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Ionic liquid templated high surface area mesoporous silica and Ru–SiO2
Microporous and Mesoporous Materials 91 (2006) 40–46 www.elsevier.com/locate/micromeso
Ionic liquid templated high surface area mesoporous silica and Ru–SiO2 Kake Zhu, Franc Pozˇgan, Lawrence D’Souza, Ryan M. Richards
*
School of Engineering and Sciences, International University Bremen, Campus Ring 8, 28759 Bremen, Germany Received 1 September 2005; received in revised form 25 October 2005; accepted 2 November 2005 Available online 4 January 2006
Abstract Imidazole type ionic liquid 1-hexadecyl-3-methylimidazolium chloride and 1-hexadecyl-3-methylimidazolium ruthenium hexachloride were used to fabricate the well-ordered hexagonal mesoporous silica (under acidic and basic conditions), and Ru–SiO2, respectively. Small angle X-ray diffraction, scanning electron microscopy, transmission electron microscopy and N2 adsorption–desorption were employed to characterize the structure. The mesoporosity of the silica matrix is preserved after calcination of organic templates. The pore diameter of silica prepared from acidic media (2.20 nm) was smaller and less ordered than that of silica from basic solution (2.74 nm). Further, the mesoporous structure of Ru–SiO2 was found to be preserved after calcination, however, RuO2 nanoparticles formed in the matrix of mesoporous silica upon calcination. Ó 2005 Elsevier Inc. All rights reserved. Keywords: High surface area; Mesoporous silica; Ru–SiO2; Ionic liquid; Imidazole; RuO2
1. Introduction The discovery of M41S silica by Mobil researchers [1] in 1992 stimulated a great deal of interest in the area of mesoporous materials. Numerous studies followed on both the mechanism of formation of mesoporous materials and the application of these materials in heterogeneous catalysis, homogeneous catalysis, chromatographic separations, etc. [2–4]. Subsequent studies showed that primary amines, quaternary ammonium ions and block polymers can template the formation of mesoporous materials, in which the mesophase is determined by the electrostatic, hydrogen-bonding, or van der Waals interactions between the amphiphilic head of the template and the silica precursors in acidic or basic conditions [5–8], according to the chargedensity-match theory [2]. Various quaternary ammonium surfactants have been employed to template the formation
*
Corresponding author. Tel.: +49 421 200 3236; fax: +49 421 200 3229. E-mail address: [email protected] (R.M. Richards).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.11.013
of mesoporous materials, it was found that the phase of these mesoporous materials are determined by a g factor, g = V/a0L, where V is the volume of surfactant chains plus co-solvent molecule, a0 is the effective head area at the micelle surface and L is the kinetic surfactant chain length. The mesophase can also be controlled by adjusting the g factor, g 1/3 is for cubic mesophase Pm3n, g 1/2 is for hexagonal p6m, g 1 is for lamellar, while 1/2 < g < 2/3 is for cubic Ia3d [2]. Room temperature ionic liquids (RTILs) with imidazole heads have a unique a0 value as compared to quaternary ammonium surfactants and possess a positively charged ring with a negatively charged anion, thus providing a way to prepare mesoporous materials with tailored pores. RTILs are organic salts with low melting points, which possess a wide temperature range of liquid phase, high conductivity, thermal stability, negligible vapor pressure, and are therefore widely used as environmentally benign ‘‘green’’ solvents [9]. Since most long chain RTILs possess both a hydrophilic ionic head and a hydrophobic organic chain, they are also one category of surfactants and can form liquid crystals in various solvents [10], which were
K. Zhu et al. / Microporous and Mesoporous Materials 91 (2006) 40–46
used as templates to prepare microporous and mesoporous materials in some recent publications. Seddon and coworkers first reported a preparation of mesoporous silica with hexagonal structure in mild basic (pH = 9) conditions by using the ionic liquid template N,N-dialkylimidazolium [11], which is similar in structure to MCM-41, however, this material was only characterized by the small angle X-ray diffraction (SAXRD) and N2 adsorption–desorption isotherms, and the mesoporosity is not as regular as that of MCM-41, as evidenced by the N2 adsorption–desorption isotherm. A recent work suggests that the silica templated by this RTIL from mild basic solution is mesoporous silica nanoparticles after RTIL extraction by ethanol and HCl mixtures [12], as confirmed by microscopic images. Zhou et al. synthesized mesoporous titania nanoparticles and silica monolith materials (mesophase of lamellar structure with pore sizes between 1 and 2 nm) using the same family of RTILs [13–15], 1-butyl-3-methylimidazolium-tetrafluoroborate and 1-alkyl-3-methylimidazolium-chlorides, the templates can be removed by acetonitrile extraction or calcination. Studies by Senadeera et al. showed that addition of ionic liquids to mesoporous oxides structure can promote the charge transfer ability of the material [16], additionally, RTIL assisted preparations of nanostructured TiO2 particles and zeolites have also been reported very recently [17,18]. In this study, RTIL 1-hexadecyl-3-methylimidazolium chloride (Fig. 1(A)) was employed to template well-ordered hexagonal mesoporous silica in both strongly acidic and basic conditions; the structures of mesoporous silica were well resolved and found to possess extraordinarily large surface area and highly regular hexagonal pore structures. Furthermore, N,N-dialkylimidazolium chloride can form chlorometalates with metal chlorides [10(b),19,20], thus providing a facile method to introduce metal centers into the mesophase material. Further, ruthenium containing N,N-dialkylimidazolium chloride was prepared and employed to template mesoporous Ru–SiO2, which is superior to previously reported tris(bipyridine) ruthenium (II) [21] or bipyridine phenanthroline complexes [22] templates in simplicity of preparation. The inclusion of ruthenium oxide nanoparticles in the mesoporous silica is particularly interesting because such systems are potentially important for a range of catalytic reactions including Fischer–Tropsch methanation of CO2, selective hydrogenation of cycloalkenes, oxidations and electrocatalysis [23].
