Homogeneous catalytic hydrogenation of CO2 – amino acid-based capture and utilization

Abstract

In this review, we provide an overview of research efforts to integrate carbon dioxide capture specifically using amino acid-based sorbents with its thermocatalytic hydrogenation promoted by homogeneous metal complexes. Carbon capture and utilization (CCU) is a promising strategy for the production of fuels, chemicals and materials using CO₂ scrubbed from point sources and the atmosphere as a C1 feedstock while mitigating CO₂ emissions. Compared to established (alkanol)amines, amino acids offer some advantages as CO₂ capture agents due to their lower volatility, higher oxygen stability and lower regeneration energies. We report how the structural diversity of amino acids and the possibility of combining them with cations in salts and ionic liquids have been exploited in the design of absorbers for improved absorption kinetics and capacity. Furthermore, we discuss selected examples from the literature illustrating the use of 1°/2° (poly)amines, since the 1°/2° amino groups are mainly responsible for CO₂ chemisorption in amino acid-based capture media, the nature of the corresponding adducts, and the most promising catalysts capable of converting the latter to formate and methanol while regenerating the scrubber. General trends regarding the influence of catalyst structure and reaction parameters on the efficiency, productivity, and selectivity of such processes will be highlighted. We will detail how this knowledge has informed the design of novel processes in which CO₂ is chemisorbed by amino acid-based solvents and hydrogenated in situ to formate and methanol, or alternatively used as a fuel to implement a “hydrogen battery” where, after metal-catalyzed H₂ release from formate, CO₂ is retained by the amino acid-based solvent in the “spent battery” which can then be recharged by hydrogenation of the retained CO₂ promoted by the same catalyst. The topic is still in its infancy, and several issues have emerged that will be critically discussed in the final section of this review. These issues need to be addressed in order to improve performance and provide a playground for researchers whose interest we hope to have aroused with this review.