Influences of nitrogen species on the hydrodesulfurization reactivity of a gas oil over sulfide catalysts of variable activity

Applied Catalysis A: General 252 (2003) 331–346

Influences of nitrogen species on the hydrodesulfurization reactivity of a gas oil over sulfide catalysts of variable activity Sri Djangkung Sumbogo Murti, Hojung Yang, Ki-Hyouk Choi, Yozo Korai, Isao Mochida∗ Institute of Advanced Material Study, Kyushu University, 6-1 Kasuga Koen, Kasuga, Fukuoka 816-8580, Japan Received 24 September 2002; received in revised form 21 May 2003; accepted 30 May 2003

Abstract The inhibition effects of nitrogen species in gas oil on staged hydrodesulfurization (HDS) over commercial NiMoS/Al2 O3 and CoMoS/Al2 O3 catalysts of variable activity were investigated at 340 ◦ C under initial 50 kg/cm2 H2 using as-distilled and nitrogen-free gas oils. Hydrogen renewal between stages was attempted to reveal an additional inhibition effects of the by-products such as H2 S and NH3 . All species in the gas oils were analyzed by GC-AED before and after hydrotreatment. NiMoS/Al2 O3 and CoMoS/Al2 O3 catalysts showed contrasting activities in HDS and susceptibility to nitrogen species according to their catalytic natures. HDS over NiMoS-2/Al2 O3 of higher activity was largely improved by removal of nitrogen species, while that over NiMoS-1 catalyst of limited activity looked insensitive. Basically, HDS of reactive sulfur species suffered less inhibitions by nitrogen species than that of refractory species. In contrast, two CoMoS catalysts showed significant influences of nitrogen species. One of the most refractory species, 4,6-DMDBT, was effectively hydrodesulfurized by the removal of nitrogen species and renewal of hydrogen in the staged HDS, showing synergy effects due to simultaneous removal of two inhibitors. It must be noted that 107 ppm level of nitrogen species, basically carbazoles, which remained after the first stage HDS, influenced still the HDS of refractory sulfur species which is most expected in the second stage for the deep desulfurization. The acidity of the catalysts is compared to their inhibitions by nitrogen species as well as H2 S and NH3 in the products. Additionally, the reactivity of nitrogen species in HDS is briefly discussed. © 2003 Elsevier B.V. All rights reserved. Keywords: Hydrodesulfurization; Nitrogen inhibition; NiMoS and CoMoS/Al2 O3 catalysts; GC-AED

1. Introduction Nitrogen species have been recognized to inhibit the hydrodesulfurization of gas oil. However, the present desulfurization process has neglected the inhibition exerted by nitrogen species so far in achieving the ∗ Corresponding author. Tel.: +81-92-583-7279; fax: +81-92-583-7798. E-mail address: [email protected] (I. Mochida).

current regulation [1–5]. The recent trend to tighten the regulation very rapidly to 10 ppm S in gas oil requires better ways to achieve such a very low level of sulfur content at least increase of cost. The major task is to hydrodesulfurize effectively and deeply the refractory sulfur species, which have been identified to be 4-methyl (4-M), 4,6-dimethyl (4,6-DM), and 4,6,X-trimethyl (4,6,X-TM) dibenzothiophenes (DBTs). Such species are of very low reactivity apparently because their methyl groups are located as

0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00468-X

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neighbor of sulfur atoms in the center ring [6,7]. In addition, they suffer marked inhibition by H2 S and NH3 produced in the HDS process as well as nitrogen and aromatic species especially at their very low concentrations below 500 ppm. Such inhibitors appear real problem for practical deep desulfurization of gas oil [8–14]. Therefore, it has been confirmed very helpful to remove produced H2 S and NH3 in situ or by two-stage reaction configuration [15]. Removal of nitrogen species, especially basic nitrogen species, was proposed prior to hydrodesulfurization to achieve deep desulfurization easily by the conventional process [16–18]. Nitrogen species found in gas oil are principally non-basic such as carbazoles [16]. LaVopa and Satterfield [19], Nagai and Kabe [20] reported strong inhibition effects of carbazole on HDS of thiophene and DBT HDS. Laredo et al. [21] reported comparable effects of non-basic nitrogen compounds indole and carbazole and basic nitrogen compounds, such as quinoline. It must be noted that the conditions for HDS of model compounds are very different from those for deep desulfurization in industrial hydrodesulfurization. Hence effects of nitrogen species need to be studied with gas oil itself. The concentration of particular sulfur species and remaining nitrogen species must be very concerned in the deep desulfurization. In addition, reactivity of nitrogen species under hydrodesulfurization must be taken into account. The objective of the present study was to evaluate the effects of nitrogen species on the deep HDS of real gas oil. Here, the hydrodesulfurization reactivities of a current gas oil and its nitrogen species-free oil were compared at 340 ◦ C, 50 kg/cm2 (initial H2 pressure) on a series of NiMoS and CoMoS sulfides (1 and 2, respectively) catalysts of different activities in term of gross and molecular-based desulfurization since the effects of nitrogen species must be very subjective to the natures of the catalysts. The reactivity of nitrogen species in the desulfurization was also concerned at every stage reaction since their relative concentration to that of refractory sulfur species may define the extent of their inhibition effects [17]. The effects of nitrogen species removal, roles of the catalysts, and the reaction pathway in the deep hydrodesulfurization of the gas oils and their respective sulfur species are discussed. Such a research is expected to clarify the