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2. Experimental 2.1. Materials 1-Methylimidazole (99%) was obtained from Aldrich. 1Chlorohexadecane (P97%) and tetraethyl orthosilicate (TEOS, P99%) were obtained from Fluka, HCl (37 wt%), NaOH (97%), RuCl3 Æ 3H2O and tetrahydrofuran (THF, 99.8%) are from Applichem Company. All materials in this work were used without further purifications. Synthesis of ionic liquid 1-hexadecyl-3-methylimidazolium chloride (116IL): 116IL was synthesized according to reported procedures [24], 1-hexadecyl chloride (65.24 g, 0.25 mol) and 1-methylimidazole (20.54 g, 0.25 mol) were mixed and refluxed at 363 K for 24 h, then cooled to room temperature to obtain a waxy white solid, which was dispersed in THF to recrystallize, then washed with THF three times. A white powder product was collected and dried under vacuum at room temperature. Mesoporous silica prepared in acidic conditions: in a typical synthesis, 0.600 g 116IL was dissolved in 20.0 ml 2 M HCl under vigorous stirring at room temperature, 2.08 g of TEOS was added to the system 2 h later. After continuously stirring for 24 h, the mixture was transferred to a teflon autoclave, which was placed in an oven at 373 K for 5 days. After filtration and washing with water, the white powder was dried at 373 K for 8 h, which is denoted as AAP (acid as prepared). Thermogravimetric (TG) and differential thermal analysis (DTA) study before calcination showed that combustion and removal of organic templates happens at around 653 K. Samples subsequently calcinated in air by a ramp of 3 K/min at 775 K for 8 h are denoted as AAC (acid as calcined). Mesoporous silica prepared in basic conditions follows the procedure: 0.30 g NaOH was dissolved in 31.86 g H2O, to which 0.6174 g 116IL was added under stirring, after 3 h 3.12 g TEOS was added dropwise under stirring at room temperature. After stirring for 24 h, the mixture was transferred to a teflon autoclave and kept in an oven at 373 K for 5 days. A white powder was filtrated and collected, after drying in an oven at 373 K. These samples were labeled as BAP (basic as prepared) and those calcined at 773 K for 8 h as BAC (basic as calcined). TG and DTA showed combustion of the template at the same temperature as of AAC sample.
Fig. 1. (A) Molecular structure of 1-hexadecyl-3-methylimidazolium chloride (116IL) and (B) 1-hexadecyl-3-methylimidazolium ruthenium hexachloride.
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To prepare the template 1-hexadecyl-3-methylimidazolium ruthenium hexachloride (Fig. 1(B)), 1.029 g 1-hexadecyl-3-methylimidazolium and 0.261 g RuCl3 Æ 3H2O was refluxed in 50.0 ml CH3CN for 12 h, the mixture was recrystallized, filtrated and washed with THF, the solid sample was collected and dried at room temperature under vacuum. To prepare the ruthenium mesoporous silicate, 0.97 g of 1-hexadecyl-3-methylimidazolium ruthenium hexachloride was dissolved in 10.0 g of 2 M HCl aqueous solution together with 4.0 g H2O under constant stirring at 333 K for 3 h. Then, 2.08 g TEOS was added under stirring, after 24 h, the yellowish solid was filtered and washed with water to remove the free ruthenium species from the mesoporous silicate surface, then dried at 443 K for 5 h. Calcination of the solid was conducted at 673 K for 12 h in air, which removed the organic template from the mesoporous composite, as TG-DTA showed a weight loss and exothermal peak at around 653 K. 2.2. Analyses SAXRD was conducted using a Siemens Diffractometer D5000 with Cu Ka (k = 0.15406 nm, 40 kV, 40 mA) radiation, from 1.5° to 10° at a scanning speed of 0.06°/min. TG and DTA experiments were performed with a SDT Q600 from TA instruments in air at a ramp of 3 K/min from room temperature to 1273 K. N2 adsorption–desorption isotherms at 77 K were performed with a Quantachrome Autosorb1-C system, the data were analyzed by employing the BJH (Barrett–Joyner–Halenda) method, pore volume and pore size distribution curves were obtained from the desorption branch of the isotherm. All samples were heated for 8 h at 350 °C under vacuum prior to physisorption experiments. SEM was carried out on a modified JEOL JSM-5900 with a LaB6-cathode, the accelerated voltage for silica and Ru–SiO2 are 15 kV and 4 kV, respectively. High resolution TEM (HRTEM) images were obtained with a JEOL 200CX electron microscope operating at 200 kV. Inductively coupled plasma-atomic emission spectrometry (ICP-AES) was employed to determine the ruthenium content in the calcined sample.
