Ultrafast Sonochemical Synthesis of Methane and Ethane Bridged

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Ultrafast Sonochemical Synthesis of Methane and Ethane Bridged Periodic Mesoporous Organosilicas Paritosh Mohanty, Nyi Myat Khine Linn, and Kai Landskron* Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015 Received June 22, 2009. Revised Manuscript Received August 24, 2009 Periodic mesoporous organosilicas (PMOs) with methane and ethane bridging groups were synthesized by the condensation of bis(triethoxysilyl)methane and bis(triethoxysilyl)ethane, respectively, in an ultrafast sonochemical method with a short reaction time of 30 min using a cationic template (1-hexadecyl)trimethylammonium bromide (HTABr). Subsequently, the template HTABr was extracted by another 30 min of sonication in an acetone/HCl mixture. The whole experimental process for the synthesis and extraction of the PMOs took about 1 h, which is much shorter than any other reported methods. Both the PMOs have very high surface areas of 1200-1390 m2 g-1 with a narrow pore size distribution of ∼3 nm. This sonochemical method further extended for the synthesis of large pore (pore size of ∼5 nm) methane and ethane bridged PMOs using a triblock copolymer Pluronic P123 as template. The methane and ethane bridged PMO materials thus obtained were characterized by small-angle X-ray scattering, transmission electron microscopy, nitrogen sorption, and solid-state NMR techniques.

Introduction Periodic mesoporous organosilicas (PMOs) have emerged as a diverse class of mesoporous materials with uniformly distributed organic groups inside the channel walls of high-surface-area siliceous frameworks.1-5 The combination of the organic and inorganic components in the PMOs tailors the macroscopic properties of porous solids. The high content and the homogeneous distribution of organic groups in the channel walls of these materials render them with attractive mechanical,6 catalytic,7 sorption,8 and dielectric6 properties. A typical sol-gel route for the synthesis of the PMOs is the hydrolysis of organosilane precursors under acidic or basic conditions in the presence of surfactants or templates as structure directing agents. The hydrolysis and the polycondensation normally take from several hours to days. This is followed by the hydrothermal treatment for another 24-48 h. Afterward, the extraction of the templates is carried out by stirring the PMOs with appropriate solvents which further takes several hours. Attempts have been made to reduce the experimental time for the synthesis of PMOs. Recently, microwave was used for the synthesis of PMOs. Ahn and co-workers9,10 synthesized ethane bridged PMO by a microwave-assisted synthesis. *Corresponding author. E-mail: [email protected]. (1) Hatton, B. D.; Landskron, K.; Whitnall, W.; Perovic, D. D.; Ozin, G. A. Acc. Chem. Res. 2005, 38, 305–312. (2) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 1999, 121, 9611–9614. (3) Melde, B. J.; Holland, B. T.; Blanford, C. F.; Stein, A. Chem. Mater. 1999, 11, 3302–3308. (4) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867–871. (5) Yoshina-Ishii, C.; Asefa, T.; Coombs, N.; MacLachlan, M. J.; Ozin, G. A. Chem. Commun. 1999, 2539–2540. (6) Landskron, K.; Hatton, B. D.; Perovic, D. D.; Ozin, G. A. Science 2003, 302, 266–269. (7) Dube, D.; Rat, M.; Beland, F.; Kaliaguine, S. Microporous Mesoporous Mater. 2008, 111, 596–603. (8) Wu, H.; Liao, C.; Pan, Y.; Yeh, C.; Kao, H. Microporous Mesoporous Mater. 2009, 119, 109–116. (9) Kim, D. J.; Chung, J. S.; Ahn, W. S.; Kang, G. W.; Cheong, W. J. Chem. Lett. 2004, 33, 422–423. (10) Yoon, S. S.; Son, W. J.; Biswas, K.; Ahn, W. S. Bull. Korean Chem. Soc. 2008, 29, 609–614.