feasibility of the present process to meet the coming regulation. 2. Experimental 2.1. Gas oil sample A gas oil and its nitrogen species-free oil used in this study were supplied from Haldor Topsoe Co. Some representative properties of gas oils are summarized in Table 1. Commercially available NiMo/Al2 O3 -1,2 and CoMo/Al2 O3 -1,2 catalysts were provided also from Haldor Topsoe Co. The composition and loading were not uncovered by the catalysts supplier. The catalyst was presulfided before reaction by H2 S (5 vol.%)/H2 flow at 360 ◦ C for 2 h. 2.2. Hydrotreatment Hydrotreatment of gas oils was performed in a 100 ml autoclave-type reactor equipped with a sampling port. Gas oil (10 g) was hydrotreated at 340 ◦ C under 50 kg/m2 H2 (initial hydrogen pressure at room temperature), over 1 g of presulfided catalyst by single and two-stage reaction configurations, as illustrated in Fig. 1. The heating time to the reaction temperature was always 40 min. The speed of agitator was 1000 rpm to prevent the mass-transfer problems. The reaction times for the single stage reaction were 30 and 60 min, after the reaction temperature was achieved, while the two-stage reaction consisted of two 30 min reactions. After the first stage reaction, the reactor was cooled down rapidly to room temperature within 5 min by dipping the reactor into cool water. The second stage was operated after refreshing the reaction atmosphere with fresh H2 at room temperature. The desulfurization taking place before reaching the

Table 1 Composition of original and N-free GOs

Carbon (wt.%) Hydrogen (wt.%) Sulfur (wt.%) Nitrogen (ppm)

Gas oil (GO)

N free gas oil (NF-GO)

85.95 12.30 1.64 300

86.08 12.30 1.60 0

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333

Single Stage 30 min 60 min First Stage

Gas Oil

Second Stage 30 min

30 min 10 g gas oil, 1 g catalyst, 50 kg/cm2 H2 (charging pressure at room temperature)

Fresh 50 Kg/cm2 of H2

Fig. 1. Hydrotreatment configuration.

reaction temperature was found about within 5–10% of total conversion to be neglected in kinetic analysis.

2.4. NH3 -TPD (temperature-programmed desorption)

2.3. Analysis of feed and products

NH3 -TPD was carried out on by a TPD-I-AT (Nippon Bell Co.). Hundred milligrams of fine catalysts were dried at 110 ◦ C for 24 h. The catalysts were treated under high purity He at 400 ◦ C (from 25 to 400 ◦ C in the rate of 10 ◦ C/min) for 30 min before the adsorption of the ammonia. The sample was then cooled to 25 ◦ C in He flow, then exposed to NH3 flow (5% NH3 balanced with He) for 30 min at 25 ◦ C. The NH3 -TPD was run between 25 and 625 ◦ C at 10 ◦ C/min after removing weakly held NH3 by heating the catalyst to 100 ◦ C then cooling to 25 ◦ C in He flow.

Carbon, sulfur and nitrogen species in the gas oil were monitored by gas chromatography with an atomic emission detector (GC-AED). A HewlettPackard gas chromatograph (HP6890) coupled with an atomic emission detector (G2350A) was used in this study. GC-AED has been known as a very powerful technique to analyze the element of interest [23–26]. The detailed conditions for GC-AED are listed in Table 2. Table 2 Condition of gas chromatography and atomic emission detector Injection port temperature AED transfer line/cavity temperature Oven temperature program Column pressure Carrier gas Split ratio Injection volume Data rate Sulfur, monitoring 181 nm Reagent gases Make-up flow Nitrogen, monitoring 288 nm Reagent gases Make-up flow

280 ◦ C 320 ◦ C 40–320 ◦ C at 10/min, holding at 320 ◦ C for 5 min 12 psi He (>99.9999) 50/1 (sulfur), 10/1 (nitrogen) 1 ␮l 2.5 Hz Oxygen: 55 psi, hydrogen: 45 psi 100 ml/min Oxygen: 80 psi, hydrogen: 40 psi, methane: 50 psi 230 ml/min

