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Nano-MgO supported CrOx catalysts applied in propane oxidative dehydrogenation: Relationship between active chromium phases and propane reaction pathway
Applied Surface Science 313 (2014) 654–659
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Nano-MgO supported CrOx catalysts applied in propane oxidative dehydrogenation: Relationship between active chromium phases and propane reaction pathway Fei Ma, Shu Chen, Yanhua Li, Hang Zhou, Aixin Xu, Weimin Lu ∗ Institute of Catalysis, Zhejiang University (Xixi Campus), Hangzhou 310028, PR China
a r t i c l e
i n f o
Article history: Received 22 April 2014 Received in revised form 22 May 2014 Accepted 7 June 2014 Available online 13 June 2014 Keywords: Nano materials Oxidative dehydrogenation Active phase Propane reaction pathway
a b s t r a c t Propane oxidative dehydrogenation was investigated on nano-MgO supported Cr catalysts. Cr-5.0 wt.% sample exhibited the best catalytic performance with a propylene selectivity of 84.1% and propane conversion of 10.8% at 450 ◦ C. Cr5 O12 and MgCrO4 were proved to be formed, and they could be consecutively reduced as shown by temperature programmed reduction results. The impacts of different Cr phases on propane reaction pathway were also investigated. Fourier transform infrared spectroscopy reflects that Cr5 O12 is favorable for the propane dehydrogenation process, while MgCrO4 would activate propane but lead to the COx outcome. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Propylene has many important industrial derivatives such as polypropylene, acrylonitrile, propylene oxide, acrylic acids, and oligomers [1]. Propane oxidative dehydrogenation (PODH) is an attractive exothermic, on-demand propylene production route. The exploration of a good catalyst that satisfies the industrial demand has never been stopped [2–6]. Supported chromium materials are among the most promising catalysts for PODH reaction, mainly because of its efficient ability in propane C–H bond activation [7]. Moreover, it has been proved that CrOx ’s interact with the carrier determines Cr species’ distribution and the formation of active sites [8]. Consequently, the choice of an ideal carrier becomes crucial. On account of the property of characteristic interaction with the doping metal oxide, and the modification of the acid property on the surface, MgO goes for a good candidate [9–11]. V-Mg-O material was the first and classic catalyst that performed well in several catalytic reactions [12]. For Cr-MgO material, the physical property, polymerization degree of chromium, Cr valance states have already been studied, and it was concluded that MgCrO4 (Cr(VI)) was formed at
∗ Corresponding author. Tel.: +86 57188273283; fax: +86 57188273283. E-mail address: [email protected] (W. Lu). http://dx.doi.org/10.1016/j.apsusc.2014.06.042 0169-4332/© 2014 Elsevier B.V. All rights reserved.
around 400 ◦ C calcination, while treatment at over 600 ◦ C would generates MgCr2 O4 (Cr(III) [13,14]. There are also examples of Cr-MgO applied in the catalytic reaction of cyclohexanol to 2,6dimethyphenol; isobutane dehydrogenation [15,16]. However, to the best of our knowledge, only one report is available for CrMgO materials directly used in PODH reaction [8], and the catalytic performance was unsatisfied. It needs to be mentioned that in their experiment the calcination temperature was 600 ◦ C, and the incorporation amount was pretty high (10.0 wt.%). Although XRD and XPS analysis were not given, the chromium species in their catalysts might be inappropriate for the PODH process. To sum up: modifications in the preparation procedure, Cr doping amount are needed to improve the catalytic behavior. Moreover, the active phase of Cr-MgO in PODH reaction has not been defined until now, and the catalyst structure–activity relationship remains unclear. Recently, nano-scale catalysts drive a lot of attention, as they possess surprisingly high reactivity not observed in their bulk analogues, besides only a mild reaction condition is needed [17,18]. In this paper, nano-MgO supported chromium catalysts were prepared and evaluated in PODH reaction. Through an integrated analysis, two chromium phases were detected and proved to ameliorate the catalytic performance. The relationship between chromium phases and propane reaction pathway was also tentatively given, which might give insight for the development of a new class of catalysts.
