The roles of CO and CO2 in high pressure methanol synthesis over Cu-based catalysts

Abstract

The roles of CO and CO2 in Cu-catalyzed methanol synthesis from syngas were evaluated in experiments with gas switching between CO/H2 and CO2/H2 feeds and between a CO2/CO/H2 feed and a corresponding CO-free CO2/inert/H2 feed. Switching between CO/Ar/H2 (3/29/68) and CO2/N2/H2 (3/29/68) for Cu/Al2O3 showed that the rate of methanol synthesis on Cu is more than an order of magnitude higher from CO2 compared to CO. Experiments switching between CO2/CO/H2 and CO2/inert/H2 showed that at low conversion conditions with negligible product formation, CO is purely inhibiting for Raney Cu and for a range of supported (on SiO2, TiO2, Al2O3 and ZnO) Cu-catalysts including an industrial-type Cu/ZnO/Al2O3 catalyst. Given the generality across Cu-based samples the mechanism of this inhibition is most likely competitive adsorption of CO on the Cu surface. However, as conversion is increased by lowering the gas space velocity there is a sharp transition from an inhibiting to a beneficial role of CO relative to a CO-free feed. With increasing conversion more water is formed, and as water is a far stronger inhibitor to Cu-based catalysts than CO, the beneficial effect of CO arises from the removal of water through the water–gas shift reaction. At low conversion the methanol synthesis rate is thus highest for a CO-free feed that minimizes CO inhibition, whereas the rate at high conversion is optimal with a CO-rich syngas that minimizes water inhibition. The transition between these two types of behavior occurs around the conversion range leading to 1 mol% of methanol in the effluent gas. Hence, CO has a beneficial role at commercial high conversion conditions. The ZnO support exerts a strong, beneficial support effect at low conversion conditions, where the strong reductant CO has a purely negative effect. This could suggest that reduced Zn-sites (oxygen vacancies in ZnO or Cu-Zn surface alloy sites), whose concentration are expected to depend on the reductive potential of the atmosphere, are not critical to the support effect from ZnO. At both industrial conditions (523 K, 50 bar), mild conditions (448 K, atm. pressure) and in a nominally oxidizing gas (498 K, 20 bar with CO2 > H2) the addition of CO to the feed is detrimental to the activity of Cu/ZnO/Al2O3 at low conversion conditions. This supports that CO plays no beneficial role by facilitating ZnO-reduction and possibly that Zn alloyed into the Cu surface is unimportant for catalytic activity.