SYNTHESIS AND CHARACTERIZATION OF UNSUPPORTED CATALYST FOR GAS OIL DESULFURIZATION

Unsupported MoS2 catalysts were synthesized for the hydrodesulfurization (HDS) of real feed gas oil using different temperatures and pressures. Hydrothermal method was utilized to prepare by using molybdenum trioxide and sodium sulfide. The characterization of the catalyst was identified by XRD, SEM, and BET techniques. It was found that BET surface and pore volume were positively affected by pressure and temperature that could improve the activity of MoS2. Kinetic analysis showed that HDS reaction over MoS2 follow pseudo-first order kinetics. Experimental results revealed that the HDS activity of the unsupported MoS2 catalyst was better than supported CoMo/Al2O3 catalyst under the same operating conditions.


INTRODUCTION
Recently, the request of fuel has been rising highly due to increase of automobile engines.However, the environmental regulation laws require motor fuels of low sulfur content which needs efficient and feasible hydrodesulfurization process.Consequently, petroleum refineries pay more attention to lower sulfur level in their products.The synthesis of new catalysts and utilizing modern refineries is the most acceptable solution to attain lowest sulfur levels.Most MoS2).The other objective was to evaluate the performance of MoS2 catalyst for HDS process of gas oil.

CATALYST PREPARATION
MoS2 was synthesized by a hydrothermal method using 1L stainless steel autoclave reactor (Model: Kurla (W), Mumbai-400070, India).A schematic of the synthesis setup was seen in Figure (1).

EXPERIMENTAL SETUP FOR KINETIC STUDY AND CATALYST ACTIVITY
The reactor was charged with MoS2 (0.5 gm), and 100 ml of gas oil (GO).Seal test was conducted on reactor by purging several times with hydrogen and then raised pressure to 35 atm with stirring 600 rpm which ensure of getting rid of mass transfer resistance.Operating Temperature was varied at 300-360oC.H2 was fed continuously during the test in order to shun the decreasing of H2 pressure due to the reaction.As the reaction continued samples were drawn periodically.The influence of sampling on mixture volume was neglected because of the small sample amounts (≤ 1.0 ml per sample).Sulfur analyzer (XOS, Sindie OTG, USA) was used to measure sulfur concentration in drawn samples.The sulfur removal is calculated from equation (1).
Where Cf and C(t) are initial and instantaneous sulfur concentrations respectively.
(1)   As seen in Figure 4, the higher and acute peaks depict that the sample was quite crystallized.The higher the imposed pressure is, the better the crystallized products will be.Sulfur concentration on Mo films is related to the dynamic pressure within the reactor.By increasing the pressure, the concentration of sulfur on Mo surface increases, therefore, a higher nucleation density of MoS2 was expected on the Mo film.These nucleation sites cannot proceed further when low pressure applied because of low sulfur concentration at Mo surface.The high and sharp diffraction peak of (Fig. 4b) of the as-prepared MoS2 samples indicates the formation of well-stacked layered structure of MoS2 during the hydrothermal process.Our results agree well with findings of (Wang et al., 2017).

Intensity 2ϴ
Copyright  2018 Al-Qadisiyah Journal For Engineering Sciences.All rights reserved.

EDX, SEM, and BET Measurements
EDX, SEM, and BET measurements were conducted after the best synthesized-temperature and pressure determined (as seen in Fig. 3b).
increase in value of the term (-ln (1 -xS) value) whose slope represents the specific reaction rate over MoS2 catalyst at the studied reaction temperatures.It can also be seen in Figure (6) and Table 3, that the reaction rate constant raises as the catalyst activity increases.As a consequence, a higher conversion at various temperatures was found.The activation energies of HDS reactions over MoS2 catalyst were calculated from the Arrhenius expression in Equ.(11).Slope of the line in Figure

CONCLUSIONS
Unsupported MoS2 catalysts were synthesized for the (HDS) of real feed gas oil using different temperatures and pressures in the hydrothermal method by utilizing molybdenum trioxide and sodium sulfide.The characterization of the catalyst was identified by XRD, SEM, and BET techniques.It was found that the BET surface and the pore volume were positively affected by pressure and temperature, which could improve the activity of MoS2.Kinetics analysis of the studied system depicted that the HDS reaction of gas oil over MoS2 unsupported catalyst behaved as a pseudo-first order with the rate constant at 300,340, and 360 o C equals to 1.288,1.96,and 2.14 hr-1, respectively and has activation energy = 26.36kJ/mol.Experimental results revealed that the HDS activity of the unsupported MoS2 catalyst was predominant over supported CoMo/Al2O3 catalyst under the same operating conditions.

