J. Phys. Chem. B 1997, 101, 7087-7094
7087
EPR Studies of the Formation Mechanism of the Mesoporous Materials MCM-41 and MCM-50 Jingyan Zhang, Zeev Luz, and Daniella Goldfarb* Department of Chemical Physics, Weizmann Institute of Science, RehoVot 76100, Israel ReceiVed: March 17, 1997; In Final Form: June 12, 1997X
The formation mechanism of the hexagonal, MCM-41, and the lamellar, MCM-50, mesoporous materials, prepared at room temperature with the surfactant cetyltrimethylammonium chloride (CTAC) and tetraethylorthosilicon (TEOS), was studied by in situ EPR spectroscopy using the spin probe 4-(N,N-dimethyl-Nhexadecyl)ammonium-2,2,6,6-tetramethylpiperidinyloxy iodide (CAT16). This probe has a structure similar to that of the surfactant molecules with the nitroxyl radical situated at the head group. Accordingly, it probes the interface between the organic and inorganic phases during the formation of M41S materials. The EPR spectrum of CAT16 in the reaction gel, prior to the addition of TEOS, consists of a superposition of two subspectra due to spin probe molecules in micelles and in the aqueous phase, respectively. For a gel composition which forms MCM-41, the addition of TEOS leads to a gradual transformation of the micellar subspectrum into a characteristic rigid limit spectrum. This observation provides direct evidence that micellar structures present in the initial reaction mixture serve as precursors for the final mesoporous product. The temporal evolution of the spectrum is characteristic of an isotropic system undergoing a gradual increase in the microviscosity. The isotropic nature of the spectrum is a consequence of the specific geometry of the CAT16 head group and its motion in the interface region. Comparison of the temporal evolution of the EPR spectrum with that of the X-ray diffraction pattern indicates that the hexagonal long-range order is formed already 5-8 min after mixing the reagents, whereas the formation of the inorganic phase, which is apparently responsible for the slowdown of the spin probe motion, is considerably slower (>1.5 h). The latter process begins only after a critical amount of TEOS is added to the mixture. These results are consistent with a mechanism whereby the addition of TEOS initially forms clusters of rodlike micelles coated with silicate anions, followed by the condensation of the silicate anions at the interface to yield the final product. By monitoring the peak height of the central EPR line, phenomenological kinetic profiles of the reaction were obtained. These curves were quite different for MCM-41 and MCM-50 and they provide qualitative information regarding the sequence of transformations which occur during the reaction. Specifically, these curves show that while no intermediate phases occur during the formation of MCM-41, several phase transformations take place when MCM-50 is formed and the reaction is significantly slower.
Introduction The M41S family of mesoporous materials have been attracting worldwide interest since their first synthesis in 19921-4 due to their potential as catalysts and adsorbents.5 These applications are a consequence of their unique structural properties, which exhibit a regular array of uniform pore openings. The new synthetic approach that led to the formation of the M41S materials opened up new research directions that resulted in a series of other novel mesoporous materials.6-10 Zeolites, which are microporous materials, are commonly synthesized using individual molecules as templating agents. These are usually small amines or salts of tertiary amines, CH3(CH2)nN+(CH3)3, with short alkyl chains (n e 6).11 In contrast, in the synthesis of the M41S materials, aggregates of surfactant molecules play the role of the templating agents and determine the phase structure and the pore sizes of the product.2 The most commonly used surfactants are long alkyl chain ammonium salts, (9 e n e 18),1,6 although other types of surfactants have been used as well.12-15 The detailed formation mechanism of the M41S materials is still unclear. Initially, the strong resemblance of the final structure to the liquid crystalline phases observed in solutions of the neat surfactant led to the formulation of a liquid crystal X
Abstract published in AdVance ACS Abstracts, August 1, 1997.
