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Hydrotreatment of light cycle oil over a dispersed MoS 2 catalyst Haiping Zhang, Hongfei Lin, Ying Zheng* Department of Chemical Engineering, University of New Brunswick, 15 Dineen Drive, Fredericton, NB, E3B 5A3, Canada *Ying Zheng, Corresponding author, [email protected] , 1 (506) 447-3329 Abstract Examination of the hydrotreatment of light cycle oil over a dispersed MoS 2 catalyst was conducted in a batch reactor at 375°C and 1500psi. Hydrodesulfurization, hydrodenitrogenation, hydrogenation of aromatics, and hydrocracking activity were all analyzed. Diaromatics are more reactive than monoaromatics. Different sulfur compounds have experimental rates of elimination following the trend of BT>1MBT>> 2MBT>3MBT>4MBT>>DBT≈1MDBT≈2MDBT≈3MDBT. BT and its derivatives are very reactive and easy to eliminate, but become harder to convert as they gain methyl groups. DBTs are harder to eliminate than BTs, and don’t experience significant changes in reactivity as they gain methyl groups. This difference indicates that steric hindrance is more significant in supported catalysts than in unsupported catalysts. Different nitrogen compounds have experimental rates of elimination following the trend of Anilines> Indoles> Carbazoles. Keywords: Kinetics; hydrotreatment; hydrodesulfurization; light cycle oil; dispersed MoS 2 . 1

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Page 1: Introduction - Edinburgh Research Explorer · Web viewThe amounts of aliphatic and aromatic hydrocarbons over time during hydrotreatment are shown in Fig. 8. The content of aliphatic

Hydrotreatment of light cycle oil over a dispersed MoS2

catalyst

Haiping Zhang, Hongfei Lin, Ying Zheng*

Department of Chemical Engineering, University of New Brunswick, 15 Dineen Drive, Fredericton, NB, E3B 5A3, Canada

*Ying Zheng, Corresponding author, [email protected], 1 (506) 447-3329

Abstract

Examination of the hydrotreatment of light cycle oil over a dispersed MoS 2 catalyst was conducted in a batch reactor at 375°C and 1500psi. Hydrodesulfurization, hydrodenitrogenation, hydrogenation of aromatics, and hydrocracking activity were all analyzed. Diaromatics are more reactive than monoaromatics. Different sulfur compounds have experimental rates of elimination following the trend of BT>1MBT>> 2MBT>3MBT>4MBT>>DBT≈1MDBT≈2MDBT≈3MDBT. BT and its derivatives are very reactive and easy to eliminate, but become harder to convert as they gain methyl groups. DBTs are harder to eliminate than BTs, and don’t experience significant changes in reactivity as they gain methyl groups. This difference indicates that steric hindrance is more significant in supported catalysts than in unsupported catalysts. Different nitrogen compounds have experimental rates of elimination following the trend of Anilines> Indoles> Carbazoles.

Keywords: Kinetics; hydrotreatment; hydrodesulfurization; light cycle oil; dispersed MoS2.

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Page 2: Introduction - Edinburgh Research Explorer · Web viewThe amounts of aliphatic and aromatic hydrocarbons over time during hydrotreatment are shown in Fig. 8. The content of aliphatic

1 IntroductionLight cycle oil (LCO) obtained from a fluid catalytic cracking unit (FCCU) is a blend to straight run gas oil used for diesel production [1, 2]. An increasing demand for diesel fuels requires more LCO to be added to the diesel pool [3]. However, LCO has a high density and contains large amounts of sulfur, nitrogen, and aromatics [4] – its quality needs to be improved. Hydrotreating is a common approach in upgrading LCO for diesel production.

Previous research has demonstrated that sulfur containing compounds in LCO include alkyl-substituted benzothiophenes and dibenzothiophenes, such as BT, MBT, DBT, MDBT, and 4,6-DMDBT [5]. In the literature, model compounds are normally used for kinetics studies due to the complexity of LCO [7, 8]. The hydrodesulfurization (HDS) reaction of the hard sulfur compounds (DBT and its alkyl-substitutes) occurs mainly through two parallel routes: the hydrogenation of one phenyl ring prior to hydrogenolysis (HYD) and the direct desulfurization of C-S bonds (DDS) [9, 10]. Alkyl-substituted DBTs, especially ones with methyl groups at the 4 and 6 positions, have lower reaction rates compared to DBT. This is due to a strong steric hindrance and electronic effect [11, 12]. Moreover, deep desulfurization is greatly inhibited in the presence of nitrogen containing compounds [13, 14]. Nitrogen containing compounds have a higher affinity for active sites and relatively lower reaction rates when compared to sulfur containing compounds – they therefore take up active sites and stay there for long periods of time, thereby reducing HDS rates. Similarly, aromatic compounds in LCO may also inhibit the HDS by taking up hydrogenation active sites on the MoS2 catalysts [11, 15].

