Crystal‐Plane‐Controlled Selectivity of Cu2O Catalysts in Propylene Oxidation with Molecular Oxygen

The selective oxidation of propylene with O₂ to propylene oxide and acrolein is of great interest and importance. We report the crystal‐plane‐controlled selectivity of uniform capping‐ligand‐free Cu₂O octahedra, cubes, and rhombic dodecahedra in catalyzing propylene oxidation with O₂: Cu₂O octahedra exposing {111} crystal planes are most selective for acrolein; Cu₂O cubes exposing {100} crystal planes are most selective for CO₂; Cu₂O rhombic dodecahedra exposing {110} crystal planes are most selective for propylene oxide. One‐coordinated Cu on Cu₂O(111), three‐coordinated O on Cu₂O(110), and two‐coordinated O on Cu₂O(100) were identified as the catalytically active sites for the production of acrolein, propylene oxide, and CO₂, respectively. These results reveal that crystal‐plane engineering of oxide catalysts could be a useful strategy for developing selective catalysts and for gaining fundamental understanding of complex heterogeneous catalytic reactions at the molecular level.     

Mechanistic Aspects of a Highly Active Dinuclear Zinc Catalyst for the Co‐polymerization of Epoxides and CO2

The dinuclear zinc complex reported by us is to date the most active zinc catalyst for the co‐polymerization of cyclohexene oxide (CHO) and carbon dioxide. However, co‐polymerization experiments with propylene oxide (PO) and CO₂ revealed surprisingly low conversions. Within this work, we focused on clarification of this behavior through experimental results and quantum chemical studies. The combination of both results indicated the formation of an energetically highly stable intermediate in the presence of propylene oxide and carbon dioxide. A similar species in the case of cyclohexene oxide/CO₂ co‐polymerization was not stable enough to deactivate the catalyst due to steric repulsion.     

A new copolymerization process leading to poly(propylene carbonate) with a highly enhanced yield from carbon dioxide and propylene oxide

Using excessively loaded propylene oxide (PO) as a solvent, the copolymerization of carbon dioxide (CO₂) and PO was carried out with zinc glutarate catalyst, consequently producing poly(propylene carbonate) of high molecular weight in a high yield (64–70 g polymer per gram of catalyst) never achieved before. Both the PO used as solvent and the excessively loaded CO₂ were fully recoverable, respectively, and reusable for their copolymerization, indicating that this is a clean, green polymerization process to convert CO₂ to its polycarbonate. The polymer yield was further improved by scaling up the copolymerization process. Among zinc glutarate catalysts prepared through several synthetic routes, one from zinc oxide delivered the highest yield in the copolymerization. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1863–1876, 1999     

Fluorinated surfactants in supercritical CO2

Liquid or supercritical carbon dioxide has important environmental and economic advantages over petrochemical solvents currently used for industrial processes. However, low solubility in CO₂, particularly of polar compounds, is a hurdle to its implementation as an acceptable alternative. These solubility problems have been overcome by employing specialised fluorinated surfactants to stabilise water nano-droplets as water-in-supercritical/liquid CO₂ microemulsions. Such novel microemulsions can now facilitate innovative ‘green-and-clean’ applications of carbon dioxide technology.     

Selective oxidation of propylene over gold deposited on titanium-based oxides

Gold supported on titanium-based metal oxides can assist the selective partial oxidation of propylene at temperatures from 313 K to 573 K in a gas containing both H₂ and O₂. The preparation method was found to be crucial in controlling the selectivities. In general, impregnation and chemical vapor deposition methods do not produce selective catalysts. Only the deposition-precipitation method makes gold selective to propylene oxide or propanal, suggesting that a strong contact between the gold particles and the titanium ion sites on the support is important. The effect of changing the support was also dramatic; the use of the anatase form of TiO₂ and Ti-MCM-41 results in propylene oxide production, while the rutile structure of TiO₂ caused complete oxidation to CO₂. Microporous crystalline titanium silicates such as TS-1, TS-2, and Ti-β zeolite make gold relatively selective to propanal and of the three TS-1 gives the highest selectivity. These results indicate that the oxidation of propylene in the copresence of H₂ must involve the surface of the supports and that the reaction takes place at the interface perimeter around the gold particles.     

