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Comparison of the thermal and hydrothermal stabilities of ethylene, ethylidene, phenylene and biphenylene bridged periodic mesoporous organosilicas
Materials Letters 65 (2011) 1460–1462
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Comparison of the thermal and hydrothermal stabilities of ethylene, ethylidene, phenylene and biphenylene bridged periodic mesoporous organosilicas Dolores Esquivel, César Jiménez-Sanchidrián, Francisco J. Romero-Salguero ⁎ Departamento de Química Orgánica, Universidad de Córdoba, Campus de Rabanales, Edificio Marie Curie, Ctra. Nnal. IV, km 396, 14014 Córdoba, Spain
a r t i c l e
i n f o
Article history: Received 14 October 2010 Accepted 10 February 2011 Available online 15 February 2011 Keywords: PMOs Mesostructure Characterization Thermal stability Hydrothermal stability
a b s t r a c t The nature of the organic bridges in the framework of periodic mesoporous organosilicas (PMOs) has been shown to determine their thermal and hydrothermal stabilities. Thus, the aromatic bridges were decomposed at higher temperatures than the aliphatic ones but afterwards the mesostructure of the resulting silica remained intact. Except for the ethylidene bridged PMO, the rest of these materials have been resistant to aqueous media. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Since they were reported in 1999 [1–3], the so-called periodic mesoporous organosilicas (PMOs), that is, hybrid organic-inorganic materials synthesized from bridged organosilanes of the type (R′O)3SiR-Si(OR′)3 in the presence of a surfactant, have attracted great interest due to their potential application in different fields. Thus, they have been used as adsorbents [4], catalysts [5], films [6], etc. The presence of both organic and inorganic moieties in the pore walls of these hybrid materials make them extraordinarily versatile. In particular, the organic fragments (R) give them a more hydrophobic character [7] and allow their functionalization [8,9]. These features are of a huge importance when they interact with organic molecules for their above-mentioned uses. However, the organic groups are considered to be more labile than the inorganic ones. In order to establish the conditions under which PMOs can be used, herein we report the thermal and hydrothermal stabilities of such materials with ―CH2―CH2―, ―CH=CH―, ―C6H4− and ―C12H8― bridges.
2. Experimental Ethylene, ethylidene and phenylene bridged PMOs were synthesized by using Brij 76 as surfactant under acidic conditions according to previously reported procedures [10]. Biphenylene bridged PMO was obtained under basic conditions in the presence of OTAC
⁎ Corresponding author. Tel.: + 34 957212065; fax: + 34 957212066. E-mail address: [email protected] (F.J. Romero-Salguero). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.02.037
(octadecyl trimethyl ammonium chloride) as structure-directing agent [11]. The thermal stability of PMOs was checked by calcination in air at different temperatures (i.e., 200, 350, 450 and 600 °C) for 4 h. Their hydrothermal behavior was tested by stirring a suspension of the samples in water (0.005 g/mL) for 8 h at either room temperature or reflux. PMOs were characterized by different techniques. X-ray powder diffraction (XRD) patterns were recorded on a Siemens D-5000 powder diffractometer (Cu-Kα radiation). N2 isotherms were determined on a Micromeritics ASAP 2010 analyzer at − 196 °C. The 29Si and 13C MAS NMR spectra were recorded at 79.49 MHz and 100.61 MHz, respectively, on a Bruker ACP-400 spectrometer at room temperature. An overall 1000 free induction decays were accumulated. The excitation pulse and recycle time for 29Si MAS NMR were 6 μs and 60 s, respectively, and those for 13C MAS NMR spectra 6 μs and 2 s. FT-IR spectra were collected on a Bomen MB-100 spectrometer, using KBr pellets of the solid samples. Simultaneous thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed using a thermobalance Setaram Setsys 12 under an air flow (40 mL min− 1) heating at 10 °C min− 1 from room temperature up to 1000 °C. 3. Results and discussion XRD measurements confirmed the formation of PMOs with twodimensional hexagonal (p6mm) symmetry, whose physicochemical properties are given in Table 1. All materials showed type IV adsorption–desorption isotherms (i.e. typical of mesoporous solids), high specific surface areas (N800 m2 g− 1) and narrow pore size
D. Esquivel et al. / Materials Letters 65 (2011) 1460–1462
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Table 1 Physicochemical properties of periodic mesoporous organosilicas. Bridge
a0a/Å BET surface Total pore volume/ area /m2 g− 1 cm3 g− 1
Pore sizeb/Å
Wall thicknessc/Å
Ethylene Ethylidene Phenylene Biphenylene
68 67 63 52
39 32 32 35
39 35 28 17
a b c
1145 929 977 810
1.06 0.74 0.70 0.57
Unit-cell dimension calculated from a0 = (2d100/√3). Calculated from the desorption branch. Estimated from (a0 — pore size).
