Article pubs.acs.org/Langmuir
Synergic Effects of Imidazolium Ionic Liquids on P123 Mixed Micelles for Inducing Micro/Mesoporous Materials Feng Gao, Jun Hu,* Changjun Peng, Honglai Liu, and Ying Hu State Key Laboratory of Chemical Engineering and Department of Chemistry, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: A series of micro/mesoporous silica composites were synthesized with P123 and imidazolium ILs ([Cnmim]X) as the cotemplates. [Cnmim]X showed notable synergic interaction with P123. By changing the alkyl chain length n in methylimidazolium, ring-like micropores were observed in the wall of the mesoporous materials when n = 4. While increasing n to 10, micropores and mesopores were found in different separated regions. Various anions of Cl−, Br−, and BF4− of ILs have little effect on the aggregation behavior of P123/C4X mixed micelles. The strong hydrogen bonding effect of BF4− has resulted in the ordered mesoporous channels with numerous micropores in the wall at a low temperature of 313 K. Hydrophobic C4PF6 can only be solubilized in the core of P123 micelles, which resulted in the swelling of P123/C4PF6 mixed aggregates and the ordered hexagonal porous silica materials at 313 K. The fundamental understanding of the synergic interaction and formation mechanisms of various porous silica materials can provide a general convenient way toward a rational design and synthesis of the micro/mesoporous composites.
1. INTRODUCTION At the beginning of the 1990s, ordered mesoporous materials (OMMs)1,2 were discovered to reduce diffusion constraints encountered in microporous materials. However, OMMs have not succeeded in replacing common zeolites as catalysts or adsorbents owing to the lower acidity and hydrothermal stability. Over the past decade, research on the exploration of new routes to synthesize zeolite materials combining micropores with mesopores have attracted intensive attentions.3,4 Various synthetic approaches can be generalized as the template synthesis or the postsynthetic demetalation process. Some specially designed and/or synthesized bifunctional agents have been introduced in the templating synthesis. The cooperative interaction between templates and siliceous precursors is crucial for obtaining the desired structures. There is still a great challenge for exploring alternative structure-directing agents with improved template properties. Recently, ionic liquids (ILs) have attracted considerable interest due to the special structure of the inorganic anion and organic cation.5 Among various ILs, those derived from 1-alkyl3-methylimidazolium ([Cnmim]) are of particular interest, since by changing the alkyl chain length or the type of the anion, a wide variation of properties such as solubility, amphiphilicity, viscosity, density can be obtained.6 Moreover, they can play multiple roles in the synthesis of porous materials, such as the template, solvent, and cosurfactant.7 Long-chain amphiphilic ILs can self-assemble into ordered structures and display both lyotropic and thermotropic liquid crystals through hydrogen bond-co-π−π stack.8 Hence, they can induce micro or mesoporous materials.9−12 More recently, Bernd13 synthesized a high-quality cubic gyroid mesoporous silica (MCM-48-type) © 2012 American Chemical Society
with the long-chain IL of [C16mim]Cl as the template under basic conditions. The high binding density of [C16mim]Cl prefers a mesostructure with a low average curvature; consequently, a bicontinuous cubic phase such as the gyroid phase could be obtained. Zhou14 reported that a highly ordered monolithic supermicroporous lamellar silica were induced by a series of [Cnmim]Cl templates via a nanocasting technique. They found that the average pore size of these materials increases with the increase of the alkyl chain length in methylimidazolium. Moreover, ILs could significantly change the self-assembly behavior of common anionic, cationic and nonionic surfactants,15−17 which results in some novel phenomena in templating materials.18 Zheng19 reported a mixed template of an anionic surfactant of sodium dodecyl sulfate (SDS) and a long-chain IL of [C12mim]Br for the synthesis of hollow silica spheres; meanwhile, the phase behavior and the vesicle formation of SDS /[C12mim]Br in aqueous solution were also investigated to illuminate the formation mechanism of hollow silica spheres. In our previous work,20 we adopted [C4mim]Br and cetyltriethylammnonium bromide (CTAB) as the co-template, at lower temperature, a mesoporous material with extremely large BET surface area was synthesized; while at higher temperature, the composite material with distinctly different micropores and mesopores coexisted was obtained since microphase separation occurs in the co-templates. Received: October 26, 2011 Revised: December 27, 2011 Published: January 12, 2012 2950
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JEM-2010. The size distributions of micelles were determined by dynamic laser light scattering (DLS) on a Malvern Nano-ZS, with a detecting angle of 173°.
