Highly selective hydrogenation of CO2 into C2+ alcohols by homogeneous catalysis

Hydrogenation of CO2 into Value-added Chemicals Using Solid-Supported Catalysts

Reducing CO2 emissions is an urgent global priority. In this context, several mitigation strategies, including CO2 tax and stringent legislation, have been adopted to halt the deterioration of the natural environment. Also, carbon recycling procedures undoubtedly help reduce net emissions into the atmosphere, enhancing sustainability. Utilizing Earth's abundant CO2 to produce high-potential green chemicals and light fuels opens new avenues for the chemical industry. In this context, many attempts have been devoted to converting CO2 as a feedstock into various value-added chemicals, such as CH4 , lower methanol, light olefins, gasoline, and higher hydrocarbons, for numerous applications involving various catalytic reactions. Although several CO2 -conversion methods have been used, including electrochemical, photochemical, and biological approaches, the hydrogenation method allows the reaction to be tuned to produce the targeted compound without significantly altering infrastructure. This review discusses the numerous hydrogenation routes and their challenges, such as catalyst design, operation, and the combined art of structure-activity relationships for the various product formations.

Ga-Promoted CuCo-Based Catalysts for Efficient CO2 Hydrogenation to Ethanol: The Key Synergistic Role of Cu-CoGaOx Interfacial Sites

Currently, direct catalytic CO₂ hydrogenation to produce ethanol is an effective and feasible way for the resource utilization of CO₂. However, constructing non-precious metal catalysts with satisfactory activity and desirable ethanol selectivity remains a huge challenge. Herein, we reported gallium-promoted CuCo-based catalysts derived from single-source Cu-Co-Ga-Al layered double hydroxide precursors. It was manifested that the introduction of Ga species could strengthen strong interactions between Cu and Co oxide species, thereby modifying their electronic structures and thus facilitating the formation of abundant metal-oxide interfaces (i.e., Cu⁰/Cu⁺-CoGaO_x interfaces). Notably, the as-constructed Cu-CoGa catalyst with a Ga:Co molar ratio of 0.4 exhibited a high ethanol selectivity of 23.8% at a 17.8% conversion, along with a high space-time yield of 1.35 mmol_EtOH·g_cat^-1·h^-1 for ethanol under mild reaction conditions (i.e., 220 °C, 3 MPa pressure), which outperformed most non-noble metal-based catalysts previously reported. According to the comprehensive structural characterizations and in situ diffuse reflectance infrared Fourier transform spectra of CO₂/CO adsorption and CO₂ hydrogenation, it was unambiguously revealed that CH_x could be formed at oxygen vacancies of defective CoGaO_x species, while CO could be stabilized by Cu⁺ species, and thus the catalytic synergistic role of Cu⁰/Cu⁺-CoGaO_x interfacial sites promoted the generation of CH_x and CO intermediates to participate in the CH_x-CO coupling process and simultaneously inhibited alkylation reactions. The present work points out a promising new strategy for constructing CuCo-based catalysts with favorable interfacial sites for highly efficient CO₂ hydrogenation to produce ethanol.

Thermodynamic Analysis of CO2 Hydrogenation to Higher Alcohols (C2-4OH): Effects of Isomers and Methane

Synthesis of higher alcohols (C_2-4OH) by CO₂ hydrogenation presents a promising way to convert CO₂ into value-added fuels and chemicals. Understanding the thermodynamics of CO₂ hydrogenation is of great importance to tailor the reaction network toward synthesis of higher alcohols; however, the thermodynamic effects of various alcohol isomers and methane in the reaction system have not yet been fully understood. Thus, we used Aspen Plus to perform thermodynamic analysis of CO₂ hydrogenation to higher alcohols, studying the effects of alcohol isomers and methane. Thermodynamically, methane is the most favorable product in a reaction system containing CO, CO₂, and H₂, as well as C_1-4 alkanes, alkenes, and alcohols. The thermodynamic favorability of alcohol isomers varies significantly. The presence of methane generally deteriorates the formation of higher alcohols. However, low temperature, high pressure, high H₂/CO₂ ratio, and formation of alcohols with a longer carbon chain can reduce the effects of methane. Our current study, therefore, provides new insights for enhancing the synthesis of higher alcohols by CO₂ hydrogenation.

