ZnO–Cr2O3 catalyst for low-temperature dehydrogenation of isobutane

ZnO–Cr2O3 catalyst for low-temperature dehydrogenation of isobutane

Applied Catalysis A: General 266 (2004) 229–233 Additive effect of Sn in Pt–Sn/ZnO–Cr2 O3 catalyst for low-temperature dehydrogenation of isobutane M...

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Applied Catalysis A: General 266 (2004) 229–233

Additive effect of Sn in Pt–Sn/ZnO–Cr2 O3 catalyst for low-temperature dehydrogenation of isobutane Michitaka Ohta∗ , Yoshihiro Ikeda, Akira Igarashi Department of Environmental Chemical Engineering, Faculty of Engineering, Kogakuin University 2665-1 Nakano-machi, Hatiouji-shi, 192-0015, Japan Received 20 November 2003; received in revised form 3 February 2004; accepted 12 February 2004 Available online 9 April 2004

Abstract The performance of a Pt/ZnO–Cr2 O3 catalyst, which has comparatively excellent performance for the low-temperature dehydrogenation of isobutane, improved upon addition of Sn. The additive effect of Sn was characterized by temperature-programmed reaction (TPR), TPD, chemisorption of CO or H2 , and TG-DTA. As a result of these characterizations, details of the Sn addition to catalysts became clear. Sn 0.5–1.0 wt.% induced improved isobutene selectivity and stabilized catalytic activity over time. These effects are thought to occur because addition of optimal amounts of Sn suppressed coke deposition on the catalyst. However, due to excess Sn on the Pt surface (decreased Pt surface area), addition of Sn 3.0 wt.% resulted in a decline and ultimate deactivation of catalytic activity over time. Furthermore, results was suggested that addition of Sn facilitated the desorption of H2 , isobutene, and 1,3-butadiene, thereby suppressing side reactions such as hydrocracking and isomerization. © 2004 Elsevier B.V. All rights reserved. Keywords: Pt catalyst; Effect of Sn; Dehydrogenation; Isobutane

1. Introduction A Pt/ZnO catalyst has high initial catalytic activity and selectivity for low-temperature dehydrogenation of isobutane to isobutene in the absence of steam with a conventional reactor and a membrane reactor. The catalytic activity of the Pt/ZnO catalyst, however, is gradually deteriorated by coke deposition on the catalyst over the course of the reaction [1]. Deterioration of catalytic activity has been inhibited by both the introduction of steam into the reaction system and the addition of a third component such as Cr, Al, and Ga to the Pt/ZnO catalyst. However, the introduction of steam resulted in a decrease of the isobutene selectivity, regardless of whether or not the third component was added. The steam

∗ Corresponding author. Present address: Fine Chemicals Group, R and D Laboratories, Nippon Steel Chemical Company Limited, 46-80 Nakabaru Sakinohama, Tobata-ku, Kitakyushu-shi, 804-8503, Japan. Tel.: +81-93-884-1622; fax: +81-93-884-1642. E-mail address: [email protected] (M. Ohta).

0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.02.011

reaches a state of equilibrium with the surfaces of the ZnO or ZnO–Cr2 O3 as shown in reaction (1) below. ZnO + H2  Zn + H2 O

(1)

It has been confirmed that H2 generated by reaction (1) promotes side reactions such as hydrocracking and isomerization [2,3]. Therefore, in order to improve the isobutene selectivity in the presence of steam, the absorption of hydrogen on the catalyst must be weakened. Pt catalysts containing Sn promoters have been employed to dehydrogenate alkanes, and the additive effect of Sn has been thoroughly investigated in several studies. Results of many of these studies have indicated that Sn weakened the adsorption of hydrogen on the Pt surface and improved the selectivity of dehydrogenation [4–10]. In this study, Sn was added as a fourth component to the Pt/ZnO–Cr2 O3 catalyst in order to improve catalytic performance, and the amount of Sn additive was optimized. The additive effects of Sn were evaluated by TPR, TPD, chemisorption of CO or H2 , and TG-DTA.

