8172
Langmuir 2001, 17, 8172-8177
Deposition of Self-Assembled Monolayers in Mesoporous Silica from Supercritical Fluids Thomas S. Zemanian, Glen E. Fryxell,* Jun Liu, Shas Mattigod, James A. Franz, and Zimin Nie Pacific Northwest National Laboratories, Richland, Washington 99352 Received May 18, 2001. In Final Form: August 23, 2001 Deposition of silane-based self-assembled monolayers onto mesoporous ceramic substrates has been found to be substantially faster from supercritical fluids than from typical condensed phase solutions. In addition, the monolayers so prepared display higher silane population density (6.5 vs 4.6 silanes/nm2), fewer defects, and greater stability than those prepared via standard solution methods. A mechanistic rationale for the pressure-induced rate enhancement and reduced defect density is presented.
Introduction The ability to coat mesoporous materials1,2 with highquality functionalized monolayers is a powerful synthetic tool in the development of high surface area heterogeneous catalysts,3 environmental sorbents,4-8 sensors,9-11 and molecular recognition materials.12 We have been interested in using Mobil Catalytic Material-41 (MCM-41) as a foundation upon which to build high-capacity environmental sorbent materials called Self-Assembled Monolayers on Mesoporous Supports (SAMMS), for the selective removal of heavy metals,4,6 tetrahedral oxoanions,7 and actinides8 from groundwater and waste streams. In the course of these studies we have learned that achieving fully dense monolayer coverage requires the appropriate level of interfacial hydration (approximately 2 to 2.5 mol of water per mole of alkoxy monomer added) on the MCM41, as well as the ability to drive the siloxane condensation equilibria to eliminate monolayer defects.13 While this traditional solution-phase methodology is an effective * To whom correspondence should be addressed. P.O. Box 999 Richland, WA. Telephone: (509) 375-3856. Facsimile: (509) 3752186. E-mail:
[email protected]. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature, 1992, 359, 710-712. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834-10842. (3) Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950-2963. (4) Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner, K. Science 1997, 276, 923-926. (5) Mercier, L.; Pinnavaia, T. Envir. Sci. Technol. 1998, 32, 27492754. Mercier, L.; Pinnavaia, T. J. Chem. Mat. 2000, 12, 188-196. (6) Liu, J.; Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Gong, M. Adv. Mat. 1998, 10, 161-165. (7) Fryxell, G. E.; Liu, J.; Hauser, T. A.; Nie, Z.; Ferris, K. F.; Mattigod, S.; Gong, M.; Hallen, R. T. Chem. Mater. 1999, 11, 2148-2154. (8) Feng, X.; Rao, L.; Mohs, T. R.; Xu, J.; Xia, Y.; Fryxell, G. E.; Liu, J.; Raymond, K. N. Self-Assembled Monolayers on Mesoporous Silica, a Super Sponge for Actinides. Ceramic Transactions, Volume 93, Environmental Issues and Waste Management Technologies in the Ceramic and Nuclear Industries (IV); Marra, J. C.; Chandler, G. T., Ed.; 1999; pp 35-42. (9) Olson, D. H.; Stucky, G. D.; Vartuli, J. C.; Sensor Device Containing Mesoporous Crystalline Material, U.S. Patent No. 5,364,797, issued November 15, 1994. (10) Li, G. J.; Kawi, S. Talanta 1998, 45, 759-766. (11) Gimon-Kinsel, M. E.; Balkus, K. J., Jr. Stud. Surf. Sci. Catal. 1998, 117, 111-118. (12) “Molecular Assembly in Ordered Mesoporosity: A New Class of Highly Functional Nanoscale Materials” (an invited feature article, submitted to J. Phys. Chem. March 2000), Liu, J.; Shin, Y.; Wang, L. Q.; Nie, Z.; Chang, J. H.; Fryxell, G. E.; Samuels, W. D.; Exarhos, G. J. J. Phys. Chem., in press.
