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mesoporous material templated by CTAB and imidazole ionic liquid in aqueous solution
Microporous and Mesoporous Materials 142 (2011) 268–275
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
One-step synthesis of micro/mesoporous material templated by CTAB and imidazole ionic liquid in aqueous solution Jun Hu, Feng Gao, Yazhuo Shang, Changjun Peng, Honglai Liu ⇑, Ying Hu State Key Laboratory of Chemical Engineering and Department of Chemistry, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 7 September 2010 Received in revised form 29 November 2010 Accepted 9 December 2010 Available online 14 December 2010 Keywords: Micro/mesoporous material Micro-phase separation Imidazole ionic liquid CTAB Synergistic interactions
a b s t r a c t Based on the micro-phase separation of the dual templates consisting of an alkyl imidazole ionic liquid [Cnmim]Br and a surfactant cetyltriethylammonium bromide (CTAB), the micro/mesoporous silicate materials were synthesized. At room temperature, the cations [C4mim]+ and CTA+ aggregated into mixed micelles and acted as co-templates for synthesis of mesoporous materials which possessed an extremely large BET surface area of 1719 m2 g1. While at a hydrothermal temperature of 373 K, the mixed micelles separated into CTA+ mesoscale micelles and [C4mim]+ micro scale aggregates which were served as mesoporous templates and microporous templates, respectively. With a total BET surface area of 1016 m2 g1, the resulted materials had distinctly different micropore range and mesopore range of pores of 0.6 and 2.7 nm, respectively. The formation mechanism was tentatively elucidated by studying the interaction between CTAB and [C4mim]Br based on the measurement of the cmc of the CTAB/[C4mim]Br mixture aqueous solutions at different compositions and temperatures. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction During the past decades, significant efforts have been devoted to generate mesoporosity in zeolite materials by using different approaches [1]. Exhibiting at least two types of pores, the designed micro/mesoporous materials can further improve their structure order, as well as the thermal, hydrothermal and mechanical stability. The approach using templates has been considered as one of the most promising methods to prepare this kind of hierarchical porous materials [2]. As turn out to be the state-of-art in the design of various hierarchical porous materials by different templates, it provides many inspirations for the development of new and improved methodologies. The direct templates could be the solid materials, such as carbon nanoparticles [3–5], polystyrene beads [6] and ion-exchange resins [7], etc., or the soft materials, such as organized assemblies of surfactants and supramolecules. The indirect templates are also known as the partially crystallizing ordered mesoporous silica [8]. One strategy for the preparation of micro/mesoporous materials is that using the nanosized zeolite seeds as building blocks, which can be assembled into mesoporous frameworks by surfactant templates [9–11]. In this method, the zeolitic seeds are firstly synthesized by using the typical molecular organic structure-directing agents, such as tetrapropylammonium (TPA+) and tetraethylammonium (TEA+) ions. Followed by crystallization in supramolecular ⇑ Corresponding author. E-mail address: [email protected] (H. Liu). 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.12.011
templates, the mesoporous phase is then formed [12]. Another novel strategy is based on the idea that the molecular templates could induce the mesoporous structure while micropores are simultaneously formed in the mesoporous walls by using dual templates in a one-step synthesis. However, the first attempt reported by Karlsson et al. [13] showed that it only resulted in the formation of bulk zeolite without any mesoporosity by using the mixed templates of alkyltrimethylammonium surfactant mixtures with different chain lengths. Later, Ryoo et al. [14] successfully synthesized the mesoporous zeolite materials by carefully selecting amphiphilic organosilanes, [(CH3O)SiC3H6N(CH3)2CnH2n+1]Cl, as supramolecular templates. The essential of the template method for micro/mesoporous material lies in that it should be hierarchical templates, i.e., two different sizes of molecular and supramolecular aggregations. The reason why Karlsson et al. did not obtain the micro/mesoporous material may be that the selected templates of two alkyltrimethylammonium surfactants were completely miscible and formed uniform mixed micelles in aqueous solution without phase separation. In nature biological systems, there is a common phenomena of the buds extruded off from the domain vesicle, which are caused by the micro-phase separation among the different components [15,16]. Based on the formation mechanism of this bud/domain structures, we can imagine, if two surfactants are different enough to result in the micro-phase separation in some specific condition, this bud/domain type of molecular/supramolecular aggregations of the dual templates may form, from which the inorganic micro/ mesoporous material could be possibly induced.
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Ionic liquids (ILs) are organic salts that are liquids at ambient temperature. Their special structure of the inorganic anion and organic cation makes them possessed the good solubility, the amphiphilic properties, the salt effect and so on [17,18]. Acting as multiple roles of cosolvent, cosurfactant as well as salt, ILs may greatly change the physicochemical properties and micellization behavior of aqueous surfactant solution. The self-assembly and the aggregation behavior of common anionic, cationic, and nonionic surfactants in ILs have been attracted considerable attention [19]. Guo et al. [20] reported the different phase behavior of P104 surfactant and [Bmim]Br mixture aqueous solution, that below a critical [Bmim]Br concentration, the [Bmim]Br embedded in the micellar core of P104, while above this concentration, P104 micelles and [Bmim]Br separated and two clusters coexisted in the system. Pandey et al. [21] studied the presence of IL [Hmim]Br in CTAB solution, and found that [Hmim]Br showed electrolytic as well as cosurfactant-type behavior in aqueous CTAB when present at low concentrations. At higher concentrations, part of [Hmim]Br switched to the role of a cosolvent in modulating the properties of aqueous CTAB. The real reason behind the diverse phenomena is the interaction among the ILs, the surfactants and the solvents [22–26]. Moreover, ILs can form associated aggregations through hydrogen bond-co-p–p stack, which can template micro or mesoporous materials by themselves [27–31]. In this work, a new one-step synthesis method of micro/mesoporous material by adopting the dual templates consisting of the conventional cationic hyamine surfactant cetyltriethylammnonium bromide (CTAB) and the alkyl imidazole ionic liquid [Cnmim]Br was presented. The interaction between [Cnmim]Br and CTAB was investigated by measuring the critical micelle concentration (cmc) of mixed surfactant solutions to elucidate the formation mechanism of bimodal pores in distinctly different microscale and mesoscale ranges.
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77.4 K on a Micrometrics-ASAP-2020 sorptionmeter. The total surface area was determined by the BET model, the micropore area was determined by the t-plot method, and the mesopore area was calculated by the total BET surface area minus the micropore area. The mesopore size distribution was determined by BJH model, while the micropore size distribution was determined by Horvath–Kawazoe model. The scanning electron micrographs (SEM) were taken on JEOL JSM-6360-LV, while the transmission electron micrographs (TEM) were on JEOL JEM-2010.
