Vol.1 No.3 2009

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Research paper : Development of highly-active hydrodesulfurization catalyst for sulfur-free diesel production (Y. Yoshimura et al.)−163 Synthesiology - English edition Vol.1 No.3 (2009) such as benzothiophenes and dibenzothiophenes. C-S bonds in these compounds are broken over the sulfide catalyst, and sulfur is removed in the form of hydrogen sulfide (Eq. 1):Sulfur compounds + H2 → Non-sulfur compounds + H2S (1)This hydrodesulfurization reaction takes place under high-pressure and high-temperature conditions; e.g., a reaction temperature of 330 to 360 °C and a reaction pressure of 3 to 7 MPa After being loaded with the hydrodesulfurization catalyst, the reactor continues operation for about two years. Most hydrodesulfurization catalysts have metal species (mainly metal oxides) such as Mo, W, Co, and Ni on the surface of porous oxides, and are presulfided before hydrodesulfurization.The structure of active sites on a hydrodesulfurization catalyst has been a subject of discussion for many years. Today, a structure model is widely supported in which, for example, in the case of a sulfided CoMo-Al2O3 catalyst, MoS2 particles are present on the porous -Al2O3 support in a highly dispersed state, the Co species is coordinated with the S-edge of the MoS2 particles, and the Co-Mo-S phase with highly active hydrodesulfurization is formed (Fig. 4).Topsøe et al. [1] classified this Co-Mo-S phase into Type I, which has a significant interaction with the support, and Type II, which has a less significant interaction with the support, and showed that the hydrodesulfurization activity of Type II per unit weight of Co is higher than that of Type I. Consequently, in order to improve the performance of hydrodesulfurization catalysts, a catalyst preparation method was developed to selectively create a Type-II Co-Mo-S structure.Cosmo Oil [2][3] developed a catalyst preparation method to multilayer the Type-II Co-Mo-S phase (① in Fig. 5: citric acid is used as a chelating agent in the catalyst preparation process). It was found that high activity could be obtained with an average of about 3.8 layers of the MoS2 phase, and the developed catalyst was commercialized. (It is reported that an alumina support containing zeolite is used as the support for the commercialized catalyst to enhance isomerization activity, in order to avoid steric hindrance by alkyl substituents in sulfur compounds.) AIST [4]-[6] developed a catalyst preparation method to lower the number of stacked layers of the Type-II Co-Mo-S phase (② in Fig. 5). In CoMo/Al2O3 or NiMo/Al2O3 catalysts used for hydrodesulfurization for an extended period of time, hydrodesulfurization activity is maintained to some extent, but there is lower stacking of layers (mostly single layers) and many MoS2 particles with the grown (002) face of MoS2 are present. We therefore considered that they would perform well with lower stacking of layers.Furthermore, we focused on the crystallinity of the Type-II Co-Mo-S phase with the expectation of the following benefits: (1) As a result of increased crystallinity, the sulfur chemical potential of the catalyst is high and sulfur coordinatively unsaturated sites (hydrodesulfurization active sites) on the Co-Mo-S phase are less susceptible to absorptive inhibition by hydrogen sulfide. (2) The basicity of the coordinated sulfur increases, facilitating its promotion of hydrodesulfurization by removing protons from sulfur compounds and activating hydrogen. (3) Due to the increased (9)−Fig. 3 Sulfur compounds contained in gas oil (straight-run gas oil).Fig. 4 Model for the active-phase structure of a molybdenum sulfide-based hydrodesulfurization catalyst.Fig. 5 Approach to high-performance hydrodesulfurization catalysts.(Ni3S2)MoS2

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