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Silicone surfactant templated mesoporous silica
Microporous and Mesoporous Materials 172 (2013) 30–35
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Silicone surfactant templated mesoporous silica Ji Feng, Bo Sun, Yuan Yao ⇑, Shunai Che ⇑ School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Key Laboratory for Thin Film and Microfabrication Technology of the Ministry of Education, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China
a r t i c l e
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Article history: Received 30 April 2012 Received in revised form 7 January 2013 Accepted 8 January 2013 Available online 16 January 2013 Keywords: Mesoporous silica Silicone surfactant Wall thickness Functionalization
a b s t r a c t Tailoring the wall thickness and organo-functionalization are two important issues in the design and application of mesoporous materials. Herein we report a novel strategy to deal with such two issues simultaneously by using a commercially available silicone surfactant, an ABA-type triblock copolymer poly(ethylene oxide)-block-polydimethylsiloxane-block-poly(ethylene oxide) (PEO14-b-PDMS13-bPEO14). The silicone surfactant serves as the template for directing meso-structures, the source of functional groups for surface modification, and the silica source for strengthening the wall of mesochannels. Meanwhile, 1,3,5-trimethyl benzene (TMB) was employed as the pore-swelling-agent. The mesoporous silica materials with thick walls and/or hydrophobic dimethylsiloxane functionalized pores were achieved by calcination and extraction, respectively. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Mesoporous silica materials are of special interest due to their unique features [1–4] and potential applications in the fields of catalysis, sensing, optically active materials, and biomaterials [5– 8]. Commonly, mesoporous silica materials could be fabricated by using cationic [9–11], non-ionic [12–14] and anionic surfactants [15–18] as templates to obtain ordered mesostructures and wellcontrolled morphologies, pore sizes, and porosities. Tailoring the wall thickness is an important issue in the design and application of mesoporous materials. Except for changing the hydrothermal treatment and aging conditions [19–21], the wall thickness could also be adjusted by tuning the length of the hydrophilic segment of template. It was found that the wall thickness of hexagonally meso-structured silica increased with the elongation of poly(ethylene oxide) (EO)x block in the Pluronic surfactant [22]. Ryoo and co-workers reported a synthetic approach for systematically controlling the wall thickness of mesoporous silica by using a mixture of PEO-containing non-ionic surfactant and cationic surfactant [23]. Recently, alkyl alcohol ether carboxylate (AEC: CnH2n+1O(CH2CH2O)mCH2COONa) surfactant was applied to fabricate hexagonal mesoporous silica with thick walls. The two types of hydrophilic head groups led to the formation of unique double-layer silica walls and the wall thickness were obviously increased as a result [24]. Organo-functionalization is also an important issue to combine enormous functions with the high surface area, uniform pore size, and thermally stable inorganic substrates [25–28]. The ⇑ Corresponding authors. Tel.: +86 21 5474 2852; fax: +86 21 5474 5365. E-mail addresses: [email protected] (Y. Yao), [email protected] (S. Che). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.01.010
organo-functionalized mesoporous materials have been prepared through several distinct routes yet: (i) the post-grafting method [29–35]; (ii) the co-condensation method [36–40]; (iii) introduction of co-structure directing agent [15–18] and (iv) the application of special precursors and templates such as bissilylated single-source organosilica [41–46] and surfactants consisting of functionalized condensable heads and cleavable alkyl tails [47]. Here we report a facile strategy to thicken the walls of mesoporous silica materials and hydrophobically functionalize the surface of channels by using a commercially available silicone surfactant Q4-3667 as template. Q4-3667 is an ABA type triblock copolymer, PEO14-b-PDMS13-b-PEO14. The PDMS block not only acts as the templates for pore formation, but also offers dimethyl siloxane segments for obtaining hydrophobic surface. In addition, the PDMS block also provides the silica source for thickening the walls by calcination. As shown in Scheme 1b, the PDMS block could form Si– O–Si inorganic network while be calcined at a high temperature (550 °C), which offers a new pathway to thicken the silica walls of the mesoporous materials. On the other hand, the PDMS blocks could attach to the silica matrix to achieve hydrophobic functionalization on the walls. However, the disadvantage of this strategy is that PDMS might fill up the pores (Scheme 1a) and prevent other molecules from entering into the mesochannels. In order to solve this problem, 1,3,5-trimethyl benzene (TMB) was employed as the pore-swelling agent [48,49]. The hydrophobic TMB was expected to swell up the hydrophobic part of the micelles of Q43667 by stabilizing the oil/water interface due to the hydrophobic interaction with the PDMS chains (Scheme 1c), and further tune the pore size of mesoporous silica. After the silica matrix was formed, TMB could be removed by extraction to obtain accessible dimethyl siloxane functionalized mesopores (Scheme 1d).