(Salan)CrCl,an effective catalyst for the copolymerization and terpolymerization of epoxides and carbon dioxide

The selective transformation of CO₂ and epoxides to afford completely alternating copolymers remains a topic of much interest for the potential utilization of carbon dioxide in chemical synthesis. The use of salicylaldimine (salen)‐metal complexes and their saturated (salan)‐metal versions have proven to be the most effective and robust single‐site catalyst for these processes. Herein, we report on mechanistic aspects of the copolymerization of alicyclic and aliphatic epoxides with CO₂ in toluene solution and in neat epoxides in the presence of a (salan)CrCl/onium salt catalyst system. The activation barriers for both cyclohexene oxide(CHO)/CO₂ and propylene oxide(PO)/CO₂ were shown to be significantly higher in toluene solution than those previously reported for reactions carried out under solventless conditions. Terpolymerization of CHO/vinylcyclohexene oxide/CO₂ was shown via Fineman‐Ross analysis at 60 °C to proceed with little monomer selectivity, for example, r_CHO = 1.03 and r_VCHO = 0.847. On the other hand, terpolymerization of CHO/PO/CO₂ occurred at 25 °C with a propensity for incorporation of PO in the polymer. However, at 40 °C, Fineman‐Ross analysis revealed r_CHO and r_PO values of 0.869 and 1.49, thereby affording a terpolymer with a more equal distribution of monomers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

CO_2/H_2O混合绿色介质中的有机催化反应

绿色化学是化学发展的必然趋势。有效利用绿色溶剂是绿色化学的重要内容。CO2和H2O混合体系是具有很多特点的绿色反应介质,可以用于不同化学反应,特别是弱酸催化的反应,从而替代传统的有机酸和无机酸。本文讨论了CO2/H2O体系的酸性随温度和压力的变化,综述了在CO2/H2O混合绿色介质中有机化学反应研究进展,这些反应包括脱水反应、烷基化反应、香茅醛环化反应、重氮化反应、多元醇转化成环醚的反应、溴氧化反应、芳硝基化合物选择性还原、多糖水解反应、生物质转化反应、环氧丙烷水解反应、脱羧反应、醇氧化反应、对映选择氧化反应以及酮不对称还原;最后对CO2/H2O体系在化学反应中应用的发展趋势进行了探讨。     

Amino acids/superbases as eco-friendly catalyst system for the synthesis of cyclic carbonates under metal-free and halide-free conditions

An eco-friendly and efficient binary catalyst system of superbases and amino acids was developed for the synthesis of cyclic carbonates from epoxides and CO₂ under metal-free and halide-free conditions. Among the various amino acids and superbases systems tested, the L-histidine/1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) system achieved the highest conversion of propylene oxide and selectivity of propylene carbonate. The effect of various reaction parameters was evaluated. A possible catalyst mechanism for L-histidine synergized with DBU in the ring opening of epoxide and DBU introduced CO₂ activation. The process herein represents a green, simple, and cost-effective route for the chemical fixation of CO₂ into cyclic carbonates.     

Differences in Reactivity of Epoxides in the Copolymerisation with Carbon Dioxide by Zinc-Based Catalysts: Propylene Oxide versus Cyclohexene Oxide

The homogeneous dinuclear zinc catalyst going back to the work of Williams et al. is to date the most active catalyst for the copolymerisation of cyclohexene oxide and CO₂ at one atmosphere of carbon dioxide. However, this catalyst shows no copolymer formation in the copolymerisation reaction of propylene oxide and carbon dioxide, instead only cyclic carbonate is found. This behaviour is known for many zinc‐based catalysts, although the reasons are still unidentified. Within our studies, we focus on the parameters that are responsible for this typical behaviour. A deactivation of the catalyst due to a reaction with propylene oxide turns out to be negligible. Furthermore, the catalyst still shows poly(cyclohexene carbonate) formation in the presence of cyclic propylene carbonate, but the catalyst activity is dramatically reduced. In terpolymerisation reactions of CO₂ with different ratios of cyclohexene oxide to propylene oxide, no incorporation of propylene oxide can be detected, which can only be explained by a very fast back‐biting reaction. Kinetic investigations indicate a complex reaction network, which can be manifested by theoretical investigations. DFT calculations show that the ring strains of both epoxides are comparable and the kinetic barriers for the chain propagation even favour the poly(propylene carbonate) over the poly(cyclohexene carbonate) formation. Therefore, the crucial step in the copolymerisation of propylene oxide and carbon dioxide is the back‐biting reaction in the case of the studied zinc catalyst. The depolymerisation is several orders of magnitude faster for poly(propylene carbonate) than for poly(cyclohexene carbonate).     

