J. Phys. Chem. B 1998, 102, 585-590
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Creation of VOx Surface Species on Pure Silica MCM-48 Using Gas-Phase Modification with VO(acac)2 P. Van Der Voort,*,† M. Morey,‡ G. D. Stucky,‡ M. Mathieu,† and E. F. Vansant† Department of Chemistry, Laboratory of Inorganic Chemistry, UniVersity of Antwerpen (U.I.A.), UniVersiteitsplein 1, B-2610 Wilrijk, Belgium, and Department of Chemistry, UniVersity of California at Santa Barbara, Santa Barbara, California 93106 ReceiVed: August 26, 1997
Pure silica MCM-48 is prepared by a novel synthesis method, using the [C18H37N+(CH3)2-(CH2)12-N+(CH3)2C18H37]‚2Br- surfactant, abbreviated as GEMINI 18-12-18. The MCM-48, obtained after careful calcination, is a highly crystalline, mesoporous material with the characteristics of the Ia3d cubic phase, a surface area exceeding 1000 m2/g, and a narrow mesoporous pore size distribution (r ) 1.4 nm; fwhh < 0.2 nm). This MCM support is grafted with VOx species using a designed dispersion of VO(acac)2 in a gasdeposition reactor. In the first step, the complex is anchored to the support. In a subsequent step the adsorbed complex is thermolyzed to yield chemically bonded VOx surface species. The final material contains 1.7 mmol V/g (8.7 wt % V) and still has a narrow pore-size distribution and a surface area of 800 m2/g. It is observed that all silanols are consumed during the adsorption of the VO(acac)2 complex to the MCM support. Therefore, the maximum achievable number of surface V species is limited by the silanol number and not by the geometrical surface, which has a higher capacity. After calcination of the adsorbed complex, the supported VOx species are present in a strictly tetrahedral configuration, mainly as chains of linked tetrahedra and not as isolated species.
Introduction
SCHEME 1: MCM-48 Cubic Phase
In the early 1990s, the discovery of a new family of mesoporous molecular sieves (called M41S) was reported.12 The M41S family (as defined by Mobil) contains three unique members: MCM-41, having a hexagonal arrangement of unidimensional pores; MCM-48, displaying a cubic structure; MCM-50, exhibiting a pillared, layered phase. Other phases were synthesized at the University of California at Santa Barabara and are designated as SBA-1 through 8.34 The M41S materials are synthesized hydrothermally in the presence of alkyltrimethylammonium surfactant cations having an alkyl side chain of greater that six carbon atoms. A liquid-crystal templating mechanism in which surfactant liquid-crystal structures serve as organic templates (rather than single molecules commonly proposed in zeolite synthesis) has originally been proposed for the formation of these M41S type materials.1,2 It is now established, however, to be a cooperative surfactant/silicate self-assembly mechanism.5,6,8 Many publications have appeared on the synthesis, characterization, and potential uses of the hexagonal MCM-41 mesoporous substrate.7-12 The cubic MCM-48, however, has received less attention. Recently, the rather difficult and poorly reproducible alkyltrimethylammonium synthesis route for MCM48 has been replaced by a more convenient synthesis, using so-called gemini surfactants, with the general formula [CnH2n+1N+(CH3)2-(CH2)s-N+(CH3)2CmH2m+1]‚2Br-, abbreviated as gemini n-s-m.3,4 The use of gemini 16-12-16 gives MCM-48 both at room temperature and at higher synthesis temperature. Huo3 concluded that the gemini 16-12-16 surfactant favors the * Corresponding author. E-mail:
[email protected]. † University of Antwerpen (U.I.A.). ‡ University of California at Santa Barbara.
