Acidic Media Impedes Tandem Catalysis Reaction Pathways in Electrochemical CO2 Reduction

Electrochemical CO₂ reduction (CO₂R) in acidic media with Cu-based catalysts tends to suffer from lowered selectivity towards multicarbon products. This could in principle be mitigated using tandem catalysis, whereby the *CO coverage on Cu is increased by introducing a CO generating catalyst (e.g. Ag) in close proximity. Although this has seen significant success in neutral/alkaline media, here we report that such a strategy becomes impeded in acidic electrolyte. This was investigated through the co-reduction of ¹³CO₂/¹²CO mixtures using a series of Cu and CuAg catalysts. These experiments provide strong evidence for the occurrence of tandem catalysis in neutral media and its curtailment under acidic conditions. Density functional theory simulations suggest that the presence of H₃O⁺ weakens the *CO binding energy of Cu, preventing effective utilization of tandem-supplied CO. Our findings also provide other unanticipated insights into the tandem catalysis reaction pathway and important design considerations for effective CO₂R in acidic media.     

Current advances in heterogeneous catalysts for the synthesis of cyclic carbonates from carbon dioxide

The novel heterogeneous catalysts are highly demanded to perform the cycloaddition reaction of carbon dioxide with epoxide to synthesize the cyclic carbonates. The heterogeneous catalysts are more preferred than homogeneous catalysts due to the easy post reaction separation, easy to recycle, high stability and cost effective nature. In this review, we have summarized the current research progress in heterogeneous catalysis for the cycloaddition of carbon dioxide (CO₂) to synthesis of cyclic carbonates. Recent advances in the design of the heterogeneous catalysts and the understanding to the role of catalysts in reaction process are summarized and discussed.     

Supported molecular catalysts for the heterogeneous CO2 electroreduction

The thorough understanding of homogeneous catalysis has triggered intense research activities on the immobilization of molecular catalysts for the heterogeneous CO₂ electroreduction. Herein, we discuss recent advances in the heterogeneous field with focus on the intrinsic effect coming from the catalyst structure and the extrinsic effect exerted by the catalyst immobilization strategy and support material on the catalytic performance.     

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.     

Bifunctional Catalysts Containing Zn(II) and Imidazolium Salt Ionic Liquids for Chemical Fixation of Carbon Dioxide

Zn(II) can efficiently promote the catalytic performance of imidazolium salt ionic liquids (imi-ILs) for the chemical fixation of CO₂ into epoxides. To obtain sustainability, immobilized bifunctional catalysts containing both imi-ILs and Zn(II) were prepared using bimodal mesoporous silica (BMMs) as carrier, through grafting of Zn(OAc)₂ and 1-(trimethoxysilyl)propyl-3-methylimidazolium chloride (Si-imi) separately in the nanopores. The catalysts, named as BMMs−Zn&ILs, were identified as efficient catalysts for cycloaddition reaction of CO₂ into epoxides under solvent-free conditions. BMMs−Zn&ILs showed good catalytic activity, which increased with the increase of the molar ratio of Zn(II) to Si-imi. As a comparison, different catalytic systems including homogeneous imi-IL, BMMs-ILs and BMMs−Zn were studied to demonstrate different cooperation behaviors. Furthermore, the kinetics studies of homogeneous and heterogeneous bifunctional catalysts were employed to confirm the differences, as well as to support the proposed cooperative catalysis mechanism in the nanopores.     

Reversible Phase Transfer of Carbon Dots between an Organic Phase and Aqueous Solution Triggered by CO2

Carbon dots (CDs) have attracted increasing attention in applications such as bio‐imaging, sensors, catalysis, and drug delivery. However, unlike metallic and semiconductor nanoparticles, the transfer of CDs between polar and non‐polar phases is little understood. A class of amine‐terminated CDs is developed and their phase transfer behavior has been investigated. It is found that these CDs can reversibly transfer between aqueous and organic solvents by alternatively bubbling and removing CO₂ at atmospheric pressure. The mechanism of such CO₂‐switched phase transfer involves reversible acid–base reaction of amine‐terminated CDs with CO₂ and the reversible formation of hydrophilic ammonium salts. By using the CDs as catalysts, the phase transfer is applied in the Knoevenagel reaction for efficient homogeneous reaction, heterogeneous separation, and recycling of the catalysts.     

