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Chinese Journal of Catalysis 36 (2015) 1242–1248
催化学报 2015年 第36卷 第8期 | www.chxb.cn
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Article
Oxidative dehydrogenation of ethane with CO2 over Cr supported on submicron ZSM-5 zeolite
Yanhu Cheng a, Fan Zhang a, Yi Zhang a, Changxi Miao b, Weiming Hua a, Yinghong Yue a,*, Zi Gao a
ab
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, China Shanghai Research Institute of Petrochemical Technology, SINOPEC, Shanghai 201208, China
ARTICLE INFO
ABSTRACT
Article history:
Received 28 January 2015 Accepted 11 May 2015 Published 20 August 2015 Keywords:
Dehydrogenation Ethane
ZSM-5 zeolite Submicron Carbon dioxide
A series of submicron ZSM-5-supported chromium oxide catalysts were prepared and characterized by XRD, N2 adsorption, 27Al MAS NMR, SEM, XPS, laser Raman spectroscopy and diffuse reflectance UV-Vis spectroscopy. The catalytic performance of these materials during ethane dehydrogenation in the presence of CO2 was investigated. The catalysts exhibited both high activity and stability, with an ethane conversion of ～65% and ethylene yield of ～49% without any obvious deactivation fol-lowing 50 h. Characterization results show that the excellent catalytic performance results from the high degree of dispersion of CrOx species on the submicron ZSM-5 surface. Both a high Si/Al ratio and the use of the Na-form of the ZSM-5 support were found to favor CrOx dispersion. The promo-tional effect of CO2 on the dehydrogenation reaction was quite evident and can be attributed to the reverse water-gas shift reaction.
? 2015, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
The catalytic conversion of light alkanes such as ethane and propane into the corresponding value-added alkenes has gained much attention over the past several decades because of the growing demand for light alkenes. The dehydrogenation of alkanes is endothermic and is inevitably controlled by the thermodynamic equilibrium, thus relatively high temperatures are required to obtain high yield of alkenes, resulting in high energy consumption and ready deactivation of the catalyst. For this reason, oxidative dehydrogenation using oxygen has been proposed as an alternative process. However, the over-oxida-tion of alkanes to carbon dioxide is unavoidable during this process, leading to a decrease in the targeted product selectivi-ty. However, it has been reported that these disadvantages can be overcome by replacing O2 with milder oxidants, such as N2O
[1–4] and CO2 [5–11].
The dehydrogenation of ethane over In [5], Cr [6,7], Ga [8,9], Co [10] and Mn [11]-containing catalysts in the presence of CO2 has been studied intensely for some time now. Cr-based cata-lysts in particular show excellent activity for the dehydrogena-tion of ethane, and CO2 can markedly promote the reaction, leading to a significant increase in ethylene yield in the pres-ence of CO2 over Cr-based catalysts. Because of the low surface area of bulk crystalline chromium oxides, Cr species are often dispersed on supports with high surface areas, such as Al2O3 [7], SiO2 [7], ZrO2 [12,13], TS-1 [14], mesoporous silicas like SBA-1 [15], SBA-15 [16], MSU-x [17], MCM-41 [18] and oxi-dized diamond [19,20], so as to prepare a catalyst with an ab-undance of active sites.
ZSM-5 plays a very important role both in industrial processes and in academic studies as either a catalyst or a cat-
* Corresponding author. Tel: +86-21-65642409; Fax: +86-21-65641740; E-mail: yhyue@fudan.edu.cn
This work was supported by the National Natural Science Foundation of China (20773027, 20773028 and 21273043) and the Science & Technology Commission of Shanghai Municipality (08DZ2270500).
DOI: 10.1016/S1872-2067(15)60893-2 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 36, No. 8, August 2015
Yanhu Cheng et al. / Chinese Journal of Catalysis 36 (2015) 1242–1248 1243
alyst support, owing to its three-dimensional microporous structure, high surface area and high thermal and hydrother-mal stability. ZSM-5-supported metal oxide catalysts have been investigated with regard to ethane or propane dehydrogena-tion using CO2, and H-form ZSM-5 with a Si/Al ratio over 1900 has been reported to be preferred as the support [21]. More recently, Cr supported on Na-type ZSM-5 having a smaller crystal size (ca. 400 nm) was found to be more effective when applied to the dehydrogenation of propane in the presence of CO2, although the stability of the catalyst system was still not satisfactory [22].
