Fabrication of supported Au-CuOx nanohybrids by reduction-oxidation strategy for efficient oxidative esterification of 5-hydroxymethyl-2-furfural into dimethyl furan-2,5-dicarboxylate

Abstract

5-Hydroxymethyl-2-furfural (HMF) is a key platform molecule for biomass utilization. The HMF derivative 2,5-furandicarboxylic acid (FDCA) by oxidation of HMF is a promising alternative for terephthalic acid as a monomer in polyethylene terephthalate. However, due to the facile purification of dimethyl furan-2,5-dicarboxylate (DMFD), an oxidative esterification derivative of HMF, is more desirable. In this work, supported Au–Cu bimetallic catalysts with Au–CuOx (x = 0–1) nanohybrids were prepared by a reduction–oxidation pretreatment strategy. The as-prepared catalysts were used for the oxidative esterification of HMF into DMFD under base-free conditions using oxygen as sole oxidant. Activity tests showed that the optimized Au–Cu/γ-Al2O3 catalyst displayed a remarkably high DMFD yield of up to 98% and a turnover frequency (TOF) value of 243 h–1, whereas the monometallic Au and Cu catalysts exhibited poor performance. Various characterizations confirmed the strong interaction between Au nanoparticles and CuOx species, and their synergistic effect on the enhanced performance of HMF oxidative esterification. The reduction–oxidation pretreatment promoted the dispersion of CuOx and intensified the interaction of Au nanoparticles with CuOx. A plausible reaction mechanism for the oxidative esterification of HMF over Au–Cu/γ-Al2O3 was proposed. This work provided new insights into the effect of reduction–oxidation pretreatment on the fabrication of supported Au–CuOx nanohybrids for the production of DMFD by HMF oxidative esterification.