Photocatalytic synthesis of tetra-substituted furans promoted by carbon dioxide

We report a simple protocol for the transition metal-free, visible-light-driven conversion of 1,3-diketones to tetra-substituted furan skeleton compounds in carbon dioxide (CO₂) atmosphere under mild conditions. It was found that CO₂ could be incorporated at the diketone enolic OH position, which was key to enabling the cleavage of a C–O bond during the rearrangement of a cyclopropane intermediate. This method allows for the same-pot construction of two isomers of the high-value tetra-substituted furan scaffold. The synthetic scope and preliminary mechanistic investigations are presented.

A CO₂-promoted transition metal-free photocatalytic synthesis of tetra-substituted furan derivatives from 1,3-diketones as the only starting material.     

Quasi-square-shaped cadmium hydroxide nanocatalysts for electrochemical CO2 reduction with high efficiency

Powered by a renewable electricity source, electrochemical CO₂ reduction reaction is a promising solution to facilitate the carbon balance. However, it is still a challenge to achieve a desired product with commercial current density and high efficiency. Herein we designed quasi-square-shaped cadmium hydroxide nanocatalysts for CO₂ electroreduction to CO. It was discovered that the catalyst is very active and selective for the reaction. The current density could be as high as 200 mA cm⁻² with a nearly 100% selectivity in a commonly used H-type cell using the ionic liquid-based electrolyte. In addition, the faradaic efficiency of CO could reach 90% at a very low overpotential of 100 mV. Density functional theory studies and control experiments reveal that the outstanding performance of the catalyst was attributed to its unique structure. It not only provides low Cd–O coordination, but also exposes high activity (002) facet, which requires lower energy for the formation of CO. Besides, the high concentration of CO can be achieved from the low concentration CO₂via an adsorption-electrolysis device.

Quasi-square cadmium hydroxide nanocrystals (Cdhy-QS) showed outstanding performance for electroreduction CO₂ to CO.     

Probing the local activity of CO2 reduction on gold gas diffusion electrodes: effect of the catalyst loading and CO2 pressure

Large scale CO₂ electrolysis can be achieved using gas diffusion electrodes (GDEs), and is an essential step towards broader implementation of carbon capture and utilization strategies. Different variables are known to affect the performance of GDEs. Especially regarding the catalyst loading, there are diverging trends reported in terms of activity and selectivity, e.g. for CO₂ reduction to CO. We have used shear–force based Au nanoelectrode positioning and scanning electrochemical microscopy (SECM) in the surface-generation tip collection mode to evaluate the activity of Au GDEs for CO₂ reduction as a function of catalyst loading and CO₂ back pressure. Using a Au nanoelectrode, we have locally measured the amount of CO produced along a catalyst loading gradient under operando conditions. We observed that an optimum local loading of catalyst is necessary to achieve high activities. However, this optimum is directly dependent on the CO₂ back pressure. Our work does not only present a tool to evaluate the activity of GDEs locally, it also allows drawing a more precise picture regarding the effect of catalyst loading and CO₂ back pressure on their performance.

Large scale CO₂ electrolysis can be achieved using gas diffusion electrodes (GDEs), and is an essential step towards broader implementation of carbon capture and utilization strategies.     

Polymeric frustrated Lewis pairs in CO2/cyclic ether coupling catalysis

Frustrated Lewis pairs (FLPs) are now ubiquitous as metal-free catalysts in an array of different chemical transformations. In this paper we show that this reactivity can be transferred to a polymeric system, offering advantageous opportunities at the interface between catalysis and stimuli-responsive materials. Formation of cyclic carbonates from cyclic ethers using CO₂ as a C₁ feedstock continues to be dominated by metal-based systems. When paired with a suitable nucleophile, discrete aryl or alkyl boranes have shown significant promise as metal-free Lewis acidic alternatives, although catalyst reuse remains illusive. Herein, we leverage the reactivity of FLPs in a polymeric system to promote CO₂/cyclic ether coupling catalysis that can be tuned for the desired epoxide or oxetane substrate. Moreover, these macromolecular FLPs can be reused across multiple reaction cycles, further increasing their appeal over analogous small molecule systems.