In our present work, a series of Cr catalysts supported on submicron H- or Na-form ZSM-5 materials having various Si/Al ratios were prepared and characterized by X-ray diffraction (XRD), nitrogen adsorption, laser Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), diffuse reflectance UV-Vis spectroscopy (DRS) and temperature-programmed reduction (TPR). The catalytic performance of each of these submicron ZSM-5-supported Cr-based catalysts during the dehydrogena-tion of ethane to ethylene in the presence of CO2 was also in-vestigated. The relationship between catalytic behavior and physicochemical properties is discussed herein on the basis of the experimental results. 2. Experimental 2.1. Catalyst preparation
Submicron ZSM-5 zeolite was prepared using procedures previously reported in the literature [23], employing tetrapro-pylammonium hydroxide (TPAOH, 25% aqueous solution, Yix-ing Dahua) as the template. Typically, NaAlO2 (CP, Sinopharm Chemical) was dissolved in an aqueous TPAOH solution, after which tetraethylorthosilicate (TEOS, AR, Shanghai Lingfeng) was added. The resulting mixture was stirred for 6 h at room temperature, followed by heating at 50 °C with further stirring to evaporate the ethanol resulting from the reaction. A clear gel was obtained with the molar composition 120SiO2:xAl2O3:48TPAOH:3600H2O. This gel was transferred to an autoclave and crystallized by heating at 170 °C for 2 d. The obtained product was centrifuged, washed, dried at 110 °C overnight and then calcined in air at 600 °C for 6 h to remove the template.
The supported chromium oxide catalysts were prepared by impregnating ZSM-5 with an aqueous solution of Cr(NO3)3?9H2O (AR, Sinopharm Chemical) using the incipient wetness method. The impregnated samples were dried at 110 °C overnight and calcined in air at 650 °C for 6 h. The obtained catalysts are denoted as yCr/ZSM-5-c, where y represents the mass fraction of Cr2O3 in the catalysts, and c represents the Si/Al ratio in gel.
2.2. Catalyst characterization
XRD patterns were acquired with a Persee XD-2 X-ray dif-fractometer using nickel-filtered Cu Kα radiation at 40 kV and 30 mA. The BET surface areas and micropore volumes of the
catalysts were determined by N2 adsorption at –196 °C using a Micromeritics ASAP 2000 instrument. Scanning electron mi-croscopy (SEM) images were recorded digitally on a Philips XL 30 microscope operating at 30 kV. Laser Raman spectra were obtained with a Horiba JY XPloRA spectrometer using the 532 nm radiation from an air-cooled solid state laser as the excita-tion source. The other parameters included a laser power of 25 mW, a data acquisition time of 30 s, an accumulation number of 8 and a spectral resolution of 2 cm–1. The spectra were ob-tained at room temperature under ambient conditions. DRS spectra were collected on a Shimadzu UV-2450 spectrometer equipped with an integrating sphere attachment. XPS data were acquired using a Perkin-Elmer PHI 5000C spectrometer with Mg Kα radiation as the excitation source. All binding ener-gy values were referenced to the C 1s peak at 284.6 eV.
TPR profiles were obtained on a Micromeritics AutoChem II apparatus loaded with 100 mg of catalyst. The TPR experi-ments were carried out in a 30 mL/min flow of 10% H2-90% Ar with a ramp rate of 10 °C/min. H2 consumption was monitored using a thermal conductivity detector. Thermogravimetric (TG) analysis was performed under an air flow on a Perkin-Elmer 7 Series Thermal Analyzer to determine the amount of coke de-posited on the catalyst following the reaction. 2.3. Catalytic testing
Catalytic tests for ethane dehydrogenation with CO2 were performed at 650 °C in a fixed-bed flow microreactor at at-mospheric pressure. The catalyst load was 200 mg, and each sample was pretreated at 650 °C for 2 h under a nitrogen flow prior to the reaction. The gaseous reactant contained 3% ethane and 15% CO2 with the balance consisting of nitrogen at a total flow rate of 30 mL/min. The hydrocarbon reaction products were analyzed using an on-line gas chromatograph (GC) equipped with a 6 m Porapak Q packed column and a flame ionization detector (FID). The gaseous products were analyzed on-line using a second GC equipped with a thermal conductivity detector (TCD) and a carbon molecular sieve 601 column. The reverse water-gas shift reaction was performed in a fixed-bed flow microreactor at atmospheric pressure using a catalyst load of 200 mg. The H2:CO2:N2 molar ratio was 1:1:1, and the total flow rate of the gaseous reactants was 30 mL/min. The reaction temperature was in the range of 500–650 °C. The amounts of CO2 before and after the reaction were determined by on-line analysis with a GC equipped with a carbon molecular sieve 601 column and a TCD. The reaction data in this work were reproducible, with a variation of less than 5%. 3. Results and discussion 3.1. Structural Characterization
Figure 1 shows the XRD patterns of the various ZSM-5-sup-ported chromium oxide catalysts. All exhibit well-crystallized MFI structures with characteristic reflections at 2θ = 8.0°, 8.9°, 23.1°, 23.4° and 24.0° [24]. No diffraction patterns corres-ponding to chromium oxide were observed, suggesting that the
1244 Yanhu Cheng et al. / Chinese Journal of Catalysis 36 (2015) 1242–1248
Table 1
Textural properties of supported chromium oxide catalysts.
