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Al2O3-ZrO2 catalysts
Molecular Catalysis 433 (2017) 1–7
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Dehydrogenation of propane to propylene with lattice oxygen over CrOy /Al2 O3 -ZrO2 catalysts Tae Hyeop Kim a , Ki Hyuk Kang a , Minsung Baek a , Ji Hwan Song a , Ung Gi Hong b , Deuk Soo Park b , Won Choon Choi c , Yong-Ki Park c , In Kyu Song a,∗ a School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-744, South Korea b SK Gas R&D Center, Gyeonggi-do 463-400, South Korea c Green Chemistry and Catalysis Research Center, Korea Research Institute of Chemical Technology, Daejeon 305-600, South Korea
a r t i c l e
i n f o
Article history: Received 11 November 2016 Accepted 11 December 2016 Keywords: Propane Dehydrogenation Propylene Chromia catalyst Oxygen capacity
a b s t r a c t A series of CrOy /Al2 O3 -ZrO2 (denoted as Cr/AZ-X) catalysts with different Zr/Al molar ratio (X) were prepared by an epoxide-driven sol-gel method and a subsequent impregnation method. They were then applied to the dehydrogenation of propane to propylene with lattice oxygen of the catalysts. The effect of zirconia addition on the physicochemical properties and catalytic activities of Cr/AZ-X catalysts was investigated. Surface area and pore volume of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts decreased with increasing Zr/Al molar. It was found that the amount of penta-coordinated aluminum species increased with increasing Zr/Al molar ratio. It was also observed that dispersion and coordination state of chromium species could be controlled by the addition of zirconia into alumina support. Initial conversion of propane and initial yield for propylene showed volcano-shaped trends with respect to Zr/Al molar ratio. This result was well correlated with the amount of lattice oxygen of Cr/AZ-X catalysts. Among the catalysts, Cr/AZ-0.10 catalyst with the largest amount of lattice oxygen showed the best catalytic performance in the dehydrogenation of propane to propylene with lattice oxygen. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Propylene is an important material in petrochemical industries because it can be used as a precursor for valuable chemicals such as acetone, propylene oxide, and isopropanol [1–4]. Propylene is mainly produced through naphtha cracking process. Nonetheless, development of on-purpose propylene production technology is still required because global demand for propylene is continuously increased [5]. In this respect, direct or oxidative dehydrogenation of propane has attracted much attraction as an efficient process for on-purpose propylene production. For oxidative dehydrogenation of propane to propylene, various transition-metal oxide catalysts such as MoOy [6–10], VOy [11–15], and CrOy [16–20] have been investigated. Among these catalysts, Cr-based catalysts supported on ␥-alumina show an excellent catalytic activity because of their superior redox property. However, acidic property of ␥-alumina support can cause coke formation and side reactions such as cracking of propane [21]. For this reason,
∗ Corresponding author. E-mail address: [email protected] (I.K. Song). http://dx.doi.org/10.1016/j.mcat.2016.12.004 1381-1169/© 2016 Elsevier B.V. All rights reserved.
second metal oxides as a structural modifier have been added to ␥-alumina support to solve deactivation problem and to enhance catalytic activity. In an attempt to enhance catalytic performance of Cr/Al2 O3 catalysts in the dehydrogenation of propane, addition of second metal including La, V, Si, W, and Zr to ␥-alumina has been investigated [11]. SiO2 has also been used as a second metal element for Cr2 O3 /Al2 O3 catalysts due to its high ability for dispersing chromia species [22]. In particular, it has been reported that the addition of zirconia modifies physicochemical properties of alumina by forming a defective electronic structure [23]. Zirconia is also known to be responsible for neutralization of acid property of alumina, which significantly decreases coke deposition [23]. In addition, zirconia can change oxygen capacity of active Cr species of the catalysts. Thus, it is expected that dehydrogenation of propane to propylene can be promoted by the addition of second metal element into the support. Therefore, a systematic investigation on the effect of zirconium addition on the catalytic activities and physicochemical properties of CrOy /Al2 O3 catalysts would be worthwhile. In this work, a series of CrOy /Al2 O3 -ZrO2 catalysts were prepared by a sol-gel method and a subsequent wetness impregnation method with a variation of Zr/Al molar ratio, and they were applied
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to the dehydrogenation of propane to propylene with lattice oxygen of the catalysts. The effect of zirconia addition on the physicochemical properties and catalytic activities was investigated. Several characterizations such as nitrogen adsorption-desorption, ICP-AES, XRD, STEM-EDX, UV-vis-DRS, 27 Al MAS NMR, and H2 -TPR analyses were conducted to investigate the physicochemical properties of the catalysts. The reaction time of our catalytic reaction was short because the catalyst in the reaction was designed for use in the fluidized-bed reactor where the catalyst was quickly circulated from reactor to regenerator.
Table 1 Textural properties of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts. Catalyst
Surface area (m2 /g)a
Pore volume (cm3 /g)b
Pore diameter (nm)c
Cr/AZ-0 Cr/AZ-0.05 Cr/AZ-0.10 Cr/AZ-0.15 Cr/AZ-0.20
214 212 209 204 194
0.42 0.39 0.38 0.35 0.34
7.8 7.4 8.2 6.9 7.0
a b c
Calculated by the BET equation. Total pore volume at P/P0 = 0.99. Average pore diameter.
