Hydrogenation of carbon dioxide

Methanol is one of the ten largest organic chemicals, manufactured worldwide on a scale of some 30 x 106 tonnes per year it is made by hydrogenation of a mixture of carbon monoxide using a Cu ZnO/A Oa developed by ICI in 1966.63 It is now recognised that the reaction proceeds by first reducing the dioxide to the monoxide, the relevant equations being 

The equilibrium concentration of methanol therefore decreases as temperature rises and as pressure is lowered the maximum conversion using carbon monoxide at 50 bar and 533 K would be about 50%. The first of these three reactions is the reverse water-gas shift. An outline reaction scheme is shown 

On the basis of these results, it appears unlikely than any gold catalyst will supplant the widely used Cu-Zn0/Al203, unless the selectivity consideration becomes a critical factor. 

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Other potential processes for production of formic acid that have been patented but not yet commerciali2ed include Hquid-phase oxidation (31) of methanol to methyl formate, and hydrogenation of carbon dioxide (32). The catalytic dehydrogenation of methanol to methyl formate (33) has not yet been adapted for formic acid production. 

Scheme 3.4 The heterolytic splitting of dihydrogen at Ru(ll) to give a hydridic-protonic bond, as proposed by Chu et al. [55] in the mechanism of the homogeneous hydrogenation of carbon dioxide.

Hydrogenation of carbon dioxide in the presence of an epoxide generates a mixture of the diol, its formate esters, and the cyclic carbonate. While the reaction has been shown to operate in high yield (1300 TON for the cyclic carbonate Eq. (10)) [93], the fact that a mixture is generated and that the cyclic carbonate could be made more cleanly in the absence of H2 makes the reaction uninteresting for synthesis. Sasaki s group showed that this reaction in the presence of an amine base gives CO rather than cyclic carbonate (Eq. (11)) [94]. The epoxide then serves as a trap for the water. 

The rate of the catalytic hydrogenation of carbon dioxide to produce methane is... 

Abstract Theoretical works on the hydrogenation of carbon dioxide catalysed by... 

Key words hydrogenation of carbon dioxide, insertion of carbon dioxide into the metal-hydride bond, reductive elimination of formic acid, C-bond metathesis... 

Transfer hydrogenation and hydrogenolysis Hydrogenation of carbon dioxide in aqueous solution Hydrogenations of biological interest... 

An interesting application of TSIL was developed by Zhang et al for the catalytic hydrogenation of carbon dioxide to make formic acid. Ruthenium immobilized on silica was dispersed in aqueous IL solution for the reaction. H2 and CO2 were reacted to produce formic acid in high yield and selectivity. The catalyst could easily be separated from the reaction mixture by filtration and the reaction products and the IL were separated by simple distillation. The TSIL developed for this reaction system was basic with a tertiary amino group (N(CH3)2) on the cation l-(A,A-dimethylaminoethyl)-2,3-dimethylimidazolium trifluoromethanesulfonate, [mammim] [TfO]. 

Examples of the applicability of eqns. (7) and (8) are, respectively, the hydrogenation of carbon dioxide over platinum [28] and the hydrogenation of ethylene over copper [29] and platinum [30] under certain conditions. There are, of course, an infinite variety of intermediate cases lying between the extreme conditions considered above. 

Arena F, et al. Synthesis, characterization and activity pattern of Cu-ZnO/Zr02 catalysts in the hydrogenation of carbon dioxide to methanol. J Catal. 2007 249(2) 185-94. 

Another interesting result is the high yield reached with palladium on alumina. Beside the desired product dmf, trimethylamine, N-methylformamide and traces of N,N-dimethylurea were identified by GC-MS. The utilization of such a stable and disposable catalyst could be interesting for developing the solvent-free hydrogenation of carbon dioxide on an industrial scale. 

Other reactions that play a significant role include the hydrogenation of carbon dioxide (Equation 40),... 

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