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2. Experimental 2.1. Catalysts preparation Nano-MgO (Aladdin, 50 nm) was used as support for preparing the catalysts studied in this work. Chromium was deposited on the MgO surface (1.0–12.0 wt.%) by wet impregnation with an aqueous solution of Cr(NO3 )3 ·9H2 O. The samples were then dried in an oven at 80 ◦ C overnight. The obtained dried powders were calcined at 450 ◦ C in air for 4 h with a ramping rate of 10 ◦ C/min. The catalysts with different Cr contents are denoted as Cr-x in part of the text, where “x” represents the doping weight percent. Pure MgCrO4 (Sigma) was used for comparison in XRD and FT-IR experiment. 2.2. Catalysts characterization Powder XRD patterns of the samples were obtained on an X-ray diffractometer (XRD, Rigaku-D/Max-B automated powder X-ray diffractometer) operating at 45 kV and 40 mA using Cu K␣ radiation ( = 0.15418 nm). X-ray photoelectron spectroscopy (XPS) experiments were carried out on a Thermo ESCALAB 250 system with Al K␣ radiation (h = 1486.6 eV). The X-ray anode was run at 150 W, and binding energies were calibrated using the C 1s line as a standard (284.8 eV). H2 -temperature programmed reduction (TPR) experiments were carried out in a flow reactor system using 20 mg of catalysts. The sample was reduced in a 10% H2 /Ar stream (40 ml/min) with a heating rate of 10 ◦ C/min from 100 to 600 ◦ C. Propane FT-IR spectra were recorded at selected temperature on a Nicolet Magna 750 Fourier transform instrument with a liquidnitrogen-cooled CCD detector. Before measurement, the catalysts were pressed in self-supporting discs and pretreated in the IR cell at 250 ◦ C for 4 h under vacuum. Isopropanol oxidation was carried out in a fixed-bed quartz reactor. The mixture flow of oxygen/Ar (controlled by two mass flow controllers) was transferred through a saturator with isopropanol to bring it to the reaction bed. EPR experiment was operated to investigate the local environment of chromium. The electron paramagnetic resonance (EPR) measurements were performed at room temperature using Bruker spectrometers operating in the X-band frequency range (∼9.5 GHz) with 100 kHz field modulation.
Fig. 1. XRD patterns of different catalysts: (a) MgO, (b) Cr-1.0, (c) Cr-2.5, (d) Cr-5.0, (e) Cr-8.0, (f) Cr-12.0 and (g) MgCrO4 , inset is the zoom out of the area between 2 = 23◦ and 26◦ .
of Cr5 O12 on different samples, the relative intensity of the peak 25.4◦ versus 23.8◦ was given (I25 /I23 , inset of Fig. 1). If there is no contribution from Cr5 O12 for the 25.4◦ diffraction, the I25 /I23 value should be consistent with MgCrO4 . As we can see, I25 /I23 on Cr-5.0 is 0.8154, which is quite larger than 0.6170 of MgCrO4 , implying the coexistence of Cr5 O12 with MgCrO4 . From Cr-5.0 to Cr-12.0, I25 /I23 decreases from 0.8154 to a minimum of 0.6312, close to that of MgCrO4 (0.6170). It manifests that Cr5 O12 is hardly detectable on 12.0 wt.% by XRD. At high chromium doping amount (8.0–12.0 wt.%), diffractions attributing to Cr2 O3 (2 = 33.58, 36.28◦ , JCPDS No.: 70-3765) appear. The crystal structure of Cr5 O12 has been clearly discussed [19], and Cr5 O12 was characterized on supported cerium and tin oxide through EPR technique [20,21]. In our experiment (Fig. 2), the carrier (line a) gives no signal in the detection field, excluding the interference with Cr peak. The pattern of Cr-1.0 catalyst (line b) has one isotropic peak centered at 3557 G, g = 1.97. On 2.5 wt.%, a superimposed small shoulder at 3590 G, g = 1.96 shows up, and it
2.3. Catalytic performance The typical PODH reaction was tested at 450 ◦ C, GHSV = 4500 ml (g h)−1 , propane/O2 molar ratio = 2:1. The feedstock and reaction products were analyzed on-line by gas chromatography with two column types: Porapak QS (4.0 m × 1/8 in.) and TDX-01 (2.0 m × 1/8 in.). 3. Results and discussion 3.1. Characterization of Cr phase and its redox property XRD patterns of calcined samples are shown in Fig. 1. The diagram of the carrier (line a) gives diffractions at 2 = 36.8◦ , 42.8◦ , 62.1◦ , 74.5◦ , 78.4◦ , which can be indexed to characteristic MgO phase (JCPDS No.: 74-1225). For the samples incorporating with Cr between 1.0 and 2.5 wt.% (lines b and c), only MgO were detected. Increasing the doping amount between 5.0 and 12.0 wt.% (patterns d–f), new phases of MgCrO4 (2 = 23.8◦ , 25.4◦ , 32.5◦ , JCPDS No.: 721606) and Cr5 O12 (2 = 25.48◦ , JCPDS No.: 73-1787) appear. The profile of pure MgCrO4 (lineg) corresponds well with the above MgCrO4 diffractions. It needs to be mentioned that both MgCrO4 and Cr5 O12 are diffractive at 2 = 25.4◦ . To verify the existence
Fig. 2. EPR spectra for MgO supported chromium catalysts: (a) MgO, (b) Cr-1.0, (c) Cr-2.5, (d) Cr-5.0, (e) Cr-8.0 and (f) Cr-12.0.
