In situdiffuse reflectance infrared Fourier transform spectroscopy of MCM-41 mesoporous silica: mechanistic consideration on the chemical fixation of CO2 with N,N′-dimethylethylenediamine to 1,3-dimethyl-2-imidazolidinone

In situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy has been applied to evaluate the thermally activated MCM-41 mesoporous silica as a catalyst for CO2 chemical fixation using N,N′-dimethylethylenediamine (DMEDA), and to elucidate the adsorption states of DMEDA, CO2 and H2O on the activated silica surface at room temperature or 300 °C and at a pressure close to the atmospheric one. The thermal activation at 500 °C under a flow of dry air led to a slight loss of the hydrogen-bonded vicinal and a part of geminal silanols as well as physically adsorbed water, whereas the free isolated and most of the geminal silanols were not influenced, possibly due to the absence of neighboring silanols for condensation to form water and the unfavorable dehydration of geminal silanols. Respective adsorptions of Lewis acidic CO2 and basic DMEDA revealed that the free silanols are rather basic, whereas the hydrogen-bonded silanols act as Brønsted acid sites. Treatment of the activated silica with a CO2 flow containing DMEDA at 300 °C revealed that the diamine does not form the corresponding DMEDA–CO2 adducts such as carbamic acids and carbamic acid salts under sufficiently CO2-diluted conditions. In addition, DMEDA was virtually unable to interact with the silanols under these conditions, due to the severe constraints imposed on these silanols by the surrounding dense CO2 molecules. Based on these results, two plausible mechanisms were proposed for the CO2 chemical fixation with DMEDA to 1,3-dimethyl-2-imidazolidinone (DMI): one involves the reaction of free DMEDA with the adsorbed CO2 interacting with the surface silanols, while the other involves the formation of a cyclic carbamic acid salt which subsequently undergoes dehydration over the silica to give DMI. Finally, the spectroscopy revealed that H2O, a byproduct of the DMI formation, is smoothly extracted from the surface at 300 °C under gaseous CO2 flow, and does not influence the parent surface structure of the silica under the conditions applied.