3. Results and discussion 3.1. The properties of micro/mesoporous materials CTAB and IL-C4 mixtures are used as the dual templates to synthesize a series of porous materials. Fig. 1 is the powder Xray diffraction patterns of the calcined samples induced by CTAB and IL-C4 co-template at different IL-C4 M fractions (aIL-C4 = 0.5, 0.6, 0.7, 0.8, 0.9) and reaction temperatures. All the samples prepared at 373 K with different IL-C4 M fractions have similar XRD patterns as shown in curves (a)–(e) of Fig. 1(A). Take the sample of aIL-C4 = 0.8 for an example, the diffraction curve (c) has an intense reflection peak and two small peaks at 2h = 2.25°, 3.95° and 4.54°, corresponding to the (1 0 0), (1 1 0) and (2 0 0) reflections, respectively, which is a typical pattern characterizing the hexagonal arrangement of MCM-41. The curves (d) and (e) with higher ILC4 content of aIL-C4 = 0.8 and 0.9 have relative lower diffraction intensity, however, the enlargement of curve (e) still possesses three distinguish peaks suggesting the existence of the less ordered hexagonal structure.
2. Experimental 2.1. Synthesis In a typical synthesis, an aqueous solution of mixed CTAB and 1butyl-3-methyl-imidazolium bromine (IL-C4) was used as a template solution and its pH was controlled by ethylamine (EA). The sol–gel precursor tetraethyl orthosilicate (TEOS) was then dripped slowly with stirring. The molar composition of the mixed gel was (TEOS:CTAB:IL-C4:EA:H2O) = (1:0.2:aIL-C4:0.6:100), where aIL-C4 was the molar fraction of IL-C4 determined as aIL-C4 ¼ nIL-C4 = ðnIL-C4 þ nCTAB Þ. After stirring for extra 120 min, the whole solution was transferred into an autoclave for hydrothermal reaction at different temperatures from 303 to 373 K for 48 h. The white assynthesized solid powders were then calcined at 823 K for 6 h in ambient air, with a heating rate of 2 K min1. 2.2. Characterization The cmc of surfactant solutions at different temperatures were determined by plotting the ratio of the emission fluorescence intensity I1/I3 of pyrene against total concentrations of CTAB and [Cnmim]Br. The steady-state fluorescence measurements were recorded on a F4500 fluorescence spectrophotometer of HITACHI using saturated pyrene as the probe (excited at kEX = 335 nm). The size distribution of micelles was determined by dynamic laser light scattering (DLS) technique on Malvern Nano-ZS, backscatter detection was used with a detecting angle of 173°. The powder X-ray diffraction (XRD) patterns were recorded on a D/Max2550 VB/PC spectrometer using Cu Ka radiation (40 kV and 200 mA). Nitrogen adsorption measurements were conducted at
Fig. 1. The XRD patterns of the samples induced by CTAB and IL-C4 co-template (A) with various IL-C4 M fractions at 373 K. (B) at various reaction temperatures with the molar fraction of aIL-C4 = 0.5.
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Fig. 1(B) shows the XRD of the samples prepared in the reaction temperature range of 303–373 K with the molar fraction of aIL-C4 = 0.5. When the reaction temperature are at 373 and 343 K, the corresponding curves (a) and (b) clearly display four and three distinct diffraction peaks in the low 2h region of 1.5–10°, respectively, which are the characteristic peaks of hexagonal MCM-41 mesoporous phase. However, except the (1 0 0) peak, the other peaks in the curve (c) with the reaction temperature of 323 K are gradually blurred. When the temperature continues decreasing to 303 K, it completely loses its ordered hexagonal structure, although the mesopores may still exist because we can find a weak broad peak within the low 2h region at curve (d). Meanwhile, the (1 0 0) peaks shifts towards the larger angle with the decrease of reaction temperature, indicating the lattice space d100 of the pores change to smaller. Although the XRD diffraction in the wide-angle range does not show the characteristic peaks of zeolite type, the TEM images of the calcined sample prepared at 373 K does exhibit the monolithic microporous lamellar structure (Fig. 2a), as well as the highly ordered mesoporous structure (Fig. 2b). With the aid of the auto correlation treatment, the average diameter of micropores is estimated as about 0.56 nm, which is compatible with the molecular mechanics calculation result (MM2) of 0.77 nm for the fully extended chain length of [C4mim]+. In terms of the hydrogen bondco-p–p stack interaction [27], the microporous lamellar structure might be initiated by the template of IL-C4. Furthermore, estimated from the TEM image of Fig. 2b, the average interspaces of the periodical mesoporous channels is about 3.7 nm, consistent with the XRD result of 4.1 nm for the lattice space d100. The morphology of the micro/mesoprous material particles prepared at 373 K, as shown in the SEM image of Fig. 2c, exhibits worm-like shapes, the periodical lamellar terraces grow up cyclically in each particle, and some terraces twist in one direction forming torsions as marked with the arrows. Fig. 3(A) shows the N2 adsorption–desorption isothermals of calcined materials at different reaction temperatures. All the isothermals exhibit the typical IV adsorption curve with obvious shoulder step in the range of 0.3 < p/p0 < 0.4, which are attributed to the predominant mesoporous structure. The presences of a pronounced hysteresis loop in the isothermals (b–d) indicate the intersection network of porous structures. At very low relative pressure p/p0, a sharp increase at the adsorption isothermal of (d) at 373 K indicates the presence of micropores. The characteristic microporous adsorption gradually disappeared with decreasing reaction temperature. Fig. 3(B) shows the pore size distribution calculated by BJH model based on desorption curves. The average diameter of the mesopore is in a range of 2.0–2.7 nm. The diameter of the pore decreases with the decrease of reaction temperature, which is consistent with the XRD results. At 373 K, the analyzed result of average diameter of the mesopore is 2.7 nm. From the insertion of Fig. 3(B) of the pore distribution by micropore measurement method, the average diameter of mircopores is about 0.6 nm. According pffiffiffi to its XRD result, the crystal lattice parameter a (ao ¼ 2d100 = 3) is calculated as 4.8 nm, hence the thickness of the walls is about 2.1 nm, which is thick enough to form the mircopores inside the wall. The characteristic data of the surface area (S), the microporous surface area (Smicro), the BJH cumulative pore volume (VBJH), the average pore diameter (Dp), and the microporous pore diameter (Dmicro) are summarized in Table 1. It is shown that the total surface area of the sample prepared at aIL-C4 = 0.5 and room temperature is extremely large, as high as 1719 m2 g1. However, there are no micropores detected in this sample. As shown in the TEM images of Fig. 4(A), there are large amounts of pore arrays closely pack together, and the pores are wormlike without long-range order in the enlargement image of Fig. 4(B). Changing the reaction
Fig. 2. The images of the calcined sample induced by CTAB and IL-C4 co-template with the molar fraction of aIL-C4 = 0.5 at hydrothermal reaction temperature of 373 K (a) the TEM image of the micropores with an insertion of auto correlation image, (b) the TEM image of the mesopores and (c) the SEM image of the particles.
temperature to 323, 343 and 373 K, the total BET surface area are all very high, although there is a decrease trend as 1525, 1159 and 1016 m2 g1, respectively. At 373 K, the micropores appear with the BET surface area of 41 m2 g1. Increasing the adding amount of IL-C4 to aIL-C4 = 0.7, correspondingly, the micropores
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Fig. 3. (A) The N2 adsorption–desorption isothermals of the samples induced by CTAB and IL-C4 co-template with the molar fraction of aIL-C4 = 0.5 at different temperatures. The scale is shifted by 5 cm3 g1 per curve. (B) The pore size distribution calculated by BJH model based on desorption curves. The insertion is the pore distribution of the sample obtained at 373 K by micropore measurement method.
