Methodologies for chemical utilization of CO2 to valuable compounds through molecular activation by efficient catalysts

The reactions of CO₂ with oxirane to produce cyclic carbonate, and with aziridine to afford oxazolidine have been of interest as a useful method for its fixation by a chemical process. Highly efficient processes employing recyclable CO₂-phlilic homogeneous catalyst were devised for environmentally benign synthesis of cyclic carbonates and oxazolidinones under supercritical CO₂ without any organic solvent. These processes represent pathways for greener chemical fixations of CO₂ to afford industrial useful materials such as organic carbonates and oxazolidinones with great potential applications.     

2,2',2'-Terpyridine-Catalyzed Synthesis of Cyclic Carbonates from Epoxides and Carbon Dioxide under Solvent-Free Conditions

An efficient coupling reaction of epoxides with CO₂ affording cyclic carbonates with the use of 2,2'',2''''-terpyridine as catalyst under solvent-free conditions has been developed.     

Metal Acetylacetonates as Highly Efficient and Cost Effective Catalysts for the Synthesis of Cyclic Carbonates from CO<Subscript>2</Subscript> and Epoxides

Abstract  

Metal acetylacetonates were found to be efficient and cost effective catalysts for the formation of cyclic carbonates by cycloaddition of carbon dioxide with epoxides, providing high to excellent yields of the corresponding carbonates. Among the various catalysts such as acetylacetonates of Co, Ni, Cu, Zn, Fe, Cr and VO studied, Ni(acac)₂ was found to be promising catalyst for this reaction. The present methodology was found to be superior due to the easy accessibility and comparatively inexpensive nature of metal acetylacetonates than salen complexes.     

Porosity and surface area of monolithic carbon aerogels prepared using alkaline carbonates and organic acids as polymerization catalysts

Carbon aerogels were prepared by polymerization of a resorcinol-formaldehyde mixture using different polymerization catalysts such as: sodium or potassium carbonates, oxalic acid or para-toluenesulfonic acid. The carbon aerogel obtained with this last acid was further CO₂-activated to 8.5% and 22% burn-off. All samples were characterized by N₂ and CO₂ adsorption at −196 and 0 °C, respectively, and by mercury porosimetry, scanning electron microscopy, and thermogravimetric analysis. Samples prepared using Na₂CO₃ were denser than those prepared using K₂CO₃. In addition, the density of samples prepared under acidic conditions was greater than that of samples prepared using alkaline carbonates as catalysts. Most of the carbon aerogels prepared were mesoporous with narrow pore size distributions. Results obtained showed that the nature of the acid used in the preparation of these aerogels only affected the gelation process. Finally, it is noteworthy that CO₂ activation of the carbon aerogel prepared with para-toluenesulfonic acid as catalyst only increased and widened the microporosity and had virtually no effect on the mesoporosity.     

Two dimensional Zn-stilbenedicarboxylic acid (SDC) metal-organic frameworks for cyclic carbonate synthesis from CO<Subscript>2</Subscript> and epoxides

A two-dimensional Zn-based metal-organic framework has been synthesized by using Zn(II) ions and H₂SDC (4,4′-stilbenedicarboxylic acid) under solvothermal conditions. The framework having a trinuclear Zn₃-(RCO₂)₆ SBUs connected by the 4,4′-stilbenedicarboxylic acid to form a hexagonal network, shows a two-dimensional structure and displays high thermal stability up to approximately 330 °C. The role of Zn²⁺ (from Zn-SDC) for epoxide activation and Br-ion (from TBABr) for ring opening of epoxide was studied for the cycloaddition reaction of CO₂ and propylene oxide (PO) under ambient conditions. Zn-SDC was found catalytically efficient towards CO₂-epoxide coupling under ambient reaction conditions with high selectivity towards the desired cyclic carbonates under solvent-free conditions. The effects of various reaction parameters such as catalyst loading, temperature, CO₂ pressure, and time were evaluated. Zn-SDC was easily separable and reusable at least five times without any considerable loss in the initial activity. A plausible reaction mechanism for the cycloaddition reaction was also proposed based on literature and experimental inferences.     

