Study On Aqueous-phase Reforming Of Low-boiling Fraction Of Bio-oil For Hydrogen Production

The rapid growing consumption of fossil fuels and depletion of total crude-oil reservations lead to global energy crisis.The utilization of bio-oil as a potential fuel substitute is an effective energy alternative to satisfy human society’s need for energy.Bio-oil is a multicomponent oxygenated mixture,containing alcohols,acids,ketones,aldehydes,phenolic derivatives and water.However,because of its poor homogeneity,thermal instability,corrosiveness and low heating value,further modification and upgrading are essential for its utilization as fuel.Low-boiling fraction（LBF）was obtained by distilling the crude bio-oil at reduced pressure.The water content of LBF can be as high as 70-85 wt.%,and the oxygenated organics in LBF have low boiling points which mainly consist of acids,aldehydes and esters.Considering the large content of water in LBF,it is quite difficult to remove the water completely because some of these compounds may form azeotropes and their boiling points are lower than water.Nevertheless,abandoning it is not only a poor efficiency use of biomass but also a pollutant to the environment.Our previous studies have showed that producing hydrogen through aqueous phase reforming（APR）of LBF was a promising way to categorize it as resource rather than waste.Pt supported on alumina was considered to be one of the best monometallic catalysts in terms of activity and selectivity for APR of LBF.In this thesis,exploratory studies have been conducted on the APR of LBF for hydrogen production from the following aspects:1.We investigated the particle size effects of Pt/Al₂O₃ on the reactivity for APR of LBF.A series of Al₂O₃ supported Pt nanoparticles catalysts with different particle size were successfully synthesized.Results showed that the activity of this reaction and selectivity to hydrogen increased as the Pt particle size decreased which meant this reaction was structure sensitive.Base on the full-shell cuboctahedron model,the numbers of surface,vertex and edge atoms of different Pt/Al₂O catalysts were estimated.It was found that the activity of the catalyst was related to the number of platinum atoms on the edge of the particle,and the normalized turnover frequency was a constant,indicating that edge sites were the main active sites for APR reaction.Because they could provide more stable Pt-C adsorption bonds and facilitate the cleavage of C-C bonds to form H₂ and CO₂.2.Though Pt/Al₂O₃ catalysts showed excellent activity and selectivity for APR of LBF,in the second run its activity was only 40%of the fresh ones.So,it was really important to study the reason for its deactivation.The used catalyst was characterized by X-ray diffraction（XRD）,transmission electron microscopy（TEM）,scan electron microscope（SEM）and thermogravimetric analysis（TG）.It demonstrated under aqueous phase conditions alumina would undergo a phase transformation from γ-Al₂O₃ to AlOOH,which promoted the formation of coke species and accompanied with great decrease in its surface area,and the surface area of Pt decreased greatly because its active surface was covered by boehmite and coke deposits.After being calcined in air,the structure of carrier could be restored and most of the coke deposits could be eliminated.However,there were still some coke deposits that clog the micropores of Al₂O₃ and the particle size experienced apparent increase.With the increase of the number of recycle runs,the catalytic activity and the selectivity to hydrogen decreased obviously,showing that the reusability of Pt/Al₂O₃ is poor.3.In order to explore platinum-based catalysts with excellent catalytic activity,selectivity and stability,a series of CeO₂-ZrO₂,CeO₂-TiO₂ and TiO₂-ZrO₂ materials with different composition were prepared.With the help of BET and XRD analysis,their hydrothermal stability was studied by subjecting the samples to HAc solutions under aqueous phase reforming conditions.Most of them,especially C1Z1（50mol%CeO₂ with 50mol%ZrO₂）and C1T1（50mol%CeO₂ with 50mol%TiO₂）,exhibited superior stability with no phase transformation and only minimal decrease in their surface area was observed even after 16 hours.4.After being loaded by Pt,these catalysts were used in the APR of LBF to investigate their reducibility and catalytic performance.Among all the three Pt/CZ and Pt/CT catalysts,Pt/CIZ1 and Pt/C1T1 showed lowest reduction temperature and the best catalytic activity respectively.The reason might be that during the reduction process some well-dispersed CeO₂ on the surface of catalysts were also reduced because of the hydrogen spill-over effect,which generated a great number of O vacancies.These O vacancies could promote the adsorption and conversion of polar oxygen molecules in LBF,so as to improve the conversion of reactants.Then,Pt/C1Z1 and Pt/CITl were chosen to investigate their recyclability.The catalytic activity and H2 selectivity of Pt/C1Z1 and Pt/C1T1 could be almost recovered after being calcined in air at 773K for regeneration.After three cycles,the particle size of Pt/C1Z1 and Pt/CIT1 only experienced a slight increase,and formed trace amount of carbon deposition.So,compared with Pt/Al₂O₃ catalysts,Pt/C1Z1 and Pt/C1T1 catalysts revealed comparative catalytic activity and H₂ selectivity,but much better hydrothermal stability and recyclability.5,Using the main typical components in LBF as follows:acetic acid,methyl acetate,acetol,furfural and guaiacol as model compounds and analyzing the results of gas and liquid products,the possible reaction pathways of the individual components were deduced.While in the liquid products only incompletely reacted reactants were detected.The real gas phase product results were also different from theoretical values.Then,our experimental results showed that there was a gap between the individual model reactions and the actual reactions of the LBF which was a complex system.Therefore,it was quite meaningful to directly study the LBF complex system as reactants.