| With the increasing shortage of fossil fuel and the pressure from the environmental pollution,the seeking and development of environmentally friendly renewable energy has become a research focus all over the world.As one of the ideal alternative energy resources biomass has the advantages of abundance in reserves,varity in sources,and being the only carbon source in renewable energy.Biomass can be effectively converted into liquid fuels such as biofuels or into platform chemicals.Meanwhile,the CO2 released from the biofuel combustion can be absorbed by biomass again so that carbon recycle can be achieved.Therefore,it is prospective for the development of biofuels.In this paper,the process of making bio-oils from lignocellulose was studied The effect of reaction conditions on biomass liquefaction was examined,and the liquefaction mechanism was proposed.The bio-oils obtained were also hydrodeoxygenated and purified.The followings are the detailed research contents.First,a new liquefaction method was proposed.It was found that the catalytic system with Raney nickel and NaOH as co-catalysts and isopropanol/water(7:3,v:v)as the solvent could effectively liquefy various biomass,such as fir,pine,corncob,bamboo powder,etc.The oil yields were all higher than 60 wt%,while the yield of solid residue was less than 3 wt.%.Among which,the oil yield of fir is the highest,reaching 64.0 wt.%,and its HHV reaching 30.1 MJ/kg.GC-MS analysis indicated that the main components of the oil phase in bio-oil were phenolic compounds,mainly 4-ethyl-2-methoxyphenol,2-methoxyphenol,4-methyl-2-methoxyphenol,followed by acids,ketones,alcohols,esters and hydrocarbons.The components for different bio-oils obtained are varous because of the different structures of biomass.Thermogravimetric analysis showed that most of the bio-oils obtained under this catalytic system were light components with low boiling point.Secondly,the effects of reaction conditions such as catalyst,solvent,reaction temperature and residence time on biomass liquefaction were studied.The results showed that NaOH could significantly increase the yield of bio-oil,while Raney nickel was an efficient hydrogenation catalyst,which could transfer hydrogen from the solvent to the biomass degraded products to achieve the hydrogenation and hydrodeoxidation of bio-oil and reduce the oxygen contentin bio-oils effectively.Another feature is that Raney nickel catalyst can be recycled for at least five times without the obvious loss of its activity.In addition,the yield of bio-oil obtained using isopropanol/water as mixed solvent was much higher than that of using isopropanol or water alone.Blt was found that the yield of bio-oil increased with the temperature at the beginning and then decreased,reaching a maximum of 67.0 wt.%at 240℃.When the residence time is varied,the yield of bio-oil increased with the increase of residence time,reaching the maximum of 88.9 wt.%at 12 h.GC-MS analysis also indicated that isopropanol itself could be converted into bio-oil and this phenomenon was the severest at 12 h,causing the yield of oil to reach the highest.Thus,in order to avoid the conversion of isopropanol,the optimal residence time should be 2 h.Thirdly,to study the liquefaction mechanism of biomass,the catalytic degradation of lignin,cellulose and hemicellulose was studied,respectively.Because of the serious polycondensation of traditional Klason lignin,which is not conducive to the study of liquefaction mechanism,the method of extracting lignin from biomass by acidic lithium bromide solution was used to hydrolyze cellulose and hemicellulose in biomass with the lignin as the insoluble product.The two-dimensional 1H-13C(2D-HSQC)NMR analysis of lignin showed that the acidic lithium bromide solution method was effective.The obtained lignin could keep most chemical bonds,which indicates that lignin has almost no or less condensation.Based on the catalytic liquefaction of lignin,cellulose,glucose and xylose separatively,the degradation mechanism of biomass liquefaction using this innovative method was proposed.It was concluded that almost all the phenols,cyclohexanols and benzenes came from the degradation of lignin,while pentacyclic,acetic acid,macromolecular cyclic hydrocarbons,and the straight-chain hydrocarbons,ketones and alcohols came from the hydrogenation of cellulose or hemicelluloseFinally,for the catalytic degradation of lignin of corn straw,it was found that the oil yield was over 80 wt.%.The conversion rate was close to 100%.The effect of different volume ratios of isopropanol to water on lignin liquefaction was also studied It was demonstrated that when the volume ratio of isopropanol to water was 7:3,the yield of bio-oil reached 91.5 wt.%,which is the highest.GC-MS analysis showed that phenolic monomers and methoxybenzenes were the main components of bio-oils from lignin.In-situ hydrogenation of lignin bio-oil was carried out with the isopropanol and Raney nickel catalyst.It was indicated that phenols were basically converted into the corresponding cyclohexanols,while methoxybenzenes had nearly no change,and with the above changes,the quality of bio-oil was greatly improved through in-situ hydrogenation. |