synthetic strategy, it is rather time-consuming, particularly in the final drying of the coated mesoporous product. Supercritical fluids (SCFs) have been used as low viscosity, low surface tension reaction media in which kinetic rate enhancements are frequently observed.14 The source of these kinetic enhancements varies from reaction to reaction, but they have been attributed to such phenomena as enhanced mass transfer, a change in local dielectric constant, or enhanced solute-solute interactions (or reactant aggregation).14 High pressure has also been found to increase the reaction rate of associative processes.15,16 High-pressure chemistry has been used in the synthesis of complex molecules where traditional thermal methods failed or damaged the products.17 The low viscosity and high diffusivity inherent to SCFs are ideally suited for rapid transport of reagents into a nanostructured ceramic phase. Siloxanes are known to be very soluble in supercritical carbon dioxide (SCCO2),18 making this a logical candidate for reaction medium for the high-pressure deposition of siloxane-based monolayers within a mesoporous framework. We have chosen mercaptopropyltrimethoxysilane (MPTMS) as our baseline model system in the development of novel synthetic methods since thiol-SAMMS have proven to be extremely effective in the sequestration of heavy metals,4-6,8,19 as well as providing key precursors to nanostructured materials with sulfonic acid functionality.19,20 This manuscript describes the use of SCFs as (13) Fryxell, G. E.; Liu, J. An invited contribution to Designing Surface Chemistry in Mesoporous Silica. Adsorption at Silica Surfaces; Papirer, E., Ed.; Marcel Dekker: New York, 2000, in press. (14) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. AIChE J. 1995, 41, 1723-1778. (15) For reviews of organic reactions under high pressures, see: Organic Synthesis at High Pressures; Matsumoto, K., Acheson, R. M., Eds.; Wiley: New York, 1991. Organic High-Pressure Chemistry; Le Noble, W. J., Ed.; Elsevier: Amsterdam, 1988. Van Eldik, R.; Asano, T.; le Noble, W. J. Chem. Rev. 1989, 89, 549-688. Isaacs, N. S. Tetrahedron 1991, 47, 8463-8497. (16) For an excellent summary of how high-pressure affects certain kinetic and thermodynamic reaction parameters, see: Diedrich, M. K.; Klarner, F. G. J. Am. Chem. Soc. 1998, 120, 6212-6218. Diedrich, M. K.; Hochstrate, D.; Klarner, F. G.; Zimny, B. Angew. Chem., Int. Ed. Engl. 1994, 33, 1079-1081. (17) For representative examples of the use of high-pressure in the synthesis of complex organic molecules, see: Back, T. G.; Gladstone, P. L.; Parvez, M. J. Org. Chem. 1996, 61, 3806-3814. Kotsuki, H.; Hayashida, K.; Shimanouchi, T.; Nishizawa, H. J. Org. Chem. 1996, 61, 984-990. Araki, Y.; Konoike, T. J. Org. Chem. 1997, 62, 5299-5309. (18) Fink, R.; Hancu, D.; Valentine, R.; Beckman, E. J. J. Phys. Chem. B 1999, 103, 6441-6444.