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They used conventional method for the hydrolysis and polycondensation steps; however, the hydrothermal treatment and the template extraction were done by microwave. The total experimental time for the synthesis of the PMO was about 35 h.9,10 Jaroniec et al.11 used the microwave technique for both the synthesis and template extraction for ethane and disulfide bridged PMOs. The synthesis time in this case was varied between 10 and 72 h and the extraction step between 6 and 36 h.11 More recently, Smeulders et al. reported a microwave-assisted synthesis of benzene PMO with an experimental time of as little as 5 h.12 Vercaemst et al. used a modified sol-gel method for the synthesis (8 h for synthesis and 5 h for template extraction process) of diastereoselective ethene bridged PMOs using P123 as template at a pH of 0.5 using butanol as a cosolvent.13 In this article, we report an ultrafast sonochemical synthesis of methane and ethane bridged PMOs with a total experiment time of 1 h (30 min for synthesis and 30 min for template extraction process) using a cationic template (1-hexadecyl)trimethylammonium bromide (HTABr). Furthermore, we have extended the sonochemical synthesis to the large pore PMOs using P123 as template with the synthesis time of 30 min. However, in this case the template was removed using the conventional extraction method of 24 h stirring in a mixture of acetone and HCl. Sonochemical methods are more and more often used for the synthesis of novel nanomaterials with unusual properties.14-18 In the sonochemical synthesis, a chemical effect called acuostic cavitation, which is a process of creation, growth, and collapse of bubbles formed in the liquid, generates localized hot spots with (11) Grabicka, B. E.; Jaroniec, M. Microporous Mesoporous Mater. 2009, 119, 144–149. (12) Smeulders, G.; Meynen, V.; Baelen, G. V.; Mertens, M.; Lebedev, O. I.; Tendeloo, G. V.; Maes, B. U. W.; Cool, P. J. Mater. Chem. 2009, 19, 3042–3048. (13) Vercaemst, C.; Ide, M.; Allaert, B.; Ledoux, N.; Verpoort, F.; Van Der Voort, P. Chem. Commun. 2007, 2261–2263. (14) Xiong, H. M.; Shchukin, D. G.; Mohwald, H.; Xu, Y.; Xia, Y. Y. Angew Chem., Int. Ed. 2009, 48, 2727–2731. (15) Son, W.; Kim, J.; Kim, J.; Ahn, W. Chem. Commun. 2008, 6336–6338. (16) Li, H.; Han, C. Chem. Mater. 2008, 20, 6053–6059. (17) Lee, S. K.; Lee, J.; Joo, J.; Hyeon, T.; Ahn, W. S.; Lee, H.; Lee, C.; Choi, W. J. Ind. Eng. Chem. 2003, 9, 83–88. (18) Wang, Y.; Yin, L.; Gedanken, A. Ultrason. Sonochem. 2002, 9, 285–290.

Published on Web 09/09/2009

DOI: 10.1021/la902239m

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a very high transient temperature of 20 000 K, pressures of several thousand bar, and heating/cooling rates of >1012 K s-1.19-21 However, the reaction temperature of the reaction mixture is moderately elevated with the pressure remaining at atmospheric level. Very little is known about the synthesis of periodic mesoporous materials by sonochemical methods.17,18 Choi and coworkers reported the synthesis of ordered mesoporous silica SBA15 and Ti-incorporated SBA-15 materials using a sonochemical method.17 Gedanken and co-workers have synthesized hexagonal mesoporous Y and Er oxides with a sonication time of 6 h.18 However, to the best of our knowledge, no report is available for the synthesis of PMOs using sonochemical methods. In this article, we report a facile ultrafast sonochemical synthesis of methane and ethane bridged PMOs using bis(triethoxysilyl)methane (BTSM) and bis(triethoxysilyl)ethane (BTSE) as precursors.

