Influence of Synthesis Conditions on Properties of Ethane-Bridged

Oct 11, 2013 - Laboratory of Adsorption and Catalysis, Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610. Antwerp-Wilrijk, ...
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Influence of Synthesis Conditions on Properties of Ethane-Bridged Periodic Mesoporous Organosilica Materials as Revealed by SpinProbe EPR Feng Lin,†,‡ Myrjam Mertens,§ Pegie Cool,*,† and Sabine Van Doorslaer*,‡ †

Laboratory of Adsorption and Catalysis, Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp-Wilrijk, Belgium ‡ Laboratory for Spectroscopy in Biophysics and Catalysis, Department of Physics, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp-Wilrijk, Belgium § VITO, Flemish Institute for Technological Research−NV, Boeretang 200, 2400 Mol, Belgium S Supporting Information *

ABSTRACT: A thorough investigation of the formation mechanism and surface properties of periodic mesoporous organosilicas (PMOs) is of crucial importance for further tuning and improving of the structural characteristics and applications of these promising meso-structured materials. In the present paper, the effects of the synthesis conditions on the properties of ethane-bridged PMOs were studied by means of spin-probe electron paramagnetic resonance (EPR) spectroscopy complemented with standard characterization techniques for porous materials. When spin probes were dissolved in the synthesis mixture, the influence of the precursor type on the formation kinetics of ethane-bridged PMOs could be tested. The use of the precursor 1,2-(tris(triethoxysilyl)ethane instead of bis(trimethoxysilyl)-ethane significantly slows the pore formation, leading to materials with larger pore diameters. Furthermore, different spin probes with varying sizes and polarities were adsorbed onto two types of ethane-bridged PMOs synthesized at room temperature or at 95 °C. The effect of surface polarity, surface water, and pore size on the incorporation and mobility of molecules in the PMO pores was thus monitored. Ethane-bridged PMO materials synthesized at room temperature were found to have a smaller pore size and a larger amount of physisorbed water than those synthesized at 95 °C, influencing strongly the insertion of molecules in the pores as observed by spin-probe EPR. distributed in the channel walls.11,12 Apart from the uniform pore size (between 2 and 50 nm) and the high surface areas, PMO materials have several other unique properties compared to the conventional SiO2-based mesoporous materials. The organic functionalization of their framewall not only allows tailoring of the properties of PMOs, such as the surface hydrophobicity and hydrophilicity, mechanical and hydrothermal stability,13 and adsorption capacity, but also offers the possibility for further chemical modification.14 So far, a variety of organic groups has been incorporated into the framework of PMOs, leading to materials with promising potential in many fields, e.g., the chemical industry (catalysis), environmental applications (metal scavenging), and medical applications (controlled drug release). Considerable research interest had been focused on ethane-bridged PMOs because of their high rigidity and ordering and the availability of the

1. INTRODUCTION The discovery of ordered mesoporous siliceous materials1−4 by the hydrolysis and condensation of inorganic precursors in the presence of surfactant micelles has opened up a whole new field of research in material science. It has also resulted in an increasing interest in the fundamental research on the synthesis and applications of mesoporous materials as well as the selfassembly phenomena, which occur in the mixture of the surfactant solution and silica precursor. In the last few decades, the original surfactant-templatingbased synthesis approach has been extended by numerous variations, leading to a large number of mesoporous materials of varying composition, pore size, and morphology.5−10 Among the most recent innovations, the use of bridged silsesquioxane [(R′O)3Si−R−Si(OR′)3] as precursors in the surfactanttemplating synthesis created a breakthrough in the research of mesoporous siliceous materials. This exciting development led to a novel class of organic−inorganic hybrid materials, referred to as periodic mesoporous organosilicas (PMOs), wherein the organic functionalities are homogeneously © 2013 American Chemical Society

Received: June 20, 2013 Revised: October 11, 2013 Published: October 11, 2013 22723