2. Experimental 2.1. Catalyst preparation A series of Al2 O3 -ZrO2 supports with different Zr/Al molar ratio were obtained by a sol-gel method. Known amounts of Al(NO3 )3 ·9H2 O (Sigma-Aldrich) and ZrO(NO3 )2 ·xH2 O (SigmaAldrich) were dissolved in an anhydrous ethanol (30 ml) at room temperature with stirring for 5 h. Zr/Al molar ratio was adjusted to be 0, 0.05, 0.10, 0.15, and 0.20. Propylene oxide (15 ml) was introduced into the solution as a gelation agent. White gel was obtained within a few minutes after the addition of propylene oxide. After gel was aged for 1 day, it was treated with ethanol in order to maintain pore structure of the support by removing impurities. The gel was then dried at 80 ◦ C in an oven for 4 days. The resultant was grinded and calcined at 700 ◦ C for 4 h to yield a Al2 O3 -ZrO2 support. The obtained supports were denoted as AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20), where X represented the Zr/Al molar ratio. Chromia catalysts supported on zirconia-modified alumina (AZ-X) were prepared by a wetness impregnation method. CrO3 (Sigma-Aldrich) was dissolved in deionized water (10 ml) and it was impregnated on AZ-X supports with stirring at 60 ◦ C. The supported chromia catalysts were dried at 80 ◦ C overnight in a convection oven. The prepared catalysts were finally calcined at 700 ◦ C for 4 h to yield chromia catalysts supported on AZ-X supports. The prepared catalysts were denoted as Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20). The Cr loading was fixed at 17.5 wt% in all Cr/AZ-X catalysts. 2.2. Characterization of catalysts Textural properties of Cr/AZ-X catalysts were obtained by nitrogen adsorption-desorption measurements (BELSORP-mini II, BEL Japan). Before the analyses, all the catalysts were pretreated at 150 ◦ C for 3 h with a rotary vacuum pump to remove moisture and impurities. Surface areas of the catalysts were calculated by the BET method. Nitrogen adsorption-desorption isotherms were obtained at −196 ◦ C. Metal contents of the catalysts were determined by inductively coupled plasma atomic emission spectroscopy (ICPAES) analyses (ICPS-7500, Shimadzu). Crystalline structures of the catalysts were investigated by X-ray diffraction (XRD) measurements (D-Max2500-PC, Rigaku). XRD patterns were obtained under Cu-K␣ radiation ( = 1.541 Å) operated at 50 kV and 100 mA. In order to confirm dispersion of metal species, scanning transmission electron microscopy (STEM) analyses (JEM-2100F, JEOL) were conducted with energy dispersive X-ray spectroscopy (EDX) mapping. Coordination states of chromium species were examined by ultraviolet visible (UV-vis) spectroscopy analyses (Lambda-35, Perkin-Elmer) within the range of 200–800 nm. Chemical states of aluminum species in the catalysts were examined with a magic angle spinning nuclear magnetic resonance (27 Al MAS NMR) spectrometer (Bruker Avance II, Bruker) operated at a resonance frequency of 104 MHz for 27 Al. Temperature-programmed reduction (TPR) measurements were performed in a U-shaped quartz
reactor in order to calculate the amount of lattice oxygen of the catalysts. For this, each catalyst was reduced under a flow of H2 (2 ml/min) diluted with N2 (20 ml/min). TPR profile was collected using a thermal conductivity detector (TCD) at a heating rate of 5 ◦ C/min from room temperature to 700 ◦ C. 2.3. Dehydrogenation of propane to propylene Propylene production by dehydrogenation of propane with lattice oxygen of the catalysts was carried out in a continuous flow fixed-bed quartz reactor at 630 ◦ C under atmosphere pressure. Each catalyst (0.2 g) was charged into the reactor, and it was pretreated under O2 flow (6 ml/min) diluted with He flow (24 ml/min) at 630 ◦ C. The reactor was then purged with He flow (24 ml/min) for 20 min, and the dehydrogenation of propane was conducted at 630 ◦ C for 45 s in the absence of oxygen feed. The products were sequentially sampled in 16-position valve at a 3 s interval. Feed composition of inlet gas was fixed at C3 H8 :N2 = 40 ml:40 ml, and total feed rate with respect to catalyst weight was maintained at 12,000 ml/(h·903;gcat ). After the reaction, products were analyzed using an on-line gas chromatograph (ACME 6000, Younglin) equipped with a thermal conductivity detector (TCD). Conversion of propane, selectivity for propylene, and yield for propylene over Cr/AZ-X catalystswere calculated according to the following equations. Conversion forpropane (%) =
mole of propane reacted × 100 (1) mole of propane supplied
Selectivity for propylene (%) =
mole of propylene formed × 100 mole of propane reacted (2)
Yield for propylene(%) = (Conversion of propane) × (Selectivity for propylene)/100
(3)
3. Results and discussion 3.1. Textural properties of Cr/AZ-X catalysts Nitrogen adsorption-desorption measurements were conducted to examine textural properties of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts as shown in Fig. 1. It was found that all the catalysts exhibited IV-type isotherms, which were attributed to well-developed mesopores [24]. H2-type hysteresis loops were also observed in the isotherms of the catalysts, indicating the presence of mesopores with bottle-necks. Detailed textural properties of the catalysts are summarized in Table 1. All the catalysts retained high surface area (>190 m2 /g), large pore volume (>0.3 cm3 /g), and large pore diameter (>6 nm). This indicates that well-developed mesopores were formed. It is interesting to note that surface area
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Fig. 1. Nitrogen adsorption-desorption isotherms of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts. Table 2 ICP-AES results of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts. Catalyst
Cr loading (wt%)a
Zr/Al molar ratioa
Cr/AZ-0 Cr/AZ-0.05 Cr/AZ-0.10 Cr/AZ-0.15 Cr/AZ-0.20
14.4 12.4 14.6 14.9 16.9
0 0.04 0.08 0.14 0.21
a
Determined by ICP-AES measurement.
