Novel bis(methylimidazolium)alkane bolaamphiphiles as templates for supermicroporous and mesoporous silicas

Microporous and Mesoporous Materials 148 (2012) 62–72

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Novel bis(methylimidazolium)alkane bolaamphiphiles as templates for supermicroporous and mesoporous silicas Alexander K.L. Yuen, Falk Heinroth, Antony J. Ward, Anthony F. Masters, Thomas Maschmeyer ⇑ Laboratory of Advanced Catalysis for Sustainability, School of Chemistry F11, The University of Sydney, Sydney 2006, Australia

a r t i c l e

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Article history: Received 28 February 2011 Received in revised form 1 June 2011 Accepted 17 June 2011 Available online 28 July 2011 Keywords: Bolaamphiphile Mesoporous Supermicroporous Silica Ionic liquid

a b s t r a c t A series of bis-cationic bolaamphiphiles were synthesized containing N-methylimidazolium head-groups, separated by alkyl chains of 12, 16 and 24 methylene units. Their critical micelle concentrations were determined with subsequent flooding experiments revealing the tendency for the formation of hexagonal mesophases at high surfactant concentrations. The surfactants were employed as templates for silica formation, at ca. four times their critical micelle concentrations, permitting super-microporous and nanoparticulate mesoporous silicas to be prepared. Silica surface areas ranged from 500 to 1000 m2 g1, and pore diameters (DTEM) from 10 to 22 Å. Analysis of the materials by small angle X-ray diffraction indicated that, in the case of the template possessing 24 methylene units, the silica obtained exhibited a 2D hexagonal pore arrangement. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction Porous inorganic materials are of ever increasing importance, especially in the fields of adsorption, separation, catalysis, drug delivery, and nanodevices. The usefulness of such inorganic materials for a particular purpose is often determined by the size of the pores they possess, with a great deal of research effort directed towards preparation methods that allow control over the size and morphology of the pore systems [1]. Many of these materials are classified as either microporous (pore diameter of less than 20 Å) or mesoporous (pore sizes in the range 20–500 Å) [2]. The preparation of high surface area mesoporous silicas has attracted much attention in the last 20 years. Various nanoporous silicas (such as M41S [3], SBA [4–11], HMS [12], TUD [13], MSU [14], KIT [15,16] and FDU [17–22]) have been obtained by using cationic and neutral surfactants. In such cases the mesoporous materials are prepared through organic–inorganic interactions including direct (e.g. S+I, S0I0) and indirect interactions with the aid of a bridging ion (e.g. S+XI+, S0X+I, [S0H]+XI+) (where S, I and X represent surfactant, inorganic species, and counter ions, respectively) [4]. Ionic liquids (ILs) are a class of compounds that has recently received much attention [23]. In particular, many ILs resemble cationic surfactants and are able to form liquid–crystalline phases in various solvents due to their molecular structure, which often includes a rigid hydrophilic head group and a hydrophobic organic domain. This property, in addition to the commonly postulated ⇑ Corresponding author. Tel.: +61 2 9351 2581; fax: +61 2 9351 3329. E-mail address: [email protected] (T. Maschmeyer). 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.06.036

partially-ordered solution structures of ILs [24], has resulted in many groups investigating their ability to act as templates to prepare mesoporous and microporous materials [25–27]. The ILs most widely investigated as templates are the 1-Cn-3-methylimidazolium halides ([CnMIM]X; n = 12–18). With these templates various silica morphologies have been obtained depending on the conditions used: such morphologies range from ‘‘worm-like’’ to hexagonal MCM-41-like and even helical channels [28–31]; all have surface areas in excess of 900 m2 g1. Although ordered mesoporous materials have been widely investigated for a variety of applications since their discovery, especially as supports in the preparation of heterogeneous catalysts, their pore sizes of >20 Å are too large to influence shape- and size-selectivities in most commonly catalyzed reactions [32]. By contrast, materials with a narrow pore-size distribution in between the micro- and mesopore ranges, are expected to have potential selectivity for size or shape in the fields of separation [33,34] and catalysis [5–9]. Improved catalytic activity based purely on pore size and shape manipulation has already been demonstrated for these materials [18,35]. Such materials, with pores sizes between 10 and 20 Å, have come to be referred to as ‘‘super-microporous’’, a term originally coined by Dubinin as early as 1974 [36]. Over the last decade there have been a number of examples of super-microporous materials prepared by a range of different methods. The earliest example of the preparation of such materials involved the post-synthetic modification of MCM41 by the selective removal of surfactant molecules at the pore openings, followed by the deposition of tetraethoxysilane (TEOS) and hydrolysis. The resulting constriction of the pores afforded a