Revisiting Reduction of CO2 to Oxalate with First-Row Transition Metals: Irreproducibility, Ambiguous Analysis, and Conflicting Reactivity

Construction of higher C_≥2 compounds from CO₂ constitutes an attractive transformation inspired by nature's strategy to build carbohydrates. However, controlled C-C bond formation from carbon dioxide using environmentally benign reductants remains a major challenge. In this respect, reductive dimerization of CO₂ to oxalate represents an important model reaction enabling investigations on the mechanism of this simplest CO₂ coupling reaction. Herein, we present common pitfalls encountered in CO₂ reduction, especially its reductive coupling, based on established protocols for the conversion of CO₂ into oxalate. Moreover, we provide an example to systematically assess these reactions. Based on our work, we highlight the importance of utilizing suitable orthogonal analytical methods and raise awareness of oxidative reactions that can likewise result in the formation of oxalate without incorporation of CO₂. These results allow for the determination of key parameters, which can be used for tailoring of prospective catalytic systems and will promote the advancement of the entire field.

A Cu2 O-derived Polymeric Carbon Nitride Heterostructured Catalyst for the Electrochemical Reduction of Carbon Dioxide to Ethylene

The electroreduction of carbon dioxide to hydrocarbons has been proposed as a promising way to utilize CO₂ and maintain the ecosystem carbon balance. However, the selective reduction of CO₂ to C₂ hydrocarbons is still challenging. In this study, a highly efficient heterostructured catalyst has been developed, composed of a carbon nitride (CN)-encapsulated copper oxide hybrid (Cu_x O/CN). The interaction between the metal and carbon nitride in the heterostructured catalysts improves the intrinsic electrical conductivity and the charge transfer processes at metal-support interfaces. A remarkable enhancement in the selectivity of hydrocarbons is achieved with these modified Cu-based electrocatalysts, with an onset potential of -0.4 V and high C₂ H₄ faradaic efficiency of 42.2 %, and these catalysts can also effectively suppress H₂ evolution during the CO₂ reduction reaction. This work provides a simple and cost-effective method for synthesizing CN-encapsulated catalysts that provides the possibility of efficiently converting CO₂ into C₂ hydrocarbons.

Synthesis of C2+ Chemicals from CO2 and H2 via C-C Bond Formation

ConspectusThe severity of global warming necessitates urgent CO₂ mitigation strategies. Notably, CO₂ is a cheap, abundant, and renewable carbon resource, and its chemical transformation has attracted great attention from society. Because CO₂ is in the highest oxidation state of the C atom, the hydrogenation of CO₂ is the basic means of converting it to organic chemicals. With the rapid development of H₂ generation by water splitting using electricity from renewable resources, reactions using CO₂ and H₂ have become increasingly important. In the past few decades, the advances of CO₂ hydrogenation have mostly been focused on the synthesis of C1 products, such as CO, formic acid and its derivatives, methanol, and methane. In many cases, the chemicals with two or more carbons (C₂₊) are more important. However, the synthesis of C₂₊ chemicals from CO₂ and H₂ is much more difficult because it involves controlled hydrogenation and simultaneous C-C bond formation. Obviously, investigations on this topic are of great scientific and practical significance. In recent years, we have been targeting this issue and have successfully synthesized the basic C₂₊ chemicals including carboxylic acids, alcohols, and liquid hydrocarbons, during which we discovered several important new reactions and new reaction pathways. In this Account, we systematically present our work and insights in a broad context with other related reports.1.We discovered a reaction of acetic acid production from methanol, CO₂ and H₂, which is different from the well-known methanol carbonylation. We also discovered a reaction of C₃₊ carboxylic acids syntheses using ethers to react with CO₂ and H₂, which proceeds via olefins as intermediates. Following the new reaction, we realized the synthesis of acetamide by introducing various amines, which may inspire the development of further catalytic schemes for preparing a variety of special chemicals using carbon dioxide as a building block.2.We designed a series of homogeneous catalysts to accelerate the production of C₂₊ alcohols via CO₂ hydrogenation. In the heterogeneously catalyzed CO₂ hydrogenation, we discovered the role of water in enhancing the synthesis of C₂₊ alcohols. We also developed a series of routes for ethanol production using CO₂ and H₂ to react with some substrates, such as methanol, dimethyl ether, aryl methyl ether, lignin, or paraformaldehyde.3.We designed a catalyst that can directly hydrogenate CO₂ to C₅₊ hydrocarbons at 200 °C, not via the traditional CO or methanol intermediates. We also designed a route to couple homogeneous and heterogeneous catalysis, where exceptional results are achieved at 180 °C.