3. Results 3.1. Analysis of feed gas oils Fig. 2 illustrates carbon, sulfur and nitrogen chromatograms of feed gas oil (GO) and its nitrogen species-free derivative (NF-GO). GO consisted basically of paraffinic hydrocarbons, as shown by a series of spike peaks (Fig. 2a). Sulfur species found in GO were alkylbenzothiophenes (BTs) carrying C2–C5 alkyl chains, dibenzothiophene (DBT), and alkylated-dibenzothiophenes. Considerable amounts of 4-DBT, 4,6-DMDBT and 4,6,X-TMDBT were found in the oil. These refractory species have been reported to make deep desulfurization of the gas oil difficult to meet the strict regulation. Most of the nitrogen species in the oil were non-basic species such as carbazole (Cz), monomethylated carbazoles (C1–Cz) and dimethylated carbazoles (C2–Cz). No hump on

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C14 C15

Carbon

1500

C13

C16

C17

C12

C18

C11

1000

C10 500

C19 C20 C21 C22

AED responds [-]

0

4-MDBT

C4-BT C3-BT

750

C2-BT

500

4,6-DMDBT DBT

Sulfur

C3-DBT

C5-BT

250

C2-DBT

0

1-Cz

100

Nitrogen

1,8-Cz C3-Cz

Cz

50

0

C2-Cz 5

10

15

20

25

30

25

30

Retention time [min]

(a) 2000

C13

Carbon

1500

C14 C15 C16

C12

1000

C11 C10

500

C17

C18 C19 C20 C21 C22

AED responds [-]

0

4-MDBT C4-BT C3-BT 4,6-DMDBT DBT C2-BT C3-DBT C5-BT

750 500

Sulfur

250 0

C2-DBT

100

Nitrogen 50

0

5

(b)

10

15

20

Retention time [min]

Fig. 2. Carbon, sulfur and nitrogen chromatograms of gas oil measured by GC-AED. FiCn: normal paraffin having n carbons. Cn-BT and Cn-DBT: BT and DBT having Cn alkyl substituents: (a) original gas oil (GO) and (b) nitrogen species-free gas oil (NF-GO).

the nitrogen chromatogram was observed, indicating little or no basic nitrogen species in the GO [24]. Carbon and sulfur chromatograms of NF-GO (Fig. 2b) were almost the same as those of the mother GO.

The sulfur contents of GO and NF-GO were also almost the same at 16,400 and 16,025 ppm S, respectively. No nitrogen peaks was observed in NF-GO, meaning the complete removal of nitrogen species from GO.

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3.2. Reactivities of GO and its N-free oil over NiMoS-1 and 2 catalysts Fig. 3 compares the reactivities of GO and NF-GO over NiMoS-1 catalyst. The reaction of 30 min removed most of BTs and DBT, leaving alkylated DBT such as 4-MDBT, 4,6-DMDBT, unidentified DM- and TM-DBTs. The sulfur contents of the desulfurized GO and NF-GO oils were 2957 and 2231 ppm S, respectively, at this stage. The rather small difference due to the nitrogen removal was remarked over this particular catalyst as shown in Fig. 3. A longer reaction time of 60 min reduced slightly the sulfur contents of the both oils to 2189 and 1991 ppm S, respectively. Two-stage HDS with renewal of hydrogen atmosphere between the first and second stages enhanced the desulfurization, leaving only 4,6-DMDBT and 4,6, X-TMDBT in the product oils. The sulfur contents of the oils decreased to 830 and 540 ppm S, respectively, as shown in Fig. 3, suggesting strong inhibition of produced H2 S and significant increase of reactivity on NiMoS-1 by removal of nitrogen species at this stage. Fig. 4 compares the reactivity of GO and NF-GO over NiMoS-2 of higher activity. After 30 min of hydrotreatment, the sulfur contents were 2245 and 1079 ppm S, as shown in Fig. 4b. Prolonging reaction time to 60 min enhanced significantly the HDS of both oils, decreasing the sulfur contents to 901 and 260 ppm S, respectively. Marked influences of nitrogen species were observed at this stage over this NiMo catalyst. The second stage with fresh H2 further enhanced the HDS of both oils, the sulfur contents being reduced to 349 and 129 ppm S, respectively. It is definite that nitrogen species removal enhanced very markedly the desulfurization over this particular NiMoS catalyst at every stage of the reaction. The effects became more significant with the progress of HDS. Favorable effects of H2 S removal for the second stage were also marked on this catalyst. 3.3. Reactivity of respective sulfur species in GO and NF-GO over NiMoS-1 and 2 catalysts Considerable amounts of refractory sulfur species such DBT and alkylated DBTs contained in the starting GO and NF-GO still remained in HDS product after 60 min of reaction time for both single and two-stage hydrotreatment. Fig. 5a and b illus-

