JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY

Volume 36, Issue 6, December 2008 Online English edition of the Chinese language journal

Received: 09-May-2008; Revised: 13-Sep-2008 Corresponding author. Tel: 0532-86981716, E-mail: cgliu@hdpu.edu.cn Foundation item: Supported by the Major State Basic Development Program of China (973 Program, 2004CB217807). Copyright 2008, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

RESEARCH PAPERCite this article as: J Fuel Chem Technol, 2008, 36(6), 684 690

Mutual influences of hydrodesulfurization of dibenzothiophene and hydrodenitrogenation of indole over NiMoS/ -Al2O3 catalyst XIANG Chun-e, CHAI Yong-ming, LIU Yun-qi, LIU Chen-guang State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis, China National Petroleum Corporation, College of Chemistry & Chemical Engineering, China University of Petroleum (East China), Qingdao 266555, China

Abstract: The influence of indole on the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and the influence of DBT on the hydrodenitrogenation (HDN) of indole were investigated over a presulfided NiMoS/ -Al2O3 catalyst in a fixed-bed high-pressure microreactor. A significant negative effect of indole on the HDS of DBT was observed. The inhibitory effect of indole on the hydrogenation route (HYD) was stronger than on the direct desulfurization route (DDS). Indole and its HDN intermediate products suppressed HDS of DBT through the competitive adsorption on active sites of the catalyst. DBT and H2S produced in situ promoted the conversion of coordinatively unsaturated sites (CUS) to Brønsted acid sites on the catalyst surface, which in turn facilitated the cleavage of C(sp3)—N bond in indoline; the conversion of indole and the relative concentration of o-ethylanline (OEA) then increased. Although the presence of sulfur atoms is essential for the formation of active sites on the catalyst for HDN, a small amount of sulfur species is sufficient to maintain the HDN active sites; higher content of sulfides may bring on a negative influence on the HDN of indole. Keywords: NiMoS/ -Al2O3; hydrodesulfurization; hydrodenitrogenation; dibenzothiophene; indole; in situ produced H2S; mutual

influence

Stringent diesel and gasoline specifications are being introduced in many countries to minimize the environmental pollution. However, the activity of a catalyst for hydrodesulfurization (HDS) in the hydrotreating process of diesel is depressed by organic heterocompounds and polyaromatic hydrocarbons, especially by nitrogen containing compounds[1,2].

The poisoning effects of basic N-compounds such as quinoline and pyridine on HDS reaction have been widely reported. However, the effects of nonbasic N-compounds such as indole and carbazole on HDS are not well understood, though 70% of the total N-compounds in the feed are nonbasic. In addition, the nonbasic N-compounds can be converted into basic ones after hydrogenation, which have higher inhibiting effects on the HDS reaction[2,3]. To improve the performance of catalysts for HDS, it is necessary to investigate the influence of nonbasic N-compounds and its intermediates on HDS reaction. Meanwhile, it is also essential to study the effects of S-compounds on hydrodenitrogenation (HDN) reaction in order to improve the

HDN activity. Therefore, in this study, the mutual influences between the HDS of dibenzothiophene (DBT) and the HDN of indole were investigated over NiMoS/Al2O3 catalyst. 1 Experimental 1.1 Chemicals and catalyst preparation

Indole, CS2, and cyclohexane of analytic pure grade were used. DBT (purity >99.7%) was synthesized by the procedure advocated by Xu[4]. Ammonium tetrathiomolybdate (ATTM) was prepared according to a patent[5].

NiMoS/ -Al2O3 catalyst was prepared by successive incipient wetness impregnation of -Al2O3 with an aqueous solution of ATTM, followed by an aqueous solution of nickel nitrate. After each impregnation step, the catalyst was dried in an oven at 60°C for 12 h, and then calcined at 250°C for the first step and 500°C for the second step under nitrogen stream for 3 h. The loading calculated in the form of

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XIANG Chun-e et al. / Journal of Fuel Chemistry and Technology, 2008, 36(6): 684 690

oxide precursor (MoO3+NiO) was 24% and the atomic ratio of Ni/(Ni+Mo) was 0.3. 1.2 Catalytic tests

HDS and HDN reactions were carried out in a fixed-bed high pressure microreactor (a stainless steel pipe of 7 mm i.d., 420 mm long). 2 mL of NiMoS/ -Al2O3 catalyst (20–40 mesh) was diluted with 3 mL of quartz sand to achieve plug-flow conditions in the reactor.

