Catalytic Conversion of CO2 to Formate with Renewable Hydrogen Donors: An Ambient-Pressure and H2-Independent Strategy

Tuning metal-support interactions in MOF-derived CoNi@NC supported Pd catalysts for efficient hydrogenation of bicarbonate using glycerol as a hydrogen donor

The hydrogenation of bicarbonate, a byproduct of CO2 captured in alkaline solutions, into formic acid (FA) using glycerol (GLY) as a hydrogen source offers a promising carbon-negative strategy for reducing CO2 emissions. While Pd-based catalysts are effective in this reaction, they often require high temperatures, leading to low FA yield due to strong hydrogen adsorption on Pd surfaces. In this work, metal-organic framework derived N-doped carbon encapsulated CoNi alloy nanoparticles (CoNi@NC) were prepared, acid-leached, and employed as a support to modulate the electronic structure of Pd-based catalysts. The electron transfer driven by the Mott-Schottky effect increases the electron density of Pd, lowers its d-band center, and thereby weakens hydrogen adsorption on Pd. Consequently, Pd/CoNi@NC achieved a full conversion of GLY with 87.2% yield of lactic acid and 57.3% yield of FA, outperforming Pd/NC and Pd/C. Additionally, Pd/CoNi@NC demonstrated high catalytic stability and was magnetically separable for ease of use.

Catalytic Hydrogenation of CO2 by Direct Air Capture to Valuable C1 Products Using Homogenous Catalysts

Growing atmospheric CO2 concentrations are a global concern and a primary factor contributing to global warming. Development of integrated CO2 capture and conversion protocols is necessary to mitigate this alarming challenge. Though CO2 hydrogenation to produce formic acid and methanol has seen many strides in the past decades, most studies utilize pure CO2 for this transformation. The CO2 concentration in the atmosphere stands at 400 ppm and reports that utilize direct air capture as the strategy to capture CO2 and utilize it for production of formic acid and methanol have only been reported in the past few years. This perspective summarizes such reports with a focus on the CO2-capturing additive, reaction solvent, and the molecular catalyst used to affect the transformation.

Roles of carbon dioxide in the conversion of biomass or waste plastics

Under the current trend of pursuing sustainable development and environmental protection, the important application of carbon dioxide (CO2) in the conversion process of biomass or waste plastics has become a research direction of concern. The goal of this conversion process is to achieve the efficient use of carbon dioxide, providing a process for the efficient use of biomass, and solving the environmental problems caused by plastics. Remarkable progress has been made in the study of the reaction of CO2 with other substances to produce methane, low-carbon hydrocarbons, methanol, formic acid, and its derivatives, as well as ethers, aldehydes, gasoline, low-carbon alcohols, and other chemicals. In this paper, the important role of CO2 in the conversion of alcohol, sugar, cellulose, and waste plastics was reviewed, with emphasis on the important applications of CO2 as a carbon source, reactant, reaction medium, enhancing agent, solvent, and carrier gas in the conversion of biomass or waste plastics and the basic insights of the reaction mechanism. The emerging CO2 new roles not only put forward the green application of CO2 but also have guiding significance for the utilization of biomass resources and the treatment of waste plastics.

Efficient co-upcycling of glycerol and CO2 into valuable products enabled by a bifunctional Ru-complex catalyst

Glycerol and CO2 are largely produced as by-products in modern industry. Herein, three Ru complexes bearing air- and water-stable NHC-nitrogen-nitrogen ligands are designed as bifunctional catalysts to upcycle glycerol and CO2 simultaneously. Among them, Ru complex 2 featuring an N-H structure shows the highest efficiency with TONs up to 300 000 and 387 000 for formate and lactate, respectively. 13C labelling experiments clearly manifest that formate is primarily derived from CO2. Furthermore, in situ FTIR spectra suggest that glyceraldehyde from glycerol might serve as a key intermediate to form lactate, while DFT calculations indicate that Ru complex 2 possesses the lowest reaction barriers and can form the RuN intermediate, contributing to its higher activity.

