Proceedings of the 4 th Slovenian-Croatian Symposium on Zeolites 5 IMPROVING THE ACCESSIBILITY AND CATALYTIC ACTIVITY IN ZEOLITES THROUGH THE GENERATION OF HIERARCHICAL POROSITY David P. Serrano ,1,2 , José Aguado 3 , José M. Escola 3 1 Department of Chemical and Energy Technology, ESCET, Universidad Rey Juan Carlos, c/ Tulipán s/ n, 28933, Móstoles, Madrid (Spain) 2 IMDEA Energy Institute, c/Tulipán s/n, 28933, Móstoles, Madrid (Spain) 3 Department of Chemical and Environmental Technology, ESCET, Universidad Rey Juan Carlos, c/ Tulipán s/n, 28933, Móstoles, Madrid (Spain) E-mail: david.serrano@imdea.org ABSTRACT A great interest has grown in recent years in the scientific community on the synthesis and application of hierarchical zeolites, i.e. zeolites having at least two levels of porosity. The presence of a secondary mesoporosity, in addition to the typical zeolite micropores, in these materials provides them with singular properties and, therefore, with a high potential for novel applications in different fields. Hierarchical zeolites possess enhanced textural properties and increased mass transfer rates. As a consequence, they suffer of quite lower steric and diffusion limitations than conventional zeolites, which is especially important properties in the processing of bulky molecules. Hierarchical zeolites have been also reported to show a better resistance to deactivation by coking. In addition, the availability of the additional mesoporosity provides an ideal space for supporting other active phase with a high dispersion and strong interaction with the support, opening new ways for the development of improved bifunctional zeolitic catalysts. Although selectivity towards the target products may be negatively affected by the occurrence of non-desired reactions over the mesopore surface of hierarchical zeolites, the results so far reported indicate that this does not occur in many cases, hence it is possible to achieve zeolitic catalysts with enhanced accessibility and catalytic activity while preserving at the same time a remarkable selectivity. Keywords: hierarchical zeolites, mesoporous zeolites. INTRODUCTION Zeolites have been traditionally defined as microporous crystalline aluminosilicates, being usually obtained in the form of crystals/particles within the micrometer range. The occurrence of uniform micropores with molecular dimensions (generally 0.4 – 0. 75 nm) turns them into molecular sieves. This unique property allows the zeolites to discriminate among different reactants, products or even transition states according to their shape and size and has led to the remarkable selectivities exhibited by zeolites in a large number of reactions. However, zeolites fail when dealing with bulky substrates, showing low activities, since the latter cannot access the active sites located inside the zeolite micropores. Thus, only those sites situated over the external surface of the zeolite crystals, or close to the micropore openings, are accessible for large molecules. However, these sites represent usually a low share (< 5%) of the total content of active sites. On the other hand, even when the substrate can enter into the zeolite micropores, the diffusion rate is usually too slow, which causes the appearance of intracrystalline mass transfer constraints. One of the most successful strategies that have been developed in recent years for improving the accessibility in zeolites is likely the case of hierarchical zeolites [1, 2]. These