Table 3 Crystallite sizes of chromia of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts. Catalyst
Crystallite size of chromia (nm)a
Cr/AZ-0 Cr/AZ-0.05 Cr/AZ-0.10 Cr/AZ-0.15 Cr/AZ-0.20
9.7 11.7 14.5 16.2 20.3
a Calculated by the Scherrer equation form ␣-Cr2 O3 (1 0 4) diffraction peak broadening in Fig. 2.
and pore volume of the catalysts decreased with increasing Zr/Al molar ratio. This might be because the addition of zirconium hindered the development of pore structure of alumina support during the preparation step [25–27]. However, average pore diameter of Cr/AZ-X catalysts showed no consistent trend with regard to the amount of zirconium added to the support. Actual Cr loading (wt%) and Zr/Al molar ratio of the catalysts were similar to the designed values as listed in Table 2. 3.2. XRD and STEM-EDX analyses of Cr/AZ-X catalysts Crystalline structures of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts were confirmed by X-ray diffraction (XRD) measurements as shown in Fig. 2. All the catalysts exhibited several diffraction peaks attributed to chromium species. Peak intensity of chromia gradually increased with increasing Zr/Al molar ratio of the catalysts. Crystallite sizes of chromia species calculated by the Scherrer equation using ␣-Cr2 O3 (1 0 4) peak broadening are listed in Table 3. Crystallite size of chromia increased with increasing Zr/Al molar
Fig. 2. X-ray diffraction (XRD) patterns of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts.
ratio. This is because the decrement of surface area by zirconium addition caused aggregation of chromia. It was found that intensity of diffraction peaks for ␥-alumina decreased with increasing Zr/Al molar ratio. This means that the addition of zirconia led to the formation of amorphous alumina. Diffraction peaks for zirconia were not detected in the Cr/AZ-X (X = 0.05 and 0.10) catalysts because zirconia was highly dispersed on the catalyst surface [28]. On the other hand, diffraction peaks for zirconia appeared in the Cr/AZ-X (X = 0.15 and 0.20) catalysts and peak intensity for zirconia increased with increasing Zr/Al molar ratio. In order to confirm distribution of aluminum, chromium, and zirconium species, STEM-EDX analyses were conducted. Fig. 3 shows the STEM and EDX mapping images of Cr/AZ-0.10 catalyst. Each metal was distinguishable in the EDX mapping images. It is noticeable that zirconium species (red dots in Fig. 3) and aluminum species (blue dots in Fig. 3) were co-presented in the catalyst. STEM-EDX mapping images revealed that zirconium species were homogeneously distributed in the alumina support without aggregation. It was also found that chromium species (yellow dots in Fig. 3) were well dispersed on the support. This result demonstrates that Cr/AZ-X catalysts were successfully prepared in this work. 3.3. UV–vis and 27 Al MAS NMR analyses of Cr/AZ-X catalysts In order to elucidate coordination state of chromium species, UV–vis analyses for Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts were conducted as represented in Fig. 4. All the catalysts exhibited four absorption bands at 267, 373, 470, and 614 nm. The bands at 267 and 373 nm correspond to Cr6+ species with a tetrahedral symmetry [29]. The bands at 475 and 614 nm are indicative of Cr3+ species with octahedral symmetry in the bulk ␣-Cr2 O3 phase [30]. It was observed that all the catalysts retained both Cr6+ and Cr3+ species. However, peak intensity for Cr3+ increased with increasing Zr/Al molar ratio. This indicates that the coordination state of chromium species could be modified by the addition of zirconia into the support. It has been reported that the amount of Cr3+ species increased with increasing particle size of chromia on the alumina support [29]. Thus, it can be inferred that the amount
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Fig. 3. STEM and EDX images of Cr/AZ-0.10 catalyst obtained by mapping on chromium, aluminum, and zirconium.
Fig. 4. UV–vis spectra of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts.
of Cr3+ species of Cr/AZ-X catalysts increased due to aggregation of chromium species as revealed from XRD results (Fig. 2 and Table 3). Chemical states of aluminum species in the Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts were investigated by 27 Al MAS NMR analyses as shown in Fig. 5. It has been reported that there are three kinds of aluminum species; octa-coordinated aluminum (AlVI ), penta-coordinated aluminum (AlV ), and tetracoordinated aluminum (AlIV ) [31]. It is known that penta- and tetra-coordinated aluminums are formed by cross-linking between two octa-coordinated aluminums [32]. The catalysts showed three
Fig. 5.
27
Al MAS NMR spectra of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts.
peaks at 6, 30, and 75 ppm, corresponding to octa-, penta-, and tetra-coordinated aluminum, respectively. It was observed that peak intensity for penta-coordinated aluminum gradually increased with increasing Zr/Al molar ratio. It is believed that introduction of metal species into alumina support causes vacant and defective structure in alumina, which leads to the formation of penta-coordinated aluminum species [33]. Thus, it can be inferred that the addition of zirconia was responsible for the formation of unsaturated penta-coordinated aluminum. It is noteworthy that chemical shift of penta- and tetra-coordinated aluminum species
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Fig. 6. H2 -TPR profiles of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts.
Fig. 7. Catalytic activities of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts in the dehydrogenation of propane to propylene with lattice oxygen of the catalysts at 630 ◦ C.
moved to lower chemical shift, indicating that electron density of aluminum atom increased by electron transfer from zirconium atom in the Al-O-Zr structure. This means that chemical binding between aluminum species and anchored chromium species decreased with increasing Zr/Al molar ratio. Thus, zirconia addition significantly affected structural and electronical properties of alumina support. 3.4. H2 -TPR analyses of Cr/AZ-X catalysts In order to measure the amount of lattice oxygen, H2 -TPR experiments were conducted. As shown in Fig. 6, all the catalysts showed a reduction peak at around 340–350 ◦ C which was associated with the reduction of chromia species. Thus, total amount of lattice oxygen can be calculated from the amount of hydrogen
reacted with the catalysts. The peak area of TPR profile was different with a variation of Zr/Al molar ratio. The amount of lattice oxygen (oxygen capacity) calculated from the peak area of TPR profile is summarized in Table 4. The oxygen capacity increased in the order of Cr/AZ-0 < Cr/AZ-0.05 < Cr/AZ-0.20 < CrAZ-0.15 < Cr/AZ0.10. Among the catalysts, Cr/AZ-0.10 catalyst showed the largest oxygen capacity. This indicates that the addition of appropriate amount of zirconia can enhance the amount of lattice oxygen of Cr/AZ-X catalysts. It has been reported that oxygen capacity of the catalysts was closely related to the catalytic performance in the partial oxidation using lattice oxygen of metal oxide [34]. Thus, it is expected that Cr/AZ-0.10 catalyst would serve as an efficient catalyst in the dehydrogenation of propane to propylene with lattice oxygen.