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Fig. 3. TEM image of Cr-5.0 sample.
becomes more apparent from Cr-2.5 to Cr-12.0 (lines c–f). The two peaks have been assigned as  (g = 1.96) and ␥ (g = 1.97) signals of Cr species [20]. The  signal is referred to clusters of Cr(III), while the ␥ signal is attributed to “Cr(VI)-O-Cr(III)-O-Cr(VI)” mixed-valence trimmers, which belongs to Cr5 O12 structure [21,22]. In order to give a visualized image of the catalyst, TEM experiment was conducted and the results are shown in Fig. 3. The as-synthesized Cr-5.0 sample was mainly constituted of irregular spheroidal particles with an average diameter of 50 nm. XPS results of the different catalysts are displayed in Fig. 4. The deconvoluted Cr 2p3/2 profiles indicate that Cr exists as two oxidation states. Binding energies of 579.2 and 576.5 eV could be accounted for Cr (VI) and Cr(III), respectively [23]. The proportion of Cr(VI) or Cr(III) versus (Cr(VI) + Cr(III)) on the surface are also given. Cr(VI) accounts for over 95% on Cr-1.0. At increasing doping
amount, the percentage of Cr(III) increases, but it is below 20% even on Cr-12.0. The XRD and XPS analysis above tells that both Cr(VI) and Cr(III) were formed on MgO surface. Cr(VI) could be reduced to Cr(III) by H2 [24], and the H2 -TPR results in our experiment are shown in Fig. 5(1). No reduction in the region of 200–600 ◦ C was detected on MgO (line a). For pure MgCrO4 (line g), the reduction peak at around 500 ◦ C shows up, which is in agreement with previous report [25]. When Cr was incorporated into MgO, a peak at around 400 ◦ C was detected on the Cr-1.0 sample (line b). Between Cr-2.5 and Cr-12.0 (lines c–f), another reduction at over 500 ◦ C occurred. The existence of two reduction peaks might originate from Cr(VI) species having different local environment [8]. By referring to the profile of MgCrO4 (line g), the higher reduction peak on supported samples might be associated with MgCrO4 ’s reduction. Moreover, as MgCrO4 could not be reduced at 400 ◦ C and no other Cr species existed in our experiment, the 400 ◦ C peak may be caused by Cr5 O12 ’s reduction. To verify it, the Cr-5.0 catalyst was subjected to an in situ, controlled reduction step by H2 , which is widely used to study the different reduction degree outcome of transition metal oxides [26]. The H2 -TPR profiles of the different reduction process on Cr-5.0 sample are shown in Fig. 5(2). The one defined as R480 was firstly reduced to 480 ◦ C, and then hydrogen supply was cut off and the sample was cooled to r.t. under Ar. Pristine Cr-5.0 catalyst reduced to 600 ◦ C was named R600 . The three samples (Cr-5.0, R480 and R600 ) were characterized by XRD and EPR techniques. As shown in Fig. 6, typical MgCrO4 diffractions at 2 = 23.81◦ , 25.48◦ and 32.55◦ (JCPDS No.: 72-1606) are observable on R480 , consolidating that MgCrO4 was retained. From the inset image, I25 /I23 decreases from 0.8154 of Cr-5.0 to 0.6190 of R480 , which is almost the same as MgCrO4 (0.6170). It indicates that Cr5 O12 was reduced on R480 , corroborating the H2 -TPR conclusion above. In R600 sample, MgCrO4 was also largely reduced as its typical diffractions all disappeared.
Fig. 4. XPS results of the Cr 2p3/2 region.
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Fig. 5. (1) H2 -TPR profiles of: (a) MgO, (b) Cr-1.0, (c) Cr-2.5, (d) Cr-5.0, (e) Cr-8.0, (f) Cr-12.0 and (g) MgCrO4 . (2) H2 -TPR profiles of Cr-5.0 sample reduced at different conditions: (a) H2 -reduction stopped at 480 ◦ C (R480 ) and (b) H2 -reduction stopped at 600 ◦ C (R600 ).
EPR spectra of pristine Cr-5.0, R480 and R600 are displayed in Fig. 7. As has been discussed (Fig. 2), the spectrum of Cr-5.0 catalyst comprises  (g = 1.96) and ␥ (g = 1.97) signals. The sample of R480 brings about the totally disappearance of ␥, and the enhancement of  signal. It means the trimmers structure of Cr(VI)-O-Cr(III)O-Cr(VI) belonging to Cr5 O12 was destroyed. The  signal of R600 becomes stronger, which could result from more quantity of Cr(III) reduced by MgCrO4 . The above analysis also supplies a proof for Fig. 5 that the peak at around 400 ◦ C was caused by the reduction of Cr(VI) to Cr(III) in Cr5 O12 . As Cr5 O12 and MgCrO4 are consecutively reduced, the crystal phase-activity relationship could be studied tentatively by the activity comparison of Cr-5.0, R480 , R600 and MgCrO4 .
Fig. 6. XRD patterns of samples: (a) Cr-5.0, (b) R480 and (c) R600 .
3.2. Catalytic activity Table 1 shows the catalytic performance in PODH reaction on different catalysts. It could be read that on nano-MgO, the propane conversion is negligible. With the incorporation of Cr, propane conversion increases from 4.8% to 10.8% as the doping increases from 1.0 to 5.0 wt.%, and then the conversion was attenuated. The selectivity of propylene reaches the highest value of 84.0% on Cr-5.0 catalyst. Between Cr-8.0 and Cr-12.0, the decreased selectivity of propylene seems to be compromised by the generation of COx . Fig. 8 shows propylene selectivity as a function of propane conversion varied by contact time for the four samples: pristine Cr-5.0, R480 , R600 and MgCrO4 . Propylene selectivity drops with the
Fig. 7. EPR spectra of Cr-5.0, R480 and R600 .