10.1021/la0107401 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/20/2001
Self-Assembled Monolayers in Mesoporous Silica
reaction media for the deposition of self-assembled monolayers within a mesoporous matrix. We present results demonstrating that the SCF methodology provides considerably enhanced deposition kinetics with MCM-41, higher quality monolayer coatings, lower defect densities, and greater stability toward alkalinity. Experimental Section BET. The N2 adsorption/desorption isotherms were obtained from a Quantachrome Brunauer-Emmett-Teller instrument (Autosorb-6). The Brunauer-Emmett-Teller (BET) surface area of the pure silica is 886 m2/g, and the BET surface of the functionalized silica is approximately half this value, depending upon the degree of coverage. The pore size is estimated from the desorption branch, using a standard Barrett-Joyner-Halenda (BJH) method. NMR. 29Si and 13C NMR spectra were determined at 19.944 and 25.2458 MHz, respectively using a Chemagnetics CMX-100 NMR quadruple channel spectrometer system. The probe was a 7-mm pencil-type probe with magic angle spinning at 4 kHz. Four-microsecond 90-degree pulses were utilized for both 29Si and 13C, with proton decoupling power at 62.5 kHz. 29Si spectra were acquired using the Freeman-Hill T1 (t1fh) pulse sequence with a short recovery time, 50 µs, to suppress acoustic ringing. Pulse delays of 60 s were employed. Samples were prepared by loading a spinner with a Teflon spacer followed by ca. 65 mg of sample, 15 mg of tetrakis(trimethylsilyl)silane (TTMS), followed by an additional 65 mg of sample and a sample end spacer. Chemical shifts were referenced to internal TTMS. For 29Si, acoustic transients were not always completely suppressed by the t1fh sequence. Distortions in the second to sixth data points in the FID were removed using back-projection methods. Determination of Surface Coverage. Two different methods were used to cross-check one another in the determination of surface coverage: solid-state 29Si NMR spectroscopy and gravimetric analysis. These two methods both depend on prior knowledge of the surface area of the starting material (which is readily available from BET analysis). The two methods of surface coverage determination are in good agreement for the monolayers deposited from toluene. However, wet SCCO2 tended to degrade the silica structure over time, causing inaccuracy in the surface area determination. This behavior is not yet fully understood. 29Si NMR Method. Samples of SAMMS were spiked with carefully weighed amounts of tetrakis(trimethylsilyl)silane (TTMS). The 29Si spectra were recorded using suitably long recycle times to permit quantitative determination of integrated silicon regions by comparison of the integral of the internal standard peak with the total silane integral. From the total number of moles of TTMS and the ratio of TTMS to the mass of SAMMS, the integral ratio revealed the total number of moles of surfacebound silane in the sample. Dividing the number of moles of surface-bound silane by the surface area of the original MCM-41 provides an estimate of the population density at the silica interface. Because of the signal-to-noise limitations of solid-state 29Si NMR spectroscopy, this method of estimating population density is felt to be accurate to within (15%. Deconvolution of the silane portion of the spectrum also allowed for the determination of silane speciation within the monolayer. Gravimetric Method. Knowledge of the mass of the initial MCM-41 sample provided information on the total surface area of that sample since the surface area-to-mass ratio was accurately known (886 m2/g in this case). Likewise, the change in mass from the starting silica to the final dried SAMMS can be correlated to the silane component added to the porous substrate. It is important to recognize that this analysis does not distinguish between monolayer siloxane and adventitious byproducts (e.g. (19) Burkett, S. L.; Sims, S. D.; Mann, S. Chem. Commun. 1996, 1367-1368. Fowler, C. E.; Burkett, S. L.; Mann, S. Chem. Commun. 1997, 1769-1770. Fowler, C. E.; Lebeau, B.; Mann, S. Chem. Commun. 1998, 1825-1826. (20) Jones, C. W.; Tsuji, K.; Davis, M. E. Microporous Mesoporous Mater. 1999, 33, 223-240. Jones, C. W.; Tsuji, K.; Davis, M. E. Nature 1998, 393, 52-54. Van Rhijn, W. M.; De Vos, D. E.; Sels, B. F.; Bossaert, W. D.; Jacobs, P. A. Chem. Commun. 1998, 317-318.
