J. Phys. Chem. C 2009, 113, 5111–5119
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Periodic Mesoporous Benzene- and Thiophene-Silicas Prepared Using Aluminum Chloride as an Acid Catalyst: Effect of Aluminum Salt/Organosilane Ratio and Stirring Time Eun-Bum Cho,† Dukjoon Kim,*,† Joanna Go´rka,‡ and Mietek Jaroniec*,‡ Department of Chemical Engineering, Polymer Technology Institute, Sungkyunkwan UniVersity, Suwon, Gyeonggi-do 440-746, Korea and Department of Chemistry, Kent State UniVersity, Kent, Ohio 44242 ReceiVed: NoVember 19, 2008; ReVised Manuscript ReceiVed: January 20, 2009
Periodic mesoporous benzene-silica and thiophene-silica with two dimensional (2D) p6mm symmetry were synthesized in the presence of a nonionic poly(ethylene oxide)-poly(DL-lactic acid-co-glycolic acid)poly(ethylene oxide) (PEO-PLGA-PEO) block copolymer template using aluminum chloride hexahydrate (AlCl3 · 6H2O) as an acid catalyst instead of the proton-containing hydrochloric acid (HCl). 1,4-Bis(triethoxysilyl)benzene (BTEB) and 2,5-bis(triethoxysilyl)thiophene (BTET) were used as organosilica precursors. The effect of the AlCl3 · 6H2O/organosilane ratio and the stirring time was investigated to find the optimum experimental conditions. It was shown that the highly ordered 2D hexagonal mesostructures of aromaticsilicas were formed using relatively small amounts of AlCl3 ([AlCl3 · 6H2O]/[BTEB] and [AlCl3 · 6H2O]/[BTET] g 1) and a stirring time of about 20 h. The BET surface areas, pore volumes, and pore diameters of the benzene- and thiophene-silicas studied varied from 505 to 1215 m2 g-1, from 0.55 to 1.62 cm3 g-1, and from 7.55 to 9.23 nm, respectively, depending on the aluminum salt concentration and the stirring time. In the case of thiophene-silica, the optimum experimental conditions for the formation of ordered mesostructure were [AlCl3 · 6H2O]/[BTET] ) 1 and a stirring time of 20 h. Also, the chemistry and crystallinity of aromaticsilicas were investigated using solid-state 13C-, 29Si-, and 27Al-NMR and wide-angle X-ray scattering methods. Introduction Since the discovery of periodic mesoporous organosilicas (PMOs) in 1999,1-3 a broad assortment of these materials has been synthesized by using various organosilica precursors and structure directing agents. Namely, PMOs with enhanced surface, optical, and mechanical properties4-13 have been designed for versatile applications such as catalysis, adsorption, separations, and sensing. Moreover, the use of block copolymer templates affording PMOs with specific functionality and accessible large pores can be regarded as a crucial factor in achieving the desired performance of these materials.14-18 PMOs templated by nonionic block copolymers are usually obtained at weaker acidic conditions than those used for the synthesis of purely siliceous mesostructures. This acidity depends on the kind of organic bridging groups; for example, higher acidity is required for the introduction of ethane groups than for that of aromatic bridges.10,17,19-21 According to Monnier et al.22 and Huo et al.,23 the change in the Gibbs free energy associated with the formation of templated mesostructures can be represented as follows:
∆G ) ∆Ginter + ∆Gwall + ∆Gintra + ∆Gsol where ∆Ginter accounts for interactions between inorganic precursors (walls) and polymer surfactant, ∆Gwall refers to the inorganic framework, ∆Gintra accounts for van der Waals force and conformational energy of the surfactant template, and ∆Gsol reflects the chemical potential associated with the species present in solution. So far, several attempts have been made to control the ∆Gwall term; namely, ∆Gwall was lowered by adjusting * Corresponding author. Phone: 82-31-290-7250. E-mail:
[email protected] (D.K.). Phone: 1-330-672 3790. E-mail:
[email protected] (M.J.). † Sungkyunkwan University. ‡ Kent State University.