Experimental Section Materials. BTSM and BTSE were purchased from Gelest Inc., 1-hexadecyltrimethylammonium bromide (HTABr) from Alfa Aesar, and Pluronic P123 from BASF. All the chemicals were used as received without any purification. The sonicator used in the present research was Cavitator Ultrasonic ME 11 (Mettler Electronics) with a maximum power output of 200 W at 67 kHz. Synthesis of PMOs Using Cationic Template HTABr. A base-catalyzed surfactant-templating sonochemical method was used for the synthesis of the PMO materials with methane and ethane bridging groups designated as Me-PMO-i and Et-PMO-i, respectively. In a typical synthesis, 1.65 g of HTABr was dissolved in a mixture of 10.8 g of NH4OH (35 wt %) and 19.8 g of deionized water. To the above solution, 7.5 mmol of BTSM or BTSE was added dropwise (within about 1 min) for the synthesis of Me-PMO-i and Et-PMO-i, respectively. The reaction mixture was sonicated for 30 min at room temperature. The precipitated PMOs were then filtered, washed with distilled water, and dried in vacuum. The total yield was about 2.5-2.6 g. The extraction of the template was carried out by sonicating the as-synthesized powder in a mixture of 75 g of acetone and 7.5 g of 2 N HCl for another 30 min. The powders were collected by filtration.

Figure 1. TEM images of (a, b) Me-PMO-i and (c, d) Et-PMO-i along [110] and [001] projections.

Synthesis of PMOs Using Pluronic P123 as Template. Methane and ethane bridged PMOs designated as Me-PMO and Et-PMO, respectively, were synthesized by an acid-catalyzed surfactant-templating sonochemical method using P123 as template and BTSM and BTSE as precursors. In a typical synthesis, 0.83 g of NaCl and 0.18 g of P123 were dissolved in a solution of 4.67 g of 2 N HCl and 1.56 g of H2O. To the above solution, 0.85 mmol of BTSM or BTSE was added dropwise (within about 1 min) for Me-PMO or Et-PMO, respectively. Precipitation started as early as 5 min after the sonication, and the sonication was continued for 30 min. The precipitated PMOs were then filtered and washed with distilled water. The total yield was about 0.35-0.4 g. The extraction of the template was carried out by stirring the as-synthesized powder in a mixture of 50 g of acetone and 5 g of 2 N HCl for 24 h at room temperature. Characterization. The TEM of the specimens was studied by a JEOL JEM-2000 electron microscope operated at 200 kV. Sample for the TEM analysis was prepared by dispersing the particles in acetone and dropping a small volume of it onto a holey carbon film on a copper grid. SAXS patterns of the specimens were obtained using a Rigaku Rotaflex diffractometer with a Cu KR radiation source (λ = 0.154 05 nm). The N2 adsorption/ desorption isotherm was measured at 77 K using an Autosorb-1 instrument (Quantachrome). Prior to the measurement, the speci(19) Suslick, K. S. Science 1990, 247, 1439–1445. (20) Suslick, K. S.; Price, G. J. Annu. Rev. Mater. Sci. 1999, 29, 295–326. (21) Suslick, K. S.; Flannigan, D. J. Annu. Rev. Phys. Chem. 2008, 59, 659–683.

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Figure 2. SAXS patterns of (a) Me-PMO-i and (b) Et-PMO-i showing periodic order of the mesopores.

mens were outgassed at 120 °C for 2 h. The 13C and 29Si NMR spectra were obtained at 59.616 MHz (silicon-29) or 75.468 MHz (carbon-13) on a General Electric NMR Instruments model GN300 equipped with a Doty Scientific 5 mm MAS probe. One pulse spectra were measured with a 1.0 μs pulse length (corresponding to a 20° tip angle) and a relaxation delay of 5.0 s (silicon) or 10 s (carbon) for 16 000-29 000 acquisitions while spinning at typically 5.0 kHz. Proton decoupling during the 40 ms acquisition time was performed with a continuous 70 kHz radio-frequency field at 300.107 MHz. The time domain signal was conditioned with a Gaussian line-broadening function equivalent to 50 Hz prior to Fourier transformation.

Results and Discussion Traditionally, PMOs were synthesized by stirring an organosilane precursor in an acidic or basic solution in the presence of a surfactant template for several hours to few days. Thereafter, the template is removed by stirring the as-synthesized products in a mixture of solvents. In the present work, we report a facile Langmuir 2010, 26(2), 1147–1151

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Figure 3. N2 adsorption/desorption isotherm of (a) Me-PMO-i and (b) Et-PMO-i. The PSD of Me-PMO-i and Et-PMO-i calculated from the desorption branch of the isotherms are shown in the inset.