distributions in the range 32–39 Å. The integrity of the organic bridges after their preparation was proven by FTIR and MAS NMR. Thus, bands at 1273, 1427, 2899 and 2948 cm− 1 were assigned to C―H vibrations of the ethylene bridges. The ethylidene bridged PMO exhibited the characteristic C=C stretching vibration at 1630 cm− 1, whereas the phenylene and biphenylene bridged PMOs showed the C―H and C―C vibrations at 3060 and 1600, 1536 and 1490 cm− 1, respectively. 29Si MAS NMR spectra had three bands which can be attributed to Tn sites, i.e. those coming from the precursors (organotrialkoxysilanes): (C-Si (OSi)3) (T3), (C-Si(OSi)2(OH)) (T2) and (C-Si(OSi)(OH)2) (T1). These signals appeared at −64, − 57 and −50 ppm for the ethylene, at −82, −73 and −64 ppm for the ethylidene, at −80, − 71 and −62 ppm for the phenylene, and at −80, − 70 and −61 ppm for the biphenylene units. Qn sites (Si(OSi)n(OH)4-n) were not detected, thus
Fig. 2. X-ray diffraction patterns of extracted ethylene (a), ethylidene (b), phenylene (c) and biphenylene (d) bridged PMOs after thermal treatment at different temperatures. The unit cell contractions (%) are given in parentheses.
indicating the absence of C―Si bond cleavage. 13C MAS NMR spectra revealed a broad signal corresponding to the carbon atoms of the organic bridges at 4 ppm for ethylene, 146 ppm for ethylidene and
Fig. 1. Thermogravimetric curves (—— TGA) and (–––DTA) of extracted ethylene (a), ethylidene (b), phenylene (c) and biphenylene (d) bridged PMOs.
Fig. 3. X-ray diffraction patterns of extracted ethylene (a), ethylidene (b), phenylene (c) and biphenylene (d) bridged PMOs before (solid line) and after hydrothermal treatments at room temperature (dashed line) and reflux (dotted line).
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139 ppm for phenylene. Four signals at 126, 134, 136 and 142 ppm were observed for biphenylene. All in all the as-synthesized samples were high quality PMOs. The thermal behavior of PMOs was determined by TGA/DTA, as depicted in Fig. 1. The ethylene bridges decomposed between 200 and 350 °C (as revealed also by the exothermic peak), although the corresponding weight loss was even extended up to 400 °C. The ethylidene groups were lost at somewhat higher temperatures in the range 300–500 °C. However, the aromatic bridges were the most thermally stable because they decomposed above 500 °C. The structure of these PMOs was followed by XRD after their calcination at different temperatures (Fig. 2). In general, the ordered hexagonal structure remained even at 600 °C, that is, once the organic fraction of these materials was decomposed. However, the mesostructure of the biphenylene bridged PMO was damaged at 450 °C and particularly at 600 °C, probably due to its longer bridge. In all cases, as the calcination temperature increased, the dimensions of the unit cell decreased. The mesostructure of ethylene, phenylene and biphenylene bridged PMOs was preserved in water both at room temperature and reflux (Fig. 3). Even, the molecular-scale periodicity (11.9 Å) in the pore walls of the biphenylene sample, revealed by the (200) reflection, remained unaltered after the treatment [11]. On the contrary, the existence of ethylidene units in the PMO made it very sensible to the presence of water even at room temperature. 4. Conclusion Several periodic mesoporous organosilicas with different bridges behaved very differently after thermal or hydrothermal treatments. Those containing aliphatic bridges (ethylene and ethylidene) decom-
