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Al2O3 catalysts in catalytic dehydrogenation of propane
Catalysis Communications 60 (2015) 42–45
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Effect of sulfate addition on the performance of Co/Al2O3 catalysts in catalytic dehydrogenation of propane Ya-nan Sun, Ya-nan Gao, Yimin Wu, Honghong Shan, Guowei Wang, Chunyi Li ⁎ State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, PR China
a r t i c l e
i n f o
Article history: Received 3 August 2014 Received in revised form 2 November 2014 Accepted 20 November 2014 Available online 21 November 2014 Keywords: Metallic Co species Cracking Dispersion Electronic effect Propylene SO2
a b s t r a c t The effect of sulfate species in propane dehydrogenation over Co/Al2O3 catalysts was studied. On the one hand, the interaction between SO2− 4 and cobalt species leads to better dispersed cobalt oxide and restrains the reduction to metallic Co species, thereby inhibiting cracking reaction. On the other hand, the initial C–H bond activation and scission are facilitated on the electron-deficient cobalt and sulfate sites, resulting in dramatically improved dehydrogenation performance. XPS, MS and XRD results show that the reduction of sulfate species to SO2 and consequently the loss of active cobalt species contribute to the irreversible deactivation of catalyst. Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.
1. Introduction Catalytic dehydrogenation of propane is an excellent way to convert cheaper and available propane into high value-added propylene. Currently, most researchers are focused on commercial Cr-based [1–3] and Pt-based [4–6] catalysts. However, due to the serious pollution of Cr6+ and the high cost of Pt, their applications are largely restricted. Besides, Pt-promoted Ga/Al2O3 [7], Zn-MOR [8] and HNO3-activated mesoporous carbon [9] catalytic systems are reported to be promising catalysts in propane dehydrogenation. Nevertheless, little attention has been paid to the application of cobalt-supported catalysts. Cobalt-based catalysts are known to be efficient in many reactions such as F–T synthesis [10,11] and partial oxidation of alkanes [12,13]. Freas and Campana [14] reported that the terminal cobalt atoms of oxygen-deficient cobalt cluster ions can insert into hydrocarbon bonds and catalyze the dehydrogenation of isobutene in the gas phase. Furthermore, van Koppen et al. [15] investigated the reaction of Co+ with propane, proposing that C–H activation only occurs on the surface of ground state Co+ ions while C–C activation is prone to occur on the surface of excited-state Co+ ions, consequently, dehydrogenation reaction and cracking reaction proceed respectively. Despite the great difference between the homogeneous gas-phase reaction and the heterogeneous gas–solid reaction, these findings provide deep insight into the nature of dehydrogenation reaction.
⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (C. Li).
http://dx.doi.org/10.1016/j.catcom.2014.11.024 1566-7367/Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.
According to our previous experiment results, the breakage of C–C bond occurred preferentially in propane dehydrogenation over Co/ Al2O3 catalysts with high loadings of Co3O4 (above 10 wt.%), generating abundant methane. Therefore, great effort should be made to achieve higher selectivity of propylene. In previous studies, we found that the introduction of sulfate to Ni-based [16] and Fe-based [17] catalysts dramatically inhibited cracking reaction and improved the selectivity of alkenes in alkane dehydrogenation, eg: up to 20 wt.% propylene yield with 80% selectivity were obtained over Fe2O3/Al2O3 catalysts [17]. Herein, in this paper, sulfate species were introduced to Co/Al2O3 catalyst with 20 wt.% loading of Co3O4 by the impregnation of ammonium sulfate. The effect of sulfate addition on the structural and electronic properties of Co/Al2O3 catalyst was investigated and correlated with the catalytic performance in propane dehydrogenation. Finally, the reasons for the catalyst deactivation were also discussed. 2. Experimental Detailed description of the synthesis, characterization and catalytic test was presented in Supporting material S1 experimental. 3. Results and discussion 3.1. Effect of sulfate addition on the performance in propane dehydrogenation The reaction results over 20Co/Al and 20Co–5S/Al catalysts at 560 °C in propane dehydrogenation were displayed in Fig. 1(a) and (b). With regard to 20Co/Al, only 3 wt.% propylene yield at an initial conversion
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Fig. 1. Reaction results over (a) 20Co/Al, (b) 20Co–5S/Al (reaction conditions: T = 560 °C, P = 0.1 MPa, m cat = 2 g, FC3H8 = 12 mL/min) and (c) 20Co–5S/Al catalysts (reaction conditions: T = 600 °C, P = 0.1 MPa, m cat = 3 g, FC3H8 = 12 mL/min).
