Stable CO2 Hydrogenation to Methanol by Cu Interacting with Isolated Zn Cations in Zincosilicate CIT-6

Abstract

The catalytic conversion of carbon dioxide (CO₂) to methanol over Cu/ZnO catalysts is expected to become valuable for recycling CO₂. The nature of the Cu–Zn interplay remains a subject of intense debate due to many different Zn species encountered in Cu/ZnO catalysts. In this study, we designed a Cu–Zn catalyst by ion-exchanging Cu into CIT-6, a crystalline microporous zincosilicate with the BEA* topology. The catalyst exhibited high and stable CO₂ hydrogenation rate to methanol. In contrast, its aluminosilicate counterparts Cu-Beta and CuZn-Beta mainly converted CO₂ to CO. Operando X-ray absorption spectroscopy combined with X-ray diffraction confirmed the stability of Zn cations in the zincosilicate framework during reduction in H₂ and reaction in CO₂/H₂. The active phase consisted of highly dispersed Cu particles. These particles located near isolated Zn²⁺ species represent a different type of active site for methanol synthesis than the active phases proposed for Cu–Zn catalysts, such as Cu–Zn alloy particles and Cu particles decorated with ZnO_x. In situ IR spectroscopy showed the formation of Zn-formate species during CO₂ hydrogenation, indicating that Zn²⁺ ions stabilize formate as a reaction intermediate in the hydrogenation of CO₂ to methanol.