ACKNOWLEDGEMENT
Authors thank the department of chemical engineering, the University of Technology for supporting this work.Thanks to the petroleum research and development center, Iraqi Ministry of Oil for their valuable assistance.
of the classical catalysts utilize in the hydrodesulfurization reactions are CoMoS2, MoS2 or NiMoS2 supported on alumina (Girgis and Gates, 1991; Speight and Ozum, 2001).In 2001, a new type of catalyst is synthesized in the market which is the unsupported catalyst (Eijsbouts et al., 2007).Such catalysts have higher concentration of active sites per unit surface area of the catalyst, thus offer more activity than supported catalysts.Consequently, the synthesis of new unsupported sulfided catalyst appears to be a good required research trend.Improvement of catalyst activity depends on knowing the connection between the active sites and the framework of MoS2 and MoS2 catalysts.Although some published data have notified the framework-activity connections for MoS2 and CoMoS2 catalysts, they essentially concentrated upon the HDS process (Hensen et al., 2001; Schweiger et al., 2002).These reports have depicted that the catalytic performance is quite related to the rims of the levels of the MoS2 layers, and particularly with the sulfur-free places that are created over the rim sites.Many methods have emerged to synthesize MoS2 or CoMoS2 with controlled surface-characteristics, such as solvothermal (Duphil et al., 2002), sonochemical (Dhas and Suslick, 2005) or biotemplate (Chang et al., 2006) syntheses.Théodet (2010) indicated that the activity of supported catalyst would be decreased due to the interference effect of the support with the active phase.Additionally, the concentration of active sites per unit volume is decreased thus high amounts of catalysts are required to attain the wanted fuel properties.The author depicted that bulk catalysts are the "wave of the future" in many industrial applications.Avarez et al. (2008) prepared unsupported NiMoS2 catalyst from ammonium and C16H37NO, (NH₄)₂MoS₄ fattened with Ni(NO3)2.Authors depicted that the alkyl group in the C16H37NO precursor had a direct effect on surfacecharacteristics of catalyst.The extent of the alkyl concatenation from C1 to C4 showed a very high HDS activity.Gaojun et al. (2010) prepared bulk Ni-Mo-S2 catalyst.Their outcomes depicted that the bulk NiMo catalyst has excellent hydrogenation performance to produce fuel with sulfur content ≤ 10 ppm.He and Que (2016) provided a thorough review of the bulk MoS2, briefing updated studies on framework, characteristics, preparation methods.The main aim of this study was to prepare and identify an unsupported catalyst (e.g.,

Figure ( 2 ) 1 -
represents a block diagram of the synthesis procedure.0.0378 moles MoO3 and 0.1415 moles Na2S.9H2O were dissolved in 0.3L distilled water by stirring for 10 minutes to ensure homogeneous solution formation and then slowly 0.0425 ml of 4 M HCl solution was added.A black solution was formed by adding HCl.The solution was putted into an autoclave reactor and reacted at 280-320 ˚C and 25-35 bars at 500 rpm for 120 minutes.Thereafter, the reactor was instantly cooled down by using a chiller.Black solid particles resulted from the synthesis was filtered and washed several times with deionized water and ethanol, and dried under nitrogen of atmosphere pressure at 160 °C for 240 minutes (Zhang et al., 2015).N2 gas; 2-Gas pressure regulator; 3-sampling valve; 4-Batch reactor with heat and mixing component; 5-Temperature and speed control system; 6-chiller; 7-motor stirrer.
XRD analysis of MoS2 was conducted using a diffraction unit [Shimadzu-6000, Japan].X-Ray diffractometer (XRD) with a 2Ɵ range from 10o to 80o with scan rate 2 (deg/min) and Cu-kα (λ = 1.541Å) as radiation source was applied.The analysis was carried out at the central service Laboratory in the University of Baghdad.Morphology analysis of MoS2 was performed using SEM instrument [VEGA 3 LM, Germany] at the University of Technology.The specific surface area and pore volume of MoS2 catalyst were determined utilizing Brunauer Emmett and Teller (BET) method using analyzer (Q Surf 1600, USA).The instrument is available in the Petroleum Research and Development Centre in Baghdad.

