance Of Hierarchical Zeolites

Zeolites are crystalline microporous aluminosilicate materials and widely used in catalysis, purification, adsorption and separation field, especially in the refining and (petro)chemical industries due to high surface area, intrinsic acidity and high (hydro)thermal and chemical stability. However, the sole presence of micropores in zeolites strongly hinder the diffusion of reactants and products, resulting in the low catalytic efficiency and the fast deactivation of the zeolites by the frequent blocking of the diffusion path. Hierarchical zeolites, combining both the advantages of mesoporous materials and microporous zeolites, have been considered as novel catalytic materials. Therefore, the design and synthesis of hierarchical zeolites have been drawn a great deal of attention. Based on the fundamental questions in hierarchical zeolite synthesis such as high synthesis costs and complicated synthesis procedure. In this thesis, we hope to overcome these problems by using commercial porogen (low-cost) or simplying the synthesis processes of hard templating method. The main research results are achieved as follows:(1) The hollow zeolite microspheres were synthesized in the presence of organosilanes via a dissolution-recrystallization procedure. The amount of organosilanes, crystallization temperature and time have great inluence on the porous structures and morphology of the final product. The formation mechanism of hollow zeolite microspheres was carefully studied by tracking the synthesis process and found that the formation of the hollow structure was attributed to the amorphous framework in the core rather than the Al distribution gradient existed in zeolite crystals. In addition, hollow Silicalite-1and hollow TS-1were also prepared via this similar synthesis method.(2) The hierarchical ZSM-5zeolites with intra-and inter-crystalline porous structures were synthesized via a steam-assisted conversion in the presence of polyethylene glycol (PEG). The mesopore volume of Hier-ZSM-5can be adjusted by changing the amount of PEG. The results of catalytic reactions revealed that all of the samples Hier-ZSM-5exhibted superior catalytic activities for conversion of bulky substrates. Importantly, a complete conversion into such hierarchical ZSM-5zeolites with high crystallinity was achieved in only12h at160℃. In addition, such hierarchical ZSM-5zeolites exhibited high mechanical stability even if subject to ultrasonic treatment for1h. The high catalytic activity and easy separation from liquid mixture of Hier-ZSM-5facilitate its practical application in chemical industry.(3) The synthesis of nanorods oriented-assembled hierarchical MFI zeolite microspheres was realized for the first time via hydrothermal treatment of amorphous silica in the confined-space of Carbon/Silica monolith. Several parameters (surface property of carbon, the weight ratio of SiO2/C, the amount of H2O) would influence the morphology and pore structures of the final product. The growth of such unique zeolite microspheres undergoes a reversed crystal-growth route:Crystallization starts on the surface of the amorphous ellipsoidal particles to form a large amount of nanoparticles, then these nanoparticles can further grow into nanorods by consuming the aluminosilicate species in the core, which results into nanorods oriented-assembled structures, due to the confined-space effectment of carbon in the carbon/silica monolith. In addition, Hier-ZSM-5and Hier-TS-1were aslo synthesized via this similar synthesis method. All of these zeolite microspheres show high mechanical stability. Thus, such novel hierarchical zeolite microspheres show great potential for industrial applications.(4) Hierarchical Beta zeolite microspheres were synthesized via a steam-assisted conversion of Carbon/Aluminosilicate monolith. Such hierarchical Beta zeolite microspheres are composed of Beta zeolite nanocrystals and show superior catalytic activity in the dehydration of fructose into HMF than conventional Beta zeolite. Thus, such hierarchical Beta zeolite microspheres which combine both the advantages of mesoporous materials and microporous zeolites show great potential for biomass conversion.