hydrophobic and π-π interactions between organic bridging groups. This adjustment also caused lowering of ∆Ginter; consequently, PMOs could be prepared under milder acidic conditions than under those used for the synthesis of mesoporous silicas. The ∆Gsol term is regarded as a constant for a given solution. Therefore, the remaining variable in the aforementioned equation, which can be lowered, is the ∆Gintra term associated with the conformational energy of the polymeric template. Poly(ethylene oxide)-poly(DL-lactic acid-co-glycolic acid)poly(ethylene oxide) (PEO-PLGA-PEO) triblock copolymer template has been reported as one of the most effective templates for the synthesis of ordered mesostructures especially PMOs.10,20,21 The presence of the highly hydrophobic PLGA block seems to be an effective factor for lowering ∆Gintra because of an enhancement of the hydrophobic interactions in the polymeric micelles. Therefore, it would be interesting to study the variation of ∆Ginter for the PEO-PLGA-PEO system in the presence of a catalyst, like AlCl3, that is able to ensure weak acidity during the self-assembly process. In a similar manner to the preparation of SBA-15 mesoporous silica, which has been carried out in the presence of nonionic Pluronic P123 triblock copolymer template under strongly acidic conditions, the synthesis of PMOs has usually been performed according to the S0H+X-I+ route24,25 using strong acid catalysts containing hydronium ions and anions in the sol-gel mixture. Moreover, inorganic salts have often been employed to improve the porosity and mesostructural ordering in the mesoporous (organo-)silicamaterials.26-38 Thereareafewrecentpublications39,40 showing the formation of the PMO structures in the presence of mixed inorganic salts (e.g., Ni, Zn, and Mg salts and other anion (F-)-containing salts) under neutral conditions using oligomeric nonionic surfactant Brij-76. Also, a trivalent aluminum chloride salt was employed as an efficient acid catalyst to prepare ordered benzene-silica using a PEO-PLGA-PEO poly-
10.1021/jp810179g CCC: $40.75 2009 American Chemical Society Published on Web 03/11/2009
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TABLE 1: Physicochemical Properties of 2D Hexagonally Ordered Benzene-Silicas Prepared by Using PEO-PLGA-PEO Block Copolymer Template and Aluminum Chloride as an Acid Catalysta sample
[AlCl3 · 6H2O]/ [BTEB]
tstirring (h)
SBET (m2 g-1)
Vt (cm3 g-1)
Vc (cm3 g-1)
d100 (nm)
a (nm)
Dp (nm)
W (nm)
CAl (ppm)
LBA-1-20 LBA-2-5 LBA-2-20 LBA-3-5 LBA-3-20
1.0 2.0 2.0 3.0 3.0
20 5 20 5 20
1215 505 1165 960 1100
1.62 0.55 1.32 0.83 1.29
0.26 0.08 0.20 0.22 0.16
10.2 8.9 10.5 9.9 10.2
11.7 10.2 12.1 11.4 11.7
8.88 7.55 8.85 8.85 8.83
2.89 2.72 3.27 2.58 2.94
1241.0 3588.5 1079.0 1404.9 1492.1
a Notation: [AlCl3 · 6H2O]/[BTEB], molar ratio of aluminum chloride hexahydrate to BTEB; tstirring, time of magnetic stirring of the synthesis gel at 313 K; SBET, BET specific surface area determined in the range of relative pressures from 0.05 to 0.2; Vt, single-point pore volume; Vc, volume of complementary pores estimated by subtracting the mesopore volume obtained by integration of PSD above 3 nm from the single-point pore volume; Dp, mesopore diameter at the maximum of the PSD curve obtained by the improved KJS method;41 W, pore wall thickness ) a - Dp; and CAl, aluminum content in the final product.
mer template.21 These results suggest that the aforementioned route (S0H+X-I+) can be replaced by S0M+X-I+ (e.g., S0Al3+Cl-I+) using a suitable inorganic acidic medium without a proton (H+)-containing strong acid. Since the specific inorganic salts seem to be effective for the formation of ordered ethaneand benzene-PMO mesostructures, it would be interesting to study in details the formation of aromatic thiophene-PMOs as well as benzene-PMOs in the presence of inorganic salts that can act as acid catalysts in order to take a full advantage of the salt effect and weak acidity. Here we explore the effect of aluminum chloride hexahydrate (AlCl3 · 6H2O) and the stirring time on the properties of aromaticPMOs with 2D hexagonal structures obtained by employing 1,4bis(triethoxysilyl)benzene (BTEB), 2,5-bis(triethoxysilyl)thiophene (BTET) organo-silanes, and a PEO-PLGA-PEO triblock copolymer, which were used to lower the structural and conformational free energy terms in the aforementioned ∆G expression. Aluminum chloride hexahydrate was used as an acidic catalyst since it was already proven as an efficient substitute for acid21 and it seems to be more effective at achieving the optimal acidity of the synthesis gel. Moreover, it is safer to handle instead of the rather toxic aluminum chloride (AlCl3). In the case of using aluminum chloride hexahydrate alone, that is, without adding a strong acid, the interactions between PEO chains and organosilica precursors would be dependent upon the ionic strength and polarity of the gel; thus, the amount of aluminum salt will be varied to find the optimum composition and stirring time. It is noteworthy that, in the case of using