formation of the cubic (Ia3d) phase, irrespective of the synthesis temperature and even without adding an organic additive. The three-dimensional pore structure of the MCM-48 substrates makes them potentially very interesting and promising supports for heterogeneous catalysts by grafting the surface of the support with catalytic active species.13 The typical cubic structure of an ideal MCM-48 support is visualized in Scheme 1.8,14 The structure is consistent with the Q230 model proposed by Mariani for water-surfactant systems.15 The use of VO(acac)2 (vanadyl acetylacetonate) to graft VOx groups on a catalytic support by means of the molecular designed dispersion method has been described in detail in some of our previous publications.16-18 In principle, the complex is anchored to the hydroxyl groups of the support by either a hydrogen-bonding or by a ligand-exchange mechanism. The adsorbed complex is called the precursor. Treatment in air at elevated temperatures converts the adsorbed acetylacetonate complex into metal oxide species that are chemically bonded to the surface. The process and the reagent are visualized in Scheme 2. The reaction mechanisms occurring during this process are now fairly well understood.16-19 In the liquid-phase synthesis, by the stirring of the dissolved complex with the substrate at room temperature, the VO(acac)2 bonds almost exclusively to
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SCHEME 2: (Left) VO(acac)2. (Right) General Overview of the Molecular Designed Dispersion Process
the silica surface by means of hydrogen-bonding interactions between the pseudo π system of the acac ligand and the silanols. In the gas-phase deposition, the complex sublimes in the vacuum deposition reactor and contacts the heated substrate. Some ambiguity exists in the literature on the exact reaction mechanism. Little information is available on the gas-phase modification of silica supports with VO(acac)2, but we can demonstrate the confusion in the literature for the case of Cr(acac)3. Haukka et al.20,21 reported in 1994 on the gas-phase reaction of Cr(acac)3 with silica. They stated that the reaction mechanism is an exclusive ligand-exchange mechanism in which SiO-Cr(acac)2 are formed with the loss of one ligand as Hacac. Babich22 stated in 1997 exactly the opposite: using a comparable reactor, he observed that the Cr(acac)3 bonds exclusively to the silica support by means of a hydrogen-bonding interaction. He observed that the three acac ligands were still associated with the Cr center after the reaction and that therefore a ligand exchange mechanism was impossible. When these precursors are calcined to yield the final catalysts, two mechanisms occur. At temperatures less than 200 °C, all hydrogen-bond-interacting ligands leave the surface as acetylacetone (Hacac), consuming a silanol. This process does not require oxygen and has been referred to as “proton-assisted thermolysis”. At higher temperatures, the remaining (nonhydrogen-bond-interacting) ligands are oxidized, yielding typical oxidation products such as CO2, acetone, acetic acid, 2,3butanedione, 2-butanone, and water. Experimental Section The MCM-48 is this study was prepared using the 18-12-18 gemini surfactant, with the formula [C18H37-N+(CH3)2C12H25-N+(CH3)2-C18H37]‚2Br-. This surfactant is prepared by refluxing 1,12-dibromodocane and N,N-dimethyloctadecylamine in ethanol for 4 days followed by several recrystallizations from acetone. To synthesize MCM-48, 1.2 g of surfactant is dissolved in 60 g of hot water. A 7.5 g sample of a 2 M solution of TMAOH (tetramethylammonium hydroxide) is added and stirred for several minutes. A 5.8 g sample of TEOS (tetraethoxy orthosilicate, Si(OC2H5)4) is added rapidly and the solution is stirred vigorously for several hours. Afterward, the entire solution is put in an autoclave at 100 °C for 10 days. The resulting white solid is then obtained by vacuum filtration. A 20 g sample of fresh water is added per gram of product and returned to the autoclave at 100 °C for several days. This additional treatment is often found to enhance the “crystallinity” of the product in terms of the degree of long-range ordering of the pores as seen by X-ray diffraction.4 When the XRD patterns are satisfactory, the product is calcined by first heating the product to 500 °C with a heating rate of 5 °C/min under flowing nitrogen. Subsequently, the material is kept isothermal at 500 °C for 6 h in flowing oxygen.