One of the most important parameters for preparing diverse porous materials is the interactions among ILs, surfactants, and solvents.21−25 Pandey et al.26 studied the properties of IL [C6mim]Br in CTAB aqueous solution, and found that [C6mim]Br showed both electrolytic and cosurfactant characteristics at lower concentration. However, at higher concentration, [C6mim]Br partially switched to playing the role of a cosolvent, and the properties of CTAB aqueous solution changed correspondingly. Guo et al.27 reported the different phase behaviors of the mixture of P104 surfactant and [C4mim]Br. Specifically, below a critical concentration, [C4mim]Br embeds in the micellar core of P104, while above this concentration, P104 micelles and [C4mim]Br aggregates separate, and two clusters coexist in the system. Bhattacharyya et al.28−30 explored the interaction between triblock copolymer P123 and ionic liquids at room temperature using femtosecond technology. The solvation dynamics and marcus-like invertion in electron transfer in the P123/ILs mixed micelles were studied. Timothy et al.31 recently found that the P123 micelle shuttles could reversibly transfer many times between water and the IL [C4mim][PF6] phases by the cyclical temperature swing. These studies have provided a good fundamental understanding of the potential co-template mechanism of ILs and Planck triblock copolymers for hierarchical porous materials. In the present work, the porous silica materials were synthesized with P123 and imidazolium ILs [Cnmim]X as the co-templates. A series of samples were obtained by changing the alkyl chain length n in methylimidazolium and the anion X. Moreover, the interactions between [Cnmim]X and P123 were investigated by measuring their aggregation behavior. Combining the above two aspects together, we tentatively elucidated the formation mechanisms of various ordered porous silica materials by P123/[Cnmim]X co-template, which would give a fundamental understanding for establishing a rational way to synthesis the micro/mesoporous composites.
3. RESULTS AND DISCUSSION 3.1. Effect of the Alkyl Chain Length of Imidazolium Bromine (CnBr). Figure 1 shows the XRD patterns of the
Figure 1. XRD patterns of the samples induced by P123/CnBr cotemplates at the aging temperature of 393 K.
calcined samples induced by P123/CnBr co-templates at the aging temperature of 393 K. When the alkyl chain length of CnBr n = 4, three well-resolved diffraction peaks are observed in its curve (a), which can be indexed as (100), (110), and (200) diffraction peaks associated with p6mm hexagonal symmetry. When n = 6 and 8, three characteristic diffraction peaks are also observed in their corresponding curves (b) and (c). However, when n = 10, only one broad diffraction peak with a much lower intensity is preserved in its curve (d), indicating its less ordered structure. N2 adsorption isotherms of the calcined samples in Figure 2A exhibit the typical IV adsorption. Each has an obvious welldefined hysteresis attributed to the predominant mesoporous structure. With the increase of the alkyl chain length n, the pressure onset of the hysteresis gradually shifts to lower values, from 0.7 (n = 4) to 0.48 (n = 10), indicating pore size shrinkage. When n = 10, the characteristic hysteresis loop distorts a little, indicating the loss of ordered pore structure, which agrees with the XRD result. The size distributions calculated by BJH model based on desorption curves in Figure 2B give more clear evidence that the average pore size of calcined samples decreases with the increase of n. The characteristic data on the samples are summarized in Table 1. From Table 1, we can see that with increasing the cationic alkyl chain length of imidazolium (n = 4−10), the average pore size of each sample is 7.1, 5.6, 5.0, 3.9 nm, respectively; and the BET surface area increases accordingly. The pore volumes of all samples nBr-393 are larger than that of the pure SBA-15 of 0.94 cm3 g−1, indicating the positive contributions of CnBr cotemplates to the volume. Among them, the sample of 4Br-393 possesses an extremely large pore volume of 1.42 cm3 g−1. TEM images in Figure 3 give intuitive evidence that P123/ CnBr co-templates can induce micro/mesoporous composites. With increasing the alkyl chain length n, the mesoporous structure becomes less ordered and the microporous structure changes greatly. The image (A) of the sample 4Br-393 shows very regular ordered structure. In contrast to SBA-15, the wall is
