Process And Mechanism Study On Conversion Of Cellulose To C2-C3 Alcohol/Acid Chemicals

With the increase in energy consumption in the industrial field,the development of renewable biomass energy has stimulated greater interest.Cellulose is the most abundant biomass resource in nature,with annual yields of 10¹¹-10¹² tons.By catalytic conversion（hydrolysis,hydrogenation,hydrogenolysis,and oxidation）,cellulose is converted to various chemicals,such as ethylene glycol,propylene glycol,and lactic acid,etc.Currently,researchers have designed a series of the proposal for cellulose conversion.However several problems are remained in the process of cellulose converting to C2-C3 alcohols/acids:（i）The cellulose concentration is limited to a low level（<5%）.The product selectivity decreases as the cellulose concentration is increased（such as 10%）.（ii）The catalyst gradually deactivates in high-temperature hydrothermal conditions.（iii）The mechanism of C-C and C-O bond crack still unclear which are the two most important processes in biomass conversion.（iv）Cellulose hydrogenolysis products cannot be adjusted according to market demand.For this reason,we design a switchable reaction route that change the selectivity of the product by adjusting the reaction conditions.The main contents of this article are as follows:1.We synthesized Ni-WO_x/Si O₂ nano-sheet catalysts by an improved deposition precipitation method.The improved materials with large specific surface area（46m²/g）and Ni particles with a diameter of about 18 nm are well dispersed on supports.Furthermore,the catalyst surface contains the right amount of active sites(W⁵⁺)and has high hydrothermal stability.Corn stover cellulose at high concentrations（10 wt%）was completely converted to ethylene glycol with 57 C%selectivity over Ni-W nano-sheet under the conditions of 4 MPa H₂ and 245 ^oC.The cellulose is continuously squeezed into the slurry bed reactor.Eventually,the cumulative concentration of cellulose reached 30 wt%,and ethylene glycol solution with a concentration of 16.9 wt%was obtained.2.The mechanism of the retro-aldol condensation reaction of glucose/fructose on WO_x was investigated.The activity test shows that the W⁵⁺/W⁶⁺ratio is directly proportional to the yield and conversion rate of ethylene glycol.It proves that W⁵⁺is the active center that catalyzes the breaking of the C-C bond.By observing the UV spectrum of glucose on WO_x,we infer that the coordination bond connected W⁵⁺with the aldehyde group of glucose.W⁵⁺and-CHOβ-OH are formed with such a complex possessed a six-membered ring-like structure involving coordination bond.Due to the electron-withdrawing effect of W⁵⁺,the positive charge or acidity of C₃-OH increases.Then proton transfers from C₃-OH to C₁=O oxygen,which then causes the C₂-C₃bond to break.DFT calculated the activation energy barrier（E_a）is 17.69 kcal/mol for this process and the kinetic experiment got 17.37 kcal/mol of E_a.The values obtained by the two methods are close.It shows that the reactive center,adsorption model and reaction mechanism proposed in this work are accurate and reasonable.3.Cellulose is hydrogenolysis to biofuel ethanol in 4 MPa H₂ and 300 ^oC over a multifunctional catalyst（contains Pd,Cu,and WO_x）.And on the optimized catalyst,42.5 C%yields of bioethanol are obtained.Kinetic studies showed that the cellulose conversion to ethanol followed the consecutive steps:cellulose hydrolysis to glucose with H⁺(k₁=1.06 h^-1),glucose conversion to glycolaldehyde over W species(k₂=0.37 h^-1),glycolaldehyde hydrogenation to EG over Pd(k₃=1464 h^-1),and EG hydrogenolysis to ethanol over Cu(k₄=0.23 h^-1).The individual rates were well balanced over the three metal components Pd,Cu,and WO_x,to achieve an ethanol formation rate of 0.163 g·g^-1_Cat.·h^-1.Characterization studies such as CO-DRIFTS and Cu LMM-XAES prove that the electronic properties of Cu⁺and the ratio of Cu⁺/Cu⁰ both contributed to EG hydrogenolysis.4.The C-C bond and C-O bond cleavage reactions are two extremely important processes in biomass conversion.The Cu catalyst has the function of selectively breaking the C-O bond without damaging the C-C bond.However,the instability of Cu in high-temperature hydrothermal conditions restricts its practical application.By incorporate Ni and Au,the dispersion of Cu⁺/Cu⁰ active sites and the ability of the sample to dissociate H₂ are improved.In the EG hydrogenolysis reaction,the conversion reaches up to 73.1%with an ethanol selectivity of 85.1 C%over the modified Au-Cu-Ni/Si O₂ catalyst.In addition,the stable（Au-Cu⁺）-Cu⁰ active sites are formed by embedding Au on the surface of the catalyst,thus the hydrothermal resistance of the sample greatly enhance.This work provides a highly stable Au-（Cu-Ni）/Si O₂ catalyst with the ability of selective cleavage of C-O bonds in the process of biomass conversion.5.Polyols/organic acids are important chemicals with sufficient market requirements.Herein,we design a switchable reaction route for cellulose conversion to 1,2-propanediol or methyl lactate by adjusting the H₂ pressure over the Ru Sn catalyst.In 4 MPa H₂,the Ru Sn catalyst converts cellulose to acetol with a yield of53.7 C%.And after further hydrogenation by Ru/C,1,2-propanediol was obtained（yield 57.1 C%）.In the absence of H₂,cellulose/glucose are converted to methyl lactate over the Ru Sn catalyst with yields of 28.2 C%and 51.4 C%,respectively.By comparing the product composition with physicochemical properties of the sample（XRD,H₂-TPR,CO-IR,N₂ adsorption,NH₃-TPD,Pyridine-FTIR,etc.）,we analyzed the relationship between the catalyst structure and activity.The Sn O_x with Lewis acid site participates in the isomerization of glucose to fructose,and the Ru₃Sn₇ alloy is the main active center for breaking the C-C bond（Retro-aldol reaction）.When cellulose hydrogenolysis in H₂,the Ru_1.5Sn_3.0catalyst has the best catalytic activity（the main composition is Ru₃Sn₇+Ru）.Excess Ru in the catalyst provides extra active sites for hydrogenation,which is beneficial to increase the selectivity of acetol and1,2-propanediol.When cellulose conversion in N₂,the excess Sn O_x in Ru1.5Sn6.0improves the ability of isomerization,thereby increasing the selectivity of the methyl lactate.