Supported Bimetallic Au-Cu Catalysts For The Oxidative Esterification Of 5-hydroxymethylfurfural

Biomass is a renewable resource that is widely distributed in nature.Through catalysis technology,biomass can be converted to chemical fines and fuels.5-Hydroxymethy 1-2-furfural(5-HMF)is a key platform molecule for biomass utilization.Its derivative,2,5-furandicarboxylic acid(FDCA),is a promising alternative for terephthalic acid as a monomer in polyethylene terephthalate.In contrast to FDCA,dimethyl furan-2,5-dicarboxylate(DMFD),an oxidative-esterified derivative of 5-HMF,is more easily purified.In this dissertation,a ?-Al2O3 supported Au-Cu bimetallic catalyst was prepared by using urea assisted deposition-precipitation method followed by reduction-oxidation strategy.The catalyst was applied to the selective oxidative esterfication of 5-HMF to DMFD using oxygen as the sole oxidant.The structure-performance correlation of the catalyst was inverstigated by means of various characterizations and kinectic studies.The main results are as follows.The catalytic performance of bimetallc Au catalysts with different transition metals(Fe,Co,Ni,Cu)and supports was evaluated,the result suggested Au-Cu bimetallic catalyst exhibited the best catalytic performance using ?-Al2O3 as the support.A series of Au-Cu catalysts with different Au/Cu molar ratio,pre-treatment methods were prepared.The reaction temperature,pressure and reaction time were carefully investigated and optimized.The results suggested bimetallic Au-Cu,/?-Al2O3 exhibited excellent activity,obtaining above 98%field to DMFD at 105? and 1.0 MPa O2 after 5 h,while only 47.9%DMFD was obtained over Au/?-Al2O3 catalyst.This finding demonstrated the synergistic effect between Au and Cu.Further study suggested the catalyst pre-treatment significantly influenced the catalytic performance,the directly oxidized Au-Cu/?-Al2O3-A only showed 28.4%DMFD yield,which was much less than that of Au-Cu1/?-Al2O3 pre-treated by reduction-oxidation strategy.The reaction pathway of 5-HMF oxidative esterification was explored,indicating the aldehyde group of 5-HMF was oxidized and esterified to esters at first,after that the hydroxyl group on the other side was oxidized into aldehyde group,and then further oxidized and esterified to generate DMFD.The oxidation of hydroxyl group is the rate-determining step in the whole reaction process.In addition,the stability of Au-Cu1/?-Al2O3 was explored,catalyst was used and regenerated for 5 cycles,the yield to DMFD decreased from 98.2%to 70.8%,indicating the deactivation of Au-Cu1/?-Al2O3.The structure-performance of Au-Cu catalysts was studied in detail by several spectroscopic characterizations.XPS and UV-Vis DRS confirmed the strong interaction between Au and Cu after the reduction-oxidation pre-treatment,while a weaker Au-Cu interaction was observed over Au-Cu1/?-A12O3-A.TEM,H2-TPR and N2O-H2 titration evidenced that the Cu species were highly dispersed on the surface of Au nano-particles as CuOx entities,forming uniform Au-CuOx hybride structures.The dispersion of Cu was as high as 31.4%.The direct oxidized Au-Cu catalyst only showed a Cu dispersion of 14.5%,and mainly bulk CuOx species deposited on the surfaces of the catalyst.Based on the above results,the formation process of Au-CuOx was supposed:in the reduction process,and Au and Cu were reduced to metallic state and mutually diffused,forming homogeneous AuCu alloy.Then in the oxidation process,Cu slowly segregated from AuCu alloy to the surface of Au and oxidized to CuOx,thus the uniform Au-Cux hybrid was obtained.According to the activity experiments,this highly dispersed and uniformily Au-CuOx hybrid structure significantly facilitated the catalytic oxidative esterification of 5-HMF to DMFD.