Study on Sulfidation Degree and Morphology of MoS 2 Catalyst Derived from Various Molybdate Precursors

Transcription

1 Catalyst Research China Petroleum Processing and Petrochemical Technology 2014, Vol. 16, No. 2, pp 1-7 June 30, 2014 Study on Sulfidation Degree and Morphology of MoS 2 Catalyst Derived from Various Molybdate Precursors Zhang Le; Li Mingfeng; Nie Hong (Research Institute of Petroleum Processing, SINOPEC, Beijing100083) Abstract: The MoS 2 catalysts were prepared from various molybdate precursors including inorganic and organic molybdate compounds. The sulfidation degree and morphology of active phases of MoS 2 activated by various molybdate precursors in H 2 S/H 2 stream at different temperatures were studied by X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The organic molybdate precursors lead to MoS 2 catalysts with higher sulfidation degree and smaller active phases to demonstrate higher catalytic activity during hydrodesulfurizaiton (HDS) of 4,6-DMDBT. Key words: MoS 2 ; molybdate precursors; sulfidation degree; morphology 1 Introduction Environmental concerns lead to increasingly tightening regulations on sulfur, nitrogen and aromatics content in fuels [1]. Conversion of these compounds is therefore of paramount importance, and such an objective needs the development of more active catalysts. Molybdenum sulfide materials have emerged as a class of promising catalysts for hydrotreating reactions. Many researches have dealt with the activation treatment by means of gas phase (H 2 /H 2 S) and liquid phase (DMDS) activation of catalysts, with the effects on morphological and catalytic properties of treated catalysts properly reported [2-3]. Especially, the decomposition of thiosalts has been widely used in preparing molybdenum or tungsten disulfide with high surface area [4-5]. L. Alvarez [6] reported that the method of activation (in-situ or ex-situ) of tetraalkylammonium thiomolybdates and the nature of the thiosalt precursor (with or without C) can influence strongly the textural and catalytic properties of the final MoS 2 and Co/MoS 2 catalysts. The use of a tetraalkylammonium thiomolybdate precursor (with C) reduces significantly the formation of a MoS 2 -like intermediate and can lead to a final meso-structure of MoS 2. H. Nava and co-workers [7] prepared unsupported nickel-molybdenum-tungsten sulphide catalysts from tri-metallic NiMoW alkyl precursors with tetraalkylammonium thiomolibdotungstates salts, (R 4 N) 4 MoWS 8 (where R=H, methyl, propyl, butyl, or cetyl-trimethyl). The nature of the alkyl group can strongly affect both the specific area and the HDS activity. The catalytic activity is strongly enhanced when the carbon-containing precursors are used. So the effect of molybdate precursors (with or without C) was warmly discussed with respect to their influence on the structure, morphology and activity of MoS 2 catalysts. However, this investigation is mainly aimed at thiosalt precursor and sulfidation degree, and the morphology of active phases of MoS 2 activated by various molybdate precursors at different temperatures is less studied. In the present work, the activation law, the difference in morphology, and performance of MoS 2 activated by various molybdate precursors have been studied and the effect of carbon contained in the molybdate precursors has been discussed. 2 Experimental 2.1 Catalyst preparation MoS 2 catalysts were prepared from five different molybdates precursors listed in Table 1. The molybdates precursors Mo-1, Mo-2 and Mo-4 are chemical reagents. The molybdate precursor Mo-3 was prepared by heating a solution of citric acid and molybdenum trioxide (at a molar ratio of 1:1). The molybdate precursor Mo-5 was obtained by the following experiment. Under heating and stirring, Recieved date: ; Accepted date: Corresponding Author: Dr. Zhang Le, Telephone: ; zhangle.ripp@sinopec.com. 1

