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Synthesis of Periodic Large Mesoporous Organosilicas and Functionalization by Incorporation of Ligands into the Framework Wall Haoguo Zhu, Deborah J. Jones,* Jerzy Zajac, Roger Dutartre, Mohammed Rhomari, and Jacques Rozie`re Laboratoire des Agre´ gats Mole´ culaires et Mate´ riaux Inorganiques, UMR CNRS 5072, Universite´ Montpellier II, Place Euge` ne Bataillon, 34095 Montpellier Cedex 5, France Received December 27, 2001. Revised Manuscript Received June 11, 2002
Highly ordered large mesopore organosilicas have been obtained by direct liquid crystal templating in acid media using bridged silsesquioxane (EtO)3Si-CH2-CH2-Si(OEt)3 [bis(triethoxysilyl)ethane, BTSE] precursor and triblock copolymers as structure-directing species. The degree of long-range ordering of the structure as determined from X-ray diffraction and transmission electron microscopy, and the most probable pore diameter, in the range 4-8 nm, were observed to depend on the concentration of triblock copolymer used in the synthesis. Further pore-wall functionalization was achieved by co-condensation with Cu(II)-complexed N,N′-bis[3-(trimethoxysilyl)propyl]ethylenediamine (BTSPED). Surfactant extraction produces periodic mesoporous organosilicas functionalized with this complex in the framework, from which the Cu(II) can then be removed by acid leaching. Such hybrid bridged bifunctional organosilicas are homogeneously mesoporous, and the pore diameter increases in the range 11-21 nm as the mole ratio of BTSPED to BTSE is increased from 0.1 to 0.3. 29Si MAS NMR shows that under the conditions used, no cleavage of the Si-C bond occurs, and suggests that the degree of condensation is higher in the bridged bifunctional organosilicas than in the bridged monofunctional organosilicas.
Introduction The properties of ordered mesoporous materials and hybrid organic-inorganic frameworks have recently been combined in a novel class of materials possessing organic fragments as wall components of a mesoporous framework.1-3 These materials, denoted unified organically functionalized mesoporous networks (UOFMN)2 or periodic mesoporous organosilicas (PMOs),3 are prepared through the surfactant-templated condensation of bifunctional organosiloxane presursors (R′O)3SiRSi(OR′)3 with R ) -CH2-, -CH2-CH2-, -CH2d CH2-, etc.1-9 The elaboration of PMOs represents an exciting new development, as the choice of the organic groups incorporated, and the synthesis conditions employed, * To whom correspondence should be addressed. E-mail: debtoja@ univ-montp2.fr. (1) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 1999, 121, 9611. (2) Melde, B. J.; Hollande, B. T.; Blanford, C. F.; Stein, A. Chem. Mater. 1999, 11, 3302. (3) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867. (4) MacLachlan, M. J.; Asefa, T.; Ozin, G. A. Chem. Eur. J. 2000, 6, 2507. (5) Asefa, T.; MacLachlan, M. J.; Gondley, H.; Coombs, N.; Ozin, G. A. Angew. Chem., Int. Ed. 2000, 39, 1808. (6) Sayari, A.; Hamoudi, S.; Yang, Y.; Moudrakovski, J. L.; Ripmeester, J. R. Chem. Mater. 2000, 12, 3857. (7) Asefa, T.; Yoshina-Ishii, C.; MacLachlan, M. J.; Ozin, G. A. J. Mater. Chem. 2000, 10, 1751. (8) Lu, Y.; Fan, H.; Doke, N.; Loy, D. A.; Assink, R. A.; LaVan, D. A.; Brinker, C. J. J. Am. Chem. Soc. 2000, 122, 5258. (9) Dag, O ¨ .; Yoshina-Ishii, C.; Asefa, T.; MacLachlan, M. J.; Grondey, H.; Coombs, N.; Ozin, G. A. Adv. Funct. Mater. 2001, 11, 213.
