A short review of recent advances in CO2 hydrogenation to hydrocarbons over heterogeneous catalysts

CO₂ hydrogenation to hydrocarbons is a promising way of making waste to wealth and energy storage, which also solves the environmental and energy issues caused by CO₂ emissions. Much efforts and research are aimed at the conversion of CO₂via hydrogenation to various value-added hydrocarbons, such as CH₄, lower olefins, gasoline, or long-chain hydrocarbons catalyzed by different catalysts with various mechanisms. This review provides an overview of advances in CO₂ hydrogenation to hydrocarbons that have been achieved recently in terms of catalyst design, catalytic performance and reaction mechanism from both experiments and density functional theory calculations. In addition, the factors influencing the performance of catalysts and the first C–C coupling mechanism through different routes are also revealed. The fundamental factor for product selectivity is the surface H/C ratio adjusted by active metals, supports and promoters. Furthermore, the technical and application challenges of CO₂ conversion into useful fuels/chemicals are also summarized. To meet these challenges, future research directions are proposed in this review.

CO₂ hydrogenation to hydrocarbons over heterogeneous catalysts.     

Nitrogen-decorated,porous carbons derived from waste cow manure as efficient catalysts for the selective capture and conversion of CO2

The catalytic conversion of CO₂ is a promising solution to the greenhouse effect and simultaneously recycles the carbon sources to produce high value-added chemicals. Herein, we demonstrated a class of nanoporous carbons, which were synthesized by the direct carbonization of bio-waste cow manure, followed by activation with KOH and NaNH₂. Various characterizations indicate that the resultant nanoporous carbons have abundant nanopores and nitrogen sites. As a result, their performances for the capture and catalytic conversion of CO₂ were investigated. The synthesized nanoporous carbons exhibited superior properties for the selective capture and catalytic cycloaddition of CO₂ to propylene oxide as compared to various solid materials.

Nitrogen-doped, hierarchically porous carbons were prepared by the activation of waste cow manure at 600 °C, which acted as efficient catalysts for the highly selective capture and conversion of CO₂ into valuable chemicals.     

Atomically dispersed Cu and Fe on N-doped carbon materials for CO2 electroreduction: insight into the curvature effect on activity and selectivity

CO₂ electroreduction reaction (CO₂ER) by single metal sites embedded in N-doped graphene (M@N-Gr, M = Cu and Fe) and carbon nanotubes (M@N-CNT, M = Cu and Fe) has been explored by extensive first-principles calculations in combination with the computational hydrogen electrode model. Both atomically dispersed Cu and Fe nanostructures, as the single atom catalysts (SACs), have higher selectivity towards CO₂ER, compared to hydrogen evolution reduction (HER), and they can catalyze CO₂ER to CO, HCOOH, and CH₃OH. In comparison with Cu@N-Gr, the limiting potentials for generating CO, HCOOH, and CH₃OH are reduced obviously on the high-curvature Cu@N-CNT. However, the curvature effect is less notable for the single-Fe-atom catalysts. Such discrepancies can be attributed to the d-band center changes of the single Cu and Fe sites and their different dependences on the curvature of carbon-based support materials.

Atomically dispersed Cu/Fe catalysts have high selectivity toward CO₂ER and the curvature of the catalyst support influences their activity.     

Auxiliary-directed etherification of sp2 C–H bonds under heterogeneous metal–organic framework catalysis: synthesis of ethenzamide

An efficient protocol for 8-aminoquinoline assisted alkoxylation and phenoxylation of sp² C–H bonds under heterogeneous catalysis was developed. The optimal conditions employed Cu-MOF-74 (20%), K₂CO₃ base, pyridine ligand or dimethyl formamide solvent, and O₂ oxidant at 80 °C or 100 °C for 24 hours. Cu-MOF-74 revealed remarkably higher activity when compared with other previously commonly used Cu-MOFs in cross coupling reactions, supported copper catalysts, and homogeneous copper salts. The reaction scope with respect to coupling partners included a wide range of various substrates. Interestingly, the developed conditions are applicable for the synthesis of high-profile relevant biological agents from easily accessible starting materials. Furthermore, a leaching test confirmed the reaction heterogeneity and the catalyst was reused and recycled at least 8 times with trivial degradation in activity.