3. Results and discussion SAXRD of the as prepared and calcined silica from acidic solutions are shown in Fig. 2(A), three prominent peaks in the range 2h = 2.0–5.0° can be identified and indexed to a two-dimensional hexagonal structure (p6m), which is similar to MCM-41 [1], after calcination the peaks shifted slightly to higher angles, due to shrinkage of the crystal structure during removal of the organic templates. The diffractions of d100, d110 and d200 planes for the calcined sample are listed in the inset, a unit cell of a0 = 3.76 nm can be referred from a0 = 2d100/31/2, where d100 was obtained from the first peak of the XRD pattern, d100 = k/2 sin h, k is the wavelength for Cu Ka. For the mesoporous silica synthesized from basic media (Fig. 2(B)), four peaks are found and characterize a hexagonal structure (p6m) similar to that prepared from acidic solutions, one more peak corresponding to d210 can be identified for both the as prepared and calcined samples, this suggest that 116IL templated silica in basic media is more ordered than that prepared in acidic conditions. After removal of the 116IL template by calcination at 773 K, the peaks of BAC shifted to higher angles and broadened as compared with BAP, which indicate shrinkage and less ordering of the mesoporous structure by calcination. a0 = 4.32 nm can be deduced for the BAC sample, which is 0.56 nm larger than the AAC sample, this shows mesoporous silica prepared in basic media has a much bigger unit cell value than that of those synthesized in acidic conditions from the same imidazole ionic liquid template. The small angle X-ray diffraction pattern obtained from the mesoporous Ru–SiO2 can be indexed to a two-dimensional p6m structure, which shows d100 and d110 diffractions at 2h = 2.09° and 3.64°, respectively (shown in Fig. 2(C)), corresponding to a d100 = 4.23 nm, a pore-to-pore distance of 4.88 nm (a = 2d100/31/2) is deduced. This indexation clearly shows the existence of a hexagonal mesoporous phase in the three samples described above both before and after calcination. N2 adsorption–desorption curve at 77 K for calcined mesoporous silica prepared in acidic and basic media are shown in Fig. 3(A) and (B), respectively, which exhibit a transition between type I and IV isotherms without
Fig. 2. Small angle X-ray diffraction pattern of as prepared and calcined mesoporous silica in both (A) acidic, (B) basic conditions and (C) mesoporous Ru–SiO2.
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Fig. 3. N2 adsorption–desorption isotherms of the 116IL templated mesoporous silica from (A) acidic, (B) basic conditions and (C) N2 adsorption– desorption isotherm of the mesoporous Ru–SiO2 and pore size distribution.
obvious hysteresis loops, which suggests that the adsorption of nitrogen is near to a monolayer and the adsorption curve overlaps with the desorption branch very well, indicating a high degree of pore structure ordering and no blocking effect during the desorption process. Pore size distributions (inset of Fig. 3(A) and (B)) from the Barrett– Joyner–Halenda (BJH) method by the desorption branch of the isotherm are pretty sharp with a mean pore diameter of 2.20 and 2.74 nm for AAC and BAC, respectively, indicating regular pore structure of mesoporosity in both cases. From the unit cell value referred from SAXRD, pore wall thicknesses of 1.56 and 1.58 nm can be deduced, respectively, for AAC and BAC samples. The corresponding Brunauer–Emmett–Teller (BET) and BJH surface areas are listed in Table 1, which shows that silica synthesized from acidic media (>1600 m2/g) has a larger surface area than the one from basic media (1032 m2/g). The pore volume for AAC (0.96 cm3/g) is smaller than that of BAC (0.99 cm3/g) due to the pore size difference between AAC and BAC. These results are similar to Zhou’s work on ionic liquid casted supermicroporous lamellar phase silica [14], Table 1 Small angle X-ray diffraction and N2 adsorption–desorption analysis of pore structure and surface area Sample
d100, nm
SBET, m2/g
SBJH, m2/g
Pore vol., cm3/g
Pore size, nm
AAC BAC Ru–SiO2
3.26 3.74 4.23
1641 1032 478.0
1691 1400 627.9
0.96 0.99 0.65
2.20 2.74 3.44
which show imidazole ionic liquids can template ordered mesoporous silica as well as supermicroporous lamellar silica. However, Seddon and co-workers [11] found a hysteresis loop at P/P0 > 0.5 for the mesoporous silica templated from 116IL in basic media, because they adopted a different composition of starting materials in mild basic media (pH = 9), and as a result, the mesoporous silica produced is not as ordered as that from present method, the surface area obtained by them was 992 m2/g. The nitrogen adsorption–desorption curve (Fig. 3(C)) for Ru–SiO2 shows that the calcined sample has a BET surface area of 478.0 m2/g and a BJH desorption curve gives a pore volume of 0.65 cm3/g with a narrow pore size distribution of around 3.44 nm, which confirmed the regularity of the mesostructure within the Ru–SiO2. Scanning electron microscopy (SEM) was employed to show the morphological difference between the mesoporous silica prepared in both acidic and basic conditions. As shown in Fig. 4, there is no obvious difference that can be discerned for the mesoporous silica prepared in acidic condition (Fig. 4(A) and (B)) and basic condition (Fig. 4(C) and (D)). Both have a ‘‘cauliflower’’ like structure of less than 100 lm upon aggregating together, which is quite different from the typical morphology of either MCM-41 or SBA-15 [1,8]. For Ru–SiO2, instead of a ‘‘cauliflower’’ morphology, sponge-like layered sheets were observed in a typical image, as shown in Fig. 4(E) and (F). Transmission electron microscopy (TEM) images of calcined silica samples are shown in Fig. 5. Regular hexagonal mesopores can be identified perpendicular (Fig. 5(D)) to
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K. Zhu et al. / Microporous and Mesoporous Materials 91 (2006) 40–46
Fig. 4. Scanning electron microscopy images of samples prepared in (A) and (B) acidic, and (C) and (D) basic conditions and (E) and (F) Ru–SiO2 after calcination.