2. EXPERIMENTAL SECTION 2.1. Chemicals. P123(PEO20PPO70PEO20) was purchased from Sigma-Aldrich, and all ionic liquids were purchased from Chengjie Chemical Inc. (Shanghai, China) with a purity of 99%. 2.2. Synthesis. In a typical synthesis, P123 (8 g) and an appropriate amount of 1-alkyl-3-methyl-imidazolium bromine [C4mim]Br were completely dissolved in water (186 g) first, and 12 mol L−1 HCl solution (6.25 g) was then added. Under strong stirring, tetraethyl orthosilicate (TEOS) (8.8 g) was added dropwise into the above solution. After stirring at 313 K for 24 h, the whole slurry was transferred into an autoclave for aging at 313 K or 393 K for another 24 h. The white as-synthesized solid powders were then obtained. After washing, the samples were calcined at 823 K for 6 h in the ambient air, with a heating rate of 2 K min−1. Similarly, a series of porous materials were synthesized by changing the alkyl chain length in methylimidazolium and the anion of ILs. The resulting products were denoted as nX-T, in which n represents the alkyl chain length in methylimidazolium; X represents the anion of ILs. T represents the aging temperature, respectively. 2.3. Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a D/Max2550 VB/PC spectrometer using Cu Kα radiation (40 kV and 200 mA). Nitrogen adsorption measurements were conducted at 77.4 K on a Micrometrics ASAP-2020 sorptionmeter. The specific surface area and the pore size distribution were calculated by Brunauer−Emmett−Teller (BET) method and Barrett−Joyner−Halenda (BJH) model, respectively. The scanning electron micrographs (SEM) were taken on JEOL JSM-6360-LV, while the transmission electron micrographs (TEM) were on JEOL 2951
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Figure 2. (A) N2 adsorption−desorption isotherms, (B) pore size distributions calculated by BJH model based on desorpotion curves of the calcined samples induced by P123/CnBr co-templates at an aging temperature of 393 K.
TEM image (C), the mesopores become less ordered and bigger compared with 4Br-393 and 6Br-393 samples. Meanwhile, the micropores not only randomly distributed in the wall of the mesoporous channels as shown in the enlargement of the region (a), but also separately coexisted in the different region (b), although most of them are disordered, there are still some arrays which can be picked out in the insert of the enlargement region (b). For the sample 10Br-393, the ordered mesopores totally disappear; only disordered worm-like mesopores can be observed in the TEM image (D), which is consistent with the XRD result. Besides, as shown in the enlargement, there are numerous micropores everywhere in the sample. Different from the results of SBA-15 microporosity by Fajula,31 that the microporous volume of SBA-15 would be at a maximum in the hydrothermal temperature range of 35−80 °C and then decrease to zero when at 130 °C; herein, the samples of nBr-
Table 1. Characteristic Data of Samples Induced by the P123/CnBr Co-Templates at an Aging Temperature of 393 K sample name
IL types
BET surface area (m2 g−1)
diameter (nm)
pore volume (cm3 g−1)
SBA-15 4Br-393 6Br-393 8Br-393 10Br-393
None C4Br C6Br C8Br C10Br
732 720 754 761 833
5.7 7.1 5.6 5.0 3.9
0.94 1.42 1.12 1.12 1.07
much thicker, approximately 7 nm. If we make a local enlargement, we can find the wall is completely filled with the cycle-like micropores. The TEM image (B) of the sample 6Br-393 shows almost the same characteristics as SBA-15, with micropores randomly distributed all over the wall of the mesoporous channels. For the sample 8Br-393, as shown in the
Figure 3. TEM images of the calcined samples induced by P123/CnBr co-templates with the CnBr concentration of 100 mmol L−1. (A) 4Br-393, (B) 6Br-393, (C) 8Br-393, (D) 10Br-393 at the temperature of 313 K. The insets are the local enlargements with the same enlargement scale. 2952
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Figure 4. Size distributions of P123/CnBr aqueous solutions determined by DLS. (A) C4Br, (B) C6Br, (C) C8Br, (D) C10Br.
