The Methanol-to-Olefins Reactions: the Influence of Water Studied by in situ Spectroscopy
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Operando UV-Vis has been used and analyzed to elaborate on the influence of water on the methanol-to-olefins reaction catalyzed by H-SAPO-34. It was shown that methanol concentration, not influenced by the nature of diluent, influences the catalytic performance. A lower methanol concentration gave rise to a higher total active time, a higher olefin yield and improved light olefin selectivity. Deactivation rates decreased and the contribution of active species was larger. This could be explained by an increased diffusion of methanol into the catalyst particles, giving rise to a higher amount of total species in the spent catalyst. When the diluent was changed from only helium to helium + water (at different ratios) the effect of water was observed. Water was shown to have no influence on the product selectivity and no clear effect on the active period or reaction time. However, the contribution of active species and the total olefin yield increased. The amount of deactivated species present in the spent catalysts was similar for all methanol / water ratios. Using NNMF analysis, it became clear water influences the reactivity of the reagent / product feed with the active species. A propylene feeding experiment confirms this. The lower reactivity explains a decreased rate of formation for deactivating species and the higher olefin yield. The slower formation of active and deactivating species together with the higher contribution of active species at their maxima implies the reactivity of the reagent / product feed is lowered due to competitive adsorption with water.