the channels (along (0 0 1) direction), together with the selected area electron diffraction (SAED) (inset) clearly demonstrating the structure of the mesoporosity. A poreto-pore distance of around 3.8 nm can be calculated from the scale bar for the AAC sample, which is consistent with the SAXRD results. Images taken from the direction parallel to the pore channels for both AAC and BAC samples possess the same regular mesoporous channels. Moreover, no mesoporous nanoparticles of silica were found from these samples, which suggest that the samples prepared by the present method are quite different from previously reported ones using the same template [11,12]. This can be explained in terms of pH values, under strongly acidic/basic conditions the chances of forming nanoparticles are lower, while under mild basic conditions only mesoporous silica nanoparticles are prepared, as confirmed by both N2 adsorption–desorption and TEM. It seems to be that the imidazole head in the template is sensitive to the pH value of the media in tailoring the morphologies. Fig. 6 shows the morphology of the Ru–SiO2 structure, hexagonal pore arrays of p6m can be identified (Fig. 6(A)), together with the regular channels of mesoporous pores
and pore wall structures (Fig. 6(B)) for the as prepared sample. An approximate 5.0 nm pore-to-pore distance can be calculated from the images, which is consistent with the SAXRD results. After calcinations, the mesoporosity became less obvious and there are RuO2 nanoparticles ranging between 6 and 10 nm formed during the calcination, as shown in Fig. 6(C) and (D). The aggregation of RuO2 to form nanoparticles between the channels partially destroyed the regularity of the pore system, thus, the nanoparticles are embedded inside the silica matrix. However, the mesoporosity of the silica remained after calcinations, as evidenced by SAXRD and N2 adsorption–desorption curves. In order to confirm that the nanoparticles formed inside silica after calcinations are RuO2, a wide angle X-ray diffraction was conducted, as shown in Fig. 7. The peaks observed in the pattern confirmed the structure as RuO2 (as referenced to JCPDS 88-0323). From the Scherrer equation a mean particle size of 8 nm can be obtained, which is consistent with the TEM results. Elemental analysis by ICP-AES gave a ruthenium content of 1.21 wt% in the calcined mesoporous silicate.
K. Zhu et al. / Microporous and Mesoporous Materials 91 (2006) 40–46
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Fig. 5. Transmission electronic microscopy images from calcined mesoporous silica (A) and (B) AAC and (C) and (D) BAC, respectively.
Fig. 6. TEM images of the morphology and pore structure of the as prepared (A) and (B) mesoporous Ru–SiO2 and (C) and (D) calcined Ru–SiO2: (C) inset is a HRTEM of RuO2 crystal nanoparticle inside the silica matrix.
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K. Zhu et al. / Microporous and Mesoporous Materials 91 (2006) 40–46
Acknowledgements Authors thank Mr. A. Hoppe, International University Bremen (IUB) for helping with SEM facility. K.Z. and L.D. thank IUB for the fellowship; RR thanks IUB for start-up funds. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.micromeso.2005.11.013. Fig. 7. Wide angle X-ray diffraction of Ru–SiO2 after calcination at 673 K for 12 h.
In this method, the ruthenium species are introduced into silica as a template, as such, the loading can only be adjusted over a small range because altering the ruthenium content also means altering the surfactant/silica ratio, which determines the formed mesophase in the self-assembly process. The particle size can also be influenced by the calcination process, however this is also limited to small range as the organic species can only be removed above 380 °C and temperatures exceeding 600 °C may damage the mesostructure. 4. Summary and conclusion In summary, an imidazole type ionic liquid was employed to template mesoporous silica in both strongly acidic and basic conditions to produce hexagonal mesoporous silica with a high surface area and ordered mesoporous pores. In acidic conditions, the template interacts with the inorganic precursor as S+X I+, where S is the positively charged surfactant (imidazole head), X is the Cl and I+ is a positively charged inorganic precursor after TEOS hydrolysis. In basic conditions, however, they interact with each other as S+I . We believe the observed difference in surface area is because the mesoporous silica from acidic media is less well ordered and the pores are smaller, as shown by the SAXRD (only three peaks can be identified from acidic media synthesized silica), which means the inner surface of the pore wall is less ordered than that of the basic media synthesized silica and hence has lower surface area. Further, a ruthenium containing imidazole ionic liquid was also used to form ruthenium–silicate in acidic media with TEOS, the mesoporous ruthenium silicate formed RuO2 nanoparticles inside the mesoporous silica matrix after calcination, which provides a new way to include metal oxide nanoparticles in mesoporous silica.