Zirconium-based isoreticular metal-organic frameworks for CO<Subscript>2</Subscript> fixation via cyclic carbonate synthesis

Two highly stable isoreticular metal-organic frameworks comprising chains of zirconium coordinated with linkers of 1,4-H₂BDC (1,4-benzenedicarboxylic acid) and 4,4′-H₂BPDC (4,4′-biphenyldicarboxylic acid), denoted as MIL-140A and MIL-140C, were synthesized. The catalytic activity of these frameworks was studied for the coupling reaction of CO₂ and epoxides to produce cyclic carbonates under solvent-free conditions. Excellent activity was observed for both catalysts: they yielded high epoxide conversion with >99% selectivity toward the cyclic carbonate, and were fully reusable even after four cycles without any considerable loss of initial activity. The enhancement in the catalytic activity was explained based on acidity/basicity studies. The influence of various reaction parameters such as catalyst amount, reaction time, reaction temperature, and CO₂ pressure was also investigated. Reaction mechanism was proposed on the basis of experimental evidence and our previous DFT (density functional theory) studies.     

Dioxomolybdenum(VI) complexes with amino acid functionalized N-salicylidene hydrazides: Synthesis,structure and catalytic activity

A cis-dioxomolybdenum(VI) complex with an amino acid functionalized hydrazone ligand derived from salicylaldehyde and Boc-protected β-alanine hydrazide (Boc = t-butyloxycarbonyl) has been synthesized. The X-ray crystal structure reveals a distorted octahedral geometry at the molybdenum atom with an O₅N donor set and vast hydrogen bonding interactions including the amino acid side chain of the ligand. The new cis-dioxomolybdenum(VI) complex is an efficient catalyst for the peroxidic oxidation of sulfides, but does not show any activity towards the oxidation of bromide.     

Mesoporous Ta–W Composite Oxides: A Highly Effective and Reusable Acid–Base Catalysts for the Cycloaddition Reaction of Carbon Dioxide with Epoxides

Cycloadditions of epoxides and carbon dioxide into corresponding cyclic carbonates were performed over mesoporous Ta–W composite oxides prepared by a modified hydrolytic method. The best yields of styrene carbonate were obtained when the Ta/W mole ratio was 2:1 (labeled as Ta_0.67W_0.33O_s). Under optimal reaction conditions, the conversion of styrene oxide and selectivity of styrene carbonate reached 95 and 97%, respectively. These Ta–W composite oxides have been extensively characterized by several techniques. X-ray diffraction (XRD) patterns and Transmission Electron Microscope (TEM) revealed that Ta₂O₅ was completely dispersed in WO_x. Scanning electron microscopy (SEM) exhibited that the particle size distributions become more and more uniform with increment of tungsten content. CO₂ and NH₃ temperature-programmed desorption (CO₂ and NH₃-TPD) revealed that Ta_0.67W_0.33O_s catalyst had the strongest acid and base strength. X-ray photoelectron spectroscopy (XPS) shows that the strongest acid–base sites of Ta_0.67W_0.33O_s catalyst origin from its highest lattice oxygen concentration and W 4f_5/2 species with Bronsted acidity. We discussed the reaction kinetics and proposed a possible mechanism, indicating the excellent catalytic activity is attributed to the cooperative action of acidic and neighboring basic sites on the catalyst surface.

Graphic Abstract

Cycloaddition reactions of carbon dioxide with epoxides into corresponding cyclic carbonates were performed over mesoporous Ta–W composite oxides prepared by a modified hydrolytic method. The best yields for cyclic carbonates were obtained when the Ta/W mole ratio was 2:1 (denoted as Ta_0.67W_0.33O_s). Acid-base synergy, specific surface area and mesoporous structure could be ascribed to the main reasons for the highest catalytic activity of Ta_0.67W_0.33O_s catalyst. Meanwhile, reaction kinetics was discussed and a possible reaction pathway was proposed.