Fig. 4. The TEM images of the calcined samples induced by CTAB and IL-C4 cotemplate with aIL-C4 = 0.5 at the reaction temperature of 303 K. (A) the overall image, and (B) the enlargement image.
Table 1 Characteristic data of samples induced by CTAB and ILs co-template at various reaction temperatures and the molar composition of the reaction gel is (TEOS:CTAB:ILs:EA:H2O) = (1:0.2:aIL-C4(or C6):0.6:100), where aIL-C4(or C6) is the molar fraction of IL-C4(or C6). IL type
aIL-C4
Reaction temperature (K)
STot (m2 g1)
SMicro (m2 g1)
VBJH (cm3 g1)
Dp (nm)
DMicro (nm)
IL-C4 IL-C4 IL-C4 IL-C4 IL-C4 IL-C4 IL-C6 IL-C6
0.5 0.5 0.5 0.5 0.7 0.9 0.4 0.6
373 343 323 303 373 373 373 373
1016 1159 1525 1719 913 812 1001 1003
41 – – – 51 60 25 37
0.91 1.09 0.92 0.87 0.82 1.12 0.89 0.91
2.7 2.5 2.2 2.0 2.7 2.6 2.6 2.6
0.58 – – – 0.58 0.58 0.58 0.59
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surface area increase to 51 m2 g1, and the mesoporous BET surface area decrease to about 900 m2 g1. 3.2. The interaction between [Cnmim]Br and CTAB In order to explore the formation mechanism of micro/mesoporous materials induced by CTAB/IL-C4 mixtures, the interaction between CTAB and IL-C4 are investigated by measuring the cmc of the CTAB/IL-C4 mixture aqueous solutions. The cmc can be determined by testing the changes of the emission fluorescence intensity ratio of I1/I3 of pyrene in the CTAB/IL-C4 mixture aqueous solutions. As shown in Fig. 5, each plot of I1/I3 with the total CTAB/ IL-C4 mixture concentrations has a distinguished inflexion corresponding to the cmc of solution. The curve (a) is the pure CTAB solution with the cmc of 1.01 mmol L1, curves (b–e) are the CTAB/IL-C4 mixture aqueous solutions with different compositions. It shows that when the molar fraction aIL-C4 of IL-C4 in the CTAB/IL-C4 mixtures are 0.2, 0.5, 0.8 and 0.9, the corresponding cmc increases along the direction of arrow as 0.42, 1.63, 2.44 and 4.08 mmol L1, respectively. It is noticeable that at the low molar fraction of aIL-C4 = 0.2, the cmc is lower than that of pure CTAB. Ionic liquid is a type of organic salt, which can be dissociated as the cation and anion under lower concentration [17,18]. At this condition, the dominant role of IL-C4 in changing cmc of surfactant is similar to that of common electrolytes. With increasing the molar fraction aIL-C4, the role of co-solvent of IL-C4 surpasses. Generally, as co-solvents, ILs can dramatically decrease the polarity and dielectric constant of water, and increase the solubility of surfactants, etc. [32,33], which would prohibit the formation of micelles. Thus when aIL-C4 P 0.5, the addition of IL-C4 benefit for the increase of the cmc of CTAB. Moreover, because of the special structure of IL-C4 molecules of the hydrophobic 4C-alkyl group appending to the imidazole ring, they have a certain surface activity. There is an attractive interaction between [C4mim]+ and CTA+, which makes the IL-C4 behave like a co-surfactant. A slight increase of the fluorescence intensity ratios I1/I3 with the increase of aIL-C4 in Fig. 5 indicates the increase of the micropolarity intensity of circumstance of micelles. Because the polarity of the imidazole cation [C4mim]+ is stronger than that of the CTA+ cation, the increase of the micropolarity of the mixed micelles would be caused by the more ILs inserted into the barrier of the micelles. Fig. 6 is the concentration dependence of the size of micelles of CTAB/IL-C4 mixture solution. There is an obvious increase in the size of the mixed micelles of CTAB/IL-C4 with the increase of IL-
Fig. 6. The change of average diameter of the mixed micelles of CTAB/IL-C4 with aIL-C4 at 303 K.
C4 concentration, which gives a further evidence that the higher concentration of IL-C4, the more [C4mim]+ cations inserted into the mixed micelles of CTAB/IL-C4. The cmc of the CTAB/IL-C4 mixture aqueous solution also increases with increasing temperature. Here, we only present two cases with molar fraction of (a) aIL-C4 = 0.5 and (b) aIL-C4 = 0.8 as shown in Fig. 7. Similar as the common features of the solubility, and consequence the cmc of an ionic surfactant increase with the temperature, when the temperature increases from 303, 323 to 343 K, for serials samples with aIL-C4 = 0.5, the cmcs increase from 1.63, 1.77 to 2.00 mmol L1, while for aIL-C4 = 0.8, they are 2.44, 3.32 and 4.00 mmol L1, respectively. The cmcs of the mixtures with higher IL-C4 concentration are more sensitive to temperature. However, the fluorescence intensity ratio I1/I3 of both serials samples, indicating the micropolarity of micelles, decreases at higher temperature. This could be explained by the lowering the interaction between the hydrophobic alkyl tails of [C4mim]+ and CTA+, which causes the dissociation of [C4mim]+ cations from the mixed micelles, namely the micro-phase seperation of mixed CTAB/IL-C4 micelles at higher temperature. The further evidence is shown in Fig. 8, in which the average diameter of two mixed CTAB/IL-C4 micelles with different IL-C4 concentrations decreases with the temperature. At room temperature, the size of the mixed micelles with the molar fraction aIL-C4 of 0.7 is about 22 nm, it sharply decreases to 7 nm as the temperature increases to 323 K and maintains almost as a constant with further temperature increasing. The variation of the cmc with the molar fraction of IL-C4 and the temperature is shown in Fig. 9. The results indicate that when IL-C4 is added into CTAB solution, the cmc of the mixture solutions decreases first; then it increases gently with further adding of IL-C4, finally, it increases shapely when the molar fraction aIL-C4 approaches 1. Different temperatures have almost the same cmc change trends. For the same molar fraction aIL-C4, the higher the temperature is, the higher the cmc of the mixture solutions. 3.3. The template mechanism of [Cnmim]Br and CTAB to the micro/mesoporous material
Fig. 5. The concentration dependence of the emission fluorescence intensity ratio I1/I3 of pyrene in the CTAB/IL-C4 mixture aqueous solutions with different molar fractions of IL-C4 at 303 K.
