The question on electro-catalytic reduction of carbon-di-oxide

Four groups of metals for CO2 reduction based on high hydrogen overvoltage, CO adsorption strength, high hydrogen producing metals and HC forming CopperThe three class of metals are understandable but why copper behaves differently and also why this metal shows phase specificityWhat makes copper to promote C-C coupling reactionThe answer is not yet known

Cyclic CarbonatesEthene carbonate (EC) propene carbonate (PC), Styrene carbonate solvents, precursor for polycarbonates, electrolyte in Li batteries, Pharmaceuticals and chemical reaction raw

materials. The reaction shown is atom economy and green process carboxylation of epoxides example Reproduced from J Chem Technol.Biotechnol,89,334 (2014)

Other attempts include starting from olefins without intermediate formation of epoxideDMF dialkylacetamide (DAA) is used as solvent since promote carboxylationPd catalyzed fixation of CO2 cobalt complexes coupling of CO2 with epoxideReproduced from J Chem Technol.Biotechnol,89,334 (2014)

Use of ionic liquidsthermal and chemical stability

selective solubility for org and inorgreusability of catalyst

carbon dioxide solubilitywater Lewis base catalysts show high

activity

Super critical carbon dioxideanother reaction medium no flammability, non toxic, absence of gas liquid phase boundary and easy work upmetalloporphrins reusableTriazine high nitrogen centres to inorganice carbonatespolymer supported IL epoxide to cyclic carbonates

Reproduced from J Chem Technol.Biotechnol,89,334 (2014)

Cobalt complex active for cyclic carbonate and polycarbonate synthesis.

Reproduced from J Chem Technol.Biotechnol,89,334 (2014)

Other options for cyclic carbonate synthesis are the reactions of CO2 with cyclic ketals, propargylic alcohols, diols and the direct oxidative carboxylation of olefins. The latter appears to be a very interesting synthetic methodology to synthesize cyclic carbonatesstarting from cheap and easily available reagents such as CO2 and O2

The direct oxidative carboxylation of olefins has great potential and has many advantages. It does not require carbon dioxide free of dioxygen. This feature makes it attractive because of the purification cost of carbon dioxide, which may discourage its use. Moreover, the direct oxidative carboxylation of olefins can couple two processes, the epoxidation of the olefins and the carboxylation of the epoxides. The process makes direct use of olefins which are available on the market at a low price, and are abundant feedstock.

Reproduced from J Chem Technol.Biotechnol,89,334 (2014)

Only a few examples are reported in the literature of the direct oxidative carboxylation of olefins such as the direct functionalization of propene and styrene. Using RhClP3 as catalyst, under homogeneous conditions, it was demonstrated that two classes of compounds are formed: the first one is due to ‘one oxygen’ transfer to the olefin with formation of epoxide and its isomerization products and carbonate ; the second class of products is due to ‘two oxygen’ transfer to the olefin with formation of aldehydes, as effect of the addition of the oxygen to the C–C double bond with cleavage of the double bond of the olefin, and the relevant acids

Using heterogeneous conditions it has been demonstrated that oxidation of the olefin does not follow the peroxocarbonate pathway, more likely it is a radical process which can be started by the catalyst which plays a very important role in the carbonation step. The carbonate yield depends on the catalyst used. The selectivity of the process (that reaches a maximum of 50% with respect to the olefin) is still affected by the formation of by-products such as benzaldehyde, benzoic acid, acetophenone, phenylacetaldehyde, 1,2-ethanediol-1-phenyl anda benzoic acid ester. After a short induction time, benzaldehyde is formed in higher amounts than the epoxide which becomes the predominant product after 45 min. The carbonate formation starts after 1 h and steadily increases with time, while the concentrationof the epoxide and benzaldehyde reach a steady status. The life of the catalyst is of days and the catalyst is easily recovered at the end of the catalytic run.

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By reacting cyclic ketals with carbon dioxide under supercritical conditions in organic solvents a cyclic carbonate has been obtained under relatively mild conditions (10 MPa and 370 K)using a suitable catalyst

Reproduced from J Chem Technol.Biotechnol,89,334 (2014)

The coproduct cyclohexanone may react with 1,2-ethane-diol in the presence of FeCl3 to afford, with almost quantitative yield, the cyclic ketal (Equation 16) which can be reused.