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precursor 1,2-bis(trimethoxysilyl)ethane (BTME) and 1,2bis(triethoxysilyl)ethane (BTEE). Furthermore, many studies were directed at elucidating the formation mechanism of micelle-templated mesoporous materials in which electron paramagnetic resonance (EPR) was shown to be an excellent tool to obtain mechanistic insight into the mesoporous materials.15 When an EPR probe is dissolved in the micelle region of the system, the motional changes that the surfactant molecules experience during the synthesis process can be directly monitored. This can provide information on the evolution of the micelle structure as well as the interactions between surfactants and silica species during the formation of self-assembled mesostructures. Goldfarb and co-workers applied this technique to investigate the formation of MCM-41 at room temperature.16,17 They discovered that the formation of MCM-41 can be divided into two stages: a fast formation of the hexagonal structure in the first 12−13 min of the synthesis and a slow reorganization of the silica framework in the second stage. A similar two-step formation mechanism of micelle-templated silica was also found by Ottaviani and coworkers.18 Furthermore, the formation of SBA-15 was investigated by in situ EPR spectroscopy with Pluronic spin probes.19,20 Besides its suitability for investigating the formation mechanism of micelle-templated silica materials, spin-probe EPR has also revealed itself to be a useful tool for analyzing pore size, pore accessibility, and surface properties.21−23 The analysis of the EPR spectra allows the extraction of information about the mobility of the spin probes and the polarity of the adsorption sites of the silica materials.24 In a recent study,25 spin-probe EPR has been used to probe functional-groupspecific surface interactions in porous solids, including benzenebridged PMO materials. Here, we study the formation of ethane-bridged PMO materials at room temperature with two different precursors by in situ spin-probe EPR. Our prime interest in this part of the work concerns the interface of the surfactant micelle and the silica precursor. For this reason, 5-doxyl stearic acid (5-DSA) in which the nitroxide radical is located close to the polar head was chosen as the spin probe (Scheme 1). In a second part of the work, we reveal the surface properties of two types of ethane-bridged PMO materials that were synthesized at room temperature and 95 °C, respectively. For this, four different spin probes (Scheme 1) are adsorbed on the PMOs and studied by EPR. Complemented by standard techniques such as N2 adsorption−desorption, X-ray powder diffraction, and FT-IR spectroscopy, this approach allows the probing of pore size, surface polarity, and pore accessibility in these materials.

Scheme 1. Chemical Formulas of the Spin Probes Used in This Work: 5-Doxylstearic Acid (5-DSA), 16-Doxylstearic Acid (16-DSA), 3-Carboxy-proxyl (3-CP), and 4-HydroxyTEMPO Benzoate (4-HTB)

composition: BTME (or BTEE)/C16TMACl/NaOH/H2O (1.0:0.12:1.0:231). In a typical application of method I, C16TMACl (0.299 g) was added under vigorous stirring to a solution of 0.309 g of NaOH in 31.68 g of water. After complete dissolution of the surfactant, 2.15 g of BTME was added. The stirring was continued for 24 h at room temperature while a precipitate formed. It was separated by filtration, washed thoroughly with deionized water, and dried at room temperature. The second approach (method II) is based on a different mixture composition: BTME(or BTEE)/ C16TMACl/NaOH/H2O (1.0:0.57:2.36:353). After the samples were mixed as described above, the solution was aged under stirring for 24 h at room temperature and then heated at 95 °C for 20−24 h without stirring. For both cases, the surfactant was removed by solvent extraction as reported by Inagaki et al.27 Typically, 1 g of as-made organosilica was treated under vigorous stirring for 6 h at 50 °C in a mixture of 150 mL of ethanol (95%) and 3.8 g of concentrated HCl (37%), followed by filtration, washing with ethanol, and drying. 2.3. Synthesis of PMO Materials for in Situ EPR Detection. The synthesis of the ethane-bridged PMO materials was carried out with method I as described above, except for the addition of the spin probe. The components in the synthesis had a BTME (or BTEE)/C16TMACl/NaOH/ H2O/ethanol/spin probe molar ratio of 1.0:0.12:1.0:231:3.3:1.12 × 10−3. The reaction mixture without BTME (or BTEE) was first prepared; then BTME (or BTEE) was added with vigorous stirring for about 2 min. Part of the mixture was then quickly transferred into a capillary, which was kept in the EPR cavity until the end of the measurement. 2.4. Spin-Probe Adsorption Procedure. Solutions of four different spin probes were prepared in CHCl3 to a concentration of 5 × 10−4 M. The ethane-bridged PMOs were