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material with a surface area of 886 m2 g1 and an effective pore size of 13–14 Å [37]. The constriction of pores has also been achieved by co-condensing triethoxyvinylsilane (TEVS) with TEOS, affording silicas with pore sizes as small as 17 Å when the TEVS loading was 65% [38]. A preparation of silica using adamantanamine as the template produced amorphous material with randomly ordered pores, a reported surface area of 737 m2 g1, and a pore size of 13–17 Å [39]. The use of the gemini-surfactants as templating agents has yielded both ordered and disordered super-microporous silicas, with a surface area of 730–1450 m2 g1 and pore sizes of 13–22 Å [32,40,41]. Alkyltrimethylammonium halide salts have been used as templates for super-microporous silicas with 2D hexagonal structures, surface areas in the range of 500–1000 m2 g1 and pore sizes of 15–18 Å [42,43]. Furthermore, using highly concentrated solutions of [CnMIM]Cl (n = 10, 14, 16, 18) as the templating agent, a series of super-microporous, lamellar silicas with pore sizes in the range 12–15 Å and surface areas ranging from 1314–1382 m2 g1, were prepared [44,45]. Finally, super-microporous silicas with surface areas between 624 and 729 m2 g1, with ordered hexagonal pores of 13 Å, and disordered micropores of 7 Å, have been prepared at low temperatures (0 to 20 °C) using the semi-fluorinated surfactant CF3(CF2)5(EO)10 in an acidic synthesis medium [46]. An alternative to the above approach is the use of salts with two hydrophilic heads connected by a hydrophobic linker as templates [47–49]. Such compounds are referred to as bolaamphiphiles or bolaform surfactants. The self-assembly behaviors of bolaamphiphiles in aqueous solutions are known for a large number of examples which show a diverse range of morphologies [47,50] such as vesicles [51,52], lamellae [53], disks [54], rods [55], tubules [55], ribbons, and fibers [56] in the nanometer and micrometer ranges. To date however, there have only been a few examples of the use of bolaamphiphiles as templates for the production of porous silica. Tanev and Pinnavaia used neutral diamines with alkyl chain lengths from C8 to C12 as the structuredirecting agents to form a series of silicates [57]. The resulting materials had surface areas in the range 756–984 m2 g1 and exhibited intergallery framework microporosity (pores of 6 Å) and interparticle mesoporosity (pores of 12 Å). The use of a bolaform surfactant containing a rigid spacer unit [(CH3)3NC12H24O4,40 -biphenyl-OC12H24N(CH3)3]Br2 for the basic hydrolysis of TEOS resulted in the formation of a silica with a 2D pore structure with a surface area of 1020 m2 g1 and a uniform pore size of 20 Å [58]. Bagshaw and Hayman used the unsymmetrical x-hydroxytetraalkylammonium bolaamphiphiles [HO(CH2)nN(CH2CH3)3]Br (n = 12, 16) as templates to prepare super-microporous silicates from sodium silicate or TEOS under basic hydrolysis conditions [59–61]. Super-microporous silicas with pore sizes of 16 and 21 Å, accompanied by surface areas of 860 and 870 m2 g1 were obtained using the C12 and C16 bolaform surfactants respectively. In this work we showcase the synthesis of nanoporous materials, combining the use of imidazolium ionic liquids and bolaamphiphiles as templates, enabled by our novel synthesis of a series of new methylimidazolium-based bolaamphiphiles (1–3) with different chain lengths. Hence, one of the main objectives of this work was to investigate the effects of increasing alkyl-chain length on the morphology and pore size of templated silica. Amphiphiles (1–3) consist of two cationic N-methylimidazolium head-groups separated by a straight-chain alkane spacer of 12, 16, or 24 methylene units, and bromide counter-ions (Fig. 1). As well as being ‘‘bolaform’’ surfactants, the C12- and C16-members are also able to be classified as ‘‘ionic liquids’’ (which are arbitrarily defined as salts containing an organic cation and a melting point below 100 °C [62]). Due to the characteristic two head-groups in the bolaamphiphiles, different behavior in terms of micelle formation and templating can be expected when compared to their single-headed

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Fig. 1. Bolaamphiphiles used in this study.

analogues. In contrast to single-headed N-methylimidazolium surfactants, detailed studies of the aggregation behavior and surfactant properties of these new bolaamphiphiles (1–3) are yet to be reported. Several groups however, have published comparative studies between bis(trialkylammonium)-bolaamphiphiles and single-headed analogues [63,64]. Given the similarities between methylimidazolium and trimethylammonium head-groups, in terms of critical micelle concentration (CMC) and aggregation behavior, [65] for the single-headed surfactants, it seemed reasonable to expect comparable behavior for the corresponding two series of bolaamphiphiles. Analysis of the trends in CMC and aggregation number for both bolaform and single-headed trialkylammonium bromide surfactants indicates that they are distinct series. Generally bis(trialkylammonium)-bolaform surfactants exhibit higher CMCs but lower aggregation numbers than the single-headed surfactants containing the same number of carbons in the hydrophobic domain [66]. Both series however, display clear trends towards increasing aggregation number with increasing length of the hydrophobic chain, and hence increasing micelle size [63,64,67,68]. In addition, for a given carbon chain, a single-headed surfactant will always form larger micelles than the corresponding bolaamphiphile. For bolaamphiphiles in general, the length of the alkyl linker, and hence the hydrophobic effect, is the critical parameter which determines surfactant folding in both the pre-micellar and micellar states. Within the bis(trimethylammonium) bromide series, members containing a linker of at least 12 carbons adopt a ‘‘U-shape’’ conformation at the air–water interface [69]. Within micelles however, there is convincing evidence in support of a transition from stretched to U-shape behavior as the linker increases from 16 to 22 carbons [67,70]. In the context of silica templating, it is expected that a silica templated by bolaamphiphiles (1–3), with a given hydrophobic domain, should contain smaller pores (either small mesoporous or supermicroporous), than one templated by an analogous singleheaded surfactant ([CnMIM]Br). Surfactants containing short chain lengths (612 for trialkylammonium bolaforms [71] and 68 for single-headed methylimidazoliums [65]) tend to form rather loose aggregates and may not result in highly ordered silica. Therefore, larger and more ordered pores are expected with increasing chain length, as a result of tighter aggregation of surfactant molecules within micelles.

2. Experimental methods 2.1. Characterization Melting points were recorded on a Gallenkamp melting point apparatus in air and are uncorrected. FT-IR spectra were recorded using a Varian Scimitar 800 FT-IR spectrometer over the range of 4000–400 cm1; samples were suspended in an anhydrous potassium bromide matrix. 1H NMR (200 MHz) and 13C{1H} NMR (50 MHz); (300 MHz) and 13C{1H} NMR (75 MHz); 1H NMR (400 MHz) and 13C{1H} NMR (100 MHz) spectra were recorded using Bruker DPX200; Bruker DPX300; and Bruker DRX400 NMR spectrometers respectively. Spectra were referenced internally to

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residual solvent: CDCl3 d 7.26 (1H NMR) and d 77.0 (13C{1H} NMR); CD3OD d 3.31 (1H NMR) and d 49.0 (13C{1H} NMR). Mass spectra (ESI & APCI) were recorded on a Finnigan LCQ Ion Trap Mass Spectrometer. Elemental analyses were performed by the Microanalytical Unit at the Australian National University, Canberra. Conductometry was performed at 25 °C using solutions of surfactants dissolved in MilliQ water at various concentrations. Specific conductivities were measured using an EDT Instruments FE280 conductivity meter, calibrated with 0.01 M potassium chloride. Flooding experiments were performed with a small amount of surfactant sandwiched between a glass slide and a cover slip; observation by optical microscopy was performed using a Leica DMLB microscope equipped with crossed polarizers, quarter-wave plate and DC 300 digital camera. A Linkam heating plate (Tmax 62.5 °C) was placed underneath the sample on the microscope stage when heating was required. Small angle X-ray scattering (SAXS) were measured by using an Anton Paar SAXSess line collimation instrument with a sealed tube source and 1D elliptical focusing graded multilayer mirror optics (d spacing = 4 ± 0.07 nm) with a Max Flux Osmic block collimator. Data were recorded using 2D position-sensitive image plates (42.3  42.3 lm2 pixel size; sample-to-image plate distance, 264.5 mm). Image plates were read using a Perkin Elmer Cyclone phosphor storage system driven with OptiQuant software. Anton Parr SAXSquant2D software was used to average the 2D images. Transmission electron micrographs were recorded digitally with a Gatan slow-scan charge-coupled device (CCD) camera on a Phillips CM120 Biofilter electron microscope operating at 120 kV. The samples were prepared by depositing an ethanolic suspension of calcined silica onto a 200-mesh copper grid, coated with a holey-carbon film, followed by drying in air. Nitrogen adsorption and desorption isotherms at 77 K were measured using a Micromeritics ASAP2020 Surface Area and Porosity Analyzer. Samples were degassed for at least 4 h at 473 K before analysis. The specific surface area SBET was obtained from the adsorption data applying the BET equation in the relative pressure range from 0.04 to 0.2. The microporous surface area St-plot was calculated using the t-plot method and the Harkins and Jura thickness curve. The total pore volume VP was determined from the nitrogen adsorbed at a relative pressure of 0.96. The pore size-distribution was calculated using either the BJH (Barrett–Joyner–Halenda) method and the non-local density functional theory (NLDFT) method, embedded in the software (ASAP 2020 ver. 3). For both, analysis was applied to the adsorption branch of the isotherms to estimate pore size-distributions using the model for cylindrical pore geometry. Additionally, the pore size-distribution DKJ was evaluated using equation (1) introduced by Kruk and co-workers [72], where c is a geometrical factor (1.1213 for cylindrical pores), d100 is the interplanar spacing, q is the pore wall density (2.2 g cm3) equal to that of amorphous silica, and VP the pore volume.

