CNT catalyst in hydro-cracking of Gudao vacuum residue

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 35, Issue 4, August 2007 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2007, 35(4), 407−411

RESEARCH PAPER

Application of Co-Mo/CNT catalyst in hydro-cracking of Gudao vacuum residue LI Chuan1,*, SHI Bin1, CUI Min2, SHANG Hong-yan3, QUE Guo-he1 1

State Key Laboratory of Heavy Oil Processing, College of Chemistry and Chemical Engineering, China University of Petroleum (East

China), Dongying 257061, China 2

College of Chemistry & Chemical Engineering, China University of Petroleum (East China), Dongying 257061, China

3

Key Laboratory of Catalysis, CNPC, Dongying 257061, China

Abstract:

Carbon nanotube-supported Co-Mo catalysts with different Co/Mo atomic ratio were prepared by pore volume

impregnation. These catalysts were used in the hydro-cracking reaction of Gudao vacuum residue, and the catalytic properties were compared with γ-Al2O3-supported Co-Mo catalysts under the same reaction conditions. It was found that the catalytic properties of Co-Mo/carbon nanotube (CNT) catalysts are inferior to Co-Mo/γ-Al2O3. However, the Co/Mo atomic ratio has great effect on the catalytic activity of Co-Mo/CNT; the Co-Mo/CNT catalyst with Co/Mo atomic ratio of 0.5 has the best catalytic properties, whereas for Co-Mo/γ-Al2O3 catalyst the best Co/Mo atomic ratio is 0.35. Key Words: carbon nanotube-supported Co-Mo catalysts; γ-Al2O3-supported Co-Mo catalysts; hydro-cracking of vacuum residue; Co/Mo atomic ratio; catalytic properties

It is well-known that Co-Mo catalysts, typical for hydrodesulfurization (HDS), are widely used in petroleum processing. γ-Al2O3 is a kind of common support, which has good characteristics such as high mechanical strength and rich pore structures. There are currently many studies on γ-Al2O3 support[1−3]. However, metal catalyst supported on γ-Al2O3 has limitations in HDS and anti-coking performance. In order to improve this state, new catalyst supports were being developed. In recent years, oxides of metal used as HDS catalysts such as Mo, W, Co or Ni supported on activated carbon have received much attention because high HDS activities have been reported. In addition, carbons have better features than γ-Al2O3 such as reduced coking activity[4]. Carbon nanotube (CNT, namely buckytube) is a material with especial nano-meter structure and pore diameter. With the discovery of CNT, much attention was focused on it[5]. Planeix[6] prepared Ru/carbon nanotube catalyst first, and studied the composition of products for cinnamyl alcohol in the hydrogenation of cinnamaldehyde. It appears that Ru/carbon nanotube catalyst is better than Ru/Al2O3 or Ru/carbon catalyst in activity, selectivity and stability. Zhang[7] investigated propene hydroformylation catalyzed by Rh-phosphine complex catalysts supported by carbon

nanotubes, and compared with that supported by SiO2 (a silica gel), TDX-601 (a carbon molecular sieve), AC (an active carbon), and GDX-102 (a polymer carrier). The results showed that the carbon nanotubes-supported Rh-phosphine complex catalysts displayed not only high activity of propene conversion, but also excellent region selectivity to the product. In 1994, the catalysts of Fe-Cu and Fe particles supporting on carbon nanotubes were prepared by Rodriguez et al.[8] and heated in hydrocarbon environments. The catalytic activity of these catalysts, for the conversion of hydrocarbons, was considerably higher than those which supported on either active carbon or γ-Al2O3 treated under the same conditions. Moreover, these catalysts had better heat stability. The HDS activity of several Co-Mo catalysts was investigated using dibenzothiophene (DBT) as model compound by Shang[9]. The results indicated that the Co-Mo/CNT catalysts were extremely active than Co-Mo/γ-Al2O3 in HDS of DBT. Up to now, CNT has been applied in the field of physics, chemistry, material science, and catalysis. However, only a few studies have focused on the application of CNT in heavy oil processing. In this study, some application prospect of CNT used in heavy oil hydro-cracking was explored. The

