The Preparation Of Polyoxometalates Catalysts And Its Application In Cellulose Hydrolysis Reaction

Nowadays,the demand for resources is increasing and the limited fossil reserves are decreasing.Biomass is the only renewable and abundant carbon energy in the world.Cellulose,which occupies an important proportion,has also been paid attention to because of its renewability and degradability.However,many catalysts used in cellulose conversion have some problems such as high cost,harsh reaction conditions and difficult recovery.Therefore,how to efficiently decompose cellulose into high value-added chemicals at low cost has become an important issue today.Polyoxometalates(POMS)are widely used in many fields such as industry,medicine,catalysis and materials science due to their excellent Br?nsted acid,oxidation stability and thermal stability.In this paper,polyoxmetalate H₅PV₂Mo₁₀O₄₀(HPVMO)and H₅PW₁₁Ti O₄₀(HPWTi)with diacid center were used as the research objects to prepare two heterogeneous composite catalysts,and their activities were explored in the hydrolysis reaction of cellulose.The specific research contents are as follows:(1)Catalysts Bet_nH_5-nPV₂Mo₁₀(n=1,2,3,4 and 5)were designed and synthesized.Their structures were characterized by elemental analysis,IR,XRD and DR-UV-vis.Their activities were investigated in the catalytic hydrolysis of cellulose.It was found that Bet H₄PV₂Mo₁₀ had the strongest Br?nsted acid and showed the best catalytic activity.The optimal reaction conditions were as follows:0.05 g cellulose and 0.24 g catalyst,reaction temperature of 160°C for 5 h,cellulose conversion rate was 93.2%,glucose yield was 50.3%,fructose yield was 12.9%,5-HMF yield was 8.9%.Its activity is due to the Br?nsted acidity and temperature control properties of the catalyst,and its stability is due to the interaction between betaine and HPVMo.The activity of the catalyst did not decrease significantly after 6 cycles,and the loss rate was 5.8%.IR,XRD and DR-UV-vis tests showed that the catalyst structure remained unchanged after the reaction.(2)Pickering emulsion(PE)catalysts HPVMo(n)/SiO₂(8)(n=10,15,20,25,and30 wt%)were synthesized and characterized by elemental analysis,IR,XRD,DR-UV-vis,and ³¹P MAS NMR.It was found that the acidity of PE catalyst Br?nsted increased with the increase of HPVMo loading,while the hydrophilicity decreased with the increase of HPVMo loading.In the catalytic hydrolysis of cellulose,HPVMo(25)/SiO₂(8)showed good activity,and the reaction conditions were optimized as follows:in the aqueous phase system with cellulose 0.05 g and catalyst 0.32 g and reaction temperature 160°C for 5 h,the cellulose conversion rate was 82.0%,glucose yield was 46.2%,fructose yield was 17.2%,5-HMF yield was 17.5%.Its activity was due to the Br?nsted acidity of the catalyst,and its stability was due to the interaction between SiO₂(8)and HPVMo.The activity of the catalyst did not decrease after 6 cycles,and the loss rate was 6.8%.IR,XRD and DR-UV-vis tests showed that the catalyst structure remained unchanged after the reaction.(3)PE type catalyst HPWTi(n)/SiO₂(8)(n=10,15,20,25 and 30 wt%)was designed and synthesized,and its structure was characterized by elemental analysis,IR,XRD,DR-UV-vis and ³¹P MAS NMR.It was found that the Br?nsted acid and Lewis acid of PE catalyst increased with the increase of HPWTi loading,while the hydrophilicity decreased with the increase of HPWTi loading.In the catalytic hydrolysis of cellulose,it was found that HPWTi(25)/SiO₂(8)showed good activity,and the reaction conditions were optimized as follows:when 0.1 g cellulose and 0.2 g catalyst were reacted at 130°C for 8 h,the cellulose conversion,glucose yield and Levulinic acid(LA)yield reached 86.0%,15.2%and 13.0%in aqueous phase.In order to obtain better effect,the reaction system was changed to H₂O/MIBK double solvent system,and the cellulose conversion rate reached 92.5%,and the LA yield reached 73.6%.Its activity is due to the Br?nsted and Lewis acidity of the catalyst,the ability of the catalyst to adsorb cellulose,and the ability of the catalyst to form a PE system in H₂O/MIBK,overcoming the organic-water mass transfer barrier.The stability of the catalyst was due to the interaction between SiO₂(8)and HPWTi.The activity of the catalyst did not decrease significantly after 6 cycles,and the loss rate was 5.9%.The results showed that the structure of the catalyst remained unchanged after the reaction by IR,XRD and DR-UV-vis.