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Preparation of Porous Aminopropylsilsesquioxane by a Nonhydrolytic Sol-Gel Method in Ionic Liquid Solvent Yang Liu,†,‡ Meijia Wang,† Zhiying Li,† Hongtao Liu,† Ping He,† and Jinghong Li*,†,‡ State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China, and Department of Chemistry, Tsinghua University, Beijing 100084, China Received September 30, 2004. In Final Form: October 22, 2004 The synthesis of porous aminopropylsilsesquioxane (APSS) has been achieved with 1-butyl-3methylimidazolium hexafluorophosphate (BMIM+PF6-) ionic liquid (IL) as a template solvent by a nonhydrolytic sol-gel method of 3-aminopropyltrimethoxysilane under mild temperature. By the alteration of the amount of IL or the catalyst, the gels with various specific surface areas ranged from ∼600 to ∼1500 m2 g-1 were obtained after the remove of IL by calcination, and their N2 sorption isotherms were typical IV-like isotherms with H2 hysteresis. The average Battett-Jouner-Halenda pore diameters ranged from 3.2 to 5.6 nm. The results demonstrated that BMIM+PF6- was physically embodied in the silsesquioxane bulk instead of the chemical bonding. Silsesquioxane has promise in catalysis, biosensors, etc. This work would also provide a way for us to prepare a wide variety of materials by using IL as a template solvent.
1. Introduction Since the discovery of surfactant-template porous materials in the early 1990s,1,2 porous materials have attracted much attention of chemists and material scientists due to commercial interest in their application in chemical separations, heterogeneous catalysis, and chemical sensing3,4 as well as the interest in the challenges posed by their synthesis, processing, and characterization.5,6 Various strategies have been employed to prepare the porous materials. The sol-gel technique, which consists of inorganic polymerization leading to a highly cross-linked solid through a hydrolytic condensation process, has been established as an important route to prepare porous materials in mild conditions. Areogel is a novel class of porous materials and is known to exhibit the lowest density, dielectric permittivity, and thermal conductivity of any other solid materials.7 The synthesis of silica aerogel has been achieved mainly through the controlled condensation of colloidal particles produced by sol-gel processing in alcoholic aqueous solutions, followed by drying under supercritical conditions in order to prevent the collapse of the tenuous solid network. Actually, this process is expensive and dangerous. Recently, templatebased sol-gel processing has rapidly gained popularity. In this method, the silicate matrix is assembled around a suitable template, and cavities with a specific size and * To whom correspondence should be addressed. Tel: 86-1062776949. Fax: 86-10-62782485. E-mail:
[email protected]. † State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry. ‡ Department of Chemistry, Tsinghua University. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. C. Nature 1992, 359, 710. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; Higgins, S. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Corma, A. Chem. Rev. 1997, 97, 2373. (4) Collinson, M. M. Crit. Rev. Anal. Chem. 1999, 29, 289. (5) Khramov, A. N.; Munos, J.; Collinson, M. M. Langmuir 2001, 17, 8112. (6) Kruk, M.; Jaroniec, M.; Pena, M. L.; Rey, F. Chem. Mater. 2002, 14, 4434. (7) Anderson, M. L.; Morris, C. A.; Stroud, R. M.; Merzbacher, C. I.; Rolison, D. R. Langmuir 1999, 15, 674.
