Methanol‐Assisted Autocatalysis in Catalytic Methanol Synthesis

Abstract

Catalytic methanol synthesis is one of the major processes in the chemical industry and may grow in importance, as methanol produced from CO2 and sustainably derived H2 are envisioned to play an important role as energy carriers in a future low‐CO2‐emission society. However, despite the widespread use, the reaction mechanism and the nature of the active sites are not fully understood. Here we report that methanol synthesis at commercially applied conditions using the industrial Cu/ZnO/Al2O3 catalyst is dominated by a methanol‐assisted autocatalytic reaction mechanism. We propose that the presence of methanol enables the hydrogenation of surface formate via methyl formate. Autocatalytic acceleration of the reaction is also observed for Cu supported on SiO2 although with low absolute activity, but not for Cu/Al2O3 catalysts. The results illustrate an important example of autocatalysis in heterogeneous catalysis and pave the way for further understanding, improvements, and process optimization of industrial methanol synthesis.

Evolution of pyrolysis and gasification as waste to energy tools for low carbon economy

“…Syngas to methanol synthesis takes place over copper oxide, zinc oxide, or chromium oxide‐based catalysts. Although the mentioned technology is around one century old, the catalytic pathways of the used reaction have been understood only in recent years (Laudenschleger et al, 2020; Thrane et al, 2020). Methanol synthesis catalysts also are sensitive to syngas pollutants, especially sulfur‐organic and chlorine‐organic compounds (Quinn et al, 2004).…”

Probing the methanol-assisted autocatalytic formation of methanol over Cu/ZnO/Al2O3 by high-pressure methanol and methyl formate pulses

“…10,11 Recently, the same group managed to confirm their findings also in the gas phase by continuously dosing methanol as promoter at low temperatures. 12 In addition, also the Christensen group 13 reported on the autocatalytic influence of methanol on high-conversion gas-phase methanol synthesis. By lowering the space velocity causing high conversion over Cu/ZnO/Al 2 O 3 , it was found that the rate of methanol synthesis increased exponentially in agreement with other studies reporting on an increased methanol formation rate with increasing conversion.…”

“…Additionally, temperatureprogrammed reduction (TPR) experiments predicted methyl formate to be a probable intermediate of the assumed pathway. According to the Tsubaki group 10,11 and Christensen group 13 methyl formate ester is formed from methanol and formate (HCOO*) (eqn (1)):…”

“…Additionally, temperature-programmed reduction (TPR) experiments predicted methyl formate to be a probable intermediate of the assumed pathway. According to the Tsubaki group 10,11 and Christensen group 13 methyl formate ester is formed from methanol and formate (HCOO*) (eqn (1)):CH 3 OH + HCOO*/HCOOH ⇌ HCOOCH 3 + OH*/H 2 OIn the following hydrogenation step the methyl formate ester is hydrogenated forming two molecules of methanol (eqn (2)):HCOOCH 3 + 2H 2 ⇌ 2CH 3 OHIn this way, the rate of methanol synthesis is accelerated, because the hydrogenation of methyl formate is faster than the hydrogenation of formate. This effect was only observed in CO-containing syngas, because CO removes water by the water gas shift reaction (WGSR), allowing the beneficial effect of methanol to dominate over the inhibiting effect of water.…”

“…Fig.5The proposed methanol-assisted autocatalytic cycle for high pressure-methanol synthesis over Cu/ZnO/Al 2 O 3 catalysts including the methyl formate ester as intermediate according to Shi et al11 and Thrane et al13 …”