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Table 1 Catalytic performance of propane oxidative dehydrogenation on nano-MgO supported Cr catalysts at 450 ◦ C, GHSV = 4500 ml (g h)−1 , propane/oxygen molar ratio = 2:1. Samples
MgO Cr-1.0 Cr-2.5 Cr-5.0 Cr-8.0 Cr-12.0 R480 R600 MgCrO4 Cr2 O3
Catalytic activity Propane conversion (%)
Propylene selectivity (%)
COx selectivity (%)
0.7 4.8 6.5 10.8 7.2 4.1 9.5 3.8 13.6 0.9
65.8 73.0 75.1 84.1 62.9 37.4 34.0 21.0 18.4 26.0
34.2 28.8 24.9 15.9 37.1 62.6 66.0 79.0 82.6 74.0
increase of propane conversion on all samples. At the same propane conversion, the propylene selectivity on R480 decreased apparently compared with Cr-5.0 catalyst, indicating that Cr5 O12 is essential for the selective outcome. It should be noted that under the same contact time, propane conversion almost does not descend from Cr-5.0 to R480 , demonstrating that the step of propane activation was not affected too much. Meanwhile, the decrease of propylene selectivity indicates that absorbed propane on R480 mainly forms deep oxidized products like COx . The profile of MgCrO4 was quite similar with that of R480 , consolidating that MgCrO4 was active for propane activation but nonselective for propylene generation. The catalytic performance on R600 was much poorer compared with R480 , illustrating the handicap in the activation of propane. 3.3. Relationship between crystal phase and propane reaction network In order to give a track on how the reaction route was affected by different Cr species, IR experiment was implemented. The IR spectra of adsorbed propane on different catalysts (after purging with Ar) are shown in Fig. 9. For the situation on Cr-5.0 (band c), a double peak at 1677 and 1637 cm−1 were observed due to C C stretching; other less intense peaks at 1440 and1350 cm−1
Fig. 8. Propylene selectivity as a function of propane conversion on Cr-5.0, R480 , R600 and MgCrO4 (evaluated at 450 ◦ C, molar ratio of C3 H8 :O2 = 2:1). Propane conversion increases with the contact time (W/F) of 0.08, 0.12, 0.16, 0.24 s.
Fig. 9. FT-IR spectra of adsorbed propane on: (a) Cr2 O3 , (b) R600 , (c) Cr-5.0, (d) R480 and (e) MgCrO4 .
associating to asymmetric and symmetric CH3 deformation coexist in the band [27,28]. It indicates the generation of propylene product, similar to the report of adsorbed propane on supported molybdenum catalysts [29]. Another peak at around 1290 cm−1 was also detected, and it was assigned to C O stretching vibration in CO3 (precursor of COx ) [28]. On R480 , new peaks of 1688 and 1706 cm−1 showed up (band d), which have been attributed to be VC=O in adsorbed acetone [29]. The appearance of acetone absorption indicates that propane reaction network has been transformed. The IR spectrum of adsorbed propane on R480 was very similar to that of on MgCrO4 (band e), illustrating it might be MgCrO4 that leads to the acetone outcome. Moreover, in case of the stronger intensity of the 1290 cm−1 peak on R480 and MgCrO4 (compared with pristine Cr-5.0 catalyst), it could be concluded that larger quantity of CO2 precursors were formed on R480 and MgCrO4 . Weak and broad IR peaks were detected(bands b and a) on R600 andCr2 O3 . It means there might be little active sites on Cr2 O3 for propane adsorption, in keeping with others’ reports [20]. It is generally believed that propane is activated from abstracting a hydrogen atom by surface nucleophilic oxygen, and the alkoxy groups formed in step one either dehydrate to propylene or form acetone [29]. Isopropanol oxidation has been commonly adopted as a probe reaction to test the reaction network of alkoxy species [30]. The isopropanol conversion results in our experiment are shown in Fig. 10. Propylene and acetone were the products detected. Propylene selectivity accounts for over 90% on Cr-5.0 catalyst, and it was transformed largely to acetone (acetone selectivity over 80%) on R480 and MgCrO4 . It testifies that MgCrO4 is favorable for the dehydrogenation process of alkoxy groups, in line with FT-IR analysis. In case of the analysis above, Cr5 O12 seems favorable for propylene formation route, while MgCrO4 alone will lead to the acetone generation process, and finally a lot of COx would be generated in PODH reaction.
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Acknowledgement The authors gratefully thank the financial support of the National Natural Science Foundation of China (Grant No. 21173186). References
Fig. 10. Reaction results of isopropanol oxidation on different samples.