VOx-supported catalysts were prepared in a vacuum deposition reactor, as shown in Figure 1. A high vacuum in the reactor is ensured by a combination of a rotation and a diffusion vacuum pump. The sublimed VO(acac)2 complex reacts with the heated substrate. Reaction products are collected in a cryotrap. Reaction is performed until there is saturation of the substrate, which is visible by the formation of crystals of the complex on top of the colder parts of the reactor. At this stage, the precursor is formed. Calcination of this precursor to form the catalyst was performed in a programmable oven (25-500 °C, heating rate of 5 °C/min, isothermal period of 17 h) in ambient air. X-ray diffraction patterns were collected with a Philips PW1840 powder diffractometer (45 kV, 30 mA), using Nifiltered Cu KR radiation. Porosity and surface-area studies were performed on a Quantachrome Autosorb-1-MP automated gasadsorption system. Infrared spectra were recorded at room temperature on a Nicolet 5 DBX Fourier transform spectrometer with photoacoustic detection.23 Thermogravimetric measurements were performed on a Mettler TG50 thermobalance, equipped with a M3 microbalance and connected to a TC10A processor. UV-vis diffuse reflectance measurements were recorded on a Unicam 8700 spectrometer equipped with a specially designed diffuse-reflectance accessory. To determine the V-loading of the catalysts, the samples were stirred with hot sulfuric acid (2 M) for 30 min. After filtration, H2O2 was added to the solution, forming the reddish-brown compound VO2(SO4)3. The vanadium concentration is then determined colorimetrically at 450 nm.24 Sporadic electronmicroprobe surface analysis (JEOL SX 733 Superprobe) confirmed that no residual V species remain on the surface after destruction. The surface concentration of acac ligands was determined from the weight loss in the respective thermograms, after heating to 500 °C in an oxygen atmosphere. We have shown in a previous publication that sublimation of anchored VO(acac)2 does not occur in an oxygen-rich atmosphere.29
Figure 1. Vacuum deposition reactor.
VOx Surface Species
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Figure 3. X-ray diffractrograms of the GEMINI 18-12-18 MCM-48 after 10 days in autoclave (A) and 21 days in autoclave (B) at 100 °C. Figure 2. (Top) nitrogen isotherm of the 18-12-18 MCM-48, recorded at 77 K, after calcination at 500 °C and degassing for 17 h at 200 °C. (Bottom) BJH pore-size distribution of the same sample.
The ratio of adsorbed acac ligands to the adsorbed vanadium species is expressed as the R value:
R)
mmol acac/g sample nacac ) mmolV/g sample nV
Results and Discussion Characterization of the Support. The nitrogen isotherm and the pore-size distribution of the synthesized MCM-48 are presented in Figure 2. The two most important features in the isotherm are the very sharp increase in the adsorbed gas volume in the region (p/p0) ) 0.3-0.4 and the absence (almost) of hysteresis in the region (p/p0) ) 0.4-1.0. The sharp increase in adsorbed gas volume already suggests a narrow pore-size distribution; in this region the smaller mesopores are filled by capillary condensation. Hysteresis in the higher-pressure region (discrepancy between the adsorption and the desorption branches of the isotherm) is caused by particle-particle porosity or by significant larger pores. Particle-particle porosity does not create a sharp pore distribution because of the different sizes of the particles that create the mesopores between them. Generally speaking, the ideal MCM material does not show hysteresis at all. The pore-size distribution in the mesoporous region was calculated using the well-established method of Barret, Joyner, and Halenda.25 The pore-size distribution in Figure 2 shows a narrow, mesoporous distribution, with a pore maximum of r ) 1.4 nm. A T-plot analysis confirmed the absence of micropores (r < 1 nm). The total surface area, as calculated by the BET method, is 1025 m2/g and the total pore volume at p/p0 ) 0.98 is 0.95 mL/g. Curves A and B of Figure 3 show the X-ray diffractograms of the MCM-48 sample that had been autoclaved prior to calcination for 10 and 21 days, respectively. It was already mentioned in the Introduction that the aging time is a very important parameter in the MCM synthesis. Figure 3 illustrates nicely how the “crystallinity” improves as a function of time.
