Bioinspired Organization Of Hierarchical Zeolites With A Nanotube-trimodal Network

Biomineralization involves the selective extraction and uptake of elements from the local environment and their incorporation into functional structures under strict biological control. This process is controlled by organic matrix as template for inorganic material precipitation. Moreover specific sites on surface of template control nucleation, growth and assembly of minerals. Biominerals have sophisticated hierarchical architecture that meets the needs of the survival of the organism.This thesis is devoted to develop a general strategy for rational organization of hierarchical zeolites inspired by biomineralization. Zeolites are microporous aluminsilicates of paramount importance in refineries and the petrochemical industry as shape-selective catalysts due to their ordered micropores, acid strength and thermal/hydrothermal stability. But the sole presence of micropores with window openings smaller than 2 nm often causes restricted access and diffusion limitations that adversely affects their catalytic activity. Hierarchical zeolites that integrate the catalytic activity of zeolitic micropores with the transport advantage exhibit higher catalytic activity and longer catalytic lifetime. This paper developed bio-polysaccharide scaffolding strategy for hierarchical zeolite with nanotube-trimodal inspired by biomineralization. Detailed research works are summarized as followings:(1) Hierarchical ZSM-5 with nanotube-trimodel network was first synthesized by BC scaffolding strategy. The diameter of inter-connected nanotubes is uniform(~300 nm). The wall of nanotube is made of a single layer of intergrown nanocrystals of ZSM-5 with an approximate size of 70 nm. The mesopore(~2 nm) formed by ZSM-5 nanocrystals can directly connect the space between internal and external of the nanotubes. The hollow nanotubes were self-generated via the Kirkendall effect. Based on Kirkendall effect, the inner diameter of the ZSM-5 nanotube can be tuned between 30-90 nm by varying the thickness of the aluminosilicate layer. (2) The ZNTS ZSM-5 exhibits superior activity in method-to-hydrocarbon(MTH) reaction compared to conventional ZSM-5. ZNTS ZSM-5 shows longer catalyst lifetimes(80 h) and lower coke formation rate under the same weigh hourly space velocity(WHSV=8.0 h-1). We believe that the unique nanotube scaffolding architecture is the key reason for the excellent catalyst stability of ZNTS ZSM-5.(3) The feasibility of the current strategy is manifested by the successful fabrication of ZNTS TS-1 and ZNTS S-1 from the silicalite-1 seeded precursor scaffolds of BC@TSO and BC@SO respectively. Ti4+ ions in ZNTS TS-1 are found exclusively in tetrahedral environment based on UV-vis spectrum. The scaffolding template can be replaced by other kinds of polysaccharide such as chitosan and the macropore size of the product ZNTS S-1(Ch) is 2000 nm. Therefore, the current polysaccharide scaffolding strategy for hierarchical zeolite is versatility.(4) The macroporous structure can be tuned by adjusting structure of scaffolding template. We prepare regenerated cellulose aerogel with different macroporous size by adjust concentration of cellulose solution(2 wt%, 3 wt%, 5 wt%). Templated by these regenerated cellulose aeraogel, ZNTS S-1(RC) with different macroporous size(1-10 μm) can be obtained.(5) The mesoporous structure can be tuned by changing synthesis strategy. Changing the vapor phase treatment to hydrothermal treatment, ZNTS S-1(L) without pores on tubes wall was obtained. As a result, the mesoprous volume of ZNTS S-1(L) is reduced and furthermore, the space inside and outside tubes of ZNTS S-1(L) can’t interconnect directly. Changing amorphous silica to zeolite precursor that contained MFI nanoclusters as silica source, NP S-1 with size of 400 nm is obtained. The uniform mesopores of NP S-1 with size of 15-35 nm are templated by fibers of cellulose.(6) Inspired by functional structure of BC, that confines bacteria in it to protect bacteria from harsh environment, catalase was immobilized on zeolite with similar structure as BC. Loading amount of catealase was associated with the external surface area and mesoporous structure of carriers. Loading amount of catalase on ZNTS S-1 was higher than other carriers. Catalase immobilized on ZNTS S-1 was more stable and was convenience to recycle. The activity was still above 50% after 12 numbers of recycles.