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.

In-Situ-Formed Potassium-Modified Nickel-Zinc Carbide Boosts Production of Higher Alcohols beyond CH4 in CO2 Hydrogenation

Ni-based catalysts have been widely studied in the hydrogenation of CO2 to CH4 , but selective and efficient synthesis of higher alcohols (C2+ OH) from CO2 hydrogenation over Ni-based catalyst is still challenging due to successive hydrogenation of C1 intermediates leading to methanation. Herein, we report an unprecedented synthesis of C2+ OH from CO2 hydrogenation over K-modified Ni-Zn bimetal catalyst with promising activity and selectivity. Systematic experiments (including XRD, in situ spectroscopic characterization) and computational studies reveal the in situ generation of an active K-modified Ni-Zn carbide (K-Ni3 Zn1 C0.7 ) by carburization of Zn-incorporated Ni0 , which can significantly enhance CO2 adsorption and the surface coverage of alkyl intermediates, and boost the C-C coupling to C2+ OH rather than conventional CH4 . This work opens a new catalytic avenue toward CO2 hydrogenation to C2+ OH, and also provides an insightful example for the rational design of selective and efficient Ni-based catalysts for CO2 hydrogenation to multiple carbon products.

Nanomaterials as catalysts for CO2 transformation into value-added products: A review

Carbon dioxide (CO₂₎ is the most important greenhouse gas (GHG), accounting for 76% of all GHG emissions. The atmospheric CO₂ concentration has increased from 280 ppm in the pre-industrial era to about 418 ppm, and is projected to reach 570 ppm by the end of the 21^st century. In addition to reducing CO₂ emissions from anthropogenic activities, strategies to adequately address climate change must include CO₂ capture. To promote circular economy, captured CO₂ should be converted to value-added materials such as fuels and other chemical feedstock. Due to their tunable chemistry (which allows them to be selective) and high surface area (which allows them to be efficient), engineered nanomaterials are promising for CO₂ capturing and/or transformation. This work critically reviewed the application of nanomaterials for the transformation of CO₂ into various fuels, like formic acid, carbon monoxide, methanol, and ethanol. We discussed the literature on the use of metal-based nanomaterials, inorganic/organic nanocomposites, as well as other routes suitable for CO₂ conversion such as the electrochemical, non-thermal plasma, and hydrogenation routes. The characteristics, steps, mechanisms, and challenges associated with the different transformation technologies were also discussed. Finally, we presented a section on the outlook of the field, which includes recommendations for how to continue to advance the use of nanotechnology for conversion of CO₂ to fuels.

Hydrogenation of Carboxylic Acids, Esters, and Related Compounds over Heterogeneous Catalysts: A Step toward Sustainable and Carbon-Neutral Processes

The catalytic hydrogenation of esters and carboxylic acids represents a fundamental and important class of organic transformations, which is widely applied in energy, environmental, agricultural, and pharmaceutical industries. Due to the low reactivity of the carbonyl group in carboxylic acids and esters, this type of reaction is, however, rather challenging. Hence, specifically active catalysts are required to achieve a satisfactory yield. Nevertheless, in recent years, remarkable progress has been made on the development of catalysts for this type of reaction, especially heterogeneous catalysts, which are generally dominating in industry. Here in this review, we discuss the recent breakthroughs as well as milestone achievements for the hydrogenation of industrially important carboxylic acids and esters utilizing heterogeneous catalysts. In addition, related catalytic hydrogenations that are considered of importance for the development of cleaner energy technologies and a circular chemical industry will be discussed in detail. Special attention is paid to the insights into the structure-activity relationship, which will help the readers to develop rational design strategies for the synthesis of more efficient heterogeneous catalysts.

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.

Highly Efficient Photothermal Reduction of CO2 on Pd2Cu Dispersed TiO2 Photocatalyst and Operando DRIFT Spectroscopic Analysis of Reactive Intermediates

The photocatalytic conversion of CO₂ to fuels using solar energy presents meaningful potential in the mitigation of global warming, solar energy conversion, and fuel production. Photothermal catalysis is one promising approach to convert chemically inert CO₂ into value-added chemicals. Herein, we report the selective hydrogenation of CO₂ to ethanol by Pd₂Cu alloy dispersed TiO₂ (P25) photocatalyst. Under UV-Vis irradiation, the Pd₂Cu/P25 showed an efficient CO₂ reduction photothermally at 150 °C with an ethanol production rate of 4.1 mmol g⁻¹ h⁻¹. Operando diffuse reflectance infrared Fourier transform (DRIFT) absorption studies were used to trace the reactive intermediates involved in CO₂ hydrogenation in detail. Overall, the Cu provides the active sites for CO₂ adsorption and Pd involves the oxidation of H₂ molecule generated from P25 and C–C bond formation.