TABLE 1: XRD Characteristics of MCM-48 Prepared by Cetyltrimethylammonium (CTAOH) Cation26 and by GEMINI 18-12-18 Surfactant reflection (hkl) 211 220 321 400 420 332 422 431 521 611 541 631 543
2θ (deg) 2.67 3.08 4.07 4.35 4.87 5.10 5.34 5.56
CTAOH d (nm) 3.31 2.86 2.17 2.03 1.81 1.73 1.65 1.59
a (nm)
2θ (deg)
8.1 8.1 8.1 8.1 8.1 8.1 8.1 8.1
2.45 2.83 3.73 3.98 4.42 4.63 4.85 5.04 5.39 6.08 6.38 6.71 6.98
18-12-18 d (nm) a (nm) 3.62 3.12 2.37 2.23 2.00 1.90 1.82 1.75 1.64 1.45 1.38 1.31 1.27
8.9 8.8 8.9 8.9 8.9 8.9 8.9 8.9 9.0 8.9 8.9 8.9 9.0
The upper diffractogram shows reflections that are typical for MCM-48 materials. Table 1 compares the literature data for MCM-48 prepared by cetyltrimethylammonium cation26 and for MCM-48 prepared with a gemini surfactant in this study. In this table, d is the interplanar spacing, obtained by Bragg’s Law, and a is the cubic lattice parameter, obtained by the formula: a ) d(h2 + k2 + l2)1/2. All important “cubic” reflections are seen in XRD and indicate a unit-cell dimension of 8.9 nm, which is slightly larger than in the case of the smaller cetyltrimethylammonium cation, which has a chain length of only 16 carbons. Characterization of the Precursor. The precursors were prepared by subliming an excess of VO(acac)2 in the vacuum deposition reactor at 150 °C. Saturation of the substrate was achieved after almost 16 h of reaction. The catalyst has a V-loading of 1.7 mmol/g V or 8.7 wt % V. Figure 4 shows the FTIR-PAS of the blank MCM-48 (thermally treated at 500 °C, spectrum a), the MCM-48 saturated with VO(acac)2 (the precursor, spectrum b), and the same sample after calcination at 500 °C (the catalyst, spectrum c). Spectrum a shows a narrow band at 3745 cm-1, assigned to free silanols, and a broader feature at 3600-3200 cm-1, attributed to the presence of some bridged silanols or possibly physisorbed water trapped in the pore system of the MCM.27
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Figure 4. FTIR photoacoustic spectra of the blank MCM-48 calcined at 500 °C (a); the MCM-VO(acac)2 precursor (b); the MCM-VOx catalyst (c). Left side shows the hydroxyl region; right side shows the acac and skeleton region.
After the gas-phase reaction, all isolated silanols have disappeared, owing to either a ligand-exchange mechanism or a hydrogen-bond interaction (spectrum b). In the case of hydrogenbond interaction, the silanol vibration shifts to a lower wavenumber (about 3400 cm-1). The complete disappearance of the isolated silanol band leads to the conclusion that the saturation of the surface is governed by the availability of surface hydroxyls and not by geometric, steric considerations. The VO(acac)2 complex has a mean crosssectional area of 0.60 nm2. A full monolayer would therefore correspond to a V-loading of 2.8 mmol/g V. The actual loading is only 60% of this monolayer capacity. The silanol content of the blank MCM-48 support can be estimated, based on theoretical considerations. Alami et al.28 have calculated that the “headgroup area” for a gemini surfactant with a spacer of 12 C atoms amounts to 2.26 nm2. Each headgroup participates in two Si-O--NR4+ functions during the synthesis of the MCM that are converted to Si-OH functions upon calcination. The theoretical silanol density of a MCM48, prepared by the 18-12-18 gemini surfactant and a 100% silica source, is therefore 2OH per 2.26 nm2 or 0.9OH/nm2. The vanadium-loading of the precursor sample is 1.7 mmol V/g sample or 1.0 V/nm2, while the BET surface area of the blank MCM-48 material is 1025 m2/g. Therefore, within a margin of error of 10%, the VO(acac)2 complex interacts in a 1:1 ratio with the surface silanols of the silica support. We have demonstrated in a previous publication that this 1:1 interaction also occurs when amorphous silica gel is used as the support.29 Figure 4 also shows the acac region of the precursor. Spectrum 4b shows a very nice pattern of infrared bands in the 1600-1300 cm-1 region, which are very characteristic for acac ligands. The exact peak assignments have been published previously.17,29 This pattern of infrared bands shows that the acac ligands are intact and not partially decomposed to acetate species, which would result in an entirely different infrared spectrum. The knowledge that the acac ligands are intact justifies the use of thermogravimetric analysis to calculate the R value, as explained in the experimental section.