2. EXPERIMENTAL SECTION 2.1. Materials. All starting materials were used as purchased without further purification: 1,2-bis(trimethoxysilyl)-ethane (BTME, 96% Sigma-Aldrich), 1,2-bis(triethoxysilyl)ethane (BTEE, 96%, Sigma-Aldrich), cetyltrimethylammoniumchloride (CTAC, 98%, Sigma-Aldrich), NaOH (98.5%, Acros Organic), ethanol (99.9%, Merck KGaA), CH2Cl2 (99.9%, Acros Organics), and toluene (99.5%, Merck Eurolab). The spin probe 5-doxyl stearic acid (5-DSA), 16-doxyl stearic acid (16DSA), 3-carboxy proxyl (3-CP), and 4-hydroxy tempo benzoate (4-HTB) were purchased from Sigma-Aldrich. 2.2. Synthesis of PMO Materials. Two synthesis methods reported by Inagaki et al.12 and Sayari et al.26 were employed to prepare the ethane-bridged PMO materials, termed method I and II, respectively. Method I uses mixtures with the following 22724

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used after twice extraction of acidic ethanol as described above. A 300 mg sample of solvent-extracted PMOs was stirred for 10 h in 5 mL of chloroform solution of spin probe (5 × 10−4 M), filtered, and dried at room temperature. 2.5. Characterization. N2 Adsorption−Desorption Isotherms were obtained at liquid-N2 temperature (77 K) using a Quantachrome Quadrasorb-SI automated gas-adsorption system. Prior to adsorption, the samples were outgassed under high vacuum for 16 h at 100 °C. The specific surface area was calculated using the Brunauer−Emmet−Teller (BET) method, between a relative pressure of 0.05 and 0.35. The pore size distributions were deduced from the desorption branches of the isotherms using the Barrett−Joyner−Halenda (BJH) method. The total pore volumes were calculated from the amount of N2 vapor adsorbed at a relative pressure of 0.95. X-ray Dif f raction (XRD) measurements were recorded on a Pananalytical X’PERT PRO MPD diffractometer with filtered Cu Kα radiation. The measurements were performed in the 2θ mode using a bracket sample holder with a scanning speed of 0.04°/4 s continuous mode. Thermogravimetric Analysis (TGA) results were recorded on a Mettler Toledo TGA/SDTA851. The analyses were performed in an oxygen atmosphere, whereby the samples were heated from 30 to 700 °C with a heating rate of 5 °C/min. FT-IR (DRIFT) measurements were recorded on a Nicolet 20 DXB Fourier Transform IR spectrometer equipped with a DTGS detector. Pure KBr was measured as a reference for the background. The samples were diluted with KBr (2% sample, 98% KBr). The resolution was set to 4 cm−1 and 200 scans were averaged. All measurements were performed under a flow of dry air. X-band CW-EPR measurements were performed on a Bruker ESP 300E instrument, equipped with a liquid helium cryostat (Oxford Inc.), working at a microwave (mw) frequency of about 9.5 GHz. A microwave power of 1 mW, modulation frequency of 100 kHz, and modulation amplitude of 0.05 mT were applied. The EasySpin program28 was utilized to simulate the CW-EPR spectra.

Figure 1. X-band CW-EPR spectra of 5-DSA in H2O/EtOH (70:1) recorded at room temperature (a), 5-DSA in CH2Cl2 with a drop of toluene recorded at 50 K (b), and of a 5-DSA/C16TMACl/H2O/ EtOH mixture (1.12 × 10−3:0.12:231:3.3) recorded at room temperature (c). The microwave frequency was 9.439 GHz (a, c) and 9.442 GHz (b).