S1089-5647(97)00962-0 CCC: $14.00
templating mechanism where the silicate polymerizes at the surface of the aggregates in a preformed liquid crystalline phase.16 This, however, could not explain the fact that the synthesis of the mesostructure is usually carried out under conditions where the silicates alone do not condense (pH of 12-14) and the concentration of the surfactant, 0.5-5%, is below that leading to the formation of liquid crystalline phases.17,18 Therefore, an alternative mechanism was suggested whereby the interaction between the silicate and the surfactant serves as a driving force for the generation of the structures which subsequently determines the nature of the final mesoporous materials. Three phenomena, essential for the formation of the surfactant-silicate mesostructure, were identified by Monnier et al:19 (i) multidentate binding of silicate oligomers, (ii) preferred polymerization of silicates at the surfactant-silicate interface, and (iii) charge density matching across the interface. The realization of these phenomena led to the synthesis of mesoporous materials with a variety of compositions using different types of surfactant molecules.6 Two other suggested mechanisms for the formation of MCM-41 include the “puckering layer model” by Steel et al.20 and the “silicate rod assembly model” by Chen et al.21 According to the first model,20 the silicate species in the aqueous solution initially form a layered structure, and further ordering results in puckering of the silicate layers and to the formation of the hexagonal channels which constitute the MCM-41 material. According to the model of © 1997 American Chemical Society
7088 J. Phys. Chem. B, Vol. 101, No. 36, 1997 Chen et al.,21 randomly disordered rodlike micelles interact with silicate species to yield two or three monolayers of silica encapsulating the external surfaces of the micelles. Subsequently, these composite species spontaneously assemble to produce the hexagonal MCM-41 structure. Several studies were carried out in order to determine the intermediate structures generated during the synthesis of the M41S materials. Nitrogen-14 NMR measurements were performed on reaction gels which produced, after heating at 90 °C, MCM-41 and MCM-50. The spectra of both gels were similar and corresponded to a partially averaged 14N quadrupole tensor. It was ascribed to an hexagonal mesophase which was assumed to be a precursor for both the hexagonal and lamellar mesostructures.20 Evidence for the liquid crystalline nature of the silicate-surfactant mixture was obtained also by in situ 2H NMR.22 Averaged powder patterns with different splittings were obtained for gels with Si/surfactant ratios of 1.4/1 and 2.8/1 and were assigned to hexagonal and lamellar mesophases, respectively. Heating the lamellar mesophase induced a transition to the hexagonal mesophase. The hexagonal MCM-41 phase was then obtained after prolonged heating at 70 °C. In this study the Si source was an aqueous silicate solution which is not very reactive so that the self-assembly process could be separated from the much slower polymerization process. Real time in situ FT-IR measurements, monitoring the high-temperature formation of MCM-41, were recently reported.23 In these experiments the IR spectrum was measured as a function of temperature and it was found that the head group ordering increased upon heating. Cryotransmission electron microscopy was also used to study the intermediates found in the solution during the synthesis.24 This study showed that the primary building units are clusters of elongate micelles which appear already 3 min after mixing the gel components.24 Although in general these results and others25 support the model of Monnier et al.,19 certain details, related for example to the growth mechanism and the transformation between phases, remain unclear. Hexagonal, lamellar, and cubic silica mesostructures can also be obtained with nonionic surfactants.13 In these cases the required surfactant concentration is high and a liquid crystalline phase exists prior to the addition of the silica source. No mesostructures could be obtained at low surfactant concentrations where the solutions consists of micellar aggregates. Consequently, it was concluded that a liquid crystalline phase directs the formation of the mesoporous material and it was suggested that at high surfactant concentrations charge density matching and multidentate binding of charges at the interface are not essential for the process. Considering the overall results obtained so far, it is not clear whether one universal mechanism applies for the formation of all the mesostructural materials or whether under different conditions different mechanisms are in effect. That is, under certain conditions the structure directing agents can be composite inorganic ions-surfactant micelles or a composite liquid crystalline phase whereas under other conditions a preformed liquid crystalline phase is required for the synthesis. In this work we investigate the formation mechanism of MCM-41 and MCM-50 by monitoring the microviscosity of the surfactant using EPR of spin probes. While X-ray diffraction provide information regarding the setting-in of long-range ordering of the mesoporous lattice, the EPR spectrum of the spin probe is most sensitive to mobility of the organic phase.26 Changes occurring in the environment of the surfactant molecules are manifested in the spectrum of the spin probe and thus provide an insight into the formation mechanism. The high sensitivity of the EPR method allows the addition of very small