Catalysts play a key role in catalytic hydrodesulfurization. In the past decades, studies have focused on conventional sulfided catalysts supported by γ-alumina [16, 17]. Al2O3 is widely applied because of its stability, acidity, and high surface area [18]. However, it is reported that Al2O3 supported Mo catalysts are not efficient in eliminating heteroaromatic sulfur containing compounds such as DBTs [19]. Dispersed MoS2 own several advantages over the supported MoS2. They show good performance in terms of activity and thermo stability, compared to the traditional supported sulfide catalysts. One of such catalysts, NEBULA (brand name), has been successfully applied in refineries, exhibiting 3 to 4 times higher activity than conventional supported catalysts [20]. such as higher activity and lower steric hindrance. the bulk sulfide-based Mo catalysts Farag et al. pointed out that dispersed MoS2 desulfurizes DBTs primarily over HYD pathway and the supported MoS2 desulfurizes DBTs via both HYD and DDS pathways [21, 22]. Liu and Ng reported that dispersed MoS2 were effective in hydrodesulfurization of hard sulfur such as DMDBT [23]. Jansen et al. studied recycling of the dispersed catalyst in treating heavy hydrocarbons [24].

In this work, light cycle oil was hydrotreated by an unsupported dispersed nano-catalyst MoS 2, which was synthesized through a reported novel hydrothermal method [25]. The feed oil and liquid products were extensively analyzed. The hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activities were evaluated by analyzing sulfur and nitrogen amounts in the original LCO and hydrotreated products. The HDS and HDN kinetics were demonstrated by analyzing the sulfur and nitrogen compound compositions. Hydrogenation ability was

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demonstrated by evaluating the variation of aromatic compounds during the reaction. A hydrocracking reaction was indicated at by the distribution of boiling points and liquid densities in the original and hydrotreated products.

2 Experimental

2.1 Catalyst synthesis and characterization

A series of molybdenum sulfided catalysts were synthesized through the hydrothermal method [25] using MoO3 (STEM Scientific) and Na2S·9H2O (Fisher Scientific) as precursors. MoO3 and Na2S·9H2O were first dissolved in deionized water and then HCl was slowly added to the solution. The mixture was placed into an autoclave reactor for 2h at 320˚C and 500rpm. The resultant black solid was washed with deionized water and ethanol.

The synthesized catalysts were identified by X-ray diffraction (XRD) pattern, which were recorded on a diffractometer using CuKα radiation with a 2θ range of 5-85° and scan speed of 1°/min. The morphology of catalysts was also measured using a transmission electron microscope (JEOL 2011 STEM, JEOL Ltd., Tokyo, Japan). The length and the number of layers of the MoS2

crystal were analyzed using imaging analysis software. The specific surface area of the catalyst powder was calculated using the Brunauer-Emmett-Teller (BET) method, based on the nitrogen adsorption-desorption isotherm measured by an Autosorb-1 (Quantachrome Instruments). The total pore volume was calculated from the volume of nitrogen adsorbed at the relative pressure p/p0 0.995. Pore size distribution was analyzed from the isotherms, by the Barrett-Joyner-Halenda (BJH) method.

2.2 Hydrotreating process

A 1L batch reactor from Autoclave Engineering was employed in this experiment. Many trials were conducted and a typical procedure is shown below. Light cycle oil (LCO) containing 1.45% sulfur and 159ppm nitrogen was used as feedstock. The specifications of LCO are listed in Table1. Catalysts that had been filtered through 200 meshes were added into the batch reactor. After the reactor was heated to 375ºC, 120g of LCO was introduced with different catalyst-to-oil weight ratios (COR). Samples were taken during the reaction. The effect of sampling is assumed to be negligible due to the small sample amounts (<1.0 gram per sample). Hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) took place at 375ºC under 1500psi hydrogen pressure and at a stirring speed of 1000rpm. The mass transfer resistance was ignored, due to the nano size of the catalyst [26].