Prior to the reaction, the catalyst was treated in H2 (2.0 MPa) to reduce the active metal sulfides at a heating rate of 40ºC/h from room temperature to 120ºC. This was held for 1 h, followed by heating to 300ºC. Finally, this temperature was maintained for 2 h. After the treatment, the temperature was decreased to 200°C and the liquid reactant was fed to the reactor by a pump.

The inhibitory effects of indole on HDS of DBT were studied at the following conditions: 300–340°C, 2.0 MPa H2, H2/feed volume ratio of 300, and liquid hour space velocity (LHSV) of 10.0 h–1. 2.3% of DBT dissolved in cyclohexane was used as the reaction feed, equivalent to the sulfur content in the feed of weight (wS) 4000 μg/g, in which indole was added at a concentration of wN = 500–2000 μg/g (the nitrogen content in the feed by weight).

The effects of DBT and CS2 on the HDN of indole were studied at the following conditions: 300°C, 3.0 MPa H2, H2/feed volume ratio of 300, and LHSV of 10.0 h–1. DBT and CS2 with a sulfur content of 1000–4000 μg/g were added to the indole-cyclohexane (nitrogen content of 500 μg/g) feed. 1.3 Analytical procedures

The reaction products were cooled down and separated into gaseous and liquid fractions in a high-pressure separator. Identification of liquid products was carried out with a Varian 3800 gas chromatograph and a Finnigan SSQ710 mass spectrometry. The quantitative analysis of products was performed with an Agilent HP 6820 GC equipped with a flame ionization detector and CP-5 capillary column.

The activities of total, direct desulfurization route (DDS) and hydrogenation route (HYD) were calculated by the following equations:

ATotal = F0x/m (1) ADDS = ATotal sBP (2) AHYD = ATotal sCHB (3)

Where ATotal, ADDS, and AHYD are the total, DDS, and HYD activity, respectively; F0 is the flow rate of DBT (mol/s); x is the conversion of DBT; m is the mass of catalyst (kg); sBP and sCHB are the selectivity of biphenyl (BP) and cyclohexylbenzene (CHB), respectively.

2 Results and discussion 2.1 Effects of indole on the HDS of DBT

The reaction products of the HDS of DBT indicate that the HDS of DBT goes via two parallel pathways[6–8]. The first one is called hydrogenation (HYD), which consists of a preliminary hydrogenation of one aromatic ring, giving tetrahydro- and hexahydro-dibenzothiophenes that can be desulfurized to form CHB. The second pathway is called direct desulfurization (DDS), which yields BP. Under moderate conditions, the transformation of BP to CHB can be negligible[8], thus the selectivity of BP and CHB is supposed to be a good representation of the reaction activity of DDS and HYD route.

The HDS of DBT was studied in the presence of indole with different concentrations at 300 C and 340 C, 2.0 MPa, LHSV of 10.0 h–1, H2/Oil (V/V) of 300. The conversion of DBT, HDS activity, and selectivity of BP and CHB are listed in Table 1. In the absence of indole, the HDS of DBT mainly follows the DDS route, whereas the contribution of HYD route is about 21.0%. However, in the presence of indole, the HYD pathway for the HDS of DBT is strongly suppressed, as AHYD decreases from 70 μmol/(Kg·s) to 14 μmol/(Kg·s) and sCHB is lower than 5.0%. ADDS increases from 258 μmol/(Kg·s) to 309 μmol/(Kg·s) at first; after reaching a maximum, it decreases with the increase of nitrogen content, indicating that indole not only suppresses the HYD route, but inhibits the DDS pathway as well.

The effects of indole on the HDS of DBT at 340oC are similar to those at 300oC. HYD route is also intensely suppressed by indole at 340oC, whereas the inhibition of the DDS route becomes obvious with high nitrogen content. As a consequence, the conversion of DBT decreases gradually with the increase of nitrogen content; the sCHB declines even as the sBP increases simultaneously.

