Room-Temperature Ionic Liquids as Template to Monolithic

as template to monolithic mesoporous silica with wormhole framework via a convenient nanocasting technique. In contrast with the applied liquid crysta...
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NANO LETTERS

Room-Temperature Ionic Liquids as Template to Monolithic Mesoporous Silica with Wormlike Pores via a Sol−Gel Nanocasting Technique

2004 Vol. 4, No. 3 477-481

Yong Zhou,*,† Jan H. Schattka, and Markus Antonietti Max-Planck Institute of Colloids and Interfaces, Research Campus, Golm, D-14424 Potsdam, Germany Received October 25, 2002

ABSTRACT This paper reports a novel room-temperature ionic liquid (RTIL), [C4mim]+ BF4-, as template to monolithic mesoporous silica with wormhole framework via a convenient nanocasting technique. In contrast with the applied liquid crystal self-assembly of long-chain surfactants on the preparation of mesoporous nanostructures, a new so-called hydrogen bond-co-π−π stack mechanism was proposed to be responsible for the present self-assembly of the RTIL in the reaction system for the formation of the wormlike mesopore, in which both the hydrogen bonds formed between the [BF4]- and silano group of silica gel and the π−π stack interaction of the neighboring imidazolium rings play crucial roles in the formation of the wormhole framework of mesporous silica. The proposed hydrogen bond-co-π−π stack mechanism with the RTIL as template may open a new pathway to prepare mesoporous materials.

As pore-upper extension of microporous crystalline compounds (e.g., zeolites and aluminum phosphates of pore size up to 1.2 nm), mesoporous materials have attracted much attention of chemists and material scientists for a long time due to commercial interest in their application in chemical separations and heterogeneous catalysis and due to scientific interest in the challenges posed by their synthesis, processing, and characterization.1 Stimulated by the exciting discovery of the M41S family of mesoporous silica in 1992,2 there has been growing interest over the past few years in the use of surfactants and block polymers as structure-directing agents for the production of mesoporous materials. The mesoporous materials of the classic M41S family are generally obtained using surfactants such as primary amines and quaternary ammonium ions.3 These products were formed via the attractive electrostatic interaction between a ceramic precursor and the surfactant, upon which the surfactant-rich gel phase precipitates from a heterogeneous reaction mixture in the form of small micron-scale powder.4 The mesoporous materials thus produced exhibit potential for application in shape-selective heterogeneous catalysis, homogeneous catalysis, biosensors, chromatographic separations, and microelectronic and optoelectronic technologies.5 * Corresponding author. E-mail: [email protected]. Tel: (+49) 331 567 9515. Fax: (+49) 331 567 9502. † Current address: Advanced Materials Laboratory, National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan. E-mail: [email protected]. 10.1021/nl025861f CCC: $27.50 Published on Web 02/12/2004

© 2004 American Chemical Society

In recent years, our group has contributed much effort on exploiting diversities of both lyotropic liquid-crystalline phase, derived from surfactants or later-amphiphilic block copolymers,6-8 and some relatively small organic molecules such as cyclodextrins (CDs),9,10 as templates to prepare the mesoporous silica of wormlike structure via a nanocasting technique. The nanocasting technique is an efficiently highconcentration way to prepare monolithic porous materials, first introduced by Attard, Glyde, and Goltner et al.11 It is based on the premise that hydrolysis and condensation of inorganic precursors are strictly limited to take place in an aqueous domain of the microphase-separated medium, derived from the self-assembly phase of the template used. As a result, the solidified inorganic compound is thought to be a replication of the original phase structure, predetermined by selection of the template phase. Due to the solidification of the inorganic precursor in a homogeneous bulk liquid crystal, this nanocasting method allows the preparation of large monoliths of porous materials. In this letter, we report a novel room-temperature ionic liquid (RTIL) as template to monolithic mesoporous silica with wormhole framework via the nanocasting technique. RTILs are salts with melting points of below ca. 100 °C, and sometimes as low as -96 °C.12 They possess a wide liquidus, in some cases in excess of 400 °C. The first finding of an RTIL with a melting point of 12 °C was reported in 1914.13 Most of the investigated RTILs consist of the organic

