Hydrogenation of CO2 to formic acid

Hydrogenation of CO2 to formic acid and its derivatives such as methyl formate and N,N-dimethylformamide is an attractive process. Among transition metal catalysts, homogeneous mthenium catalysts are especially effective for these reactions. 

As described in Section 11.1, the transition metal-catalyzed hydrogenation of CO2 to formic acid, methyl formate and N,N-dimethylformamide is a very attractive reaction with regard to CO2 fixation to produce valuable chemicals on a large scale [4,122], Formic acid is a very important industrial chemical that is used as the simplest carboxylic acid and an organic reducing agent. Among transition metal complexes, ruthenium complexes have been found to be very efficient catalysts for the conversion for CO2 to formic acid or formate. 

Equation 1 Thermodynamic parameters for the hydrogenation of CO2 to formic acid,... 

Carbon dioxide in its supercritical state is a reaction medium of great interest. Noyori et. al. recently detected that Ruthenium(II)-phosphine-complexes of typ [(X)2Ru(PMe3)4] 15 (X = H) and 16 (X = Cl) can act as highly active catalysts for an effective transition metal catalysed hydrogenation of CO2 to formic acid in a supercritical mixture of CO, H, and NEt, without use of any further solvent. 

There now exists evidence for the extension of two-phase catalysis into the new area of Ci-chemistry. Thus, Leitner an co-workers [206] described the biphase hydrogenation of CO2 to formic acid (cf. Section 3.3.4). Two-phase hydrogenations of aromatic nitro compounds with Pd or Rh catalysts are examined by Tafesh and co-workers [207] and others [212 f, 218 d, 226]. 

The accessible molecular surface (AMS) model is introduced as a unique approach for the description of steric ligand effects in homogeneous rhodium-catalyzed hydrogenation of CO2 to formic acid [33]. 

Scheme 5. Postulated mechanism for the hydrogenation of CO2 to formic acid with the most active catalytic system dppb/Rh with a ligand/metal ratio of 1 1. P = PPh2.

H2 is proposed to displace H2O and transfer a proton to an a-fluorine to eliminate HF and form a carbene ligand that is then hydrogenated to give fluorohydrocarbon products. Heterolysis of H2 is also a key step in hydrogenation of CO2 to formic acid in water catalyzed by [( -C6Me6)Ru(bipy)(H20)]S04. ... 

Formic acid is produced mainly by carbonylation of methanol to methyl formate followed by hydrolysis of this ester to formic acid and methanol [route (d) in Topic 5.3.3]. The applied reaction sequence represents formally the hydrolysis of carbon monoxide to formic acid. Owing to the growing worldwide interest in converting CO2 into useful chemicals, the catalytic hydrogenation of CO2 to formic acid has been investigated intensively but no commercial processes has been realized yet. Formic acid is also obtained as one of the side products in the catalytic oxidation of butane and light naphtha to acetic acid (see Section 6.15 for details). 

Recently, in a theoretical study [99] of the hydrogenation of CO2 to formic acid using Ru catalysts in presence of water, no coordination of CO2 to the metal centre was identified, but low energy assemblages with the C and O atoms of CO2 interacting with H bound to the metal or the HT of H2O. In the absence of water, CO2 directly coordinates to the Ru center to afford Ru(H)2(ti -C02) (PMe3)3. The Ru-(q -formate) intermediate is produced via CO2 interaction with the M-H bond more than attack by the H on coordinated CO2. 

Hydrogenation of CO2 to formic acid (H2 -I- CO2 -< HC(O)OH) catalyzed by transition metal complexes is one of interesting and attractive reactions for COj fixation. Experimentally, various metal complexes, especially those of Ru and Rh, have been found to be able to efficiently catalyze this reaction [10]. Studies show that metal hydride complexes display excellent catalytic activity. This is understandable as reactions of metals with Hj easily generate metal hydride species. Computationally, hydrogenation of COj to formic acid is also among one of the most studied reactions involving COj [11,12]. 

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