Langmuir, Vol. 17, No. 26, 2001 8173 residual polysiloxanes) trapped within the mesoporous matrix. It is therefore very important to cross-check these samples with NMR and BET to confirm that the pores are still open and not plugged with byproducts (this sort of contamination was not encountered with any of the samples described in this paper). In addition, it is important to realize that there are two components to this weight-gain term, the water added and the siloxane deposited. Since the exact surface silanol population varied from sample to sample, the fraction of the final product’s weight attributed to the interfacial water (i.e. the oxygen atoms in the siloxane anchor) was unknown. Therefore, the following approximation was adopted to eliminate the need for that information and simplify the analysis: the reaction was viewed as a direct condensation between the silica surface and a silane with the (admittedly unrealistic) silicone structure HO-Si(O)-(CH2)3SH, which has a “molecular weight” of 136.24 g/mol. Thus, the weight gain can be related to the number of moles of siloxane deposited, and normalization based on surface area provides an estimate of the population density at the silica interface. This method of estimating population density is thought to be accurate to within (10%. Preparation of MCM-41. Mesoporous silica of 60-65 Å was made as following procedure: Eighty grams of the C16H33(CH3)3NOH/Cl was combined with 1.65 g of sodium aluminate. Forty grams of a tetramethylammonium silicate solution and 10 g of HiSil were then added. While the gel was stirred at room temperature, the auxiliary organic mesitylene(MES) was added as the last ingredient. The gel was then loaded into an autoclave and heated to 105 °C with stirring. After 24 h of heating, the reaction was quenched with cold water, and the contents were removed. The resulting solid products were recovered and calcined at 540 °C for 10 h. BET analysis of the product revealed structure consisting primarily of 65 Å pores and affording 886 m2/g. Standard SAMMS Procedure. The following is representative of the standard SAMMS procedure: 5.0 g of MCM-41 (corresponding to 4430 m2 of surface area) was suspended in 150 mL of toluene in a three-necked 500-mL round-bottomed flask, which was fitted with a stopper, a rubber septum, and a reflux condenser, and maintained under a static dry nitrogen atmosphere. To this mixture was added 1.6 mL of water (90 mmol, which corresponds to approximately 1.2 × 1019 water molecules/ m2, or slightly more than twice the number of silanols present on a fully hydroxylated silica surface). The water addition caused immediate aggregation of the silica. The mixture was then stirred for 1-2 h at ambient temperature to allow the water to disperse throughout the mesoporous matrix, during which time it became a thick, homogeneous slurry. At this point, 8.5 mL (45 mmol) of mercaptopropyltrimethoxysilane (MPTMS) was added via syringe and the mixture taken to reflux for 6 h (addition of the silane caused an immediate reduction in the viscosity of the slurry). During the course of heating, the mixture turned from a milky off-white color to a translucent light brown. The mixture was allowed to cool to room temperature and the product collected by vacuum filtration. Washing with three 50-mL portions of 2-propanol caused the product to change color from light brown to off-white (because of the change in refractive index). The SAMMS were then air-dried for 3 days in a fume hood and then under vacuum until they reached a constant weight of 8.2 g (corresponding to a coverage of 3.2 silanes/nm2). As determined by NMR, the coverage is 4.64 silanes/nm2. The NMR spectrum of a typical sample is shown in Figure 3a. It was found that the quality of coverage could be improved by driving the condensation equilibria via removal of water. A similar procedure using 468.5 g of MCM-41 (900 m2/g) was suspended in 3.5 L of toluene and treated with 150 mL of water. This suspension was stirred for 2 h and then treated with 660 mL (3.5 mol) of MPTMS. The reaction mixture was taken to reflux for a total of 6 h. After the reflux period, the reflux condenser was replaced with a still-head, the heating was continued, and the methanol and water azeotrope were removed by distillation (the distillation was terminated when the head temperature rose to 110 °C). The mixture was cooled to room temperature and the product collected by vacuum filtration and washed copiously with 2-propanol. The damp product was air-dried for 2 days in a fume hood and then dried to constant weight of 943.0 g in a vacuum desiccator. This corresponds to a coverage of 4.97 silanes/nm2.
8174
Langmuir, Vol. 17, No. 26, 2001
Zemanian et al.
Figure 1. A schematic of the sample holder used to contain the MCM-41 during the deposition of MPTMS in the SCCO2 reaction medium.