393 synthesized at 120 °C still possess a lot of various micropores, indicating the effect of CnBr co-templates. To understand the co-template function of CnBr, especially the effect of the alkyl chain length n, the aggregation behaviors of mixed P123/CnBr in aqueous solutions were investigated. Figure 4 shows the influence of CnBr concentration on the aggregation behavior of Pluronic P123 in aqueous solutions (1% (w/v)) at room temperature. Only one peak can be observed in each of size distributions of P123/C4Br aqueous solutions in Figure 4A. With increasing the C4Br concentration, the average hydrodynamic diameter (Dh) of P123/C4Br mixed aggregates shifts from 20 nm to a little higher value. In the case of P123/C6Br, as shown in Figure 4B, only one kind of P123/ C6Br mixed aggregates with Dh ≈ 22 nm exist when the C6Br concentration is in the range of 50−800 mmol L−1; when the concentration is above 800 mmol−1, another new kind of aggregate with much smaller Dh (about 1 nm) appeared. When n increases to 8, very complex behavior is observed in P123/ C8Br aqueous solutions. As shown in Figure 4C, at low C8Br concentration, such as 50 and 100 mmol L−1, P123 and C8Br molecules aggregate together, and only one peak is observed centered at 21 and 18 nm, respectively; with increasing C8Br concentration, multidistribution occurs, suggesting the aggregates separate into several phases with different scales. The higher the C8Br concentration, the stronger the intensity of the peak at 1.8 nm; in other words, more of the smaller aggregates appear. While Dh of the original aggregates shifts to the lower value of 5.7 nm. Moreover, extremely large-scale aggregates (∼1 um) also appear. Figure 4D of P123/C10Br aqueous solutions reveals a simple relation that the size of aggregates
decreases with increasing the concentration of C10Br, from 13.5 nm for 50 mmol L−1 C10Br to 1.6 nm for 300 mmol L−1; meanwhile, there are also some extremely large scale aggregates formed. Generally, a P123 micelle is composed of a core of relatively hydrophobic PPO blocks and a shell of hydrophilic PEO blocks. The organic cation of ILs [Cnmim]Br consists of an alkyl chain and an imidazole cation, just like the corresponding tail and head of a cationic surfactant. When P123 and [Cnmim] Br are mixed together, the alkyl chain tail would tend to insert in the core of PPO blocks due to the hydrophobic attraction, and the imidazole head would be embedded among PEO blocks through hydrogen bonding. Because interactions between P123 and [Cnmim]Br are different with the alkyl chain length of the ILs, the size of P123/CnBr micelle aggregations in the solution, and the corresponding average pore size of the induced porous materials decreased with increasing in alkyl chain length. With the case of adding 100 mmol·L−1 CnBr into P123 solution as an example, the average sizes of P123/CnBr aggregations are similar to each other when n = 4, 6, and 8, with little difference in the sequence of P123/ C6Br (24.3 nm) > P123/C4Br (21.0 nm) > P123/C8Br (18.5 nm) ≈ P123 (18.3 nm), confirming the alkyl chains of ILs inserted in P123 micelles, or even replaced some P123 molecules to form mixed micelles. However, when n = 10, the size of P123/CnBr aggregations has an abnormal decrease to 3.6 nm, suggesting C10Br can dissociate P123 micelle and form new aggregations with much smaller diameter. When P123/CnBr aggregations were adopted as the co-template, undergoing the hydrothermal process, the average pore size of 2953
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SiO2 wall hinders the further transfer of C4Br molecules into the bulk solution; instead, C4Br molecules assemble themselves through the hydrogen bond-co-π−π stack in the loose amorphous wall. C4Br stacks can be ring-like or worm-like, which is verified in the enlargement of TEM image of Figure 3A. Through P123/C4Br co-template, composites with micropores existing in the wall of mesoporous channels are produced. It seems that the channels with meso-size are mainly determined by the mixed P123/C4Br micelles while the micropores originate from C4Br molecules escaped from the mixed micelles into the channel wall. When n = 6, C6 chains insert into the core of P123 micelles due to the strong hydrophobic attraction with PPO block and cationic imidazole rings embedded in PEO blocks. The synergistic interaction between [C6mim]+ and P123 molecules makes [C6mim]+ act in a similar role as P123 surfactant in the mixed micelles. With increasing C6Br concentration, [C6mim]+ can replace some P123 molecules in the micelles, which keeps the average size of the micelles unchanged; when C6Br concentration is high enough, the fraction of [C6mim]+ in the mixed micelles reaches saturation, C6Br molecules will aggregate to form ring-like small assemblies by themselves, as shown in Figure 4B, and a new peak with the size of 1 nm appears. When P123/C6Br acts as the co-template, even at the hydrothermal temperature, the strong attraction forbids the microphase separation occurring between P123 and C6Br, and the general SBA-15-type pore structure is obtained finally. When n = 8, when C8Br concentration is above 100 mmol·L−1, there are mainly three kinds of aggregates in the P123/C8Br aqueous solution. P123/ C8Br mixed micelles still exist with middle size of 15−20 nm. Differing from C6Br, the C8 chain is long enough to behave like a surfactant to form small micelles of 1.5−1.8 nm. Moreover, the strong interaction and intertwist between C8 chain and PPO block can greatly change the hydrophilicity of PPO block due to the attached imidazole cations, even result in the dissociation of P123 micelles. The dissociated P123 molecules extend randomly; on some occasions, one PEO block end may insert in a micelle and the other end in another micelle, which links several micelles together to produce extremely large aggregates such as 1 μm. When C8Br/P123 acts as the co-template, the micro/mesoporous composites can be produced, in which micropores exist in the wall of mesoporous channels and also in different regions separately. When n = 10, at low C10Br concentration, C10Br can insert into PPO blocks to form mixed micelles; with increasing C10Br concentration,
