Design, Synthesis And Catalytic Applications Of Hierarchically Porous Zeolites

Diffusion performance,the main mechanism of catalysts,is of crucial importance for their application in industrial catalysis,especially for the diffusion-controlled process.Zeolites,the most widely used heterogeneous catalysts,are often victims of their own success.The sole presence of micropores impose severe mass-transfer constraints,leading to low catalytic efficiency and short catalytic life.Hierarchically porous zeolites,integrating multiple porosity,emerged as an efficient method to solving this dilemma and attracted growing attentions.Guided by the requirements of industrial catalytic applications and the scientific thought of ‘need-problem-design-synthesize-optimize’,this doctor thesis,aims at realizing the fully interconnectivity between the various levels of pores,designing and synthesizing hierarchically porous zeolites with intercrystalline and intracrystalline porous structure,investigating the crystalline mechanism of chemically crystallizing process and steam-assisted crystallizing process respectively and optimally tune the pore structures and properties.The as-synthesized hierarchically porous zeolites are characterized by a serious of testing techniques and tested in model catalytic reactions for studying the influence of ordered macro-meso-microporous hierarchy on their catalytic performances.The optimal design and synthesis of hierarchically porous zeolites: well-organized MFI zeolitic nanocrystal aggregates with an interconnected hierarchically ordered porous system,are achieved.Multipole levels of ordered porosity from micro-,meso-and macropore being appropriately connected in one solid body are successfully fabricated.The advantages of both hierarchical porous structure and nanosized effect are present in the resultant one solid body including enhanced mass transport and short effective diffusion path which resulting the superior catalytic property in the cracking of bulky 1,3,5-triisopropylbenzene(TIPB)compared to traditional zeolite catalysts.Meanwhile,the chemical crystallization transformation processes from amorphous silica nanoparticles to crystalline zeolite nanocrystals are investigated in detail.This strategy could be extended to prepare various hierarchically ordered porous zeolites(Silicalite-1,ZSM-5,TS-1,etc.)with micro-meso-macroporous structures,as well as tunable porosity.In order to further improve the stability of hierarchically porous zeolites,we further dedicated to introduce intracrystalline hierarchically porous structure into zeolites.We successfully developed a strategy to synthesize zeolite single crystals with a highly interconnected and ordered intracrystalline macro-mesoporous hierarchy by a templating method via a steam-assisted crystallization(SAC)process.The mesopore and macropore diameter is tailorable according to the size of the building blocks in the resulting catalysts.The high interconnectivity of the intracrystalline porous hierarchy results in a substantially improved diffusion performance for various organic reactants including bulky molecules compared to commercial zeolites,strongly promoting the catalytic performance.This suggests that the hierarchically porous carbon templating method combined with the SAC process that targets the intracrystalline hierarchically macro-mesoporous system in the zeolite,is fairly general and can be extended to other zeotype materials,such as Beta,TS-1,et al.