Hydrodesulfurization of Thiophene on Activated Carbon Fiber

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Hydrodesulfurization of Thiophene on Activated Carbon Fiber-supported NiMo Catalysts Yogendra Nath Prajapati, and Nishith Verma Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03407 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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Highlights •

ACF-supported NiMo catalysts were prepared for the HDS of thiophene.

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High degree of sulfidability was achieved in the synthesized catalysts.

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The catalytic activity remained constant up to 30 h of the test run.

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Reasonably good kinetic parameters were determined for the HDS of thiophene.

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Table of Contents


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Hydrodesulfurization of Thiophene on Activated Carbon Fiber-supported NiMo Catalysts Yogendra Nath Prajapati†, Nishith Verma†, §, * †

Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur208016, India

§

Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur- 208016, India.


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ABSTRACT Activated carbon microfiber (ACF)-supported NiMo-based catalysts were prepared for the hydrodesulfurization (HDS) of thiophene. The catalysts were characterized for their physicochemical properties, using the BET, SEM, HR-TEM, XPS, XRD, TPR, and TPD analysis. The effects of the NiO (promoter) to MoO3 (active metal oxide) ratios were experimentally determined on the HDS of thiophene (3% v/v in hydrogen) over the temperature range of 300-370 °C. The catalytic activity tests showed the optimum reaction temperature and weight-ratio of NiO to the binary oxide mixture to be 350 °C and 0.25 at 12% (w/w)-loading of MoO3, respectively. The pseudo first order rate constant and activation energy for the HDS reaction were determined to be 980×10-6 mole-thiophene/(g of catalyst-min) and 46.3 kJ/mole, respectively, which were greater and smaller, respectively, than those for most of the supported metal catalysts discussed in literature for the HDS of thiophene. The prepared NiMo-based catalyst was found to be stable, with negligible loss of activity observed up to 30 h of the test run.

1. INTRODUCTION Petroleum feed-stocks contain significant amounts of sulfur (0.03-7.89% w/w) and nitrogen (0.3-5% w/w) in the form of organosulfur and organonitrogen compounds.1 These compounds are responsible for the generation of atmospheric air pollutants such as SOx, NOx, and particulate matter during the combustion of the feed-stocks. Further, the presence of sulfur in fuels adversely affects the performance of catalysts through poisoning and/or deactivation in many applications such as automobile convertors and fuel cells.2-4 In recent years, strict environmental regulations have been imposed worldwide for producing clean transportation fuels.


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At present, European standard organization and the U.S. Environmental Protection Agency (EPA) have restricted sulfur content in diesel below 10 and 15 ppmw-S, respectively.5,

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In future, even more stringent regulations are expected to be in place.

Developing countries such as India, China, and Mexico have also imposed stringent regulations for sulfur in diesel. In India, the maximum permissible sulfur in diesel for transportation vehicles is 50 ppmw, which will be reduced to 10 ppmw (BS VI standard) by 2020.7 Generally, hydrodesulfurization (HDS) reaction is carried out at high temperatures (> 300 °C) and pressures (> 30 bar), using the Co(Ni)Mo(W)-based catalysts to produce sulfur-free diesel. Owing to its excellent textural and mechanical properties, alumina (Al2O3) is commonly used as a support material. However, interaction between Al2O3 and metal is relatively stronger, and therefore, promoter ions (Ni and Co) react with Al2O3 during calcination, and occupy tetrahedral and/or octahedral sites over the external surface of the material, resulting in small HDS activity of the catalysts.8,

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Several studies have been

performed to synthesize new supports, or use various additives in the support materials to control the metal-support interaction and achieve high catalytic activity.10-12 Carbon-based materials such as activated carbon (AC), activated carbon microfibers (ACFs), carbon nanofibers (CNFs), carbon nanotubes, and aerographite are stable under severe acidic and basic conditions. These materials possess large BET surface area, and are amenable to surface functionalization, which make them a suitable candidate for various applications.13-19 The ACs have been used as an effective support for the HDS reaction, because of (i) small metal-support interaction, (ii) low coke formation, and (iii) high metal recovery from the spent catalysts.20 Prabhu et al.21 studied the HDS of light gas oil over mesoporous carbon (CMK-3) and Al2O3-supported NiMo catalysts. The study showed the higher HDS activity (~4%) over the mesoporous carbon-supported NiMo than the γ-Al2O3-


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supported catalysts having similar metal loading on equivalent mass basis. The high activity of the carbon-supported catalysts was attributed to the relatively smaller metal-support interaction and higher specific surface area. Similar results have been reported by Sakanishi et al.22 for the HDS of 4,6-dimethyldibenzothiophene (4,6-DMDBT) over the AC-supported NiMo catalysts, and by Dong et al.23 and Shang et al.24 for the HDS of thiophene and dibenzothiophene (DBT), respectively, over the multi-walled carbon nanotube-supported CoMo catalysts. Farag et al.25 studied the HDS of DBT, 4,6-DMDBT, and commercial diesel fuels over the temperature range of 300-380 °C and at 29 bar-H2 pressure, using CoMo catalysts supported on AC and Al2O3. The results showed that the AC-supported catalysts were more active (approximately three times) than the Al2O3-supported catalyst. Over the last decade, ACFs have been extensively used for many applications including adsorbents for various gaseous and liquid pollutants,26-28 and a substrate for different metal catalysts in several chemical reactions.29, 30 A literature review clearly shows that ACF has not been studied as a support to the HDS catalysts. Interestingly, a few studies performed on the powdered activated carbon (PAC)-based HDS catalysts show higher catalytic activities than the conventional support-based catalysts. Therefore, owing to the ease of handling and excellent textural properties of ACF, the material is a preferable choice over PAC and granular activated carbon (GAC).31 In many catalytic applications ACF has also shown better performance than PAC.30,

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Therefore, the present study was performed with a view to

exploring potential application of ACF as a substrate for the HDS catalysts. To the authors' knowledge the use of ACFs as a support for the HDS catalysts has not been studied, except by Sakanishi et al.22 These authors performed the HDS of 4,6-DMDBT over the NiMo catalysts dispersed in the pitch precursor-based ACF over the H2 pressure range of 30-150 bar and the reaction temperature range of 340-380 °C. The comparative data showed that the catalytic activity of the ACF-supported catalyst was greater than that of the AC-supported


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catalyst at 380 °C. Owing to small pore-diffusion resistance, high surface area, amenable to surface functionalization including acidification, and low pressure drop through a bundle of fibers, ACF may be preferred over its counterpart AC as a potentially effective support for the HDS catalysts, and is the focus of the present study. A recent study performed on the adsorption of different sized molecules has also shown a significant adsorption capacity of the microporous carbon for the small sized thiophene molecules, indicating the insignificant pore diffusion resistance in the catalyst.5 In the present study, the phenolic resin precursor-based ACF was used to disperse NiMo and the prepared ACF-supported NiMo catalysts were tested for the HDS of thiophene which was used as a model sulfur compound in the diesel oil. The effects of reaction temperatures and the weight-ratios of the promoter (NiO) to the active metal (MoO3), on the HDS of thiophene were experimentally demonstrated. A comparison of the kinetic parameters (rate constant and activation energy) was made with those for the supported catalysts discussed in literature for HDS.