Zhang et al. amounts of spin probes without perturbing the system. Moreover, measurements can be performed in situ, thus providing direct information on the propagation of the reaction. Besides demonstrating the feasibility of the method we show that the formation of the mesoporous material occurs in two stages: a relatively rapid formation of a long-range lattice order followed by a much slower process during which the mesoporous structure is consolidated. Moreover, we show that a minimum Si/surfactant ratio is required to initiate the silica polymerization at the interface of the micelles. Experimental Section Synthesis. The reagents used for the synthesis were tetraethylorthosilicon (TEOS, Aldrich 98%), cetyltrimethylammonium chloride (CTAC, 25 wt % aqueous solution, Aldrich), cetyltrimethylammonium bromide (CTAB, Aldrich), and NaOH (2N solution). The spin probe 4-(N,N-dimethyl-N-hexadecyl)ammonium-2,2,6,6-tetramethyl piperidinyloxy iodide (CAT16) was purchased from Molecular Probe Europe BV Co. All compounds were used without further purification. The synthesis procedures for MCM-41 and MCM-50 were similar to those described in the literature,1,27 except for the addition of the spin probe. A typical MCM-41 synthesis was as follows: To a 10 mL aqueous solution containing 0.72 mg (0.55 mmol) of CTAC, 2.7 µmol spin probe was added and the mixture was stirred for several minutes until all the spin probe dissolved. Then, 1.32 mL of NaOH (2N) (2.5 mmol) were added, the mixture was stirred for an additional 10 min, followed by the addition of 1.02 mL (4.6 mmol) of TEOS. The resulting mixture was left under stirring for 48 h at room temperature. The solid precipitate was recovered by filtration, washed with distilled water, and dried in air at ambient temperature. The molar compositions of the synthesis gels of MCM-41 and MCM50 (given in parentheses) were TEOS:0.12 CTAC:0.52 (1.23) NaOH:114.87 H2O:1.12 × 10-3 spin probe. Three methods were tested for the incorporation of the spin probe: (i) addition during the synthesis (see above); (ii) adsorption onto the final product; in this method, as-synthesized MCM-41 was stirred for 10 h in a chloroform solution of the spin probe followed by filtration; and (iii) impregnation, which is similar to the adsorption method, except that the solvent was removed by evaporation under reduced pressure. The EPR spectra exhibited by the products of the various methods were quite similar. We chose the first method because it does not require the addition of other solvents, the spin probe is well dispersed, and it allows in situ studies. In situ EPR experiments were carried out as follows: The reaction mixture without TEOS was first prepared and then TEOS was added with vigorous stirring for about 2 min. Part of the mixture was then quickly transferred into an EPR flat cell which was kept in the measuring cavity until the end of the measurement. The solid which formed during the experiment remained within the active part of the cavity throughout the measuring period. The time-dependent XRD measurements were carried out on a series of samples prepared by dividing the starting mixture into a number of flasks and the solid formed in each of the flasks was filtered and washed at different times after the initiation of the reaction (addition of TEOS). Sample Characterization. Powder X-ray diffraction patterns of the final products were prepared as thin layers on glass slides and measured on a Rigaku D/Max-B diffractometer, using Cu KR radiation. The samples obtained after different reaction times were measured in capillary on a small-angle X-ray (SAX) diffractometer, equipped with a Franks mirror and one-
Formation of Mesoporous Materials MCM-41 and MCM-50
J. Phys. Chem. B, Vol. 101, No. 36, 1997 7089
Figure 1. Molecular structure of CAT16 and 16DSA.
Figure 3. Room temperature EPR spectra of CAT16 in the different components of the reaction gel: (a) neat H2O, (b) aqueous NaOH (0.25 M), (c) same as (b) plus 0.057 M CTAC, (d) same as (c) plus 0.48 M TEOS, recorded 15 min after mixing, (e) same as (d) but recorded 19 h later. The dashed trace corresponds to the spectrum of the final MCM41 product after filtration.