signal indicates that 5-DSA is incorporated into the CTAC micelles, which decrease the mobility of the spin probe compared to that of the free radicals in the H2O/EtOH solution. The spectrum in Figure 1c can be simulated using a correlation time τc of 1.2 ns (Supporting Information). The addition of NaOH further decreases the mobility of 5-DSA (Figure S2 of Supporting Information). This is probably due to the swelling of the micelles caused by NaOH.18 In a next step, BTME is added to the solution, and the spectral changes are monitored from 3 min to 64 h after mixing (Figure 2a). Comparison with the EPR spectrum recorded for

3. RESULTS 3.1. Effect of Silica Precursor Material: In Situ EPR. The EPR spectral line shape of samples containing spin probes directly reflects the movement of the spin-probe molecule. The spectrum of 5-DSA in a H2O/EtOH (70:1) solution (5 × 10−4 M) recorded at room temperature (Figure 1a) consists of a sharp triplet, reflecting fast motion. Figure 1b shows the EPR spectrum at 50 K of a CH2Cl2 solution of 5-DSA with a drop of toluene to improve glass formation. The spectrum exhibits a powder line shape, characteristic of a rigid-limit EPR spectrum. Both EPR spectra can be simulated using the parameters gx = 2.0086(1), gy = 2.0068(1), gz = 2.0028(1), Ax = 13.8(5) MHz, Ay = 13.8(5) MHz, and Az = 96.0(5) MHz, whereby a rotational correlation time τc of 0.1 ns is found for the EPR spectrum in Figure 1a (see also Supporting Information). The EPR parameters are similar to those reported earlier for these probes.29 Prior to the in situ EPR experiment, the spectrum of 5-DSA in the different components of the synthesis mixture was examined. The spectrum of 5-DSA in the mixture without NaOH and silica precursor (Figure 1c) shows a triplet structure similar to that in the H2O/EtOH solution (70:1) (Figure 1a), but with a clear broadening of the three peaks. Considering that 5-DSA is not well-soluble in pure water, the well-resolved EPR

Figure 2. Room-temperature CW-EPR spectra of the ethane-bridged synthesis mixture with (a) BTME and (b) BTEE recorded at different times after mixing as indicated in the graph. The synthesis method was method I (see Materials).

the reaction mixture lacking the silica precursor (Figure 1c and Figure S2 of Supporting Information) shows that a significant change in the EPR line shape has occurred during the first 3 min after the addition of BTME. The spectrum does not change in the subsequent 64 h (Figure 2a). The spectrum is typical for a large immobilization of the spin probe. This indicates that the use of the BTME precursor causes a fast condensation of the silicate. This reaction is clearly slower when BTEE is used (Figure 2b). With time, the spectra exhibit 22725

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Figure 3. Evolution of PMO structure formation observed by XRD during PMO synthesis with BTME (left) and BTEE (right).

different properties of the PMO materials. To this end, ethanebridged PMOs were synthesized at room temperature (method I) and at 95 °C (method II); in both cases, BTME was used. Prior to the spin-label adsorption experiments, information about the textural characteristics of the two types of PMO materials was obtained from XRD (Supporting Information) and the low temperature N2-adsorption−desorption isotherms (Figure 4a). Both solvent-extracted (surfactant-free) PMOs exhibit typical type IV isotherms (IUPAC classification) with a nitrogen capillary condensation step characteristic of mesoporous materials. For PMOs synthesized at room temperature (method I), the capillary condensation step happens in the relative pressure range of 0.15−0.30. However, the capillary condensation step for the PMO materials synthesized at 95 °C is shifted to higher relative pressures, ranging from 0.25 to 0.40. The position of the inflection point is directly related to the diameter of the mesopore, which indicates that the pore size of PMOs synthesized with method II is larger than that of those synthesized using method I (Figure 4b). The specific surface area, pore volume, and pore diameter of the two samples are presented in Table 1. It can be seen that method I results in PMOs with a higher surface area and larger pore volume, but smaller pore size than those synthesized with method II. In a subsequent experiment, spin probes were adsorbed on the template-free PMOs synthesized by methods I and II (see details in Materials). The four spin labels depicted in Scheme 1 were used. After spin-label adsorption, the PMOs were filtrated, dried, and measured with EPR at room temperature (Figure 5). We first consider the adsorption experiments using the bulky spin probes 5-DSA and 16-DSA. From Figure 5, it is clear that while a considerable amount of 5-DSA or 16-DSA spin probes remains after filtration in the PMOs synthesized with method II (large EPR intensity), this is not the case for the PMOs synthesized using method I. This is most likely due to the difference in the pore size (Figure 4b): the average pore size (3.4 nm) of PMO(method II) is approximately twice as large as that of PMO(method I). The bulky spin probes will thus be more easily incorporated in the larger pores. The difference in the EPR spectra of 16-DSA and 5-DSA adsorbed on PMO(method II) stems from a different spin mobility. The nitroxyl fragment of 16-DSA exhibits a faster motion than that

