Study On Hydrotreating Upgrading Of Bio-oil And Its Reaction Mechanism Based On The Supported Noble Metal Catalyst

Fast pyrolysis technology is a renewable energy technology that can convert biomass into bio-oil for energy use.At present,the main problems are the high oxygen content and low heat value of bio-oil,which limits its practical applications.Therefore,it is of great theoretical and practical significance to carry out researches on hydrotreating processes and reaction mechanisms for improving the quality of bio-oil and promoting the development of biomass pyrolysis technology.This study mainly focuses on the catalytic hydrodeoxygenation(HDO)of bio-oil and its model compound(guaiacol),as well as the catalytic hydropyrolysis of different kinds of biomass,in order to improve bio-oil quality by reducing its oxygen content.The main results obtained are as follows:(1)A batch HDO reactor with a specialized catalyst fixed-bed was designed,and a supported noble metal catalyst was also prepared and well characterized.According to the technical requirements of bio-oil HDO,the reactor construction and operation methods,and the reaction conditions were well studied and established for ensuring stability.The convenient operation and control required for conducting mechanism research are the independent design of feeding and sampling lines and the spin-fixed removable catalytic basket design.At the same time,a supported noble metal catalyst Rh/ZrO₂was prepared via incipient wetness impregnation technique by using the stable zirconia support with good catalytic potential,and the rhodium metal loading was 0.5 wt%.Results of characterization,recycling,and regeneration experiments showed that the prepared catalyst,Rh/ZrO₂ has a good catalytic effect on the HDO of the bio-oil model compound,and the conversion of guaiacol can reach 100%.When the Rh/ZrO₂ catalyst was used repeatedly for the fifth time,the Rh/ZrO₂ catalyst deactivated to a certain extent,and the conversion of guaiacol could still reach 50.3%.The deactivated catalyst can be regenerated by the thermal removal of coke and reduction with hydrogen.After regeneration,the conversion of guaiacol can be restored to 92.4%.In order to further verify the excellent HDO performance of the supported noble metal Rh/ZrO₂ catalyst,the bio-oil produced by a large-scale pyrolysis plant was selected as feedstock with industrial significance and bio-oil HDO experiments were carried out in the batch fixed-bed HDO reactor with the self-synthesized Rh/ZrO₂ catalyst and commercial Rh/Al₂O₃ and Pd/Al₂O₃ catalysts.Results showed that Rh/ZrO₂ catalyst showed a better catalytic effect,which gained the highest yield of liquid product.For Rh/ZrO₂,Rh/Al₂O_3,and Pd/Al₂O₃catalytic groups,the oxygen contents of bottom oil decreased to41.3%,39.4%,and 37.1%compared to bio-oil,respectively.And their O/C ratios are all close to that of bituminous coal while the H/C ratios are better than bituminous coal.Each catalytic group attained a suitable deoxygenation extent(DE),and the DE of Rh/ZrO₂catalytic group at 300°C reached 58.49%.In summary,the products obtained in the Rh/ZrO₂catalytic HDO group only has a slight advantage in terms of composition and physicochemical properties of upgraded bio-oil compared to the commercial Rh/Al₂O₃ and Pd/Al₂O₃catalysts with 5 wt%metal loading.However,its 0.5 wt%metal loading is much lower than that of similar commercial catalysts.As for noble metal catalysts with higher cost,the Rh/ZrO₂ catalyst with a low metal loading rate synthesized in this study has apparent advantages.(2)Using the synthesized zirconia-supported Rh catalyst,the HDO experiments of the typical bio-oil model compounds(guaiacol)under different conditions were carried out,and its optimized condition and reaction pathway explored for the subsequent kinetics research.HDO of guaiacol was performed at 150?350 ^oC under 3?7 MPa(H₂)in the batch fixed-bed reactor,with insight into the effects of different reaction parameters,including temperature,initial reactant concentration,initial H₂ pressure and reaction time,on the guaiacol conversion,the whole process of HDO and the formation of undesired oxygen-contained product.The good HDO effect of Rh/ZrO₂ catalyst verifies that the 5 wt%guaiacol can be completely converted.Reaction temperature plays a decisive role and significantly affects the degree of hydrogenation.Rh/ZrO₂ catalyst has favourable performance on hydrogenation of benzene ring and mainly produced oxygen-containing compounds at 150?250 ^oC.Only at higher temperatures(?250 ^oC),guaiacol could completely deoxygenated by Rh/ZrO_2,mainly produced hydrocarbon products.The optimum reaction conditions were 5 wt%guaiacol,300 ^oC,and 7 MPa in which gained 87.7 mol%oxygen-free product of cyclohexane in 180 min.A specific reaction pathway which includes three main steps:“guaiacol?1-methyl-1,2-cyclohexanediol?cyclohexanone and cyclohexanol?completely deoxygenated compounds of cycloalkanes”was deduced by a comprehensive study on different reaction parameters,and a typical key reaction network for HDO of lignin-derived bio-oil is proposed based on the studies of the bio-oil model compound.