Construction Of Catalysts For The Selective Hydrogenolysis Of Dibenzofuran To Chemicals

With the decrease of fossil-derived resources and the increase of economic demand for energy,the unconventional oils(bio-oil,coal tar,shale oil,etc.)have been developed to gradually replace fossil-derived resources.The highly oxygenated compounds in the unconventional oils,that are the undesirable fuel characteristics,can be converted to chemicals by catalytic hydrogenolysis.In this thesis,the various catalysts were designed and used to the selective hydrogenolysis of dibenzofuran(DBF)to aromatic compounds with desirable selectivity.The great efforts were to produce the o-phenylphenol(OPP)or biphenyl(BP)by controlling selective cleavage of C-O bond with unsaturation of aromatic ring.The main research contents and results of this thesis are listed as follows:(1)In order to explore the influence factors of reaction pathways,including the hydrogenation pathway(HYD)and direct deoxygenation pathway(DDO),the hydrogenation of DBF was performed over high-surface-area magnesia supported Pt,Pd,Ru catalysts under 360-400℃C and 0.1-3.0 MPa.The catalysts could obviously decrease the activate barrier of reaction and the intrinsic activity(TOF)decreased in the trend:Pt/MgO(0.36 s-1)>Ru/MgO(0.29 s-1)>Pd/MgO(0.09 s-1).Pt had a high activity in the hydrogenation of DBF,and exhibited a perfect deoxygenation activity followed by hydrogenation(HYD)pathway.Ru showed better cleavage ability of Caryl-O bond and the removal of oxygen from DBF mainly occurred via direct deoxygenation(DDO)pathway.However,the intrinsic activity(TOFDDo)of DDO pathway on Ru was similar as the Pt catalyst.The increased Pt loadings largely promoted the conversion of DBF by enhancing both HYD and DDO pathways.In addition,more direct cleavage of Caryi-O bond occurred at higher temperature and the production of aromatics by the DDO pathway preferred the relatively low reaction pressure.Therefore,a suitable metal site,together with the favorable temperature and H2 pressure were crucial factors for controlling multiple steps to achieve target routes.(2)By designing the Pt catalysts with different acid-base supports,we explored the effect of acid-base property on the hydrogenolysis of DBF to OPP.The OPP selectivity increased with the improvement of supports basicity:Al2O3<MgO/Al2O3<SiO2<MgO<MgO-900.Highly efficient Pt/MgO catalysts had been developed for selective hydrogenolysis of DBF to OPP with high selectivity(80%)and yield(48%)at 400℃ and 1.0 MPa.The basic sites promoted the formation of OPP and acidic sites promoted the further cleavage of C-O bond to BP.Besides,the dehydrogenation of OPP to DBF was observed on basic catalysts and the stronger basicity promoted the dehydrogenation phenomenon,meanwhile increasing reaction temperature also accelerated dehydrogenation of OPP.(3)By Mg modified Pt/SiO2 catalyst to adjust acid-base property,we investigated the promotional effect of varying Mg loading on Pt/SiO2 catalyst for the hydrogenolysis of DBF to BP.Small amounts of Mg enriched the electron density of Pt surface due to charge transfer and increased the Lewis acidity due to the formation of Si-O-Mg bond,leading to significant improvement of the reactivity with an excellent turnover frequency(TOF)of 0.33 s-1 on Pt/3MgO/SiO2 compared with 0.22 s-1 on Pt/SiO2.Moreover,the addition of Mg also increased the basicity of catalyst,promoted the desorption of BP,and then improved the selectivity for BP with 68%.With the same moles of alkaline earth dopants,the activity of dopants decreased in the order:Mg>Ca>Sr>Ba,which was attributed to the decrease in Pt dispersion resulting from the structural change and the decrease in electronegativity of alkaline earth metals.Additionally,the addition of Mg effectively stabilized Pt nanoparticles and the Pt/3MgO/SiO2 catalyst was stable during the 240 h run.(4)The Pt/MgO catalyst was modified by oxophilic Mo,aiming at improving the deoxygenation activity and BP selectivity.The addition of Mo species could significantly improve the reducibility of the Pt surface species and increase the acidity of catalysts.The existence of synergic effect between Pt species in affording hydrogen species and neighboring MoOa species in adsorbing/activating C-O bond promoted the formation of BP and then improved the HDO reaction rate by 26 times.Moreover,the catalytic activity can be controlled by MoOx surface density and vary with the increased MoOx surface density in a volcano-shape manner.At low Mo coverage(<2.27 Mo/nm2),the dispersed MoOx species were dominant that promoted the reaction activity,whereas the formation of crystallite MoO3 at higher MoOx surface density(>6,19 Mo/nm2)reduced the availability of reactive MoOx species and then decreased the reaction activity.The highest BP yield(100%)was obtained with Pt/7MoOx/MgO in which the molybdate species formed a near-perfect monolayer on the support.(5)The Mo species have high deoxygenation ability but the low reaction activity.Therefore,the MoO3 was used as the catalyst and Co(Ni)as the modifier,aiming at the identification/elucidation of the active sites and deactivation of Co(Ni)/MoO3 for high-yield production of BP.The activity of Mo species in MoO3 catalyst decreased as follows:Mo5+>Mo6+>>Mo4+>Mo0.A strong promotion effect in activity between Co(Ni)and Mo in Co(Ni)/MoO3 catalysts was observed,resulted from specific Co(Ni)-MoO3 interaction in which Co(Ni)facilitated the reductio1 of Mo6+to Mo5+species and the formation of acidic Co(Ni)MoO4 species.The best catalytic activity was realized with a 100%yield of BP over Co/MoO3.During 300 h catalyst life test,the MoO3 and Co/MoO3 catalysts underwent a gradual deactivation resulted from the over-reduction of Mo species to less active MoO2 and carbon deposition.While Co/MoO3 showed better stability than MoO3 due to the formation of more MoOxCyHz species arising from the acidic CoMoO4 which prevented the further reduction of Mo species.Importantly,the deactivation of catalysts can be regenerated by calcination without losing its initial activity.