Figure 2. X-band EPR spectra of as-synthesized MCM-41(CAT16) (A) and MCM-41(16DSA) (B) as a function of temperature.
dimensional position sensitive detector (homemade), using Cu KR radiation with a Ni filter.28 The EPR spectra were recorded at X-band (∼9 GHz) using either a Varian E-12 or a Bruker ER200D-SRC spectrometer. Temperature-dependent measurements were done using a Bruker variable-temperature assembly. Liquid mixtures were measured in flat cells and solid samples in 3 mm o.d. quartz tubes. Results Selection of a Proper Spin Probe. The success of the EPR method depends primarily on the proper choice of the spin probe. Our prime interest in this study is the interface of the organic and inorganic phases and therefore the nitroxyl radical should be located at this region. Among the many spin probes which we have tried,29 CAT16 was found most suitable due to its structural similarity with the surfactant used (see Figure 1) and the nitroxyl group is located near the polar head where the silicate anions bind. Figure 2A shows the temperature dependence of the EPR spectra of as-synthesized MCM-41 containing CAT16, henceforth referred to as MCM-41(CAT16). The spectra show that over the temperature range of 273-373 K a gradual transformation from a rigid limit spectrum to that of a low-viscosity isotropic liquidlike spectrum takes place. These results show that CAT16 is a suitable probe for monitoring the dynamic properties of the organic/inorganic interface. For comparison, when 16-doxylstearic acid (16DSA), where the nitroxyl group is close to the hydrophobic end of the alkyl chain (see Figure 1), is used as a spin probe, the spectrum exhibits solidlike features only well below room temperature and a liquidlike spectrum is observed already at 343 K (see Figure 2B). As expected, the higher mobility of the spin label region at room temperature makes this probe less sensitive to changes in the microviscosity at the interface.
We also compared the motional characteristic of CAT16 in as-synthesized MCM-41 (CAT16) with that of CAT16 which was adsorbed on calcined MCM-41 by monitoring the temperature dependence of the overall hyperfine splitting (2Amax, see inset in Figure 2). This parameter behaved significantly different in the two samples. First, its rigid limit value, which corresponds to 2A| of the spin probe26 is larger (by 2.5 G) in the calcined sample compared to MCM-41(CAT16) and, second, upon heating, 2Amax decreased more steeply in the latter. This is consistent with the CAT16 residing within the organic layer in MCM-41(CAT16) where it is subjected to somewhat more motional freedom that is responsible for the decrease in 2Amax. Formation Mechanism of MCM-41. Prior to the in situ measurements, the EPR spectrum of CAT16 in the various components of the reaction mixture was examined. The spectrum in water consists of a relatively sharp hyperfine triplet with a splitting of 16.26 G (Figure 3a). The addition of NaOH (Figure 3b) results in a slight increase in the splitting to 16.55 G, attributed to a small change in the solution polarity. The appearance of a sharp triplet indicates that the spin probe is homogeneously distributed in the solution. The addition of CTAC causes a significant broadening of all three peaks, and the high-field component split into two (Figure 3c). The CTAC concentration in this solution was 4.24 × 10-2 M, which is above the critical micellar concentration.31 Accordingly, the splitting is interpreted in terms of a superposition of two subspectra due to spin probe molecules in the micelles and in the aqueous environments, respectively.32 The fact that two subspectra are observed indicates that the exchange of CAT16 between the two environments is slow on the EPR time scale. The spectrum observed did not vary in time, indicating that the spin probe enters the micellar arrangement rapidly. This is expected due to the similar structure of CAT16 and the CTAC molecules. An increased resolution of the two spectral components was observed immediately after the addition of TEOS (Figure 3d), and after 19 h, during which the formation of MCM-41 took place, the spectrum shown in Figure 3e was obtained. It corresponds to a superposition of a motionally averaged spectrum due to CAT16 in the aqueous solution, and a solidlike powder spectrum attributed to MCM-41(CAT16). A more detailed description of the line-shape evolution during the formation of MCM-41(CAT16) is shown in the left, column
7090 J. Phys. Chem. B, Vol. 101, No. 36, 1997
Zhang et al.
Figure 4. (A) EPR spectra of the reaction mixture of MCM-41(CAT16) as a function of time. (B) Same spectra as in (A) after subtraction of the aqueous component.
