Research Article pubs.acs.org/acscatalysis
Structure−Activity Correlations in Hydrodesulfurization Reactions over Ni-Promoted MoxW(1−x)S2/Al2O3 Catalysts
Lennart van Haandel,† Marien Bremmer,‡ Patricia J. Kooyman,§ J. A. Rob van Veen,† Thomas Weber,† and Emiel J. M. Hensen*,† †
Laboratory of Inorganic Materials Chemistry, Schuit Institute of Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, De Rondom 70, 5612 AP Eindhoven, The Netherlands ‡ Huygens-Kamerlingh Onnes Laboratory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands § Department of Chemical Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands S Supporting Information *
ABSTRACT: In this work, we have investigated the activation process and structure of Ni-promoted MoxW(1−x)S2/Al2O3 hydrodesulfurization (HDS) catalysts. Conversion of Mo and W oxides to the catalytically active MS2 (M = Mo, W) phase by sulfidation in gaseous H2S/H2 proceeded via different pathways, as found by XPS and EXAFS. The slower sulfidation kinetics of W on the alumina support formed NiMoxW(1−x) sulfides with a two-dimensional core−shell structure. Mo was mostly located in the core and W in the shell, as evidenced by EXAFS. Increasing the H2S/H2 pressure during sulfidation distributed Mo and W more homogeneously in the metal sulfide particles. This was attributed to the more favorable sulfidation of W under these conditions (i.e., below the temperature of MoS2 formation). Catalytic testing was consistent with these findings and demonstrated that a core−shell structure is the active phase in thiophene HDS (1 atm), whereas a homogeneously mixed MS2 phase catalyzes the HDS of dibenzothiophene at 40 bar. This is the first example of a core−shell structure in promoted MoxW(1−x)S2 catalysts. Support interactions in the oxidic precursor, which affect the sulfidation kinetics, were determined to play a key role in the formation of these structures. KEYWORDS: hydrodesulfurization, metal sulfides, alumina support, catalyst activation, core−shell on their edges.2 This model was later confirmed for both MoS2 and WS2 model systems by scanning tunneling microscopy (STM) and TEM studies.3 Despite the structural similarities of MoS2 and WS2-based HDT catalysts, some differences are also apparent. Conversion of tungsten oxide to the catalytically active sulfide phase by sulfidation is more difficult than for molybdenum oxide, because of the stronger W−O bond4 and stronger tungsten-support interaction.5 Complete sulfidation of tungsten generally requires more severe conditions (i.e., high temperature or H2S partial pressure),6 or the use of less-conventional and more-expensive metal precursors, such as thiosalts or molecular complexes.7 Second, while both Co and Ni are efficient promoters to MoS2, Ni is a better promoter for WS2 than Co. Vissenberg et al., using Mössbauer emission spectroscopy, determined that cobalt is sulfided at lower temperatures than tungsten and has a tendency to form stable Co9S8 particles, which are much less active for HDS reactions.8 In contrast, Ni sulfide particles formed at low temperature are redispersed at ∼300 °C to form the “Ni-W-S”
1. INTRODUCTION The removal of sulfur and other impurities from crude oil is an essential step in oil refineries for the production of ultralow sulfur diesel (ULSD). The organosulfides in oil, amongst others (substituted) thiophenes, and dibenzothiophenes, react with hydrogen to form hydrogen sulfide (H2S) and desulfurized products, according to reaction 1. H2S can then be converted to elemental sulfur using the Claus process:1
Although the technology for hydrodesulfurization (HDS) is well-established in industry, increasingly stringent fuel specifications and the simultaneous decrease of crude oil quality prove to be a challenging paradox for refineries to cope with. The development of more active catalysts is an economic way to increase HDS capacity and satisfy the increasing demand for ultralow sulfur diesel. Present-day industrial HDS catalysts are composed of Ni- or Co-promoted MS2 (M = Mo, W) nanoparticles supported on γ-alumina. The “Co-Mo-S” model, proposed by Topsøe and co-workers, describes these catalysts as two-dimensional MS2 nanosheets with the promoter dispersed © XXXX American Chemical Society
Received: August 16, 2015 Revised: October 9, 2015
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DOI: 10.1021/acscatal.5b01806 ACS Catal. 2015, 5, 7276−7287
Research Article