     

Hydrogen bond activation strategy for cyclic carbonates synthesis from epoxides and CO2: current state-of-the art of catalyst development and reaction analysis

Chemical fixation of CO₂ into useful organic compounds has been attracting much attention from the viewpoint of CO₂ emission reduction and energy structure reformation. A useful and widely investigated chemical utilization of CO₂ is the cycloaddition of CO₂ to epoxides for the synthesis of cyclic carbonates. Efforts have been paid to the design and preparation of various catalyst systems that are active and selective to the production of the desired products under mild conditions and in green processes. This article is to review the current state of the catalyst development and the experimental and theoretical analysis of reaction mechanism for the cyclic carbonate synthesis from epoxides, one of currently important reactions involving CO₂ as a reactant with 100% atom economy. Particular attention is given to the catalysis of multifunctional catalyst systems such as metal- and hydrogen-bond donor (HBD)-based catalysts.     

Synthesis of organic carbonates from alcoholysis of urea: A review

Organic carbonates are green compounds with a wide range of applications. They are widely used for the synthesis of important industrial compounds including monomers, polymers, surfactants, plasticizers, and also used as fuel additives. They can be divided into two main classes: cyclic and linear carbonates. Dimethyl carbonate (DMC) and diethyl carbonate (DEC) are the important linear carbonates. Carbonyl and alkyl groups present in DMC and DEC make them reactive and versatile for synthesizing various other important compounds. Ethylene carbonate (EC), glycerol carbonate (GC) and propylene carbonate (PC) are well-known cyclic organic carbonates. Phosgenation of alcohols was widely used for synthesis of organic carbonates; however, toxicity of raw materials restricted use of phosgenation method. A number of new non-phosgene methods including alcoholysis of urea, carbonylation of alcohols using CO₂, oxy-carbonylation of alcohols, and trans-esterfication of alcohols and carbonates have been developed for synthesizing organic carbonates. Carbonylation of alcohols is preferred as it helps in utilization and sequestration of CO₂, however, poor thermodynamics due to high stability of CO₂ is the major obstacle in its large scale commercialization. Oxy-carbonylation of alcohols offers high selectivity but presence of oxygen poisons the catalyst. Recently, alcoholysis of urea has received more attention because of its inexpensive abundant raw materials, favorable thermodynamics, and no water-alcohol azeotrope formation. Also, ammonia evolved in this synthesis route can be recycled back to urea by reacting it with CO₂. In other words, this method is a step towards utilization of CO₂ as well. This article reviews synthesis of DMC, DEC, GC, PC, and EC from urea by critically examining various catalysts used and their performances. Mechanisms have been reviewed in order to give an insight of the synthesis routes. Research challenges along with future perspectives have also been discussed.     

The effect of reaction conditions on montmorillonite-catalysed peptide formation

Oligomerization of glycine (gly) and diglycine (gly₂) on montmorillonite was performed as cyclic, drying-wetting process at temperatures below 100°C, under varying reaction conditions. The influence of substrate/clay ratio, temperature and pH was found to be different for amino acid (AA) dimerization, cyclic anhydride (CA) formation and peptide chain elongation. High temperatures and neutral pH favour CA formation over diglycine production. An AA/catalyst ratio of 0.2 mmol/g leads to optimal yields. This supports the assumption that amino acid dimerization and CA formation take place at the edges of clay particles. Peptide chain elongation, starting from gly₂, produces higher yields at higher temperatures and neutral pH.     

Palladium‐Catalyzed Carbonylation of Diols to Cyclic Carbonates

The catalytic alkoxycarbonylation of 1,2‐diols by (neocuproine)palladium(II) acetate (neocuproine=2,9‐dimethyl‐1,10‐phenanthroline) or palladium(II) acetate/(−)‐sparteine using N‐chlorosuccinimide as the oxidant affords cyclic carbonates. The oxidative carbonylation of diols proceeds under mild conditions, requiring only 1 atm of carbon monoxide, and produces cyclic carbonates in moderate to good yields. Both 1,2‐ and 1,3‐diols can be carbonylated using (neocuproine)Pd(OAc)₂ and sodium dichloroisocyanuric acid, which serves as a competent oxidant and base for this system, to yield 5‐ and 6‐membered cyclic carbonates.     