References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] (a) Q.S. Huo, D.I. Margolese, U. Ciesla, P.Y. Feng, T.E. Gier, P. Sieger, R. Leon, P.M. Petroff, F. Schuth, G.D. Stucky, Nature 368 (1994) 317; (b) Q.S. Huo, D.I. Margolese, G.D. Stucky, Chem. Mater. 8 (1996) 1147; (c) S.T. Hyde, Pure Appl. Chem. 64 (1992) 1617. [3] D. Trong On, D. Desplantier-Giscard, C. Danumah, S. Kaliaguine, Appl. Catal. A-Gen. 222 (2001) 299. [4] A. Corma, Chem. Rev. 97 (1997) 2373. [5] S. Oliver, A. Kuperman, N. Coombs, A. Lough, G. Ozin, Nature 378 (1995) 47. [6] P.T. Tanev, T.J. Pinnavaia, Science 271 (1996) 1267. [7] S.S. Kim, W.Z. Zhang, T.J. Pinnavaia, Science 282 (1998) 1302. [8] D.Y. Zhao, J.L. Feng, Q.S. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [9] T. Welton, Chem. Rev. 99 (1999) 2071. [10] (a) T.A. Bleasdale, G.J.T. Tiddy, E. Wyn-Jones, J. Phys. Chem. 95 (1991) 5385; (b) C.K. Lee, H.H. Peng, I.J.B. Lin, Chem. Mater. 16 (2004) 530. [11] C.J. Adams, A.E. Bradley, K.R. Seddon, Aust. J. Chem. 54 (2001) 679. [12] B.G. Trewyn, C.M. Whitman, V.S.Y. Lin, Nano Lett. 4 (2004) 2139. [13] Y. Zhou, J.H. Schattka, M. Antonietti, Nano Lett. 4 (2004) 477. [14] Y. Zhou, M. Antonietti, J. Am. Chem. Soc. 125 (2003) 14960. [15] Y. Zhou, M. Antonietti, Chem. Mater. 16 (2004) 544. [16] G.K.R. Senadeera, K. Nakamura, T. Kitamura, Y. Wada, S. Yanagida, Appl. Phys. Lett. 83 (2003) 5470. [17] K. Yoo, H. Choi, D.D. Dionysiou, Chem. Commun. (2004) 2000. [18] E.R. Cooper, C.D. Andrews, P.S. Wheatley, P.B. Webb, P. Wormald, R.E. Morris, Nature 430 (2004) 1012. [19] P.J.E.L. Dullius, A.Z. Suarez, S. Einloft, R.E. De Souza, J. Dupont, Organometallics 17 (1998) 815. [20] P.B. Hitchcock, K.R. Seddon, T. Welton, J. Chem. Soc., Dalton Trans. 17 (1993) 2639. [21] H.B. Jervis, M.E. Raimondi, R. Raja, T. Maschmeyer, J.M. Seddon, D.W. Bruce, Chem. Commun. (1999) 2031. [22] V.W.W. Yam, B. Li, N. Zhu, Adv. Mater. 14 (2002) 719. [23] R.M. Richards, in: J.A. Schwarz, C. Contescu, K. Putyera (Eds.), Metal Oxide Nanoparticles, Encyclopedia of Nanoscience and Nanotechnology, Marcel Dekker Inc., New York, 2004, p. 1905. [24] K.R. Seddon, A. Stark, M.J. Torres, Pure Appl. Chem. 72 (2000) 2275.
Ionic liquid templated high surface area mesoporous silica and Ru–SiO2 Kake Zhu, Franc Pozˇgan, Lawrence D’Souza, Ryan M. Richards
*
School of Engineering and Sciences, International University Bremen, Campus Ring 8, 28759 Bremen, Germany Received 1 September 2005; received in revised form 25 October 2005; accepted 2 November 2005 Available online 4 January 2006
Abstract Imidazole type ionic liquid 1-hexadecyl-3-methylimidazolium chloride and 1-hexadecyl-3-methylimidazolium ruthenium hexachloride were used to fabricate the well-ordered hexagonal mesoporous silica (under acidic and basic conditions), and Ru–SiO2, respectively. Small angle X-ray diffraction, scanning electron microscopy, transmission electron microscopy and N2 adsorption–desorption were employed to characterize the structure. The mesoporosity of the silica matrix is preserved after calcination of organic templates. The pore diameter of silica prepared from acidic media (2.20 nm) was smaller and less ordered than that of silica from basic solution (2.74 nm). Further, the mesoporous structure of Ru–SiO2 was found to be preserved after calcination, however, RuO2 nanoparticles formed in the matrix of mesoporous silica upon calcination. Ó 2005 Elsevier Inc. All rights reserved. Keywords: High surface area; Mesoporous silica; Ru–SiO2; Ionic liquid; Imidazole; RuO2
1. Introduction The discovery of M41S silica by Mobil researchers [1] in 1992 stimulated a great deal of interest in the area of mesoporous materials. Numerous studies followed on both the mechanism of formation of mesoporous materials and the application of these materials in heterogeneous catalysis, homogeneous catalysis, chromatographic separations, etc. [2–4]. Subsequent studies showed that primary amines, quaternary ammonium ions and block polymers can template the formation of mesoporous materials, in which the mesophase is determined by the electrostatic, hydrogen-bonding, or van der Waals interactions between the amphiphilic head of the template and the silica precursors in acidic or basic conditions [5–8], according to the chargedensity-match theory [2]. Various quaternary ammonium surfactants have been employed to template the formation
*
Corresponding author. Tel.: +49 421 200 3236; fax: +49 421 200 3229. E-mail address: [email protected] (R.M. Richards).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.11.013
of mesoporous materials, it was found that the phase of these mesoporous materials are determined by a g factor, g = V/a0L, where V is the volume of surfactant chains plus co-solvent molecule, a0 is the effective head area at the micelle surface and L is the kinetic surfactant chain length. The mesophase can also be controlled by adjusting the g factor, g 1/3 is for cubic mesophase Pm3n, g 1/2 is for hexagonal p6m, g 1 is for lamellar, while 1/2 < g < 2/3 is for cubic Ia3d [2]. Room temperature ionic liquids (RTILs) with imidazole heads have a unique a0 value as compared to quaternary ammonium surfactants and possess a positively charged ring with a negatively charged anion, thus providing a way to prepare mesoporous materials with tailored pores. RTILs are organic salts with low melting points, which possess a wide temperature range of liquid phase, high conductivity, thermal stability, negligible vapor pressure, and are therefore widely used as environmentally benign ‘‘green’’ solvents [9]. Since most long chain RTILs possess both a hydrophilic ionic head and a hydrophobic organic chain, they are also one category of surfactants and can form liquid crystals in various solvents [10], which were
K. Zhu et al. / Microporous and Mesoporous Materials 91 (2006) 40–46