of 50 wt.% was observed. Besides, a large amount of oxidation product COx (16.6 wt.% yield) was detected, suggesting that the oxidative reaction predominates in the initial stage. After 1 h, propane conversion increased to almost 100 wt.% while methane yield to 92 wt.% without the formation of propylene. Meanwhile, the yield of COx decreased to 0.8 wt.%, indicating a fast consumption of surface oxygen species. Thus, it is concluded that the dramatically enhanced cracking reaction is probably resulted from the changes in cobalt species during the reaction, hence generating abundant methane. In order to get more information about the changes in cobalt species during the reaction, XRD characterization of fresh and spent 20Co/Al, which exhibited high cracking activity was carried out and shown in Fig. 2. As for fresh 20Co/Al, diffraction peaks centered at 2θ 31.2, 36.8, 59.3 and 65.2 indicative of Co3O4 consistent with reported values (JCDPS 01-081-1543) were observed, which disappeared after reaction and accompanied by the appearance of metallic Co phase. As reported in literature [18], metallic Co species were very active in C–C rupture through catalyzing the hydrogenolysis of alkanes. Therefore, it is deduced that as the reaction proceeded, Co3O4 was reduced to metallic Co species in the presence of generated hydrogen, resulting in dramatically increased yield and selectivity of methane.
Compared with 20Co/Al, 20Co–5S/Al showed a much lower initial propane conversion of about 22 wt.%. With prolonged reaction time, both propane conversion and propylene yield increased in the first 2 h, eg: 22.4 wt.% propylene yield and 80% selectivity were obtained at TOS (time on stream) = 2 h. Though the activity is lower than that reported for Cr-based and Pt-based catalysts, the catalyst is environment-friendly compared with Cr-based catalysts and cheaper than Pt-based catalysts. Moreover, the yield of methane was maintained at a rather low level during the investigated 6 h, TG–DTA results also showed that the carbon deposition for 20Co/Al after reaction for 6 h was 482 mg/gcat while that of 20Co–5S/Al was 91 mg/gcat, demonstrating that the cracking reaction is effectively inhibited with sulfate addition. Furthermore, the reaction was performed at 600 °C using 3 g of 20Co–5S/Al to get an identical conversion (Fig. 1(c)). At TOS = 2 h, propane conversion increased to 50 wt.% with propylene selectivity to 70.2%, which is much higher than that over 20Co/Al (TOS = 0.16 h), demonstrating the improved catalytic performance with sulfate addition. Based on these results, it is concluded that sulfate addition significantly altered the reaction route over 20Co/Al catalyst, turning dehydrogenation reaction into the predominant reaction. For detailed elaboration, we will discuss it in the following sections. 3.2. The role of sulfate species Based on the above results, it is concluded that the metallic Co species generated from the reduction of Co3O4 during the reaction are the active sites for cracking reaction over 20Co/Al catalyst. Whereas the introduction of sulfate species dramatically inhibits the cracking reaction and promotes propane dehydrogenation, leading to much higher yield and selectivity of propylene. Thus, it is of great interest to explore the origin of the promoting effect of sulfate species.
Fig. 2. XRD patterns of (a) fresh 20Co/Al, (b) fresh 20Co–5S/Al, (c) 20Co–5S/Al after five reaction–regeneration cycles, (d) spent 20Co/Al, (e) reduced 20Co–5S/Al, and (f) reduced 20Co/Al.
3.2.1. Dispersion effect As shown in Fig. 2, the XRD pattern of 20Co–5S/Al was very similar to that of 20Co/Al, predominantly exhibiting diffraction lines corresponding to Co3O4 with weaker intensity, suggesting that sulfate addition effectively suppresses the aggregation of cobalt oxide, which is consistent with the increased surface area (from 111.3 to 121.0 m2/g) and decreased average size (from 49.5 nm to 35.6 nm) of Co3O4 crystals calculated from the (311) line using Scherrer equation (Table S1). The SEM images (Fig. 3) of the catalysts using the oxide precursors also showed that after the introduction of sulfate species, the relatively large catalyst particles (300–400 nm) were divided into small particles of about 100 nm, further verifying that sulfate addition effectively prevents the formation of large ensembles of cobalt metals which cause undesirable cracking reaction.
