Investigation of the Surfactant Role in the Synthesis of Mesoporous

Dec 16, 2009 - Romanian Academy. .... A template-free synthesis of mesoporous crystalline γ-alumina through partial hydrolysis of aluminum nitrate aq...
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J. Phys. Chem. C 2010, 114, 28–35

Investigation of the Surfactant Role in the Synthesis of Mesoporous Alumina Agneta Caragheorgheopol,*,† Adina Rogozea,† Rodica Ganea,‡ Marc Florent,§ and Daniella Goldfarb§ “Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Spl Independentei 202, 060021 Bucharest, Romania, ZECASIN, Spl Independentei 202, 060021 Bucharest, Romania, and Department of Chemical Physics, Weizmann Institute of Science, RehoVot, Israel ReceiVed: August 3, 2009; ReVised Manuscript ReceiVed: NoVember 24, 2009

Mesoporous alumina with a narrow pore size distribution can be synthesized by hydrolysis of aluminum alkyl ethers in an organic solvent in the presence of an ionic or nonionic surfactant. However, the pores are not ordered and the role of the surfactant as a possible template is not clear as it is in the synthesis of the mesoporous silica of the M41S family and of the SBA-15, 16 type materials. In this work we use continuous wave electron paramagnetic resonance (CW-EPR) spectroscopy, in combination with electron spin-echo envelope modulation (ESEEM) measurements, to provide experimental evidence for the interaction between the surfactant and the alumina species during synthesis. This is achieved by using spin-labeled analogs of the surfactants and following their interaction with 27Al at different stages of the alumina formation. The results show that in sec-butanol solutions the surfactants do not form micellar aggregates. Instead, the anionic surfactant, lauric acid, has a strong tendency to bind to alumina precursors, hydrolysis products of the aluminum alkyl ethers in the starting solution, and remains bound until the final product. Here the surfactant decorates the surface of the alumina particles with an average extended configuration with the carboxylate being the closest to the alumina surface. The interaction with aluminum increases during precipitation as the density of the alumina increases. By contrast, the nonionic polyethyleneoxide type surfactants: Tergitol 15-S-12 and Pluronics P123 and L64 seem to remain unbound even in the as-synthesized precipitates. In this last case, EPR spectra of a small, hydrophobic spin probe have shown that solvent evaporation by room temperature drying brings about the formation of liquid-like “organic zones” confined in the alumina structure, which presumably are at the origin of the pores. Introduction Alumina is a heavily used industrial support and catalyst and there is a continuous search for improving its quality. Surface area, pore size, and size distribution width are basic characteristics which are important for its catalytic activity. The discovery that silica precipitation using templates represented by surfactant assemblies1 yields materials with an ordered pore system with narrow size distribution (the M41S family) has been a breakthrough in the synthesis of mesoporous silica. An analog procedure was sought for the case of alumina. It was soon observed, however, that approaches similar to those used in the synthesis of mesoporous silica do not work for alumina. Nevertheless, many synthesis routes have been investigated and a large body of results emerged, the products of which, with suitable catalytic properties, are the so-called “organized mesoporous aluminas”. These are characterized by a high surface area and narrow pore size distribution in the mesopores region (2-10 nm), but with no long-range order of the pores. All syntheses and their results are examined in an exhaustive review published in 2008 by Ma´rquez-Alvarez at al.2 Most of the reactions that gave good products represent the controlled hydrolysis of an aluminum alkoxide in an organic solvent with a (sub)stoichiometric quantity of water, in the presence of a surfactant (anionic, nonionic, or cationic).3-12 After examining * To whom correspondence should be addressed. E-mail: [email protected]. † Romanian Academy. ‡ ZECASIN. § Weizmann Institute of Science.

these data it becomes obvious that, as opposed to silica, the role of the surfactant, while essential, is less clearly defined in the alumina syntheses. For example, the pore sizes obtained mostly do not reflect the surfactant size and do not follow its variation.11,13 Also, the TEM (transmission electron microscopy) images of the pores are in some cases of the “wormhole” type but, in others, reflect agglomeration of pellets.2,11 More research is necessary for rationalizing the role of the surfactant in the formation of the pore system in specific cases. A significant volume of research has been devoted to improving the synthesis of mesoporous alumina, yet we are not aware of studies investigating the primary steps, at molecular level, of alumina formation in organic solvents in the presence of surfactants. While many spectroscopic or imaging methods have been successfully used to describe the final products, EPR of spin probes seems best suited to report on local events at the molecular level during precipitate formation, as was shown in the studies of mesoporous silica formation.14-17 Kim et al.5 reported that the porosity of the final material depends on the way the hydrolysis of aluminum compound is carried out: before addition of the surfactant (textural porosity) or after addition of surfactant (framework porosity). We chose to concentrate on the synthesis procedures where the precipitation takes place in the presence of the surfactant. Accordingly, alumina with wormhole porosity has been obtained. It is the aim of this work to closely examine the initial steps in the formation of mesoporous alumina by hydrolysis of an aluminum alkoxide in an organic solvent with controlled amounts of water, in the presence of certain surfactants, using EPR of surfactant

