Benzene-Silica with Hexagonal and Cubic Ordered Mesostructures

Jul 3, 2012 - *(E.-B.C.) Phone: 82-2-970-6729. E-mail: ... (M.J.) Phone: 1-330-672 3790. ... Bulletin of the Korean Chemical Society 2014 35, 2342-234...
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Benzene-Silica with Hexagonal and Cubic Ordered Mesostructures Synthesized in the Presence of Block Copolymers and Weak Acid Catalysts Eun-Bum Cho,*,† Dukjoon Kim,‡ Manik Mandal,§,∇ Chamila A. Gunathilake,∥ and Mietek Jaroniec*,∥ †

Department Department § Department ∥ Department ‡

of of of of

Fine Chemistry, Seoul National University of Science and Technology, Seoul 139-743, Korea Chemical Engineering, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, Korea Chemistry, College of Staten Island, City University of New York, New York 10314, United States Chemistry, Kent State University, Kent, Ohio 44242, United States

ABSTRACT: Highly ordered mesoporous benzene−silicas with 2D hexagonal (p6mm) and 3D cubic (Im3m) symmetry were prepared using weaker acids than hydrochloric acid in the presence of Pluronic P123 and F127 triblock copolymer templates. The resulting ordered mesostructures of 1,4-bis(triethoxysilyl)benzene (BTEB) were well-organized when iron(III) chloride hexahydrate (weak Brönsted acid with Ka = 6.3 × 10−3) was employed; 2D hexagonal structure was obtained at the concentration exceeding 0.064 M (pH ≤1.76) and 3D cubic structure was obtained at the concentration about 0.112 M (pH ≤1.63). The specific surface areas and pore diameters of the hexagonal benzene−silicas studied were in the range from 707 to 966 m2 g−1 and from 7.5 to 8.2 nm, respectively; the corresponding values for cubic benzene−silicas were 514−604 m2 g−1 and 8.5−9.0 nm, respectively. Also, type IV nitrogen adsorption−desorption isotherms and very narrow pore size distributions were obtained for these materials, which are similar to those for mesoporous benzene−silica prepared in the presence of strong hydrochloric acid.

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INTRODUCTION Silica and organosilica materials with large mesopores have been prepared by using commercial Pluronic triblock copolymers (i.e., nonionic poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide), PEO−PPO−PEO) as soft templates.1−4 The representative 2D hexagonal (p6mm) SBA-15 and 3D cubic (Im3m) SBA-16 mesoporous silica materials were obtained in the presence of Pluronic P123 (EO20PO70EO20) and F127 (EO106PO70EO106) nonionic block copolymers, respectively, under strongly acidic conditions.1,2 Interactions between silica precursors and poly(ethylene oxide) (PEO) are attributed to hydrogen bonding between hydronium ions and oxygen atoms in PEO blocks and to electrostatic ion− ion interactions.5,6 Recently, a few reports indicated that a highly ordered 2D p6mm mesostructure can be obtained even under weaker acidic conditions, especially in the case of periodic mesoporous organosilica (PMO) materials because additional π−π and related hydrophobic interactions facilitate the self-assembly process.7−31 Longer hydrophobic chains in block copolymers are thought to form a more stable platform for the reaction domain composed of PEO chains and (organo-)silane precursors and facilitate the supramolecular interactions. Several metallic salts and weaker Lewis acids have been reported as acidic media suitable for the mesostructure formation via sol−gel reactions involving organosilane precursors.31−33 In the case of more hydrophobic triblock © 2012 American Chemical Society