335

trates quantitatively reactivities of DBT, 4-MDBT and 4,6-DMDBT in the single and two-stage hydrotreatment over NiMoS-1 and NiMoS-2 catalysts, respectively. The single stage of 60 min HDS left 7 and 12 ppm S of DBT over NiMoS-1 and -2 catalysts, respectively, while the two-stage HDS completely removed DBT over both catalysts. 4-MDBT was much less reactive than DBT. The single stage removed 47 and 70% of initial 4-MDBT, whereas the two-stage process left 14 and 10% over NiMoS-1 and -2 catalysts, respectively. Nitrogen removal enhanced the HDS markedly, leaving 110 and 19 ppm S of 4-MDBT by 60 min single stage over the catalysts. The second stage reaction left only 13 and 5 ppm S of 4-MDBT, in desulfurized NF-GO. 4,6-DMDBT was most unreactive. Only 20% (leaving 94 ppm S) and 50% (54 ppm S) of initial content were removed by 60 min single stage. The two-stage process removed 40% (67 ppm S) and 70% (37 ppm S), respectively, over NiMoS-1 and -2 catalysts. N-removal enhanced HDS, removing 30 and 80% of the species by 60 min single stage, and 70% (36 ppm S) and 85% (17 ppm S) over the catalyst, respectively, by the two-stage reaction. It must be noted that removal of nitrogen species is more effective than gas refreshment by H2 before the second stage in the HDS of 4-MDBT and 4,6-DMDBT over NiMoS-2, suggesting larger inhibition on this particular catalyst by the remaining nitrogen species in the second stage. 4,6,X-TMDBTs were another refractory sulfur species, as much as 129 and 104 ppm S in GO were left by HDS of any possible configuration. After the second stage 52 and 39 ppm S in NF-GO were left over NiMoS-1 and -2 catalysts, respectively. Removal of nitrogen species was also very effective to reduce these refractory species. 3.4. HDS reactivity of GO and its N-free oil over CoMoS-1 and -2 catalysts Fig. 6 compares the reactivities of GO and NF-GO over CoMoS-1 catalyst. HDS over CoMoS-1 catalyst proceeded more rapidly than those over NiMoS catalysts in the single stage. The 30 and 60 min of the single stage decreased the sulfur content to 1654 and 680 ppm S in GO, respectively, as shown in Fig. 6. Renewal of reaction atmosphere by fresh hydrogen in two-stage reaction enhanced HDS, reducing sulfur

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250 GO Feed 16425 ppm

C4-BT C2-BT C3-BT

4-MDBT DBT 4,6-DMDBT

C5-BT

C3-DBT

0

AED Responds [-]

C2-DBT

250 30 min 2957 ppm 0 250 60 min 2189 ppm 0 250 2-stage 830 ppm 0 5

10

15

20

25

30

25

30

Retention time [min]

(a)

C4-BT C3-BT C2-BT

250 NF-GO 16042 ppm

4-MDBT 4,6-DMDBT

DBT

C3-DBT

C5-BT

0 C2-DBT

AED Responds [-]

250 30 min 2231

0 250 60 min 1991 ppm

0 250 2-stage 540 ppm

0 5

(b)

10

15

20

Retention time [min]

Fig. 3. Hydrodesulfurization over NiMoS-1/Al2 O3 catalyst of several reaction configurations: (a) GO and (b) NF-GO.

S.D. Sumbogo Murti et al. / Applied Catalysis A: General 252 (2003) 331–346 C4-BT C3-BT C2-BT

600

4-MDBT DBT 4,6-DMDBT C3-DBT

C5-BT

GO Feed 16425 ppm

300 0

AED Responds [-]

337

C2-DBT

600 30 min 2245 ppm

300 0 600

60 min 901 ppm

300 0 600

2-stage 349 ppm

300 0 5

10

15

20

25

30

25

30

Retention time [min]

(a)

C4-BT C3-BT C2-BT

600

C3-DBT

C5-BT

NFGO Feed 16042 ppm

300

4-MDBT DBT 4,6-DMDBT

0

C2-DBT

AED Responds [-]

600 30 min 1079 ppm

300 0 600

60 min 260 ppm

300 0 600

2-stage 129 ppm

300 0 5

(b)

10

15

20

Retention time [min]

Fig. 4. Hydrodesulfurization over NiMoS-2/Al2 O3 catalyst of several reaction configurations: (a) GO and (b) NF-GO.

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DBT(ppm)

4-MDBT(ppm)

4,6-DMDBT (ppm)

Feed

30 min

60 min

Feed

30 min

60 min

Feed

30 min

60 min

205

18

7

234

163

125

114

107

94

2 stage N Removal

2 stage

0

15

2 stage

N Removal

32

140

12

N Removal

110

97

2 stage

2 stage

67 76 2 stage

13

0

36

(a)

DBT(ppm)

4-MDBT (ppm)

4,6-DMDBT (ppm)

Feed

30 min

60 min

Feed

30 min

60 min

Feed

30 min

60 min

205

54

12

234

134

70

114

74

54

2 stage

2 stage N Removal

0

20

N Removal

2

2 stage 23

83

19

N Removal 62

2 stage

2 stage 0

37 24 2 stage

5

17

(b) Fig. 5. Reactivities of sulfur species over NiMoS/Al2 O3 catalysts: (a) NiMoS-1 and (b) NiMoS-2.

