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Effect of the evacuation mode of solvent on the textural, structural and catalytic properties of sulfated zirconia doped with cerium
Zeolites and Related Materials: Trends, Targets and Challenges Proceedings of 4th International FEZA Conference A. Gédéon, P. Massiani and F. Babonneau (Editors) © 2008 Elsevier B.V. All rights reserved.
493
Effect of the evacuation mode of solvent on the textural, structural and catalytic properties of sulfated zirconia doped with cerium I. Mejria, M. K. Younesa, A. Ghorbela, P. Eloyb and E. M. Gaigneauxb a
Laboratoire de Chimie des Matériaux et Catalyse, Faculté des Science de Tunis,Campus Universitaire Tunis Elmanar 2092 Tunis, Tunisia b Unité de catalyse et chimie des matériaux divisés,Université catholique de Louvain, Louvain-la-neuve, Belgium
Abstract This work studies the evacuation mode of the solvent to optimise the catalytic properties of sulfated zirconia doped with cerium prepared by the sol-gel method. The xerogel solid obtained by ordinary gel drying and calcined at different temperatures exhibits a very low surface area. On the contrary, the aerogel obtained by solvent evacuation under supercritical conditions has a more developed surface. Both aerogel and xerogel exhibit the tetragonal phase of zirconia and/or the zirconium-cerium solid solution phase. Aerogel exhibits more developed superficial Ce4+ and higher acidity. The latter confers it a good reactivity in n-hexane isomerization in the whole temperature range investigated. Keywords: Sulfated zirconia doped cerium, sol gel route, acidity.
1. Introduction Since the discovery of sulfated zirconia by Hino and Arata [1], this solid attracts the attention of many researchers because it exhibits exceptional acidity and a high catalytic activity in many reactions such as n-paraffin isomerization. In order to improve the activity, selectivity and stability of sulfated zirconia many alternatives were used as the addition of transition metal [2] or the preparation of mixed oxide as SiO2-ZrO2 [3] or Al2O3-ZrO2 [4]. Among these metal oxides, CeO2-ZrO2 attracts the attention of many researchers because it exhibits oxygen storage [5] and acid properties (as precisely Zr4+ and Ce4+ act as Lewis acid sites). Here, the sulfated zirconia doped with cerium is prepared by the sol gel route, while the effect of two solvent evacuation methods of the wet gel is investigated. The physicochemical and catalytic properties of the resulting materials are compared.
2. Experimental 2.1. Catalysts preparation Sulfated zirconia doped cerium is prepared as follows. First zirconium (IV) propoxide ( 70% wt in propanol) was dissolved in 1-propanol. (NH4)2[Ce(NO3)6] was then added to obtain a molar ratio Ce/Zr = 0.4. The solution is sulfated with concentrated H2SO4 to obtain a molar ratio S/Zr = 0.5 and finally water was slowly added to obtain a gel with a hydrolysis ratio (nH2O/nZrO2) of 3. The wet gel was dried either by simple evaporation in an oven over night (T= 120 °C, P = 1 bar) to give xerogel called XZC or under the supercritical conditions of the solvent (T = 263.6 °C, P = 51 bar) to give aerogel called
494
I. Mejri et al.
AZC. The resulting solids are then calcined under pure oxygen at 560 and 650 °C with a heating rate of 3°C/min during 3 hours. 2.2. Catalysts characterization and catalytic test X-ray diffraction, physisorption of N2, FT-IR and Raman spectroscopies, and X-ray photoelectron spectroscopy were carried out as described in [6]. TPD-NH3 was additionally followed with a quadrupole Balzers spectrometer QMS 311 and was performed between 100 and 500 C at 10C/min after adsorption of ammonia at 100 °C by flowing 0.5% NH3 in He over the catalyst for 30 min until saturation and then elimination of physisorbed NH3 by treatment under He at 100°C for 1 h. The n-hexane isomerization reaction was performed in a U-shaped tubular reactor operating at atmospheric pressure. A mechanical mixture of 100 mg of aerogel or xerogel and 100 mg of Pt/Al2O3 (0.35% wt Pt) prepared by impregnation were exposed to a stream of nhexane (20 torr) diluted in hydrogen (total flow rate: 30 cc/min). Reaction products were analysed online using a gas chromatograph with a flame ionization detector.
