Relation between sulfur coordination of active sites and HDS activity for Mo and NiMo catalysts

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Abstract

The structure of sulfidic Type I and Type II Mo and NiMo alumina supported hydrodesulfurization (HDS) catalysts was studied using infrared (IR) analysis of adsorbed CO. The results were compared to the catalytic activity in gas phase thiophene and liquid phase dibenzothiophene HDS reactions. IR analysis confirmed the presence of MoS2- and NiMoS-type phases. The Type II NiMo catalyst (prepared using NTA as chelating agent) had a fully promoted edge structure, whereas the Type I (calcined) NiMo catalyst exhibited both promoted and unpromoted edge sites, confirming that NTA enhances the Ni-decoration of the MoS2 edges. The active phase in both types of catalysts exhibited a dynamic and reversible behavior in terms of sulfur coordination. The catalysts exposed a higher number and/or a higher degree of coordinative unsaturation of vacant sites under gas phase thiophene HDS conditions than directly after sulfidation. The NiMo catalysts could be fully restored to their initial state by resulfiding after thiophene HDS, whereas some sites of the Mo catalysts were irreversibly blocked. Apparently, the NiMo catalysts were able to suppress coke deposition on the active sites under the conditions applied, in contrast to the Mo catalysts. An inverse correlation was found between the gas phase thiophene HDS conversion and the number of vacant sites present on the catalyst surface. A similar trend was observed for the ratio between hydrogenation and direct desulfurization of dibenzothiophene in the liquid phase HDS reaction. These correlations show that the thiophene HDS reaction and the dibenzothiophene hydrogenation pathway are catalyzed by sulfided species on the active phase, while the direct desulfurization pathway is catalyzed by vacant sites.

Graphical abstract

The structure of Mo and NiMo HDS catalysts was studied using IR analysis of adsorbed CO. The Type II NiMo catalyst had a fully promoted structure, whereas the Type I NiMo catalyst exhibited both promoted and unpromoted sites. An inverse correlation was found between thiophene conversion and the number of vacant sites present on the catalyst surface. A similar trend was observed for the ratio between hydrogenation and direct desulfurization of dibenzothiophene, showing that the hydrogenation pathway is catalyzed by sulfided species on the active phase, while the direct desulfurization pathway is catalyzed by vacant sites.

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Introduction

CoMo and NiMo based hydrodesulfurization (HDS) catalysts have been used in industrial hydrotreating processes for decades [1]. Although many improvements have been made in the composition, metal dispersion and support material of these catalysts, their formulation has basically remained unchanged. In order to further improve these catalysts, fundamental insights in the structure of these catalysts and the nature of the active sites are needed. Although great progress has been made over the years, crucial details regarding the structure of this type of catalysts on the atomic scale have not been fully uncovered. The observation of unpromoted and promoted MoS2 slabs under the STM microscope [2], [3] inspired several research groups to perform new modeling studies on the edge structure of MoS2 slabs in HDS catalysts, see e.g. [4], [5], [6]. Comprehensive reviews on this topic were published by Raybaud [7] and Sun et al. [8]. An example of different edge configurations for a MoS2 slab is shown in Fig. 1. The coordination number or degree of coordinative unsaturation of the edge Mo atoms changes as a function of sulfur content [3]. Sulfur atoms on the edges of the active phase can be removed by reacting with hydrogen, yielding coordinatively unsaturated (cus) Mo sites. The case of promoted HDS catalysts is more complex [9]. Most researchers have accepted the CoMoS/NiMoS model, in which the promoter atoms are atomically dispersed over the MoS2 edges, forming the so-called CoMoS or NiMoS phase. In this model the promoter atoms are either the active centers or able to enhance (promote) the activity of the MoS2 edge sites [1]. It has been well established that at least two different types of active sites are involved in HDS reactions and it is generally believed that vacant (cus) sites are responsible for the removal of hetero-atoms [1]. Furthermore, several researchers propose that fully sulfided species (i.e. sulfur that is a part of the catalyst active phase) may play a role in the HDS reaction [10], [11], [12], [13]. These suggestions are mostly based on the observed selectivity of the catalyst, which was recently reported again for the HDS of dibenzothiophene [14].

In the present study, the active phase of Mo and NiMo based HDS catalysts was characterized using infrared (IR) analysis of adsorbed CO. Furthermore, these results were correlated to the thiophene and dibenzothiophene HDS activity of the different catalysts. The adsorption of probe molecules like CO, coupled with vibrational spectroscopy is a widely used and powerful tool for the identification of the surface structure and properties of various inorganic materials, including HDS catalysts [15], [16], [17]. Adsorbed CO molecules interact with transition metals through their molecular orbitals, which affects the CO bond strength. The resulting CO bond stretching frequency observed in the IR spectrum is a function of the type of metal on which it adsorbs, the oxidation state of the metal and the coordination number of the metal center. For this study, Mo and NiMo based catalysts were used with different morphologies (so-called Type I and Type II). It is generally believed that the Type II active phase has improved properties, resulting in a higher HDS activity (e.g. see [18], [19], [20]). Dried Type II catalysts were prepared using nitrilo triacetic acid (NTA) as a complexing agent and calcined Type I catalysts were prepared without NTA. The catalysts were subjected to various treatments including HDS reactions, in order to gain further insight in the active phase structure and its catalytic function.

Section snippets

Catalysts

Four different catalysts were prepared, as summarized in Table 1. A solution of ammonium dimolybdate (NH4)2Mo2O7 and nickel nitrate Ni(NO3)2 was impregnated onto 1.5 mm cylindrical extrudates of a high purity γ-Al2O3 support (Ketjen CK 300). The resulting catalysts were dried at 393 K for 1 h and calcined at 723 K for 1 h (heating rate 5 K/min). This procedure leads to the formation of typical Type I HDS catalysts. Their so-called Type II counterparts were prepared using a different procedure.

Infrared analysis

The resulting IR spectra of CO adsorbed on the catalysts after sulfiding and treatment in H2 for 0.5 h at 673 K are shown in Fig. 2, Fig. 3. To help the reader, the different peak positions have already been labeled according to the corresponding active phase type (Mo or NiMo) and the type of adsorption site (S, A, B or C), throughout Section 3 and in the figures. The assignment of these peaks will be described in Section 4; see also Table 2, Table 3. In all spectra, two features can be observed,

Effects of NTA

The band attributed to CO adsorbed on the support (Al–OH sites at 2154 cm−1) has a higher intensity in the spectra of both NTA based catalysts than in the spectra of their calcined counterparts (Fig. 2, Fig. 3; note that the figures are plotted on the same scale). This suggests that the active phase covers less support surface area in the case of the NTA catalysts and, because the support material and the metal loading are the same, the stacking degree of the MoS2 slabs is higher (and the

Conclusions

IR analysis of adsorbed CO proved to be an excellent technique to study the active phase of HDS catalysts. The differences between Mo and NiMo based catalysts were clearly visible using this method, probing both the unpromoted MoS2 and the NiMoS phase. The use of NTA as chelating agent had a marked effect on the active phase of the NiMo catalyst: the NTA based catalyst (Type II) had a fully promoted edge structure, whereas the Type I catalyst exhibited both promoted and unpromoted edge sites.

Acknowledgements

Mr. S. Driessen is gratefully acknowledged for experimental support. Albemarle Catalysts and NWO are gratefully acknowledged for their financial support.

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