Methanol Synthesis

Abstract

Methanol, like ammonia, is one of the key industrial chemicals produced by heterogeneous catalysis. As with the original ammonia catalyst (Fe/K/Al₂O₃), so with methanol, the original methanol synthesis catalyst, ZnO, was discovered by Alwin Mittasch. This was translated into an industrial process in which methanol was produced from CO/H₂ at 400 °C and 200 atm. Again, as with the ammonia catalyst where the final catalyst which is currently used was achieved only after exhaustive screening of putative “promoters”, so with methanol, exhaustive screening of additives was undertaken to promote the activity of the ZnO. Early successful promoters were Al₂O₃ and Cr₂O₃ which enhanced the stability of the ZnO but not its activity. The addition of CuO was found to increase the activity of the ZnO but the catalyst so produced was short lived. Current methanol synthesis catalysts are fundamentally Cu/ZnO/Al₂O₃, having high CuO contents of ~60 % with ZnO ~ 30 % and Al₂O₃ ~ 10 %. Far from promoting the activity of the ZnO by incorporation of CuO, the active component of these Cu/ZnO/Al₂O₃ catalysts is Cu metal with the ZnO simply being involved as the preferred support. Other supports for the Cu metal, e.g. Al₂O₃, MgO, MnO, Cr₂O₃, ZrO₂ and even SiO₂ can also be used. In all of these catalysts the activity scales with the Cu metal area. The original feed has now changed from CO/H₂ to CO/CO₂/H₂ (10:10:80), radiolabelling studies having provided the unlikely discovery that it is the CO₂ molecule which is hydrogenated to methanol; the CO molecule acts as a reducing agent. The CO₂ is transformed to methanol on the Cu through the intermediacy of an adsorbed formate species. These Cu/ZnO/Al₂O₃ catalysts now operate at ~230° and between 50 and 100 atm. This important step change in the activity of methanol synthesis has resulted in a significant reduction in the energy required to produce methanol. The “step change” however has been incremental. It has been obtained on the basis of fundamental knowledge provided by a combination of surface science techniques, e.g. LEED, scanning tunnelling microscope, TPD, temperature programmed reaction spectroscopy, combined with catalytic mechanistic studies, including radiolabelling studies and chemisorption studies including reactive chemisorption studies, e.g. N₂O reactive frontal chromatography.