PEO-PLGA-PEO triblock copolymers and proton (H+)containing hydrochloric acid, the precipitation of highly ordered organosilica powders have been achieved after one-half to a couple of hours of stirring.10,20,21 Experimental Section Materials. A poly(ethylene oxide)-poly(DL-lactic acid-coglycolic acid)-poly(ethylene oxide) triblock copolymer (EO16(L29G7)EO16, LGE538) was synthesized in the laboratory as reported elsewhere.21 The average molecular weight of the resulting LGE538 batch was 5,310, and the polydispersity index was 1.28; these estimations were done by using a GPC-RI (Waters HPLC) system. The molecular weight estimated by 1H NMR was 4,220 Daltons, and the volume fraction of the PEO blocks (ΦPEO), calculated by the group contribution method on the basis of the molar composition obtained by the NMR analysis, was 0.38. 1,4-Bis(triethoxysilyl)benzene (BTEB, Aldrich) and 2,5bis(triethoxysilyl)thiophene (BTET, JSI silicone) were used as organosilica precursors, respectively, and aluminum chloride
hexahydrate (AlCl3 · 6H2O, Aldrich) was used as an substitute of acid catalysts instead of typical hydrochloric acid and sulfuric acids. Preparation of Periodic Mesoporous Benzene- and Thiophene-Silicas Using Aluminum Chloride. In a typical synthesis of benzene-silica (e.g., LBA-1-20 in Table 1), 0.5 g of LGE538 triblock copolymer was dissolved in a mixture of 21.78 g of distilled water and 0.5 g of ethanol, and after stirring the polymer solution for 2 h, the mixture of BTEB (0.80 g) and 0.48 g of AlCl3 · 6H2O was added. Precipitates were obtained after stirring the mixture for 20 h at 313 K followed by aging for 24 h at 373 K. A 60 g sample of acetone was used as a washing solvent under magnetic stirring for 5 h at 329 K to remove the block copolymer template and remaining inorganic materials. An analogous route was used for the synthesis of other samples listed in Table 1, except for the molar ratio of aluminum chloride to organosilica precursor (e.g., [AlCl3 · 6H2O]/ [BTEB]) and the stirring time which were varied as shown in Table 1. The numbers in the sample codes indicate the molar ratio of [AlCl3 · 6H2O]/ [BTEB] and the stirring time, respectively. In a typical synthesis of thiophene-silica (e.g., LTA-1-20 in Table 2), 0.5 g of LGE538 triblock copolymer, 21.80 g of distilled water, 0.5 g of ethanol, 0.44 g of AlCl3 · 6H2O, and 0.75 g of BTET were used. Precipitates were obtained after stirring the mixture for about 20 h at 313 K followed by aging for 24 h at 368 K. Washing and drying procedures were the same as in the case of benzene-silica. The sample names are listed in Table 2; the numbers in the sample codes indicate the molar ratio of [AlCl3 · 6H2O]/ [BTET] and the stirring time, respectively. Measurements and Calculations. The small-angle X-ray scattering experiments were performed using a Synchrotron radiation with λ ) 1.608 Å at the 4C1 lines of Pohang Accelerator Laboratory in POSTECH. The distance between each sample and detector was fixed as 50 cm, and the sample exposure time was in the range of 5-25 s. Data were obtained by converting signals collected by a 2D detector using 2D data processing software. The wide-angle X-ray measurements were performed using Bruker WAXS analyzer with a goniometer of 600 mm and a 2D area detector. The WAXS data were collected and taken by the CCD camera and converted to spectra versus 2θ from 5 to 40°. The TEM images were obtained with an FE-TEM (JEOL JEM2100F) operated at an accelerating voltage of 200 kV. The samples were sonicated for 30 min in an adequate quantity of ethanol, and the solution was dropped onto a porous carbon film on a copper grid and then dried. Nitrogen adsorption-desorption isotherms were measured on a Micromeritics 2020 analyzer. The samples were degassed at
Periodic Mesoporous Benzene- and Thiophene-Silicas
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TABLE 2: Physicochemical Properties of 2D Hexagonally Ordered Thiophene-Silicas Prepared by Using PEO-PLGA-PEO Block Copolymer Template and Aluminum Chloride as an Acid Catalysta sample
[AlCl3 · 6H2O]/ [BTET]
tstirring (h)
SBET (m2 g-1)
Vt (cm3 g-1)
Vc (cm3 g-1)
d100 (nm)
a (nm)
Dp (nm)
W (nm)
CAl (ppm)
LTA-1-20 LTA-2-20 LTA-2.33-7 LTA-3-7 LTA-3-20
1.0 2.0 2.33 3.0 3.0
20 20 7 7 20
1070 995 890 760 895
1.24 1.10 0.96 0.82 0.92
0.17 0.19 0.16 0.13 0.17
9.6 9.6 9.4 9.3 9.3
11.0 11.0 10.8 10.7 10.7
8.41 9.23 8.75 8.53 8.20
2.67 1.85 2.10 2.20 2.53
986.1 718.9 1145.4 3798.4 1118.4
a Notation: [AlCl3 · 6H2O]/[BTET], molar ratio of aluminum chloride hexahydrate to BTET; tstirring, time of magnetic stirring of the synthesis gel at 313 K; SBET, BET specific surface area determined in the range of relative pressures from 0.05 to 0.2; Vt, single-point pore volume; Vc, volume of complementary pores estimated by subtracting the mesopore volume obtained by integration of PSD above 3 nm from the single-point pore volume; Dp, mesopore diameter at the maximum of the PSD curve obtained by the improved KJS method;41 W, pore wall thickness ) a - Dp; and CAl, aluminum content in the final product.