Role of single-site catalysts in the hydrogenation of thiophenes: from models systems to effective HDS catalysts

The hydrogenation of thiophenes, leading to cyclic thioethers, thiols or hydrocarbons and H₂S, can be achieved by single-site metal catalysts in both homogeneous and heterogeneous phase. An overview of the processes that have provided useful mechanistic information on hydrodesulfurization catalysis, is presented and commented.     

Electrocatalytic CO2 Reduction: from Discrete Molecular Catalysts to Their Integrated Catalytic Materials

Molecular catalysts (metal complexes), with molecularly defined uniform active sites and atomically precise structural tailorability allowing for regulating catalytic performance through metal- and ligand-centered engineering and elucidating reaction mechanisms via routine photoelectrochemical characterizations, have been increasingly explored for electrocatalytic CO₂ reduction (ECR). However, their poor stability and low catalytic current density are undesirable for practical applications. Heterogenizing discrete molecular catalysts can potentially surmount these issues, and the resulting integrated catalysts largely share catalytical properties with their discrete molecular counterparts, which bridge the gap between heterogeneous and homogeneous catalysis and combine their advantages. This minireview surveys advances in design and regulation of molecular catalysts such as porphyrin, phthalocyanine, and bipyridine-based metal complexes and their integrated catalytic materials for selective ECR.     

Liquid–liquid equlibria of polyethylene glycol (PEG) 400 and CO2 with common organic solvents

Liquid polyethylene glycol (PEG), in combination with carbon dioxide (CO₂) and common organic solvents, enables the coupling of a homogeneous reaction with a heterogeneous separation. This is important for the application of homogenous catalysts, which offer superior reactivity but are difficult to separate and recycle. CO₂ can act as a miscibility switch to shift the system from homogeneous at atmospheric conditions to heterogeneous under CO₂ pressure. This allows for extraction of the products into the organic solvent phase and immobilization of the homogeneous catalyst in the PEG phase. This work examines the phase behavior of PEG and carbon dioxide with 1,4-dioxane and acetonitrile at 25 and 40 °C and pressures ranging from 5 to 8 MPa. The experimental data are compared to theoretical calculations using the Sanchez–Lacombe equation of state.     

Heterogeneous soft acid catalysis of the sucrose hydrolysis

Using a micro-calorimetrical DSC we have compared the acid-catalyzed inversion of sucrose in homogeneous and heterogeneous systems. Acetic acid was chosen as catalyst for homogeneous system, and several carboxylic cationites were used as heterogeneous catalysts. The kinetic apparent parameters (A, E, k _ap) for all the systems were calculated from DSC data with Friedmann’s method and catalytic constant, k₃₂₃^cat, was further inferred. We found that the specific catalyst efficiency, q _cat, in heterogeneous system is over 5000 times higher than in case of homogeneous ones. The activity of heterogeneous carboxylic systems is still about 30 times larger than those of a strong mineral acid in homogeneous catalysis. The results indicate the high efficiency of heterogeneous systems for soft acid catalysis of the sucrose hydrolysis.     

Direct Synthetic Processes for Cyclic Carbonates from Olefins and CO<Subscript>2</Subscript>

This short review presents the recent developments in the direct synthesis of cyclic carbonates from olefins and CO₂. The straightforward synthesis of cyclic carbonates from olefins instead of epoxides, also called one-pot “oxidative carboxylation” of olefins, can be viewed as the coupling of two sequential reactions of epoxidation of olefins and CO₂ cycloaddition to epoxides formed. The facile synthetic approach would make carbonate synthesis simpler and even cheaper with industrial potential from environmental and economic points of view. Some progresses have been made on this direct synthetic reaction for cyclic carbonates with homogeneous and heterogeneous catalysts, however, this reaction system is still at a preliminary stage. Among the catalysts reported, only a few can be considered as effective for the direct oxidative carboxylation of olefins to cyclic carbonates. Thus active and selective catalysts should be explored to put the direct synthesis of cyclic carbonates into practical applications.     