 DkJ ¼ cd100

qV p 1 þ qV p

1=2 ð1Þ

2.2. Experimental section The following chemicals were used as received: tetraethoxysilane (TEOS), triethanolamine (TEA), 1-methylimidazole (99% redistilled), 1,12-dodecanediol, 3,4dihydro-2H-pyran (all Sigma Aldrich), palladium on activated charcoal (10% Pd), lithium aluminium hydride (tablets), tetrabromomethane, (all Merck), magnesium turnings, hydrobromic acid, sodium hydroxide (Ajax), 1,12-dibromododecane (Fluka), absolute ethanol (CSR), and x-6hexadecenelactone (SAFC). Triphenylphosphine (Aldrich) was recrystallised from hexanes prior to use. Solvents (dichloromethane, diethyl ether, hexane) were deoxygenated and dried over

activated alumina using an apparatus derived from that described in literature [73]. Tetrahydrofuran was distilled and dried from sodium/benzophenone ketyl. Silver bromide was prepared from silver nitrate and sodium bromide, dried under vacuum and stored at 60 °C. 2.3. Template synthesis 2.3.1. Hexadec-6-ene-1,16-diol 4 A solution of x-6-hexadecenelactone (10.0, 37.9 mmol) in tetrahydrofuran (50 mL) was slowly added to a suspension of LiAlH4 (4.20 g, 110 mmol) in tetrahydrofuran (150 mL) at 0 °C with stirring under nitrogen. The reaction mixture was stirred overnight at room temperature, re-chilled in an ice-bath and quenched sequentially with water (4.20 mL), aqueous sodium hydroxide (8.40 ml, 10% w/v), then more water (12.6 mL). The reaction mixture was filtered and the solvent removed in vacuo. Recrystallisation from ethanol afforded the product as a white powder (9.31 g, 92%), m.p. 62–63 °C. mmax (KBr)/cm1: 3410, 3345, 2923, 2851, 1462, 1361, 1060, 964, 729, 624. 1H NMR (CDCl3, 200 MHz): d 5.48–5.32 (2H, m, 2 C = CH), 3.63 (4H, app t, 3 JHH = 6.5 Hz, 2 CH2OH), 1.98 (4H, br m, 2 C = CHCH2), 1.65 to 1.53 (6H, m, 2 CH2CH2OH and CH2(CH2)2OH), 1.47 to 1.23 (14H, m, 7 CH2) ppm. 13C{1H} NMR (CDCl3, 100 MHz): d 130.4, 130.2, 63.0, 32.7(3), 32.7(1), 32.5, 32.4, 29.5(3), 29.5(1), 29.4, 29.3(6), 29.0, 28.8, 25.7, 25.6 ppm, 1 signal obscured. m/z (APCI): 257 ([M+H]+, 100%). Anal. Calcd. for C16H32O2: %C 74.9; %H 12.6. Found: %C 74.8; %H 12.6. 2.3.2. 1,16-hexadecanediol 5 To a solution of 4 (9.60 g, 37.5 mmol) in ethanol (200 mL) was added 10% w/w Pd/C (800 mg, 750 lmol Pd) and the resulting suspension was placed under 1 atm of hydrogen gas. The reaction mixture was stirred for 4 h at room temperature, filtered and the solvent was removed in vacuo to afford a white powder (9.58 g, 98%), m.p. 93–94 °C. mmax (KBr)/cm1: 3415, 3354, 2922, 2849, 1737, 1462, 1058, 729, 612. 1H NMR (CDCl3, 300 MHz): d 3.64 (4H, t, 3JHH = 6.6 Hz, 2 CH2OH), 1.59  1.53 (8H, m, 2 CH2CH2OH and 2 CH2(CH2)2OH), 1.31  1.26 (22H, m, 10 CH2 and 2 OH) ppm. 13C{1H} NMR (CDCl3): d 63.1, 32.8, 31.9, 29.6(4), 29.6(1), 29.5(8), 29.4, 25.8 ppm. m/z (APCI): 258 (M+, 100%). Anal. Calcd. for C16H34O2: %C 74.4; %H 12.6. Found: %C 74.1; %H 12.6. 2.3.3. 1,16-Dibromohexadecane 6 1,16-Dibromohexadecane 6 was prepared according to the procedure of Moss and co-workers [74]. A mixture of diol 5 (1.70 g, 6.58 mmol) and 48% w/v aqueous hydrobromic acid (4, ca. 36 mmol) were heated at 150 °C in a sealed tube for 14 h. The mixture was allowed to cool to r.t., diluted with 2:1 dichloromethane:hexane (30 mL), separated, and the organic phase was washed with water (30 mL), followed by saturated, aqueous sodium thiosulfate solution (2 10 mL). The organic phase was dried over sodium sulfate, filtered and concentrated, affording a brown oil. Purification through a 10 cm column of flash silica, eluting with hexanes, afforded the desired compound as a white, crystalline solid (1.72 g, 68%), m.p. 57–58 °C. mmax (KBr)/cm1: 2917, 2851, 1472, 1327, 1292, 1250, 1210, 1182, 718, 643. 1H NMR (CDCl3, 400 MHz): d 3.41 (4H, t, 3JHH = 6.9 Hz, 2 CH2Br), 1.85 (4H, m, 2 CH2CH2Br), 1.42 (4H, br m, 2 CH2(CH2)2Br), 1.29 to 1.25 (20H, m, 10 CH2) ppm. 13C{1H} NMR (CDCl3, 100 MHz): d 34.1, 32.8, 29.6(2), 29.6(0), 29.5, 29.4, 28.8, 28.2 ppm. m/z (EI, positive ion): 384, (1%, M+), 303–305 (2%, [MBr]+), 135–137 (17%, [Br(CH2)4]+), 111–113 (10%), 97–99 (30%), 83–85 (42%), 69–71 (77%), 55–57 (100%,). Anal. Calcd. for C16H34Br2: %C 50.0; %H 8.4; %Br 41.6. Found: %C 50.4; %H 8.5; %Br 41.4.

