- Email: [email protected]
Calcination mechanism of block-copolymer template in SBA-15 materials
Mesostructured Materials Recent Progress in Mesostructured D. Zhao, S. Qiu, Y. Tang and C. Yu (Editors) © 2007 Elsevier B.V. All rights reserved.
189 189
Calcination mechanism of block-copolymer template in SBA-15 materials Francois Berube and Serge Kaliaguine Chemical Engineering Department, Universite Laval, Ste-Foy, Que, Canada. G1K 7P4 1. Introduction
Since poly(alkeneoxide) triblock copolymers have been used as templates for the synthesis of ordered mesoporous silica [1, 2], several methods have been developed to remove the template such as oxidative ozone treatment [3, 4], supercritical fluid extraction [5, 6], microwave digestion [7] and ether cleavage by an acid [8, 9], but the most common ones are calcination under air and extraction with an organic solvent [5, 10-14]. Several studies have been carried out to understand the influence of calcination on the physico-chemical properties of SBA-15 materials showing that a significant lattice shrinkage occurs upon high-temperature treatment [5, 7, 8, 10, 15]. In spite of the fact that calcination is very often used to remove the organic phase from SBA-15 materials, relatively little works was done to understand the influence of this process on the MMS properties and to quantify and locate the residual template. The present work is aiming at understanding the template degradation reaction mechanism during calcination under oxygen. 2. Experimental Section SBA-15 material was obtained following the procedure reported by Zhao et al. [1]. Tri-block copolymer (P-123) was used as the template and tetraethyl orthosilicate as the silica source. In a typical synthesis, 7.659 g of P123 was dissolved in 290 mL of a 1.6 M aqueous HC1 solution. 16 g of TEOS was then added dropwise. The synthesis was carried out for 20h at 35°C followed by 24h of hydrothermal treatment at 80°C. All the samples were dried at 80°C under vacuum prior to calcination.
190 190
Temperature programmed calcination (TPC) monitored by mass spectrometry (TPC-MS) was performed using a RXM-100 multi catalyst testing and caracterization system (Advanced Scientific Design Inc.). 70 mg of support was placed in a U-shaped reactor coupled with a quadrupole mass spectrometer. All the experiments were carried out under a flow (50 mL/min) of 10% O2 in He from 25°C to the various temperatures of interest with a 2 K/min ramp and then cooled down to room temperature under the same flow. The temperature was maintained for 120 minutes after the end of the ramp to complete the combustion except for the MMS calcined at 160°C which was immediately cooled down after the end of the ramp and the one calcined at 575°C which was maintained at this temperature for 15 min. Water and high molecular weight carbonaceous species were trapped at -80°C (dry ice in ethanol solution) before reaching the mass spectrometer leak in order to increase the monitoring resolution. As-synthesized and calcined samples were characterized using N2 adsorption, carbon elemental analysis, thermogravimetry analysis and 13C NMR. 3. Results and Discussion Table 1 and Fig. 1 respectively show the structural properties and evolution of the various pore volumes of SB A-15 calcined at different temperatures. After calcination at 160°C, the alpha-s plot shows that no micropore volume is accessible to N2. Almost all micropores and mesopores are free for the sample Table 1. Structural properties of SBA-15 after different treatments.
Samples
%C
SBET 2
Water washed C160 C175 C270 C335 C575
29.67 24.23 4.26 1.36 0.82 0.30
(m /g) 40 170 1020 1120 1080 900
DPa (nm) 7.7 8.3 8.8 9.0 8.5 8.3
vtb
(cc/g) 0.09 0.31 1.12 1.19 1.16 1.03
V v
• c-d mes
(cc/g) 0.05 0.26 0.79 0.84 0.81 0.75
V •° v mic (cc/g) 0 0 0.14 0.17 0.17 0.16
a
Pore diameter determined by the modified BJH method from the adsorption branch; b Adsorbed volume at P/Po = 0.995; c Micropore and mesopore volumes determined from alphas plot [16]; d VmeS+mi(. (sum of mesopore and micropore volumes, 1.6 < as < 2.0) — Vmjc (micropore volume, 0.9 < as < 1.2).