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Table 4 H2 -TPR results of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts. Catalyst
Oxygen capacity (mol-Olattice /gcat )a
Cr/AZ-0 Cr/AZ-0.05 Cr/AZ-0.10 Cr/AZ-0.15 Cr/AZ-0.20
243.8 249.5 288.1 265.9 254.9
a Calculated from peak area of H2 -TPR profiles in Fig. 6 on the basis of a redox mechanism: H2 + MeOx,lattice → H2 O + MeOx-1 ,lattice .
3.5. Catalytic performance in the dehydrogenation of propane to propylene Catalytic activities in the dehydrogenation of propane to propylene with lattice oxygen of the catalysts are presented in Fig. 7. It was observed that conversion of propane and yield for propylene rapidly decreased during short reaction time (45 s). This is because the limited amount of lattice oxygen in chromia is the only oxygen source in this reaction. It also means that propylene was formed via oxidative dehydrogenation of propane using lattice oxygen of the catalysts. It was observed that selectivity for propylene over Cr/AZ-X (X = 0.05, 0.10, 0.15, and 0.20) was higher than that over Cr/AZ-0 catalyst. This indicates that zirconia was effective for suppressing coke formation and side reactions such as cracking of propane. Conversion of propane and yield for propylene over Cr/AZX (X = 0.05, 0.10, 0.15, and 0.20) catalyst were also higher than those over Cr/AZ-0 catalyst. Thus, we can say that the addition of zirconia was effective for enhancing catalytic activities by modifying oxygen capacity of the catalysts. Detailed conversion of propane and yield for products obtained after 6 s-reaction and 45-s reaction are listed in Table 5. It was found that C1 -C2 compounds (methane, ethylene, and ethane), hydrogen, carbon monoxide, and carbon dioxide were produced as by-products together with a desired product (propylene). Fig. 8 shows the initial conversion of propane and initial yield for propylene over Cr/AZ-X catalysts, plotted as a function of Zr/Al molar ratio. Both conversion of propane and yield for propylene showed volcano-shaped curves with regard to Zr/Al molar ratio. Initial conversion of propane and initial yield for propylene increased in the order of Cr/AZ-0 < Cr/AZ-0.05 < Cr/AZ-0.20 < Cr/AZ0.15 < Cr/AZ-0.10. This result was well correlated with the trend of oxygen capacity of the catalysts as presented in Fig. 9. As oxygen capacity increased, initial yield for propylene over Cr/AZ-X catalysts increased. Among the catalysts, Cr/AZ-0.10 catalyst with the largest oxygen capacity showed the highest initial yield for propylene in the dehydrogenation of propane with lattice oxygen. This is because zirconia led to the enhanced oxygen capacity of the catalyst by modifying physicochemical properties of the catalyst.
Fig. 8. Correlation between initial catalytic activity and Zr/Al molar ratio of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts.
Fig. 9. Correlation between initial yield for propylene and oxygen capacity of Cr/AZX (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts.
4. Conclusions A series of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts with different Zr/Al molar ratio (X) were prepared by a sol-gel method and a subsequent wetness impregnation method for use in the dehydrogenation of propane to propylene with lattice oxygen of the catalysts. The effect of Zr/Al molar ratio
Table 5 Catalytic performance of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts in the dehydrogenation of propane to propylene (reaction temperature = 630 ◦ C, GHSV = 12,000 ml/gcat ).
a
Catalyst
Xi (%)a
Xf (%)b
Cr/AZ-0 Cr/AZ-0.05 Cr/AZ-0.10 Cr/AZ-0.15 Cr/AZ-0.20
33.1 35.3 43.8 41.0 37.0
27.0 27.6 29.3 31.8 30.3
Yi (%)a
Yf (%)b
C3 H6
C1 -C2
H2
CO
CO2
C3 H6
C1 -C2
H2
CO
CO2
25.5 27.3 36.8 33.8 30.0
3.4 3.1 3.4 2.7 3.4
1.4 1.5 1.9 1.6 1.4
1.1 1.5 0.8 1.0 1.0
0.8 0.7 0.9 1.2 1.0
21.1 23.3 26.5 26.5 25.7
2.5 2.1 2.1 1.8 2.8
0.9 1.0 1.0 1.0 1.0
0.4 0.6 0.4 0.5 0.5
0.2 0.2 0.2 0 0
Xi is initial conversion of propane and Yi is initial yield obtained after 6 s-reaction. b Xf is final conversion of propane and Yf is final yield obtained after 45 s-reaction.
