Imprinting With Phenyl Group Interactions: A Case Study of the Hybrid

Dec 2, 2010 - The sodium complex cation, {Na[Ph2P(O)−CH2−P(O)Ph2]3}+, is used as a guest species in acid-catalyzed aqueous sol−gel processes of ...
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Imprinting With Phenyl Group Interactions: A Case Study of the Hybrid Sol-Gel Encapsulation of the Complex {Na[Ph2P(O)-CH2-P(O)Ph2]3}+ Nina M. Wichner,†,§ Nugzar Ghavtadze,‡,§ Ernst-Ulrich Wu¨rthwein,‡,§ and Hubert Koller*,† Institut fu¨r Physikalische Chemie, Westfa¨lische Wilhelms-UniVersita¨t, Mu¨nster, Germany, and Organisch-Chemisches Institut, Westfa¨lische Wilhelms-UniVersita¨t, Mu¨nster, Germany ReceiVed: August 11, 2010; ReVised Manuscript ReceiVed: NoVember 2, 2010

The sodium complex cation, {Na[Ph2P(O)-CH2-P(O)Ph2]3}+, is used as a guest species in acid-catalyzed aqueous sol-gel processes of tetraethoxysilane and phenyltriethoxysilane. Raman spectroscopy in combination with band assignments from DFT calculations, solid-state NMR, and thermal analyses are applied to study the structural integrity of the sodium complex and to characterize the structural properties of the sol-gel materials. The sodium complex is decomposed at a synthesis pH value of 1, whereas it is encapsulated intact at pH ) 4. Calcination removes the complex, and micropores are observed. Phenyl group interactions between the complex surface and the phenyl-functionalized hybrid gel are suggested to take place in the encapsulation process. The spherically shaped sodium complex serves as a model compound for surface interactions between aromatic groups during the genesis of an encapsulated sol-gel hybrid material, and it can be regarded as a soft complex, whose structural stability is of the same order as the interaction energy with its environment. Introduction 1

One of the great advantages of sol-gel production of silica is that it allows one to synthesize porous functional materials with a soft chemical approach. Silica gels with tailor-made pore sizes and shapes have a large technical impact for example in catalysis, molecular sieving, or chemical sensing.2 However, the imprinting mechanism and formation of internal pore space is still not well understood. The formation of mesoscopic pore space by the use of amphiphilic surfactants is well investigated based on the phase behavior of liquid crystal templates.3-8 On the other hand, the generation of porous materials by smaller, soft molecular assemblies of nonsurfactant molecules or molecular clusters is less well established. While designing and synthesizing imprinted materials based on covalent bonds is more or less straightforward, the preparation of materials by the controlled use of van der Waals forces remains a difficult task.9,10 Nevertheless, there is a large impetus behind the application of pore-generating soft species (porogenic agents) due to their high potential in tailoring functional materials. In addition, soft templates can be recycled from the rigid host by extraction, sometimes preceded by chemical degradation.11 After removal of the encapsulated porogenic agent, pore space is formed that may correspond to the shape and size of the imprint.2,12,13 An early example of molecular imprinting was the work of Dickey (1949).14 The silica gel made in the presence of the dye molecule methyl orange exhibits an enhanced affinity for this dye molecule after its postsynthetic removal. However, a genuine imprint is often not the aim of sol-gel encapsulation because it would bury the guest completely in the matrix. A more useful target is to incorporate the guest molecule in a matrix with open pore space. The benefit of such an approach is to shape side pockets of large pores with the * To whom correspondence should be addressed. Fax: (+) 49 251 8323409 E-mail: [email protected]. † Institut fu¨r Physikalische Chemie. ‡ Organisch-Chemisches Institut. § E-mail: [email protected] (N.M.W.); [email protected] (N.G.); [email protected] (E.-U.W.).