383 K to achieve vacuum below 30 µmHg. The BET (BrunauerEmmet-Teller) specific surface area was calculated from the adsorption data in the relative pressure range from 0.05 to 0.2. The total pore volume was evaluated from the amount adsorbed at a relative pressure of 0.99. The volume of complementary (fine) pores was estimated by subtracting the mesopore volume obtained by integration of pore size distribution (PSD) above 3 nm from the single-point pore volume. The PSD curves were calculated from the adsorption branches of the isotherms by using the improved KJS (Kruk-Jaroniec-Sayari) method.41 The pore width was estimated at the maximum of PSD. The solid-state NMR spectra were obtained with a Bruker DSX400 spectrometer using a 4 mm magic angle (MAS) spinning probe at the Korea Basic Science Institute. The samples were spun at a rate of 6-8 kHz, and the chemical shifts were obtained with respect to the tetramethylsilane reference peak. The 13C CP-MAS NMR spectra were measured under the experimental conditions of a contact time of 2 ms, a recycle delay of 3 s, and 3000 scans. The 29Si MAS NMR spectra were obtained with a recycle delay of 50 s and 2000 scans. Relative quantification of 29Si MAS NMR spectra was performed by simulation, deconvolution, and integration of the spectra. The samples analyzed by 27Al MAS NMR were spun at a rate of 13 kHz, and the signals were referenced to a 1.0 M aqueous solution of aluminum chloride (AlCl3). The 27Al MAS NMR spectra were recorded with a recycle delay of 1 s and 1000 scans. Aluminum contents in the final products were quantified using ICPS-100IV (Shimadzu) inductively coupled plasma-atomic emission spectroscopy (ICP-AES) with argon plasma of 6000 K. Results and Discussion Five benzene-PMO samples with p6mm hexagonal symmetry were prepared in the presence of PEO-PLGA-PEO (EO16(L29G7)EO16, LGE538) template by varying the quantity of aluminum chloride hexahydrate catalyst and the stirring time as shown in Table 1. The ratio of [AlCl3 · 6H2O]/[BTEB] was varied from 1 to 3 and the stirring reaction time at 313 K was 5 and 20 h as listed in Table 1. The 2D Synchrotron SAXS results for benzene-PMO powder materials are shown in Figure 1. As can be seen from this figure, all template-free benzenePMOs except for the LBA-1-5 sample (curve a in Figure 1) are highly ordered 2D hexagonal (p6mm) mesostructures with at least three well-resolved peaks indexed as the (100), (110), and (200) reflections. The LBA-1-5 sample was prepared using [AlCl3 · 6H2O]/[BTEB] ) 1 and stirring time of 5 h; its SAXS pattern shows an intense peak without high-order peaks, indicating disordered mesoporosity. The most intense Bragg peak shifts causing an increase in the d spacing from 8.9 to 10.5 nm mainly with increasing stirring time; the d-spacing
values for the samples stirred for 20 h are 10.2-10.5 nm, which are larger than those (8.9-9.9 nm) for the samples stirred for 5 h. The LBA-2-20 sample (Figure 1d) exhibited the most intense high-order peaks and the largest d-spacing value among all of the benzene-PMO samples prepared in this study. A comparison of the SAXS patterns shown in Figure 1 indicates that the experimental conditions used for the synthesis of LBA2-20 are close to the optimal ones that afford highly ordered benzene-silica materials. A further increase of the [AlCl3 · 6H2O]/ [BTEB] ratio to 4 did not lead to highly ordered samples (data not shown). This study shows that the highly ordered benzenesilicas can be prepared using aluminum chloride hexahydrate catalyst (without addition of strong acids) in the appropriate range; moreover, a short stirring time of 5 h is sufficient to obtain ordered mesostructues when the ratio of [AlCl3 · 6H2O]/[BTEB] exceeds 2. Note that the ordered benzene-silica mesostructure was not obtained using a typical P123 PEO-PPO-PEO template and aluminum chloride catalyst, which indicates the importance of the PEO-PLGA-PEO template in lowering the free energy of the system to the level that assures the formation of ordered p6mm benzene-silica mesostructure. Nitrogen adsorption-desorption isotherms for 2D hexagonal (p6mm) benzene-PMOs are shown in Figure 2. As can be seen from this figure, all of the isotherms of benzene-PMO samples are type IV showing distinct steps at P/P0 of about 0.65-0.80 due to the capillary condensation of nitrogen in the mesopores. The BET surface areas for this series of PMOs are in the range from 505 to 1215 m2 g-1, and the total pore volumes vary from 0.55 to 1.62 cm3 g-1 (Table 1). These quantities are larger for the samples prepared by using longer stirring time (20 h). The ratio of the volume of complementary pores to the total pore volume varies from 12.4 to 26.5%, where the low and high values are observed for the LBA-3-20 and LBA-3-5 samples, respectively, which suggests the importance of both stirring time and the aluminum chloride amount in the formation of ordered porosity. The mesopore peaks on the pore size distributions, calculated from the adsorption isotherms by the improved KJS method,41 are narrow indicating high uniformity of mesopores (Figure 2). The pore diameters estimated at the maximum of the PSD curves are nearly the same (8.83-8.88 nm) except that for the LBA-2-5 sample, which is 7.55 nm; however, the pore wall thicknesses vary from 2.58 to 3.27 nm irrespective of the pore diameters. The LBA-2-20 sample shows the highest value ofwallthickness,3.27nm.Also,theshapeofadsorption-desorption hysteresis loops for the benzene-silica samples was improved with increasing amounts of aluminum chloride (left panels of Figure 2).