Combining the phenomena of interaction between IL-C4 and CTAB as well as the characteristics of induced mesoporous products, we tentatively suggest a template mechanism scheme as shown in Fig. 10. Having a large positive-charged imidazole ring and a hydrophobic C4 tail, IL-C4 exhibits the characteristics of triple roles of electrolyte, co-solvent and co-surfactant. At synthesis
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Fig. 9. The variation of the cmc of CTAB/IL-C4 mixture aqueous solutions with the molar fraction of IL-C4 at 303, 323 and 343 K.
Fig. 7. The temperature dependence of the emission fluorescence intensity ratio I1/I3 of the CTAB/IL-C4 mixture aqueous solutions with (A) aIL-C4 = 0.5 and (B) aIL-C4 = 0.8.
Fig. 8. The change of average diameter of mixed micelles of CTAB/IL-C4 at various temperatures.
composition of aIL-C4 = 0.5, when the temperature is low, the cosolvent and co-surfactant functions of IL-C4 are dominant, and IL-C4 and CTAB molecules are miscible and can form mixed micelles in aqueous solution. Due to the relatively smaller ratio of
[C4mim]+ in the mixed micelles, the CTA+ cations are dominated. As shown in Fig. 10(A), with the cationic imidazole rings attached at the surface of micelles and C4 tails inserted into the barrier layer, the [C4mim]+ cations synergistically aggregated with CTA+ cations together to act as co-templates for the synthesis of mesoporous materials. Because of the participation of [C4mim]+ cations, the average aggregation number of CTA+ in micelles reduces and more micelles are formed in the solution, sequentially, more mesoporous channels are induced during the ageing (hydrothermal reaction) process. Comparing to the normal MCM-41 induced by CTAB along, the mesoporous materials induced by CTAB/IL-C4 cotemplate at room temperature has extremely larger surface area. With the increase of temperature, the solubility of CTAB increase and the [C4mim]+ cations can dissociate from the mixed micelles freely in the aqueous solutions, which makes mixed micelles shrink as shown in Fig. 8. However, in the synthesis situation, the co-template of mixed micelles is fully enwrapped by the hydrolytic silica sol–gels at room temperature, during the hydrothermal reaction process, with the increase of temperature, the closely packed tails in the nucleus of micelles can extend and loose up, accordingly, the pore size of the mesoporous materials increases. Only when the temperature is high enough, the reducing of attractive interaction between CTAB and IL-C4 results in the micro-phase separation of the mixed micelles, and IL-C4 aggregations are formed through the hydrogen bond-co-p–p stack interaction nearby the CTAB-rich micelles, just like the natural domain/bud formation process. The shorter C4 tails make the IL-C4 aggregations much smaller than the CTAB micelles, as shown in Fig. 10(B). In the hydrothermal process, with the CTAB-rich micelles served as mesoporous templates and the IL-C4 aggregations as microporous templates, the micro/mesoporous materials are formed. As mentioned above, because the [C4mim]+ cations only can aggregate nearby the CTAB-rich micelles, the micropores may generate in the wall of the mesoprous material. Although the small amount of IL-C4 cannot produce enough micropores to give an clear evidence in the XRD characterization, the TEM images and the N2 adsorption and desorption isothermals give a certainty of the existing of micropores in the synthesized porous materials. It can be predicted that with the increase of IL-C4 content, the amount of the micropores, as well as the microporous BET surface area will increase correspondingly, but the sizes of micropore and mesopore will not change anymore. From Table 1, we can see that increasing the molar fraction of IL-C4 from 0.5, 0.7 to 0.9, the microporous BET surface area increases 10 m2 g1 for each, whereas the sizes of mesopore and micropore are unchanged.
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Fig. 10. The template mechanism of IL cationic [C4mim]+ and CTA+ (A) synergism for mesoporous material at lower temperature, and (B) for the micro/mesoporous materials at higher temperature.
These results give a strong support for the above presumption and discussions. To verify this mechanism, another kind of IL of [C6mim]Br (ILC6) were also used as co-template. As shown in Table 1, when the hydrothermal temperature is at 373 K, the micropores exist in the porous products. The total surface areas are almost same no matter aIL-C6 is 0.4 or 0.6, but the micropores surface area increase from 25 to 37 m2 g1. The size of micropores of 0.6 nm appears no distinct difference compared with the co-template of IL-C4. As described in Fig. 10(B), the formation of the microporous structure is mainly due to the hydrogen bond-co-p–p stack of [Cnmim]+ rings; consequently, the length of the tail of IL-C6 plays an unimportant role and almost has no effect on the lamellar microporous structure. For the same weight percentage of [Cnmim]Br, the bigger the molar mass of IL-C6, the smaller the molar fractions of IL-C6, hence, the fewer the amounts of micropores, which cause the smaller microporous surface area compared with IL-C4.