An efficient protocol for 8-aminoquinoline assisted alkoxylation and phenoxylation of sp² C–H bonds under heterogeneous catalysis was developed.     

Efficient electrocatalytic reduction of carbon dioxide by metal-doped β12-borophene monolayers

Electrochemical reduction of CO₂ to value-added chemicals and fuels shows great promise in contributing to reducing the energy crisis and environment problems. This progress has been slowed by a lack of stable, efficient and selective catalysts. In this paper, density functional theory (DFT) was used to study the catalytic performance of the first transition metal series anchored TM–B_β12 monolayers as catalysts for electrochemical reduction of CO₂. The results show that the TM–B_β12 monolayer structure has excellent catalytic stability and electrocatalytic selectivity. The primary reduction product of Sc–B_β12 is CO and the overpotential is 0.45 V. The primary reduction product of the remaining metals (Ti–Zn) is CH₄, where Fe–B_β12 has the minimum overpotential of 0.45 V. Therefore, these new catalytic materials are exciting. Furthermore, the underlying reaction mechanisms of CO₂ reduction via the TM–B_β12 monolayers have been revealed. This work will shed insights on both experimental and theoretical studies of electroreduction of CO₂.

These new TM–B_β12 monolayers will display excellent catalytic performance for electroreduction of CO₂. Primary reduction product of Sc is CO (overpotential 0.45 V). Primary product Ti–Zn is CH₄, and Fe–B_β12 has 0.45 V overpotential.     

Electroactive Co(iii) salen metal complexes and the electrophoretic deposition of their porous organic polymers onto glassy carbon

This paper reports the CO₂ electroreduction properties of three bis-bromo Co(iii) salen metal complexes and their Porous Organic Polymers (POPs) as a platform for using the salen core as a multi-electron reducing agent. Although Co(iii) salen metal complexes have been studied extensively for their chemical catalysis with CO₂, their electrochemical behaviour, particularly their reduction, in the presence of CO₂ is much less explored. The discrete Co(iii) complexes enabled the reduction of CO₂ to CO in faradaic efficiencies of up to 20%. The reductive electrochemical processes of Co(iii) salen complexes are relatively unknown; therefore, the mechanism of reduction for the complexes was investigated using IR and UV-Vis-NIR spectroelectrochemical (SEC) techniques. The discrete bis-bromo salen complexes were incorporated into POPs with tris-(p-ethynyl)-triphenylamine as a co-ligand and were characterised using solid state NMR, IR, UV-Vis-NIR and Field Emission Scanning Electron Microscopy (FE-SEM). The POP materials were electrophoretically deposited onto glassy carbon under milder conditions than those previously reported in the literature. Direct attachment of the POP materials to glassy carbon enabled improved solid state electrochemical analysis of the samples. The POP materials were also analysed via SEC techniques, where a Co(ii/i) process could be observed, but further reductions associated with the imine reduction compromised the stability of the POPs.

We report the synthesis, characterisation and electrochemistry of Co(iii) salen metal complexes and their porous organic polymers for CO₂ electroreduction.     

Recent advancements in g-C3N4-based photocatalysts for photocatalytic CO2 reduction: a mini review

Carbon dioxide (CO₂) is a very important micro-molecular resource. Using CO₂ captured from the atmosphere for high-output synthesis of chemicals as raw materials has great significance and potential for various industrial applications. Since the industrial revolution in the 18^th century, manmade CO₂ emission has increased by 45%, which negatively impacts the planetary climate by the so-called greenhouse effect. Therefore, high-efficiency photocatalysis and photocatalysts for CO₂ conversion have become the most important challenges and milestones throughout the world. In consideration of this, various catalysts have been explored. Among these, graphitic carbon nitride (g-C₃N₄) as a semiconductor is emerging as a highly promising photocatalyst for removing CO₂ from the atmosphere. Moreover, due to its excellent chemical stability and unique band structure, g-C₃N₄ has exhibited significant application potential for photocatalysis. This review summarizes the advancements that have been made in the synthesis and photocatalytic applications of g-C₃N₄-based catalysts for CO₂ reduction in recent years and explains the future challenges and prospects in this vital area of research.

g-C₃N₄-based photocatalysts for photocatalytic CO₂ reduction.     

