�������� ����� ��

Hydrothermally prepared Cr2O3-ZrO2 as a novel efficient catalyst for dehy-drogenation of propane with CO2

Runxia Wu, Pengfei Xie, Yanhu Cheng, Yinghong Yue, Songyuan Gu,Weimin Yang, Changxi Miao, Weiming Hua, Zi Gao

PII: S1566-7367(13)00167-2DOI: doi: 10.1016/j.catcom.2013.05.002Reference: CATCOM 3495

To appear in: Catalysis Communications

Received date: 24 January 2013Revised date: 2 April 2013Accepted date: 2 May 2013

Please cite this article as: Runxia Wu, Pengfei Xie, Yanhu Cheng, Yinghong Yue,Songyuan Gu, Weimin Yang, Changxi Miao, Weiming Hua, Zi Gao, Hydrothermallyprepared Cr2O3-ZrO2 as a novel efficient catalyst for dehydrogenation of propane withCO2, Catalysis Communications (2013), doi: 10.1016/j.catcom.2013.05.002

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

1

Hydrothermally prepared Cr2O3-ZrO2 as a novel efficient

catalyst for dehydrogenation of propane with CO2

Runxia Wu a, Pengfei Xie

a, Yanhu Cheng

a, Yinghong Yue

a, Songyuan Gu

b, Weimin

Yang b, Changxi Miao

b,*, Weiming Hua

a,*, Zi Gao

a Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of

Chemistry, Fudan University, Shanghai 200433, PR China

b Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, PR China

* Corresponding authors. Tel.: +86 21 65642409; fax: +86 21 65641740.

E-mail addresses: miaocx.sshy@sinopec.com (C. Miao), wmhua@fudan.edu.cn (W Hua).

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

2

Abstract A series of Cr2O3-ZrO2 mixed oxides were prepared by a hydrothermal

method and studied in relation to their performance in the dehydrogenation of propane

to propylene with CO2. The material prepared from a hydrothermal treatment at 180

oC gives an initial propane conversion of 53.3%, which is 1.6 times that of the

conventional Cr2O3-ZrO2. The enhanced activity of the hydrothermally prepared

materials is attributed to a higher concentration of Cr6+

species. CO2 can alleviate the

catalyst deactivation significantly.

Keywords: Propane dehydrogenation; Propylene; Cr2O3-ZrO2; Hydrothermal

method; Carbon dioxide

1. Introduction

Propylene is one of the most important commodity petrochemicals, which is used

as a raw material for the production of a variety of polymers and chemical

intermediates, such as polypropylene, polyacrylonitrile, acrolein and acrylic acid.

Nowadays, propylene is mainly produced as a by-product of ethylene production by

the naphtha steam-cracking process and of a fluid catalytic cracking (FCC) of

petroleum. Due to the growing demand for propylene and the shortage of petroleum

resource in the future, much effort has been dedicated to new routes of propylene

production from alternative substrates, such as dehydrogenation of propane and

natural gas-based processes [1,2].

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

3

Over the last decade, more attention has been paid to the CO2-assisted propane

dehydrogenation, which is considered as a potentially alternative approach to the

traditional dehydrogenation as well as a new attractive pathway for CO2 utilization. In

this new promising technology, cheap and abundant CO2 can act as a mild oxidant to

shift the equilibrium to the product side and/or promote the dehydrogenation through

reaction coupling between a simple dehydrogenation of propane and the reverse

water–gas shift reaction [35]. Furthermore, CO2 can act as a diluent in the

dehydrogenation process, which delivers the required heat and reduces coking of the

catalyst by coke gasification [5,6].

So far, supported gallium and chromium oxide catalysts have been documented as

the most promising ones due to their high catalytic efficiency. In the latter catalyst

systems, Al2O3, active carbon, Na-ZSM-5 and SiO2 including various mesoporous

silica materials have been studied as catalyst supports for dehydrogenation of propane

with CO2 [4,714]. The nature of the support has a significant influence on the

catalytic performance. For instance, CO2 has a promoting effect on the Cr2O3/SiO2

catalyst, while a negative effect was observed over the Cr2O3/Al2O3 catalyst [3,7,13].

In the present work, we demonstrate for the first time that, Cr2O3ZrO2 mixed oxide

materials prepared by a hydrothermal method display a significantly improved

performance for catalyzing dehydrogenation of propane with CO2.

2. Experimental

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

4

2.1. Catalyst preparation

The Cr2O3ZrO2 mixed oxide catalysts were prepared by a hydrothermal method.