Polymeric frustrated Lewis pairs catalyse the coupling of epoxides and oxetanes with CO₂ with high selectivity under mild CO₂ pressures across multiple reaction cycles.     

Spatial-confinement induced electroreduction of CO and CO2 to diols on densely-arrayed Cu nanopyramids

The electroreduction of carbon dioxide (CO₂) and carbon monoxide (CO) to liquid alcohol is of significant research interest. This is because of a high mass-energy density, readiness for transportation and established utilization infrastructure. Current success is mainly around monohydric alcohols, such as methanol and ethanol. There exist few reports on converting CO₂ or CO to higher-valued diols such as ethylene glycol (EG; (CH₂OH)₂). The challenge to producing diols lies in the requirement to retain two oxygen atoms in the compound. Here for the first time, we demonstrate that densely-arrayed Cu nanopyramids (Cu-DAN) are able to retain two oxygen atoms for hydroxyl formation. This results in selective electroreduction of CO₂ or CO to diols. Density Functional Theory (DFT) computations highlight that the unique spatial-confinement induced by Cu-DAN is crucial to selectively generating EG through a new reaction pathway. This structure promotes C–C coupling with a decreased reaction barrier. Following C–C coupling the structure facilitates EG production by (1) retaining oxygen and promoting the *COH–CHO pathway, which is a newly identified pathway toward ethylene glycol production; and, (2) suppressing the carbon–oxygen bond breaking in intermediate *CH₂OH–CH₂O and boosting hydrogenation to EG. Our findings will be of immediate interest to researchers in the design of highly active and selective CO₂ and CO electroreduction to diols.

Densely-arrayed Cu nanopyramids have spatial confinement induced by the additional Cu–O bond. This promotes C–C coupling, regulates post-C–C coupling, and retains both oxygen atoms in an alternative pathway toward ethylene glycol formation from CO.     

Amide-bridged conjugated organic polymers: efficient metal-free catalysts for visible-light-driven CO2 reduction with H2O to CO

The visible-light-driven photoreduction of CO₂ to value-added chemicals over metal-free photocatalysts without sacrificial reagents is very interesting, but challenging. Herein, we present amide-bridged conjugated organic polymers (amide-COPs) prepared via self-condensation of amino nitriles in combination with hydrolysis, for the photoreduction of CO₂ with H₂O without any photosensitizers or sacrificial reagents under visible light irradiation. These catalysts can afford CO as the sole carbonaceous product without H₂ generation. Especially, amide-DAMN derived from diaminomaleonitrile exhibited the highest activity for the photoreduction of CO₂ to CO with a generation rate of 20.6 μmol g⁻¹ h⁻¹. Experiments and DFT calculations confirmed cyano/amide groups as active sites for CO₂ reduction and second amine groups for H₂O oxidation, and suggested that superior selectivity towards CO may be attributed to the adjacent redox sites. This work presents a new insight into designing photocatalysts for artificial photosynthesis.

Amino nitrile-derived conjugated organic polymers can realize the photoreduction of CO₂ with water to CO without H₂ generation efficiently.     

Unraveling differences in aluminyl and carbene coordination chemistry: bonding in gold complexes and reactivity with carbon dioxide

The electronic properties of aluminyl anions have been reported to be strictly related to those of carbenes, which are well-known to be easily tunable via selected structural modifications imposed on their backbone. Since peculiar reactivity of gold-aluminyl complexes towards carbon dioxide has been reported, leading to insertion of CO₂ into the Au–Al bond, in this work the electronic structure and reactivity of Au–Al complexes with different aluminyl scaffolds have been systematically studied and compared to carbene analogues. The analyses reveal that, instead, aluminyls and carbenes display a very different behavior when bound to gold, with the aluminyls forming an electron-sharing and weakly polarized Au–Al bond, which turns out to be poorly modulated by structural modifications of the ligand. The reactivity of gold–aluminyl complexes towards CO₂ shows, both qualitatively and quantitatively, similar reaction mechanisms, reflecting the scarce tunability of their electronic structure and bond nature. This work provides further insights and perspectives on the properties of the aluminyl anions and their behavior as coordination ligands.

Aluminyls and carbenes as coordination ligands, although sharing similar electronic properties, reveal fundamental differences in their tunability, bonding to gold and reactivity of their complexes with carbon dioxide.     

