Unravelling Proximity-Driven Synergetic Effect within CIZO–SAPO Bifunctional Catalyst for CO₂ Hydrogenation to DME

Dimethyl ether (DME) production has been attracting significant research attention for its broad uses as an important chemical feedstock as well as a promising fuel. Herein we report CO₂ direct hydrogenation to produce DME using a new CIZO–SAPO bifunctional catalytic structure, which consists of Cu–In–Zr–O (CIZO) mixed oxide sites toward methanol synthesis and SAPO-34 zeolite sites for intermediate dehydration to DME. Compared with CIZO, a significant increase in CO₂ conversion was achieved by simply mixing CIZO and SAPO, indicating the existence of synergy within the bifunctional catalyst. The study of mixing ratios and methods further confirmed the synergetic effect being proximity dependent. Mechanistic insight was obtained by conducting in-situ DRIFTS analyses. Variation in the proximity between CIZO and SAPO was discovered to alter the reaction pathways. When CIZO and SAPO were more closely contacted, DME could be generated via a shortcut methoxy–DME pathway instead of a typical methoxy–methanol–DME route, resulting in more efficient DME formation. The shortcut pathway was suppressed with an increase in distance between the two components. Therefore, it is proposed that the synergetic effect that leads to boosted DME formation in the bifunctional catalyst is determined by the altered reaction pathway which is controlled by the proximity between CIZO and SAPO active sites. The adjacency of CIZO and SAPO facilitates migration of methoxy intermediates from the former to the latter, so that DME would be formed directly without a need of methanol formation. This study unveils the synergetic mechanism within bifunctional catalyst in DME synthesis that would provide guidance to new catalyst research.