(4)
Intensity (a.u.)(3)(2)(1)
10
20
302θ/()
Fig. 1. XRD patterns of (1) 3Cr/NaZSM-5-60, (2) 3Cr/NaZSM-5-100, (3) 3Cr/NaZSM-5-160 and (4) 3Cr/HZSM-5-160.
o
External Surface
Catalyst area surface area a
(m2/g) (m2/g)
3Cr/NaZSM-5-60 339 53 3Cr/NaZSM-5-100308 38 3Cr/NaZSM-5-160356 34 3Cr/HZSM-5-160 342 36 a Calculated by the t-plot method.
Pore volume (cm3/g) 0.27 0.21 0.24 0.24 Micropore volume a (cm3/g) 0.17 0.14 0.16 0.15
4050
(3)
(2)
chromium oxide was well dispersed on all the ZSM-5 supports. SEM images of the ZSM-5 supports are presented in Fig. 2. All samples were well crystallized without any presence of amorphous materials and exhibit a rod-like morphology with a uniform crystallite size distribution. The average crystallite size was approximately 400 nm for all the ZSM-5 zeolites.
The textural properties of the ZSM-5-supported chromium oxide catalysts are summarized in Table 1. These catalysts, regardless of whether the ZSM-5 support was the Na- or H-form or had a low or high Si/Al ratio, exhibit similar BET surface areas and micropore volumes, showing that the sup-ports had similar crystallinities and that their micropore chan-nels were not blocked by the supported CrOx species.
27Al MAS NMR spectra of the ZSM-5 supports were acquired to characterize the local coordination environment of the alu-minum atoms in the zeolites. An intense line at δ = 55, assigned to ZSM-5 framework aluminum atoms in tetrahedral coordina-tion, can be observed in the spectra of all the samples, while no discernable signal at δ = 0, attributed to extra-framework alu-minum atoms in octahedral coordination, is evident (Fig. 3). The absence of extra-framework aluminum atoms indicates that all of the Al species have been incorporated into the zeolite framework.
3.2. State of Cr species
XPS was employed to investigate the oxidation states of Cr species. The Cr 2p2/3 spectra obtained from the catalysts were deconvoluted into two bands at approximately 576.5 and 579.5 eV, assigned to Cr3+ and Cr6+, respectively [22,25,26], and the
(a)(b)(1)
-200
-100
0
Chemical shift
100
200
Fig. 3. 27Al MAS NMR spectra of (1) ZSM-5-60, (2) ZSM-5-100 and (3) ZSM-5-160.
Table 2
XPS and TPR data for supported chromium oxide catalysts. Catalyst
BE (eV) Cr6+/Cr3+ H2 consumption b
(mmol/g) Cr3+ Cr6+ atomic ratio a
3Cr/NaZSM-5-60 576.5 579.5 2.87 0.24 3Cr/NaZSM-5-100576.5 579.5 1.82 0.35 3Cr/NaZSM-5-160576.5 579.5 0.92 0.36 3Cr/HZSM-5-160 576.5 579.5 0.89 0.32 a Calculated from XPS peak areas. b Obtained by TPR.
quantitative data after fitting are listed in Table 2. All the cata-lysts were found to have similar binding energy (BE) values. The Cr6+/Cr3+ ratio evidently decreases with increasing Si/Al ratios and as the Na content is decreased, indicating that Na+ cations may have a positive effect in terms of raising the Cr6+/Cr3+ ratio, similar to the action of K+ ions in K-doped CrOx/Al2O3 catalysts [27]. Compared with the NaZSM-5-sup-ported catalyst, the HZSM-5 material having the same Si/Al ratio exhibited a lower Cr6+/Cr3+ ratio, which may also result from a lower Na content.
Additional information on the state of Cr species was ob-tained from UV-Vis diffuse reflectance measurements, and the results are summarized in Fig. 4. Two bands are observed for
(c)
Fig. 2. SEM images of (a) ZSM-5-60, (b) ZSM-5-100 and (c) ZSM-5-160.