copolymers such as PEO−PLGA−PEO, boric acid as well as aluminum chloride was proven to be a good acidic catalyst to obtain a 2D hexagonal benzene−silica PMO material.34 Naturally, Pluronic triblock copolymers exhibit small difference in hydrophobicity between hydrophobic PPO block and hydrophilic PEO block, thus they are well-soluble in aqueous solutions.35−37 Moreover, commercial Pluronics have several advantages as soft templates because of their very cheap price and nontoxicity.38−49 However, Pluronic block copolymers show a limited capability to template highly ordered mesoporous materials with nonsiliceous and bulky organosilane species, which is a big barrier in the development of novel materials. It is believed that a poor mesoporosity of the materials prepared through sol−gel assembly is caused by disharmony in the organization of self-assembled species, which is mainly attributed to the acidity of the aqueous phase. Thus, the aim of this study was to find good mild acidic catalysts to replace strong hydrochloric acid and to minimize the aforementioned disharmony in the self-assembly process. Herein, we report the study of several weak acids as catalysts for the formation of hexagonal (p6mm) and cubic (Im3m) mesostructures of benzene−silica in the presence of commercial Pluronic P123 and F127 triblock copolymer templates. Received: March 21, 2012 Revised: June 13, 2012 Published: July 3, 2012 16023

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Then 0.673 g of BTEB was added after stirring the polymer solution for 2 h, and the mixture was stirred for further 20 h at 40 °C followed by aging for 24 h at 100 °C. To remove the block copolymer template and nonreacted species, a mixture of 57 mL of ethanol and 3 mL of hydrochloric acid was used as a washing solvent under magnetic stirring for 24 h at 78 °C. An analogous route was used for the synthesis of other samples listed in Table 2, except for the molar concentrations of Iron(III) chloride hexahydrate, which were varied as shown in Table 2. In a typical synthesis of cubic benzene−silica (e.g., PBIC-1T in Table 2), 0.5 g of F127 triblock copolymer was dissolved in a mixture of 22.5 g of distilled water and 0.7 g of iron(III) chloride hexahydrate, and 1.04 g of BTEB were added after stirring the polymer solution for 2 h at 40 °C. The mixture was stirred for 20 h at 40 °C followed by aging for 24 h at 100 °C. The block copolymer template and nonreacted species were removed using ethanol/HCl mixture. Final cubic structured samples were obtained by thermal treatment at 400 °C for 10 min under nitrogen flow. Measurements and Calculations. The small-angle X-ray scattering (SAXS) experiments were performed on a Bruker Nanostar U instrument using Cu Kα radiation (with λ = 1.5418 Å) source, which was a rotating anode. Each sample was placed in an aluminum sample holder and secured on both sides using a Kapton tape. The wide-angle X-ray diffraction (WAXRD) measurements were performed using a PANalytical X’Pert Pro Multipurpose Diffractometer with Cu Kα radiation at 40 kV and 40 mA. The samples were ground manually and put on the microscope glass holder at room temperature. The spectra were collected versus 2θ from 10 to 65°. The TEM images were obtained with an FEI Tecnai G2 Twin microscope operated at an accelerating voltage of 120 kV. The samples were sonicated for 30 min in ethanol and the solution was dropped onto a porous carbon film on a copper grid and then dried. The SEM images were obtained with a field emission SEM (JEOL JSM-6700F) operated at an accelerating voltage of 15 kV. Nitrogen adsorption−desorption isotherms were measured at −196 °C on a Micromeritics 2010 analyzer. The samples were degassed at 120 °C under vacuum below 30 μmHg for at least 2 h before measurement. The BET (Brunauer−Emmet−

Metal hydrates are known as weak Brönsted acids to provide a proton from the coordinated water molecules (i.e., [M(H2O)6]n+ (aq) + H2O (l) ↔ [M(H2O)5(OH)](n‑1)+ (aq) + H3O+ (aq)). Thus, some weak Brönsted acids such as metal hydrates and acetic acid were selected to investigate the acidity effect in the self-assembly synthesis of ordered benzene−silica mesostructures. The acids studied were iron(III) chloride hexahydrate, acetic acid, copper(II) perchlorate hexahydrate, cobalt(II) chloride hexahydrate, and nickel(II) chloride hexahydrate; their acid ionization constants (Ka) are listed in Table 1. This study also shows the minimum acid amount required to prepare ordered mesostructures of benzene−silica materials. Table 1. Acids Used and Their Acid Ionization Constants reagents

molecular formula

iron(III) chloride hexahydrate acetic acid copper(II) perchlorate hexahydrate cobalt(II) chloride hexahydrate nickel(II) chloride hexahydrate