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2. Experimental

He flow. The desorbed gas was analyzed with a quadrupole mass spectrometer (AQA-100MPX, Anelva).

2.1. Catalyst preparation

2.1.2. Loading of Pt and Sn A liquid phase reduction method was used for Pt loading. ZnO–Cr2 O3 supports were immersed in an aqueous solution of H2 PtCl6 ·6H2 O and stirred for 24 h. An aqueous solution of reducing agent, NaBH4 , was added dropwise to the suspension in order to reduce Pt. After reduction, the suspension was stirred for 24 h and then filtered. The filter cake was washed three times with pure water, dried at 100 ◦ C for 12 h, and calcined under N2 flow at 500 ◦ C for 1 h. The Pt content in the resultant catalysts was 0.5 wt.%. A Pt–Sn/ZnO–Cr2 O3 catalyst was also prepared using the liquid phase reduction method. The ZnO–Cr2 O3 support was added to an aqueous solution of H2 PtCl6 ·6H2 O and the mixture was stirred for 24 h. An aqueous solution of reducing agent, SnCl2 , was then added dropwise to the suspension to reduce Pt (PtIV + 2SnII → Pt0 + 2SnIV ) and this mixture was stirred for 24 h. In addition, an aqueous solution of NaBH4 was added dropwise to the suspension to reduce Sn and this mixture was stirred for 24 h. After reduction, the suspension was treated under the same conditions as the Pt/ZnO–Cr2 O3 catalyst. 2.2. Activity test Activity tests were performed using a conventional reactor for 6 h. The catalyst was placed in the reactor and reduced under H2 –Ar mixed gas flow at 500 ◦ C for 1 h. After the reactor was purged under Ar flow, isobutane was supplied at a gas hourly space velocity of 2000 h−1 with an equivalent volume of steam at 500 ◦ C. The details have been previously described [11]. 2.3. Characterization 2.3.1. Temperature-programmed desorption (TPD) of H2 or isobutene A catalyst (0.3 g) was placed in the TPD reactor and reduced under H2 –He (1:6) flow at 500 ◦ C for 0.5 h. After the reactor was purged with He (30 ml/min) for 1 h, H2 or isobutene was adsorbed on the catalyst at 50 ◦ C for 0.5 h. After the reactor purged was under He flow (30 ml/min) for 1 h, the adsorbate was desorbed by heating at 10 ◦ C/min under

2.3.2. Temperature-programmed reaction (TPR) of 1, 3-butadiene After a catalyst was pretreated by the same method as the TPD, 1,3-butadiene was absorbed at 50 ◦ C for 0.5 h. The reactor was purged under He flow for 1 h, and a H2 –He (1:6) mixture gas was introduced at the same temperature to remove the excess 1,3-butadiene. The adsorbed 1,3-bubadiene was reacted and desorbed under H2 –He flow at 10 ◦ C/min. The desorbed gas was analyzed along with the TPD. 2.3.3. Chemisorption and TG-DTA Chemisorption measurements of CO or H2 and TG-DTA for determining the amount of the deposited coke were carried out. The details have been previously described [11]. 2.4. Deactivation rate constant The deactivation rate constant was defined as follows assuming that the deactivation rate is kinetically first-order [11]. kt = kt0 exp(−kd × t)

(2)

Here kt is the reaction rate constant (h−1 ), kt0 is the initial reaction rate constant (h−1 ), kd is the deactivation rate constant (h−1 ), and t is the time-on-stream (h).

3. Results and discussion 3.1. Effect of Sn concentration on catalyst performance Fig. 1 shows that the isobutane conversion of the Pt–Sn/ZnO–Cr2 O3 catalyst with various amounts of Sn. Sn 0.5–1.0 wt.% brought about improved isobutene selectivity and stability of catalytic activity with time. However, 50 Isobutane conversion (mol%)

2.1.1. Support preparation A ZnO–Cr2 O3 (Zn : Cr = 8 : 2, mol ratio) support was prepared by coprecipitation from an aqueous solution. A mixed aqueous solution (pH 9) of Zn(NO3 )·6H2 O, Cr(NO3 )3 ·9H2 O, and CO(NH2 )2 (urea) was heated at 95–98 ◦ C. The precipitate was filtered, washed three times with pure water, dried at 100 ◦ C for 12 h, and calcined in air flow at 500 ◦ C for 1 h.