Figure 2. A schematic of the apparatus used for the pressureenhanced deposition of MPTMS monolayers onto MCM-41 from SCCO2. Comparison of the integrated peaks of the NMR spectrum of the material (see Figure 3b) indicates a coverage of 5.7 silanes/nm2. These measurements agree within the limits of the precision of the two techniques. Repeating this procedure several times, we found that it was possible to reproducibly install approximately 4.6 silanes/nm2 in 0.5 kg to 1.0 kg batches of SAMMS. SCCO2 SAMMS Procedure and Apparatus. A 19.1-g sample of MCM-41 was stored in a 100% humidity chamber until it gained 4.2 g, at which point it was stored in a tightly capped plastic vessel. A 1.005-g sample of this hydrated silica (0.785 g silica + 0.220 g water) was placed in the sample holder shown in Figure 1 and 1.5 mL (8.0 mmol) of MPTMS was added. Attempts to add the monomer to the SCCO2 as a modifier rather than a saturant, did not yield as effective results. The stainless steel vessel and sample holder developed a coating of polymer over time, changing the amount of MPTMS available to the silica sample. The sample was then placed in the apparatus shown in Figure 2 and exposed to supercritical carbon dioxide (SCCO2) at 150 °C and 7500 psi for 30 min (other samples were exposed for the length of time described in the text, which varied from 5 min to 48 h).21 At the end of the deposition period, the SAMMS were washed with a 2-minute dynamic purge at approximately 10 mL/min liquid CO2 (i.e. there was a continuous flow of SCCO2 through the sample at a bulk flow rate of approximately 10 cm/ min), followed by a 2-minute static purge (i.e. the cell was closed and the sample soaked in SCCO2), followed by a second 2-min dynamic purge. At this point, the dry SAMMS were removed and characterized. The final weight of the dry sample was 1.831 g, corresponding to a surface coverage of 6.55 silanes/nm2. pH Stability Studies. Samples of bare MCM-41, toluene SAMMS, and SCCO2 SAMMS were taken up in a series of pH buffers and shaken for a period of 4 h. The solutions were then filtered and the supernatant subjected to ICP analysis to determine solution silicon concentration resulting from dissolution of the silica matrix (the results of these tests are summarized in Figure 6). (21) Because of the nature of our apparatus, and the time necessary to bring it up to pressure and subsequently vent pressure, we were unable to look at reaction times much shorter than 5 min. Hence we were unable to measure deposition kinetics.
Figure 3. 29Si NMR spectra of SAMMS prepared using traditional toluene-phase methodology. (a) Simple deposition from refluxing toluene. (b) A similar material, except that the water and alcohol byproducts have been removed via the toluene azeotrope (“azeotropically cured SAMMS”).
Results Thiol terminated monolayers have proven themselves to be highly effective at sequestering mercury and other soft heavy metals (such as Au, Ag, Cd, and Pb).4-6,8 Oxidation of the thiol terminus to the corresponding sulfonic acid provides additional value in the form of nanostructured size-selective acid catalysts.19,20 Therefore, we have chosen to adopt mercaptopropyltrimethoxysilane (MPTMS) as our baseline for methodology development. The traditional solution phase deposition of MPTMS, in which the hydrated MCM-41 is boiled in a toluene solution of excess silane for several hours, results in a population density of approximately 3.0-3.5 silanes/nm2 (see Figure 3 and the Experimental Section). The silane speciation of these materials is approximately 50-60% internal silanes, with the remainder being primarily terminal silanes, and a small fraction containing isolated silanes.4,6 Use of freshly prepared MCM-41 without the hydration step results in poor surface coverage (approximately 0.5 silanes/ nm2).4 By following the alkoxysilane deposition with an azeotropic removal of the methanol and water byproducts, this surface population density can be increased to approximately 4.6 silanes/nm2 (see Experimental Section). The silane speciation of these “azeotropically cured” samples was typically found to be in the range of 60% internal, 40% terminal, and little, if any, isolated silane. (See Figure 3b.) Overall, the optimized toluene-phase deposition takes approximately 10 h of laboratory manipulation time and several days drying time and provides decent surface
Self-Assembled Monolayers in Mesoporous Silica
Langmuir, Vol. 17, No. 26, 2001 8175
Figure 4. 29Si NMR spectra of SAMMS prepared in SCCO2 at 7500 psi and 150 °C as a function of time, showing the evolution of silane speciation. The disappearance of the isolated silanes and the increase in cross-linking are clearly visible.
