Zeolite Nanocrystals- Synthesis and Applications

Abstract

Zeolites, crystalline aluminosilicate materials, possess 3-dimensionaly connected framework structures constructed from corner-sharing TO4 tetrahedra, where T is any tetrahedrallycoordinated cation such as Si and Al. These framework structures are composed of n-rings, where n is the number of T-atoms in the ring (e.g. 4-, 5-, and 6-rings), and large pore openings of 8-, 10-, and 12-rings are framed by these small rings. Figure 1 shows the pore sizes and framework structures of typical zeolites. The sizes of the intracrystalline pores and nanospaces, depending on the type of zeolite providing the framework, are close to the molecular diameters of lighter hydrocarbons. Moreover, strong acid sites exist on the nanopore surfaces, enabling the zeolites to be used as shape-selective catalysts in industrial applications such as fluid catalytic cracking of heavy oil, isomerization of xylene and synthesis of ethyl-benzene. However, compared to the sizes of micropores exhibiting a molecular-sieving effect, the crystal sizes of zeolites are very large, approximately 1–3 μm. When the zeolite is used as a shape-selective catalyst, the diffusion rates of reactant molecules within the zeolite crystals are lower than the intrinsic reaction rates. This resistance to mass transfer limits the reaction rates and low selectivity of intermediates. Moreover, since effective active sites (acid sites) for catalytic reactions are distributed on the internal surfaces of the main channels and the external surfaces of the crystal, the pore mouths are easily plugged due to coke deposition, leading to short lifetimes for the catalysts.