ACS Catalysis phase,9 the analogue of the “Co-Mo-S” phase, which is assumed to be the preferred phase for HDS reactions. These redispersion effects of the promoter are believed to be less important in the formation of the “Co(Ni)-Mo-S” phase, because of the more facile sulfidation of molybdenum. Despite the numerous studies on Co- or Ni-promoted MoS2 and, to a lesser extent, Ni-WS2 catalysts, only few studies have addressed the catalytic properties of trimetallic NiMoW catalysts.10 Raybaud and co-workers found enhanced catalytic activity for Ni-promoted MoW catalysts, while no such effect was observed for unpromoted MoW sulfides.11 They ascribed the enhanced catalytic activity of NiMoW sulfides to optimum metal−sulfur bond energy in keeping with predictions of DFT calculations. Indeed, NiMoW sulfides have been commercialized as highly active unsupported catalysts12 and, as a result, most research has focused on the structure and properties of bulk mixed sulfides. Unsupported NiMoW catalysts were found to be composed of irregular MS2 sheets (M = Mo, W) and segregated Ni sulfide particles.12,13 The high catalytic activity of these materials has been ascribed to irregularities in the MS2 sheets, rather than the presence of a “Ni-Mo-W-S” phase. Supported NiMoW catalysts are promising, because of their smaller particles, compared with unsupported materials.13b The improved activity of NiMoW catalysts supported on SBA-15 was ascribed to an improved dispersion over their bimetallic counterparts.14 Thomazeau et al. characterized unsupported and γ-alumina supported NiMoW sulfides by EXAFS. They concluded that a solid solution of Mo and W in the MS2 matrix is formed upon sulfidation, but this work lacks a comparison of the catalytic performance of these materials.15 Consequently, the effects of preparation conditions (i.e., Mo:W ratio, sulfidation temperature, and pressure) on the structure and catalytic activity of γ-alumina supported NiMoW catalysts are not well understood. In this work, we have investigated the genesis of the active phase in trimetallic NiMoxW(1−x)/γ-Al2O3 catalysts (x = 0, 0.25, 0.50, 0.75, 1.0) by EXAFS, XPS, and TEM. In the light of previous findings, we have focused on three open questions. First, the sulfidation of bimetallic NiMo and NiW is compared with trimetallic NiMoxW(1−x) to evaluate whether the presence of Mo can accelerate the sulfidation of tungsten oxides. Second, the structure of sulfided NiMoxW(1−x) catalysts with varying x was characterized by Mo K and W LIII EXAFS. Since MoS2 and WS2 are isostructural, Mo and W were expected to compete for the M position in the MS2 lattice, with Ni decorating the edges of MS2 analogous to the “NiMoS” or “NiWS” structure. In view of the different sulfidation kinetics of molybdenum and tungsten, the formation of separate MoS2 and WS2 phases, atomically mixed MS2 particles, and core−shell structures (Figure 1) was considered and determined on the basis of EXAFS data. Lastly, the structure of the active phase was correlated to catalytic activity in atmospheric thiophene HDS, medium-pressure dibenzothiophene (DBT) HDS, and gas-oil HDS to evaluate the potential for supported trimetallic NiMoW catalysts in HDS applications.
Figure 1. Possible structures of the active phase in Ni-promoted MoxW(1−x) catalysts (x ≤ 1): (left) separate MS2 particles (M = Mo, W); (middle) randomly mixed MS2 particles; and (right) mixed MS2 particles with a two-dimensional core−shell structure. S atoms are represented by yellow balls, Mo atoms are blue, and W atoms are pink. For the sake of clarity, Ni has been omitted from the drawings.
tungstate ((NH4)6H2W12O40·xH2O, Sigma−Aldrich) were dissolved in water. In some cases, hydrogen peroxide was added as an oxidant to form a stable solution. In a second solution, an appropriate amount of nickel nitrate (Ni(NO3)2· 6H2O, Sigma−Aldrich) was dissolved in water. This solution was added to the first one, and the volume was adjusted to the required pore volume by adding water. The carrier was impregnated with the solution and allowed to mature for 2 h. Afterward, the precursors were dried in hot air and calcined at 450 °C for 2 h in static air. The extrudates were crushed and sieved to a fraction of 30−80 mesh. The targeted final loading of group VIB metals was 2.2 at./nm2, with a Ni/(Mo+W) molar ratio of 0.3 and Mo/(Mo+W) molar ratios of 1.0, 0.75, 0.50, 0.25, and 0.0. 2.2. Characterization. Prior to characterization, catalysts were sulfided in a flow of 60 mL/min (STP) H2/H2S (10% H2S, Scott) at a pressure of 1 or 15 bar. Catalysts were heated to 100, 200, 300, 400, or 650 °C at a ramp of 6 °C/min, followed by an isothermal period of 2 h, and then they were cooled to room temperature. The reactor was flushed with helium and transferred to a N2-flushed glovebox (