2. EXPERIMENTAL 2.1. Materials. The phenolic resin precursor-based ACFs were procured from Nippon Kynol Inc., Japan. Nickel nitrate hexahydrate (≥ 98%), ammonium heptamolybdate tetrahydrate (99%) and thiophene (99%) were purchased from Merck, Germany. All solutions were prepared in Milli-Q® water. All gases were zero grade and purchased from Sigma Gases and Services, New Delhi (India). 2.2. Pretreatment of ACFs. The as-received ACF samples were soaked in a 0.05 M HNO3 aqueous solution and pretreated at 90 °C for 4 h to remove surface contaminants and volatile impurities. The samples were then washed several times with water until pH of the washing liquid was measured to be ~7. The samples were then dried at 110 °C for 12 h.


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2.3. Catalyst Preparation. Catalysts were prepared using wet impregnation technique. Different catalyst samples were prepared having the NiO/(NiO + MoO3) weight ratios in the range of 0.2 - 0.3. The required amounts of ammonium heptamolybdate tetrahydrate and nickel nitrate hexahydrate were first dissolved in 10 mL of Milli-Q® water at room temperature (~30 °C). Approximately 1 g of the pretreated ACF sample was dipped in the prepared salt solution. Ultrasonication of the salt solution dispersed with ACF was performed for 1 h to uniformly distribute the metal salts in the ACFs. Water was removed by heating the solution at 110 °C. The salt-impregnated ACF samples were dried at 110 °C for 12 h. The dried samples were subjected to calcination in a horizontal furnace. Approximately 1 g of the sample was placed over a stainless steel (SS) mesh. The mesh was placed in the hot zone of the furnace. The sample was heated 5 °C/min to 400 °C in a N2 atmosphere (100 standard cubic centimeter per minute (sccm)-flow rate). Calcination was carried out for 4 h. The prepared catalysts (calcined samples) were named as NiMo-X, where X = 1, 2, 3, 4, and 5, for the reference purposes in this study, depending upon the amounts of MoO3 in the samples. Table 1 shows the compositions of the synthesized catalysts in this study. It may be mentioned that the NiO (or CoO) promoter has been used in the range of 2-6% (w/w), whereas MoO3 has been used in the range of 8-19% (w/w) for the synthesis of HDS catalysts in most of the studies.33-36 Therefore, these ranges were used as a guideline to synthesize the HDS catalysts in our study. Moreover, the industrial NiMo/Al2O3 catalysts also have similar compositions.36, 37 A literature review also indicates that a relatively higher HDS performance can be achieved by maintaining the Ni/(Ni+Mo) ratio in the range of 0.25-0.33.38


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Table 1. Composition, and NH3-TPD Data of the ACF-Supported NiMo Catalysts Sample NiMo-1 NiMo-2 NiMo-3 NiMo-4 NiMo-5

NiO (% w/w) 3 4 4 5 4

MoO3 (% w/w) 9.0 9.3 12.0 15.0 16.0

NiO/(NiO+MoO3) a

(Ni/Mo) b

(Ni/Mo) c

0.25 0.30 0.25 0.25 0.20

0.39 0.51 0.39 0.39 0.30

0.40 0.50 0.38 0.41 0.28

NH3 (mmol/g) 1.54 1.76 1.77 1.89 2.27

a

theoretical weight ratio of promoter to total metal oxides in the catalyst equivalent theoretical atomic ratio of promoter to active metal c atomic ratio of promoter to active metal determined using XPS analysis b

2.4. Catalyst Characterization. The BET surface area measurements of the calcined catalysts were determined based on the N2 adsorption-desorption isotherm at 77 K, using the Autosorb-iQ instrument (Quantachrome, USA). The samples were degassed before the analysis at 150 °C for 12 h. The BET surface area of the samples was calculated by using the linear segment of the isotherms (P/Po < 0.1). A high resolution field emission scanning electron microscope (FE-SEM - MIRA3 series, Tescan) was used to observe the surface morphologies of the samples. Elemental mapping of the calcined catalysts was performed using the Carl Zeiss EVO-50 energy dispersive spectroscopy (EDS). Powder X-ray diffraction (XRD) patterns of the catalysts were recorded at room temperature, using the X′Pert3, PANalytical X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). Scans were performed over the 2θ-range of 10-80° at the scanning rate of 3° per min. Thermo gravimetric analysis (TGA) was performed on the calcined samples to determine the thermal stability of the materials over the temperature range of 30 to 800 °C, using a TG analyzer (MettlerToledo, United States). Temperature of the sample was increased 10 °C/min under N2 atmosphere during the analysis. Mechanical strength of the calcined catalyst sample was performed using Universal Testing Machine (UTM: Zwick Roell- Z005, Germany). Hydrogen-temperature programmed reduction (H2-TPR) of the calcined samples was performed using the Autosorb-iQ instrument for determining the reducibility of the materials.


