Homogeneously catalysed conversion of aqueous formaldehyde to H2 and carbonate

Small organic molecules provide a promising solution for the requirement to store large amounts of hydrogen in a future hydrogen-based energy system. Herein, we report that diolefin–ruthenium complexes containing the chemically and redox non-innocent ligand trop2dad catalyse the production of H2 from formaldehyde and water in the presence of a base. The process involves the catalytic conversion to carbonate salt using aqueous solutions and is the fastest reported for acceptorless formalin dehydrogenation to date. A mechanism supported by density functional theory calculations postulates protonation of a ruthenium hydride to form a low-valent active species, the reversible uptake of dihydrogen by the ligand and active participation of both the ligand and the metal in substrate activation and dihydrogen bond formation

Iridium-Catalyzed Homogeneous Hydrogenation and Hydrosilylation of Carbon Dioxide

The knowledge of the potential of transition metal-based complexes as catalysts for the reduction of CO2 has grown significantly over the last few decades. This chapter focuses on the progress made during recent years in the field of homogeneous iridium-catalyzed reduction of CO2 by using hydrogen and/or silicon hydrides as reducing agents, comparing them with homogeneous catalysts based on other transition metals. The reported studies on iridium-catalyzed CO2 reduction processes show that an important point to keep in mind when designing a catalyst is the nature of the reducing agent (hydrogen, hydrosilanes, and/or hydrosiloxanes). Thus, iridium(III) half-sandwich complexes with 4,4′-dihydroxy-bipyridine (DHBP) or 4,7-dihydroxy-1,10-phenanthroline (DHPT) ligands, and iridium(III)-PNP pincer complexes have proven to be excellent catalysts for the hydrogenation of CO2 to formic acid. However, Ir(III)-NSiNMe (NSiN = fac-bis-(4-methylpyridine-2-yloxy)methylsilyl) and Ir(III)-NSiMe (NSiMe = 4-methylpyridine-2-yloxydimethylsilyl) species are not stable under hydrogen atmosphere but are effective catalysts for the reduction of CO2 with hydrosiloxanes to silylformate under solvent-free conditions and moderate CO2 pressures and temperatures. Moreover, while using iridium(III)-DHBP half-sandwich complexes, high CO2 and H2 pressures are required to achieve the catalytic CO2 hydrogenation to methanol; Ir-NSiMe species catalyze the reduction of CO2 to methoxysilane with hydrosiloxanes under low CO2 pressure.Peer reviewe

Selective Hydrogenation of Carbon Dioxide into Methanol

International audienceThis chapter is dedicated to methanol synthesis from carbon dioxide and hydrogen. Methanol, chemical formula CH3OH, is an important platform molecule which can be transformed into a large number of other chemicals, i.e., formaldehyde, acetic acid, dimethyl ether, methyl tert-butyl ether, and methyl methacrylate, as well as complex hydrocarbon mixtures, e.g., gasoline and diesel. Up to date, methanol is produced at industrial scale by steam reforming of natural gas, leading to high environmental impacts. The selective hydrogenation of carbon dioxide into methanol can be a good alternative since it is possible to capture carbon dioxide from industrial processes and to produce hydrogen from renewable energies, e.g., solar energy and wind energy.From a thermodynamic point of view, carbon dioxide hydrogenation is strongly influenced by the total pressure, temperature, and feeding composition. The use of a catalyst is also mandatory to control the kinetic and the selectivity into methanol. Among solid catalysts studied, copper-based catalysts have been found to be the best catalytic systems. Promoters like zinc oxide were usually used. Nickel-, palladium-, and silver-based catalysts also showed good catalytic performance compared to copper-based catalysts. Soluble catalysts have been intensively studied for this hydrogenation. Ru complexes appeared as the best homogeneous catalyst. Other metal-free homogeneous catalysts, e.g., N-heterocyclic carbenes, have been found to be active and selective in this reaction. Efforts have been made on the mechanistic study of the reaction in both the gas and liquid phases. Large industrial production has started in several countries showing the interest and the feasibility of the process