SCHEME 1: Encapsulation of the Sodium Complex and Removal of Organic Components

size and shape of the guest molecules. This allows one to characterize the imprint memory after extraction by the selectivity of readsorption, for example, for chiral substrates.10 Another situation occurs if the loading is high enough to form molecular assemblies of the encapsulated guest species. Then, the balance between the interactions of the loaded molecular species and the surface will dominate the dispersion behavior of the guest in the matrix. Under very high concentrations, the self-assembly of the guest will lead to phase separation by crystallization. In this work, we employ a sodium complex with three methylene-bis-diphenylphosphine oxide ligands, forming an unusual trigonal prismatic sodium complex (Scheme 1).15 This cluster compound adopts a nearly spherical shape with 12 phenyl groups at its external surface. Therefore, this sodium complex is an ideal model case for a soft complex with only aromatic phenyl groups exposed to the surface. The phenyl groups form a hydrophobic shell around the center, and Coulombic interactions between the complex and the silica surface are reduced

10.1021/jp107581g  2010 American Chemical Society Published on Web 12/02/2010

Imprinting With Phenyl Group Interactions by the large size of the cationic cluster. When a hybrid silica gel with phenyltriethoxysilane and tetraethoxysilane is formed, the sodium complex is able to form noncovalent interactions with the phenyl groups of the hybrid matrix and/or with neighboring complexes. In the latter case, complex assemblies may be encapsulated in the sol-gel matrix, forming either larger pores or channel structures. While the phenyl groups of the sodium ligands are relatively electron-poor due to the withdrawing effect of the PdO groups, the phenyl groups of the silica matrix are electron-rich due to the polarity of the Si-C bond. Theoretical calculations on benzene derivatives show that aromatic interactions with additional Coulombic contributions may have interaction energies of up to -20 kJ/mol.16 If all phenyl groups of the sodium complex would interact with matrix phenyl groups, such interaction energies between 12 phenyl pairs could add up to a value as high as -240 kJ/mol, which is almost as large as that for a covalent bond. Hence, van der Waals interactions between the 12 surface phenyl groups and the local environment become dominant with respect to the Coulombic interaction between the shielded sodium cation and surface SiO- groups. In addition, this interaction energy is of the same order, or even larger, than the coordinative binding of the PdO groups to the sodium cation. Hence, the complex can be regarded to be a soft template in terms of its internal stabilization energy compared to the encapsulation energy. The aim of this work is the formation of a rigid silica network around the guest complex, as described in Scheme 1, with a porous structure. It will be shown that the sodium complex is encapsulated in a phenyl-rich environment under the formation of phenyl-phenyl interactions. The structural integrity of the sodium complex is investigated by Raman spectroscopy in combination with band assignments using DFT calculations and solid-state NMR. It will also be shown that this complex is stable under sol-gel reaction conditions at pH ) 4, and it forms micropores in the hybrid gel. The organic fragments and encapsulated ligands of the hybrid gel can be removed by calcination, leaving behind a porous material. Evidence for the interaction between guest and host phenyl groups is provided by thermal analysis, and the porosity after removal of the organic fragments is studied by nitrogen adsorption. Experimental Section Synthesis. The sodium complex [Ph2P(O)-CH2-(O)Ph2]3NaBr · 2H2O · (CH3)2CO was prepared by a modified synthesis.15 While the oxidation and complex formation had been carried out in two steps in the original literature, the parent methylenebis-diphenylphoshine was oxidized here with H2O2 (30%, Aldrich) in the presence of sodium bromide (Aldrich), and the resulting raw product of the sodium complex with the phospine oxide was recrystallized in acetone. In a typical sol-gel encapsulation reaction, the sodium complex, TEOS (ABCR, 97%), PhTES (ABCR, 97%), and deionized water with the molar ratio complex TEOS/PhTES/ H2O ) 1:40:10:200 are stirred for 2-3 min, and a catalytic amount of formic acid (Fischer Scientific, 98%) is added to yield pH ) 1 or 4. The sample notations and synthesis compositions are listed in Table 1. The amount of phenyl groups in complex cations, Phcomplex, and in the matrix, Phmatrix, varies in a ratio close to 1. For the synthesis at pH ) 4, two phases were obtained at the beginning. After 20 h, a single, turbid phase was observed for all samples. The total amount of Si was 0.0225 mol. After stirring for 1-2 days at pH ) 4 or for 7 days at pH ) 1, a solid gel was obtained. Subsequently, the colorless

J. Phys. Chem. C, Vol. 114, No. 51, 2010 22591 TABLE 1: Relative Syntheses Compositions sample

TEOS

PhTES

H 2O

complex

A (pH ) 1) BC(pH ) 1) C0(pH ) 4) DC(pH ) 4) EC(pH ) 4) FC(pH ) 4) GC(pH ) 4)