A, of Figure 4, where the top spectrum was taken 3 min after the addition of TEOS. The temporal evolution of the spectrum shows that while the part corresponding to the aqueous solution remains practically invariant throughout the reaction, that of the micellar component gradually broadens until it converts completely to the rigid limit line shape, as shown in Figure 3e. The conversion process lasts about 2.5 h after which the spectrum remained practically invariant. The XRD pattern of the final product confirmed the formation of MCM-41. In order to obtain a more detailed picture of the evolution of the EPR line shape of CAT16 in the micellar component, the subspectrum of the aqueous phase was subtracted from each of the experimental spectra. The different spectra ascribed to the micellar component are shown in the right-hand side, column B, of the figure. The spectrum of the aqueous phase, which is somewhat different from that of CAT16 in neat water, was determined by recording the EPR spectrum of a filtrate obtained by filtering the final product. To discuss the character of the temporal evolution of the micellar component we compare these spectra with those of CAT16 in a glycerol solution, recorded over a wide temperature range (223-353 K) as shown in Figure 5. The line-shape evolution in both figures is quite similar, thus providing a scale for the increase in the local viscosity of the spin probe during the formation of MCM-41(CAT16). In parallel with the EPR experiments small-angle X-ray (SAX) diffraction measurements were carried out. These, however, were not in situ measurements. A reaction mixture, having the same composition as that used for the EPR experiments, was divided into 10 parts, each of which was filtered and dried at different times after the addition of the TEOS. The SAX patterns so obtained are summarized in Figure 6. Within the first few minutes, a diffraction pattern, typical of an hexagonal structure is observed, with the d100 spacings changing from 45.7 Å at 3 min to 41.8 Å at 5 min. During the next 4 min it decreased further to 40.62 Å and then it remained constant until the end of the process. These solid samples were also measured by EPR. The solids filtered after 3, 10, and 20 min of reaction exhibited essentially identical spectra of a rigid limit powder. This shows that the solid formed during the initial stages of the reaction is different from the one obtained after filtering and drying. A phenomenological kinetic profile for the formation of MCM-41(CAT16) was obtained by monitoring the intensity of
Figure 5. EPR spectra of a solution of CAT16 (5 × 10-4 M) in glycerol recorded at various temperatures.
the central line of the CAT16 EPR spectrum as a function of the reaction time. The results, presented in Figure 7, show an initial fast decrease in the peak intensity, followed by a much slower decay. This, however, is not simply related to the kinetics of the MCM-41 formation. First, the signal intensity corresponds to a weighted sum from the micellar and the aqueous environments, although the latter changes very little during the reaction. Second and more important is the dependence of the peak height on the line broadening of the signal due to the spin probe in the micellar environment. This broadening reflects the increase in the microviscosity of the organic phase, but the relation between two is not simple. In general the spectrum broadens as the correlation time for the motion of the spin probe, τc, increases.26 The extent of broadening, however, is dependent on the motional regime. As τc of the spin probe in the micelles becomes longer, the amplitude of the triplet first deceases monotonically until it becomes of the order of the inverse of the hyperfine interaction (A|-1, in frequency units). Thereafter, the effect of slowing
Formation of Mesoporous Materials MCM-41 and MCM-50
Figure 6. Powder X-ray diffraction patterns of the solid obtained during the formation of MCM-41 at room temperature at various times after the addition of TEOS.
Figure 7. The change of the intensity of the central EPR line (indicated by the arrow in the inset) during the course of the formation of MCM41(CAT16) and MCM-50(CAT16) at room temperature.
down of the motion is manifested in more subtle changes of the spectrum line shape with a much weaker effect on the line intensity. Therefore, the “break” observed in Figure 7 reflects, at least partially, the transition from a liquidlike to a solidlike spectrum, rather than a clear kinetic step in the formation of MCM-41. A quantitative analysis of the curve to extract the time dependence of the correlation time, which can provide direct kinetic information, would require a comprehensive analysis of the dynamics line shape of the spin probe in the micellar environment. We have not carried out such an analysis. For the sake of comparison with MCM-50(CAT16) we refer to the intensity curve for MCM-41(CAT16) as the “standard profile”. Attempts to slow down the reaction by lowering the temperature to 0 °C were not successful and resulted in an amorphous solid rather than in MCM-41. We found, however, that at room temperature a critical ratio of TEOS to surfactant is required to initiate a broadening in the spectrum of the micellar component. A reaction mixture (5 mL) containing all components, except for TEOS, was prepared and TEOS was added in steps of 0.02 mL. After each step the mixture was allowed to react for 15 min and then a small amount was transferred to the EPR flat cell for measurements. After the measurement the amount taken was returned to the reaction mixture. The results, presented in Figure 8, show that up to the addition of 0.12 mL of TEOS
J. Phys. Chem. B, Vol. 101, No. 36, 1997 7091
Figure 8. EPR spectra of the reaction mixture of MCM-41 after the addition of different amounts of TEOS as indicated in the figure. (a) The reaction solution without TEOS. The asterisk indicates the mixture at which solid was first observed.