The R value (mmole acac per gram sample/mmole V per gram sample) for these precursors is 1.5 ( 0.1. This means that on average 1.5 acac ligands are associated with each grafted V center. In other words, 50% of the VO(acac)2 is bonded by means of the ligand-exchange mechanism (only one acac associated with a V center; the other evolved as Hacac) and 50% is bonded by the hydrogen-bonding interaction mechanism (both acac ligands associated with the V center). The infrared spectrum in Figure 4b indeed shows a strong contribution of hydrogen-bond-interacting species, that were not present in the blank MCM-48. Characterization of the Grafted VOx Surface Species (Catalyst). Figure 4c shows the infrared spectrum of the final catalyst after calcination of the precursor at 500 °C in ambient air. All acac vibration have disappeared because of thermolysis or oxidation. A new band has appeared at 3660 cm-1, assigned to VO-H surface species.19 We have shown in a previous publication that these species are created distinctly after the oxidation of the acac ligands, owing to the reaction of ambient water at high temperatures with a V-O-V bond to create two VOH species, the VdO bond to create V(OH)2, or a Si-O-V species to create V-OH + Si-OH.19 In addition, a band has appeared at 930 cm-1. We have good indication30 that this band in the result of contributions of both Si-O-V bonds and the V-O-V bridges of linked VOx tetrahedra. Additional structural information on the grafted VOx species can be obtained from UV-vis diffuse reflectance measurements of the catalyst (Figure 5). It is obvious from this spectrum that there is no absorption in the visible range of the spectrum (>400 nm), meaning that the catalyst has a perfectly white color. It is well-known that the position of the O f V chargetransfer bands provides a very good indication of the coordination of the central V5+ center. Tetrahedrally surrounded V5+ centers have main absorptions in the region 240-350 nm, square pyramidal configurations in the region 350-450 nm, and octahedral (or pseudo-octahedral as in V2O5) in the region 450600 nm.17,19 Extreme caution is necessary, since it is now wellknown that the colorless tetrahedral V5+ centers quickly turn orange when in contact with wet air, owing to the coordinative
VOx Surface Species
Figure 5. UV-vis diffuse reflectance spectrum of the MCM-VOx catalyst.
SCHEME 3
bonding of H2O.13,19 Therefore, the spectra are recorded in extreme dry conditions. It can be inferred from Figure 5 that this particular catalyst is exclusively covered with tetrahedral species. No crystallites or square-pyramidal multilayers are present at the surface. Moreover, nonoxidized VIV centers should give strong d-d transition bands, which are not observed. Although UV-vis has established that the surface of the MCM is exclusively covered with tetrahedrally surrounded VOx centers, it cannot differentiate between monomers, onedimensional chains and two-dimensional chains (Scheme 3). It can be reasoned though that a surface coverage with exclusively tetrahedral monomers (Scheme 3a) is impossible, since such a coverage would require (1.7 × 3) ) 5.1 mmol OH/g, whereas the actual silanol concentration is only 0.9 OH/nm2 or 1.5 mmol OH/g. Moreover, the infrared spectrum of Figure 4c indicates that a large fraction of the original isolated silanols (3745 cm-1) are restored after calcination of the precursor at 500 °C in ambient (wet) air. This observation suggests that the formation of V-O-V bonds during the calcination of the precursor is favored relative to the formation of Si-O-V bonds. An objective criterion for evaluating the clustering of the grafted VOx species is the ensemble size, defined as
E.S. )
V(mmol/g) total number of V centers ) total number of reacted silanols OHr(mmol/g)
If the surface were covered excusively with tetrahedral monomers (Scheme 3a), the ensemble size would be 1/3. When used with extreme care, the FTIR-PAS band of the isolated silanols can be integrated and normalized to the 19501766 cm-1 silica structure vibration to assess the fraction of silanols that has disappeared owing to the formation of SiO-V bonds. The exact procedure has been elaborated in detail
J. Phys. Chem. B, Vol. 102, No. 3, 1998 589
Figure 6. Pore-size distribution of blank MCM-48 (a), MCM-VO(acac)2 precursor (b), MCM-VOx catalyst (c).