Table 1 Effects of indole content on the reaction activity of the HDS of DBT

Activity /

μmol· (kg·s)–1

Selectivity

smol/ % t

/oC

wN

μg/g

DBT conversion

xmol/% ATotal ADDS AHYD BP CHB

0 92.0 331 258 70 77.8 21.0

500 90.2 325 309 14 95.3 4.4

1000 80.3 289 281 6.6 97.1 2.3

1500 68.7 247 242 3.8 97.7 1.5

300

2000 64.2 231 227 2.7 98.2 1.2 0 99.9 360 266 88 73.8 24.5

500 99.7 359 317 41 88.2 11.4

1000 87.9 351 330 21 93.9 6.0

1500 76.6 317 303 13 95.6 4.0

340

2000 70.8 300 290 9 96.5 3.1

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In general, the HYD route is inhibited even under low nitrogen content, whereas the inhibition for the DDS route can only be observed at quite high nitrogen content. The inhibitory effects of indole on both HYD and DDS route become weaker at higher temperature. 2.2 Inhibition mechanism of indole on the HDS of DBT

To get a better understanding of the inhibition mechanism, the product distribution of the indole HDN was analyzed. A rapid hydrogenation of indole (IND) to indoline (HIN) is followed by two different paths. The first is direct hydrogenation of the aromatic ring of indoline to form octahydroindoline (OHI), which is rapidly transformed to ethylcyclohexylamine (OECHA). Another is C(sp3)—N bond rupture to form orthoethylaniline (OEA)[9]. The reaction network is represented in Fig. 1.

The indole conversion, HDN ratio, and relative products concentration with varied nitrogen contents in the feed are listed in Table 2. It is evident that with increasing nitrogen content, the conversion, HDN ratio, the relative concentration of ECH, ECHE, and OEA decrease, whereas the relative concentration of HIN increases.

It is commonly assumed that Mo atom at the edges and corners of MoS2 crystallites in Mo/ -Al2O3 catalyst are catalytically active sites. At least one sulfur vacancy at a site is needed to allow the reacting molecule to bind chemically with Mo atom[10,11]. The promoter atom (Ni or Co) substitutes the Mo atom, forming so-called Ni-Mo-S or Co-Mo-S structure. The sulfur atom between Ni and Mo atom is less strongly bonded than the sulfur atom between two Mo atoms and can be more easily removed. Under current situation, indole strongly inhibits the HYD route for the HDS of DBT, whereas the DDS pathway is only slightly suppressed. This indicates that the hydrogenolysis and hydrogenation of DBT may proceed on different active sites.

As shown in Table 1, the HYD route in HDS of DBT is inhibited even under low nitrogen concentration, which can be explained by the competitive adsorption between DBT

and indole. When DBT hydrogenation occurs, at least two neighboring vacancies are needed for adsorbing DBT molecule through benzene ring in the side-on mode[12–15]. For the HDN of indole, the reaction begins with the hydrogenation of pyrrole ring and most likely is adsorbed in a flat conformation through pyrrole ring at the same DBT active sites. The electron cloud density of N-ring in indole ( 5

6) is greater than that of benzene ring in DBT ( 66); thus,

the indole molecule can adsorb on the catalytic sites for hydrogenation prior to the DBT molecule.

One vacancy at a metal atom is needed to perform the DDS reaction of DBT, in which case the molecule is adsorbed via end-on mode perpendicular to the catalyst surface[16]. HIN may be adsorbed in the same mode as DBT molecule on the vacancy through lone-pair electron of nitrogen atom because the conjunctive effect of nitrogen atom disappears. Indole has a negative effect on the DDS route, but it is observed only at high nitrogen content; the apparent activity of DDS route was even enhanced at low nitrogen content (Table 1). This can be explained by the competitive adsorption of DBT and N-compounds. With low nitrogen content, the amount of N-containing intermediates is too small to cause significant inhibitory effect on DDS route. As the HYD route is largely blocked, more DBT molecules can react through DDS route. The inhibition on DDS route becomes obvious with increasing nitrogen content.

Fig. 1 Reaction network for HDN of indole on NiMoS/ -Al2O3 catalyst

IND: indole; HIN: indoline; OHI: octahydroindole; OEA: o-ethylanline; EB:

ethylbenzene; OECHA: o-ethylcyclohexylamine; ECHE: ethylcyclohexene;

ECH: ethylcyclohexane

Table 2 Relative products concentration of indole HDN reaction

Relative products concentration wmol/% t

/oC

wN

μg/g ECH ECHE EB OECHA OEA HIN IND

Conversion

xmol/%

HDN

xmol/%

500 11.56 10.57 0.90 0.30 28.70 4.29 43.68 56.31 23.02

1000 2.44 5.29 0.23 0.20 15.09 6.80 69.95 30.05 7.96

1500 1.10 3.59 0.13 0.17 9.90 7.84 77.27 22.73 4.82

300

2000 0.58 2.50 0.09 0.14 7.43 8.47 80.78 19.22 3.18

500 58.03 25.91 0.00 0.00 14.15 0.00 1.92 98.08 83.94

1000 20.46 9.40 5.23 0.00 32.98 0.84 31.09 68.91 35.10

1500 6.65 7.00 1.69 0.00 22.16 1.87 60.62 39.38 15.34

340

2000 3.16 4.67 0.84 0.00 13.26 2.24 75.82 24.18 8.67

XIANG Chun-e et al. / Journal of Fuel Chemistry and Technology, 2008, 36(6): 684 690