create a broad opportunity for fine-tuning of porous structure, surface and framework characteristics, reactivity and functionality, for example in catalysis, separations, and advanced materials design. Since the first three papers devoted to PMOs were published in 1999,1-3 periodic mesoporous organosilicas have, to the best of our knowledge, been synthesized in solutions of low surfactant concentration, with surfactant liquid crystal phases and mesoporous organosilicas being formed in a cooperative manner driven by charge density matching between surfactant assemblies and inorganic precursors; and in all cases the pore diameters of well-ordered materials are limited to less than 4 nm. However, PMOs with larger pores are desirable for a variety of possible applications such as hosts for chemical reactions, uses in separations, immobilization or encapsulation involving large molecules, etc.10-12 Generally speaking, the family of nonionic block copolymers can template the synthesis of large mesoporous silica in strong acid (2 M HCl).13 In bridged silsesquioxane systems, however, the charge density on organosilicate species formed by hydrolysis and oligomerization of organosiloxanes under acid or base conditions is lower than that on a silicate species formed from monosilane (10) Rao, M. S.; Dave, B. C. J. Am. Chem. Soc. 1998, 120, 13270. (11) Han, Y. J.; Stucky, G. D.; Butler, A. J. Am. Chem. Soc. 1999, 121, 9897. (12) Matos, J. R.; Kruk, M.; Mercuri, L. P.; Jaroniec, M.; Asefa, T.; Coombs, N.; Ozin, G. A.; Kamiyama, T.; Terasaki, O. Chem. Mater. 2002, 14, 1903. (13) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024.
10.1021/cm011742+ CCC: $22.00 © 2002 American Chemical Society Published on Web 11/01/2002
Synthesis of Periodic Large Mesoporous Organosilicas
compounds, such as tetraethoxysilane (TEOS). No inorganic-organic interaction-induced long-range ordering of block polymers into templates occurs to help organize polymerizing inorganic species,13-15 due to the separation of silyl groups by organic fragments.1 An alternative route toward preparation of ordered mesoporous oxides is the use of preformed lyotropic liquid crystal phases as structure-directing media.16-24 For direct liquid crystal templating, surfactant concentrations are high enough to form long-range ordered assemblies even in the absence of silica-surfactant interactions. Thus, it is not necessary to create strong interactions between surfactant arrays and inorganic species, indeed such interactions may disrupt the existing long-range ordering of surfactant arrays.22-25 As we reported in a preliminary communication,26 nonionic block copolymers are very suitable for use in preparing ordered large mesoporous PMO materials by direct liquid crystal templating. Below we describe more fully our findings and an extension of this method to facilitate the synthesis of functional PMOs, class IV in the classification scheme of Ozin et al.7 Although a variety of organic groups (-NH-CH2-CH2-NH2, -SH, -CH2d CH2, etc.) have been used in the preparation of PMOs,27-29 the organic function of the channel wall component has not, to date, incorporated free ligands, which can act to complex with metal ions and so afford functional mesoporous materials. In precursors to bridged silsesquioxanes containing ligands for binding metal ions, the end groups are usually separated by lengthy flexible organic spacers. The polymers formed from these precursor units lack sufficient structural rigidity to form ordered mesoporous structures, and removal of surfactant from the ordered mesoporous composites generally results in matrix collapse, giving disordered materials of very low surface area.29 One means to circumvent this collapse is to ensure that the free ligands first bind to metal ions, thereby shrinking the flexible organic spacers, and to then incorporate the metal-ligand complex into the channel walls. The metal ions can subsequently be removed from the framework by acid leaching. This (14) Muth, O.; Schellbach, C.; Fro¨ba, M. Chem. Commun. 2001, 2032. (15) Burleigh, M. C.; Markowitz, M. A.; Wong, E. M.; Lin, J.-S.; Gaber, B. P. Chem. Mater. 2001, 13, 4411. (16) Attard, G. S.; Glyde, J. C.; Go¨ltner, C. G. Nature 1995, 378, 366. (17) Coleman, N. R. B.; Attard, G. S. Microporous Mesoporous Mater. 2001, 44-45, 73. (18) McGrath, K. M.; Dabbs, D. M.; Yao, N.; Aksay, I. A.; Gruners, S. M. Science 1997, 277, 552. (19) Rozie`re, J.; Brandhorst, M.; Dutartre, R.; Jacquin, M.; Jones, D. J.; Vitse, P.; Zajac, J. J. Mater. Chem. 2001, 11, 3264. (20) Go¨ltner, C. G.; Henke, S.; Weissenberger, M. C.; Antonietti, M. Angew. Chem., Int. Ed. 1998, 37, 613. (21) Blin, J. L.; Le´onard, A.; Su, B. L. Chem. Mater. 2001, 13, 3542. (22) Feng, P.; Bu, X.; Stucky, G. D.; Pine, D. J. J. Am. Chem. Soc. 2000, 122, 994. (23) Feng, P.; Bu, X.; Pine, D. J. Langmuir 2000, 16, 5304. (24) Melosh, N. A.; Lipic, P.; Bates, F. S.; Wudl, F.; Stucky, G. D.; Fredrickson, G. H.; Chmelka, B. F. Macromolecules 1999, 32, 4332. (25) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152. (26) Zhu, H.; Jones, D. J.; Zajac, J.; Rozie`re, J.; Dutartre, R. Chem. Commun. 2001, 2568. (27) Burleigh, M. C.; Dai, S.; Hagaman, E. W.; Lin, J. S. Chem. Mater. 2001, 13, 2537. (28) Asefa, T.; Kruk, M.; MacLachlan, M. J.; Coombs, N.; Grondey, H.; Jaroniec, M.; Ozin, G. A. J. Am. Chem. Soc. 2001, 123, 8520. (29) Burleigh, M. C.; Markowitz, M. A.; Spector, M. S.; Gaber, B. P. J. Phys. Chem. B 2001, 105, 9935.
