Recent advances in metal-free catalysts for the synthesis of cyclic carbonates from CO_2 and epoxides

The aim of ＂green chemistry＂ and ＂atom economy＂ is to utilize carbon dioxide and replace harmful reactants such as CO and phosgene for the production of cyclic carbonates. In this paper, metal-free catalysts including organic bases, ionic liquids, supported catalysts, organic copolymers and carbon materials for the synthesis of cyclic carbonates by the cycloaddition of carbon dioxide to epoxides are reviewed. Recent advances in the design of the catalysts and the understanding of the reaction mechanism are summarized and discussed. The synergistic effects of organic bases and hydrogen bond donors, organic bases and nucleophilic anions, hydrogen bond donors and nucleophilic anions and active components and supports are highlighted. The challenge is to develop metal-free catalysts suitable for carbon dioxide capture and fixation. The ultimate goal is to synthesize cyclic carbonates in a flow reactor directly using carbon dioxide from industrial flue gas at ambient temperature and atmospheric pressure. By using synergetic effects, a multi-functional approach can meet the design strategy of metal-free catalysts for carbon dioxide adsorption and activation as well as epoxide ring opening.

Synthesis of cyclic carbonates from epoxides or olefins and CO_2 catalyzed by metal-organic frameworks and quaternary ammonium salts

Catalytic properties of the metal-organic framework Cr-MIL-101 in solvent-free cycloaddition of CO2 to epoxides to produce cyclic carbon- ates using tetrabutylammonium bromide as co-catalyst have been explored under mild reaction conditions （8 bar CO2, 25 ~C）. Styrene and propylene carbonates were formed with high yields （95% and 82%, respectively）. Catalytic performance of Cr-MIL-101 was compared with other MOFs： Fe-MIL-101, Zn-MOF-5 and HKUST-1, The catalytic properties of different quaternary ammonium bromides, Cr-MIL-101 as well as PW12/Cr-MIL-101 composite material have been assessed in oxidative carboxylation of styrene in the presence of both tert-butyl hydroperoxide and H202 as oxidants at 8-100bar CO2 and 25-80 ~C with selectivity to styrene carbonate up to 44% at 57% substrate conversion.

Efficient homogenous catalysis of CO_(2) to generate cyclic carbonates by heterogenous and recyclable polypyrazoles

The cycloaddition between CO_(2)and epoxides to produce cyclic carbonate is an attractive and efficiency pathway for the utilization of CO_(2)as C1 source.The development of catalyst to mediate cycloaddition between CO_(2)and epoxides at low temperature and pressure is still a challenge.Herein,a series of polypyrazoles with glass transition temperature(T_(g))in the range of 42.3-52.5℃ were synthesized and served as catalyst to mediate the cycloaddition of CO_(2)and epoxides by the assistant of tetrabutylammonium bromide.The catalytic behaviors of polypyrazole on the model cycloaddition of CO_(2)to epichlorohydrin,including the reaction parameters optimization and versatility were investigated in detail,and excellent yield(99.9%)and selectivity(99%)were obtained under the optimized reaction conditions of70℃ and 1.0 MPa for 6.0 h.Noteworthily,the polypyrazole acts as homogeneous catalyst during reaction(higher than T_(g)).And under room temperature,polypyrazoles can be easily separated and recovered,which is a promising feature of a heterogeneous catalyst.Furthermore,the reaction mechanism was proposed.The DFT calculation suggested that the formation of hydrogen bond between pyrazole and epoxide greatly reduced the energy barrier,which play an important role in promoting CO_(2)cycloaddition.

Synthesis of Cyclic Carbonates from CO_2 and Epoxides Catalyzed by Hexaalkylguanidinium Halides

Hexaalkylguanidinium halides exhibit an efficient catalytic activity in the synthesis of cyclic carbonates from epoxides and carbon dioxide. By this method cyclic carbonates can be obtained in a high yield and a high selectivity at a low temperature and atmospheric pressure. This procedure is easy for the product isolation and recycling of the catalyst.

Chemical fixation of carbon dioxide to cyclic carbonates catalyzed by zinc(Ⅱ) complex bearing 1,2-disubstituted benzimidazole ligand

A new zinc(II)complex of formula[ZnCl2(L1)2](1)[L1=2‐(2‐thienyl)‐1‐(2‐thienylmethyl)‐1Hbenzimidazole]was synthesized and fully characterized by nuclear magnetic resonance and infrared spectroscopy,elemental analysis,electrospray ionization high‐resolution mass spectrometry,and thermogravimetric analysis.The molecular structure was confirmed by single‐crystal X‐ray diffraction.Complex1consists of mononuclear tetrahedral zinc(II)units with a locked geometry resulting from weak intramolecular S···?and?–?interligand interactions.The benzimidazole ligand and its zinc(II)complex were readily obtained through a simple synthetic route.The catalytic activity of1was investigated in the coupling of carbon dioxide with epoxides to produce cyclic carbonates,and a series of parameters were evaluated.The complex efficiently catalyzed the transformation of various epoxides under solvent‐free conditions,with good conversions,turnover numbers,and turnover frequencies.