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2.3.4. 1-Bromo-12-dodecanol 7 [75] and 2-((12-bromododecyl) oxy)tetrahydro-2H-pyran 8 [76] 1-Bromo-12-dodecanol 7 [75] and 2-((12-bromododecyl)oxy)tetrahydro-2H-pyran 8 [76] were prepared according to literature conditions and found to be identical to reported spectroscopic data. 2.3.5. 1,24-bis((tetrahydro-2H-pyran-2-yl)oxy)tetracosane 9 To a solution of protected bromoalkanol 8 (1.60 g, 4.60 mmol), in tetrahydrofuran (50 mL), under nitrogen, was added pre-activated magnesium turnings (182 mg, 7.49 mmol). The reaction was refluxed for several h, allowed to cool to room temperature and a slurry of silver bromide (940 mg, 5.00 mmol) in tetrahydrofuran (10 mL) was added via cannula. The resulting reaction was stirred and briefly reheated. The resulting black suspension was stirred overnight at room temperature, filtered through Celite and concentrated. The residue was suspended in concentrated aqueous citric acid (20 mL), extracted with ethyl acetate (3 20 mL), the organic extracts dried over sodium sulfate, filtered and concentrated. Purification by flash chromatography, eluting with 1:1 dichloromethane:hexanes, followed by 0–5% methanol in dichloromethane (gradient) afforded the desired compound as a white solid (455 mg, 37%), m.p. 55–56 °C. 1H NMR (CDCl3, 200 MHz): d 4.58 (2H, m, 2 pyranC2-H), 3.93 to 3.82 (2H, m, 2 pyranC6-H), 3.73 (2H, dt, 3JHH = 9.6, 6.8 Hz, 2 OCHH0 ), 3.55 to 3.44 (2H, m, 2 pyranC6-H0 ), 3.38 (2H, dt, 3JHH = 9.6, 6.6 Hz, 2 OCHH0 ), 1.93 to 1.54 (14H, m, pyranylCH2 + 2 CHH0 ), 1.43–1.13 (42H, m, 20 CH2 + 2 CHH0 ). Found to be identical to reported spectroscopic data [77]. 13C{1H} NMR (CDCl3, 50 MHz): d 98.8, 67.7, 62.3, 30.8, 29.8, 29.7 (7 resonances), 29.6, 29.5, 29.5, 26.2, 25.5, 19.7 ppm. 2.3.6. 1,24-Dibromotetracosane 10 [77] To a solution of bis-THP ether 9 (455 mg, 8.84 mmol) and carbon tetrabromide (784 mg, 2.36 mmol) in dichloromethane (20 mL), under nitrogen, was added triphenylphosphine (1.24 g, 4.73 mmol) the resulting solution was stirred at room temperature overnight, concentrated and adsorbed onto flash silica. Purification by flash chromatography (10 cm column), eluting with 1:1 dichloromethane: hexanes, afforded the desired compound as a lustrous solid (416 mg, 100%), m.p. 77.5–78.5 °C. 1H NMR (CDCl3, 400 MHz): d 3.41 (4H, t, 3JHH = 6.9 Hz, 2 CH2Br), 1.85 (4H, m, 2 CH2CH2Br), 1.42 (4H, br m, 2 CH2(CH2)2Br), 1.29–1.25 (20H, m, 18 CH2) ppm. 13C{1H} NMR (CDCl3, 100 MHz): d 34.1, 32.8, 29.7 (5 resonances), 29.6, 29.5(3), 29.5, 29.4, 28.8, 28.2 ppm. 2.3.7. 1,10 -(dodecane-1,12-diyl)bis(3-methyl-1H-imidazol-3-ium) bromide 1 A mixture of 1,12-dibromododecane (10.0 g, 30.5 mmol) and 1-methylimidazole (37.5 g, 457 mmol) was heated at 60 °C under nitrogen for 24 h. The resulting orange oil was then triturated with ether (3 100 mL) and then dried in vacuo. The resulting oil was then dissolved in water (50 mL) heated at reflux in the presence of activated charcoal overnight. The solution was cooled and filtered through Celite and the solvent removed. The product was dried in vacuo at 70 °C overnight to afford a viscous pale yellow oil that crystallized on standing (14.9 g, 99%), m.p. 47–49 °C. mmax (NaCl)/cm1: 3427, 3143, 3078, 2926, 2853, 1628, 1570, 1462, 1427, 1380, 1336, 1168, 847, 761, 653, 622. 1H NMR (CDCl3 + 3 drops MeOD, 200 MHz): d 9.79 (2H, s, 2 Im2H), 7.51–7.47 (4H, m, 2 Im4H and 2 Im5H), 4.29 (4H, t, 3JHH = 7.4 Hz, 2 NCH2), 4.01 (6H, s, 2 NCH3), 1.91 (4H, m, 2 NCH2CH2), 1.33–1.26 (16H, m, 8 CH2) ppm. 13C{1H} NMR (MeOD, 75 MHz): d 137.8, 124.9, 123.6, 50.8, 36.7, 31.1, 30.5, 30.4, 30.0, 27.2 ppm. m/z (ESI, positive ion): 411 ([MBr]+, 49%), 331 ([MHBr2]+, 14%), 249 ([MC4H7N2Br2]+, 13%), 166 (M2+, 100%). HRMS (ESI, positive ion): Found 413.2097, [C20H36N4(81Br)]+ requires 413.2097.