4. Conclusions Nano-MgO supported Cr catalysts exhibit two characteristics: (a) from 1.0 to 12.0 wt.%, Cr(VI) species are more favorably generated compared with Cr(III); (b) Cr5 O12 and MgCrO4 were formed, and they have different redox properties. In propane oxidative dehydrogenation, Cr-5.0 sample gives the best catalytic performance. To explore the active Cr phase for PODH reaction, the consecutively reduction outcomes of Cr-5.0 sample were extensively studied. It was revealed that Cr5 O12 is favorable for propylene generation route, while MgCrO4 alone will lead to the acetone formation process. In order to give a more specific picture of the catalytic behavior, studies on molecular structure of the active centers are in process.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Nano-MgO supported CrOx catalysts applied in propane oxidative dehydrogenation: Relationship between active chromium phases and propane reaction pathway Fei Ma, Shu Chen, Yanhua Li, Hang Zhou, Aixin Xu, Weimin Lu ∗ Institute of Catalysis, Zhejiang University (Xixi Campus), Hangzhou 310028, PR China
a r t i c l e
i n f o
Article history: Received 22 April 2014 Received in revised form 22 May 2014 Accepted 7 June 2014 Available online 13 June 2014 Keywords: Nano materials Oxidative dehydrogenation Active phase Propane reaction pathway
a b s t r a c t Propane oxidative dehydrogenation was investigated on nano-MgO supported Cr catalysts. Cr-5.0 wt.% sample exhibited the best catalytic performance with a propylene selectivity of 84.1% and propane conversion of 10.8% at 450 ◦ C. Cr5 O12 and MgCrO4 were proved to be formed, and they could be consecutively reduced as shown by temperature programmed reduction results. The impacts of different Cr phases on propane reaction pathway were also investigated. Fourier transform infrared spectroscopy reflects that Cr5 O12 is favorable for the propane dehydrogenation process, while MgCrO4 would activate propane but lead to the COx outcome. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Propylene has many important industrial derivatives such as polypropylene, acrylonitrile, propylene oxide, acrylic acids, and oligomers [1]. Propane oxidative dehydrogenation (PODH) is an attractive exothermic, on-demand propylene production route. The exploration of a good catalyst that satisfies the industrial demand has never been stopped [2–6]. Supported chromium materials are among the most promising catalysts for PODH reaction, mainly because of its efficient ability in propane C–H bond activation [7]. Moreover, it has been proved that CrOx ’s interact with the carrier determines Cr species’ distribution and the formation of active sites [8]. Consequently, the choice of an ideal carrier becomes crucial. On account of the property of characteristic interaction with the doping metal oxide, and the modification of the acid property on the surface, MgO goes for a good candidate [9–11]. V-Mg-O material was the first and classic catalyst that performed well in several catalytic reactions [12]. For Cr-MgO material, the physical property, polymerization degree of chromium, Cr valance states have already been studied, and it was concluded that MgCrO4 (Cr(VI)) was formed at
∗ Corresponding author. Tel.: +86 57188273283; fax: +86 57188273283. E-mail address: [email protected] (W. Lu). http://dx.doi.org/10.1016/j.apsusc.2014.06.042 0169-4332/© 2014 Elsevier B.V. All rights reserved.
around 400 ◦ C calcination, while treatment at over 600 ◦ C would generates MgCr2 O4 (Cr(III) [13,14]. There are also examples of Cr-MgO applied in the catalytic reaction of cyclohexanol to 2,6dimethyphenol; isobutane dehydrogenation [15,16]. However, to the best of our knowledge, only one report is available for CrMgO materials directly used in PODH reaction [8], and the catalytic performance was unsatisfied. It needs to be mentioned that in their experiment the calcination temperature was 600 ◦ C, and the incorporation amount was pretty high (10.0 wt.%). Although XRD and XPS analysis were not given, the chromium species in their catalysts might be inappropriate for the PODH process. To sum up: modifications in the preparation procedure, Cr doping amount are needed to improve the catalytic behavior. Moreover, the active phase of Cr-MgO in PODH reaction has not been defined until now, and the catalyst structure–activity relationship remains unclear. Recently, nano-scale catalysts drive a lot of attention, as they possess surprisingly high reactivity not observed in their bulk analogues, besides only a mild reaction condition is needed [17,18]. In this paper, nano-MgO supported chromium catalysts were prepared and evaluated in PODH reaction. Through an integrated analysis, two chromium phases were detected and proved to ameliorate the catalytic performance. The relationship between chromium phases and propane reaction pathway was also tentatively given, which might give insight for the development of a new class of catalysts.
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2. Experimental 2.1. Catalysts preparation Nano-MgO (Aladdin, 50 nm) was used as support for preparing the catalysts studied in this work. Chromium was deposited on the MgO surface (1.0–12.0 wt.%) by wet impregnation with an aqueous solution of Cr(NO3 )3 ·9H2 O. The samples were then dried in an oven at 80 ◦ C overnight. The obtained dried powders were calcined at 450 ◦ C in air for 4 h with a ramping rate of 10 ◦ C/min. The catalysts with different Cr contents are denoted as Cr-x in part of the text, where “x” represents the doping weight percent. Pure MgCrO4 (Sigma) was used for comparison in XRD and FT-IR experiment. 2.2. Catalysts characterization Powder XRD patterns of the samples were obtained on an X-ray diffractometer (XRD, Rigaku-D/Max-B automated powder X-ray diffractometer) operating at 45 kV and 40 mA using Cu K␣ radiation ( = 0.15418 nm). X-ray photoelectron spectroscopy (XPS) experiments were carried out on a Thermo ESCALAB 250 system with Al K␣ radiation (h = 1486.6 eV). The X-ray anode was run at 150 W, and binding energies were calibrated using the C 1s line as a standard (284.8 eV). H2 -temperature programmed reduction (TPR) experiments were carried out in a flow reactor system using 20 mg of catalysts. The sample was reduced in a 10% H2 /Ar stream (40 ml/min) with a heating rate of 10 ◦ C/min from 100 to 600 ◦ C. Propane FT-IR spectra were recorded at selected temperature on a Nicolet Magna 750 Fourier transform instrument with a liquidnitrogen-cooled CCD detector. Before measurement, the catalysts were pressed in self-supporting discs and pretreated in the IR cell at 250 ◦ C for 4 h under vacuum. Isopropanol oxidation was carried out in a fixed-bed quartz reactor. The mixture flow of oxygen/Ar (controlled by two mass flow controllers) was transferred through a saturator with isopropanol to bring it to the reaction bed. EPR experiment was operated to investigate the local environment of chromium. The electron paramagnetic resonance (EPR) measurements were performed at room temperature using Bruker spectrometers operating in the X-band frequency range (∼9.5 GHz) with 100 kHz field modulation.