Received: 2007-01-15; Revised: 2007-04-11 * Corresponding author. Tel: +86-546-8399373; Fax: +86-546-8396054; E-mail: [email protected] Foundation item: Supported by the Innovation Fund of China University of Petroleum (East China) (b2006-10). Copyright©2007, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

LI Chuan et al. / Journal of Fuel Chemistry and Technology, 2007, 35(4): 407−411

reaction activities of Co-Mo/CNT catalysts were evaluated in an autoclave and compared with that of the conventional Co-Mo/γ-Al2O3 under the same reaction condition.

1 1.1

Table 2 List of catalysts and their properties Catalysts

Properties

Co-Mo/γ-Al2O3*

Co/Mo atomic ratio = 0.20, 0.35, 0.50, 0.70

Experimental Materials

Co-Mo/CNT*

Co/Mo atomic ratio = 0.20, 0.35, 0.50, 0.70, 1.0

Mo/CNT*

content of MoO3 = 10.00%, 11.10%, 12.60%, 13.60%

Co/CNT

content of CoO = 10.00%

* content of MoO3 in Co-Mo/γ-Al2O3 or Co-Mo/CNT was 10 wt%

Characterizations of Gudao vacuum residue (GDVR) are included in Table 1. Carbon nanotube was obtained from Tsinghua University.

Table 3 Physical properties of various catalysts Co/Mo Sample

Table 1 Properties of Gudao vacuum residue Property

atomic

MoO3

area

ratio

w/%

A / m2⋅g−1

GDVR

Density (20°C) ρ / g⋅cm

−3

Viscosity (100°C) µ / mm²⋅s−1

CNT

0.990 2057

Average pore diameter d / nm

Pore volume v / cm3⋅g−1

–

–

193.6

16.7

0.83

0.20

10.00

181.2

15.1

0.70

0.35

10.00

179.9

14.7

0.67

0.50

10.00

178.8

14.2

0.65

0.70

10.00

176.3

13.5

0.58

1.0

10.00

173.2

12.6

0.50

Carbon residue w / %

15.67

Sulfur w / %

2.30

Nitrogen w / %

1.15

H/C atomic ratio

1.57

10.00

183.1

15.7

0.72

Condensation point t / °C

8.0

11.10

181.4

15.5

0.71

wNi / µg⋅g−1

46.4

12.60

179.0

14.5

0.65

13.60

177.2

13.8

0.61

Co-Mo/CNT

Mo/CNT

–

−1

8.8

−1

14.9

Co/CNT*

–

–

185.4

15.8

0.75

82.6

γ-Al2O3

–

–

281.5

8.8

0.79

0.20

10.00

209.4

7.1

0.51

0.35

10.00

205.7

6.5

0.45

0.50

10.00

200.2

5.8

0.40

0.70

10.00

197.5

5.1

0.36

wV / µg⋅g

wFe / µg⋅g

−1

wCa / µg⋅g

Saturates w / %

1.2

Content of BET surface

17.8

Aromatics w / %

31.4

Resins w / %

48.9

C7-asphaltene w / %

1.9

Catalysts preparation and characterization

A series of mono- and bimetallic Co-Mo catalysts supporting on carbon nanotube or γ-Al2O3 (140 screen meshes) were prepared by wetness impregnation. The bimetallic samples, prepared by co-impregnation of aqueous ammonium heptamolybdate and Co-nitrate solutions, had Co/Mo atomic ratios of 0.2, 0.35, 0.5, 0.7, and 1.0. In this series, the amount of MoO3 was kept constant for all catalysts at 10 wt%, while the amount of Co was changed accordingly. The monometallic catalysts were prepared with loadings of 10 wt% Co and 10 wt%, 11 wt%, 12.6 wt%, and 13.6 wt% Mo, respectively. After impregnation, the solids were dried 24 h at 120°C and then cooled to room temperature. Once cooled, the samples were calcined for 8 h at 500°C in N2 flow. Properties of the catalysts are included in Table 2. The results of BET surface area, average pore diameter and pore volume for the prepared catalysts are given in Table 3.