40

30

20

10

0 0

1

2

3 4 Time-on-stream (h)

5

6

Fig. 1. Catalytic performance of Pt–Sn/ZnO–Cr2 O3 catalyst with various Sn contents: (䊊) Sn = 0.0 wt.%; ( ) Sn = 0.5 wt.%; (䊐) Sn = 1.0 wt.%; and (䉫) Sn = 3.0 wt.%.

0.05

80

0.04

60

0.03

40

0.02

20

0.01

0 0.0

0.12

CO/Pt (-)

0.10

1.0 1.5 2.0 Sn content (wt%)

2.5

0.08 0.06 0.04 0.02

0.00 0.5

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0.14 -1

100

Deactivation rate constant (h )

Isobutane conversion and isobutene selectivity (mol%)

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0.00 0.0

3.0

0.5

1.0

1.5

2.0

2.5

3.0

Sn content (wt%)

Fig. 2. Effect of Sn content on catalytic performance of Pt–Sn/ZnO–Cr2 O3 catalyst (reaction time: 6 h): (䊊) conversion; (䊐) selectivity; and ( ) deactivation rate constant.

Fig. 4. Effect of Sn content on amount of CO adsorbed on Pt–Sn/ZnO–Cr2 O3 catalyst: (䉱) without O2 treatment; and (䊊) with O2 treatment.

addition of Sn 3.0 wt.% resulted in a decline in the initial catalytic activity and deactivation of catalytic activity with time. Fig. 2 shows the effect of Sn content on the catalytic performance after 6 h. In catalysts containing Sn 0.5–1.0 wt.% the isobutane conversion reached its maximum and the deactivation rate constant was at the minimum. Isobutene selectivity improved as the Sn content increased. The Sn 3.0 wt.% catalyst in particular, had an isobutene selectivity of 99.4%. The sharp increase in the deactivation rate constant of the catalyst with Sn 3.0 wt.% will be discussed in detail later.

ing the Pt surface area. When the catalyst was not treated with O2 , the deposited coke remained on the catalyst, Sn 0.5–1.0 wt.% resulted in an increase in the amount of adsorbed CO, whereas Sn 3.0 wt.% led to a decrease in the amount of adsorbed CO. However, when the catalyst was treated with O2 , the deposited coke was removed from the catalyst; the amount of adsorbed CO decreased with an increase in the Sn content. The amount of CO adsorbed to the catalyst was higher, however, upon treatment with O2 compared with that of catalysts not treated with O2 , with the exception of the Sn 3.0 wt.% catalyst. Fig. 5 shows the relationship between the amount of deposited coke and the difference in the amount of CO adsorbed before and after O2 treatment. There is a clear correlation between them. This observation suggests that coke has been selectively deposited on the Pt surface, since CO selectively chemisorbed onto the Pt surface. Fig. 6 shows the relationship between the Sn content and the turnover number frequency (TOF) of isobutane dehydrogenation, calculated from the amount of adsorbed CO on the catalyst without O2 treatment, assuming the stoichiometry is CO/ Pt = 1. As can be seen in the figure, the

3.2. Cause of deterioration of Pt–Sn/ZnO–Cr2 O3 catalysts Fig. 3 shows the relationship between the Sn content in the Pt–Sn/ZnO–Cr2 O3 catalyst and the amount of coke deposited on the catalyst after reaction for 6 h. The amount of deposited coke decreased with an increase in Sn content. Fig. 4 shows the relationship between the Sn content and the amount of CO adsorbed on the catalyst surface, indicat0.5

Amount of deposited coke (wt%)

Amount of deposited coke (wt%)

0.6

0.4

0.3

0.2

0.1

0.0 0.0

0.5

1.0

1.5 2.0 Sn content (wt%)

2.5

3.0

Fig. 3. Effect of Sn content on amount of coke deposited on Pt–Sn/ZnO–Cr2 O3 catalyst.