Direct air capture of CO2via crystal engineering

This article presents a perspective view of the topic of direct air capture (DAC) of carbon dioxide and its role in mitigating climate change, focusing on a promising approach to DAC involving crystal engineering of metal–organic and hydrogen-bonded frameworks. The structures of these crystalline materials can be easily elucidated using X-ray and neutron diffraction methods, thereby allowing for systematic structure–property relationships studies, and precise tuning of their DAC performance.

A perspective view of direct air capture (DAC) of CO₂ and its role in mitigating climate change is presented. The article focuses on a promising approach to DAC involving crystal engineering of metal–organic and hydrogen-bonded frameworks.     

Recent advances in single atom catalysts for the electrochemical carbon dioxide reduction reaction

The electrochemical carbon dioxide reduction reaction (CO₂RR) offers a promising solution to mitigate carbon emission and at the same time generate valuable carbonaceous chemicals/fuels. Single atom catalysts (SACs) are encouraging to catalyze the electrochemical CO₂RR due to the tunable electronic structure of the central metal atoms, which can regulate the adsorption energy of reactants and reaction intermediates. Moreover, SACs form a bridge between homogeneous and heterogeneous catalysts, providing an ideal platform to explore the reaction mechanism of electrochemical reactions. In this review, we first discuss the strategies for promoting the CO₂RR performance, including suppression of the hydrogen evolution reaction (HER), generation of C₁ products and formation of C₂₊ products. Then, we summarize the recent developments in regulating the structure of SACs toward the CO₂RR based on the above aspects. Finally, several issues regarding the development of SACs for the CO₂RR are raised and possible solutions are provided.

The electrochemical carbon dioxide reduction reaction (CO₂RR) offers a promising solution to mitigate carbon emission and at the same time generate valuable carbonaceous chemicals/fuels.     

A sustainable approach for the synthesis of recyclable cyclic CO2-based polycarbonates

It is highly desirable to reduce the environmental pollution related to the disposal of end-of-life plastics. Polycarbonates derived from the copolymerization of CO₂ and epoxides have attracted much attention since they can enable CO₂-fixation and furnish biorenewable and degradable polymeric materials. So far, only linear CO₂-based polycarbonates have been reported and typically degraded to cyclic carbonates. Here we synthesize a homogeneous dinuclear methyl zinc catalyst ((BDI-ZnMe)₂, 1) to rapidly copolymerize meso-CHO and CO₂ into poly(cyclohexene carbonate) (PCHC) with an unprecedentedly cyclic structure. Moreover, in the presence of trace amounts of water, a heterogeneous multi-nuclear zinc catalyst ((BDI-(ZnMe₂·xH₂O))_n, 2) is prepared and shows up to 99% selectivity towards the degradation of PCHC back to meso-CHO and CO₂. This strategy not only achieves the first case of cyclic CO₂-based polycarbonate but also realizes the complete chemical recycling of PCHC back to its monomers, representing closed-loop recycling of CO₂-based polycarbonates.

It is highly desirable to reduce the environmental pollution related to the disposal of end-of-life plastics.     

CO reductive oligomerization by a divalent thulium complex and CO2-induced functionalization

The divalent thulium complex [Tm(Cp^ttt)₂] (Cp^ttt = 1,2,4-tris(tert-butyl)cyclopentadienyl) reacts with CO to afford selective CO reductive dimerization and trimerization into ethynediolate (C₂) and ketenecarboxylate (C₃) complexes, respectively. DFT calculations were performed to shed light on the elementary steps of CO homologation and support a stepwise chain growth. The attempted decoordination of the ethynediolate fragment by treatment with Me₃SiI led to dimerization and rearrangement into a 3,4-dihydroxyfuran-2-one complex. Investigation of the reactivity of the C₂ and C₃ complexes towards other electrophiles led to unusual functionalization reactions: while the reaction of the ketenecarboxylate C₃ complex with electrophiles yielded new multicarbon oxygenated complexes, the addition of CO₂ to the ethynediolate C₂ complex resulted in the formation of a very reactive intermediate, allowing C–H activation of aromatic solvents. This original intermolecular reactivity corresponds to an unprecedented functionalization of CO-derived ligands, which is induced by CO₂.