2 China Petroleum Processing and Petrochemical Technology 2014,16(2):1-7 the ammonium heptamolybdate tetrahydrate solution was added to a solution of hexadecyl trimethyl ammonium bromide (CTAB) prior to being refluxed at 373 K for 4 h. The white precipitate formed thereby was isolated by filtration, washed with water and dried at 393 K for 3 h to obtain Mo-5. The carbon content in the molybdates precursors Mo-3 and Mo-5 was analyzed by a carbon-sulfur analyzer. Then the molybdate precursors are all sulfided in a flow of 15% H 2 S/H 2 mixture at 473 K and 573 K, respectively, for 4 h to produce the MoS 2 catalysts (Table 2). Table 1 The properties and resources of molybdate precursor Molybdate precursor Name Molecular formula Manufacturer Carbon content, % Mo-1 Ammonium heptamolybdate tetrahydrate, AR (NH 4 ) 6 Mo 7 O 27 4H 2 O Tianjin Sifang Chemical Company China National Pharmaceutical Mo-2 Phosphomolybdic acid, AR H 3 PMo 12 O 40 0 Group Corporation Mo-3 Polymolybdate-citric acid 19.5 Mo-4 Molybdenyl acetyl acetonate, AR C 10 H 14 MoO 6 ACROS 36.8 Mo-5 Polymolybdate-CTAB Table 2 MoS 2 catalysts prepared by various molybdate precursors under different activation conditions MoS 2 catalysts Molybdate precursor Sulfidizing temperature, K MoS Mo MoS Mo MoS Mo MoS Mo MoS Mo MoS Mo MoS Mo MoS Mo MoS Mo MoS Mo The MoS 2 catalysts were characterized by X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The XPS experiments were performed in a VG Scientific ESCALab 220i- XL spectrometer, with source of X-rays, Al Ka ( ev) anode and 300 W of power. HRTEM was carried out on a TECNAI F20 G2 apparatus, made by the FEI Company with a resolution of 0.24 nm. The hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene (4,6-DMDBT) was carried out in a fixed-bed micro-reactor made by the American Autoclave Engineers Company. The molybdate precursors were in-situ sulfided with a solution of 5% CS 2 in cyclohexane at a flow rate of 0.4 ml/min at 360, 4.14 MPa of H 2 pressure and a H 2 flow rate of 400 ml/min for 3 h. Then the reactor was switched to treatment of the reactant feed (0.45% of 4,6-DMDBT in decane) at a flow rate of 0.2 ml/min in the same hydrogen atmosphere. The reaction products were analyzed by the on-line GC-FID directly. The conversion of 4,6-DMDBT was calculated using the internal standard method. The HDS activities were calculated using the following equation: Total HDS activity: A HDS =F 0 conversion/m where F 0 is the molar flow rate of the reactant (mol/s) and m is the mass of the catalyst (g). 3 Results and Discussion 3.1 The sulfidation laws of the catalysts The XPS spectra of the MoS 2 catalysts were collected. Table 6 gives the binding energies of S2p and Mo3d derived from decomposition of the XPS spectra of MoS 2 catalysts. The XPS spectra of S2p on the MoS 2 catalysts exhibit only one peak at about ev, which corresponds to S 2- [8-9]. The absence of any signal at ev after sulfidation indicates that no oxidation of the catalysts occurs during the transfer of the solid from the sulfidizing reactor to the XPS spectrometer. In the Mo3d spectra, the peaks are attributed to Mo 4+ species (229 ev and 232 ev for the 3d 5/2 and 3d 3/2, respectively) and Mo 6+ species (233 ev and 236 ev) [10]. The sulfidation degree of surface species has been calculated based on the area of the XPS peaks of the various species as shown in Table 3. The peaks at around 2

3 Zhang Le, et al. Study on Sulfidation Degree and Morphology of MoS 2 Catalyst Derived from Various Molybdate Precursors Table 3 Binding energy of elements and sulfidation degree of molybdenum in MoS 2 catalysts Sulfidation Binding energy, ev degree MoS 2 catalysts Mo 3d 5/2 S 2p Mo 4+ /Mo, % Mo 4+ Mo 6+ MoS , MoS , MoS , MoS , MoS , MoS , MoS , , MoS , MoS , MoS , ev and 232 ev are attributed to Mo 4+ species and can be used to calculate the sulfidation degree of surface Mo species. There are great difference in the sulfidation degree for the molybdate precursors at a sulfidation temperature of 473 K. The sulfidation degree of inorganic molybdate precursors (Mo-1 and Mo-2) dips to as low as 50% 60% upon activation at 473 K. The organic molybdate precursors (Mo-3, Mo-4 and Mo-5) have much higher sulfidation degree than inorganic molybdate precursors, and especially Mo-4 and Mo-5 are almost totally sulfided upon activation at 473 K. Interestingly, the sulfidation degree increases with the increase in carbon content of the molybdate precursors (Figure 1). Upon sulfidation at 573 K, the sulfidation degree of Figure 1 Carbon content of molybdate precursors versus molybdenum sulfidation degree of MoS 2 upon activation at 473 K catalysts activated by different molybdate precursors shows no large difference. The sulfidation degree of catalysts activated by organic molybdate precursors is still higher than that of catalysts activated by inorganic molybdate precursors. The sulfidation degree of catalysts activated by inorganic molybdate precursors reach up to 82% under this condition, and the catalysts are almost totally sulfided by organic molybdate precursors. 