40 40 40 40 40 40 40

10 10 10 10 10 10 10

200 200 200 200 200 200 200

0 1.0 0 0.64 0.80 1.0 1.6

0

Phcomplex/Phmatrix 1.20 0.77 0.96 1.20 1.80

products were dried in an oven at 323 K. The organic components were removed by calcination in air at 923 K. To this end, the temperature was increased from room temperature to 373 K with a heating rate of 1 K min-1. After 2 h of holding at 373 K, the temperature was increased to 923 K with a heating rate of 2 K min-1. The final temperature of 923 K was held for 6 h. Instrumentation. The N2 sorption isotherms of the silica gels were measured with a Micromeritics ASAP 2010 apparatus at 77 K. The uncalcined samples were evacuated at 393 K and the calcined samples at 523 K. The standard BET method was used to determine the specific surface area. The obtained values are reported in parentheses for when the C value was negative. The t-plot method was applied to measure micropore volumes, external surface areas, and micropore areas, and the BJH method was employed for determination of the mesopore diameter. The Raman spectra were recorded by a LabRAM spectrometer of Horiba Jobin Yvon with the He-Ne laser at 633 nm. The 29Si MAS NMR measurements were recorded at a Bruker CXP 300 FT Spectrometer (B ) 7.04 T). A 90° pulse with a relaxation delay of 60 s was chosen. The rotation frequency was 4 kHz, and the resonance frequency was 59.6 MHz. Tetramethylsilane was used as the external standard. The signals were fitted by the software dmfit. 23Na MAS NMR spectra were recorded at a larmor frequency of 105.9 MHz on a Bruker DSX 400 spectrometer (9.4 T) using a 4 mm MAS probe and a spinning rate of 7.5 kHz. A 1 mol/L solution of NaCl was used as the external reference (0 ppm). A 45° pulse with a relaxation delay time of 1 s was chosen. The 23Na{31P} REDOR experiments were carried out on a Bruker DSX 400 spectrometer (9.4 T) using the standard pulse sequence of Gullion and Schaefer.17 A 4 mm MAS probe was used with a spinning frequency of 12 kHz. The Larmor frequencies were 105.9 MHz for 23Na and 162.0 MHz for 31P. The radio frequency power was 71 kHz for 23 Na and 35 kHz for 31P. A 1 mol/L solution of NaCl or 85% H3PO4 was used as the external reference. Thermogravimetric and differential thermal analyses (TGA and DTA) were performed with a Netzsch STA 409 C thermal analyzer. About 20 mg of sample was heated at a rate of 10 K min-1 between 298 and 1173 K in oxygen flow. The baseline correction was made using Al2O3 as the reference sample. Results and Discussion Two different pH values, pH ) 1 and 4, have been chosen for the synthesis of the hybrid gel in the presence of the sodium complex. Figure 1 shows the Raman spectra of the phosphine oxide ligand, the bulk crystalline sodium complex, and the gels containing the complex synthesized at different pH values. The most dominant band arises from the C-C vibrations of the phenyl groups at 1000 cm-1.18,19 The phenyl groups of the ligand and the hybrid gel cannot be distinguished. Additional vibrations of the phenyl groups are observed at 1030, 1130, 1160, and 1190 cm-1. These bands originate from different modes with pronounced C-H bending vibrations of the phenyl

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Figure 1. Raman spectra of the ligand phosphine oxide, pure sodium complex, and the sodium complex incorporated in silica gels (complex/ Si ) 1:50) synthesized at pH ) 1 and 4.

Figure 2. Raman measurement of the sol-gel process of silica gel FC(pH ) 4); (a) upper phase after 2 h, 20 min; (b) lower phase after 2 h, 30 min; (c) single phase after 20 h and (d) after 48 h; (e) dried gel; * is an instrumental artifact.

groups. Their intensities vary in Figure 1, which we propose to occur due to different intermolecular interactions on the complex surface. The most important band is observed at 1170 cm-1 with low intensity (Figures 1 and 2). DFT calculations20 at the B3LYP/6-31G(d) level for the simplified model system, in which only one ligand is coordinated to the sodium cation, and at the M06/6-31G(d) level for the C3 symmetrical complex have shown that this band can be assigned to a combination of several vibrational modes with a pronounced contribution of the PdO stretching vibration of the phosphine oxide in the sodium complex. This signal is absent in the pure phosphine oxide ligand without interaction with sodium cations. Unfortunately, due to the high CPU demand, we were not able to use basis sets higher than 6-31G(d) for the calculation of the Raman