shape remained in the cross-linked host.8 Moreover, a wide variety of templates such as organic ligands, salts, latex spheres, and surfactants have been used and surfactants, including cationic, anionic, neutral, zwitterionic, and bolaamphiphile, are the most popular ones.9-13 It was suggested that hydrogen bonding and electrostatic interactions directed the phase formation of ionic or neutral surfactant templated porous materials, respectively.1,11,15 However, the toxicity of amines in the surfactants was a potential danger.11 Room temperature ionic liquids (RTILs), which are generally exemplified by the combination of a large organic cation with a weakly coordinating anion, have attracted significant attention in many fields of chemistry and industry as environmentally benign solvents.16-22 RTILs are commonly considered to be polar solvents but can be noncoordinating and mainly depend on the IL’s anion. By changing the nature of the ions present in an IL, it is possible to change the resulting properties of the ILs markedly. For example, the miscibility with water can be varied from complete miscibility to almost immiscible by changing the anion form. Good examples are BMIM+BF4and BMIM+PF6-. Though they have the same cation, BMIM+BF4- is miscible to water but BMIM+PF6- is (8) Schottner, G. Chem. Mater. 2001, 13, 3422. (9) Khramov, A. N.; Collinson, M. M. Chem. Commun. 2001, 767. (10) Imhof, A.; Pine, D. J. Nature 1997, 389, 948. (11) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242. (12) Wei, Y.; Jin, D.; Ding, T. Adv. Mater. 1998, 3, 313. (13) Yim, J. H.; Seon, J. B.; Jeong, H. D.; Pu, L. S.; Baklanov, M. R.; Gidley, D. W. Adv. Funct. Mater. 2004, 14, 277. (14) Green, W. H.; Le, K. P.; Grey, J.; Au, T. T.; Sailor, M. J. Science 1997, 276, 1826. (15) Behrens, P. Angew. Chem., Int. Ed. Engl. 1995, 34, 317. (16) Bagshaw, S. A.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 1102. (17) Dupont, J.; D Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002, 102, 3667. (18) Welton, T. Chem. Rev. 1999, 99, 2071. (19) Handy, S. T. Chem. Eur. J. 2003, 9, 2938. (20) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68, 351. (21) Hayakawa, Y.; Yanagi, T.; Kunimori, M. J. Am. Chem. Soc. 2001, 123, 8165. (22) Schofer, S. H.; Kaftzik, N.; Wasserscheidt, P.; Kragl, U. Chem. Commum. 2001, 425.
10.1021/la0475829 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/22/2005
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immiscible to water. Normally, ILs with anions of Cl-, BF4-, or NO3- are mostly miscible in water and those with anions of PF6- or (CF3SO2)2N- are mostly immiscible in water. Furthermore, the lipophilicity of an IL can also be modified by the degree of cation substitution. The primary solvent features of ILs usually include the capability for H-bond donation from the cation to polar solutes, H-bond accepting functionality in the anion, and π-π or C-H‚‚‚π interactions. Since RTILs have advantageous chemical and physical properties, such as negligible vapor pressure, low toxicity, low melt points, and high chemical and thermal stability, they have been widely used in organic synthesis, electrochemistry, separations, sonochemistry, biopolymer, molecular self-assembly, and biocatalysis.22-28 Recently, RTILs were also used to prepare porous silica materials; Dai et al. prepared silica aerogel using 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] amide (EtMeIm+Tf2N-) as solvent under mild chemical conditions.29 Moreover, highly ordered monolithic super-microporous lamellar silica and monolithic mesoporous silica with wormlike porous were prepared by a nanocasting technique with the templates 1-hexadecyl-3-methyl imidazolium chloride (C16MIM+Cl-) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM+BF4-), respectively, by Antonitti’s group.30,31 In this paper, aminopropylsilsesquioxane (APSS) is prepared without shrinking or cracking, which is of great interest owing to their potential applications in environmental, biochemical, and industrial processes, by a nonhydrolytic sol-gel method employing 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM+PF6-) as template solvent. BMIM+PF6- decomposes at a temperature of over 200 °C and is an environmentally benign solvent with high hydrophobicity showing a weaker hydrogen bond ability than that of BMIM+BF4- and a smaller anionic dimension than that of EtMeIm+Tf2N-. The functionalized porous silsesquioxane materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) surface and pore volume analysis. For the weak hydrogen bond ability of the BMIM+PF6-, a mechanism of physical embodiment of the RTIL in the silsesquioxane bulk was proposed. 