level to 401 ppm S. The effects of nitrogen removal were very significant, the sulfur level achieving 320 and 220 ppm S by 60 min single stage and two-stage reactions, respectively, over the CoMoS-1 catalysts. Fig. 7 compares HDS of two oils over CoMoS-2 catalyst of higher activity. The single stage reaction of 60 min removed most of BTs and DBTs in GO, leaving refractory species such as 4-MDBT, 4,6-DMDBT and 4,6,X-TMDBTs. The sulfur level achieved 421 ppm S by single stage and 333 ppm S by two stage, respectively. NF-GO was more reactive, achieving 194 ppm S by single stage and 161 ppm S by two-stage reaction.

figuration. Only one exception was observed with NFGO in two-stage HDS. The conversions of 4-MDBT and 4,6-DMDBT were 96 and 97% (leaving 9 and 7 ppm S, respectively) and 71 and 80% (leaving 33 and 23 ppm S, respectively) over CoMoS-1 and -2, respectively. The activity of CoMoS catalysts was less than that of NiMoS catalysts for the HDS of 4,6-DMDBT. NiMoS-2 catalyst appears more active for HDS of refractory sulfur species under H2 S-free conditions.

3.5. Reactivity of respective sulfur species in GO and NF-GO over CoMoS-1 and -2 catalysts

The significant hydrodenitrogenation (HDN) progressed concurrently with HDS. The total content of nitrogen species decreased to 60% (180 ppm N) of initial nitrogen content by 30 min single stage reaction over NiMoS-1 catalyst. It reached to 47% (141 ppm N) by an additional 30 min reaction of single stage. Two-stage hydrotreatment by renewal of hydrogen atmosphere between the stages enhanced the HDN, the nitrogen level achieving 33% (24 ppm N).

Fig. 8 summarizes the reactivities of sulfur species over CoMoS catalysts. All DBT were clearly removed by 60 min of single stage over both CoMoS-1 and -2 catalysts. More 4-MDBT and 4,6-DMDBT were also removed over CoMoS catalysts than over NiMoS catalysts at every stage of the present hydrotreatment con-

3.6. Reactivities of nitrogen species over NiMoS and CoMoS catalysts

S.D. Sumbogo Murti et al. / Applied Catalysis A: General 252 (2003) 331–346

250 GO Origin 16425 ppm

339

4MDBT C4-BT DBT C3-BT 46DMDBT C2-BT C5-BT C3-DBT

0

C2-DBT

AED Responds [-]

250 30 min 1654 ppm

250 60 min 680 ppm 0 250 2-stage 401 ppm 0 5

10

15

20

25

30

Retention time [min]

(a)

4MDBT

250 NF-GO 16042 ppm

C3-BT C2-BT C4-BT

DBT

C5-BT

4,6-DMDBT C3-DBT

0

C2-DBT

AED Responds [-]

250 30 min 1298 ppm 0 250 60 min 320 ppm 0 250 2-stage 220 ppm 0 5

(b)

10

15

20

25

Retention time [min] Fig. 6. Hydrodesulfurization over CoMoS-1/Al2 O3 catalyst: (a) GO and (b) NF-GO.

30

340

S.D. Sumbogo Murti et al. / Applied Catalysis A: General 252 (2003) 331–346 4-MDBT C4-BT DBT 4,6-MDBT C3-BT C2-BT C5-BT C3-DBT

600 GO Feed 16425 ppm

300 0

C2-DBT

AED Responds [-]

600 30 min 1177 ppm

300 0 600

60 min 421 ppm

300 0 600

2-stage 333 ppm

300 0 5

10

15

20

25

30

25

30

Retention time [min]

(a)

4-MDBT 4,6-DMDBT DBT C3-DBT C5-BT

C4-BT C3-BT C2-BT

600 NFGO Feed 16042 ppm

300 0

C2-DBT

AED Responds [-]

600 30 min 686 ppm

300 0 600

60 min 194 ppm

300 0 600

2-stage 161 ppm

300 0 5

(b)

10

15

20

Retention time [min]

Fig. 7. Hydrodesulfurization over CoMoS-2/Al2 O3 catalyst: (a) GO and (b) NF-GO.

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341

Fig. 8. Reactivities of sulfur species over CoMoS/Al2 O3 catalysts: (a) CoMoS-1 and (b) CoMoS-2.

