Controllable thermal conversion of thiomolybdate to active few-layer MoS2 on alumina for efficient hydrodesulfurization
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Sulfur removal is the most challenging task in petroleum refineries for upgrading clean fuel from crude oil. In this regard, an efficient Ni–MoS2 catalyst was prepared using ammonium tetrathiomolybdate (ATM), (NH4)2MoS4 as a precursor for hydrodesulfurization (HDS) reaction. ATM was derived by sulfiding an aqueous ammoniacal ammonium heptamolybdate and impregnated on alumina support with and without nickel promoter to form a series of MoS2/Al2O3, and Ni–MoS2/Al2O3 catalysts with controlled calcination temperature. The prepared ATM and catalysts were characterized by X-ray diffraction (XRD), Infrared spectroscopy and nitrogen adsorption–desorption. The phase transformation of ATM to MoS2 was observed by TGA, and number of MoS2 layer formation was calculated from the frequency difference between the redshift and blueshift by Raman spectrum. The catalytic properties of MoS2/Al2O3, and Ni–MoS2/Al2O3 were investigated using thiophene as the model compound for HDS reaction. The HDS rate, C4 hydrocarbon product distribution and selectivity were quantified by microreactor with refinery gas analyzer system. The prepared MoS2 catalysts (MoS and NiMoS) at 350 °C showed a better conversion rate than another catalyst due to the synergy between the promoter and few-layered MoS2 on support. Thus, ATM is a suitable candidate for the formulation of Ni–MoS2 catalyst at 350 °C for improving thiophene HDS activity.
KeywordsMolybdenum disulfide Ammonium tetrathiomolybdate Hydrodesulfurization Thiophene
Recent literature reveals that one-third to one-half of the total global energy demand has been fulfilled by fossil fuels [1, 2]. Perhaps the other striking sources across the world, fuels are derived from petroleum refinings such as gasoline, diesel, fuel oil and aviation fuel. Petroleum feedstocks are expected to emit a significant amount of sulfur as SOx during combustion that has a direct negative impact on the environment. Thus, environmental objectives have been imposed to implement stringent regulations to control the sulfur level in fuels. Based on the aforementioned, many countries have started focusing on reducing the sulfur content of diesel fuel to an ultra-low level of about 10 ppm . However, the refineries are facing a significant challenge in achieving the required sulfur specification , because the sulfur level in the petroleum fraction varies, and depends on the source of crude oils . The hydrodesulfurization (HDS) is part of the hydrotreating process for removing organosulfur compounds from heavy crude oils using the metal disulfide catalyst. Generally, molybdenum and tungsten are used as active metals along with the promoter nickel/cobalt, which may be obtained by two approaches, which are supported [6, 7] and unsupported catalyst [8, 9]. Notably, the supported catalysts were widely used for HDS process, and prepared with different methodologies such as sulfiding metal oxide [3, 6], thermal decomposition of thiomolybdate [10, 11], hydrothermal and solvothermal processes .
The conventional method reveals that the sulfidation of molybdenum oxides under hydrogen sulfide flow at high-temperature is the critical step, because it gives a mixture of partially converted MoS2 and unconverted MoOx , and it may decrease HDS activity due to the strong interaction between metal and support . However, the support plays a crucial role in the dispersion of active metals for enhancing the dispersion of surface metals . Hence, a new formulation approach is required to produce an even dispersion of active metal on the support for the production of sulfur-free clean fuel from crude oil. In this regard, the direct sulfided catalyst has been prepared using the thermal decomposition of thiomolybdates and recognized as a simple route than the conventional sulfidation of molybdenum oxides. Thiomolybdates possess tetrahedrally coordinated sulfur atom with molybdenum atom that undergoes a topotactic reaction during the decomposition, as resultant, the sulfide structure is retained in the same precursor  that enhances HDS activity.
The unsupported trimetallic HDS catalyst is prepared using different thiometallates precursor with better catalytic activity [17, 18, 19, 20]. However, the unsupported catalysts tend to have increased pressure drop, reduced lifespan, and less economic feasibility, whereas, the supported catalyst can be recycled and possesses a long life due to the synergistic properties of active metal and support . The porous support materials are considered as an efficient candidate for uniform dispersion of active constituents over high surface area and suitable pore size for the mobility of reactants inside. Currently, there are a number of supports reported for dispersion of an active metal, such as alumina, silica, carbon, titanium oxide and carbon . Especially, alumina has some advantages like high surface area, good strength, tailoring pore structure, and low cost , and more acid sites are available that offers the strong interaction between metal and support, which promotes the hydrotreating and hydrogenation reactions [24, 25]. Hence, the highly dispersed active MoS2 or WS2 supported catalyst was prepared from thiometallates by thermal decomposition for better catalytic activity and which provides excess sulfur for the simultaneous sulfidation of the promoter without H2S sulfidation process [26, 27].