posed at lower temperatures than those with aromatic bridges (phenylene and biphenylene), which also exhibited less unit cell contraction than the former ones. The mesostructure of these PMOs remained intact even above the decomposition temperature, particularly for PMOs with ethylene and phenylene bridges. All of them, except the ethylidene bridged PMO, were stable in aqueous media. Acknowledgements The authors wish to acknowledge funding of this research by Ministerio de Ciencia e Innovación of Spain (project MAT2010-18778) and Junta de Andalucía (project P06-FQM-01741). D.E. thanks Ministerio de Educación y Ciencia for a teaching and research fellowship. References [1] Inagaki S, Guan S, Fukushima Y, Ohsuna T, Terasaki O. J Am Chem Soc 1999;121: 9611–4. [2] Melde BJ, Holland BT, Blandford CF, Stein A. Chem Mater 1999;11:302–8. [3] Asefa T, MacLachlan MJ, Coombs N, Ozin GA. Nature 1999;402:867–71. [4] Borghard WG, Calabro DC, DiSanzo FP, Disko MM, Diehl JW, Fried JC, Markowitz MA, Zeinali M, Melde BJ, Riley AE. Langmuir 2009;25:12661–9. [5] Cho W, Park J, Ha C. Mater Lett 2004;58:3551–4. [6] Ohtani O, Goto Y, Okamoto K, Inagaki S. Mater Lett 2006;60:177–9. [7] Karam A, Alonso JC, Ivanova T, Ferreira P, Bion N, Barrault J, Jérôme F. Chem Commun 2009:7000–2. [8] Inagaki S, Guan S, Ohsuna T, Terasaki O. Nature 2002;416:304–7. [9] Nakajima K, Tomita I, Hara M, Hayashi S, Domen K, Kondo JN. Adv Mater 2005;17: 1839–42. [10] Burleigh MC, Jayasundera S, Thomas CW, Spector MS, Markowitz MA, Gaber BP. Colloid Polym Sci 2004;282:728–33. [11] Kapoor MP, Yang Q, Inagaki S. J Am Chem Soc 2002;124:15176–7.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Comparison of the thermal and hydrothermal stabilities of ethylene, ethylidene, phenylene and biphenylene bridged periodic mesoporous organosilicas Dolores Esquivel, César Jiménez-Sanchidrián, Francisco J. Romero-Salguero ⁎ Departamento de Química Orgánica, Universidad de Córdoba, Campus de Rabanales, Edificio Marie Curie, Ctra. Nnal. IV, km 396, 14014 Córdoba, Spain
a r t i c l e
i n f o
Article history: Received 14 October 2010 Accepted 10 February 2011 Available online 15 February 2011 Keywords: PMOs Mesostructure Characterization Thermal stability Hydrothermal stability
a b s t r a c t The nature of the organic bridges in the framework of periodic mesoporous organosilicas (PMOs) has been shown to determine their thermal and hydrothermal stabilities. Thus, the aromatic bridges were decomposed at higher temperatures than the aliphatic ones but afterwards the mesostructure of the resulting silica remained intact. Except for the ethylidene bridged PMO, the rest of these materials have been resistant to aqueous media. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Since they were reported in 1999 [1–3], the so-called periodic mesoporous organosilicas (PMOs), that is, hybrid organic-inorganic materials synthesized from bridged organosilanes of the type (R′O)3SiR-Si(OR′)3 in the presence of a surfactant, have attracted great interest due to their potential application in different fields. Thus, they have been used as adsorbents [4], catalysts [5], films [6], etc. The presence of both organic and inorganic moieties in the pore walls of these hybrid materials make them extraordinarily versatile. In particular, the organic fragments (R) give them a more hydrophobic character [7] and allow their functionalization [8,9]. These features are of a huge importance when they interact with organic molecules for their above-mentioned uses. However, the organic groups are considered to be more labile than the inorganic ones. In order to establish the conditions under which PMOs can be used, herein we report the thermal and hydrothermal stabilities of such materials with ―CH2―CH2―, ―CH=CH―, ―C6H4− and ―C12H8― bridges.