3. Results and discussion 3.1. Textural properties The surface area of the aerogel calcined at 560 °C is 119 m2/g. At higher calcination temperatures, the surface area decreases due to the loss of sulfur which is known to preserve the surface area as was previously obtained in the case of the non doped sulfated zirconia [6]. The xerogel calcined at the same temperature exhibits a very low surface area (Table 1). This result is probably related to the formation of cerium oxide agglomerates. The average pore diameter of the aerogel is higher than the xerogel one and remains constant in the whole calcination temperature range. This difference may be due to the solvent evacuation mode. In fact, the solvent evacuation by simple evaporation in an oven is susceptible to form a liquid-vapor interface within the gel, creating a surface tension which acts on the pores and causes their shrinkage. On the contrary, when the solvent is removed under supercritical conditions, the liquid-vapor interface is avoided and the absence of surface tension allows the gel to dry without pore shrinkage. 3.2. Structural properties The XRD patterns of the aerogel and the xerogel calcined at different temperatures are given in Figure 1. Both the aerogel and the xerogel solids exhibit peaks at 2 in the range of 30, 40 and 60° attributed to the tetragonal phase of zirconia (JCPDS 17-0923) and/or the solid solution Zr0.84Ce0.16O2 phase (JCPDS 38-1437). At higher temperatures, only the aerogel exhibits peaks at 2 = 29; 48 and 57° which are attributed to the cubic phase of cerium oxide (JCPDS 43-1002). Table 1: Textural properties of the solids Samples AZC-560 AZC-650 XZC-560 XZC-650
2
SBET (m /g) 119 87 <5 23
Dmoy (Å) 213 202 0 69
Table 2: Surface XPS results of the solids Samples AZC-560 AZC-650 XZC-560 XZC-650
S/Zr 0.40 0.40 0.75 0.80
% Ce4+ 56 66 39 50
%Ce3+ 44 34 61 50
Effect of the evacuation mode of solvent on the textural, structural and catalytic properties of sulfated zirconia doped with cerium
495
3.3. RAMAN spectroscopy The RAMAN spectra of both aerogel and xerogel show a band at 268 cm-1 assigned to the tetragonal ZrO2 phase [6], a band at 465 cm-1 attributed to the symmetric stretching mode of Ce-O-Ce and a band at 1011 cm-1 ascribed to the stretching mode of Zr-O-S [7]. The xerogel exhibits also bands at 1068 and 1079 cm-1 attributed to stretching mode of surface sulfate groups connected to cerium [7] indicating a different cerium environment in this two kinds of solids. 3.4. IR spectroscopy The FTIR spectra of the aerogel and xerogel calcined at 560 °C exhibit bands at 1071 and 1131 cm-1 attributed to sulfate groups [6]. At higher temperatures, these bands decrease slightly due to the loss of sulfur. In our conditions no bands characteristics of Ce-O and Ce-S are detected. 3.5. XPS spectroscopy XPS analyses show that the amount of sulfur on the surface of the xerogel is more important than for the aerogel. This result can be explained by the presence of cerium connected to sulfur on the surface of the xerogel as shown by RAMAN spectroscopy. The superficial amount of sulfur remains constant at higher temperatures for the aerogel, but it increases slightly for the xerogel (Table 2). The Ce4+ and Ce3+ species are present at the surface of the two kinds of solids and their relative percentage is determined as described by Hilaire et al [8]. In the present case the Ce4+ percentage is always higher in the case of the aerogel. This means that the aerogel possesses more Lewis acidity. 3.6. TPD-NH3 The TPD-NH3 shows that the aerogel and xerogel present three types of acidity (strong, moderate and weak acidity). The aerogel exhibits an important strong acidity (41%), while the xerogel presents only a small amount of strong acid sites (10 %). Besides, the total acidity is more important for the aerogel (Table 3). This result seems to be related to the important superficial amount of Ce4+ on the surface and thus allows to expect a higher isomerization activity for the aerogel. 