calcined at 175°C. After a 2 h. isotherm at 175°C, 84% of the micropore volume and 94% of the mesopore volume are free compared to their maximal values, obtained at 270°C. These results showed that larger mesopores are first emptied followed by intrawall porosity. For higher temperature treatments, mesopore volume decreased suggesting that lattice shrinkage occurred in agreement with the XRD results of F. Kleitz et al. [15]. Surprisingly, the micropore volume
191 191
d%/d °C
W eight loss (%)
V (cc/g)
remained approximately unchanged for treatment temperatures higher than 270°C even if lattice shrinkage occurred indicating that for these temperature, template oxidation was located all within micropores. TGA and DTG of PI23 and SB A-15 material both under air 1,0 and nitrogen are presented in Fig.2. Under air, TGA of the 0,8 organic template inside the MMS showed a major weight loss of 100 nm 0,6 37% between 130 and 190°C as previously reported [10, 15]. 0,4 TGA of PI23 copolymer under air also shows that oxidation of the polyalkoxide begins at the 0,2 same temperature, but at a lower rate than the copolymer inside 0,0 0 100 200 300 400 500 600 700 SBA-15 materials. Both Temperature (°C) experiments showed a subsequent weight loss above 190°C Fig 1. Comparative plots of micropore volumes indicating that the template (solid circles) and mesopore volumes (open oxidation takes place in two steps. circles) of SB A-15 calcined at different Under flowing nitrogen, temperatures. decomposition of the P123/SBA15 composite occurs at a lower 200 6 rate in comparison to PI23, that SBA-15 5 + 100 takes place in a single step 150 between 300 and 375°C. 4 Temperature programmed + 50 calcination (TPC) monitored with 100 3 MS under 10% O2 of the SBA-15 P123 2 material washed with water is 50 presented in Fig. 3. As reported 1 before, the MS signals recorded 0 0 between 140 and 190°C show a 0 100 200 300 400 500 rich spectrum including masses °C} Temper3ture{(°C) Temperature 14, 26, 27, 29, 30, 43 and 58 in Fig 2. TGA and DTG curves under air (solid addition to masses 44, 28 and 12 ijnes) and under N2 (dotted lines) of P123 and that suggests VOC production [15]. Mass 29 is the most important VOC fragment indicating that carbonaceous species formed during that step have carbonyl group terminations. Several molecules such as alchools and aldehydes have been identified among the products of combustion of diethyleneglycol [17]. As demonstrated with the TGA experiments, TPC followed with m/z = 44 also showed a small shoulder between 190 and 300°C due to oxidation of the polyalkoxide fragmentation
192 192
products formed at lower temperature. Above 300°C, mass spectra showed a broad CO2 m/z = 12 peak that continued until 500°C. m/z = 28 m/z = 44 Interestingly, degradation of the copolymer inside the SB A-15 in absence of oxygen occurs within this temperature range (see Fig.l) and also continued at m/z = 14 m/z = 26 higher temperature. Thus, one m/z = 29 m/z = 30 may suggest that this step is 11 jj associated with the template located inside the ultramicroporosity (<10 A) that limits the accessibility of oxygen. Tem perature (°C) (°C) Temperature Fig.4 shows 13C NMR spectra Fig 3. MS monitored temperature programmed of the same sample. As reported oxidation of SB A-15 washed with water. The before, chemical shifts of 19.3, bottom graph scale reduced by a factor of two 73 and 75 ppm are associated compared to the upper one. with polyoxopropylene (PPO) and the shoulder at 72 ppm is attributed to the PEO chains of \ the template [8, 9]. The sample calcined at 160°C clearly shows a decrease of PPO signal in L J comparison to PEO lines proving that an ether cleavage of the PI23 occurs during the first calcination step and first empties |< «j"°
k
3000 •
2000 •
1000 1000 •
0
:>
SUf
Intensity (A.U.)
T-
m /z = 12 m /z = 28 m /z = 44
i
1600
1200 1200
m /z m /z m /z m /z
= = = =
14 26 29 30
400 •
0
0
100
200
300
400
500
74.3 ppm
(a)
"I
1 163 ppm
600
19.3 ppm
72.0 ppm 69.1 ppm 9.1 p p m 66.2 66.2 p pppm m
62.3 ppm
49 ppm pm
Signal (U.A.)