Yanhu Cheng et al. / Chinese Journal of Catalysis 36 (2015) 1242–1248 1245
272
370468
605
(4)(3)(2)(1)
H2 consumption (a.u.)(4)
Absorbance (3)(2)(1)
100
200
300
400500600
o
Temperature (C)
700
800
200300
400500Wavelength (nm)
600700
Fig. 4. Diffuse reflectance UV-Vis spectra of (1) 3Cr/NaZSM-5-60, (2) 3Cr/NaZSM-5-100, (3) 3Cr/NaZSM-5-160 and (4) 3Cr/HZSM-5-160. Fig. 6. H2-TPR profiles of (1) 3Cr/NaZSM-50, (2) 3Cr/NaZSM-5-100, (3) 3Cr/NaZSM-5-160 and (4) 3Cr/HZSM-5-160.
each catalyst, at approximately 272 and 370 nm, assigned to the O2–→Cr6+ charge transfer transition of chromate species in tetrahedral coordination [25,28–32]. Bands at 468 and 605 nm, corresponding to octahedral Cr3+ species in Cr2O3 or CrOx clus-ters, are not present, indicating that all Cr species were well dispersed on the submicron ZSM-5 support.
Laser Raman spectra were acquired to identify the molecu-lar nature of the Cr species dispersed on the ZSM-5 supports, and the results are depicted in Fig. 5. A band at 551 cm–1, as-signed to crystalline Cr2O3, appears in the spectra of the 3Cr/HZSM-5-160 catalyst [18,22,25,30–33], while the band at the same position is much weaker in the case of the NaZSM-5 supported catalysts. The intensity of the 551 cm–1 band de-creases in the order 3Cr/HZSM-5-160 > 3Cr/NaZSM-5-60 > 3Cr/NaZSM-5-100 ≈ 3Cr/NaZSM-5-160. These results demon-strate that the use of a high Si/Al ratio and the Na-form of the zeolite favor CrOx dispersion.
Two intense bands at approximately 980 and 1000 cm–1 are present in the spectrum of each of the catalysts, and can be ascribed to the symmetric vibrational modes of the terminal Cr=O bonds of monochromates and polymeric chromates, re-spectively [18,22,25,30–33]. Meanwhile, a weak, broad band at 810 cm–1, attributed to the bending mode of the Cr–O–Cr lin-
kage of polymeric chromates, also appears. The intensity of the band at 1000 cm–1 increases as the Si/Al ratio is increased, in-dicating that the dispersed Cr(VI) species on the catalyst sur-faces gradually transition from monochromates to polymeric chromates with increasing Si/Al ratios.
H2-TPR was carried out to characterize the redox ability of Cr species on the catalysts, which has a very important effect on the dehydrogenation activity. The results are presented in Fig. 6 and Table 2. It is evident that there is only one reduction peak at 370 °C, with a shoulder at 270 °C, that can be attributed to the reduction of Cr6+ to Cr3+ (and/or Cr2+) [30–32]. H2 con-sumption is higher over the NaZSM-5-supported catalysts as compared with HZSM-5 material, and increases as the Si/Al ratio is increased, indicating increasing quantities of reducible surface Cr6+. All these findings are consistent with the data ob-tained from laser Raman studies, showing that high Si/Al ratios and the Na-form are favorable for CrOx dispersion. 3.3. Catalytic activity
The prepared Cr/ZSM-5 catalysts were evaluated during ethane dehydrogenation in the presence of CO2, and the results are shown in Table 3. For each catalyst, the ethane conversion drops with reaction time while the selectivity for ethylene in-creases. There are only minimal differences in the initial activi-ties as well as the initial ethylene yields among the Cr/NaZSM-5 catalysts, although the stability is improved with increasing Si/Al ratios in the ZSM-5 support, resulting in a slight increase in the plateau value for the yield of ethylene. While all the sub-micron particle-supported catalysts exhibit superior perfor-mance, the 3Cr/NaZSM-5-160 shows higher activity than the 3Cr/HZSM-5-160.
Cr-based catalysts have been reported to represent one of the most promising catalysts for light alkane dehydrogenation reactions because of their high catalytic efficiency both in the absence and presence of CO2. There are two types of coordina-tively-unsaturated Cr(III) in chromium species, Cr(III) ions formed from the reduction of Cr(VI) and dispersed on fresh catalysts. Both are generally considered as active sites during non-oxidative dehydrogenation and oxidative dehydrogenation
(4)
Intensity (a.u.) (3)(2)
(1)
500
600
700
8009001000Raman shift (cm?1)
1100
1200
Fig. 5. Laser Raman spectra of (1) 3Cr/NaZSM-5-60, (2) 3Cr/NaZSM-5-100, (3) 3Cr/NaZSM-5-160 and (4) 3Cr/HZSM-5-160.