0.25 M aqueous zirconium oxynitrate solution containing the required amount of

Cr(NO3)3, corresponding to 10% Cr in the final product, and 6 M ammonia solution

were simultaneously added dropwise to a pH = 10 ammonia solution under vigorous

stirring at room temperature. During the entire course of co-precipitation the pH value

was kept constant at 10. After the addition of two solutions, the hydroxide suspension

was stirred for 1 h. Then it was transferred into a Teflon-lined stainless steel autoclave

and hydrothermally treated under static condition for 24 h at 110, 150 and 180 oC,

respectively. The precipitate was filtered, washed, dried at 100 oC overnight, and

calcined at 600 °C in air for 4 h to yield the final product. The obtained catalysts were

denoted as CZx, where the number x represents the hydrothermal temperature. For

comparison purpose, a conventional Cr2O3ZrO2 mixed oxide sample (labeled as CZ)

was prepared in the same way as CZx, except that there was no hydrothermal

treatment for the preparation of CZ.

2.2. Catalyst characterization

The X-ray powder diffraction (XRD) measurements of the samples were carried

out on a Bruker D8 Advance X-ray diffractometer using nickel-filtered Cu K

radiation at 40 kV and 40 mA. The crystalline size of tetragonal ZrO2 phase was

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

5

determined using Scherrer equation. The BET specific surface areas of the samples

were determined by N2 adsorption using a Micromeritics ASAP 2010 instrument.

Raman spectra were measured on a HORIBA Jobin Yvon LabRAM HR spectrometer

using a laser at 514.5 nm line as the excitation source. The samples were loaded in an

in situ cell and were pretreated in Ar flow at 500 oC for 1 h for dehydration. All

spectra were recorded at 300 oC. X-ray photoelectron spectroscopy (XPS)

measurements were carried out on a Perkin–Elmer PHI 5000C spectrometer with

MgK radiation as the excitation source. All binding energy values were referenced

to the C 1s peak at 284.6 eV. Temperature-programmed reduction (TPR) profiles were

obtained on a Micromeritics AutoChem II apparatus loaded with 100 mg of catalyst.

The catalysts were pretreated at 500 oC for 1 h in N2 flow. The TPR experiments were

carried out in 10 vol.% H2/Ar flowing at 30 mL min1

, with a ramping rate of 10 oC

min1

. H2 consumption was monitored using a TCD. Thermogravimetric (TG)

analysis was performed in air flow on a Perkin–Elmer 7 Series Thermal Analyzer

apparatus to determine the amount of coke deposited on the catalyst after reaction,

with a ramping rate of 10 oC min

1 from room temperature to a final temperature of

800 oC.

2.3. Activity measurement

Catalytic tests were performed at 550 oC in a fixed-bed flow microreactor at

atmospheric pressure, with a catalyst load of 0.2 g. The catalysts were pretreated at

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

6

550 oC for 1 h in nitrogen flow prior to the reaction. The total flow rate of the gas

reactant is 20 mL min1

. For dehydrogenation of propane in the presence of CO2, the

gas reactant contained 2.5 vol% propane, 5 vol% CO2, and the balance nitrogen. For

dehydrogenation of propane in the absence of CO2, the gas reactant contained 2.5

vol% propane and the balance nitrogen. The hydrocarbon products were analyzed

with an on-line GC equipped with a 6-m packed column of Porapak Q and a FID. The

conversion and selectivity were calculated as follows:

C3H8 conversion = (n(C3H8)in n(C3H8)out) 100% / n(C3H8)in

C3H6 selectivity = n(C3H6) 100% / (n(C3H8)in n(C3H8)out)

where n(C3H8)in and n(C3H8)out are the moles of propane from inlet and outlet,

respectively; n(C3H6) is the moles of propylene formed from outlet.

3. Results and discussion

Fig. 1 shows the XRD patterns of the Cr2O3ZrO2 mixed oxide samples calcined at

600 oC. The XRD observation demonstrates that zirconia in all samples exists in a

pure tetragonal phase. The wider peaks observed for the samples prepared by a

hydrothermal method than the conventional one suggests a smaller crystalline size of

ZrO2 in the former samples, as shown in Table 1. No crystalline peaks of chromia

were observed, suggesting that chromia is sufficiently homogeneously mixed with

zirconia. As presented in Table 1, the BET surface areas of the Cr2O3ZrO2 mixed

oxide samples prepared by a hydrothermal method are higher than that of the

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

7

conventional one, which is caused by the smaller crystalline domain size of the former

samples.

