Removal of refractory S-containing compounds from liquid fuels on novel bifunctional CoMo/HMS catalysts modified with Ti T.A. Zepeda a , B. Pawelec b, * , J.L.G. Fierro b , T. Halachev a a Centro de Fı ´sica Aplicada y Tecnologı ´a Avanzada-UNAM, Campus Juriquilla, Quere ´taro, A.P. 1-1010, 76230, Me ´xico b Instituto de Cata ´lisis y Petroleoquı ´mica, CSIC, c/Marie Curie 2, Cantoblanco, 28049 Madrid, Spain Received 17 May 2006; received in revised form 8 September 2006; accepted 17 September 2006 Available online 17 October 2006 Abstract This study shows that titanium incorporation into hexagonal mesoporous silica (HMS) material has a positive effect on the activity of supported CoMo catalysts in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4-ethyl,6-methyl-dibenzothiophene (4E6MDBT). All catalysts showed the highest activity in the HDS of DBT than in the HDS of 4E6MDBT. The low reactivity observed in the HDS of 4E6MDBT is caused by the steric hindrance of the two alkyl groups at positions 4 and 6. The HDS of DBT over Ti-free catalyst proceeds exclusively via the direct desulfurization (DDS) route whereas over Ti-containing catalysts proceed via DDS (main route) and hydrogenation (HYD) pathway. The catalyst with a Si/Ti = 40 (molar ratio) was the most active in the HDS of DBT. A further increase in the Ti-content led to a decrease in Brønsted acidity and the S BET specific area of the catalysts, which implies a decrease in the bifunctional character of the catalysts. Raman spectroscopy demonstrated that Ti-incorporation into HMS material leads to a decrease in the degree of polymerization of Mo species, and this implies a better dispersion of MoS 2 , in good agreement with the XPS measurements. Regarding the HDS-resistant 4E6MDBT, the HDS reaction over the Ti-free catalyst was found to proceed exclusively via the dealkylation (DA) route. After Ti-incorporation into HMS material, additional acid-catalyzed isomerization occurs. With respect to industrial sample, the catalyst with Si/Ti = 40 showed lower intrinsic activity as well as greater selectivity toward isomerization route products. # 2006 Elsevier B.V. All rights reserved. Keywords: Supported CoMo catalyst; Hydrodesulfurization; 4-Ethyl,6-methyl dibenzothiophene; Dibenzothiophene 1. Introduction Severe environmental regulations on the sulfur contents of diesel fuels is leading refiners to develop novel catalysts for the production of ultra-low sulfur diesel. In this sense, the threshold limit for sulfur in diesel fuel is expected to be regulated to 50 ppm of weight (ppmw) or less within the next few years [1]. To satisfy this demand, it is important to remove most refractory alkyl-substituted dibenzothiophene (DBT) com- pounds from the middle distillates. However, this is impossible to achieve with a commercial Co(Ni)Mo HDS catalyst. Thus, the challenge is to develop catalysts that are more efficient at removing of refractory compounds than Co(Ni)–Mo-based sulfides supported on g-Al 2 O 3 [2–4]. In order to develop highly active catalysts, many studies have been carried out in the last few years to gain a better understanding of the mechanistic aspects of the HDS reaction of 4,6-dialkyl-substituted dibenzothiophenes (having alkyl groups adjacent to the sulfur atom), which are the most refractory to desulfurize compounds [3–20]. Actually, most of the results are in accordance with the early proposal of Kabe et al. [8] that factors other than the steric hindrance of the reactant adsorption originated the difference in reactivity between 4,6-dialkyl-substituted dibenzothiophenes and DBT. The factors to be considered as responsible of the low reactivity of 4,6-dialkyl-dibenzothiophene compounds are: (i) a steric hindrance by the alkyl groups to the adsorption of the dihydrointermediates; (ii) a steric hindrance by the alkyl groups of the C–S scission (elimination step) in the dihydrointermedi- ates; (iii) the presence of only one H-atom available for the elimination step; (iv) the effect of the alkyl groups on the acidity of the H-atom [19]. As a consequence of the steric hindrance of the C–S bond scission in the partially www.elsevier.com/locate/apcatb Applied Catalysis B: Environmental 71 (2007) 223–236 * Corresponding author. Tel.: +34 915855475; fax: +34 915854760. E-mail address: bgarcia@icp.csic.es (B. Pawelec). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.09.011