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follows the imprinting concept developed by other authors.27,30,31 The presence of coordinatively unsaturated sites in the framework of organic silica should allow for high degrees of functionalization, while simultaneously achieving large mesopores and ordered structures. In the second part of the report below, we describe the synthesis of large mesoporous PMOs from cocondensation of 1,2-bis(triethoxysilyl)ethane and Cu(II)bound, N,N′-bis[3-(trimethoxysilyl)propyl]ethylene-diamine by lyotropic liquid crystal polymer templating. Experimental Section Chemicals. 1,2-Bis(triethoxysilyl)ethane (BTSE), hydrochloric acid (37 wt %), and Pluronic P123 (Mv ) 5800), EO20PO70EO20 were obtained from Aldrich; N,N′-bis[3-(trimethoxysilyl)propyl]ethylenediamine (BTSPED) [60-65% (NMR) in methanol], and cupric acetate monohydrate were obtained from Fluka. All chemicals were used as received. Synthesis. Mesoporous organosilica phases were synthesized at room temperature by using nonionic surfactant P123 as the structure-directing agent. In a typical synthesis, colloidal ethanesilica was prepared by mixing an aqueous dilute HCl solution (pH 2) with BTSE. Pluronic P123 was then added to the mixture, and ethanol from the hydrolysis of BTSE was removed under vacuum. The mass ratio was H2O/HCl (1.0)/ P123 (0.4-1.2)/BTSE (3.2). Materials so-prepared are denoted ESx, where x is the mass ratio of P123 to H2O + P123 expressed as a percentage: x ) 30, 39, 44, 48, or 55. The fluid mixture usually rigidified within a few days at room temperature, with the optically transparent monolithic gel taking the shape of the reaction vessel. The monolith was then hydrothermally treated at 100 °C for 24 h, and the product was recovered and dried at 60 °C for 24 h prior to further characterization. The surfactant was removed by twice stirring 1.0 g of as-synthesized material in 150 mL of ethanol and 2 g of 37% HCl aqueous solution at 50 °C for 6 h. Bifunctional PMO mesophases were synthesized as follows: the precursor complex32 Cu(BTSPED)22+ was prepared by mixing BTSPED in methanol solution and the corresponding amount of cupric acetate. The complex was added to the above liquid mixture of P123, BTSE, and H2O (mass ratio P123/BTSE/H2O, 0.8:3.2:1.0). The molar ratio of BTSPED to BTSE was varied between 0 and ca. 0.3. The corresponding products are denoted EBPEDS-x, [ethane-bis(propyl)ethylenediamine silica] where x is the mole ratio BTSE/BTSPED. The blue colloidal organosilicas solidified after several minutes, were then hydrothermally treated at 100 °C for 24 h, and the surfactant was removed using the aforementioned procedure. Metal Ion Sorption. The adsorption capacity was determined by measuring the change in metal ion content of solutions contacted with solid samples for given periods of time. Surfactant was extracted from EBPED-x with ethanol/ HCl, and the samples were washed with dilute NH4OH and water. Analysis of metal ion contents was made by atomic absorption spectrophotometry using a Pye Unicam instrument model SP9. In a typical procedure, 0.1 g of EBPED-0.2 was dispersed in 10 mL of aqueous solution of metal (Cu2+, Zn2+) acetate of concentration 10-4-10-2 mol dm-3. The temperature of the contents was maintained at 25 °C by means of a thermostated water bath. After a given time period, the solid was separated by centrifugation and the supernatant was recovered. The PMO was washed several times with water, and the water washings were added to the supernatant. The overall capacity of the sorbent for a given metal ion was (30) Dai, S.; Burleigh, M. C.; Shin, Y.; Morrow, C. C.; Barnes, C. E.; Xue, Z. L. Angew. Chem., Int. Ed. 1999, 38, 1235. (31) Dai, S.; Ju, Y. H.; Burleigh, M. C.; Gao, H. J.; Lin, J. S.; Pennycook, S.; Barnes, C. E.; Xue, Z. L. J. Am. Chem. Soc. 2000, 122, 292. (32) De, G.; Epifani, M.; Licciulli, A. J. Non-Cryst. Solids 1996, 201, 250.
