Preparation of Ni3P-Based Catalysts and Their Catalytic Performances in Phenol Hydrodeoxygenation

Author:Yu Zhi Quan

Supervisor:wang an jie wang yao


Degree Year:2019





Bio-oil is the only liquid fuel,which can be used to produce engine fuel as an alternative to fossil fuels,among the renewable energies.Nevertheless,bio-oil has a high oxygen content,which leads to undesired properties such as low heating value,high viscosity,low thermal stability and poor miscibility with petroleum fractions.It is,therefore,essential to remove oxygen to upgrade bio-oil.Hydrodeoxygenation(HDO)is regarded as one of the effective routes for oxygen removal from bio-oil,and it is of significant importance to develop high-performance HDO catalysts.Metal phosphides,which exhibit comparable hydrogenation activity to noble metals,show great promise in hydrotreating catalysis,including hydrodesulfurization,hydrodenitrogenation and HDO.Among them,nickel phosphides are reported to be most active in hydrotreating catalysis.In the present study,a novel phase(Ni3P)was synthesized and tested in phenol HDO both in aqueous phase and in oil phase.Acid was introduced by adding promoters or by depositing Ni3P over acidic supports in order to enhance the dehydration of cyclohexanol and thus the overall HDO activity.An electroless plating approach was developed to prepare highly dispersed Ni3P crystallites on various supports,including alumina.The kinetics of phenol hydrogenation phenol and hydrogenation of cyclohexanol were investigated separately so as to establish the reaction network.In addition,the structure-performance relationship of Ni3P in phenol HDO was investigated by charactering the catalysts by means of XRD,N2 physisorption,XPS,TEM,NH3-TPD,Py-FTIR,CO chemisorption,H2-TPR,TG-DSC,and ICP analysis.It is found that bulk Ni2P,which was prepared by temperature-programmed reduction,was not stable in aqueous phase HDO of phenol.When the reactor was heated in a specific program,Ni2P was transferred into Ni3P,which exhibited higher hydrogenation activity than Pd/SiO2(1.0 wt.%)and markedly higher than Ni at low temperatures(150-200℃).When decalin was used as the solvent in phenol HDO,Ni3P was superior to Ni2P or Ni12P5.XPS characterization revealed that the electron transfer from Ni to P was weakest and thus exhibited highest ability of hydrogen dissociation.In the HDO products catalyzed by Ni3P,Ni2P and Ni12P5,no benzene was detected,suggesting that phenol HDO proceeded exclusively by the hydrogenation(HYD)path.It is shown that cyclohexanol was predominant in HDO products(>80%)at low temperatures,suggesting that cyclohexanol conversion,dehydration followed by hydrogenation,was rate-determining.The kinetics of phenol hydrogenation and cyclohexanol conversion were investigated separately.Ni3P-CePO4 catalysts were prepared by a "one-pot" procedure.It is shown that the presence of CePO4 improved the dispersion of Ni3P particles,leading to enhanced hydrogenation activity.Measurements of NH3-TPD and Py-FTIR displayed that medium Lewis acid,which catalyzes the dehydration of cyclohexanol,was present on the surface of CePO4.As a result,Ni3P-CePO4 catalysts showed enhanced HDO performance.XRD characterization demonstrated that CePO4 was precipitated when Ce(NO3)3 and(NH4)2HPO4 were mixed in the solution.CePO4 survived in the subsequent calcination and temperature-programmed reduction in the preparation of Ni3P.Nickel phosphides supported on various carriers(SiO2,γ-Al2O3 Hβ and HZSM-5)were prepared by conventional impregnation method.It was found that the support significantly affected the formation of nickel phosphide phases.When SiO2 was used as the support,Ni3P,Ni12P5 and Ni2P were generated by temperature-programmed reduction from precursors with initial Ni/P mole ratios of 3,2 and 1,respectively.When γ-Al2O3 was the support,only Ni was obtained from precursors with various Ni/P ratios.The absence of nickel phosphides was linked with the large amount loss of P due to the reaction of P species with Al at elevated temperature in calcination.When Hβ was used as the support,Ni,Ni-Ni3P and Ni2P were produced from the precursors with Ni/P ratios of 3,2 and 1,respectively.When HZSM-5 was the support,Ni3P-Ni,Ni12P5 and Ni2P were generated from the precursors with Ni/P ratios of 3,2 and 1,respectively.Interestingly,when the procedure was modified by eliminating the drying of the precursor,Ni3P/HZSM-5 was obtained from the precursor with Ni/P ratio of 3.Highly dispersed Ni3P crystallites were deposited on various supports,including γ-Al2O3,SiO2,TiO2 and HZSM-5,by means of electroless plating under mild conditions.The ICP analysis indicated that the Ni/P mole ratio in the prepared catalysts was around 3,and nano-sized crystallites(4.3~8.2 nm)of Ni3P and their characteristic lattice fringes were observed in HRTEM images.It is found that the time in deposition-precipitation was the main factor in determining the loading and particle size of supported Ni3P particles.Within 16-h deposition-precipition time,the loading of the Ni3P/HZSM-5 catalyst was the highest,while the particle size of the catalyst was the smallest(~4.3 nm),leading to the highest activity in phenol HDO.