HYDRODESULFURIZATION OF THIOPHENE OVER BIMETALLIC ...

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58 Hamid A. Al-Megren Normally, the flow rate of hydrogen was 20 ml/min at atmospheric pressure. The activity of nickel–molybdenum supported catalysts (MECH, IMPR, and OMXC), and thiosophene conversion were studied at a temperature of 350°C Blank reactor runs were conducted as described above in the absence of catalysts. No significant conversions of thiophenes were observed under the conditions of each experiment. Carbon mass balancing closed within 8%, most within 5%. 2.2.1. Gas Chromatography The analysis of the reaction mixture was carried out using, a Hewlett–Packard 5890 series II. The instrument equipped with 1 × 1/8" O.D. stainless steel packed column (80 – 100 mesh range Porpak S); and flame ionization detector (FI), and thermal conductivity detector (TCD). The instrument was calibrated by external standardization method using standard mixture of (CH4, C2H6, C2H4, C3H8, C3H6, C4H10, C4H8, and C4H8). 2.3. Catalyst Characterization Electron probe micro-analyzer (EPMA) (JEOL JXA-8900R/RL) was used for identification of constituent elements in the prepared samples and to study the elements distribution over the support. Analysis was performed by illuminating the sample surface with a finely focused electron beam and measuring the wavelengths and intensities of characteristic X-rays from the specimen and the quantities of secondary electrons and backscattered electrons from it. The X-ray diffraction (XRD) was carried out in Philips PW1710 diffractometer with CuKα radiation. The data were collected at room temperature, in θ/θ reflection mode, scanning the specimen between 3º–70º in 2θ, using steps of 0.05º, with a time per step of 1.25 s. The structure of the samples was studied using laser Raman spectroscopy from a Jobin Yvon Labram spectrometer with a 632.8 nm HeNe laser, run in a back-scattered confocal arrangement. The samples were pressed in a microscope slide; the scanning time was set to 30s. Raman spectra were recorded in air at room temperature with a resolution of 2 cm –1 and a scanning range of 100 to 2500 cm –1 . 2.3.1. Physisorption Nitrogen adsorption–desorption isotherm of MECH, IMPR, and OMXC catalyst samples were measured on a Micromeretics ASAP2010 system using nitrogen sorption at 77K. Prior to the experiments, approximately 0.2 g of sample was degassed at 523K for 16h. The surface areas were calculated using a multipoint Brunauer–Emmett– Teller (BET) model. The pore size distribution was obtained using BJH and BET models and the total pore volumes were estimated at a relative pressure of 0.99 atm, assuming full surface saturation with nitrogen. For characterization purpose, samples of the fresh and used catalyst were passivated at room temperature in flow of 1% O2/He before they were exposed to air. 3. RESULTS AND DISCUSSION The surface area measurements for the three prepared bimetallic catalyst samples (MECH, IMPR, and OMXC) in oxide phase, before sulfurization, are shown in Table 1. The surface areas for the samples increase in following order: MECH (125.5 m 2 /g) > OMXC (122.5 m 2 /g) > IMPR (119.3 m 2 /g); the differences are relatively small. The highest surface area for the sample prepared by mechanical method could be attributed to less dispersion between the active component and surface of the support. In Table 1, the average pore diameter and the total volume, measured for the three samples, were the highest for the samples prepared by mechanical and the lowest for the sample prepared by combustion method. The particle size distribution analysis shows that the samples prepared by IMPR and OMXC have almost similar size (~60μm), much higher than the particle size for sample prepared by MECH (~27μm). Table 1. Physical Properties of NiMoOx Catalyst Prepared by Impregnation, Mechanical, and Organic Matrix Methods Preparation Surface Method Area m 2 g -1 Total Pore Volume Cm 3 g -1 Average Pore Particle Size Diameter A º Distribution μm IMPR 119.27 0.16529 55.436 59.81 OMXC 122.50 0.16420 53.617 63.53 MECH 125.51 0.18621 59.343 26.81 January 2009 The Arabian Journal for Science and Engineering, Volume 34, Number 1A
Count/arb, units 500 400 300 200 100 Hamid A. Al-Megren 0 0 10 20 30 40 50 60 70 2 q Figure 1. XRD pattern for Ni3Mo7Ox/Al2O3 catalysts prepared by Impregnation, Mechanical and OMXC methods The Arabian Journal for Science and Engineering, Volume 34, Number 1A January 2009 59 Al Al Al Al Al OMXC Al The X-ray diffraction patterns for the three samples in oxide phase are shown in Figure 1. Sample MECH shows an appearance of MoO3 peaks at 2θ values of 14.4, 23.6, and 26.8º. Sample IMPR also shows slight appearance of MoO3 peak at 2θ values of 23.6º, while there is no appearance of these peaks on the sample prepared by OMXC method. These appearances of the MoO3 peaks suggest that both methods, mechanical and impregnation, lead to lower dispersion of the active component compared to OMXC method, which enables a high dispersion of Ni3Mo7Ox. Raman spectroscopy is a powerful tool to study the structure of catalytically active phases on a support. Typical support such as alumina shows weak Raman scatters, with consequence that adsorbed species can be measured at frequencies as low as 50 cm –1 [10]. Figures 2, 4, and 8 show the Raman spectra of the oxides and sulfided phase for the catalysts in the 200–2000 cm –1 range. The spectra of Ni3Mo7Ox/Al2O3 catalysts do not exhibit the features of Ni3Mo7Sx/Al2O3 catalysts. The Raman spectra of NiMoOx/Al2O3 bimetallic supported oxide samples prepared using the three methods are shown in Figure 2. It shows that the main phase on the surface of samples IMPR and MECH is MoO3. The Raman band on these samples at 995 cm –1 corresponds to the stretching mode of Mo=O, and the band at 830 cm –1 corresponds to the anti-symmetric stretching mode of Mo–O–Mo, and the band at 295 cm –1 corresponds to the bending mode of Mo=O in the polycrystalline MoO3. The strong band at 965 cm –1 on sample MECH is due to the A1 mode of Mo=O in the MoO4 tetrahedral units. The broad band for samples IMPR and OMXC at 955 cm –1 , which can be unfolded into two peaks at 955 and 965 cm –1 , highlights the band characteristic of NiMoO4. The Raman results suggest that the main oxide present clearly on the surface of sample MECH and IMPR is MoO3. Our findings agree with those reported by Sergio et al. [46, 47]. In addition, the shoulder of 950 cm –1 and the weak Raman bands at about 825 cm –1 are associated with the Mo–O–Mo for the polymolybdate species [48–51]. On the other hand, the weak intensity band at 950 cm –1 is characteristic of Ni–Mo oxide. Ramman Pand Intincety 4500 4000 3500 3000 2500 2000 1500 1000 500 MECH OMXC I MPR Al MECH I MPR 0 0 500 1000 1500 2000 Ramman Shift, cm -1 Figure 2. Laser Raman spectra for Ni 3Mo 7O x/Al 2O 3 catalysts prepared by Impregnation, Mechanical and OMXC methods
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