10.1021/jp907478v  2010 American Chemical Society Published on Web 12/16/2009

Surfactant Role in Synthesis of Mesoporous Alumina likes spin probes. CW (continuous wave) EPR has been previously successfully used to describe micellization processes as well as micelle interaction with inorganic material.18,19 A first approach to elucidate the template-alumina interaction using spin probes has been made earlier by one of us20 but without spin-labeled surfactants and without resort to pulse EPR. The results were concentrated on other aspects of the synthesis, that is, the effect of washing with different solvents on the surfactant in as-synthesized alumina and its influence on the pore size of the final product.20 The more advanced techniques of pulse EPR broaden the scope of the application of the spin probe methodology. Specifically, ESEEM (electron spin echo envelope modulation) is a well-established method for measuring distances between an electron spin and nearby nuclear spins.21-23 The experiment consists of the application of a series of microwave pulses that generate an echo, and the echo decay is followed as a function of one of the time intervals between the pulses. When an anisotropic hyperfine interaction is present, the echo decay is modulated. In the case of a very weak anisotropic hyperfine interaction, the modulation frequency is the nuclear Larmor frequency, νI, and the modulation depth (k) is a function of the electron-nuclear distance, the number of interacting nuclei, and their nuclear spin.21 In the case of I > 1/2, the nuclear quadrupole interaction contributes to the modulation depth,22 but the electron-nuclear distance, however, has the strongest effect. Accordingly, k provides direct information on the close environment of the unpaired electron. Here we make use of the nonzero nuclear moment of 27Al atoms in alumina to examine the interaction of the surfactant (via spin labeling) with the aluminum compounds in different stages of the syntheses of mesoporous alumina. We focus on two types of syntheses: (i) the method of Vaudry, Khodobandeh, and Davies13 based on an anionic surfactant, dodecanoic (lauric) acid, using as spin probes the closely related doxyl-stearic acid free radicals and (ii) the method of Bagshaw and Pinnavaia,10 which employs nonionic polyoxyethylene type surfactants, using Tergitol 15-S-12, Pluronic P123, and L64. In these cases, the spin probes used were the spin-labeled surfactants or closely related compounds. In both methods, sec-butanol was used as solvent and the aluminum source was Al(sec-BuO)3 (AlsB). Experimental Section Materials. sec-Butanol anhydrous, lauric acid (LA), Al(s-BuO)3, and Tergitol 15-S-12 ((C15H31-(EO)12) were obtained from Aldrich. Pluronic L64 ((EO)13(PO)30(EO)13; where EO and PO denote ethyleneoxide and propyleneoxide units, respectively) was obtained from BASF. All these were used as received. Pluronic P123 ((EO)20(PO)70(EO)20) from BASF, was dried before use by azeotropic distillation with anhydrous benzene. Alumina Syntheses. Mesoporous alumina syntheses were performed with three nonionic surfactants, that is, the triblockcopolymers Pluronic L64 and P123 as well as the diblockcopolymer Tergitol 15-S-12 in sec-butanol, according to the procedure of Bagshaw and Pinnavaia10 and with the anionic surfactant lauric acid (LA), according to that of Vaudry, Khnodabandeh, and Davis13 and as modified by Ray et al.3 Generally, the preparations consisted of a number of stages such as separate preparation of surfactant and of AlsB solutions in anhydrous sec-butanol, mixing the solutions, slow addition of a stoichiometric quantity of water in sec-butanol, aging, thermal treatment (100 °C in autoclave) in the case of lauric acid synthesis, precipitate separation by centrifugation, washing with

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Figure 1. Spin probes used in this work: (a) spin-labeled P123 (P123NO; x ) 70, n ) 20); spin-labeled L64 (L64-NO; x ) 30, n ) 13); (b) spin-labeled TX-100 (TX-NO; n ) 10); (c) x-doxyl stearic acids (xDSA, x ) 5, 7, 10, 16); (d) 3-carboxy-PROXYL (3CP); (e) 4-hydroxy TEMPO benzoate (HTB); and (f) TEMPO-laurate (C12-NO).

