J. Phys. Chem. B 2001, 105, 9935-9942
9935
Direct Synthesis of Periodic Mesoporous Organosilicas: Functional Incorporation by Co-condensation with Organosilanes Mark C. Burleigh, Michael A. Markowitz,* Mark S. Spector, and Bruce P. Gaber Laboratory for Molecular Interfacial Interactions, Code 6930, Center for Bio/Molecular Science and Engineering, NaVal Research Laboratory, Washington, DC 20375 ReceiVed: May 11, 2001; In Final Form: August 17, 2001
Mesoscopic organosilicas were synthesized with bis(triethoxysilyl)ethane (BTSE) and cetyltrimethylammonium chloride (CTAC) under basic conditions. Further functionalization was achieved by co-condensation with trialkoxyorganosilanes. Surfactant extraction produced periodic mesoporous organosilicas (PMO’s) functionalized with the respective organosilane pendent groups. Organosilanes used in this study include: 3-aminopropyltrimethoxysilane, n-(2-aminoethyl)-3-aminopropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane, 2-(trimethoxysilylethyl)pyridine, n-(3-triethoxysilylpropyl)-4,5-dihydroimidazole, phenethyltrimethoxysilane, and benzyltriethoxysilane. These materials have been characterized by nitrogen gas adsorption, powder X-ray diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), elemental analysis (EA), and high-resolution thermogravimetric analysis (TGA). The effect of organosilane incorporation on the porous structure of these materials is examined.
Introduction The recent development of periodic mesoporous organosilicas (PMO’s) has led to great interest in materials science.1-6 These unique materials combine the structural characteristics of ordered mesoporous silica7-11 with the chemical functionality of organic polymers. Different organic bridging groups have been used to give PMO’s a favorable combination of properties normally associated with both organic and inorganic materials. Structural rigidity on the molecular scale allows for the formation of an ordered mesoporous matrix using the surfactant template method (Figure 1). The presence of organic functional groups within the matrix gives these materials many of the favorable properties associated with organic polymers, but with improved accessibility to functional sites due to their open pore structure. The silica component of the PMO’s gives them a degree of hydrophilic character useful for applications in aqueous systems. Organic functional groups have been incorporated into mesoporous silica by both post-synthetic grafting and cocondensation of trialkoxyorganosilanes with tetraethyl orthosilicate (TEOS). The latter approach, also referred to as direct synthesis, has been the preferred route for most researchers.12-19 Advantages of co-condensation over postsynthetic grafting include the easier synthetic protocols involved with a single pot synthesis and better control of the loading of organosilanes. A decided disadvantage of postsynthetic surface grafting of mesoporous silicas is the uneven distribution of organic species throughout the material. High ligand loading is accomplished only by placing monolayers on the mesopore surfaces. This high concentration of organic groups leads to hydrophobic materials with poor wettability in aqueous environments. The interactions that can occur between neighboring organic groups with each other or with analyte species is undesirable for many adsorbent and catalytic applications. Co-condensation achieves an even distribution of organic groups throughout the mesoporous matrix17 and, therefore, achieves a relatively high ligand loading * Corresponding author.