A fully bio-based monomer derived from undecylenic acid,glycerol, and CO2 useful for the synthesis of polyhydroxyurethanes

Undecylenic acid, glycerol, and CO₂ were used as building blocks for obtaining a fully bio-based carbonated monomer, useful for polyurethanes. The functionality of the monomer was close to 3 cyclic carbonates/mol, located in terminal positions. In a first stage, a synthetic triglyceride was obtained with 99% selectivity by esterification of glycerol and undecylenic acid at 160°C. The triglyceride was then epoxidized using H₂O₂ and Amberlyst 15 or Amberlite IR-120 acidic exchange resins at 57°C. The selectivity to epoxide was kept constant at 98% using Amberlite IR-120. Terminal cyclic carbonates were then inserted through epoxide moieties under mild conditions by the chemical fixation of CO₂ at 80°C and 6 MPa in 6 h. A complete conversion was obtained in 6 h reaction while the selectivity to carbonate groups was near to 99% during all the reaction time. An elastomeric polyhydroxyurethane was obtained by aminolysis of the carbonated monomer with ethylenediamine at 70°C, affording a Young's modulus of 22.6 MPa and T_g of −15.2°C. The material showed a good thermal stability below 240°C.     

In Situ Generation of Strong Bases from Alkaline and Alkaline–Earth Carbonates

Strong bases from alkaline and alkaline-earth metal carbonates were generated in situ by adding a small amount of acetic acid at reflux in toluene under water-free conditions. Their basic strength reached superbasicity thus changing the color of 4-chloroaniline (H₋=26.5). The high conversion of ethyl acetate in its self-condensation over decomposed carbonates, which require strong basicity to abstract protons from ethyl acetate (pK_a=25), also confirmed the formation of strong bases. Adding acetic acid during the reaction indicated that metal oxides—decomposed materials from carbonates—were responsible for their high catalytic activity. The lack of sufficient coordination of in situ generated metal oxides was considered to be a plausible cause for their strong basicity.     

Cyclic fatty acids from linolenic acid

Linolenic acid of 95% purity was heated with excess alkali in ethylene glycol to produce cyclic fatty acids. Reaction variables, which are associated with the cyclization reaction and which were investigated, included solvent-to-fatty-acid ratio, catalyst concentration, and reaction temperature, headspace gas (N₂ or C₂H₄), and head-space gas pressure. Yields of cyclic acids were improved by increasing solvent ratio (1.5–6 wt basis), reaction temperature (225–295C), and catalyst concentration (10–100% excess). With nitrogen the optimum catalyst concentration was about 100% excess, but when ethylene was used, no increase was obtained beyond 50% excess catalyst. Yields of polymeric acids produced in the reaction generally decreased as cyclic acid yields increased, except in one instance. Higher yields of cyclic fatty acids were obtained with ethylene than with nitrogen under all comparable conditions, and increasing the ethylene pressure to as high as 500 psi improved the yield. Ethylene adds to the conjugated double bonds and is believed to give C₂₀ fatty acids having a 1,4-disubstituted monoene ring in the chain. The maximum yield of monomeric cyclic acids from 95% linolenic acid was 84.6%, the balance being polymeric and unreacted monomeric acids. Monomeric acids from this test contained 95% cyclic acids. Presented at AOCS meeting, New Orleans, 1962. No. Utiliz. Res. & Dev. Div., ARS, U.S.D.A.     

Oxidative Dehydrogenation of <Emphasis Type="Italic">n</Emphasis>-Butene to 1,3-Butadiene over Sulfated ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Catalyst

Oxidative dehydrogenation of n-butene to 1,3-butadiene over sulfated ZnFe₂O₄ catalyst was carried out in a continuous flow fixed-bed reactor. The effect of sulfation on the catalytic performance of ZnFe₂O₄ was investigated. Sulfated ZnFe₂O₄ catalyst showed a better catalytic performance than ZnFe₂O₄ catalyst in the oxidative dehydrogenation of n-butene. Acid–base property of sulfated ZnFe₂O₄ catalyst was measured by TPD experiment, with an aim of correlating the catalytic performance with the surface acid–base property of the catalyst. It was revealed that the catalytic performance of sulfated ZnFe₂O₄ catalyst was closely related to the surface weak-acid density of the catalyst. The enhanced acidity of sulfated ZnFe₂O₄ catalyst was responsible for its high catalytic performance in the oxidative dehydrogenation of n-butene. Thus, sulfation served as an efficient method for improving catalytic performance of ZnFe₂O₄ in the oxidative dehydrogenation of n-butene.     