2. Experimental Ethylene, ethylidene and phenylene bridged PMOs were synthesized by using Brij 76 as surfactant under acidic conditions according to previously reported procedures [10]. Biphenylene bridged PMO was obtained under basic conditions in the presence of OTAC
⁎ Corresponding author. Tel.: + 34 957212065; fax: + 34 957212066. E-mail address: [email protected] (F.J. Romero-Salguero). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.02.037
(octadecyl trimethyl ammonium chloride) as structure-directing agent [11]. The thermal stability of PMOs was checked by calcination in air at different temperatures (i.e., 200, 350, 450 and 600 °C) for 4 h. Their hydrothermal behavior was tested by stirring a suspension of the samples in water (0.005 g/mL) for 8 h at either room temperature or reflux. PMOs were characterized by different techniques. X-ray powder diffraction (XRD) patterns were recorded on a Siemens D-5000 powder diffractometer (Cu-Kα radiation). N2 isotherms were determined on a Micromeritics ASAP 2010 analyzer at − 196 °C. The 29Si and 13C MAS NMR spectra were recorded at 79.49 MHz and 100.61 MHz, respectively, on a Bruker ACP-400 spectrometer at room temperature. An overall 1000 free induction decays were accumulated. The excitation pulse and recycle time for 29Si MAS NMR were 6 μs and 60 s, respectively, and those for 13C MAS NMR spectra 6 μs and 2 s. FT-IR spectra were collected on a Bomen MB-100 spectrometer, using KBr pellets of the solid samples. Simultaneous thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed using a thermobalance Setaram Setsys 12 under an air flow (40 mL min− 1) heating at 10 °C min− 1 from room temperature up to 1000 °C. 3. Results and discussion XRD measurements confirmed the formation of PMOs with twodimensional hexagonal (p6mm) symmetry, whose physicochemical properties are given in Table 1. All materials showed type IV adsorption–desorption isotherms (i.e. typical of mesoporous solids), high specific surface areas (N800 m2 g− 1) and narrow pore size
D. Esquivel et al. / Materials Letters 65 (2011) 1460–1462
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Table 1 Physicochemical properties of periodic mesoporous organosilicas. Bridge
a0a/Å BET surface Total pore volume/ area /m2 g− 1 cm3 g− 1
Pore sizeb/Å
Wall thicknessc/Å
Ethylene Ethylidene Phenylene Biphenylene
68 67 63 52
39 32 32 35
39 35 28 17
a b c
1145 929 977 810
1.06 0.74 0.70 0.57
Unit-cell dimension calculated from a0 = (2d100/√3). Calculated from the desorption branch. Estimated from (a0 — pore size).
distributions in the range 32–39 Å. The integrity of the organic bridges after their preparation was proven by FTIR and MAS NMR. Thus, bands at 1273, 1427, 2899 and 2948 cm− 1 were assigned to C―H vibrations of the ethylene bridges. The ethylidene bridged PMO exhibited the characteristic C=C stretching vibration at 1630 cm− 1, whereas the phenylene and biphenylene bridged PMOs showed the C―H and C―C vibrations at 3060 and 1600, 1536 and 1490 cm− 1, respectively. 29Si MAS NMR spectra had three bands which can be attributed to Tn sites, i.e. those coming from the precursors (organotrialkoxysilanes): (C-Si (OSi)3) (T3), (C-Si(OSi)2(OH)) (T2) and (C-Si(OSi)(OH)2) (T1). These signals appeared at −64, − 57 and −50 ppm for the ethylene, at −82, −73 and −64 ppm for the ethylidene, at −80, − 71 and −62 ppm for the phenylene, and at −80, − 70 and −61 ppm for the biphenylene units. Qn sites (Si(OSi)n(OH)4-n) were not detected, thus
Fig. 2. X-ray diffraction patterns of extracted ethylene (a), ethylidene (b), phenylene (c) and biphenylene (d) bridged PMOs after thermal treatment at different temperatures. The unit cell contractions (%) are given in parentheses.
indicating the absence of C―Si bond cleavage. 13C MAS NMR spectra revealed a broad signal corresponding to the carbon atoms of the organic bridges at 4 ppm for ethylene, 146 ppm for ethylidene and
Fig. 1. Thermogravimetric curves (—— TGA) and (–––DTA) of extracted ethylene (a), ethylidene (b), phenylene (c) and biphenylene (d) bridged PMOs.