Catalytic valorisation of biomass levulinic acid into gamma valerolactone using formic acid as a H2 donor: a critical review

This review sheds light on the catalytic valorisation of agroforestry biomass through levulinic acid and formic acid towards γ-valerolactone and other higher-value chemicals. γ-Valerolactone is produced by the hydrogenation of levulinic acid, which can be achieved through an internal hydrogen transfer reaction with formic acid in the presence of catalyst. By reviewing corresponding catalysts, the paper underlines the most efficient steps constituting an integrated sustainable process that eliminates the need for external H₂ sources while producing biofuels as an alternative energy source. Furthermore, the review emphasizes the role of catalysts in the hydrogenation of levulinic acid, with special focus on heterogeneous catalysts. The authors highlighted the dual role of different catalysts by comparing their activity, morphology, electronic structure, synergetic relation between support and doped species, as well as their deactivation and recyclability. Acknowledging the need for green and sustainable H₂ production, the review extends to cover the role of photo catalysis in dissociating H₂-donor solvents for reducing levulinic acid into γ-valerolactone under mild temperatures. To wrap up, the critical discussion presented enables readers to hone their knowledge about different schools and emphasizes research gaps emerging from experimental work. The review concludes with a comprehensive table summarizing the recent catalysts reported between the years 2017–2021.

This review sheds light on the catalytic valorisation of agroforestry biomass through levulinic acid and formic acid towards γ-valerolactone and other higher-value chemicals.     

Recent advances in liquid hydrosilane-mediated catalytic N-formylation of amines with CO2

Carbon dioxide is an ideal raw material for the synthesis of complex organic compounds because of its rich, non-toxic, and good physical properties. It is of great significance to transform CO₂ into valuable fine chemicals and develop a green sustainable cycle of carbon surplus. Based on hydrosilane as a reducing agent, this work summarizes the recent applications of reductive amidation of CO₂ using different catalysts such as organocatalysts, ionic liquids (ILs), salts, transition metal complexes, and solvents. The main factors affecting the reductive amidation of CO₂ and the possible reaction mechanism are discussed. Moreover, the future orientation and catalytic systems of the formylation of amines with CO₂ and hydrosilane are prospected.

This review depicts different types of catalyst systems developed for upgrading of amines and carbon dioxide into N-formylated products in the presence of hydrosilane, with attention on reaction mechanism and process optimization.     

Eucalyptus red grandis pretreatment with protic ionic liquids: effect of severity and influence of sub/super-critical CO2 atmosphere on pretreatment performance

Deconstruction of lignocellulosic biomass with low-cost ionic liquids (ILs) has proven to be a promising technology that could be implemented in a biorefinery to obtain renewable materials, fuels and chemicals. This study investigates the pretreatment efficacy of the ionoSolv pretreatment of Eucalyptus red grandis using the low-cost ionic liquid triethylammonium hydrogen sulfate ([N₂₂₂₀][HSO₄]) in the presence of 20 wt% water at 10% solids loading. The temperatures investigated were 120 °C and 150 °C. Also, the influence of performing the pretreatment under sub-critical and supercritical CO₂ was investigated. The IL used is very effective in deconstructing eucalyptus, producing cellulose-rich pulps resulting in enzymatic saccharification yields of 86% for some pretreatment conditions. It has been found that under a CO₂ atmosphere, the ionoSolv process is pressure independent. The good performance of this IL in the pretreatment of eucalyptus is promising for the development of a large-scale ionoSolv pretreatment processes.

Deconstruction of lignocellulosic biomass with low-cost ionic liquids (ILs) has proven to be a promising technology that could be implemented in a biorefinery to obtain renewable materials, fuels and chemicals.     

