Opening up a Radical Cross-Coupling Etherification Path by a Defect-Rich Cu/ZrO₂ Catalyst for a High-Value Transformation of HMF

In this work, a radical coupling path was opened up for the construction of C–O bonds in the reduction–etherification of 5-hydroxymethylfurfural (HMF) by designing the unpaired electron defect-rich catalyst Cu/ZrO2. A decent 2,5-bis(isopropoxymethyl)furan (BPMF) yield (86.3%) was obtained under the absence of H2 and external pressure. A radical quenching experiment proved that the etherification process, which is the key step for achieving BPMF, indeed belongs to a radical-related route. Subsequently, a series of in situ FTIR, radical capture experiment, and electron spin density analyses were used to elucidate the formation of key radical intermediates at the molecular level in which the O–H dissociation of 2.5-dihydroxymethylfuran and C–O dissociation of isopropanol occurred on the zirconium-vacancy-adjacent lattice oxygen atom (VZr–O–) and oxygen vacancy (VO) sites, respectively. Furthermore, we revealed the induction mechanism of the alkoxy radical intermediate at the electronic level and the electron structure cycle process of the unpaired electron VZr–O– sites via a combination of density functional theoretical calculations and isotope labeling. More importantly, the calculation result from Gaussian showed that radical intermediates do not occur in a chain reaction with unactivated molecules; thus, the optimal path of radical cross-coupling to produce BPMF was clarified. The findings reported by this article reveal the role of unpaired electron defects in opening up a radical coupling path and thus broaden the downstream production of the high-value transformation of HMF. Furthermore, this study could provide a reference for the construction of C–O bonds in other heterogeneous reaction systems.