Hierarchically structured zeolites: synthesis, mass transport properties and applications

Zeolites with hierarchically porous structures have garnered much attention due to their highly attractive properties. Hierarchically structured zeolites integrate at least two levels of porosity and present the advantages associated with each level of porosity, from selectivity to mass transport. They are categorized into three distinctly different types according to their hierarchical porosities: mesostructured zeolites, macrostructured zeolites, and micro-meso-macroporous structured zeolites. Most importantly, hierarchically structured zeolites offer an effective solution to the mass transport problem associated with conventional zeolites in catalysed reactions because they combine the catalytic features of micropores and the improved accessibility and increased molecular transport related to the addition of several porosities within a single body. In recent years, many strategies have been successfully developed to synthesize hierarchically structured zeolitic materials. This feature article thoroughly summarizes recent developments that have been achieved in the field of hierarchically structured zeolites, with the main focus on the synthesis strategies that are available, with examples given from the literature. Available approaches are reviewed for the preparation of micro-mesoporous structured zeolites, micro-macroporous structured zeolites and micro-meso-macroporous structured zeolites. Furthermore, the enhanced mass transport properties of hierarchically structured zeolites, featuring additional larger pores in addition to the crystalline micropores, have also been described. The significant improvement in catalytic properties in a range of important reactions resulting from enhanced mass transport properties have also been discussed through several representative cases. It is the intent of this work to stimulate intuition into the optimal design of related hierarchically organized zeolites with desired characteristics.