HDN of GO over NiMoS-2 proceeded faster than that over NiMoS-1. The nitrogen species decreased to 107 ppm N (36%) and 48 ppm N (16%) of original content of original content by 30 and 60 min of the single stage, respectively. Further enhancement of HDN was achieved by two-stage hydrotreatment, the nitrogen level achieving 24 ppm N (8% of original content). Hydrotreatment over CoMoS catalysts removed less nitrogen species than that over NiMoS catalysts. Total nitrogen content decreased to 72% (78 ppm N) and 36% (109 ppm N) by 60 min single stage over CoMoS-1 and 2, respectively. Two-stage enhanced denitrogenation over both catalysts, reducing nitrogen level to 60 and 75 ppm N, respectively. Fig. 9 illustrates N chromatograms of GO after stages of HDS over NiMoS-2 and CoMoS-2 catalysts. The present study identified 29 carbazoles, mono-, di- and trimethyl-carbazoles contained in GO feed as shown in Fig. 10. Their contents are summarized in Table 3. Their reactivity order appeared to be basically the same over both catalysts. Carbazole being the most reactive species, was denitrogenated completely by 60 min of the single stage. Substituted methyl groups on the carbazole retarded the HDN reactivity. A par-

ticular trimethyl-carbazole (1,4,8-Cz, 1,5,7-Cz) were very refractory. Reactivities of a series of methyl-carbazoles are summarized as follows: • Number of methyl groups: Cz > C1–Cz > C2–Cz > C3–Cz > C4–Cz • Among monomethyl-carbazoles: 3-Cz > 4-Cz > 2-Cz > 1-Cz • Among dimethyl-carbazoles: 2,3-Cz > 1,4-Cz, 1,5-Cz > 1,8-Cz • Among trimethyl-carbazoles: 1,4,5-; 2,3,5-; 2,3,6-Cz > 3,4,6-Cz > 1,5,7-Cz > 1,4,8-Cz Some particular locations of methyl groups retarded more than other locations although the influences were much smaller than those observed in the HDS of alkylated DBTs. 1 and 8 positions appear lower the reactivity most significantly. 1 and 4 or 5 positions

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Fig. 9. Hydrodenitrogenation of GO over NiMo/Al2 O3 and CoMo/Al2 O3 catalysts.

around 200 ◦ C, and then decreased rather slowly to be zero at around 600 ◦ C. NiMo catalysts showed the higher peak maxima around 200 ◦ C, while the desorption above 380 ◦ C or more markedly above 400 ◦ C was smaller than that of CoMo catalysts. NiMo catalysts have more acidic site than CoMo catalyst. But their acidic strength is found to be lower than that of CoMo catalysts. The amount of rather strong acidic site, which could be estimated by NH3 desorption between 400 and 600 ◦ C, are in the order of CoMo-2 >

appears also retard markedly HDN. Another low reactivity of 3,4,6-Cz suggests additional retardation by substituted methyl groups at 3 and 6 positions of carbazole. 3.7. Acidity of catalysts Fig. 11 illustrates the NH3 -TPD profile of sulfide catalysts used in the present study. NH3 desorption started from around 80 ◦ C, reached its maximum at 2

4

10 5

13

9

3

1

8 6

7

15

16

18

12 11

19 14

21 17

19

20

21

22

23

20 22

25 26 27 24 28

23

Retention time (min) Fig. 10. Nitrogen chromatogram of GO.

24

29

25

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343

Table 3 Nitrogen species in GO and their reactivities over NiMoS-2 and CoMoS-2 catalysts (ppm) Compounds

Peak no.

Carbazole (Cz) 1-Cz 3-Cz 2-Cz 4-Cz 1,8-Cz 1,3-Cz 1,6-Cz 1,7-Cz 1,4- and 1,5-Cz 3,6-Cz 2,6-, 3,5-, 2,7-Cz 2,4- and 1,2-Cz 2,5-Cz 2,3-Cz 1,4,8-Cz 3,4-Cz 1,3,5-Cz 1,5,7-Cz 2,4,6-Cz 1,3,4- and 2,4,7-Cz 1,4,5-, 2,3,5-, 2,3,6-Cz 3,4,6-Cz C3–Cz C4–Cz C4–Cz C4–Cz C4–Cz C4–Cz

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Total

Feed

NiMoS-2 30 min

60 min

1.2 4.1 1.5 2.2 2.9 4.8 2.8 2.7 3.6 8.0 1.9 2.8 3.0 3.1 4.2 8.2 2.0 7.7 7.8 2.1 5.1 4.1 4.6 2.7 3.0 2.7 3.7 2.6 1.9

0.7 2.0 0.5 1.2 1.1 2.0 0.9 1.0 1.3 3.2 0.9 1.0 1.5 1.1 1.9 3.7 1.1 3.1 3.3 1.1 1.9 1.9 2.3 1.5 2.0 1.4 1.8 1.2 1.4

11.2 18.5 8.9 12.9 15.2 10.3 7.8 8.2 11.2 19.1 5.9 9.2 11.5 9.0 11.5 18.0 5.8 19.9 15.0 6.0 12.8 10.1 8.5 5.7 6.8 5.6 5.8 5.5 3.6 300