2.3 Product analysis

The sulfur and nitrogen amounts in the original LCO and hydrotreated products were determined using a Sulfur/Nitrogen analyzer (9000 series, Antek Instruments Inc). The hydrodesulfurization conversion and hydrodenitrogenation conversion (xHDS and xHDN) is the ratio of removed sulfur and nitrogen to their respective amounts in the feed. A first-order reaction was assumed for the calculation of the HDS and HDN rate constant k.

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Page 4: Introduction - Edinburgh Research Explorer · Web viewThe amounts of aliphatic and aromatic hydrocarbons over time during hydrotreatment are shown in Fig. 8. The content of aliphatic

xHDS=SFeed−SProduct

SFeed×100 %

Equation 1

xHDN=N Feed−N Product

N Feed×100 %

Equation 2

The distribution of sulfur compounds in the feed and products was analyzed using a gas chromatograph (Varian 450) equipped with a non-polar VF-1ms capillary column (15m x 0.25mm x 0.25μm, max temperature: 325°C) and a pulsed flame photometric detector (PFPD). Benzothiophene (BT), dibenzothiophene (DBT), and 4, 6-dimethyldibenzothiophene (4, 6-DMDBT) were used to represent easy and hard sulfur compounds. The distribution of nitrogen compounds in the feed and products was analyzed using a gas chromatograph (GC-950, Shanghai Haixin Chromatographic Instrument Co. LTD) equipped with a nitrogen phosphorus detector (NPD). Anilines, indole, quinoline, tetrahydroquinoline, and carbazole were used to represent the different nitrogen compounds. The parameter setup followed the literature [27]. The column was heated up at 4°C/min from 40°C to 265°C. The distribution of aromatics in the collected samples was determined by high performance liquid chromatography (HPLC, Agilent 1200 series) according to ASTM 6591. Simulated distillation of the products was carried out using Shimadzu GC-2010 according to ASTM-2887. The density of the oil was measured by portable density meter (DMA 35N, Anton Paar GmbH, Graz, Austria) following ASTM 4052.

3 Results and discussion

3.1 Catalyst characterization

Dispersed MoS2 crystallines are successfully synthesized. It is seen from the XRD spectra (Fig.1). Characteristic MoS2 diffraction peaks are observed attributing to the planes (002), (100), (103), (110), (105), and (201), respectively. The broad peaks indicate a nano crystalline structure, which is verified from TEM images. Fig. 2 shows MoS2 consists of layered sheets forming S–Mo–S sandwiches. In the pure compounds the sheets are stacked and held together by van der Waals forces. The synthesized MoS2 has a highly dispersed dendritic morphology and a layered nanocrystal with an average length of 17.43nm and width of 2.93nm (Table 2). Most of the crystals shown are curved and associated with each other, leading to a high BET surface area of 236.3m2/g and a high pore volume of 2.276 cm3/g. The isotherm curve shows a type V shape, and the hysteresis loop between adsorption and desorption isotherms is associated with mesoporosity (Fig. 3a). Pore size distribution was determined by the isotherm curve by BJH analysis (Fig. 3b). A bimodal distribution was observed with a sharp peak centered at 2.46nm with a small shoulder of 3.84nm and a broad peak at about 12nm. The spent catalyst were also characterized and found all the properties are similar to the fresh catalyst.

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3.2 Hydrodesulfurization activity

The graph of sulfur conversion versus hydrotreatment time is shown in Fig. 4. Three catalyst-to-oil (COR) ratios were used to investigate the effect of catalyst amounts on HDS performance. At a very low ratio (e.g. 1:800), the HDS rate increases gradually with time. Addition of more catalysts results in a sharp increase in sulfur conversion during the initial reaction. When the COR reaches 1:100, the conversion rate increases dramatically during the first half hour, followed by a relatively slow increase from 0.5 hours to approximately 2 hours. From 2 hours to 8 hours, the conversion rate only increases very slowly (by approximately 10%). A higher COR is related to a higher initial reaction speed. Sulfur conversion reaches almost 60% within the first half hour when the COR is at 1:100, whereas the amounts of sulfur conversion for CORs 1:200 and 1:800 during the same time period are 30% and 15%, respectively. This indicates that the use of more catalysts can significantly increase the removal rate of easy sulfurs. Between 2 hours and 8 hours, the sulfur conversion rates between all CORs remain similar. This suggests that different CORs exhibit similar removal rates for hard sulfurs. At the end of the reaction (8 hours), the higher COR samples contain lower amounts of sulfur. This may be explained by the fact that when more catalysts are involved, the time required for the depletion of easy sulfur is reduced, and therefore the catalyst gains more time for the removal of hard sulfurs.