0.5 0.4 0.3 0.2 0.1 0 0.00

0.02

0.04

0.06

0.08

0.10

0.12

CO/Ptwith O2 treatment - CO/Ptwithout O2 treatment (-)

Fig. 5. Relationship between amount of deposited coke and amount of adsorbed CO before and after treatment with O2 .

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M. Ohta et al. / Applied Catalysis A: General 266 (2004) 229–233 10

12

8

-1

Turn over frequency (s )

Sn=3.0wt% (Pt=0.0wt%)

M/e=2

9

10

7 Intensity (-)

8 6 4

6 Sn=3.0wt%

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Sn=1.0wt%

3 Sn=0.5wt%

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Sn=0.0wt%

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0 0.0

0.5

1.0

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2.0

2.5

0

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0

Sn content (wt%) Fig. 6. Effect of Sn content on turn over frequency based on amount of CO absorped on Pt.

TOF decreased significantly upon addition of slight amounts of Sn. Collectively, these results lead to the conclusion that, although the addition of Sn decreases the TOF, it inhibits coke deposition on the Pt surface, thereby improving the stability of catalyst over time. 3.3. Effect of Sn in Pt–Sn/ZnO–Cr2 O3 catalyst There has been much discussion about how Sn exists in Pt–Sn bimetallic catalysts. Most investigations have concluded that Pt and Sn form an alloy [4–10]. Upon formation of the Pt–Sn alloy, the amount of adsorbed H2 will decrease, due to the overall decrease in the Pt surface concentration [5]. In this study, the ratio of adsorbed H2 to CO decreased as the Sn content of the catalyst increased, as shown in Fig. 7. This decrease in H2 /CO ratio adsorbed indicated that Pt and Sn had formed an alloy in Pt–Sn/ZnO–Cr2 O3 catalyst. Fig. 8 shows the H2 –TPD results of Pt–Sn/ZnO–Cr2 O3 catalysts at various Sn content wt.%, in addition to the Pt/ZnO–Cr2 O3 and Sn/ZnO–Cr2 O3 catalysts. The catalyst

100

M/e=41

169˚C 164˚C 155˚C 154˚C

14 12

Intensity (-)

H2/CO(-)

0.8

10 8 Sn=0.0wt% 6 Sn=0.5wt%

4

0.2

Sn=1.0wt%

2

0.0 0.0

0.5

1.0 1.5 2.0 Sn content (wt%)

2.5

3.0

Fig. 7. Effect of Sn content in the catalyst on H2 /CO ratio adsorbed on Pt–Sn/ZnO–Cr2 O3 catalyst.

500

containing purely Sn or Pt showed a large hydrogen desorption peak at about 150 ◦ C. However, this peak became small when Pt and Sn coexisted. Furthermore, the Pt/ZnO–Cr2 O3 catalyst had a broad desorption peak at 380 ◦ C that disappeared upon addition of Sn to the Pt/ZnO–Cr2 O3 catalyst. The disappearance of the hydrogen desorption peaks indicate that the amount of adsorbed H2 decreases upon addition of Sn to the Pt/ZnO–Cr2 O3 catalyst. This observation is consistent with that obtained from Fig. 7. In addition, the amount of CO adsorbed to the catalyst with Sn 3.0 wt.% was significantly decreased (Fig. 4). This decrease is thought to be due to excess Sn on the Pt surface, which did not form an alloy with the Pt. The catalytic activity of the catalyst with Sn 3.0 wt.% gradually deteriorated, as shown in Figs. 1 and 2, in spite of the non-deposition of coke on the catalyst. This deterioration is also attributed to Sn on the Pt surface over the course of the reaction. Fig. 9 shows the results of the isobutene-TPD measurement of Pt–Sn/ZnO–Cr2 O3 catalyst with various Sn contents. The desorption peak of isobutene around 160 ◦ C shifted to lower temperatures as the Sn content increased. 16

0.4

400

Fig. 8. H2 –TPD profiles of Pt–Sn/ZnO–Cr2 O3 catalyst at various Sn contents.