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2.3.8. 1,10 -(hexadecdecane-1,16-diyl)bis(3-methyl-1H-imidazol-3ium) bromide 2 1,10 -(Hexadecdecane-1,16-diyl)bis(3-methyl-1H-imidazol-3ium) bromide 2 is prepared using the method described for compound 1, using 6 on a 3.90 mmol scale. The crude product was recrystallised from methanol in acetone (ca. 2% v/v) to afford a white crystalline powder (1.88 g, 88%), m.p. 77–78 °C. mmax (KBr)/ cm1: 3431, 3226, 3146, 3081, 3051, 2918, 2850, 2078, 1615, 1576, 1471, 1330, 1174, 1109, 846, 760, 718, 623, 557. 1H NMR (CDCl3 + 3 drops MeOD, 300 MHz): d 9.51 (2H, s, 2 Im2H), 7.40 (2H, m, 2 Im4H or Im5H), 7.34 (2H, m, 2 Im5H or Im4H), 4.18 (4H, t, 3JHH = 7.4 Hz, 2 NCH2), 3.96 (6H, s, 2 NCH3), 1.81 (4H, m, 2 NCH2CH2), 1.44–1.16 (24H, m, 12 CH2) ppm. 13C{1H} NMR (CDCl3 + 3 drops MeOD, 75 MHz): d 136.6, 123.5, 121.9, 49.9, 36.3, 30.0, 29.2 (2 resonances), 29.1(4), 29.0(6), 28.7, 26.0 ppm. m/z (ESI, positive ion): 467 ([MBr]+, 44%), 387 ([MHBr2]+, 7%), 194 (M2+, 100%). HRMS (ESI, positive ion): Found 469.2722, [C24H44N4(81Br)]+ requires 469.2723. Anal. Calcd. for C24H44N4Br2MeOH: %C 51.7; %H 8.3; %N 9.6. Found: %C 51.6; %H 8.6; %N 9.9. 2.3.9. 1,10 -(tetracosane-1,24-diyl)bis(3-methyl-1H-imidazol-3-ium) bromide 3 1,10 -(tetracosane-1,24-diyl)bis(3-methyl-1H-imidazol-3-ium) bromide 3 is prepared using the method described for compound 1, using 10 on a 0.85 mmol scale. The crude product was recrystallised from methanol in acetone (ca. 2% v/v) to afford a white crystalline powder (0.55 g, 98%), m.p. 104–105 °C. mmax (KBr)/cm1: 3480, 3429, 3387, 3227, 3148, 3080, 3052, 2917, 2850, 2076, 1673, 1615, 1576, 1472, 1330, 1174, 1109, 849, 789, 761, 71, 623, 566. 1H NMR (CDCl3 + 3 drops MeOD, 400 MHz): d 9.54 (2H, s, 2 Im2H), 7.51 (2H, s, 2 Im4H or Im5H), 7.44 (2H, s, 2 Im5H or Im4H), 4.26 (4H, t, 3JHH = 7.4 Hz, 2 NCH2), 4.04 (6H, s, 2 NCH3), 1.90 (4H, m, 2 NCH2CH2), 1.34 (4H, m, 2 N(CH2)2CH2), 1.29–1.15 (36H, m, 18 CH2) ppm. (CDCl3 + 3 drops MeOD, 100 MHz): d 136.4, 123.5, 121.9, 49.9, 36.2, 29.9, 29.4 (5 resonances), 29.3, 29.2, 29.1, 28.7, 26.0 ppm. m/z (ESI, positive ion): 579 ([MBr]+, 66%), 250 (M2+, 100%). HRMS (ESI, positive ion): Found 581.3990, [C32H60N4Br(81Br)]+ requires 581.3975. Anal. Calcd. for C32H60N4Br21.5H2O: %C 55.9; %H 9.2; %N 8.2. Found: %C 55.9; %H 9.4; %N 8.3. 2.4. General procedures for silica syntheses 2.4.1. 2 h, 80 °C A solution of 15 mM aqueous sodium hydroxide (100 mL) and bolaform surfactant (1, 6.70 g; 2, 3.68 g; 3, 181 mg) were heated to 80 °C (reflux condenser), with stirring, under ambient atmosphere. Tetraethoxysilane (1.045 mL, 4.68 mmol) was added dropwise and the reaction was stirred at 700 rpm for 2 h. The reaction was allowed to cool to room temperature and the pH adjusted to 6–7 with several drops of 3 M aqueous hydrobromic acid. The suspension was centrifuged, the silica isolated and resuspended in water (2 30 mL) then ethanol (30 mL). For template recovery the washings were combined with the supernantant and concentrated to dryness, taken up in 5% v/v methanol in dichloromethane, filtered through a 0.45 lm syringe filter and concentrated. Decolorization with charcoal in water was performed on samples before reuse. The isolated silica was dried at 60 °C, then calcined at 550 °C for 4 h (ramp rate 5 °C/min), affording silicas (200–250 mg), denoted 1–2 h-80 °C, 2–2 h-80 °C, and 3–2 h-80 °C. 2.4.2. 72 h, 100 °C and hydrothermal treatment The reaction was performed as described above. After 2 h at 80 °C, the reaction was split into 2 portions and either stirred at 100 rpm for 72 h at 100 °C, or was transferred (45 mL) to a 50 mL

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The general strategy for the preparation of 1–3 involved double SN2 reaction of long-chain a,x-dibromoalkanes, with concomitant salt formation, in the presence of excess N-methylimidazole (see experimental section). The a,x-dibromoalkanes themselves were either commercially available (C12) or synthesized from readily available starting materials. The high cost of 1,16-dibromohexadecane (6) prompted the synthesis of this material using a simple 3step sequence, starting from x-6-hexadecenelactone (Scheme 1). Reductive opening of the macrolactone using lithium aluminiumhydride, followed by palladium-catalyzed hydrogenation of the double-bond to afford the 1,16-diol (5) were achieved cleanly and efficiently using standard conditions. Conversion of diol 5 to dibromoalkane 6, using excess hydrobromic acid, was achieved in good yield using the conditions of Moss and co-workers [74] which were easy to perform and readily scalable. The above sequence thus permitted synthetically useful amounts of 6 to be prepared from readily available, inexpensive starting materials. A Grignard fragment-coupling method was used to prepare 1,24-dibromotetracosane (10). Mohr and co-workers previously prepared this material using a copper-catalyzed cross-coupling of two C9-organomagnesium bromides to a C6-dibromoalkane coupling-partner [77]. Alternatively, we chose the method of Gardner and co-workers to dimerize a C12-organomagnesium bromide using stoichiometric silver bromide [78] (Scheme 2). Monobromination [75] and protection [76] of 1,12-dodecanol were achieved using literature syntheses and allowed the preparation of ample amounts of coupling precursor 8. For the dimerization reaction, a catalytic amount of iodine was used for in situ activation of excess magnesium followed by addition of the starting material (7). The C12-organomagnesium intermediate was then reacted with a slurry of excess silver bromide in tetrahydrofuran. Although only moderately yielding, enough of the desired product (9) could be obtained for further elaboration to the C24-dibromide (10) directly, using the method of Mioskowski and co-workers [79]. With compounds 1–3 in hand, attention turned to their relevant surfactant properties. For this purpose the CMCs of 1–3 were measured, and flooding experiments to determine mesophase behavior were performed. Subsequent silica syntheses were carried out to test the capability of 1–3 for templating. Successful silica templating required that reactions were conducted at surfactant concentrations suitable to yield high surface area silicas. Initial explorations conducted, led to four times the measured CMCs for 1–3 being adopted as standard. Conductometry studies of all three surfactants were performed in distilled water at 25 °C and the change in slope [80] of the specific conductivity vs. surfactant