Coupling of cyclohexene oxide (CHO) and carbon disulfide (CS2) catalyzed by the asymmetric bis-Schiff-base Zn(II) complex

Based on the self-assembly of the asymmetric bis-Schiff-base ligand H₂L (H₂L = 4-((E)-(2-((E)-3,5-dibromo-2-hydroxybenzylideneamino)phenylimino)(phenyl)methyl-1-(4-chlorophenyl)-3-methyl-1H-pyrazol-5-ol) and Zn(OAc)₂·2H₂O, a new [Zn(L)] (1) was obtained and shown to efficiently catalyze the coupling of CHO (cyclohexene oxide) and CS₂ (carbon disulfide) in activation with [PPN]Cl (PPN⁺ = bis(triphenylphosphoranyidene)-ammonium), n-Bu₄NBr or n-Bu₄NI, where both poly[thio]carbonates and cyclic [thio]carbonates were produced, and the strong O/S exchange afforded the limited formation of trithiocarbonate in the cyclic [thio]carbonate byproducts.     

Poly(4‐vinylphenol)/tetra‐n‐butylammonium iodide: Efficient organocatalytic system for synthesis of cyclic carbonates from CO2 and epoxides

An efficient polymer‐based catalytic system of poly(4‐vinylphenol) and tetra‐n‐butylammonium iodide was developed for the synthesis of cyclic carbonates from epoxides and CO₂. Owing to the synergistic effects of hydroxyl groups and iodide anions, this commercially available and metal‐free system was highly active for the reaction of various terminal epoxides under environmentally benign conditions, at 25 to 60 °C and atmospheric pressure of CO₂, without the use of any organic solvents. The catalyst system can be easily separated by adding ether, and its ability was recovered by treating it with 40% CH₃CO₂H aq. The recyclability was investigated in detail for three substrates, epichlorohydrin, 1,2‐epoxyhexane, and styrene oxide, using ¹H nuclear magnetic resonance analysis. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45189.     

Supported imidazole as heterogeneous catalyst for the synthesis of cyclic carbonates from epoxides and CO2

Imidazole anchored onto a silica matrix, by means of a propyl linkage, is found to be an effective heterogeneous catalyst for the synthesis of cyclic carbonates from epoxides and CO₂ in near quantitative yield. The versatility of this catalyst is demonstrated by using different substrates (epichlorohydrin, propylene oxide, butylene oxide and styrene oxide) for this cycloaddition reaction. These CO₂ insertion reactions were typically carried out in the temperature range of 343 to 403 K at 0.6 MPa CO₂ pressure under solvent-free conditions. Several spectroscopic methods were used to characterize the catalyst and study the integrity of the fresh and spent catalysts.     

Synthesis of Polycarbonate Precursors over Titanosilicate Molecular Sieves

A novel catalytic application of titanosilicate molecular sieves (TS-1 and TiMCM-41), in the synthesis of polycarbonate precursors like cyclic carbonates and dimethyl and diphenyl carbonates, avoiding toxic chemicals like phosgene or CO, is reported. Cyclic carbonates were prepared, over TS-1 and TiMCM-41, in high yields, by cycloaddition of CO₂ to epoxides, like epichlorohydrin, propylene oxide and styrene oxide, at low temperatures and pressures. Further, transesterification of the cyclic carbonates with methanol and phenol, over TiMCM-41, yielded other polycarbonate precursors like dimethyl carbonate (DMC) and diphenyl carbonate (DPC). The cyclic carbonates could also be synthesized from the olefins in the same reactor by reacting the olefins, in the presence of TiMCM-41, with a mixture of an epoxidizing agent (like H₂O₂ or tert-butyl hydroperoxide) and CO₂.