Transforming Highly Oxidized and Reduced Carbon Feedstocks: Strategies for Catalytic CO2 and CH4 Valorization

Abstract

Carbon is a ubiquitous element in society as a critical component of materials, medicines, commodity chemicals, and fuels. A primary reason for the broad utility of carbon-containing compounds is the redox and chemical versatility of carbon. Remarkably, a carbon atom can span 9 oxidation states (+4 to −4), with fuels typically consisting of highly reduced carbon-based compounds (−1 to −4), (1) biological metabolites concentrating carbon in intermediate oxidation states (−1 to 2), (2) and plastics such as polycarbonates featuring some carbon atoms in their highest oxidation state (+4). (3) The anthropogenic use of carbon-containing compounds to generate energy typically relies on combustion, resulting in the accumulation of CO₂, the most oxidized form of carbon. In contrast, both biotic and abiotic anoxic processes use oxidized forms of carbon as an electron sink, forming CH₄, the most reduced form of carbon, as the ultimate product. While CO₂ and CH₄ span the extreme ends of the viable oxidation states of carbon, they share two important chemical attributes: (i) they have high global warming potentials and contribute substantially to climate change (4) and (ii) they have thermodynamically strong C–O or C–H bonds, respectively, and the kinetic barriers to convert these abundant gases into other compounds are typically large. As a result, despite their abundance and low cost, neither CO₂ nor CH₄ is currently used as a carbon feedstock at scale. (4) This special issue of Accounts of Chemical Research is centered on research that addresses the global challenge of CO₂ and CH₄ upgrading. Specifically, it focuses on using homogeneous molecular catalysts or enzymes to valorize these under utilized C₁ gases. For CO₂, these studies are motivated by the opportunity to replace the fossil-fuel-based carbon feedstocks that are currently used in fuels, chemicals, and materials with a nonfossil source, with global ramifications for climate, the environment, and the economy. (5) For CH₄, these studies are motivated by the opportunity to limit flaring and instead use it to generate chemicals, which will also impact the global climate. (6) Accounts articles that describe research on the development of heterogeneous catalysts for CO₂ reduction, which is a complementary approach to the molecular-based systems described in this special issue, are already available, (7) as well as a collection that focuses on photo- and electrochemical approaches. (8) Chemists have an expansive toolbox for finding systems for the conversion of CO₂ and CH₄. Biology offers important templates based on billions of years of evolution selecting for the transformation of inert carbon compounds into value-added products. The methods described in this issue include the use of enzymes that activate CH₄ and CO₂ with high efficiency and selectivity. For example, Allen and co-workers describe the factors that enable methyl-coenzyme M reductase to anaerobically oxidize methane and also study related enzymatic methane formation. (9) They use calculations to show that an enzyme which optimizes the electric field is more active for methane formation than an enzyme which does not maximize electric field effects. The end goal of their work is the development of enzymes for microbial biomanufacturing, and they discuss the strengths and weaknesses of current systems for methane oxidation in this context. In related work on enzymatic systems, Wagner and co-workers discuss the highly processive formylmethanofuran dehydrogenases, which contain interconnected active sites and directional substrate channels. (10) One can envision leveraging nature’s pathway for forming specific C–H and C–N bonds through sequential CO₂ reduction and formate condensation to drive atom-by-atom construction of precise organic building blocks. These studies complement several other studies on valuable enzymatic reactivity that have recently been published in Accounts of Chemical Research, including an article describing the biochemical upconversion of C₁ precursors to energy-conserving thioester compounds (11) and another on the selective activation of alkanes to alcohols that connects recently elucidated structural information to extensive functional and mechanistic studies. (12) The active sites of enzymes and their mechanistic details also serve as a source of inspiration in the design of more efficient and/or selective synthetic molecular catalysts. For example, Ishizuka and Kojima describe the development of iron catalysts for the selective oxidation of methane to methanol that feature hydrophobic pockets similar to those found in metalloenzymes. (13) They also discuss their work using Lewis acids to improve the selectivity for CO₂ reduction to CO in nickel catalysts akin to the mode of action of carbon monoxide dehydrogenase. Similarly, Dutta and co-workers describe how the enzyme formate dehydrogenase has inspired molecular systems for CO₂ electroreduction to formate and discuss the challenges that remain to develop large-scale electrolyzers for this process. (14) Enzymes are effective in redox processes in part because they are tuned for controlling the rate of electron transfer. Machan and Moberg describe the optimization of novel chromium-based catalysts for CO₂ reduction through the use of redox-active ligands and redox mediators, which allow precise control over the movement of both electrons and protons. (15) The careful control of the secondary and outer coordination environment is another important feature of natural enzymes. Warren describes his group’s research exploring the impact of the number and position of hydrogen bonding groups on the periphery of iron tetraarylporphyrin complexes for CO₂ electroreduction. (16) Surprisingly, even the interactions between the solvent and the hydrogen bonding group can impact the catalytic activity. Although the secondary ligand environment is important, the primary coordination sphere around the metal center is even more crucial in the design of catalysts for CO₂ reduction. Choudhury et al. demonstrate that the use of strongly donating N-heterocyclic carbene (NHC) ligands provide metal complexes with increased stability in CO₂ hydrogenation reactions and show that metal hydrides supported by NHC ligands are more likely to transfer a hydride. (17) Their studies on metal hydrides form the basis for the discovery of an organic hydride for catalytic CO₂ electroreduction to formate. In more fundamental stoichiometric studies, Lee and co-workers demonstrate that an unusual acridine-based pincer ligand is crucial for enabling a nickel complex to break a C–O bond in CO₂. (18) This motif provides a new strategy for activating CO₂. In general, the two-electron reduction of CO₂ to either CO or formate is easier to achieve than six-electron reduction to methanol. Onishi and Himeda carefully design an iridium complex to facilitate H₂ heterolysis and then show that by performing a reaction between the solid complex and gaseous CO₂ they can generate methanol. (19) This work is an example of homogeneous studies guiding the development of a heterogeneous system. Whereas the focus of most of the studies in this issue is the generation of commodity chemicals, Yu and co-workers use electrocatalysis to facilitate the coupling of organic molecules such as aryl and alkyl halides with CO₂ to generate carboxylic acids. This new synthetic methodology may enable the late-stage incorporation of carboxylic acids in complex molecules. (20) Finally, in many transition-metal-mediated transformations of CO₂, the insertion of CO₂ into a metal hydride or metal alkyl is a crucial step. Yang describes thermodynamic considerations for selective CO₂ insertion into metal hydrides. (21) Hazari describes fundamental kinetic studies exploring how the ancillary ligand, solvent, and additives such as Lewis acids influence the rate of CO₂ insertion. (22) Both of these studies provide guidelines for the design of catalysts for CO₂ valorization reactions. The articles described in this special issue highlight recent progress tackling CO₂ and CH₄ valorization to different products through a suite of creative approaches. However, these accounts also describe the challenges associated with these reactions that still need to be addressed. It is our hope that the compilation of these diverse strategies will provide context and inspiration for future work. The significance of the carbon management problem is increasing at an alarming rate, and effective large-scale strategies for activating carbon in both its highest and lowest oxidation states are essential for sustaining today’s societal demands. This article references 22 other publications. This article has not yet been cited by other publications.