Progress in application of binary composite oxides as supports for hydrodesulfurization catalysts

Hydrodesulfurization (HDS) technique has been considered to play a crucial role in the clean, low-carbon, and diverse effective utilization of inferior crude distillates. The key to this technology is the development of catalyst with excellent catalytic performance. After decades of development, although the HDS performances for most sulfides of the non-noble metal supported catalysts have been greatly improved, but their catalytic activities for highly refractory sulfides are still limited due to the over-strong metal and support interaction (MSI), insufficient acidity, poor textural properties and damnable surface environments. Researches across the world made a lot of efforts to solve the above problems and the developing of novel support candidates is considered as the most efficient solution. In this review, we summarized the developments for the applications of binary Al₂O₃ based composite oxides and binary TiO₂ based composite oxides as support for hydrodesulfurization catalyst and systematically analyzed the effect of the second component on both the properties of the catalyst, mainly focused on the acidity property, MSI, pore structures and the catalytic performances, and the applications of the corresponding catalysts in thiophene, dibenzoethiophene, 4,6-dimethyldibenzothiophene and inferior diesel fuels. It was concluded that both the MSI and the acidity can be effectively modulated after incorporation of appropriate amount of SiO₂ into Al₂O₃ support, which can be attributed to the successful formation of Al−OH−Si linkages over the support surface and thus prevented the formation of excessive Mo−O−Al bonds, resulted in the enhanced hydrogenation activity of the corresponding catalyst which further contributed to the excellent HDS performance. The introduction of ZrO₂ into Al₂O₃ support can also modulate the MSI and the acidity due to the similar reasons, except for that, researchers also found that the reducibility of the active phase precursors can be effectively enhanced, which is favorable for the formation of more active phases. Introducing small amounts of MgO into Al₂O₃ can effectively enhance the dispersion of active metals over the support surface and promote the formation of Ni(Co)−O−Mo(W) precursors, then acquiring more Ni(Co)Mo(W)S active phases. The introducing of B₂O₃ can effectively lower the density of hydroxyls and promote the formation of octahedral coordinated Mo species which can be easily sulfided. In summary, the introduction of the second component into Al₂O₃ successfully overcame the disadvantages such as the solely acid type and the strong interaction between the metal and the support materials over Al₂O₃ based hydrodesulfurization catalyst, and the advantage of high specific surface area remained. The addition of SiO₂ into TiO₂ support can effectively improve both the acidity property and the stability of the catalyst, moreover, the specific surface area of the catalyst can also be enlarged after SiO₂ addition. After introduction of ZrO₂ into TiO₂, the density of hydroxyl groups over the support surface decreased, the dispersion of active metals improved and the high stacking Mo(W)S₂ slabs formed, thus enhanced the direct desulfurization pathway selectivity. Addition of basic MgO into TiO₂ support can enhance the MSI and thus improve the dispersion of active metals over the support surface due to the strong interaction between the basic-acidic pairs. In summary, the introduction of the second component not only improved the thermal stability and the specific surface area, but also modulated the acidity properties. The main factor causing these changes is that the introduction of the second component profoundly changed the hydroxyl environments. Which further improved the anchorage and dispersion of the precursors over the support surface and promoted the formation of more NiMo(W)S active phase, resulted in the enhanced hydrodesulfurization performances of the corresponding catalysts.