Wichner et al. frequencies. Because this peak is also present in the gel which has been made at pH ) 4, it follows that the sodium complex is stable upon encapsulation in the gel host. However, this peak is not observed for the silica gel synthesized at pH ) 1. This is due to the decomposition of the complex under this synthesis pH, as will be shown in more detail below. Hydrolysis of the precursors is fast at pH ) 1, as indicated by in situ Raman spectroscopy (not shown), and a homogeneous liquid mixture is formed immediately when all components are combined in the sol-gel reaction. The reaction mixture of the synthesis at pH ) 4 has two liquid phases in the early stages of the sol-gel process. The hydrolysis of the precursors is slow at this pH value. The lower phase is turbid, and the upper phase is clear. The Raman spectra in Figure 2a,b show both phases after a reaction time of about 2 h, 30 min. The upper phase shows a deficiency of phenyl groups which can be clearly seen by the lower intensities of the corresponding Raman vibrations (Figure 2a). The broad, intense band at 655 cm-1 in Figure 2a is assigned to a Si-O vibration of tetraethoxysilane. The lower phase (Figure 2b) contains the phenyl vibrations with higher intensities. Interestingly, the PdO stretching of the sodium complex at 1170 cm-1 is extremely weak or even absent in this Raman spectrum (Figure 2b). For longer sol-gel reaction times, the two liquid phases mix, and a turbid solid-liquid mixture is formed. After 20 h (Figure 2c), ethanol is formed (880 cm-1) as the hydrolysis product of the two precursors, and the intensity of the characteristic ethanol band increases after 48 h (Figure 2d). The mixing of the two immiscible liquid phases at the beginning of the sol-gel reaction seems to be facilitated by the hydrolysis of both precursors. After 20 h (Figure 2c), the characteristic band of the sodium complex is observed again at 1170 cm-1. For comparison, the final dried gel is included in Figure 2e. This band can also be found in the gel after drying at 323 K, and it indicates that the sodium complex is encapsulated at pH ) 4. The structural integrity of the sodium complex is of crucial importance here, and its characterization deserves special attention. Therefore, we will now present conclusive evidence for the prevailing Na coordination environment after the sol-gel process. In 1989, Gullion and Schaefer developed the rotational double resonance (REDOR) NMR technique, which provides information on spatial atomic proximity beyond the first coordination sphere.17 The 23Na{31P} REDOR measurements can prove the proximity of the sodium and the phosphorus. The method is based on the 23Na-31P heteronuclear dipole interaction, which is a function of the internuclear distance and the number of neighbors. For the complex under investigation, 23Na is surrounded by six 31P nuclei with the same Na-P distances. The calculated second moment of the 23Na-31P heteronuclear dipolar interaction of this sodium complex compound, using the Van Vleck equation,21,22 is 3.1 × 106 s-2. The REDOR experiment consists of two individual measurements, first, the detection of the 23Na MAS NMR intensity in a spin echo experiment and, second, the measurement of the intensity of the echo under 31P pulses during the echo period. These 31P pulses reintroduce the heteronuclear 23Na-31P dipole interaction in the MAS evolution of the echo period, leading to a damped echo intensity corresponding to the strength of the dipole interaction. The normalized intensity difference between the two experiments, ∆S/S0, is shown in Figure 3 as a function of echo evolution time for the crystalline sodium complex and the two gels made at different pH values of 1 and 4.

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Figure 3. 23Na{31P} REDOR data of the pure sodium complex and of the sodium complex in silica gels BC(pH ) 1) and FC(pH ) 4); the dashed line is an analytical fit (see text).