Immobilization of Glucoamylase onto Novel Porous Supports Containing Cyclic Carbonate

Glucoamylase was immobilized onto novel porous polymer supports containing cyclic carbonate. The relationship between activity of immobilized glucoamylase and the properties of porous polymer supports was investigated. The operation stability and storage stability of immobilized glucoamylase were studied.

Ru(II)-Catalyzed ortho C-H Allylation of N-Aryl-7-azaindoles with 2-Methylidene Cyclic Carbonate

A Ru(II)-catalyzed ortho allylation reaction of N-aryl-7-azaindole with readily available 2-methylidene cyclic carbonate has been developed.This reaction is an effective pathway for synthesizing 7-azaindole derivatives with a wide scope of substrates and high yields.In addition,the method can be extended to the allylation of other heterocyclic compounds and several cyclic carbonates,highlighting the practicality of this strategy for synthesis.

N-Heterocyclic Carbene-Pyridine Molybdenum Complex Supported over SBA-15 for Converting of Carbon Dioxide into Cyclic Carbonates

Synthesis of cyclic carbonates from carbon dioxide(CO_(2))and epoxides is an effective pathway for the CO_(2) utilization.Although various metal catalysts have been reported,it is highly desirable to develop a method for the reuse or recycling of catalysts.Herein,an N-heterocyclic carbene-pyridine molybdenum complex supported over SBA-15(Mo@SBA-15)was used as an efficient and recyclable catalyst for converting CO_(2) and epoxides into cyclic carbonates.Mo@SBA-15 in combination with tetra-butylammonium bromide(TBAB)shows high catalytic activity in the synthesis of cyclic carbonates under 100℃and 1 MPa CO_(2) pressure.In addition,Mo@SBA-15 was reused seven times without any significant activity loss.

Synthesis of Cyclic Carbonate Catalyzed by DBU Derived Basic Ionic Liquids

In this work, 1,8-diazabicyclo[5.4.0]undec-7-ene （DBU）, 1,5-diazabicyclo[4.3.0]-5-nonene （DBN）, and imidazole （MIM）-derived bromide ionic liquids （ILs） were synthesized and used to catalyze the cycloaddition reactions of carbon dioxide （CO_2） with several kinds of epoxides to form cyclic car- bonates. The DBU derived bromide ionic liquid system was found to have the best catalytic activity among all the tested ILs. The influences of reaction conditions （including temperature, pressure and reaction time） on the reaction of CO_2 to propylene oxide （PO） were studied to show the best conditions of 120℃, 1 MPa, 2.5 h catalyzed by 2 mol% DBU-derived bromide ionic liquid, with the conversion of PO and the selectivity of propylene carbonate （PC） reaching 99% and 99%, respectively. Under the optimum reaction conditions, the ionic liquid system could be reused at least five times without decrease in selectivity and conversion. NMR spectroscopy and DFT calculations were used to reveal the hydrogen-bond interaction between ionic liquids and rea- gent, based on which the reaction mechanism was proposed.

Tannic Acid as a Polyphenol Material-Assisted Synthesis of Cyclic Carbonates Using CO2 as a Feedstock： Kinetic Characteristic and Mechanism Studies

Tannic acid （TA）/KI as a cheap,green,highly-efficient,biocompatible,and recyclable catalytic system represents the excellent synergistic effect in the cycloaddition reactions of CO2 and epoxides under solvent-free conditions,which is originated from hydrogen bonding between the hydrogen atoms of phenolic hydroxyl groups within TA and the oxygen atom of the epoxide.Therefore,it is considerable to be a sustainable methodology for the utilization of bio-resources.