3.7. Catalytic test The catalytic properties of the aerogel and xerogel calcined at different temperatures are tested in the n-hexane isomerization between 170 and 220 °C. All catalysts are mechanically mixed with Pt/Al2O3 in the ratio 1/1 (w/w). The presence of platinum is necessary to form the hydrogen species which clean the acid sites and inhibit the formation of coke (responsible for a rapid deactivation of the catalysts [6]). The corresponding results show that the evacuation mode of the solvent has an impact on the catalytic properties of the catalysts. The aerogel calcined at 560 °C is active in the whole reaction temperature range. However, the xerogel calcined at the same temperature is inactive (Table 4). This result can be explained by the presence of cerium oxide agglomerates on the surface of the xerogel which reduce the SBET area (Table 1) and thus probably block the access of the active site. Although the superficial amount of sulfur is more important in the case of the xerogel, the latter does not exhibit important acidity, and in particular shows a low total acidity and a low strong acidity percentage compared to the aerogel (Table 3). It is probable that the sulfur present on the surface of the xerogel (which is connected to the cerium as shown by RAMAN spectroscopy) does not allow the presence of active acid sites. Furthermore, the more marked presence of less superficial Ce4+ (which probably acts as a Lewis sites) in the xerogel than in the aerogel (Table 2) can explain the different behavior of the two solids in terms of their
496
I. Mejri et al.
(in)activity in the n-hexane isomerization reaction. At higher temperature, the xerogel becomes active for the higher temperature reaction range. However, this activity still remains lower than that of the aerogel. This activity increase for the xerogel can be explained by its development of surface area and the increase of its superficial Ce4+percentage.
intensity (a.u.)
Table 3: TPD-NH3 results (W: weak, M: moderate, S: strong) (d)
(b) (a) 5
Total acidity (mol.g-1)
Strength acidity (%)
(c)
15
(a) AZC-560
25
35
(b) AZC-650
45
55
(c) XZC-560
65 2 (degree) (d)CZC-650
Transmittance (%)
Figure1: XRD patterns of the solids
(c) (d (a) (b)
3400
(a) AZC-560 (c) AZC-650
2400
(b) XZC-560 (d) XZC-650
1400
400 Wavenumber (cm-1)
Figure2: IR spectra of the solids
AZC-560 XZC-560
W 19 51
M 40 39
S 41 10
280 50
Table 4: Reactivity results Activity (10-8 mol.g-1.s-1) 170 200 220 AZC560 AZC650 XZC560 XZC650
Selectivity (%) 170 200 220
7
50
113
88
96
98
3
45
142
96
99
99
0
0
0
0
0
0
0
1
9
0
90
91
4. Conclusion The effect of the evacuation mode of the solvent in preparation step of solids based on sulfated zirconia doped with cerium was investigated. The aerogel and the xerogel exhibit different textural, structural and acid properties. This difference has an impact on the catalytic properties of the solids in the isomerization of n-hexane. The aerogel exhibits higher activity due to its more developed surface area properties and Lewis acidity due to the presence of more Ce4+ species on its surface.
References [1] [2] [3] [4] [5] [6]
M. Hino, S. Kobayashi, K. Arata, J. Am. Chem. Soc., 101 (1979) 6439. C. R. Vera, J. C. Yori, J. M. Parera, Appl. Catal., 167 (1998) 75. D. J. Rosenberg, F. Coloma, A. J. Anderson, J. Catal., 210 (2002) 218. B. M. Reedy, P. M. Sreekanth, Y. Yamada, T. Kobayashi, J. Mol. Catal., 227 (2005) 81. L. Can, L. Meijun, J. Raman. Spectrosc. 33 (2002) 301. I. Mejri, M. K. Younes, A. Ghorbel, P. Eloy, E. M. Gaigneaux, Stud. Surf. Sci. Catal., 162 (2006) 953. [7] B. R. Reddy, P. M. Sreekanth, P. Lakshmanan, A. Khan, J. Mol.Catal., 244 (2006) 1. [8] S. Hilaire, X. Wang, T. Luo, R. J. Gorte, J. Wagner, Appl. Catal. A., 215 (2001) 271.