Sol–Gel Entrapped Lewis Acids as Catalysts for Biodiesel Production

Replacing fossil fuels with biodiesel enables the emission of greenhouse gases to be decreased and reduces dependence on fossil fuels in countries with poor natural resources. Biodiesel can be produced by an esterification reaction between free fatty acids (FFAs) and methanol or by transesterification of triglycerides from oils. Both reactions require homogeneous or heterogeneous catalysis. Production of biodiesel catalyzed by heterogeneous catalysts seems to be the preferred route, enabling easy product separation. As we have previously shown, the Lewis acids AlCl₃ and BF₃ can serve as highly efficient catalysts under ultrasonic activation. The present study focused on the development of oleic acid (OA) esterification with methanol by the same catalysts immobilized in silica matrices using the sol–gel synthesis route. During the course of immobilization, AlCl₃ converts to AlCl₃ × 6H₂O (aluminite) and BF₃ is hydrolyzed with the production of B₂O₃. The immobilized catalysts can be reused or involved in a continuous process. The possibility of biodiesel production using immobilized catalysts under ultrasonic activation is shown for the conversion of FFAs into biodiesel in batch and continuous mode.     

Selective Hydrogenation of CO2 Dictated by Isomers in Au28(SR)20 Nanoclusters: Which One is Better?

It is a challenge to make clear how isomerism in a heterogeneous catalyst induces distinct differences in catalytic properties, as attainment of the structural isomerism in a conventional catalyst is difficult. By successfully identifying the isomerism in the atomically precise Au nanoclusters, an exciting opportunity for unravelling catalysis of isomeric catalysts is opened up. Herein, we report that the isomerism in the Au₂₈(SR)₂₀ nanoclusters with different surface atom arrangements can indeed render different catalytic behaviors in the selective hydrogenation of CO₂. We anticipate that our studies will serve as a starting point for fundamental investigations about how to control the catalytic activity and selectivity by the isomerism-induced catalysis.     

Hydroformylation of 1-hexene by soluble and zeolite-supported rhodium species part II

The hydroformylation of 1-hexene at 50 and 125°C and 300 psig CO:H₂ (1:1) using soluble and zeolite-supported rhodium species is reported. The presence of excess phosphine during homogeneous catalysis in shown to inhibit isomerization of 1-hexene and thus give high normal/branched aldehyde ratios for all levels of conversion. However, the absence of phosphine allows significant isomerization, causing the normal/branched ratio to vary with conversion. The activity of the immobilized catalysts is affected by the type and amount of phosphine. The homogeneous and heterogeneous catalysts can be poisoned by mercaptans, provided an excess of phosphine is not present. The data from the immobilized catalysts suggest that claims of intrazeolitic hydroformylation with rhodium-containing faujasites must be taken with caution.     

Advances in the electrochemical catalytic reduction of CO2 with metal complexes

Electrocatalytic CO₂ reduction has emerged as a promising strategy to effectively produce fuels and chemicals sustainably. In this regard, the study of electrochemical catalytic reduction of CO₂ with metal complexes is a powerful tool for both the development of catalysts that operated under desired conditions (low overpotentials, high catalytic rates and selectivity, and extended durability) and the understanding of basic principles in catalysis. To illustrate the state-of-the-art, this revision presents a selection of the most recent and remarkable findings reported in terms of key strategies to improve reaction rates, selectivity and mechanism understanding for the leading families of homogeneous catalysts.     

A Water-Soluble Sodium Pectate Complex with Copper as an Electrochemical Catalyst for Carbon Dioxide Reduction

A selective noble-metal-free molecular catalyst has emerged as a fruitful approach in the quest for designing efficient and stable catalytic materials for CO₂ reduction. In this work, we report that a sodium pectate complex of copper (PG-NaCu) proved to be highly active in the electrocatalytic conversion of CO₂ to CH₄ in water. Stability and selectivity of conversion of CO₂ to CH₄ as a product at a glassy carbon electrode were discovered. The copper complex PG-NaCu was synthesized and characterized by physicochemical methods. The electrochemical CO₂ reduction reaction (CO₂RR) proceeds at −1.5 V vs. Ag/AgCl at ~10 mA/cm² current densities in the presence of the catalyst. The current density decreases by less than 20% within 12 h of electrolysis (the main decrease occurs in the first 3 h of electrolysis in the presence of CO₂). This copper pectate complex (PG-NaCu) combines the advantages of heterogeneous and homogeneous catalysts, the stability of heterogeneous solid materials and the performance (high activity and selectivity) of molecular catalysts.     

