Oxide clusters as source of the third oxygen atom for the formation of carbonates in alkaline earth dehydrated zeolites

摘要

In our paper, we show that carbonates can be formed with almost no energetic barrier from CO_2 and metal-oxide binuclear MOXM species (M = Mg, Ca, Sr, Ba, with X = 1-4, depending on the cation) in alkaline earth zeolites, mordenite (MOR) and phillipsite (PHI), on the basis of quantum mechanical density functional theory (DFT) calculations at both isolated cluster and 3D periodic levels. The participation of MOXM species (X = 1 and 3) explains the source of the third O atom in CO_3 species in dehydrated zeolites, on the basis of a good agreement between the calculated and experimental positions of the asymmetric and symmetric CO_3 vibration bands, of the ratio of their intensities, and of the weak dependence versus the cation and framework type. The reaction of formation of dimethylcarbonate from CaCO_3Ca in the 8-membered (8R) ring of MOR and methanol has also been considered, suggesting the carbonate activity as the source of CO_2 at elevated temperatures.