The Reaction Mechanism of Methanol Synthesis

Gas feeds for industrial methanol synthesis usually contain both CO and C02. Which carbon oxide serves as the primary source for methanol formation has been arguably the most important question pertaining to the reaction mechanism. Early work by Klier and coworkers assumed that CO was the primary source, and that the active site consisted of Cu+ species dissolved in ZnO. However, Klier s model 

In situ spectroscopic studies have identified a variety of species, such as formate, dioxymethylene, carbonate, and methoxide, to coexist under methanol synthesis conditions on Cu/ZnO-based catalysts [22, 23], Fourier transform infrared spectroscopy studies of CuZn-based catalysts under H2/C02 identified the presence of formate bound to both Cu and ZnO, whereas methoxide was found on ZnO only. Carbonates were found to form via C02 adsorption on ZnO [24] and partially oxidized Cu [23], and were quickly converted into formate via Cu-activated hydrogen. Upon exposure to CO mixtures, only zinc-bound formate was observed [22], The hydrogenation of these formates to methoxide is thought to be rate determining in methanol synthesis. 

5 Methanol Synthesis from CO2 Thermodynamic and Kinetic Considerations 

When comparing the intrinsic methanol synthesis activity of both catalysts (turnover frequency [TOF] per surface Cu site measured with N20-reactive frontal chromatography) at the space velocity of 20 mmol/gcatmin, the HT catalyst (0.54 min-1) appears twice as active as the MA catalyst (0.24 min-1). Although both catalysts are exposed to nearly the same reactant concentration, the MA catalyst operates at 11% CO2 

This unique micro structure can be described as an intermediate stage between a supported catalyst and a bulk metallic sponge or skeletal Raney-type catalyst. It enables a reasonably high dispersion of Cu and exposure of many Cu-ZnO interfaces at a high total Cu content. The specific Cu surface area (SACu) of methanol catalysts can be determined by reactive N20 titration [59, 60], which causes surface oxidation of the Cu particles and allows calculation of SAcu from the amount of evolved N2. The SACu of state-of-the-art methanol synthesis catalysts measured by this method 

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The reaction mechanism of methanol synthesis is complex since two processes are involved and coupled. Formally, the reaction can be written as the hydrogenation of CO by the overall reaction ... 

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