Reduction of CO2 to Formic Acid and its Derivatives

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. 

In 1994, Noyori and coworkers discovered that RuX2(PMe3)4 (X = H or Cl) are highly active catalysts for the hydrogenation of CO2 to formic acid in a supercritical mixture of CO2 (SCCO2 Tc = 31 °C, Pc = 72.9 atm), H2 and NEt3. A turnover number (TON) of 7200 and a turnover frequency (TOF) of 1400 h at 50 °C were achieved (Eq. 11.69) [123aj. 

Noyori s report had a major impact on research into the hydrogenation of CO2, and many papers and reviews have subsequently been published on the subject. It should be noted that a trace amount of water or alcohol accelerates the reaction. This water-effecf - which was first reported by Inoue and coworkers [124] - is often observed in the catalytic reduction of CO2 with H2. 

The hydrogenation of CO2 in water is an important topic in both industrial and environmental chemistries. Leitner and coworkers reported that RhCl(tppts)3 (tppts = (C6H4-m-S03 Na )3P) is an efficient catalyst for the hydrogenation of CO2 in water to form formate, with an initial TOF of 7260 h at 81 °C and 1365 h at 23 °C in the presence of HNMe2 under 40 atm (CO2/H2 = 1/1) [122b,c], 

Methyl formate has been proposed to be a versatile intermediate in the synthesis of oxygenated base chemicals [128, 129]. One of the most interesting synthetic routes to methyl formate is the reduction of CO2 with hydrogen in the presence of methanol. This reaction is exothermic, and has been referred to as the hydrocondensation of CO2 with methanol. Since the first report of a successful transition metal-catalyzed reaction by a Russian group [130], several other reports have been published. However, the catalytic activity (i.e., the TOF) has not been satisfactory. 

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