1-alkyl-3-methylimidazolium (abbreviated [Cnmim]+, where n ) number of carbon atoms in a linear alkyl chain). The cations are combined with either organic or inorganic anions. RTILs are a particularly unique class of solvents that have virtually negligible vapor pressure and possess versatile solvent and thermal stability properties,14 widely used as environmentally friendly organic solvent alternatives for catalysis, electrochemistry, liquid/liquid extractions, and organic liquid-phase reactions under conventional conditions. Herein we choose 1-butyl-3-methyl-imidazolium-tetrafluoroborate, [C4mim]+ BF4- (1), as template, by which a monolithic mesoporous silica with wormhole framework can be prepared. The chemical structure of 1 is shown schematically below. It is obviously that 1, a short-chain organic molecular, is not supposed to preferentially self-assemble into an ordered micelle structure or liquid crystalline phase by the rearrangement of the hydrophobic and hydrophilic molecular chains in solution, such as long-chain alkylammonium surfactants and amphiphilic block copolymers. Based on the special molecular structure and properties of 1, we propose a novel so-called hydrogen bond-co-π-π stack mechanism responsible for the self-assembly of 1 in the reaction system for the formation of the wormlike mesopore, in which both the hydrogen bond formed between the [BF4]and silano group of silica gel and the π-π stack interaction of the neighboring imidazolium rings play crucial roles in the formation of the wormhole framework of mesporous silica. The proposed hydrogen bond-co-π-π stack mechanism with the RTIL as template may open a new pathway to prepare mesoporous materials.

In a typical synthetic procedure of the mesoporous silica with 1 as template via the nanocasting technique, tetramethyl orthosilicate (TMOS) was used as the sol-gel precursor. A 0.6 mL sample of 1 was mixed with 0.5 mL of TMOS under mild magnetic stirring. After homogenization of the mixture solution occurred, to the solution was added dropwise 0.25 mL of aqueous solution of 0.01 M HCl as an acid catalyst. The resultant mixture solution was stirred at room temperature for 2 h, allowing the sol-gel reaction of the silica, followed by exposure under vacuum at 60 °C for removal of the formed methanol during hydrolysis of TMOS. The complete gelation process was accomplished by standing the sample in an open flask at 60 °C for 48 h. A transparent 1-displaying pale yellow silica monolith was obtained with no visible cracks. The transparency of the hybrid materials strongly suggests the homogeneity of 1 in the silica matrix and no phase separation occurred between the RTIL and silica. The incorporated 1 was removed from the silica by extracting the sample with acetonitrile in an autoclave at 90 °C. The extraction process was repeated several times with the fresh solvent until complete removal of the RTIL from the silica, determined with the Fourier transform infrared (FTIR) spectrum of the hybrid materials, in which imidazolium ν(C-H) stretching region (3200-3000 cm-1) of the RTIL 478

Figure 1. TEM image of the pore morphology and structure of the synthesized 1-templated mesoporous silica with the nanocasting technique.

disappears completely.18 The final product was ground into powders for further characterization. The FT-IR spectrum of the sample was recorded at room temperature with a Bio-Rad 6000 FT-IR spectrometer equipped with a single reflection diamond ATR. The transmission electron microscopy (TEM) image was acquired on a Zeiss EM 912 Ω at an acceleration voltage of 120 kV. The small-angle X-ray scattering (SAXS) curve was recorded on a rotating-anode instrument with pinhole collimation. A Nonius rotating anode device (P ) 4 kW, Cu KR) and an image-plate detector system were used. With the image plates placed at a distance of 40 cm from the sample, a scattering vector ranging from s ) 0.05 to 1.0 nm-1 was available [s ) (2 sin θ)/λ; 2θ scattering angle, λ ) 0.154 18 nm]. Nitrogen sorption data was obtained with a Micromeritics Tristar 3000 automated gas adsorption analyzer. Before sorption measurement, the sample was degassed in a Micromeritics VacPrep061 degasser overnight at 150 °C under 100 µTorr pressure. Figure 1 shows the TEM image of the pore morphology and structure of the synthesized 1-templated mesoporous silica with the nanocasting technique. The micrograph demonstrates clearly that the mesoporous structure consists of regularly bicontinuous wormlike mesopore of ca. 2.5 nm in diameter and silica wall system of ca. 2.5-3.1 nm in thickness, respectively. The formed mesopore is of narrow size distribution and no large pore is observed in the image, indicating the homogeneity of 1 in the silica matrix and no phase separation when the sol-gel process of silica took place. The homogeneous distribution of the pores in the matrix is the basis of the high mechanical stability of the materials, an essential prerequisite for the use of such systems as long-lasting catalyst supports or solid inorganic membrane systems. The bulk structure of the mesoporous silica prepared with 1 as template via the nanocasting technique was characterized Nano Lett., Vol. 4, No. 3, 2004

Figure 2. Small-angle X-ray scattering (SAXS) pattern of the synthesized 1-templated mesoporous silica with the nanocasting technique.