J. Feng et al. / Microporous and Mesoporous Materials 172 (2013) 30–35
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Scheme 1. Schematic illustration of the synthetic routes to the formation of Q4-3667 directed mesostructure (a), mesoporous silica with thick wall (b), organo-functionalized mesoporous silica with addition of TMB (c) and subsequent extraction (d). The black part of the cross section view in (b) denoted the deoxidized product of the PDMS chains.
2. Experimental 2.1. Materials Q4-3667 was provided by Dow Corning Corporation. 1,3,5-trimethyl benzene (TMB) and tetraethoxysilane (TEOS) were purchased from Sinopharm Chemical Reagent Co., Ltd. All these reagents were used as received.
Barett-Joyner-Halenda (BJH) method using both the desorption and adsorption branch of the isotherm. The pore volumes of the mesopores and micropores were estimated by BJH and t-plot method base on the desorption branch of the isotherm. Solid-state 13 C CP/MAS NMR spectra were collected on an Oxford AS400 NMR spectrometer at 100 MHz and a sample spinning frequency of 3 kHz.
2.2. A representative synthetic method
3. Results and discussion
1.0 g of Q4-3667, 0, 0.4 or 0.8 g of TMB and 108.6 g of deionized water were mixed to form a homogeneous solution. The solution was heated to 35 °C prior to the addition of TEOS. After the addition of TEOS (2.97 g), the mixture was stirred for 30 min, followed by reaction under the static condition for one day at 35 °C. The mixture was sealed in a Teflon autoclave and aged at 100 °C for two days. The reaction was carried out in a sealed vessel throughout the whole synthetic process. The solid product was collected by filtration or centrifugation, washed with water, and dried in air at 60 °C. The resultant powder was either calcined at a 2 °C/min heating rate from room temperature to 550 °C and held for 6 h in air to simultaneously remove the template and TMB, or extracted in cyclohexane at 80 °C for 48 h to remove TMB. The feed mass composition was Q4-3667:H2O:TMB:TEOS = 1:108.6:x:2.97. The asmade samples were denoted as SMS-0-as (with no TMB), SMS0.4-as (with TMB/Q4-3667 (w/w) = 0.4/1) and SMS-0.8-as (with TMB/Q4-3667 (w/w) = 0.8/1), respectively. The corresponding calcined samples and extracted samples were denoted as SMS-0-cal, SMS-0.4-cal and SMS-0.8-cal, and SMS-0.4-ex and SMS-0.8-ex.
The XRD patterns of the as-made, extracted and calcined samples synthesized with different TMB/Q4-3667 ratios are shown in Fig. 1. All samples show three well-resolved reflection peaks with p a d-spacing ratio of 1: 3:2, which can be indexed as (10), (11) and (20) reflections of the two dimensional (2D)-hexagonal p6mm structure. The unit cell parameter a of the SMS-0-as was 8.9 nm. The diffraction peaks of the samples SMS-0.4-as and SMS-0.8-as shifted toward low angle and their unit cell parameters increased to 9.3 and 9.7 nm, respectively because TMB swelled the hydrophobic cores of the micelles [50–53]. As shown in Fig. 1B, the diffraction peaks of all calcined samples shifted toward high angles. The unit cell parameters of the SMS-0-cal, SMS-0.4-cal and SMS-0.8-cal decreased to 8.5, 8.6, and 8.9 nm respectively because of the shrinkage of the silica frameworks. Compared to the asmade samples, the unit cell parameters of the SMS-0.4-ex and SMS-0.8-ex that were calculated from the XRD patterns shown in Fig. 1C also decreased to 9.0 and 9.3 nm respectively due to the shrinkage of the silica matrix. As shown in Fig. 2, the corresponding high-resolution transmission electron microscopy (HRTEM) images with the typical hexagonal p6mm array of (10) plane of the calcined samples, which were consistent with the XRD results. The center-to-center distances of pores in these three samples were 8.5, 8.7, and 8.9 nm for SMS0-cal, SMS-0.4-cal and SMS-0.8-cal, respectively, which were close to the unit cell parameters calculated based on the XRD patterns. The isotherms of the calcined samples displayed type IV curves with H2-type hysteresis loops, indicating the existence of ink-bottle style mesopore channel structure within the materials (Fig. 3A). It can be considered that the pores were not perfectly cylindrical because the PDMS chains were oxidized into silica after calcination, which may form ink-bottle style pore channel structure, and result in the H2-type hysteresis loops, although the XRD patterns and TEM images revealed the silica materials possessed ordered
2.3. Characterization Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku X-ray diffractometer D/MAX-2200/PC equipped with Cu Ka radiation (40 kV, 20 mA) at the rate of 0.1 deg./min over the range of 1–6°(2h). HRTEM was performed with a JEOL JEM-3010 microscope operating at 300 kV (Cs = 0.6 mm, resolution 1.7 nm). Images were recorded with the Kodak electron film SO-163 using low-electron-dose conditions. The nitrogen adsorption/desorption isotherms were measured at 77 K with the Quantachrome Nova 4200E. The surface area was calculated by the Brunauer-EmmettTeller (BET) method and the pore size was obtained from the maxima of the pore size distribution curve calculated by the
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J. Feng et al. / Microporous and Mesoporous Materials 172 (2013) 30–35
Fig. 1. XRD patterns of three silica samples: as-made (A), calcined at 550 °C (B), and extracted in cyclohexane (C). The XRD patterns of the SMS-0.4-as, SMS-0.8-as, SMS-0.4cal, SMS-0.8-cal, and SMS-0.8-ex were shifted along the y axis.