Hydride Transfer-Based CO2 Reduction Catalysis: Navigating Metal Hydride to Organic Hydride in the Catalytic Loop

ConspectusThe reductive conversion of carbon dioxide (CO2) into value-added products is a process of immense importance. In the context of rising CO2 concentration in the atmosphere and the detrimental effects it is having on the biosphere, use of alternative fuels which can offer a low-carbon or carbon-neutral pathway for storage and utilization of low-carbon energy by maintaining the net atmospheric CO2 concentration might be a prospective solution. Among the wide variety of reduced products that can be obtained from CO2, formic acid and formate salts are particularly important due to their ability to be used as an alternative fuel or a reversible hydrogen storage material. Utilization of molecular catalysts for CO2 conversion offers several advantages such as high selectivity, mechanistic clarity, versatility, and stability, making them attractive for thermochemical and electro/photochemical CO2 reduction processes. The presence of N-heterocyclic carbene (NHC) ligands in transition-metal-based molecular catalysts enhances the stability of the catalysts under harsh reaction conditions, such as high pressure, high temperature, and reductive environments, providing crucial benefits for sustained catalytic activity and longevity. Though the development of metal complex-based catalysts is essential to addressing the challenge of CO2 reduction, the possibility of using purely organic compounds as catalysts for this transformation is lucrative from the aspect of developing a truly sustainable protocol with photosynthesis being its biggest inspiration. We begin this Account by examining our systematic development of molecular metal complexes based on NHC ligands for the chemical upgradation of CO2 to formic acid/formate salt. In such cases, the ability of NHCs to act as strong σ-donor ligands for a greater hydride transfer propensity is discussed and analyzed. The reports range from catalytic ambient- and high-pressure CO2 hydrogenation to CO2 transfer-hydrogenation. Coupling of CO2 capture methodologies with CO2 conversion is also discussed. A case is made for the heterogenization of one of the highly efficient metal-NHC catalysts to develop a self-supported single-site catalyst for practical applications. Finally, our recent success of developing a novel organic catalyst system inspired from the natural NADP+/NADPH-based hydride-transfer redox couple that is active in photosynthetic CO2 reduction has been discussed. This catalyst is designed based on a bis-imidazolium-embedded heterohelicene with a central pyridine ring and is capable of electrocatalytically converting CO2 to HCO2H with TON values 100-1000 times greater than the existing reported values achieved so far by organic catalysts. Overall, we believe that the results of hydride transfer-based CO2 reduction catalysis presented in this Account hold significant implications beyond our work and have the potential for motivating future research toward further development in this important field.

A phosphine-free molecularly-defined Ni(II) complex in catalytic hydrogenation of CO2

The development of base metal catalysts capable of CO2 hydrogenation is a challenge and a necessity to progress from the scarce noble metal catalysts. In this regard, we report herein the first non-phosphine-based Ni complex, supported by a "carbazolato-bis-NHC" pincer ligand framework, for efficient catalytic hydrogenation of CO2 to formate. A tailored combination of the Ni complex as a catalyst, DBU as a base, and Zn(OAc)2 as an additive offered enhanced activity leading to a TON up to 5476 and an excellent yield up to 92% for the formate product from a reaction on ∼27 mmol scale.

Directly Knitted Hierarchical Porous Organometallic Polymer-Based Self-Supported Single-Site Catalyst for CO2 Hydrogenation in Water

In recent times, heterogenization of homogeneous molecular catalysts onto various porous solid support structures has attracted significant research focus as a method for combining the advantages of both homogeneous as well as heterogeneous catalysis. The design of highly efficient, structurally robust and reusable heterogenized single-site catalysts for the CO2 hydrogenation reaction is a critical challenge that needs to be accomplished to implement a sustainable and practical CO2 -looped renewable energy cycle. This study demonstrated a heterogenized catalyst [Ir-HCP-(B/TPM)] containing a molecular Ir-abnormal N-heterocyclic carbene (Ir-aNHC) catalyst self-supported by hierarchical porous hyper-crosslinked polymer (HCP), in catalytic hydrogenation of CO2 to inorganic formate (HCO2 - ) salt that is a prospective candidate for direct formate fuel cells (DFFC). By employing this unique and first approach of utilizing a directly knitted HCP-based organometallic single-site catalyst for CO2 -to-HCO2 - in aqueous medium, extremely high activity with a single-run turnover number (TON) up to 50816 was achieved which is the highest so far considering all the heterogeneous catalysts for this reaction in water. Additionally, the catalyst featured excellent reusability furnishing a cumulative TON of 285400 in 10 cycles with just 1.6 % loss in activity per cycle. Overall, the new catalyst displayed attributes that are important for developing tangible catalysts for practical applications.

