HYDRODESULFURIZATION OF THIOPHENE OVER BIMETALLIC ...

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Hamid A. Al-Megren
HYDRODESULFURIZATION OF THIOPHENE OVER BIMETALLIC Ni–Mo SULFIDE
CATALYSTS PREPARED BY DIFFERENT METHODS
1. INTRODUCTION
At present, the hydrodesulfurization (HDS) process is an efficient method for sulfur removal from gasoline and
diesel of high sulfur content [1,2]. In the HDS process, organic sulfur compounds in the liquid fuels are broken
down to H2S using NiMo–CoMo/Al2O3 catalysts [3–6], followed by the removal of H2S, and the sulfur content in
fuels can be reduced to about 10 ppm. Sulfided Ni–Mo catalysts are widely used in hydrotreating reactions [7,8].
Several studies have shown that monometallic cobalt catalyst are active for hydrodesulfurization (HDS) reactions
[9–11]. For example, Duchet et al. investigated carbon supported molybdenum and tungsten catalysts for HDS of
thiophenes [12]. In some cases, however, their HDS activities are low and unimportant due to a low degree of cobalt
dispersion and/or strong interactions with some support materials like alumina [13]. Therefore it is of great interest
to examine the influence of the preparation method on the properties and activity of the catalyst, as well as on the
interaction between promoters and supports [14]. In the present investigations, emphasis was focused on studying
the effect of the preparation method on the hydrodesulfurization activity of Ni–Mo catalysts.
To improve the performance of the HDS catalyst, all steps in the preparation procedure should be considered.
The key parameters are the choice of a precursor of the active species, support, synthesis, and post-treatment of the
synthesized catalyst. The nature of the active phase can be modified by changing the amount of active component
[15], and by the introduction of additives. Numerous additives have been studied, and phosphorus [16, 17] and
fluorine [18–21] have received special attention. In some studies, the sulfides of transition metals were replaced by
nitrides [22] or carbides [23, 24]. Noble metals are also used as the active phase in hydrotreating catalysts for second
stage HDS [25].
Various supports have been used to enhance the catalytic activity in HDS: carbon [26–28], silica [29, 30],
zeolites [31–34], titania and zirconia [35–37], and silica–alumina [38, 39]. Combining new types of catalytic species
with advanced catalyst supports such as ASA (amorphous silica–alumina) can result in an extremely high
desulfurization performance. The application of ASA-supported noble metal-based catalysts for the second-stage
deep desulfurization of gas oil is an example [25]. The Pt and Pt–Pd catalysts are very active in the deep HDS of prehydrotreated
straight run gas oil under industrial conditions. These catalysts are able to reduce the sulfur content to 6
ppm, while simultaneously reducing the aromatics to 75% of their initial amount. At high sulfur levels, the ASA
supported noble metal catalysts are poisoned by sulfur and Ni–W–ASA catalysts become preferable for deep HDS
and dearomatization.
It has long been recognized that the optimal performance of a catalyst depends on its preparation method [40].
The common preparation methods require a combination of different unit operations. Some of these are not entirely
understood. They can be described as: (i) introduction of the metal precursor on the support by impregnation or ionexchange
[41], co-precipitation and deposition precipitation [42]; (ii) drying and calcination, and (iii) reduction. The
size and distribution of metal crystallites in the support, the homogeneity of the multi-component catalyst, the
porosity, surface area, and pore size distribution are all functions of the precursors used in the preparation. Some
preparation parameters such as temperature of treatment, pH of the solution and the aqueous or organic medium also
contribute to the catalyst preparation. Recently, significant progress has been made towards understanding the
relationship between the preparation method and final properties of the catalyst and supports [41]. Generally, it is
believed that the catalyst selectivity depends on the particle size of the dispersed metal and spatial distribution of the
active component in the porous support. The stability of the catalyst depends on the interaction of the metal and the
supports.
The controlled preparation of heterogeneous catalysts has been studied for more than 100 years [42]. However,
perfectly designed catalysts still remain a challenge. Impregnation, sol–gel, ion-exchange, precipitation, and
deposition-precipitation are the commonly used methods [43].
The impregnation method requires the support to be contacted with a certain amount of solution of a metal
precursor, usually a salt, and then it is aged, usually for a short time, dried, and calcined [41]. This method appears to
be simple, and economical (especially when using solutions of costly active components), and able to yield a
reproducible catalyst. However, when drying, the dissolved metal compounds often crystallize in the support, which
leads to larger crystallites and thus leading to low metal dispersion.
The ion–exchange method consists of exchanging either hydroxyl groups or protons from the supports, with
cations and anions in the solutions [41]. In this method, it is important to adjust the pH values to control the
interaction between the support and metal precursors. A knowledge of the isoelectric point is essential if meaningful
dispersions are to be obtained.
January 2009 The Arabian Journal for Science and Engineering, Volume 34, Number 1A