CoMoS-2

107

Two-stage 0 1.0 0 0 0 1.5 0.5 0.3 0.7 1.1 0.5 0 0.5 0.5 1.1 2.1 0.6 2.0 2.0 0.7 1.2 1.2 1.6 1.0 1.1 0.9 0.6 0.7 0.8

48

24

30 min

60 min

3.4 8.0 3.9 4.7 6.0 6.8 4.0 4.1 5.9 11.3 3.0 4.0 5.3 4.8 5.8 10.5 2.8 10.4 10.3 3.0 7.4 5.6 5.7 3.5 3.8 3.3 3.5 3.3 2.6

1.2 4.1 1.9 2.2 2.9 4.8 2.8 2.7 3.6 8.2 1.9 2.8 3.0 3.1 4.2 8.6 2.0 7.7 8.2 2.1 5.1 4.1 5.2 2.8 3.4 2.6 2.8 2.9 2.3

157

109

Two-stage 0.9 2.6 1.2 1.7 1.3 3.4 1.4 1.3 1.7 4.5 1.3 1.7 1.9 1.5 3.0 6.4 1.4 4.7 6.1 1.3 3.1 3.1 4.4 2.5 2.7 2.3 2.6 2.6 2.1 75

Table 4 Some properties of catalysts used

areaa

(m2 /g)

Specific surface Specific pore volumea (m3 /g) Acidityb (␮mol/gcat ) (total) Acidityc (␮mol/gcat ) (>400 ◦ C)

NiMo-1

NiMo-2

CoMo-1

CoMo-2

165 0.34 351 20

175 0.36 531 39

139 0.30 327 44

191 0.41 415 63

a

Measured by N2 physisorption. Measured by NH3 -TPD, desorbed amount of NH3 at 25–625 ◦ C in TPD. c Measured by NH -TPD, desorbed amount of NH at 400–625 ◦ C in TPD. 3 3 b

CoMo-1 > NiMo-2  NiMo-1. Total acidity to reflect the total amount of desorbed NH3 was in order of NiMo-2 > CoMo-2 > NiMo-1 > CoMo-1 as summarized in Table 4.

4. Discussion The present study attempted to clarify the inhibition effects of nitrogen species in the HDS of GO by

S.D. Sumbogo Murti et al. / Applied Catalysis A: General 252 (2003) 331–346

QMS Intensity at amu 16(A)

344

3.5x10

-6

3.0x10

-6

2.5x10

-6

2.0x10

-6

1.5x10

-6

1.0x10

-6

5.0x10

-7

NiMo-1 NiMo-2 CoMo-1 CoMo-2

0.0 0

100

200

300

400

500

600

700

o

Temperature( C) Fig. 11. NH3 -TPD spectra of sulfide catalysts.

comparing the reactivities of GO and that of NF-GO over NiMoS and CoMoS catalysts. The inhibition effects of nitrogen species were found to vary their degree, reflecting some other factors such as other inhibitors, catalyst natures and the inhibitor/substrate ratio, which vary with the progress of reaction. The boundary conditions framed by the autoclave batch reactor allow the participation of other inhibitors by keeping all products and unreacted substrates in the same vessel, unless the atmosphere is renewed as attempted by the staged reaction in the present study. Inhibition roles of nitrogen species varied markedly according to the kinds of catalysts. The effects of their removal are more enhanced than those of hydrogen renewal on the more active catalysts as observed on NiMoS-2 and CoMoS-2 catalysts. Desulfurization of reactive sulfur species accumulates H2 S in the autoclave, which inhibits the desulfurization of less reactive sulfur species. Strong inhibition by H2 S must be taken into account when inhibition by nitrogen species is discussed. CoMoS/Al2 O3 suffers less inhibition by H2 S than NiMoS/Al2 O3 ; the former exhibits a significantly higher activity in the HDS of GO in single stage reaction. The activity order of the catalyst for the HDS of GO in this study can be summarized as CoMoS-2 > CoMoS-1 > NiMoS-2 > NiMoS-1, for both 30 and 60 min reaction times in the single stage.

The inhibition effects of nitrogen species are competitive with those of H2 S for HDS. Nitrogen removal is effective in HDS of GO in both first and second stages, but become more effective by atmosphere renewal in the second stage or by longer reaction time. The order of catalysts activity is CoMoS-2 (single stage of nitrogen-free GO 194 ppm S < renewal 333 ppm S) > NiMoS-2 (260 < 349 ppm S) > CoMoS-1 (320 < 410 ppm S) > NiMoS-1 (1991 > 830 ppm S) of the lowest activity. Nitrogen removal is more effective than the renewal with more active catalysts. In contrast, NiMoS-1 showed a different trend, where the H2 change is more effective than N-removal. Combination of nitrogen removal and H2 change in the second stage is very effective over all catalysts. The highest activity with NiMoS-2 is remarked under such conditions; the activity order of the catalyst is NiMoS-2 (129 ppm S) > CoMoS-2 (161 ppm S) > CoMoS-1 (220 ppm S) > NiMoS-1 (540 ppm S). HDS of refractory sulfur species was noted to proceed more rapidly over NiMoS than over CoMoS under no inhibition by either H2 S or nitrogen species, as observed with the model reactions [2]. The inhibitions of nitrogen species over the sulfide catalysts may reflect the acidic natures of the catalysts. The acidity measured by NH3 -TPD (Fig. 11 and Table 4) appeared to correspond to the extent of inhibition by nitrogen species. CoMo catalysts which