In this work, a well-dispersed MoS2 on alumina support was synthesized using ammonium tetrathiomolybdate (ATM) as a precursor for MoS2, and Ni as a promoter by incipient wetness impregnation method. The metal sulfide formation was studied as a function of calcination temperature under an inert medium in order to diffuse excess ‘S’ to convert the promoter as in sulfide form. The physicochemical properties of synthesized ATM and series of the catalyst with and without promoter (mole ratio 0.3 (Ni/(Ni + Mo))) were characterized using several analytical techniques. The hydrodesulfurization activity of the catalyst was investigated using thiophene as a model compound at atmospheric pressure, and calculated reaction rate and selectivity of C4 hydrocarbons. Further, the effect of calcination temperature was also studied, which plays a significant role in the catalytic activity. The synthesized MoS2/Al2O3 and Ni–MoS2/Al2O3 catalysts reveal a higher thiophene HDS conversion rate and selectivity, and hence can contribute as efficient catalysts for HDS reactions.
2 Experimental section
2.1 Synthesis of ammonium tetrathiomolybdate
Ammonium tetrathiomolybdate (ATM) was prepared by sulfidation of ammonical molybdate salt as reported elsewhere . Typically, ammonium heptamolybdate (8.09 mmol, (NH4)6Mo7O24·4H2O) dissolved in the mixture of ammonium hydroxide (60 ml) and water (5 ml). The reaction mixture was purged with hydrogen sulfide (H2S) for sulfidation at 25 °C for 60 min, and the temperature was slowly raised to 60 °C with continuous H2S purging until turn a dark red color. The reaction mixture was kept in an ice bath for crystallization of red ATM crystals, and followed by vacuum filtration, washed several times with cold isopropyl alcohol and dried under vacuum. The prepared ATM was stored under an inert atmosphere and subjected to further characterization and used as a precursor for catalysts preparation.
2.2 Preparation of MoS and NiMoS catalysts
The commercial alumina (γ-Al2O3) extrudates (Sasol, Germany) was used as a support and synthesized ATM as a precursor for MoS2. The stoichiometric molar ratio of Ni/(Ni + Mo) = 0.3 of ATM precursor and promoter nickel as nitrate salt were dispersed on γ-Al2O3 by incipient wetness impregnation method. The promoter nickel was dispersed on γ-Al2O3 by wet impregnation using nickel, dried at 120 °C. Similarly, ATM/Al2O3 was prepared without a promoter to evaluate the role of the promoter. The prepared nickel promoted ATM/Al2O3 was calcined at 300 °C, 350 °C, 400 °C and 450 °C under inert medium and corresponding catalyst were denoted as NiMoS-1, NiMoS-2, NiMoS-3, and NiMoS-4, respectively. Likewise, the other catalyst also prepared from ATM/Al2O3 by calcined as above temperatures and labeled as MoS-1, MoS-2, MoS-3, and MoS-4, respectively. The prepared series of MoS and NiMoS catalysts were characterized and evaluated for their catalytic activity.
2.3 Characterization of ATM and catalysts
The textural properties of prepared catalysts were analyzed by N2 adsorption–desorption at 77 K on Micromeritics ASAP 2020. The crystal structure of ATM and catalyst were collected using powder X-ray diffraction (XRD) patterns with Cu-Kα radiation (λ = 0.1542 nm) on PANalytical PW3040, and unit cells quantified with X’pert PRO software. ATM symmetric and asymmetric vibration modes were recorded with potassium bromide pellet method by Fourier-transform infrared spectroscopy (FTIR), Vertex 70 spectrometer, Bruker. The Raman spectroscopy was recorded for active modes of MoS42− and MoS2 structure by Senterra, Bruker. The thermal decomposition pathway of ATM → MoS3 → MoS2 and prepared catalysts were performed under N2 medium at a ramping rate of 10 °C/min from 40 to 800 °C by thermogravimetric analyzer (TGA), Mettler. H2-temperature-programmed reduction (TPR) profile of the MoS and NiMoS catalyst were carried out by AMI-300S, Altamira Inc, USA. About 100 mg of each catalyst was placed in a quartz cell, pretreated at 150 °C for 2 h under Ar medium, and cooled to room temperature. TPR was performed using 10% H2/Ar flow (30 ml/min), and the sample was increased up to 1000 °C at a ramp rate of 10 °C/min . Thermal conductivity detector (TCD) was used to measure the hydrogen consumption during TPR run.