TABLE 2: Surface Area and Total Pore Volume of the Blank MCM-48, the MCM-48 with Adsorbed VO(acac)2 Species (the Precursor), and the MCM-48 with Grafted VOx Species (after Calcination of the Precursor) sample
surface area (m2/g)
total pore volume (mL/g)
MCM MCM-VO(acac)2 MCM-VOx
1025 530 800
0.95 0.45 0.67
in two earlier publications.31,32 Applying this procedure to spectra a and c of Figure 4 indicates that 80% of the silanols of the blank MCM-48 (spectrum a of Figure 4) are restored after calcination of the precursor (spectrum c of Figure 4). Therefore, the number of silanols that have disappeared is 20% of 1.5 mmol/g ) 0.3 mmol/g. The vanadium loading is 1.7 mmol/g, and thus, the ensemble size is 1.7/0.3 ) 5.6. This means that on average 5.6V centers are bonded to one silanol anchor. Van Hengstum et al.33 have calculated the cross-sectional area of a VO5/2 group to be 0.103 nm2. This would mean that only 20% of the geometrical MCM surface is covered with VOx species and provides an additional geometrical argument for the formation of linked tetrahedral VOx chains: the crosssectional area of a VO5/2 group is obviously too small to form bidentate or tridentate bondings with the silanols, having a density of only 0.9 OH/nm2. Figure 6 shows the pore-size distributions of the original MCM-48, the precursor, and the catalyst. The surface area and pore volume are listed in Table 2. When the MCM is loaded with the bulky VO(acac)2 complex, the porosity and surface area are drastically reduced, but after calcination and oxidation of the acac ligands, the surface area is restored to almost 80% of the original value. Although the pore-size distribution of the final catalyst has broadened at little, the fwhh is still smaller than 0.3 nm. The slight broadening of the PSD might be explained by the fact that this particular sample is only covered with a submonolayer of vanadium species. Therefore, uncovered patches will be altered with monomers and oligomers of tetrahedral VOx species, inducing a broadening of the overall pore-size distribution. Owing to the sharp PSD of the original MCM and the pore radius of almost 1.5 nm, pore-blocking effects can be neglected. Conclusions MCM-48 has been prepared by a novel synthesis route, using the [C18H37N+(CH3)2-(CH2)12-N+(CH3)2C18H37]‚2Br- (GEMINI 18-12-18) surfactant. The resulting support has a surface area of 1025 m2/g and a very narrow mesoporous pore distribution around a radius of 1.4 nm. VO(acac)2 adsorbs onto the MCM-48 support in a gas-phase deposition reaction by a combination of a ligand-exchange
590 J. Phys. Chem. B, Vol. 102, No. 3, 1998 mechanism (evolving one ligand as Hacac) and a hydrogenbond interaction between a surface silanol and the pseudo π system of the acac ring. The saturation of this reaction is not governed by the surface area but by the availability of silanols at the surface. After calcination of the adsorbed VO(acac)2 species (called the precursor), chemically bonded VOx species are grafted to the surface in a concentration of 1.7 mmol V/g (8.7 wt % V). The VOx species are present on the surface of the MCM-48 support as tetrahedral-coordinated oligomers (linked tetrahedra) with an average “ensemble size” of 5.6. The original surface area of 1025 m2/g is reduced to 800 m2/g by this treatment. Even at this high loading, still only 20% of the geometrical surface is covered with VOx species. For the precursor, a maximum of 60% of the geometrical surface was covered with VO(acac)2 species. Acknowledgment. P. Van Der Voort acknowledges the F.W.O. Vlaanderen (Fund for Scientific Research, Flanders, Belgium) for financial support. The authors thank Mrs. F. Quiroz for her aid in the experimental work. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C. U.S. Patent 5098684. (3) Huo, Q.; Leon, R.; Petroff, P. M.; Stucky, G. D. Science 1995, 268, 1324. (4) Huo, Q.; Margolese, D. E.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (5) Firouzi, A.; Atef, F.; Oertli, A. G.; Stucky, G. D. J. Am. Chem. Soc. 1997, 119, 3595. (6) Firouzi, A.; Kumar, D.; Bull, L. M.; Besier, T. Science 1995, 267, 1138. (7) Cheng, C. F.; Park, D. H.; Klinowski, J. J. Chem. Soc., Faraday Trans. 1997, 93, 193. (8) Monnier, A.; Schu¨th, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krisnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (9) Edler, K. J.; Reynolds, P. A.; White, J. W.; Cookson, D. J. Chem. Soc., Faraday Trans. 1997, 93, 199.