Figure 5. Kinetics plot showing the speciation (obtained from the 29Si NMR spectra) as a function of time, demonstrating the pressure-induced cross-linking (results obtained at 2000 psi in CO2 and 7500 psi in CO2 and N2 at 150 °C).
coverage, but still leaves a significant defect density (in the form of dangling hydroxyls, i.e. terminal silanes). When the same reaction was carried out under conditions of high pressure (7500 psi), using SCCO2 as reaction solvent, several notable observations were readily apparent. The deposition chemistry is much more rapid (see Figures 4 and 5), resulting in complete surface coverage in less than 5 min.21 In addition, the surface population density is noticeably higher (6.5 vs 4.6 silanes/nm2) than the toluene solution phase deposition. Following the reaction by 29Si NMR as a function of time, a slow evolution
Figure 6. pH stability plot of bare MCM-41, toluene SAMMS, and SCCO2 SAMMS. Solution silicon concentration (resulting from dissolution of the silica matrix) after a 4-h contact with a series of pH buffers. The passivating effect of the SCCO2 monolayer is clearly evident from the data.
of silane speciation was apparent, with increasing crosslinking as a function of time (see Figures 4 and 5). This is the highest degree of surface coverage and silane crosslinking ever demonstrated for a siloxane-based monolayer. One might argue that the gravimetric measurement of the surface coverage is inflated by formation of mercaptopropylsilane polymer in the pores of the material. The NMR spectra of the samples do not indicate the presence of polymer, however. Moreover, as noted above, long-term exposure to the SCCO2 resulted in structural degradation
8176
Langmuir, Vol. 17, No. 26, 2001
Figure 7. Kinetics plot showing the hydration of the silica surface (obtained from the 29Si NMR spectra) as a function of time, revealing the slow hydration process (results obtained at 8000 psi and 100 °C).
of the mesoporous substrate, thereby introducing indeterminacy into the NMR coverage measurements. The speciation measurements are unaffected, however. In addition, the SCCO2 deposited monolayers were found to display significantly greater stability toward hydroxide ion than do those SAMMS made by the standard toluene procedure (see Figure 6). While these enhancements may be partially attributed to the higher temperatures afforded by the use of a pressurized system (150 °C vs a maximum of 110 °C in the depositions using toluene as the reaction medium), other workers have reported similar rate enhancements when using supercritical and liquid CO2 at more modest temperatures (70 °C).22 The calcined MCM-41 interface is known to be silanol depleted.23 It is possible that the observed rate enhancement of silane deposition might be attributed to a rapid increase in the surface population of surface silanols, resulting from a pressure-enhanced interfacial hydration process. To probe this possibility, control experiments examining siloxane bridge hydrolysis under these conditions were performed. While cleavage of surface siloxane bridges was indeed found to be facilitated slightly by SCCO2 at 7500 psi and 150 °C, the kinetics of SCCO2 hydration were found to be considerably slower than monolayer deposition (see Figure 7). Indeed, other work has shown that it is possible to remove bulk water from a silica surface by treatment with SCCO2.24 Clearly, the enhanced rate of monolayer deposition is not due to the rapid installation of a silanol interface as a result of facile hydration of the silica surface. Discussion SCFs have been used to prepare porous aerogel ceramics,25 to carry particles and metal clusters into macroporous substrates,26 and to functionalize the interior of zeolites.27 To the best of our knowledge, this report represents the (22) Chuntao, C.; Fadeev, A. Y.; McCarthy, T. J. Langmuir 2001, 17, 757-761. (23) Cauvel, A.; Brunel, D.; DiRenzo, F.; Garrone, E.; Fubini, B. Langmuir 1997, 13, 2773-2778. (24) Tripp, C. P.; Combes, J. R. Langmuir 1998, 14, 7348-7352. (25) Loy, D. A.; Russick, E. M.; Yamanaka, S. A.; Baugher, B. M. Chem. Mater. 1997, 9, 2264-2268. (26) Watkins, J. J.; Blackburn, J. M.; McCarthy, T. J. Chem. Mater. 1999, 11, 213-215. (27) Shin, Y.; Zemanian, T. S.; Fryxell, G. E.; Wang, L. Q.; Liu, J. Supercritical Processing of Functionalized Size Selective Microporous Materials. Microporous Mesoporous Mater. 2000, 37, 49-56.
