Influence of decomposition time and H2 pressure on properties of unsupported ammonium tetrathiomolybdate-derived MoS2 catalysts
© Whelan et al.; licensee Springer. 2015
Received: 6 April 2014
Accepted: 12 December 2014
Published: 25 February 2015
Molybdenum sulfide (MoS2) catalysts to be used for hydrodesulfurization (HDS) processes were prepared via the reductive thermal decomposition of ammonium tetrathiomolybdate at fixed temperature (653 K) by varying decomposition times and H2 pressures. Both parameters were found to strongly influence textural and catalytic properties of the resulting MoS2 catalysts.
Nitrogen sorption, FT-IR, and XRD analyses revealed the effect of varying decomposition times (3 to 7 h) and H2 pressure (20 to 1,000 psig) on the morphology and structure of the catalysts. Dibenzothiophene (DBT) was used to assess catalytic efficiency for HDS reactions.
The influence of time on specific surface was minimal at low pressures but increased at higher decomposition pressures. Vibrational energies of Mo-S bonds in FT-IR indicate that MoS2 catalysts prepared at higher pressures exhibit weaker Mo-S bonds. Analysis of XRD patterns point towards an increase in stacking and crystallite size with increasing pressure; interlayer rotation about both the a- and c-axes of the stacks was also observed. Catalytic testing results show that conversion increases at higher values of decomposition time and pressure. Partially hydrogenated products were also observed at higher pressures, and the ratio of partially to fully hydrogenated DBT was calculated as an additional measure of catalytic efficiency.
Decomposition time and H2 pressure during ammonium tetrathiomolybdate (ATM) thermal decomposition have a significant impact on the morphological and catalytic properties of the derived MoS2 catalysts. Samples prepared for 5 h at 1,000 psig exhibited the highest conversion of DBT and the lowest ratio of partially to fully hydrogenated products.
With environmental concerns continually on the rise, greater demand for fuels with low-sulfur content has increased the focus on hydrotreating (HDT) catalysts (Topsøe et al. 1996). Molybdenum sulfide (MoS2)-based catalysts, varyingly promoted with cobalt or nickel, are one of the most common metal sulfides used in HDT, with a strong emphasis on hydrodesulfurization (HDS) reactions (Brunet et al. 2005; Egorova and Prins 2006; Álvarez et al. 2008; Breysse et al. 2008; Chianelli et al. 2009; Klimov et al. 2010). There are several common methods to synthesize this catalyst such as the direct sulfidation of molybdenum oxide, hydrothermal and sonochemical synthesis, and the relatively simple procedure of thermally decomposing ammonium tetrathiomolybdate (ATM) (Camacho-Bragado et al. 2005; Devers et al. 2002; Mdleleni et al. 1998; Afanasiev 2008; Polyakov et al. 2008).
While several studies of note have already examined the effect of pre-treatment of ATM and the correlation between MoS2 HDS activity by varying gaseous environment (e.g., H2S/H2) and Mo precursors as well as reducing and sulfiding conditions, little work has focused on the influence of decomposition pressure (Liang et al. 1986; Zhang and Vasudevan 1995; Alonso et al. 1998; Afanasiev 2010). As a general rule, pressure increases are known to decrease the rate of thermal decomposition; however, other influences have been observed. In their study of nitramine compounds, Piermarini et al. found a change in decomposition mechanism above a certain pressure (Piermarini et al. 1987). In addition, Criado et al. studied the decomposition of CaCO3 and observed a shift towards higher decomposition temperatures with increasing CO2 pressure (Criado et al. 1995).
Though some examples in literature report pressure-induced effects on MoS2 properties such as electrical conductivity and critical current density, little is known regarding the pressure effects on decomposition of ATM and HDS activity of unsupported MoS2 catalyst (Sánchez et al. 2006; Alekseevskii et al. 1977). While it could be argued that the impact of decomposition may be small relative to other ‘stronger’ experimental parameters, clearly, the studies on decomposition of nitramine compounds and CaCO3 show it to be a potentially fruitful exercise. Thus, the present study focuses on pressure and time effects on reductive thermal decomposition of ATM and the catalytic activity of the resulting MoS2. It is shown that by varying the decomposition parameters, resulting structural and chemical differences prove insightful towards designing an economical catalyst for the effective removal of sulfur. The degree of influence these experimental parameters have on the morphology and activity of MoS2 catalysts is described below.
Reagents and solutions
The following chemicals were used as purchased: ATM (Aldrich, Dorset UK, 99.97%), dibenzothiophene (DBT) (Aldrich, 98%), hexadecane (Aldrich, 99%), hydrogen gas (Air Products, 99.992%), and nitrogen gas (Air Products, Allentown, PA, USA; 99.995%).