Non-thermal plasma technology for the conversion of CO2

The conversion of carbon dioxide is vital if we are to avoid the catastrophic consequences that will result from further global temperature rise as a result of burning fossil fuels. Current techniques, such as catalytic conversion and biochemical processes, are each associated with their own drawbacks such as catalyst deactivation and high energy input. Plasma processes are gaining increasing interest as they have the potential to reduce a greater amount of atmospheric environmental pollutants at any one time due to an increased throughput, whilst using a smaller reactor with improved energy efficiency and near-zero emissions. Non-thermal plasma can dissociate stable molecules, such as CO₂, at temperatures as low as room temperature. It is this key feature which makes plasma conversion such a promising technology in the conversion and utilisation of CO₂. Furthermore, possible products from plasma processes include fuels and chemicals, such as methanol and syngas, which have a high market value; hence potentially making the process feasible on an industrial scale. This paper discusses recent advances in the use of plasma processes for carbon dioxide conversion, along with the future outlook of this technology and the impact these techniques could have on the chemical and energy industries.     

Biodegradable Polycarbonate Synthesis by Copolymerization of Carbon Dioxide with Epoxides Using a Heterogeneous Zinc Complex

As a means for the chemical fixation of carbon dioxide and the synthesis of biodegradable polycarbonates, copolymerizations of carbon dioxide with various epoxides such as cyclohexene oxide (CHO), cyclopetene oxide, 4-vinyl-1-cyclohexene-1,2epoxide, phenyl glycidyl ether, allyl glycidyl ether, propylene oxide, butene oxide, hexene oxide, octene oxide, and 1-chloro-2,3-epoxypropane were investigated in the presence of a double metal cyanide catalyst (DMC). The DMC catalyst was prepared by reacting K₃Co(CN)₆ with ZnCl₂, together with tertiary butyl alcohol and poly(tetramethylene ether glycol) as complexing reagents and was characterized by various spectroscopic methods. The DMC catalyst showed high activity (526.2 g-polymer/g-Zn atom) for CHO/CO₂ (P_CO2 = 140 psi) copolymerization at 80 °C, to yield biodegradable aliphatic polycarbonates of narrow polydispersity (M_w/M_n = 1.67) and moderate molecular weight (M_n = 8900). The DMC catalyst also showed high activities with different CO₂ reactivities for other epoxides to yield various aliphatic polycarbonates with narrow polydispersity.     

Emergence of CO2 electrolyzers including supported molecular catalysts

With access to cheap, sustainable electricity, electrocatalysis is a promising technology for converting electric power into storable chemical fuels or value-added chemical compounds. This has sparked the development of electrocatalysts that need to operate at high product selectivity and high energy efficiency. Electrocatalytic alcohol oxidation, oxygen activation and nitrogen and carbon dioxide (CO₂) reduction are examples of reactions with a huge industrial potential. Notably, electrocatalytic reduction of CO₂ has recently developed as a favourable pathway to convert this greenhouse gas into value-added chemicals and fuels. Earth-abundant metals stabilized by carbon/nitrogen macrocycle ligands are well-known efficient and selective catalysts for the electrochemical reduction of CO₂ in homogeneous conditions. Recently, such catalysts have also been used in supported conditions and implemented in flow cell electrolyzers, showing promising performances. This review provides a synopsis for the evolution of CO₂ electrolyzers using molecular catalysts.     

Study of electronic effect in bifunctional catalysts for the copolymerization of CO2 and PO/CHO

Carbon dioxide (CO₂) is an easily available renewable carbon source that can be used as a comonomer in the catalytic ring-opening polymerization of epoxides to form aliphatic polycarbonates. Herein, a series of new Salen-Co(III) bifunctional catalysts were synthesized for the first time, and they were studied to catalyze the copolymerization of CO₂ and propylene oxide (PO)/cyclohexene oxide (CHO). At the same time, the effects of reaction conditions (electronic effect, temperature, time) on catalytic activity and selectivity were investigated. The results show that the Salen-Co(III) complexes with electron-withdrawing groups have higher selectivity and activity for propylene carbonate (PPC)/cyclohexylene carbonate (PCHC). At the same time, the Salen-Co(III) complexes can better catalyze the copolymerization of CHO and CO₂ than that of PO and CO₂. The catalytic efficiency of the four complexes increased with increasing temperature, and the best reaction condition is 80°C, 30 min and 2 MPa of CO₂.     

