Structural Stability of SiâC Bonds in Periodic Mesoporous Thiophene

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Structural Stability of Si-C Bonds in Periodic Mesoporous Thiophene-Silicas Prepared under Acidic Conditions Eun-Bum Cho, Jeonghyun Park, and Mietek Jaroniec J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp408385t • Publication Date (Web): 23 Sep 2013 Downloaded from http://pubs.acs.org on September 28, 2013

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The Journal of Physical Chemistry

Structural Stability of Si-C Bonds in Periodic Mesoporous Thiophene-Silicas Prepared under Acidic Conditions Eun-Bum Cho,*,† Jeonghyun Park,† and Mietek Jaroniec*,‡ † ‡

Department of Fine Chemistry, Seoul National University of Science and Technology, Seoul 139-743, Korea Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio 44242, USA

ABSTRACT: Periodic mesoporous thiophene-silicas with hexagonal (p6mm) symmetry were synthesized using 2,5-bis(triethoxysilyl)thiophene (BTET) precursor in the presence of Pluronic P123 (EO20PO70EO20) and PLGE (EO17(L28G7)EO17) triblock copolymers at different acidic conditions. P123templated mesoporous thiophene-silicas with p6mm ordered structure were prepared in the presence of hydrochloric acid and iron(III) chloride hexahydrate used as acid catalysts. However, it was found that a relatively large fraction of the Si-C bonds in thiophene-bridging groups were decomposed during synthesis process. On the other hand, thiophene-silicas synthesized at lower acidic conditions were disordered and non-porous structures. In contrast, PLGE-templated thiophene-silicas with p6mm ordered mesostructure were prepared using copper(II) perchlorate hexahydrate and boric acid as well as hydrochloric acid. Importantly, up to 97.3% of the Si-C bonds in mesoporous thiophene-silica prepared in the presence of boric acid were retained. Solid state 29Si MAS NMR clearly showed that the structural stability of Si-C bond is dependent on the acidity and time of the initial self-assembly stage. Also, thermal stability of thiophene-bridging groups was shown to be dependent on the acidity of the synthesis gel. Keywords: periodic mesoporous organosilica, thiophene-silica, Si-C bond, boric acid, block copolymer INTRODUCTION Since the first publications1-3 in 1999 on the periodic mesoporous organosilicas (PMO), a large variety of PMO structures have been reported.4-6 The main advantage of the one-pot sol-gel synthesis of PMO materials is the possibility of functionalization of mesoporous frameworks that assures a homogeneous distribution of organic functional groups without significant alteration of porosity because this synthesis employs bridged silsesquioxanes [(R’O)3Si-R-Si-(OR’)3] as PMO precursors.1-7 At the early stage of the development of PMO materials, it was expected that a lot of PMOs with versatile organic bridging groups could be easily obtained. However, it was quickly realized that the synthesis of PMO materials with complex organic functionalities such as long, bulky, cyclic and reactive organic groups is not easy.8-38 So, it is necessary to develop new strategies for the synthesis of PMO materials with multi-functional groups as well as new functional mesostructures.32,35,38 In the case of non-ionic block copolymer templating, which affords PMOs with larger mesopores, it is more difficult to obtain highly ordered PMOs with complex bridging groups because of possible structural hindrance and weak interactions between bock copolymer and the precursors used. The only controllable parameters are the concentration of hydronium ion (i.e., acidic strength) and salts as additives to facilitate interactions between organosilica precursors and poly(ethylene oxide) (PEO) chains for a given Pluronic PEO-PPO-PEO template.11-17 Recently, highly ordered 2D hexagonal (p6mm) and 3D cubic (Im3m) mesostructured benzene-silicas were

developed under weak acidic conditions using iron(III) chloride38 as well as strong acidic conditions with hydrochloric acid33 in the presence of Pluronic P123 (EO20PO70EO20) and F127 (EO106PO70EO106) block copolymers, which shows that additional π-π interactions facilitate the self-assembly process.38 Similarly, there are several reports on the synthesis of thiophene-containing PMO materials in the presence of Pluronic P123 block copolymer under acidic conditions.4,22,28,29 However, cyclic bridging groups with reactive atoms such as thiophenebridging group are likely to be decomposed because sulfur atom can act as an electron donor to hydronium ion in acidic aqueous environment and thiophene rings can be opened. Since integrity of PMOs depends on the stability of Si-C bonds, there is a need to investigate these bonds in all kinds of PMO materials, including thiophene-silica. For instance, the integrity of the Si-C bonds was studied by using Raman and IR vibrational spectroscopy for four kinds of PMO materials prepared with bis(triethoxysilyl)-ethane (BTEE), -thiophene (BTET), -benzene (BTEB), and biphenyl (BTEBP) precursors.29 However, it was concluded that vibrational spectroscopy is not a reliable method for monitoring the integrity of the Si-C bond quantitatively and the solid-state NMR spectroscopy was suggested to estimate the extent of Si-C bond cleavage.29 Our previous publications report the synthesis of PMOs under mild acidic conditions and show the behavior of different block copolymer templates at these conditions.20,28,31-33,35,38 A more detailed discussion of the self-assembly synthesis of PMOs under mild acidic conditions can be found in our previous work.32 However, there is no report on quantification of the Si-C bond