2. Experiments 2.1. Chemicals. 3-Aminopropyltrimethoxysilane (97%) (APTMOS) was purchased from Aldrich Chemical Co. and used as received. Acetic acid (Beijing Chemical Co., 99.5%) was distilled before using. 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM+PF6-) was purchased from Acros Organics and was dried under vacuum overnight at 70 °C. 2.2. Preparation of Porous Aminopropylsilsesquioxane Gel. In a typical synthesis, APTMOS was used as the sol-gel precursor. BMIM+PF6- (0.5 mL) was mixed with 1 mL of APTMOS under mild magnetic stirring. After homogenization of the mixture, 1 mL of acetic acid was added. The mixture was (23) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. Engl. 2000, 39, 3772. (24) Hultgren, V. M.; Mariotti, A. W. A.; Bond, A. M.; Wedd, A. G. Anal. Chem. 2002, 74, 3151. (25) Pez, G. P.; Carlin, R. T.; Laciak, D. V.; Sorenson, C. US Patent 4761164. (26) Kavan, L.; Dundch, L. ChemPhysChem 2003, 4, 944. (27) Zhou, Y.; Antonietti, M. J. Am. Chem. Soc. 2003, 125, 14960. (28) Oxley, J. D.; Prozorov, T.; Suslick, K. S. J. Am. Chem. Soc. 2003, 125, 11138. (29) Dai, S.; Ju, Y. H.; Gao, H. J.; Lin, J. S.; Pennycook, S. J.; Barnes, C. E. Chem. Commun. 2000, 243. (30) Zhou, Y.; Schattka, J. H.; Antonietti, M. Nano Lett. 2004, 4, 477. (31) Zhou, Y.; Antonietti, M. Adv. Mater. 2003, 17, 1452.
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Figure 1. Typical N2 adsorption-desorption isotherm for the silsesquioxane gel of sample 3 calcined at 500 °C. stirred for 24 h and cured at ambient temperature in a simple glovebox with N2 flu. The sol-gel reaction can be described as previously reported.14 A transparent pale yellow monolith gel with no cracks was obtained after 4 weeks. BMIM+PF6- was removed by the calcination of the gel at 500 °C for 3 h or washing by ethanol. The final products were characterized as well as those containing BMIM+PF6- by crushing to powder with a mortar and pestle. For the preparation of sample 1, sample 3, and sample 5, 1, 0.5, and 0.25 mL of IL were added to the solutions of APTMS (1 mL) and HAc (1 mL), respectively. Sample 2, sample 4, and sample 6 were prepared by dropping 0.5, 1, and 0.25 mL of IL to the mixtures of APTMS (1 mL) and HAc (2 mL), respectively. 2.3. Characterization. SEM measurements were made on a XL30 ESEM FEG scanning electron microscope at an accelerating voltage of 20 kV. TEM graphs were taken with a JEOL-JEM-2010 operating at 200 kV (JEOL, Japan). Samples for TEM were prepared by dropping a diluted suspension of the sample powders onto a standard carbon-coated (20-30 nm) Formvar film on a copper grid (230 mesh). BET surface and pore volume were measured on a Quantachrome NOVA 1000 Ver 6.11 system at 77.4 K. All samples were first degassed in a vacuum at 200 °C for 2 h before analysis. The BET specific area was calculated from the nitrogen adsorption data in the relative pressure range from 0.01 to 0.3. The total volume was estimated from the amount adsorbed at a relative pressure of about 0.99. FT-IR was conducted using a FTS135 infrared spectroscopy (BIO-RAD, USA). FT-IR (4000-400 cm-1) of the gels was obtained by forming thin transparent KBr pellets containing the samples and that of the BMIM+PF6- was obtained by brushing the sample on the pure KBr slice. XPS was conducted using a VG ESCALAB MK II spectrometer (VG Scientific, U.K.) employing a monochromatic Mg KR X-ray source (hν ) 1253.6 eV). Peak position was internally referenced to the C1s peak at 284.6 eV. XRD was recorded on a PW1710 BASED X-ray diffractometer using Cu KR radiation (1.5406 Å) of 40 kV and 30 mA. TGA was carried out using a Perkin-Elmer 7 series TGA system. Measurements were conducted by heating the sample from 20 to 700 °C at a heating rate of 10 °C/min under N2 atmosphere.
