Hydrodeoxygenation Of Dibenzofuran Over Supported Noble Metal Catalysts

With the decline of fossil fuels and increasing demands of energy resources, upgrading of unconventional oils (i.e. biomass-derived oils, shale oil, coal tar, et al.) has been proved to lower dependence on the fossil fuels and relieve the pressure of greenhouse gas emission. The high oxygen content in the unconventional oils results in poor thermal stability and low heating value. Catalytic hydrodeoxygenation (HDO) is an efficient way to remove the heteroatom oxygen and transform unconventional oils into high-quality fuels. Noble metal catalysts are preferably used in the HDO due to their superior low-temperature activity and deoxygenation performance. In this thesis, the HDO performance of supported Pt, Pd, and Ru catalysts are tested in the conversion of dibenzofuran (DBF). The effect of metal and support acidity on the HDO activity and selectivity toward cleavage of C-O bond are discussed, also the function of metal and acidic sites in the HDO reactions are explained. Considering the application of noble metal catalysts in the hydrotreating of real oils, the introduction of fluorene and indole into the HDO reaction over supported Pt catalysts is conducted. The main contents and results of this thesis are listed below.First, in order to screen suitable noble metals for HDO catalysts, HDO reactions are conducted over mesoporous silica supported Pt, Pd, Ru catalysts under 280-300 ? and 3.0 MPa. The HDO reaction mainly goes through hydrogenation reaction route. After a comparative study the HDO performance over supported Pt, Pd, Ru catalysts, Pt catalyst shows superior HDO activity with a higher TOF value and selectivity toward ring hydrogenated species, while Ru catalyst is highly selective in the deoxygenation. Based on the qualitative analysis of HDO products by GC-MS, new reaction intermediates dodecahydrodibenzofuran and 2-cyclohexylcyclohexanone are detected. A more detailed HDO reaction network of DBF over mesoporous silica supported noble metal catalysts is schemed, which makes an contribution to the HDO research of furans.Then, by introducing alumina in the synthesis of mesoporous silica to adjust the support acidity, the purpose of designing and preparing silica-alumina supports for enhanced HDO performance is achieved. In combination with the quantitative analysis of support acidity by NH3-TPD and FT-IR study of pyridine adsorption, it is clear that enhanced acidity in the Pt catalysts contributes the HDO conversion of DBF, where the kHDO increases from 7.0×104 mol molsurf. pt min-1 (Pt/SiO2) to 19.1 ×104 mol molsurf. pt min-1 (Pt/Al-mSiO2-20). Correspondingly, a higher selectivity toward deoxygenated products (12% vs.62%) is observed. Comparing the HDO results in the absence and presence of cyclohexanol, it is claimed that metal sites are responsible for hydrogenation of aromatic rings and hydrogenolysis of C-O bond. Whereas, the Br(?)nsted acidic sites promote the cleavage of C-O bond and catalyze the dehydration of hydroxyl groups, while the Lewis acidic sites are related to the activation of oxygenates.Next, due to that the useful acidic sites for the HDO reaction are existed on the surface of support, surface modification of mesoporous silica supports by depositing oxides is an efficient way to improve the HDO performance of Pt catalysts. Depositing Al2O3 or ZrO2 over SiO2 via NH2/water vapor-induced internal hydrolysis method largely promotes the dispersion of Pt nanoparticles with reduced particle size from 4.5 nm to ?2.5 nm, which accelerates the hydrotreating of dibenzofuran. The surface acidic sites over Al2O3/SiO2 and ZrO2/SiO2 help the cleavage of C-O bond with raised selectivity to deoxygenated products. It is apparent that highly dispersed Pt nanoparticles and acidic sites function cooperatively to transform the hexahydrodibenzofuran preferably via hydrogenolysis of C-O bond to 2-cyclohexylphenol over Pt/Al2O3/SiO2. The selectivity toward hydrogenolysis route exceeds 60%, which is well above that over Pt/ZrO2/SiO2(?25%) and Pt/SiO2(<22%). In the pseudo-first-order reaction kinetic analysis, the HDO activity of prepared Pt catalysts enhances after surface modification of SiO2 (Pt/Al2O3/SiO2>Pt/ZrO2/SiO2>Pt/SiO2). The apparent activation energy of HDO reaction is?70 kJ mol-1.Finally, regarding to the application of noble metal catalysts in the hydrotreating of real unconventional oils, it is desired to explain the influence of fluorene and indole to the HDO of DBF over supported Pt catalysts. The hydrotreating of fluorene involves ring hydrogenation, hydroisomerization, and hydrocracking. The Pt/Y zeolite catalysts shape-selectively catalyze the hydroisomerization of perhydrofluorene to dodecahydrocyclopenta[a]naphthalene and perhydrophenalene, leading to 63% yield of isomers. After introducing fluorene and indole in the HDO reaction over Pt/Al2O3, the hydrotreating of fluorene competes with the hydrogenation of aromatic rings of DBF, leading to a decreased selectivity (43%vs.38%) to ring hydrogenated intermediates. Whereas, the hydrodenitrogenation of indole strongly inhibits the hydrogenolysis of C-O bond, consequently the selectivity of oxygen-containing intermediates and deoxygenated products decrease from 22% and 35% to 13% and 17%, respectively. The transformation of DBF is inhibited by the competitive adsorption behaviors of reaction species over the surface of catalyst. The above research guides the application of Pt catalysts in the refinery of unconventional oils.