Decoding the Role of Isolated Ga+ in PdGa@MFI Catalyst Promoting a Direct CO2 Hydrogenation Path to DME.

This work presents a strategy to control not only the distance and proximity of active sites at the atomic or nanoscale but also the nature of sites in zeolite-based catalysts, promoting the coupling rate of surface intermediate species and accordingly the formation rate of dimethyl ether (DME) by a direct CO2 hydrogenation path. We use a one-pot synthesis strategy and a thermal-induced detachment process of framework elements, such as Ga3+ ions, to stabilize PdGa alloys and Ga+ sites in close proximity to Brønsted acid sites under reductive conditions. Using this strategy, a production of oxygenates of up to 42,864 gMeOH+DME·kgPd-1·h-1 at 45 bar, 260 °C, and WHSV = 15,000 mL·gcat-1·h-1, is obtained with 80% selectivity to oxygenated (19% methanol/61% DME), outperforming the most active Pd-based CO2 hydrogenation catalysts in the literature. Time-resolved kinetic studies, in situ X-ray adsorption, and in situ and operando IR offer strong proof of the key role of isolated Ga+ Lewis acid sites in close proximity to Brønsted acid sites in stabilizing monoformate intermediate species and facilitating the direct production of DME. Finally, this work highlights the key role of the zeolite in metal confinement, conferring excellent stability, oxidation resistance, hydrophilicity, and close proximity of active sites together with the stabilization of low-coordinated Lewis acid sites.