In Situ Bismuth Nanosheet Assembly for Highly Selective Electrocatalytic CO2 Reduction to Formate

The reduction of carbon dioxide (CO₂ ) into value-added fuels using an electrochemical method has been regarded as a compelling sustainable energy conversion technology. However, high-performance electrocatalysts for CO₂ reduction reaction (CO₂ RR) with high formate selectivity and good stability need to be improved. Earth-abundant Bi has been demonstrated to be active for CO₂ RR to formate. Herein, we fabricated an extremely active and selective bismuth nanosheet (Bi-NSs) assembly via an in situ electrochemical transformation of (BiO)₂ CO₃ nanostructures. The as-prepared material exhibits high activity and selectivity for CO₂ RR to formate, with nearly 94% faradaic efficiency at -1.03 V (versus reversible hydrogen electrode (vs. RHE)) and stable selectivity (>90%) in a large potential window ranging from -0.83 to -1.18 V (vs. RHE) and excellent durability during 12 h continuous electrolysis. In addition, the Bi-NSs based CO₂ RR/methanol oxidation reaction (CO₂ RR/MOR) electrolytic system for overall CO₂ splitting was constructed, evidencing the feasibility of its practical implementation.

Selectivity controlled transformation of carbon dioxide into a versatile bi-functional multi-carbon oxygenate using a physically mixed ruthenium–iridium catalyst

The efficiency of supercritical CO₂ (scCO₂) as a reactant was successfully unfolded in the synthesis of a high-value C₂₊ oxygenate via hydrogenation and C–C bond formation under comparatively mild conditions.

Ruthenium-catalyzed hydrogenation of CO2 as a route to methyl esters for use as biofuels or fine chemicals

A novel robust diphosphine–ruthenium(ii) complex has been developed that can efficiently catalyze both the hydrogenation of CO₂ to methanol and its in situ condensation with carboxylic acids to give methyl esters.

Cyclometalation of lanthanum(iii) based MOF for catalytic hydrogenation of carbon dioxide to formate

A novel metal–organic framework JMS-1 with rare topology zaz shows catalytic activity towards conversion of carbon dioxide to formate.

Synthesis of higher carboxylic acids from ethers, CO2 and H2

Synthesis of higher carboxylic acids using CO2 and H2 is of great importance, because CO2 is an attractive renewable C1 resource and H2 is a cheap and clean reductant. Herein we report a route to produce higher carboxylic acids via reaction of ethers with CO2 and H2. We show that the reaction can be efficiently catalyzed by an IrI4 catalyst with LiI as promoter at 170 °C, 5 MPa of CO2 and 2 MPa of H2. The catalytic system applies to various ether substrates. The mechanistic study indicates that the ethers are converted to olefins, which are further transformed into alkyl iodides. The higher carboxylic acids are produced by carbonylation of alkyl iodides with CO generated in situ via RWGS reaction. This report offers an alternative strategy of higher carboxylic acid synthesis and CO2 transformation.

Synthesis of Asymmetrical Organic Carbonates using CO2 as a Feedstock in AgCl/Ionic Liquid System at Ambient Conditions

Synthesis of asymmetrical organic carbonates from the renewable and inexpensive CO₂ is of great importance but also challenging, especially at ambient conditions. Herein, we found that some metal salt/ionic liquid catalyst systems were highly active for the synthesis of asymmetrical organic carbonates from CO₂ , propargylic alcohols, and primary alcohols. Especially, the AgCl/1-butyl-3-methylimidazolium acetate ([Bmim][OAc]) system was very efficient for the reactions of a wide range of substrates at room temperature and atmospheric pressure, and the yields of the asymmetrical organic carbonates could approach 100 %. The catalyst system could be reused at least five times without changing its catalytic performance, and could be easily recovered and reused. A detailed study indicated that AgCl and [Bmim][OAc] catalyzed the reactions cooperatively, resulting in unique catalytic performance.

