The hydrodesulfurization (HDS) of a diesel oil was carried out in a batch autoclave reactor over the temperature range 280-420 °C for 0-90 min under a total pressure of 2.9 MPa, using CoMo and NiMo catalysts in both one and two stages. The HDS reactivities of benzothiophenes, dibenzothiophenes (DBTs), and their alkylated homologes existing in the diesel fuel were examined in detail by means of respective quantitative analyses. The sulfur compounds can be classified into four groups according to their HDS reactivities which were described by their pseudo-first-order rate constants. DBTs carrying two alkyl substituents at the 4-and 6-positions, respectively, were the most resistant to desulfurization. H2S produced from reactive sulfur compounds in the early stage of the reaction is one of the main inhibitors for HDS of the unreactive species. A second stage using fresh hydrogen solved this inhibition problem, with NiMo achieving deeper desulfurization.

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Biohydrogenated diesel (BHD) and liquefied petroleum gas (LPG) fuel were produced by the hydrotreatment of vegetable oils over Ni–Mo-based catalysts in a high-pressure fixed-bed flow reaction system at 350 °C under 4 MPa of hydrogen. Because triglycerides and free fatty acids underwent the hydrogenation and deoxidization at the same time during the reaction, various vegetable oils (jatropha oil, palm oil, and canola oil) were converted to mixed paraffins by the one-step hydrotreatment process although they contained quite different amounts of free fatty acids. Ni-Mo/SiO2 formed n-C18H38, n-C17H36, n-C16H34, and n-C15H32 as predominant products in the hydrotreatment of jatropha oil. These long normal hydrocarbons had high melting points and thus gave the liquid hydrocarbon product over Ni-Mo/SiO2 a high pour point of 20 °C. Either Ni-Mo/H-Y or Ni-Mo/H-ZSM-5 was not suitable for producing BHD from jatropha oil because a large amount of gasoline-ranged hydrocarbons was formed on the strong acid sites of zeolites. When SiO2-Al2O3 was used as a support for the Ni-Mo catalyst, the pour point of the liquid hydrocarbon product decreased to −10 °C by converting some C15–C18 n-paraffins to iso-paraffins and light paraffins on SiO2-Al2O3. Because SiO2-Al2O3 had a proper solid acidic strength, both the chemical composition and the pour point of liquid hydrocarbon product over Ni-Mo/SiO2-Al2O3 were similar to those of a normal diesel bought from a petrol station. Meanwhile, the glycerin groups in the vegetable oils were converted to propane over Ni-Mo/SiO2-Al2O3 by the hydrogenation and deoxidization. Therefore, the liquid hydrocarbon product can be directly used as a BHD fuel for the current diesel engines, and the gas hydrocarbon product can be used as a liquefied petroleum gas (LPG) fuel in the hydrotreatment of vegetable oils over Ni-Mo/SiO2-Al2O3.

The hydrodesulfurization (HDS) of a vacuum gas oil (VGO) was performed at 360 °C (6.9 MPa) over a commercial NiMo catalyst to examine the HDS reactivities of various sulfur compounds which exist in the VGO by means of quantitative pseudo-first-order kinetic analysis. Four representative types of aromatic−skeleton sulfur compounds were observed in the VGO:  alkylbenzothiophenes (BTs), alkyldibenzothiophenes (DBTs), alkylphenanthro[4,5-b,c,d]thiophenes (PTs), and alkylbenzonaphthothiophenes (BNTs). Among these, alkyl-BTs exhibited the highest HDS reactivity, whereas alkyl-DBTs with alkyl substituents at the 4 and/or 6 positions appeared to have the least reactivity even though their aromatic−skeleton is smaller than those of both alkyl-PTs and -BNTs. Steric hindrance of alkyl groups at specific positions appears to be a major reason for the low reactivity. Quantum chemical calculations on representative sulfur compounds were carried out to compare molecular parameters with their different HDS reactivities.

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