493
Effect of the evacuation mode of solvent on the textural, structural and catalytic properties of sulfated zirconia doped with cerium I. Mejria, M. K. Younesa, A. Ghorbela, P. Eloyb and E. M. Gaigneauxb a
Laboratoire de Chimie des Matériaux et Catalyse, Faculté des Science de Tunis,Campus Universitaire Tunis Elmanar 2092 Tunis, Tunisia b Unité de catalyse et chimie des matériaux divisés,Université catholique de Louvain, Louvain-la-neuve, Belgium
Abstract This work studies the evacuation mode of the solvent to optimise the catalytic properties of sulfated zirconia doped with cerium prepared by the sol-gel method. The xerogel solid obtained by ordinary gel drying and calcined at different temperatures exhibits a very low surface area. On the contrary, the aerogel obtained by solvent evacuation under supercritical conditions has a more developed surface. Both aerogel and xerogel exhibit the tetragonal phase of zirconia and/or the zirconium-cerium solid solution phase. Aerogel exhibits more developed superficial Ce4+ and higher acidity. The latter confers it a good reactivity in n-hexane isomerization in the whole temperature range investigated. Keywords: Sulfated zirconia doped cerium, sol gel route, acidity.
1. Introduction Since the discovery of sulfated zirconia by Hino and Arata [1], this solid attracts the attention of many researchers because it exhibits exceptional acidity and a high catalytic activity in many reactions such as n-paraffin isomerization. In order to improve the activity, selectivity and stability of sulfated zirconia many alternatives were used as the addition of transition metal [2] or the preparation of mixed oxide as SiO2-ZrO2 [3] or Al2O3-ZrO2 [4]. Among these metal oxides, CeO2-ZrO2 attracts the attention of many researchers because it exhibits oxygen storage [5] and acid properties (as precisely Zr4+ and Ce4+ act as Lewis acid sites). Here, the sulfated zirconia doped with cerium is prepared by the sol gel route, while the effect of two solvent evacuation methods of the wet gel is investigated. The physicochemical and catalytic properties of the resulting materials are compared.
2. Experimental 2.1. Catalysts preparation Sulfated zirconia doped cerium is prepared as follows. First zirconium (IV) propoxide ( 70% wt in propanol) was dissolved in 1-propanol. (NH4)2[Ce(NO3)6] was then added to obtain a molar ratio Ce/Zr = 0.4. The solution is sulfated with concentrated H2SO4 to obtain a molar ratio S/Zr = 0.5 and finally water was slowly added to obtain a gel with a hydrolysis ratio (nH2O/nZrO2) of 3. The wet gel was dried either by simple evaporation in an oven over night (T= 120 °C, P = 1 bar) to give xerogel called XZC or under the supercritical conditions of the solvent (T = 263.6 °C, P = 51 bar) to give aerogel called
494
I. Mejri et al.
AZC. The resulting solids are then calcined under pure oxygen at 560 and 650 °C with a heating rate of 3°C/min during 3 hours. 2.2. Catalysts characterization and catalytic test X-ray diffraction, physisorption of N2, FT-IR and Raman spectroscopies, and X-ray photoelectron spectroscopy were carried out as described in [6]. TPD-NH3 was additionally followed with a quadrupole Balzers spectrometer QMS 311 and was performed between 100 and 500 C at 10C/min after adsorption of ammonia at 100 °C by flowing 0.5% NH3 in He over the catalyst for 30 min until saturation and then elimination of physisorbed NH3 by treatment under He at 100°C for 1 h. The n-hexane isomerization reaction was performed in a U-shaped tubular reactor operating at atmospheric pressure. A mechanical mixture of 100 mg of aerogel or xerogel and 100 mg of Pt/Al2O3 (0.35% wt Pt) prepared by impregnation were exposed to a stream of nhexane (20 torr) diluted in hydrogen (total flow rate: 30 cc/min). Reaction products were analysed online using a gas chromatograph with a flame ionization detector.