Fig. 2. TEM images of three calcined silicas: SMS-0-cal (A), SMS-0.4-cal (B) and SMS-0.8-cal (C).
Fig. 3. N2 adsorption/desorption isotherms (A) and pore size distribution curves calculated based on the desorption (B) and adsorption (C) branches of the SMS-0-cal, SMS0.4-cal and SMS-0.8-cal. The isotherms of SMS-0.4-cal and SMS-0.8-cal are shifted by 100 and 240 cm3/g, respectively. The pore distribution curves of SMS-0.4-cal and SMS0.8-cal are shifted by 0.2 and 0.4 cm3/nm/g (B), and 0.03 and 0.06 cm3/nm/g (C), respectively.
p6mm symmetry. The bulk pore diameter and the narrowed pore entrance were calculated based on adsorption and desorption branches, respectively (Fig. 3C and B) [54]. As listed in Table 1, the size of the bulk pore and pore entrance of SMS-0-cal were 5.8 and 3.7 nm respectively. With the amount of TMB increasing, the bulk pore diameters of the SMS-0.4-cal and SMS-0.8-cal increased to 6.6 and 6.7 nm. Meanwhile, their pore entrances increased to 3.9 and 4.0 nm. The isotherms of the calcined samples
showed a sharp increase in adsorption at p/p0 < 0.1 and a plateau at p/p0 > 0.1, suggesting the existence of micropores in the silica matrix. The formation of micropores was attributed to the PEO segments embedded in the silica walls [55,56]. The t-plot calculation revealed that the volumes of mesopores increased from 0.60 cm3/ g to 0.67 cm3/g and 0.85 cm3/g with the increase of TMB in calcined SMS-0-cal, SMS-0.4-cal and SMS-0.8-cal samples respectively while the volumes of micropores in these three samples
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J. Feng et al. / Microporous and Mesoporous Materials 172 (2013) 30–35 Table 1 Structural properties of all samples. Sample SMS-0-as SMS-0.4-as SMS-0.8-as SMS-0-cal SMS-0.4-cal SMS-0.8-cal SMS-0.4-ex SMS-0.8-ex
TMB/Q4 0 0.4/1 0.8/1 0 0.4/1 0.8/1 0.4/1 0.8/1
a (nm)
d1 (nm)
d2 (nm)
Vp (cm3/g) Vmicro
Vmeso
SBET (m2/g)
w1 (nm)
w2 (nm)
8.9 9.3 9.7 8.5 8.6 8.9 9.0 9.3
– – – 3.7 3.9 4.0 3.8 3.9
– – – 5.8 6.6 6.7 5.8 6.6
– – – 0.03 0.02 0.02 – –
– – – 0.60 0.67 0.85 0.54 0.68
– – – 567 476 447 255 300
– – – 4.8 4.7 4.9 5.1 5.3
– – – 2.7 2.0 2.2 3.2 2.7
Note: a, unit cell parameter; SBET, BET specific surface area; Vp, pore volume calculated by BJH method on the basis of adsorption branch of N2 desorption isotherm curves; Vmicro, micropore volume and Vmeso, mesopore volume calculated by t-plot method on the basis of adsorption branch of N2 desorption isotherm curves; d1 and d2, BJH pore diameter calculated on the basis of desorption branch and adsorption branch of N2 sorption isotherm curve, respectively; w1 and w2, wall thickness evaluated with the relation w1 = ad1 and w2 = ad2, respectively.