Several different sulfur species were identified in the LCO (Fig. 5), including benzothiophene (BT), methyl- benzothiophene (1MBT-4MBT), dibenzothiophene (DBT), and methyl-dibenzothiophene (1MDBT-3MDBT). At hour 2 of the reaction, easy sulfurs such as BT, 1MBT-4MBT were almost removed, whereas hard sulfurs remained largely unreacted. This is consistent with the observation that only easy sulfurs are quickly removed within the first 2 hours. At the end of the reaction, no easy sulfurs were detected in the final products. The amounts of hard sulfurs were also greatly reduced but not totally removed, indicating that hard sulfurs require a significantly longer time to react compared to easy sulfurs. This is probably because hard sulfurs like 1-3MDBT have an additional aromatic ring, which greatly increases steric hindrance and decreases contact efficiency between the substance and the active sites on catalysts. This observation is agreed upon with other researchers.

The amounts of main sulfur-containing compounds vary with reaction time and are shown in Fig. 6; the calculated rate constants are listed in Table 3. The compounds are catalogued into two groups: easy sulfurs (Fig. 6a) and hard sulfurs (Fig. 6b). In Fig 6a, BT and 1MBT show the highest reaction rates, and are completely removed within the first 2 hours. Compounds from 2MBT to 4MBT show slightly lower reaction speeds, but more than 90% of these substances are still converted at the 2nd hour. The reaction becomes more difficult due to additional alkyls on the aromatic ring. Computational study [28] showed that charges of the methyl-substituted sulfur-containing compounds become more concentrated on the sulfur atom. Hydrogen needs to use methyl groups as bridge to transfer to sulfur atom and the sulfur atoms are more refractory in the hydrodesulfurization process. This give a possible electronic explanation to the experimentally observed. In Fig. 6b, all the compounds exhibit similar reaction rates, indicating that the steric effect is not significant among these compounds. Compared easy sulfurs that showed a decreasing reaction rates against reaction time, hard sulfurs remains a similar conversion rate throughout. This contrast exists probably due to the fact that easy sulfurs can utilize more active sites on

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catalysts than hard sulfurs can. With stronger steric effects, hard sulfurs can only be absorbed on certain sites, and so their reaction rates are determined by the amount of active sites specific for hard sulfurs on the catalysts.

3.3 Hydrodenitrogenation activity

The LCO feedstock used in this study contains small amounts of nitrogen compounds, which are categorized into 3 species: anilines, indoles and carbazoles. The nitrogen components in the feed and product were determined using GC-NPD (Fig. 7). In LCO feedstock, the content of indole is significantly higher than its alkyl derivatives. For carbazoles, the highest peak refers to C1 carbazole. Fig. 7 shows that all anilines and the majority of indoles and carbazoles were removed within 6 hours of hydrotreatment. The unreacted nitrogen containing compounds were mainly C4 indoles and C1-C2 carbazoles. The nitrogen distribution in the feed and in the products at hours 2 and 6 are listed in Table 4. In the reaction, anilines were the easiest nitrogen compound in LCO and were quickly removed within the first 2 hours. Indoles were harder to remove, the indole concentration only dropping from 73.73 ppm to 20.2 ppm in the first 2 hours. Carbazoles were the hardest nitrogen compounds to remove, only reducing by about 50% in the first 2 hours. The dynamics of different nitrogen types were simulated by pseudo-first-order and their rate constants are listed in Table 5. Only indole and carbazole are provided since anilines were not detectable after 2 hours of reaction.

3.4 Hydrogenation and hydrocracking activities

The amounts of aliphatic and aromatic hydrocarbons over time during hydrotreatment are shown in Fig. 8. The content of aliphatic hydrocarbons was almost unchanged, indicating that the conversion from aromatic to aliphatic hydrocarbons was insignificant. However, considerable diaromatics were reduced to monoaromatics: the percent fraction of diaromatics decreased from 52.3% to 10.3% while the percent fraction of monoaromatics increased from 12.9% to 59.6%. Polycyclic aromatics were very hard to eliminate.