ethanol, and drying at RT for several days. Calcination was carried out by heating the as-synthesized material to 500 °C for 6 h with a temperature ramp of 0.5 °C/min from room temperature to the final calcination temperature. Two procedures were used in the case of the anionic surfactant, differing mainly in the order of the mixing of the various components. In procedure I, water is the last to be added and initialize the hydrolysis of the organo-aluminum, and in procedure II, it is the organo-aluminum that is the last to be added and initialize the reaction. Procedures. (I) The following three solutions have been prepared: (a) 0.6 g LA in 6 g sec-butanol; (b) 3.48 g AlsB in 6 g sec-butanol; (c) 0.55 g H2O in 3.5 g sec-butanol. After dissolution, solutions (a) and (b) were mixed and stirred overnight, then solution (c) was added very slowly with stirring. The gel formed was aged at room temperature for 2 h, then subjected to thermal treatment in autoclave at 100 °C for 24 h. After that the precipitate was separated by centrifugation, washed with ethanol, and dried at room temperature for several days. (II) Same as I, except that water was added to LA solution prior to the addition of AlsB and the thermal treatment at 100 °C lasted for 2 days. (III) (a) A total of 6.1 g of a nonionic surfactant (Tergitol 15-S-12, Pluronic P123 or L64) and 5.166 g AlsB were dissolved in 25 mL of sec-butanol. (b) A total of 0.756 g H2O was dissolved in 10 mL of sec-butanol. Solution (b) was added very slowly, under stirring, to solution (a); the mixture was stirred for another 3 h and then aged 16 h at RT. The precipitate was then separated by centrifugation, washed with ethanol, and dried at RT for several days. Some samples were dried for 20 h at 50 and 100 °C. Spin Probes. Spin-labeled analogs of the surfactants were used as spin probes in the syntheses with the corresponding surfactants, as follows: TX NO (spin-labeled TRITON X-100) for Tergitol 15-S-12, L64-NO for L64, and P123-NO for P123, 3-carboxy-PROXYL (3CP), and the series of 5-, 7-, 10-, 16doxyl stearic acids, all for lauric acid (Figure 1). The doxyl radicals, due to their specific structure are well suited to report on organized assemblies of molecules. Besides these, 4-hydroxy-

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Caragheorgheopol et al.

TEMPO-benzoate (HTB) and TEMPO-laurate (C12NO) were used as hydrophobic probes. The series of doxyl stearic acids and 3-CP and HTB were commercial samples from Aldrich, while the P123-NO, L64NO, C12-NO, and TX-NO were synthesized as reported earlier..16,18,24 Samples for EPR Measurements. The spin probe spectra were measured at different stages of the preparation, as follows: (1) In the initial separate solutions of the surfactants and of Al(sec-BuO)3 in anhydrous sec-butanol. (2) In the mixture of the aluminum and the surfactant solutions according to the specific procedure. (3) In the previous mixture after addition of 1/4, 1/2, and the entire quantity of water, according to the described procedure. (4) In the precipitate obtained after centrifugation. (5) In the solid product after thermal treatment, measured wet and after drying at RT or 50 and 100 °C. Suitable amounts of a 5 mmol stock solutions of the spin probes in ethanol were evaporated on small vials. Aliquots of the examined solutions, suspensions, or gels have then been added to the vials so as to yield final spin probe concentrations of about 0.5 mM. When following the precipitation process the spin probe was introduced in the starting solutions and observed in the subsequent stages (gradual water addition, precipitate separation, drying). The samples were mixed with a vortex and left in the closed vials overnight before measuring. Dry samples were obtained from the corresponding precipitates. In thermally treated samples the spin probes were introduced in the same way but after the heating stage. The resulting samples were subsequently examined by CW-EPR at RT and by ESEEM at 50 K. Gel samples were introduced (with the aid of centrifugation) into small cylindrical (4 mm OD, 2-3 mm ID, 20 mm height) Teflon containers attached to 5 mm OD rods, which could be connected to the liquid Helium cavity sample holder. The same sample holders were used also for measuring liquids or powders. The samples were frozen in liquid nitrogen before ESEEM measurements. In most cases it was the same sample that was examined by ESEEM and by CW-EPR. Characterization of Products. Small-angle X-ray scattering (SAXS) measurements on dry or calcined products were carried out on a SAX diffractometer, equipped with a Franks mirror and one-dimensional position sensitive detector (homemade), using Cu KR (1.54 Å) with a Ni filter.25 Transmission electron micrographs (TEM) were obtained with a Philips 120 microscope operated at 120 kV and high resolution transmission electron micrographs (HRTEM) on a FEI Tecnai F30 instrument operated at 300 kV on samples dissolved in EtOH, followed by a few minutes of sonication, after which they were deposited on a carbon/collodion-coated 300 mesh copper grid (TEM) or lacey carbon grid (HRTEM), respectively. EPR Spectroscopy. Continuous wave (CW) X-band (9.2 GHz) EPR measurements were carried out at room temperature on a modified Varian E12 spectrometer and on a FA-100 JEOL spectrometer. The ESR spectra yielded parameters related to the local structure at the location of the spin probes, that is, the 14N isotropic hyperfine splitting, aN, which represents a measure of local polarity and the rotational correlation time, τC, whose approximate value can be calculated by the formula26

τC ) (6.51 × 10-10)∆H(0){[h(0)/h(-1)]1/2 + [h(0)/h(1)]1/2} s where ∆H(0) is the line width (in Gauss) of the central line and h(-1), h(0), and h(1) are the peak-to-peak heights of the