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Fig. 3. SEM images of (a) 20Co/Al and (b) 20Co–5S/Al catalysts.
In order to further authenticate above speculations, 20Co/Al and 20Co–5S/Al were reduced in hydrogen at 560 °C for 2 h before reaction and the reaction results were displayed in Fig. S1. As expected, propane was converted to methane without the formation of propylene over reduced 20Co/Al, meanwhile, much higher propane conversion of 99 wt.% was obtained. According to XRD results, obvious diffraction peaks characteristic of metallic Co species were observed for reduced 20Co/Al, demonstrating the high cracking activity of metallic Co species. In comparison, propylene was still the main product with the formation of a small amount of methane during the investigated 6 h over reduced 20Co–5S/Al. Besides, no diffraction lines of metallic Co were detected, confirming that sulfate addition significantly restrains the reduction of Co3O4 to metallic Co species. 3.2.2. Effect on the acidity and electronic properties As discussed above, sulfate addition not only improves the dispersion of cobalt species but also strongly inhibits the reduction to large metallic Co particles, resulting in the transformation from cracking reaction to dehydrogenation reaction. To get a deeper understanding of the favorable interaction, IR, H2-TPR and XPS measurements were carried out. The FT-IR spectra of Al2O3 and 20Co/Al only exhibited a strong band at around 1450 cm−1 attributed to the chemisorption of pyridine on Lewis acid sites [19]. After sulfate addition, Bronsted acid sites represented by the absorption band at 1540 cm−1 appeared, meanwhile, the intensity of the band at 1490 cm−1 indicative of the total amount of acid significantly increased, suggesting an enhanced acidity, which is supposed to be responsible for the improved dehydrogenation performance. Fig. 4 showed the Co 2p spectra of all samples. As for 20Co/Al, the Co 2p peak located at 779.6 eV with an associated satellite peak at 785.8 eV, ascribing to the presence of Co3O4 [20,21]. After sulfate addition, the band position shifted to higher binding energy (780.2 eV), indicating an electron transfer from Co atoms, which can be attributed to the strong electron-withdrawing ability of SO24 as evidenced by the S 2p peak centered at around 169.3 eV (Fig. S2) [22]. On the basis of aforementioned results, it is concluded that the impregnation of alumina by ammonium sulfate significantly improved the interaction between the Co precursor and the support, leading to increased cobalt dispersion and enhanced acidity. That is, the generated sulfate species interact with Co atoms via Co–O–S bond and confer on Co atoms at electronefficient state. For one thing, the Co–O bonds are strengthened and make the reduction to metallic Co species more difficult, consequently, inhibiting the cracking reaction. This can be further verified by the H2TPR results as shown in Fig. S4. 20Co/Al mainly exhibited two reduction peaks centered at about 520 °C and 580 °C, which were attributed to the reduction process of Co3O4 → Co2+ and Co2+ → Co. After sulfate addition, only one peak in the range of 500–600 °C ascribing to the reduction
from Co3O4 to Co2+ was observed, demonstrating above speculations. For another, the initial C–H scission was facilitated at the sulfate site in a manner similar to that proposed by Burch et al. for propane activation − over Pt/Al2O3 catalysts [23,24], forming C3H+ 7 and H which will intern+ 2− act with CoLC and OLC (LC denotes low coordination) respectively. According to this mechanism, the lower coordinated Co sites (probably Co2 +) are in favor of propane dehydrogenation, agreeing with the observed “induction period” of activity over 20Co–5S/Al. All in all, the sulfate addition to 20Co/Al catalyst exerts a dispersion effect and obviously improves both the acidity and electronic properties. In this regard, the reduction to large metallic cobalt species which are responsible for cracking reaction is strongly inhibited, meanwhile, C–H bond activation and subsequent rupture are facilitated, leading to higher propylene yield and selectivity. 3.3. Catalyst deactivation Fig. S5 exhibited the reaction results over 20Co–5S/Al catalyst during the five reaction–regeneration cycles. The activity gradually decreased with increased cycles, eg: after five cycles, only 14.8 wt.% propylene yield was retained at a conversion of 19.4 wt.%, suggesting that in addition to carbon deposition, there are other reasons for the irreversible deactivation of catalyst. According to XPS results (Table S2), the sulfur content of 20Co–5S/Al (Table S2) decreased from 2.0 wt.% to 1.0 wt.% after five cycles, suggesting a loss of sulfate species. Fig. 5 also showed that the content of SO2 during the reaction–regeneration cycles dramatically decreased,
Fig. 4. Co 2p spectra for 20Co/Al, fresh 20Co–5S/Al, regenerated 20Co–5S/Al (after five reaction–regeneration cycles) of oxide precursors.