Ammonium tetrathiomolybdate was weighted on an aluminum boat and placed into a 300-cm3 batch reactor at room temperature. The reactor was sealed, flushed with N2 and H2, and then heated to 623 K (10 K min−1). Upon reaching the reaction temperature, H2 was injected into the reactor at the required pressure, taken as time zero. After the chosen heating duration was completed, heating would be stopped and the reactor would be allowed to cool naturally while maintaining a reductive (H2) atmosphere. Upon reaching room temperature, the reactor was degassed, flushed with N2, opened, and the resulting product (MoS2) was removed and weighted. Immediately afterwards, the specimen was ground using a pestle and mortar and used for characterization and HDS testing. MoS2 specimens prepared by this method were named as follows: number (time in hours)-number (hydrogen pressure, psig); decomposition times were 3, 5, and 7 h and H2 pressures were 20 (0.138 MPa), 500 (3.447 MPa), and 1,000 psig (6.895 MPa). For example, 3-1,000 implies MoS2 prepared from ATM thermally decomposed at 623 K for 3 h at 1,000 psig H2.
Textural characterization was carried out on all catalysts with N2 sorption at 77 K with a Quantachrome Autosorb-1 (Quantachrome Instruments, Boynton Beach, FL, USA). Prior to analysis, each sample was degassed under vacuum at 573 K for 2 h; the BET surface area (S BET) was determined from the resulting isotherm. FT-IR spectra were obtained using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy on a Bruker Vertex 70 (Bruker AXS, Inc., Madison, WI, USA). The dark-colored catalysts were mixed with KBr at a ratio MoS2:KBr = 1:100 to improve infrared transmission; spectra were collected in the 4,000 to 350-cm−1 range. Samples for XRD were mounted on a zero background holder, and spectra were collected with a X'Pert PRO Panalytical Powder diffractometer (PANalytical, Almelo, The Netherlands), using Cu Kα radiation (45 kV and 40 mA) in the 2-theta range 10° to 80° with a step time of 0.01 s. Temperature-programmed reduction (TPR) analyses were carried out under continuous H2 flow (5% H2 in He) in a quartz cell at 10 K min−1 using a Quantachrome ChemBET 3000 (Quantachrome Instruments, Boynton Beach, FL, USA) coupled with a Hiden Analytical HPR 20 QIC mass spectrometry detector (Hiden Analytical, Warrington, UK). It should be mentioned that in no case was any specimen exposed to air for more than 2 min, thus avoiding oxidation of the sulfide catalysts.
Freshly synthesized and ground MoS2 (0.050 g, 3.1 × 10−4 mol) was added to a 100-cm3 batch reactor. Feedstock used was 1% DBT in hexadecane (25 cm3). The reactor was sealed, flushed with N2, and then heated to 573 K under stirring (1,000 rpm). Upon reaching the reaction temperature, H2 (500 psig) was injected into the reactor (time zero). Heating and stirring would be stopped after 3 h, and the reactor would be left to cool naturally, while retaining a reductive (H2) atmosphere. Upon reaching room temperature, the reactor was degassed and a sample of the reaction mixture was removed, centrifuged, and the supernatant diluted and analyzed on a gas chromatograph coupled with a sulfur chemiluminescence detector (GC-SCD - Agilent 6C 6980 and SCD 335 (Agilent Technologies, Inc., Santa Clara, CA, USA)). The column was 100% dimethylpolysiloxane, 30 m × 0.32 mm × 1 μm, with a maximum temperature of 598 K. In this study, conversion of DBT was calculated based on the decrease in DBT signal (and increase in partially hydrogenated DBT (HYD)) from the GC-SCD compared to the initial concentration following calibration.
Results and discussion
According to the reaction mechanism, for every mole of ATM that decomposes, four moles of gaseous products are produced (2NH3 and 2H2S). Based on the starting concentration of ATM, it was calculated that there was a pressure increase of approximately 15 psig above the H2 injected upon reaching decomposition temperature; no attempt was made to correct for this during catalyst preparation. In general, throughout this paper, the H2 decomposition pressures applied are termed as low (20 psig), medium (500 psig), and high (1,000 psig), and it is only against the pressure trend that meaningful conclusions are obtained, not the actual value itself. It should be noted that the actual pressure ‘felt’ by the ATM is not exactly the same pressure as mentioned above; variation of pressure around the decomposition reaction zone lead to different values than the set point of the external pressure regulator.
