SBA-15 as Support for MoS2 and Co-MoS2 Catalysts Derived from Thiomolybdate Complexes in the Reaction of HDS of DBT

Z.-D. Huang 0 1 2 3 W. Bensch 0 1 2 3 L. Kienle 0 1 2 3 S. Fuentes 0 1 2 3 G. Alonso 0 1 2 3 C. Ornelas 0 1 2 3 0 S. Fuentes Centro de Ciencias de la Materia Condensada , UNAM, Ensenada, Baja California C.P. 22860, Mexico 1 L. Kienle Max-Planck-Institut fur Festkorperforschung , Heisenbergstr. 1, 70506 Stuttgart, Germany 2 Z.-D. Huang W. Bensch (&) Institut fur Anorganische Chemie, University of Kiel , Olshausenstrae 40-60, 24098 Kiel, Germany 3 G. Alonso C. Ornelas Centro de Investigacion en Materiales Avanzados S. C. , Chihuahua, Chih. C.P. 31109, Mexico Molybdenum sulfide and cobalt-molybdenum sulfide catalysts supported on mesoporous SBA-15 were prepared by thermal decomposition of ammonium thiomolybdate (ATM). SBA-15 was synthesized at 353 K and 413 K to obtain pore diameters of about 6 and 9 nm, respectively. The (Co)-MoS2/SBA-15 catalysts were characterized with X-ray diffraction (XRD), N2-physisorption and high-resolution transmission electron microscopy (HRTEM). HRTEM images give evidence for the presence of a poorly dispersed MoS2 phase with long MoS2 slabs and a pronounced MoS2 stacking. The catalytic performance in the hydrodesulfurization (HDS) reaction of dibenzothiophene (DBT) was examined at T = 623 K and P = 3.4 MPa. The Co-MoS2/SBA-15 materials show a relatively high catalytic activity with a strong preference for the direct desulfurization (DDS) pathway. This is an interesting result in view of the significant stacking of MoS2 particles and the size of the slabs. The generation of the catalytically active CoMoS phase and a large number of coordinately unsaturated sites (CUS) may explain the high performance of Co promoted MoS2/SBA-15 catalysts in the HDS reaction. 1 Introduction More stringent limitations to sulfur contents of transportation fuels lead to continuing quest for better performing hydrodesulfurization (HDS) catalysts. One of the successful ways to improve the catalyst activity is to synthesize transition metal sulfides on a porous support material which can well control the structure of the catalyst. Most commercially available HDS catalysts are supported on alumina with a surface area of about 200 m2/g. Mesoporous silica materials such as MCM-41 [1, 2] and SBA-15 [3] with their well ordered hexagonal pore structures, narrow pore diameters (330 nm) and higher surface areas (more than 700 m2/g) offer several advantages in comparison with alumina. MCM-41 supported Co(Ni)Mo catalysts were investigated in the last decade [47]. The catalysts exhibited higher activities for conversion of dibenzothiophene (DBT) and petroleum residues than Co(Ni)-Mo/cAl2O3 at high molybdenum loading [4] or high atomic ratio Co(Ni)-Mo [5, 6]. More recently, mesoporous SBA-15 has been in the focus of research interests due to the larger pores, thicker pore walls and higher hydrothermal stability compared to MCM-41. Indeed, Vradman et al. [8, 9] demonstrated the excellent potential of high loading Ni-W/ SBA-15 catalysts for deep hydrotreatment of petroleum feedstocks. The catalysts prepared by an ultrasonication route showed a high HDS activity. Mesoporous SBA-15 was also found to be a suitable support for Mo, CoMo, and NiMo catalysts as reported by Rao and co-workers [10]. Furthermore, Al, Ti and Zr containing SBA-15 supported NiMo or CoMo catalysts were prepared by Klimova and co-workers [11, 12], Rao and co-workers [13] and Fierro and co-workers [14]. They observed that SBA-15 modified with these hetero-atoms (Al, Ti, Zr) provides better dispersion for the deposited Ni(Co) and Mo active species and yields considerably high activity for HDS of DBT. In the majority cases these hydrotreating catalysts were prepared via an oxide route, i.e. the MCM-41 or SBA-15 materials containing transition metal salts like ammonium heptamolybdate, nickel (cobalt) nitrate or nickel (cobalt) acetate were calcined to produce stable oxides. The oxides must then be sulfided either prior to or during the start-up of the hydrotreatment process in a stream of H2S (15%) in H2. Catalysts processed with such a procedure exhibit HDS activity being strongly influenced by the calcination temperature, the temperature and pressure for transformation to the sulfide [15]. On the other hand, molybdenum and tungsten sulfide catalysts for hydrotreating reactions can be obtained directly by decomposition of the corresponding thiometallates. It was observed that the thermal decomposition of molybdenum and tungsten thiosalts is an effective method for the production of improved unsupported catalysts. One reason is that no metal-sulfur bond formation is required which is necessary applying the oxide route [1619]. However, to the best of our knowledge only a few studies have been performed to investigate supported MoS2 catalysts derived from ammonium thiomolybdate (ATM) as precursor [20, 21]. The aim of this work was to study the SBA-15 support with different pore sizes (6 nm and 9 nm) for MoS2 and MoS2 promoted with Co catalysts using ATM as molybdenum source in the HDS of DBT reaction. The supported catalysts were characterized with X-ray diffractometry (XRD), nitrogen physisorption using the Brunauer, Emmett and Teller (BET) method and high-resolution transmission electron microscopy (HRTEM). 2 Experimental 2.1 Sample Preparation 2.1.1 Synthesis of Pristine Mesoporous SBA-15 [2224] Eight gram of poly(ethyleneglycol)-block-poly(propyleneglycol)-block-poly(ethyleneglycol) triblock copolymer (Aldrich, pluronic, P-123) was dissolved in 240 g of water and 28.6 g of concentrated hydrochloric acid at 303 K on a water bath. After drop wise addition of 16 g of tetraethylorthosilicate (TEOS), the reaction mixture was stirred for 24 h at 303 K. The resulting gel was transferred into a Teflon bottle and heated to 353 K and 413 K for 24 h to obtain SBA-15 with 6 nm and 9 nm pores, respectively. The resulting white powder was filtered and washed with deionized water, and the surfactant was removed by Soxleth extraction at 351 K with a mixture of 970 mL of ethanol and 30 mL of concentrated hydrochloric acid. After washing with ethanol, the white powder was dried at room temperature under vacuum for about one week. 2.1.2 Synthesis of MoS2/SBA-15 Typically, 2 g of SBA-15 was stirred in aqueous solutions of ammonium thiomolybdate (NH4)2MoS4 (ATM) with different concentrations. After stirring at room temperature for 20 h the products were filtered without washing. Materials containing different Mo concentrations were prepared using appropriate concentrations of ATM and the final products were stored in dry air. The synthesized samples are denoted as Mo(X)/SBA-15(Y) in the figure legends, where X is the Mo loading in wt.% and Y is the pore size of SBA-15. 2.1.3 Synthesis of Co-MoS2/SBA-15 Supported Co-Mo catalysts were prepared by impregnation of SBA-15 with Co(CH3COO)2 4H2O followed by evaporation of H2O and CH3COOH at 773 K for 3 h. At this temperature the pink (CH3COO)2Co 4H2O/SBA-15 material turned blue (...truncated)