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SO2− 4 to SO2 and thus the loss of active cobalt species contribute to the irreversible deactivation of catalyst. These findings can help to improve the dehydrogenation performance of cobalt-based catalysts by introducing electron-withdrawing promoters and increasing the dispersion of cobalt species. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant No. U1362201) and the Fundamental Research Funds for the Central Universities (No. R1404019A). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.11.024. Fig. 5. MS signal of SO2 during reaction–regeneration cycles over 20Co–5S/Al.
References confirming the reduction of sulfate species and release in the form of SO2. Besides, the intensity of Co3O4 significantly decreased after five cycles (Fig. 2). Combining with the decreased surface molar ratio of Co/Al, it is concluded that with the loss of sulfate species and consequently weakened Co–O–S bond, active cobalt ions may migrate and incorporate into the octahedral sites of alumina, causing the formation of CoAl2O4-like phase as evidenced the Co 2p peak at around 781 eV [25] and thus the decreased activity. Besides, though the activity decreased gradually with prolonged reaction time in each cycle, the selectivity of propylene exhibited little changes, suggesting that the loss of sulfate species does not change the reaction route over 20Co–5S/Al. 4. Conclusions The effect of sulfate addition on the performance of Co/Al2O3 catalysts in catalytic dehydrogenation of propane was studied. It is found that the generated sulfate species improve the dispersion, acidity and electronic properties of cobalt species. The dispersion effect restrains the aggregation of cobalt oxide and the reduction to metallic Co species, thus inhibiting the cracking reaction. Meanwhile, the electronic effect facilitates C–H bond activation and scission, leading to significantly improved dehydrogenation performance. However, the reduction of
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Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Effect of sulfate addition on the performance of Co/Al2O3 catalysts in catalytic dehydrogenation of propane Ya-nan Sun, Ya-nan Gao, Yimin Wu, Honghong Shan, Guowei Wang, Chunyi Li ⁎ State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, PR China
a r t i c l e
i n f o
Article history: Received 3 August 2014 Received in revised form 2 November 2014 Accepted 20 November 2014 Available online 21 November 2014 Keywords: Metallic Co species Cracking Dispersion Electronic effect Propylene SO2
a b s t r a c t The effect of sulfate species in propane dehydrogenation over Co/Al2O3 catalysts was studied. On the one hand, the interaction between SO2− 4 and cobalt species leads to better dispersed cobalt oxide and restrains the reduction to metallic Co species, thereby inhibiting cracking reaction. On the other hand, the initial C–H bond activation and scission are facilitated on the electron-deficient cobalt and sulfate sites, resulting in dramatically improved dehydrogenation performance. XPS, MS and XRD results show that the reduction of sulfate species to SO2 and consequently the loss of active cobalt species contribute to the irreversible deactivation of catalyst. Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.
1. Introduction Catalytic dehydrogenation of propane is an excellent way to convert cheaper and available propane into high value-added propylene. Currently, most researchers are focused on commercial Cr-based [1–3] and Pt-based [4–6] catalysts. However, due to the serious pollution of Cr6+ and the high cost of Pt, their applications are largely restricted. Besides, Pt-promoted Ga/Al2O3 [7], Zn-MOR [8] and HNO3-activated mesoporous carbon [9] catalytic systems are reported to be promising catalysts in propane dehydrogenation. Nevertheless, little attention has been paid to the application of cobalt-supported catalysts. Cobalt-based catalysts are known to be efficient in many reactions such as F–T synthesis [10,11] and partial oxidation of alkanes [12,13]. Freas and Campana [14] reported that the terminal cobalt atoms of oxygen-deficient cobalt cluster ions can insert into hydrocarbon bonds and catalyze the dehydrogenation of isobutene in the gas phase. Furthermore, van Koppen et al. [15] investigated the reaction of Co+ with propane, proposing that C–H activation only occurs on the surface of ground state Co+ ions while C–C activation is prone to occur on the surface of excited-state Co+ ions, consequently, dehydrogenation reaction and cracking reaction proceed respectively. Despite the great difference between the homogeneous gas-phase reaction and the heterogeneous gas–solid reaction, these findings provide deep insight into the nature of dehydrogenation reaction.
⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (C. Li).
http://dx.doi.org/10.1016/j.catcom.2014.11.024 1566-7367/Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.
According to our previous experiment results, the breakage of C–C bond occurred preferentially in propane dehydrogenation over Co/ Al2O3 catalysts with high loadings of Co3O4 (above 10 wt.%), generating abundant methane. Therefore, great effort should be made to achieve higher selectivity of propylene. In previous studies, we found that the introduction of sulfate to Ni-based [16] and Fe-based [17] catalysts dramatically inhibited cracking reaction and improved the selectivity of alkenes in alkane dehydrogenation, eg: up to 20 wt.% propylene yield with 80% selectivity were obtained over Fe2O3/Al2O3 catalysts [17]. Herein, in this paper, sulfate species were introduced to Co/Al2O3 catalyst with 20 wt.% loading of Co3O4 by the impregnation of ammonium sulfate. The effect of sulfate addition on the structural and electronic properties of Co/Al2O3 catalyst was investigated and correlated with the catalytic performance in propane dehydrogenation. Finally, the reasons for the catalyst deactivation were also discussed. 2. Experimental Detailed description of the synthesis, characterization and catalytic test was presented in Supporting material S1 experimental. 3. Results and discussion 3.1. Effect of sulfate addition on the performance in propane dehydrogenation The reaction results over 20Co/Al and 20Co–5S/Al catalysts at 560 °C in propane dehydrogenation were displayed in Fig. 1(a) and (b). With regard to 20Co/Al, only 3 wt.% propylene yield at an initial conversion
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Fig. 1. Reaction results over (a) 20Co/Al, (b) 20Co–5S/Al (reaction conditions: T = 560 °C, P = 0.1 MPa, m cat = 2 g, FC3H8 = 12 mL/min) and (c) 20Co–5S/Al catalysts (reaction conditions: T = 600 °C, P = 0.1 MPa, m cat = 3 g, FC3H8 = 12 mL/min).
of 50 wt.% was observed. Besides, a large amount of oxidation product COx (16.6 wt.% yield) was detected, suggesting that the oxidative reaction predominates in the initial stage. After 1 h, propane conversion increased to almost 100 wt.% while methane yield to 92 wt.% without the formation of propylene. Meanwhile, the yield of COx decreased to 0.8 wt.%, indicating a fast consumption of surface oxygen species. Thus, it is concluded that the dramatically enhanced cracking reaction is probably resulted from the changes in cobalt species during the reaction, hence generating abundant methane. In order to get more information about the changes in cobalt species during the reaction, XRD characterization of fresh and spent 20Co/Al, which exhibited high cracking activity was carried out and shown in Fig. 2. As for fresh 20Co/Al, diffraction peaks centered at 2θ 31.2, 36.8, 59.3 and 65.2 indicative of Co3O4 consistent with reported values (JCDPS 01-081-1543) were observed, which disappeared after reaction and accompanied by the appearance of metallic Co phase. As reported in literature [18], metallic Co species were very active in C–C rupture through catalyzing the hydrogenolysis of alkanes. Therefore, it is deduced that as the reaction proceeded, Co3O4 was reduced to metallic Co species in the presence of generated hydrogen, resulting in dramatically increased yield and selectivity of methane.