M ) -1, 0, and +1 derivative lines. The above relationship is valid in the fast motion domain, where the τc value does not significantly exceed 10-9 s. ESEEM experiments were carried out on a Bruker ELEXSYS E580 spectrometer operating at ∼9.5 GHz at 50 K to slow down the echo decay. The three-pulse ESEEM sequence, π/2-t-π/ 2-T-π/2-t-echo, was employed with a four-step phase cycling. The measurements were carried out at a magnetic field where the echo intensity is maximum, and the length of the π/2 pulses was 16 ns. The pulse interval t was selected as ∼130 ns (depending on the magnetic field) to maximize the 27Al modulation. The number of accumulations was 20-90, depending on the modulation depth. The ESEEM trace was processed as follows: the traces were normalized to unity, the background decay was subtracted using an exponential fit, then the data were apodized with a Sine Bell window, zero filling was performed followed by Fourier transformation (FT), cross term averaging,27 and finally the FT magnitude spectrum was calculated. All ESEEM traces were treated identically. This treatment ensures that the modulation depth can be compared and it is independent of the number of accumulations. When the hyperfine interaction is very small, the modulation frequency is the corresponding Larmor frequency and the modulation depth is the property that reflects the average electron-nuclear interactions. So far the majority of the works that have used the modulation depth as a characteristic parameter of the system used 2H modulation and in a recent report 15N and 23Na modulation were used.28 Several approaches are commonly used to describe the modulation depth in ESEEM experiments: (i) k, the ratio between the extrapolated echo intensity between the first two extremis and at the first minimum.21,29 (ii) When the background decay is negligible, it is also possible to describe the modulation depth as the echo intensity at long T, where it has reached a leveling off value given by d. Here, prior to the determination of the modulation depth, the background should be removed by division followed by renormalization.30 (iii) The intensity of the 2H peak in the Fourier transform (FT) of the time domain ESEEM trace, I.28,31 Methods (i) and (ii) are preferred when the modulation depth is high, but it is sensitive to contributions from residual protons.30 Method (iii) does not suffer from this problem and is preferred when the proton modulation is significant and when the modulation is shallow. In this work we have chosen the amplitude of the peak at the Larmor frequency, I(27Al), to describe the modulation depth (method (iii)) for the reasons stated above. We have not observed significant variation in the line width of this peak and therefore used the amplitude and not the intensity that requires integration of the peak area. In our case, the interaction between the 27Al nuclei and the NO group of the nitroxide on the surfactant is weak and FT of the ESEEM trace yields a peak at the 27Al Larmor frequency. In this case, the isotropic hyperfine is zero and the point dipole approximation applies. Accordingly, all the above considerations made in the case of 2H hold for Al as well, except for the nuclear quadrupole interaction that is known to be small for 2H, but for 27 Al it can vary. In experiments where the comparison is between I(27Al) of different spin probes within the same system, the effect of the quadrupole interaction is not important because it is constant for all spin probes. The quadrupole interaction does matter when the sample changes, like wet and dried samples, because it can change as well. This will be further discussed in the text when needed. Figure 2 shows a representative ESEEM trace and its FT, with the peak at 3.81 MHz coming

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Figure 2. Example of an ESEEM trace and its Fourier transform (inset): 5-DSA in the precipitate of procedure III (with P123; see below), after addition of 1/2 of the required water quantity.

from interaction of 5-DSA with 27Al, while the one at 14.6 MHz comes from 5-DSA interaction with 1H. Results Characterization of Products. SAXS measurements on dry or calcined products have shown the low angle broad line characteristic of mesoporous materials (see Figure S1 in the Supporting Information (SI)). The value of the repeat units is, as expected, much larger for P123-alumina (d ) 6.24 nm) than for LA-alumina (d ) 3.5 nm). TEM images obtained with both anionic and neutral templates are characteristic of the wormhole-type materials10 (for example. Figure S2 in SI). Synthesis with an Anionic Surfactant, Lauric Acid (LA). Starting Solutions. An important question in the mesoporous alumina synthesis in organic solvents is whether organized colloidal mesophases of the surfactant pre-exist, are formed during the precipitation of the inorganic component, or may not be formed at all. Therefore, we examined the initial surfactant solutions and their mixture with the AlsB solution prior to hydrolysis, following procedure I, looking for possible aggregation of the surfactant. Later we checked whether micelles are formed by LA in sec-butanol in the presence of water using the slightly modified procedure II. In the following we will report both sets of data (I and II) as they illustrate the common trend as well as the scatter of the results. We used anhydrous sec-butanol throughout the experiments and recorded CW-EPR spectra of 5-DSA, a spin probe sensitive to the presence of surfactant aggregates. The spectra of 5-DSA in a lauric acid solution of sec-butanol are practically the same as those in the pure solvent (τC ) 5 × 10-10 s). Similar results were obtained in 1,4-dioxane. Thus, we have not found any evidence for the existence of surfactant aggregates in the pure anhydrous solvents or by addition of a small quantity of water (procedure II). However, one has to keep in mind that, in the case of a nonselective solvent, such as butanol (or dioxane), distinctions between core and corona, both solvated, are blurred and the existence of aggregates is hard to certify. Addition of the AlsB solution brought about very significant increases in the rotational correlation time τC of the doxyl stearic acid spin probes (for 5-DSA τC ∼ 24 × 10-10 s; Figure S3 in SI). However, this does not result from micelle formation because it was also observed in AlsB solutions in the absence of surfactants. Thus, it has to be attributed to the association of the doxyl-stearic acid spin probe with small particles, which result from incipient AlsB hydrolysis (even in anhydrous solvent and anhydrous surfactant (P123), probably from the AlsB

Figure 3. EPR spectra of 5-DSA during precipitation of a solution containing LA and AlsB in sec-butanol (solutions a + b, procedure I) after addition of different amounts of water.