the induced porous materials, as listed in the Table 1, decreased in the sequence of 4Br-393 (7.1 nm) > 6Br-393 (5.6 nm) ≈ SBA-15 (5.7 nm) > 8Br-393 (5.0 nm) > 10Br-393 (3.9 nm), quite compatible with DLS results. We tentatively elucidate the effect of alkyl chain length on both formations of P123/CnBr aggregations in aqueous solutions and corresponding porous materials. As shown in Scheme 1, when n = 4, C4 chains are relatively shorter, they can Scheme 1. Variations of the P123/CnBr Aggregates and Their Template Functions for the Porous Products
only insert into the exterior of the P123 micelles, with cationic imidazole rings embedded at the surface layer. With increasing C4Br concentration, more and more [C4mim]+ cations are embedded into P123 micelles so that the aggregates grow bigger and bigger (Figure 4A). When P123/C4Br is adopted as the co-template, the hydrolysates of TEOS would wrap around the mixed micelles at 313 K. During the hydrothermal process, with the increase of the temperature, the reduction of the interaction between P123 and [C4mim]+ results in the escape of C4Br molecules from the micelles.19 However, the sol−gel
Figure 5. Size distributions of P123/C4X aggregates in aqueous solutions determined by DLS. (A) C4Cl, (B) C4BF4. 2954
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small C10Br micelles are dominant in the solution. Since the peaks of P123/C10Br aggregations almost disappear in the size distribution curves in Figure 4D, most P123/C10Br mixed micelles have already dissociated. When C10Br/P123 acts as the co-template, the sample is full of micropores, with disordered mesopores coexisting with random aggregate containing micropores. 3.2. Effect of the Anion Moiety of Imidazolium IL. The structure-directing template strategy is based on its interaction with the hydrolysates of TEOS. For nonionic surfactant P123 templates, the anions in the bulk solution play an important role in the acidic synthesis procedure. When we fixed the alkyl chain length as 4, the imidazolium ILs has the same 1-butyl-3methylimidazolium cationic moieties. The effect of anionic moieties of the imidazolium C4X on the properties of both P123/C4X mixed aggregates in aqueous solutions and the corresponding induced porous products were investigated. Figure 5 shows the size distributions of P123/C4X mixed aggregates in aqueous solutions with a certain concentration of 1% (w/v) P123 but with different concentrations of C4X at room temperature. At low C4X concentration, no matter what kind of anions are, the sizes of P123/C4X are all about 20 nm, similar to each other. With increasing the C4X concentration, different anions have different effects. In the case of the anion Br−, as we mentioned above, the size of P123/C4Br (Figure 4A) only has a small increase; for example, the Dh increment is merely 7 nm when the C4Br concentration increases from 50 mmol L−1 to 900 mmol L−1. In the case of anion Cl−, the size change of P123/C4Cl (Figure 5A) shows a similar trend to C4Br, that a small increase can be observed with increasing C4Cl concentration, from 20 nm (100 mmol L−1) to 26 nm (900 mmol L−1). Because the anions of Br− and Cl− are homotopes, and the size of the Br− anion is bigger than that of Cl−, the size of P123/C4Br is bigger than that of P123/C4Cl, and their change is almost the same. However, in the case of the anion BF4−, the Dh of P123/C4BF4 (Figure 5B) has few changes before the C4BF4 concentration increases to 500 mmol L−1. With the concentration further increasing to 800 mmol L−1, a type of small-scale aggregate with Dh of about 3.5 nm is found, and the pristine aggregates swell to 27 nm. Because of the relatively lower solubility of C4BF4, C4BF4 molecules can assemble to form small aggregates when they are saturated in P123/C4BF4 mixed micelles. When the concentration increases to 900 mmol L−1, the size distribution becomes much more complex; the formation of the larger aggregates (100 nm) is probably due to the cross-linking of the pristine aggregates, while the aggregates of C4BF4 micelles (1.6 nm) and some oligomers of P123/C4BF4 (5.8 nm) are also produced. With P123/C4X acting as co-templates, the corresponding XRD patterns of the induced samples are illustrated in Figure 6. Because of the relatively low solubility of C4BF4, all the C4X concentrations in co-templates were chosen as50 mmol L−1, and the aging temperatures were changed to as low as 313 K. One intense diffraction peak (100) can be observed in each pattern, and there are three well-resolved diffraction peaks in the pattern of the sample 4BF4-313, indicating that mesopores were produced in all samples and the highly ordered hexagonal network was obtained in 4BF4-313. In fact, the high-order peaks also can be found in the enlargement of each pattern of pristine SBA-15−313, 4Br-313, and 4Cl-313; namely, the hexagonal network is still preserved in these samples, but lacking orderliness.