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cleavage, specifically in relation to thiophene-bridged PMO materials. Herein, we report the synthesis of mesoporous thiophene-silicas using several weak acids as well as a typical strong acid in the presence of the commercial Pluronic P123 and poly(ethylene oxide)poly(DL-lactic acid-co-glycolic acid)-poly(ethylene oxide) triblock copolymer (EO17(L28G7)EO17, PLGE). A minimum acidic strength required to obtain hexagonally ordered mesostructure of thiophene-silicas was determined in relation to two different P123 and PLGE triblock copolymer templates. Hydrated metal salts like iron(III) chloride were proven to be used as Brönsted acids for the synthesis of ordered benzene-bridged PMO materials even in the presence of P123.38 Also, it was possible to obtain highly ordered mesoporous benzene-silicas under much lower acidity using boric acid as Lewis acid in the presence of a PEO-PLGA-PEO triblock copolymer.35 Thus, the aforementioned acids and copper(II) perchlorate hexahydrate were selected to investigate the acidity effect in the self-assembly synthesis of ordered thiophene-silica mesostructures. The acids studied and their acid ionization constants (Ka) are listed in Table 1. To investigate in details the structural stability of Si-C bonds, mesoporous thiophene-silicas were prepared by using different times of initial self-assembly under stirring at a constant speed of 500 rpm as well as by varying acidity of the synthesis gel. In this preparation acetone was employed as a solvent instead of the typical mixed solution of ethanol/HCl used in the extraction step. The decomposition of Si-C bonds in various mesoporous thiophene-silicas was quantified by using solid state 29Si MAS NMR measurements.

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P123 (P) or PLGE (L); Y denotes the catalyst: hydrochloric acid (H), iron(III) chloride hexahydrate (I), copper(II) perchlorate hexahydrate (C), and boric acid (B); T refers to thiophene bridging group; z denotes the molar ratio of the catalyst and BTET; and t denotes the initial self-assembly time under magnetic stirring (see Table 2). For instance, LBT-20-20 denotes thiophene-silica sample prepared using PLGE template (L) and boric acid (B) with the molar ratio of B to BTET = 20 and the initial self-assembly time = 20 h. Synthesis of Periodic Mesoporous Thiophene-Silica. In a typical synthesis of p6mm hexagonally structured thiophene-silica (e.g., LBT-20-20 in Table 2), 0.5 g of PLGE was dissolved completely in 0.5 g of ethanol and 22.0 g of deionized water under vigorous stirring. After stirring the solution with magnetic bar in a glass bottle of 125 mL for 2 h at 40 oC, 2.27 g of boric acid and 0.75 g of BTET were added to the solution. After stirring the solution at a constant speed of 500 rpm for further 20 h (initial selfassembly stage), the mixture was aged for 24 h at 100 oC in a convection oven under quiescent conditions (hydrothermal treatment stage). To remove the block polymer, solvents, and unreacted species, the product was stirred in 60 mL of acetone solvent for 24 h at 56 oC, followed by filtering using a suction flask and an adequate amount of ethanol and acetone. An analogous procedure was used for the synthesis of other samples listed in Table 2, except for varying the template, catalyst, the amount of reactants, and stirring time. The sample names and the other experimental conditions are listed in Table 2. Table 1. Acid Ionization Constants of the Acids Used.

EXPERIMENTAL SECTION

Acids

Molecular formula

Ka 8

Hydrochloric acid HCl 1.0 × 10 Materials. Poly(ethylene oxide)-poly(DL-lactic acid-coglycolic acid)-poly(ethylene oxide) triblock copolymer Iron(III) chloride hexahydrate FeCl3·6H2O 6.3 × 10-3 (EO17(L28G7)EO17, PLGE) was synthesized in laboratory Copper(II) perchlorate hexahydrate Cu(ClO4)2·6H2O 1.6 × 10-7 through ring-opening polymerization in anhydrous toluene Boric acid B(OH)3 5.9 × 10-10 at 120 oC using 3,6-dimethyl-1,4-dioxane-2,5-dione (DLlactide) (Aldrich), glycolide (Polyscience), and poly(ethylene glycol) methyl ether (Mn = 750 g/mol, Table 2. Synthesis Conditions for the Periodic Mesoporous Aldrich) supported by stannous octoate (Aldrich) and Thiophene-Silicas. Tstirrb hexamethylene diisocyanate (Sigma).20 The average sample template catalyst fcat/BTET a (h) molecular weight of PLGE block copolymer was obtained as 4,709 and the polydispersity index was 1.24 by using a PIT-1-20 P123 FeCl3·6H2O 1 20 GPC system (EcoSEC HLC 8320 GPC, Tosoh). The PIT-2-20 P123 FeCl3·6H2O 2 20 average molecular weight was estimated as 4,055 Daltons PIT-3-20 P123 FeCl ·6H O 3 20 3 2 and the volume fraction of the PEO blocks (ΦPEO) was 1 calculated as 0.38 by H-NMR and the group contribution PIT-4-20 P123 FeCl3·6H2O 4 20 method, respectively. A commercial Pluronic P123 triblock PIT-4-3 P123 FeCl3·6H2O 4 3 copolymer (EO20PO70EO20, Aldrich) and PLGE were used PHT-1.5-20 P123 HCl 1.5 20 as polymer templates. 2,5-bis(triethoxysilyl)thiophene (BTET, JSI Silicone) LHT-1.3-2 PLGE HCl 1.3 2 was used as an organosilica precursor. Hydrochloric acid LCT-3-20 PLGE Cu(ClO4)2·6H2O 3 20 (37%), iron(III) chloride hexahydrate, copper(II) LBT-20-20 PLGE B(OH)3 20 20 perchlorate hexahydrate, and boric acid were used as acid a Molar ratio of the catalyst/2,5-bis(triethoxysilyl)thiophene used catalysts and purchased from Aldrich. Ethanol (> 99.5%, in the synthesis gel. bInitial self-assembly time under magnetic Aldrich) and deionized water were used as solvents for stirring. preparation of the samples studied. The following notation was used for the samples studied: XYT-z-t where X denotes the polymeric template used: ACS Paragon Plus Environment