increasing anisotropy, which is characteristic of decreased rotational diffusion rates, and an increasing order parameter (analysis shown in Supporting Information). The major changes in the EPR spectral line shape occur during the first hour after the addition of BTEE, and the spectrum undergoes slight variations with time until 24 h after mixing, after which it remains invariant. The reaction rate when BTEE is used is slower than that when BTME is used. The EPR spectrum observed 3 min after the addition of BTME (Figure 2a) is similar to that measured >17 min after the addition of BTEE (Figure 2b). The XRD patterns of the as-synthesized ethane-bridged PMO materials and N2-sorption experiments of the final ethane-bridged PMOs obtained after template removal show a larger pore size when BTEE is used instead of BTME (Supporting Information). In a parallel experiment, the evolution of the PMO structure formation with BTME or BTEE was also followed with XRD. To this end, parts of the reaction mixture were filtered off after 1 h and after 24 h of reaction, respectively. After being dried at room temperature, the powders were characterized by X-ray powder diffraction (Figure 3). For all samples, only one diffraction peak (100) can be observed in the low-angle region. However, the diffraction peak of the samples taken after 24 h of reaction was much sharper than the one after 1 h, indicating a more ordered structure (Figure 3). Although the EPR spectral line shape, sensitive to the initial pore formations, remains constant from 1 h to 24 h of synthesis (Figure 2), the low-angle XRD patterns indicate that the pore wall formation continues more than 1 h after the addition of BTME or BTEE. Furthermore, the diffraction peak one hour after the addition of BTME is sharper than that after the addition of BTEE (Figure 3), which is consistent with the faster reaction rate observed with EPR (Figure 2). The main difference between both materials obtained is the considerable difference in the framework formation time, whereas the surface properties of the materials are mainly unaffected (as was evidenced by IR experiments not shown here). 3.2. Effect of Synthesis Temperature: Spin-Probe Adsorption Experiment. In this part of the work, we aim at probing the effect of the synthesis temperature on the 22726