Fig. 3. X-ray diffraction patterns of extracted ethylene (a), ethylidene (b), phenylene (c) and biphenylene (d) bridged PMOs before (solid line) and after hydrothermal treatments at room temperature (dashed line) and reflux (dotted line).
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139 ppm for phenylene. Four signals at 126, 134, 136 and 142 ppm were observed for biphenylene. All in all the as-synthesized samples were high quality PMOs. The thermal behavior of PMOs was determined by TGA/DTA, as depicted in Fig. 1. The ethylene bridges decomposed between 200 and 350 °C (as revealed also by the exothermic peak), although the corresponding weight loss was even extended up to 400 °C. The ethylidene groups were lost at somewhat higher temperatures in the range 300–500 °C. However, the aromatic bridges were the most thermally stable because they decomposed above 500 °C. The structure of these PMOs was followed by XRD after their calcination at different temperatures (Fig. 2). In general, the ordered hexagonal structure remained even at 600 °C, that is, once the organic fraction of these materials was decomposed. However, the mesostructure of the biphenylene bridged PMO was damaged at 450 °C and particularly at 600 °C, probably due to its longer bridge. In all cases, as the calcination temperature increased, the dimensions of the unit cell decreased. The mesostructure of ethylene, phenylene and biphenylene bridged PMOs was preserved in water both at room temperature and reflux (Fig. 3). Even, the molecular-scale periodicity (11.9 Å) in the pore walls of the biphenylene sample, revealed by the (200) reflection, remained unaltered after the treatment [11]. On the contrary, the existence of ethylidene units in the PMO made it very sensible to the presence of water even at room temperature. 4. Conclusion Several periodic mesoporous organosilicas with different bridges behaved very differently after thermal or hydrothermal treatments. Those containing aliphatic bridges (ethylene and ethylidene) decom-
posed at lower temperatures than those with aromatic bridges (phenylene and biphenylene), which also exhibited less unit cell contraction than the former ones. The mesostructure of these PMOs remained intact even above the decomposition temperature, particularly for PMOs with ethylene and phenylene bridges. All of them, except the ethylidene bridged PMO, were stable in aqueous media. Acknowledgements The authors wish to acknowledge funding of this research by Ministerio de Ciencia e Innovación of Spain (project MAT2010-18778) and Junta de Andalucía (project P06-FQM-01741). D.E. thanks Ministerio de Educación y Ciencia for a teaching and research fellowship. References [1] Inagaki S, Guan S, Fukushima Y, Ohsuna T, Terasaki O. J Am Chem Soc 1999;121: 9611–4. [2] Melde BJ, Holland BT, Blandford CF, Stein A. Chem Mater 1999;11:302–8. [3] Asefa T, MacLachlan MJ, Coombs N, Ozin GA. Nature 1999;402:867–71. [4] Borghard WG, Calabro DC, DiSanzo FP, Disko MM, Diehl JW, Fried JC, Markowitz MA, Zeinali M, Melde BJ, Riley AE. Langmuir 2009;25:12661–9. [5] Cho W, Park J, Ha C. Mater Lett 2004;58:3551–4. [6] Ohtani O, Goto Y, Okamoto K, Inagaki S. Mater Lett 2006;60:177–9. [7] Karam A, Alonso JC, Ivanova T, Ferreira P, Bion N, Barrault J, Jérôme F. Chem Commun 2009:7000–2. [8] Inagaki S, Guan S, Ohsuna T, Terasaki O. Nature 2002;416:304–7. [9] Nakajima K, Tomita I, Hara M, Hayashi S, Domen K, Kondo JN. Adv Mater 2005;17: 1839–42. [10] Burleigh MC, Jayasundera S, Thomas CW, Spector MS, Markowitz MA, Gaber BP. Colloid Polym Sci 2004;282:728–33. [11] Kapoor MP, Yang Q, Inagaki S. J Am Chem Soc 2002;124:15176–7.