3.2 Morphology of active phases of MoS 2 catalysts Figure 2 presents the HRTEM photographs of the sulfided MoS 2 catalysts prepared from various molybdate precursors upon activation at different temperatures. Interpretation of the HRTEM pictures of Mo sulfidebased catalysts was well documented [11-12]. The black lines in the pictures correspond to the lattice images of MoS 2 fragments. These short black lines might be either single or in packets of up to about seven parallel threads which correspond to the lamellar structure of the Mo dichalcogenides. Figure 2 presents some short black lines in the MoS 2 catalysts corresponding to the lamellar structure of random MoS 2. It is difficult to find the black lines in the MoS and MoS and there is a small amount of black lines in the MoS However, in the HRTEM pictures of MoS and MoS-5-473, a large amount of black lines can be observed. So it means that under mild activation at a low temperature of 473 K, seldom MoS 2 lamellar structure can be identified in precursors Mo-1, Mo-2 and Mo-3, while a large amount of MoS 2 layer stacks can be formed in precursors Mo-4 and Mo-5. Upon sulfidation at 573 K, it can be seen clearly that the number of MoS 2 layer stacks in MoS-1-573, MoS and MoS increases significantly. There are also many MoS 2 layer stacks in MoS and MoS In addition, the number of MoS 2 layer stacks in MoS-3-573, MoS and MoS is remarkably more than that in MoS and MoS In another word, MoS-3-573, MoS and MoS have a much higher density of active sites than that in MoS and MoS According to the literature [13], the length L of the black lines, which roughly corresponds to the lateral dimension of the observed MoS 2 platelets, and the number N of three-dimensional stacked layers can be calculated as 3

4 China Petroleum Processing and Petrochemical Technology 2014,16(2):1-7 where L - is the mean length of the observed slabs; N - is the mean number of the stacked layers; and n i is the number of the elemental particles having the parameters of L i and N i. We assume here that each slab with several layers has the same dimension. The statistical results of the length and number of the MoS 2 stacked layers are shown in Table 4. Table 4 Mean length (L - ) and number (N - ) of the stacked layers MoS 2 catalysts L -, nm N - Average number of MoS 2 stacks, (1 000 nm 2 ) -1 MoS MoS MoS MoS MoS MoS MoS Figure 2 High-resolution TEM images of sulfided MoS 2 catalysts follows. In addition, the average number of MoS 2 stacks in an area of nm 2 was also calculated. More than 200 slabs were examined on several HRTEM pictures taken from different parts of the same sample dispersed on the microscope grid. Because there are seldom MoS 2 layer stacks in MoS-1-473, MoS and MoS-3-473, the length and number of the MoS 2 stacked layers are not calculated. After sulfidation at 473 K, MoS and MoS have 1 3 layers of MoS 2 slabs with a thickness of about 5 nm, and their average number of MoS 2 stacks in nm 2 is high. After sulfidation at 573 K, MoS and MoS have a large MoS 2 layer stacks with 3 4 layers in thickness and 7 8 nm in length. After sulfidation at 573 K, the length and number of the MoS 2 stacked layers of MoS-3-573, MoS and MoS derived from the organic molybdate precursors are all smaller than those of MoS and MoS derived from the inorganic molybdate precursors. On the other hand, the average number of MoS 2 stacked layers in MoS-3-573, MoS and MoS is much greater. Especially, MoS has the smallest MoS 2 slabs, albeit with a highest density. It is strange that the MoS and MoS derived from molybdate precursors with high carbon content have larger MoS 2 slabs and lower density of active sites than that derived from the molybdate precursor Mo-3 with low carbon content. It may cause a lower activity of MoS and MoS as compared to MoS

