Surface reaction kinetics of the methanol synthesis on Cu/Zn-based catalysts

Introduction
Catalytic methanol synthesis is a relevant process in the chemical industry and its importance has increased in recent years, since it is an intermediate step in the production of liquid fuels and chemicals from sustainably derived hydrogen and CO/CO2. Despite being a well-developed process in the industry, the detailed reaction mechanism is still in debate, which is crucial for further optimization of the methanol synthesis. Although several models have been published, a reliable microkinetic model is still missing. In this contribution, a new multiscale microkinetic mechanism of the methanol synthesis is presented and validated over a wide range of experimental conditions. In this model, it is considered the methanol formation from both CO and CO2, and the water-gas shift reaction (WGSR). The nature of the different active sites and its dynamic changes are also taken into account.

Methodology
In the methanol synthesis from H2/CO/CO2, there are three main reactions: CO hydrogenation (CO + 2 H2 ⇄ CH3OH), CO2 hydrogenation (CO2 + 3 H2 ⇄ CH3OH + H2O), and WGSR (CO + H2O ⇄ CO2 + H2). In this model, a three-site assumption was made: Cu (211) (site a), Cu/ZnO (211) (site b), and a third site for hydrogen and water adsorption (site c). ... mehrThe surface reactions and their corresponding activation energies and entropy barriers were based on the DFT studies published by Studt et al. [1-2]. The values were slightly corrected to ensure thermodynamic consistency of the model. In order to estimate the fraction of each different active site in the catalyst surface, the method proposed by Kuld et al. [3] for quantifying the zinc coverage is implemented in the model. A sizable database of 687 experimental points was used for the model validation, which corresponds to a wide range of operating conditions.

Results and Discussion
A microkinetic model for the methanol synthesis with 25 reversible reactions based on DFT-derived data has been successfully developed. An extensive validation with own experiments (359) and experiments from the literature (328) [4-6] was performed, and the model predicted the experiments quantitatively, with a CO2 yield relative error of 3.3%, and a methanol yield relative error of 21.5%.
The method of the degree of rate control (DRC) [7] showed that formic acid (HCOOH*) hydrogenation on the Cu/Zn surface is the most sensitive step. The DRC also showed that formate (HCOO*) is the most sensitive intermediate, inhibiting CO hydrogenation and the reverse WGSR to some extent.
It is known that, for CO2-rich feeds, the reverse WGSR is significantly increased at higher temperature, which is normally credited to the favored thermodynamics, since the reverse WGSR is endothermic. However, according to the model, there is also a relevant kinetic reason: an increase in temperature reduces the coverage of formate, decreasing its inhibition of the reverse WGSR, which is then naturally faster. This suggests that a catalyst with a stronger bond to formate could be more effective in slowing down the non-desirable reverse WGSR, increasing methanol selectivity.