Reactive Capture and Conversion of Carbon Dioxide to Methanol with ZnZrO2 and Alkali-Promoted Mg3AlOx Mixed Oxide Catalytic Sorbents

Reactive capture and conversion (RCC) explores the use of a single-unit process to capture CO2 and produce a product, in this case, methanol (MeOH). In this study, different configurations of a catalytic sorbent (CS) composed of ZnZrO2 catalyst and Mg3AlOx sorbent with and without alkali modification are evaluated for CO2 adsorption, steady-state catalysis with cofed CO2 and H2, and transient RCC performance. A catalyst composed of a physical mixture of Mg3AlOx with ZnZrO2 resulted in a slight increase in CO2 uptake, with a low impact on the catalytic activity and RCC of the materials compared to ZnZrO2 alone. In contrast, Na impregnation significantly increased the level of CO2 uptake from 0.28 mmol/g (ZnZrO2 alone) to 0.6 and 1.1 mmol/g for the CS with Na on the catalyst or Mg3AlOx, respectively. However, Na impregnation reduced the CO2 conversion rate and MeOH selectivity during steady-state cofeed experiments at 300 °C and 6 bar. In contrast to steady-state catalysis conditions, RCC, which is a cyclic capture and conversion process, creates dynamic CO2 and H2 surface coverages, favoring CH4 in the early stages of the conversion step and then CO and MeOH as the catalyst CO2 coverage reduces. The highest MeOH productivity during RCC was achieved with CS that balanced the CO2 uptake with only moderate catalyst rate reductions caused by Na addition. The optimal material, ZnZrO2+10%Na/Mg3AlOx, achieved a CO2 uptake of 0.8 mmol/g and a MeOH productivity of 0.5 mmol/g with 100% selectivity at 260 °C and 6 bar during RCC. This marks the highest RCC MeOH productivity reported to date, although the process needs further optimization and even with optimization, may remain impractical. The results further demonstrate that optimization of catalytic sorbents under steady-state flow conditions does not easily correlate to transient capture and conversion cycles for methanol synthesis from CO2.