Figure 4. EPR spectra of the various x-DSA spin probes in the LAalumina precipitate (I), separated by centrifugation.

product itself). A similar increase in τC was observed with 3CP, a short chain carboxylic derivative of PROXYL. A poor reproducibility of the τC values from batch to batch was observed and attributed to the variable size of the hydrolysis products. These observations demonstrate that there is a strong tendency of carboxylic derivatives to bind to alumina precursors. ESEEM measurements of 3CP and 5-DSA in AlsB solution gave rather small values of I(27Al) (0.55 and 0.25, respectively). As expected, 3CP shows a deeper modulation due to the shorter distance of the NO• group to the Al atom in the alumina precursors, presumably bonded to the carboxylic group. Precipitation Process. In procedure I the addition of 1/4 of the water quantity produces a slight turbidity, while at 1/2 the amount a rather rigid gel is formed. After the whole quantity of water is added, the gel becomes softer as more butanol is released. The precipitates in samples with 1/2 and with all water were separated by centrifugation. The effect of the amount of water added was examined through the spectra of 5-, 7-, 10-, and 16-DSA in the mixture of AlsB and LA solutions (solutions a + b, procedure I) in anhydrous sec-butanol. Figure 3 shows that as the water content increases the motional freedom of 5-DSA is reduced. The spectra depicted in Figure 4 show that, after the addition of the whole amount of water, 5-DSA, as well as 7- and 10-DSA appear strongly immobilized, with a certain decrease of the immobilization degree (reflected in ) as the radical moiety is further from the binding headgroup (the

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Figure 5. 2 of 5-, 7-, 10-, and 16-DSA vs number of bonds separating the radical from the Al atom in the LA-alumina II, thermally treated (TT) precipitate (triangles) and the same dried at RT (diamonds; the dashed lines are regression type, 2nd degree polynomials).

Figure 6. I(27Al) of 5-, 7-, 10-, and 16-DSA vs number of bonds separating the radical from the Al atom in (a) as-synthesized LA precipitates and (b) in RT dried precipitates: alumina (diamonds), LAalumina I (squares), LA-alumina I, TT (circles), LA-alumina II, TT (triangles; the dashed lines are regression type 2nd degree polynomials).

carboxyl group). 16-DSA, where the nitroxide is furthest from the binding point remains mobile, with a rapid, isotropic movement. The variation of with the position of the spin label along the fatty acid chain is shown in Figure 5. The I(27Al) values of the precipitate were significantly higher than the solution values and decreased strongly in the 5-, 7-, and 10-DSA series, reflecting increasing distances to the Al atom (Figure 6a). The gradual mobility increase in the 5-, 7-, 10-, and 16-DSA series results from the binding of the x-DSA spin probe via the carboxyl group to AlsB hydrolysis products, maybe even to unsaturated Al atoms. This explanation is supported by the stepwise decrease of I(27Al) values in the same series. This conclusion can be safely extended to lauric acid, the surfactant. Thermal Treatment. The separated precipitate was subjected to thermal treatment (TT) at 100 °C in an autoclave, for 2 days. Unfortunately, the spin probes do not survive the hydrothermal process and therefore they were added post treatment. There were no significant changes in the EPR spectra or in the I(27Al)