Polymers without Petrochemicals: Sustainable Routes to Conventional Monomers

Access to a wide range of plastic materials has been rationalized by the increased demand from growing populations and the development of high-throughput production systems. Plastic materials at low costs with reliable properties have been utilized in many everyday products. Multibillion-dollar companies are established around these plastic materials, and each polymer takes years to optimize, secure intellectual property, comply with the regulatory bodies such as the Registration, Evaluation, Authorisation and Restriction of Chemicals and the Environmental Protection Agency and develop consumer confidence. Therefore, developing a fully sustainable new plastic material with even a slightly different chemical structure is a costly and long process. Hence, the production of the common plastic materials with exactly the same chemical structures that does not require any new registration processes better reflects the reality of how to address the critical future of sustainable plastics. In this review, we have highlighted the very recent examples on the synthesis of common monomers using chemicals from sustainable feedstocks that can be used as a like-for-like substitute to prepare conventional petrochemical-free thermoplastics.

An all-aqueous and phosphine-free integrated amine-assisted CO2 capture and catalytic conversion to formic acid

A phosphine-free Ir(III)-NHC-based efficient catalytic system is developed for integrated CO₂ capture with tetramethylguanidine as a capturing agent and conversion to formate with H₂ gas, conducting both the steps in water, affording product yield up to 85% and TON up to 19 171 in just 12 h. In the segment of "integrated CO₂-capture and conversion to formate", this system represents not only the first phosphine-free module, but also one of the few best known homogeneous catalysts.

Towards Sustainable Oxalic Acid from CO2 and Biomass

To quickly and drastically reduce CO2 emissions and meet our ambitions of a circular future, we need to develop carbon capture and storage (CCS) and carbon capture and utilization (CCU) to deal with the CO2 that we produce. While we have many alternatives to replace fossil feedstocks for energy generation, for materials such as plastics we need carbon. The ultimate circular carbon feedstock would be CO2 . A promising route is the electrochemical reduction of CO2 to formic acid derivatives that can subsequently be converted into oxalic acid. Oxalic acid is a potential new platform chemical for material production as useful monomers such as glycolic acid can be derived from it. This work is part of the European Horizon 2020 project "Ocean" in which all these steps are developed. This Review aims to highlight new developments in oxalic acid production processes with a focus on CO2 -based routes. All available processes are critically assessed and compared on criteria including overall process efficiency and triple bottom line sustainability.

Reduction of Carbon Dioxide with Ammonia-Borane under Ambient Conditions: Maneuvering a Catalytic Way

In the development of alternatives to the traditional catalytic hydrogenation of CO₂ with gaseous H₂, employing nongaseous H₂ storage compounds as potential reductants for catalytic transfer hydrogenation of CO₂ is promising. Ammonia-borane, due to its high hydrogen storage capacity (19.6 wt %), has been used for catalytic transfer hydrogenation of several organic unsaturated compounds. However, a similar protocol involving catalytic transfer hydrogenation of less reactive CO₂ with NH₃BH₃ is yet to be realized experimentally. Herein, we demonstrate the first catalytic CO₂ transfer hydrogenation process for generating formate salt with NH₃BH₃ under ambient conditions (1 atm and 30 °C) employing a cationic "Ir(III)-abnormal NHC" catalyst via an electrophilic NH₃BH₃ activation route. It exhibited an initial turnover frequency of 686 h^-1 and a high turnover number (TON) of ≈1300 in just 4 h. Most significantly, the catalyst was durable enough to maintain long-term activity, and upon only periodic recharging of NH₃BH₃, it furnished a total TON of >4200 in 10 h.

Iridium(NHC)-Catalyzed Sustainable Transfer Hydrogenation of CO2 and Inorganic Carbonates

Iridium(NHC)-catalyzed transfer hydrogenation (TH) of CO2 and inorganic carbonates with glycerol were conducted, demonstrating excellent turnover numbers (TONs) and turnover frequencies (TOFs) for the formation of formate and lactate. Regardless of carbon sources, excellent TOFs of formate were observed (CO2: 10,000 h−1 and K2CO3: 10,150 h−1). Iridium catalysts modified with the triscarbene ligand showed excellent catalytic activity at 200 °C and are a suitable choice for this transformation which requires a high temperature for high TONs of formate. On the basis of the control experiments, the transfer hydrogenation mechanism of CO2 was proposed.

Transfer hydrogenation of CO2 into formaldehyde from aqueous glycerol heterogeneously catalyzed by Ru bound to LDH

Aqueous glycerol was used in this study as a liquid-phase hydrogen source for the hydrogenation of CO₂. It was found that hydrogen could be efficiently evolved from aqueous glycerol upon highly dispersed Ru on layered double hydroxide (LDH), inducing the transformation of CO₂ into formaldehyde under base-free conditions at low temperature.