Figure 4. Solid-state

13

C MAS NMR spectra of (a) Me-PMO-i and (b) Et-PMO-i samples.

Table 1. Physicochemical Properties of Methane and Ethane Bridged PMOs Synthesized by HTABr and P123 Templates sample

BET surface area pore volume pore diameter wall thickness (nm) (nm)b (m2 g-1) (cm3 g-1)a

Me-PMO-i 1390 Et-PMO-i 1201 Me-PMO 666 Et-PMO 669

0.986 0.987 0.742 0.692

3 3.3 4.3 4.9

3.5 3.2 6.4 7

a Total pore volume obtained from the volume of N2 adsorbed at P/P0 of 0.95. b Wall √ thickness estimate=lattice parameter a0 - pore diameter; a0=2d100/ 3.

ultrafast sonochemical synthesis of methane and ethane bridged PMOs with high surface areas and narrow pore size distributions. The reaction was carried out for only 30 min, which is far less than the conventional sol-gel methods (about 24-48 h). In the present research, the whole process of synthesis and template removal took only 1 h, which is substantially less than the reported methods, and we achieved high surface areas in a much less reaction time. The extracted Me-PMO-i and Et-PMO-i show good periodic order of the mesopores as can be seen by the TEM images. Figures 1a and 1b show the TEM images of the Me-PMO-i along the [110] and [001] projections, respectively. The pore diameter and wall thickness calculated from the TEM images are 3 and 3.2 nm, respectively. The pore diameter and wall thickness for the Et-PMO-i sample was calculated to be 2.9 and 3.2 nm as can be seen with TEM images in Figure 1c,d. The order of the specimens is further studied by SAXS. Observation of a sharp peak at 1.55 in the 2θ scale and two small higher order peaks at 2.7 and 3.1 in the Me-PMO-i sample further confirms the hexagonal structure. The lattice parameter calculated to be 6.5 nm is little higher than the value obtained from the TEM images. Observation of smaller lattice constant from TEM can be attributed to the comparatively lower accuracy of the TEM technique compared to the SAXS technique for lattice parameter estimations as well as possible lattice contractions in the electron beam. In the Et-PMO-i sample a similar sharp peak was observed at 1.56 in the 2θ scale (Figure 2b); however, the higher order peaks are not well resolved. The N2 adsorption/desorption isotherms of the Me-PMO-i and Et-PMO-i samples are shown in Figure 3. The BET surface area and the total pore volume for the Me-PMO-i are calculated to be 1390 m2 g-1 and 0.986 cm3 g-1, respectively. A narrow pore size distribution (inset of Figure 3) which centered at 3 nm was Langmuir 2010, 26(2), 1147–1151

Figure 5. 29Si MAS NMR spectra of (a) Me-PMO-i and (b) EtPMO-i. Deconvolution of the 29Si MAS NMR spectrum is shown in the inset.

observed from the BJH analysis. The observation of high surface area and large pore volume indicate that the template was completely removed by the sonochemical extraction procedure used in the present research. Similarly, the Et-PMO-i sample has a surface area of 1201 m2 g-1 and a total pore volume of 0.987 cm3 g-1. The BJH pore size distribution centered at 3.3 nm is shown in the inset of Figure 3. The observation of higher surface area for the Me-PMO-i compared to the Et-PMO-i sample may be attributed to the smaller pore size and thinner wall thickness. Details of the physicochemical properties of these samples are summarized in Table 1. The existence of the organic groups and the effectiveness of the extraction of the template in these samples after the template removal were studied by solid-state 13C and 29Si magic angle spinning (MAS) NMR spectroscopy. Figure 4 shows the 13C NMR spectrum of (a) Me-PMO-i and (b) Et-PMO-i. The spectrum in Figure 4a shows the presence of a signal at -1 ppm. This is due to the -CH2- groups covalently linked to two Si atoms. Such a signal was also observed in the methane bridged PMO synthesized by sol-gel methods.22,23 It is noted that an additional very weak signal was also observed at ∼30 ppm due to the presence of trace amount of acetone, which was used for the extraction of the template. The absence of any other signal further confirms that the template was completely removed by the sonochemical extraction process. Observation of similar 13C signal at 5 ppm in the Et-PMO-i sample (Figure 4b) confirms the existence of the -CH2-CH2- groups. Signals observed (22) Bao, X. Y.; Li, X.; Zhao, X. H. J. Phys. Chem. B 2006, 110, 2656–2661. (23) Burleigh, M. C.; Jayasundera, S.; Thomas, C. W.; Spector, M. S.; Markowitz, M. A.; Gaber, B. P. Colloid Polym. Sci. 2004, 282, 728–733.

