Reactive Captureand Conversion of Carbon Dioxideto Methanol with ZnZrO₂ and Alkali-Promoted Mg₃AlO_<i>x</i> 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 Mg₃AlO_<i>x</i> 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 Mg₃AlO_<i>x</i> 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 Mg₃AlO_<i>x</i>, 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.