used as templates to prepare microporous and mesoporous materials in some recent publications. Seddon and coworkers first reported a preparation of mesoporous silica with hexagonal structure in mild basic (pH = 9) conditions by using the ionic liquid template N,N-dialkylimidazolium [11], which is similar in structure to MCM-41, however, this material was only characterized by the small angle X-ray diffraction (SAXRD) and N2 adsorption–desorption isotherms, and the mesoporosity is not as regular as that of MCM-41, as evidenced by the N2 adsorption–desorption isotherm. A recent work suggests that the silica templated by this RTIL from mild basic solution is mesoporous silica nanoparticles after RTIL extraction by ethanol and HCl mixtures [12], as confirmed by microscopic images. Zhou et al. synthesized mesoporous titania nanoparticles and silica monolith materials (mesophase of lamellar structure with pore sizes between 1 and 2 nm) using the same family of RTILs [13–15], 1-butyl-3-methylimidazolium-tetrafluoroborate and 1-alkyl-3-methylimidazolium-chlorides, the templates can be removed by acetonitrile extraction or calcination. Studies by Senadeera et al. showed that addition of ionic liquids to mesoporous oxides structure can promote the charge transfer ability of the material [16], additionally, RTIL assisted preparations of nanostructured TiO2 particles and zeolites have also been reported very recently [17,18]. In this study, RTIL 1-hexadecyl-3-methylimidazolium chloride (Fig. 1(A)) was employed to template well-ordered hexagonal mesoporous silica in both strongly acidic and basic conditions; the structures of mesoporous silica were well resolved and found to possess extraordinarily large surface area and highly regular hexagonal pore structures. Furthermore, N,N-dialkylimidazolium chloride can form chlorometalates with metal chlorides [10(b),19,20], thus providing a facile method to introduce metal centers into the mesophase material. Further, ruthenium containing N,N-dialkylimidazolium chloride was prepared and employed to template mesoporous Ru–SiO2, which is superior to previously reported tris(bipyridine) ruthenium (II) [21] or bipyridine phenanthroline complexes [22] templates in simplicity of preparation. The inclusion of ruthenium oxide nanoparticles in the mesoporous silica is particularly interesting because such systems are potentially important for a range of catalytic reactions including Fischer–Tropsch methanation of CO2, selective hydrogenation of cycloalkenes, oxidations and electrocatalysis [23].
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2. Experimental 2.1. Materials 1-Methylimidazole (99%) was obtained from Aldrich. 1Chlorohexadecane (P97%) and tetraethyl orthosilicate (TEOS, P99%) were obtained from Fluka, HCl (37 wt%), NaOH (97%), RuCl3 Æ 3H2O and tetrahydrofuran (THF, 99.8%) are from Applichem Company. All materials in this work were used without further purifications. Synthesis of ionic liquid 1-hexadecyl-3-methylimidazolium chloride (116IL): 116IL was synthesized according to reported procedures [24], 1-hexadecyl chloride (65.24 g, 0.25 mol) and 1-methylimidazole (20.54 g, 0.25 mol) were mixed and refluxed at 363 K for 24 h, then cooled to room temperature to obtain a waxy white solid, which was dispersed in THF to recrystallize, then washed with THF three times. A white powder product was collected and dried under vacuum at room temperature. Mesoporous silica prepared in acidic conditions: in a typical synthesis, 0.600 g 116IL was dissolved in 20.0 ml 2 M HCl under vigorous stirring at room temperature, 2.08 g of TEOS was added to the system 2 h later. After continuously stirring for 24 h, the mixture was transferred to a teflon autoclave, which was placed in an oven at 373 K for 5 days. After filtration and washing with water, the white powder was dried at 373 K for 8 h, which is denoted as AAP (acid as prepared). Thermogravimetric (TG) and differential thermal analysis (DTA) study before calcination showed that combustion and removal of organic templates happens at around 653 K. Samples subsequently calcinated in air by a ramp of 3 K/min at 775 K for 8 h are denoted as AAC (acid as calcined). Mesoporous silica prepared in basic conditions follows the procedure: 0.30 g NaOH was dissolved in 31.86 g H2O, to which 0.6174 g 116IL was added under stirring, after 3 h 3.12 g TEOS was added dropwise under stirring at room temperature. After stirring for 24 h, the mixture was transferred to a teflon autoclave and kept in an oven at 373 K for 5 days. A white powder was filtrated and collected, after drying in an oven at 373 K. These samples were labeled as BAP (basic as prepared) and those calcined at 773 K for 8 h as BAC (basic as calcined). TG and DTA showed combustion of the template at the same temperature as of AAC sample.
Fig. 1. (A) Molecular structure of 1-hexadecyl-3-methylimidazolium chloride (116IL) and (B) 1-hexadecyl-3-methylimidazolium ruthenium hexachloride.
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To prepare the template 1-hexadecyl-3-methylimidazolium ruthenium hexachloride (Fig. 1(B)), 1.029 g 1-hexadecyl-3-methylimidazolium and 0.261 g RuCl3 Æ 3H2O was refluxed in 50.0 ml CH3CN for 12 h, the mixture was recrystallized, filtrated and washed with THF, the solid sample was collected and dried at room temperature under vacuum. To prepare the ruthenium mesoporous silicate, 0.97 g of 1-hexadecyl-3-methylimidazolium ruthenium hexachloride was dissolved in 10.0 g of 2 M HCl aqueous solution together with 4.0 g H2O under constant stirring at 333 K for 3 h. Then, 2.08 g TEOS was added under stirring, after 24 h, the yellowish solid was filtered and washed with water to remove the free ruthenium species from the mesoporous silicate surface, then dried at 443 K for 5 h. Calcination of the solid was conducted at 673 K for 12 h in air, which removed the organic template from the mesoporous composite, as TG-DTA showed a weight loss and exothermal peak at around 653 K. 2.2. Analyses SAXRD was conducted using a Siemens Diffractometer D5000 with Cu Ka (k = 0.15406 nm, 40 kV, 40 mA) radiation, from 1.5° to 10° at a scanning speed of 0.06°/min. TG and DTA experiments were performed with a SDT Q600 from TA instruments in air at a ramp of 3 K/min from room temperature to 1273 K. N2 adsorption–desorption isotherms at 77 K were performed with a Quantachrome Autosorb1-C system, the data were analyzed by employing the BJH (Barrett–Joyner–Halenda) method, pore volume and pore size distribution curves were obtained from the desorption branch of the isotherm. All samples were heated for 8 h at 350 °C under vacuum prior to physisorption experiments. SEM was carried out on a modified JEOL JSM-5900 with a LaB6-cathode, the accelerated voltage for silica and Ru–SiO2 are 15 kV and 4 kV, respectively. High resolution TEM (HRTEM) images were obtained with a JEOL 200CX electron microscope operating at 200 kV. Inductively coupled plasma-atomic emission spectrometry (ICP-AES) was employed to determine the ruthenium content in the calcined sample.