Co-Mo/γ-Al2O3

* content of CoO in Co/CNT was 10.00 wt%

1.3

Reaction and analysis

GDVR (240 g), catalyst with granularity of 140 meshes and mass concentration of catalyst from 0.075% to 0.5% and sulfur (0.2 g) were put in 0.5 L autoclave, respectively. Air was then expelled from the autoclave using 2 MPa H2 to wash autoclave thrice and the autoclave was weighed (w1). The initial H2 pressure in autoclave was 8 MPa and was heated at 7°C⋅min−1 up to 300°C to carry out the sulfuration reaction for 2 h. After that, the temperature was increased to 430°C and kept for 1.5 h to finish the hydro-cracking reaction. When the reaction was complete, the autoclave was cooled quickly in cold water to room temperature. The gas was then put out and the autoclave was weighed again (w2). The weight of gas is the margin of w1 and w2. The weight of gasoline, diesel oil, VGO and VR of liquid product without coke are obtained by atmospheric and vacuum distillation. The weight of coke is obtained by centrifugal method and vacuum drying. The weight of sulfur of the liquid product without coke is determined by WK-3 type

LI Chuan et al. / Journal of Fuel Chemistry and Technology, 2007, 35(4): 407−411

2.2 Effect of catalyst concentration on hydro-cracking of GDVR

micro-coulometer.

2

Results and discussion

2.1 Catalytic effect of Co-Mo/CNT with different Co/Mo atomic ratio on GDVR hydro-cracking reaction Figure 1 shows the properties of GDVR hydro-cracking products over Co-Mo/CNT catalysts with different Co/Mo atomic ratios (the mass percentage of catalyst is 0.1%).

Figure 2 shows the properties of GDVR hydro-cracking products with Co-Mo/CNT catalyst with different concentration (Co/Mo atomic ratio is 0.35). When the concentration of the catalyst increases, yields of coke, gas and light oil (gasoline and diesel oil) decrease, and heavy oil yield (VGO and VR) increases. This shows that increasing concentration of the catalyst improves its catalytic effect on GDVR hydro-cracking reaction. And the result is applicable for other Co-Mo/CNT catalysts.

60

(4)

55

(5)

50

(4)

40

45 (2)

Analysis of products w / %

Analysis of products w / %

50

30

20 (3)

10

(1)

0 0.2

0.4

0.6

0.8

(3)

40 35 30 25 20 15 10

1.0

(2)

5

Co/Mo atomic ratio

(1)

0.0

0.1

0.2

0.3

0.4

0.5

Content of catalyst w / %

Fig. 1 Effect of different Co/Mo atomic ratios on products distribution

Fig. 2 Effect of different contents catalyst on the products

(1): coke yield; (2): desulfurization ratio; (3): gas yield; (4): yield of gasoline

distribution

and diesel oil; (5): yield of VGO and VR

(1): coke yield; (2): gas yield; (3): yield of gasoline and diesel oil; (4): yield

When the Co/Mo atomic ratio increases, the yield of coke, gas and light oil (gasoline and diesel oil) decreases first, and then increases, whereas heavy oil yield (VGO and VR) and the ratio of desulfurization increase first, and then decrease. This illustrates that the catalytic effect of Co-Mo/CNT catalysts was enhanced first, and then weakened with the increasing Co/Mo atomic ratio. And when the Co/Mo atomic ratio is 0.5, the Co-Mo/CNT catalyst has the best catalytic effect in hydro-cracking reaction of GDVR.

of VGO and VR

2.3 Effect of compound metal Co on the catalytic effect of CNT-supported metal catalysts In order to confirm the function of compound metal Co, a series of experiments for CNT-supported single metal catalysts were processed. The results are listed in Table 4.

