Correlation between synthesis pH, structure and Cu/MgO/Al2O3 heterogeneous catalyst activity and selectivity in CO2 hydrogenation to methanol

Abstract

Cu/MgO/Al2O3 catalysts were synthesized by a co-precipitation preparation method, varying pH, while their catalytic performance was evaluated in a continuous-flow packed-bed reactor to relate the catalytic material activity, selectivity and stability with its surface structure. The major reaction products were methanol and water with CO as a sole process by-product. Alkaline synthesis conditions (pH = 8) positively affected CO2 conversion and CH3OH yield (< 250 °C), as CuO and Cu2O phases were present, since Cu2+ was only partially reduced. The hydrogen consumption variation also indicated that these different Cu species changed the interaction with magnesium and alumina. An increase in active basic sites was ascribed to the formation of spinel CuAl2O4 and MgAl2O4. Alongside with H2 TPD, XRD and SEM data suggested that a higher precursor solution pH enhanced the dispersion of CuO, enlarged Cu and BET areas, and improved the adsorption of H2. The XPS profiles of fresh, pre-treated and spent heterogeneous catalysts indicated that the dispersed copper moieties were reduced (to Cu0 or Cu+) during CO2 hydrogenation. CO2 turnover was consistent with metallic atom distribution, derived from N2O measurements. A maximum TOF of 13.2 × 10–4 s–1 was obtained over the catalyst, prepared at the pH of 8, whereas a minimum TOF of 5.5 × 10–4 s–1 corresponded to the pH of 5. A volcano plot-type shape of the productivity and basicity versus pH was observed as well, as the highest methanol productivity was attained using the precipitation pH of 8 (200 °C operation).