This paper is a selective review on design approaches and associated catalysis and chemistry for deep desulfurization and deep dearomatization (hydrogenation) of hydrocarbon fuels, particularly diesel fuels. The challenge for deep desulfurization of diesel fuels is the difficulty of removing the refractory sulfur compounds, particularly 4,6-dimethyldibenzothiophene, with conventional hydrodesulfurization processes. The problem is exacerbated by the inhibiting effect of polyaromatics and nitrogen compounds, which exist in some diesel blend stocks on deep HDS. With the new Environmental Protection Agency (EPA) Tier II regulations to cut the diesel sulfur from current 500 ppmw down to 15 ppmw by June 2006, refineries are facing major challenges to meet the fuel sulfur specification along with the required reduction of aromatics contents. The principles and problems for the existing hydrodesulfurization processes, and the concepts, advantages and disadvantages of various new approaches will be discussed. Specifically, the following new design approaches for sulfur removal will be discussed: (1) novel catalysts for ultra-deep hydrodesulfurization under conventional HDS process conditions; (2) new design concept for sulfur-tolerant noble metal catalysts for low-temperature hydrogenation; (3) new desulfurization process by sulfur adsorption and capture under H2; (4) new desulfurization process by selective adsorption at ambient temperature without H2 and a related integrated process concept; (5) oxidative desulfurization in liquid-phase; and (6) biodesulfurization.

New design approaches to ultra-clean diesel fuels by deep desulfurization and deep dearomatization

High aromatic content in diesel fuel has been recognized both to lower the fuel quality and to contribute significantly to the formation of undesired emissions in exhaust gases [1, 2]. Because of the health hazards associated with these emissions, environmental regulations governing the composition of diesel fuels are being tightened in both Europe and the United States, leading to limitations on aromatics [3, 4].

Aromatic Hydrogenation Catalysis: A Review

Present State of the Art and Future Challenges in the Hydrodesulfurization of Polyaromatic Sulfur Compounds

The hydrodeoxygenation of 2-ethylphenol was carried out under 7 MPa of total pressure and at 340 °C in a fixed-bed reactor over unpromoted Mo/Al2O3 catalyst and over two promoted catalysts (CoMo/Al2O3 and NiMo/Al2O3). For all experiments, dimethyldisulfide was added to the feed to maintain the sulfidation state of the catalysts. On these sulfided catalysts, the transformation of 2-ethylphenol is considered to proceed by three pathways: (1) prehydrogenation of the aromatic ring followed by a dehydration reaction leading to a mixture of alkenes (1-ethylcyclohexene and 3-ethylcyclohexene) and after hydrogenation leading to ethylcyclohexane (HYD pathway); (2) direct cleavage of the Csp2–O bond leading to ethylbenzene (DDO pathway); (3) disproportionation and isomerization reactions leading to oxygenated products (phenol, isomers of 2-ethylphenol and diethylphenols) and their deoxygenated products (ACI pathway). The production of those oxygenated compounds mainly involved the support acidity. The presence of nickel and cobalt allowed an increase of the deoxygenation rate. Nickel only promoted the HYD pathway whereas cobalt promoted both the HYD and DDO pathways. Consequently, the DDO/HYD selectivity was very dependent on the catalyst used. The highest DDO/HYD selectivity was obtained for CoMo/Al2O3. Sulfur vacancies are proposed as active sites for both deoxygenation pathways of 2-ethylphenol, although other active sites (e.g. brim sites) could be involved to explain that the HYD pathway was always predominant. For both deoxygenation pathways, two probable mechanisms are described. The adsorption mode of the molecule most likely determines the deoxygenation route.

Hydrodeoxygenation of 2-ethylphenol as a model compound of bio-crude over sulfided Mo-based catalysts: Promoting effect and reaction mechanism

Hydroprocessing challenges in biofuels production

Structure-activity relationships in hydroprocessing of aromatic and heteroaromatic model compounds over sulphided NiO-MoO3/γ-Al2O3 and Nio-WO3/ γ -Al2O3 catalysts; chemical evidence for the existence of two types of catalytic sites

Factors affecting the hydrogenation of substituted benzenes and phenols over a sulfided NiOMoO3γ-Al2O3 catalyst

Hydroprocessing of dibenzothiophene, phenothiazine, phenoxathiin, thianthrene, and thioxanthene on a sulfided NiOMoO3γ-Al2O3 catalyst

The rate constants of hydrodesulfurization of phenothiazine, phenoxathiine thianthrene and thioxanthene on sulfided NiO-MoO3/γ-Al2O3 catalyst, at 340°C under 70 bar H2, are similar to one another and about 10-fold higher than that of dibenzothiophene.



From comparison with HDS of dibenzothiophene, the presence of a second heteroatom, N,S,O or of a methylene group has no or little effect on the HDS rates of the title compounds. The observed rate enhancement could result from differences in the strength of adsorption between the adsorbed state and the transition state (anchor effect) which depends on whether the molecules are planar or folded.



The comparison of the rate constants of hydrodeoxygenation of homologous oxygenated compounds, xanthene and phenoxazine, with that of dibenzofurane leads to the same conclusion.

Claudine Aubert

Papers

Structure-activity relationships in hydroprocessing of aromatic and heteroaromatic model compounds over sulphided NiO-MoO3/γ-Al2O3 and Nio-WO3/ γ -Al2O3 catalysts; chemical evidence for the existence of two types of catalytic sites

Factors affecting the hydrogenation of substituted benzenes and phenols over a sulfided NiOMoO3γ-Al2O3 catalyst

Hydroprocessing of dibenzothiophene, phenothiazine, phenoxathiin, thianthrene, and thioxanthene on a sulfided NiOMoO3γ-Al2O3 catalyst

Anchor Effect in HDS of Phenothiazine, Phenoxathiine, Thianthrene and Thioxanthene on Sulfided Nio‐Moo3/γ‐Al2o3 Catalyst