concentration was used to determine the CMCs (shown in Table 1). Consistent with the trends reported for single-headed surfactants, increasing the hydrophobic domain (1 < 2 < 3) also led to a decrease in the CMC, reflecting the increased entropy cost of solvating longer hydrocarbon chains by water [81]. It has been shown by Vanyúr and co-workers, that classical micelle formation and behavior are observed in cationic surfactants ([CnMIM]Br) containing a methylimidazolium head-group and a single hydrocarbon tail of at least nine carbons (Table 1). [65] The two-headed compounds 1–3 display much higher CMC values than the corresponding single-headed methylimidazolium surfactants, principally due to the additional hydrophilic head-group which increases their relative solubilities in water. A similar trend has been observed for the bis(trialkylammonium) series [67,82]. For example, the introduction of two head-groups to a C16-alkyl chain resulted in bolaform surfactant 2, halving the CMC compared with 1, but increasing the CMC by more than 25 times, compared with [C16MIM]Br, which contains only one head-group. The latter indicates a significant decrease in the relative thermodynamic stabilization of micelle formation. The C24-bolaamphiphile 3 was observed to have a CMC very similar to [C16MIM]Br (i.e. a singleheaded surfactant with 8 fewer methylenes). As 3 contains a 24 carbon linker, it can be expected to adopt a U-shape conformation within a micelle, as was shown for the bis(trimethylammonium) bromide surfactants containing a linker of at least 22 methylenes [70]. Furthermore, 3 will have a lower aggregation number per micelle and looser packing than [C16MIM]Br, for the reasons outlined in the introduction. It is expected that under similar conditions, the micelles formed from 3 will possess a smaller diameter than those formed from [C16MIM]Br. Having determined the minimum concentrations required for micelle formation, the effect of increasing surfactant concentration on mesophase structure was focused upon. Further analysis of surfactant behavior was performed by means of flooding experiments, monitored by polarized-light microscopy [83]. For this purpose, a 15 mM aqueous sodium hydroxide solution was used in order to observe the types of mesophases formed under a concentration gradient (snapshots are shown in Fig. 2) as a first indication of how the surfactant might behave. As these flooding experiments involved all the reagents used for the silica syntheses except for TEOS, there may be significant differences in the surfactant aggregation behavior under the actual reaction conditions. At 25 °C surfactant 1 was so highly soluble that the birefringent hexagonal phase (H1) could only be observed at very high concentrations, proximal to undissolved solid material. At lower concentrations a micellar solution (L1) formed (Fig. 2a). Surfactant 2 (Fig. 2b) was also highly soluble in the flooding solution at 25 °C. However, the formation of an H1-phase was readily apparent and persisted between undissolved crystals and the surrounding L1-phase. Due to the relative insolubility of 3 at 25 °C in the flooding solution, a heating stage was used. A disordered H1-phase gradually formed at 45 °C (Fig. 2c) which took on the typical ‘‘fan-like’’ appearance [83] by 50 °C and persisted at 60 °C (Fig. 2d), the size of each domain increasing with increasing temperature. Although all three surfactants had the propensity to assemble into hexagonal mesophases, a distinct dependency on the

Scheme 1.

Scheme 2.

PTFE-lined steel autoclave and heated to 100 °C over 72 h without stirring. Work-up and isolation was performed as described above, affording silicas (80–100 mg), denoted 1–72 h-100 °C, 2–72 h100 °C, 3–72 h-100 °C; or 1–72 h-100 °C-HT, 2–72 h-100 °C-HT, 3–72 h-100 °C-HT. 3. Results and discussion 3.1. Template preparation and behavior

A.K.L. Yuen et al. / Microporous and Mesoporous Materials 148 (2012) 62–72 Table 1 Chain-length and CMC values of surfactants 1–3 vs. classical imidazolium bromide surfactants.

a

Compound

Surfactant

NC

CMCa(mM)

1 2 3 – – – –

[MIM2C12]Br2 [MIM2C16]Br2 [MIM2C24]Br2 [C10MIM]Br [C12MIM]Br [C14MIM]Br [C16MIM]Br

12 16 24 10 12 14 16

35 17 0.63 41 9.8 2.5 0.61

1–3 This work, all others from Vanyúr and co-workers [65].

chain-length was observed during the flooding experiments. The longer the hydrophobic linker, the more pronounced the resulting hexagonal phase, under the chosen conditions: being stable over a greater temperature range and at lower concentration. Such behavior indicated that under these conditions, hexagonally-ordered porous materials could be formed when aqueous solutions of these surfactants were used as templates for silica polymerization.

3.2. Silica characterization The influence of the surfactants 1–3 on the morphology of silica prepared from the hydrolysis of TEOS under basic conditions was

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investigated using three different sets of conditions for each surfactant. These different conditions were chosen so as to assess the effect of reaction time and hydrothermal conditions on the ordering of the silicas produced. Hydrolyses of TEOS were carried out at surfactant concentrations 4-fold that of the determined CMCs, to ensure that micellar solutions were present, for 2 h and 72 h at ambient atmosphere or 72 h under autogenous pressure. None of the silicas produced by the conditions described here formed stable colloidal dispersions, although distinct nanoparticle boundaries could be observed by TEM, as discussed below. In Fig. 3 the SAXS patterns for all the silica samples, denoted by synthetic conditions, using templates 1, 2 and 3 are shown. All the patterns exhibit at least one prominent reflection and their intensities and resolutions increase with increasing linker length (1 < 2 < 3). The effect of the synthetic conditions on the ordering of the material could not be determined using the SAXS data: within each set of experiments, there was no obvious difference in the intensities of the main reflections. When template 3 was used, two additional reflections were observable in the material resulting from the hydrothermal approach (Fig. 3c). These reflections can be indexed as (1 0 0), (1 1 0) and (2 0 0): corresponding to a 2D hexagonal pore arrangement (p6mm) with a lattice constant of 40 Å. SAXS patterns from uncalcined silica samples (Fig. 3d), prepared with 3, reveal additional reflections (unable to be indexed) for all samples that are independent of which method is used. The disappearance of the minor reflections after calcination for samples 3–2 h-80 °C and 3–72 h-100 °C indicates a less condensed and cross-linked

Fig. 2. Polarized-light microscopy at 5 magnification: (a) 1 at 25 °C, (b) 2 at 25 °C, (c) 3 at 45 °C, (d) 3 at 60 °C. Direction of flooding from right to left for all.

