Photocatalytic Oxidation of 5-Hydroxymethylfurfural Selectively into 2,5-Diformylfuran with CdS Nanotube

Conversion of 5-hydroxymethylfurfural (HMF) into high value-added chemicals to take place of carbon resources relying on non-renewable fossil reserves has attracted much attention in recent years. Besides, in a system of water splitting for hydrogen production, HMF oxidation can be incorporated to provide electrons instead of the kinetically sluggish water oxidation. In this paper, hollow ZnS@CdS/Ni nanotubes were prepared for photocatalytic oxidation of HMF coupled by catalytic hydrogen generation. ZnO was employed as template to form ZnO@ZnS@CdS first. The ZnO template was removed in the presence of strong basic solution. Due to the excess of S2- used, ZnS defect state exsited in ZnS@CdS, which was confirmed by ultraviolet-visible (UV-Vis) spectroscopy and X-ray photoelectron spectroscopy (XPS). The ZnS defect state had no effect on the conduction band edge potential of ZnS but reduced the band gap of ZnS. In combination of the absorption edge and the electrochemical measurements, the potentials for valence band edges of CdS and ZnS defect state were determined, respectively. Under light irradiation, the photogenerated electrons could be accumulated on the surface of CdS, and the photogenerated holes could migrate from CdS to ZnS defect, reducing the carrier recombination and improving the photocatalytic performance. With further deposition of Ni nanoparticles on the surface of CdS nanotube, the ZnS@CdS/Ni structure could accomplish the oxidation of HMF into 2,5-diformylfuran (DFF) selectively and hydrogen production at the both reaction sites. Under optimal amount of the Ni deposition, conversion of 36% for HMF oxidation was accomplished by ZnS@CdS/Ni-30 after 1 h photo reaction. At this stage, DFF as the main product with selectivity of 99% was obtained. Besides, H-2 was detected in the head space of the reaction system, suggesting that HMF can be functioned as electron donor in stead of an H2O molecule in photo water splitting. The photo-generated holes were found to be the active species for the conversion of HMF into DFF. Under optimal photo reaction time (1 h), the prepared catalyst can be recycled three times without obvious decrease in catalytic activity. And the X-ray diffraction (XRD) pattern for the catalyst after photo reaction kept similar to that before reaction.