3. 1
Figures (3a) and (3b) demonstrate XRD images for the effect of temperature (300 and 320 oC), at constant pressure (35 bars), on crystalline structure and phase purity of MoS2 nanostructures.As could be seen, all peaks presented in the two images have nearly the same locations on the 2 -axis.However, at 300 oC, a small peak appears at 56.6•, seen as left shift of 110 lattice facets.The spectra indicate a more amorphouslike MoS2 (Fig.3a-MoS2) structure at low temperature.When the synthesis temp increases to 320•C, a crystalline structure (denoted as Fig.3b-MoS2 hereafter) starts to develop, manifested by the characteristic peaks for 002, 100, 103, and 110 facets (Fig.3a-b).The broad peaks, on the other hand, also reveal poor crystallite structure.The peak intensities were enhanced with increased synthesis temperature.As the temperature increased from 300 to 320 oC, the corresponding 2θ reflections became sharper and could be clearly observed.Moreover, sizes of peaks in Fig.3aare smaller than in Fig.3b, indicating that Mo and S powders could not completely react at the lower temperature.This confirmed the predominant effect of temperature on the yield of MoS2.As can be observed in Figure3b, the XRD peaks can be recorded to those of the perspicuous hexagonal phase of MoS2 with lattice coefficients a = 3.161 Å, c = 12.84 Å, which are agree well with the amounts of standard card (JCPDS No. 37-1492).No featured peaks were revealed from other impurities, pointing out that the sample has high purity.The application of XRD showed that the crystal structure of the particles was hexagonal.Additionally, the comparison of the obtained peaks in Figures3a and 3bindicated that the MoS2 nanoparticles average particle size was calculated, according to Scherer's equation (Eq.2), approximately as 32 and 21nm at temperatures of 300 and 320 oC respectively.

Figure
Figure (4a) and (4b) demonstrates XRD images for the effect of pressure (25 and 35 bar) at constant temperature (280 o C).The main perceptible XRD peaks can be easily recorded to the hexagonal phase of MoS2 compatible with the standard powder XRD folder of MoS2 (JCPDS 37-1492), and there is peak from impurity due to incomplete MoO3 conversion.Moreover, the strength of the XRD peaks of MoS2 varied significantly under different imposed pressures.With further increasing of the reaction pressure to 35 bars, XRD pattern (Figure.4b) shows that the intensities of XRD peaks of MoS2 increase.As seen in Figure4, the higher and acute peaks depict that the sample was quite crystallized.The higher the imposed pressure is, the better the crystallized products will be.Sulfur concentration on Mo films is related to the dynamic pressure within the reactor.By increasing the pressure, the concentration of sulfur on Mo surface increases, therefore, a higher nucleation density of MoS2 was expected on the Mo film.These nucleation sites cannot proceed further when low pressure applied because of low sulfur concentration at Mo surface.The high and sharp diffraction peak of (Fig.4b) of the as-prepared MoS2 samples indicates the formation of well-stacked layered structure of MoS2 during the hydrothermal process.Our results agree well with findings of (Wang et al., 2017).

( i )
Copyright  2018 Al-Qadisiyah Journal For Engineering Sciences.All rights reserved.

( 7 )
represents the value of activation energy for HDS reaction over unsupported MoS2 catalyst in terms of (-Ea/R).According to Figure(7), the activation energy (Ea) = 26.36kJ/mol.

Figure ( 8 )
Figure(8) illustrates the effects of temperature on sulfur removal from gas oil while other operating parameters were kept constant at (P=35 bar, WMoS2 = 0.5 gm).It can be observed that a positive relationship was established between sulfur removal and operating temperature.The sulfur removal, after 60 min of HDS reaction of gas oil, is 96.2, 95.8, and 88.0% at temperatures 360, 340, and 300 o C respectively.This may be because of the equilibrium limitations at higher reaction temperatures for reversible HDS reactions.Moreover, Figure(8) illustrates a comparison between unsupported MoS2 catalyst and supported CoMo/Al2O3 composed of 15.5 wt% Mo and 5.5 wt% Co, catalyst for HDS of Iraqi gas oil produced by Al-Daura Refinery, Baghdad (Abid et al., 2018).As can be observed, MoS2 catalyst offers 7.3% increasing in sulfur removal over CoMo/Al2O3 although the later catalyst has a higher weight percentage of Mo with 5.5 wt% of the promoter (i.e., Co).

Table ( 2
): BET measurements of MoS2 catalyst synthesized at different synthesis conditions

Table ( 3
): Values of kHDS at different operating conditions