FeCl3·6H2O CH3COOH Cu(ClO4)2·6H2O CoCl2·6H2O NiCl2·6H2O

Ka 6.3 1.8 1.6 1.3 2.5

× × × × ×

10−3 10−5 10−7 10−9 10−11

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EXPERIMENTAL SECTION Materials. Commercial Pluronic P123 (EO20PO70EO20) and F127 (EO106PO70EO106) (Aldrich) triblock copolymers were used as soft templates. 1,4-Bis(triethoxysilyl)benzene (BTEB, Aldrich) was used as an organosilica precursor. Iron(III) chloride hexahydrate, acetic acid, copper(II) perchlorate hexahydrate, cobalt(II) chloride hexahydrate, and nickel(II) chloride hexahydrate (Aldrich) were used as substitute acid catalysts instead of a typical proton-containing strong acid. Acids used in this study are listed in Table 1. All chemicals were used as purchased. Preparation of Periodic Mesoporous Benzene−Silicas Using Weak Acids as Catalysts. In a typical synthesis of hexagonally structured benzene−silica (e.g., PBIH-1 in Table 2) in the presence of Pluronic P123 polymer template, 0.66 g of P123 and 0.452 g of iron(III) chloride hexahydrate were dissolved in 24 mL of distilled water, and the mixture was stirred with magnetic bar in a glass bottle of 125 mL at 40 °C.

Table 2. Physicochemical Properties of 2D Hexagonal (p6mm) and 3D Cubic (Im3m) Benzene−Silicas Prepared by Using P123 and F127 PEO−PPO−PEO Block Copolymer Templates and Iron(III) Chloridea sample

f ICH/BTEB

[Fe(III)Cl3] (M)

SBET (m2 g−1)

Vt (cm3 g−1)

Vmicro (cm3 g−1)

Vmeso (cm3 g−1)

Vc (cm3 g−1)

Vo (cm3 g−1)

DKJS (nm)

DXRD‑ads (nm)

a (nm)

W (nm)

PBIH-1 PBIH-2 PBIH-3 PBIH-4 PBIC1T PBIC2T

1 2 3 4 1

0.064 0.127 0.187 0.246 0.112

753 707 783 966 604

0.88 0.87 0.87 1.10 0.40

0.06 0.05 0.07 0.11 0.11

0.82 0.82 0.80 0.99 0.25

0.14 0.12 0.16 0.23 0.19

0.56 0.54 0.51 0.56 0.19

7.8 7.5 7.8 8.2 5.7

n/a n/a n/a n/a 8.5

11.1 10.9 11.3 11.7 15.3

3.3 3.4 3.5 3.5

2

0.224

514

0.58

0.07

0.33

0.15

0.22

5.7

9.0

15.3

a

Notation: f ICH/BTEB, molar ratio of iron(III) chloride hexahydrate to 1,4-bis(triethoxysilyl)benzene used in the synthesis gel; [Fe(III)Cl3], molar concentration of iron(III) chloride hexahydrate in the synthesis gel; SBET, BET specific surface area determined in the range of relative pressures from 0.04 to 0.2; Vt, single-point pore volume; Vmicro, volume of micropores obtained by integration of PSD up to 2 nm; Vmeso, volume of mesopores obtained by integration of PSD from 2 to 30 nm; Vc, volume of complementary pores obtained by integration of PSD up to 4 nm; Vo, volume of ordered mesopores (pore volume at 0.8P/P0 − Vc); DKJS, mesopore diameter at the maximum of the PSD curve obtained by the improved KJS method;50 DXRD‑ads, pore width calculated using the following equation: 0.985a[Vo/(1/ρ + Vo + Vc)]1/3, where ρ = 1.52 g cm−3 (approximate density of the organosilica framework); a, unit cell parameter (=2d/√3 for hexagonal and √2d for cubic mesostructure, respectively); W, pore wall thickness = a − DKJS for hexagonal structure. 16024