Tuning nanocavities of Au@Cu2O yolk–shell nanoparticles for highly selective electroreduction of CO2 to ethanol at low potential

The electrosynthesis of high-value ethanol from carbon dioxide and carbon monoxide addresses the need for the large-scale storage of renewable electricity and reduction of carbon emissions. However, the electrosynthesis of ethanol by the CO₂ reduction reaction (CO₂RR) has suffered from low selectivity and energy efficiency. Here, we report a catalyst composed of Au nanoparticles in Cu₂O nanocavities (Au@Cu₂O) that is very active for CO₂ reduction to ethanol through the confinement of the CO intermediate. The architecture shows tandem catalysis mechanisms in which CO₂ reduction on Au yolks produces CO filling Cu nanocavities, where a sufficiently high CO concentration due to the confinement effect promotes ethanol formation and then results in an ethanol faradaic efficiency of 52.3% at −0.30 V versus the reversible hydrogen electrode (vs. RHE) via regulating the hollow size of the Cu₂O nanocavities. Such a strategy provides a new way of fabricating various tandem catalysts with high selectivity and efficiency for the CO₂RR.

The confinement effect is applied to tandem catalysis in Au@Cu₂O yolk–shell nanoparticles to promote the efficient and selective reduction of CO₂ to ethanol at low potential.     

Molybdenum disulfide (MoS2) based photoredox catalysis in chemical transformations

Photoredox catalysis has been explored for chemical reactions by irradiation of photoactive catalysts with visible light, under mild and environmentally benign conditions. Furthermore, this methodology permits the activation of abundant chemicals into valuable products through novel mechanisms that are otherwise inaccessible. In this context, MoS₂ has drawn attention due to its excellent solar spectral response and its notable electrical, optical, mechanical and magnetic properties. MoS₂ has a number of characteristic properties like tunable band gap, enhanced absorption of visible light, a layered structure, efficient photon electron conversion, good photostability, non-toxic nature and quantum confinement effects that make it an ideal photocatalyst and co-catalyst for chemical transformations. Recently, MoS₂ has gained synthetic utility in chemical transformations. In this review, we will discuss MoS₂ properties, structure, synthesis techniques, and photochemistry along with modifications of MoS₂ to enhance its photocatalytic activity with a focus on its applications and future challenges.

Photoredox catalysis has been explored for chemical reactions by irradiation of photoactive catalysts with visible light, under mild and environmentally benign conditions.     

A new iron-based metal–organic framework with enhancing catalysis activity for benzene hydroxylation

A new Fe-based metal–organic framework (MOF), termed Fe-TBAPy Fe₂(OH)₂(TBAPy)·4.4H₂O, was solvothermally synthesized. Structural analysis revealed that Fe-TBAPy is built from [Fe(OH)(CO₂)₂]_∞ rod-shaped SBUs (SBUs = secondary building units) and 1,3,6,8-tetrakis(p-benzoate)pyrene (TBAPy⁴⁻) linker to form the frz topological structure highlighted by 7 Å channels and 3.4 Å narrow pores sandwiching between the pyrene cores of TBAPy⁴⁻. Consequently, Fe-TBAPy was used as a recyclable heterogeneous catalyst for benzene hydroxylation. Remarkably, the catalysis reaction resulted in high phenol yield and selectivity of 64.5% and 92.9%, respectively, which are higher than that of the other Fe-based MOFs and comparable with those of the best-performing heterogeneous catalysts for benzene hydroxylation. This finding demonstrated the potential for the design of MOFs with enhancing catalysis activity for benzene hydroxylation.

A new Fe-based MOFs catalyst was used for benzene hydroxylation with the high phenol yield (64.5%) and selectivity (92.9%).     

