Research On Unsupported NiAlZrW Hydrodesulfurization Catalyst Based On LDHs

The development of more active hydrodesulfurization(HDS)catalyst is considered to be the most effective and economic way to achieve clean diesel.However,it is difficult to achieve the deep desulfurization over the conventional Ni(Co)-W(Mo)catalysts supported on Al2O3 due to their poor activity for the refractory sulfides,such as dibenzothiophenes(DBTs),which may be caused by the difficulty in the combination of high metal loading and high dispersion.Layered double hydroxides(LDHs)are the versatility of metal cations in the layers joined with the exchange ability of anions in the interlayer.Therefore,the most common method to introduce active metals into LDHs was to incorporate Ni in the layers and to intercalate tungstate into the interlayer,which would be beneficial to HDS.After calcination,the unsupported HDS catalysts with high active metal content and high dispersion could be achieved.First,a series of tungstate intercalated NiAlZr LDHs with different Zr contents were synthesized by an ion exchange(IE)method.They were further calcined and used as HDS catalysts.A reference catalyst with the same metal contents was prepared by the conventional impregnation(IM)method.The as-synthesized catalysts were characterized by XRD,FT-IR,nitrogen adsorption/desorption,Raman,TG-DSC,SEM,H2-TPR and H2S-TPS.The influences of Zr contents on the property and activity of catalysts were studied.Second,the NiAlZrW HDS catalysts were prepared by IE and a co-precipitation(CP)methods in order to study the influences of catalyst preparation methods on the property and activity of catalysts.The results were listed as follows.1.Tungstates were sucessfully intercalated into the interlayer of NiAlZr LDHs by the IE method.The well ordered LDHs were obtained in Zr/Al ratio up to 0.4/0.6.Aftrer calcination at 450?,tungsten species were highly dispersed on the catalysts even at the tungsten content above 54 wt%,owing to the peculiar layered structure of LDHs,which facilitates the uniform dispersion of tungsten species on both the surface and edge of layer.The controlled calcination temperature of LDHs offers the well retained layer structure,which could act as separator to allow W spcies highly dispersed on catalysts.2.Highly dispersed NiWO4 phases were formed on the catalysts because of the reaction between Ni in the layer and tungstate in the interlayer upon calcination.The incorporation of Zr partly substituted Al in the layer and hence could weaken the strong interaction between W and Al,which may facilitate the intimate interaction between W and Ni to form more NiWO4 phases and further improve their reducibility and sulfidability.Such well dispersed and easily reducible NiWO4 phases may be the precursors of active Ni-W-S species.Therefore,the catalysts containing Zr exhibit the superior HDS activity for both DBT and diesel than the catalyst without Zr.Moreover,bulk WO3 and NiO particles are formed on the reference NiW/AlZr catalyst prepared by the IM method,leading to lower HDS activity.3.The NiAlZrW HDS catalysts were prepared by the IE method and the CP method,respectively.For the ion exchange method,well ordered LDHs were achieved.After calcination,the layers can be retained and act as support and separator to allow amorphous NiWO4 highly dispersed on catalysts.The substitution of Zr for Al in the brucite-like layers decreases the amount of Al neighboring around W and hence decreases strong W-O-Al linkages,improving the reducibility and sulfidability.Therefore,the DBT HDS activity and HYD selectivity were considerably enhanced.For the coprecipitation method,less ordered LDHs were formed and crystalline NiWO4 particles were present on catalysts after calcination.The introduction of Zr further stimulates the crystal growth and aggregation of NiWO4.Such NiWO4 crystallites were difficult to reduce and sulfurize,leading to the decreased HDS activity.