HYDRODESULFURIZATION OF THIOPHENE OVER BIMETALLIC ...

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60 Hamid A. Al-Megren Figure 3 shows the X-ray diffraction patterns for the three samples after the sulfidation reaction. It is clear that the main peaks for the oxide phase, i.e. MoO3 at 2θ values 14.4, 23.6, and 26.8, have disappeared from IMPR and OMXC samples while they have weakened on the MECH sample. This disappearance was attributed to the higher formation of sulfide phase on IMPR and OMXC, while some oxides still remain on MECH sample. Our results agreed with those reported by some investigators [48]. They have shown that during the primary period of thiophenes hydrodesulfurization on MoO3, a definite amount of sulfur was consumed by sulfidation of the oxide. This means that oxidic molybdenum catalyst can be sulfided by thiophenes, or by the hydrogen sulfide produced by DHO of thiophenes. Count/unit 400 300 200 100 0 10 20 30 40 50 60 70 January 2009 The Arabian Journal for Science and Engineering, Volume 34, Number 1A 2 q MECH OMXC I MPR Figure 3. XRD pattern for Ni 3Mo 7S x/Al 2O 3 catalysts prepared by Impregnation, Mechanical and OMXC methods The laser Raman spectra of the three samples (IMPR, MECH, and OMXC) after converting them from oxide to sulfide phase by sulfurization reaction are shown in Figure 4. The spectrum intensity of oxide phase (Figure 2) was highly reduced after sulfurization reaction. The vibration frequency at band 965 cm –1 Mo=O in the MoO4 tetrahedral units is strongly reduced after sulfurization. This band, i.e. 965 cm –1 disappeared completely on OMXC sample but it was partially reduced on rest of the samples. This result suggests that the molecular symmetry mode in the surface of MoO3 was changed after sulfurization [46–47]. In addition the spectrum of Figure 4 shows two bands at about 440 and 450 cm –1 which could be attributed to MoS2 structure [13] 1000 Raman band intensity / Arb. units 800 600 400 200 0 MECH OMXC IMPR 500 1000 1500 2000 Raman shift, cm -1 Figure 4. Laser Raman spectra for Ni 3Mo 7S x/Al 2O 3 catalysts prepared by Impregnation, Mechanical and OMXC methods
Hamid A. Al-Megren In the present study, the catalytic activity of sulfided NiMo/Al2O3 catalysts (IMPR, OMXC, MECH) were examined in the hydrodesulfurization of thiophenes at a temperature of 350°C and under atmospheric pressure. The conversion of thiophenes is shown in Figure 5. It can be observed that thiophenes conversion is constant over about 8 h time on stream. Thus, high catalytic activity (57% of thiophenes conversion) is reached for the catalyst OMXC. Catalytic activities of IMPR and MECH catalysts are lower (45% and 21% of thiophene conversion respectively). The rate of reactions also show in order 73.80, 58.16, and 26.05 µmol (g min) –1 for OMXC, IMPR, and MECH respectively. The superior activity of OMXC catalyst is attributed to the incorporation of Ni–Mo onto the support surface. This provides a stronger metal–support interaction allowing a better dispersion of nickel and molybdenum oxidic and sulfided phases. On the other hand, too strong metal–support interaction makes the reduction and sulfidation of Ni and Mo species more difficult. Conversion, % 60 50 40 30 20 10 OMXC 0 0 1 2 3 4 5 6 7 8 Time, hour The Arabian Journal for Science and Engineering, Volume 34, Number 1A January 2009 61 IMPR MECH Figure 5. HDS of thiophene over Ni 3Mo 7S x/Al 2O 3 catalysts at 350ºC, atmospheric pressure and 20ml/min flow of hydrogen This is in agreement with the results of Klimova et al. [53] who found NiMo catalysts supported on Al-SBA-16, prepared by grafting with AlCl3, show high activity in HDS of 4,6-DMDBT which attributed a good dispersion of Ni and Mo active phases. The catalytic activity for these catalysts was tested on hydrodesulfurization of thiophene at 350 o C and atmospheric pressure. Figure 5 shows that OMXC catalyst has superior activity (57%) compared to IMPR (45%) and MECH (21%) catalysts after seven hour of the reaction time. The rates of reaction also show in order 73.80, 58.16, and 26.05 umol (g min) –1 for OMXC, IMPR, and MECH respectively. This superior activity for OMXC sample attributed to the well dispersion of Ni–Mo over alumina as well as the higher sulfide formation compare to IMPR and MECH [46]. = Ratio C 4 /C 4 2.6 2.5 2.4 2.3 2.2 2.1 IMPR MECH OMXC 2.0 0 1 2 3 4 5 6 7 8 Time, hour Figure 6. C4/C4= mol ratio for HDS of Thiophene over Ni3Mo7Sx/Al2O3 catalysts at 350ºC, atmospheric pressure and 20ml/min flow of hydrogen.
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