3. Results and discussion 3.1. Textural properties The surface area of the aerogel calcined at 560 °C is 119 m2/g. At higher calcination temperatures, the surface area decreases due to the loss of sulfur which is known to preserve the surface area as was previously obtained in the case of the non doped sulfated zirconia [6]. The xerogel calcined at the same temperature exhibits a very low surface area (Table 1). This result is probably related to the formation of cerium oxide agglomerates. The average pore diameter of the aerogel is higher than the xerogel one and remains constant in the whole calcination temperature range. This difference may be due to the solvent evacuation mode. In fact, the solvent evacuation by simple evaporation in an oven is susceptible to form a liquid-vapor interface within the gel, creating a surface tension which acts on the pores and causes their shrinkage. On the contrary, when the solvent is removed under supercritical conditions, the liquid-vapor interface is avoided and the absence of surface tension allows the gel to dry without pore shrinkage. 3.2. Structural properties The XRD patterns of the aerogel and the xerogel calcined at different temperatures are given in Figure 1. Both the aerogel and the xerogel solids exhibit peaks at 2 in the range of 30, 40 and 60° attributed to the tetragonal phase of zirconia (JCPDS 17-0923) and/or the solid solution Zr0.84Ce0.16O2 phase (JCPDS 38-1437). At higher temperatures, only the aerogel exhibits peaks at 2 = 29; 48 and 57° which are attributed to the cubic phase of cerium oxide (JCPDS 43-1002). Table 1: Textural properties of the solids Samples AZC-560 AZC-650 XZC-560 XZC-650
2
SBET (m /g) 119 87 <5 23
Dmoy (Å) 213 202 0 69
Table 2: Surface XPS results of the solids Samples AZC-560 AZC-650 XZC-560 XZC-650
S/Zr 0.40 0.40 0.75 0.80
% Ce4+ 56 66 39 50
%Ce3+ 44 34 61 50
Effect of the evacuation mode of solvent on the textural, structural and catalytic properties of sulfated zirconia doped with cerium
495
3.3. RAMAN spectroscopy The RAMAN spectra of both aerogel and xerogel show a band at 268 cm-1 assigned to the tetragonal ZrO2 phase [6], a band at 465 cm-1 attributed to the symmetric stretching mode of Ce-O-Ce and a band at 1011 cm-1 ascribed to the stretching mode of Zr-O-S [7]. The xerogel exhibits also bands at 1068 and 1079 cm-1 attributed to stretching mode of surface sulfate groups connected to cerium [7] indicating a different cerium environment in this two kinds of solids. 3.4. IR spectroscopy The FTIR spectra of the aerogel and xerogel calcined at 560 °C exhibit bands at 1071 and 1131 cm-1 attributed to sulfate groups [6]. At higher temperatures, these bands decrease slightly due to the loss of sulfur. In our conditions no bands characteristics of Ce-O and Ce-S are detected. 3.5. XPS spectroscopy XPS analyses show that the amount of sulfur on the surface of the xerogel is more important than for the aerogel. This result can be explained by the presence of cerium connected to sulfur on the surface of the xerogel as shown by RAMAN spectroscopy. The superficial amount of sulfur remains constant at higher temperatures for the aerogel, but it increases slightly for the xerogel (Table 2). The Ce4+ and Ce3+ species are present at the surface of the two kinds of solids and their relative percentage is determined as described by Hilaire et al [8]. In the present case the Ce4+ percentage is always higher in the case of the aerogel. This means that the aerogel possesses more Lewis acidity. 3.6. TPD-NH3 The TPD-NH3 shows that the aerogel and xerogel present three types of acidity (strong, moderate and weak acidity). The aerogel exhibits an important strong acidity (41%), while the xerogel presents only a small amount of strong acid sites (10 %). Besides, the total acidity is more important for the aerogel (Table 3). This result seems to be related to the important superficial amount of Ce4+ on the surface and thus allows to expect a higher isomerization activity for the aerogel. 