3. Results and Discussion Figure 1 shows the typical N2 adsorption-desorption isotherm for the APSS of sample 3 calcined at 500 °C. The samples prepared with BMIM+PF6- as template exhibited
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Table 1. Properties of the Porous APSS Gels after the Removal of BMIM+PF6- by Calcination av pore special surface APTMS HAc IL (P) total pore diameter 3 (nm) area (m2/g) sample (mL) (mL) (mL) vol (cm /g) 1 2 3 4 5 6
1 1 1 1 1 1
1 2 1 2 1 2
1 0.5 0.5 1 0.25 0.25
1.25 0.76 1.27 0.45 1.21 1.10
3.2 5.1 5.6 3.4 3.2 3.3
1565 600 901 527 1524 1335
type VI isotherms with type H2 hysteresis loops.32 These characteristics were similar to those observed for aerogels prepared by solution extraction,29,33 which can be attributed to the capillary condensation within the pores of the gels. The inset in Figure 1 presents Barrett-JounerHalenda (BJH) pore sizes and their distribution for sample 3. On comparison of the surfactant-template mesoporous materials such as MCM-41, a relatively wide pore size distribution was observed, which was in accord with the relatively smooth increase in the N2 absorbing process of the isotherm. However, pore sizes about 3 and 5 nm were prominent in the porous material. Table 1 lists the BET surface area data, total pore volumes, and pore parameters of the samples after calcination. As the structures and properties of silica gel depended strongly on the processing parameters, such as amount of the template and the nature and amount of the catalyst used, and so on, several samples with different volume ratio of catalyst to silicon and amount of BMIM+PF6- were investigated and were summarized in Table 1. In general, an increase in the ratio of catalyst (acetic acid) to silicon resulted in a large BET surface area and pore volume of the sample powder after calcinations. In the case of sample 1 and sample 2, as the volume ratios of catalyst to silicon decreased from 2 to 1, the surface area increased from 600 to 901 m2 g-1 and the pore volume increased from 0.76 to 1.27 cm3 g-1. For samples 2, 4, and 6, which have the same ratio of catalyst to silicon but different amounts of BMIM+PF6-, the surface areas increased from 527 to 1335 m2 g-1 and the pore volumes increased form 0.45 to 1.04 cm3 g-1 as the amounts of BMIM+PF6- decreased from 1 to 0.25 mL. It was thought that as the amount of BMIM+PF6decreased, smaller RTIL “microspheres” formed and larger surface areas were obtained. Figure 2 shows the pore diameter of the gels prepared with different amount of BMIM+PF6-. It was shown that pore diameters of ca. 3 nm dominated in all the samples except that the pores with diameter of ca. 3 nm had a more important function in the APSS gels prepared with smaller BMIM+PF6-. The APSS gels prepared at ambient temperature and calcined at 500 °C were characterized. Figure 3a presents the typical SEM image of the bulk morphology of the sample 3 calcined at 500 °C. Morphology of an interconnected network of islandliked nanopaticles was observed. The diameter of the nanopaticles in the monolith was about 10 nm. Figure 3b shows the TEM image of the morphology and structure of sample 6 prepared with the template of BMIM+PF6- and calcined at 500 °C. On comparison with that of Figure 3a, denser and smaller pores were observed. This result was in accord with that of the distribution of pore diameter. Figure 4 shows the X-ray diffractograms of the BMIM+PF6- (a) and the BMIM+PF6- templated APSS gel (32) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J. Pure Appl. Chem. 1985, 57, 603. (33) Morris, C. A.; Anderson, M. L.; Stroud, R. M.; Merzbacher, C. I.; Rolison, D. R. Science 1999, 284, 622.