The divalent thulium complex [Tm(Cp^ttt)₂] activates CO to form reductive CO dimerization or trimerization products. These complexes further react with electrophiles, including CO₂, yielding multicarbon oxygenates and original C–H activation products.     

Stereodivergent synthesis of β-iodoenol carbamates with CO2via photocatalysis

Photocatalytic conversion of carbon dioxide (CO₂) into value-added chemicals is of great significance from the viewpoint of green chemistry and sustainable development. Here, we report a stereodivergent synthesis of β-iodoenol carbamates through a photocatalytic three-component coupling of ethynylbenziodoxolones, CO₂ and amines. By choosing appropriate photocatalysts, both Z- and E-isomers of β-iodoenol carbamates, which are difficult to prepare using existing methods, can be obtained stereoselectively. This transformation featured mild conditions, excellent functional group compatibility and broad substrate scope. The potential synthetic utility of this protocol was demonstrated by late-stage modification of bioactive molecules and pharmaceuticals as well as by elaborating the products to access a wide range of valuable compounds. More importantly, this strategy could provide a general and practical method for stereodivergent construction of trisubstituted alkenes such as triarylalkenes, which represents a fascinating challenge in the field of organic chemistry research. A series of mechanism investigations revealed that the transformation might proceed through a charge-transfer complex which might be formed through a halogen bond.

Stereodivergent synthesis of β-iodoenol carbamates was achieved via a photocatalytic three-component coupling reaction of ethynylbenziodoxolones, CO₂ and amines.     

The Mechanistic Study of Methane Reforming with Carbon Dioxide on Ni/α-Al2O3

The reactions of oxidized and reduced 6 wt % NiO/-Al₂O₃ with H₂, CH₄, CO₂, O₂, and their mixtures are studied in flow and pulse regimes using a setup equipped with a differential scanning calorimeter DSC-111 and a system for chromatographic analysis. It is shown that treatment with hydrogen at 700° results in the partial reduction of NiO to Ni. Methane practically does not react with oxidized Ni/-Al₂O₃ but it does react actively with the reduced catalyst to form H₂ and surface carbon. The latter is capable of reacting with lattice oxygen of Ni/-Al₂O₃ (slowly) and with adsorbed oxygen (rapidly). Carbon dioxide also reacts with surface carbon to form CO (rapidly) and with metallic Ni to yield CO and NiO (slowly). Thus, the main route of methane reforming with carbon dioxide on Ni/-Al₂O₃ is the dissociative adsorption of CH₄ to form surface carbon and H₂ and the reaction of this carbon with CO₂ resulting in the formation of CO by the reverse Boudouard reaction. Side routes are the interaction of the products of methane chemisorption with catalyst oxygen and the dissociative adsorption of CO₂ on metallic nickel. A competitive reaction of surface carbon with adsorbed oxygen results in a decrease in the CO₂ conversion in methane reforming with carbon dioxide. Therefore, the presence of gaseous oxygen in the reacting mixture decelerates methane reforming (catalyst poisoning by oxygen).     

Effect of pore diameter and length on electrochemical CO2 reduction reaction at nanoporous gold catalysts

In this work, we employ differential electrochemical mass spectrometry (DEMS) to track the real-time evolution of CO at nanoporous gold (NpAu) catalysts with varying pore parameters (diameter and length) during the electrochemical CO₂ reduction reaction (CO₂RR). We show that due to the increase in the local pH with increasing catalyst roughness, NpAu catalysts suppress the bicarbonate-mediated hydrogen evolution reaction (HER) compared to a flat Au electrode. Additionally, the geometric current density for CO₂RR increases with the roughness of NpAu catalysts, which we attribute to the increased availability of active sites at NpAu catalysts. Together, the enhancement of CO₂RR and the suppression of competing HER results in a drastic increase in the faradaic selectivity for CO₂RR with increasing pore length and decreasing pore diameter, reaching near 100% faradaic efficiency for CO in the most extreme case. Interestingly, unlike the geometric current density, the specific current density for CO₂RR has a more complicated relation with the roughness of the NpAu catalysts. We show that this is due to the presence of ohmic drop effects along the length of the porous channels. These ohmic drop effects render the pores partially electrocatalytically inactive and hence, they play an important role in tuning the CO₂RR activity on nanoporous catalysts.