were same and equal to 0.03 cm3/g. The micropore volumes of these materials were much smaller than those of the conventional mesoporous materials synthesized by Pluronic surfactants with PEO as hydrophilic segments. It was because that the PEO segments of the Q4-3667 retrieved from the silica wall due to the dehydration effect during the aging process at 100 °C [57]. And also, the micropores might be blocked by the silica originated from the PDMS segment of Q4-3667 during calcination. On the other hand, the Brunauer-Emmett-Teller specific surface areas (SBET) of SMS-0-cal, SMS-0.4-cal and SMS-0.8-cal decreased from 567 m2/g to 476 m2/g and 447 m2/g, respectively. The increase of pore sizes and volumes could be totally attributed to the addition of TMB. TMB swelled the hydrophobic part of the micelles of Q4-3667, resulting in the formation of larger mesochannels with the increase of TMB. For the mesoporous silica materials with p6mm structures, the average pore wall thickness, w, is generally evaluated using the equation w = ad, where a represents the unit cell parameter calculated from the positions of the peaks in XRD patterns, and d is the pore diameter defined as the position of the maximum on pore size distribution. The wall thicknesses according to the bulk pores were calculated to be in the range of 2.0–2.7 nm based on the adsorption branches (See Table 1). However, such data might be inaccurate because that the channels of the calcined samples were not perfectly cylindrical according to the H2 type hysteresis loops. Otherwise, the real pore entrances may be much smaller than the data calculated based on desorption branches (See Table 1) due to the tensile strength effect of nitrogen [54]. Therefore, it was considered that the average pore wall thicknesses of the mesoporous silica materials would be thicker than those calculated based on the adsorption branches of their isotherms. As listed in Table 1, the wall thicknesses of the calcined samples were calculated to be between 4.7 nm and 4.9 nm according to desorption branches, similar to the wall thickness of the mesoporous silica synthesized from the Pluronic P84 (PEO19-PPO43-PEO19) template [22]. Zhao et al. also reported the fabrication of mesoporous silica of 2D hexagonal symmetry with a pore diameter of 4.6 nm and the wall thickness of 4.7 nm by using PEO17-PPO55-PEO17 template [58]. It had been thoroughly confirmed that the wall thicknesses of mesoporous silica materials were mainly determined by the length of the hydrophilic block when nonionic surfactants were applied as template [24,59]. The hydrophilic block of the Q4-3667 (14 PEO segments) is shorter than that of the Pluronic P84 (19 PEO segments) and PEO17-PPO55-PEO17.Moreover, these mesoporous materials were prepared under almost identical hydrothermal treatment and aging conditions. It indicates that the PDMS blocks of the Q4-3667 actually thicken the silica walls of the mesoporous materials as shown in Scheme 1b.
The selective removal of TMB by extraction of the as-synthesized materials in cyclohexane retained the PDMS chains as hydrophobic functional groups on the pore surface. The solid-state 13 C NMR (Fig. 4) spectroscopy showed that the resonance peaks of CI (methyl of PDMS block) and CII (methylene of PEO block) still presented at 0.4 and 70.3 ppm after 48 h extraction, while the resonance peaks of 1,3,5-TMB were not observed, indicating the removal of the 1,3,5-TMB and the remaining of Q4-3667 within the mesoporous silica materials. The isotherm of the SMS-0-as was shown in Fig. 5A, which displayed a H3-type hysteresis loop in the relative pressure range from 0.7 to 1.0, indicating a wide distribution of the mesopores arising from the stacking of the silica particles. There was no capillary condensation loop in the relative pressure range of 0.2–0.7, because the mesopores were filled with surfactants. The isotherms of the extracted samples SMS-0.4-ex and SMS-0.8-ex showed type IV curves and H2-type hysteresis loops, suggesting the existence of the mesopores within the silica materials. The PDMS chains within the mesochannels might accumulate together and form ink-bottle pore structures which lead to the H2-type hysteresis loop. Notably, the adsorption and desorption isotherms of the SMS-0.4-ex and SMS-0.8-ex did not match at low relative pressures, which is a typical nature of mesopolymers [60–62]. It is reasonable to consider that the unclosed hysteresis loop was caused by the PDMS chains within the pore channels. The structural properties of the extracted samples were also listed in Table 1. With the addition of TMB, the
Fig. 4. Solid-state
13
C NMR spectrum of SMS-0.4-ex.
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J. Feng et al. / Microporous and Mesoporous Materials 172 (2013) 30–35
Fig. 5. N2 adsorption/desorption isotherms (A) and pore size distribution curves calculated based on the desorption (B) and adsorption (C) branches of the SMS-0-as, SMS-0.4ex and SMS-0.8-ex. The isotherm of SMS-0.8-ex is shifted by 80 cm3/g.