Synthesis of Cyclic Carbonates from Alkenyl and Alkynyl Substrates

As an extensively used chemical product,cyclic carbonate was generally synthesized by transesterification,or the cycloaddition of epoxides,diols with CO2.To reduce the production costs and expand the raw materials,alkenyl and alkynyl substrates have caused much attention in the synthesis of cyclic carbonates,such as olefins,allyl alcohols and propargylic alcohols.Based on the alkenyl substrate,the synthetic process involves a continuous reaction of oxidative carboxylation,with epoxide or halohydrin as an intermediate usually.Therefore,peroxides and nucleophiles （halogens or organic bases） are often necessary in the conversion.Using propargylic alcohols to produce a-alkylidene cyclic carbonates,noble metal catalysts play crucial roles in alkynyl activation,and organic bases are considered to assist the intramolecular and intermolecular proton transfer and combine CO2 molecular.As the carboxyl sources in products,inorganic carbonates and organic carboxylic acids also have some applications instead of CO2.In this review,we summarized the synthetic routes of cyclic carbonates from alkenyl and alkynyl substrates in the aspect of catalyst,mechanism and the development tendency.

Histidine-catalyzed synthesis of cyclic carbonates in supercritical carbon dioxide

The coupling reaction of carbon dioxide with epoxides was investigated using naturally occurring α-amino acids as the catalyst in supercritical carbon dioxide and it was found that L-histidine is the most active catalyst.In the presence of 0.8 mol% of L-histidine at 130°C under 8 MPa of CO_(2),the reaction of carbon dioxide with epoxides proceeded smoothly,affording corresponding cyclic carbonates in good to excellent yields.

Recent advances in the coupling of CO2 and epoxides into cyclic carbonates under halogen-free condition

The chemical transformation of CO2 and epoxides into cyclic carbonates has been receiving much attention and is one of the successful examples for CO2 utilization as carbon resource.Many catalysts containing halide anions have been explored and exhibit excellent catalytic activity.However,halogen salt is generally toxic and corrosive to reactors.From a green chemistry perspective,it is more attractive to develop a halogen-free catalyst with excellent performance.Herein,a review of recent research progress of halogen-free catalysts in the cycloaddition of CO2 and epoxide is presented.According to previous experimental and theoretical works,two possible strategies for achieving the halogen-free process were summarized.The relationship between catalytic activity and catalyst structure,the mechanism of CO2 activation should be both studied deeply combined with experimental results and DFT calculation,which can guide the design of new catalysts and realize halogen-free process under mild reaction conditions.

Synthesis of cyclic carbonates and dimethyl carbonate using CO_(2) as a building block catalyzed by MOF-5/KI and MOF-5/KI/K_(2)CO_(3)

The synthesis of cyclic carbonates or dimethyl carbonate(DMC)using CO_(2) as a building block is a very interesting topic.In this work,we found that the metalorganic framework-5(MOF-5)/KI was an active and a selective catalytic system for the synthesis of cyclic carbonates from CO_(2) and epoxides,and MOF-5/KI/K_(2)CO_(3) was efficient for the preparation of DMC from CO_(2),propylene,and methanol by a sequential route.The impacts of temperature,pressure,and reaction time length on the reactions were investigated,and the mechanism of the reactions is proposed on the basis of the experimental results.

Polystyrene-supported Phenol/DMAP： an Efficient Binary Catalyst System for CO2 Fixation to Give Cyclic Carbonates

Polystyrene-supported phenol （PS-PhOH） was successfully synthesized by alkylation reaction of phenol with 2% DVB cross-linked chloromethylated polystyrene and characterized by IR spectra and elemental analysis. In conjunction with an organic base such as DMAP, DBU, triethylamine （Et3N）, diethylamine （Et2NH） or pyridine, the PS-PhOH could effectively catalyze the coupling reaction of carbon dioxide with epoxides to give cyclic carbonates in high yield and selectivity under mild conditions. The binary catalyst system of the PS-PhOH/DMAP was found to be the most active. The influence of reaction temperature, carbon dioxide pressure and reaction time on the yield of product was carefully investigated. The PS-PhOH could be recycled by simple filtration for at least up to ten times without loss of catalytic activity.

A mini review on chemical fixation of CO_(2):Absorption and catalytic conversion into cyclic carbonates

In this article,we present our research results on chemical fixation of CO_(2) using organobismuth compounds.We fabricated bismuth biphenoate complex,Zn-Mg-Al composite oxides,and SBA-15 or Al-SBA-15 immobilized hydroxyl ionic liquid for CO_(2) cycloaddition onto epoxides.The hypervalent bismuth compounds show good ability for association and dissociation with CO_(2).The bismuth biphenolate complexes are catalytically effective for the cycloaddition reaction.The heterogeneous catalysts,viz.Zn-Mg-Al oxides and SBA-15 or Al-SBA-15 immobilized ionic liquid,are efficient for the synthesis of cyclic carbonate from CO_(2) and epoxide.It is found that the presence of a trace amount of water can improve the catalytic activity of the immobilized ionic liquid.