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Figure 3. Representative TEM images for benzene-PMO (LBA-3-20 in Table 1) prepared using LGE538 triblock copolymer, BTEB, and aluminum chloride hexahydrate without HCl addition.
Figure 1. Two dimensional SAXS patterns for benzene-PMOs prepared using LGE538 triblock copolymer, BTEB, and aluminum chloride hexahydrate. The sequence of the SAXS patterns from b to f correspond to the list of samples in Table 1: (b) LBA-1-20, (c) LBA2-5, (d) LBA-2-20, (e) LBA-3-5, and (f) LBA-3-20, respectively. The pattern (a) was obtained for the sample prepared using [AlCl3 · 6H2O]/ [BTEB] ) 1 and stirring time of 5 h.
Figure 4. Solid state 13C CP-MAS NMR spectra for the benzenePMOs studied; (a) LBA-1-20, (b) LBA-2-5, (c) LBA-2-20, (d) LBA3-5, and (e) LBA-3-20 correspond to the samples in Table 1.
Figure 2. Nitrogen adsorption-desorption isotherms and the corresponding pore size distributions for benzene-PMOs prepared using LGE538 triblock copolymer, BTEB, and aluminum chloride hexahydrate. The sequence of adsorption isotherms and PSDs agrees with the list of the samples in Table 1: (a) LBA-1-20, (b) LBA-2-5, (c) LBA2-20, (d) LBA-3-5, and (e) LBA-3-20, respectively.
Analysis of adsorption and structural parameters listed in Table 1 shows that aluminum chloride hexahydrate is an effective acid catalyst for the preparation of highly ordered hexagonal benzene-silica materials in the presence of a PEOPLGA-PEO block copolymer template. In addition, the use of
longer stirring time improved mesoporosity in the samples studied as evidenced by visible increase in the surface area (Table 1). As shown in Table 1 and Figures 1 and 2, the highly ordered 2D hexagonal mesostructures of benzene-PMO were obtained by adjusting the amount of aluminum chloride and the stirring time. The formation of well-defined 2D hexagonal mesostructures of benzene-PMO was also confirmed by the TEM imaging (Figure 3). Solid-state 13C CP-MAS NMR measurements for 2D hexagonal (p6mm) benzene-PMOs were performed to verify the presence of covalently bonded organic groups in the PMO framework. An intense peak at 133 ppm, observed on the 13C CP-MAS NMR spectra for five benzene-PMO samples (Figure 4), is attributed to benzene-bridging group. The other peaks are typical spinning sidebands for aromatic benzene groups except for a weak peak at around 70 ppm caused by a small quantity of nonextracted polymer template. Solid-state 29Si MAS NMR measurements for 2D hexagonal (p6mm) benzene-PMOs were performed to investigate the linkage between silicon atoms and benzene-bridging groups in the PMO framework. The characteristic signals on the 29Si MAS NMR spectra of benzene-PMOs (Figure 5) are assigned to C-Si(OSi)3 (T3, δ ) -78), C-Si(OSi)2(OH) (T2, δ ) -69),
Periodic Mesoporous Benzene- and Thiophene-Silicas
Figure 5. Solid state 29Si MAS NMR spectra for the benzene-PMOs studied; (a) LBA-1-20, (b) LBA-2-5, (c) LBA-2-20, (d) LBA-3-5, and (e) LBA-3-20 correspond to the samples listed in Table 1.