Copper(I) Complexes Containing PCP Ligand Catalyzed Hydrogenation of Carbon Dioxide to Formate under Ambient Conditions

The five new copper(I) complexes [Cu₂(μ-Cl)₂(κ¹-PCP^t-Bu)] (1), [Cu₂(μ-Br)₂(κ¹-PCP^t-Bu)] (2), [Cu₂(μ-I)₂(κ¹-PCP^t-Bu)] (3), [Cu₂(μ-CN)₂(κ¹-PCP^t-Bu)] (4), and [Cu₄(μ₃-SCN)₄(κ¹-PCP^t-Bu)₂]·CH₂Cl₂ (5) bearing a 1,3-bis[(di-tert-butylphosphino)methyl]benzene ligand were synthesized and characterized spectroscopically, and the molecular structures of 1, 3, and 5 were determined by single-crystal X-ray diffraction techniques. Structural studies for 1 and 3 revealed their binuclear structures with Cu···Cu separations of 2.609(3) and 2.6359(19) Å, respectively. However, 5 has a tetranuclear cubane structure with an 18-electron configuration at each copper without any metal-metal bonds. The two copper centers in 1 and 3 are bonded to one bridging PCP^t-Bu ligand in a κ¹-manner and two bridging (pseudo)halido ligands in a μ₂-bonding mode to generate a nonplanar Cu₂(μ-X)₂ framework. The four copper centers in 5 are at the vertices of a tetrahedron. Each copper center has pseudo-tetrahedral coordination provided by two bridging PCP^t-Bu ligands in a κ¹-manner and the four bridging thiocyanate groups in a μ₃-manner. These complexes were used as catalysts for the hydrogenation of CO₂ to formate in the presence of DBU as a base to produce valuable energy-rich chemicals, and therefore it is a promising, safe, and simple strategy to conduct reactions under ambient pressure at room temperature. Among all of the five copper(I) complex based catalysts, 3 displayed the best catalytic performance with turnover number (TON) values of 38-8700 in 12-48 h of reaction at 25-80 °C. The outstanding catalytic performance of [Cu₂(μ-I)₂(κ¹-PCP^t-Bu)] (3) makes it a potential candidate for realizing the large-scale production of formate by CO₂ hydrogenation.

Sustainable Chemistry - An Interdisciplinary Matrix Approach

Within the framework of green chemistry, the continuous development of new and advanced tools for sustainable synthesis is essential. For this, multi-facetted underlying demands pose inherent challenges to individual chemical disciplines. As a solution, both interdisciplinary technology screening and research can enhance the possibility for groundbreaking innovation. To illustrate the stages from discovery to the implementing of combined technologies, a SusChem matrix model is proposed inspired by natural product biosynthesis. The model describes a multi-dimensional and dynamic exploratory space where necessary interaction is exclusively provided and guided by sustainable themes.

Ester Hydrogenation with Bifunctional Metal-NHC Catalysts: Recent Advances

Hydrogenation of ester to alcohol is an essential reaction in organic chemistry due to its importance in the production of a wide range of bulk and fine chemicals. There are a number of homogeneous and heterogeneous catalyst systems reported in the literature for this useful reaction. Mostly, phosphine-based bifunctional catalysts, owing to their ability to show metal-ligand cooperation during catalytic reactions, are extensively used in these reactions. However, phosphine-based catalysts are difficult to synthesize and are also highly air- and moisture-sensitive, restricting broad applications. In contrast, N-heterocyclic carbenes (NHCs) can be easily synthesized, and their steric and electronic attributes can be fine-tuned easily. In recent times, many phosphine ligands have been replaced by potent σ-donor NHCs, and the resulting bifunctional metal-ligand systems are proven to be very efficient in several important catalytic reactions. This mini-review focuses the recent advances mainly on bifunctional metal-NHC complexes utilized as (pre)catalysts in ester hydrogenation reactions.

