F. Gasc, S. Thiebaud-Roux, R. Valentin, Z. Mouloungui
Carbon dioxide is one of the greenhouse gases (together with CH4, NOx and CFCs) responsible for global warming. By 2005, CO2 concentration in the atmosphere had increased from the 280ppm level, thought to be present before the Industrial Revolution, to 379 ppm.
Over the last 10 years (1995–2005) it has increased substantially, at a mean rate of
1.9ppm.year
−1
[1]. Several studies on the synthesis of dimethyl carbonate from alcohol and
CO2 have shown the major problem to be the rate of conversion of methanol, due to the thermodynamic limitations of the reaction and deactivation of the homogeneous catalysts (e.g. tincomplexes: distannoxanes) [2,3] and heterogeneous catalysts (e.g. zirconium oxide: ZrO2)
[4]. Fujita et al. [5] reported a study of the reaction conditions required for the synthesis of
DMC and proposed a reaction mechanism in which DMC and dimethyl ether (DME) were produced by parallel pathways.
We investigated, here [6], carbon dioxide sequestration by synthesizing diethyl carbonate
(DEC) from ethanol and CO2 under supercritical conditions in the presence of inorganic basic catalysts and co-reagent like ethyl iodide or a concentrated strong acid.
We showed that this reaction occurred in two steps. The first step involves the formation of an intermediate, potassium ethyl carbonate (PEC) [7], which was characterized by infrared spectrometry and 13C NMR. This hybrid carbonate can be used as nucleophilic agent in for the second step of the reaction.
The second step is the O-alkylation of PEC by ethyl iodide. Its study demonstrated that it is the limiting step. During this step etherification of ethanolate (produced by the reaction of ethanol with the base K2CO3) and the EtI dehydrohalogenation reaction, according to an E2 elimination mechanism are identified to be in competition with the O-alkylation reaction of
PEC (SN2).
We showed that the reaction carried out in concentrated alcoholic solution of a strong acid as a co-reactant was a way of sequestration of CO2 into potassium bicarbonate.
Finally the one pot procedure involving two consecutive steps was the more efficient procedure to synthesize DEC. Indeed, the O-alkylation of the hybrid organic-inorganic carbonate is favored when the reaction occurs in the swelling solvent supercritical carbon dioxide. The use of a phase transfer agent increased the production yield of DEC without more side reactions.
The supercritical reactor developed in this study allowed the characterization of the reaction intermediate and DEC by the FTIR online analysis of these organic carbonates compounds.
This work constitutes advance in CO2 capture in two forms: an organic carbonate and an organic–inorganic hybrid that could be reused without purification for the synthesis of organic carbonates.
[1] B. Metz, IPCC special report. IPCC, 2006.
[2] S. Sakai, T. Fujinami, T. Yamada, S. Furusawa, Nippon Kagaku Kaishi (1975) 1789–1794.
[3] D. Ballivet-Tkatchenko, T. Jerphagnon, R. Ligabue, L. Plasseraud, D. Poinsot, Applied Catalysis A: General
255 (2003) 93–99.
[4] K. Tomishige, Y. Ikeda, T. Sakaihori, K. Fujimoto, Journal of Catalysis 192 (2000) 355–362.
[5] S. Fujita, B.M. Bhanage, M. Arai, Y. Ikushima, Green Chemistry 3 (2001) 87–91.
[6] F. Gasc, S. Thiebaud-Roux, Z. Mouloungui , J. of Supercritical Fluids 50 (2009) 46–53
[7] R. Howe, Elsevier Publishing Co, New York, 1965, 268