Microkinetic modeling of the methanol synthesis on Cu/Zn-based catalysts

Introduction
Methanol is a key intermediate for numerous process chains and a viable high-value energy carrier itself [1]. Considering the ever-increasing interest in sustainable industrial processes, its synthesis from rich-CO2 syngas is unquestionably in the focus of academia and industry. Although the methanol synthesis from CO-rich syngas is a widespread and proficiently developed process in the industry, the detailed reaction mechanism is still in debate, being essential to further optimize the process efficiency. Different models have been proposed, but a reliable microkinetic model is still missing, model which should include many aspects like e.g. the methanol formation from CO and CO2, the water-gas shift reaction (WGSR), the nature of the different active sites, and its validation over a wide range of operating conditions. In this contribution, a new multiscale microkinetic mechanism of the methanol synthesis including the WGSR is presented and validated over a wide range of experiments.

Methodology
In the methanol synthesis from H2/CO/CO2, three reactions principally occur: CO hydrogenation (CO + 2 H2 ⇄ CH3OH), CO2 hydrogenation (CO2 + 3 H2 ⇄ CH3OH + H2O), and WGSR (CO + H2O ⇄ CO2 + H2). ... mehrIn this study, three active sites were considered: Cu (211) (site a), Cu/ZnO (211) (site b) and a third site for hydrogen and water adsorption (site c). The facet 211 was chosen for the modeling, since its surface is more active for the methanol synthesis [3-4]. Five reaction paths were considered: CO hydrogenation on site (a) [2], CO2 hydrogenation on sites (a) and (b) [2], and the water-assisted carboxyl mechanism during WGSR on sites (a) and (b) [3]. Activation energies and entropy barriers were calculated from DFT studies proposed by Studt et al. [2-3], and were used in the calculation of surface reaction rates. The thermodynamic consistency of the model was ensured using a modified
version of a method proposed in the literature [5], whereas experimental data from our group and from the literature [6] were used for the validation.

Results and Discussion
A microkinetic model with 25 reversible reactions has been successfully developed, based on first principles DFT-derived kinetic data for methanol synthesis and WGSR on Cu (211) and Cu/Zn (211). Performing an extensive validation combining own experiments (371) and experiments from the literature (140), the model predicts well the process over a wide range of relevant operating conditions, with a methanol yield relative error around 14%. Difficulties in the predictions were only found for the combination of low temperature (T ≤ 220 °C) and high CO2/COx feed concentration (≥ 80%).
The most relevant reaction path is the CO2 hydrogenation on the Cu/Zn surface followed by the WGSR on the Cu surface. Formate (HCOO*) plays thereby an important role as intermediate, inhibiting to some extent other reactions. Formate lowest coverage occurs logically at reduced CO2 content and higher temperature, conditions in which CO hydrogenation on Cu occurs reasonably.

References
1. N. Dahmen et al. Energy Sustainability and Society 2:3 (2012).
2. F. Studt et al. ChemCatChem 7 (2015), 1105-1111.
3. F. Studt et al. Catalysis Letters 144 (2014), 1973-1977.
4. M. Behrens et al. Science 336 (2012), 893-897.
5. K. Herrera Delgado et al. Catalysis 5 (2015), 871-904.
6. C. Seidel et al. Chemical Engineering Science 175 (2018), 130-138.