Caragheorgheopol et al. values before and after thermal treatment. In fact, the data are practically identical. The precipitate obtained by procedure II gives somewhat different values for I(27Al), but the range of values as well as their descending trend are very similar. These reflect the extended conformation of the stearic acid backbone of the probes. The same is observed in the case of an alumina precipitate in the absence of any surfactant (Figure 6a). Drying at RT increased , that is, increased immobilization (Figure 5) along with a significant increase of I(27Al) for all spin probes, compared to the wet precipitate (Figure 6b). Both precipitates I and II show in the dry state the same trend of decreasing values in the 5-, 7-, and 10-DSA series (i.e., increasing distance to Al), but followed by a higher value for 16-DSA. It is not unusual for 16-DSA to have the chain in a bent conformation,32 especially when the surfactant has (as in our case) a shorter chain. Alternatively, the drying can bring the alumina closer together such that the end of the chain can “see” Al from the neighboring particle. Finally we observed (Table S1 in SI) that, in the absence of the surfactant, the spin probes exhibit lower I(27Al) values in dry alumina as compared to synthesis with surfactant, without any clear trend. A summary of the I(27Al) and τc values of the various preparations is given in Table S1 in SI. Syntheses with Nonionic Surfactants. We have looked at synthesis mixtures and intermediate products prepared with Tergitol 15-S-12, Pluronic P123, or L64 as surfactants. Solution Spectra. According to the phase diagram of the ternary system P123/n-butanol/water,33 P123 forms micelles in water, which persist at the addition of up to ∼10% butanol, but there is no micelle phase over the whole range of P123 concentrations in anhydrous butanol. EPR data confirm these observations, as the spectra of 5-DSA in P123 solutions in butanol are almost identical to those in pure solvent. A small τC increase (from 5 to 7 × 10-10 s) was observed and may be caused by a solution viscosity increase due to the presence of large surfactant molecules or may result from their partial aggregation. This is true for Tergitol as well. As observed in the LA synthesis, the addition of AlsB produces a significant increase of the rotational correlation time of x-DSA (Table S2) due to the strong interaction with the evolving hydrolysis products. As with 5-DSA, the spin labeled analog of the P123 did not show in P123/sec-butanol solution any difference compared to pure solvent spectra. However, unlike 5-DSA (in fact x-DSA), it did not show any changes upon addition of AlsB either (Table S2 in SI), meaning that it does not bind to the alumina precursor species in solution. Precipitation Process. We have followed in some detail the precipitation of alumina in the reaction with Tergitol or P123, by gradual addition of water. The 5-DSA probe has shown definite changes in the CW spectrum (immobilization after addition of 1/2 and all the water) and considerably increasing I(27Al) values in the series: anhydrous solution, 1/4 water, 1/2 water, all the water. As the density of the precipitate increases, the amount of precipitated alumina in the closed environment of the attached doxyl probe is also increased. By contrast, with P123-NO, there are no changes in the spectrum even in the centrifuged precipitate, compared to the pure solvent. ESEEM data parallel the CW EPR data and there was no change in I(27Al) compared to anhydrous solution (I ) 0.20; Figure 7). Thus, there is no binding of the P123 to the alumina species even in the separated precipitate. The same behavior was observed also with L64 and Tergitol.

Surfactant Role in Synthesis of Mesoporous Alumina

Figure 7. I(27Al) during water addition to a solution containing P123 and AlsB in sec-butanol (solution a, procedure III), as measured with 5-DSA (diamonds) and with P123-NO (squares).

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Figure 9. EPR spectra of the indicated spin probes in Tergitol-alumina precipitate dried at RT.

Figure 10. I(27Al) vs number of bonds from radical moiety to Al atom, in spectra of 5-, 7-, 10-, and 16-DSA in Tergitol-alumina precipitate, as well as of TX-NO in Tergitol-alumina and P123-NO in P123-alumina precipitates, all dried at RT. Figure 8. EPR spectra of P123-NO and 5-DSA in P123-alumina precipitate wet and dried at 50 °C.

Dry Precipitates. The EPR spectra show a progressive immobilization of all probes in all samples during drying. However, there is a clear distinction between the immobilization of spin probes bound closely to alumina species and those that are free in the gel. In Figure 8 a comparison is made of the effect of drying at 50 °C on the spectra of P123-NO and of 5-DSA in the P123-alumina precipitate. The 5-DSA spectrum shows the presence of a single strongly immobilized species, whereas P123-NO has a composite spectrum consisting of a fast and a slow component. Separation of the two components was possible by subtraction of spectra for samples dried at 50 and 100 °C, respectively, consisting of the same components in different proportions. The fast component represents about 25% in the sample dried at 50 °C and 10% in the sample dried at 100 °C. In Figure 9 the spectra of a variety of probes are compared in the dry (RT) precipitate of Tergitol-alumina. These show that 5-and 10-DSA have immobilized spectra, while 16-DSA and TX-NO still exhibit isotropic diffusions with increased rotational correlation times. We have also looked at the spectra of HTB, a small spin probe known to localize in hydrophobic zones. The nitrogen hyperfine splitting in the spectrum of the Tergitol preparation (aN ) 15.5 G) is well below the value in sec-butanol (16.0 G) or carbowax 200 (15.90 G), decreasing toward hexane (15.0 G). This indicates the segregation of a hydrophobic environment, like an aggregate “core”, which forms as the solvent evaporates during drying. The effect is most pronounced with Tergitol,