1.0

0.6

200 300 Temperature (˚C)

Sn=3.0wt%

0 0

100

200 300 Temperature (˚C)

400

500

Fig. 9. iso-C4 H8 –TPD profiles of Pt–Sn/ZnO–Cr2 O3 catalyst at various Sn contents.

M. Ohta et al. / Applied Catalysis A: General 266 (2004) 229–233

tion products are immediately hydrogenated and desorbed from the catalyst even if they had initially formed on the catalyst. However, in the Pt/ZnO–Cr2 O3 catalyst, the higher-order dehydrogenation products were strongly absorbed, and were unable to desorb from the catalyst without hydrocracking. Thus, addition of Sn promotes the desorption of products such as H2 , isobutane, and higher-order dehydrogenation products, and improves the isobutene selectivity and the stability of catalyst over time.

3.0 a) P t/ZnO-Cr 2 O 3 M/e=43

2.5 2.0 Intensity (-)

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4. Conclusions

0.0 3.0 b) P t-Sn(0.5)/ZnO-Cr 2 O 3 2.5 M/e=43

Intensity (-)

2.0 1.5 1.0 0.5

M/e=44 0.0 0

50

100

150

200

250

300

350

400

Temperature (˚C)

Fig. 10. 1, 3-C4 H6 –TPR profiles of Pt and Pt–Sn/ZnO–Cr2 O3 catalysts under H2 + He stream.

Furthermore, the isobutene desorption peak of the catalyst with Sn 3.0 wt.% became small. This shift in the desorption peak indicates that the addition of Sn weakened the adsorption of isobutane thereby, decreasing the amount of adsorbed isobutene. It is concluded that the decrease in the amount of deposited coke upon addition of Sn is caused by the promotion of isobutene desorption as a result of dehydrogenation. This enhanced isobutene desorption prevents isobutene from converting into higher-order dehydrogenation products, which are coke precursors. Furthermore, by assuming the formation of higher-order dehydrogenation products on the catalyst, one can conclude that 1, 3-butadiene was adsorbed on the catalyst as a model compound, thus a 1, 3-butadiene-TPR measurement in H2 air flow was conducted. When hydrogen was introduced at 50 ◦ C, a small amount of 1,3-butadiene remained on the catalyst, although most it was hydrogenated and desorbed from the catalyst. As shown in Fig. 10, the m/e = 43 peak, which corresponds to n-butane as a hydrogenation product, was observed in spite of the existence of Sn in the catalyst. However, the m/e = 44 peak, which corresponds to propane, a hydrocracking product, was observed in only the Pt/ZnO–Cr2 O3 catalyst. This observation led to the conclusion that, in the Pt–Sn/ZnO–Cr2 O3 catalyst, the higher-order dehydrogena-

The catalytic performance of Pt/ZnO–Cr2 O3 catalyst for low-temperature dehydrogenation of isobutane was improved upon addition of Sn. Addition of Sn 0.5–1.0 wt.% unquestioningly improved the isobutene selectivity and stability of the catalyst over time. However, addition of excess Sn deteriorated catalytic activity and stability. Measurements of the amount of deposited coke and the amount of adsorbed CO revealed that addition of Sn 0.5–1.0 wt.% inhibited the deposition of coke and prevented coke from covering the Pt surface. Excessive addition of Sn, decreased the surface area of Pt, and thus caused catalytic activity and stability to deteriorate. Measurement of the amount of adsorbed H2 suggested that Pt and Sn formed an alloy. H2 –TPD and isobutene–TPD analysis revealed that addition of Sn promoted the desorption of both H2 and isobutene. The promotion of H2 desorption inhibited hydrocracking and isomerization, resulting in improved the selectivity. The promotion of isobutene desorption inhibited the formation of higher-order dehydrogenation products, which are coke precursors, preventing coke deposition on the catalyst surface. In addition, the 1,3-butadiene TPR measurement in H2 flow revealed that the adsorption of the higher-order dehydrogenation products was weakened by the addition of Sn.

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