Formic acid, from hydrogenation carbon dioxide

As shown in Figure 1, the next step in the catalytic cycle of carbon dioxide hydrogenation is either reductive elimination of formic acid from the transition-metal formate hydride complex or CT-bond metathesis between the transition-metal formate complex and dihydrogen molecule. In this section, we will discuss the reductive elimination process. Activation barriers and reaction energies for different reactions of this type are collected in Table 3. 

The catalytic addition of hydrogen on COj presents also an important starting point for the utilization of CO2 as a couple of technical important basic chemicals can be produced on this way (Scheme 7). The formation of formic acid from carbon dioxide and dihydrogen is an exothermic but strongly endergonic process under standard conditions. 

Jessop and co-workers have pointed out that homogeneous catalysis in supercritical fluids can offer high rates, improved selectivity, and elimination of mass-transfer problems.169 They have used a ruthenium phosphine catalyst to reduce supercritical carbon dioxide to formic acid using hydrogen.170 The reaction might be used to recycle waste carbon dioxide from combustion. It also avoids the use of poisonous carbon monoxide to make formic acid and its derivatives. There is no need for the usual solvent for such a reaction, because the excess carbon dioxide is the solvent. If the reaction is run in the presence of dimethy-lamine, dimethylformamide is obtained with 100% selectivity at 92-94% conversion.171 In this example, the ruthenium phosphine catalyst was supported on silica. Asymmetric catalytic hydrogenation of dehydroaminoacid derivatives (8.16) can be performed in carbon dioxide using ruthenium chiral phosphine catalysts.172... 

In an attempt to elucidate the mechanism of these peroxide-carbohydrate reactions, a carefril study of products, particularly of gaseous ones, from various model compoimds has been undertaken. - Thus, formaldehyde gives hydrogen, carbon dioxide, and formic acid by the steps shown in equations (5) to (8). Using deuterated hydrogen peroxide, Bonhoeffer... 

Hydrogen can be generated from formic acid-amine adducts at room temperature, and used directly in fuel cells [126,127]. Ruthenium metal carbonyl and hydrido carbonyl complexes exhibit a catalytic activity in the decarboxylation of formic acid. Hence, [Ru4(CO)i2H4] prepared from RUCI3 and formic acid can decompose formic acid to hydrogen and carbon dioxide [126]. 

When a photosynthetic organism is omitted, the addition of a photosensitizer is necessary. The methods use light energy to promote the transfer of an electron from a photosensitizer to NAD(P) via an electron transport reagent [6g]. Recently, carbon dioxide cvas reduced to formic acid with FDH from Saccharomyces cerevisiae in the presence of methylviologen (MV ) as a mediator, zinc tetrakis(4-methylpyridyl) porphyrin (ZnTMPyP) as a photosensitizer, and triethanolamine (TEOA) as a hydrogen source (Figure 8.8) [6h]. 

Carbon dioxide is known to readily insert into a metal-hydride bond to give a metal formate [57, 58] this forms the first step in insertion mechanisms of C02 hydrogenation (Scheme 17.2). Both this insertion step and the return path from the formate complex to the hydride, generating formic acid, have a number of possible variations. 

Transfer hydrogenations are typically equilibrium reactions however, when formic acid (49) is utilized as the hydrogen donor, carbon dioxide (50) is formed which escapes from the reaction mixture [61-64]. 

Hydrogen transfer reactions are highly selective and usually no side products are formed. However, a major problem is that such reactions are in redox equilibrium and high TOFs can often only be reached when the equilibria involved are shifted towards the product side. As stated above, this can be achieved by adding an excess of the hydrogen donor. (For a comparison, see Table 20.2, entry 8 and Table 20.7, entry 3, in which a 10-fold increase in TOF, from 6 to 60, can be observed for the reaction catalyzed by neodymium isopropoxide upon changing the amount of hydrogen donor from an equimolar amount to a solvent. Removal of the oxidation product by distillation also increases the reaction rate. When formic acid (49) is employed, the reduction is a truly irreversible reaction [82]. This acid is mainly used for the reduction of C-C double bonds. As the proton and the hydride are removed from the acid, carbon dioxide is formed, which leaves the reaction mixture. Typically, the reaction is performed in an azeotropic mixture of formic acid and triethylamine in the molar ratio 5 2 [83],... 

Photolytic. A carbon dioxide yield of 46.5% was achieved when aniline adsorbed on silica gel was irradiated with light (X >290 nm) for 17 h (Freitag et al., 1985). Products identified from the gas-phase reaction of ozone with aniline in synthetic air at 23 °C were nitrobenzene, formic acid, hydrogen peroxide, and a nitrated salt having the formula [CeHsNHsl NOs" (Atnagel and Himmelreich, 1976). A second-order rate constant of 6.0 x 10 " cmVmolecule-sec at 26 °C was reported for the vapor-phase reaction of aniline and OH radicals in air at room temperature (Atkinson, 1985). 

Photolytic. Methyl formate, formed from the irradiation of dimethyl ether in the presence of chlorine, degraded to carbon dioxide, water, and small amounts of formic acid. Continued irradiation degraded formic acid to carbon dioxide, water, and hydrogen chloride (Kallos and Tou, 1977 Good et al, 1999). 

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]. 

The efficient prodnction of formic acid (HCOOH) from carbon dioxide (CO2) has been reported. HCOOH was produced in a snpercritical mixture of CO2 and hydrogen containing a catalytic mthenium(ll)-phosphine complex. An initial rate of reaction of up to 1,400 moles of HCOOH per mole of catalyst per hour was achieved, giving a final yield of 3,700 moles HCOOH per mole of catalyst after several honrs. The rate of reaction in the snpercritical flnid was 18 times faster than that in tetrahydrofuran... 

The methyl ester has also been obtained by esterification of cyclopentanecarboxylic acid.8 The acid, in turn, has been prepared by the Favorskii rearrangement,6 7 9-11 by the reaction of cyclopentyl Grignard reagent with carbon dioxide,12 by the carbonylation of cyclopentyl alcohol with nickel carbonyl13 or with formic acid in the presence of sulfuric acid,14 and by the hydrogenation of cyclopentene-1-carboxylic acid prepared from ethyl cyclopentanone-2-carboxylate 15 or from cyclopentanone cyanohydrin.16... 

The same workers have more recently used a similar catalyst system to produce formic acid (formate ion) from hydrogen and carbon dioxide (160). The catalyst system is again Pd(diphos)2 and Et3N in benzene solution under 25 atm each of C02 and H2. The reaction is run at room temperature or higher. A small amount of water dramatically accelerates the rate of the reaction, and a mechanism is proposed to account for this effect. Control experiments are stated to rule out the initial reduction of C02 to CO by H2, followed by reaction with water to yield the formic acid. 

---