1. Generation of dihydrogen via electrolysis of water using electric energy from nuclear power plants coupled with CO2 reduction (the synthesis of methanol is exoergonic);
2. Coupling water oxidation with CO2 reduction using solar energy.
In this case, a new route can be developed for recycling carbon through the conversion of captured CO2 into methanol, which may find a practical application during the transition period to the 'H2-economy'. In fact, such methanol could be used either as a fuel or as a starting material in the chemical industry.
NADH NAD+ NADH
Interestingly, CO2 can be converted into methanol at room temperature in an aqueous environment by using a cascade of reactions catalysed by enzymes under electron transfer conditions (see Eq. 7.6).
CO2 is first reduced to HCOOH, which is converted into formaldehyde (CH2O), and eventually into methanol (Obert and Dave, 1999; Jiang et al., 2003). Such research is still
There is industrial interest in more selective and efficient technologies with respect to the use of CO2 for the direct synthesis of ethanol (C2) or higher (Cn) alcohols. Efficient catalysts for ethanol formation from H2-CO2 have been developed by Takagawa et al. (1998), while higher alcohols have been produced by Kieffer et al. (1997), but with a yield too low for practical application.
The catalytic hydrogenation of CO2 has also been applied to the selective synthesis of C5+ olefins. Iron carbide (Fe5C2) has been patented by the Exxon Corporation (Fiato et al., 1992; Choi et al., 2000). The C2-C4 olefin synthesis is made using a Fe-K/alumina catalyst with a selectivity of about 44%, at a CO2 conversion of 68% under 2 MPa at 400°C. Alternatively, and more efficiently, methanol produced from H2-CO2 can be selectively converted using solid acid catalysts. Inui (1996) has shown that in a two-stage reactor, C2-C4 olefins are produced with >90% selectivity using the same Cu-ZnO catalyst as in the synthesis of methanol in the first stage and a solid acid catalyst in the second stage.
The photochemical conversion of H2O-CO2 into fuels is of interest if solar energy can be used effectively. Several authors (Inoue et al., 1979; Mackor et al., 1987; Arakawa, 2003) have reported that HCOOH, HCHO and CH3OH are produced in the reduction of CO2 with H2O under solar irradiation of an aqueous suspension of a variety of semicon ductors such as TiO2 and SrTiO3. The barrier to this application is the low quantum yield that is achieved (only 0.1%). In order to find a practical application new selective photocatalysts must be developed that are characterized by a light efficiency higher than 2% (Halmann and Steinberg, 1999). This efficiency may be improved by using sacrificial hole traps or electron donors, such as n-propanol, tertiary amines or eth-ylenediaminetetraacetic acid (EDTA), but this approach is not economic. An interesting photocatalytic system for the reduction of CO2 with H2O has been reported by Anpo et al. (1997), who have described a selectivity of 30% for ethanol production using a Ti-modified mesoporous silica catalyst, compared with just 1.4% over bulk TiO2.
Overall though, the electrochemical reduction of CO2 to hydrocarbons is not yet practicable given the low selectivity and the cost, and demands further research (Fujishima, 1998).
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