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Teller) specific surface area was calculated from the adsorption data in the relative pressure range from 0.04 to 0.2. The total pore volume was evaluated from the amount adsorbed at the relative pressure of 0.99. The volumes of micropores and complementary (fine) pores were estimated by integration of pore size distribution (PSD) up to 2 and 4 nm, respectively and the mesopore volume was obtained by taking PSD from 2 to 30 nm. Also, the volume of ordered mesopores was obtained from the volume at 0.8P/P0 − Vc to calculate the pore diameter of cubic structured materials. The PSD curves were calculated from the adsorption branches of the isotherms by using the improved KJS (Kruk−Jaroniec−Sayari) method.50 The mesopore diameter was obtained at the maximum of PSD for hexagonal mesoporous PMO samples and calculated using the following equation: 0.985a[Vo/(1/ρ + Vo + Vc)]1/3, where ρ = 1.52 g cm−3 (approximate density of the organosilica framework) for cubic samples.51,52 The pore wall thickness was estimated from the pore size obtained at the maximum of PSD and the unit cell parameter (a) obtained by SAXS. The solid-state 29Si CP MAS 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 kHz, and the chemical shifts were obtained with respect to the tetramethylsilane (TMS) reference peak. Thermogravimetric (TG) measurements were performed using a high-resolution mode of the TA Instrument TGA 2950 analyzer. The TG profiles were recorded up to 800 °C in flowing nitrogen with a heating rate of 10 °C min−1.

Figure 1. SAXS patterns for benzene−silicas prepared using P123 triblock copolymer, BTEB, and iron(III) chloride hexahydrate. The sequence of the SAXS patterns from (b) to (e) corresponds to the list of samples in Table 2: (b) PBIH-1, (c) PBIH-2, (d) PBIH-3, and (e) PBIH-4, respectively. The pattern (a) is for the reference sample prepared using P123, BTEB, and HCl.

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brown synthesis gel possessed 2D hexagonally ordered mesostructure showing that iron(III) chloride hexahydrate can be used as an acidic catalyst instead of strong acids. Similarly to the hexagonally ordered benzene−silica materials prepared in the presence of weak acids, cubic (Im3m) mesostructured benzene−silica samples were obtained using iron(III) chloride hexahydrate as an acidic catalyst. Figure 2 shows the SAXS patterns for the 3D cubic (Im3m) benzene− silica prepared by using F127 triblock copolymer template and iron(III) chloride hexahydrate to ensure the 1−3 molar ration of F127 to BTEB. Unlike in the case of hexagonal mesostructure, the SAXS patterns show distinct reflections for cubic mesostructures obtained only up to the molar ratio of iron(III) chloride to BTEB = 2; see Figure 2a−c. Since the body-centered cubic (Im3m) is not a close-packed structure, it seems that it is more difficult to form this type of structure under weakly acidic conditions. However, as can be seen in Figure 2a,b, one intense Bragg peak and five high order wellresolved peaks corresponding to (110), (200), (211), (310), (222), and (200) reflections, respectively, are present. The Bragg’s spacing values obtained for the Im3m cubic samples show the same value of 10.8 nm for PBIC-1T and PBIC-2T, which is slightly higher than those (10.3−10.5 nm) for the cubic samples prepared with HCl and H2SO4 as reported previously.52 Nitrogen adsorption−desorption isotherms for hexagonal p6mm benzene−silicas prepared in the presence of P123 block copolymer and iron(III) chloride hexahydrate are shown in Figure 3. All of the isotherms are type IV showing distinct hysteresis in a range of P/P0 from 0.60 to 0.70 except for PBIH4 with rather broad hysteresis from 0.5 to 1.0. The BET surface areas for the series of PBIH benzene−silica samples are in the range from 707 to 966 m2 g−1, and the total pore volumes (Vt) vary from 0.87 to 1.10 cm3 g−1 (see Table 2). The pore sizes calculated by the improved KJS method shows very narrow