The 23Na{31P} REDOR measurement of the pure sodium complex shows a clear REDOR effect, and a second moment of 1.9 × 106 s-2 is obtained by fitting the initial slope of the data curve up to ∆S/S0 ) 0.2.23 This value is smaller than expected from the Van Vleck calculation (see above). The most likely explanation for this deviation is an influence of the 31P chemical shift anisotropy.24 Next, the REDOR curves of the two hybrid gels, synthesized at pH ) 1 and 4, will be shown. Decomposition of the sodium complex will inevitably lead to larger Na-P distances and, consequently, smaller second moments. Therefore, the 23Na{31P} REDOR method is an excellent method to probe the structural integrity of the Na coordination with the phosphine oxide ligands. The REDOR experiment for the gel synthesized at pH ) 1 shows a much smaller slope of the experimental data point compared to that of the crystalline reference (Figure 3). This observation is strong evidence for the decomposition of the sodium complex under these synthesis conditions at pH ) 1, as it was already suggested from Raman spectroscopy. This is different for the silica gel synthesized at pH ) 4. A REDOR effect is observed that compares well with the crystalline sample, at least for small evolution times (Figure 3). The deviation of the data for higher evolution times can easily be explained by higher coordination effects due to long distance interactions or dynamic influences (molecular or spin dynamics) that become effective for higher evolution times. From the similar REDOR data at low evolution times of the crystalline complex and the encapsulated complex, it can be concluded that the sodium complex is stable at pH ) 4, and it is encapsulated intact in the hybrid gel. These conclusions are further corroborated by the 23 Na MAS NMR spectra which are shown in Figure 4. Similar spectra are obtained for the crystalline complex and the encapsulated sample synthesized at pH ) 4 with a chemical shift of δ ) -5.6 ppm, whereas the gel made at pH ) 1 shows a clear change in the 23Na NMR peak position (δ ) 0.46 ppm). Quadrupolar broadening is not observed for these samples, despite the unusual trigonal prismatic sodium coordination. Calculations of quadrupole coupling constants with a simple point charge model25 have shown that this small quadrupole coupling constant for the sodium complex is accidentally due to the specific geometric properties, that is, the ratio of edge lengths of the trigonal prismatic coordination. 31 P NMR data confirm that the complex is stable at pH ) 4 but not at pH ) 1 (see Supporting Information). The pure complex shows one line at 23.9 ppm; the pH ) 4 gel yields a similar signal at 24.5 ppm, but the gel prepared at pH ) 1 shows

Figure 4. 23Na MAS NMR spectra of the pure sodium complex and silica gels synthesized at pH ) 1 and 4.

Figure 5. Thermogravimetric analyses of (a) silica gels A, B, C, and F and (b) the complex.

a different peak at 30.4 ppm. A solution NMR spectrum in methanol surprisingly yields at least three lines at around 28.4 ppm. This observation indicates that the complex ligands are replaced by solvent molecules on the sodium cations, and the coordination of Na+ is suggested to be in dynamic equilibrium with exchanging ligands. Having this in mind, it is even more striking that the sodium environment with three phosphine oxide ligands prevails when it is encapsulated in the amorphous gel at pH ) 4. Thermogravimetric analyses provide information on the encapsulation of the sodium complex. Figure 5 illustrates the mass losses of the pure complex, the pure hybrid gels, and the hybrid gels with an encapsulated sodium complex. Figure 6

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Figure 6. Differential thermogravimetric analyses of silica gels A, B, C, and F and the complex.

Figure 7. Differential thermal analyses of silica gels A, B, C, and F and the complex.

shows the first derivatives of these curves which show additional fine structure information. The corresponding DTA curves are assembled in Figure 7. The bulk complex shows five mass loss steps, which are best resolved in Figure 6. Step I at 485 K (3%) is assigned to the endothermic weight loss step for encapsulated solvent molecules (water and acetone) for the pure complex crystals. All other steps are exothermic and associated with combustion of the organics. Steps II-IV represent the main mass loss between 610 and 742 K of 77% for the main combustion process of the phosphine oxide, followed by a final combustion process (step V, 787 K, 3%). The pure hybrid gels without complex (A, C) mainly have mass losses and thermal events for the combustion of the phenyl groups, in addition to smaller mass losses in a broad low-temperature range (298-473 K) for water. A striking difference between the hybrid gels is a considerable difference in the temperature of the phenyl group combustion process depending on the synthesis pH value. The gel made at pH ) 1 (Figures 5A, 6A, and 7A) shows the maximum of the mass loss event at 893 K, whereas the gel made at pH ) 4 (Figures 5C, 6C, and 7C) shows the maximum of the same combustion already at 836 K. As will be shown below, this difference can be readily interpreted by the difference in the sample porosity, causing different diffusion limitations in the samples; at pH ) 4, the gel shows a higher porosity than the gel made at pH ) 1. Silica gels with the encapsulated complex show a combination of the events of the pure complex and the hybrid gels. However, the temperatures are not the same as the ones of the components. The combustion of the complex in the hybrid gels, at pH ) 1 or 4, takes place at lower temperatures with less resolution (550-660 K; Figure 6, samples BC and FC) than that for steps

Wichner et al.