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Measurements and Calculations. The small angle Xray scattering (SAXS) experiments were performed using synchrotron radiation source (E = 10.5199 keV, λ = 1.1785 Å) of a 4C beam line in Pohang Accelerator Laboratory (PAL). Each sample was placed in a copper-alloy sample holder and secured on both sides using a Kapton tape. Nitrogen adsorption-desorption isotherms were measured at -196 oC on a Micromeritics 2420 analyzer. The samples were degassed at 120 oC under vacuum below 30 µmHg for at least 2 h before each measurement. The BET (Brunauer– Emmet–Teller) specific surface area was calculated from adsorption data in the relative pressure (P/P0) 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 were obtained by subtracting the mesopore volumes from the single-point pore volumes. The pore size distributions (PSD) were calculated from adsorption branches of the isotherms by using the improved KJS (Kruk–Jaroniec–Sayari) method.25 The pore wall thickness (w) was estimated from the pore size (DKJS) obtained at the maximum of PSD and the unit cell parameter (a) obtained by SAXS (i.e., w = a - DKJS).

The solid-state 29Si MAS NMR spectra were obtained with a Bruker AVANCE II+ (400 MHz) spectrometer using a 4 mm magic angle (MAS) spinning probe at the KBSI Daegu Center. The samples were spun at a spinning rate of 6 kHz and the spectra were obtained with a recycle delay of 50 s and 2,000 scans. The chemical shifts were obtained with respect to the tetramethylsilane (TMS) reference peak. The simulation, deconvolution, and integration of NMR peaks were performed using Gaussian multi-peak fitting method. The TEM images were obtained with 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. The SEM images were obtained with a field emission SEM (JEOL JSM-4300F) operated at an accelerating voltage of 15 kV. Thermogravimetric (TG) measurements were performed using a TA Instrument TGA Q50 analyzer. The TG and DTG profiles were recorded up to 900 oC in flowing air of 60 mL/min and nitrogen of 40 mL/min with a heating rate of 10 oC/min.

Table 3. Physicochemical Properties of 2D Hexagonal (p6mm) Thiophene-Silicas Prepared in the Presence of Block Copolymers under Acidic Conditions.a sample

SBET (m2 g-1)

Vt (cm3 g-1)

Vmicro (cm3 g-1)

DKJS (nm)

d-spacing (nm)

a (nm)

w (nm)

PIT-1-20

202

0.30

0.01

7.6

9.70

11.20

3.6

PIT-2-20

205

0.34

0.01

7.6

9.98

11.52

3.9

PIT-3-20

247

0.39

0.01

7.6

9.88

11.41

3.8

PIT-4-20

247

0.35

0.01

7.7

9.86

11.38

3.7

PIT-4-3

378

0.33

0.02

7.1

9.67

11.17

4.1

PHT-1.5-20

402

0.59

0.02

6.9

9.14

10.56

3.7

LHT-1.3-2

629

0.64

0.05

6.3

7.76

8.96

2.6

LCT-3-20

652

0.69

0.06

6.5

7.76

8.96

2.4

LBT-20-20

624

0.66

0.05

6.2

7.71

8.90

2.7

Notation: SBET - BET specific surface area determined in the range of relative pressures from 0.04 to 0.2; Vt - single-point pore volume at P/Po = 0.99; Vmicro - volume of micropores obtained by subtracting the volumes of mesopores from single-point pore volume; DKJS mesopore diameter at the maximum of the PSD curve obtained by the improved KJS method;25 d-spacing - Bragg’s spacing (= 2π/q*: q* is q value at maximum (100) peak for p6mm); a - unit cell parameter (= 2d100/√3); w - pore wall thickness (= a – DKJS). a

