Synthesis Of Mesoporous Molybdenum Based Metal Oxides And Their Hydrodesulfurization Performance

Smoggy weather has become more frequent in China during recent years,not only bringing detrimental effects to peoples’health,but also affecting their daily life and work.Researches reveal that emissions from vehicles are one of the main causes for the smoggy weather in big cities,and that sulfur-containing compounds play a key role in its formation.Therefore,the whole world emphasizes a strict legislation on the sulfur contents of fossil fuels.Hydrodesulfurization（HDS）process is the prior choice for most refineries to remove sulfur,and HDS catalysts are regarded as the core.Traditional HDS catalysts encounter challenging tasks when enhanced activities are required,due to the limited loadings of active components and to the strong interaction between active components and support.Therefore,unsupported HDS catalysts with high activities emerged,because of high contents of active components and no interaction between active components and support.Among them Nebula is the typical catalyst.This paper focuses on the preparations of unsupported HDS catalysts by sulfiding the mesoporous molybdenum-containing metal oxide precursors,and the HDS activities towards varioussulfur-containingcompounds,likedibenzothiophene（DBT）,4,6-dimethyl-dibenzothiophene（4,6-DMDBT）and diesel,were evaluated.The physicochemical properties of the unsupported HDS catalysts were fully characterized in order to be correlated with the HDS activities,thus providing theoretical suggestions for industrial applications.A nickel-and molybdenum-containing compound was synthesized by a chemical precipitation method,and various phases of the catalyst precursors were obtained by calcining the NiMo compounds under different temperatures.Results reveal that with the increasing calcination temperature the NiMo precursor with a phase of ammonium nickel molybdate（ANM,（NH₄）HNi₂（OH）₂（MoO₄）₂）began to physically decompose into smaller nanoparticles,and a phase transformation from ANM to nickel molybdate（NiMoO₄）occurred.A further increase of the calcination temperature made the nickel molybdate nanoparticles agglomerate apparently.As for the sulfided unsupported catalysts,the MoS₂ nanoparticles deriving from the ANM precursors had lower stacking numbers and shorter slabs than those from the nickel molybdate precursors.Catalytic results showed that the unsupported catalyst with its precursor calcined at 350 ^oC,which was composed of small ANM nanoparticles,had a higher activity towards DBT than the other catalysts.Besides,based on differences of adsorption/desorption abilities of various compounds onto the catalyst surface and their mutual inhibiting effects,the formation mechanism of bicyclohexane（BCH）was proposed.We suppose that BCH was produced by the sulfur removal of hydrogenated intermediate,not by the hydrogenation of desulfurized product.A scheme of HDS of DBT on the unsupported catalyst was proposed.Effects of zinc promoter on the physicochemical and catalytic properties of unsupported nickel-molybdenum catalysts were studied.A series of Ni ZnMo precursors with various Ni/Zn molar ratios were synthesized by a chemical precipitation method.Based on isostructural replacement Ni could be replaced by Zn,maintaining its original structure of ANM.Incorporations of Zn improved thermal stabilities of the NiZnMo precursors,and the reductions of molybdate shifted to higher temperatures with the increasing contents of zinc in the precursors.Besides,the molybdate in octahedral site was also observed due to the zinc incorporations.The plate-like morphology of the Zn10Mo10 and Ni5Zn5Mo10 precursors changed to the plate-and three-dimensional-morphology of the Ni8Zn2Mo10 precursor.The Ni9.5Zn0.5Mo10 precursor was composed of small nanoparticles about 50 nm attached to large ones.The MoS₂ and Ni₃S₂ nanoparticles were detected on the sulfided unsupported NiZnMo catalysts.Compared to nickel promoter,zinc was not an efficient promoter,and a low activity was observed on a Zn10Mo10 catalyst in HDS of DBT.A Ni9.5Zn0.5Mo10catalyst had a higher specific reaction rate than the other NiZnMo catalysts,attributed to its low reduction temperatures both in the oxide precursor and sulfided catalyst,and also to the complete sulfidation of molybdate.Incorporations of copper promoter into the NiMo component to adjust the physicochemical and catalytic performances were investigated.A series of NiCuMo precursors with various Ni/Cu molar ratios were synthesized by chemical precipitation reactions.Results reveal that due to the similarities of lattice parameters and ionic radii between nickel and copper,nickel could be replaced by copper while maintaining its original ANM structure,indicating that there also existed an isostructural replacement of nickel by copper.Some uniform nanoparticles with sizes of 50 and 30 nm forming a net structure were detected on the Cu10Mo10 and Ni5Cu5Mo10 precursors,respectively.As for the Ni9.5Cu0.5Mo10 precursor,some irregular geometric structure was confirmed with sizes from 100 to 300 nm.For sulfided unsupported NiCu Mo catalysts,a hydrogen consumption peak assigned to the nickel-containing species forming the?NiMoS’active phase shifted to lower temperatures due to the incorporation of copper.A Ni9.5Cu0.5Mo10 catalyst had a higher HDS activity in the conversions of 4,6-DMDBT than the other Ni CuMo catalysts,due to the synergetic effect between the highly stacked MoS₂ nanoparticles and small nickel sulfides.A series of binary transition metal molybdenum/tungsten based precursors（NiMo,NiW,CoMo,and CoW）were synthesized by hydrothermal reactions,and unsupported catalysts were prepared by mixing the precursors with an alumina gel.As for the NiMo and CoMo precursors ammonium nickel（cobalt）molybdate were detected,respectively,while the phases of the CoW and NiW precursors were almost amorphous.MoS₂/WS₂,Co₉S₈,and Ni₃S₂nanoparticles were observed in the sulfided catalysts by HRTEM and XRD characterizations due to the specific spacings and XRD peaks,respectively.The CoMo catalyst had a high HDS activity towards DBT,and direct desulfurization pathway was predominant.In HDS of fluided catalytic cracking（FCC）diesel the NiMo catalyst was very active.The differences between HDS reactions of DBT and FCC diesel were discussed by considering the promoter effect,mechanism of HDS,and the reactivities of various sulfur-containing species.Production of ultra-low sulfur diesel and one step hydrogenation of naphthalene to decalin were studied on the unsupported NiMoW catalysts.The NiMoW precursors with various Mo/W molar ratios were synthesized by a hydrothermal reaction.Increasing molar ratios of Mo/W led to phase transformations from WO₃·0.5H₂O to amorphous and finally to ANM,and an unsupported Ni2Mo0.4W1.6 catalyst had a high activity in HDS of DBT.An unsupported Ni2Mo1W1 catalyst was very active in HDS of FCC diesel,and the contents of sulfur and nitrogen could be decreased to the ultra-low level（no more than 10mg/g）.The mechanisms of ultra-deep HDS and hydrodenitrogenation were discussed.The unsupported Ni2Mo1W1catalyst was also very active in hydrogenation of naphthalene to decalin.At low temperature（200 ^oC）a high conversion of naphthalene（more than 99%）and high selectivity to decalin（more than 98%）were observed,implying the extraordinary high hydrogenation ability of the unsupported catalyst.Besides,product distributions of decalin were far away from equilibrium,and the ratios between trans-/cis-decalin were almost equal.We propose a possible mechanism of naphthalene hydrogenation on the unsupported catalyst.