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Several metal systems were tested, either oxides [ZnO, Nb2O5, ZrO2, TiO2], or metal halides [ZnCl2, FeCl2], or else metal complexes [FeCl2 · 1.5 THF], CuL2, FeClL. The most active catalysts have been found to be CuL2 and FeClL (L=C11H7F4O2), i.e. those bearingperfluoro alkyl groups, which are soluble in sc-CO2 under the reaction conditions

Reproduced from J Chem Technol.Biotechnol,89,334 (2014)

Cyclic carbonates have also been synthesized from propargylic alcohol derivatives and CO2 as the starting materials. This synthetic approach (Equation 17) is based on the cyclizationof the propargylic carbonate moiety (HC≡CCH2OCO2 –) into the corresponding α-alkylidene cyclic carbonate in the presence of a suitable catalyst such as ruthenium, cobalt, palladium,copper, or phosphine.

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Ikarya has reported the use of imidazolin-2-ylidenes with N-alkyl and N-aryl substituents and their CO2 adducts as catalyst of the carboxylative cyclization of internal and terminalpropargylic alcohols. The reaction of internal propargyl alcohols with CO2 has been carried out also under supercritical conditions. Ikariya et al. have developed a synthetic process toafford Z-alkylidene cyclic carbonates promoted by P(n-C4H9)3 with high efficiency.

Reproduced from J Chem Technol.Biotechnol,89,334 (2014)

Oxidative carboxylation of styrene under homogeneous conditions.Reproduced from J Chem Technol.Biotechnol,89,334 (2014)

Reproduced from J Chem Technol.Biotechnol,89,334 (2014)

Ionic liquid (1-butyl-3-methylimidazolium benzene sulfonate ([BMIm][PhSO3])) has also been used as reaction medium for the synthesis of α-methylene cyclic carbonates from CO2and propargyl alcohols using transition metal salts as catalyst

Reproduced from J Chem Technol.Biotechnol,89,334 (2014)

Among the catalysts used, CuCl was revealed to be the most efficient. On the contrary, when Pd(II), Rh(III), Ru(III), and Au(III) salts were used as catalysts no carbonate was produced, also if the substrate has been converted. This is due to the formation of the kind of polymer (black tar is found on the inner wall of the reactor) that occurs when the noble metal salts/ [BMIm] [PhSO3] systems are used. In the absence of metal salt as catalyst, the reaction did not yield any product even after a long reaction time

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Starting from propargyl alcohols using supercritical carbon dioxide in the presence of bicyclic guanidines as catalystsα-methylene cyclic carbonates is obtained

Reproduced from J Chem Technol.Biotechnol,89,334 (2014)

Cyclic carbonates can be produced from diols and carbon dioxide in the presence of suitable catalysts

The thermodynamics of this reaction are not very favourable and the major drawback is related to the coproduction of water, which may involve modification or deactivation of the catalyst with negative effects on the conversion rate.

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Ceria based catalysts and CeO2–ZrO2 solid solution catalysts have been reported to be very efficient catalyst for the synthesis of ethene carbonate and propene carbonate by reaction of CO2 with ethene glycol and propene glycol, respectively.The catalytic activity has been shown to be dependent on the composition and the calcination temperature of catalysts

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Different metallic acetates have been used in acetonitrile which acts not only as solvent but also as dehydrating agent to eliminate the effect of the water produced during the reaction. In this way, the thermodynamic equilibrium is shifted and the yield of cyclic carbonates improved. Organic super bases such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), or 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) havealso been used as effective promoters in the synthesis of propene carbonate from propene glycol and carbon dioxide in the presence of acetonitrile (yield 15.3%, selectivity 100% under the optimal conditions

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The reaction of polyols with urea is a recent strategy to afford cyclic carbonates. Efficient catalysts have been used for the synthesis of glycerol carbonate that has been used asplatform molecule for the synthesis of several chemicals, including epichlorohydrin.

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SYNTHESIS OF LINEAR CARBONATES