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A mixture of 5% (v/v) H2 in N2 was used as the reducing agent. Approximately 100 mg of the sample loaded in a U-shaped quartz cell was degassed at 110 °C for 1 h in a helium stream. The sample was then cooled to 40 °C. The H2-reduction was performed using the reducing gas at 30 sccm, with the reduction temperature increased 10 °C/min from 40 to 900 °C. The consumption of H2 was monitored using the TCD attached to the instrument. The NH3temperature programmed desorption (NH3-TPD) of the calcined samples was performed using the Autosorb-iQ instrument. Approximately 100 mg of the sample, contained in the Ushaped quartz reactor, was first outgassed at 150 °C for 2 h under helium stream (40 sccm) to remove moisture. The temperature was then decreased to 40 °C. A mixture of 20% (v/v) NH3 in helium was flowed at 40 sccm over the sample for 1 h. The sample was purged with pure helium for 1 h at the same flow rate to remove physisorbed NH3. The sample was then heated 10 °C/min from 40 to 1000 °C. The desorbed NH3 from the sample was measured using the thermal conductivity detector (TCD). The X-ray photoelectron spectra of the catalysts (oxide and sulfide forms) were recorded using an X-ray photoelectron spectroscopy (XPS) (PHI 5000 Versa Prob II, FEI Inc. US) with AlKα X-ray source (1486.6 eV) and a hemispherical analyzer. Binding energies (BEs) of various elements were referenced to the C 1s level of carbon support at 384.6 eV. The spectra were analyzed using the Casa XPS software. High-resolution transmission electron microscopy (HR-TEM) analysis of the freshly sulfided catalysts was carried out on a FEI Titan G2 60-300 (300 kV) HR-TEM equipped with the EDS detector. A small amount of fine catalyst powder was dispersed in ethanol. Few micro-drops of the samples were then loaded on the carbon-coated copper grid (300 mesh) and dried at room temperature for 12 h before the analysis. The average slab length (Lavg) and stacking number (Navg) of the MoS2 particles selected from multiple (3-4) images were calculated using the following equations.38


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=

∑ (1) ∑

=

∑ (2) ∑

where ni is the number of stacks of length li and Ni is the number of the layers in a cluster. 2.5. HDS Tests. The prepared NiMo catalysts were sulfided before the HDS tests in a common SS tubular reactor (15 mm ID × 18 mm OD × 100 mm length) mounted vertically in a furnace (Figure 1). Calcined sample in required amount was supported between quartz wools in the reactor. The reactor was heated 5 °C/min up to 150 °C under N2 flow (100 sccm). Sulfidation was performed using a H2S/H2 gaseous mixture (10% (v/v) H2S) at 50 sccm. Reactor temperature was increased 5 °C/min from 150 to 400 °C. Sulfidation was performed at 400 °C for 2 h. The HDS tests were performed at ~1 bar by bubbling H2 at 50 sccm, using a mass flow controller (Eureka MFC), in liquid thiophene (~40 cc volume) contained in a glass tube. Temperature of the glass tube was maintained constant at 0 °C. The thiophene-laden H2 (~3% v/v) gas was flowed into the reactor. Exit concentration of thiophene in the gaseous mixture exiting from the reactor was determined using a gas chromatograph equipped with the flame ionization detector (Nucon gas chromatograph 5700, Delhi, India). The HDS tests used different catalyst amounts (0.05 - 0.5 g), and reaction temperatures over 300 – 370 °C which is the commonly used temperature range for the HDS of commercial diesel oil.6, 25 Kinetic parameters (rate constant and activation energy) were determined from the HDS measurements at low thiophene conversions (5-20%), using 5 to 15 mg of the catalyst diluted in 1 g of SiC. Steady state conversion was determined after ~5 h of the experimental runs. The rate constant was determined using the plot between ln(1/(1-XA)) vs. W/FAO. The tests were also carried out at different temperatures (310, 330, 350, and 370 °C) using a fixed


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amount of catalyst (10 mg) to determine activation energy calculated from the slope of the Arrhenius plot.

Figure 1. Experimental setup used for the HDS of thiophene.

3. RESULTS AND DISCUSSION 3.1. Textural Properties of the Catalysts. The N2 adsorption-desorption isotherms of the calcined catalysts were observed to be type-I according to IUPAC classification (Figure S1). The type-I isotherm indicates the microporous characteristics of the sample. In such samples capillary condensation occurs in the micropores. Table 2 shows the BET surface area and pore size distribution (PSD) of the calcined samples. The BET surface area of the samples, based on the total weight (support + metals) decreased with increasing amounts of the total metal oxides (12-20% w/w) in the samples. However, the BET surface areas, based on the ACF support alone, were calculated to be approximately constant at 1448 ± 14 m2/g. Average pore diameter was calculated to be 1.7 ± 1 nm, which is sufficiently larger than molecular


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size of most of the large sized sulfur compounds present in diesel oil, viz, thiophene (0.47 nm × 0.47 nm), DBT (0.49 nm × 0.987 nm) and 4, 6-DMDBT (0.78 nm × 1.23 nm), indicating that the thiophene molecules can penetrate the pores of the catalysts.39, 40 Total pore volume of the samples was determined to be approximately 0.53 ± 0.03 cc/g. As expected the total pore volume of the ACF support decreased by 16% -24% after metal impregnation. Micropore volume of the samples slightly decreased, whereas a significant increase in mesopore volume was determined with increasing MoO3 contents. Increase in mesoporosity is attributed to widening of micropores by metals during the calcination step of the synthesis.

Table 2. BET Surface Area and Pore Size Distribution (PSD) of the Catalysts Sample ACF NiMo-1 NiMo-2 NiMo-3 NiMo-4 NiMo-5 Spent NiMo-3

SBET (m2/g) 1592 1280 1249 1205 1176 1152 1200

davg (nm) 1.9 1.8 1.6 1.8 1.9 1.8 1.6

Vtotal (cc/g) 0.66 0.50 0.51 0.54 0.55 0.53 0.48

Vmicro (cc/g) 0.61 0.49 0.49 0.46 0.45 0.44 0.47

Vmeso (cc/g) 0.05 0.01 0.02 0.08 0.10 0.09 0.01

%Vmicro %Vmeso 92.4 98.0 96.1 85.2 81.8 83.0 97.9

7.6 2.0 3.9 14.8 18.2 17.0 2.1

3.2. SEM and EDX Analysis. Figure 2a shows the SEM images of the calcined samples and the spent NiMo-3 catalyst. Surface coverage of the ACFs, including pores, increased with increasing amounts of MoO3 in the samples. At 15 and 16% (w/w) active metal loadings in the NiMo-(4-5) samples, the ACF surface was completely covered with the metal oxides, with significant agglomeration of the particles. Particle size in the NiMo-(1-3) samples was measured to be in the range of 20-60 nm. Large particles (> 60 nm) were formed over the NiMo-(4-5) surface because of the agglomeration and/or sintering of small particles. No distinct change in the morphology of the spent NiMo-3 catalyst was observed, indicating that no coke formation occurred during the HDS reaction over the ACF surface. The SEM images of the ACF support are shown in Figure S2, which indicate the presence of micro-mesopores


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Figure 2. (a) SEM images of the calcined samples, and spent NiMo-3 sample, (b) EDX mapping of the calcined NiMo-3 sample.

