Preparation Of Highly Dispersed Ni₂P Embedded In Carbon Framework And Its Catalytic Performance For Dehydrogenation Of Isobutane

Highly dispersed nickel phosphide catalysts supported on active carbon and nickel phosphide catalysts embedded in carbon framework were prepared by citric acid sol-gel and alginic acid sol-gel method,respectively.The morphology,composition and structure,hydrogen adsorption performance,specific surface area and pore texture were characterizated by X-ray diffraction（XRD）,transmission electron microscopy（TEM）,infrared spectroscopy（IR）,X-ray photoelectron spectroscopy（XPS）,thermogravimetry（TG）,hydrogen temperature programmed desorption（H2-TPD）and BET.The effects of complexing agent content,calcination temperature and time,phosphidation temperature and time,etc.on catalytic performance for the dehydrogenation of isobutane to isobutene in the presence of hydrogen were investigated.Citric acid could chelate with N i₂+ ion in the impregnation liquid to form a stable gel state.With solvent evaporation a steep increase in viscosity occurs,which inhibits redistribution of impregnated solution during drying of the support,resulting a highly dispersed and small particle size catalyst.Meanwhile,calcination conditions have an obvious influence on the morphology and catalytic performance of the catalyst.The lower calcination temperature could cause citric acid decomposition incompletely and a higher calcination temperature or a longer calcination time could increase the N i₂ P particle size,resulting a lower catalytic performance.In the liquid phosphidation process,a lower phosphidation temperature favors the formation of N i12P5 phase which has a inferior catalytic activity,a higher phosphidation temperature or a longer phosphidation time could also increase the N i₂ P particle size,resulting a lower catalytic performance.CA-Ni₂P/AC catalyst with N i/CA = 1:2.5,calcined at 773 K for 3 h and then phosphorized by PPh3 in liquid phase at 578 K for 3 h has the smallest N i₂ P particle size（6⁷ nm）and the highest catalytic performance for isobutane dehydrogenation.According to the characterization of spent catalysts,t he N i₂ P catalyst prepared by the conventional citric acid sol-gel method is found to be sintered during the reaction,and the size of N i₂ P particle grows.The chelating reaction between N i₂+ and alginic acid could form a N i-alginate gel with three dimensional network structures,which could be carbonized at high temperature in inert atmosphere and then phosphorized by PPh3 in liquid phase at a relatively low temperature to form a highly dispersed N i₂ P embedded in carbon framework.This catalyst posseses not only a higher N i₂ P dispersion but also a good inhibition effect on the sintering of N i₂ P particels.Activity evalution results reveal that the Ni₂P-ADC catalyst derived from N i-alginate gel calcined at 973 K for 5 h and then phosphorized by PPh3 at 578 K for 3 h exhibits the best catalytic performace,selectivity of isobutene higher than 90% and isobutane conversion beyond 16% was achieved under the reaction conditions of 833 K,H2/i-C4H10=1 and SV=1600 h^-1.