without some of the disadvantages associated with the postsynthetic grafting process. While organic functional groups must be added to mesoporous metal oxides when needed, periodic mesoporous organosilicas already contain organic moieties. The techniques necessary for the synthesis of organic-inorganic polymers containing a wide variety of organic groups are well documented.20-25 In many cases, the sol-gel synthesis of many polysilsesquioxane materials can be directly applied with the surfactant template approach to synthesize PMO’s. Although a wide variety of organic groups have been incorporated into PMO’s, some very useful ones have not. Initial PMO devlopment has been limited largely to those made from commercially available precursors, precluding inclusion of primary amine or thiol groups, which require timeconsuming precursor synthesis. Also, many of the bis trialkoxyorganosilanes containing amines or sulfides (e.g., bis[3-(trimethoxysilyl)propyl]ethylenediamine, bis[3-(triethoxysilyl)propyl]tetrasulfide) have flexible organic spacers. Polymers formed from these precursors lack the structural rigidity to form ordered mesoporous structures. Removal of surfactants from the ordered mesoscopic composites of these monomers results in matrix collapse. These materials are disordered and exhibit relatively low surface areas.26 For these reasons, co-condensation has been used to further functionalize PMO’s. The co-condensation of bis(triethoxysilyl)ethane with N-[3(trimethoxysilyl)propyl] ethylenediamine has been used to synthesize PMO’s containing the ethylenediamine moiety for metal ion sorbent applications.27 During the course of this work a number of interesting properties of the ethanesilica matrix were discovered that we feel make it suitable as a support for functionalized mesoporous materials. Unlike TEOS, bis(triethoxysilyl)ethane mixes very well with many trialkoxyorganosilanes. Also, many organosilanes can disrupt the silica matrix, which contains only relatively short siloxane bridges between silicon atoms. This results in a noticeable decrease in the surface area and porosity of these materials.28 With its ethylene bridges, bis(triethoxysilyl)ethane can co-condense with organosilanes
10.1021/jp011814k CCC: $20.00 © 2001 American Chemical Society Published on Web 09/21/2001
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Figure 2. Trialkoxyorganosilanes used in this study. Figure 1. Schematic diagram of the synthesis of periodic mesoporous organosilica.
without exhibiting such decreases. These favorable properties of ethanesilica should allow for higher degrees of functionalization than can be achieved with silica-based materials, while simultaneously achieving the large surface areas and ordered pore structures that are advantageous for adsorbent and catalytic applications. Herein, we describe the synthesis of periodic mesoporous organosilicas from bis(triethoxysilyl)ethane under basic conditions. Further functionalization is achieved by co-condensation with trialkoxyorganosilanes. Structural characterization of these materials has been performed with X-ray diffraction, nitrogen gas adsorption, and high-resolution thermogravimetric analysis. The incorporation of functional groups is confirmed by diffuse reflectance infrared Fourier transform spectroscopy and elemental analysis. The effect of organosilane incorporation on the structural ordering, surface areas, and porosity of these materials is reported. Experimental Section Chemicals. BTSE and all trialkoxyorganosilanes (see Figure 2) were obtained from Gelest, Inc. NaOH, HCl, CTAC, and EtOH were obtained from Aldrich. All chemicals were used as
received. Water used in all synthetic procedures was deionized to 18.0 MΩ cm. Synthesis. A protocol similar to that used by Inagaki et al.1 to obtain 3-D hexagonal PMO’s was followed, with the addition of functional organosilanes. Supramolecular assemblies of surfactant were formed under basic conditions, followed by the addition of BTSE and the functional silane. In a typical procedure, CTAC (25% w/w) was added to deionized water under stirring in a polypropylene vessel and NaOH was added dropwise to give a clear solution. BTSE (75 mol %) and the functionalized silane (25 mol %) were stirred in a separate vial for 10 min. The silanes were then added to the surfactant solution and the mixture was covered and stirred at room temperature for 12 h. The molar composition of the original synthetic mixtures was 0.75 BTSE:0.25 R′Si(OR)4:0.12 CTAC: 1.0 NaOH:230 H2O. The white precipitates were recovered by filtration, washed with deionized water, and dried under vacuum at 60 °C for 12 h. A control sample was prepared by following the procedure described above, without any added functional silane. A second control blank, containing neither surfactant nor functional silane, was also synthesized. NaOH was added to deionized water and stirred prior to the addition of BTSE. This mixture was stirred for 12 h. Hydrochloric acid was then added dropwise to lower the pH ∼ 8.0. The solid product was then dried in vacuo at 60 °C for 24 h prior to characterization.