A highly active copper catalyst for the hydrogenation of carbon dioxide to formate under ambient conditions

Carbon dioxide (CO2) is an important reactant and can be used for the syntheses of various types of industrially important chemicals. Hence, investigation concerning the conversion of CO2 into valuable energy-rich chemicals is an important and current topic in molecular catalysis. Recent research on molecular catalysts has led to improved rates for conversion of CO2 to energy-rich products such as formate, but the catalysts based on first-row transition metals are underdeveloped. Copper(i) complexes containing the 1,1'-bis(di-tert-butylphosphino) ferrocene ligand were found to promote the catalytic hydrogenation of CO2 to formate in the presence of DBU as the base, where the catalytic conversion of CO2via hydrogenation is achieved using in situ gaseous H2 (granulated tin metal and concentrated HCl) to produce valuable energy-rich chemicals, and therefore it is a promising, safe and simple strategy to conduct reactions under ambient pressure at room temperature. Towards this goal, we report an efficient copper(i) complex based catalyst [CuI(dtbpf)] to achieve ambient-pressure CO2 hydrogenation catalysis for generating the formate salt (HCO2-) with turnover number (TON) values of 326 to 1.065 × 105 in 12 to 48 h of reaction at 25 °C to 80 °C. The outstanding catalytic performance of [CuI(dtbpf)] makes it a potential candidate for realizing the large-scale production of formate by CO2 hydrogenation.

Electrocatalytic CO2 Reduction with a Half-Sandwich Cobalt Catalyst: Selectivity towards CO

We present herein a Cp*Co(III)-half-sandwich catalyst system for electrocatalytic CO₂ reduction in aqueous acetonitrile solution. In addition to an electron-donating Cp* ligand (Cp*=pentamethylcyclopentadienyl), the catalyst featured a proton-responsive pyridyl-benzimidazole-based N,N-bidentate ligand. Owing to the presence of a relatively electron-rich Co center, the reduced Co(I)-state was made prone to activate the electrophilic carbon center of CO₂ . At the same time, the proton-responsive benzimidazole scaffold was susceptible to facilitate proton-transfer during the subsequent reduction of CO₂ . The above factors rendered the present catalyst active toward producing CO as the major product over the other potential 2e/2H⁺ reduced product HCOOH, in contrast to the only known similar half-sandwich CpCo(III)-based CO₂ -reduction catalysts which produced HCOOH selectively. The system exhibited a Faradaic efficiency (FE) of about 70% while the overpotential for CO production was found to be 0.78 V, as determined by controlled-potential electrolysis.

Transfer hydrogenation of carbon dioxide via bicarbonate promoted by bifunctional C-N chelating Cp*Ir complexes

Metal-ligand cooperative Cp*Ir(iii) complexes derived from primary benzylic amines effectively promote transfer hydrogenation of atmospheric CO2 using 2-propanol at 80 °C. Isotope-labelling experiments strengthen that active Ir species can preferentially reduce bicarbonate congeners formed from CO2. The powerful transfer hydrogenation catalyst exhibits remarkable activity for the conversion of bicarbonates into formate salts with a turnover number up to 3200, even without H2 and CO2.

Selective and high yield transformation of glycerol to lactic acid using NNN pincer ruthenium catalysts

A sterically less hindered 2,6-bis(benzimidazol-2-yl)pyridine based pincer–ruthenium complex has been used here to accomplish the catalytic conversion of glycerol selectively to lactic acid in high yield.

Study on the Adsorption and Activation Behaviours of Carbon Dioxide over Copper Cluster (Cu4) and Alumina-Supported Copper Catalyst (Cu4/Al2O3) by means of Density Functional Theory

The adsorption and activation of carbon dioxide over copper cluster (Cu4) and copper doped on the alumina support (Cu4/Al2O3) catalytic systems have been investigated by using density functional theory and climbing image nudged elastic band. The adsorption energies, geometrical configurations, and electronic properties are analysed. The results show the strong chemical interaction between the copper cluster and the alumina support. Both the Cu4 cluster and Cu4/Al2O3 systems have a high adsorption ability for CO2, and the adsorption process is of chemical nature. The role of the alumina support in the adsorption and activation of CO2 has been addressed. The calculated results show that the “synergistic effect” between Al2O3 and Cu4 is the key factor in the activation of CO2.

Catalytic recycling of NAD(P)H

A large number of industrially relevant enzymes depend upon dihydronicotinamide adenine dinucleotide (NADH) and dihydronicotinamide adenine dinucleotide phosphate (NADPH) cofactors, which are too expensive to be added in stoichiometric amounts. Existing NAD(P)H-recycling systems suffer from low activity, or the generation of side products. This review focuses on NAD(P)H cofactor regeneration catalyzed by transition metal complexes such as rhodium, ruthenium and iridium complexes using cheap reducing agents such as hydrogen (H₂) and ethanol, which have attracted increasing attention as sustainable energy carriers. The catalytic mechanisms for the regioselective reduction of NAD(P)⁺ are discussed with emphasis on identification of catalytically active intermediates such as transition metal hydride complexes. Applications of NAD(P)H-recycling systems to develop artificial photosynthesis are also discussed.