4. Conclusions In summary, the IL [Cnmim]Br play different roles such as the electrolytes, co-surfactant and co-solvent at different composition regions in surfactant CTAB mixture aqueous solutions; also there are different synergistic interactions between [Cnmim]Br and CTAB at different temperatures. At room temperature, they can form mixed micelles as co-templates and induce mesoprous materials with extremely large BET surface area of 1719 m2 g1. While at a higher temperature 373 K, micro-phase separation occurs between [Cnmim]Br and CTAB, the mixed micelles are separated into CTAB dominant micelles and [Cnmim]Br molecular aggregations nearby. With CTAB micelles served as mesoporous templates and [Cnmim]Br molecular aggregations as microporous templates, respectively, the micro/mesoporous materials with distinctly different in the micropore range of 0.6 nm and the mesopore range of 2.7 nm were prepared. This strategy provides a new convenient method to prepare the micro/mesoporous materials. The key point
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is that the micro-phase separation is triggered between two components of the dual templates, from which the domain/bud formations of the suitable molecular and supramolecular aggregations successfully induce the micro/mesoporous materials in the hydrothermal process. Acknowledgements This work is supported by the National Natural Science Foundation of China (Nos. 20736002, 20776045), the National High Technology Research and Development Program of China (No. 2008AA062302), National Basic Research Program of China (2009CB219902) and Program for Changjiang Scholars and Innovative Research Team in University of China (No. IRT0721). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2010.12.011. References [1] [2] [3] [4] [5] [6] [7]
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
One-step synthesis of micro/mesoporous material templated by CTAB and imidazole ionic liquid in aqueous solution Jun Hu, Feng Gao, Yazhuo Shang, Changjun Peng, Honglai Liu ⇑, Ying Hu State Key Laboratory of Chemical Engineering and Department of Chemistry, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 7 September 2010 Received in revised form 29 November 2010 Accepted 9 December 2010 Available online 14 December 2010 Keywords: Micro/mesoporous material Micro-phase separation Imidazole ionic liquid CTAB Synergistic interactions
a b s t r a c t Based on the micro-phase separation of the dual templates consisting of an alkyl imidazole ionic liquid [Cnmim]Br and a surfactant cetyltriethylammonium bromide (CTAB), the micro/mesoporous silicate materials were synthesized. At room temperature, the cations [C4mim]+ and CTA+ aggregated into mixed micelles and acted as co-templates for synthesis of mesoporous materials which possessed an extremely large BET surface area of 1719 m2 g1. While at a hydrothermal temperature of 373 K, the mixed micelles separated into CTA+ mesoscale micelles and [C4mim]+ micro scale aggregates which were served as mesoporous templates and microporous templates, respectively. With a total BET surface area of 1016 m2 g1, the resulted materials had distinctly different micropore range and mesopore range of pores of 0.6 and 2.7 nm, respectively. The formation mechanism was tentatively elucidated by studying the interaction between CTAB and [C4mim]Br based on the measurement of the cmc of the CTAB/[C4mim]Br mixture aqueous solutions at different compositions and temperatures. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction During the past decades, significant efforts have been devoted to generate mesoporosity in zeolite materials by using different approaches [1]. Exhibiting at least two types of pores, the designed micro/mesoporous materials can further improve their structure order, as well as the thermal, hydrothermal and mechanical stability. The approach using templates has been considered as one of the most promising methods to prepare this kind of hierarchical porous materials [2]. As turn out to be the state-of-art in the design of various hierarchical porous materials by different templates, it provides many inspirations for the development of new and improved methodologies. The direct templates could be the solid materials, such as carbon nanoparticles [3–5], polystyrene beads [6] and ion-exchange resins [7], etc., or the soft materials, such as organized assemblies of surfactants and supramolecules. The indirect templates are also known as the partially crystallizing ordered mesoporous silica [8]. One strategy for the preparation of micro/mesoporous materials is that using the nanosized zeolite seeds as building blocks, which can be assembled into mesoporous frameworks by surfactant templates [9–11]. In this method, the zeolitic seeds are firstly synthesized by using the typical molecular organic structure-directing agents, such as tetrapropylammonium (TPA+) and tetraethylammonium (TEA+) ions. Followed by crystallization in supramolecular ⇑ Corresponding author. E-mail address: [email protected] (H. Liu). 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.12.011
templates, the mesoporous phase is then formed [12]. Another novel strategy is based on the idea that the molecular templates could induce the mesoporous structure while micropores are simultaneously formed in the mesoporous walls by using dual templates in a one-step synthesis. However, the first attempt reported by Karlsson et al. [13] showed that it only resulted in the formation of bulk zeolite without any mesoporosity by using the mixed templates of alkyltrimethylammonium surfactant mixtures with different chain lengths. Later, Ryoo et al. [14] successfully synthesized the mesoporous zeolite materials by carefully selecting amphiphilic organosilanes, [(CH3O)SiC3H6N(CH3)2CnH2n+1]Cl, as supramolecular templates. The essential of the template method for micro/mesoporous material lies in that it should be hierarchical templates, i.e., two different sizes of molecular and supramolecular aggregations. The reason why Karlsson et al. did not obtain the micro/mesoporous material may be that the selected templates of two alkyltrimethylammonium surfactants were completely miscible and formed uniform mixed micelles in aqueous solution without phase separation. In nature biological systems, there is a common phenomena of the buds extruded off from the domain vesicle, which are caused by the micro-phase separation among the different components [15,16]. Based on the formation mechanism of this bud/domain structures, we can imagine, if two surfactants are different enough to result in the micro-phase separation in some specific condition, this bud/domain type of molecular/supramolecular aggregations of the dual templates may form, from which the inorganic micro/ mesoporous material could be possibly induced.
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Ionic liquids (ILs) are organic salts that are liquids at ambient temperature. Their special structure of the inorganic anion and organic cation makes them possessed the good solubility, the amphiphilic properties, the salt effect and so on [17,18]. Acting as multiple roles of cosolvent, cosurfactant as well as salt, ILs may greatly change the physicochemical properties and micellization behavior of aqueous surfactant solution. The self-assembly and the aggregation behavior of common anionic, cationic, and nonionic surfactants in ILs have been attracted considerable attention [19]. Guo et al. [20] reported the different phase behavior of P104 surfactant and [Bmim]Br mixture aqueous solution, that below a critical [Bmim]Br concentration, the [Bmim]Br embedded in the micellar core of P104, while above this concentration, P104 micelles and [Bmim]Br separated and two clusters coexisted in the system. Pandey et al. [21] studied the presence of IL [Hmim]Br in CTAB solution, and found that [Hmim]Br showed electrolytic as well as cosurfactant-type behavior in aqueous CTAB when present at low concentrations. At higher concentrations, part of [Hmim]Br switched to the role of a cosolvent in modulating the properties of aqueous CTAB. The real reason behind the diverse phenomena is the interaction among the ILs, the surfactants and the solvents [22–26]. Moreover, ILs can form associated aggregations through hydrogen bond-co-p–p stack, which can template micro or mesoporous materials by themselves [27–31]. In this work, a new one-step synthesis method of micro/mesoporous material by adopting the dual templates consisting of the conventional cationic hyamine surfactant cetyltriethylammnonium bromide (CTAB) and the alkyl imidazole ionic liquid [Cnmim]Br was presented. The interaction between [Cnmim]Br and CTAB was investigated by measuring the critical micelle concentration (cmc) of mixed surfactant solutions to elucidate the formation mechanism of bimodal pores in distinctly different microscale and mesoscale ranges.
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77.4 K on a Micrometrics-ASAP-2020 sorptionmeter. The total surface area was determined by the BET model, the micropore area was determined by the t-plot method, and the mesopore area was calculated by the total BET surface area minus the micropore area. The mesopore size distribution was determined by BJH model, while the micropore size distribution was determined by Horvath–Kawazoe model. The scanning electron micrographs (SEM) were taken on JEOL JSM-6360-LV, while the transmission electron micrographs (TEM) were on JEOL JEM-2010.