RESULTS AND DISCUSSION Ordered mesoporous thiophene-silica powders were synthesized in the presence of Pluronic P123 triblock copolymer under acidic conditions using several acid catalysts. Similarly as in the case of mesoporous benzenesilica prepared in the presence of Pluronic P123 block copolymer template,38 ordered mesoporous thiophene-silica was not be obtained when weak acidic catalysts such as acetic acid, aluminum chloride, and copper(II) perchlorate hexahydrate were employed. However, p6mm hexagonally ordered mesostructure of thiophene-silica was obtained

using iron(III) chloride hexahydrate catalyst in the presence of P123 template. The amount of acid catalyst (i.e., iron(III) chloride hexahydrate) was controlled by varying molar ratio of catalyst/BTET (i.e., Fe(III)/Si) from 1 to 4 as listed in Table 2. To compare the stability of Si-C bond in thiophene ring, the other experimental parameter in the preparation of thiophene-silica was the time of stirring. The standard stirring time was fixed as 20 h and the minimum stirring time was set as 3 h. The SAXS patterns of mesoporous thiophene-silica samples obtained by using different Fe(III)/Si ratios are shown in Figure 1. As can be seen from this figure, all


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thiophene-silica samples were highly ordered with 2D hexagonal (p6mm) mesostructure giving three wellresolved peaks indexed as (100), (110), and (200) according to p6mm symmetry group. Figure 1(a,b,c) shows a weak trace of Im3m cubic structure, however, it seems the main mesostructure is hexagonal phase for the samples studied. The d-spacing (d100) values obtained from the most intense Bragg peaks varied slightly from 9.67 to 9.98 nm, not showing any correlation with the amount of catalyst. The unit cell parameter (a) was obtained in the range of 11.17 – 11.52 nm from d-spacing values. SAXS analysis data for p6mm structured thiophene-silica samples are listed in Table 3 as well as Figure 1.

Figure 2. Synchrotron SAXS patterns of mesoporous thiophenesilicas prepared using hydrochloric acid, copper(II) perchlorate hexahydrate, and boric acid. The sequence of the SAXS patterns from (a) to (d) corresponds to the list of samples in Table 2: (a) PHT-1.5-20, (b) LHT-1.3-2, (c) LCT-3-20, and (d) LBT-20-20, respectively.

Figure 1. Synchrotron SAXS patterns of mesoporous thiophenesilicas prepared using iron(III) chloride hexahydrate. The sequence of the SAXS patterns from (a) to (e) corresponds to the list of samples in Table 2: (a) PIT-1-20, (b) PIT-2-20, (c) PIT-320, (d) PIT-4-20, and (e) PIT-4-3, respectively.

Furthermore, ordered mesoporous thiophene-silicas were synthesized in the presence of PLGE triblock copolymer using different weak acids. It was found that 2D hexagonal (p6mm) mesostructure of thiophene-silica was obtained when very weak copper(II) percolate hexahydrate (Ka = 1.6 × 10-7) and boric acid (Ka = 5.9 × 10-10) were used (see Figure 2(c, d)). The diffracted peaks are indexed as (100), (110), and (200), and the d-spacing values were obtained as 7.76 and 7.71 nm for LCT-3-20 and LBT-20-20 samples, respectively. The unit cell parameters were calculated as 8.96 and 8.90 nm, respectively. To compare the structure and porosity, the reference samples were prepared using hydrochloric acid in the presence of respective P123 and PLGE templates. Figure 2(a) is a SAXS pattern of a reference thiophene-silica prepared using P123 and HCl, while Figure 2(b) shows the pattern of the corresponding sample obtained in the presence of PLGE and HCl. The structural results from SAXS analysis are listed in Table 3.

Based on the SAXS patterns in Figures 1 and 2, one can conclude that hexagonally ordered mesoporous thiophenesilica samples can be synthesized easily under weak acidic conditions. In the case of Pluronic P123 template, the acidity stronger than iron(III) chloride hexahydrate is needed to obtain hexagonally ordered mesostructures of thiophene-silica. However, even very weak boric acid is sufficient to obtain the ordered structure when more hydrophobic PLGE triblock copolymer was employed as a structure-directing agent. Nitrogen adsorption-desorption isotherms and pore size distributions for p6mm hexagonally ordered thiophenesilicas prepared in the presence of iron(III) chloride hexahydrate and Pluronic P123 polymer template are shown in Figure 3. All the isotherms are type IV isotherms showing sharp condensation/evaporation steps and H1 hysteresis loops in a range of P/P0 from 0.55 to 0.73. These steps reflect the capillary condensation of nitrogen in hexagonally ordered mesopores. PIT-4-3 shows a two-step desorption isotherm indicating the formation of plugged pores (i.e. a small fraction of pores with constrictions) as shown in Figure 3(a-E), which can be attributed to the formation of somewhat perturbed mesostructures due to short stirring time of 3 h. The BET surface areas for the series of the PIT type thiophene-silica samples are in the range from 202 to 378 m2g-1 and the total pore volumes (Vt) vary from 0.31 to 0.39 cm3g-1 (see Table 3). The pore sizes calculated by the improved KJS method for thiophenesilica samples exhibit very narrow distributions as shown in Figure 3(b). The pore diameters obtained at the maximum of the PSD curves are in the range of 7.1 – 7.7 nm and the wall thickness values are calculated in the range of 3.6 –


