Preparation Of Highly Dispersed NM/HY-TiO₂ Bifunctional Catalysts And Catalytic Conversion Of Cellulose

In order to meet the demand of energy,food and environment with a growing global population,research efforts are increasing to find alternatives to energy and chemical production.Biomass resources are a major renewable source of carbon necessary for the production of fuels and chemicals,being widely available in the natural environment,and biomass reservations are currently abundant and do not compete with food production.Cellulose is the most abundant biomass on earth and its catalytic conversion to high value-added chemicals has great application prospects.Therefore,the development of a catalyst with high catalytic activity is greatly important to catalyse the conversion of cellulose to polyols.In this thesis,bifunctional catalysts with"metal-zeolite-fiber"composite structures were prepared using TiO₂ nanofibers as the supports,HY nanozeolite as the acidic site active centers,and noble metal（NM）nanoparticles as the hydrogenation active center,and the HY nanozeolites were grafted onto the TiO₂ nanofiber surface（HY-TiO₂）by the"secondary grafting"method,and the size of the HY nanozeolite on the TiO₂ nanofiber surface was controlled by different crystallization temperatures.The NM/HY-TiO₂ bifunctional catalysts（Pt/HY-TiO₂ and Ru/HY-TiO₂）were prepared by using the"sol-gel deposition"method to solidify the noble metal Pt and Ru nanoparticles onto the HY-TiO₂ supports.The morphology,crystalline structure,acidity,size and distribution of the prepared catalysts were analyzed by XRD,SEM,TEM,FT-IR,NH₃-TPD and Py-FTIR.The characterization results of the prepared HY-TiO₂ catalysts and the experimental results of cellulose hydrolysis to glucose showed that the size of HY nanocrystals in the prepared HY-TiO₂ catalysts was controlled at 40-60 nm and uniformly distributed on the TiO₂ nanofibers,and the new HY-TiO₂ catalysts showed higher TONs and glucose selectivity compared with the large supports-free HY nanozeolite particles（500 nm-2μm）catalysts due to the higher accessibility of Br（?）nsted acid sites on the smaller HY nanocrystals,which was more favorable for the breakage ofβ-1,4-glycosidic bonds to generate cellobiose intermediates and subsequent glucose products.The characterization results of the prepared Pt/HY-TiO₂ and Ru/HY-TiO₂ bifunctional catalysts and the experimental results of cellulose hydrogenation/hydrolysis to polyol showed that,in the prepared highly dispersed Pt/HY-TiO₂ and Ru/HY-TiO₂ catalysts,the noble metal Pt NPs and Ru NPs were uniformly dispersed on the HY-TiO₂ supports,and the Pt NPs size was kept around 4 nm,while the size of Ru NPs was kept around 2.4 nm;the noble metal Pt NPs and Ru NPs increased the strong acid amount of HY-TiO₂ catalyst,which may be due to the increase of Br（?）nsted acid site on the supports by the hydroxyl group attached to the metal,thus increasing the strong acid content of the catalyst;and the selectivity of different noble metal Pt NPs and Ru NPs for various reaction products was different,for Pt/HY-TiO₂ catalyst,when the reaction temperature was 160^oC,the H₂ pressure was 20 bar and the mass ratio of cellulose and catalyst was 1:1,the selectivity of ethylene glycol was 14.8%at maximum.For Ru/HY-TiO₂ catalyst,increasing the catalyst amount and increasing the H₂ pressure were favorable to improve the selectivity of C5 alcohol while increasing the reaction temperature was favorable to improve the selectivity of 1,2-propanediol.In addition,the reaction process of cellulose hydrogenation/hydrolysis was explored in this thesis by using Pt/HY-TiO₂ and Ru/HY-TiO₂ catalysts with glucose as the initial model reaction.The results showed that cellulose was first hydrolyzed by HY-TiO₂ to generate small molecules of glucose mo NMer,and then the reaction mechanisms of the conversion of glucose to polyol catalyzed by noble metal Pt and Ru active sites were different:the C-C bond breaking activity catalyzed by noble metal Pt was greater than the hydrogenation activity,and glucose firstly underwent hydrogenolysis to generate glycol aldehyde and erythrose,then was further hydrogenated to generate ethylene glycol;while the hydrogenation activity of noble metal Ru was greater than the C-C bond-breaking activity,glucose was first hydrogenated to produce C6alcohol,and then C6 alcohol was further hydrogenolysis to produce low carbon alcohol.