RESULTS AND DISCUSSION Using weak acids listed in Table 1, benzene−silica powder samples were prepared without the other strong acids (e.g HCl and H2SO4) or additive salts (e.g., KCl and NaF) in the presence of commercial P123 and F127 PEO−PPO−PEO triblock copolymer templates. The acids were selected according to their acidity (i.e., sequential values of acid ionization constants) and good solubility in water. First, the molar ratio = 1 of five kinds of acids to BTEB organosilica precursor was used to obtain ordered mesostructures. However, only in the presence of iron(III) chloride hexahydrate hexagonal and cubic ordered mesostructures were formed. Thus, additional powder materials were prepared with the molar ratios = 2, 3 and 4 of iron(III) chloride hexahydrate to BTEB precursor and these samples are discussed in the current paper. The SAXS patterns for benzene−silicas prepared in the presence of P123 and iron(III) chloride hexahydrate are shown in Figure 1. As can be seen from this figure, all template-free benzene−silica samples show highly ordered 2D hexagonal (p6mm) mesostructure with similar Bragg’s spacings. The reference pattern is curve (a) obtained for the benzene−silica sample prepared by using hydrochloric acid. All the samples show three well-resolved peaks indexed according to p6mm symmetry group as (100), (110), and (200). The d-spacing values obtained from the most intense Bragg peaks vary slightly from 9.44 to 10.13 nm, which seems to correlate well with the concentration of iron(III) chloride hexahydrate except for the PBIH-2 sample (curve c); in all cases the d-spacings show a little higher values of unit cell as compared with that for the reference sample (i.e., d-spacing of 9.43 nm, curve a) prepared with hydrochloric acid at the same conditions. The resultant white solid product obtained after final washing from the dark16025

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Figure 2. SAXS patterns for benzene−silicas prepared using F127 triblock copolymer, BTEB, and iron(III) chloride hexahydrate. The SAXS patterns (a) and (b) correspond to the samples listed in Table 2: (a) PBIC-1T and (b) PBIC-2T. The pattern (c) is for the sample prepared using F127, BTEB, and the molar ratio = 3 of iron(III) chloride to 1,4-bis(triethoxysilyl)benzene.

Figure 4. Pore size distributions for benzene−silicas prepared using P123 triblock copolymer, BTEB, and iron(III) chloride hexahydrate; panels a−d refer to the following samples: (a) PBIH-1, (b) PBIH-2, (c) PBIH-3, and (d) PBIH-4.

Table 2). The wall thickness values are about 3.3−3.5 nm. The micropore (≤2 nm; Vmicro) and complementary pore (≤4 nm; Vc) volumes for PBIH-1, PBIH-2, and PBIH-3 are in the range of 0.05−0.07 and 0.12−0.16 cm3 g−1, respectively, which are smaller than those of PBIH-4 and typical mesoporous organosilicas prepared with polymer templates. The mesopore volumes corresponding to the range of (2 nm ≤ DKJS ≤ 30 nm) and (4 nm ≤ DKJS ≤ 20 nm) vary in the range of 0.80−0.99 and 0.51−0.56 cm3 g−1, respectively. The overall physicochemical properties of hexagonally ordered PBIH samples are summarized in the upper part of Table 2. Nitrogen adsorption−desorption isotherms for cubic Im3m mesostructured benzene−silica samples are type IV with small hysteresis in the range of P/P0 from 0.40 to 0.60, which is characteristic for the cage-like mesoporous materials (Figure 5). The BET surface areas for the PBIC-1T and PBIC-2T benzene−silica samples are in the range from 514 to 604 m2 g−1 and the total pore volumes vary from 0.40 to 0.58 cm3 g−1, respectively (Table 2). The pore size distributions (PSDs) obtained from the adsorption branches of the isotherms by the KJS method50 are narrow indicating uniformity of mesoporous structure with significant amount of micropores (Figure 6). Since the KJS method is applicable for cylindrical mesopores, the pore diameter was also estimated using the equation mentioned in the Experimental Section, which relates the SAXS unit cell and adsorption pore volume for the cubic Im3m symmetry.53 The KJS method was used only to get some information about the mesopore and micropore volumes and to estimate the volumes of complementary pores. The resulting pore diameters for PBIC-1T and PBIC-2T samples are 8.5 and 9.0 nm, respectively (Table 2); these values are larger by about 2.8−3.3 nm than the corresponding values obtained at the