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4.1 nm (see Table 3). The PIT-4-3 sample shows the smallest pore diameter and the largest wall thickness. Figure 4 shows nitrogen adsorption-desorption isotherms and pore size distributions for mesoporous thiophene-silica reference samples prepared in the presence of hydrochloric acid. Figure 4(a-A, b-A) refers to the PHT-1.5-20 sample prepared in the presence of P123 triblock copolymer, while Figure 4(a-B, b-B) presents data for the LHT-1.3-2 sample templated by PLGE triblock copolymer. Adsorption isotherms for the aforementioned samples are also type IV with sharp steps of capillary condensation and show distinct hysteresis loops in the range of P/P0 from 0.45 to 0.70. The BET surface areas for the PHT-1.5-20 and LHT1.3-2 samples are 402 and 629 m2g-1, and the total pore volumes (Vt) are 0.59 and 0.64 cm3g-1, respectively (see Table 3). The pore sizes calculated by the improved KJS method show very narrow distributions for the reference thiophene-silica samples (see Figure 4(b)). The pore diameters obtained at the maximum of the PSD curves show values of 6.9 and 6.3 nm for the PHT-1.5-20 and LHT-1.3-2 samples, as shown in Figure 4(b) and Table 3. The corresponding wall thickness values are 3.7 and 2.6 nm, respectively.

the LCT-3-20 and LBT-20-20 samples are 652 and 624 m2g-1, and the total pore volumes are 0.69 and 0.66 cm3g-1, respectively (see Table 3). The PSD curves calculated by the improved KJS method for the LCT-3-20 and LBT-2020 samples are very narrow and the pore diameters obtained at the maximum of these curves are 6.5 and 6.2 nm, respectively, as shown in Figure 5(b) and Table 3. The wall thickness values for these samples are 2.4 and 2.7 nm, respectively. The overall physicochemical properties of the hexagonally ordered thiophene-silica samples are summarized in Table 3.

Figure 4. Nitrogen adsorption-desorption isotherms (a) and the corresponding pore size distributions (b) for mesoporous thiophene-silicas prepared in the presence of hydrochloric acid. Pattern (A) is for PHT-1.5-20 and (B) is for LHT-1.3-2 samples, respectively.

Figure 3. Nitrogen adsorption-desorption isotherms (a) and the corresponding pore size distributions (b) for mesoporous thiophene-silicas prepared using iron(III) chloride hexahydrate. The sequence of the patterns from (A) to (E) corresponds to the following samples: (A) PIT-1-20, (B) PIT-2-20, (C) PIT-3-20, (D) PIT-4-20, and (E) PIT-4-3, respectively.

Figure 5 shows nitrogen adsorption-desorption isotherms and the corresponding pore size distributions for mesoporous thiophene-silica samples prepared in the presence of weak acids such as copper(II) perchlorate hexahydrate and boric acid using PLGE as a template. Figure 5(a-A, b-A) is for the LCT-3-20 sample and Figure 5(a-B, b-B) is for the LBT-20-20 sample, respectively. Adsorption isotherms are type IV with capillary condensation steps and show distinct hysteresis loops in the range of P/P0 from 0.45 to 0.70. The BET surface areas of

Figure 5. Nitrogen adsorption-desorption isotherms (a) and the corresponding pore size distributions (b) for mesoporous thiophene-silicas prepared using copper(II) perchlorate hexahydrate and boric acid. Pattern (A) is for LCT-3-20 and (B) is for LBT-20-20, respectively.

TEM images for PIT-4-20 (Figure 6(a,b)) and LBT-2020 (Figure 6(c,d)) clearly represent the p6mm hexagonal mesostructure of thiophene-silica samples. The SAXS and TEM analysis clearly show that the highly ordered p6mm mesoporous thiophene-silica samples can be prepared even using weak acids such as iron(III) chloride, copper(II) perchlorate hexahydrate, and boric acid in the presence of a suitable block copolymer template. The PLGE polymer ACS Paragon Plus Environment

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template, having stronger hydrophobic block than that in P123, seems to be more efficient under weak acidic conditions in the formation of ordered mesoporous structure of thiophene-silica. SEM images were obtained to observe the variation of morphology in the mesoporous thiophene-silica samples depending on the acidity (see Supporting Information Figure S1). Figure S1(a-c) shows the images of LHT-1.3-2, LCT-3-20, and LBT-20-20, respectively. The SEM images exhibit irregular shapes of particular aggregates and the average particle size of the LBT-20-20 sample was estimated to be below 10 µm, while the average particle diameters of the LHT-1.3-2 and LCT-3-20 samples exceeded 10 µm. The SEM images suggest that the morphology of thiophene-silica does not depend on the acidity except for the samples prepared in the presence of boric acid.

Figure 6. TEM images for hexagonally (p6mm) mesostructured thiophene-silicas prepared under acidic conditions. The images (a) and (b) are for PIT-4-20 and (c) and (d) are for LBT-20-20 samples, respectively.