3.7. Catalytic test The catalytic properties of the aerogel and xerogel calcined at different temperatures are tested in the n-hexane isomerization between 170 and 220 °C. All catalysts are mechanically mixed with Pt/Al2O3 in the ratio 1/1 (w/w). The presence of platinum is necessary to form the hydrogen species which clean the acid sites and inhibit the formation of coke (responsible for a rapid deactivation of the catalysts [6]). The corresponding results show that the evacuation mode of the solvent has an impact on the catalytic properties of the catalysts. The aerogel calcined at 560 °C is active in the whole reaction temperature range. However, the xerogel calcined at the same temperature is inactive (Table 4). This result can be explained by the presence of cerium oxide agglomerates on the surface of the xerogel which reduce the SBET area (Table 1) and thus probably block the access of the active site. Although the superficial amount of sulfur is more important in the case of the xerogel, the latter does not exhibit important acidity, and in particular shows a low total acidity and a low strong acidity percentage compared to the aerogel (Table 3). It is probable that the sulfur present on the surface of the xerogel (which is connected to the cerium as shown by RAMAN spectroscopy) does not allow the presence of active acid sites. Furthermore, the more marked presence of less superficial Ce4+ (which probably acts as a Lewis sites) in the xerogel than in the aerogel (Table 2) can explain the different behavior of the two solids in terms of their
496
I. Mejri et al.
(in)activity in the n-hexane isomerization reaction. At higher temperature, the xerogel becomes active for the higher temperature reaction range. However, this activity still remains lower than that of the aerogel. This activity increase for the xerogel can be explained by its development of surface area and the increase of its superficial Ce4+percentage.
intensity (a.u.)
Table 3: TPD-NH3 results (W: weak, M: moderate, S: strong) (d)
(b) (a) 5
Total acidity (mol.g-1)
Strength acidity (%)
(c)
15
(a) AZC-560
25
35
(b) AZC-650
45
55
(c) XZC-560
65 2 (degree) (d)CZC-650
Transmittance (%)
Figure1: XRD patterns of the solids
(c) (d (a) (b)
3400
(a) AZC-560 (c) AZC-650
2400
(b) XZC-560 (d) XZC-650
1400
400 Wavenumber (cm-1)
Figure2: IR spectra of the solids
AZC-560 XZC-560
W 19 51
M 40 39
S 41 10
280 50
Table 4: Reactivity results Activity (10-8 mol.g-1.s-1) 170 200 220 AZC560 AZC650 XZC560 XZC650
Selectivity (%) 170 200 220
7
50
113
88
96
98
3
45
142
96
99
99
0
0
0
0
0
0
0
1
9
0
90
91
4. Conclusion The effect of the evacuation mode of the solvent in preparation step of solids based on sulfated zirconia doped with cerium was investigated. The aerogel and the xerogel exhibit different textural, structural and acid properties. This difference has an impact on the catalytic properties of the solids in the isomerization of n-hexane. The aerogel exhibits higher activity due to its more developed surface area properties and Lewis acidity due to the presence of more Ce4+ species on its surface.
References [1] [2] [3] [4] [5] [6]
M. Hino, S. Kobayashi, K. Arata, J. Am. Chem. Soc., 101 (1979) 6439. C. R. Vera, J. C. Yori, J. M. Parera, Appl. Catal., 167 (1998) 75. D. J. Rosenberg, F. Coloma, A. J. Anderson, J. Catal., 210 (2002) 218. B. M. Reedy, P. M. Sreekanth, Y. Yamada, T. Kobayashi, J. Mol. Catal., 227 (2005) 81. L. Can, L. Meijun, J. Raman. Spectrosc. 33 (2002) 301. I. Mejri, M. K. Younes, A. Ghorbel, P. Eloy, E. M. Gaigneaux, Stud. Surf. Sci. Catal., 162 (2006) 953. [7] B. R. Reddy, P. M. Sreekanth, P. Lakshmanan, A. Khan, J. Mol.Catal., 244 (2006) 1. [8] S. Hilaire, X. Wang, T. Luo, R. J. Gorte, J. Wagner, Appl. Catal. A., 215 (2001) 271.