Novel Heterogeneous Catalysts for CO2 Hydrogenation to Liquid Fuels

Carbon dioxide (CO₂) hydrogenation to liquid fuels including gasoline, jet fuel, diesel, methanol, ethanol, and other higher alcohols via heterogeneous catalysis, using renewable energy, not only effectively alleviates environmental problems caused by massive CO₂ emissions, but also reduces our excessive dependence on fossil fuels. In this Outlook, we review the latest development in the design of novel and very promising heterogeneous catalysts for direct CO₂ hydrogenation to methanol, liquid hydrocarbons, and higher alcohols. Compared with methanol production, the synthesis of products with two or more carbons (C₂₊) faces greater challenges. Highly efficient synthesis of C₂₊ products from CO₂ hydrogenation can be achieved by a reaction coupling strategy that first converts CO₂ to carbon monoxide or methanol and then conducts a C-C coupling reaction over a bifunctional/multifunctional catalyst. Apart from the catalytic performance, unique catalyst design ideas, and structure-performance relationship, we also discuss current challenges in catalyst development and perspectives for industrial applications.

Hydroxyl-mediated ethanol selectivity of CO2 hydrogenation

Oxide-supported Rh nanoparticles have been widely used for CO₂ hydrogenation, especially for ethanol synthesis. However, this reaction operates under high pressure, up to 8 MPa, and suffers from low CO₂ conversion and alcohol selectivity. This paper describes the crucial role of hydroxyl groups bound on Rh-based catalysts supported on TiO₂ nanorods (NRs). The RhFeLi/TiO₂ NR catalyst shows superior reactivity (≈15% conversion) and ethanol selectivity (32%) for CO₂ hydrogenation. The promoting effect can be attributed to the synergism of high Rh dispersion and high-density hydroxyl groups on TiO₂ NRs. Hydroxyls are proven to stabilize formate species and protonate methanol, which is easily dissociated into *CH _x , and then CO obtained from the reverse water-gas shift reaction (RWGS) is inserted into *CH _x to form CH₃CO*, followed by CH₃CO* hydrogenation to ethanol.

Efficient conversion of CO2 to methane using thin-layer SiOx matrix anchored nickel catalysts

Thin-layer SiO_x matrix anchored nickel catalysts with high specific surface area and a unique electronic/geometric structure were fabricated for efficient CO₂ methanation.

Carbonate-Promoted Hydrogenation of Carbon Dioxide to Multicarbon Carboxylates

CO₂ hydrogenation is a potential alternative to conventional petrochemical methods for making commodity chemicals and fuels. Research in this area has focused mostly on transition-metal-based catalysts. Here we show that hydrated alkali carbonates promote CO₂ hydrogenation to formate, oxalate, and other C₂₊ carboxylates at elevated temperature and pressure in the absence of transition-metal catalysts or solvent. The reactions proceed rapidly, reaching up to 56% yield (with respect to CO₃^2-) within minutes. Isotope labeling experiments indicate facile H₂ and C-H deprotonations in the alkali cation-rich reaction media and identify probable intermediates for the C-C bond formations leading to the various C₂₊ products. The carboxylate salts are in equilibrium with volatile carboxylic acids under CO₂ hydrogenation conditions, which may enable catalytic carboxylic acid syntheses. Our results provide a foundation for base-promoted and base-catalyzed CO₂ hydrogenation processes that could complement existing approaches.

Selective hydrogenation of CO2 to methanol catalyzed by Cu supported on rod-like La2O2CO3

Cu-based catalysts have long been applied to convert CO₂ and H₂ into methanol, and their performances are well known to be markedly influenced by the support and promoter.

Abiotic Synthesis with the C-C Bond Formation in Ethanol from CO2 over (Cu,M)(O,S) Catalysts with M = Ni, Sn, and Co

We demonstrate copper-based (Cu,M)(O,S) oxysulfide catalysts with M = Ni, Sn, and Co for the abiotic chemical synthesis of ethanol (EtOH) with the C-C bond formation by passing carbon dioxide (CO₂) through an aqueous dispersion bath at ambient environment. (Cu,Ni)(O,S) with 12.1% anion vacancies had the best EtOH yield, followed by (Cu,Sn)(O,S) and (Cu,Co)(O,S). The ethanol yield with 0.2 g (Cu,Ni)(O,S) catalyst over a span of 20 h achieved 5.2 mg. The ethanol yield is inversely proportional to the amount of anion vacancy. The kinetic mechanism for converting the dissolved CO₂ into the C₂ oxygenate is proposed. Molecular interaction, pinning, and bond weakening with anion vacancy of highly strained catalyst, the electron hopping at Cu⁺/Cu²⁺ sites, and the reaction orientation of hydrocarbon intermediates are the three critical issues in order to make the ambient chemical conversion of inorganic CO₂ to organic EtOH with the C-C bond formation in water realized. On the other hand, Cu(O,S) with the highest amount of 22.7% anion vacancies did not produce ethanol due to its strain energy relaxation opposing to the pinning and weakening of O-H and C-O bonds.

Aryl appended neutral and cationic half-sandwich ruthenium(ii)–NHC complexes: synthesis, characterisation and catalytic applications

Aryl appended half-sandwich Ru(ii)–NHC complexes were synthesised and their use as selective catalysts for transfer hydrogenation of aromatic ketones was demonstrated.