Solid-state 29Si MAS NMR measurements were performed to identify the chemical nature of the thiophenebridging groups and to investigate the integrity of the C-Si covalent bonds within the framework of mesoporous thiophene-silica. Figure 7 shows 29Si NMR spectra for representative six kinds of mesoporous thiophene-silica samples divided into three groups according to the strength of acidity of the synthesis gel. The spectra shown in Figure 7(a) are for PHT-1.5-20 (A) and LHT-1.3-2 (B) samples, which were prepared using the strongest acid studied, hydrochloric acid, but different polymer template (i.e. P123 and PLGE) and stirring for 20 h and 2h, respectively. Figure 7(b) refers to PIT-4-20 (A) and PIT-4-3 (B) samples, which were prepared using the same amount of iron(III) chloride hexahydrate as an acidic catalyst and P123 polymer template but different stirring time (i.e. 20 h

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and 3 h). The 3rd group shown in Figure 7(c) refers to LCT-3-20 (A) and LBT-20-20 (B) samples prepared using the same PLGE polymer template and the same stirring time of 20 h but different acidic catalysts such as copper(II) perchlorate hexahydrate and boric acid, respectively. The characteristic signals on the 29Si MAS NMR spectra of thiophene-silicas are assigned to C-Si(OSi)(OH)2 (T1, δ = 64.8 ppm), C-Si(OSi)2(OH) (T2, δ = -74.5 ppm), C-Si(OSi)3 (T3, δ = -83.0 ppm), respectively, as shown in Figure 7. However, additional signals appeared in the range of -90 – 120 ppm, which are assigned as Q peaks related to noncarbon bonds in silica (i.e. -O-Si-O-). The appearance of Q peaks is attributed to the decomposition of Si-C bonds inside the mesoporous thiophene-silica frameworks during synthesis carried out under acidic conditions; note that additional precursor such as triethoxysilyl orthosilicate (TEOS) was not used in this study. The characteristic signals of Q peaks can be assigned to –O2Si(OH)2 (Q2, δ = 91), –O3Si(OH) (Q3, δ = -101), and –SiO4 (Q4, δ = -110.5), respectively, as shown in Figure 7. The relative molar contents of Si-C bonds (T peaks) and non Si-C bonds (Q peaks) were calculated by simulation, deconvolution, and integration using a Gaussian multiple fitting method. The simulated peaks are shown in Figure 8 and the corresponding data are listed in Table 4. Figure 8(a) shows NMR peak analysis for PHT-1.5-20 (A) and LHT-1.3-2 (B) prepared in the presence of hydrochloric acid; in the case of these systems the total molar fractions of Q peaks (i.e. ∑Q/∑(T + Q)) are 40.5 and 17.8%, respectively (see Table 4). The result shows that the longer initial self-assembly time under stirring and acidic conditions enhances the decomposition of Si-C bond; note that for PHT-1.5-20 (fHCl/BTET ~ 1.5) and LHT-1.3-2 (fHCl/BTET ~ 1.3) samples the difference in pH was not so big. Figure 8(b) shows the peak analysis for PIT-4-20 (A) and PIT-4-3 (B) samples; in this case the fractions of Q peaks are 33.2 and 16.5%, respectively. The result shows also that the initial self-assembly time under stirring may be a possible factor affecting the integrity of the Si-C bond because in both cases the same amounts of the P123 polymer template and iron(III) chloride hexahydrate (weak acid) were used. The final Figure 8(c) shows the peak analysis for LCT-3-20 (A) and LBT-20-20 (B) samples; in this case the fractions of Q peaks are 21.1 and 2.7%, respectively. Note that the Si-C bond of LBT-20-20 thiophene-silica seems to be intact because no meaningful cleavage occurred. This result shows that the acidity is the most important factor controlling the decomposition of SiC bonds in thiophene-bridged organosilica. A comparison of the respective Q fractions (40.5, 33.2, 21.1, and 2.7%) of PHT-1.5-20, PIT-4-20, LCT-3-20, and LBT-20-20 at the same stirring time of 20 h shows that the acidity is one of the key factors deciding about the stability of Si-C bond. The 29Si NMR analysis shows that the structural stability of Si-C bond in thiophene-silica is mostly dependent on the acidity. Namely, the relative amount of cleavage of Si-C bonds increases with increasing acidity of the synthesis gel.


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Figure 7. Solid state 29Si MAS NMR spectra of hexagonally (p6mm) mesostructured thiophene-silicas prepared under acidic conditions. 29 Si NMR patterns (a) to (c) correspond to the following samples: (a-A) PHT-1.5-20, (a-B) LHT-1.3-2, (b-A) PIT-4-20, (b-B) PIT-4-3, (cA) LCT-3-20, and (c-B) LBT-20-20, respectively.

