An insight into possibility of chemical reaction between dense carbon dioxide and methanol

Possibility of chemical reaction between components of binary mixture of CO2-MeOH was demonstrated by analyzing in situ IR spectroscopy data. The kinetics of this reaction was studied in a wide range of temperatures under isochoric conditions showing that the process is considerably slow. Dependences of product and intermediates yields on temperature were obtained by means of qualitative analysis of in situ IR spectroscopy data as well. Temperature growth leads to an increase in the overall reaction speed, although its influence on various process stages is not straightforward. It has been found that the reaction between MeOH and CO2 is a two-stage process. The intermediate formed at the first stage is monomethyl carbonate which then reacts with methanol producing dimethyl carbonate and water. We believe that this synthesized water further partly reacts with CO2 producing carbonic acid which is responsible for the increased acidity of CO2-MeOH mixtures. Quantum chemical calculations reveal that the first stage is only possible when some sort of catalyst is involved. We assume that since all experiments with supercritical carbon dioxide are performed in a stainless steel vessel, the inner surface of the latter might serve as a catalyst. Fe(OH)2 was chosen as a model compound representing the type of acidic catalytic centre on the surface of stainless steel that might promote interaction between CO2 and methanol. With Fe(OH)2 involved, the potential barrier of the reaction first stage dramatically decreases, from 192 kJ/mol to 8.5 kJ/mol. And the energy of the complex of reaction products with Fe(OH)2 is lower than the energy of the complex of Fe(OH)2 with initial components, which confirms the hypothesis of vessel walls being able to catalyze a reaction between CO2 and methanol.