Effect of the support on the high activity of the (Ni)Mo/ZrO2–SBA-15 catalyst in the simultaneous hydrodesulfurization of DBT and 4,6-DMDBT
Graphical abstract
The hydrodesulfurization of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) on Mo- and NiMo-catalysts supported on pure oxides and ZrO2–SBA-15 indicated that the active phase morphology was the dominant factor during the reaction. Monolayers of MoS2 in Mo catalysts had low activity for the HDS of both DBT compounds. On NiMo catalysts, DBT reacted on monolayered and stacked NiMoS clusters, but 4,6-DMDBT was converted only on the latter.
Highlights
► The HDS of DBT and 4,6-DMDBT was studied on Mo and NiMo catalysts supported on ZrO2–SBA-15, ZrO2, Al2O3, and SBA-15. ► Characteristics of Mo oxide precursors determine the morphology of MoS2 and NiMoS clusters. ► Monolayers of MoS2 in Mo catalysts have low activity for HDS of both dibenzothiophenes. ► In the case of NiMo catalysts, DBT was able to transform on both monolayered and stacked NiMoS particles. ► The HDS of 4,6-DMDBT requires the presence of short but stacked NiMoS clusters as those obtained on ZrO2–SBA-15.
Introduction
The requirements that a fuel must meet in transport applications have become more stringent in the last decade. The concentration of sulfur, a major impurity in crude oil, has been limited to few parts per million in many countries. However, removing sulfur from diesel by the standard hydrodesulfurization (HDS) process is a major challenge because of the presence of low reactivity compounds so-called refractory. The response of refineries to the demand of high-quality diesel has been to increase the severity of the process conditions or to implement several reaction stages [1]. Sulfur is removed from the most reactive species in the first reactor or catalytic bed, whereas the refractory compounds react in subsequent stages on a second catalyst or at different conditions. However, the use of catalysts active in the HDS of refractory compounds is a more convenient option to improve the entire HDS process because it minimizes the investment needed to upgrade the product. Thus, the fundamental study of the interaction between the sulfur containing molecules and the catalysts has been the aim of many groups in the last years. The understanding of this issue would allow designing catalysts able to remove sulfur from refractory and not refractory compounds in a single reaction stage.
It is possible to use novel supports, active phases, promoters, or additives to upgrade the performance of the HDS catalyst, typically MoS2 supported on alumina and promoted with Co or Ni [2], [3], [4], [5]. The use of an alternative material as carrier is a promising option because the support has a key effect in the catalytic performance. A diversity of materials has been tested as HDS supports, i.e., pure oxides, mixed oxides [6], [7], [8], [9], [10], [11], [12], carbon [13], and mesostructured silicas such as MCM-41 and SBA-15 [14], [15], [16], [17], [18], [19].
The application of mesostructured silicas as HDS supports is of special interest due to their outstanding textural characteristics, e.g., pore volume and surface area [20]. Pure siliceous mesostructured materials were successfully applied as supports for W and Mo catalysts in the HDS of dibenzothiophene (DBT) and thiophene and hydrogenation of cyclohexene [18], [21]. It was shown afterward that adding metal oxides to the mesostructured silica led to more active catalysts for the same reactions [22]. The use of pure siliceous and modified SBA-15 materials as HDS supports has been extensively studied by our group. Mo and NiMo catalysts supported on SBA-15 materials modified with Ti, Zr or Al showed high activity in the HDS of 4,6-dimethyldibenzothiophene (4,6-DMDBT) [23], [24], [25], [26]. NiMo catalysts supported on SBA-15 modified with different metal oxides (MgO, CaO, BaO, TiO2, or ZrO2) were tested in the HDS of DBT [27]. The common observation in previous works was that adding ZrO2 to the SBA-15 led to the most remarkable improvement in the activity of the catalysts in the HDS of both DBT-type compounds, which was correlated with the morphology of the active phase.