Carbon Dioxide Hydrogenation into Higher Hydrocarbons and Oxygenates: Thermodynamic and Kinetic Bounds and Progress with Heterogeneous and Homogeneous Catalysis

Under specific scenarios, the catalytic hydrogenation of CO₂ with renewable hydrogen is considered a suitable route for the chemical recycling of this environmentally harmful and chemically refractory molecule into added-value energy carriers and chemicals. The hydrogenation of CO₂ into C₁ products, such as methane and methanol, can be achieved with high selectivities towards the corresponding hydrogenation product. More challenging, however, is the selective production of high (C₂₊ ) hydrocarbons and oxygenates. These products are desired as energy vectors, owing to their higher volumetric energy density and compatibility with the current fuel infrastructure than C₁ compounds, and as entry platform chemicals for existing value chains. The major challenge is the optimal integration of catalytic functionalities for both reductive and chain-growth steps. This Minireview summarizes the progress achieved towards the hydrogenation of CO₂ to C₂₊ hydrocarbons and oxygenates, covering both solid and molecular catalysts and processes in the gas and liquid phases. Mechanistic aspects are discussed with emphasis on intrinsic kinetic limitations, in some cases inevitably linked to thermodynamic bounds through the concomitant reverse water-gas-shift reaction, which should be considered in the development of advanced catalysts and processes.

Bromide promoted hydrogenation of CO2 to higher alcohols using Ru-Co homogeneous catalyst

Iodides are commonly used promoters in C2+OH synthesis from CO2/CO hydrogenation. Here we report the highly efficient synthesis of C2+OH from CO2 hydrogenation over a Ru3(CO)12-Co4(CO)12 bimetallic catalyst with bis(triphenylphosphoranylidene)ammonium chloride (PPNCl) as the cocatalyst and LiBr as the promoter. Methanol, ethanol, propanol and isobutanol were formed at milder conditions. The catalytic system had a much better overall performance than those of reported iodide promoted systems because PPNCl and LiBr cooperated very well in accelerating the reaction. LiBr enhanced the activity and PPNCl improved the selectivity, and thus both the activity and selectivity were very high when both of them were used simultaneously. In addition, the catalyst could be reused for at least five cycles without an obvious change of catalytic performance.

Selective Catalytic Synthesis Using the Combination of Carbon Dioxide and Hydrogen: Catalytic Chess at the Interface of Energy and Chemistry

The present Review highlights the challenges and opportunities when using the combination CO2 /H2 as a C1 synthon in catalytic reactions and processes. The transformations are classified according to the reduction level and the bond-forming processes, covering the value chain from high volume basic chemicals to complex molecules, including biologically active substances. Whereas some of these concepts can facilitate the transition of the energy system by harvesting renewable energy into chemical products, others provide options to reduce the environmental impact of chemical production already in today's petrochemical-based industry. Interdisciplinary fundamental research from chemists and chemical engineers can make important contributions to sustainable development at the interface of the energetic and chemical value chain. The present Review invites the reader to enjoy this exciting area of "catalytic chess" and maybe even to start playing some games in her or his laboratory.

Synthesis of acetic acid via methanol hydrocarboxylation with CO2 and H2

Acetic acid is an important bulk chemical that is currently produced via methanol carbonylation using fossil based CO. Synthesis of acetic acid from the renewable and cheap CO₂ is of great importance, but state of the art routes encounter difficulties, especially in reaction selectivity and activity. Here we report a route to produce acetic acid from CO₂, methanol and H₂. The reaction can be efficiently catalysed by Ru–Rh bimetallic catalyst using imidazole as the ligand and LiI as the promoter in 1,3-dimethyl-2-imidazolidinone (DMI) solvent. It is confirmed that methanol is hydrocarboxylated into acetic acid by CO₂ and H₂, which accounts for the outstanding reaction results. The reaction mechanism is proposed based on the control experiments. The strategy opens a new way for acetic acid production and CO₂ transformation, and represents a significant progress in synthetic chemistry.

Industrial routes to acetic acid use carbon monoxide for the carbonylation of methanol. Here, the authors report a hydrocarboxylation method that instead uses carbon dioxide and hydrogen for the conversion of methanol into acetic acid.

Direct synthesis of ethanol via CO2 hydrogenation using supported gold catalysts

Au nanoclusters (NCs) supported on anatase TiO₂ can achieve the high performance for direct synthesis of EtOH via CO₂ hydrogenation.