Great interest has arisen in the past years in the development of hierarchical zeolites, having at least two levels of porosities. Hierarchical zeolites show an enhanced accessibility, leading to improved catalytic activity in reactions suffering from steric and/or diffusional limitations. Moreover, the secondary porosity offers an ideal space for the deposition of additional active phases and for functionalization with organic moieties. However, the secondary surface represents a discontinuity of the crystalline framework, with a low connectivity and a high concentration of silanols. Consequently, hierarchical zeolites exhibit a less “zeolitic behaviour” than conventional ones in terms of acidity, hydrophobic/hydrophilic character, confinement effects, shape-selectivity and hydrothermal stability. Nevertheless, this secondary surface is far from being amorphous, which provides hierarchical zeolites with a set of novel features. A wide variety of innovative strategies have been developed for generating a secondary porosity in zeolites. In the present review, the different synthetic routes leading to hierarchical zeolites have been classified into five categories: removal of framework atoms, surfactant-assisted procedures, hard-templating, zeolitization of preformed solids and organosilane-based methods. Significant advances have been achieved recently in several of these alternatives. These include desilication, due to its versatility, dual templating with polyquaternary ammonium surfactants and framework reorganization by treatment with surfactant-containing basic solutions. In the last two cases, the materials so prepared show both mesoscopic ordering and zeolitic lattice planes. Likewise, interesting results have been obtained with the incorporation of different types of organosilanes into the zeolite crystallization gels, taking advantage of their high affinity for silicate and aluminosilicate species. Crystallization of organofunctionalized species favours the formation of organic–inorganic composites that, upon calcination, are transformed into hierarchical zeolites. However, in spite of this impressive progress in novel strategies for the preparation of hierarchical zeolites, significant challenges are still ahead. The overall one is the development of methods that are versatile in terms of zeolite structures and compositions, capable of tuning the secondary porosity properties, and being scaled up in a cost-effective way. Recent works have demonstrated that it is possible to scale-up easily the synthesis of hierarchical zeolites by desilication. Economic aspects may become a significant bottleneck for the commercial application of hierarchical zeolites since most of the synthesis strategies so far developed imply the use of more expensive procedures and reagents compared to conventional zeolites. Nevertheless, the use of hierarchical zeolites as efficient catalysts for the production of high value-added compounds could greatly compensate these increased manufacturing costs.

Research activities and recent developments in the area of three-dimensional zeolites and their two-dimensional analogues are reviewed.

Sunlight plays a critical role in the development of emerging sustainable energy conversion and storage technologies. Light-induced CO₂ reduction by artificial photosynthesis is one of the cornerstones to produce renewable fuels and environmentally friendly chemicals. Interface interactions between plasmonic metal nanoparticles and semiconductors exhibit improved photoactivities under a wide range of the solar spectrum. However, the photo-induced charge transfer processes and their influence on photocatalysis with these materials are still under debate, mainly due to the complexity of the involved routes occurring at different timescales. Here, we use a combination of advanced in situ and time-resolved spectroscopies covering different timescales, combined with theoretical calculations, to unravel the overall mechanism of photocatalytic CO₂ reduction by Ag/TiO₂ catalysts. Our findings provide evidence of the key factors determining the enhancement of photoactivity under ultraviolet and visible irradiation, which have important implications for the design of solar energy conversion materials.