S.D. Sumbogo Murti et al. / Applied Catalysis A: General 252 (2003) 331–346

carried more site of the strong acid suffered more HDS inhibition by nitrogen species. NiMo-1 without strong acid sites suffered a little inhibition while NiMo-2 with strong acid site suffered significant inhibition by nitrogen species. The acidity has been recognized to accelerate the HDS through enhancing H2 S desorption, hydrogenation and isomerization of sulfur species [22]. NiMo-2 showed certainly much higher activity and less inhibition with H2 S than NiMo-1. Both CoMo catalysts showed high resistivity against H2 S. Two-stage HDS eliminates NH3 as well as H2 S, which are inhibitor for HDS while they must show contrast behaviors with the acidity of the catalyst. Hence, the most appropriate acidity must be designed to the catalysts by selecting forms of sulfide, support and additive, although the catalyst preparation procedure has not been disclosed from the catalyst manufacturers. All catalysts also removed nitrogen species along with HDS, as shown in Fig. 11. NiMoS-2 catalyst showed the largest activity among the catalysts used at the same level of desulfurization. Such high concentration of H2 S did not kill the hydrogenation activity of NiMoS-2 under the present conditions. Hence, basic nitrogen species are removed first and carbazoles are slowly eliminated under the present conditions.

About 100–180 ppm N nitrogen species (basically carbazoles) were left in the GO after the 30 min of the first stage. This level of the nitrogen species is found to inhibit more markedly the desulfurization of 700–3000 ppm S level of principally refractory sulfur species in the second stage. Refractory sulfur species of lower concentration may suffer more inhibition by lower concentration of nitrogen species. Less adsorption ability of refractory sulfur species is suggested to cause more inhibition of inhibitors, which compete for the active sites of sulfide catalysts. The reactivity order of carbazoles could be also confirmed in the present study, although a number of nitrogen species were not fully identified yet. Very refractory nitrogen species were found in the highest boiling range. The influence of methyl group substitution in carbazoles in HDN as well as HDS has been discussed (Fig. 12). Their steric hindrance is the principal factor, although their electronic roles cannot be neglected. The influences of methyl groups on the HDN reactivity of carbazoles observed in the present study are much smaller than those on HDS of dibenzothiophenes, although methyl groups on the neighbor carbons appear to inhibit the HDN of carbazoles (Cz). Thus, methyl groups on 1 and 8 positions in the Cz rings hinder more than those at other locations. In

1.E+05 CoMoS-1 CoMoS-2 NiMoS-1 NiMoS-2 S content (ppm)

1.E+04

1.E+03

1.E+02 0

50

1 00

345

150 200 N content (ppm)

250

Fig. 12. Comparison of HDS and HDN reactivities.

300

350

346

S.D. Sumbogo Murti et al. / Applied Catalysis A: General 252 (2003) 331–346

addition, methyl groups on 1 and 4 or 5 positions appear also to retard the HDN reaction. The active site approaches close to N atom at the center of Cz rings, being sterically interfered with the methyl groups on the 1 and 8 positions as observed with 4,6-DMDBT, although the higher polarity of heterocyclic nitrogen atom may suffer less hindrance from the methyl groups. The steric hindrance may occur at the activated adsorption of N atom or during substitution of C–N bond with the active metal of the catalyst. The methyl groups at 4 and 5 positions may un-stabilize an intermediate coordinated to the active site to retard HDN. Additionally, co-presence of methyl groups at both 3 and 6 positions further retards the HDN. More details of the HDN scheme including the structure of the intermediate coordinated to the active site must be clarified. References [1] M.J. Girgis, B.C. Gates, Ind. Eng. Chem. Res. 30 (1991) 2021. [2] D.D. Whitehurst, T. Isoda, I. Mochida, Adv. Catal. 42 (1998) 345. [3] R.M. Laine, Catal. Rev.-Sci. Eng. 25 (1983) 459. [4] T.C. Ho, Catal. Rev.-Sci. Eng. 30 (1988) 117. [5] E. Furimsky, F.E. Massoth, Catal. Today 52 (1999) 381. [6] X. Ma, K. Sakanishi, I. Mochida, Ind. Eng. Chem. Res. 33 (1994) 218. [7] X. Ma, K. Sakanishi, I. Mochida, Ind. Eng. Chem. Res. 35 (1996) 2487.

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