Figure 3. Nitrogen adsorption−desorption isotherms for benzene− silicas prepared using P123 triblock copolymer, BTEB, and iron(III) chloride hexahydrate; panels a−d refer to the following samples: (a) PBIH-1, (b) PBIH-2, (c) PBIH-3, and (d) PBIH-4.

distributions for all hexagonally ordered benzene−silica samples (see Figure 4). The pore diameters obtained at the maximum of the PSD curves are in the range between 7.5 and 8.2 nm (see 16026

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Figure 5. Nitrogen adsorption−desorption isotherms for benzene− silicas prepared using F127 triblock copolymer, BTEB, and iron(III) chloride hexahydrate; panels a and b refer to the following samples: (a) PBIC-1 and (b) PBIC-2.

Figure 7. Representative TEM and SEM images of benzene−silicas prepared using P123 triblock copolymer, BTEB, and iron(III) chloride hexahydrate: (a) PBIH-1 and (b) PBIH-4.

Figure 6. Pore size distributions for benzene−silicas prepared using F127 triblock copolymer, BTEB, and iron(III) chloride hexahydrate; panels a and b refer to the following samples: (a) PBIC-1 and (b) PBIC-2.

maximum of the KJS PSD curves shown in Figure 6 and also larger than those (7.5 and 7.7 nm) obtained for the cubic benzene−silica samples prepared with HCl and H2SO4 as reported previously.52 The SAXS and nitrogen adsorption analysis clearly demonstrates that the ordered p6mm and Im3m mesoporous benzene−silica materials can be prepared using a small amount of 0.064 and 0.112 M of iron(III) chloride hexahydrate in the presence of P123 and F127 PEO-PPO-PEO block copolymer templates, respectively. The p6mm hexagonally ordered mesostructures of PBIH benzene−silica prepared in the presence of iron(III) chloride hexahydrate were confirmed by the TEM images. Figure 7a shows the mesostructured pattern of parallel channels for PBIH-1. The SEM image (Figure 7b) shows the overall morphology of this benzene−silica sample in the form of cylindrical rods and bundles. Solid-state 29Si CP MAS NMR measurements for 2D hexagonal (p6mm) and 3D cubic (Im3m) benzene−silicas were performed to confirm benzene bridging groups in the silica framework. The three peaks present on the 29Si CP MAS NMR spectra, as shown in Figure 8, are characteristic signals

Figure 8. Representative solid state 29Si CP MAS NMR spectra for PBIH-3 (a), PBIH-4 (b), PBIC-1T (c), and PBIC-2T (d) benzene− silica samples prepared using P123 and F127 triblock copolymers, BTEB, and iron(III) chloride hexahydrate.

which can be assigned to C−Si(OSi)(OH)2 (T1, δ = −61), C− Si(OSi)2(OH) (T2, δ = −70), and C−Si(OSi)3 (T3, δ = −79), respectively. Figure 8 clearly shows that the benzene bridging groups are covalently bonded to −O−Si−O− species. The relative amounts of T2 + T3 and the T3/T2 ratio are nearly the same for four representative samples and they are also comparable to the respective values for benzene−silicas prepared with strong acids such as HCl. The Q peaks were not observed between −90 and −120 ppm, which confirms that 16027

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the benzene bridging groups in the silica framework are stable under acidic conditions. Figure 9 shows the WAXRD patterns for the PBIH-2, PBIH3, and PBIH-4 benzene−silicas prepared in the presence of

Figure 10. Themogravimetric (TG) and the corresponding differential TG profiles for the representative PBIH benzene−silica samples studied in flowing nitrogen. The reference sample was prepared using 0.1 M HCl acidic solution in the presence of the P123 template.

