Modern synthesis strategies for hierarchical zeolites: Bottom-up versus top-down strategies

Zeolites are well known for their ordered microporous networks, good hydrothermal stability, large surface area, high acidity and selectivity. These excellent properties make zeolites extremely useful for petrochemical processes and refining. However, the presence of only microporous channels also restricts the diffusion of reactants and products into and out of the microporous networks, especially limiting zeolite applications involving bulky molecules. The importance of developing hierarchical zeolites has attracted great attention in recent years due to the prospect of increased accessibility for bulky molecules. Introducing additional mesoporosity, and even macroporosity, into conventional zeolites produces a combination of three different size scales of porosity. It expands the original zeolite hierarchical structure and greatly enhances the mass transport of molecules while maintaining the intrinsic size, shape and transition state selectivity of zeolite. The promising applications of this new zeolite architecture have prompted a multitude of efforts to develop a variety of different synthesis strategies. In this review, we summarized and evaluated the modern synthesis strategies (bottom-up and top-down) for introducing additional meso/macroporosity into microporous zeolites. The advantages and limitations of these different strategies were discussed in detail.