CO2 can advantageously substitute phosgene in the synthesis of carbonates, carbamates, isocyanates, polycarbonates and polyure-thanes. The following scheme (Scheme 7.2) shows the network of reactions based on CO2 that correlates the chemicals mentioned previously. The carboxylation of epoxides and the aminolysis of carbonates are already well established. Other processes are at different states of development (Aresta et al., 2003c). Carbonates are good target molecules due to their favourable thermodynamics and their widespread use as solvents or reagents:
COCI2 + 2ROH + 2NaOH ® (RO)2CO + 2NaCl + 2H2O (7.10)
Equations 7.10-7.12 represent the first, second and third generation processes, respectively, for the synthesis of dimethylcarbonate (DMC). The utilization of CO2 (Eq. 7.12) is a winning strategy as it is a clean process that also complies with the atom economy principle (for R = CH3, the atom efficiency is 54.5% for Eq. 7.10, and 80% for Eqs 7.11 and 7.12). Water is the only co-product, and the reaction is selective (100%). For ther-modynamic isues, the reaction is slightly shifted to the left. New catalysts are needed and new reactors for shifting the equilibrium to the right (Aresta and Dibenedetto, 2002; Aresta and Dibenedetto, 2003; Aresta et al., 2003a,b, 2005a; Ballivet-Tkatchenko and Sorokina, 2003; Ricci, 2003).
The production of polycarbonates by reacting epoxides and CO2 is well understood and industrially exploited. Only a limited number of epoxides have been used so far (essentially propene oxide and cyclohexene oxide) whereas many others could be used to generate materials with new properties (Beckman et al., 1997). Polycarbonates find a large utilization in
several industrial sectors, with an expanding market (Burridge, 2002a).
Urea could find a new use as a chemical building block for the synthesis of urethanes and carbonates, with a significant utilization of CO2 as a result (Eqs 7.13 and 7.14). Ammonia can be recycled and converted into urea:
Similarly, carbonates can be used as starting materials for the synthesis of carbamates by aminolysis with primary or secondary amines (Aresta et al., 2004).
Other applications with large potential include the production of polyurethanes from isocyanates (Burridge, 2002b,c) and polyesters (co-polymers CO2-ethene or propene), the polymers with the largest market.
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