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on the catalytic activities and physicochemical properties of the catalysts was investigated. Surface area and pore volume of the catalysts decreased with increasing Zr/Al molar ratio. 27 Al MAS NMR analyses showed that the catalyst with high Zr/Al molar ratio retained much amount of penta-coordinated aluminum species. In the STEM-EDX mapping analyses, it was found that aluminum and zirconium species were co-existed in the Cr/AZ-0.10 catalyst. XRD results revealed that crystallite size of chromia increased with increasing Zr/Al molar ratio because of the aggregation of chromia species. It was also found that oxygen capacity of the catalysts was different depending on Zr/Al molar ratio. In the dehydrogenation of propane to propylene, initial conversion of propane and initial yield for propylene showed volcano-shaped trends with respect to Zr/Al molar ratio. Thus, an optimal amount of zirconia addition to alumina support was required for efficient propylene production. Initial yield for propylene increased with increasing oxygen capacity of the catalysts, indicating that oxygen capacity played an important role in determining the catalytic performance in the reaction. Among the catalysts, Cr/AZ-0.10 catalyst with the largest oxygen capacity served as the most efficient catalyst in the dehydrogenation of propane to propylene. Acknowledgment The authors wish to acknowledge support from the Korea Research Institute of Chemical Technology (KRICT) (045820160024). References [1] [2] [3] [4]
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Editor’s choice paper
Dehydrogenation of propane to propylene with lattice oxygen over CrOy /Al2 O3 -ZrO2 catalysts Tae Hyeop Kim a , Ki Hyuk Kang a , Minsung Baek a , Ji Hwan Song a , Ung Gi Hong b , Deuk Soo Park b , Won Choon Choi c , Yong-Ki Park c , In Kyu Song a,∗ a School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-744, South Korea b SK Gas R&D Center, Gyeonggi-do 463-400, South Korea c Green Chemistry and Catalysis Research Center, Korea Research Institute of Chemical Technology, Daejeon 305-600, South Korea
a r t i c l e
i n f o
Article history: Received 11 November 2016 Accepted 11 December 2016 Keywords: Propane Dehydrogenation Propylene Chromia catalyst Oxygen capacity
a b s t r a c t A series of CrOy /Al2 O3 -ZrO2 (denoted as Cr/AZ-X) catalysts with different Zr/Al molar ratio (X) were prepared by an epoxide-driven sol-gel method and a subsequent impregnation method. They were then applied to the dehydrogenation of propane to propylene with lattice oxygen of the catalysts. The effect of zirconia addition on the physicochemical properties and catalytic activities of Cr/AZ-X catalysts was investigated. Surface area and pore volume of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts decreased with increasing Zr/Al molar. It was found that the amount of penta-coordinated aluminum species increased with increasing Zr/Al molar ratio. It was also observed that dispersion and coordination state of chromium species could be controlled by the addition of zirconia into alumina support. Initial conversion of propane and initial yield for propylene showed volcano-shaped trends with respect to Zr/Al molar ratio. This result was well correlated with the amount of lattice oxygen of Cr/AZ-X catalysts. Among the catalysts, Cr/AZ-0.10 catalyst with the largest amount of lattice oxygen showed the best catalytic performance in the dehydrogenation of propane to propylene with lattice oxygen. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Propylene is an important material in petrochemical industries because it can be used as a precursor for valuable chemicals such as acetone, propylene oxide, and isopropanol [1–4]. Propylene is mainly produced through naphtha cracking process. Nonetheless, development of on-purpose propylene production technology is still required because global demand for propylene is continuously increased [5]. In this respect, direct or oxidative dehydrogenation of propane has attracted much attraction as an efficient process for on-purpose propylene production. For oxidative dehydrogenation of propane to propylene, various transition-metal oxide catalysts such as MoOy [6–10], VOy [11–15], and CrOy [16–20] have been investigated. Among these catalysts, Cr-based catalysts supported on ␥-alumina show an excellent catalytic activity because of their superior redox property. However, acidic property of ␥-alumina support can cause coke formation and side reactions such as cracking of propane [21]. For this reason,
∗ Corresponding author. E-mail address: [email protected] (I.K. Song). http://dx.doi.org/10.1016/j.mcat.2016.12.004 1381-1169/© 2016 Elsevier B.V. All rights reserved.
second metal oxides as a structural modifier have been added to ␥-alumina support to solve deactivation problem and to enhance catalytic activity. In an attempt to enhance catalytic performance of Cr/Al2 O3 catalysts in the dehydrogenation of propane, addition of second metal including La, V, Si, W, and Zr to ␥-alumina has been investigated [11]. SiO2 has also been used as a second metal element for Cr2 O3 /Al2 O3 catalysts due to its high ability for dispersing chromia species [22]. In particular, it has been reported that the addition of zirconia modifies physicochemical properties of alumina by forming a defective electronic structure [23]. Zirconia is also known to be responsible for neutralization of acid property of alumina, which significantly decreases coke deposition [23]. In addition, zirconia can change oxygen capacity of active Cr species of the catalysts. Thus, it is expected that dehydrogenation of propane to propylene can be promoted by the addition of second metal element into the support. Therefore, a systematic investigation on the effect of zirconium addition on the catalytic activities and physicochemical properties of CrOy /Al2 O3 catalysts would be worthwhile. In this work, a series of CrOy /Al2 O3 -ZrO2 catalysts were prepared by a sol-gel method and a subsequent wetness impregnation method with a variation of Zr/Al molar ratio, and they were applied
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to the dehydrogenation of propane to propylene with lattice oxygen of the catalysts. The effect of zirconia addition on the physicochemical properties and catalytic activities was investigated. Several characterizations such as nitrogen adsorption-desorption, ICP-AES, XRD, STEM-EDX, UV-vis-DRS, 27 Al MAS NMR, and H2 -TPR analyses were conducted to investigate the physicochemical properties of the catalysts. The reaction time of our catalytic reaction was short because the catalyst in the reaction was designed for use in the fluidized-bed reactor where the catalyst was quickly circulated from reactor to regenerator.
Table 1 Textural properties of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts. Catalyst
Surface area (m2 /g)a
Pore volume (cm3 /g)b
Pore diameter (nm)c
Cr/AZ-0 Cr/AZ-0.05 Cr/AZ-0.10 Cr/AZ-0.15 Cr/AZ-0.20
214 212 209 204 194
0.42 0.39 0.38 0.35 0.34
7.8 7.4 8.2 6.9 7.0
a b c
Calculated by the BET equation. Total pore volume at P/P0 = 0.99. Average pore diameter.