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of the 5-DSA (Supporting Information). Most likely this is because of the different location of the nitroxyl group (EPRactive part) within the spin-label molecule. Generally speaking, the driving force for 5-DSA or 16-DSA adsorption on the PMO wall will be the interaction between the acidic parts of the spin probe and the silanol groups of the pore wall. In 5-DSA, the nitroxyl group is located close to the acidic group, and anchoring of the probe will restrict the motion of the nitroxyl site. In 16-DSA, the EPR-active part is located at the end of the alkyl chain, and after spin-probe adsorption via the acidic group, this part will have a degree of motional freedom larger than that in the 5-DSA case. The spin probes 3-CP and 4-HTB are considerably smaller than 5-DSA and 16-DSA (Scheme 1) and are in principle sufficiently small to enter the pores of both types of PMO materials. In the case of 3-CP, we indeed observe a very intense EPR spectrum for both materials. However, while the spin probe is very mobile inside the pores of the PMO synthesized using method I, a fraction of 3-CP is immobilized in PMO(method II), as can be discerned from the shoulders left and right from the EPR contribution of the mobile probes (asterisk in Figure 5). The behavior of the 4-HTB spin probe is even more surprising. Although the probe is small enough to enter the pores of both PMO materials, a significant EPR contribution can be detected only for the case of PMO synthesized via route II (Figure 5), and this contribution is weaker than that observed for the other spin labels in the same material (see reduced signal-to-noise ratio). Because the pore size is not the limiting factor in this case, it seems probable that the spin-probe behavior is reflecting the specific surface properties of the material. To probe possible surface differences, FT-IR experiments were performed on the solvent-extracted PMO materials synthesized via methods I and II (Figure 6). The bands at 1426, 1165, and 783 cm−1 suggest the presence of organic fragments related to the CH2−CH2 units in the framework positions.30 The bands at 693 and 1270 cm−1 can be assigned to the C−Si stretching vibrations, while the sharp peak at about 1089 cm−1 confirms the formation of siloxane bands.31 In addition, the band at 910 cm−1 shows the presence of residual silanol, ν(Si−OH), stretching vibrations, and the broad band between 3000 and 3700 cm−1 can be assigned to the adsorbed water (or water of crystallization) and O−H vibrations.32 From Figure 6, it is obvious that the intensity of the bands at 910 and 1089 cm−1 and in the 3000−3700 cm−1 region of PMO(method I) is much higher than those of PMO(method II), indicating the presence of more silanol groups and adsorbed water on the pore walls. This finding coincides with the TGA results, for which a distinct weight loss was observed for PMOs synthesized using route I below 110 °C, corresponding to the loss of the physisorbed water molecules (Supporting Information). To check whether physisorbed water plays a role in the spinlabel adsorption and motional behavior of the spin probes 3-CP and 4-HTB on the PMO materials, the materials were outgassed under high vacuum for 24 h at 100 °C prior to the adsorption experiments in order to remove the water component. The resulting EPR spectra are depicted in Figure 7. We now notice that the spin probes remain adsorbed in the pores and that the EPR spectra for a specific probe are almost independent of the PMO synthesis route. This shows that surface water was indeed playing an essential part in the earlier observed spin-probe behavior. When we compare the EPR

Figure 4. (a) N2 sorption isotherms of the template-free PMO materials synthesized using methods I and II and (b) the corresponding pore diameter distribution.

Table 1. Surface Area, Pore Size, and Pore Volume Determined for the Ethane-Bridged PMOs Synthesized Using Methods I and II synthesis method

I

II

surface area (m2 g−1) pore size (nm) total pore volume (cm3 g−1)

1458 2.1 0.74

758 3.4 0.64

Figure 5. Room-temperature EPR spectra of 5-DSA, 16-DSA, 3-CP, and 4-HTB spin probes adsorbed on ethane-bridged PMOs synthesized using method I and II. For each of the spin probes, the spectra are shown such that they reflect the relative intensity of the two spectra. For 16-DSA and 5-DSA, the spectra are also shown magnified 15× to enlarge the small signal of the spin probe.

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Figure 6. FT-IR spectrum of ethane-bridged PMOs obtained using synthesis method I (solid line) and method II (dotted line).

the reaction rate was too fast to be monitored by the used EPR methodology (Figure 2a), indicating that the surfactant micelles are immediately encapsulated by the silica molecules, leading to larger entities that tumble more slowly in the solvent as is reflected by the reduced spin-probe mobility. In the case of BTEE, the slower hydrolysis rate (and thus slower condensation of the silica walls) allows us to follow this motional change during the first hour of the reaction. After the initial condensation, the pore wall formation continues as shown by XRD (Figure 3), but it is no longer detectable by EPR. Indeed, once the condensed structure becomes too large, its motion becomes so slow that it no longer dominates the spin-probe motion. The residual spin-probe motion detected by EPR stems from the motional freedom of the label in the micelles. The N2-sorption characteristics of the final PMO solids after template removal indicate a larger pore size when BTEE is used as a precursor (Supporting Information). Because the two PMO materials differ only in their silica precursor, the pore expansion seems to be linked to the different reaction rate of the two systems. Because BTEE is more hydrophobic than BTME and its hydrolysis rate is slower than that of BTME, unhydrolyzed BTEE may more readily penetrate into the core of template micelles and act as a swelling agent, resulting in larger pore size. This has been earlier suggested in the context of studies on MCM-41 materials.35,36 BTEE may thus act both as silica source and as swelling agent in the formation of ethanebridged PMOs as follows from the slightly larger pore size in the PMOs synthesized using BTEE (Supporting Information). Note that the reduced hydrolysis rate in the case of the BTEE precursor can give the silica species more time to condense properly around the template and also provides more possibility for the incorporation of heteroatoms into the framework of ethane-bridged PMO materials. The latter may be exploited for framework modifications of these PMO materials. 4.2. Characterization of the Surface Properties of PMOs from Spin-Probe EPR. All potential applications of mesoporous materials involve insertion and/or immobilization of molecules in the pores of the material. Spin-probe EPR gives a unique tool to monitor the incorporation of molecules in the pores of such systems.15−17 The spin probes can be considered to be mimics of the active molecules that one would like to insert in the material. Here, four different spin probes were