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calculated by the change in concentration between the filtrate and the initial metal ion solution. Characterization. X-ray powder diffraction data were recorded on an automated Philips X’Pert diffractometer with Cu KR radiation. Cross polarization (CP) MAS spectra were recorded on a Bruker ASX300 spectrometer with a 1H frequency of 300 MHz and 29Si frequency of 59.6 MHz. For the 1 H to 29Si CP MAS experiments, the π/2 pulse length was 4.5 µsec, and the contact time, recycle delay, and spinning speed were 5 ms, 10 s, and 5 kHz, respectively. Transmission electron micrographs were obtained with a JEOL 1200 EX microscope operating at 100 kV. Adsorption-desorption of nitrogen at 77 K was investigated using an automated volumetric Analsorb 9011 apparatus. Prior to the adsorption measurements, all samples were evacuated to 10-5 Torr at 140 °C for 3 h. A complete adsorption isotherm, followed by a desorption isotherm, was obtained with 20-30 points on each branch, and BET plots were constructed for the relative pressure ranging from 0.05 to 0.3. These lines were subsequently used to evaluate the BET specific surface area, SBET, taking the crosssectional area of 0.162 nm2 per nitrogen molecule. Evaluation of pore size distribution was made using the BJH method. All distribution functions were normalized to unity by dividing the values of differential volume by the corresponding pore volume. Thermogravimetric analyses were performed in air on a Stanton Redcroft STA 781 thermobalance. Infrared spectra were measured on KBr disks with a Bomem DA8 FTIR spectrometer.
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Figure 1. XRD patterns for as-synthesized and extracted hexagonal mesophases prepared from P123/water system (55 wt %) (a) as-synthesized ethanesilica-surfactant mesophase before hydrothermal treatment; (b) hydrothermally treated at 100 °C for 24 h; and (c) hydrothermally treated at 100 °C for 24 h and then solvent extracted.
Results and Discussion Synthesis of Periodic Large Mesoporous Organosilicas. The direct liquid crystal templating approach for silica and aluminosilicate is well documented.16-24 The porous product is obtained by solidifying a homogeneous bulk liquid crystal phase, as opposed to being precipitated from a heterogeneous mixture as in the case of M41S and SBA-15. The resulting inorganic oxide is a direct cast of the liquid crystal phase under relatively high surfactant concentrations (usually >20 wt %), which indicates a high degree of control over the structure. Further, the inorganic oxide prepared in liquid crystalline phases can be obtained as monoliths.16-20,22,33 As expected, the use of lyotropic polymer templates for the synthesis of organosilicas has the advantage that macroscopic, optically transparent monoliths are obtainable in considerable size. The powder X-ray diffraction analyses performed on as-synthesized, hydrothermally treated, and extracted ethanesilica samples prepared in the presence of 55% P123 are shown in Figure 1, where it may be seen that the diffraction patterns of the surfactant-extracted materials exhibit an intensity greater than that of the corresponding as-synthesized sample. This is probably due to the larger contrast in density between the matrix and open pores (extracted sample) relative to that between the matrix and surfactant assemblies (assynthesized material). Hydrothermal treatment also leads to a striking enhancement of the intensity of the lowest angle diffraction peaks, as a result of an improvement in mesoscopic ordering. Such hydrothermal treatment also reduces shrinkage of the organosilica after removal of surfactant.22,23 The sample templated by surfactant concentration of 55 wt % shows three clear diffraction peaks, which can be indexed to a 2D hexagonal lattice with lattice constant ao ) 10.3 nm as (33) Lebeau, B.; Fowler, C. E.; Hall, S. R.; Mann, S. J. Mater. Chem. 1999, 9, 2279.