mesopore volume increased from 0.54 cm3/g (SMS-0.4-as) to 0.68 cm3/g (SMS-0.8-as). The SBET of the SMS-0.4-ex and SMS-0.8ex were 255 and 300 m2/g, respectively. The values were much smaller than those of calcined samples because that the PDMS chains still retained in the pores after extraction. 4. Conclusion In conclusion, the application of an ABA-type silicone surfactant (Q4-3667) as the template provided a new strategy to tailor the wall thickness and functionalize the surface of mesoporous materials. The PDMS chain of Q4-3667 could be oxidized to form a part of the silica walls by calcination at high temperature (550 °C), which offers a novel pathway to thicken the silica walls of the mesoporous materials. Furthermore, mesoporous silica materials with ordered 2D hexagonally symmetric structures and PDMS modified channels were rationally designed and facily achieved by using Q43667 as the template and TMB as the pore-swelling agent. These would open up new applications for mesoporous silica such as recyclable adsorption and separation materials, and nanocasting devices taking the advantage of low surface energy and hydrophobic properties of PDMS.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Silicone surfactant templated mesoporous silica Ji Feng, Bo Sun, Yuan Yao ⇑, Shunai Che ⇑ School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Key Laboratory for Thin Film and Microfabrication Technology of the Ministry of Education, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China
a r t i c l e
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Article history: Received 30 April 2012 Received in revised form 7 January 2013 Accepted 8 January 2013 Available online 16 January 2013 Keywords: Mesoporous silica Silicone surfactant Wall thickness Functionalization
a b s t r a c t Tailoring the wall thickness and organo-functionalization are two important issues in the design and application of mesoporous materials. Herein we report a novel strategy to deal with such two issues simultaneously by using a commercially available silicone surfactant, an ABA-type triblock copolymer poly(ethylene oxide)-block-polydimethylsiloxane-block-poly(ethylene oxide) (PEO14-b-PDMS13-bPEO14). The silicone surfactant serves as the template for directing meso-structures, the source of functional groups for surface modification, and the silica source for strengthening the wall of mesochannels. Meanwhile, 1,3,5-trimethyl benzene (TMB) was employed as the pore-swelling-agent. The mesoporous silica materials with thick walls and/or hydrophobic dimethylsiloxane functionalized pores were achieved by calcination and extraction, respectively. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Mesoporous silica materials are of special interest due to their unique features [1–4] and potential applications in the fields of catalysis, sensing, optically active materials, and biomaterials [5– 8]. Commonly, mesoporous silica materials could be fabricated by using cationic [9–11], non-ionic [12–14] and anionic surfactants [15–18] as templates to obtain ordered mesostructures and wellcontrolled morphologies, pore sizes, and porosities. Tailoring the wall thickness is an important issue in the design and application of mesoporous materials. Except for changing the hydrothermal treatment and aging conditions [19–21], the wall thickness could also be adjusted by tuning the length of the hydrophilic segment of template. It was found that the wall thickness of hexagonally meso-structured silica increased with the elongation of poly(ethylene oxide) (EO)x block in the Pluronic surfactant [22]. Ryoo and co-workers reported a synthetic approach for systematically controlling the wall thickness of mesoporous silica by using a mixture of PEO-containing non-ionic surfactant and cationic surfactant [23]. Recently, alkyl alcohol ether carboxylate (AEC: CnH2n+1O(CH2CH2O)mCH2COONa) surfactant was applied to fabricate hexagonal mesoporous silica with thick walls. The two types of hydrophilic head groups led to the formation of unique double-layer silica walls and the wall thickness were obviously increased as a result [24]. Organo-functionalization is also an important issue to combine enormous functions with the high surface area, uniform pore size, and thermally stable inorganic substrates [25–28]. The ⇑ Corresponding authors. Tel.: +86 21 5474 2852; fax: +86 21 5474 5365. E-mail addresses: [email protected] (Y. Yao), [email protected] (S. Che). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.01.010
organo-functionalized mesoporous materials have been prepared through several distinct routes yet: (i) the post-grafting method [29–35]; (ii) the co-condensation method [36–40]; (iii) introduction of co-structure directing agent [15–18] and (iv) the application of special precursors and templates such as bissilylated single-source organosilica [41–46] and surfactants consisting of functionalized condensable heads and cleavable alkyl tails [47]. Here we report a facile strategy to thicken the walls of mesoporous silica materials and hydrophobically functionalize the surface of channels by using a commercially available silicone surfactant Q4-3667 as template. Q4-3667 is an ABA type triblock copolymer, PEO14-b-PDMS13-b-PEO14. The PDMS block not only acts as the templates for pore formation, but also offers dimethyl siloxane segments for obtaining hydrophobic surface. In addition, the PDMS block also provides the silica source for thickening the walls by calcination. As shown in Scheme 1b, the PDMS block could form Si– O–Si inorganic network while be calcined at a high temperature (550 °C), which offers a new pathway to thicken the silica walls of the mesoporous materials. On the other hand, the PDMS blocks could attach to the silica matrix to achieve hydrophobic functionalization on the walls. However, the disadvantage of this strategy is that PDMS might fill up the pores (Scheme 1a) and prevent other molecules from entering into the mesochannels. In order to solve this problem, 1,3,5-trimethyl benzene (TMB) was employed as the pore-swelling agent [48,49]. The hydrophobic TMB was expected to swell up the hydrophobic part of the micelles of Q43667 by stabilizing the oil/water interface due to the hydrophobic interaction with the PDMS chains (Scheme 1c), and further tune the pore size of mesoporous silica. After the silica matrix was formed, TMB could be removed by extraction to obtain accessible dimethyl siloxane functionalized mesopores (Scheme 1d).