where the hydrophobic part is a hydrocarbon, thus, better differentiated from PEO than PPO in Pluronics (in dry L64alumina HTB has aN ) 15.8 G). In all RT dried samples there is a significant increase of I(27Al) versus the respective wet precipitates for all spin probes. For the Tergitol synthesis we have also followed how this value diminishes with the distance from Al by measuring I(27Al) in the doxyl series. Indeed, there is a steep decrease from 5- to 10-DSA, but for 16-DSA, this value increases again (Figure 10). In the case of the spin-labeled surfactants, the increase of I(27Al) after drying is, however, much smaller. Figure 10 shows the dependence of I(27Al) for 5-, 7-, and 10-DSA versus the number of bonds connecting the spin probe moiety to Al in the precipitate dried at RT. Using this graph as a calibration curve, one may conclude from the corresponding I(27Al) values that the nitroxide group in 16-DSA lies on average at a distance of about 10 bonds, in TX-NO at a distance of 14 bonds, and in P123-NO at 15 bonds distance. Using hydrophobic spin probes, we have also checked whether there is an organic/inorganic segregation, that is, whether the L64 molecules form organic microphases in the wet and dry precipitates obtained with Pluronic L64. As with Tergitol, we find that HTB has a somewhat lower aN value, compared to sec-butanol, and that a number of spin probes (HTB, C12-NO, L64-NO, 16-DSA) report on a high viscosity, liquid-like (isotropic) environment. This fact points to the presence of an organic “core” in the dry material. A summary of all EPR and ESEEM data is given in Table S2 in SI.

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Discussion Introducing spin probes that are analogs of the surfactant used in the preparation has allowed us to follow the interactions in which the surfactants are involved during different stages of the preparation. Thus, lauric acid was represented by the series of doxyl stearic acid spin probes, while spin-labeled analogues of Tergitol 15-S-12 (TX-NO), Pluronic P123 (P123-NO), and Pluronic L64 (L64-NO) were used in syntheses involving the corresponding surfactants. We have examined two families of syntheses, one with neutral surfactants and the other with anionic surfactant. In the latter, we have looked at two close procedures, for which no significant differences were observed. In the following, we first summarize the experimental observations and then discuss their implications: (1) Micelles were not detected in the starting surfactant (ionic and nonionic) solutions in any of the synthesis routes. (2) The carboxylic group of the anionic surfactant, LA, binds to alumina precursor species formed by hydrolysis of AlsB, already in the starting solution and remains bounded in all following stages of the synthesis. In the precipitates, the surfactant adopts a rather extended conformation of the stearic acid chain, bound at the carboxy headgroup; the thermal treatment seems to have no effect on this arrangement. (3) In dry samples of LA-alumina, all doxyl probes, as well as HTB become completely immobilized. The I(27Al) values are considerably larger than in the gels. This increase is attributed to the increase in Al density near the spin probe and not to an increase in the quadrupole interaction, which is expected to reduce the modulation depth due to broadening.34 Furthermore, the doxyl probes follow the same trends as in the gel, except for 16-doxyl, which appears to have adopted a bent configuration. A similar bending of 16-DSA was observed also in Tergitol-alumina. (4) The spin labels in the nonionic surfactants, located at the PEO chain ends, do not experience detectable interaction with alumina precursor species neither in the starting solution nor in the as-synthesized precipitate. This suggests negligible effect from hydrogen bonding via the many etheric oxygen atoms of the PEO chain, meaning weak interaction of the Tergitol, P123, and L64 with alumina precursors. (5) Spin-labeled nonionic surfactants show much smaller increase of I(27Al) after drying, as compared to doxyls. I(27Al) of TX-NO and P123-NO correspond to 14 and 15 bonds, respectively, distance to Al, quite a large distance for the surfactants that are supposed to be the templates. (6) There appears to be a viscous liquid-like, isotropic core in the dry precipitates of alumina with nonionic surfactants. With Tergitol, the polarity of this core is close to values in hydrocarbon solvents. Vaudry, Khodabandeh, and Davis13 presented the first successful synthesis of mesoporous alumina in an organic solvent, by controlled hydrolysis of aluminum sec-butoxide in the presence of long chain carboxylic acids. In the absence of surfactants, the following reactions occur:

Al(OR)3 + 3H2O f Al(OH)3 + 3ROH Al(OR)3 + 2H2O f AlOOH + 3ROH The amorphous precipitate is converted into pseudoboehmite in a few hours. It has been shown that carbonate and carboxylate ions hinder this process. When carboxylic acid interacts with freshly precipitated aluminum hydroxide the following reaction occurs:13

>Al-OH + R1COOH f >Al-OOC-R1 + H2O The authors believe that the high affinity of the carboxylate ions to complex with one or two aluminum sites on very small