over the entire surface. The elemental mapping and corresponding EDS spectrum of the NiMo-3 catalyst confirmed the presence of Mo and Ni uniformly distributed over the ACF


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surface (Figure 2b). A uniform distribution of Mo and Ni was also observed over other catalyst surface (Figure S3). 3.3. XRD Characterization. Figure 3a shows the XRD patterns of the different calcined samples. The XRD pattern of the spent catalyst (spent NiMo-3) is also presented to show structural change, if any, in the catalyst during the HDS reaction. All samples showed similar XRD patterns. The characteristics peaks at 26 and 44° were observed in all samples, corresponding to the diffraction from (0 0 2) graphitic basal plane and (1 0 0) plane of carbon, respectively.12 The characteristics peaks of MoO3/NiMoO4 overlapped with the C(0 0 2) and C(1 0 0) peaks. No significant peaks of MoO3/NiMoO4 were observed at 2θ of ~26 and 44° in NiMo-1, NiMo-2, and NiMo-3 samples, which confirm that the metals were well dispersed in the support. However, small intensities peaks of MoO3/NiMoO4 observed at 2θ of 26.1, 37.18, and 53.73° in the NiMo-4 and NiMo-5 samples were attributed to relatively higher amounts of MoO3 present in the samples. Similar XRD patterns were observed for the CNF-supported NiMo catalysts.41 Also, the diffraction pattern of the spent NiMo-3 catalysts was found to be similar to that of the fresh (calcined) catalyst sample, indicating that no structural changes occurred in the material during the HDS reaction. 3.4. Thermal Stability. Figure 3b shows the TGA data for the calcined samples. The corresponding data for the ACF substrate is also included for the comparison purposes. Approximately 1-8% weight-loss was observed in all samples over the temperature range of 30 to 90 °C, because of the removal of moisture from the samples. Approximately 10% weight-loss was observed in the samples over the temperature range of 100 to 700 °C because of the decomposition of MoO3 into the MoO2/Mo4O11 intermediate compounds42 and/or carbothermal reductive decomposition of NiO or NiMoO4 at high temperatures (> 520 °C).43 A sharp weight-loss (~5%) was observed over the temperature range of 700 to 800 °C, attributed to the formation of Mo2C.44 The NiMo-1 sample showed the highest thermal


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stability (overall weight loss ~15%), whereas the NiMo-4 and 5 showed the least stability (overall weight loss ~25%). The ACF sample showed thermally stability over 30-800 °C, with a maximum of 10% weight-loss.

Figure 3. Physico-chemical characterization of the prepared catalysts: (a) XRD, (b) TG, (c) TPR and (d) TPD patterns.

3.5. H2-TPR Analysis. Figure 3c shows the TPR patterns of the calcined samples. All samples showed similar reduction behavior. The H2 consumption occurred over a wide temperature range (400-800 °C), with the principal reduction peaks observed over 400-435 °C, 475-540 °C, and at ~800 °C. The reduction peaks observed in the sample over the low temperature range are attributed to the reduction of Ni2+ and a part of Mo6+ to the respective metallic state, and the remaining Mo6+ reduced to Mo4+.45 The peaks observed over the ACS Paragon Plus Environment

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intermediate temperature range (475-540 °C) are assigned to the reduction of MoO3 and/or NiMoO4. The peaks shifted towards low temperatures with increasing amounts of MoO3 in the samples. The shift is attributed to the easy reduction of MoO3 and/or NiMoO4.46 The NiMo-5 sample, which contained the largest amount of MoO3, showed an additional peak at 642 °C, which is assigned to the reduction of MoO3 and/or NiMoO4 strongly bound with the support. The broad and low intense reduction peaks at 800 °C in all samples are attributed to the reduction of oxygen containing functional groups at the ACF surface. The TPR data clearly showed that the metal oxides could be reduced at relatively lower temperatures (~450 °C), indicating the weak interactions of the metal catalyst particles with the support. Relatively higher reduction temperatures have been associated with a lower catalytic activity of the metal-based catalysts.47 3.6. Surface Acidity. Acidic strength of the calcined catalysts was determined using the NH3-TPD tests (Figure 3d). The TPD patterns of the plain ACF sample (without metals) are also included in the figure for the comparison purposes. All catalyst samples showed desorption peaks over the broad temperature range of 100-800 °C. The strengths of both, Brønsted and Lewis acid sites are related to the desorption temperature (Tdes) of NH3. Based on the strength of acid sites, the temperature range can be divided into three sub-temperature ranges, viz. Tdes < 300 °C, 300 < Tdes < 450 °C, and Tdes > 450 °C, for weak, medium, and strong acid sites, respectively.12 All samples showed relatively low intensity-peaks at 120 °C, assigned to the weak acid sites in the samples. NiMo-1 showed an additional peak at 270 °C, which was almost absent (i.e., insignificant) in the other samples. The NiMo-2 and NiMo-3 samples showed peaks at 388 °C and 422 °C, respectively, attributed to the medium acid sites present in the samples. The NiMo-4 sample showed a peak at 475 °C, which was just outside the temperature range for the medium acid sites. Such peak was, however, absent in the NiMo-5 sample. The TPD data clearly showed that medium acid sites shifted towards


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relatively higher temperatures with increasing amounts of MoO3 in the catalyst samples. Also, the corresponding peak intensity decreased with increasing MoO3, and the peak was absent in the NiMo-5 sample containing the largest amount of MoO3. All samples showed the presence of strong acid sites indicated by the peaks over the temperature range of 570-730 °C. The peak at 788 °C in all samples is attributed to the decomposition of the support during Mo2C formation. The ACF sample showed desorption peaks at 360 °C and 850 °C, attributed to the evolution of NH3 and/or CO and CO2. Therefore, the desorption peaks observed in the catalyst samples at temperatures below 730 °C is attributed to the evolution of NH3 from the catalyst samples and not from the decomposition of the ACF support or metallic oxide precursors. This is consistent with the earlier discussed TGA data that the catalyst samples showed weight-loss at temperatures greater than 730 °C because of the formation of MO2C. The NH3-TPD tests have also been performed on Mo-based catalysts at temperatures greater than 600 °C, even up to 900 °C.12, 48, 49 The respective TPD pattern in these studies attributed the peaks appearing over the entire temperature range to the evolution of NH3 from the acidic sites in the catalyst samples. The area under the TPD patterns was used to determine/estimate the surface acidity. Amounts of acidic sites in the calcined samples, attributed to the metals only, were determined by subtracting the area under the TPD patterns (up to 700 °C) of the plain ACF sample from that under the corresponding TPD pattern of the catalyst samples (Table 1). The acidity of the samples increased with increasing MoO3 content: NiMo-1 < NiMo-2 ≈ NiMo-3 < NiMo-4 < NiMo-5. Activity of a HDS catalyst depends on its surface acidity.50 High acidity may cause the protonation of the reacting species, which migrate to the sulfide phase and are easily hydrogenated. It may be mentioned that the effects of the surface acidity on the HDS


