Catalytic Transfer Oxidation Of Xylose To Renewable Chemicals Over Bimetallic PtAu Catalysts

Lignocellulosic biomass is considered to be the most abundant and cheapest renewable carbon resource on earth.Lignocellulose,can be transformed into transportation fuels and various bio-based commodity and specialty chemicals,providing an alternative route to fossil-based products.Xylose accounts for about 18%-30%of lignocellulosic resources,and is the second largest abundant sugar in nature.Xylose can be oxidized to a variety of value-added products such as xylonic acid and xylaric acid.Due to safety concerns and challenges in temperature control due to strong exothermic reaction,it is difficult to further commercialize xylose oxidation techniques using nitric acid.Catalytic oxidation of xylose to xylaric acid using molecular O₂ is regarded to be much more environmental-friendly method.Very limited work,however,has been published in this area.Therefore,in this paper,we reported synergistic bimetallic Pt Au catalysts supported on TiO₂ for facile oxidation of xylose to xylonic acid（68%）and xylaric acid（38%）at 60-100°C,1 MPa O₂ pressure.The proposed Pt Au/TiO₂ catalysts display two to six-fold enhancement in catalytic activity(TOF:～500 h^-1 at 60^oC),compared with mono and other bimetallic catalysts including Au/TiO₂,Pt/TiO₂,Pt M/TiO₂（M:Cu,Co,Fe,Mn）.One of the key findings in this work is that,the formation of bimetallic Pt Au sites mediates the-C=O passivation effect on Pt surface,thus xylose（with-CH=O as main functional group）inhibition is significantly reduced with enhanced oxidation kinetics over bimetallic Pt Au/TiO₂ catalyst.According to surface analysis using X-ray photoelectron spectroscopy（XPS）and UV-visible spectroscopy（UV-vis）,it is found that the presence of Au alters the surface electronic density of Pt sites,thus tunable oxidation rates of xylose to xylonic acid,as well as further oxidation of xylonic acid to xylaric acid can be achieved on bimetallic Pt Au/TiO₂ catalysts.Four common catalyst preparation methods are used to synthesize bimetallic nanoparticles,including immersion method,deposition precipitation method,hydrothermal synthesis method,and liquid phase reduction method.We explored and optimized the reaction conditions in the preparation of catalysis by the liquid-phase reduction method.Temperature affects the particle size of the bimetallic catalyst.As the preparation temperature increases,the particle size of the sample gradually increases,and the distribution becomes more uneven.The particle size（2.41～5.90 nm）of Pt Au/TiO₂ bimetallic nanoparticles was adjusted by different catalyst preparation methods.Xylose conversion has significant particle size sensitivity,and there is a"volcanic"trend between particle size and xylose conversion rate.The oxidation path of xylose was studied systematically.The oxidative dehydrogenation of the-CH₂OH group to-CHO is slow.The rate of formation of xylonic acid is much higher than xylaric acid.We also observed other co-products such as tartaric,glyceric,glycolic and formic acids,suggesting that xylose and xylonic acid might undergo C₁-C₄ and C₂-C₃ cleavage during oxidation reactions.Meanwhile,the effects of reaction temperature,O₂ pressure,reaction time,initial p H and other experimental parameters were also disscussed.Experimental results obtained from this work show that,the overall selectivity towards xylonic acid and xylaric acid obtained is 84%with 78%xylose conversion at 80^oC after 10h under 1.0MPa oxygen pressure.The stability studies demonstrated that,the bimetallic Pt Au catalyst exhibits better stability than the monometallic Pt catalyst,and the catalyst still has a higher reaction activity after four cycles.