Fig. 2 shows the Raman spectra of the samples which were dehydrated in situ in Ar

at 500 oC for 1 h. The band at 1034 cm

1 is associated with the vibration of terminal

Cr=O of monochromate, while the 1012 and 860 bands are assigned to the terminal

Cr=O and bridging CrOCr vibrations of polychromate, respectively [15,16]. The

Raman spectra demonstrate the copresence of two surface Cr6+

species, i.e.

monomeric and polymeric species. Judging from the intensities of the Raman bands at

1034 and 1012 cm1

, the hydrothermally prepared samples possess a higher

concentration of surface Cr6+

species than the conventional one. As shown in Table 1,

the intensity ratio of the band at 1012 cm1

to that at 1034 cm1

is 1.51.6 for the

hydrothermally prepared samples, which is higher than that of the conventional one

(1.3). This suggests that the proportion of polymeric Cr6+

species in the former

samples is higher.

The XPS spectra of Cr 2p on the fresh and used CZ180 as a representative sample

were deconvoluted into two bands at about 576 and 579 eV (Fig. S1) which can be

assigned to Cr3+

and Cr6+

ions, respectively [17,18]. The XPS data obtained by

applying a peak-fitting program are listed in Table 2. It is clear that the ratio of Cr6+

to

Cr3+

is significantly higher on the fresh CZ180 sample than on the used one, which

indicates that most of the surface Cr6+

species were reduced to Cr3+

species during the

dehydrogenation of propane, even if under a CO2 atmosphere. It is noteworthy that

the ratio of Cr6+

to Cr3+

on the used CZ180 sample after the dehydrogenation reaction

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

8

for 6 h is greater in presence of CO2 than in the absence of CO2, suggesting that a

higher amount of surface Cr6+

species were remained under a CO2 atmosphere during

the dehydrogenation reaction. This is reasonably related to weak oxidizing property of

CO2.

The redox properties of the Cr2O3ZrO2 mixed oxide samples were investigated by

H2-TPR and the resulting profiles are depicted in Fig. 3. Only one sharp reduction

peak with the maximum in the temperature range of 261272 oC appears on the

profiles of all samples, corresponding to the reduction of Cr6+

in the oxide [10,19].

The mixed oxide samples prepared by a hydrothermal method exhibit a bit lower peak

temperature than the conventional one, which is caused by a higher proportion of

polymeric Cr6+

species in the former samples, as evidenced by the Raman result.

The dehydrogenation of propane to propylene was carried out at 550 oC using CO2

as a mild oxidant over Cr2O3ZrO2 mixed oxide catalysts. The results are given in Fig.

4 and Table 3. The propane conversion increases with raising the hydrothermal

temperature. When the temperature increases from 150 to 180 oC, the improvement in

conversion becomes smaller. Obviously, the catalysts prepared by a hydrothermal

method exhibit obviously higher activity than the conventional one. The CZ180

catalyst gives an initial conversion of 53.3%, which is 1.6 times that of the CZ

catalyst (33.6%). It is also found that there is a positive correlation between the initial

conversion of Cr2O3ZrO2 mixed oxide catalysts for propane dehydrogenation with

CO2 and the amount of Cr6+

in the fresh catalysts determined by the TPR method (Fig.

5), assuming that Cr6+

species in the catalysts are reduced to Cr3+

[10,19]. This

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

9

strongly suggests that a high abundance of Cr6+

species in the calcined catalysts is the

key factor in achieving high catalytic activity of Cr2O3ZrO2 mixed oxide catalysts.

Combined with the Cr K-edge XANES result, Shishido et al. proposed that the

oxidative dehydrogenation of propane with CO2 took place over Cr6+

in the initial

stage of the reaction [13]. Therefore, it can be concluded that the better catalytic

performance observed on the Cr2O3ZrO2 mixed oxide catalysts prepared by a

hydrothermal method than the conventional one is caused by a higher concentration of

Cr6+

species present on the former catalysts, as revealed by Raman and H2-TPR

results, which could be related to the smaller crystalline domain size of the former

catalysts. A comparison of TOF shows that the hydrothermally prepared catalysts

have a similar initial TOF, which is higher than that of the conventional one (Table 3).

This is caused by a higher proportion of polymeric Cr6+

species in the former catalysts

(Table 1). Based on in situ XANES result, Kumar et al. considered that polymeric Cr

species supported on γ-Al2O3 were more active than monomeric Cr sites for propane

dehydrogenation [20].

CO was detected in the reaction products, suggesting that CO2 was converted into

CO during the reaction. As shown in Fig. S2, the ratio of CO formation rate to CO2

consumption rate on the CZ180 catalyst is 1.51.7, which is always higher than 1.