Figure 6. XRD patterns of the samples induced by P123/C4X cotemplates at the aging temperature of 313 K.
N2 adsorption/desorption isotherms of samples are shown in Figure 7A, which exhibits an H1 type hysteresis loop characterizing the cylindrical mesopores. The loops associated with the capillary condensation commence at the relative pressure p/p0 ≈ 0.45 for the samples of pristine SBA-15−313, 4Br-313, and 4Cl-313, respectively, whereas for 4BF4-313, the corresponding p/p0 increases significantly to 0.7. The cause of this difference lies in their different pore sizes. The pore size distributions shown in Figure 7B give clearer evidence that 4BF4-313 possesses much larger average BJH pore size of 7.8 nm, whereas the others have similar average pore size of 3.6− 4.0 nm. Their characteristic data such as the BET surface area, average pore size, and volume are listed in Table 2. In our synthesis process, hydrochloric acid was used as the pH controller; therefore, Cl− anions always exist in the reaction solution. That is to say, the synthesis systems of the pristine SBA-15−313 and 4Cl-313 have the same anions, so the characteristic differences such as BET surface area of the induced porous products come from the 1-butyl-3-methylimidazolium cationic moieties of C4Cl, i.e., adding ILs can effectively increase the surface area. In the Hofmeister series of anions, Cl− and Br− are the neighbors; furthermore, we have already found that P123/C4Cl and P123/C4Br mixed micelles have almost the same properties, hence the porous 4Br-313 has no obvious changes compared with 4Cl-313. However, BF4− anions have quite different properties from Cl− and Br− anions. There are 4 F atoms, namely, 4 hydrogen bonding acceptors, in one BF4− anion; therefore, it can interact with water molecules.33 As we described in Scheme 1, C4 imidazolium cations will insert into the exterior of the P123 micelles with the imidazole head embedded among PEO blocks, so the counterion BF4− anions nearby would change the water environment around the outer layer of PEO blocks in P123 micelles. As shown in Figure 8A, a highly ordered mesoporous structure already forms in 4BF4-313 even at a low aging temperature of 313 K. In addition, numerous micropores in the wall of the mesoporous channels can be observed in the enlarged insert. Among them, most are random worm-like, while some are cycle-like. In acidic synthesis conditions, the cationic silica species can assemble themselves on the surface of P123/C4BF4 aggregates through the strong electrostatic interaction with BF4− anion at a lower temperature, so the ordered structure can be obtained at 313 K. Furthermore, PEO blocks, imidazolium cations, and BF4− anions can embed into the amorphous silica wall and induce the micropores together. 2955
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Figure 7. (A) N2 adsorption−desorption isotherms of the samples induced by P123/C4X co-templates at the temperature of 313 K. (B) The corresponding pore size distribution calculated by BJH model based on the desorption curves.
aging temperature of 313 K. The TEM image as shown in Figure 9A gives a clear confirmation. However, the ordered
Table 2. Characteristic Data of the Samples Induced by P123/C4X Co-Templates at the Aging Temperature of 313 K sample name
ILs Types
BET surface area (m2/g)
diameter (nm)
pore volume (cm3/g)
SBA-15− 313 4Br-313 4Cl-313 4BF4−313
None
346
4.0
0.42
C4Br C4Cl C4BF4
602 512 710
3.7 3.6 7.1
0.60 0.51 1.37
Besides, the BJH average mesopore diameter and the pore volume are much larger than that of others due to the hydrated BF4− anions. However, 4Cl-313 did not possess highly ordered structure at low temperature. When the temperature increased to the normal aging temperature of 393 K, as shown in Figure 8B, the 100 planar image of 4Cl-393 shows its highly ordered hexagonal porous structure. Similar to 4Br-393, we can see that the wall is much thicker compared with its pore diameter from the local enlarged insert, and the walls are fully filled with the small cycle-like micropores. 3.3. Effect of the Hydrophobicity of Imidazolium IL. Different from the above C4X ILs (X = Cl, Br, and BF4), C4PF6 is a hydrophobic IL, which will be preferentially soluble in the hydrophobic core of P123 micelle in the mixed P123/ C4PF6 aqueous solution. When we added a small amount of C4PF6 (50 mmol L−1) as the co-template, the product of 4PF6313 possesses perfectly hexagonal symmetry structure at the
Figure 9. TEM images of the calcined samples induced by P123/ C4PF6 co-template with the C4PF6 concentration of 50 mmol L−1 at various aging temperatures: (A) 313 K, (B) 333 K.