J. Feng et al. / Microporous and Mesoporous Materials 172 (2013) 30–35
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Scheme 1. Schematic illustration of the synthetic routes to the formation of Q4-3667 directed mesostructure (a), mesoporous silica with thick wall (b), organo-functionalized mesoporous silica with addition of TMB (c) and subsequent extraction (d). The black part of the cross section view in (b) denoted the deoxidized product of the PDMS chains.
2. Experimental 2.1. Materials Q4-3667 was provided by Dow Corning Corporation. 1,3,5-trimethyl benzene (TMB) and tetraethoxysilane (TEOS) were purchased from Sinopharm Chemical Reagent Co., Ltd. All these reagents were used as received.
Barett-Joyner-Halenda (BJH) method using both the desorption and adsorption branch of the isotherm. The pore volumes of the mesopores and micropores were estimated by BJH and t-plot method base on the desorption branch of the isotherm. Solid-state 13 C CP/MAS NMR spectra were collected on an Oxford AS400 NMR spectrometer at 100 MHz and a sample spinning frequency of 3 kHz.
2.2. A representative synthetic method
3. Results and discussion
1.0 g of Q4-3667, 0, 0.4 or 0.8 g of TMB and 108.6 g of deionized water were mixed to form a homogeneous solution. The solution was heated to 35 °C prior to the addition of TEOS. After the addition of TEOS (2.97 g), the mixture was stirred for 30 min, followed by reaction under the static condition for one day at 35 °C. The mixture was sealed in a Teflon autoclave and aged at 100 °C for two days. The reaction was carried out in a sealed vessel throughout the whole synthetic process. The solid product was collected by filtration or centrifugation, washed with water, and dried in air at 60 °C. The resultant powder was either calcined at a 2 °C/min heating rate from room temperature to 550 °C and held for 6 h in air to simultaneously remove the template and TMB, or extracted in cyclohexane at 80 °C for 48 h to remove TMB. The feed mass composition was Q4-3667:H2O:TMB:TEOS = 1:108.6:x:2.97. The asmade samples were denoted as SMS-0-as (with no TMB), SMS0.4-as (with TMB/Q4-3667 (w/w) = 0.4/1) and SMS-0.8-as (with TMB/Q4-3667 (w/w) = 0.8/1), respectively. The corresponding calcined samples and extracted samples were denoted as SMS-0-cal, SMS-0.4-cal and SMS-0.8-cal, and SMS-0.4-ex and SMS-0.8-ex.
The XRD patterns of the as-made, extracted and calcined samples synthesized with different TMB/Q4-3667 ratios are shown in Fig. 1. All samples show three well-resolved reflection peaks with p a d-spacing ratio of 1: 3:2, which can be indexed as (10), (11) and (20) reflections of the two dimensional (2D)-hexagonal p6mm structure. The unit cell parameter a of the SMS-0-as was 8.9 nm. The diffraction peaks of the samples SMS-0.4-as and SMS-0.8-as shifted toward low angle and their unit cell parameters increased to 9.3 and 9.7 nm, respectively because TMB swelled the hydrophobic cores of the micelles [50–53]. As shown in Fig. 1B, the diffraction peaks of all calcined samples shifted toward high angles. The unit cell parameters of the SMS-0-cal, SMS-0.4-cal and SMS-0.8-cal decreased to 8.5, 8.6, and 8.9 nm respectively because of the shrinkage of the silica frameworks. Compared to the asmade samples, the unit cell parameters of the SMS-0.4-ex and SMS-0.8-ex that were calculated from the XRD patterns shown in Fig. 1C also decreased to 9.0 and 9.3 nm respectively due to the shrinkage of the silica matrix. As shown in Fig. 2, the corresponding high-resolution transmission electron microscopy (HRTEM) images with the typical hexagonal p6mm array of (10) plane of the calcined samples, which were consistent with the XRD results. The center-to-center distances of pores in these three samples were 8.5, 8.7, and 8.9 nm for SMS0-cal, SMS-0.4-cal and SMS-0.8-cal, respectively, which were close to the unit cell parameters calculated based on the XRD patterns. The isotherms of the calcined samples displayed type IV curves with H2-type hysteresis loops, indicating the existence of ink-bottle style mesopore channel structure within the materials (Fig. 3A). It can be considered that the pores were not perfectly cylindrical because the PDMS chains were oxidized into silica after calcination, which may form ink-bottle style pore channel structure, and result in the H2-type hysteresis loops, although the XRD patterns and TEM images revealed the silica materials possessed ordered
2.3. Characterization Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku X-ray diffractometer D/MAX-2200/PC equipped with Cu Ka radiation (40 kV, 20 mA) at the rate of 0.1 deg./min over the range of 1–6°(2h). HRTEM was performed with a JEOL JEM-3010 microscope operating at 300 kV (Cs = 0.6 mm, resolution 1.7 nm). Images were recorded with the Kodak electron film SO-163 using low-electron-dose conditions. The nitrogen adsorption/desorption isotherms were measured at 77 K with the Quantachrome Nova 4200E. The surface area was calculated by the Brunauer-EmmettTeller (BET) method and the pore size was obtained from the maxima of the pore size distribution curve calculated by the
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Fig. 1. XRD patterns of three silica samples: as-made (A), calcined at 550 °C (B), and extracted in cyclohexane (C). The XRD patterns of the SMS-0.4-as, SMS-0.8-as, SMS-0.4cal, SMS-0.8-cal, and SMS-0.8-ex were shifted along the y axis.