Fig. 1. XRD patterns of different catalysts: (a) MgO, (b) Cr-1.0, (c) Cr-2.5, (d) Cr-5.0, (e) Cr-8.0, (f) Cr-12.0 and (g) MgCrO4 , inset is the zoom out of the area between 2 = 23◦ and 26◦ .
of Cr5 O12 on different samples, the relative intensity of the peak 25.4◦ versus 23.8◦ was given (I25 /I23 , inset of Fig. 1). If there is no contribution from Cr5 O12 for the 25.4◦ diffraction, the I25 /I23 value should be consistent with MgCrO4 . As we can see, I25 /I23 on Cr-5.0 is 0.8154, which is quite larger than 0.6170 of MgCrO4 , implying the coexistence of Cr5 O12 with MgCrO4 . From Cr-5.0 to Cr-12.0, I25 /I23 decreases from 0.8154 to a minimum of 0.6312, close to that of MgCrO4 (0.6170). It manifests that Cr5 O12 is hardly detectable on 12.0 wt.% by XRD. At high chromium doping amount (8.0–12.0 wt.%), diffractions attributing to Cr2 O3 (2 = 33.58, 36.28◦ , JCPDS No.: 70-3765) appear. The crystal structure of Cr5 O12 has been clearly discussed [19], and Cr5 O12 was characterized on supported cerium and tin oxide through EPR technique [20,21]. In our experiment (Fig. 2), the carrier (line a) gives no signal in the detection field, excluding the interference with Cr peak. The pattern of Cr-1.0 catalyst (line b) has one isotropic peak centered at 3557 G, g = 1.97. On 2.5 wt.%, a superimposed small shoulder at 3590 G, g = 1.96 shows up, and it
2.3. Catalytic performance The typical PODH reaction was tested at 450 ◦ C, GHSV = 4500 ml (g h)−1 , propane/O2 molar ratio = 2:1. The feedstock and reaction products were analyzed on-line by gas chromatography with two column types: Porapak QS (4.0 m × 1/8 in.) and TDX-01 (2.0 m × 1/8 in.). 3. Results and discussion 3.1. Characterization of Cr phase and its redox property XRD patterns of calcined samples are shown in Fig. 1. The diagram of the carrier (line a) gives diffractions at 2 = 36.8◦ , 42.8◦ , 62.1◦ , 74.5◦ , 78.4◦ , which can be indexed to characteristic MgO phase (JCPDS No.: 74-1225). For the samples incorporating with Cr between 1.0 and 2.5 wt.% (lines b and c), only MgO were detected. Increasing the doping amount between 5.0 and 12.0 wt.% (patterns d–f), new phases of MgCrO4 (2 = 23.8◦ , 25.4◦ , 32.5◦ , JCPDS No.: 721606) and Cr5 O12 (2 = 25.48◦ , JCPDS No.: 73-1787) appear. The profile of pure MgCrO4 (lineg) corresponds well with the above MgCrO4 diffractions. It needs to be mentioned that both MgCrO4 and Cr5 O12 are diffractive at 2 = 25.4◦ . To verify the existence
Fig. 2. EPR spectra for MgO supported chromium catalysts: (a) MgO, (b) Cr-1.0, (c) Cr-2.5, (d) Cr-5.0, (e) Cr-8.0 and (f) Cr-12.0.
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Fig. 3. TEM image of Cr-5.0 sample.
