In situ diffuse 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.