Crystalline order along the basal direction and apparent stacking heights in the c -axis direction
SA (m 2 /g)
SA (m 2 /g)
SA (m 2 /g)
FT-IR catalyst characterization
Catalytic activity of MoS 2 samples
Diffusion rates of gaseous products from thermal decomposition of solids are known to decrease with an increase in pressure (Oyumi and Brill 1987). In the case of ATM decomposing, gas is released from each step (see Equations 1 and 2) leading to an increase in pressure which could slow down diffusion of gaseous by-products. This is expected to have an impact on the resulting MoS2 formed.
Analysis of results
A general increase in S BET with an increase in H2 pressure (Figure 2) was observed across all decomposition times, though to varying degrees. The influence of decomposition time on S BET at low pressure was minimal, though as decomposition pressure increased, the influence of decomposition time became more pronounced. Based on the experimental parameters studied herein, it appears that 5 h was the optimal time and 1,000 psig the optimal pressure to produce a catalyst with the largest specific surface area. Increasing H2 pressure was also found to produce larger pore volumes in the catalyst (Figure 3) leading to the conclusion that textural properties of the catalyst can be controlled by optimizing ATM decomposition time and pressure.
While increasing H2 decomposition pressure increased S BET within each decomposition time series, S BET did not increase within each decomposition pressure series, particularly from 5 to 7 h. Intuitively, it is not unreasonable to expect an increase in pressure to result in a greater propensity for pore collapse (and subsequent decrease in surface area) which we see in 7-500 and 7-1,000, when compared against 5-500 and 5-1,000; however, the fact that pore collapse only happens at longer decomposition times again highlights the importance of the combined and sometimes conflicting roles of time and pressure and the necessity of varying ATM decomposition parameters to ensure greater control of the resulting MoS2. A similar trend was observed for TPV of MoS2 samples (Figure 3) in that the greatest increase in TPV was for the 5-h series and the lowest was for the 7-h series and likewise a decrease in TPV for the high-pressure 7 h series compared to that for the 5-h series (e.g., compare 5-500 with 7-500 and 5-1,000 with 7-1,000).
The change in stacking height with a change in pressure is not a novel concept having been noted before on supported hydrotreating catalysts, with HDS process conditions determined as the main cause of destacking (De la Rosa et al. 2004). Hydrothermal synthesis of MoS2 in an autoclave was also found to result in destacking, and in that study, the presence of hydrocarbons was presented as a possible contributor (Peng et al. 2001) while, in a separate study, Chianelli et al. note that possible intercalation of H2 at high pressures could result in MoS2 layers simply sliding apart over time (Chianelli et al. 2006). However, all of these analyses were conducted on catalysts under HDS conditions whereas, in this case, stacking is not measured as a function of reaction conditions but as a function of thermal decomposition parameters on the original ATM starting material.
For any crystalline solid (in this case poorly crystalline), it can be expected that with an increase in stacking there should be a corresponding increase in S BET; however, the degree to which S BET increases would not be by the same factor as the increase in stacking, simply on the basis that it is only the exposed edge surface which will add to S BET whereas the basal planes are ‘covered’ by the extra stacks (this of course infers perfect alignment of stacks). The use of S BET, however, assumes that there is no agglomeration. Iwata et al. have observed vastly different S BET from the theoretical values of surface area obtained from utilizing data from XRD (Iwata et al. 2001); applying their formula, a similar pattern is evident in our samples, i.e., agglomeration is occurring (compare Figure 2 and SA values in Table 1). While S BET increases with an increase in decomposition pressure, it can be seen from Table 1 that the XRD-determined surface area decreases, appearing to level off close to 270 m2 g−1. Thus, the pattern emerges within each time series such that the extent of agglomeration is decreasing with an increase in decomposition pressure. Agglomeration is assumed to be prevalent in the solid state, i.e., during N2 sorption, though less so during activity measurements due to the presence of solvent and high temperatures.
Previously reported modelling studies of poorly crystalline MoS2 have shown that interlayer rotation about the a-axis results in little change for the 110 peak but in a shift of the 002 peak towards lower angles (Liang et al. 1986). The same authors model interlayer rotation about the c-axis, with changes observed in the 100-103-105 region, with only a small effect on the 002 and 110 peaks. XRD patterns of our samples display slight variations in the angle of 002, as well changes in I 103:I 002 and I 105:I 002 indicating interlayer rotation about the a- and c-axes, respectively, though by how much exactly is unknown.
