Study On The Production Of Liquid Fuels From Bio-oil By Mild Hydrogenation

As the only renewable energy in nature that can be directly converted into carbon-containing liquid fuels,the conversion and utilization of biomass energy has attracted extensive attention from researchers.The liquid products obtained by rapid pyrolysis of lignocellulosic biomass is called bio-oil,which has the potential to be used as aviation fuel,additives and to produce platform compounds,but its application was limited by the high oxygen content.In order to carry out catalytic hydrodeoxygenation and achieve upgrading of bio-oil,our research proposed to produce hydrogen under mild conditions（230℃,1 atm N₂）by methanol-aqueous phase reforming,which used cheap methanol as a hydrogen source,and coupled with the hydrodeoxygenation（HDO）of oxygenated compounds,especially phenolic substances,to improve the quality of bio-oil and make it more valuable.In this study,1% Ru/C and 3% Ru/C catalysts were prepared by an incipient-wetness impregnation method,and carried on catalytic experiment with 5%Ru/C catalysts separately.Under the catalysis of 5%Ru/C,phenol was almost completely converted,and the conversion rate was close to 100%;at the same time,the yield of benzene was increased to 60%,and the yield of cyclohexane was also increased to 27%.The HDO effect was improved significantly.For this experimental system,the hydrogen production by methanol aqueous phase reforming and phenolic HDO both need to be carried out at the metal active sites,as the loading increases,more noble metal introduced more active sites.So higher metal loading and better dispersion can better promote the reaction to a more complete direction.Then three typical phenolic model compounds（phenol,guaiacol,2,6-dimethoxyphenol）were used for HDO experiment to explore the influence of reaction time and the amount of methanol on the experiment.Under the optimal experimental condition（atmospheric pressure N₂,reaction temperature 230°C,the molar ratio of methanol/oil 2.5:1,reaction time 12 h）,all the three model compounds were close to or reached the full conversion.The yield of benzene was 81%and Cyclohexane was 11%.According to the experimental results,a possible reaction path was proposed,that is,the model compounds containing methoxy phenol were first removed from methoxy group to form phenol,then the benzene ring was hydrogenated to form cyclohexanol,and then further HDO to produce deoxidized products.It was found that there has a competition between the conversion path of cyclohexanol to benzene and cyclohexane,and the number of active hydrogen atoms in the system has an important influence on the distribution of the two deoxygenated products.Too many active hydrogen atoms will compete with the reaction product for adsorption,thereby affecting the deoxygenation effect.At the same time,the selectivity of benzene may be reduced due to the further over-hydrogenation.This article also proposed that the presence of methoxy functional groups reduced the difficulty of hydrogenation of phenolic substances,and at the same time,the removed methoxy groups would partially generate methanol,which participated in the hydrogen production and hydrodeoxygenation reaction.On the experimental basis of the previous model compounds,taking the model bio-oil as the research object,we proposed an in-situ hydrodeoxygenation experimental condition that was better for bio-oil（atmospheric pressure N₂,reaction temperature 230℃,model oil 0.2g,catalyst0.1g,methanol 300μL,reaction time 20h）.Under this condition,the selectivity of benzene can reach 24.39%,and a little cyclohexanol and cyclohexanone were also found in the product.In addition,the system still has a good ability to remove the methoxy functional groups in the model oil,and the selectivity was as low as 29.79%.After catalytic HDO,the oxygen content of model oil was reduced to 7.03%.Finally,the complex real bio-oil was used to further study the in-situ hydrogenation effects of aldehydes,acids,ketones and other small molecules and different phenolics in the bio-oil under different reaction conditions.Under the conditions of in-situ hydrogen supply,there was still a certain ability to remove the methoxy functional groups in the bio-oil,but there was no further hydrodeoxygenation products.It was speculated that the reason was the complex composition of the actual bio-oil,that is,various substances were competitive adsorption on the catalyst surface,and there were more side reactions in the system and more interference,which affected the effect of HDO.