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framework structure than that for 3–72 h-100 °C-HT. The d100 values corresponding to the most prominent XRD reflections for all the calcined samples are listed in Table 2. The d100 values of the samples studied are wide-ranging: from around 21–23 Å for silicas templated by 1; to 26 Å for those by 2; to 33–35 Å for those by 3, and are consistent with the increase in the length of the alkyl linker in 1–3. In contrast to the lack of variation in the intensities of the (1 0 0) reflections within each set of experiments, small shifts in reflectance, indicating slight increases in the d100 values, are observable, resulting from increasing temperature and pressure during the syntheses. The d100 value for 1-templated material is remarkable, being amongst the smallest of those reported for surfactanttemplated silica [40]. In general the d100 values are in good agreement with other published porous silicas on the border between super-microporous and mesoporous materials [32,40,46]. As a positive control, silica templated with single-headed surfactant [C16MIM]Br, was prepared using the method of Trewyn and coworkers [29]. XRD analysis of this sample (not shown), indicated that the d100 value was 13 Å larger than that for silica templated with 2. This result is consistent with the arguments presented in the introduction concerning the differences in micelle size between bolaamphiphiles and single-headed surfactants containing the

same hydrophobic domain. As expected, even the d100 values for silicas templated by 3, containing 24 methylenes, were smaller than those prepared using [C16MIM]Br. Fig. 4 shows representative TEM images of the resulting pore structures when 1 and 2 were used as templates. It is apparent that the pore structures formed correspond to typical ‘‘worm-like’’ morphologies with small pore sizes. The pore size for samples prepared using 1 was estimated to be 10 Å, whereas the pore size for samples prepared with 2 was approximately 15 Å. TEM images of samples prepared using 3 as a template, with three reaction conditions, are displayed in Fig. 5. In contrast to the TEM images of materials templated by 1 and 2, there is a remarkable increase in the quality of the pore arrangement. This result is in good agreement with the previously discussed SAXS data, which revealed a higher degree of pore ordering for all samples templated with 3. For the sample 3–2 h-80 °C (Fig. 5a) mainly spherical, highly porous nanoparticles were observed. In the case of sample 3–72 h-100 °C (Fig. 5b) there is a tendency towards larger, less spherical particles. In both cases the samples exhibit pores radiating from the center of these particles, a feature also found in spherical, mesoporous silica, classified as MCM-41-type material.[84] The TEM images of sample 3–72 h-100 °C-HT (Fig. 5c

Fig. 3. SAXS patterns of calcined silica samples prepared with (a) 1, (b) 2, (c) 3, (d) uncalcined silica prepared with 3.

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A.K.L. Yuen et al. / Microporous and Mesoporous Materials 148 (2012) 62–72 Table 2 Structural parameters of calcined silica samples prepared using templates 1–3 and different preparation conditions. Surfactant

SBET [m2 g1]

St-plot [m2 g1]

VP [cm3 g1]

ads. DBJH [Å]

DKJ [Å]

DDFT [Å]

d100 [Å]

1–2 h-80 °C 1–72 h-100 °C 1–72 h-100 °C-HT 2–2 h-80 °C 2–72 h-100 °C 2–72 h-100 °C-HT 3–2 h-80 °C 3–72 h-100 °C 3–72 h-100 °C-HT

549 604 560 760 832 731 963 1047 1001

477 530 485 597 639 517 N/A N/A N/A

0.26 0.28 0.28 0.39 0.38 0.39 0.42 0.63 0.63

N/A N/A N/A 8.1 9.2 9.3 13.1 16.9 19.6

15.4 16.1 17.4 21.8 21.3 22.1 27.7 30.9 32.1

15.2 15.1 15.6 17.6 18.4 18.4 23.9 29.9 33.2

21.1 21.5 23.3 26.5 26.0 26.8 32.9 33.4 34.7

and d) reveal a more bulky textural character and only small particles (<50 nm), other than the bulk material were detected. For both samples 3-72 h-100 °C as well as 3–72 h-100 °C-HT, the trend towards bulk material can be explained by Ostwald ripening. Despite the bulk character of the material, radiating pore channels from several centers (formerly individual nanoparticles) were seen in sample 3-72 h-100 °C-HT. In terms of pore arrangement, domains close to the rim of the bulk material were found approaching the typical hexagonal pore structure with parallel channels. This observation verifies the SAXS results, which suggests an ordered hexagonal pore arrangement, based on three resolved reflections. Pore sizes were determined to be 20–22 Å for this sample by analysis of the TEM images. Similar analysis of samples 3–2 h-80 °C and 3–72 h-100 °C, gave pore sizes in the range of 14 Å and 17 Å respectively. Nitrogen sorption measurements for all samples are presented in Fig. 6 and the calculated structural parameters are listed in Table 2. In the case of silica templated with 1, the associated isotherms can be described as standard Type I curves, in which the nitrogen adsorption isotherm levels off at a very low relative pressure of <0.1, characteristic of the presence of micropores. When 3 was used as template however, the nitrogen sorption isotherms obtained exhibited the shape of Type IV curves, typical of mesoporous material. There is a noticeable difference in the shape of the isotherms for the samples 3–72 h-100 °C and 3–72 h-100 °C-HT compared to sample 3–2 h-80 °C, attributed to filling of pores with a relatively narrow size-distribution. The isotherms for the samples where 2 was used as a template, show a transitional curve type between standard Type I and IV, which indicates pores smaller than 20 Å. Such isotherms have been reported for other super-microporous materials [40]. Only the samples 1–72 h-100 °C-HT and 2– 72 h-100 °C-HT show H4-type hysteresis loops, occurring in the relative pressure range of 0.4 to 0.9, characteristic for cavitation