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Mohanty et al. Table 2. Position of the T Sites in Different Samples position of T sites (ppm)

sample Me-PMO-i Et-PMO-i Me-PMO Et-PMO

T

1

-52 -50 -52 -47

T2 -62 -59 -63 -55

T3 -70 -67 -70 -62

Figure 7. TEM images of (a, b) Me-PMO and (c, d) Et-PMO along [110] and [001] projections. Figure 6. SAXS patterns of (a) Me-PMO and (b) Et-PMO showing periodic order of the mesopores.

around that chemical shift were also observed for the ethane bridged PMO synthesized by sol-gel methods.24-26 The 29Si MAS NMR spectra of the extracted specimens are shown in Figure 5. Three signals (inset of Figure 5) were observed in both the samples correspond to T1 [C-Si(OH)2(OSi)], T2 [C-Si(OH)(OSi)2], and T3 [C-Si(OSi)3] sites.22-26 The observation of these T sites in the 29Si MAS NMR spectrum confirms the presence of the -CH2- moieties in the framework. Details of the peak positions for all the specimens are summarized in Table 2. Furthermore, the absence of any other signal for the Qn [Si(OSi)n(OH)4-n] species between -90 and -120 ppm confirms that practically no silicon-carbon bond was cleaved either during the synthesis or during the template removal process. We further extended this sonochemical method for the synthesis of large pore methane and ethane bridged PMO designated as Me-PMO and Et-PMO, respectively, using Pluronic P123 triblock copolymer as template and the BTSM and BTSE as precursors. However, in these samples the template could not be extracted satisfactorily by 30 min sonication using an HCl/ acetone mixture. So, we have extracted these Me-PMO and EtPMO by stirring a mixture of 50 g of acetone and 5 g of 2 N HCl for 24 h. After extraction by stirring, the samples show very good periodic order of the mesopores. This was studied by the TEM images and SAXS patterns. Figure 6a shows the SAXS pattern of Me-PMO. An intense reflex was observed at 0.95° in the 2θ scale and two additional weak higher order peaks were observed at 1.86° and 1.64°. These three peaks can be assigned to (100), (110), and (200) planes in the p6mm hexagonal symmetry. The lattice parameter, a, was calculated to be 10.7 nm. Similar peaks were also observed for Et-PMO at slightly different positions (Figure 6b). The lattice parameter for the Et-PMO was calculated (24) Zhang, Z.; Yan, X.; Tian, B.; Shen, S.; Chen, D.; Zhu, G.; Qiu, S.; Zhao, D. Chem. Lett. 2005, 34, 182–183. (25) Cho, E.; Kim, D.; Jaroniec, M. J. Phys. Chem. C 2008, 112, 4897–4902. (26) Pan, Y.; Wu, H.; Jheng, G.; Tsai, H. G.; Kao, H. J. Phys. Chem. C 2009, 113, 2690–2698.