5 Zhang Le, et al. Study on Sulfidation Degree and Morphology of MoS 2 Catalyst Derived from Various Molybdate Precursors When the MoS 2 layer stacks of MoS are compared with MoS and MoS-5-473, it can be found out that MoS and MoS have higher density of active sites than that of MoS with almost the same size of MoS 2 slabs. So there is a corresponding suitable sulfidation temperature for the different molybdate precursors. For molybdate precursors Mo-4 and Mo- 5, a temperature of 473 K is more appropriate for their sulfidation which can produce smaller size of MoS 2 slabs and higher density of active sites. However, if molybdate precursors Mo-4 and Mo-5 are transformed to sulfides at a temperature which is higher than their proper temperature, the active phases will grow into slightly larger slabs and the average number of MoS 2 stacks will decrease. In general, if the molybdate precursors are all transformed catalysts have the lowest HDS activity, while the HDS activity of MoS-3 increases by about 100% as compared to that of MoS-1 catalyst. Among all these catalysts, the HDS activity of sulfidized MoS-4 and MoS-5 catalysts are the highest which is about 6 times higher than that of MoS-1 catalyst. By taking into account the above evaluation results, the carbon content in the molybdate precursors is quite consistent with their measured catalytic activity (Figure 3). The MoS 2 catalyst prepared from the organic molybdate precursor Mo-5 has a highest activity along with a highest carbon content in the precursor at the same time. Comparably, the MoS 2 catalysts prepared from the inorganic molybdate precursors Mo-1 and Mo-2 have the lowest activity. into sulfides at a suitable temperature, the size of MoS 2 slabs will decrease and the density of active sites will increase with an increasing carbon content in the molybdate precursors. Therefore, the organic molybdate precursors will promote a small size of MoS 2 stacking layers and a much high density of active sites after proper sulfidation. 3.3 Evaluation of the catalysts It is generally accepted that the HDS of 4,6-DMDBT occurs through two parallel reaction pathways as follows: (i) direct desulfurization (DDS) which gives 3,3 -dimethlybiphenyl (DMBiPh); and (ii) desulfurization through hydrogenation (HYD) which yields 3-(3 -methylcyclohexyl)-toluene (DMCHT) with a tetrahydrogenated compound as an intermediate [14]. In all reactions for HDS of 4,6-DMDBT over MoS 2 catalysts derived from various molybdate precursors, there is no DDS product, so only the total HDS activity is given in Table 5. Table 5 Catalytic activity of MoS 2 catalyst for HDS of 4,6-DMDBT MoS 2 catalyst Molybdate precursors A HDS, 10-6 (mol g -1 s -1 ) MoS-1 Mo MoS-2 Mo MoS-3 Mo MoS-4 Mo MoS-5 Mo The results show that the sulfidized MoS-1, MoS-2 Figure 3 Activity of MoS 2 catalysts for HDS of 4,6-DMDBT as a function of carbon content of their molybdate precursors 4 Discussion The nature of the molybdate precursor influences strongly the sulfidation degree, morphology of active sites and catalytic properties of the final MoS 2 catalysts. The presence of initial carbon in the molybdate precursor has a beneficial effect on the final MoS 2 structure and performance. Compared with the inorganic molybdate precursor, the carbon-containing molybdate precursor (with C) can form MoS 2 catalyst with higher sulfidation degree, smaller MoS 2 slabs and higher HDS activity. With the increase of the carbon content in the molybdate precursor, firstly the activation of catalyst would become easier because the temperature of activation decreases greatly. The Mo and Mo have almost the same sulfidation degree while their activation temperature differs by 100 K. Secondly, the MoS 2 slabs 5