Figure 2. XRD patterns of as-synthesized samples after hydrothermal treatment and surfactant removal: (a) ES30, (b) ES39, (c) ES44, (d) ES48, and (e) ES55.
shown in Figure 1(c). Such materials with one-dimension channels arranged in a hexagonal net are defined as 2D-hexagonal because the diffraction pattern shows 2D P6mm symmetry. A series of samples templated by different surfactant concentrations under mild acidic conditions were prepared. Figure 2 shows the variation of the XRD patterns for samples ES30, ES39, ES44, ES48, and ES55, prepared with a surfactant weight percentage in the aqueous solution from 30 to 55%. All samples exhibit a prominent peak in the diffraction pattern at 2θ ≈ 1°. With decreasing surfactant concentration, the position of the sharp lowest angle peak remains almost constant, indicating that a0 only slightly varies. Nevertheless, the reflections at higher angle become less well resolved and their intensity decreases and disappears completely for a surfactant weight percentage equal to 39%. This suggests that the channel arrays lose regularity and that long distance hexagonal symmetry is progressively lost below ca. 40 wt % surfactant. Importantly, all the N2 adsorption isotherms of the ethanesilicas at 77 K are of type IV with a clear H2- or H1-type hysteresis loop at high relative pressures with
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Figure 3. N2 adsorption-desorption isotherms of the samples (a) ES30, (b) ES39, (c) ES44, (d) ES48, and (e) ES55.
the capillary condensation step becoming increasingly vertical as the synthesis weight percent of surfactant increases. Nitrogen gas adsorption isotherms of the extracted samples are shown in Figure 3 for ethanesilicas ES30-ES55. For the sample prepared with 55 wt % P123, the vertical step at P/Po ) 0.75 suggests that the materials have very regular mesoporous channels despite their large pore size. H1-type hysteresis is evidence for the presence of cylindrical pore geometry and a high degree of pore size uniformity. Quantitative analysis for this sample shows that the BrunauerEmmett-Teller (BET) surface area is 957 m2g-1, the total pore volume is 1.37 cm3g-1, and the most probable pore diameter is 7.7 nm (BJH model). Textural data for the other samples prepared using lower surfactant concentrations are provided in Table 1. It is interesting to note that the adsorption isotherms considered here have quite steep initial portions which may no longer be attributed to the presence of micropores but rather to layer-by-layer building up of a N2 film on the extended surface of mesopores.
Table 1. Values of d Spacing, BET Surface Area, Pore Volume, and Pore Diameter of Ethanesilica Samples Obtained using Different Weight Percent Surfactant in the Syntheses surfactant wt %
d spacing (nm)
SBET (m2 g-1)
pore volume (cm3 g-1)
BJH pore diameter (nm)
30 39 44 48 55
8.5 8.6 8.7 8.8 9.0
868 943 817 882 957
0.76 0.81 0.82 0.97 1.37
4.0 5.0 5.5 6.2 7.7
When the surfactant concentration is varied from 30 to 55%, results obtained by nitrogen adsorption-desorption analysis show that the pore diameter is very sensitive to the weight percent surfactant in the synthesis gel undergoing marked increase with increasing amounts of template, while, in addition, the normalized pore size distribution appears to become more homogeneous. With the ratio of propylene oxide/ethylene oxide units >1, the increase in the hydrophobic volume exceeds the increase in hydrophilic volume, and the pore size can be varied over a broad range. Thus, the BJH
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Figure 4. BJH pore-size distributions (normalized to unity) obtained from N2 adsorption isotherms for the samples (a) ES30, (b) ES39, (c) ES44, (d) ES48, and (e) ES55.
