Challenges on molecular aspects of dealumination and desilication of zeolites

Zeolites are widely used heterogeneous catalysts in the field of chemistry and refining. These microporous and crystalline aluminosilicates exhibit a strong Bronsted acidity making them attractive for processes such as hydrocracking and fluid catalytic cracking. However, micropores can induce diffusion limitations and confinement effects resulting in the formation of undesired side products. Posterior introduced mesopores, leading to the so called "hierarchical zeolites" can overcome these phenomena. They contain bi- or multimodal pore structures which can be post-synthetically introduced by partial dealumination and/or desilication of the framework. In the last years these hierarchical zeolites became of great interest for the scientific community. Synthesis procedures and their consequences on the properties of zeolites were optimized in a great extent at a mesoscale. However, at a molecular scale the mechanisms of this mesopore formation and their impact on the nature of the resulting active sites were much less debated in the literature. In the present review, we focus on state-of-the-art knowledge of these aspects at the molecular scale. Spectroscopy techniques (such as NMR, FTIR, XAS), microscopy and density functional theory approaches, dealing with the mechanism of Si/Al removal and nature of the resulting extra-framework species are gathered and discussed. This analysis of the literature reveals that the atomic scale's understanding of the mesopore formation during desilication and dealumination remains a current scientific challenge. (C) 2014 Elsevier Inc. All rights reserved.