3. Results and discussion 3.1. The properties of micro/mesoporous materials CTAB and IL-C4 mixtures are used as the dual templates to synthesize a series of porous materials. Fig. 1 is the powder Xray diffraction patterns of the calcined samples induced by CTAB and IL-C4 co-template at different IL-C4 M fractions (aIL-C4 = 0.5, 0.6, 0.7, 0.8, 0.9) and reaction temperatures. All the samples prepared at 373 K with different IL-C4 M fractions have similar XRD patterns as shown in curves (a)–(e) of Fig. 1(A). Take the sample of aIL-C4 = 0.8 for an example, the diffraction curve (c) has an intense reflection peak and two small peaks at 2h = 2.25°, 3.95° and 4.54°, corresponding to the (1 0 0), (1 1 0) and (2 0 0) reflections, respectively, which is a typical pattern characterizing the hexagonal arrangement of MCM-41. The curves (d) and (e) with higher ILC4 content of aIL-C4 = 0.8 and 0.9 have relative lower diffraction intensity, however, the enlargement of curve (e) still possesses three distinguish peaks suggesting the existence of the less ordered hexagonal structure.
2. Experimental 2.1. Synthesis In a typical synthesis, an aqueous solution of mixed CTAB and 1butyl-3-methyl-imidazolium bromine (IL-C4) was used as a template solution and its pH was controlled by ethylamine (EA). The sol–gel precursor tetraethyl orthosilicate (TEOS) was then dripped slowly with stirring. The molar composition of the mixed gel was (TEOS:CTAB:IL-C4:EA:H2O) = (1:0.2:aIL-C4:0.6:100), where aIL-C4 was the molar fraction of IL-C4 determined as aIL-C4 ¼ nIL-C4 = ðnIL-C4 þ nCTAB Þ. After stirring for extra 120 min, the whole solution was transferred into an autoclave for hydrothermal reaction at different temperatures from 303 to 373 K for 48 h. The white assynthesized solid powders were then calcined at 823 K for 6 h in ambient air, with a heating rate of 2 K min1. 2.2. Characterization The cmc of surfactant solutions at different temperatures were determined by plotting the ratio of the emission fluorescence intensity I1/I3 of pyrene against total concentrations of CTAB and [Cnmim]Br. The steady-state fluorescence measurements were recorded on a F4500 fluorescence spectrophotometer of HITACHI using saturated pyrene as the probe (excited at kEX = 335 nm). The size distribution of micelles was determined by dynamic laser light scattering (DLS) technique on Malvern Nano-ZS, backscatter detection was used with a detecting angle of 173°. The powder X-ray diffraction (XRD) patterns were recorded on a D/Max2550 VB/PC spectrometer using Cu Ka radiation (40 kV and 200 mA). Nitrogen adsorption measurements were conducted at
Fig. 1. The XRD patterns of the samples induced by CTAB and IL-C4 co-template (A) with various IL-C4 M fractions at 373 K. (B) at various reaction temperatures with the molar fraction of aIL-C4 = 0.5.
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Fig. 1(B) shows the XRD of the samples prepared in the reaction temperature range of 303–373 K with the molar fraction of aIL-C4 = 0.5. When the reaction temperature are at 373 and 343 K, the corresponding curves (a) and (b) clearly display four and three distinct diffraction peaks in the low 2h region of 1.5–10°, respectively, which are the characteristic peaks of hexagonal MCM-41 mesoporous phase. However, except the (1 0 0) peak, the other peaks in the curve (c) with the reaction temperature of 323 K are gradually blurred. When the temperature continues decreasing to 303 K, it completely loses its ordered hexagonal structure, although the mesopores may still exist because we can find a weak broad peak within the low 2h region at curve (d). Meanwhile, the (1 0 0) peaks shifts towards the larger angle with the decrease of reaction temperature, indicating the lattice space d100 of the pores change to smaller. Although the XRD diffraction in the wide-angle range does not show the characteristic peaks of zeolite type, the TEM images of the calcined sample prepared at 373 K does exhibit the monolithic microporous lamellar structure (Fig. 2a), as well as the highly ordered mesoporous structure (Fig. 2b). With the aid of the auto correlation treatment, the average diameter of micropores is estimated as about 0.56 nm, which is compatible with the molecular mechanics calculation result (MM2) of 0.77 nm for the fully extended chain length of [C4mim]+. In terms of the hydrogen bondco-p–p stack interaction [27], the microporous lamellar structure might be initiated by the template of IL-C4. Furthermore, estimated from the TEM image of Fig. 2b, the average interspaces of the periodical mesoporous channels is about 3.7 nm, consistent with the XRD result of 4.1 nm for the lattice space d100. The morphology of the micro/mesoprous material particles prepared at 373 K, as shown in the SEM image of Fig. 2c, exhibits worm-like shapes, the periodical lamellar terraces grow up cyclically in each particle, and some terraces twist in one direction forming torsions as marked with the arrows. Fig. 3(A) shows the N2 adsorption–desorption isothermals of calcined materials at different reaction temperatures. All the isothermals exhibit the typical IV adsorption curve with obvious shoulder step in the range of 0.3 < p/p0 < 0.4, which are attributed to the predominant mesoporous structure. The presences of a pronounced hysteresis loop in the isothermals (b–d) indicate the intersection network of porous structures. At very low relative pressure p/p0, a sharp increase at the adsorption isothermal of (d) at 373 K indicates the presence of micropores. The characteristic microporous adsorption gradually disappeared with decreasing reaction temperature. Fig. 3(B) shows the pore size distribution calculated by BJH model based on desorption curves. The average diameter of the mesopore is in a range of 2.0–2.7 nm. The diameter of the pore decreases with the decrease of reaction temperature, which is consistent with the XRD results. At 373 K, the analyzed result of average diameter of the mesopore is 2.7 nm. From the insertion of Fig. 3(B) of the pore distribution by micropore measurement method, the average diameter of mircopores is about 0.6 nm. According pffiffiffi to its XRD result, the crystal lattice parameter a (ao ¼ 2d100 = 3) is calculated as 4.8 nm, hence the thickness of the walls is about 2.1 nm, which is thick enough to form the mircopores inside the wall. The characteristic data of the surface area (S), the microporous surface area (Smicro), the BJH cumulative pore volume (VBJH), the average pore diameter (Dp), and the microporous pore diameter (Dmicro) are summarized in Table 1. It is shown that the total surface area of the sample prepared at aIL-C4 = 0.5 and room temperature is extremely large, as high as 1719 m2 g1. However, there are no micropores detected in this sample. As shown in the TEM images of Fig. 4(A), there are large amounts of pore arrays closely pack together, and the pores are wormlike without long-range order in the enlargement image of Fig. 4(B). Changing the reaction
Fig. 2. The images of the calcined sample induced by CTAB and IL-C4 co-template with the molar fraction of aIL-C4 = 0.5 at hydrothermal reaction temperature of 373 K (a) the TEM image of the micropores with an insertion of auto correlation image, (b) the TEM image of the mesopores and (c) the SEM image of the particles.
temperature to 323, 343 and 373 K, the total BET surface area are all very high, although there is a decrease trend as 1525, 1159 and 1016 m2 g1, respectively. At 373 K, the micropores appear with the BET surface area of 41 m2 g1. Increasing the adding amount of IL-C4 to aIL-C4 = 0.7, correspondingly, the micropores
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Fig. 3. (A) The N2 adsorption–desorption isothermals of the samples induced by CTAB and IL-C4 co-template with the molar fraction of aIL-C4 = 0.5 at different temperatures. The scale is shifted by 5 cm3 g1 per curve. (B) The pore size distribution calculated by BJH model based on desorption curves. The insertion is the pore distribution of the sample obtained at 373 K by micropore measurement method.
