CO2 reduction to methanol

Scheme 9.18 CO2 reduction to methanol in water promoted by Fa,eDH, FaidDH, and ADH...

Dibenedetto A, Stufano P, Baran T, Macyk W, Aresta M (2011) Hybrid technologies for an enhanced carbon recycling based on enzymatic CO2 reduction to methanol in water. International conference on carbon dioxide utilization ICCDU XI, Dijon, FR... 

Dibenedetto A, Angelini A, Aresta M, Macyk W, Batan T (2013) Nanomaterials as photocatalysts for the CO2 reduction to methanol in water. Intemational conference on carbon dioxide utilization, ICCDU XII, Alexandria, VA... 

CO2 reduction to acetate and CO2 reduction to CH4 involve two energetically analogous sites (1) the endergonic reduction of CO2 to the formate level ( CO2 activation ), and (2) the exergonic conversion of a coenzyme-bound formaldehyde to the methanol level. 

Fig. 9. Proposed function of electrochemical and Na potentials in energy conservation coupled to methanol disproportionation to CH4 and CO2- It is assumed that prior to oxidation methanol binds first to coenzyme M and that the oxidation is mechanistically and energetically the reversal of CO2 reduction to methyl-coenzyme M. The Na /H" antiporter is involved in the generation of A/iNa Ifom AjlVt. CHO-MFR, formyl-methanofuran CH2=H4MPT, methylene-tetrahydromethanopterin CH3-H4MPT, methyl-tetrahydromethanopterin CH3-S-C0M, methyl-coenzymeM. The hatched boxes indicate membrane-bound electron transport chains or membrane-bound methyltransferase catalyzing either Na or H translocation (see Figs. 5, 6 and 12). ATP is synthesized via membrane-bound H -translocating ATP synthase. The stoichiometries of Na and of translocation were taken from refs. [105,107,167]. x, y and z are unknown stoichiometric factors.

The net reaction is the conversion of 1 mole of glucose to 2 moles each of ethanol and CO2 with balanced consumption of the reducing power generated during the formation of pyruvic acid. The analogous biochemical conversion of glucose to methanol would involve formation of formaldehyde and its reduction to methanol ... 

The two equations above illustrates that the reduction of CO2 to methanol is a scientifically severe challenge. Inoue et al. (Inoue et al., 1979) [5] were the first to report CO2 photoreduction to methanol in 1979 over binary inorganic materials with a... 

Aresta M (2010) CO2 enzymatic carhoxylation and reduction to methanol. International Scientific Forum on CO2 chemistry and biochemistry, CO2 Challenge Forum, Lyon... 

Based on natural photosynthesis in where H2O is the electron donor molecule for CO2 reduction to sugars performed by aU the complex machineiy present in photosynthetic systems I and II and enzymatic reduction, the ideal photocatalytic process will be the reduction of CO2 by H2O. In this case there are several possibilities depending on the final product, either formic acid, methanol, or methane (Eqs. 1.6, 1.7, and 1.8). 

As already mentioned before the elaitrochemical reduction of CO2 at a metal electrode leads only to the formation of formic acid. Recently it has been reported by Ogura et al. (see and literature cited therein), however, that at a Pt-electrode coated by a layer of Everitt s salt (ES), K2Fe(II)[Fe(II) (CNg)], CO2 is selectively reduced to methanol in the presence of metal complexes as homogeneous catalysts and a primary alcohol. The overall reaction is given by... 

Carbon dioxide has been reduced to methanol with the Everitt s salt (ES)-mediated electrode in the presence of l,2-dihydroxybenzene-3,5-disul-phonato(iron) ferrate(III) complex (Scheme 103) [408, 409]. The reduction proceeds as follows a weak coordination bond is first formed between the central metal of ES and ethanol, then the subsequent insertion of CO2 onto... 

More complex reductions of CO2 by enzyme cascades have also been achieved. A combination of an electron mediator and two enzymes, formate dehydrogenase and methanol dehydrogenase, was used to reduce CO2 to methanol. This system operates with current efficiencies as high as 90% and low overpotentials (approximately —0.8 V vs. SCE at pH 7) [125]. The high selectivity and energy efficiency of this system indicate the potential of enzyme cascades. There are also drawbacks to these systems. In general, enzymes are... 

It has also been found that mixtures of CO and CO2 undergo hydrogenation faster than either component individually. The role of CO2 here is not clear but some studies have concluded that the actual CO hydrogenation catalytic cycle may involve carbon dioxide which is first converted into formate on the catalyst surface before reduction to surface methoxy and methanol (Equations 42a-c). 

Carbon dioxide, 0=C=6 , mp —57°C (5.2 atm), bp —79 °C (sublimes), is obtained from the combustion of carbon and hydrocarbons in excess air or oxygen or by the pyrolysis ( calcination ) of CaCOs (limestone). The photosynthesis in plants reduces CO2 to organic matter, but the similar reduction of CO2 in a nonliving system ( in vitro ) appears to be very difficult. However, CO2 can be reduced electrochemically to methanol, formate, oxalate, methane, and/or CO depending upon the conditions. Numerous transition metal complexes of CO2 are known,which exhibit the modes of metal-C02 bonding depicted in Figure 2. 

Biocatalytic synthetic reactions also include carbon dioxide fixation with the production of methanol in artificial multi-enzyme systems [188]. Formate dehydrogenase (FDH, EC 1.2.1.2) can catalyze the reduction of carbon dioxide to formate, and methanol dehydrogenase (MDH, EC 1.1.99.8) can catalyze the reduction of formate to methanol. Both of these enzymes require NAD+-NADE1 cofactor, and in the presence of the reduced dimethyl viologen mediator (MV+), they can drive a sequence of enzymatic reactions. The cascade of biocatalytic reactions results in the reduction of CO2 to formate catalyzed by FDEI followed by the reduction of formate to methanol catalyzed by MDH. A more complex system composed of immobilized cells of Parococcus denitrificans has been demonstrated for the reduction of nitrate and nitrite [189]. 

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