Catalytic Conversion Of Lignocellulosic Biomass Into Liquid Fuels

With the current environmental pollution,greenhouse effect and energy shortages increasing,the search for a clean renewable energy is imminent.Biomass,as the only renewable organic carbon source on earth,has large reserves and wide distribution,and has the potential to replace fossil energy.Among them,lignocellulosic biomass is the most abundant biomass resource,mainly composed of cellulose,hemicellulose and lignin,which can be converted and depolymerized into C5/C6 alkanes and monomers/dimers small molecule products,respectively used as the liquid transportation fuel.It is of great significance to alleviate the current energy crisis.However,both cellulose and lignin are natural polymers,which have the characteristics of complicated structure and insoluble in water.Therefore,their conversion to liquid fuels is faced with harsh conversion conditions,complicated products,unclear paths and high temperatures that easily cause increased by-products and difficulties such as product polycondensation.In this paper,different high-efficient catalystic systems were developed according to the characteristics of lignin and cellulose.On the basis of improving product yieldthe reaction conditions were lowered and the mechanism in the conversion process was thoroughly explored.Based on the above research background,on the premise of using cellulose-derivative sorbitol,microcrystalline cellulose and kraft lignin as the raw materials,the relevant research work on the preparation of liquid fuel was carried out using different composite catalytic systems as follows:(1)Using V-modified Ir/SiO2 catalyst(Ir-VOx/SiO2 catalyst)combined with molecular sieve HZSM-5 to catalyze the conversion of cellulose derivative sorbitol to prepare C5/C6 liquid alkane fuel,and explore its reactivity for catalytic conversion of sorbitol.When the V/Ir molar ratio is 0.13,the total yield of alkanes reaches 96.8%,and the yield of liquid C5/C6 alkanes is 88.5%.When the concentration of sorbitol reaches 20wt%,the yield of C5/C6 alkanes was still above 80%.The mechanism study shows that hydrogen overflow occurs between the added VOx and Ir metals,which improves the hydrogenolysis ability of the Ir-VOx/SiO2 catalyst for the C-O bond.At the same time,more acid active sites were provided by the VOx in the lower valence state forms on the Ir-VOx/SiO2 catalyst.These activity changes promoted the intramolecular dehydration of sorbitol and further hydrogenolysis reactions,and finally accelerated the conversion of sorbitol to C5/C6 alkanes.(2)Directly using cellulose as the raw material,the reactivity of the composite catalyst Ir-VOx/SiO2 and HZSM-5 to convert cellulose to C5/C6 alkane was investigated.Compared with sorbitol,there are a large number of hydrogen bonds in the cellulose and the structure of cellulose is complicated.Therefore,It is more difficult to directly convertion of cellulose into C5/C6 alkanes.The study found that Ir-VOx/SiO2 and HZSM-5 achieved the efficient conversion of cellulose,a yield of 85.1%of C5/C6 alkanes was obtained.Through the comparative experimental study of Ir-VOx/SiO2 and HZSM-5 on cellulose,glucose,sorbitol and HMF,proving that sorbitol is the key intemediate during the conversion of cellulose to C5/C6 alkanes.Glucose from the hydrolysis of cellulose can be immediately hydrogenated to sorbitol on Ir-VOx/SiO2 catalyst to start the next reaction,which was also an important reason why the composite catalysts Ir-VOx/SiO2 and HZSM-5 can equally efficiently convert cellulose.It is also found that the addition of HZSM-5 can not only accelerate the hydrolysis of cellulose,but also improve the hydrogenolysis activity of Ir-VOx/SiO2 catalyst for the C-O bond.(3)Based on the research of Ir-VOx/SiO2 catalyst,it was found that the reaction time can be greatly shortened by transition metal Mo modified Ir/SiO2(Ir-MoOx/SiO2)during the catalytic conversion of cellulose to C5/C6 alkanes.Using Ir-MoOx/SiO2(Mo/Ir=0.5)catalyst,91.7%of C5/C6 alkanes was obtained under the optimal conditions.It is shown that when the Mo/Ir molar ratio is 0.5,the average particle size of Ir is least at about 3 nm.At the same time,the addition of Mo also increased the amount of the Lewis acid(L acid)of the Ir-MoOx/SiO2 catalyst,which promoted the cyclic dehydration reaction of the intermediate sorbitol.In addition,the Ir-MoOx/SiO2 catalyst had no obvious B acid,therefore the formation of by-products can be suppressed,so the activity is better than Ir-VOx/SiO2 catalyst.Electron transfer Ir?Mo occurs between a small amount of Mo species and Ir particles reduces the reducibility of Mo,and improves the hydrogenation activity of Ir-MoOx/SiO2 catalyst,which is conducive to the formation of C5/C6 alkane.(4)Ir-WOx/SiO2 catalyst was prepared by replacing V and Mo with transition metal W to modify Ir/SiO2 catalyst.It was found that when W was added,(Ir?W)electron transfer occurred between W and Ir particles.A series of characterization indicated that the added W is highly dispersed on the surface of the carrier in the form of isolated or oligomeric WOx.It has been found that there is balance between the acid sites and the hydrogenation active sites in the Ir-WOx/SiO2 catalyst.Excessive addition of tungsten will increase the amount of acid,thereby aggravating the isomerization and the retra-aldol condensation reaction of the intermediate glucose,generating a large number of by-products.When tungsten is added too little,the amount of acid sites and the amount of hydrogenation active sites are correspondingly small,resulting in a slow reaction.When the W/Ir molar ratio is 0.06,the Ir-WOx/SiO2 catalyst has the highest hydrogenolysis activity for the C-O bond.Under optimal conditions,92.1%C5/C6 alkane can be obtained,which is higher than the modification activity of V and Mo.At the same time,trying to use corn stover as a raw material to prepare liquid alkane can achieve effective conversion of cellulose and hemicellulose in the straw,but it produces a large amount of lignin residue.(5)In order to solve the problem of lignin residues during the conversion of biomass raw materials,research on the catalytic depolymerization of lignin continued.Using Indulin AT lignin as the main depolymerization object and Rh/C as the hydrogenation catalyst,the depolymerization activity of four layered layered solid acids HTAMoO6,HTaMoO6,HTaMoO6 and HTaMoO6 on lignin was compared.It is found that HTaMoO6 exhibits the best reactivity,which can be due to its strongest acidity,on the other hand,the layered structure of HTaMoO6 can also expose more active sites to lignin.Under the optimal conditions of 320? and 24h,a higher petroleum ether soluble fraction yield of 58.7%is obtained.In addition,under the depolymerization conditions of Indulin AT lignin at 290 ? and 2h,the yield of liquid product reached 95.6%.Among them,the main components of the solubles in petroleum ether are monomers and dimers,with the high heat value of 30.2MJ/Kg.