structures, whereas the other weak acids studied did not work well. The molar concentration and the resulting pH of iron(III) chloride hexahydrate in the synthesis gel were estimated to be above 0.064 M and below 1.76, respectively, for the hexagonal mesostructure and around 0.112 M and 1.63 for cubic structure, respectively. The resulting samples showed high BET surface areas in the range of 707 − 966 m2 g−1 and large pore diameters of 7.5−8.2 nm for hexagonal (p6mm) benzene− silica and 514−604 m2 g−1 and 8.5−9.0 nm for cubic (Im3m) analogues, respectively. The existence of benzene bridging groups covalently linked with −O−Si−O− was confirmed by 29 Si CP MAS NMR spectra, wide-angle X-ray spectra, and TG profiles. Wide angle X-ray and TG patterns suggest that the hexagonally mesostructured benzene−silica prepared in the presence of iron(III) chloride hexahydrate does not contain iron and related nanoparticles, which indicates that the solvent extraction performed with HCl/ethanol solution was very effective to remove the remaining iron species. Note that the use of another organic solvent such as acetone was insufficient to fully remove the template and obtain high surface area for the samples studied. This study shows clearly that the highly ordered benzene−silicas can be prepared using small amount of weak iron(III) chloride hexahydrate and commercial Pluronic P123 and F127 PEO−PPO−PEO block copolymer templates under weak acidic conditions. Moreover, these results clearly show that the supramolecualr interactions between reactants can be finely controlled by subtle variation of synthetic conditions especially pH, which concurs with previous studies in this area.32−34

Figure 9. Wide angle X-ray diffraction patterns of the PBIH-2 (a), PBIH-3 (b), and PBIH-4 (c) benzene−silica samples prepared using P123 triblock copolymer, BTEB, and iron(III) chloride hexahydrate.

iron(III) chloride hexahydrate, providing information about structural ordering of molecules inside the pore walls framework. Two typical diffraction peaks correspond to the d-spacings of 0.74 nm (2θ = 12°) and 0.37 nm (2θ = 24.0°), which are assigned to a periodic arrangement of benzene bridging groups linked with silica within the pore walls.10,32,34 Also, very weak reflections around 2θ = 44° and 60° might be attributed to trace amount of iron(III)-containing catalyst. However, the weak and broad peaks represent the lack of the crystalline phase, which is typical for the benzene−silica prepared in the presence of nonionic block copolymer templates under acidic conditions. Also, the amount of crystalline iron particles is negligible in the benzene−silica framework. Figure 10 shows the thermal degradation of benzene−silica materials in flowing nitrogen. The TG and DTG profiles for the representative PBIH-2 and PBIH-4 samples were measured and compared with the reference benzene−silica sample prepared using hydrochloric acid (Figure 10). The residue at 800 °C is nearly constant for all three samples, 73% in relation to the sample weight at 100 °C, which suggests there is no profound trace of iron species inside the samples. Two-steps in the degradation process are also similar for three benzene−silica samples, which clearly shows that the thermal stability of the benzene−silica prepared with iron(III) chloride hexahydrate is not affected much by acidic conditions.

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AUTHOR INFORMATION

Corresponding Author

CONCLUSIONS Several weak acids have been investigated to explore the possibility of obtaining hexagonal and cubic mesoporous benzene−silica in the presence of Pluronic P123 and F127 PEO-PPO-PEO block copolymers and without using typical strong acids. It was found that iron(III) chloride hexahydrate shows efficient acidity for the preparation of highly ordered hexagonal (p6mm) and cubic (Im3m) benzene−silica meso-

*(E.-B.C.) Phone: 82-2-970-6729. E-mail: [email protected]. kr. (M.J.) Phone: 1-330-672 3790. E-mail: [email protected]. Present Address ∇

Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania, 18015 USA.

Notes

The authors declare no competing financial interest. 16028

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ACKNOWLEDGMENTS This work was supported partially by Seoul National University of Science and Technology, the New & Renewable Energy R&D program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20113020030040), and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2012000855). M.J. acknowledges support by the National Science Foundation under CHE-0848352 grant. E.-B.C. thanks Mr. H. Lee and Mr. P. Sung for assistance in materials synthesis. The authors also thank Prof. Michal Kruk for discussion and the use of Bruker Nanostar Instrument funded by NSF CHE-0723028.

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