This result implies that CO2 took part in the Boudouard’s reaction during the

dehydrogenation process. The catalytic performance of the CZ180 catalyst for

propane dehydrogenation in the presence and in the absence of CO2 is also compared

in Fig. 4. At the beginning of the reaction, the propane conversion is higher in the

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

10

absence of CO2 than in the presence of CO2, which could be ascribed to the

competitive adsorption of CO2 and C3H8 on the catalyst [13]. Analogous phenomenon

was reported by Shishido et al. for -Al2O3-supported chromia catalyst [13]. The

catalyst deactivated much slowly in the presence of CO2 than in the absence of CO2.

Thus, the propane conversion after 6 h on stream is obviously higher in the presence

of CO2 than in the absence of CO2 (27.8% vs 3.0%). The enhanced catalyst stability is

due to decreased amount of carbon deposited on the catalyst surface (Table 3, 2.7% vs

9.0%) in the presence of CO2, which results from the elimination of coke by the

Boudouard’s reaction (CO2 + C 2CO) [5,21]. The lower initial activity in presence

of CO2 leads also probably to lower coke amount. The other reason is that a higher

amount of surface Cr6+

species were remained in the presence of CO2 than in the

absence of CO2 during the dehydrogenation reaction (Table 2), since Cr6+

species

were suggested to be more active than Cr3+

species in propane dehydrogenation

[9,13,22].

4. Conclusions

A series of Cr2O3-ZrO2 mixed oxides were prepared by a hydrothermal method.

These materials exhibit obviously higher catalytic activity than the conventional

Cr2O3-ZrO2. Combined Raman and H2-TPR results reveal that the former catalysts

possess a higher number of Cr6+

species, which accounts for their superior catalytic

performance. The present findings demonstrate that a high concentration of Cr6+

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

11

species in the calcined catalyst is the key factor in achieving high activity of

Cr2O3-ZrO2 mixed oxides in the titled reaction. From the results of dehydrogenation

in the presence and in the absence of CO2, it is revealed that the addition of CO2 can

slow down the catalyst deactivation substantially.

Acknowledgments

This work was financially supported by the NSFC (20773027, 21273043), the

Research Fund for the Doctoral Program of Higher Education (20100071110008), the

Shanghai Research Institute of Petrochemical Technology, and STCSM

(08DZ2270500).

References

[1] M. Saito, S. Watanabe, I. Takahara, M. Inaba, K. Murata, Catalysis Letters 89

(2003) 213217.

[2] J.Q. Chen, A. Bozzano, B. Glover, T. Fuglerud, S. Kvisle, Catalysis Today 106

(2005) 103107.

[3] P. Michorczyk, J. Ogonowski, Reacion Kinetics and Catalysis Letters 78 (2003)

4147.

[4] S.B. Wang, Z.H. Zhu, Energy Fuels 18 (2004) 1126–1139.

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

12

[5] B.J. Xu, B. Zheng, W.M. Hua, Y.H. Yue, Z. Gao, Journal of Catalysis 239 (2006)

470477.

[6] J. Ogonowski, E. Skrzyńska, Catalysis Letters 111 (2006) 79–85.

[7] I. Takahara, M. Saito, Chemistry Letters 25 (1996) 973–974.

[8] X.Z. Zhang, Y.H. Yue, Z. Gao, Catalysis Letters 83 (2002) 19–25.

[9] K. Takehira, Y. Ohishi, T. Shishido, T. Kawabata, K. Takaki, Q.H. Zhang, Y.

Wang, Journal of Catalysis 224 (2004) 404–416.

[10] L.C. Liu, H.Q. Li, Y. Zhang, Catalysis Communications 8 (2007) 565–570.

[11] P. Michorczyk, J. Ogonowski, K. Zeńczak, Journal of Molecular Catalysis A

349 (2011) 1.

[12] F. Zhang, R.X. Wu, Y.H. Yue, W.M. Yang, S.Y. Gu, C.X. Miao, W.M. Hua, Z.

Gao, Microporous and Mesoporous Materials 145 (2011) 194–199.

[13] T. Shishido, K. Shimamura, K. Teramura, T. Tanaka, Catalysis Today 185

(2012) 151–156.

[14] F. Zhang, Y.Y. Nie, C.X. Miao, Y.H. Yue, W.M. Hua, Z. Gao, Chemical Journal

of Chinese Universities 33 (2012) 96–101.

[15] B.M. Weckhuysen, I.E. Wachs, Journal of Physical Chemistry 100 (1996)

14437–14442.

[16] T.V.M. Rao, G. Deo, J.M. Jehng, I.E. Wachs, Langmuir 20 (2004) 7159–7165.