Compared with 20Co/Al, 20Co–5S/Al showed a much lower initial propane conversion of about 22 wt.%. With prolonged reaction time, both propane conversion and propylene yield increased in the first 2 h, eg: 22.4 wt.% propylene yield and 80% selectivity were obtained at TOS (time on stream) = 2 h. Though the activity is lower than that reported for Cr-based and Pt-based catalysts, the catalyst is environment-friendly compared with Cr-based catalysts and cheaper than Pt-based catalysts. Moreover, the yield of methane was maintained at a rather low level during the investigated 6 h, TG–DTA results also showed that the carbon deposition for 20Co/Al after reaction for 6 h was 482 mg/gcat while that of 20Co–5S/Al was 91 mg/gcat, demonstrating that the cracking reaction is effectively inhibited with sulfate addition. Furthermore, the reaction was performed at 600 °C using 3 g of 20Co–5S/Al to get an identical conversion (Fig. 1(c)). At TOS = 2 h, propane conversion increased to 50 wt.% with propylene selectivity to 70.2%, which is much higher than that over 20Co/Al (TOS = 0.16 h), demonstrating the improved catalytic performance with sulfate addition. Based on these results, it is concluded that sulfate addition significantly altered the reaction route over 20Co/Al catalyst, turning dehydrogenation reaction into the predominant reaction. For detailed elaboration, we will discuss it in the following sections. 3.2. The role of sulfate species Based on the above results, it is concluded that the metallic Co species generated from the reduction of Co3O4 during the reaction are the active sites for cracking reaction over 20Co/Al catalyst. Whereas the introduction of sulfate species dramatically inhibits the cracking reaction and promotes propane dehydrogenation, leading to much higher yield and selectivity of propylene. Thus, it is of great interest to explore the origin of the promoting effect of sulfate species.
Fig. 2. XRD patterns of (a) fresh 20Co/Al, (b) fresh 20Co–5S/Al, (c) 20Co–5S/Al after five reaction–regeneration cycles, (d) spent 20Co/Al, (e) reduced 20Co–5S/Al, and (f) reduced 20Co/Al.
3.2.1. Dispersion effect As shown in Fig. 2, the XRD pattern of 20Co–5S/Al was very similar to that of 20Co/Al, predominantly exhibiting diffraction lines corresponding to Co3O4 with weaker intensity, suggesting that sulfate addition effectively suppresses the aggregation of cobalt oxide, which is consistent with the increased surface area (from 111.3 to 121.0 m2/g) and decreased average size (from 49.5 nm to 35.6 nm) of Co3O4 crystals calculated from the (311) line using Scherrer equation (Table S1). The SEM images (Fig. 3) of the catalysts using the oxide precursors also showed that after the introduction of sulfate species, the relatively large catalyst particles (300–400 nm) were divided into small particles of about 100 nm, further verifying that sulfate addition effectively prevents the formation of large ensembles of cobalt metals which cause undesirable cracking reaction.
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Fig. 3. SEM images of (a) 20Co/Al and (b) 20Co–5S/Al catalysts.
In order to further authenticate above speculations, 20Co/Al and 20Co–5S/Al were reduced in hydrogen at 560 °C for 2 h before reaction and the reaction results were displayed in Fig. S1. As expected, propane was converted to methane without the formation of propylene over reduced 20Co/Al, meanwhile, much higher propane conversion of 99 wt.% was obtained. According to XRD results, obvious diffraction peaks characteristic of metallic Co species were observed for reduced 20Co/Al, demonstrating the high cracking activity of metallic Co species. In comparison, propylene was still the main product with the formation of a small amount of methane during the investigated 6 h over reduced 20Co–5S/Al. Besides, no diffraction lines of metallic Co were detected, confirming that sulfate addition significantly restrains the reduction of Co3O4 to metallic Co species. 3.2.2. Effect on the acidity and electronic properties As discussed above, sulfate addition not only improves the dispersion of cobalt species but also strongly inhibits the reduction to large metallic Co particles, resulting in the transformation from cracking reaction to dehydrogenation reaction. To get a deeper understanding of the favorable interaction, IR, H2-TPR and XPS measurements were carried out. The FT-IR spectra of Al2O3 and 20Co/Al only exhibited a strong band at around 1450 cm−1 attributed to the chemisorption of pyridine on Lewis acid sites [19]. After sulfate addition, Bronsted acid sites represented by the absorption band at 1540 cm−1 appeared, meanwhile, the intensity of the band at 1490 cm−1 indicative of the total amount of acid significantly increased, suggesting an enhanced acidity, which is supposed to be responsible for the improved dehydrogenation performance. Fig. 4 showed the Co 2p spectra of all samples. As for 20Co/Al, the Co 2p peak located at 779.6 eV with an associated satellite peak at 