and C-Si(OSi)(OH)2 (T1, δ ) -61), respectively, which reflect the structure of the Si species covalently bonded to the carbon atoms. As can be seen from Figure 5a-e, the T2 peaks show highest intensity, which indicates that the condensation between ethoxy groups in siloxane precursors was not fully developed; however, this type of pattern is typical for several other benzenePMO samples prepared under acidic conditions.17 The Q peaks such as Si(OSi)4 and Si(OSi)3(OH) were not observed between -90 and -120 ppm, which confirms that the cleavage of the C-Si bonds in the BTEB precursors did not occur. The relative amounts of T2 + T3 and the T3/T2 ratio are nearly the same for the five benzene-PMOs, and these values are not so high in comparison to those observed for mesoporous benzene-silicas reported elsewhere,17 which suggests that the condensation reaction in the system studied does not depend strongly on the aluminum chloride quantity and the stirring time. Aluminum contents in five 2D hexagonal (p6mm) benzenePMO samples were investigated by 27Al NMR and ICP-AES. Figure 6 shows the 27Al NMR spectra recorded for the benzenePMOs studied. As can be seen from Figure 6, there are one or two weak resonances, which correspond to tetrahedral (∼60 ppm) and octahedral (∼1 ppm) coordinated aluminum, respectively. Aluminum content in the benzene-PMO samples estimated by ICP-AES was about 1.0-3.5 mg per gram of the solid as listed in Table 1, and there is no visible dependence on the aluminum quantity and the stirring reaction time. The 27Al NMR and ICP results suggest that there was no significant incorporation of aluminum into the PMO framework. Thus, aluminum chloride acted mainly as a catalyst mediating the interactions between poly(ethylene oxide) blocks of the polymer template and ethoxy groups of the BTEB precursor as well as facilitating the condensation reaction between BTEB precursors. The 2D WAXS patterns for benzene-PMOs shown in Figure 7 indicate the structural ordering of the pore walls. The patterns display two discrete and broad peaks corresponding to the d spacings of 0.76 nm (2θ ) 11.5°) and 0.38 nm (2θ ) 23.0°), and they are assigned to a periodic arrangement of benzene bridges within the pore walls;6 however, weak intensities of these broad peaks suggest the lack of highly crystalline phase, which
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Figure 6. Solid state 27Al MAS NMR spectra for the benzene-PMOs studied; (a) LBA-1-20, (b) LBA-2-5, (c) LBA-2-20, (d) LBA-3-5, and (e) LBA-3-20 correspond to the samples listed in Table 1.
Figure 7. Two dimensional WAXS patterns for the benzene-PMOs studied; (a) LBA-1-20, (b) LBA-2-5, (c) LBA-2-20, (d) LBA-3-5, and (e) LBA-3-20 correspond to the samples listed in Table 1.
is typical for the wall crystallinity of the benzene-PMOs prepared in the presence of nonionic block copolymer templates under acidic conditions. Thiophene-PMO samples with p6mm symmetry (hexagonally ordered mesostructures) were prepared similarly to the aforementioned benzene-PMO samples. The [AlCl3 · 6H2O]/[BTET] ratio was varied from 1 to 3, and the time of stirring at 313 K was 7 and 20 h (see Table 2). It should be mentioned that in the case of thiophene-silica the synthesis gel was stirred for 7 h because 5 h was insufficient for precipitation of the sample. The 2D Synchrotron SAXS patterns for thiophene-PMO powders are shown in Figure 8. As can be seen from this figure, the SAXS patterns of the thiophene-PMO samples (except those for LTA-1-7 and LTA-2-7 shown Figure 8a and Figure 8c, respectively) are characteristic for highly ordered mesostructures because they possess at least three well-resolved peaks indexed
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Figure 8. Two dimensional Synchrotron SAXS patterns for thiophenePMOs prepared using LGE538 triblock copolymer, BTET, and aluminum chloride hexahydrate. The sample names and synthetic conditions correspond to the sequence of samples listed in Table 2 except for a and c: (b) LTA-1-20, (d) LTA-2-20, (e) LTA-2.33-7, (f) LTA-3-7, and (g) LTA-3-20, respectively. The SAXS patterns a and c were recorded for thiophene-PMOs prepared using 7 h of stirring at 313 K and [AlCl3 · 6H2O]/[BTET] ) 1 and 2, respectively.