Fig. 4. The TEM images of the calcined samples induced by CTAB and IL-C4 cotemplate with aIL-C4 = 0.5 at the reaction temperature of 303 K. (A) the overall image, and (B) the enlargement image.
Table 1 Characteristic data of samples induced by CTAB and ILs co-template at various reaction temperatures and the molar composition of the reaction gel is (TEOS:CTAB:ILs:EA:H2O) = (1:0.2:aIL-C4(or C6):0.6:100), where aIL-C4(or C6) is the molar fraction of IL-C4(or C6). IL type
aIL-C4
Reaction temperature (K)
STot (m2 g1)
SMicro (m2 g1)
VBJH (cm3 g1)
Dp (nm)
DMicro (nm)
IL-C4 IL-C4 IL-C4 IL-C4 IL-C4 IL-C4 IL-C6 IL-C6
0.5 0.5 0.5 0.5 0.7 0.9 0.4 0.6
373 343 323 303 373 373 373 373
1016 1159 1525 1719 913 812 1001 1003
41 – – – 51 60 25 37
0.91 1.09 0.92 0.87 0.82 1.12 0.89 0.91
2.7 2.5 2.2 2.0 2.7 2.6 2.6 2.6
0.58 – – – 0.58 0.58 0.58 0.59
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surface area increase to 51 m2 g1, and the mesoporous BET surface area decrease to about 900 m2 g1. 3.2. The interaction between [Cnmim]Br and CTAB In order to explore the formation mechanism of micro/mesoporous materials induced by CTAB/IL-C4 mixtures, the interaction between CTAB and IL-C4 are investigated by measuring the cmc of the CTAB/IL-C4 mixture aqueous solutions. The cmc can be determined by testing the changes of the emission fluorescence intensity ratio of I1/I3 of pyrene in the CTAB/IL-C4 mixture aqueous solutions. As shown in Fig. 5, each plot of I1/I3 with the total CTAB/ IL-C4 mixture concentrations has a distinguished inflexion corresponding to the cmc of solution. The curve (a) is the pure CTAB solution with the cmc of 1.01 mmol L1, curves (b–e) are the CTAB/IL-C4 mixture aqueous solutions with different compositions. It shows that when the molar fraction aIL-C4 of IL-C4 in the CTAB/IL-C4 mixtures are 0.2, 0.5, 0.8 and 0.9, the corresponding cmc increases along the direction of arrow as 0.42, 1.63, 2.44 and 4.08 mmol L1, respectively. It is noticeable that at the low molar fraction of aIL-C4 = 0.2, the cmc is lower than that of pure CTAB. Ionic liquid is a type of organic salt, which can be dissociated as the cation and anion under lower concentration [17,18]. At this condition, the dominant role of IL-C4 in changing cmc of surfactant is similar to that of common electrolytes. With increasing the molar fraction aIL-C4, the role of co-solvent of IL-C4 surpasses. Generally, as co-solvents, ILs can dramatically decrease the polarity and dielectric constant of water, and increase the solubility of surfactants, etc. [32,33], which would prohibit the formation of micelles. Thus when aIL-C4 P 0.5, the addition of IL-C4 benefit for the increase of the cmc of CTAB. Moreover, because of the special structure of IL-C4 molecules of the hydrophobic 4C-alkyl group appending to the imidazole ring, they have a certain surface activity. There is an attractive interaction between [C4mim]+ and CTA+, which makes the IL-C4 behave like a co-surfactant. A slight increase of the fluorescence intensity ratios I1/I3 with the increase of aIL-C4 in Fig. 5 indicates the increase of the micropolarity intensity of circumstance of micelles. Because the polarity of the imidazole cation [C4mim]+ is stronger than that of the CTA+ cation, the increase of the micropolarity of the mixed micelles would be caused by the more ILs inserted into the barrier of the micelles. Fig. 6 is the concentration dependence of the size of micelles of CTAB/IL-C4 mixture solution. There is an obvious increase in the size of the mixed micelles of CTAB/IL-C4 with the increase of IL-
Fig. 6. The change of average diameter of the mixed micelles of CTAB/IL-C4 with aIL-C4 at 303 K.
C4 concentration, which gives a further evidence that the higher concentration of IL-C4, the more [C4mim]+ cations inserted into the mixed micelles of CTAB/IL-C4. The cmc of the CTAB/IL-C4 mixture aqueous solution also increases with increasing temperature. Here, we only present two cases with molar fraction of (a) aIL-C4 = 0.5 and (b) aIL-C4 = 0.8 as shown in Fig. 7. Similar as the common features of the solubility, and consequence the cmc of an ionic surfactant increase with the temperature, when the temperature increases from 303, 323 to 343 K, for serials samples with aIL-C4 = 0.5, the cmcs increase from 1.63, 1.77 to 2.00 mmol L1, while for aIL-C4 = 0.8, they are 2.44, 3.32 and 4.00 mmol L1, respectively. The cmcs of the mixtures with higher IL-C4 concentration are more sensitive to temperature. However, the fluorescence intensity ratio I1/I3 of both serials samples, indicating the micropolarity of micelles, decreases at higher temperature. This could be explained by the lowering the interaction between the hydrophobic alkyl tails of [C4mim]+ and CTA+, which causes the dissociation of [C4mim]+ cations from the mixed micelles, namely the micro-phase seperation of mixed CTAB/IL-C4 micelles at higher temperature. The further evidence is shown in Fig. 8, in which the average diameter of two mixed CTAB/IL-C4 micelles with different IL-C4 concentrations decreases with the temperature. At room temperature, the size of the mixed micelles with the molar fraction aIL-C4 of 0.7 is about 22 nm, it sharply decreases to 7 nm as the temperature increases to 323 K and maintains almost as a constant with further temperature increasing. The variation of the cmc with the molar fraction of IL-C4 and the temperature is shown in Fig. 9. The results indicate that when IL-C4 is added into CTAB solution, the cmc of the mixture solutions decreases first; then it increases gently with further adding of IL-C4, finally, it increases shapely when the molar fraction aIL-C4 approaches 1. Different temperatures have almost the same cmc change trends. For the same molar fraction aIL-C4, the higher the temperature is, the higher the cmc of the mixture solutions. 3.3. The template mechanism of [Cnmim]Br and CTAB to the micro/mesoporous material
Fig. 5. The concentration dependence of the emission fluorescence intensity ratio I1/I3 of pyrene in the CTAB/IL-C4 mixture aqueous solutions with different molar fractions of IL-C4 at 303 K.