pore diameter is almost doubled from 4.0 to 7.7 nm when the P123 wt % is increased from 30 to 55% (Figure 4). The 2D symmetries inferred from XRD were confirmed by transmission electron microscopy, as shown in Figure 5. The TEM images of the sample ES55 show a clear hexagonal arrangement of pores with uniform size (Figure 5a), while well-aligned channels running parallel to organosilica rods are also observed (Figure 5b). Despite the large pore size, this high degree of order is maintained over extensive regions of observation (>1 µm). These observations confirm that the pore structure consists of a hexagonal array of uniform 1D channels for the 2D-hexagonal sample. The wall thickness can be measured from the micrograph of Figure 5a as varying between 1.6 and 2.4 nm. The sum of the pore diameter and the wall thickness corresponds to the unit cell parameter, and the value so-estimated [(7.8 +1.6) - (7.8 + 2.4) nm, i.e. 9.4-10.2 nm] is in good agreement with that calculated from the d100 diffraction line for this sample. However, the TEM of materials prepared with a lower concentration of surfactant display, in contrast to the circular pores of Figure 5a, a marked directional anisotropy. As may be seen in Figure 5c, the pores of the sample ES48 have an ellipsoidal appearance, and the resulting hexagonal structure is distorted. As it may be considered that the BJH pore diameter corresponds to an approximate average value of the principal axes of the ellipsoids, the use of the above relation to estimate the wall thickness will tend to skew the number obtained to artificially high values. The average unit cell parameter appears to be virtually invariant for all samples throughout the surfactant concentration range, and the average BJH cylindrical pore diameter seems to decrease markedly as the surfactant concentration decreases, but it may be assumed that the average wall thickness remains in the range of 2-3 nm. It is important to demonstrate that the Si-C bond is maintained in the solvent-extracted organosilicas. The 29Si MAS NMR spectrum shows a broad 29Si resonance at -56.5 ppm and a shoulder at -64.2 ppm, corresponding to trifunctional (-56.5 ppm, RSiO2(OH); -64.2 ppm, RSiO3) silicon, as shown in Figure 6.1-3,6 There was no evidence of tetrafunctional Q4 silicon atoms between
Figure 5. Transmission electron micrographs of the mesoporous ethanesilica ES55 after surfactant extraction, showing well-ordered hexagonal symmetry (a) and aligned cylindrical structure (b). Sample ES48 prepared using 48 wt % P123, showing directional anistropy (c).
-90 and -120 ppm, and the absence of SiO4 species such as Si(OH)(OSi)3 and Si(OSi)4 confirms that no cleavage of carbon-silicon bond of the BTSE starting organosilane compound has occurred during hydrolysis and polymerization. The presence of T2 Si atoms at -56.5 ppm was indicative of incomplete condensation of the organosilane precursor, with unreacted silanol groups still being present. A slow and moderate condensation of organosilica units in acid medium probably facilitates the ordering of the organic fragments due to
Synthesis of Periodic Large Mesoporous Organosilicas
Figure 6. 29Si MAS NMR spectrum of as-synthesized extracted ethanesilica sample.
the rigidity of the siloxane network: lower when it consists of T2 units than when it contains fully condensed T3 substructures.34 FT-IR spectroscopy also supported this result, showing the appearance of an absorption band assigned to CH2-Si stretching (ν(CSi), 690 cm-1), while the broad absorption at 1000-1200 cm-1 [ν(Si-O-Si)] indicates the formation of siloxane bonds, and residual silanol groups are evidenced by the ν(Si-OH) stretching vibration at 900 cm-1.35 Condensation reactions are therefore not complete in agreement with the conclusion from 29Si NMR.33 Three recent communications12,14,15 describe the use of block copolymers in micellar concentrations to template organosilica, leading to high-surface-area, large-pore-diameter materials, having no14,15 or limited12 long-range order. The organosilicas reported14 have textural properties similar to those reported here for ES30 and ES39 (i.e., those prepared using 30 and 39 wt % P123), while materials prepared using P123 modified by the addition of trimethylbenzene show very high surface areas and large pore diameters.15 Functionalized Periodic Large Mesoporous Organosilicas. An extension of the above method leads to the synthesis of Class IV multifunctional hybrid materials7 containing more than one type of bridging functional group. Bifunctional PMOs reported to date contain bridging organic moieties in the framework and terminal organic functional groups protruding into the channel pore, but no incorporation of ligands into the framework.28,29 In fact, BTSE can co-condense with another bis-organosilane precursor N,N′-bis[3-(trimethoxysilyl)propyl]ethylenediamine (BTSPED), and the structural similarities between BTSE and BTSPED favor mixing and homogeneous incorporation of the functional ligands into the mesoporous silsesquioxane framework. This is represented schematically in Figure 7. The co-condensation process was investigated over the range BTSPED/BTSE ) 0-0.3 (molar ratio). Because the materials are obtained by hydrolysis-condensation-drying of the synthesis mixture, and not as precipitates from solution, this ratio is conserved in the product materials. After surfactant extraction in etha(34) Moreau, J. J. E.; Vellutini, L.; Wong Chi Man, M.; Bied, C.; Bantignies, J.; Dieudonne´, P.; Suvajol, J. J. Am. Chem. Soc. 2001, 123, 7957. (35) Franville, A. C.; Zambon, D.; Mahiou, R.; Troin, Y. Chem. Mater. 2000, 12, 428.