Synthesis of Ag nanoparticles/ordered mesoporous carbon as a highly efficient catalyst for the electroreduction of benzyl bromide

To develop efficient catalysts for the electroreduction of organic halides, a facile one-pot synthesis of Ag nanoparticles/ordered mesoporous carbon electrode materials via the self-assembly of CH₃COOAg and resol in the presence of triblock copolymer is proposed. The resultant electrode materials possess uniform mesopore sizes (3.3 nm) and pore volumes (∼0.28 cm³ g⁻¹), high specific surface areas (∼500 m² g⁻¹), and uniformly dispersed Ag nanoparticles (12–36 nm) loaded within the carbon matrix. Cyclic voltammetry, measurements of electrochemically active surface area, and electrolysis experiments were conducted to understand the correlations between the catalytic ability and the structural and textural features of the catalysts. Excellent bibenzyl yield (98%) and remarkable reusability were obtained under mild conditions. The results confirm that the prepared nanocomposites show outstanding performance in the electroreduction degradation of PhCH₂Br to bibenzyl.

To develop efficient catalysts for the electroreduction of organic halides, a facile one-pot synthesis of Ag nanoparticles/ordered mesoporous carbon electrode materials via the self-assembly is proposed.     

Nitrogen-doped mesoporous carbon supported CuSb for electroreduction of CO2

The construction of an efficient catalyst for electrocatalytic reduction of CO₂ to high value-added fuels has received extensive attention. Herein, nitrogen-doped mesoporous carbon (NMC) was used to support CuSb to prepare a series of materials for electrocatalytic reduction of CO₂ to CH₄. The catalytic activity of the composites was significantly improved compared with that of Cu/NMC. In addition, the Cu content also influenced the activity of electrocatalytic CO₂ reduction reaction. Among the materials used, the CuSb/NMC-2 (Cu: 5.9 wt%, Sb: 0.49 wt%) catalyst exhibited the best performance for electrocatalytic CO₂ reduction, and the faradaic efficiency of CH₄ reached 35%, and the total faradaic efficiency of C1–C2 products reached 67%.

CuSb anchored onto nitrogen-doped mesoporous carbon (CuSb/NMC) were prepared for electroreduction of CO₂ to CH₄, C₂H₄ and CO.     

Recent advances in metal-free heteroatom-doped carbon heterogonous catalysts

The development of cost-effective, efficient, and novel catalytic systems is always an important topic for heterogeneous catalysis from academia and industrial points of view. Heteroatom-doped carbon materials have gained more and more attention as effective heterogeneous catalysts to replace metal-based catalysts, because of their excellent physicochemical properties, outstanding structure characteristics, environmental compatibility, low cost, inexhaustible resources, and low energy consumption. Doping of heteroatoms can tailor the properties of carbons for different utilizations of interest. In comparison to pure carbon catalysts, these catalysts demonstrate superior catalytic activity in many organic reactions. This review highlights the most recent progress in synthetic strategies to fabricate metal-free heteroatom-doped carbon catalysts including single and multiple heteroatom-doped carbons and the catalytic applications of these fascinating materials in various organic transformations such as oxidation, hydrogenation, hydrochlorination, dehydrogenation, etc.

Recent advances in metal-free heteroatom-doped carbon heterogeneous catalysts including the preparation methods and their catalytic applications in various organic reactions have been reported.     

Anodic SnO2 porous nanostructures with rich grain boundaries for efficient CO2 electroreduction to formate

Formic acid (HCOOH), the acidic form of formate, is an important hydrogen carrier which can be directly used in fuel cells. Development of earth-abundant element-based catalysts to convert carbon dioxide (CO₂) into HCOOH or formate with high selectivity and high efficiency has been a vigorous research activity in recent years but remains an unsolved challenge. In this contribution, using one-step anodization, we prepare nanotubular SnO₂ porous nanostructures with high surface area (90.1 m² g⁻¹), large porosity (0.74 cm³ g⁻¹), and rich grain boundaries for electrochemical CO₂ reduction (CO₂RR). They exhibit stable 95% faradaic efficiency (FE) towards CO₂RR and 73% FE for formate at −0.8 V_RHE. The notable performance of such SnO₂ nanostructures can be attributed to their unique structural and chemical properties, which provide active sites for CO₂ adsorption and conversion, and easy access for CO₂ to the active sites. The insights gained from the structure/property relationships might be beneficial for designing superior electrocatalysts for CO₂ electroreduction into formate.