Figure 2. The BJH pore diameter distribution of the APSS gels prepared with different amounts of BMIM+PF6-: 0.5 mL in sample 2, 0; 1 mL in sample 4, O; and 0.25 mL in sample 6, 3.
Figure 3. The typical SEM (a, sample 3) and TEM (b, sample 6) images of the APSS gel after the remove of RTIL by calcinations at 500 °C. The scale bars are 200 nm (a) and 10 nm (b), respectively.
without calcinations (b) and with calcinations (c). In Figure 4a, two peaks at 2θ ) 19.5° and 2θ ≈ 14° of BMIM+PF6were observed. This result was in accord with that of
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Langmuir, Vol. 21, No. 4, 2005 1621 Table 2. The Main IR Bands of BMIM+PF6- a wavelength of the bands (cm-1)
vibration
3170, 3125 2965, 2940, 2878 1574,1466 1432, 1387 1169 1113 837 747
ν(C-H) aromatic, str ν(C-H) aliphatic, str ν(ring, C-N), str sym δ(Me C-H), asym ring, str sym ring, C-H, i/p, bend ν(P-F) ring, o/p, b, asym
a Abbreviations for the tables: str, stretching; b, bending; i/p, in plane; o/p, out of plane; sym, symmetrical; asym, asymmetrical; as, association.
Figure 4. The XRD patterns of BMIM+PF6- (a) and the BMIM+PF6- templated APSS gel before (b) and after (c) the remove of BMIM+PF6-.
Figure 5. The FT-IR spectra of BMIM+PF6- (a), the BMIM+PF6- templated APSS gel (b), and the APSS gel calcined at 500 °C (c).
Billard et al.32 It was proposed that BMIM+PF6- was purely monophasic, although it presented a local organization and the result was consistent with the hypothesis that the ions were associated as ion pairs in the liquid state. As the gel formed, a novel peak of BMIM+PF6- at 2θ ) 20.5° was observed. After the removal of BMIM+PF6-, a wide peak at ca. 22.5° (Figure 4c) was shown for the amorphous structure of the porous APSS gel. The results indicated the formation of APSS gel in the BMIM+PF6-. Moreover, the silsesquioxane framework coexisted with BMIM+PF6- before the remove of RTIL. Figure 5 shows a comparison of the typical FTIR spectra of BMIM+PF6- (a), the BMIM+PF6- templated APSS gel (b), and the APSS gel calcined at 500 °C (c). The peaks at 3170 and 3125 cm-1 in Figure 5a were attributed to C-H stretch of the imidazole ring, while those below 3000 cm-1 were attributed to aliphatic stretches. The main IR spectra of BMIM+PF6- are given in Table 2. The strong peaks at ∼1094 and ∼797 cm-1 in Figure 5b and Figure 5c were attributed to the Si-O-Si asymmetric vibration and the O-Si-O vibration of the as-synthesized and calcined APSS gels respectively, indicating the formation of the APSS framework. In Figure 5c, a band
at 3429 cm-1 was observed for the stretching vibrations of NH2 on the silsesquioxane gel bulk. The Si-O asymmetric stretching bands (1094 and 1048 cm-1) illustrated the long-chain linear siloxanes.34 The presence of bands at 1634 cm-1(-NH2 bending vibration), 1457 cm-1, and 1331 cm-1 (C-H bending vibration) also indicated the existence of -(CH2)3NH2 in the silsesquioxane gel bulk. The bonding energies of N1s (399.2 eV) in XPS spectra further confirmed the existence of NH2, and no bonding energies of fluorine were observed (not shown). It is believed that the anion of the RTIL has great influence on the structure of the porous silica gel. Dai et al. synthesized the silica aerogel in 1-ethyl-3methylimidazolium bis[(trifluoromethylsulfonyl)]amide (EMIM+Tf2N-) solution, and a wide pore size distribution was obtained as well with our experiments. It was supposed that the EMIM+Tf2N- was entrapped in the silica network instead of chemically bound to the APSS bulk.29 On the other hand, a hydrogen bond-co-π-π stack mechanism was proposed by Antonitti’s group for monolithic mesoporous silica prepared by using BMIM+BF4- as a template solution. Considering the distinction with that of Dai, Zhou et al. believed that the large space structure of the Tf2N- blocking formation of the π-π stack resulted in