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reactions is, however, still subject to debate among many researchers.12 The other factors such as total metal oxides, metal ratios, and surface area also affect the catalytic activity. 3.7. HR-TEM Analysis of Sulfided Catalysts. Figure 4a-e shows the representative HRTEM images of the sulfided catalyst samples. All samples show homogeneity and characteristic structure of MoS2 layers. The fringes observed in the images were approximately 0.65 ± 0.02 nm apart, which is the characteristics of the (0 0 2) plane of the crystallite MoS2 nanoparticles. Average slab length of MoS2 varied in the following order: NiMo-1 (3.64 nm) > NiMo-2 (3.54 nm) > NiMo-5 (3.50) > NiMo-4 (3.39 nm) > NiMo-3 (2.41 nm). Average slab length decreased with increasing MoO3 contents in the catalyst from 9 to 12%. Further increase of MoO3 content in the catalyst resulted in the formation of relatively larger slabs. Moderate stacking (3-5 layers) of MoS2 was observed in all of the samples because of lower metal-support interaction, which leads to the type-II MoS2 phase. Average number of stacking in the catalysts varied in the following order: NiMo-5 (4.80) > NiMo-4 (4.67) > NiMo-2 (4.04) > NiMo-1 (3.90) > NiMo-3 (3.80). As reported in the literature, moderate slab layers (average 3.1 layers) was suitable for higher catalytic activity because of higher density of multivacancies.51 NiMo-3 showed relatively less stacked and smaller particles of MoS2, indicating the relatively better dispersion of metal particles in the NiMo-3 catalyst. The selected area electron diffraction (SAED) patterns of the NiMo-3 catalyst confirms the polycrystalline characteristics of the sample (Figure 4f). Elemental mapping of the sulfided catalysts was also performed to observe the elemental distributions. The mapping showed a uniform homogeneous distribution of different elements (S, Mo, Ni, C, and O) over the catalyst surface. Figure 4g shows the elemental mapping of the representative NiMo-3 catalyst sample.


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Figure 4. HR-TEM images of (a) NiMo-1, (b) NiMo-2, (c) NiMo-3, (d) NiMo-4, and (e) NiMo-5 sulfided catalysts; (f) SAED pattern and (g) EDX mapping of the sulfided NiMo-3 catalyst.


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3.8. XPS Analysis of Sulfided Catalysts. Figure 5 shows the XPS profiles of sulfided catalysts in the Mo 3d and Ni 2P3/2 regions. Insignificant shift in binding energy (B.E.) was observed for a particular peak in all samples. The maximum standard deviation for the Mo or Ni peak was calculated to be 0.2 and 0.4 eV, respectively, indicating that similar chemical species were present in the samples. The registered peaks were deconvoluted. The data are presented in Table 3. The Mo signals were fitted considering doublets (3d5/2 and 3d3/2 contributions) of Mo4+ (from MoS2) and Mo6+ (from oxidic molybdenum).52, 53 The first peak centered at 226.5 eV is attributed to the B.E. of the 2s electron of the sulfur. The components located at 229.2 and 232.4 eV correspond to the 3d5/2 and 3d3/2 peaks of MoS2. The weak components located around 233.6 and 235.5 eV correspond to the 3d5/2 and 3d3/2 peaks of Mo6+ oxides. The Mo5+ peak was not observed in the XPS spectra, attributed to the insignificant contribution of Mo5+. The Ni 2p3/2 main components located at 853.9 and 856.4 eV indicate the presence of Ni2+ in the form of Ni-S (probably Ni3S2) and Ni-O oxides, respectively on the catalysts surface.10 Degree of sulfidability in the samples (Mo6+ → Mo4+, and NiO → Ni-S) was calculated using the following equations.51, 54 Sulfidability =

!" #$ (3) (!" #$ + !" &$ )

Sulfidability( ) =

(* ∗

* ∗ (4) + * # )

where Mo4+, Mo6+, Ni*, and Ni# are the areal intensities of the Mo4+, Mo6+, Ni-S, and Ni-O contributions, respectively. The percentage areal intensities of the XPS peaks of Mo 3d and Ni 2p3/2 components are presented in Table 3. A high degree of sulfidability was observed in all samples, ranging from 0.81-0.94 for MoS2 and 0.63-0.73 for Ni-S (Table 3). The reason for varying degrees of sulfidability is attributed to the relatively lower or higher metal-support interactions in the samples, depending on the metal loading in the support. The degree of sulfidability is, however, higher


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in ACF than in the conventional supports such as Al2O3, SiO2, and zeolite,51, 55 because of metal-support interaction being lower in the former material.

Figure 5. Mo 3d, S 2s and Ni 2p 3/2 core level XPS spectra of the sulfided (a-a’) NiMo-1, (bb’) NiMo-2, (c-c’) NiMo-3, (d-d’) NiMo-4, and (e-e’) NiMo-5.


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Table 3. XPS B.E. of Core Electrons and Percentage Peak Area of Mo 3d and Ni 2p3/2 Components of Freshly Sulfided Catalysts. Mo 3d Catalyst

NiMo-1 NiMo-2 NiMo-3 NiMo-4 NiMo-5

Mo4+ (5/2) B.E. (eV) 229.26 229.14 229.24 229.22 229.19

% Area 51.58 52.23 53.07 48.42 56.39

Mo4+ (3/2) B.E. (eV) 232.36 232.25 232.35 232.32 232.32

% Area 34.35 34.78 35.35 32.25 37.55

Ni 2p3/2 Mo6+ (5/2)