Figure 8. N2 adsorption of silica gels A and B synthesized at pH ) 1. Sample B before and after calcination at 923 and 1173 K does not show any adsorption.

II-V of the pure complex (Figure 6). Although both samples have the same loading, the smaller peak area for sample BC compared to that for sample FC indicates that the combustion of the complex in sample BC is incomplete, which was also concluded from a gray color after calcination. Sample FC is colorless after calcination, as are A0 and C0. Due to the low sensitivity of the DTA signals of the pure complex (Figure 7), the DTA peaks of the complex combustion are not observed in the encapsulated samples. Interestingly, the combustion temperature of the matrix phenyl groups (>700 K) is also influenced by the encapsulation of the complex. This is most clearly observed for the sample made at pH ) 1 (Figures 5, 6, and 7, samples A0 and BC). The thermal decomposition temperature of phenyl groups for the gel made at pH ) 4 shows a lower dependence on the presence of the complex, although it can also be identified. Figures 6 and 7, samples C0 and FC, show a slightly higher peak temperature for the unloaded gel, C0. These combustion peak dependences are indicative of encapsulation of the complex in the hybrid gels. After combustion of the encapsulated complex, new porosity is generated, as will be shown below, and the oxidation of the matrix phenyl groups as well as the diffusion of the combustion products is kinetically less restricted than that for the unloaded hybrid gels. N2 physisorption experiments of the silica gels before and after removal of the organic components show the strong poregenerating effect of the sodium complex at higher pH values. Figure 8 shows the adsorption isotherm of an uncalcined reference hybrid gel, A0(pH ) 1) without a complex. This blank experiment yields a high microporosity of a PhTES/TEOS hybrid gel. The silica gel with the guest complex, BC (pH ) 1), shows no porosity before and after calcination at 923 and 1173 K. The calcined sample was black, indicating that the combustion of organic components was incomplete. These observations indicate that the sodium complex or, more precisely, its decomposition fragments has a strong influence on the pore generation of the hybrid gel. At this pH value, the guest complex is decomposed, and the fragments are firmly encapsulated in the silica matrix. Diffusion of oxygen and/or calcination products through the dense silica material is severely hindered. The blank sample without the complex remains colorless and shows no porosity after calcination (not shown). The collapse of the sol-gel matrix of hybrid gels made at such acidic pH values is not unusual.26,27 The N2 sorption isotherms of silica gels synthesized at pH ) 4 are shown in Figure 9. The blank gel, C0(pH ) 4), contains micropores and mesopores, the latter detected by the long type

Imprinting With Phenyl Group Interactions

Figure 9. N2 sorption of silica gels C and F synthesized at pH ) 4 before and after calcination.

H3 hysteresis loop. Calcination leads to shrinking, yielding a lower physisorption isotherm, but a substantial porosity is retained in sample C0(pH ) 4, calc). Adding the complex to the gel synthesis yields an almost dense material, FC(pH ) 4). However, in contrast to the lower synthesis pH value, the sample with the complex synthesized at pH ) 4 remains colorless and has increased microporosity after calcination at 923 K. The changes in porosities are not reflected in particle shapes and morphologies (see SEM images for samples C0(pH )4) and FC(pH )4) in the Supporting Information). N2 adsorption probes the porosity at least 1-2 orders of magnitude smaller in size than SEM. The increased porosity after calcination shows that the complex has generated stable micropores, which are characterized by a type I isotherm in Figure 9 for sample FC(pH ) 4, calc). The small hysteresis at high relative pressures indicates very large mesopores with a much smaller number density than that for the blank sample. These adsorption data are strong evidence for complex encapsulation in the sol-gel matrix. It is remarkable that all four isotherms in Figure 9 show a mixture of H3 and H4 hystereses. Type H3 loops are usually produced by aggregates of platy particles or adsorbents containing slitshaped pores.28 Hysteresis loops of Type H4 are observed for slit-shaped pores, but in this case, the pore sizes are mainly in the micropore range. The pore-generating effect of phenyl groups in hybrid gels with the same sol-gel reaction conditions is currently under investigation. It can already be concluded from these ongoing systematic studies that slit pores are quite common for hybrid systems with phenyl groups. Obviously, such aromatic functionalities generate layer-type micro/mesopores in disordered, amorphous sol-gel materials. The influence of the complex concentration at pH ) 4 on pore generation is shown in Figure 10. When more sodium complex is added to the sol-gel process, mesoporosity is reduced, and the microporous character of the materials is more pronounced. This is observed by the variation of hystereses in Figure 10, which shows mixed type I/type IV isotherms. The gel, GC(pH ) 4, calc), with the highest loading shows a nearly perfect type I isotherm for a microporous system. The isotherms for the lower loadings in Figure 9 show additional type IV behavior with H3/H4 hystereses for slit-shaped pores. The micropore volumes of these calcined silica gels are listed in Table 2. The complex-free silica gel, C0(pH ) 4, calc), has

