Gas-phase carbonylation of methanol to dimethyl carbonate on chloride-free Cu-precipitated zeolite Y at normal pressure

Chloride-free Cu/zeolite Y catalysts with Cu loading of 2–14% were prepared by precipitation from aqueous copper(II) acetate solutions and inert activation with an Ar flow at 700–750 °C for 15 h. This inert activation resulted in a considerable activity of the catalyst for the oxidative carbonylation of methanol (MeOH) to dimethyl carbonate (DMC) under normal pressure at 140–160 °C at 10–12 wt% Cu loading. Space-time yields (STY) of DMC up to 100 gDMC l−1Cat h−1 were achieved with a feed composed of 36% MeOH, 48% CO, 6% O2, and balance He at a gaseous hourly space velocity (GHSV) of 3000 h−1. A threshold of copper loading (5–6 wt%) was found to exist before catalysts became active. This is associated with the preferential location of copper at ion-exchange positions of the zeolite structure Y not accessible for the reactants. After saturation of these sites, the placement of copper ions within the supercage led to active catalysts. Characterization of samples at various stages of preparation by N2 adsorption, XRD, XPS, ESR, 27Al-MAS-NMR, and TPR analysis revealed that the solid-state ion exchange during inert activation is accompanied by reduction of Cu2+ to Cu+. Copper ions exert a stabilizing effect on the crystallinity of the zeolite (in situ XRD, 27Al-MAS-NMR). No crystalline metallic copper, cuprous oxide, or cupric oxide were formed (XRD), but melting occurred at 750 °C for catalysts with 14% copper loading, resulting in the formation of a glassy amorphous copper silicate/aluminate phase. The latter effect can be prevented by applying lower activation temperatures. The catalysts were prepared without using chloride, and the reaction did not require co-feeding of HCl for maintaining activity, as is needed for CuCl/zeolite catalyst formulations.