Figure 7. Room-temperature EPR spectra of 3-CP and 4-HTB spin probes adsorbed on outgassed ethane-bridged PMOs synthesized using method I and II.

spectra of the 3-CP probes adsorbed on the original PMO materials (Figure 5) and on the degassed materials (Figure 7), we notice a clear reduction of the nitroxyl mobility. This indicates that while the small polar 3-CP probe can freely move in the adsorbed water layer on the pore walls, removal of this layer leads to a stronger immobilization of the 3-CP probe on the pore walls. The mobility of the 4-HTB spectrum is higher than that of 3-CP in the outgassed PMOs. This is most probably due to the lower polarity and higher hydrophobicity of the former molecule, which will lead to a reduced adsorption of the molecule on the polar pore surface. It also explains why the 4-HTB probe was not retained, or was retained less, in the adsorption experiments shown in Figure 5. The presence of the surface water will have hampered 4-HTB entering the pores.

4. DISCUSSION 4.1. Formation of Ethane-Bridged PMOs with Different Precursor Materials. We here probed the influence of two different organosilica precursors, BTME and BTEE, on the formation mechanism of ethane-bridged PMOs at room temperature. For this, the time evolution of the EPR spectra of a 5-DSA spin probe mixed with the template molecules was recorded during PMO synthesis. At any time, the spin-label motion will be the resultant of its own internal motion within the template micelle and the motion of the micelle as a whole. This label motion is probed by the EPR spectral line shape. In the case of the BTME precursor, the major change of the EPR spectral line shape occurred during the first 3 min of reaction, whereas in the BTEE case, the changes happened at a slower rate during the first hour. The differences reflect the different hydrolysis rates of BTME and BTEE under strong basic conditions. The hydrolysis of BTEE is slower than that of BTME owing to steric hindrance at the larger ethoxide moieties and reduced solvation of the resulting ethanol.33,34 For BTME, 22728

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Scheme 2. Schematic Representations of the Surface Properties of the Ethane-Bridged PMOs Synthesized Using Method I and II

hydrophobic than that of the MCM-41 silica material, in which the hydrophobic parts are constituted by siloxane groups with a few isolated silanols.37 Therefore, 5-DSA with its long hydrocarbon side chain is more easily adsorbed onto the PMO materials, resulting in a lower mobility of the nitroxyl moiety. Besides the smaller pore size, the larger amount of physisorbed water in the PMO materials synthesized at room temperature will also disfavor insertion of the DSA labels. A fraction of the probes may self-aggregate to minimize the repulsion between them and the polar environment,24 leading to larger entities that cannot enter the pores.