Synthesis of Periodic Mesoporous Organosilicas
J. Phys. Chem. B, Vol. 105, No. 41, 2001 9937
Surfactant Extraction. The as-synthesized powders were placed in excess (350 mL/g) acidified ethanol (1 M HCl) and refluxed for 6 h. The products were recovered by filtration, washed with ethanol, and dried under vacuum at 60 °C for 10 h. The extraction procedure was then repeated. Characterization. X-ray diffraction measurements were made on an Enraf-Nonius FR591 rotating-anode using a bent graphite monochromator that selected Cu KR radiation and provided in-plane resolution of 0.014 Å-1 full-width at halfmaximum. Powder samples were placed in 1.0 mm quartz capillary tubes. Gas sorption experiments were performed using a Micromeritics ASAP 2010. Nitrogen gas was used as the adsorbate at 77 K. Infrared spectra were measured with a Nicolet Impact 400 spectrophotomer equipped with a Spectra-Tech diffuse reflectance attachment. Analyses were performed using KBr as the blank. The IR cell was purged with nitrogen gas. All elemental analyses were conducted by Oneida Research Services, Whitesboro, NY. Thermogravimetric analyses were performed with a TA Instruments TGA 2950 thermogravimetric analyzer. All measurements were made in high-resolution dynamic mode. Results and Discussion X-ray Diffraction. Powder X-ray diffraction analyses were performed on as-synthesized, extracted, and blank samples (Figures 3 and 4). The diffraction patterns of the extracted materials exhibit larger intensities than the corresponding assynthesized samples. This is likely due to the larger contrast in density between the matrix and open pores (extracted samples) relative to that between the matrix and surfactant assemblies (as-synthesized materials). All samples exhibit a prominent peak in the diffraction pattern at ∼ 2θ ) 2°. The diffraction pattern of extracted ethanesilica (Figure 3a) contains secondary reflections similar to those reported by Inagaki1,6 for 3-D hexagonal materials, suggesting that our 2θ peaks correspond to the d100 spacing in hexagonal systems. Extracted thiol, amine, diamine, and imidazole-containing PMO’s also show evidence of secondary reflections which are not resolvable with our instrumentation. Diffraction patterns for the three PMO’s that contain aromatic functionalities are shown in Figure 4. These materials do not exhibit any secondary reflections and their patterns resemble those of mesoporous materials with wormhole-like structures.10,29 The phenethyl-functionalized PMO may be the most disordered, showing the lowest d100 intensity and exhibiting little difference between the as-synthesized and extracted samples. Although most of the PMO’s exhibit no appreciable difference in d100 spacing (Table 1), the functionalized PMO’s that contain aromatics show a small (2-3 Å) matrix contraction following surfactant extraction. X-ray analysis of the blank material synthesized without any surfactant indicated that it is amorphous, as no peaks were resolved. Nitrogen Sorption. Nitrogen gas sorption isotherms and corresponding pore size distributions (insets) of the extracted samples are shown in Figure 5. The blank, amine, diamine, thiol, and imidazole-functionalized PMO’s exhibit type IV isotherms30 and pore size distributions with sharp peaks ∼25 Å. The pyridine-, benzyl-, and phenethyl-functionalized PMO’s exhibit type I isotherms characteristic of materials that are primarily microporous. Pore size distributions of these materials indicate that the maxima may indeed be less than 20 Å. The desorption branches of the benzyl- and phenethyl-containing materials show a broad hysteresis. Although the bottlenecking of pore openings
Figure 3. Powder X-ray diffraction patterns of (a) BTSE blank extracted, (b) BTSE blank as-synthesized, (c) SHE extracted, (d) SHE as-synthesized, (e) NHE extracted, (f) NHE as-synthesized, (g) EN extracted, (h) EN as-synthesized, (i) IMID extracted, (j) IMID assynthesized.