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Figure 8. N2 adsorption/desorption isotherm of (a) Me-PMO and (b) Et-PMO. The PSD of Me-PMO and Et-PMO calculated from the desorption branch of the isotherms are shown in the inset.

as 11.9 nm. TEM images as shown in Figure 7a-d along the [110] and [001] projections for both the samples were further confirm the hexagonal structure. The pore size measured from the TEM images are 4.2 and 5 nm with wall thickness of 6.2 and 6.5 nm for the Me-PMO and Et-PMO, respectively. The lattice parameter calculated from the SAXS patterns is slightly higher than that obtained from the TEM images. The N2 sorption (Figure 8) of both the samples shows type IV isotherms typically for the mesoporous materials. The BET surface area and the pore volumes for the Me-PMO are 666 m2 g-1 and 0.742 cm3 g-1, respectively. The Et-PMO has a surface area of 669 m2 g-1 and a pore volume of 0.692 cm3 g-1. Both of the samples show very narrow pore size distributions (calculated from the desorption branch of the isotherm) (inset of Figure 8) which are centered at 4.3 and 4.9 nm for the Me-PMO and Et-PMO, respectively. These values are in good accordance with the values obtained from the DFT and Monte Carlo method (5 and 5.5 nm) and also are in good accordance with the TEM data. However, calculation of the PSD from the adsorption branch shows good matching for the EtPMO (5.6 nm) but does not match well for the Me-PMO (6.5 nm). This indicates that the calculation from the desorption branch Langmuir 2010, 26(2), 1147–1151

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Figure 9. Solid-state 13C MAS NMR spectra of (a) Me-PMO and (b) Et-PMO samples.

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shows the presence of a signal at -1 ppm.22,23 This is due to the -CH2- group between the two Si atoms. In the Et-PMO sample a similar signal was observed at 5 ppm, which confirms the presence of the -CH2-CH2- groups in the specimen.24-26 It is noted that an additional weak signal was also observed at ∼71 ppm in both the samples. This was due to the presence of a small amount of surfactant that remains after the extraction.23,27 The 29 Si MAS NMR spectrum of the extracted specimen is shown in Figure 10. Three signals (inset of Figure 10) were observed which correspond to T1 [C-Si(OH)2(OSi)], T2 [C-Si(OH)(OSi)2], and T3 [C-Si(OSi)3] sites. The positions of the different T sites are summarized in Table 2. The observation of these T sites and the absence of any other signal between -90 and -120 ppm in the 29Si MAS NMR spectra confirms the presence of the -CH2- moieties in the framework.

Conclusions

Figure 10.

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Si MAS NMR spectra of (a) Me-PMO and (b) Et-PMO. Deconvolution of the 29Si MAS NMR spectrum is shown in the inset.

and the DFT and Monte Carlo calculation reflecting the pore size more accurately. The desorption isotherm closes at P/P0 =0.45, which is above 0.42 indicating that the closing of the hysteresis is not due to the tensile strength effect. Figures 9a and 9b show the 13C NMR spectra of the Me-PMO and Et-PMO, respectively. The spectrum in Figure 9a (27) Bao, Y. K.; Han, O. H. Microporous Mesoporous Mater. 2007, 106, 304– 307.

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A facile ultrafast sonochemical method was developed for the synthesis of highly ordered methane and ethane bridged PMOs using a cationic surfactant template. Both the synthesis and the template removal steps can be completed in 1 h, which is much shorter compared to the conventional sol-gel methods, where the reaction and template removal processes take up to a few days. The sonochemical method works also for the synthesis of PMOs using block copolymer templates. However, the template removal process cannot be speeded up by sonochemical methods. The physicochemical properties of the obtained PMOs are as good as the PMOs obtained by the sol-gel methods. The method works for gram scale production of highly ordered PMOs. The extremely short reaction time which yields high-quality products could be interesting for the industrial scale productions of PMOs. This sonochemical method for the synthesis of PMO can likely be further extended for the synthesis of many other PMOs with various bridging groups. Acknowledgment. The present work was supported by Lehigh University start-up funds and faculty grants. Dr. James E. Roberts is gratefully acknowledged for MAS NMR measurements. We further thanks Dr. Chris Kiely and Dr. Dave Ackland for generously supporting our TEM investigations. Dr. G. Slade Cargill is gratefully acknowledged for supporting our X-ray diffraction experiments.

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