Combining the phenomena of interaction between IL-C4 and CTAB as well as the characteristics of induced mesoporous products, we tentatively suggest a template mechanism scheme as shown in Fig. 10. Having a large positive-charged imidazole ring and a hydrophobic C4 tail, IL-C4 exhibits the characteristics of triple roles of electrolyte, co-solvent and co-surfactant. At synthesis
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Fig. 9. The variation of the cmc of CTAB/IL-C4 mixture aqueous solutions with the molar fraction of IL-C4 at 303, 323 and 343 K.
Fig. 7. The temperature dependence of the emission fluorescence intensity ratio I1/I3 of the CTAB/IL-C4 mixture aqueous solutions with (A) aIL-C4 = 0.5 and (B) aIL-C4 = 0.8.
Fig. 8. The change of average diameter of mixed micelles of CTAB/IL-C4 at various temperatures.
composition of aIL-C4 = 0.5, when the temperature is low, the cosolvent and co-surfactant functions of IL-C4 are dominant, and IL-C4 and CTAB molecules are miscible and can form mixed micelles in aqueous solution. Due to the relatively smaller ratio of
[C4mim]+ in the mixed micelles, the CTA+ cations are dominated. As shown in Fig. 10(A), with the cationic imidazole rings attached at the surface of micelles and C4 tails inserted into the barrier layer, the [C4mim]+ cations synergistically aggregated with CTA+ cations together to act as co-templates for the synthesis of mesoporous materials. Because of the participation of [C4mim]+ cations, the average aggregation number of CTA+ in micelles reduces and more micelles are formed in the solution, sequentially, more mesoporous channels are induced during the ageing (hydrothermal reaction) process. Comparing to the normal MCM-41 induced by CTAB along, the mesoporous materials induced by CTAB/IL-C4 cotemplate at room temperature has extremely larger surface area. With the increase of temperature, the solubility of CTAB increase and the [C4mim]+ cations can dissociate from the mixed micelles freely in the aqueous solutions, which makes mixed micelles shrink as shown in Fig. 8. However, in the synthesis situation, the co-template of mixed micelles is fully enwrapped by the hydrolytic silica sol–gels at room temperature, during the hydrothermal reaction process, with the increase of temperature, the closely packed tails in the nucleus of micelles can extend and loose up, accordingly, the pore size of the mesoporous materials increases. Only when the temperature is high enough, the reducing of attractive interaction between CTAB and IL-C4 results in the micro-phase separation of the mixed micelles, and IL-C4 aggregations are formed through the hydrogen bond-co-p–p stack interaction nearby the CTAB-rich micelles, just like the natural domain/bud formation process. The shorter C4 tails make the IL-C4 aggregations much smaller than the CTAB micelles, as shown in Fig. 10(B). In the hydrothermal process, with the CTAB-rich micelles served as mesoporous templates and the IL-C4 aggregations as microporous templates, the micro/mesoporous materials are formed. As mentioned above, because the [C4mim]+ cations only can aggregate nearby the CTAB-rich micelles, the micropores may generate in the wall of the mesoprous material. Although the small amount of IL-C4 cannot produce enough micropores to give an clear evidence in the XRD characterization, the TEM images and the N2 adsorption and desorption isothermals give a certainty of the existing of micropores in the synthesized porous materials. It can be predicted that with the increase of IL-C4 content, the amount of the micropores, as well as the microporous BET surface area will increase correspondingly, but the sizes of micropore and mesopore will not change anymore. From Table 1, we can see that increasing the molar fraction of IL-C4 from 0.5, 0.7 to 0.9, the microporous BET surface area increases 10 m2 g1 for each, whereas the sizes of mesopore and micropore are unchanged.
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Fig. 10. The template mechanism of IL cationic [C4mim]+ and CTA+ (A) synergism for mesoporous material at lower temperature, and (B) for the micro/mesoporous materials at higher temperature.
These results give a strong support for the above presumption and discussions. To verify this mechanism, another kind of IL of [C6mim]Br (ILC6) were also used as co-template. As shown in Table 1, when the hydrothermal temperature is at 373 K, the micropores exist in the porous products. The total surface areas are almost same no matter aIL-C6 is 0.4 or 0.6, but the micropores surface area increase from 25 to 37 m2 g1. The size of micropores of 0.6 nm appears no distinct difference compared with the co-template of IL-C4. As described in Fig. 10(B), the formation of the microporous structure is mainly due to the hydrogen bond-co-p–p stack of [Cnmim]+ rings; consequently, the length of the tail of IL-C6 plays an unimportant role and almost has no effect on the lamellar microporous structure. For the same weight percentage of [Cnmim]Br, the bigger the molar mass of IL-C6, the smaller the molar fractions of IL-C6, hence, the fewer the amounts of micropores, which cause the smaller microporous surface area compared with IL-C4.
4. Conclusions In summary, the IL [Cnmim]Br play different roles such as the electrolytes, co-surfactant and co-solvent at different composition regions in surfactant CTAB mixture aqueous solutions; also there are different synergistic interactions between [Cnmim]Br and CTAB at different temperatures. At room temperature, they can form mixed micelles as co-templates and induce mesoprous materials with extremely large BET surface area of 1719 m2 g1. While at a higher temperature 373 K, micro-phase separation occurs between [Cnmim]Br and CTAB, the mixed micelles are separated into CTAB dominant micelles and [Cnmim]Br molecular aggregations nearby. With CTAB micelles served as mesoporous templates and [Cnmim]Br molecular aggregations as microporous templates, respectively, the micro/mesoporous materials with distinctly different in the micropore range of 0.6 nm and the mesopore range of 2.7 nm were prepared. This strategy provides a new convenient method to prepare the micro/mesoporous materials. The key point
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is that the micro-phase separation is triggered between two components of the dual templates, from which the domain/bud formations of the suitable molecular and supramolecular aggregations successfully induce the micro/mesoporous materials in the hydrothermal process. Acknowledgements This work is supported by the National Natural Science Foundation of China (Nos. 20736002, 20776045), the National High Technology Research and Development Program of China (No. 2008AA062302), National Basic Research Program of China (2009CB219902) and Program for Changjiang Scholars and Innovative Research Team in University of China (No. IRT0721). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2010.12.011. References [1] [2] [3] [4] [5] [6] [7]
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