Preparation and reactivity of hierarchical catalysts in catalytic cracking

The objective of this review is to describe recent advances in preparation and reactivity of hierarchical catalysts for catalytic cracking of carbonaceous materials consisting of not only simple straight-chained and substituted aromatic hydrocarbons but also heavy molecules such as vacuum gas oil, atmospheric residue and biomass like vegetable oil. Catalysts for catalytic cracking are mainly composed of matrix and zeolite. It has been well-known that, since the content of zeolite is usually smaller than that of matrix, zeolite is approximately surrounded by matrix particles. This means that the catalysts of catalytic cracking forms a hierarchical structure even if major component of matrix would be kaolin which has not significant pores. Zeolite has micropores and very high activity for cracking while kaolin does not have the activity for cracking but the catalysts have the very large size and wide range of pores from micropores to mesopores and macropores which promote the diffusion of not only reactants but also products. Preparation, characterization and reactivity of zeolite with the high catalytic activity have extensively been reported, however it has not necessarily been clarified for a long time how the amounts and the sizes of pores in matrix affect the reactivity in catalytic cracking. Recent advances in preparation methods of not only zeolite but also mesoporous materials are remarkable and the advantages of hierarchical structure in catalysts for catalytic cracking have extensively been reported. These include novel mesoporous zeolites, the combination of zeolite and advanced mesoporous materials, and zeolite surrounded by mesoporous materials, which increased the activity and the selectivity for products in catalytic cracking. The review describes how the hierarchical catalysts are made, how the hierarchical structure act for the reaction and what are the roles of matrix and zeolite in detail.