(b) (b)
(c) (c)
(d)
(e) (e)
J|| '
[CH22CHCH CHCH33O]m O]m
[
CH22CH CH22 O]nn CH22 CH2 [CH 2 OH
HOCH HOCH2CH2OH 2CH2OH
O=CHOCH2CH2O
200
150
OCH3
[CH22CHCH33O]m O]m
100
50
0
4. Conclusion The above results lead to a better understanding of the combustion mechanism of the template in the MMS. N2 sorption and 13C NMR experiments
193 193
of these materials calcined at different temperatures showed that larger mesopores are first emptied followed by intrawall porosity. Further on going studies will investigate the relation between the SBA-15 porosity and the template combustion pattern. 5. References [1] D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka and G. D. Stucky, Science 279 (1998) 548. [2] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka and G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [3] G. Buchel, R. Denoyel, P. L. Llewellyn and J. Rouquerol, J. Mater. Chem. 11 (2001) 589. [4] M.TJ. Keene, R. Denoyel and P.L. Llewellyn Chem. Commun. (1998) 2203. [5] R. van Grieken, G. Calleja, G. D. Stucky, J. A. Melero, R. A. Garcia and J. Iglesias, Langmuir 19 (2003) 3966. [6] S. Kawi and M. W. Lai, Chem. Commun. (1998) 1407. [7] B. Tian, X. Liu, C. Yu, F. Gao, Q. Luo, S. Xie, B. Tu and D. Zhao Chem. Commun. (2002) 1186. [8] C.-M. Yang, B. Zibrowius, W. Schmidt and F. Schuth, Chem. Mater. 16 (2004) 2918. [9] C.-M. Yang, B. Zibrowius, W. Schmidt and F. Schuth, Chem. Mater. 15 (2003) 3739. [10] M. Kruk, M. Jaroniec, C. H. Ko and R. Ryoo, Chem. Mater. 12 (2000) 1961. [11] C.-Y. Chen, H.-X. Li and M.E. Davis Microporous Mater. 2 (1993) 17. [12] A.G.S. Prado and C. Airoldi J. Mater. Chem. 12 (2002) 3823. [13] R. Mokaya and W. Jones, J. Mater. Chem. 8 (1998) 2819. [14] S. Hitz and R. Prins, J. Catal. 168 (1997) 194. [15] F. Kleitz, W. Schmidt and F. Schuth, Micropor. Mesopor. Mater. 65 (2003) 1. [16] M. Jaroniec, M. Kruk and J. P. Oliver, Langmuir 15 (1999) 5410. [17] C. Decker and J. Marchal, Die Makromolekulare Chemie 166 (1973)117.
189 189
Calcination mechanism of block-copolymer template in SBA-15 materials Francois Berube and Serge Kaliaguine Chemical Engineering Department, Universite Laval, Ste-Foy, Que, Canada. G1K 7P4 1. Introduction
Since poly(alkeneoxide) triblock copolymers have been used as templates for the synthesis of ordered mesoporous silica [1, 2], several methods have been developed to remove the template such as oxidative ozone treatment [3, 4], supercritical fluid extraction [5, 6], microwave digestion [7] and ether cleavage by an acid [8, 9], but the most common ones are calcination under air and extraction with an organic solvent [5, 10-14]. Several studies have been carried out to understand the influence of calcination on the physico-chemical properties of SBA-15 materials showing that a significant lattice shrinkage occurs upon high-temperature treatment [5, 7, 8, 10, 15]. In spite of the fact that calcination is very often used to remove the organic phase from SBA-15 materials, relatively little works was done to understand the influence of this process on the MMS properties and to quantify and locate the residual template. The present work is aiming at understanding the template degradation reaction mechanism during calcination under oxygen. 2. Experimental Section SBA-15 material was obtained following the procedure reported by Zhao et al. [1]. Tri-block copolymer (P-123) was used as the template and tetraethyl orthosilicate as the silica source. In a typical synthesis, 7.659 g of P123 was dissolved in 290 mL of a 1.6 M aqueous HC1 solution. 16 g of TEOS was then added dropwise. The synthesis was carried out for 20h at 35°C followed by 24h of hydrothermal treatment at 80°C. All the samples were dried at 80°C under vacuum prior to calcination.