3. Results and discussion SAXRD of the as prepared and calcined silica from acidic solutions are shown in Fig. 2(A), three prominent peaks in the range 2h = 2.0–5.0° can be identified and indexed to a two-dimensional hexagonal structure (p6m), which is similar to MCM-41 [1], after calcination the peaks shifted slightly to higher angles, due to shrinkage of the crystal structure during removal of the organic templates. The diffractions of d100, d110 and d200 planes for the calcined sample are listed in the inset, a unit cell of a0 = 3.76 nm can be referred from a0 = 2d100/31/2, where d100 was obtained from the first peak of the XRD pattern, d100 = k/2 sin h, k is the wavelength for Cu Ka. For the mesoporous silica synthesized from basic media (Fig. 2(B)), four peaks are found and characterize a hexagonal structure (p6m) similar to that prepared from acidic solutions, one more peak corresponding to d210 can be identified for both the as prepared and calcined samples, this suggest that 116IL templated silica in basic media is more ordered than that prepared in acidic conditions. After removal of the 116IL template by calcination at 773 K, the peaks of BAC shifted to higher angles and broadened as compared with BAP, which indicate shrinkage and less ordering of the mesoporous structure by calcination. a0 = 4.32 nm can be deduced for the BAC sample, which is 0.56 nm larger than the AAC sample, this shows mesoporous silica prepared in basic media has a much bigger unit cell value than that of those synthesized in acidic conditions from the same imidazole ionic liquid template. The small angle X-ray diffraction pattern obtained from the mesoporous Ru–SiO2 can be indexed to a two-dimensional p6m structure, which shows d100 and d110 diffractions at 2h = 2.09° and 3.64°, respectively (shown in Fig. 2(C)), corresponding to a d100 = 4.23 nm, a pore-to-pore distance of 4.88 nm (a = 2d100/31/2) is deduced. This indexation clearly shows the existence of a hexagonal mesoporous phase in the three samples described above both before and after calcination. N2 adsorption–desorption curve at 77 K for calcined mesoporous silica prepared in acidic and basic media are shown in Fig. 3(A) and (B), respectively, which exhibit a transition between type I and IV isotherms without
Fig. 2. Small angle X-ray diffraction pattern of as prepared and calcined mesoporous silica in both (A) acidic, (B) basic conditions and (C) mesoporous Ru–SiO2.
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Fig. 3. N2 adsorption–desorption isotherms of the 116IL templated mesoporous silica from (A) acidic, (B) basic conditions and (C) N2 adsorption– desorption isotherm of the mesoporous Ru–SiO2 and pore size distribution.
obvious hysteresis loops, which suggests that the adsorption of nitrogen is near to a monolayer and the adsorption curve overlaps with the desorption branch very well, indicating a high degree of pore structure ordering and no blocking effect during the desorption process. Pore size distributions (inset of Fig. 3(A) and (B)) from the Barrett– Joyner–Halenda (BJH) method by the desorption branch of the isotherm are pretty sharp with a mean pore diameter of 2.20 and 2.74 nm for AAC and BAC, respectively, indicating regular pore structure of mesoporosity in both cases. From the unit cell value referred from SAXRD, pore wall thicknesses of 1.56 and 1.58 nm can be deduced, respectively, for AAC and BAC samples. The corresponding Brunauer–Emmett–Teller (BET) and BJH surface areas are listed in Table 1, which shows that silica synthesized from acidic media (>1600 m2/g) has a larger surface area than the one from basic media (1032 m2/g). The pore volume for AAC (0.96 cm3/g) is smaller than that of BAC (0.99 cm3/g) due to the pore size difference between AAC and BAC. These results are similar to Zhou’s work on ionic liquid casted supermicroporous lamellar phase silica [14], Table 1 Small angle X-ray diffraction and N2 adsorption–desorption analysis of pore structure and surface area Sample
d100, nm
SBET, m2/g
SBJH, m2/g
Pore vol., cm3/g
Pore size, nm
AAC BAC Ru–SiO2
3.26 3.74 4.23
1641 1032 478.0
1691 1400 627.9
0.96 0.99 0.65
2.20 2.74 3.44
which show imidazole ionic liquids can template ordered mesoporous silica as well as supermicroporous lamellar silica. However, Seddon and co-workers [11] found a hysteresis loop at P/P0 > 0.5 for the mesoporous silica templated from 116IL in basic media, because they adopted a different composition of starting materials in mild basic media (pH = 9), and as a result, the mesoporous silica produced is not as ordered as that from present method, the surface area obtained by them was 992 m2/g. The nitrogen adsorption–desorption curve (Fig. 3(C)) for Ru–SiO2 shows that the calcined sample has a BET surface area of 478.0 m2/g and a BJH desorption curve gives a pore volume of 0.65 cm3/g with a narrow pore size distribution of around 3.44 nm, which confirmed the regularity of the mesostructure within the Ru–SiO2. Scanning electron microscopy (SEM) was employed to show the morphological difference between the mesoporous silica prepared in both acidic and basic conditions. As shown in Fig. 4, there is no obvious difference that can be discerned for the mesoporous silica prepared in acidic condition (Fig. 4(A) and (B)) and basic condition (Fig. 4(C) and (D)). Both have a ‘‘cauliflower’’ like structure of less than 100 lm upon aggregating together, which is quite different from the typical morphology of either MCM-41 or SBA-15 [1,8]. For Ru–SiO2, instead of a ‘‘cauliflower’’ morphology, sponge-like layered sheets were observed in a typical image, as shown in Fig. 4(E) and (F). Transmission electron microscopy (TEM) images of calcined silica samples are shown in Fig. 5. Regular hexagonal mesopores can be identified perpendicular (Fig. 5(D)) to
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Fig. 4. Scanning electron microscopy images of samples prepared in (A) and (B) acidic, and (C) and (D) basic conditions and (E) and (F) Ru–SiO2 after calcination.
