Rationally designed Au-ZrOx interaction for boosting 5-hydroxymethylfurfural oxidation

Upgrading bioderived 5-hydroxymethylfurfural (HMF) to high value-added bioplastic monomer 2,5-furandicarboxylic acid (FDCA) is a promising route for biomass conversion. Developing an efficient supported metal catalyst and clarifying the structure–activity relationship between the support and active sites is the key to boost FDCA production. Herein, ZrO2 coated on natural and abundant halloysite nanotubes (HNTs) was synthesized and employed as a support for Au nanoparticles loading. The Au-ZrOx interface active sites were successfully engineered, and the active site amount can be rationally tailored by adjusting the crystal phases of ZrO2 via changing calcination temperature. A combination study of catalysts surface physicochemical properties and catalytic activity reveals that oxygen vacancies (Ov) play a vital role in enhancing the adsorption of O2. Meanwhile, interface active sites facilitate the chemisorption and activation of O2 molecules, which makes it easier to produce oxygen active species (superoxide radicals). More importantly, in-situ infrared spectroscopy and adsorption experiments also demonstrate that the catalyst with a higher Ov concentration not only contributes to a stronger Lewis acid intensity, but also enhances the adsorption capacity of a key intermediate 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), thus leading to an acceleration of the rate-determining step of hydroxyl group oxidation. The highest 99.4 % of FDCA yield can be obtained in 3.0 h. This newly proposed model of interface active sites may shed a light on the design of novel heterogeneous catalysts for the related biomass oxidation reactions.