It was interesting that the same system presented the highest activity for the HDS of DBT and 4,6-DMDBT because some studies suggested that improving the activity of the Mo-based catalysts for the HDS of DBT decreases the activity in the HDS of 4,6-DMDBT and vice versa [28]. For instance, incorporating SiO2 to the alumina support decreased the activity in the HDS of DBT but led to higher activity in the HDS of 4,6-DMDBT [29], [30]. Thus, the aim of this work was to understand the high HDS activity of the (Ni)Mo/ZrO2–SBA-15 system by combining detailed physicochemical characterization of the catalysts with the kinetics of the simultaneous HDS of DBT and 4,6-DMDBT. Mo- and NiMo catalysts supported on Al2O2, ZrO2, and SBA-15 were also investigated to achieve the goal.
The materials were characterized by N2 physisorption, X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy (DRS), temperature-programmed reduction and sulfidation (TPR and TPS), high-resolution transmission electron microscopy (HRTEM), and adsorption of NO. The catalytic performance of the sulfide catalyst was evaluated in the simultaneous HDS of DBT and 4,6-DMDBT. The results showed a qualitatively clear relationship between the oxide and sulfide form of the catalysts explained in terms of support-active-phase interaction strength. Furthermore, the analysis provided quantitative evidence of the key role of the MoS2 phase morphology in the performance of the catalysts. The (Ni)Mo/ZrO2–SBA-15 system is postulated as an alternative catalyst to remove sulfur from refractory and non-refractory compounds in a single reaction stage.
Section snippets
Support and catalyst preparation
SBA-15 was synthesized according to the procedure reported in literature [31]. The triblock copolymer Pluronic P123 (Mav = 5800, EO20PO70EO20) and tetraethyl orthosilicate (≥99%, Aldrich) were used as structure-directing agent and silica source, respectively. ZrO2-modified supports were prepared by incipient wetness impregnation of the siliceous SBA-15 with solutions of zirconium (IV) propoxide (70 wt.% in 1-propanol, Aldrich) in dry ethanol (Aldrich, 99.999%). The solids were dried at room
Characterization of oxide precursors
The effect of the ZrO2 loading into the structure of SBA-15 by incipient wetness impregnation has been described in previous works [25], [40]. The characteristics of Mo and NiMo catalyst supported on ZrO2–SBA-15 modified by chemical grafting (ZrO2 loadings <25 wt.%) were discussed in [41], [42]. The application of impregnated ZrO2–SBA-15 as HDS support was studied only for NiMo catalysts in [25]. Here, we report the characterization results that complement previous collaborations and allow us to
Effect of the oxide precursor on the sulfide catalysts
Qualitative relations between the oxide precursors and the corresponding sulfide catalysts can be stated from our results. Td and Oh Mo species were observed on the siliceous SBA-15, the later being the most abundant species; indeed, a fraction of Oh Mo species was agglomerated to MoO3 (XRD). In line with this heterogeneous distribution of oxide species, the sulfide catalyst supported on SBA-15 exhibited some monolayers of MoS2 as well as the longest and most stacked MoS2 crystals. Some MoS2
Conclusions
According to the above-mentioned results, the following conclusions can be stated.
The characteristics of the oxide Mo species depend on the interaction strength with the support. The increasing proportion of Td Mo species indicates stronger interactions. The highest proportion of Oh Mo species with the maximum dispersion was found among the studied catalysts on ZrO2(x)SBA supports.
The characteristics of the oxide species on ZrO2(x)SBA conduct to MoS2 clusters with short length (36 and 31 Å on Mo
Acknowledgments
Financial support by CONACYT-Mexico (Grant 100945) is gratefully acknowledged. The authors thank M. Aguilar Franco, C. Salcedo Luna and I. Puente Lee and for their technical assistance with XRD and HRTEM characterizations. The authors also thank Professor J.A. Lercher for providing laboratory facilities for the characterization of sulfided materials at the Catalysis Research Center in the Technische Universität München.
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Present address: Lehrstuhl II für Technische Chemie, Department Chemie, Technische Universität München, Garching, Germany.