structure is gradually destroyed by increasing the aging temperature. Figure 9B is the TEM image of the sample of 4PF6-333, which indicates that the mesoporous structure is well preserved, but the order-degree decreases. Figure 10A shows N2 adsorption/desorption isotherms of samples induced by P123/C4PF6 co-template at various aging temperatures. Except for the sample of 4PF6-393, they all exhibit a hysteresis loop characterizing the cylindrical mesopores. Moreover, the loop shifts to higher p/p0 values with the increase of the aging temperature, respectively. Although the 4PF6-393 sample also has a hysteresis loop, it extends to a pressure close to the vapor
Figure 8. TEM images of the calcined samples induced by P123/C4X co-templates with the C4X concentration of 100 mmol L−1. (A) 4BF4-313, (B) 4Cl-393. The inserts are the local enlargements with the same enlarged scale. 2956
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Figure 10. (A) N2 adsorption−desorption isotherms of the samples induced by P123/C4PF6 co-template at various aging temperatures. (B) The corresponding pore size distribution calculated by BJH model based on the desorpotion curves.
Figure 11. (A) Average diameter of P123/addition mixed aggregates and (B) size distribution of P123/C4PF6 mixed aggregates at various temperatures.
pressure, which is attributed to large pores formed through randomly aggregating silica particles. Furthermore, the amount of adsorbed N2 in the 4PF6-333 sample levels off. The narrow pore size distribution of 4PF6-313 shown in Figure 10B reveals that it possesses a quite uniform BJH average pore size of 7 nm, whereas the broadened distributions of 4PF6-333 and 4PF6-353 suggest that they possess a polydispersed pore structure, with some new, different scales of porous phases appearing. When the aging temperature increases to 393 K, no obvious peak exists in the pore size distribution curve; only the amorphous silica is obtained. The behavior of P123/C4PF6 aggregates is analogous to that of the pore diameter expander, reagent 1,3,5-trimethylbenzene (TMB) and P123 aggregates. Figure 11A shows the temperature effect on size changes of P123/additions mixed aggregates. In the case of the pristine P123 micelles, the size remains almost a constant 20−30 nm because the aggregate number of the micelles remains almost constant in this temperature range.34 Furthermore, with increasing temperature, dehydration of the PEO block takes place, and PEO blocks become somewhat hydrophobic which leads to the partial miscibility in the core, consequently resulting in the size slightly increasing. In the case of P123/TMB mixed micelles, TMB molecules are totally solubilized in the core of P123 micelle, which obviously swells the P123 micelle. The slight change caused by PEO blocks has little effect on the relatively large
mixed micelles; therefore, the size of the P123/TMB mixed micelles maintains a constant 60−70 nm within the temperature range. However, in the case of P123/C4PF6 mixed aggregates, they are remarkably sensitive to temperature.35 Below 313 K, the mixed aggregates exhibit only a slight increase in the average size; however, in the temperature range between 313 and 333 K, they grow up rapidly from the average size of 70 to 450 nm, and then maintain at about 420 nm with further increase in temperature. As shown in Figure 11B, in the temperature where the size increases rapidly, such as 323 K, the P123/C4PF6 mixed aggregates are polydisperse, with different aggregates coexisting in the aqueous solution. With increasing temperature, the hydrophobicity of both PEO blocks and PPO blocks is greatly enhanced due to dehydration. However, the hydrophobic C4PF6 is a good solvent for PPO and PEO blocks. As a result, microphase separation may occur, which means P123 and C4PF6 can aggregate to form microemulsions rather than micelles at high temperature. This also can be a reason for the formation of amorphous silica, 4PF6-393, in which without suitable mesoporous templates, only a disordered porous silica sample can be obtained. The results from increasing the C4PF6 concentration in the synthesis system show, furthermore, that C4PF6 is able to affect the aggregation behavior of the P123 micelles. As shown in Figure 12, when the C4PF6 concentration increases to 100 mmol L−1 and the aging temperature is fixed at 313 K, the 2957