as (100), (110), and (200) according to the p6mm symmetry group. The aforementioned two samples were prepared by using the [AlCl3 · 6H2O]/[BTET] ratio ) 1 and 2, respectively, and the stirring time of 7 h. The lack of high order reflections on the SAXS patterns of these two samples indicates that their porous structures are rather disordered. For highly ordered thiophene-PMOs the d-spacing values show only a small variation, from 9.3 to 9.6 nm; a slight decrease in this value is observed with an increasing amount of aluminum chloride in the initial synthesis gel. The SAXS pattern of LTA-1-20 (Figure 8b) exhibits the most intense high-order peaks among all thiophene-PMO samples studied; thus, the experimental conditions used for the synthesis of this sample are close to optimal ones. Analysis of the SAXS patterns shown in Figure 8 suggests that the highly ordered p6mm thiophene-PMO powders can be prepared using aluminum chloride hexahydrate catalyst in the presence of a PEO-PLGA-PEO triblock copolymer template as specified in Table 2. Moreover, it should be noted that the highly ordered thiophene-silicas were obtained for [AlCl3 · 6H2O]/[BTET] > 1; this value is twice smaller than that required for the synthesis of highly ordered benzene-silicas; in the latter case [AlCl3 · 6H2O]/[BTEB] should be greater than 2. Thus, in comparison with benzene-silica, the lower amount of acidic catalyst is required for the synthesis of ordered thiophene-silica mesostructure. The nitrogen adsorption-desorption isotherms for 2D hexagonal (p6mm) thiophene-PMOs are type IV with steep condensation steps at P/P0 of about 0.70-0.80 (Figure 9). The BET surface areas for thiophene-PMOs are in the range from 760 to 1070 m2 g-1, and the corresponding total pore volumes vary from 0.82 to 1.24 cm3 g-1 (Table 2). It is shown that the highest
Cho et al.
Figure 9. Nitrogen adsorption-desorption isotherms and the corresponding pore size distributions for thiophene-PMOs prepared using LGE538 triblock copolymer, BTET, and aluminum chloride hexahydrate. The sample names and synthetic conditions correspond to the sequence of samples listed in Table 2: (a) LTA-1-20, (b) LTA-2-20, (c) LTA-2.33-7, (d) LTA-3-7, and (e) LTA-3-20, respectively.
values of the aforementioned parameters were obtained for [AlCl3 · 6H2O]/[BTET] ) 1 and the stirring time of 20 h (LTA1-20 sample). The relative ratios of the volume of complementary pores to the total pore volume vary from 13.7 to 18.5%. The pore size distributions obtained by the improved KJS method41 are very narrow indicating a high uniformity of porous structures (Figure 9). The pore diameters estimated at the maximum of the PSD curves vary from 8.20 to 9.23 nm, and the pore wall thicknesses vary from 1.85 to 2.67 nm. There is a trend of improvement of the shape of the adsorption-desorption hysteresis loop with increasing the stirring time (see loops in Figure 9 for the samples obtained by stirring for 20 h). The largest value of the wall thickness, 2.67 nm, was obtained for LTA-1-20, while the smallest one, 1.85 nm, was evaluated for LTA-2-20. Analysis of the adsorption and structural parameters listed in Table 1 and Figure 9 show that aluminum chloride hexahydrate is an effective acid catalyst for the synthesis of highly ordered hexagonal thiophene-silica materials in the presence of a PEO-PLGA-PEO block copolymer template. Also, the largest pore volume was obtained for the LTA-1-20 sample, which contrasts this system from that of benzene-silica. A comparison of the experimental conditions for the synthesis of PMOs with aromatic benzene- and thiophene-bridging groups shows that in the latter case a smaller amount of aluminum hexahydrate catalyst is required to obtain highly ordered mesostructure; however, in both cases, the longer stirring time (at least ∼20 h) is required. The 2D hexagonal ordering of thiophene-PMOs is also confirmed by the representative TEM images for the LTA-1-20 and LTA-3-20 samples shown in Figure 10. Solid-state 13C CP-MAS NMR data for 2D hexagonal thiophene-PMOs are shown in Figure 11. An intense peak at 138 ppm, characteristic for thiophene groups, is observed for the five thiophene-PMO samples. The other peaks reflect
Periodic Mesoporous Benzene- and Thiophene-Silicas
Figure 10. Representative TEM images for thiophene-PMOs, LTA1-20 (a, b) and LTA-3-20 (c, d) prepared using LGE538 triblock copolymer, BTET, and aluminum chloride hexahydrate without HCl addition.
Figure 11. Solid state 13C CP-MAS NMR spectra for the thiophenePMOs studied; (a) LTA-1-20, (b) LTA-2-20, (c) LTA-2.33-7, (d) LTA-3-7, and (e) LTA-3-20 correspond to the samples listed in Table 2.
spinning sidebands of aromatic thiophene and a negligible amount of residual polymer template. Solid-state 29Si MAS NMR data for 2D hexagonal thiophenePMOs are shown in Figure 12. The characteristic signals on the 29Si MAS NMR spectra of thiophene-PMOs are assigned to C-Si(OSi)3 (T3, δ ) -83), C-Si(OSi)2(OH) (T2, δ ) -74), and C-Si(OSi)(OH)2 (T1, δ ) -66), respectively, which reflect the structure of the Si species covalently bonded to the carbon atoms of thiophene-bridging groups. As can be seen from Figure 12a-e, the T2 peaks show highest intensity, which resembles the NMR spectra of benzene-silicas shown in Figure 5. However, the presence of Q peaks such as Si(OSi)4 (Q4, δ ) -110), Si(OSi)3(OH) (Q3, δ ) -100), and Si(OSi)2 (OH)2 (Q2, δ ) -91) observed between -90 and -120 ppm indicates a
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Figure 12. Solid state 29Si MAS NMR spectra for the thiophene-PMOs studied; (a) LTA-1-20, (b) LTA-2-20, (c) LTA-2.33-7, (d) LTA3-7, and (e) LTA-3-20 correspond to the samples listed in Table 2.