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nol, nitrogen adsorption isotherms for the bifunctional PMOs are still of type IV, but the amounts adsorbed at low relative pressures are shifted toward smaller adsorption values, as shown in Figure 8 (compare, e.g., isotherms of materials EBPEDS-0.1 and EBPEDS-0.3). Because the ratio between the overall mesopore volume and the surface area is modified significantly from one system to another, the scale on the adsorption axis is increased, and the initial isotherm regions are less steep than those reported in Figure 3. This may give an erroneous impression that these isotherms exhibit a tendency to change from type IV to V as the amount of BTSPED in the synthesis gel is increased. In fact, such behavior cannot be attributed to changes in the lateral interactions between the adsorbed nitrogen molecules, as has been suggested in the literature.36 The initial portions of all the isotherms of Figures 3 and 8, plotted in terms of the relative amount adsorbed, i.e., V/Vm (where Vm is the respective monolayer capacity) as a function of P/Po, are almost identical. This means that the “enhancement” or “impoverishment” of the amount adsorbed at low relative pressures is only due to the absolute value of the specific surface area of the adsorbent. The interpretation of the adsorption data on the basis of the BET model provides another argument for this hypothesis because the interaction BET constants are very similar in all cases. An opposite tendency to change from the isotherm type IV to I has been reported for other bifunctional PMOs containing organic functional groups protruding into the pore void space.28 Removal of surfactants from the ordered mesoscopic composites of higher BTSPED ratio led to matrix collapse, and the resulting extracted materials were disordered and showed low surface areas and small pore volumes. For the Cu2+-containing bifunctional materials, the BET specific surface area ranges from 575 (EBPEDS0.1) to 346 (EBPEDS-0.3) m2g-1, Table 2. These values are much smaller than those provided by large mesoporous ethanesilica PMOs (Table 1). This is not surprising, as the pore diameter for the bridging bifunctional PMO samples approaches the upper end of the mesopore size range. The variation of surface area among the samples also may be caused by the presence of residual surfactant (about 10% as determined from TGA, see below) in the samples with lowest surface areas. Surfactant was removed more thoroughly by washing with acidified ethanol when the pore diameter calculated from the adsorption branch of the nitrogen isotherm accordingly increases slightly. Washing with acidified ethanol also leads to protonation of the amine functions, which then lose their ability to complex copper ions, and the pale blue materials become pale yellow as Cu2+ is released into solution. In materials washed with ethanol only, the atomic ratio N/Cu is 4.7, consistent with complexation of almost all of the nitrogen atoms with copper ions in a square planar arrangement. Complexation capability is restored on rinsing materials first with dilute ammonium hydroxide and then liberally with water, as shown in Figure 9a, which expresses the percent uptake of Cu2+ and Zn2+ from dilute solutions equimolar in both ions. This figure indicates that the (36) Kruk, M.; Jaroniec, M.; Guan, S.; Inagaki, S. J. Phys. Chem. B 2001, 105, 681.
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Figure 7. Schematic representation of the synthesis of bridge-bonded bifunctional organosilica. Table 2. Specific Surface Area, Pore Volume, and Pore Diameter of Bridged Bifunctional Ethane-Bis(propyl)Ethylenediamine Silica sample
Figure 8. N2 adsorption-desorption isotherms for bifunctional PMOs: (a) EBPEDS-0.1; (b) EBPEDS-0.2; and (c) EBPEDS-0.3. The inset shows the BJH pore-size distribution calculated from the corresponding adsorption branch of the isotherm, and normalized to unity.