Influence of different substitution in pyrazolium ionic liquids on catalytic activity for the fixation of CO2 under solvent- and metal-free conditions

Three pyrazolium ionic liquids, 1,2-diethylpyrazolium bromide (DEPzBr), 1,2-diethyl-3-methylpyrazolium bromide (DEMPzBr), and 1,2-diethyl-3,5-dimethylpyrazolium bromide (DEDMPzBr), are firstly applied as catalysts for coupling reaction of carbon dioxide (CO₂) and propylene oxide (PO) with the propylene carbonate (PC) yields in a range of 82.7%–88.7% under a benign condition, 120?°C, 2.0?MPa initial CO₂ pressure and 4?h with 0.5?mol% catalysts loading. The relationship between structure and catalytic activity of pyrazolium ionic liquids are investigated by two different theoretical models, which indicates that both electrostatic interaction and hydrogen bond promote the ring-opening of PO. Both the theoretical and experimental results present that the catalytic activity decreases in the sequence of DEPzBr?>?DEDMPzBr?>?DEMPzBr. Pyrazolium ionic liquids would be employed as a novel efficient single-component catalyst without solvent and co-catalyst. It is expected that we would open an express pathway to develop new catalysts with the desired properties.     

Electrochemically Assisted Cycloaddition of Carbon Dioxide to Styrene Oxide on Copper/Carbon Hybrid Electrodes: Active Species and Reaction Mechanism

A novel electrochemically assisted cycloaddition process is proposed, in which highly efficient coupling of CO₂ with styrene oxide (SO) can be achieved to form styrene carbonate (SC) as a high-value-added product. A series of Cu catalysts with different morphologies and chemical states were fabricated on carbon paper (CP) by using in-situ electrodeposition, and the sample with nano-dendrimer structure was found to exhibit a relatively high activity of 74.8 % SC yield with 92.7 % SO conversion under gentle reaction conditions, thus showing its potential for practical applications. The relatively high electrochemically active surface area and charge transfer ability of dendrimer-like Cu benefited the electrochemical reaction. In particular, the Cu²⁺ species that were formed in situ during the reaction played a vital role in enhancing the activity and selectivity of the proposed Cu/CP hybrid catalyst. Cu²⁺ atoms served as active sites that can not only electrochemically activate CO₂ but also facilitate the ring opening of SO. Mechanistic analysis suggested that the reaction followed electrochemical and liquid-phase heterogeneous paths, which provide a new green and sustainable route for efficient utilization of CO₂ resources for fine chemical electrosynthesis.     

Co‐Catalyst‐Free Chemical Fixation of CO2 into Cyclic Carbonates by using Metal‐Organic Frameworks as Efficient Heterogeneous Catalysts

The concentration of carbon dioxide (CO₂) in the atmosphere is increasing at an alarming rate resulting in undesirable environmental issues. To mitigate this growing concentration of CO₂, selective carbon capture and storage/sequestration (CCS) are being investigated intensively. However, CCS technology is considered as an expensive and energy‐intensive process. In this context, selective carbon capture and utilization (CCU) as a C1 feedstock to synthesize value‐added chemicals and fuels is a promising step towards lowering the concentration of the atmospheric CO₂ and for the production of high‐value chemicals. Towards this direction, several strategies have been developed to convert CO₂, a Greenhouse gas (GHG) into useful chemicals by forming C?N, C?O, C?C, and C?H bonds. Among the various CO₂ functionalization processes known, the cycloaddition of CO₂ to epoxides has gained considerable interest owing to its 100% atom‐economic nature producing cyclic carbonates or polycarbonates in high yield and selectivity. Among the various classes of catalysts studied for cycloaddition of CO₂ to cyclic carbonates, porous metal‐organic frameworks (MOFs) have gained a special interest due to their modular nature facilitating the introduction of a high density of Lewis acidic (LA) and CO₂‐philic Lewis basic (LB) functionalities. However, most of the MOF‐based catalysts reported for cycloaddition of CO₂ to respective cyclic carbonates in high yields require additional co‐catalyst, say tetra‐n‐butylammonium bromide (TBAB). On the contrary, the co‐catalyst‐free conversion of CO₂ using rationally designed MOFs composed of both LA and LB sites is relatively less studied. In this review, we provide a comprehensive account of the research progress in the design of MOF based catalysts for environment‐friendly, co‐catalyst‐free fixation of CO₂ into cyclic carbonates.     