Figure 3. Typical N2 gas adsorption-desorption isotherm of 1-templated mesoporous silica with the nanocasting technique.

by small-angle X-ray scattering (SAXS). SAXS has turned out to be a powerful technique with which to investigate the structure of two-phase systems,15 (i.e., nanostructure materials that can be supposed to be composed of two domains of constant electron density). The SAXS diffractogram of the present sample is shown in Figure 2. It exhibits a significantly main peak at s ≈ 0.20 nm,-1 which corresponds to the packing of the wormlike pores into a certain distorted 3D alignment. The position of the peak is related to a length scale of approximately 5 nm, the averaged pore-to-pore distance, in good agreement with the TEM observation. Figure 3 shows a typical N2 gas adsorption-desorption isotherm of 1-templated mesoporous silica with the nanocasting technique. It is clear that the isotherm exhibits the typical IV curve, which is attributed to a predominantly mesoporous structure. The presence of a pronounced hysteresis loop in the isotherm curve is indicative of a 3D intersection network of porous structure.16 The product exhibits 801 m2/g of surface area by nitrogen BrunauerEmmett-Teller (BET) measurement and a pore volume of 1.27 cm3/g. Nano Lett., Vol. 4, No. 3, 2004

It has been proposed that the formation mechanism of the M41S family of mesoporous molecular sieves follows an electrostatic charge matching assembly pathway between the structure-directing alkylammonium ion surfactants (S+) and the framework-forming silicate oligomers (I-) present in the reaction solution, minimizing the electrostatic energy.3 The pore structures of these materials are largely determined by the structure and polarity of the organic groups. In addition to the S+I- pathway, the complementary S-I+ anionic surfactant and the assembly of surfactants and inorganic species bearing identical charge (i.e., S+ and I+ or S- and I-) via counterion-mediated binding S+X-I+ (where X- ) Cl- or Br-) or S-M+I- (where M+ ) Na+ or K+) have also been employed to produce the analogues materials. In addition to these electrostatic assembly strategies, nonelectrostatic assembly pathways were also utilized to produce the denoted HMS and MSU-X family of oxides with wormhole frameworks by using neutral amine and alkylpoly(ethylene oxide) surfactants as structure-directing agents, respectively. For both HMS and MSU-X silicas, the assembly processes involve the association of partially hydrolyzed silica precursors (I0) to the surfactant (S0 or N0) headgroups by hydrogen bonds, followed by their cross-linking around the micellar assembly. The formation mechanism of the present wormhole-like mesoporous silica with 1 as template via the nanocasting technique is intriguing. The reason is that 1 is a short-chain organic molecular, which is not supposed to preferentially self-assemble into an ordered micelle structure or LC phase by the rearrangement of the hydrophobic and hydrophilic molecular chains in solution, such as long-chain surfactants.17 Based on the special molecular structure and property of the RTIL, a new so-called hydrogen bond-co-π-π stack mechanism, responsible for the formation of the wormhole-like mesoporous silica with 1 as template via the nanocasting technique, is proposed. The schematic illustration of the mechanism is presented in Scheme 1. For the stabilization of such a structure, a combination of energy contributions has to be employed. Cammarata et al. has studied the molecular states of water in a series of RTILs.18 They found that water molecules dissolved in RTILs are present mostly in the “free” state (not self-associated into clusters or pools of water), interacting via hydrogen bonds with the anions of RTILs in a symmetric complex A-‚‚‚H-O-H‚‚‚A- (both protons of water bound to two discrete anions A-). The strength of the hydrogen bonds between water molecules and anions increases in the order [PF6]- < [SbF6]- < [BF4]- < [(CF3SO2)2N]- < [ClO4]- < [CF3SO3]- < [NO3]- < [CF3CO2]-. It has been also found that the imidazolium ring of 1 does not form hydrogen bonds with water. We believe that in the present case the [BF4]- anion should interact in a similar fashion with the silanol groups and form hydrogen bonds in the reaction system, which may induce the oriented arrangement of the [BF4]- anion along the pore walls, as shown in Scheme 1. Note that a structurally significant presence of water can be excluded by IR-measurements shown below. Along with the [BF4]-, [C4mim]+ will be also aligned and arrayed along the silica sol-gel phase, driven 479

Scheme 1. Schematic Illustration of the Proposed Hydrogen Bond-co-π-π Stack Mechanisma

a Note that for simplifying the schematic illustration, one silano group of silica is displayed to correspond to one [BF ]- for the formation 4 of hydrogen bonds.