B.E. (eV) 233.16 233.20 233.28 233.11 233.20

% Area 8.45 7.80 6.95 11.60 3.64

Mo6+ (3/2) B.E. (eV) 235.71 235.29 236.00 235.63 235.72

Ni-S % Area 5.62 5.19 4.63 7.73 2.42

B.E. (eV) 853.93 853.54 853.88 853.95 853.85

Sulfidability Ni-O

% Area 72.82 64.67 66.06 70.79 63.25

B.E. (eV) 856.42 855.53 856.33 856.62 855.94

% Area 27.18 35.33 33.94 29.21 36.75

MoS2

Ni-S

0.86 0.87 0.89 0.81 0.94

0.73 0.65 0.66 0.71 0.63

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3.9. HDS Test Results. Figure 6a shows the effects of catalyst amounts (0.05, 0.10, 0.25, and 0.50 g) on thiophene conversion at 350 °C. The conversion expectedly increased with increasing amounts of the catalyst in the sample. Thiophene conversion was measured to be approximately constant over 5 h of the reaction time. Steady state conversion increased from ~35 to 90% with the increase in the amount of the catalyst from 0.05 to 0.5 g. Increase in conversion was, however, measured to be negligible beyond 0.5 g of the catalyst, attributed to mass transport limitations in the reactor used in the study. Figure 6b shows the effects of reaction temperatures (300, 325, 350 and 370 °C) on the conversion of thiophene over the NiMo-3 catalyst. In each test run, ~0.5 g of the catalyst was used. The conversions increased with increasing reaction temperatures. The conversion increased from 60 to 97% with the increase in the temperature from 300 to 370 °C. Figure 6c shows the effect of the amounts (9.3, 12, and 16% w/w) of MoO3 in the catalyst, at a fixed amount of NiO (4% w/w). Thiophene conversion increased from 76 to 90% with the increase in the amount of MoO3 from 9.3 to 12% or decrease in the weight-ratio from 0.3 to 0.25. Despite having smaller BET surface area, approximately the same acidity and equal amount of the promoter, NiMo-3 showed a higher thiophene conversion than NiMo-2, attributed to the relatively larger amounts of MoO3, and thus, more number of active MoS2 sites per unit weight of the catalyst. The relatively smaller and less stacked MoS2 particles (probably most suitable stacking layers for higher catalytic activity) enhanced the higher HDS activity of NiMo-3 than that of NiMo-2 catalyst. The thiophene conversions in NiMo-3 and NiMo-5 catalysts were measured to be approximately equal (90%), although the latter catalyst was more acidic and contained higher amount of MoO3. The agglomeration and/or sintering of the metal oxide particles covering the pores of the NiMo-5 surface reduced the catalytic activity of the material (Figure 2a).


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Figure 6. HDS tests: effects of (a) amounts of NiMo-3 (temperature = 350 °C), (b) reaction temperatures (amount of NiMo-3 = 0.5 g), (c) loading of MoO3 in the catalysts at the constant amount of NiO, and (d) the loading of total metal oxides in the catalysts at the constant ratio of NiO to the total oxides (catalyst amount = 0.5 g, temperature = 350 °C).

Figure 6d shows the effects of the total metal oxide (NiO + MoO3) loadings on the catalytic activity of the samples at a constant NiO/(NiO + MoO3) ratio of 0.25. Total metal oxide (NiO + MoO3) loadings in the NiMo-1, NiMo-3, and NiMo-4 catalysts were determined to be 12, 16, and 20%, respectively (Table 1). Thiophene conversions were measured to be approximately the same (~90%) in NiMo-1 and NiMo-3. The conversion, however, significantly decreased in NiMo-4, re-attributed to the blocking of the pores in the ACF substrate because of the relatively larger amounts of total metal oxides in the catalyst (Figure 2a).


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The data presented in Figure 6c and d show that thiophene conversion increased in the following order: NiMo-2 ≈ NiMo-4 < NiMo-1 < NiMo-3 ≈ NiMo-5. Interestingly, the catalytic activity of NiMo-1 is greater than that of NiMo-2, despite the former material having smaller total metal oxide contents and acidity. The effect is attributed to the ratio of NiO/(NiO + MoO3) in the catalyst. In the previous section it was mentioned that the thiophene conversions increased with decreasing ratios. Therefore, NiMo-1 showed higher catalytic activity than NiMo-2. Also, NiMo-3 showed higher catalytic activity than NiMo-4, but similar activity as that of NiMo-5 in spite of having lower metal oxides-loading. Therefore, an appropriate ratio of promoter and active metal, total metal oxides, BET surface area, acidity of the catalyst, and sulfidability, all must be considered while synthesizing a suitable HDS catalyst. Figure 7a-b shows the effects of the BET surface area and surface acidity of the catalyst on thiophene conversion via the MoO3 contents in the catalysts (Figure 7b). At the fixed amount of the NiO promoter in the catalyst, BET surface area and surface acidity decreased and increased, respectively, with increasing amounts of MoO3 (% w/w) in the NiMo-3 catalyst. The thiophene conversions increased with increasing amounts of MoO3 effectively up to 12% of the MoO3 content. A high degree (≥ 0.81) of sulfidability of catalysts, and relatively smaller MoS2 particle sizes with moderate number of stacking made the ACF-based catalysts more active towards the HDS of thiophene. Among the synthesized catalysts, NiMo3 showed better performance because of approximately the same degree of sulfidability, relatively smaller MoS2 particles which helped to increase stacking layers, and comparable BET surface area and acidity. Therefore, the ratio of NiO to (NiO+MoO3) in the material should be considered to be important for synthesizing an active HDS catalyst.


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Figure 7. Effects of MoO3 (% w/w) on (a) the BET surface area and surface acidity and (b) thiophene conversion. NiMo-3 (12% MoO3) is the recommended catalyst.

The reaction rate constant and activation energy should be determined to evaluate the performance of the catalysts. Assuming the HDS reaction for thiophene to be pseudo-first order, the apparent rate constant can be related to the conversion:

.