(Si/CTAC ) 5.4 mole ratio) there were practically no changes in the EPR spectrum. In fact, in samples containing less than 0.14 mL of TEOS no change in the EPR spectrum were observed even 3 h after the preparation of the samples. Significant changes in the spectrum appeared only when 0.14 mL of TEOS (Si/CTAC ) 6.3 mole ratio) were added to the reaction mixture. At this TEOS content turbidity in the reaction mixture was also observed. The intensity of the signal decreased abruptly after the addition of 0.16 mL of TEOS (Si/CTAC ) 7.168 mole ratio). This experiment indicates that changes in the TEOS content up to Si/CTAC ) 6.3 does not alter significantly the motional characteristic of the surfactant within the micelles. Formation of MCM-50(CAT16). Similar experiments were performed on reaction gels leading to the formation of the lamellar material MCM-50. The motional characteristics of CAT16 in as-synthesized MCM-50(CAT16) correspond to a much higher mobility as compared with MCM-41(CAT16). The fast motion spectra in MCM-50(CAT16) are observed already at 333 K, whereas in MCM-41(CAT16) they appear at 373 K, and the overall width of the spectrum at room temperature (2Amax) is considerably smaller than in the rigid limit spectrum of the spin probe. Also, 2Amax in MCM-50(CAT16) decreases faster with increasing temperature. In situ experiments showed that at room temperature the formation of the lamellar phase is significantly slower than that of the hexagonal phase. Figure 9 presents the evolution of the EPR spectrum during the course of the reaction. It may be seen that a significant broadening of micellar component of the spectrum starts only 1.5-2 h after the addition of TEOS, whereas turbidity in these samples showed up only 30 min after the addition of TEOS. The evolution of the XRD patterns for the MCM-50 reaction mixture over a period of 24 h is shown in Figure 10. It is considerably more complex than that for MCM-41 (Figure 6). Thirty-five minutes after the reaction is initiated, weak peaks due to an hexagonal (d ) 42.3 Å) and a lamellar (L1, d ) 31.7 Å) phases may be detected. With time, the intensity of the latter increases, whereas the peak intensity due to the hexagonal phase passes through a maximum at ∼75 min, followed by a gradual decay (after 4 h). Concomitantly, a new set of reflections attributed to a different lamellar phase, L2 (d ) 28.3 Å), show
7092 J. Phys. Chem. B, Vol. 101, No. 36, 1997
Figure 9. EPR spectra of the reaction mixture of MCM-50(CAT16) as function of time. Spectrum a corresponds to the reaction mixture without TEOS.
Figure 10. Powder X-ray diffraction patterns of the solid obtained during the room temperature formation of MCM-50 at various times after the addition of TEOS. * and O indicate the lamellar phases; 3 corresponds to the hexagonal phase.
up. After 24 h the product consists primarily of a mixture of L1 and L2 with a small amount of a third, unidentified phase. The same XRD pattern was obtained for a final MCM-50(CAT16) product. The complexity of the formation of MCM-50 is also manifested in the EPR reaction profile, shown in Figure 7, which was recorded as explained for MCM-41(CAT16). It can roughly be described as a sequence of two standard profiles with the first and the second associated with the appearance of the hexagonal and L1 phases and the L2 phase, respectively (cf. Figure 10). Discussion Application of the EPR spin probe methodology allows the in situ investigation of the room temperature formation of MCM-
Zhang et al.
Figure 11. Room temperature EPR spectra of CAT16 in aqueous solutions of CTAB as function of the concentration (weight percent) of the latter.