becomes more apparent from Cr-2.5 to Cr-12.0 (lines c–f). The two peaks have been assigned as  (g = 1.96) and ␥ (g = 1.97) signals of Cr species [20]. The  signal is referred to clusters of Cr(III), while the ␥ signal is attributed to “Cr(VI)-O-Cr(III)-O-Cr(VI)” mixed-valence trimmers, which belongs to Cr5 O12 structure [21,22]. In order to give a visualized image of the catalyst, TEM experiment was conducted and the results are shown in Fig. 3. The as-synthesized Cr-5.0 sample was mainly constituted of irregular spheroidal particles with an average diameter of 50 nm. XPS results of the different catalysts are displayed in Fig. 4. The deconvoluted Cr 2p3/2 profiles indicate that Cr exists as two oxidation states. Binding energies of 579.2 and 576.5 eV could be accounted for Cr (VI) and Cr(III), respectively [23]. The proportion of Cr(VI) or Cr(III) versus (Cr(VI) + Cr(III)) on the surface are also given. Cr(VI) accounts for over 95% on Cr-1.0. At increasing doping
amount, the percentage of Cr(III) increases, but it is below 20% even on Cr-12.0. The XRD and XPS analysis above tells that both Cr(VI) and Cr(III) were formed on MgO surface. Cr(VI) could be reduced to Cr(III) by H2 [24], and the H2 -TPR results in our experiment are shown in Fig. 5(1). No reduction in the region of 200–600 ◦ C was detected on MgO (line a). For pure MgCrO4 (line g), the reduction peak at around 500 ◦ C shows up, which is in agreement with previous report [25]. When Cr was incorporated into MgO, a peak at around 400 ◦ C was detected on the Cr-1.0 sample (line b). Between Cr-2.5 and Cr-12.0 (lines c–f), another reduction at over 500 ◦ C occurred. The existence of two reduction peaks might originate from Cr(VI) species having different local environment [8]. By referring to the profile of MgCrO4 (line g), the higher reduction peak on supported samples might be associated with MgCrO4 ’s reduction. Moreover, as MgCrO4 could not be reduced at 400 ◦ C and no other Cr species existed in our experiment, the 400 ◦ C peak may be caused by Cr5 O12 ’s reduction. To verify it, the Cr-5.0 catalyst was subjected to an in situ, controlled reduction step by H2 , which is widely used to study the different reduction degree outcome of transition metal oxides [26]. The H2 -TPR profiles of the different reduction process on Cr-5.0 sample are shown in Fig. 5(2). The one defined as R480 was firstly reduced to 480 ◦ C, and then hydrogen supply was cut off and the sample was cooled to r.t. under Ar. Pristine Cr-5.0 catalyst reduced to 600 ◦ C was named R600 . The three samples (Cr-5.0, R480 and R600 ) were characterized by XRD and EPR techniques. As shown in Fig. 6, typical MgCrO4 diffractions at 2 = 23.81◦ , 25.48◦ and 32.55◦ (JCPDS No.: 72-1606) are observable on R480 , consolidating that MgCrO4 was retained. From the inset image, I25 /I23 decreases from 0.8154 of Cr-5.0 to 0.6190 of R480 , which is almost the same as MgCrO4 (0.6170). It indicates that Cr5 O12 was reduced on R480 , corroborating the H2 -TPR conclusion above. In R600 sample, MgCrO4 was also largely reduced as its typical diffractions all disappeared.
Fig. 4. XPS results of the Cr 2p3/2 region.
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Fig. 5. (1) H2 -TPR profiles of: (a) MgO, (b) Cr-1.0, (c) Cr-2.5, (d) Cr-5.0, (e) Cr-8.0, (f) Cr-12.0 and (g) MgCrO4 . (2) H2 -TPR profiles of Cr-5.0 sample reduced at different conditions: (a) H2 -reduction stopped at 480 ◦ C (R480 ) and (b) H2 -reduction stopped at 600 ◦ C (R600 ).
EPR spectra of pristine Cr-5.0, R480 and R600 are displayed in Fig. 7. As has been discussed (Fig. 2), the spectrum of Cr-5.0 catalyst comprises  (g = 1.96) and ␥ (g = 1.97) signals. The sample of R480 brings about the totally disappearance of ␥, and the enhancement of  signal. It means the trimmers structure of Cr(VI)-O-Cr(III)O-Cr(VI) belonging to Cr5 O12 was destroyed. The  signal of R600 becomes stronger, which could result from more quantity of Cr(III) reduced by MgCrO4 . The above analysis also supplies a proof for Fig. 5 that the peak at around 400 ◦ C was caused by the reduction of Cr(VI) to Cr(III) in Cr5 O12 . As Cr5 O12 and MgCrO4 are consecutively reduced, the crystal phase-activity relationship could be studied tentatively by the activity comparison of Cr-5.0, R480 , R600 and MgCrO4 .
Fig. 6. XRD patterns of samples: (a) Cr-5.0, (b) R480 and (c) R600 .
3.2. Catalytic activity Table 1 shows the catalytic performance in PODH reaction on different catalysts. It could be read that on nano-MgO, the propane conversion is negligible. With the incorporation of Cr, propane conversion increases from 4.8% to 10.8% as the doping increases from 1.0 to 5.0 wt.%, and then the conversion was attenuated. The selectivity of propylene reaches the highest value of 84.0% on Cr-5.0 catalyst. Between Cr-8.0 and Cr-12.0, the decreased selectivity of propylene seems to be compromised by the generation of COx . Fig. 8 shows propylene selectivity as a function of propane conversion varied by contact time for the four samples: pristine Cr-5.0, R480 , R600 and MgCrO4 . Propylene selectivity drops with the
Fig. 7. EPR spectra of Cr-5.0, R480 and R600 .
