CO2 to formic acid

One attractive approach to photochemical conversion and storage of solar energy is photofixation of carbon dioxide to C-1 organic compounds (formic acid, formaldehyde, methanol, and methane). Photoreduction of CO2 to formic acid and formaldehyde has been demonstrated by using n-type Bi2S3 and CdS semiconductor powders (particle size 300 00 mesh) as photoelectrocatalysts in emulsions... 

The cationic complex [Rh(nbd)(PMe2Ph)3](BF4) catalyzes the oxygenation of several ethers (Eq. 7) by O2/CO2 mixtures with moderate turnovers (1,000 in 8 days at 60 °C for y-butyrolactone) [36], accompanied by the reduction of CO2 to formic acid. In the absence of CO2, 2-hydroperoxidetetrahydrofuran is the major product. 

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. 

Of considerable importance are the reports on the photochemical fixation of CO2 and the photoreduction of N2 that have appeared this year. In many respects, these reduction processes offer more potential for the storage of solar energy than does the photoreduction of water to H2. Several developments in this area have been noted recently. Thus, it has been reported that certain zinc porphyrins can fix CO2 upon irradiation with visible light. It has been claimed that reduced will reduce CO2 to formic acid with an overall quantum... 

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.

Iwakura et al. reduced CO2 to formic acid at a palladized Pd/CO2 interface in a gas compartment without any protic solvent. CO2 is reduced by atomic hydrogen permeating through the membrane, which is produced by electrolysis of aqueous KOH solution at the back side of the Pd membrane. The reaction proceeds chemically in gas solid surface, not electrochemically. The current density of the electrolysis of water was 10 mA cm and the faradaic efficiency of HCOOH formation corresponded to 10 to 20%. 

The solubility of carbon dioxide in aqueous and non-aqueous solutions depends on its partial pressure (via Henry s law), on temperature (according to its enthalpy of solution) and on acid-base reactions within the solution. In aqueous solutions, the equilibria forming HCO3 and CO3 depend on pH and ionic strength the presence of metal ions which form insoluble carbonates may also be a factor. Some speculation is made about reactions in nonaqueous solutions, and how thermodynamic data may be applied to reduction of CO2 to formic acid, formaldehyde, or methanol by heterogenous catalysis, photoreduction, or electrochemical reduction. 

Since the overall reaction is a two-electron reduction, it is desirable for the oxidized metal complex to undergo a reversible two-electron reduction as shown in Step 4 of Scheme 1. At pH 7 the standard potential for the reduction of CO2 to formic acid is -0.61 vs NHE ( ) or -0.85 vs SCE. Since it would be desirable if the reduction of CO2 could ultimately be carried out in aqueous... 

Since that time there have been many reports of intramolecular hydridic-protonic bonds [78-86]. Recently an intermediate with an intramolecular RuH—HN interaction has been implicated in the catalytic asymmetric reduction of ketones [87] and another, in the reduction of CO2 to formic acid [83]. In the last process, the RuH—HN bond is proposed to form via the heterolytic splitting of dihydrogen (see Scheme 4, Section 1.9). 

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. 

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