2.4 Evaluation of catalysts for HDS activity
The prepared MoS and NiMoS catalysts were evaluated for HDS activity using thiophene, as reported elsewhere . Typically, about 100 mg of each catalyst was placed in the glass microreactor under continuous purging of hydrogen gas (50 ml/min) into thiophene. The flow of thiophene (feed) was kept at constant by controlling saturator temperature at 5 °C, and thiophene containing hydrogen gas was injected into the reactor at 350 °C over 4.5 h for achieving steady state average values. The reactor outlet was connected to online gas chromatography (Agilent, USA) equipped with a capillary column for flame ionization detector and packed column for thermal conductivity detector for quantification hydrocarbons and gaseous products, respectively. Gas chromatography (GC) analyzed data were collected at a constant interval to calculate the reaction rate and C4 product selectivity.
3 Results and discussion
3.1 Characterization of ammonium tetrathiomolybdate
In addition, the stability of ATM was also evaluated with different laser power at 532 nm (Fig. 3B), as MoS42− ion is unstable at high laser power. Photochemical decomposition of ATM is minimized at a low power of 0.2 mW and shows all Raman active modes. However, ATM decomposes to gas products as ammonia and hydrogen sulfide, and partial conversion of MoS2 to MoO3 under atmospheric air environment at 2.0 mW laser power. The formation of MoS2 was confirmed by the characteristics vibration bands of A1g (402 cm−1), E2g (387 cm−1) and E1g (378 cm−1). Further, it was also verified by surface oxidization of MoS2 to stable MoO3 with higher laser power (5 mW) under atmospheric condition. The peaks observed at 158, 291, 666, 820, and 990 cm−1 due to symmetric and asymmetric stretch of the terminal oxygen atom of Mo–O complies with the pure MoO3 spectrum (2 mW-MoO3, Fig. 3b) . Therefore, the Raman spectrum reveals minimum laser power is a suitable condition for ATM without oxidation at atmospheric environment (for example, 0.2 mW at 532 cm−1).
3.2 Characterization of MoS and NiMoS catalysts
Fig. S4 and S5 shows the thermal stability of prepared MoS and NiMoS catalysts determined by a thermogravimetric analyzer (TGA). The TG profiles inferred different structural changes of catalysts through a mass change in the range of 40 °C to 800 °C. MoS and NiMoS supported catalysts show about 5–8% weight loss below 180 °C due to the loss of physisorbed moisture from the atmosphere. Also, the TGA curve exhibits a descending order in the weight loss of catalysts with increasing calcination temperature from 300 to 450 °C, due to decomposition of bounded ammonium hydrogen sulfide. MoS-1 and MoS-2 show higher weight loss about 7% than MoS-3 and MoS-4 in this region of 180–480 °C, which is mainly due to the undecomposed intermediate or incomplete MoS3 phases (Fig. S4). NiMoS catalyst shows the minimum mass loss (~ 3%) in the same temperature range, which attributes to the low retaining capacity of humidity (Fig. S5). Above 500 °C, the weight loss observed in NiMoS catalyst may be due to the oxidation of NixSx to nickel oxide on support.
Raman spectra confirm the formation of MoS2 from ATM at different calcination temperatures. The Raman analysis is an indicative technique to identify the formation of hexagonal MoS2 geometry, which consists of D6h (P63/mmc) space group symmetry on the support surface . The structural characteristics of supported MoS2 and Ni–MoS2 catalyst indicated a significant amount of molybdenum-oxygen stretching at 300 °C. The peak intensity of Mo–O was decreased on calcination at 350 °C and disappeared on calcination temperatures at 400 °C and 450 °C calcined catalyst. On the other hand, the presence of strong bands at 402 and 378 cm−1 belong to MoS2.
The wavelength difference between E 2g 1 and A1g are fingerprint modes that correspond to the number of layer formation of MoS2. Typically, monolayer formation is denoted when the difference between E 2g 1 and A1g is 18 cm−1, and the few-layer formation for the difference of 19 cm−1 to 26 cm−1 in bulk crystal . The wavelength difference between A1g and E 2g 1 for NiMoS-1, NiMoS-2, NiMoS-3, and NiMoS-4 catalysts were found to be 24.0, 23.3, 23.0 and 22.8 cm−1, respectively. Thus, Raman spectra revealed that the NiMoS catalyst, prepared at calcination temperatures from 300 to 450 °C under inert medium, possesses few-layer MoS2. Further, layer formation depends on a function of calcination temperature, which gives descending order of MoS2 layer with increasing calcination temperature.