adsorbed onto two types of ethane-bridged PMO materials synthesized at room temperature (method I) or at 95 °C (method II) (using BTME). The high-temperature synthesis route is found to lead to materials with a larger pore diameter (Figure 4). The probes differ in size and hydrophobicity (Scheme 1). The 3-CP and 4-HTB probes are sufficiently smaller than the pore sizes, so that pore size effects can be excluded as a limiting factor for molecule uptake and adsorption. Nevertheless, a significant difference in the spin-probe adsorption was observed for the PMO materials determined by the two synthesis routes (Figure 5). Outgassing of the PMO materials prior to spinprobe adsorption (Figure 7) revealed that the difference is due to the large amount of adsorbed water and silanol groups in the case of the room-temperature synthesis (Figure 6). In these materials, the presence of the water layer in the PMO materials synthesized at room temperature leads to a large motional freedom of the 3-CP probe and a repulsion of the more hydrophobic 4-HTB probe. High-temperature synthesis leads to pore walls that are less covered by water (see pictorial representation in Scheme 2). This explains why a fraction of the 3-CP probes are more immobilized in this PMO material (Figure 5, asterisks); it consists of 3-CP molecules adsorbed to water-free areas in the PMO material. Similarly, the presence of these water-free sections explains why the hydrophobic 4-HTB is (to a small degree) adsorbed to the high-temperature PMO material (Figure 5). In accordance with our results, ethanoldissolved 3-CP showed a mobility at RT in porous silica-based solids with polar surfaces larger than that in those with nonpolar, hydrophobic surfaces.25 The 5-DSA and 16-DSA spin probes are quite hydrophobic and have a larger molecular size due to the C16 hydrocarbon chain. In this case, pore size dimensions will also start to play an important role, in addition to surface polarity and surface water. Indeed, the DSA probes are found not to enter in the PMO materials synthesized at low temperature (Figure 5), which have smaller pore sizes (Figure 4). 5-DSA has been adsorbed earlier to MCM-41 silica materials.18 Interestingly, the reported EPR spectrum of 5DSA adsorbed onto MCM-4118 is typical of a spin probe with a very large mobility, only slightly reduced compared to the free radicals in solution. This contrasts our finding for 5-DSA adsorbed onto the ethane-bridged PMO synthesized by method II (Figure 5), which is typical of a strongly reduced motion (Figure 5, Supporting Information). Both the PMOs studied here and the MCM-41 exhibit two well-defined patches, one hydrophilic and one hydrophobic. The hydrophilic patch is similar in both cases (silanols). However, because of the ethyl group present in the wall of PMOs, its surface is more

5. CONCLUSION Spin-probe EPR has been proven to be a powerful tool for obtaining information from mesoporous materials. It not only allows monitoring the formation of ethane-bridged PMOs but also allows probing the surface properties in a quite simple way. In situ spin-probe EPR reveals that during the synthesis of ethane-bridged PMOs, the reaction rate when BTEE is used is slower than that when BTME is used. BTEE acts as both silica precursor and swelling agent in the synthesis process. The evolution of the EPR and XRD patterns shows that the initial structure of ethane-bridged PMOs synthesized using BTEE is formed in the first hour of synthesis, followed by a slow condensation step in which the pore wall is consolidated. Spin-probe EPR also reveals the influence of the synthesis temperature on the surface properties of the ethane-bridged PMO materials. The incorporation and mobility of the different probes in the porous materials are governed by different factors. For large molecular spin probes like 5-DSA and 16-DSA, the PMO pore size dominantly influences the adsorption and mobility of spin probes in the pores. In contrast, for small molecular spin probes, such as 3-CP and 4-HTB, the surface water and surface polarity play a decisive role in probe insertion and immobilization. Ethane-bridged PMO materials synthesized at room temperature have smaller pore size and a larger amount of physisorbed water than those synthesized at 95 °C. This alters totally the incorporation of the molecules in the porous material.

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ASSOCIATED CONTENT

S Supporting Information *

S1, simulations of spectra in Figure 1; S2, effect of addition of NaOH to mixture; S3, evaluation of EPR spectra during PMO synthesis; S4, N2-sorption experiments of ethane-bridged PMO with BTEE or BTME precusors; S5, XRD patterns of ethanebridged PMOs synthesized with method I or method II; S6, motional behavior of spin labels adsorbed to PMOs; and S7, 22729

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TGA results for ethane-bridged PMOs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 032 (0)3 265.23.55. Fax: 032 (0)3 265.23.74. *E-mail: [email protected]. Phone: 032 (0)3 265.24.61. Fax: 032 (0)3 265.24.70. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Erasmus Mundus CONNEC program is acknowledged for PhD funding of F.L. S.V.D. and P.C. acknowledge the GOABOF project funding of the University of Antwerp.

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