41 and MCM-50. The spin probe, CAT16, has a structure similar to that of CTAC and is incorporated into the micellar structure just as the surfactant molecules are. Therefore, the continuous changes in the environment of the surfactant molecules during the course of the reaction are well reflected in the EPR line shape of the spin probe. Prior to the addition of the silica source, the spin probe is distributed between two different environments, the aqueous phase and the micelles. The reaction is initiated by the addition of TEOS, and the process is monitored through the temporal evolution of the EPR signal of the probe molecules in the micellar component. The spectral changes involve a continuous transformation from a relatively sharp triplet, characteristic of fast motion, into a rigid limit spectrum. This provides direct experimental evidence that micelles are indeed the precursors of the M41S final products. The EPR line shapes observed during the synthesis of MCM41 correspond to a motional slowdown similar to that observed in an isotropic environment. The reorientational motion of the micelles is too slow to average out the EPR anisotropic parameters (i.e., the g and 14N hyperfine anisotropies) and only the local motion of the spin probe need to be considered.33 At a first glance it is, therefore, somewhat surprising that the motion leads to spectra characteristic of an isotropic motion, since the local environment of the spin probe in rodlike micelles or in a liquid crystalline phase, presumed to appear in the course of the synthesis,1 is clearly not isotropic. Most likely, the geometry of the head group of the CAT16 spin probe is such that rapid reorientation about its long axis leads to a more or less complete averaging of the anisotropic magnetic interactions and hence to an isotropic spectrum. This explanation is supported by several other EPR measurements of the CAT16 spin probe in ordered environment. For example, isotropic like spectra are observed in MCM-41(CAT16) at high temperatures and in aqueous solutions with 1-37.5% CTAB. While at low CTAB concentration micelles are present, at 37.5% the system exhibits a hexagonal phase.17 These examples indicate that even in wellordered environments when the reorientation of CAT16 about its long axis is fast enough, there is essentially a complete averaging of the anisotropic g and hyperfine interactions. This is somewhat unfortunate because it renders CAT16 insensitive to the local ordering under fast reorientation conditions.
Formation of Mesoporous Materials MCM-41 and MCM-50 Nevertheless, it is still sensitive to the local microviscosity in the range where the rate of the reorientation is of the order of the anisotropic magnetic interactions. The rigid limit EPR spectrum exhibited by MCM-41(CAT16) at room temperature indicates that the reorientation of the surfactant molecules about their long axes is strongly hindered by the silica wall, suggesting a strong interaction between the two phases. The slowdown of the spin probe mobility during the reaction is in agreement with recent in situ IR measurements23 which showed that as the reaction temperature increases the head group ordering increased as well. While the EPR spectrum of CAT16 reaches its final line shape after ∼90 min, the X-ray diffraction pattern exhibits its asymptotic shape already after ∼8 min (see Figure 6). This indicates that the long-range ordering is achieved at rather early stages of the reaction, whereas other processes, essential for the formation of the final mesostructures, are significantly slower. These slow processes most likely involve the polymerization of the silicate ions at the interface and which affects the mobility of the spin probe. Silicon-29 NMR spectra, used to determine the degree of the polymerization and the concentration of silanol groups through the Q3/Q4 ratio, showed that after the long-range MCM-41 order appeared, condensation of silanol groups was still in effect.2 The EPR results also indicate that the solid which appeared 3 min after initiation of the reaction is “soft” and allows fast motion of the spin probe. As the polymerization proceeds, the motion of the spin probe slows down, leading to a solidlike rigid limit spectrum. The comparison of the X-ray results on filtered solids with those of the in situ EPR measurements is not straightforward though. The X-ray data were collected on dry samples while the in situ EPR measurements were made on the original aqueous mixtures. The EPR spectra of dry samples prepared as for the XRD measurements were quite different and exhibited rigid limit patterns even after short reaction periods. This shows that the solid observed during the initial stages of the reaction is different from that obtained after drying. In the in situ experiments the local motion is significantly less restricted. The absence of any change in the EPR spectrum up to a Si/ CTAC ) 6.3 suggests that the binding of the silicate ions to the micelles does not induce any significant changes in the mobility of the surfactant molecules at the interface region. Once the silicate concentration at the interface reaches a critical value, polymerization