Zeolites are microporous catalysts that are widely used in industry. However, for molecules with sizes similar to the micropores, mass-transfer resistance may limit the accessibility to catalytic sites. Introducing mesopores and/or macropores into microporous zeolites can reduce the size of microporous domains (i.e., shorten the diffusion lengths) and increase accessibility. Such zeolites with multilevel porous structures are commonly known as hierarchical zeolites. Opening up porosity in hierarchical zeolites increases the fraction of surface that is outside the micropores — called external surface area — and it increases the portion of active sites on the external surface relative to those within micropores.
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