Van Der Voort et al. (10) Cheng, C. F.; Zhuo, W.; Park, D. H.; Klinowksi, J.; Hargreaves, M.; Gladder, L. F. J. Chem. Soc., Faraday Trans. 1997, 93, 359. (11) Rathousky, J.; Zukal, A.; Franke, O.; Schulz-Elkoff, G. J. Chem. Soc., Faraday Trans. 1994, 90, 2821. (12) Chen, C. Y.; Burkett, S. L.; Li, H. X.; Davis, M. E. Microporous Mater. 1993, 2, 27. (13) Morey, M.; Davidson, A.; Eckert, H.; Stucky, G. D. Chem. Mater. 1996, 8, 486. (14) Alfredsson, V.; Anderson, M. W. Chem. Mater. 1996, 8, 1141. (15) Mariani, P.; Luzatti, V.; Delacroix, H. J. Mol. Biol. 1988, 204, 135. (16) Van Der Voort, P.; Possemiers, K.; Vansant, E. F. J. Chem. Soc., Faraday Trans. 1996, 92, 843. (17) Van Der Voort, P.; Babitch, I. V.; Grobet, P. J.; Verberckmoes, A. A.; Vansant, E. F. J. Chem. Soc., Faraday Trans. 1996, 92, 3635. (18) Van Der Voort, P.; White, M. G.; Vansant, E. F. Interface Sci. 1997, 5, 179. (19) Van Der Voort, P.; White, M. G.; Mitchell, M. B.; Verberckmoes, A. A.; Vansant, E. F. Spectrochim. Acta, Part A, in press. (20) Haukka, S.; Lakomaa, E. L.; Suntola, T. Appl. Surf. Sci. 1994, 75, 220. (21) Haukka, S.; Lakomaa, E. L.; Suntola, T. Appl. Surf. Sci. 1994, 82, 548. (22) Babich, I. V.; Plyuto, Yu. V.; Van Der Voort, P.; Vansant, E. F. J. Colloid Interface Sci. 1997, 189, 144. (23) Van Der Voort, P.; Geladi, P.; Swerts, J.; Vrancken, K. C.; Grobet, P. J.; Vansant, E. F. J. Chem. Soc., Faraday Trans. 1993, 89, 63. (24) Vogel, A. I. QuantitatiVe Inorganic Analysis; Longman Group Ltd.: London, 1971; p 790. (25) Barret, E. P.; Jouner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373. (26) Vartuli, J. C.; Schmitt, K. D.; Kresge, C. T.; Roth, W. J.; Leonowicz, M. E.; McCullen, S. B.; Hellring, S. D.; Beck, J. S.; Schlenker, J. L.; Olson, D. H.; Sheppart, E. W. Zeolites and Related Microporous Materials; Weitkamp, J., Karge, H. G., Pfeifer H., Ho¨lderich W., Eds.; Studies in Surface Science and Catalysis, 84; Elsevier Science: Amsterdam, 1994. (27) Vansant, E. F.; Van Der Voort, P.; Vrancken, K. C. Characterization and Chemical Modification of the Silica Surface; Studies in Surface Science and Catalysis 93; Elsevier Science: Amsterdam, 1995. (28) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 1465. (29) Van Der Voort, P.; White, M. G.; Vansant, E. F. Langmuir, in press. (30) Morey, M.; Van Der Voort, P. In preparation. (31) Van Der Voort, P.; Gillis-D’Hamers, I.; Vrancken, K. C.; Vansant, E. F.; J. Chem. Soc., Faraday Trans. 1991, 87, 3899. (32) Van Der Voort, P.; Gillis-D’Hamers, I.; Vansant, E. F. J. Chem. Soc., Faraday Trans. 1990, 86, 3751. (33) Van Hengstum, A. J.; Van Ommen, J. G.; Bosch, H.; Gellings, P. J.; Appl. Catal. 1983, 5, 207.