190 190
Temperature programmed calcination (TPC) monitored by mass spectrometry (TPC-MS) was performed using a RXM-100 multi catalyst testing and caracterization system (Advanced Scientific Design Inc.). 70 mg of support was placed in a U-shaped reactor coupled with a quadrupole mass spectrometer. All the experiments were carried out under a flow (50 mL/min) of 10% O2 in He from 25°C to the various temperatures of interest with a 2 K/min ramp and then cooled down to room temperature under the same flow. The temperature was maintained for 120 minutes after the end of the ramp to complete the combustion except for the MMS calcined at 160°C which was immediately cooled down after the end of the ramp and the one calcined at 575°C which was maintained at this temperature for 15 min. Water and high molecular weight carbonaceous species were trapped at -80°C (dry ice in ethanol solution) before reaching the mass spectrometer leak in order to increase the monitoring resolution. As-synthesized and calcined samples were characterized using N2 adsorption, carbon elemental analysis, thermogravimetry analysis and 13C NMR. 3. Results and Discussion Table 1 and Fig. 1 respectively show the structural properties and evolution of the various pore volumes of SB A-15 calcined at different temperatures. After calcination at 160°C, the alpha-s plot shows that no micropore volume is accessible to N2. Almost all micropores and mesopores are free for the sample Table 1. Structural properties of SBA-15 after different treatments.
Samples
%C
SBET 2
Water washed C160 C175 C270 C335 C575
29.67 24.23 4.26 1.36 0.82 0.30
(m /g) 40 170 1020 1120 1080 900
DPa (nm) 7.7 8.3 8.8 9.0 8.5 8.3
vtb
(cc/g) 0.09 0.31 1.12 1.19 1.16 1.03
V v
• c-d mes
(cc/g) 0.05 0.26 0.79 0.84 0.81 0.75
V •° v mic (cc/g) 0 0 0.14 0.17 0.17 0.16
a
Pore diameter determined by the modified BJH method from the adsorption branch; b Adsorbed volume at P/Po = 0.995; c Micropore and mesopore volumes determined from alphas plot [16]; d VmeS+mi(. (sum of mesopore and micropore volumes, 1.6 < as < 2.0) — Vmjc (micropore volume, 0.9 < as < 1.2).
calcined at 175°C. After a 2 h. isotherm at 175°C, 84% of the micropore volume and 94% of the mesopore volume are free compared to their maximal values, obtained at 270°C. These results showed that larger mesopores are first emptied followed by intrawall porosity. For higher temperature treatments, mesopore volume decreased suggesting that lattice shrinkage occurred in agreement with the XRD results of F. Kleitz et al. [15]. Surprisingly, the micropore volume
191 191
d%/d °C
W eight loss (%)
V (cc/g)
remained approximately unchanged for treatment temperatures higher than 270°C even if lattice shrinkage occurred indicating that for these temperature, template oxidation was located all within micropores. TGA and DTG of PI23 and SB A-15 material both under air 1,0 and nitrogen are presented in Fig.2. Under air, TGA of the 0,8 organic template inside the MMS showed a major weight loss of 100 nm 0,6 37% between 130 and 190°C as previously reported [10, 15]. 0,4 TGA of PI23 copolymer under air also shows that oxidation of the polyalkoxide begins at the 0,2 same temperature, but at a lower rate than the copolymer inside 0,0 0 100 200 300 400 500 600 700 SBA-15 materials. Both Temperature (°C) experiments showed a subsequent weight loss above 190°C Fig 1. Comparative plots of micropore volumes indicating that the template (solid circles) and mesopore volumes (open oxidation takes place in two steps. circles) of SB A-15 calcined at different Under flowing nitrogen, temperatures. decomposition of the P123/SBA15 composite occurs at a lower 200 6 rate in comparison to PI23, that SBA-15 5 + 100 takes place in a single step 150 between 300 and 375°C. 4 Temperature programmed + 50 calcination (TPC) monitored with 100 3 MS under 10% O2 of the SBA-15 P123 2 material washed with water is 50 presented in Fig. 3. As reported 1 before, the MS signals recorded 0 0 between 140 and 190°C show a 0 100 200 300 400 500 rich spectrum including masses °C} Temper3ture{(°C) Temperature 14, 26, 27, 29, 30, 43 and 58 in Fig 2. TGA and DTG curves under air (solid addition to masses 44, 28 and 12 ijnes) and under N2 (dotted lines) of P123 and that suggests VOC production [15]. Mass 29 is the most important VOC fragment indicating that carbonaceous species formed during that step have carbonyl group terminations. Several molecules such as alchools and aldehydes have been identified among the products of combustion of diethyleneglycol [17]. As demonstrated with the TGA experiments, TPC followed with m/z = 44 also showed a small shoulder between 190 and 300°C due to oxidation of the polyalkoxide fragmentation
192 192
products formed at lower temperature. Above 300°C, mass spectra showed a broad CO2 m/z = 12 peak that continued until 500°C. m/z = 28 m/z = 44 Interestingly, degradation of the copolymer inside the SB A-15 in absence of oxygen occurs within this temperature range (see Fig.l) and also continued at m/z = 14 m/z = 26 higher temperature. Thus, one m/z = 29 m/z = 30 may suggest that this step is 11 jj associated with the template located inside the ultramicroporosity (<10 A) that limits the accessibility of oxygen. Tem perature (°C) (°C) Temperature Fig.4 shows 13C NMR spectra Fig 3. MS monitored temperature programmed of the same sample. As reported oxidation of SB A-15 washed with water. The before, chemical shifts of 19.3, bottom graph scale reduced by a factor of two 73 and 75 ppm are associated compared to the upper one. with polyoxopropylene (PPO) and the shoulder at 72 ppm is attributed to the PEO chains of \ the template [8, 9]. The sample calcined at 160°C clearly shows a decrease of PPO signal in L J comparison to PEO lines proving that an ether cleavage of the PI23 occurs during the first calcination step and first empties |< «j"°
k
3000 •
2000 •
1000 1000 •
0
:>
SUf
Intensity (A.U.)