TABLE 1: d100 Spacings (Å) sample
as-synthesized
extracted
blank thiol amine diamine imidazole pyridine benzyl phenethyl
43 45 44 44 43 40 43 38
44 44 44 43 43 38 41 35
and relatively disordered pore structures may explain the hysteresis at higher relative pressures, the reasons for the hysteresis at relative pressures less than 0.5 are not clearly understood. The structural properties of the extracted mesoporous organosilicas are listed in Table 2. The effects of surfactant templating are clearly evidenced by the nitrogen sorption and X-ray data. The blank material synthesized without any surfactant exhibits a surface area of 530 m2/g, a pore volume of 0.37 cm3/g, and no local ordering. The control blank material made with CTAC exhibits more than twice the surface area and pore volume, as well as the hexagonal ordering discussed above. PMO’s with type IV isotherms all have BET surface areas ∼1000 m2/g and pore volumes much larger than the blank
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Figure 4. Powder X-ray diffraction patterns of (a) PYR extracted, (b) PYR as-synthesized, (c) BENZ extracted, (d) BENZ as-synthesized, (e) PHEN extracted, (f) PHEN as-synthesized.
TABLE 2: Structural Properties of Functionalized Periodic Mesoporous Organosilicas
sample blank (no surfactant) blank PMO thiol amine diamine imidazole pyridine benzyl phenethyl
d100 BET total pore size surface area pore volume maximum spacing (m2/g) (cm3/g) (Å) (Å) 530 1170 1180 1090 980 1170 800 840 690
0.37 0.93 0.67 0.83 0.51 0.87 0.20 0.24 0.17
27 23 25 23 26
44 44 44 43 43 38 41 35
synthesized without any CTAC. PMO’s that exhibit type I isotherms have larger surface areas than the blank BTSE, but smaller pore volumes. This indicates a relative inefficiency of surfactant templating in materials that contain aromatic groups. Nitrogen sorption analysis was performed on the as-synthesized blank PMO and yielded a surface area of 0.3 m2/g and no pore volume. This indicates that the as-synthesized composites are nonporous powders prior to ethanol extraction. Infrared Spectroscopy. Qualitative identification of the functional groups in our PMO’s has been achieved with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). All PMO’s exhibit strong bands at 2920 and 1410 cm-1, corresponding to C-H and Si-CH2 stretches, respectively. These bands are clearly exhibited by the pure BTSE sample shown in Figure 6a. The phenethyl- and benzyl-containing materials absorb strongly due to both aromatic C-H (3085, 3060, 3030 cm-1) and C-C ring stretches (1605, 1495, 1460 cm-1). All amine-containing materials exhibit a very broad N-H stretch from 2200 to 3300 cm-1 and varying degrees of N-H scissoring indicated by absorption at 1620 cm-1 (Figure 7). The C-C and C-N skeletal bands of the pyridine group (1540, 1475 cm-1) are also clearly visible. Thermogravimetric Analysis. A thermogravimetric analysis of all samples was conducted from room temperature to 1000
°C. Samples were analyzed in three forms: as-synthesized, following surfactant extraction, and after a second extraction. A typical weight loss curve and derivative plot for as-synthesized materials is shown in Figure 8. The as-synthesized PMO’s exhibited a weight loss of 2-5% at temperatures less than 120 °C, attributable to the loss of small amounts of residual water adsorbed to the material surface. This is followed by a weight loss of 35-40% from 100 to 300 °C due to surfactant decomposition. This rather large loss is characterized by three overlapping peaks in the derivative curve, indicating that this process may occur in steps and not as a concerted reaction. This is followed by an additional loss of 5-7 wt % attributed to matrix decomposition from 300 to 700 °C. It is within this broad temperature range that the functional pendent groups also decompose. This overlap, coupled with the relatively large weight loss due to surfactant decomposition make the detection of the functional groups with TGA difficult for the assynthesized PMO’s. Samples analyzed following the primary extraction show no indication of any residual surfactant (Figure 8b-d). The weight loss curve of the BTSE control sample shown in Figure 8b exhibits a very sharp decrease of about two weight percent below 80 °C, followed by a very small (