Figure 8. Gaussian deconvolution of solid state 29Si MAS NMR spectra of hexagonally (p6mm) mesostructured thiophene-silicas prepared under acidic conditions. 29Si NMR patterns (a) to (c) correspond to the following samples: (a-A) PHT-1.5-20, (a-B) LHT-1.3-2, (b-A) PIT4-20, (b-B) PIT-4-3, (c-A) LCT-3-20, and (c-B) LBT-20-20, respectively. Table 4. Solid State 29Si NMR Analysis for Hexagonally Structured Mesoporous Thiophene-Silicas. sample PHT-1.5-20 LHT-1.3-2 PIT-4-20 PIT-4-3 LCT-3-20

peak assignment

T1

T2

T3

Q2

Q3

Q4

chemical shift (ppm)

-65.57

-74.07

-82.51

-91.96

-100.50

-108.02

peak area (%)

4.32

32.67

22.50

11.34

22.72

6.46


-65.01

-73.88

-82.99

-90.84

-100.24

-108.43

peak area (%)

10.01

52.88

19.25

7.15

9.51

1.19


-66.75

-74.38

-82.54

-91.70

-100.29

-106.49

peak area (%)

7.00

35.83

23.98

10.93

17.30

4.96


-65.52

-74.03

-82.71

-91.11

-100.26

-109.01

peak area (%)

11.68

53.15

18.71

7.51

8.54

0.40


-65.15

-73.78

-82.98

-91.68

-100.25

-107.76

peak area (%)

8.59

47.13

23.16

8.82

9.74

2.56


100 × ∑Q /∑(T +Q) 40.5 17.8 33.2 16.5 21.1

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LBT-20-20

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-64.85

-73.87

-83.31

-91.35

-100.29

n/a

peak area (%)

13.44

59.65

24.21

0.82

1.88

0

The thermal stability of thiophene-silica samples was monitored to investigate the structural stability of Si-C bond in powder products. The thermogravimetric (TG) weight change profiles and the corresponding differential TG (DTG) patterns for representative LHT-1.3-2, LCT-320, and LBT-20-20 samples were recorded in flowing mixed air (i.e. air flow of 60 mL/min and nitrogen flow of 40 mL/min) as shown in Figure 9. The LHT-1.3-2 and LBT-20-20 samples were stable up to 450 °C and LCT-320 samples started to decompose at 300 °C, which indicates the thermal stability of the Si-C covalent bond is affected by the synthesis conditions such as acidity of the synthesis gel. It seems that the stability of Si-C bond of mesoporous thiophene-silica become weaker under weak acidic conditions. However, boric acid is known to be a very stable component under at elevated temperatures, which can explain that LBT-20-20 sample prepared in the presence of boric acid showed nearly the same thermal stability as compared to the LHT-1.3-2 sample prepared in the presence of hydrochloric acid. The final weights of the thiophene-silicas at 900 °C were 63.2, 62.7, and 59.1% for LHT-1.3-2, LCT-3-20, and LBT-20-20 samples, respectively (Figure 9). The observed weight losses (after correction for small weight losses in the range between 100 and 300 °C that correspond to the elimination of adsorbed water and impurities; the latter were equal to 1.6, 2.2, and 2.7% for LHT-1.3-2, LCT-3-20, and LBT-20-20, respectively) can be mainly attributed to the decomposition of thiophene-bridging groups present in the organosilica framework. Analysis of 29Si NMR spectra shows that the relatively large quantity of -Si-C4H2S-Si- inside LHT-1.3-2 and LCT-3-20 samples turned to -Si-OH and -Si-O-Siduring the synthesis. Therefore, it is reasonable to assume that the LBT-20-20 sample experienced the largest amount of degradable thiophene-bridging groups among the samples studied, which explains clearly the difference of 4 - 6 % in the final weight through thermal decomposition in the temperature range from 300 to 900 °C.

2.7

CONCLUSIONS Hexagonally (p6mm) ordered mesoporous thiophenesilicas were synthesized successfully using BTET organosilane in the presence of P123 and PLGE nonionic triblock copolymer templates under weak acidic conditions. In order to investigate the stability of the Si-C bond, the acidity of the synthesis gel was varied by using hydrochloric acid, iron(III) chloride hexahydrate, copper(II) perchlorate hexahydrate, and boric acid. The PLGE triblock copolymer template was used to compare the stability of the Si-C bond in thiophene-silica formed in the presence of weak acids like copper(II) perchlorate hexahydrate and boric acid; Pluronic P123 template was not used because the ordered thiophene-silica mesostructure was not formed in the presence of copper(II) perchlorate hexahydrate and boric acid. For all hexagonally (p6mm) ordered mesoporous thiophene-silica powders studied, the solid-state 29Si MAS NMR analysis clearly showed that the acidity is the key parameter controlling the decomposition of the Si-C bonds in thiophene-silica. Namely, the fraction of the decomposed Si-C bonds increased with increasing acidity of the synthesis gel. The synthesis of thiophene-silica in the presence of boric acid and PLGE template proved to be the most suitable method to prepare this material with fully preserved Si-C bonds. Also, the TG and DTG analysis showed that the thermal stability of thiophene-silica can be altered by the acidity of the synthesis gel. The LCT-3-20 sample prepared in the presence of copper(II) perchlorate hexahydrate and PLGE template showed very weak thermal stability, only up to 300 °C, which is much lower as compared to the samples prepared in the presence of hydrochloric acid and boric acid. Finally, this study can serve as a guide in the preparation of versatile periodic mesoporous organosilica materials, especially those with complex bridging groups, for various applications. AUTHOR INFORMATION *Corresponding author (Eun-Bum Cho) Phone: 82-2-970-6729. E-mail: [email protected]. (Mietek Jaroniec) Phone: 1-330-672 3790. E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT

E.-B. Cho was supported partially by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2012000855) and the New & Renewable Figure 9. Thermogravimetric (TG) and the corresponding Energy R&D Program of the Korea Institute of Energy differential TG patterns for mesoporous thiophene-silicas recorded Technology Evaluation and Planning (KETEP) grant in flowing air. funded by the Korea government Ministry of Trade, ACS Paragon Plus Environment

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Industry and Energy (No. 20113020030040). M. J. acknowledges the National Science Foundation support of this research under CHE-0848352 grant. Experiments at PLS were supported in part by MSIP and POSTECH. ASSOCIATED CONTENT Supporting Information SEM images of the PMOs studied. This material is available free of charge via the Internet at http://pubs.acs.org REFERENCES (1) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. Novel Mesoporous Materials with a Uniform Distribution of Organic Groups and Inorganic Oxide in Their Frameworks. J. Am. Chem. Soc. 1999, 121, 9611-9614. (2) Melde, B. J.; Holland, B. T.; Blanford, C. F.; Stein, A. Mesoporous Sieves with Unified Hybrid Inorganic/Organic Frameworks. Chem. Mater. 1999, 11, 3302-3308. (3) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Periodic Mesoporous Organosilicas with Organic Groups inside the Channel Walls. Nature 1999, 402, 867-871. (4) Yoshina-Ishii, C.; Asefa, T.; Coombs, N.; MacLachlan, M. J.; Ozin, G. A. Periodic Mesoporous Organosilicas, PMOs: Fusion of Organic and Inorganic Chemistry ‘inside’ the Channel Walls of Hexagonal Mesoporous Silica. Chem. Commun. 1999, 2539-2540. (5) Van der Voort, P.; Esquivel, D.; De Canck, E.; Goethals, F.; Van Driessche, I.; Romero-Salguero, F. J. Periodic Mesoporous Organosilicas: from Simple to Complex Bridges; a Comprehensive Overview of Functions, Morphologies and Applications. Chem. Soc. Rev. 2013, 42, 3913-3955. (6) Guan, S.; Inagaki, S.; Ohsuna, T.; Terasaki, O. Cubic Hybrid Organic−Inorganic Mesoporous Crystal with a Decaoctahedral Shape. J. Am. Chem. Soc. 2000, 122, 5660-5661. (7) Asefa, T.; MacLachlan, M. J.; Grondey, H.; Coombs, N.; Ozin, G. A. Metamorphic Channels in Periodic Mesoporous Methylenesilica. Angew. Chem. Int. Ed. 2000, 39, 1808-1811. (8) Temtsin, G.; Asefa, T.; Bittner, S.; Ozin, G. A. Aromatic PMOs: Tolyl, Xylyl and Dimethoxyphenyl Groups Integrated within the Channel Walls of Hexagonal Mesoporous Silicas. J. Mater. Chem. 2001, 11, 32023206. (9) Muth, O.; Schellbach, C.; Fröba, M. Triblock Copolymer Assisted Synthesis of Periodic Mesoporous Organosilicas (PMOs) with Large Pores. Chem. Commun. 2001, 2032-2033. (10) Sayari, A.; Hamoudi, S. Periodic Mesoporous Silica-Based Organic−Inorganic Nanocomposite Materials. Chem. Mater. 2001, 13, 3151-3168. (11) Goto, Y.; Inagaki, S. Synthesis of Large-Pore Phenylene-Bridged Mesoporous Organosilica Using Triblock Copolymer Surfactant. Chem. Commun. 2002, 2410-2411. (12) Kapoor, M. P.; Yang, Q.; Inagaki, S. Self-Assembly of Biphenylene-Bridged Hybrid Mesoporous Solid with Molecular-Scale Periodicity in the Pore Walls. J. Am. Chem. Soc. 2002, 124, 15176-15177. (13) Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. An Ordered Mesoporous Organosilica Hybrid Material with a Crystal-Like Wall Structure. Nature 2002, 416, 304-307. (14) Yang, Q.; Kapoor, M. P.; Inagaki, S. Sulfuric Acid-Functionalized Mesoporous Benzene-Silica with a Molecular-Scale Periodicity in the Walls. J. Am. Chem. Soc. 2002, 124, 9694-9695. (15) Matos, J. R.; Mercuri, L. P.; Kruk, M.; Jaroniec, M. Synthesis of Large-Pore Silica with Cage-Like Structure Using Sodium Silicate and Triblock Copolymer Template. Langmuir, 2002, 18, 884-890. (16) Yu, C.; Tian, B.; Fan, J.; Stucky, G. D.; Zhao, D. Nonionic Block Copolymer Synthesis of Large-Pore Cubic Mesoporous Single Crystals by Use of Inorganic Salts. J. Am. Chem. Soc. 2002, 124, 4556-4557.

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Graphics and Summary for Table of Contents only Highly ordered hexagonal p6mm mesoporous thiophene-silicas without Si-C bond cleavage were synthesized in the presence of PEO-PLGA-PEO triblock copolymer template using boric acid as an acidic catalyst.


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Structural Stability of SiâC Bonds in Periodic Mesoporous Thiophene

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