T-
m /z = 12 m /z = 28 m /z = 44
i
1600
1200 1200
m /z m /z m /z m /z
= = = =
14 26 29 30
400 •
0
0
100
200
300
400
500
74.3 ppm
(a)
"I
1 163 ppm
600
19.3 ppm
72.0 ppm 69.1 ppm 9.1 p p m 66.2 66.2 p pppm m
62.3 ppm
49 ppm pm
Signal (U.A.)
(b) (b)
(c) (c)
(d)
(e) (e)
J|| '
[CH22CHCH CHCH33O]m O]m
[
CH22CH CH22 O]nn CH22 CH2 [CH 2 OH
HOCH HOCH2CH2OH 2CH2OH
O=CHOCH2CH2O
200
150
OCH3
[CH22CHCH33O]m O]m
100
50
0
4. Conclusion The above results lead to a better understanding of the combustion mechanism of the template in the MMS. N2 sorption and 13C NMR experiments
193 193
of these materials calcined at different temperatures showed that larger mesopores are first emptied followed by intrawall porosity. Further on going studies will investigate the relation between the SBA-15 porosity and the template combustion pattern. 5. References [1] D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka and G. D. Stucky, Science 279 (1998) 548. [2] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka and G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [3] G. Buchel, R. Denoyel, P. L. Llewellyn and J. Rouquerol, J. Mater. Chem. 11 (2001) 589. [4] M.TJ. Keene, R. Denoyel and P.L. Llewellyn Chem. Commun. (1998) 2203. [5] R. van Grieken, G. Calleja, G. D. Stucky, J. A. Melero, R. A. Garcia and J. Iglesias, Langmuir 19 (2003) 3966. [6] S. Kawi and M. W. Lai, Chem. Commun. (1998) 1407. [7] B. Tian, X. Liu, C. Yu, F. Gao, Q. Luo, S. Xie, B. Tu and D. Zhao Chem. Commun. (2002) 1186. [8] C.-M. Yang, B. Zibrowius, W. Schmidt and F. Schuth, Chem. Mater. 16 (2004) 2918. [9] C.-M. Yang, B. Zibrowius, W. Schmidt and F. Schuth, Chem. Mater. 15 (2003) 3739. [10] M. Kruk, M. Jaroniec, C. H. Ko and R. Ryoo, Chem. Mater. 12 (2000) 1961. [11] C.-Y. Chen, H.-X. Li and M.E. Davis Microporous Mater. 2 (1993) 17. [12] A.G.S. Prado and C. Airoldi J. Mater. Chem. 12 (2002) 3823. [13] R. Mokaya and W. Jones, J. Mater. Chem. 8 (1998) 2819. [14] S. Hitz and R. Prins, J. Catal. 168 (1997) 194. [15] F. Kleitz, W. Schmidt and F. Schuth, Micropor. Mesopor. Mater. 65 (2003) 1. [16] M. Jaroniec, M. Kruk and J. P. Oliver, Langmuir 15 (1999) 5410. [17] C. Decker and J. Marchal, Die Makromolekulare Chemie 166 (1973)117.