(3)The kinetics mechanism for the Rh/ZrO₂ catalyzed HDO of bio-oil model compound was explored.HDO kinetic model was established based on the experimental data of the change in concentration of compounds in the reaction system of guaiacol HDO over Rh/ZrO₂.The kinetic parameter was calculated by the pattern search method in the MATLAB software.Results showed that the Rh/ZrO₂ catalytic HDO of guaiacol fits well to a pseudo-first-order kinetic model at four temperature(150°C,250°C,300°C,and 350°C).The proposed reaction rate equation well describes the HDO process of guaiacol and the formation mechanism of main products.The final equation fitting results show that the kinetic curves of main compounds fit well,and the corresponding R² values obtained were all?0.97,except for that of the trace of lumped products.At low temperature of 150 ^oC,the kinetically relevant step is hydrogenation of the aromatic ring,whereas,in high temperature(?300 ^oC),complete deoxygenation of oxygen-containing functional groups mainly yields cyclohexane.These results indicated that the initial hydrogenation of the benzene ring of guaiacol occur under the catalysis of Rh/ZrO₂ easily,and can be completed quickly at lower temperature.Whereas,the reaction of deep deoxygenation producing hydrocarbons requires higher reaction temperature.Results of kinetics and reaction mechanism well explained the experimental results of Rh/ZrO₂ catalyzing HDO of guaiacol.The inferred reaction mechanism further corroborates that the Rh/ZrO₂ catalyst promoted the hydrogenation of benzene ring firstly and then cleave the C-OCH₃ and C-OH bonds to remove the oxygen element of guaiacol.These results would be helpful in further understanding the mechanism of Rh/ZrO₂ catalytic HDO and provide theoretical reference for the optimization of HDO conditions and highly selective catalysts design for HDO of lignin-derived bio-oil.(4)The potential application of Rh/ZrO₂ catalyst in biomass hydropyrolysis was studied,and the corresponding reaction pathway and the catalytic mechanism was further explored by experimental results and quantum chemistry calculation of products.Firstly,a laboratory Py-GC/MS microreactor system was modified to suit biomass hydropyrolysis study.An applicable evaluation system was also established,including estimating method of product selectivity,the calculation method of O/C and H/C ratio,deoxygenation extent(DE),and hydrogen-loss extent(HLE).A comprehensive study of pyrolysis,hydropyrolysis,and catalytic hydropyrolysis of poplar sawdust and rice husk was investigated in the modified Hy Py-GC/MS microreactor system so as to explore the potential effect of Rh/ZrO₂ and the feasible method for different biomass species.The three types of pyrolysis processes have different reaction modes and product distribution.Compared with fast pyrolysis process,hydropyrolysis has an obvious deoxidization and upgrading effect,while the catalytic hydropyrolysis showed a good deoxygenation effect in which the O/C ratios are comparable to that of bituminous coal and the reported upgraded bio-oil by HDO,indicating its suitable potential for the production of low-oxygen biofuel.Various species of biomass have different effects on different pyrolysis processes.For poplar sawdust,which is typical woody biomass,the preferable solution for low-oxygen products was catalytic hydropyrolysis,resulting in the highest hydrocarbon selectivity of 49.14%and DE of 87.6%.For high-ash rice husk,which is a representative of herbaceous biomass,the hydropyrolysis process gained a similar DE(82.7%)to that of the catalytic Hy Py(83.7%),because rice husk contains a large number of mineral elements which are likely to exhibit some catalytic effects.Therefore,rice husk could be considered to be a better feedstock for the hydropyrolysis process to produce high hydrocarbons fuel.The reaction mechanism was inferred according to the product distribution of hydropyrolysis and the quantum calculation of their oxygen-containing products suggests that Rh/ZrO₂catalytic hydropyrolysis was accomplished by multistage reactions,involving initial pyrolysis of raw biomass and further deoxygenation of the oxygenated pyrolysis intermediates.The results of this research help deepen the understanding of the reaction processes and internal mechanism of the catalytic hydrodeoxygenation of bio-oil and the pyrolysis process of biomass to produce low-oxygen bio-oil products.Besides,it provides technical and theoretical support for bio-oil upgrading to reduce the oxygen content and increase hydrocarbons content,thus serving as a scientific reference for the development of pyrolysis technologies of biomass.