by the coulomb coupling force with the anion. The fluid state of 1 is believed to facilitate the proposed relocation of the molecules, which is then stabilized by additional π-π interactions between the imidazolium rings of 1. The π-π stacking interactions between aromatic motifs are not unexpected to occur, as they influence and control many natural self-assembly and molecular recognition processes.19,20 A rather rigid, oriented stacking structure of cylindrical type of the RTIL is needed to act as the replication phase for the production of the wormlike mesopore. Since the extended molecular length of 1 is calculated as ca. 1.2 nm, the deduced 2.4 nm diameter of a hypothetical cylindrical array of the RTIL molecules is consistent with the 2.5 nm mesopore size observed with the TEM or found in the nitrogen sorption curve. The proposed possible π-π stack interaction of the imidazolium rings of 1 was further confirmed by IR experiments. Figure 4 shows a comparison of the IR spectra of the neat silica (a), neat 1 (b), and the 1-templated silica nanocomposite (c). It is believed that the change of peak position of the C-N stretching vibration (νC-N) mode of the imidazolium ring at 1000 cm-1 in spectrum (b) is an effective tool to examine the π-π stack of the imidazolium ring, which normally shifts toward low wavenumbers after stacking. Unfortunately, in the present case, it is not easy to observe the shift of the νC-N mode due to some overlap from the strong and broad Si-O stretching vibration (νSi-O) mode located around 1050 cm-1. Nevertheless, as shown in the spectra (b) and (c), the π-π stacking interaction of the imidazolium ring can still be scrutinized by the change of the C-H stretching vibration (νC-H) mode of the imidazolium ring at 3200-3050 cm-1. It can be seen that one of νC-H peaks in (b), marked with a solid circle, is found to be 480

Figure 4. FT-IR spectra of the neat silica (a), neat 1 (b), and 1-incorporated silica nanocomposite (c).

considerably broadened in (c), probably originating from the π-π stack interaction of the positive-charged imidazolium ring which decreases the electronic density of C-H bond of the ring, leading to the shift of the absorption band toward low wavenumbers. In addition, the νC-H mode of C-H of the alkyl chain of [C4mim]+ around 2960-2850 cm-1 in spectrum (b) is also found to be influenced due to the mutual packing. In comparing spectrum (b) with spectrum (c), it is clear that the νC-H band structure is changed. Moreover, a new band appears at about 3013 cm-1 in spectrum (c). The observed variation of the νC-H mode of the alkyl chain can be explained in that the π-π stacking interaction of the imidazolium ring may influence the nitrogen-alkyl carbon Nano Lett., Vol. 4, No. 3, 2004

bond, changing the νC-H mode. Importantly, it should be emphasized that three strong new peaks marked with asterisks in the spectrum (c) appear compared with the spectrum (b), additionally demonstrating the change of the IR spectrum due to the mutual packing of the imidazolium rings. The proposed possible hydrogen bond-co-π-π stack mechanism responsible for the formation of the wormlike framework of mesoporous silica will, however, need further support by NMR and UV-vis experiments. The described mechanism is also supported by another set of experiments that demonstrate that the anion of the RTIL has great influence on the pore structure of the produced mesoporous silica. Despite that (trifluoromethyl)sulfonyl amide, [(CF3SO2)2N]-, has a stronger hydrogenbonding interaction to the silica gel than does [BF4]-,18 the use of the RTIL with [(CF3SO2)2N]- as a template resulted in silica aerogels with wide pore size distribution only.21 The possible interpretation for the difference of the pore structures is that the [(CF3SO2)2N]- is much larger than [BF4]-, blocking the formation of π-π stacks of imidazolium rings and resulting in a disordered arrangement of the RTIL with [(CF3SO2)2N]- in the reaction system. In conclusion, the room-temperature ionic liquid (RTIL), [C4mim]+ BF4-, has been used as template to prepare monolithic mesoporous silica via the nanocasting technique. The formed bicontinuous wormlike mesopore system possesses a narrowly distributed pore diameter of ca. 2.5 nm, separated by silica walls of ca. 2.5-3.1 nm in thickness, which is well consistent with the determined averaged poreto-pore distance from the peak position of small-angle X-ray scattering. The typical N2 gas adsorption-desorption isotherm of the produced mesoporous silica exhibits the typical IV behavior, indicative for the mesoporous structure. A novel so-called hydrogen bond-co-π-π stack mechanism was proposed to be responsible for the self-assembly of the RTIL in the reaction system for the formation of the wormlike mesopore, in which both the hydrogen bonds formed between [BF4]- and the silica gel and π-π stack interaction of the neighboring imidazolium rings contribute to the mutual packing. As the presented self-assembly does not rely on amphiphilic interactions and the presence of water, it is expected that the proposed hydrogen bond-co-π-π stack mechanism with RTILs as templates may open new pathways to synthesize mesoporous materials under different conditions than presently applied.

Nano Lett., Vol. 4, No. 3, 2004

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