2. Experimental 2.1. Catalyst preparation A series of Al2 O3 -ZrO2 supports with different Zr/Al molar ratio were obtained by a sol-gel method. Known amounts of Al(NO3 )3 ·9H2 O (Sigma-Aldrich) and ZrO(NO3 )2 ·xH2 O (SigmaAldrich) were dissolved in an anhydrous ethanol (30 ml) at room temperature with stirring for 5 h. Zr/Al molar ratio was adjusted to be 0, 0.05, 0.10, 0.15, and 0.20. Propylene oxide (15 ml) was introduced into the solution as a gelation agent. White gel was obtained within a few minutes after the addition of propylene oxide. After gel was aged for 1 day, it was treated with ethanol in order to maintain pore structure of the support by removing impurities. The gel was then dried at 80 ◦ C in an oven for 4 days. The resultant was grinded and calcined at 700 ◦ C for 4 h to yield a Al2 O3 -ZrO2 support. The obtained supports were denoted as AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20), where X represented the Zr/Al molar ratio. Chromia catalysts supported on zirconia-modified alumina (AZ-X) were prepared by a wetness impregnation method. CrO3 (Sigma-Aldrich) was dissolved in deionized water (10 ml) and it was impregnated on AZ-X supports with stirring at 60 ◦ C. The supported chromia catalysts were dried at 80 ◦ C overnight in a convection oven. The prepared catalysts were finally calcined at 700 ◦ C for 4 h to yield chromia catalysts supported on AZ-X supports. The prepared catalysts were denoted as Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20). The Cr loading was fixed at 17.5 wt% in all Cr/AZ-X catalysts. 2.2. Characterization of catalysts Textural properties of Cr/AZ-X catalysts were obtained by nitrogen adsorption-desorption measurements (BELSORP-mini II, BEL Japan). Before the analyses, all the catalysts were pretreated at 150 ◦ C for 3 h with a rotary vacuum pump to remove moisture and impurities. Surface areas of the catalysts were calculated by the BET method. Nitrogen adsorption-desorption isotherms were obtained at −196 ◦ C. Metal contents of the catalysts were determined by inductively coupled plasma atomic emission spectroscopy (ICPAES) analyses (ICPS-7500, Shimadzu). Crystalline structures of the catalysts were investigated by X-ray diffraction (XRD) measurements (D-Max2500-PC, Rigaku). XRD patterns were obtained under Cu-K␣ radiation ( = 1.541 Å) operated at 50 kV and 100 mA. In order to confirm dispersion of metal species, scanning transmission electron microscopy (STEM) analyses (JEM-2100F, JEOL) were conducted with energy dispersive X-ray spectroscopy (EDX) mapping. Coordination states of chromium species were examined by ultraviolet visible (UV-vis) spectroscopy analyses (Lambda-35, Perkin-Elmer) within the range of 200–800 nm. Chemical states of aluminum species in the catalysts were examined with a magic angle spinning nuclear magnetic resonance (27 Al MAS NMR) spectrometer (Bruker Avance II, Bruker) operated at a resonance frequency of 104 MHz for 27 Al. Temperature-programmed reduction (TPR) measurements were performed in a U-shaped quartz
reactor in order to calculate the amount of lattice oxygen of the catalysts. For this, each catalyst was reduced under a flow of H2 (2 ml/min) diluted with N2 (20 ml/min). TPR profile was collected using a thermal conductivity detector (TCD) at a heating rate of 5 ◦ C/min from room temperature to 700 ◦ C. 2.3. Dehydrogenation of propane to propylene Propylene production by dehydrogenation of propane with lattice oxygen of the catalysts was carried out in a continuous flow fixed-bed quartz reactor at 630 ◦ C under atmosphere pressure. Each catalyst (0.2 g) was charged into the reactor, and it was pretreated under O2 flow (6 ml/min) diluted with He flow (24 ml/min) at 630 ◦ C. The reactor was then purged with He flow (24 ml/min) for 20 min, and the dehydrogenation of propane was conducted at 630 ◦ C for 45 s in the absence of oxygen feed. The products were sequentially sampled in 16-position valve at a 3 s interval. Feed composition of inlet gas was fixed at C3 H8 :N2 = 40 ml:40 ml, and total feed rate with respect to catalyst weight was maintained at 12,000 ml/(h·903;gcat ). After the reaction, products were analyzed using an on-line gas chromatograph (ACME 6000, Younglin) equipped with a thermal conductivity detector (TCD). Conversion of propane, selectivity for propylene, and yield for propylene over Cr/AZ-X catalystswere calculated according to the following equations. Conversion forpropane (%) =
mole of propane reacted × 100 (1) mole of propane supplied
Selectivity for propylene (%) =
mole of propylene formed × 100 mole of propane reacted (2)
Yield for propylene(%) = (Conversion of propane) × (Selectivity for propylene)/100
(3)
3. Results and discussion 3.1. Textural properties of Cr/AZ-X catalysts Nitrogen adsorption-desorption measurements were conducted to examine textural properties of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts as shown in Fig. 1. It was found that all the catalysts exhibited IV-type isotherms, which were attributed to well-developed mesopores [24]. H2-type hysteresis loops were also observed in the isotherms of the catalysts, indicating the presence of mesopores with bottle-necks. Detailed textural properties of the catalysts are summarized in Table 1. All the catalysts retained high surface area (>190 m2 /g), large pore volume (>0.3 cm3 /g), and large pore diameter (>6 nm). This indicates that well-developed mesopores were formed. It is interesting to note that surface area
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3
Fig. 1. Nitrogen adsorption-desorption isotherms of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts. Table 2 ICP-AES results of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts. Catalyst
Cr loading (wt%)a
Zr/Al molar ratioa
Cr/AZ-0 Cr/AZ-0.05 Cr/AZ-0.10 Cr/AZ-0.15 Cr/AZ-0.20
14.4 12.4 14.6 14.9 16.9
0 0.04 0.08 0.14 0.21
a
Determined by ICP-AES measurement.