the channels (along (0 0 1) direction), together with the selected area electron diffraction (SAED) (inset) clearly demonstrating the structure of the mesoporosity. A poreto-pore distance of around 3.8 nm can be calculated from the scale bar for the AAC sample, which is consistent with the SAXRD results. Images taken from the direction parallel to the pore channels for both AAC and BAC samples possess the same regular mesoporous channels. Moreover, no mesoporous nanoparticles of silica were found from these samples, which suggest that the samples prepared by the present method are quite different from previously reported ones using the same template [11,12]. This can be explained in terms of pH values, under strongly acidic/basic conditions the chances of forming nanoparticles are lower, while under mild basic conditions only mesoporous silica nanoparticles are prepared, as confirmed by both N2 adsorption–desorption and TEM. It seems to be that the imidazole head in the template is sensitive to the pH value of the media in tailoring the morphologies. Fig. 6 shows the morphology of the Ru–SiO2 structure, hexagonal pore arrays of p6m can be identified (Fig. 6(A)), together with the regular channels of mesoporous pores
and pore wall structures (Fig. 6(B)) for the as prepared sample. An approximate 5.0 nm pore-to-pore distance can be calculated from the images, which is consistent with the SAXRD results. After calcinations, the mesoporosity became less obvious and there are RuO2 nanoparticles ranging between 6 and 10 nm formed during the calcination, as shown in Fig. 6(C) and (D). The aggregation of RuO2 to form nanoparticles between the channels partially destroyed the regularity of the pore system, thus, the nanoparticles are embedded inside the silica matrix. However, the mesoporosity of the silica remained after calcinations, as evidenced by SAXRD and N2 adsorption–desorption curves. In order to confirm that the nanoparticles formed inside silica after calcinations are RuO2, a wide angle X-ray diffraction was conducted, as shown in Fig. 7. The peaks observed in the pattern confirmed the structure as RuO2 (as referenced to JCPDS 88-0323). From the Scherrer equation a mean particle size of 8 nm can be obtained, which is consistent with the TEM results. Elemental analysis by ICP-AES gave a ruthenium content of 1.21 wt% in the calcined mesoporous silicate.
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Fig. 5. Transmission electronic microscopy images from calcined mesoporous silica (A) and (B) AAC and (C) and (D) BAC, respectively.
Fig. 6. TEM images of the morphology and pore structure of the as prepared (A) and (B) mesoporous Ru–SiO2 and (C) and (D) calcined Ru–SiO2: (C) inset is a HRTEM of RuO2 crystal nanoparticle inside the silica matrix.
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Acknowledgements Authors thank Mr. A. Hoppe, International University Bremen (IUB) for helping with SEM facility. K.Z. and L.D. thank IUB for the fellowship; RR thanks IUB for start-up funds. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.micromeso.2005.11.013. Fig. 7. Wide angle X-ray diffraction of Ru–SiO2 after calcination at 673 K for 12 h.
In this method, the ruthenium species are introduced into silica as a template, as such, the loading can only be adjusted over a small range because altering the ruthenium content also means altering the surfactant/silica ratio, which determines the formed mesophase in the self-assembly process. The particle size can also be influenced by the calcination process, however this is also limited to small range as the organic species can only be removed above 380 °C and temperatures exceeding 600 °C may damage the mesostructure. 4. Summary and conclusion In summary, an imidazole type ionic liquid was employed to template mesoporous silica in both strongly acidic and basic conditions to produce hexagonal mesoporous silica with a high surface area and ordered mesoporous pores. In acidic conditions, the template interacts with the inorganic precursor as S+X I+, where S is the positively charged surfactant (imidazole head), X is the Cl and I+ is a positively charged inorganic precursor after TEOS hydrolysis. In basic conditions, however, they interact with each other as S+I . We believe the observed difference in surface area is because the mesoporous silica from acidic media is less well ordered and the pores are smaller, as shown by the SAXRD (only three peaks can be identified from acidic media synthesized silica), which means the inner surface of the pore wall is less ordered than that of the basic media synthesized silica and hence has lower surface area. Further, a ruthenium containing imidazole ionic liquid was also used to form ruthenium–silicate in acidic media with TEOS, the mesoporous ruthenium silicate formed RuO2 nanoparticles inside the mesoporous silica matrix after calcination, which provides a new way to include metal oxide nanoparticles in mesoporous silica.
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