Recent Advances in Homogeneous/Heterogeneous Catalytic Hydrogenation and Dehydrogenation for Potential Liquid Organic Hydrogen Carrier (LOHC) Systems

Abstract

Here, we review liquid organic hydrogen carriers (LOHCs) as a potential solution to the global warming problem due to the increased use of fossil fuels. Recently, hydrogen molecules have attracted attention as a sustainable energy carrier from renewable energy-rich regions to energy-deficient regions. The LOHC system is one a particularly promising hydrogen storage system in the “hydrogen economy”, and efficient hydrogen mass production that generates only benign byproducts can be applied in the industry. Therefore, this article presents hydrogenation and dehydrogenation, using homogeneous or heterogeneous catalysts, for several types of LOHCs, including formic acid/formaldehyde/ammonia, homocyclic compounds, nitrogen- and oxygen-containing compounds. In addition, it introduces LOHC system reactor types.

Reusable Single Homogeneous Ir(III)–NHC Catalysts for Bidirectional Hydrogenation–Dehydrogenation of N-Heteroarenes in Water

“…Such simplification and practicalization of important catalytic reactions are of immense significance. Recently, in the context of reversible hydrogen-storage methods, bidirectional hydrogenation–dehydrogenation (BHD) of N-heteroarenes has been identified as an important and prospective catalytic process, and therefore a great deal of attention has been paid into this process (Figure A). − Gaseous H 2 can be loaded via hydrogenation of easily handleable unsaturated N-heteroarenes and stored therein for future use; a reverse process via dehydrogenation of the hydrogenated compounds can unload the stored H 2 gas for reutilization. With N-heteroarene platforms, many unidirectional homogeneous and heterogeneous catalysts were reported, which performed either hydrogenation or dehydrogenation under high pressure and high-temperature conditions in organic solvents. − Later, bidirectional hydrogenation–dehydrogenation using a single homogeneous and heterogeneous catalyst was developed, operating still under harsh conditions and mainly in organic solvents. − Use of water as a solvent for bidirectional hydrogenation–dehydrogenation under relatively mild conditions with just a single catalyst is a current challenge, and toward this, Albrecht and co-workers, and Fischmeister and co-workers recently discovered two significant systems based on homogeneous Ir complexes (Figure B).…”

Toward a Hydrogen Economy: Development of Heterogeneous Catalysts for Chemical Hydrogen Storage and Release Reactions

“…In the area of chemical hydrogen storage and release, many outstanding reviews, perspectives, and accounts on heterogeneous and homogeneous catalytic systems have sprung up, ,,− most of which focus on one of the chemical H 2 carriers [formic acid (including formate salts), − N -heterocycles, ,,− (aromatic) hydrocarbons, ,− ammonia, , ammonia–borane, or dimethyl ether] or on the individual hydrogen production/​release reactions. − Notably, in a large proportion of the previous reports, only one individual reaction, either H 2 storage or H 2 release, is involved or discussed rather than combining those individual reactions to demonstrate the overall concept of chemical H 2 batteries; even the research works are very often entitled to contain the topic of “H 2 storage and release”. Those examples are not considered in this Review.…”

The formation of active phases in Pt-containing catalysts for bicyclohexyl dehydrogenation in hydrogen storage

“…Noble metal-based catalysts are known to exhibit high catalytic activity in the LOHC hydrogenation, and platinum is the most active metal for dehydrogenation. 14 At the same time, due to the high cost of platinum, the strategy of modifying Pt with transition metals is of great importance for reducing the cost of catalysts. The synergistic effect of interaction between platinum and transition metals in composite active clusters can significantly increase the activity and selectivity of catalytic reactions.…”