In this work, we employ differential electrochemical mass spectrometry (DEMS) to track the real-time evolution of CO at nanoporous gold (NpAu) catalysts with varying pore parameters (diameter and length) during the electrochemical CO₂ reduction reaction (CO₂RR).     

Ensemble effects in Cu/Au ultrasmall nanoparticles control the branching point for C1 selectivity during CO2 electroreduction

Bimetallic catalysts provide opportunities to overcome scaling laws governing selectivity of CO₂ reduction (CO₂R). Cu/Au nanoparticles show promise for CO₂R, but Au surface segregation on particles with sizes ≥7 nm prevent investigation of surface atom ensembles. Here we employ ultrasmall (2 nm) Cu/Au nanoparticles as catalysts for CO₂R. The high surface to volume ratio of ultrasmall particles inhibits formation of a Au shell, enabling the study of ensemble effects in Cu/Au nanoparticles with controllable composition and uniform size and shape. Electrokinetics show a nonmonotonic dependence of C1 selectivity between CO and HCOOH, with the 3Au:1Cu composition showing the highest HCOOH selectivity. Density functional theory identifies Cu₂/Au(211) ensembles as unique in their ability to synthesize HCOOH by stabilizing CHOO* while preventing H₂ evolution, making C1 product selectivity a sensitive function of Cu/Au surface ensemble distribution, consistent with experimental findings. These results yield important insights into C1 branching pathways and demonstrate how ultrasmall nanoparticles can circumvent traditional scaling laws to improve the selectivity of CO₂R.

Bimetallic catalysts provide opportunities to overcome scaling laws governing C1 selectivity of CO₂ reduction (CO₂R).     

Next steps for solvent-based CO2 capture; integration of capture,conversion, and mineralisation

In this perspective, we detail how solvent-based carbon capture integrated with conversion can be an important element in a net-zero emission economy. Carbon capture and utilization (CCU) is a promising approach for at-scale production of green CO₂-derived fuels, chemicals and materials. The challenge is that CO₂-derived materials and products have yet to reach market competitiveness because costs are significantly higher than those from conventional means. We present here the key to making CO₂-derived products more efficiently and cheaper, integration of solvent-based CO₂ capture and conversion. We present the fundamentals and benefits of integration within a changing energy landscape (i.e., CO₂ from point source emissions transitioning to CO₂ from the atmosphere), and how integration could lead to lower costs and higher efficiency, but more importantly how CO₂ altered in solution can offer new reactive pathways to produce products that cannot be made today. We discuss how solvents are the key to integration, and how solvents can adapt to differing needs for capture, conversion and mineralisation in the near, intermediate and long term. We close with a brief outlook of this emerging field of study, and identify critical needs to achieve success, including establishing a green-premium for fuels, chemicals, and materials produced in this manner.

In this perspective, we detail how solvent-based carbon capture integrated with conversion can be an important element in a net-zero emission economy.     

Why heterogeneous single-atom catalysts preferentially produce CO in the electrochemical CO2 reduction reaction

Formate and CO are competing products in the two-electron CO₂ reduction reaction (2e CO₂RR), and they are produced via *OCHO and *COOH intermediates, respectively. However, the factors governing CO/formate selectivity remain elusive, especially for metal–carbon–nitrogen (M–N–C) single-atom catalysts (SACs), most of which produce CO as their main product. Herein, we show computationally that the selectivity of M–N–C SACs is intrinsically associated with the CO₂ adsorption mode by using bismuth (Bi) nanosheets and the Bi–N–C SAC as model catalysts. According to our results, the Bi–N–C SAC exhibits a strong thermodynamic preference toward *OCHO, but under working potentials, CO₂ is preferentially chemisorbed first due to a charge accumulation effect, and subsequent protonation of chemisorbed CO₂ to *COOH is kinetically much more favorable than formation of *OCHO. Consequently, the Bi–N–C SAC preferentially produces CO rather than formate. In contrast, the physisorption preference of CO₂ on Bi nanosheets contributes to high formate selectivity. Remarkably, this CO₂ adsorption-based mechanism also applies to other typical M–N–C SACs. This work not only resolves a long-standing puzzle in M–N–C SACs, but also presents simple, solid criteria (i.e., CO₂ adsorption modes) for indicating CO/formate selectivity, which help strategic development of high-performance CO₂RR catalysts.