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into the core of the P123 micelles due to the increasing interaction between alkyl chains and PPO blocks. P123 chains may reduce in the P123/CnBr mixed micelles, which results in a slight decrease of the size. At high concentration, when [Cnmim]+ cations in the mixed micelles reach saturation, [Cnmim]Br molecules aggregate themselves through the hydrogen bond-co-π−π stack to form ring-like small assemblies, or small micelles like a common surfactant. When P123/CnBr mixed aggregates are used as co-templates, microphase separation can occur during the aging process at high temperature. The ring-like micropores in the wall of the mesoporous materials have been observed when n = 4, with increasing n, micropores and mesopores have been found in different separated regions. Moreover, at lower [C4mim]X concentration, various anions of Cl−, Br−, and BF4− have little effect on the behavior of P123/ C4X mixed aggregates; whereas at higher concentration, C4BF4 can assemble to form small micelles. The strong hydrogen bonding effect of BF4− could be the driving force for inducing the ordered periodic silica materials at low temperature, such as 313 K. The large hydrated BF4− anions embedded in the surface layer of P123 micelles result in large mesopore diameter and pore volume. The hydrophobic C4PF6 can only be solubilized in the core of P123 micelles, so that it swells the size of P123/C4PF6 mixed aggregates at low concentration and results in the ordered hexagonal porous silica materials at 313 K. However, with increasing temperature or concentration, C4PF6 greatly changes the hydrophobicity of both PEO blocks and PPO blocks; consequently, the ordered structure gradually collapses. The future potential of this simple approach for new multiporous materials lies in the versatility of the ILs as well as that of the template surfactants. Focusing on the ILs, due to the easily changeable anions and cations, numerous new structures have been reported. It constitutes a challenge to try to use the surfactant/IL as structure-directing agent for the generation of new nanostructured materials.
Figure 12. TEM images of the calcined sample induced by P123/ C4PF6 co-template with the C4PF6 concentration of 100 mmol L−1 at 313 K.
ordered hexagonal pore structure degrades into less ordered structure (Figure 12A), whereas in the local enlargement, some ordered channel arrays can also be found (Figure 12B). A similar conclusion can be drawn from N2 adsorption isotherms and corresponding pore size distributions in Figure 13. With increasing concentration of C4PF6, more C4PF6 molecules are solubilized in the core of the P123 micelle, which results in the maximum peak value of the pore size distribution shifting to larger scale. Meanwhile, its distribution becomes broader. In the case of the concentration of 100 mmol L−1, a broad distribution with main bimodal mesopores at 7.9 and 3.5 nm is observed, while in the case of the concentration of 500 mmol L−1, a much broader distribution range from 2.5 to 20 nm with one dominant peak at 9.5 nm is found. C4PF6 can greatly change the environment around PPO and PEO blocks, and make both of them more hydrophobic. Consequently, various aggregates will coexist in the system, which induces broadening of the pore size distributions of the porous products.
4. CONCLUSIONS This research shows the synergistic effect of imidazolium ILs [Cnmim]X on P123 micelles, as well as their co-template functions for inducing porous silica materials. The alkyl chain length, the different types of anions, and the hydrophobicity of [Cnmim]X have been investigated systematically. With increasing alkyl chain length n in methylimidazolium from 4 to 10, [Cnmim]Br gradually exhibits the properties of a surfactant. At low concentration, [Cnmim]+ inserts even deeper
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Figure 13. (A) N2 adsorption−desorption isotherms of the samples induced by P123/C4PF6 co-templates with various C4PF6 concentration at the aging temperature of 313 K. (B) Corresponding pore size distributions calculated by BJH model based on the desorpotion curves. 2958
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(33) Wang, Y.; Li, H. R.; Han, S. J. J. Phys. Chem. B 2006, 110, 24646−24651. (34) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145−4159. (35) Lee, H. N.; Lodge, T. P. J. Phys. Chem. Lett. 2010, 1, 1962− 1966.
ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Nos. 21176066, 21136004), the National High Technology Research and Development Program of China (No. 2008AA062302), and National Basic Research Program of China (2009CB219902). We also appreciated the suggestions of Prof. Richard Schwenz from the University of Northern Colorado.
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