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Table 1 Catalytic performance of propane oxidative dehydrogenation on nano-MgO supported Cr catalysts at 450 ◦ C, GHSV = 4500 ml (g h)−1 , propane/oxygen molar ratio = 2:1. Samples
MgO Cr-1.0 Cr-2.5 Cr-5.0 Cr-8.0 Cr-12.0 R480 R600 MgCrO4 Cr2 O3
Catalytic activity Propane conversion (%)
Propylene selectivity (%)
COx selectivity (%)
0.7 4.8 6.5 10.8 7.2 4.1 9.5 3.8 13.6 0.9
65.8 73.0 75.1 84.1 62.9 37.4 34.0 21.0 18.4 26.0
34.2 28.8 24.9 15.9 37.1 62.6 66.0 79.0 82.6 74.0
increase of propane conversion on all samples. At the same propane conversion, the propylene selectivity on R480 decreased apparently compared with Cr-5.0 catalyst, indicating that Cr5 O12 is essential for the selective outcome. It should be noted that under the same contact time, propane conversion almost does not descend from Cr-5.0 to R480 , demonstrating that the step of propane activation was not affected too much. Meanwhile, the decrease of propylene selectivity indicates that absorbed propane on R480 mainly forms deep oxidized products like COx . The profile of MgCrO4 was quite similar with that of R480 , consolidating that MgCrO4 was active for propane activation but nonselective for propylene generation. The catalytic performance on R600 was much poorer compared with R480 , illustrating the handicap in the activation of propane. 3.3. Relationship between crystal phase and propane reaction network In order to give a track on how the reaction route was affected by different Cr species, IR experiment was implemented. The IR spectra of adsorbed propane on different catalysts (after purging with Ar) are shown in Fig. 9. For the situation on Cr-5.0 (band c), a double peak at 1677 and 1637 cm−1 were observed due to C C stretching; other less intense peaks at 1440 and1350 cm−1
Fig. 8. Propylene selectivity as a function of propane conversion on Cr-5.0, R480 , R600 and MgCrO4 (evaluated at 450 ◦ C, molar ratio of C3 H8 :O2 = 2:1). Propane conversion increases with the contact time (W/F) of 0.08, 0.12, 0.16, 0.24 s.
Fig. 9. FT-IR spectra of adsorbed propane on: (a) Cr2 O3 , (b) R600 , (c) Cr-5.0, (d) R480 and (e) MgCrO4 .
associating to asymmetric and symmetric CH3 deformation coexist in the band [27,28]. It indicates the generation of propylene product, similar to the report of adsorbed propane on supported molybdenum catalysts [29]. Another peak at around 1290 cm−1 was also detected, and it was assigned to C O stretching vibration in CO3 (precursor of COx ) [28]. On R480 , new peaks of 1688 and 1706 cm−1 showed up (band d), which have been attributed to be VC=O in adsorbed acetone [29]. The appearance of acetone absorption indicates that propane reaction network has been transformed. The IR spectrum of adsorbed propane on R480 was very similar to that of on MgCrO4 (band e), illustrating it might be MgCrO4 that leads to the acetone outcome. Moreover, in case of the stronger intensity of the 1290 cm−1 peak on R480 and MgCrO4 (compared with pristine Cr-5.0 catalyst), it could be concluded that larger quantity of CO2 precursors were formed on R480 and MgCrO4 . Weak and broad IR peaks were detected(bands b and a) on R600 andCr2 O3 . It means there might be little active sites on Cr2 O3 for propane adsorption, in keeping with others’ reports [20]. It is generally believed that propane is activated from abstracting a hydrogen atom by surface nucleophilic oxygen, and the alkoxy groups formed in step one either dehydrate to propylene or form acetone [29]. Isopropanol oxidation has been commonly adopted as a probe reaction to test the reaction network of alkoxy species [30]. The isopropanol conversion results in our experiment are shown in Fig. 10. Propylene and acetone were the products detected. Propylene selectivity accounts for over 90% on Cr-5.0 catalyst, and it was transformed largely to acetone (acetone selectivity over 80%) on R480 and MgCrO4 . It testifies that MgCrO4 is favorable for the dehydrogenation process of alkoxy groups, in line with FT-IR analysis. In case of the analysis above, Cr5 O12 seems favorable for propylene formation route, while MgCrO4 alone will lead to the acetone generation process, and finally a lot of COx would be generated in PODH reaction.
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Acknowledgement The authors gratefully thank the financial support of the National Natural Science Foundation of China (Grant No. 21173186). References
Fig. 10. Reaction results of isopropanol oxidation on different samples.
4. Conclusions Nano-MgO supported Cr catalysts exhibit two characteristics: (a) from 1.0 to 12.0 wt.%, Cr(VI) species are more favorably generated compared with Cr(III); (b) Cr5 O12 and MgCrO4 were formed, and they have different redox properties. In propane oxidative dehydrogenation, Cr-5.0 sample gives the best catalytic performance. To explore the active Cr phase for PODH reaction, the consecutively reduction outcomes of Cr-5.0 sample were extensively studied. It was revealed that Cr5 O12 is favorable for propylene generation route, while MgCrO4 alone will lead to the acetone formation process. In order to give a more specific picture of the catalytic behavior, studies on molecular structure of the active centers are in process.
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