Table 4 Catalytic properties of CNT-supported metal catalysts during hydro-cracking of GDVR Catalyst

Mo/CNT

Co-Mo/CNT

Total metal content w / %

10.00

11.10

12.60

13.60

Co/Mo atomic ratio

–

–

–

–

Catalyst content w / % Gas yield

w/%

Co/CNT

11.10

12.60

13.60

0.20

0.50

0.70

10.00 –

9.10

5.01

6.22

11.76

0.10 8.68

8.24

7.10

4.19

Yield of gasoline and diesel oil w / %

44.08

38.63

37.74

37.64

56.57

43.17

46.70

39.70

Yield of VGO and VR w / %

47.24

53.13

55.16

58.17

34.33

51.82

47.08

48.54

Coke yield w / %

7.11

6.60

5.62

4.89

5.53

4.40

4.76

5.96

Desulfurization ratio w / %

37.25

39.95

41.67

40.69

45.83

54.08

43.63

36.27

LI Chuan et al. / Journal of Fuel Chemistry and Technology, 2007, 35(4): 407−411

metal Co can improve the catalytic effect of CNT supported metal catalyst in GDVR hydro-cracking reaction.

In GDVR hydro-cracking reaction with Co/CNT or Mo/CNT, the coke yield is high and the desulfurization ratio is low, which shows that the catalytic effect of CNT-supported single metal catalyst is not good. When Co-Mo/CNT and Mo/CNT have the same concentration of total metal, compared with the Mo/CNT, Co-Mo/CNT can make the yield of coke yield, gas and the light oil (gasoline and diesel oil) decrease, and the heavy oil yield (VGO and VR) and the desulfurization ratio increase in GDVR hydro-cracking reaction. This indicates that the compound

2.4 Comparison of catalytic effect of Co-Mo/CNT and Co-Mo/γ-Al2O3 in GDVR hydro-cracking reaction Table 5 compares the products of GDVR hydro-cracking reaction with Co-Mo/CNT catalysts and Co-Mo/γ-Al2O3 catalysts.

Table 5 Comparison of catalytic properties of Co-Mo/γ-Al2O3 catalysts and Co-Mo/CNT catalysts during hydro-cracking of GDVR with the same catalyst content Catalyst

I

Co/Mo atomic ratio

II

I

II

0.20

w/%

II

I

II

0.50

Catalyst content w / % Gas yield

I

0.35

0.70

0.10 7.99

9.10

4.24

7.23

9.82

5.01

10.99

6.22

Yield of gasoline and diesel w / %

40.01

56.57

42.37

45.81

40.44

43.17

41.23

46.70

Yield of VGO and VR w / %

52.00

34.33

53.40

46.96

49.70

51.82

47.80

47.08

Coke yield w / %

3.64

5.53

3.20

5.17

4.17

4.40

3.47

4.76

Desulfurization ratio w / %

48.53

45.83

56.28

48.20

49.51

54.08

45.51

43.63

I is Co-Mo/γ-Al2O3 catalysts; II is Co-Mo/CNT catalysts

When the Co/Mo atomic ratio is the same (except 0.5), the light oil (gasoline and diesel oil) yield and coke yield in the product of GDVR hydro-cracking reaction with Co-Mo/γ-Al2O3 catalysts are lower than that with Co-Mo/CNT catalysts, whereas the heavy oil (VGO and VR) yield is more. It shows the better activity of the former catalyst. When the Co/Mo atomic ratio is 0.5, Co-Mo/CNT catalyst has the best catalytic effect, which is better than the Co-Mo/γ-Al2O3 catalyst with the same Co/Mo atomic ratio. Co-Mo/γ-Al2O3 catalyst has the best catalytic effect with Co/Mo atomic ratio of 0.35, which is the best among all the catalysts in this experiment.

3

Conclusions

the catalytic effects of Co-Mo/γ-Al2O3 catalysts and Co-Mo/CNT catalysts both improve first, and then weaken with increasing Co/Mo atomic ratio. When Co/Mo atomic ratio is 0.5, Co-Mo/CNT catalyst has the best catalytic effect, whereas Co-Mo/γ-Al2O3 catalyst has the best catalytic effect with Co/Mo atomic ratio of 0.35.

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