Table 3 Crystallite sizes of chromia of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts. Catalyst
Crystallite size of chromia (nm)a
Cr/AZ-0 Cr/AZ-0.05 Cr/AZ-0.10 Cr/AZ-0.15 Cr/AZ-0.20
9.7 11.7 14.5 16.2 20.3
a Calculated by the Scherrer equation form ␣-Cr2 O3 (1 0 4) diffraction peak broadening in Fig. 2.
and pore volume of the catalysts decreased with increasing Zr/Al molar ratio. This might be because the addition of zirconium hindered the development of pore structure of alumina support during the preparation step [25–27]. However, average pore diameter of Cr/AZ-X catalysts showed no consistent trend with regard to the amount of zirconium added to the support. Actual Cr loading (wt%) and Zr/Al molar ratio of the catalysts were similar to the designed values as listed in Table 2. 3.2. XRD and STEM-EDX analyses of Cr/AZ-X catalysts Crystalline structures of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts were confirmed by X-ray diffraction (XRD) measurements as shown in Fig. 2. All the catalysts exhibited several diffraction peaks attributed to chromium species. Peak intensity of chromia gradually increased with increasing Zr/Al molar ratio of the catalysts. Crystallite sizes of chromia species calculated by the Scherrer equation using ␣-Cr2 O3 (1 0 4) peak broadening are listed in Table 3. Crystallite size of chromia increased with increasing Zr/Al molar
Fig. 2. X-ray diffraction (XRD) patterns of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts.
ratio. This is because the decrement of surface area by zirconium addition caused aggregation of chromia. It was found that intensity of diffraction peaks for ␥-alumina decreased with increasing Zr/Al molar ratio. This means that the addition of zirconia led to the formation of amorphous alumina. Diffraction peaks for zirconia were not detected in the Cr/AZ-X (X = 0.05 and 0.10) catalysts because zirconia was highly dispersed on the catalyst surface [28]. On the other hand, diffraction peaks for zirconia appeared in the Cr/AZ-X (X = 0.15 and 0.20) catalysts and peak intensity for zirconia increased with increasing Zr/Al molar ratio. In order to confirm distribution of aluminum, chromium, and zirconium species, STEM-EDX analyses were conducted. Fig. 3 shows the STEM and EDX mapping images of Cr/AZ-0.10 catalyst. Each metal was distinguishable in the EDX mapping images. It is noticeable that zirconium species (red dots in Fig. 3) and aluminum species (blue dots in Fig. 3) were co-presented in the catalyst. STEM-EDX mapping images revealed that zirconium species were homogeneously distributed in the alumina support without aggregation. It was also found that chromium species (yellow dots in Fig. 3) were well dispersed on the support. This result demonstrates that Cr/AZ-X catalysts were successfully prepared in this work. 3.3. UV–vis and 27 Al MAS NMR analyses of Cr/AZ-X catalysts In order to elucidate coordination state of chromium species, UV–vis analyses for Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts were conducted as represented in Fig. 4. All the catalysts exhibited four absorption bands at 267, 373, 470, and 614 nm. The bands at 267 and 373 nm correspond to Cr6+ species with a tetrahedral symmetry [29]. The bands at 475 and 614 nm are indicative of Cr3+ species with octahedral symmetry in the bulk ␣-Cr2 O3 phase [30]. It was observed that all the catalysts retained both Cr6+ and Cr3+ species. However, peak intensity for Cr3+ increased with increasing Zr/Al molar ratio. This indicates that the coordination state of chromium species could be modified by the addition of zirconia into the support. It has been reported that the amount of Cr3+ species increased with increasing particle size of chromia on the alumina support [29]. Thus, it can be inferred that the amount
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Fig. 3. STEM and EDX images of Cr/AZ-0.10 catalyst obtained by mapping on chromium, aluminum, and zirconium.
Fig. 4. UV–vis spectra of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts.
of Cr3+ species of Cr/AZ-X catalysts increased due to aggregation of chromium species as revealed from XRD results (Fig. 2 and Table 3). Chemical states of aluminum species in the Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts were investigated by 27 Al MAS NMR analyses as shown in Fig. 5. It has been reported that there are three kinds of aluminum species; octa-coordinated aluminum (AlVI ), penta-coordinated aluminum (AlV ), and tetracoordinated aluminum (AlIV ) [31]. It is known that penta- and tetra-coordinated aluminums are formed by cross-linking between two octa-coordinated aluminums [32]. The catalysts showed three
Fig. 5.
27
Al MAS NMR spectra of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts.
peaks at 6, 30, and 75 ppm, corresponding to octa-, penta-, and tetra-coordinated aluminum, respectively. It was observed that peak intensity for penta-coordinated aluminum gradually increased with increasing Zr/Al molar ratio. It is believed that introduction of metal species into alumina support causes vacant and defective structure in alumina, which leads to the formation of penta-coordinated aluminum species [33]. Thus, it can be inferred that the addition of zirconia was responsible for the formation of unsaturated penta-coordinated aluminum. It is noteworthy that chemical shift of penta- and tetra-coordinated aluminum species
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Fig. 6. H2 -TPR profiles of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts.
Fig. 7. Catalytic activities of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts in the dehydrogenation of propane to propylene with lattice oxygen of the catalysts at 630 ◦ C.
moved to lower chemical shift, indicating that electron density of aluminum atom increased by electron transfer from zirconium atom in the Al-O-Zr structure. This means that chemical binding between aluminum species and anchored chromium species decreased with increasing Zr/Al molar ratio. Thus, zirconia addition significantly affected structural and electronical properties of alumina support. 3.4. H2 -TPR analyses of Cr/AZ-X catalysts In order to measure the amount of lattice oxygen, H2 -TPR experiments were conducted. As shown in Fig. 6, all the catalysts showed a reduction peak at around 340–350 ◦ C which was associated with the reduction of chromia species. Thus, total amount of lattice oxygen can be calculated from the amount of hydrogen
reacted with the catalysts. The peak area of TPR profile was different with a variation of Zr/Al molar ratio. The amount of lattice oxygen (oxygen capacity) calculated from the peak area of TPR profile is summarized in Table 4. The oxygen capacity increased in the order of Cr/AZ-0 < Cr/AZ-0.05 < Cr/AZ-0.20 < CrAZ-0.15 < Cr/AZ0.10. Among the catalysts, Cr/AZ-0.10 catalyst showed the largest oxygen capacity. This indicates that the addition of appropriate amount of zirconia can enhance the amount of lattice oxygen of Cr/AZ-X catalysts. It has been reported that oxygen capacity of the catalysts was closely related to the catalytic performance in the partial oxidation using lattice oxygen of metal oxide [34]. Thus, it is expected that Cr/AZ-0.10 catalyst would serve as an efficient catalyst in the dehydrogenation of propane to propylene with lattice oxygen.
