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56 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
Hamid A. Al-Megren The sol–gel method is the latest technique for catalyst preparation, in which the metal–organic precursors are mixed with metal precursor to form a homogeneous solution [43]. The solution is passed through the hydrolysis and polymerization process by adding water while carefully controlling the pH and reaction temperature. This results in the formation of colloidal particles and micelles of approximately 10 nm diameter. These particles continue to increase in size until a metal oxide gel is formed. However, each of these techniques is either difficult to control or the preparation process is complex and difficult to use on an industrial scale. Also, none of them ever takes into consideration the adjustment of the interaction between the active metal and support, which is regarded as one of the properties of supported catalysts. Also, for hydrotreating catalyst preparation, the content and particle sizes of the active components such as Mo or W are crucial for the catalyst performance. No methods are available to load the high content of the Mo or W needed with desirable particle size. Hence, selective deep removal HDS now needs noble metal catalysts, which are much more costly and easy to deactivate. Recently, a new catalyst preparation method, i.e. organic matrix combustion (OMXC) has been developed [44, 45]. Some organic compounds are mixed with soluble metal compounds to form a homogeneous solution with addition of minimum water (or other solvent) where necessary. The particle size of the catalyst is controlled by the ratio of the organic compounds to the metal in the solution. Interaction between the active component and support is controlled by the calcinations temperature and atmosphere. The aim of this work is to develop more robust catalysts with high activity for selective removal of sulfur from thiophene, to hydrogenate the C–S bond; and with low ability to hydrogenate the mono C=C bonds. The key to achieving this goal is to control the distribution and particle size of the active component, e.g. MoS2, and optimize the promotion of Ni and Mo in the catalysts supported over alumina. Active component precursors will be chosen from the ammonium salts. This catalyst will be prepared and compared by three different preparation methods. 2. EXPERIMENTAL 2.1. Catalyst Preparation In this work, 15% Ni–Mo bimetallic material supported over alumina was prepared using three different methods: impregnation (IMPR), mechanical (MECH), and organic matrix combustion (OMXC). All catalysts were prepared in oxide phase then sulfurized using a stream of hydrogen saturated by thiophene at room temperature. Nickel nitrate hexahydrate Ni(NO3)2.6H2O(99.0% Fluka) was mechanically mixed with molybdenum trioxide at a Ni/Mo atomic ratio of 0.43. An amount of 15% from the mixture was added to alumina (150 mesh, 150 m 2 g -1 , from Aldrich). The mixture was dried in a vacuum oven at 120ºC and calcined at 550ºC for 15 hours. The resulted powder sample was named as MECH. Nickel nitrate hexahydrate was dissolved in small amount of water, mixed with ammonium heptamolybdatetetrahydrate (NH4)6Mo7O24-4H2O (99% Riedel-de Haën) then impregnated with alumina. The mixture was dried in a vacuum oven and calcined at 550ºC for 15 hours. The resulted powder sample was named as IMPR. The same ratio of Ni/Mo supported over alumina was prepared using OMXC method by mixing calculated amount of urea (Fisher Chemicals) with ammonium heptamolybdate-tetrahydrate and nickel nitrate hexahydrate (Ca.1g/2ml = (Ni- and Mo-Precursors+urea)/ water ratio. This mixture was stirred to form a homogeneous slurry, then mixed with Al2O3 support and heated at 50ºC for 2-3 hours to obtain urea-based slurry containing molybdenum fully loaded over alumina. This paste is ignited at 500ºC in static air for 10 min. The resulted powder sample was named as OMXC. 2.2. Catalytic Tests All catalysts were activated by sulfurization using 20 ml of hydrogen through a jacketed saturator containing thiophene (10.9 kPa). The flow of hydrogen with thiophene passed through 200mg of catalyst placed in a fixed bed reactor and heated at 5ºC/min to maximum temperature 450ºC, and was then held at this temperature for 3 hours. The catalytic behavior tests of the prepared supported catalysts (MECH, IMPR, and OMXC) for the hydrodesulfurization of thiophene were carried out in a laboratory bench scale pilot plant fitted with mass flow meters and a fixed bed down-pass flow quartz reactor placed in a cylindrical furnace, equipped with a coaxial thermocouple. The inner diameter for the reactor was 4 mm and its length 300 mm. The catalyst zone in the middle of the reactor had a length of 30 mm and was filled with 200 mg of catalyst diluted with an equal amount of quartz particles (50 – 100 mesh) in order to minimize temperature gradients. In the center of the catalyst bed, a thermocouple was installed in contact with the catalyst particles to measure the reaction temperature. The heating zones at the inlet and the outlet of the reactor were measured by a thermocouple located inside the furnace and were controlled by a temperature controller (Cole Parmer Digi-Science). The Arabian Journal for Science and Engineering, Volume 34, Number 1A January 2009 57
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