a disordered arrangement and a wide pore size distribution.30 The ring symmetric stretching vibration of BMIM+PF6- was observed to shift from 1574 to 1560 cm-1. Strong and broad peaks occurred in the regions of 13501450 and 1070-1181 cm-1 corresponding to the ring stretching and bending vibration of BMIM+PF6-. A shift toward low wavelength was observed on comparison with that in Figure 5b, indicating the existence of the interaction on the ring of BMIM+PF6-. It was proposed that a π-π stacking interaction existed between the imidazolium rings of BMIM+PF6-.30 We also investigated the interaction of the BMIM+PF6- and the APSS bulk with IR spectra. Cammarata et al. has found that the strength of the hydrogen bonds between -OH and anions of RTIL increased in the order of [PF6]- < [SbF6]- < [BF4]- < [(CF3SO2)2N]- < [ClO4]- < [CF3SO3]- < [NO3]- < [CF3CO2]-.35 Moreover, BMIM+PF6- had the same cation with that of BMIM+BF4- and had a smaller anionic dimension than that of EtMeIm+Tf2N-. It was observed that the P-F vibration peak of the as-synthesized APSS (Figure 5b) was a little positive shift comparing the free BMIM+PF6- (Figure 5a), instead of the negative shift of hydrogen bond interaction, which indicated that nearly (34) Anderson, D. R. In Analysis of Silicones; Smith, A. L., Ed.; Wiley: New York, 1974; Vol. 1, pp 247-286. (35) Camarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Phys. Chem. Chem. Phys. 2001, 3, 5192. (36) Groselj, N.; Gaberrscek, M.; Krasovec, U. O.; Orel, B.; Drazic, G.; Judeinstein, P. Solid State Ionics 1999, 125, 125.
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no hydrogen bond interaction existed between BMIM+PF6and silica bulk for the as-synthesized silsesquioxane. This could be the reason why the monolithic mesoporous silica with wormlike pores cannot be obtained. It was also indicated that the silsesquioxane framework was physically embodied by BMIM+PF6- in our cases. The results showed that both the cation and anion of RTIL were important for the formation of mesoporous silica gel. On comparison with the spectra of Figure 1a, a large and wide peak in Figure 5b was observed at high wavenumber, which was attributed to the O-H of HAc and methanol, the N-H of siloxane that resided in the complex system, and the interaction of hydrogen bonds.36 A possible mechanism for the formation of the APSS gel physically enwrapped by BMIM+PF6- in the silsesquioxane bulk is as follows: The APSS sol aggregated after the formation of the sol, and then the APSS gel formed. The TGA data indicated that BMIM+PF6- in the gel began to decompose at 261 °C. The higher decomposition temperature gave the gel framework longer time for the process of condensation and densification at increased temperatures which is better for the formation of aerogel.
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4. Conclusion In conclusion, a series of porous APSS gels have been prepared in BMIM+PF6 solutions by a nonhydrolytic solgel method with 3-aminopropyltrimethoxysilane under mild temperature. By alteration of the amount of IL or catalyst, the specific surface area of the gel ranged from ∼600 to ∼1500 m2 g-1 after calcinations and the N2 sorption isotherm was a typical IV-like isotherm with H2 hysteresis. Hydrogen bond and electrostatic forces between the silicate species and IL were proposed to be responsible for the formation of the porous APSS gel. With the environmentally benign properties of the room temperature ionic liquids, porous gel shows promise for applications in the fields of catalysis, biosensors, etc., and provides an alternative way for us to synthesis novel porous materials. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20125513, No. 20435010), Heritage Prize from Li Foundation, USA, and Foundation for the Author of National Excellent Doctoral Dissertation of PR China. LA0475829