The hydrocracking activity was evaluated by simulated distillation (Fig. 9). During the reaction, the fraction of heavy components (boiling points > 250 °C) gradually decreased and the fraction of relatively lighter components (boiling points of 200-250 °C) increased significantly. This indicates that a hydrocracking reaction occurred in the heavier components of the LCO. It was noticed that the total fraction of components with boiling points above 300 °C barely changed within the first 6 hours. This indicates that the hydrocracking reaction mostly only converted the 250-300 °C fraction to the 200-250 °C fraction during the first several hours. However, it was noticed that hydrocracking on components with boiling points greater than 300 °C eventually happened during hydrotreatment. This was evidenced by a decrease in the >300 °C fraction after 8 hours of reaction.

Conclusions

Light cycle oil is a complex middle distillate, containing different kinds of sulfur, nitrogen and aromatic compounds. Hydrotreatment of this complex feedstock using a dispersed MoS2 catalyst was carried out in a batch reactor and conclusions were drawn through the extensive analysis of

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hydrotreated products. The reactivity of different sulfur compounds in the LCO followed the trend BT>1MBT>>2MBT>3MBT>4MBT>> DBT≈1MDBT≈2MDBT≈3MDBT. The rate of elimination of nitrogen compounds in the LCO followed the trend of Anilines > Indoles > Carbazoles. Hydrogenation mainly occurred on bicyclic aromatics which were reduced to monocyclic compounds; monocyclic and polycyclic aromatics were barely hydrogenated. This phenomenon confirms the weak hydrogenation ability of the unsupported catalyst. Hydrocracking activity was revealed by the distribution of boiling points and the variation of liquid densities in the feed and final products.

Acknowledgements

The authors gratefully acknowledge the financial assistance from Canada Research Chairs program, Natural Sciences and Engineering Research Council of Canada and Canada Foundation for Innovation.

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Page 8: Introduction - Edinburgh Research Explorer · Web viewThe amounts of aliphatic and aromatic hydrocarbons over time during hydrotreatment are shown in Fig. 8. The content of aliphatic

Table 1 Properties and composition of light cycle oilPropertiesDensity (g/ml, 30°C) 0.9641Sulfur content (w.t. %) 1.45Nitrogen (ppmw) 159.2

Chemical composition (w.t. %)Saturates 22.61Monoaromatics 12.89Diaromatics 52.35Polyaromatics 12.15

Boiling points (°C)Initial boiling point (IBP) 113.310% 227.020% 237.530% 254.940% 258.750% 273.560% 283.470% 293.880% 307.390% 328.4Final boiling point (FBP) 376.4

Table 2 Specifications of unsupported MoS2 catalystsPropertiesAverage Slab length (nm) 17.43Average Slab width (nm) 2.93BET surface area (m2/g) 236.3Total pore volume (cm3/g) 2.276Average pore size (nm) 9.36

Table 3 Reaction rate constant for different sulfur componentsComponents kHDS (x10-4 s-1 g cata -1)BT 20.651MBT 14.152MBT 5.143MBT 4.984MBT 4.54DBT 0.651MDBT 0.632MDBT 0.653MDBT 0.65

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Table 4 Nitrogen distribution in feed and hydrotreated productsHydrotreating time(h)

N in Alilines(ppm)

N in Indoles(ppm)

N in carbazoles(ppm) Total N

(ppm)0 (LCO2) 10.3 76.73 67.2 159.22 0 20.2 34.4 54.66 0 6.9 9.8 16.7

Table 5 Reaction rate constant for different nitrogen typeComponents kHDS (x10-4 s-1 g cata -1)Indole types 1.60Carbazole types 1.29

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Fig. 1 XRD spectrum of dispersed MoS2

Fig. 2 TEM images of dispersed MoS2

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Fig. 3 Isotherm curve and pore size distribution of dispersed MoS2 by BJH (adsorption)

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Fig. 4 HDS conversion over dispersed MoS2 at different catalyst-to-oil ratios (COR, w.t.)

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Fig. 5 Major sulfur compounds composition of LCO and hydrotreated products (GC-PFPD), COR=1:200

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Fig. 6 The conversion of individual sulfur compounds (COR 1:200)

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Fig. 7 Major nitrogen compounds composition of LCO and hydrotreated products (GC-NPD)

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Fig. 8 Hydrogenation along with hydrotreating time (COR 1:200)

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Fig. 9 Boiling point distribution of LCO and hydrotreated products (COR 1:200)

Fig. 10 Variation of density of the product with the hydrotreating time (COR 1:200)

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