This report discloses a nontrivial role of the CO₂ adsorption mode in governing the CO/formate selectivity of single-atom catalysts towards two-electron CO₂ reduction.     

Copper-catalyzed borofunctionalization of styrenes with B2pin2 and CO

The construction of structurally complexed and high-value chemical molecules from simple and readily available feedstocks is a long-standing challenge to chemists. Here, we describe a copper-catalyzed borofunctionalization of styrenes with B₂pin₂ and carbon monoxide. A set of new sodium cyclic borates were obtained with NaO^tBu as the base. These unique sodium cyclic borates can be easily converted into a variety of multifunctional β-boryl vinyl esters, boryl carbonates, β-boryl aldehydes, and boryl vinyl ether. In addition, the procedure also features good functional group tolerance and utilizes CO as the C1 source.

A new copper-catalyzed borofunctionalization of styrenes with B₂pin₂ and carbon monoxide has been developed.     

The effect of specific adsorption of halide ions on electrochemical CO2 reduction

In the electrochemical CO₂ reduction reaction (CO₂RR), halide ions could impose a significant effect on multi-carbon (C₂₊) product production for Cu-based catalysts by a combined contribution from various mechanisms. However, the nature of specific adsorption of halide ions remains elusive due to the difficulty in decoupling different effects. This paper describes a facile method to actively immobilize the morphology of Cu-based catalysts during the CO₂RR, which makes it possible to reveal the fundamental mechanism of specific adsorption of halide ions. A stable morphology is obtained by pre-reduction in aqueous KX (X = Cl, Br, I) electrolytes followed by conducting the CO₂RR using non-buffered and non-specifically adsorbed K₂SO₄ as the supporting electrolyte, by which the change of local pH and cation concentration is also maintained during the CO₂RR. In situ spectroscopy revealed that the specific adsorption of halide ions enhances the adsorption of *CO intermediates, which enables a high selectivity of 84.5% for C₂₊ products in 1.0 M KI.

Reconstruction of catalyst morphology induced by halide ions over of Cu-based catalysts during CO₂ electroreduction is suppressed using K₂SO₄ as supporting electrolyte. Adsorption of halide ions is an enabling factor to enhance CO₂ electroreduction.     

Comparing interfacial cation hydration at catalytic active sites and spectator sites on gold electrodes: understanding structure sensitive CO2 reduction kinetics

Hydrated cations present in the electrochemical double layer (EDL) are known to play a crucial role in electrocatalytic CO₂ reduction (CO₂R), and numerous studies have attempted to explain how the cation effect contributes to the complex CO₂R mechanism. CO₂R is a structure sensitive reaction, indicating that a small fraction of total surface sites may account for the majority of catalytic turnover. Despite intense interest in specific cation effects, probing site-specific, cation-dependent solvation structures remains a significant challenge. In this work, CO adsorbed on Au is used as a vibrational Stark reporter to indirectly probe solvation structure using vibrational sum frequency generation (VSFG) spectroscopy. Two modes corresponding to atop adsorption of CO are observed with unique frequency shifts and potential-dependent intensity profiles, corresponding to direct adsorption of CO to inactive surface sites, and in situ generated CO produced at catalytic active sites. Analysis of the cation-dependent Stark tuning slopes for each of these species provides estimates of the hydrated cation radius upon adsorption to active and inactive sites on the Au electrode. While cations are found to retain their bulk hydration shell upon adsorption at inactive sites, catalytic active sites are characterized by a single layer of water between the Au surface and the electrolyte cation. We propose that the drastic increase in catalytic performance at active sites stems from this unique solvation structure at the Au/electrolyte interface. Building on this evidence of a site-specific EDL structure will be critical to understand the connection between cation-dependent interfacial solvation and CO₂R performance.

Site-specific vibrational probes were used to elucidate the interfacial solvation structure between catalytic active sites and inactive sites on a Au electrode to reveal a unique, opposing cation-dependent double layer structure at active sites.