Fig. 2 Effects of the DBT content on the conversion and HDN

ratio of indole : indole conversion; : HDN ratio

Fig. 3 Effects of DBT content on the relative product

concentration of indole HDN : ECH; : ECHE; : EB; : OEA; : HIN; : IND

Some metal atoms available for DDS may be blocked simultaneously, because the amount of N-containing intermediates increases and high content of indole in the reactor may also cover the hydrogenation sites through side-on mode. 2.3 Effects of DBT on the HDN of indole

To investigate how sulfides influence HDN reaction, the HDN of indole was studied in the presence of DBT at 300 C, 3.0 MPa, LHSV of 10.0 h–1, H2/Oil (V/V) of 300. The conversion and HDN ratio of indole in the presence and absence of DBT are presented in Fig. 2. DBT has a great influence on the conversion of indole; the conversion increases from 45% in the absence of indole to 68% by adding 1000 μg/g sulfur. However, the HDN ratio is only enhanced by 4% in the presence of 1000 μg/g sulfur, and it decreases slightly with further increase of sulfur content.

The relative products concentration of indole HDN under different sulfur contents is depicted in Fig. 3. The main products are ECH, ECHE, and HIN in the absence of DBT. Hydrogenation of OEA to OECHA was very slow in the presence of large amounts of indole[17], therefore, one can deduce that indole reacted mainly through the path of HIN OHI OECHA ECHE(ECH). With the increase of sulfur content in the feed, the relative concentration of OEA increases considerably, whereas that of HIN decreases slightly, suggesting that the cleavage of the C(sp3)—N bond is promoted by DBT. However, the cleavage of the C(sp2)—N bond is not promoted by DBT as the relative amount of EB, the hydrogenolysis product of OEA, maintains at the same level. In the presence of DBT, the selectivity of OEA is the highest among all products, and OEA may cover active sites and produce a self-inhibitory effect[18]. The relative concentrations of ECH and ECHE declines slightly with the increase of sulfur content indicating that DBT has a negative effect on the

HIN OHI OECHA ECHE(ECH) path. 2.4 Effects of H2S on the HDN of indole

To decipher whether the effects of DBT on HDN of indole

results from DBT itself or from H2S released in the HDS reaction, the HDN of indole with CS2 in the feed was conducted, where H2S was generated in situ from CS2 under H2 stream. The conversion and HDN ratio of indole (wN = 500 μg/g) under different CS2 contents are shown in Fig. 4. CS2 has a positive influence on indole conversion. Low content of CS2 may also enhance the HDN ratio slightly; however, with CS2 content of wN > 1000 μg/g, the HDN ratio decreases slightly. These suggest that the effects of CS2 on HDN of indole are coherent with those of DBT.

Figure 5 shows the relative concentration of products in HDN of indole under different CS2 contents. Comparing with Fig. 3, the tendency of main products suggests again that the effect of CS2 on the HDN of indole is consistent with that of DBT. In conclusion, H2S generated in situ has the same influence as DBT on the HDN of indole; both of them may enhance the rupture of C(sp3)—N bond and improve the conversion of indole, bringing only limited effects on HDN ratio. 2.5 Roles of sulfides in the HDN of indole

The conversion and HDS products of DBT were also

analyzed in the mutual HDS and HDN process. Even the highest sulfur content of DBT can be converted completely and the main product is BP, meaning that the pressure of H2S released in the HDS of DBT was equal to that released from CS2. In situ generated H2S has the same influence as DBT on indole HDN, indicating that the influence of DBT on HDN of indole may be caused by the H2S released in the HDS reaction, rather than the presence of DBT.

XIANG Chun-e et al. / Journal of Fuel Chemistry and Technology, 2008, 36(6): 684 690

Fig. 4 Effects of CS2 content on the conversion and HDN ratio of

indole : conversion; : HDN ratio

Fig. 5 Effect of CS2 content on relative product concentration of

indole HDN : ECH; : ECHE; : EB; : OEA; : HIN; : IND

The result is in good agreement with that of Satterfield:

HDN reaction was promoted by the HDS product H2S rather than thiophene itself[19].