[17] S.D. Yim, I.S. Nam, Journal of Catalysis 221 (2004) 601–611.

[18] Q.J. Zhu, M. Takiguchi, T. Setoyama, T. Yokoi, J.N. Kondo, T. Tatsumi,

Catalysis Letters 141 (2011) 670–677.

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

13

[19] D.L. Hoang, H. Lieske, Thermochimica Acta 345 (2000) 93–99.

[20] M.S. Kumar, N. Hammer, M. Rønning, A. Holmen, D. Chen, J.C. Walmsley, G.

Øye, Journal of Catalysis 261 (2009) 116–128.

[21] P. Michorczyk, J. Ogonowski, Applied Catalysis A 251 (2003) 425–433.

[22] S. Sokolov, M. Stoyanova, U. Rodemerck, D. Linke, E.V. Kondratenko, Journal

of Catalysis 293 (2012) 67–75.

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

14

Figure captions

Fig. 1. XRD patterns of prepared samples.

Fig. 2. Raman spectra of prepared samples.

Fig. 3. H2-TPR profiles of prepared samples.

Fig. 4. Propane conversion (left) and selectivity to propylene (right) for the catalysts

as a function of time on stream. () CZ, () CZ110, () CZ150, () CZ180, ()

CZ180. Filled symbols: in the presence of CO2; open symbol: in the absence of CO2.

Reaction conditions: catalyst weight 0.2 g; P(C3H8) = 2.5 kPa, P(CO2) = 5.0 kPa,

P(N2) = 92.5 kPa; reaction temperature 550 oC; total flow rate 20 mL min

1.

Fig. 5. Correlation between initial activity and the amount of Cr6+

in the fresh

catalysts. Reaction conditions: catalyst weight 0.2 g; P(C3H8) = 2.5 kPa, P(CO2) =

5.0 kPa, P(N2) = 92.5 kPa; reaction temperature 550 oC; total flow rate 20 mL min

1.

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

15

Fig. 1

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

16

Fig. 2

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

17

Fig. 3

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

18

Fig. 4

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

19

Fig. 5

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

20

Table 1

Characteristics of the catalysts

Catalyst Surface area (m2 g1

) Crystalline size (nm) I1012/I1034

a

CZ 123 8.5 1.3

CZ110 154 7.4 1.5

CZ150 194 6.4 1.5

CZ180 178 6.9 1.6

a Ratio of integrated area of the Raman band at 1012 cm

1 to that at 1034 cm

1.

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

21

Table 2

Summary of XPS studies

Sample Sample description Binding energy (eV)

Cr 2p3/2

Cr6+

/Cr3+

Cr6+

Cr3+

A fresh CZ180 579.4 576.6 3.3

B CZ180 reacted for 6 h on stream

in the presence of CO2

578.9 576.9 0.9

C CZ180 reacted for 6 h on stream

in the absence of CO2

579.1 576.9 0.5

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

22

Table 3

Reaction data of the catalysts for propane dehydrogenation in the presence of CO2a.

Catalyst Conversion Selectivity (%) TOF104 Coke

(%) CH4 C2H4c C2H6

c C3H6 (s

1)

d (%)

CZ 33.6(12.7) 14.9(5.4) 0.5(0.6) 84.6(94.0) 7.9(3.0) 2.0

CZ110 44.3(19.8) 15.2(6.5) 0.5(0.5) 84.3(93.0) 10.1(4.5) 2.4

CZ150 51.1(25.3) 17.8(7.9) 0.5(0.4) 81.7(91.7) 10.2(5.0) 2.8

CZ180 53.3(27.8) 20.5(8.9) 0.5(0.3) 79.0(90.8) 10.3(5.4) 2.7

CZ180b 76.5(3.0) 19.9(2.6) 4.1() 15.7() 60.3(97.4) 14.8(0.6) 9.0

a The values outside and inside the parenthesis are the data obtained at 10 min and 6 h,

respectively.

b propane dehydrogenation in the absence of CO2.

c “” indicates that no C2H4 or C2H6 was detectable.

d Calculated on the basis of the amount of Cr

6+ in the catalysts obtained by the TPR

method.

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

23

Graphical Abstract

ACC

EPTE

D M

ANU

SCR

IPT

ACCEPTED MANUSCRIPT

24

Research Highlights

Cr2O3-ZrO2 mixed oxide was prepared by a hydrothermal method.

Propane dehydrogenation with CO2 over the Cr2O3-ZrO2 samples was studied.

Enhanced activity was attributed to a higher concentration of Cr6+

species.

The addition of CO2 can alleviate the catalyst deactivation substantially.