Central Doping of a Foreign Atom into the Silver Cluster for Catalytic Conversion of CO2 toward C−C Bond Formation

Clusters with an exact number of atoms are of particular interest in catalysis. Their catalytic behaviors can be potentially altered with the addition or removal of a single atom. Now the effects of doping with a single foreign atom (Au, Pd, and Pt) into the core of an Ag cluster with 25 atoms on the catalytic properties are explored, where the foreign atom is protected by 24 Ag atoms (Au@Ag₂₄, Pd@Ag₂₄, and Pt@Ag₂₄). The central doping of a single atom into the Ag₂₅ cluster has a substantial influence on the catalytic performance in the carboxylation reaction of CO₂ with terminal alkyne through C?C bond formation to produce propiolic acid. These studies reveal that the catalytic properties of the cluster catalysts can be dramatically changed with the subtle alteration by a single atom away from the active sites.     

Imidazolium Ionic Liquids,Imidazolylidene Heterocyclic Carbenes,and Zeolitic Imidazolate Frameworks for CO2 Capture and Photochemical Reduction

Imidazolium ionic liquids (ILs), imidazolylidene N‐heterocyclic carbenes (NHCs), and zeolitic imidazolate frameworks (ZIFs) are imidazolate motifs which have been extensively investigated for CO₂ adsorption and conversion applications. Summarized in this minireview is the recent progress in the capture, activation, and photochemical reduction of CO₂ with these three imidazolate building blocks, from homogeneous molecular entities (ILs and NHCs) to heterogeneous crystalline scaffolds (ZIFs). The developments and existing shortcomings of the imidazolate motifs for their use in CO₂ utilizations is assessed, with more of focus on CO₂ photoredox catalysis. The opportunities and challenges of imidazolate scaffolds for future advancement of CO₂ photochemical conversion for artificial photosynthesis are discussed.     

Hydrogen storage and delivery: immobilization of a highly active homogeneous catalyst for the decomposition of formic acid to hydrogen and carbon dioxide

The homogeneous catalytic system, based on water-soluble ruthenium(II)–TPPTS catalyst (TPPTS = meta-trisulfonated triphenylphosphine), selectively decomposes HCOOH into H₂ and CO₂ in aqueous solution. Although this reaction results in only two gas products, heterogeneous catalysts could be advantageous for recycling, especially for dilute formic acid solutions, or for mobile, portable applications. Several approaches have been used to immobilize/solidify the homogeneous ruthenium–TPPTS catalyst based on ion exchange, coordination and physical absorption. The activity of the various heterogeneous catalysts for the decomposition of formic acid has been determined. These heterogenized catalysts offer the advantage of easy catalyst separation/recycling in dilute formic acid, or for mobile, portable applications.     

Recent Advances in the Preparation and Applications of Organo‐functionalized Porous Materials

Organo‐functionalized materials with porous structure offer unique adsorption, catalytic and sensing properties. These unique properties make them available for various applications, including catalysis, CO₂ capture and utilization, and drug delivery. The properties and the performance of these unique materials can be altered with suitable modifications on their surface. In this review, we summarize the recent advances in the preparation and applications of organo‐functionalized porous materials with different structures. Initially, a brief historical overview of functionalized porous materials is presented, and the subsequent sections discuss the recent developments and applications of various functional porous materials. In particular, the focus is given on the various methods used for the preparation of organo‐functionalized materials and their important roles in the heterogenization of homogeneous catalysts. A special emphasis is also given on the applications of these functionalized porous materials for catalysis, CO₂ capture and drug delivery.