3.3 Hydrodesulfurization on MoS and NiMoS catalysts
The MoS2-based catalysts with and without promoter on alumina support were prepared at different temperatures, 300, 350, 400 and 450 °C, and tested for thiophene HDS activity at 350 °C (Fig. S6). The various products were obtained by thiophene HDS reaction such as C4 (2-butenes, 1-butene, n-butane, i-butane, and butadiene), C3 (propane and propene), C2 (ethane and ethene) and C2 (methane) compounds [45, 46]. The desulfurized products including C1–C3 and C4 hydrocarbons were quantified using online refinery gas analyzer (RGA) system with calibrated Agilent refinery gas mixture P/N 5190-0519. The initial conversion was the same for both MoS and NiMoS catalysts, later significant changes in activity on NiMoS catalyst were observed, which shows higher HDS rate. The higher activity may correspond to the role of promoter as well as higher hydrogenation function of NiMoS catalyst (Fig. S6B). Thus, the thiophene HDS rate depends on the calcination temperature of the catalyst, and the order of HDS activity at equilibrium condition as follows 350 °C > 300 °C > 400 °C > 450 °C.
Figure 9 shows each C4 hydrocarbon formation rate as a function of calcination temperature from 300 to 450 °C. It is observed that rates of formation of n-butane, 1-butene, cis-2-butene, and trans-2-butene were higher on NiMoS than MoS catalyst calcined at 350 °C catalyst and were about 1.57, 1.66, 1.37 and 1.77-fold, respectively, which might be due to the influence of Ni promoter. The butene formation rate remains constant with increasing calcination temperature and/or presence of Ni promoter. As discussed previously, the number of few-layer MoS2 and weakly bonded sulfur on the catalyst surface contributes to the rate of formation of butenes. The number of layer of MoS2 from MoS42− decreases with increasing calcination temperature as supported by Raman spectrum and the same trend is expected for HDS of thiophene. Based on this, the rate of formation of butene has to decrease, but the weakly bonded sulfur on surface catalyst stimulates the isomerization of 1-butene to cis- and trans-2-butene. Hence, the butenes formation rates were almost similar either in the presence or absence of Ni promoter for the catalysts calcined at 450 °C. Overall, the prepared MoS and NiMoS catalysts reveal that the ratio of formation rate of cis- to trans-2-butene was higher at 350 °C than other calcination temperatures due to the availability of weakly bonded sulfur on the surface of the catalyst.
The characteristics of synthesized ATM such as orthorhombic phase, the strong and stretching vibration mode of Mo–S bond, the asymmetric stretching and symmetric vibration of a Mo=S bond, and thermal conversion of ATM → MoS3 → MoS2 were confirmed by XRD, FT-IR spectra, Raman spectra, and TGA analysis, respectively. A series of few-layer MoS2 catalysts with and without nickel promoter on alumina was prepared by thermal decomposition of impregnated ATM at different calcination temperatures. The Raman spectra revealed that the number of MoS2 layer formation decreased on increasing the calcination temperatures. The surface area of MoS and NiMoS catalyst was increased about 4 and 16%, respectively, when the calcination temperature increased from 350 to 450 °C due to the thermal expansion. TPR exhibits the presence of well-defined and highly reactive sulfur species that are weakly bonded and located on the surface of NiMoS and MoS catalysts. Based on the characteristics of NiMoS catalysts, higher HDS conversion rate and selectivity for C4 hydrocarbons of n-butane and butenes were observed when compared to MoS catalyst at 350 °C, due to the presence of promoter that enhanced the hydrogen capacity on MoS2. Rates of formation of n-butane, 1-butene, cis-2-butene, and trans-2-butene were higher on NiMoS catalyst than MoS catalysts calcined at 350 °C and were about 1.57, 1.66, 1.37 and 1.77-fold, respectively. Finally, synthesis of the MoS2 catalysts by thiomolybdate precursor was found to be a simple route compared to traditional sulfided route for catalysts preparation. Thus, the NiMoS2 catalysts synthesized by thiomolybdate precursor also proves to be an efficient catalyst for HDS reaction.
This work was supported through petroleum research center funded by the Kuwait Institute for Scientific Research (KISR).
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Conflict of interest
The authors declare that they have no conflict of interest.
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