In the absence of sulfides, the HDN of indole is mainly through HIN OHI OECHA ECHE(ECH) path and the conversion of indole is low. DBT and in situ generated H2S promotes the rupture of C(sp3)—N bond of HIN, and hence the chemical equilibrium between indole and HIN shifts toward the right direction. In the presence of sulfides, the main product of indole is OEA. However, the HDN activity of OEA is very low, because the bond energy of C(sp2)—N is high and the hydrogenolysis of C(sp2)—N bond needs highly unsaturated molybdenum atoms to adsorb OEA in side-on mode[20]. These may be the reasons that sulfides cannot promote the HDN ratio of indole.

Two kinds of catalytic sites were postulated on NiMoS catalyst[21]: site I is a vacancy (CUS) in Ni-Mo-S structure responsible for hydrogenation, whereas site II is a Brønsted acid site responsible for hydrogenolysis. As shown in Fig. 6, a sulfur vacancy can be converted to a Brønsted acid site and

an -SH group in a reversible process, which includes the adsorption and dissociation of an H2S molecule. C(sp3)—N bond breaks via elimination or nucleophilic substitution followed by C—S hydrogenolysis on Brønsted acid site and an -SH group[22,23]. Therefore, the increase of Brønsted acid sites has a positive effect on the cleavage of C(sp3)—N bond, which is verified in this study. The increase of relative concentration of OEA with increasing sulfur content indicates that the amount of active sites for hydrogenolysis of C(sp3)—N bond depends on the H2S/H2 ratio.

Fig. 6 Interconversion between coordinatively unsaturated site

(CUS) and Brønsted acid site in the presence of H2S

Fig. 7 Conversion and HDN ratio of indole after CS2 being

removed from reaction system : conversion; : HDN ratio

Fig. 8 Products relative concentration of indole HDN after CS2 being removed from reaction system

: ECH; : ECHE; : OEA

XIANG Chun-e et al. / Journal of Fuel Chemistry and Technology, 2008, 36(6): 684 690

High content of DBT has an negative effect on the HIN OHI OECHA ECHE(ECH) path under current situation. In order to verify this phenomenon, the HDN of indole was studied particularly after 4000 μg/g of CS2 was withdrawn from the feed. 17 samples were collected at an interval of 1 h or 2 h and the results are depicted in Figs. 7 and 8.

Comparing the results in Figs. 7 and 4 (sulfur content 4000 μg/g), it can be seen that the conversion and HDN ratio increase immediately after the withdrawal of CS2, but declines with extending the reaction time. After the reaction lasted for 13 h, the conversion and HDN ratio of indole leveled off. Comparing the relative product concentration of the first sample and that of the sample with the sulfur content of 4000 μg/g, it can be seen that the relative concentrations of ECH and ECHE increased by 22.2% and 3.8%, respectively, whereas the relative concentration of OEA decreased by 36.9%. However, the relative concentration of ECH decreased evidently with the reaction time.

On the surface of catalyst, the active sites for hydrogenation and those for hydrogenolysis in the HDN process can convert each other reversibly (Fig. 6). In the initial stage after the withdrawal of CS2, the CUS occupied by H2S decreases; this favors the hydrogenation reaction. The inhibition of hydrogenation by H2S has also been reported[24,25]. The decrease of conversion and HDN ratio subsequently results from the sulfur atom loss on the catalyst surface. Therefore, it can be concluded that although the formation of active sites for HDN must be in the presence of sulfur atoms, the maintenance of active sites does not require too much sulfur atoms. The relative concentration of ECH increases at first and then declines with the increase of sulfur loss, whereas the relative concentration of ECHE does not change notably. Therefore, it may be concluded that ECHE and ECH are different from OECHA in the formation mechanism, which need to be further verified by more studies. 3 Conclusions

Because of the competitive adsorption between indole and

its HDN intermediates on the active sites, the HDS of DBT is suppressed. The inhibitory effect of indole on HYD route is stronger than on the DDS route, suggesting that the DDS and HYD routes in HDS of DBT may take place at different active sites. In the presence of DBT and CS2, the conversion of indole and the relative concentration of o-ethylanline (OEA) increases. The influences of DBT on HDN of indole may be caused by the H2S released in the HDS reaction. Although sulfur atoms are essential for the formation of active sites for HDN, a small amount of sulfur species is sufficient to maintain the HDN active sites; higher content of

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