Catalytic Materials

Catalytic materials exist in several forms and can be prepared using various methods involving different schemes and protocols. They can also be applied in many fields, such as environmental and sustainable catalysis, biomass valorization, renewable fuels production, CO₂ recycling, synthetic chemistry, gas storage/capture, drug delivery, catalysis, photocatalysis, chemical sensing, and so on.

The present Section on Catalytic Materials aims to advance our understanding of heterogeneous catalysis, offering a comprehensive investigation of physicochemical properties, catalytic application, and the structure–activity relationship.

Among the different catalytic materials investigated in the present Section, it is worth mentioning hybrid materials, which are composites of organic and inorganic constituents that are characterized by peculiar properties due to the synergetic effects of their organic and inorganic components. New efficient and eco-sustainable hybrid materials find application in chemical and enzymatic catalysis, photocatalysis, electrocatalysis, and catalytic transformation to target chemical compounds or key platform molecules.

Another important class of catalytic materials herein investigated is that of hybrid metal-free nanostructures (e.g., POSS organic–inorganic hybrid molecules) which are able to convert CO₂ and epoxides into cyclic carbonates that are interesting compounds finding applications as aprotic polar solvents, electrolytes for batteries, sources for reactive polymers synthesis, and precursors for pharmaceuticals.

Metal organic frameworks (MOFs) are a class of porous materials with a modular structure for advanced applications, such as adsorption, gas storage/capture, drug delivery, catalysis, photocatalysis, chemical sensing, and so on. A Special Issue of this Section is devoted to such materials.

Among the various catalytic materials herein investigated, we cannot leave out microporous zeolites and nanoporous materials, which are important broth from an academic and from an industrial research point of view, thanks to their unique properties, such as their uniform pores, channel systems, shape selectivity, resistance to coke formation, and thermal and hydrothermal stability. Furthermore, the possibility to tune the amount and strength of Brønsted and Lewis acid sites and the possibility to introduce modifications with transition and noble metals are key to the successful design of efficient, highly-selectivity, and stable systems.

Organofluorine compounds are substances of considerable interest in various industrial fields. Fluorine is now an important element thanks to the unique properties associated with the nature of this atom and its bond to carbon, its high electronegativity, and relatively small size. Due to these attractive properties, organofluorine compounds have been widely used in the design of pharmaceuticals, agrochemicals, refrigerants, dyes, liquid crystals, optical fibers, and highly-durable polymers. Moreover, due to the increasing need for fluorine-containing molecules in diverse fields of science and technology, selective synthesis of organofluorine compounds constitutes one of the most challenging issues of modern organic chemistry.

Rare earth catalysts are currently widely involved in the field of coordination polymerization, as they can produce high added-value stereoregular polymers or copolymers. In that frame, the design of well-defined ligands in order to tune the activity or selectivity of the polymerization catalysts plays a key role. Rare earth polymerization catalysts were first mainly dominated by metallocene complexes, before the more recent development of non-Cp, post-metallocene systems. The emergence of undercoordinated cationic catalytic species was also a breakthrough in the field, leading to extremely active and selective systems towards olefins and dienes. More recently, in a context of sustainable chemistry, many efforts have been made to develop ring opening polymerization (ROP) catalysts of cyclic esters to produce various biodegradable polymers.

From its first catalytic application in three-way catalysts more than 40 years ago, ceria and ceria derivate oxides have been widely employed in energy conversion and environmental issues. The ever-growing interest in these materials is due to the peculiar redox property of ceria, which can easily be reduced and oxidized without significant changes to its primitive cubic structure. Non-stoichiometry and the reducibility of ceria can be modulated and enhanced through the doping of its lattice, with thermal and redox treatments and with specific synthesis methods leading to nanostructured materials. Moreover, improvements of the oxygen storage capability of ceria may originate from a strong metal/support interaction usually established with noble metal-supported catalysts. The advances of the techniques for characterization and analysis of the defect chemistry of non-stoichiometric oxides and, especially, of ceria-based oxides, make the correlation between the catalytic properties and their defect chemistry possible, making them fundamental in the design of new active materials.

There is still a great deal of controversy over whether CO₂ conversion can be considered as a means to massively mitigate CO₂. Nonetheless, recent progress in CO₂ conversion has shown that the technology has the potential to create new industries in new chemical and energy fields. Catalysis for CO₂ conversion has been mainly focused on CO₂ hydrogenation and polymer synthesis, as it is shown in the present Section. Innovative routes are also explored to prepare environmentally friendly polymers from CO₂. On the other hand, the advances on the electrochemical CO₂ reduction deliver persuasive results that the electrochemical CO₂ conversion can be commercialized in the near future. Furthermore, enzyme and microbial electro-synthesis is studied to reduce CO₂ into valuable products. Several processes using innovative catalysts are also investigated to examine the potential of the commercialization of the CO₂ conversion. Recent progress and advances in the field of CO₂ conversion are addressed in the present Section, such as: (1) CO₂ hydrogenation, (2) monomer and polymer synthesis from CO₂, (3) electrochemical CO₂ reduction, (4) photoelectrochemical CO₂ reduction, and (5) enzyme and microbial electrosynthesis from CO₂.

Materials composed of layered silicates, boron nitride, graphene, layered clays, and layered metal oxides, such as layered titanates belonging to the class of two-dimensional (2D) materials, find application in the field of catalysis and photocatalysis and are discussed in the present Section.

In conclusion, regarding all the investigated materials, catalyst performance represents a challenge to date. With respect to the selected catalytic reactions, the papers collected in the present Special Issue aim at understanding catalyst properties and possible reaction pathways through a knowledge-driven approach. The insight into the correlation between catalyst formulation, synthesis route parameters, structural features, and catalytic performance provide the opportunity for the fine-tuning of catalytic materials.