The formate is electrochemical synthesized from a porous anodic SnO₂ with the faradic efficiency over 70% under low potential.     

Synergistic effects of Ni–Fe alloy catalysts on dry reforming of methane at low temperatures in an electric field

Dry reforming of methane (DRM) is a promising reaction able to convert greenhouse gases (CO₂ and CH₄) into syngas: an important chemical feedstock. Several difficulties limit the applicability of DRM in conventional thermal catalytic reactions; it is an endothermic reaction that requires high temperatures, resulting in high carbon deposition and a low H₂/CO ratio. Catalysis with the application of an electric field (EF) at low temperatures can resolve these difficulties. Synergistic effects with alloys have also been reported for reactions promoted by the application of EF. Therefore, the synergistic effects of low-temperature DRM and Ni–Fe bimetallic catalysts were investigated using various methods and several characterisations (XRD, XPS, FE-STEM, etc.), which revealed that Ni–Fe binary catalysts show high performance in low-temperature DRM. In particular, the Ni_0.8Fe_0.2 catalyst supported on CeO₂ was found to carry out DRM in EF effectively and selectively by virtue of its bimetallic characteristics.

Dry reforming of methane (DRM) is a promising reaction able to convert greenhouse gases (CO₂ and CH₄) into syngas: an important chemical feedstock.     

Solar driven reduction of CO2 using Pt–Cu/C as a catalyst in a photoelectrochemical cell: experiment and mechanism study

Carbon supported nano-metal catalysts are expected to improve CO₂ reduction selectivity and efficiency due to the addition of more active sites and enhancement of electron transport ability. In this study, HKUST-1 was pyrolyzed and decorated with Pt to prepare Pt–Cu/C catalysts. The catalytic effect of the catalysts with different Pt contents in the CO₂ photoeletrochemical reduction reaction (CO₂PRR) were compared. The total carbon atom conversion rate in CO₂PRR experiments using Pt–Cu/C catalysts first increased to a peak when using 1.6 wt% Pt–Cu/C catalyst and then decreased with the increase of Pt content. The 1.6 wt% Pt–Cu/C catalyst showed good hydrogen evolution reaction (HER) inhibiting ability compared with other Pt–Cu/C catalysts. Density functional theory (DFT) calculations were conducted to give an insight into the CO₂PRR mechanism on some possible active sites in Pt–Cu/C catalysts. The result demonstrated that HER was more likely to be inhibited on the Cu/Pt active surface and at the same time CO₂PRR was promoted.

Carbon supported nano-metal catalysts are expected to improve CO₂ reduction selectivity and efficiency due to the addition of more active sites and enhancement of electron transport ability.     

The pioneering role of metal–organic framework-5 in ever-growing contemporary applications – a review

MOF-5 with a Zn(ii) cluster and terephthalic acid is a distinctive porous material among the metal–organic frameworks (MOFs), with unique physical, chemical and mechanical properties. MOF-5 based composites possess ample applications in modern chemistry. Huge surface area, suitable pore dimensions and scope of tunability make MOF-5 noteworthy in advanced materials. The extensive features of MOF-5 provided an opportunity for researchers to explore atomic/molecular scale materials. Various MOF-5 based composites have been designed with revamped properties appropriate to the application by altering and fabricating MOF-5 in situ or using a post-synthetic approach. Surface modification via the dispersion and impregnation of active substances into the pores of MOF-5 enhances its applicability. The boundless topologies and morphologies of MOF-5 combined with other chemical entities has provided opportunities in various fields, including catalysis, gas storage and sensors. The present review illuminates the leading role of MOF-5 and its composites in contemporary applications based on the current literature in heterogeneous catalysis, H₂ and CO₂ storage and sensors.

MOF-5 with a Zn(ii) cluster and terephthalic acid is a distinctive porous material among the metal–organic frameworks (MOFs), with unique physical, chemical and mechanical properties.