785.8 eV, ascribing to the presence of Co3O4 [20,21]. After sulfate addition, the band position shifted to higher binding energy (780.2 eV), indicating an electron transfer from Co atoms, which can be attributed to the strong electron-withdrawing ability of SO24 as evidenced by the S 2p peak centered at around 169.3 eV (Fig. S2) [22]. On the basis of aforementioned results, it is concluded that the impregnation of alumina by ammonium sulfate significantly improved the interaction between the Co precursor and the support, leading to increased cobalt dispersion and enhanced acidity. That is, the generated sulfate species interact with Co atoms via Co–O–S bond and confer on Co atoms at electronefficient state. For one thing, the Co–O bonds are strengthened and make the reduction to metallic Co species more difficult, consequently, inhibiting the cracking reaction. This can be further verified by the H2TPR results as shown in Fig. S4. 20Co/Al mainly exhibited two reduction peaks centered at about 520 °C and 580 °C, which were attributed to the reduction process of Co3O4 → Co2+ and Co2+ → Co. After sulfate addition, only one peak in the range of 500–600 °C ascribing to the reduction
from Co3O4 to Co2+ was observed, demonstrating above speculations. For another, the initial C–H scission was facilitated at the sulfate site in a manner similar to that proposed by Burch et al. for propane activation − over Pt/Al2O3 catalysts [23,24], forming C3H+ 7 and H which will intern+ 2− act with CoLC and OLC (LC denotes low coordination) respectively. According to this mechanism, the lower coordinated Co sites (probably Co2 +) are in favor of propane dehydrogenation, agreeing with the observed “induction period” of activity over 20Co–5S/Al. All in all, the sulfate addition to 20Co/Al catalyst exerts a dispersion effect and obviously improves both the acidity and electronic properties. In this regard, the reduction to large metallic cobalt species which are responsible for cracking reaction is strongly inhibited, meanwhile, C–H bond activation and subsequent rupture are facilitated, leading to higher propylene yield and selectivity. 3.3. Catalyst deactivation Fig. S5 exhibited the reaction results over 20Co–5S/Al catalyst during the five reaction–regeneration cycles. The activity gradually decreased with increased cycles, eg: after five cycles, only 14.8 wt.% propylene yield was retained at a conversion of 19.4 wt.%, suggesting that in addition to carbon deposition, there are other reasons for the irreversible deactivation of catalyst. According to XPS results (Table S2), the sulfur content of 20Co–5S/Al (Table S2) decreased from 2.0 wt.% to 1.0 wt.% after five cycles, suggesting a loss of sulfate species. Fig. 5 also showed that the content of SO2 during the reaction–regeneration cycles dramatically decreased,
Fig. 4. Co 2p spectra for 20Co/Al, fresh 20Co–5S/Al, regenerated 20Co–5S/Al (after five reaction–regeneration cycles) of oxide precursors.
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SO2− 4 to SO2 and thus the loss of active cobalt species contribute to the irreversible deactivation of catalyst. These findings can help to improve the dehydrogenation performance of cobalt-based catalysts by introducing electron-withdrawing promoters and increasing the dispersion of cobalt species. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant No. U1362201) and the Fundamental Research Funds for the Central Universities (No. R1404019A). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.11.024. Fig. 5. MS signal of SO2 during reaction–regeneration cycles over 20Co–5S/Al.
References confirming the reduction of sulfate species and release in the form of SO2. Besides, the intensity of Co3O4 significantly decreased after five cycles (Fig. 2). Combining with the decreased surface molar ratio of Co/Al, it is concluded that with the loss of sulfate species and consequently weakened Co–O–S bond, active cobalt ions may migrate and incorporate into the octahedral sites of alumina, causing the formation of CoAl2O4-like phase as evidenced the Co 2p peak at around 781 eV [25] and thus the decreased activity. Besides, though the activity decreased gradually with prolonged reaction time in each cycle, the selectivity of propylene exhibited little changes, suggesting that the loss of sulfate species does not change the reaction route over 20Co–5S/Al. 4. Conclusions The effect of sulfate addition on the performance of Co/Al2O3 catalysts in catalytic dehydrogenation of propane was studied. It is found that the generated sulfate species improve the dispersion, acidity and electronic properties of cobalt species. The dispersion effect restrains the aggregation of cobalt oxide and the reduction to metallic Co species, thus inhibiting the cracking reaction. Meanwhile, the electronic effect facilitates C–H bond activation and scission, leading to significantly improved dehydrogenation performance. However, the reduction of
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