begins and the mobility at the interface is significantly reduced. Unfortunately, due to the isotropic character of the CAT16 spectrum in ordered systems under fast motion conditions, it is not sensitive to the shape of the micelles. In particular it does not provide information on whether rodlike micelles are induced by the addition of TEOS to the surfactant solution before polymerization sets in. To detect such details a spin probe with a different geometry will have to be used. Two pathways for the formation of the MCM-41 phase can be envisioned. In the first, the so called “silicate rod assembly model”,21 the addition of silicate generates rodlike micelles with silicate ions at the interface. These ions polymerize to give a wall of a few layers of silica, followed by condensation into the final product. The effect of silicate ions on the structure of quaternary ammonium surfactant aggregates was recently studied by dynamic light scattering and rheological techniques.34 The silicate ions were found to promote the formation of flexible wormlike micelles at dilute concentrations of CTAC through their binding at the micellar surface. It was suggested that this process is followed by intramicellar silicate polymerization and then by a gradual arrangement of the encapsulated wormlike micelles into an hexagonal array. This mechanism is, however,
J. Phys. Chem. B, Vol. 101, No. 36, 1997 7093 inconsistent with the XRD data24 and the present EPR results, which showed that the polymerizations at the interface is the slow process while the long-range order is acquired almost immediately after the initiation of the reaction. The EPR/XRD results are more consistent with the second pathway in which the addition of silicate generates domains with long-range hexagonal order. Due to the low CTAC concentration (1.6 wt %), this mixture is not a uniform lyotropic liquid crystalline phase as obtained at high CTAC concentrations. Rather, the system can be viewed as a dispersion of ordered domains in an aqueous medium, as manifested by the persistence of an aqueous CAT16 EPR signal throughout the reaction. Only after the hexagonal domain structure is formed, or simultaneously with its formation, do the silicate ions start to polymerize. This is in agreement with earlier cryogenic TEM measurements which showed that elongated micelles, arranged in small, rather ordered clusters, are present already after 3 min.24 In this scheme the polymerization of the silica is the rate-determining step. It has been suggested that the precursor to the hexagonal is a lamellar phase.6,19,22 Our EPR and XRD results for MCM-41(CAT16) and MCM-50(CAT16) do not support this suggestion. Conclusions We have demonstrated that suitable spin probes can be used to monitor the in situ formation of M41S materials by EPR. Specifically, we performed such measurements on reaction mixtures leading to the formation of the hexagonal, MCM-41, and the lamellar, MCM-50, materials at room temperatures. These in situ measurements provide direct experimental evidence that micelles serve as precursors for the mesoporous materials. The MCM-41 appears to form in two stages. The first stage, which starts immediately after the mixing of the silica source with the surfactant solution, involves the formation of domains with hexagonal order. These domains consist of micellar rods encapsulated with silicate ions oligomers. At room temperature this stage lasts 3-5 min. The second, much slower stage (11.5 h), involves polymerization of the silicate ion at the interface. It results in hardening of the inorganic phase and at the same time in restricting the motion of the surfactant molecules at the interface. The latter stage requires a minimum Si/surfactant ratio, below which the polymerization will not start. The room temperature formation of MCM-50 is considerably slower and appears to involve several stages with a number of long-range order structures. The final product is a mixture of two lamellar phases. Acknowledgment. This work was supported by a grant from the Israeli Ministry of Science and Technology. References and Notes (1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresege, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Shepard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (2) Beck, J. S.; Vartuli, J. C.; Kennedy, G. J.; Kresege, C. T.; Roth, W. J.; Schranm, S. E. Chem. Mater. 1994, 6, 1861. (3) Vartuli, J. C.; Kresege, C. T.; Leonowicz, M. E.; Chu, A. S.; McCullen, S. B.; Johnson, I. D.; Shepard, E. W. Chem. Mater. 1994, 6, 2070. (4) Vartuli, J. C.; Schmitt, K. D.; Kresege, C. T.; Roth, W. J.; Leonowicz, M. E.; McCullen, S. B.; Hellring, S. D.; Beck, J. S.; Schlenker, J. L.; Olson, D. H.; Shepard, E. W. Chem. Mater. 1994, 6, 2317. (5) Schuth, F. Ber. Bunsenges. Phys. Chem. 1995, 99, 1306. (6) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P. Y.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schuth, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (7) Sayari, A.; Moudrakovski, I.; Reddy, J. S.; Ratcliffe, C. I.; Ripmeester, J. A.; Preston, K. F. Chem. Mater. 1996, 8, 2080.
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