Caragheorgheopol et al. clusters of aluminum oxyhydroxides (monodentate or bridging coordination) would determine the formation of mesophases in which a micellar aggregate is encapsulated by hydrated alumina walls. Analysis of XRD data made the estimate of aggregate diameters possible, which proved to be compatible only with bent conformations of the surfactant chains. Our CW-EPR and ESEEM measurements provide experimental evidence for the binding of carboxylic compounds to the alumina precursors in solution, in as-synthesized precipitates and in the dry material. Although we have found that in solution and in the wet precipitate the carboxylic acid chain is in an extended form that is further bent in the dry material, we cannot positively affirm the existence of micellar aggregates in the precipitate. The spectrum of HTB, a small hydrophobic spin probe, reports in the as-synthesized precipitate the same polarity as in bulk butanol. An indication of a more hydrophobic environment would be expected if micelles would be present at this stage. In the dry sample the isotropic fast motion spectrum from the as-synthesized precipitate turns into a slow motion almost immobilized spectrum. Because the hydrocarbon chain of lauric acid is rather short and the resulting core is probably too small to be identified as such, this result does not rule out the possible separation of a proper organic core in case of longer chain carboxylic acids (such as stearic acid). Hydrolysis of Al(sec-BuO)3 in sec-butanol, with a stoichimetric quantity of water, should not produce charged entities (see reaction eq above), and therefore, the strong interaction of the carboxylic group of the surfactant with the hydrolysis products is mediated through protons introduced with the lauric acid and x-DSA. Bagshaw and Pinnavaia10 were the first to report mesoporous alumina prepared with nonionic surfactants: alkyl polyoxyethylene ethers or Pluronics. In this case, hydrogen bonding between the PEO oxygen atoms and OH or coordinatively unsaturated Al atoms from the alumina species are supposed to ensure interaction of surfactants with the alumina intermediate species. Our data hardly show any binding of the nonionic surfactants Tergitol 15-S-12, Pluronic P123, or L64 to alumina precursors or precipitates at any stage of the precipitation. In the wet precipitate, the spectrum is characteristic of a fast-limit motion. This is unlike the synthesis of mesoporous silica, SBA15 with P123 and P123-NO added as a probe that could sense the silica polymerization within the PEO region.15 In the latter, however, the reaction is done at an acidic pH that can provide ample protons to mediate the interaction between the PEO and the forming silica.28 After drying, however, a part of the P123NO probes experiences a considerable reduction in motion similar to that observed in SBA-15. Moreover, 16-DSA, C12NO, HTB appear to be in a viscous fluid-like region, as reported by their higher τC values. The aN value of HTB in dry Tergitolalumina sample becomes significantly lower (15.5 G) than in the gel or in butanol (16.0 G), getting closer to the value in hydrocarbons (Tergitol hydrophobic part is such). These data suggest the presence of segregated organic “islands” confined in the alumina framework. By contrast, with LA all regions of the sample appear to be solid-like, probably due to the much shorter chain length of this surfactant and the local rigidity of the nitroxide spin lable in 5-DSA. Thus, even if nonionic surfactants appear not to be bonded to alumina species during preparation, they are nevertheless the origin of the mesopores. These observations should be viewed in connection with previous results20 concerning the relation of as-synthesized mesoporous alumina with the template: it was found that washing the as-synthesized (in butanol or acetonitril)

Surfactant Role in Synthesis of Mesoporous Alumina

J. Phys. Chem. C, Vol. 114, No. 1, 2010 35 Supporting Information Available: SAXS patterns and TEM images of typical products; solution spectra of 5-DSA; complete tables of CW EPR and ESEEM results (aN, τC, Azz, I(27Al)) for all examined samples. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 11. Schematic model showing the interaction of alumina particles with the ionic and with the nonionic surfactants in assynthesized and in dry (RT) precipitates.

alumina with water produces a reorganization of the surfactant (P123, Tergitol) aggregates, which was accompanied by a corresponding pore size modifications. This shows that, while the surfactant is easily influenced by a change in solvent, being weakly bound, it nevertheless influences the size of the mesopores. In Figure 11 we present a schematic model showing the interaction of the alumina particles with the ionic and with the nonionic surfactants. Conclusions CW-EPR was used together with 27Al ESEEM to get information on the interaction of ionic and nonionic surfactants with the alumina species in the synthesis of organized mesoporous alumina and thereby clarify their role as templates. When lauric acid was the surfactant, EPR of doxyl stearic acid spin probes has shown that the carboxylic acids bind to alumina species already in solution at the very first hydrolysis stages and remain bound in the final product. By contrast, nonionic, polyethylene oxide type surfactants do not bind to alumina species at all, not even in the precipitated gel. However, during drying an organic/inorganic segregation of microphase was evidenced, which explains the role of the surfactant in the formation of the typical wormhole mesoporous structure of the aluminas. ESEEM data, more specific in describing close environments of free radicals, provide a measure of the distances between the radical moieties and the Al atoms, which are in total accord with the CW-EPR description. Acknowledgment. Most work in this paper has been accomplished during a Rosi and Max Vernon Visiting Professorship of A.C. in the Department of Chemical Physics at the Weizmann Institute of Science. Partial funding was provided by CNCSIS Romania (Grant 154/2007). Acknowledgment is given to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research. This research is made in part possible by the historic generosity of the Harold Perlman Family. We thank Dr. Ronit PopovitzBiro for acquiring and interpreting the (HR)TEM measurements. A.C. thanks Dr. Jiri Cejka from the Jan Heyrovsky Institute of Physical Chemistry in Prague for a preprint of the review in ref 2 and for helpful discussions. D.G. holds the Erich Klieger Professorial chair in Chemical Physics.

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