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 H-2 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-vacancyadjacent lattice oxygen atom (V-Zr-O-) and oxygen vacancy (V-o) 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 electronV(Zr)-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.