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Figure 10. N2 adsorption of silica gels C, D, E, F, and G after calcination at 923 K synthesized at pH ) 4 with different amounts of complex.

TABLE 2: BET, t-Plot, and BJH Analyses of Calcined Silica Gels

sample, complex/ siloxane derivatives

BET/(m2/g)

t-plot: micropore volume/(cm3/g); external surface area/ (m2/g); micropore area (m2/g)

GC(pH)4,calc) FC(pH )4,calc) EC(pH )4,calc) DC(pH)4,calc) C0(pH )4,calc)

(110.6 ( 1.5) (144.9 ( 1.6) 165.34 ( 0.64 201.75 ( 0.30 294.2 ( 2.0

0.04; 24.0; (86.6) 0.05; 39.1; (105.7) 0.01; 134.39; 30.65 0.003; 189.08; 12.67 0.06; 165.5; 128.7

BJHdes pore diameter/nm 4.1 3.9 8-9 8.5 3.5

a micropore volume of 0.06 cm3/g. With a small amount of complex, DC(pH ) 4, calc), a reduced micropore volume of 0.003 cm3/g is obtained. Upon further increasing of the complex concentration, the micropore volume increases for the samples EC(pH ) 4, calc) and FC(pH ) 4, calc). The mesopore diameters obtained by the BJH method are larger for the samples made in the presence of the complex than those for the complex-free silica gel (Table 2). Conclusions The encapsulation of the complex cation, {Na[Ph2P(O)CH2-P(O)Ph2]3}+, in a hybrid gel with phenyl-functionalized surface is possible at pH ) 4, but the complex is not stable at pH ) 1. Micropores are generated upon calcination of the complex ligands at 923 K for the samples made at pH ) 4. For the calcined silica gels synthesized with low complex concentration (up to the ratio of complex to siloxane derivatives of 1 to 60), mesopores are formed with a higher pore volume than that for the calcined pure silica gel that has been made in the absence of the sodium complex. The further addition of the complex mainly reduces the mesopore volume, and micropores are formed after calcination. The nature of the soft template complex cation, {Na[Ph2P(O)-CH2-P(O)Ph2]3}+, differs from the smaller molecular structure-directing agents which are usually employed for zeolite syntheses. Here, the entire surface of the cluster is decorated with hydrophobic phenyl groups, and the positive charge is buried deep inside of the cluster, with no significant polarity. The complex cation prefers to be dissolved in the

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solvent with less polarity, containing the precursor with phenyl groups. Structure-directing agents for zeolite synthesis have to show a minimum of polar behavior.29 On the other hand, the complex is also different from surfactant chemistry, which is used to form mesoporous materials based on amphiphilic behavior. Mesopores are suppressed in the presence of the complex during sol-gel synthesis. The experimental data show that the formation of micropores is supported by the presence of the complex. Detailed analyses of the thermoanalytical data reveal that an interaction takes place between the phenyl groups on the surface of the sodium complex template and the hybrid gel host. Careful analysis of the complex stability was necessary. 23Na solid-state MAS NMR, 23Na{31P} rotational echo double resonance, and Raman spectroscopy in tandem with band assignments by DFT calculations provide a conclusive picture of the stability properties of the sodium complex. Acknowledgment. We thank Dr. Klaus Bergander for his help with a solution 31P NMR spectrum and the Deutsche Forschungsgemeinschaft for funding of the International Research Training Group, IRTG 1444. A helpful comment by one referee to provide 31P solution NMR is gratefully acknowledged. Supporting Information Available: Selected SEM images, P NMR data, and quantum chemical calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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