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Table 4 H2 -TPR results of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts. Catalyst
Oxygen capacity (mol-Olattice /gcat )a
Cr/AZ-0 Cr/AZ-0.05 Cr/AZ-0.10 Cr/AZ-0.15 Cr/AZ-0.20
243.8 249.5 288.1 265.9 254.9
a Calculated from peak area of H2 -TPR profiles in Fig. 6 on the basis of a redox mechanism: H2 + MeOx,lattice → H2 O + MeOx-1 ,lattice .
3.5. Catalytic performance in the dehydrogenation of propane to propylene Catalytic activities in the dehydrogenation of propane to propylene with lattice oxygen of the catalysts are presented in Fig. 7. It was observed that conversion of propane and yield for propylene rapidly decreased during short reaction time (45 s). This is because the limited amount of lattice oxygen in chromia is the only oxygen source in this reaction. It also means that propylene was formed via oxidative dehydrogenation of propane using lattice oxygen of the catalysts. It was observed that selectivity for propylene over Cr/AZ-X (X = 0.05, 0.10, 0.15, and 0.20) was higher than that over Cr/AZ-0 catalyst. This indicates that zirconia was effective for suppressing coke formation and side reactions such as cracking of propane. Conversion of propane and yield for propylene over Cr/AZX (X = 0.05, 0.10, 0.15, and 0.20) catalyst were also higher than those over Cr/AZ-0 catalyst. Thus, we can say that the addition of zirconia was effective for enhancing catalytic activities by modifying oxygen capacity of the catalysts. Detailed conversion of propane and yield for products obtained after 6 s-reaction and 45-s reaction are listed in Table 5. It was found that C1 -C2 compounds (methane, ethylene, and ethane), hydrogen, carbon monoxide, and carbon dioxide were produced as by-products together with a desired product (propylene). Fig. 8 shows the initial conversion of propane and initial yield for propylene over Cr/AZ-X catalysts, plotted as a function of Zr/Al molar ratio. Both conversion of propane and yield for propylene showed volcano-shaped curves with regard to Zr/Al molar ratio. Initial conversion of propane and initial yield for propylene increased in the order of Cr/AZ-0 < Cr/AZ-0.05 < Cr/AZ-0.20 < Cr/AZ0.15 < Cr/AZ-0.10. This result was well correlated with the trend of oxygen capacity of the catalysts as presented in Fig. 9. As oxygen capacity increased, initial yield for propylene over Cr/AZ-X catalysts increased. Among the catalysts, Cr/AZ-0.10 catalyst with the largest oxygen capacity showed the highest initial yield for propylene in the dehydrogenation of propane with lattice oxygen. This is because zirconia led to the enhanced oxygen capacity of the catalyst by modifying physicochemical properties of the catalyst.
Fig. 8. Correlation between initial catalytic activity and Zr/Al molar ratio of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts.
Fig. 9. Correlation between initial yield for propylene and oxygen capacity of Cr/AZX (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts.
4. Conclusions A series of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts with different Zr/Al molar ratio (X) were prepared by a sol-gel method and a subsequent wetness impregnation method for use in the dehydrogenation of propane to propylene with lattice oxygen of the catalysts. The effect of Zr/Al molar ratio
Table 5 Catalytic performance of Cr/AZ-X (X = 0, 0.05, 0.10, 0.15, and 0.20) catalysts in the dehydrogenation of propane to propylene (reaction temperature = 630 ◦ C, GHSV = 12,000 ml/gcat ).
a
Catalyst
Xi (%)a
Xf (%)b
Cr/AZ-0 Cr/AZ-0.05 Cr/AZ-0.10 Cr/AZ-0.15 Cr/AZ-0.20
33.1 35.3 43.8 41.0 37.0
27.0 27.6 29.3 31.8 30.3
Yi (%)a
Yf (%)b
C3 H6
C1 -C2
H2
CO
CO2
C3 H6
C1 -C2
H2
CO
CO2
25.5 27.3 36.8 33.8 30.0
3.4 3.1 3.4 2.7 3.4
1.4 1.5 1.9 1.6 1.4
1.1 1.5 0.8 1.0 1.0
0.8 0.7 0.9 1.2 1.0
21.1 23.3 26.5 26.5 25.7
2.5 2.1 2.1 1.8 2.8
0.9 1.0 1.0 1.0 1.0
0.4 0.6 0.4 0.5 0.5
0.2 0.2 0.2 0 0
Xi is initial conversion of propane and Yi is initial yield obtained after 6 s-reaction. b Xf is final conversion of propane and Yf is final yield obtained after 45 s-reaction.
T.H. Kim et al. / Molecular Catalysis 433 (2017) 1–7
on the catalytic activities and physicochemical properties of the catalysts was investigated. Surface area and pore volume of the catalysts decreased with increasing Zr/Al molar ratio. 27 Al MAS NMR analyses showed that the catalyst with high Zr/Al molar ratio retained much amount of penta-coordinated aluminum species. In the STEM-EDX mapping analyses, it was found that aluminum and zirconium species were co-existed in the Cr/AZ-0.10 catalyst. XRD results revealed that crystallite size of chromia increased with increasing Zr/Al molar ratio because of the aggregation of chromia species. It was also found that oxygen capacity of the catalysts was different depending on Zr/Al molar ratio. In the dehydrogenation of propane to propylene, initial conversion of propane and initial yield for propylene showed volcano-shaped trends with respect to Zr/Al molar ratio. Thus, an optimal amount of zirconia addition to alumina support was required for efficient propylene production. Initial yield for propylene increased with increasing oxygen capacity of the catalysts, indicating that oxygen capacity played an important role in determining the catalytic performance in the reaction. Among the catalysts, Cr/AZ-0.10 catalyst with the largest oxygen capacity served as the most efficient catalyst in the dehydrogenation of propane to propylene. Acknowledgment The authors wish to acknowledge support from the Korea Research Institute of Chemical Technology (KRICT) (045820160024). References [1] [2] [3] [4]
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