Amongst the current developments in the field of hierarchical pore structures, the creation of mesopores in zeolite crystals is the most frequently employed way to combine micropores with mesopores in one material. In this review an overview is presented of the different approaches to generate and characterize mesopores in zeolite crystals and establish their impact on the catalytic action. Mesopores can be created via several routes from which steaming and acid leaching are the most frequently applied. Novel approaches using secondary carbon templates that are removed after synthesis have recently been launched. For the characterization of mesopores, nitrogen physisorption and electron microscopy are commonly used. More recently, it was shown that electron tomography, a form of three-dimensional transmission electron microscopy, is able to reveal the three-dimensional shape, size, and connectivity of the mesopores. The effect of the presence of mesopores for catalysis is demonstrated for several industrially applied processes that make use of zeolite catalysts: the cracking of heavy oil fractions over zeolite Y, the production of cumene and hydroisomerization of alkanes over mordenite, and synthesis of fine chemicals over Y, ZSM-5, and Beta. For these processes, the mesopores ensure an optimal accessibility and transport of reactants and products, while the zeolite micropores induce the preferred shapeselective properties.

International audienceWith the necessity for the refining industry to treat heavier feedstocks, there is a clear demand for improved zeolite materials displaying better accessible surface areas and higher pore volumes in order to capitalize on their effectiveness. To this end, over the last decade, there has been an intensification of research on the exploration of new routes to synthesize zeolite materials combining micropores with mesopores. Different synthesis strategies are used for their preparation (i.e., by structure breaking or so-called 'destructive' pathways, or structure building or so-called 'constructive' synthesis pathways). This Review discusses the variety of current synthesis strategies, while emphasizing the strengths and weaknesses of the different routes regarding material characteristics; health, safety, and environment aspects; and synthesis costs

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