or pore-blocking [85]. As a general trend, it is observed that the specific surface areas SBET and the pore volumes VP (Table 2) increase with increasing surfactant chain length from: 500– 600 m2 g1 and 0.26 cm3 g1 for 1-templated silicas; to 700–850 and 0.38 cm3 g1for 2-templated silicas; to 900–1050 and 0.62 cm3 g1 for 3-templated silicas. Interestingly, use of the 72 h-100 °C method, under ambient pressure, resulted in samples with the highest SBET generally, but not the highest pore ordering (see above). The micropore surface areas St-plot were determined, where possible, by means of the t-plot curves and are summarized in Table 2. These areas increase only slightly from 450–530 to 500– 640 m2 g1 with increasing chain length. Pore sizes were calculated from nitrogen adsorption data using the BJH method DBJH, a geometrical model DKJ, and non-local density functional theory (NLDFT) DDFT (Table 2). Due to the microporous nature of samples prepared using 1, it was not possible to apply the BJH method for calculating the pore size-distribution. The authors are aware that even for samples prepared with 2 and with 3, usage of the BJH method is questionable due to its well-known limitations for pores smaller than 25 Å [40,41,72]. Nevertheless, the BJH model was applied in order to compare the results herein with the published results of others. As can be seen from Table 2 and Fig. 7, the DBJH are approximately 9 Å for silicas prepared using 2 and vary from 13 to 20 Å for samples synthesized using 3. These pore sizes indicate that the samples synthesized with 2 are microporous and the samples prepared with 3 are supermicroporous: bordering on mesoporous. The BJH method is well-known for underestimating the pore size-distribution for small pores and hence other methods were applied for comparison. The pore sizes DKJ, calculated from the geometrical model introduced by Kruk and co-workers, are 15–17 Å when using 1, 21–22 Å when using 2 and 27–32 Å when using 3. These values are approximately 10 Å larger than the pore sizes

Fig. 4. TEM images of samples (a) 1–72 h-100 °C and (b) 2–72 h-100 °C-HT showing the ‘‘worm-like’’ pore structure.

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Fig. 5. TEM images of samples (a) 3–2 h-80 °C, (b) 3–72 h-100 °C, (c) and (d) 3–72 h-100 °C-HT showing the ordered pore structure.

determined using the BJH model. A deviation of 10 Å between the two models for porous materials in super-microporous to mesoporous regions has been previously reported [32,41]. Possible explanations for the overestimation of pore size by the geometrical model are that it relies on the density of bulk silica for the estimate of the sample density, and the assumption of a perfectly ordered array of uniform pores. Slightly smaller pore sizes were calculated using the NLDFT method, which assumes cylindrical pores. In this case, DDFT 15 Å for 1-, 17–18 Å for 2-, and 24–33 Å for 3-templated silicas were obtained. The following trend is observed for the calculation of pore sizes: DBJH < DTEM < DDFT < DKJ. Taking into account the well-known limitations of the BJH and geometrical methods however, the DTEM and DDFT values are considered the most reliable. Nevertheless the agreement between these two methods is better for (super-) micropores than for mesopores: 5 Å difference for 1-templated silicas; 3 Å difference for 2-templated silicas, compared with 610 Å difference for 3-templated silicas. In the case of silicas prepared with 3 the pore size is actually in the range where the BJH method reliable and thus there is good agreement between the values DTEM 14–22 Å, and DBJH 14–20 Å. Based on the results discussed above, it is apparent that each of the bolaform surfactants (1–3) displayed individual templating behavior, resulting in silicas with different morphologies and pore sizes. Compound 1, the bolaamphiphile with the shortest hydrophobic linker, affords only low surface area silicas with ‘‘worm-like’’ micropores. Due to the fact that 1 contains only a 12-carbon hydrophobic domain, resulting in a high CMC, it is not certain if it behaves as a classical surfactant template, since only small micelles or loose aggregates are formed in solution. This outcome is in agreement with the flooding experiments,

which revealed that the H1-mesophase formed by 1 was very sensitive to concentration and temperature. The surfactant characteristics observed for 2 were reflected in the structural parameters of the templated silica. XRD analysis of the silicas obtained using 2, revealed slightly better pore ordering than that obtained with 1. In comparison with the results of Trewyn and co-workers, who used single-headed [C16MIM]Br as a template, the BET surface area was some 100 m2 g1 lower and the BJH pore size was more than 20 Å smaller [29]. Increasing the alkyl spacer to 24 methylene units led to 3, with a CMC determined to be very similar to [C16MIM]Br (Table 1). The two surfactants result in silicas with different morphologies however, with radially orientated pores, smaller pore sizes, and >100 m2 g1 higher surface areas for those templated by 3 compared with [C16MIM]Br. The above investigations indicate that although it is possible to prepare supermicroporous materials using bis(imidazolium)-based templates, small pore sizes result at the expense of pore ordering. Increasing the length of the alkyl linker not only increases the size of the micelle, and hence the pores formed, but also increases the ordering of these pores overall. It is expected that bolaamphiphiles of the above type, with spacer lengths longer than 24 carbons, will exhibit higher-ordered materials well within the mesoporous range. 4. Conclusions New bolaamphiphile surfactants 1, 2 and 3, possessing N-methylimidazolium head-groups and alkane spacers consisting of 12, 16 or 24 methylene groups, respectively, have been successfully prepared and characterized. Both the CMC values and the propensity

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Fig. 7. Pore size-distribution data silica templated with (a) 2 and (b) 3 calculated with the BJH method from the adsorption branch of the nitrogen sorption measurements at 77 K.

Fig. 6. Nitrogen sorption measurements at 77 K for the calcined samples templated with (a) 1, (b) 2, (c) 3.

and 3 at concentrations four times their measured CMC values were used as templates for the formation of silicas. The materials formed when templated with 1 and 2, were super-microporous or bordering mesoporous respectively. These materials showed no extended order regardless of the synthetic conditions employed in this study. In the case of templating with 3, the nanoparticulate silica obtained was mesoporous, with the pore size increasing with longer reaction time and higher pressure. Silica prepared under hydrothermal conditions, using 3, was more ordered than the other samples and unambiguously assigned to a 2D hexagonal structure with p6mm symmetry. The surface areas of the silicas obtained increased with increasing chain length of the template: from 500– 600, to 700–850, to 900–1050 m2 g1 when 1, 2, or 3 were used respectively. The effect of lengthening the alkyl spacer, further study into the influence this class of surfactants has on the morphologies of siliceous materials, and potential applications for size-dependent catalysis, will be reported in due course.

Acknowledgment for hexagonal mesophase formation, observed during flooding experiments, were determined. In the case of 1 and 2, hexagonal mesophases were only observed at high concentrations at room temperature, whereas for 3 a hexagonal mesophase was observed over larger concentration and temperature ranges. Solutions of 1, 2

The authors thank the Australian Research Council for funding. Dr. Paul Fitzgerald is gratefully acknowledged for assistance with flooding experiments and discussions regarding surfactant chemistry.

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