Fig. 2. TEM images of three calcined silicas: SMS-0-cal (A), SMS-0.4-cal (B) and SMS-0.8-cal (C).
Fig. 3. N2 adsorption/desorption isotherms (A) and pore size distribution curves calculated based on the desorption (B) and adsorption (C) branches of the SMS-0-cal, SMS0.4-cal and SMS-0.8-cal. The isotherms of SMS-0.4-cal and SMS-0.8-cal are shifted by 100 and 240 cm3/g, respectively. The pore distribution curves of SMS-0.4-cal and SMS0.8-cal are shifted by 0.2 and 0.4 cm3/nm/g (B), and 0.03 and 0.06 cm3/nm/g (C), respectively.
p6mm symmetry. The bulk pore diameter and the narrowed pore entrance were calculated based on adsorption and desorption branches, respectively (Fig. 3C and B) [54]. As listed in Table 1, the size of the bulk pore and pore entrance of SMS-0-cal were 5.8 and 3.7 nm respectively. With the amount of TMB increasing, the bulk pore diameters of the SMS-0.4-cal and SMS-0.8-cal increased to 6.6 and 6.7 nm. Meanwhile, their pore entrances increased to 3.9 and 4.0 nm. The isotherms of the calcined samples
showed a sharp increase in adsorption at p/p0 < 0.1 and a plateau at p/p0 > 0.1, suggesting the existence of micropores in the silica matrix. The formation of micropores was attributed to the PEO segments embedded in the silica walls [55,56]. The t-plot calculation revealed that the volumes of mesopores increased from 0.60 cm3/ g to 0.67 cm3/g and 0.85 cm3/g with the increase of TMB in calcined SMS-0-cal, SMS-0.4-cal and SMS-0.8-cal samples respectively while the volumes of micropores in these three samples
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J. Feng et al. / Microporous and Mesoporous Materials 172 (2013) 30–35 Table 1 Structural properties of all samples. Sample SMS-0-as SMS-0.4-as SMS-0.8-as SMS-0-cal SMS-0.4-cal SMS-0.8-cal SMS-0.4-ex SMS-0.8-ex
TMB/Q4 0 0.4/1 0.8/1 0 0.4/1 0.8/1 0.4/1 0.8/1
a (nm)
d1 (nm)
d2 (nm)
Vp (cm3/g) Vmicro
Vmeso
SBET (m2/g)
w1 (nm)
w2 (nm)
8.9 9.3 9.7 8.5 8.6 8.9 9.0 9.3
– – – 3.7 3.9 4.0 3.8 3.9
– – – 5.8 6.6 6.7 5.8 6.6
– – – 0.03 0.02 0.02 – –
– – – 0.60 0.67 0.85 0.54 0.68
– – – 567 476 447 255 300
– – – 4.8 4.7 4.9 5.1 5.3
– – – 2.7 2.0 2.2 3.2 2.7
Note: a, unit cell parameter; SBET, BET specific surface area; Vp, pore volume calculated by BJH method on the basis of adsorption branch of N2 desorption isotherm curves; Vmicro, micropore volume and Vmeso, mesopore volume calculated by t-plot method on the basis of adsorption branch of N2 desorption isotherm curves; d1 and d2, BJH pore diameter calculated on the basis of desorption branch and adsorption branch of N2 sorption isotherm curve, respectively; w1 and w2, wall thickness evaluated with the relation w1 = ad1 and w2 = ad2, respectively.