1 5 (5) 2 = 344 1 / 01 617

where XA is the steady state conversion, kapp is the apparent first order rate constant, W is the amount of the catalyst, and FAO is the inlet molar flow rate of thiophene. For pseudo-first order kinetics, a plot of ln(1/(1-XA)) vs. W/FAO should be linear. As mentioned earlier, the kinetic parameters were determined from the HDS measurements at low thiophene conversions (5-20%). Figure 8a shows the plot of 1/(1-XA) vs. W/FAO at 350 °C and molar flow rate of 1.2 µmole/s, using different amounts (5, 7.5, 10, 12.5, and 15 mg) of NiMo-3. The data are well represented by a linear fitting (R2 = 0.92). The slope of the line passing through origin provides the value of the apparent rate constant. The apparent rate constant is determined to be approximately 980×10-6 mole-thiophene/g of catalyst-min. The activation


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energy is calculated using Arrhenius plot (Figure 8b). The data are represented using linear fitting (R2 = 0.99) and the activation energy is determined to be 46.3 kJ/mole. Stability of the catalyst was checked by performing the reactions over relatively longer time (~30 h), using 0.5 g of NiMo-3 at 350 °C (Figure 8c). Insignificant decrease (less than 3%) in the conversion was observed. As earlier discussed, the SEM image and XRD pattern of the spent catalyst (after 30 h) did not show any perceptible change in morphology and structure of the sample. Also, the BET surface area and average pore diameter of the spent catalyst (Table 2) were determined to be approximately the same as those of the fresh NiMo3 catalyst, indicating that no morphological, structural and textural changes occurred during the HDS of thiophene. The above-discussed data indicated that insignificant coke formation occurred, with the catalytic activity sustained (< 3% decrease in conversion) till the completion of the experimental run through 30 h. It is mentioned that coke deposition over carbon support is much slower than in alumina-supported catalysts because of the lower surface acidity of carbon than alumina.56 Consequently, the frequency for regeneration is likely to be less in the former material. Also, chemical method using organic solvents such as tetrahydrofuran can be successfully used to regenerate and remove the deposited coke from the carbon-supported NiMo catalysts.57 The regeneration study is extensive and will be addressed in a future study. Mechanical strength of ACF depends on various aspects including its precursor, weaving technique, and carbonization/activation conditions to prepare the material from its pristine non-activated form. The ACF-based materials can be effectively used in packed bed by special arrangements (e.g. wrapping of ACF over the perforated tubular reactor or placing the material between the S.S. wire meshes). Figure S4 shows the tensile stress-strain curve of the NiMo-3 catalyst. Tensile stress of the calcined NiMo-3 catalyst increased with increasing strains. The ultimate stress was determined to be ~19500 N/m2 at ~7% of strain. The


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corresponding load was measured to be ~48.70 N which is comparable to the value (43.52 N) reported in literature.58 High tensile strength have also been reported in literature for similar ACF-based materials.59, 60

Figure 8. Kinetic and deactivation study: (a) pseudo first order plot for the HDS of thiophene over NiMo-3 catalyst (reaction temperature = 350 °C), (b) Arrhenius plot for NiMo-3 catalyst, and (c) stability or deactivation test.

Table 4 presents the comparative BET surface area, acidity and kinetic data discussed in literature for the HDS of thiophene. The CNF- (fishbone and platelet types) supported NiMo catalysts43 have also been included, along with Al2O3, SiO2, AC, and CNT. Table 4 shows better kinetic parameters (i.e. higher rate constant and lower activation energy) for the ACFsupported catalyst (NiMo-3) than the other forms of carbon viz. AC, CNF, CNT, or Al2O3-


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based catalysts. Table 4 also shows that the BET surface area of NiMo-3 is higher than those reported for the other catalysts. The surface acidity is also higher than that for the γ-Al2O3based catalyst. Therefore, the ACF-based HDS catalysts may be considered to be advantageous over the other support-based catalysts. From the perspective of a plausible mechanism, sulfided form of the catalysts (e.g. MoS2 phase) is considered to be active for the HDS reactions. Studies described in literature indicate possibly two pathways for the HDS of thiophene over the sulfided catalysts, namely, direct desulfurization (DDS), and hydrogenation followed by desulfurization.61,

62

During

DDS, thiophene is first converted to 1,3-butadiene, then to butene, and finally to butane. As per the latter mechanism, thiophene is first hydrogenated to form dihydrothiophene and tetrahydrothiophene. Dihydrothiophene is then desulfurized to butene which in turn is hydrogenated to form butane. Tetrahydrothiophene is directly desulfurized to butane. Co dispersed in the catalyst increases direct desulfurization, whereas Ni is known for its good hydrogenation ability. In the present study Ni served as the promoter. Therefore, HYD followed by desulfurization is indicated to be the main pathway for the HDS of thiophene.

4. CONCLUSIONS The ACF-supported NiMo catalyst was synthesized for the effective HDS of thiophene at ~1 bar and 350 °C. Screening of the catalysts revealed that the NiMo-3 catalyst having 4% (w/w) NiO promoter and 12% (w/w) MoO3 yielded ~90% conversion at W/FAO of ~115 (kg catalyst-h)/kmole, and the catalytic activity remained approximately constant up to 30 h of the experimental run. The catalyst was thermally stable up to 800 °C, with a maximum weight-loss of 20%. The comparative data clearly showed a lower activation energy (46.3 kJ/mole) and greater reaction rate constant (980×10-6 mole thiophene/g-catalyst-min) than the similar catalysts discussed in literature using different substrate materials for the HDS


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applications. In addition to the relative amounts of the NiO promoter and MoO3 active metal oxide, BET surface area, acidic contents, and MoS2 particle size in the catalysts also affected the catalytic activity. This study has for the first time shown the successful synthesis of ACFsupported NiMo catalysts for the HDS applications under moderate pressure and temperature conditions.


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Table 4. Comparative Study of BET Surface Area, Acidity, and Kinetics Data Available in Literatures for the HDS of Thiophene

Catalyst

SBET (m2/g)

Acidity

NiMo-3

1205

1.77b

7.24% Mo3Col/CNTs 140a 7.24% Mo3Col/C 650a NiMo-80A20Z 399 NiMo/Fishbone 88 a NiMo/Platelet 172 a NiMo/γ-Al2O3 193a CoMoS–CVD/D-Al2O3 90a Pd-PtO/MSA (9% Al2O3) 564 CoMo/Al2O3 0.98PtC-HY 216a CoMoS-O/γ-Al2O3 235 CoMoS-O/TiO2 123 CoMoS-O/SiO2 261 Pd/HMS 790a 3.6c Pd(AuCo)/MCM-41 810a 5.6c a Pd(AuCo)/SiO2 316 6.5c a CoMo-S-CTH 899 5-NiSO4/γ-Al2O3 270 0.314b Mo (II) 325a MoS2 325a NiMo-AP (8 wt% Mo) NiMo-AP (15 wt% Mo) NiMo-COP (8 wt% Mo) NiMo-COP (15wt% Mo) a area of support, b acidity (mmol-NH3/g), c point of zero charge

Reaction conditions temperature (°C), pressure (bar)

Pseudo-first order rate constant [molethiophene/(g catalystmin)] ×106

Apparent activation energy (kJ/mole)

350, 1

980

46.3

340, 1 340, 1 300, 1 300, 1 300, 1 300, 1 300, 1 280, 20 280, 20 320, 20 340, 1 340, 1 340, 1 340, 1 340,1 340, 1 400, 1 350, 20 257-522, 1 257-522, 1 400, 1 400, 1 400, 1 400, 1

55.80 24.30 17 235 240 369 375 275 108 228 223 137 68 85 60 26 135 0.27 126.8 277.0 80.1 183.6

96.6 98.6 63.0 33.0 30.2 43.4 48.5 -

Ref.