Figure 13. Solid state 27Al MAS NMR spectra for the thiophenePMOs studied; (a) LTA-1-20, (b) LTA-2-20, (c) LTA-2.33-7, (d) LTA-3-7, and (e) LTA-3-20 correspond to the samples listed in Table 2.
partial cleavage of the C-Si bonds in the BTET precursor under conditions used. The relative value of T/(T + Q) varies from 57.7% (LTA-3-20 sample) to 68.5% (LTA-2.33-7 sample) as shown in Figure 12a-e, indicating that the stirring time, rather than the aluminum chloride amount, is the major factor responsible for the growth of Q peaks. The respective T and Q values were estimated by simulation, deconvolution, and integration of the NMR spectra. Similarly to the five 2D hexagonal benzene-PMO samples, the 27Al NMR spectra for thiophene-PMOs show two weak resonances, which correspond to tetrahedral (∼60 ppm) and octahedral (∼1 ppm) coordinated aluminum, respectively, as shown in Figure 13a-e. The aluminum content in the thiophene-
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Cho et al. materials is strongly influenced by the nature of polymeric template, the amount of catalyst, and the stirring time. It is shown that the use of PEO-PLGA-PEO triblock copolymer affords ordered mesostructures under milder conditions than those required for Pluronic block copolymer templates. The aluminum contents in the PMO materials were rather small, about 1.0-3.5 mg/g for benzene-PMOs and 0.7-3.7 mg/g for thiophene-PMOs, respectively. The present study shows a great potential for tailoring PMOs by using adequate combinations of inorganic acid catalysts and block copolymer templates. Acknowledgment. This work was partially supported by a Korea Research Foundation grant (KRF 2006-005-J04601) and the Korean Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST; No. R0A-2007-00010029-0). Experiments at PLS were supported in part by MEST and POSTECH. M.J. acknowledges support by the National Science Foundation under CTS-0553014 grant.
Figure 14. Two dimensional WAXS patterns for the thiophene-PMOs studied; (a) LTA-1-20, (b) LTA-2-20, (c) LTA-2.33-7, (d) LTA3-7, and (e) LTA-3-20 correspond to the samples listed in Table 2.
PMO samples estimated by ICP-AES was about 0.7-3.7 mg/g as listed in Table 2, and there is only a slight dependence of this value on the amount of added aluminum chloride and the time of stirring; the aluminum content increases with increasing the amounts of aluminum chloride and decreasing the stirring time (7 h vs 20 h). However, the 27Al NMR and ICP results suggest that the primary role of aluminum chloride was to catalyze the self-assembly of BTET and block copolymer. The 2D WAXS patterns of thiophene-PMOs shown in Figure 14 indicate the degree of structural ordering of the pore walls in these materials. The 2D WAXS patterns in Figure 14 display a broad peak corresponding to d spacing of 0.38 nm (2θ ) 23.0°), indicating the rather amorphous nature of the pore walls. Conclusions This study shows that periodic mesoporous organosilicas can be prepared using aluminum chloride as an acid catalyst instead of proton-containing strong acidic catalysts such as hydrochloric and sulfuric acids. Highly ordered 2D hexagonal PMOs with benzene- and thiophene-bridging groups were successfully prepared in the presence of PEO-PLGA-PEO triblock copolymer as a template and aluminum chloride hexahydrate as an acid catalyst. Aromatic-PMOs prepared in this study show high BET surface areas up to 1200 m2/g, large pore diameters (7.5-9.2 nm), narrow pore size distributions, and highly ordered 2D hexagonal (p6mm) mesostructures. The aluminum chloride amount needed for the synthesis of highly ordered hexagonal meostructures of both aromatic-PMOs was not less than [AlCl3 · 6H2O]/[oragnosilane] ) 1 under employing 20 h of stirring. While it was difficult to obtain the hexagonally ordered mesostructures of aromatic-PMOs for the stirring time shorter than 20 h, the use of aluminum chloride ([AlCl3 · 6H2O]/[BTEB] > 2) permitted to shorten this time to 5 h in the case of benzene-PMO and to 7 h in the case of thiophene-PMO. However, ordered mesostructures with the aforementioned bridging groups were not obtained in the presence of typical Pluronic P123 PEO-PPO-PEO template using only an aluminum chloride hexahydrate catalyst. These results show clearly that the synthesis of ordered aromatic-PMO
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