Predicting metal-organic frameworks as catalysts to fix carbon dioxide to cyclic carbonate by machine learning

The process of discovering and developing new materials currently requires considerable effort,time,and expense.Machine learning(ML)algorithms can potentially provide quick and accurate methods for screening new materials.In the present work,the features of the metal organic frameworks(MOFs)as a catalyst for fixing carbon dioxide into cyclic carbonate were extracted to build a data set,which were collected from the experimental results of approximately 100 published papers.Classifiers were trained with the data set with various ML algorithms,including support vector machine(SVM),K-nearest neighbor classification(KNN),decision trees(DT),stochastic gradient descent(SGD),and neural networks(NN),to predict the catalytic performance.The ML models were trained on 80% of the data set and then tested on the remaining 20%to predict the carbon dioxide fixation ability.The trained ML model was extended to explore 1311 hypothetical MOFs,and some structures displayed a strong catalytic ability.Finally,the six best metal ions(Mn,V,Cu,Ni,Zr and Y)and four best ligands(tactmb,tdcbpp,TCPP,H_(3)L)were determined.These six metals and four ligands could be combined into 24 MOFs,which are strongly potential catalysts for carbon dioxide fixation.Using machine learning methods can speed up the screening of materials,and this methodology is promising for application not only to MOFs as catalysts but also in many other materials science projects.

Ti and Zr amino-tris(phenolate) catalysts for the synthesis of cyclic carbonates from CO_(2) and epoxides

Herein,we report the application of four amino-tris(phenolate)-based metal complexes incorporating Ti(IV)or Zr(IV)centres(2a-3b)as homogeneous catalysts for the conversion of CO_(2)and epoxides into cyclic carbonates.The four complexes were synthesised,characterised and then evaluated in combination with tetrabutylammo-nium iodide,bromide or chloride as binary catalytic systems for the reaction of CO_(2)with 1,2-epoxyhexane as epoxide substrate at 12 bar CO_(2)pressure and 90?C for 2 h.The catalytic systems comprising the two Ti(IV)complexes(2a and 2b)showed similar performance.One notable exception was the catalytic system consisting of titanium complex 2b,bearing an axial Cl-ligand,and tetrabutylammonium chloride,which displayed higher catalytic activity compared to other titanium-based systems.Even higher activity was achieved with Zr(IV)complex 3a,bearing an axial isopropoxide ligand,which reached turnover numbers(TON metal)up to 1920 for the reaction of CO_(2)with 1,2-epoxyhexane at 12 bar CO_(2)pressure and 90?C for 2 h.This performance is comparable with that of state-of-the-art catalysts for this reaction.The catalytic system consisting of complex 3a and tetra-butylammonium bromide was explored further by investigating its applicability with a broad substrate scope,achieving quantitative conversion of several epoxides with CO_(2)into cyclic carbonate products at 90?C and 12 bar CO_(2)pressure for 18 h.The selectivity towards the cyclic carbonate products was?98%for all studied terminal epoxides and?80%for all examined cyclohexene-type epoxides.

Cationic organobismuth complex as an effective catalyst for conversion of CO_(2) into cyclic carbonates

In order to achieve high-efficiency conversion of CO_(2) into valuable chemicals,and to exploit new applications of organobismuth compounds,cationic organobismuth complex with 5,6,7,12-tetrahydrodibenz[c,f][1,5]azabismocine framework was examined for the first time for the coupling of CO_(2) into cyclic carbonates,using terminal epoxides as substrates and tetrabutylammonium halide as co-catalyst in a solvent-free environment under mild conditions.It is shown that the catalyst exhibited high activity and selectivity for the coupling reaction of CO_(2) with a wide range of terminal epoxide.The selectivity of propylene carbonates could reach 100%,and the maximum turnover frequency was up to 10740 h^(-1) at 120℃ and 3 MPa CO_(2) pressure when tetrabutylammonium iodide was used as co-catalyst.Moreover,the catalyst is environment friendly,resistant to air and water,and can be readily reused and recycled without any loss of activity,demonstrating a potential in industrial application.

Carbon dioxide fixation into cyclic carbonates at room temperature catalyzed by heteroscorpionate aluminum complexes

Novel mono-and bimetallic scorpionate aluminum compounds have been designed and characterized by spec-troscopic methods.These organometallics complexes,in combination with tetrabutylammonium bromide as cocatalyst,displayed good catalytic activity for the cycloaddition of styrene oxide and CO_(2) under ambient con-ditions.Among the compounds tested,monometallic complex 3 featuring diethylamino groups showed the highest catalytic activity.This catalytic system catalyzed the formation of cyclic carbonate products from their corresponding epoxides at room temperature and 1 bar of CO_(2) pressure in the absence of a solvent in good yields and excellent selectivities.