were same and equal to 0.03 cm3/g. The micropore volumes of these materials were much smaller than those of the conventional mesoporous materials synthesized by Pluronic surfactants with PEO as hydrophilic segments. It was because that the PEO segments of the Q4-3667 retrieved from the silica wall due to the dehydration effect during the aging process at 100 °C [57]. And also, the micropores might be blocked by the silica originated from the PDMS segment of Q4-3667 during calcination. On the other hand, the Brunauer-Emmett-Teller specific surface areas (SBET) of SMS-0-cal, SMS-0.4-cal and SMS-0.8-cal decreased from 567 m2/g to 476 m2/g and 447 m2/g, respectively. The increase of pore sizes and volumes could be totally attributed to the addition of TMB. TMB swelled the hydrophobic part of the micelles of Q4-3667, resulting in the formation of larger mesochannels with the increase of TMB. For the mesoporous silica materials with p6mm structures, the average pore wall thickness, w, is generally evaluated using the equation w = ad, where a represents the unit cell parameter calculated from the positions of the peaks in XRD patterns, and d is the pore diameter defined as the position of the maximum on pore size distribution. The wall thicknesses according to the bulk pores were calculated to be in the range of 2.0–2.7 nm based on the adsorption branches (See Table 1). However, such data might be inaccurate because that the channels of the calcined samples were not perfectly cylindrical according to the H2 type hysteresis loops. Otherwise, the real pore entrances may be much smaller than the data calculated based on desorption branches (See Table 1) due to the tensile strength effect of nitrogen [54]. Therefore, it was considered that the average pore wall thicknesses of the mesoporous silica materials would be thicker than those calculated based on the adsorption branches of their isotherms. As listed in Table 1, the wall thicknesses of the calcined samples were calculated to be between 4.7 nm and 4.9 nm according to desorption branches, similar to the wall thickness of the mesoporous silica synthesized from the Pluronic P84 (PEO19-PPO43-PEO19) template [22]. Zhao et al. also reported the fabrication of mesoporous silica of 2D hexagonal symmetry with a pore diameter of 4.6 nm and the wall thickness of 4.7 nm by using PEO17-PPO55-PEO17 template [58]. It had been thoroughly confirmed that the wall thicknesses of mesoporous silica materials were mainly determined by the length of the hydrophilic block when nonionic surfactants were applied as template [24,59]. The hydrophilic block of the Q4-3667 (14 PEO segments) is shorter than that of the Pluronic P84 (19 PEO segments) and PEO17-PPO55-PEO17.Moreover, these mesoporous materials were prepared under almost identical hydrothermal treatment and aging conditions. It indicates that the PDMS blocks of the Q4-3667 actually thicken the silica walls of the mesoporous materials as shown in Scheme 1b.
The selective removal of TMB by extraction of the as-synthesized materials in cyclohexane retained the PDMS chains as hydrophobic functional groups on the pore surface. The solid-state 13 C NMR (Fig. 4) spectroscopy showed that the resonance peaks of CI (methyl of PDMS block) and CII (methylene of PEO block) still presented at 0.4 and 70.3 ppm after 48 h extraction, while the resonance peaks of 1,3,5-TMB were not observed, indicating the removal of the 1,3,5-TMB and the remaining of Q4-3667 within the mesoporous silica materials. The isotherm of the SMS-0-as was shown in Fig. 5A, which displayed a H3-type hysteresis loop in the relative pressure range from 0.7 to 1.0, indicating a wide distribution of the mesopores arising from the stacking of the silica particles. There was no capillary condensation loop in the relative pressure range of 0.2–0.7, because the mesopores were filled with surfactants. The isotherms of the extracted samples SMS-0.4-ex and SMS-0.8-ex showed type IV curves and H2-type hysteresis loops, suggesting the existence of the mesopores within the silica materials. The PDMS chains within the mesochannels might accumulate together and form ink-bottle pore structures which lead to the H2-type hysteresis loop. Notably, the adsorption and desorption isotherms of the SMS-0.4-ex and SMS-0.8-ex did not match at low relative pressures, which is a typical nature of mesopolymers [60–62]. It is reasonable to consider that the unclosed hysteresis loop was caused by the PDMS chains within the pore channels. The structural properties of the extracted samples were also listed in Table 1. With the addition of TMB, the
Fig. 4. Solid-state
13
C NMR spectrum of SMS-0.4-ex.
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Fig. 5. N2 adsorption/desorption isotherms (A) and pore size distribution curves calculated based on the desorption (B) and adsorption (C) branches of the SMS-0-as, SMS-0.4ex and SMS-0.8-ex. The isotherm of SMS-0.8-ex is shifted by 80 cm3/g.
mesopore volume increased from 0.54 cm3/g (SMS-0.4-as) to 0.68 cm3/g (SMS-0.8-as). The SBET of the SMS-0.4-ex and SMS-0.8ex were 255 and 300 m2/g, respectively. The values were much smaller than those of calcined samples because that the PDMS chains still retained in the pores after extraction. 4. Conclusion In conclusion, the application of an ABA-type silicone surfactant (Q4-3667) as the template provided a new strategy to tailor the wall thickness and functionalize the surface of mesoporous materials. The PDMS chain of Q4-3667 could be oxidized to form a part of the silica walls by calcination at high temperature (550 °C), which offers a novel pathway to thicken the silica walls of the mesoporous materials. Furthermore, mesoporous silica materials with ordered 2D hexagonally symmetric structures and PDMS modified channels were rationally designed and facily achieved by using Q43667 as the template and TMB as the pore-swelling agent. These would open up new applications for mesoporous silica such as recyclable adsorption and separation materials, and nanocasting devices taking the advantage of low surface energy and hydrophobic properties of PDMS.
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