The presence of two distinct peaks for Mo-S bonds implies a variation in strength and/or length; based on FT-IR vibrational energies, we assign 385 cm−1 to the stronger (i.e., shorter) Mo-S bond, with the weaker (longer) Mo-S bond at 378 cm−1. Mo-S vibrations are usually assigned to the region ca. 380 cm−1, with no literature evidence on FT-IR of MoS2 found distinguishing between these peaks. In a related study, however, laser Raman studies of alumina-supported MoS2 by Payen et al. observed a shift in the Raman band from 380 to 385 cm−1 with an increase in Mo loading (Payen et al. 1987); the authors attributed their shift (and broadening) to the lateral growth of MoS2 particles without making reference to whether this ‘crystallite size’ effect is an actual broadening of the basal plane or merely side-to-side stacking of MoS2 crystals. In our samples, it can be seen that the crystallite sizes do increase with an increase in decomposition pressure though the values do not correlate well with the above conclusion - our results show a general increase in basal plane size but the results for 7-20 in particular and all samples prepared at 500 psig in general show similar crystalline orders than the samples prepared at 1,000 psig; thus at present, it is inconclusive that the peak shift is due (at least primarily) to lateral growth.
Since FT-IR spectra are of the bulk material, it could be argued that there is an increase in the concentration of weaker (longer), more reactive Mo-S bonds produced with increasing decomposition pressure. While this finding is intuitively unappealing as one would reasonably expect a shortening of bond lengths with increase in pressure (Pietosa et al. 2008), a possible explanation is that pressure induces structural changes which in turn induce creation of new catalytically active sites. Though recent investigations of MoS2 nanoclusters using direct space DFT calculations have found that an increase in S atom coordination of Mo atom results in increasing Mo-S bond lengths (McBride and Head 2009), no such investigations were carried out in this study. Work is ongoing with a view to quantifying the interlayer rotations observed using XRD and the creation of new catalytically active sites and the unique FT-IR shifts observed through the preparation of more ATM-derived MoS2 using more discreet changes in decomposition pressures.
The increase in HYD:DDS ratio, as confirmed by chromatographic analysis, can be partially explained by a possible increase in the number of rim sites available due to interlayer rotation of the layers of MoS2 in the stacks. Another possibility is that the reduced agglomeration from samples prepared at higher decomposition pressure simply exposes more rim sites to the DBT molecule. Of the samples which produce the hydrogenated DBT, 5-1,000 has one of the lowest HYD:DDS ratios (0.15) compared to say 3-500 (0.32). Thus, not only does 5-1,000 have a more active surface for HDS of DBT, but it also has a higher preference towards the DDS pathway compared to HYD which leads to the assumption that the exposure of rim sites for MoS2 prepared from reductively decomposed ATM can be optimized by varying decomposition time and pressure.
We have shown herein the role played by both time and pressure for reductive thermal decomposition of ATM for the formation of MoS2 and their subsequent impact on HDS activity. The influence of decomposition time serves to both enable an increase in surface area as well as an increased likelihood of sintering. Pressure increases slow down thermal decomposition by limiting gaseous by-product diffusion while at the same time facilitating the collapse of produced pores. Based on FT-IR vibrational energies, FT-IR spectral analysis shows that catalysts prepared at higher pressures have weaker Mo-S bonds (378 cm−1) than those prepared at lower pressure (385 cm−1). Evidence from XRD indicates both time and pressure induce interlayer rotation of stacks along both the a- and c-axes and that ATM decomposition pressure plays a role in determination of MoS2 layer stacking. Using XRD-derived values for crystallite sizes, theoretical surface area values were determined; the changes evident lead us to conclude that agglomeration is occurring. Optimal conditions were found producing a catalyst with weak Mo-S bonds, the largest (theoretical) surface area and largest pore volume which unsurprisingly resulted in the highest HDS activity of model dibenzothiophene. It was found that catalytic activity increased with a decrease in theoretical surface area, with 5-1,000 having the highest activity per unit area. Samples prepared at low pressures yielded no detectible HYD product, but this increased in samples prepared at higher decomposition pressure. An optimal system was found which gave the lowest HYD:DDS ratio, again 5-1,000. There is no doubt that there are other parameters which can impact both morphology and catalytic activity, such as ramp rates and variously substituted thiomolybdates; we have shown that the role of decomposition time and pressure for ATM-derived MoS2 can in fact be a useful tool in optimizing catalyst synthesis with certain morphologies and activities.
The authors would like to acknowledge financial support from the Abu Dhabi Oil Refining Company (TAKREER) and from the Department of Chemical Engineering at The Petroleum Institute, Abu Dhabi, United Arab Emirates.
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