6 China Petroleum Processing and Petrochemical Technology 2014,16(2):1-7 become smaller and the density of active sites increases dramatically which means that there are more active sites in the MoS 2 catalyst upon using a carbon-containing molybdate precursor. Finally, MoS 2 catalyst derived from organic molybdate precursor has good performance in 4,6-DMDBT HDS reaction because of its high sulfidation degree and the existence of more active sites. The activation condition for each molybdate precursor differs a lot. For the organic molybdate precursors, a temperature of 473 K 573 K might be the right temperature range for their total sulfidation. While for the inorganic molybdate precursors, a temperature of 573 K 623 K may be suitable for its total sulfidation. So different molybdate precursors should have their own suitable sulfidation temperature. At the suitable sulfidation temperature, the sulfidation degree of MoS 2 is high and there is a highest density of small MoS 2 slabs which also means the highest active sites. However, with activation conducted at a temperature higher than their suitable temperature range, the density of MoS 2 slabs will decrease and the size of slabs becomes bigger. So the active sites would decrease in that case, although their sulfidation degree is still high. Apparently, the activity of MoS-4 and MoS-5 catalysts for HDS of 4,6-DMDBT might not be their best performance in this reaction and could be optimized after adjusting their activation temperature. A more suitable sulfidation condition can be investigated for obtaining higher activity of MoS 2 catalyst in the future research work. 5 Conclusions The activation law and morphology of active sites of MoS 2 catalysts prepared from various molybdate precursors were studied by XPS and TEM. The nature of the molybdate precursor has an significant effect on the sulfidation situation, active phase structure and HDS activity of MoS 2 catalyst. The MoS 2 catalyst originated from organic molybdate precursor (with C) has higher sulfidation degree, more active sites and better HDS performance compared with the MoS 2 catalyst originated from inorganic molybdate precursor. With the increase of carbon content in molybdate precursor, more MoS 2 active sites will be formed in the MoS 2 catalyst and its activity in 4,6-DMDBT HDS reaction will also improve. Acknowledgments: The authors gratefully acknowledge the financial support by the National Key Basic Research Development Program 973 Project (2012CB224800) of China. References [1] Song C. An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel[j]. Catal Today, 2003, 86: [2] Alvarez L, Espino J, Ornelas C, et al. Comparative study of MoS 2 and Co/MoS 2 catalysts prepared by ex situ/in situ activation of ammonium and tetraalkylammonium thiomolybdates[j]. J Mol Catal A, 2004, 210(1/2): [3] Alonso G, Berhault G, Aguilar A, et al. Characterization and HDS activity of mesoporous MoS 2 catalysts prepared by in situ activation of tetraalkylammonium thiomolybdates[j]. J Catal, 2002, 208: [4] Alonso G, Berhault G, Chianelli R R. Synthesis and characterization of tetraalkylammonium thiomolybdates and thiotungstates in aqueous solution[j]. Inorg Chim Acta, 2001, 316: [5] Alonso G, Yang J, Siadati M H, et al. Synthesis of tetraalkylammonium thiometallates in aqueous solution[j]. Inorg Chim Acta, 2001, 325: [6] Alvarez L, Espino J, Ornelas C, et al. Comparative study of MoS 2 and Co/MoS 2 catalysts prepared by ex situ/in situ activation of ammonium and tetraalkylammonium thiomolybdates[j]. J Mol Catal A, 2004, 210(1/2): [7] Navaa H, Pedrazab F, Alonso F. Nickel-molybdenumtungsten sulphide catalysts prepared by in situ activation of tri-metallic (Ni-Mo-W) alkylthiomolybdotungstates[j]. Catalysis Letters, 2005, 99(1/2): [8] Okamoto Y, Imanaka T, Teranishi S. Surface structure of CoO-MoO 3 /Al 2 O 3 catalysts studied by X-ray photoelectron spectroscopy[j]. J Catal, 1980, 65: [9] Gajardo P, Mathieux A, Grange P, et al. Structure and catalytic activity of CoMo/γ-Al 2 O 3 and CoMo/SiO 2 hydrodesulphurization catalysts: An XPS and ESR characterization of sulfided used catalysts[j]. Appl Catal, 1982, 3: [10] Li C P, Hercules D M. A surface spectroscopic study of sulfided molybdena-alumina catalysts[j]. J Phys Chem, 1984, 88(3): [11] Sanders J V. Chapter 2: The electron microscopy of catalysts[m]// Anderson J R, Boudart M. Catalysis Science and Technology: vol. 7. Berlin: Springer-Verlag, 1985:

7 Zhang Le, et al. Study on Sulfidation Degree and Morphology of MoS 2 Catalyst Derived from Various Molybdate Precursors [12] Zaikovskii V I, Playasova L M, Burmistrov V A, et al. Sulphide catalysts on silica as a support. ii. High resolution electron microscopy data[j]. Appl Catal, 1984, 11: [13] Payen E, Hubaut R, Kasztelan S, et al. Morphology study of MoS 2 -based and WS 2 -based hydrotreating catalysts by high-resolution electron-microscopy[j]. J Catal, 1994, 147(1): [14] Bataille F, Lemberton J L, Perot G, et al. Sulfided Mo and CoMo supported on zeolite as hydrodesulfurization catalysts: Transformation of dibenzothiophene and 4,6-dimethyldibenzothio-phene[J]. Appl Catal A, 2001, 220 (1/2): The Project Development and Commercial Application of Continuous Liquid-Phase Diesel Hydrotreating Technology Passed SINOPEC s Technical Appraisal On December 12, 2013 the project Development and commercial application of continuous liquid-phase diesel hydrotreating (SLHT) technology jointly undertaken by RIPP, SEI, the Shijiazhuang Refining and Chemical Co. (SRCC) and the Anqing Petrochemical Co. has passed the technical appraisal organized by the Science and Technology Division of SINOPEC in Beijing. Technical personnel engaging in this project upon delving into the principles of continuous liquid-phase hydrotreating of diesel first of all investigated the process regulations of upflow reactor, developed the method for automatic control over the continuous phase in the upflow reactor, finalized the technical route for realization of SLHT, and worked out the SLHT process scheme featuring a simple and intrinsically safe operation. In pursuit of the Chinese content, the technical team for the first time designed an upflow reactor, developed the high-efficiency reactor internals in order to make the liquid phase inside the reactor function as the continuous phase with the gas phase serving as the dispersed phase. The space utilization rate is increased so that the internals can be easily fabricated and installed. A flow of excess hydrogen can be properly replenished at the reactor inlet and between catalyst beds to enhance the reaction rate, reduce the recycle ratio, and cut the equipment investment cost and operating cost. It is the first attempt to design the mass-transfer internals inside the high-pressure separator and reduce H 2 S concentration in the recycle oil by means of introducing a small amount of stripping hydrogen to promote the deep desulfurization reaction. A brand new process scheme for start-up and shut-down of equipment has been formulated in order to reduce the startup and shutdown time, while a new safety interlock and control theory and an emergency response program have been proposed. The interim commercial calibration tests of the SLHT process at SRCC have revealed that it is possible to manufacture a diesel fuel product meeting the national quality standard IV for diesel fuel by using a straight-run gasoil blended with 8% of coker diesel fraction. When the mixed feedstock containing μg/g of sulfur and 217 μg/g of nitrogen was hydrotreated at a reactor inlet pressure of 9.0 MPa, an average reaction temperature of 365.3, a space velocity of 1.55 h -1, and a recycle ratio of 2.0, the diesel fuel product meeting the national quality standard IV was obtained. When the reaction temperature was raised to 376.3, the sulfur content in diesel could be reduced to less than 10 μg/g. The energy consumption for manufacture of diesel fuel meeting the national quality standard IV is equal to 6.24 kg of SOE/ton (for cold feedstock), or is tantamount to 4.81 kg of SOE/ ton of feedstock after being converted to heated feed oil at the design point, which is much less than the energy consumption achieved at the traditional trickle bed for hydrotreating of diesel. The outcome of an accumulated 10 months of operation has indicated that the continuous liquid-phase diesel hydrotreating (SLHT) technology is characteristic of simple and smooth operation coupled with good catalyst stability that can meet the commercial production needs. The test results have once more verified the pilot plant testing outcome. 7