Nature uses CO2 in thousands of reactions for the synthesis of a large number of products. These reactions can be categorized into two major classes:
• The carboxylation of substrates, either inorganic or organic. Note that the 'weathering of silicates' is a special case of carboxylation that fixes CO2 into long-living compounds such as inorganic carbonates (see Chapter 6).
The reduction reactions, in which CO2 is reduced to other C (CO, CH3OH) or Cn molecules (block D in Fig. 7.1) that can be used as fuels, use dihydrogen as co-reagent. Again, such CO2 utilization may contribute to an overall reduction in CO2 emissions (Song, 2002).
The carboxylation reactions, which occur in mild conditions, are exoergonic (blocks B and C in Fig. 7.1). In such reactions the entire CO2 moiety is incorporated into molecular or polymeric compounds containing moieties like COOR (carboxylates, esters, lactones), N-COOR (carbamates) or NCO (isocyanates or ureas) and ROCOOR (carbonates).
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