Synthesis, Characterization And Hydrodesulfurization Performance Study Of Ni-Mo-W Bulk Catalyst

Environmental regulation is becoming more and more restrictive for sulfur and aromatics contents in diesel fuels. Utilizing of hydrotreating catalysts with ultra-high hydrogeneration performance would be the less costly and the most effective way for the production of clean diesel fuels. It is hard for conventional supported catalyst to enhance its activity further due to the limitation of active component capacity on the support, so the supported catalyst can not fulfill the requirement of ultra-low sufur, nitrogen and aromatics clean diesel fuels.In the dissertation, a novel preparation method of the bulk hydrotreating catalyt was developed. This method was different from the present technology, in which Ni-Mo(-W) bulk catalysts were prepared by deposition-precipitation with ammonium molybdate, ammonium metatungstate, nickel nitrate as precursors, and silicon dioxide and diatomite were employed as dispersion medium to disperse active metals. The catalyst prepared by this method can obtain excellent properties, such as high surface area, mesopore distribution, and more importantly, active components are deposited on the surface of dispersion medium and homogeneously distributed, which can improve the utilization efficiency of the active metals. And meanwhile, the loading of active components in the bulk catalyst can be arbitrarily adjusted by variations of molar ratio of SiO₂ to active metal components, thereby the cost/performance ratio is optimized.Silica-dispersed Ni-Mo-W catalyst processes high surface area and large pore volume. XRD characterization shows that the pure Ni-Mo-W bulk catalyst displays a microcrystalline structure of Ni-Mo-W components, while the silica-dispersed Ni-Mo-W trimetallic catalyst is a typical amorphous material. The reason may be that metal components present as small nano-sized particles and are well dispersed on the surface of silica (as confirmed by TEM), so they can not be detected by XRD method. Silica-dispersed Ni-Mo bimetallic catalyst displays nickel molybdate nanocrystals, which are dispersed on the silicate platelets. The surface area and pore volume of silica-dispersed bulk catalysts are much higher than that of comparative catalysts. HDS test of DBT showed that HDS activity of Ni-Mo(-W)/SiO₂ was higher than that of commercial Ni-Mo alumina-supported catalyst and comparative catalyst.The surface area and pore volume of diatomite-dispersed Ni-Mo-W catalyst prepared by deposition-precipitation method are higher than pure Ni-Mo-W bulk catalyst, indicating diatomite is an effective dispersion medium. At the same time, diatomite can improve crushing strength of the bulk catalyst, so diatomite plays a role as an effective binder. The preparation parameters of Ni-Mo-W/diatomite catalyst were investigated in detail and the optimun conditions were determined. It is found Ni-Mo-W components have a layered structure, the sheet of which is made up of Ni and W elements, yet Mo is located in the interlayer region. XRD pattern of sulfided Ni-Mo-W/diatomite catalyst exhibits an obvious peak which is characteristic of the (002) basal planes of crystalline MoS2, this indicates the high stacking mumbers of MoS2 or WS2, thus the active phase mainly exists in the form of type II Ni-Mo(W)-S with higher activity. The content of Ni in the catalyst has a great effect on catalytic activity, the reason is: firstly, Ni sulfide microcrystallites can serve as the support to support MoS2 or WS2 nanoparticles; and secondly, Ni sulfide microcrystallites can control the morphology of the MoS2 or WS2 nanoparticles to result in producing highly dispersed flakes with a high proportion of edge and corner sites, which can improve catalytic activity obviously.The activities of Ni-Mo-W/diatomite bulk catalyst were evaluated utilizing the model compounds and FCC diesel oil as the raw materials in a continuous microreactor, and comparied with a commercial supported catalyst. Qualitative analysis and quantitative analysis were carried on for sulfur compounds in the diesel and hydrogenated diesel by using GC-PFPD in order to study removal mechanism of sulfur compounds on the different catalysts. It is shown that Ni-Mo-W/diatomite bulk catalyst has much higher HDS, HDN and HDAr performances than the commercial supported catalyst. Py-IR adsorption patterns of catalysts indicate there are Br?nsted acid sites and Lewis acid sites in the comparative supported catalyst, but only Lewis acid sites exist in Ni-Mo-W/diatomite bulk catalyst, the main reason for higher HDS, HDN and HDAr performances should be the different stacking numbers and the curvature of MoS2 or WS2 in the two kinds of catalysts: Ni-Mo-W/diatomite bulk catalyst has higher stacking numbers, so the active phase mainly exists in the form of type II Ni-Mo(W)-S, and the curvature of MoS2 or WS2 can provide more active sites, these result to higher hydrogenation activity than the commercial supported catalyst. In the course of HDS of diesel oil, benzothiophene and its derivatives are firstly removed, subsequently dibenzothiophene and its derivatives. The residual sulfur compounds in the hydrogenated diesel oil are alkyl substituted dibenzothiophene derivatives. Due to the big steric hindrance of alkyl substituted dibenzothiophene derivatives, they are mainly removed by hydrogenation (HYD) pathway, and Ni-Mo-W/diatomite bulk catalyst has higher hydrogenation activity than the comparative supported catalyst, thus Ni-Mo-W/diatomite bulk catalyst displays higher ability for the removal of dibenzothiophene and its alkyl substituted derivatives.