This study 23 23 34 41 41 41 63 64 64 65 66 66 66 67 67 67 68 69 70 70 71 71 71 71

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AUTHOR INFORMATION Corresponding Author *Tel.: +91-512-2596767, Fax: +91-512-2590104, E-mail: [email protected].

REFERENCES (1) Kilbane II, J. J.; Borgne, S. L., Petroleum biorefining: the selective removal of sulfur, nitrogen, and metals. In Studies in surface science and catalysis, Vazquez-Duhalt, R.; Quintero-Ramirez, R., Eds. Elsevier: Paises Bajos; Reino Unido, 2004; Vol. 151, Chapter 2, pp 29-65. (2) Shekhawat, D.; Berry, D. A.; Haynes, D. J.; Spivey, J. J., Fuel constituent effects on fuel reforming properties for fuel cell applications. Fuel 2009, 88, (5), 817-825. (3) Kaspar, J.; Fornasiero, P.; Hickey, N., Automotive catalytic converters: current status and some perspectives. Catal. Today 2003, 77, (4), 419-449. (4) Yu, T. C.; Shaw, H., The effect of sulfur poisoning on methane oxidation over palladium supported on gamma-alumina catalysts. Appl. Catal. B Environ. 1998, 18, (1-2), 105-114. (5) Prajapati, Y. N.; Verma, N., Adsorptive desulfurization of diesel oil using nickel nanoparticle-doped activated carbon beads with/without carbon nanofibers: Effects of adsorbate size and adsorbent texture. Fuel 2017, 189, 186-194. (6) Stanislaus, A.; Marafi, A.; Rana, M. S., Recent advances in the science and technology of ultra low sulfur diesel (ULSD) production. Catalysis Today 2010, 153, (1-2), 1-68. (7) Chandramowli, P. V. All you wanted to know about BS IV emission norms. The Hindu Business Line. , 2017 (accessed 15/07/2017). (8) Vissers, J. P. R.; Mercx, F. P. M.; Bouwens, S. M. A. M.; Debeer, V. H. J.; Prins, R., Carbon-covered alumina as a support for sulfide catalysts. J. Catal. 1988, 114, (2), 291-302. (9) Zepeda, T. A.; Pawelec, B.; Díaz de León, J. N.; de los Reyes, J. A.; Olivas, A., Effect of gallium loading on the hydrodesulfurization activity of unsupported Ga2S3/WS2 catalysts. Appl. Catal. B Environ. 2012, 111-112, 10-19. (10) Zepeda, T. A.; Pawelec, B.; Obeso-Estrella, R.; Díaz de León, J. N.; Fuentes, S.; Alonso-Núñez, G.; Fierro, J. L. G., Competitive HDS and HDN reactions over NiMoS/HMSAl catalysts: Diminishing of the inhibition of HDS reaction by support modification with P. Appl. Catal. B Environ. 2016, 180, 569-579. (11) Solís-Casados, D. A.; Escobar-Alarcón, L.; Klimova, T.; Escobar-Aguilar, J.; Rodríguez-Castellón, E.; Cecilia, J. A.; Morales-Ramírez, C., Catalytic performance of CoMo/Al2O3-MgO-Li(x) formulations in DBT hydrodesulfurization. Catal. Today 2016, 271, 35-44. (12) Soghrati, E.; Kazemeini, M.; Rashidi, A. M.; Jozani, K. J., Development of a structured monolithic support with a CNT washcoat for the naphtha HDS process. J. Taiwan Inst. Chem. E. 2014, 45, (3), 887-895. (13) Fallah, R. N.; Azizian, S.; Dwivedi, A. D.; Sillanpää, M., Adsorptive desulfurization using different passivated carbon nanoparticles by PEG-200. Fuel Process. Technol. 2015, 130, 214-223. (14) Garlof, S.; Mecklenburg, M.; Smazna, D.; Mishra, Y. K.; Adelung, R.; Schulte, K.; Fiedler, B., 3D carbon networks and their polymer composites: Fabrication and


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List of figure captions Figure 1. Experimental setup used for the HDS of thiophene. Figure 2. (a) SEM images of the calcined samples, and spent NiMo-3 sample, (b) EDX mapping of the calcined NiMo-3 sample. Figure 3. Physico-chemical characterization of the prepared catalysts: (a) XRD, (b) TG, (c) TPR and (d) TPD patterns. Figure 4. HR-TEM images of (a) NiMo-1, (b) NiMo-2, (c) NiMo-3, (d) NiMo-4, and (e) NiMo-5 sulfided catalysts; (f) SAED pattern and (g) EDX mapping of the sulfided NiMo-3 catalyst. Figure 5. Mo 3d, S 2s and Ni 2p 3/2 core level XPS spectra of the sulfided (a-a’) NiMo-1, (bb’) NiMo-2, (c-c’) NiMo-3, (d-d’) NiMo-4, and (e-e’) NiMo-5. Figure 6. HDS tests: effects of (a) amounts of NiMo-3 (temperature = 350 °C), (b) reaction temperatures (amount of NiMo-3 = 0.5 g), (c) loading of MoO3 in the catalysts at the constant amount of NiO, and (d) the loading of total metal oxides in the catalysts at the constant ratio of NiO to the total oxides (catalyst amount = 0.5 g, temperature = 350 °C). Figure 7. Effects of MoO3 (% w/w) on (a) the BET surface area and surface acidity and (b) thiophene conversion. NiMo-3 (12% MoO3) is the recommended catalyst. Figure 8. Kinetic and deactivation study: (a) pseudo first order plot for the HDS of thiophene over NiMo-3 catalyst (reaction temperature = 350 °C), (b) Arrhenius plot for NiMo-3 catalyst, and (c) stability or deactivation test.

Supplementary figures Figure S1. N2 adsorption/desorption isotherms of the prepared catalysts. Figure S2. SEM images of ACF. Figure S3. Elemental mapping of the calcined samples. Figure S4. Tensile strength of the calcined NiMo-3 catalyst.


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Hydrodesulfurization of Thiophene on Activated Carbon Fiber

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