material adsorbs Cu2+ almost quantitatively, whereas uptake of Zn2+ is less than half that of Cu2+. Although this could simply reflect the respective stability constants associated with formation of ethylenediamine-
EBPEDS-0.1
EBPEDS-0.2
EBPEDS-0.3
SBET (m2 g-1) Vpore (cm3 g-1) dBJH (nm)
Ethanol Extracted 530 467 1.60 1.26 11.2 12.4
323 1.30 21.5
SBET (m2 g-1) Vpore (cm3 g-1) dBJH (nm)
Ethanol/HCl Extracted 534 1.37 11.5 -
-
copper and ethylenediamine-zinc complexes at pH 5, it may also be a result of the difference in ionic ratio between these ions, with selectivity for copper created by imprinting. Up to 80% of the initial copper content is recovered from concentrated (0.02 mol dm-3) Cu(OAc)2 solution. Figure 9b shows that uptake from solution from dilute solution reaches a maximum value after approximately 2 h of contact time. Protonated-form bifunctional PMOs also present capability for uptake of divalent cations, although the amount taken up is less than that following ammonium hydroxide treatment. In comparison to functionalized mesoporous silicates bearing the same functional, but pendant groups, the present materials show slower kinetics for divalent metal uptake but similar capability for uptake of Cu2+ from dilute solutions.27 The effect of imprinting on selectivity in the present PMOs is not entirely clarified, and further evaluation of uptake, selectivity, and exchange properties of this class of materials is in progress. The presence of propylethylenediamine and ethane functions in the framework was identified by FT-IR. After surfactant extraction, the ligands in the framework remained intact as indicated by the ν(C-N) stretching vibration at 1185 cm-1.34 This band is not exhibited by ethanesilica prepared from BTSE alone. The ν(N-H) absorption bands overlap with ν(O-H) at 3300-3500 cm-1, as shown in Figure 10. In addition, the sample extracted in ethanol only still exhibits a weak symmetrical δs(CH3) bending vibration at 1375 cm-1, which confirms that some surfactant remains (P123, EO20PO70EO20; PO: -[OCH(CH3)CH2]-). However, none of the samples extracted in ethanol/HCl show these signals. This further indicates the efficiency of the extraction in ethanol/HCl in removing the surfactant template. No evidence for the presence of acetate groups is seen in ethanol-washed materials. Solid state 29Si NMR spectra (Figure 11) displayed resonances at -58.1
Synthesis of Periodic Large Mesoporous Organosilicas
Chem. Mater., Vol. 14, No. 12, 2002 4893
Figure 10. FT-IR spectra of the EBPEDS-0.1 (a) extraction with ethanol/HCl; (b) extraction with ethanol; (c) before extraction; and (d) ethanol/HCl extracted BTSE (only) (no BTSPED).
Figure 9. (a) Cu2+ and Zn2+ uptake by EBPEDS-0.2. Solutions equimolar in Cu2+ and Zn2+. Samples are ethanol/HCl extracted, and rinsed with dilute NH4OH and water prior to equilibration for 4 h. (b) Percent copper absorbed from aqueous solutions (10-3 mol dm-3) of the corresponding acetate versus contact time for (i, iii) Cu2+; (ii) Zn2+; (i, ii) samples rinsed with NH4OH and water prior to uptake experiment; (iii) sample in protonated form.
ppm and -64.7 ppm, assigned to T2 and T3. Comparison of Figures 6 and 11 further indicates that the condensation of a mixture of BTSE and BTSPED precursors is more complete than that achieved with a BTSE precursor only. The absence of resonances corresponding to Q substructures provides evidence that no cleavage of Si-C bonds occurred. Thermogravimetric analyses of samples with ratio BTSPED/BTSE ) 0.2 were conducted in air from room temperature to 800 °C, and typical weight loss curves are shown in Figure 12a. The sample before extraction exhibits a weight loss of about 4% below 100 °C, attributable to the loss of small amounts of residual water adsorbed on the material surface. This is followed by a weight loss of 20-30% between 180 and 260 °C due to surfactant decomposition and combustion. Thereafter, an additional loss of 40 wt % is observed in the range 260-600 °C, probably resulting principally from decomposition of the organic components of the hybrid porewall. The thermogram of the material recovered after extraction in ethanol/HCl shows no indication of residual surfactant, although there are more significant
Figure 11. 29Si MAS NMR spectrum of surfactant-extracted bifunctional EBPEDS-0.1.
amounts of adsorbed residual water and ethanol (weight loss of 10% at