Progress in the Foaming of Polymer-based Electromagnetic Interference Shielding Composites by Supercritical CO2

As a critical action plan formulated for peaking carbon dioxide emissions, polymeric electromagnetic interference (EMI) shielding materials based on CO₂ foaming technology have recently been attracting widespread attention in both research and industry, attributable to their efficient use of CO₂, high specific strength, corrosion resistance and low-cost characteristics. In the past decade, the emergence of novel design concepts and preparation techniques for CO₂ foaming technology has led to the development of new high-performance EMI shielding materials in this field. This review summarizes the research progress made to date on the fabrication of EMI shielding composite foams by supercritical carbon dioxide (scCO₂) foaming. We also explore the structure-activity relationships between the component/distribution and EMI shielding properties. Additionally, the application prospects and development challenges of new EMI shielding composite foams are described.     

One‐Pot Alkoxylation of Phenols with Urea and 1,2‐Glycols

A one‐pot epoxide‐free alkoxylation process has been developed for phenolic compounds. The process involves heating phenols and urea in 1,2‐glycols at 170‐190 °C using Na₂CO₃/ZnO as co‐catalysts under atmospheric conditions. During the course of this new alkoxylation reaction, a five‐membered ring cyclic carbonate intermediate, ethylene carbonate (EC) or propylene carbonate (PPC), was produced in‐transit as the key intermediate and was subsequently consumed by phenols to form alkoxylated ether alcohols as final products in excellent yields. For instance, phenol, bisphenol A (BPA), hydroquinone and resorcinol were converted into their respective mono‐alkoxylated ether alcohols on each of their phenolic groups in 80‐95% isolated yields. In propoxylation of phenols, this approach shows great product selectivity favoring production of high secondary alcohols over primary alcohols in isomeric ratios of nearing 95/5. Since ammonia (NH₃) and carbon dioxide (CO₂) evolving from the reaction can be re‐combined in theory into urea for re‐use, the overall net‐alkoxylation by this approach can be regarded as a simple condensation reaction of phenols with 1,2‐glycols giving off water as its by‐product. This one‐pot process is simple, safe and environmentally friendlier than the conventional alkoxylated processes based on ethylene oxide (EO) or propylene oxide (PO). Moreover, this process is particularly well‐suited for making short chain‐length alkoxyether alcohols of phenols.     

Catalyst Engineering for the Selective Reduction of CO2 to CH4: A First-Principles Study on X-MOF-74 (X=Mg,Mn, Fe,Co, Ni,Cu, Zn)

The conversion of carbon dioxide (CO₂) into more valuable chemical compounds represents a critical objective for addressing environmental challenges and advancing sustainable energy sources. The CO₂ reduction reaction (CO₂RR) holds promise for transforming CO₂ into versatile feedstock materials and fuels. Leveraging first-principles methodologies provides a robust approach to evaluate catalysts and steer experimental efforts. In this study, we examine the CO₂RR process using a diverse array of representative cluster models derived from X-MOF-74 (where X encompasses Mg, Mn, Fe, Co, Ni, Cu, or Zn) through first-principles methods. Notably, our investigation highlights the Fe-MOF-74 cluster's unique attributes, including favorable CO₂ binding and the lowest limiting potential of the studied clusters for converting CO₂ to methane (CH₄) at 0.32 eV. Our analysis identified critical factors driving the selective CO₂RR pathway, enabling the formation CH₄ on the Fe-MOF-74 cluster. These factors involve less favorable reduction of hydrogen to H₂ and strong binding affinities between the Fe open-metal site and reduction intermediates, effectively curtailing desorption processes of closed-shell intermediates such as formic acid (HCOOH), formaldehyde (CH₂O), and methanol (CH₃OH), to lead to selective CH₄ formation.     

Small-molecule fluorescent probes for the detection of carbon dioxide

This review summarizes the design principles, recognition mechanisms, properties and functions of various kinds of small-molecule fluorescent probes for the detection of carbon dioxide