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.

The present review is aimed at exploring the field of the catalytic cracking of polyolefins over solid acids, focusing on the role played by the catalysts toward the synthesis of fuels and chemicals as well as on the reaction systems currently used. Initially, conventional solid acids, such as micrometer sized crystal zeolites and silica−alumina, were used to establish the relationship among their activity, selectivity, and deactivation in the polyolefin cracking and the inherent properties of the catalysts (acidity, pore structure); however, the occurrence of steric and diffusional hindrances for entering the zeolite micropores posed by the bulky nature of the polyolefins highlighted the importance of having easily accessible acid sites, either through mesopores or by a high external surface area. This fact led toward the investigation of mesoporous materials (Al-MCM-41, Al-SBA-15) and nanozeolites, which allowed increasing the catalytic activities, especially for the case of polypropylene. Further advances have come by the application of hierarchical zeolites whose bimodal micropore−mesopore size distribution has turned them into the most active catalysts for polymer cracking. In this regard, hierarchical zeolites may be regarded as a clear breakthrough, and it is expected that future research on them will bring new achievements in the field of catalytic cracking of polyolefins. In addition, other materials with high accessibility toward the active sites, such as extra-large pore zeolites, delaminated zeolites, or pillared zeolite nanosheets, can also be considered potentially promising catalysts. From a commercial point of view, two-step processes seem to be the most feasible option, including a combination of thermal treatments with subsequent catalytic conversion and reforming, which allows the catalytic activity to be preserved against different types of deactivation.

A novel method for the synthesis of hierarchical zeolites has been developed, based on perturbing the crystal growth by organo-functionalization of the zeolite seeds. These materials present unique properties, such as a secondary porosity, enhanced surface area, and high catalytic activity for the conversion of bulky molecules.

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.

Feedstock recycling of plastic waste by thermal and catalytic processes is a promising route to eliminate this refuse (which is harmful to the environment) by obtaining, at the same time, products that are useful as fuels or chemicals. During the past decade, this option has undergone an important evolution from a promising scientific idea to an alternative that is very close to reality with commercial opportunities. Thus, several commercial processes have been developed worldwide, most of them especially addressed toward the preparation of diesel fuel. The present review highlights the most remarkable achievements of the field, providing a fundamental insight into this fascinating area and highlighting the main milestones that should be achieved in the next future for this alternative to become applied commercially on a large scale.

The catalytic degradation of both low-and high-density polyethylene (LDPE and HDPE) and polypropylene (PP) has been investigated using MCM-41, a mesoporous aluminosilicate recently discovered, as catalyst. The results obtained have been compared to those of ZSM-5 zeolite and amorphous silica-alumina. For all the studied plastics, MCM-41 has been found more active than the amorphous SiO 2 -Al 2 O 3 , as a consequence of the higher surface area and the uniform mesoporosity present in the former. Compared to ZSM-5, MCM-41 exhibits a lower activity for the degradation of linear and low branched polymers (HDPE and LDPE, respectively), which can be related to the higher strength of the zeolite acid sites. However, the opposite is observed for the cracking of highly substituted plastics such as PP due to the severe steric hindrances these molecules encounter to enter into the narrow pores of the zeolite, as confirmed by molecular simulation measurements. Moreover, for the cracking of LDPE, HDPE, and PP, the selectivities toward hydrocarbons in the range of gasolines and middle distillates obtained over MCM-41 are clearly higher than those of ZSM-5. Therefore, MCM-41 is a catalyst potentially interesting for the conversion of polyolefinic plastic wastes into liquid fuels.

Surface-passivating silanization of protozeolitic units has been shown to be an effective strategy for the preparation of ZSM-5 nanocrystals, showing a controlled aggregation degree and a hierarchical porosity. ZSM-5 zeolite materials are thus obtained with adjustable and relatively uniform mesoporosities that have a strong influence on resulting macroscopic reaction properties, especially for macromolecular reagents. The mean sizes of the nanounits and, therefore, the textural and accessibility of these materials can be varied by changing the precrystallization conditions and the concentration of the seed-silanization agent. In addition to conventional characterization techniques, solid-state two-dimensional (2D) nuclear magnetic resonance (NMR) spectroscopy measurements and the application of the NLDFT model to the argon adsorption isotherms have allowed both the local and the mesoscopic compositions, as well as the structures of the hierarchically porous ZSM-5 materials, to be established. The resulting combination of mesopore sizes and exterior-nanocrystal surface properties of the hierarchically structured ZSM-5 zeolites is shown to catalyze reactions that are otherwise limited by steric and/or diffusional limitations, as demonstrated by their enhanced activity for polyethylene cracking.

Research activities and recent developments in the area of three-dimensional zeolites and their two-dimensional analogues are reviewed. Zeolites are the most important industrial heterogeneous catalysts with numerous applications. However, they suffer from limited pore sizes not allowing penetration of sterically demanding molecules to their channel systems and to active sites. We briefly highlight here the synthesis, properties and catalytic potential of three-dimensional zeolites followed by a discussion of hierarchical zeolites combining micro- and mesoporosity. The final part is devoted to two-dimensional analogues developed recently. Novel bottom-up and top-down synthetic approaches for two-dimensional zeolites, their properties, and catalytic performances are thoroughly discussed in this review.