Zemanian et al.
first use of SCFs to prepare fully dense, self-assembled monolayer coatings inside a mesoporous matrix. Recently reported n-octadecyldimethylsilyl monolayer depositions22 onto flat silica wafers, silica gels (60 Å and 300 Å pore sizes) show considerably lower coverages of 1-2 groups/ nm2. In our experience, performing the MPTMS deposition in SCCO2 results in substantially faster deposition of the silane than when the reaction is carried out in refluxing toluene. A reasonable explanation for the observed rate enhancement can be attributed to the fact that the SCCO2 deposition is carried out at significantly higher pressure than the ambient toluene deposition. An associative process will have a negative ∆vq and ∆v° (volume of activation and volume of reaction, respectively) associated with it, and increasing the reaction pressure will accelerate such a process, as well as drive any equilibrium toward the adduct (in this case, the monolayer-coated mesoporous ceramic).15,16 The formation of a silane-based self-assembled monolayer is the culmination of a complex series of equilibria: silane adsorption, hydrolysis, translocation, condensation, aggregation, and finalized with a series of several additional condensation processes. Each of these reactions is an associative process and is therefore expected to accelerate with increased reaction pressure. The observed rate enhancement might also be attributed to acid catalysis since carbonic acid would be present to some degree because of the CO2/water equilibrium (bulk water in contact with SCCO2 has a pH of approximately 3).28 The traditional toluene-phase hydrolysis/condensation chemistry of the monolayer deposition takes place in the acidic environment of a hydrated silica interface (the isoelectric point of hydrated silica is approximately 1.8).29 While comparing the exact acidities of these two systems is not possible at this time, it seems reasonable to postulate that their acidities are not significantly different. Supporting the conclusion that the observed rate enhancement is indeed a pressure effect and not due to acid catalysis is the observation that MPS monolayer deposition is almost equally facile from SCN2 as it is from SCCO2.30 (See Figure 5 and compare the speciation progress of the samples prepared in SCN2 with those prepared in SCCO2.) Thus, while general acid catalysis by carbonic acid cannot be ruled out entirely, it is thought to play, at best, a minimal role in the rate enhancement. MPTMS monolayers prepared in refluxing toluene are typically composed of approximately 3.2 silanes/nm2 to as much as 4.6 silanes/nm2 (see Experimental Section). Solidstate 29Si NMR reveals that as many as half of the silanes in these monolayers contain dangling hydroxyls. The increased population density observed in SCCO2 MPTMS depositions can be attributed to a significantly reduced defect density. In a siloxane-based monolayer, there are two primary classes of monolayer defects, pinholes and dangling hydroxyls. The higher surface population is a reflection of the filling in of pinhole defects at higher pressure. The increased cross-linking observed in the longer reaction time SCCO2 depositions is consistent with the ∆v arguments outlined above since dangling hydroxyls (which are readily monitored in the 29Si NMR) represent “bumps” and “wrinkles” in the morphology of the monolayer. By carrying out the deposition chemistry at elevated temperature and pressure, it is possible to “iron out” these wrinkles by facilitating cross-linking, thereby minimizing overall monolayer volume. A reduction in the number of (28) Toews, K. L.; Schroll, R. M.; Wai, C. M. Anal. Chem. 1995, 67, 4040-4043. (29) Iler, R. K. The Chemistry of Silica; Wiley-Interscience: New York, 1979; p 660. (30) Zemanian, T. S.; Fryxell, G. E. To be reported separately.
Self-Assembled Monolayers in Mesoporous Silica
dangling hydroxyls allows the silanes to pack in a somewhat more ordered fashion on the surface, thereby making room for more silanes on the surface. By combining these two defect reduction mechanisms, it is possible to attain higher population densities from SCCO2 than from toluene. The SCCO2 SAMMS have significantly better stability toward alkaline conditions than do those prepared in toluene. This enhanced stability is simply a reflection of the lower overall defect density. Defects provide access for hydroxide ion to penetrate to the sensitive siloxane anchor beneath the passivating hydrocarbon portion of the monolayer. Both pinholes and dangling hydroxyls provide vulnerability to hydroxide. By enhancing the degree of cross-linking, and minimizing the pinhole density, the stability of the resulting monolayer toward hydroxide ion is significantly enhanced. Perhaps the most intriguing observation is the slow evolution of silane speciation as a function of time when exposed to elevated pressure and temperature. This observation is also in accord with the ∆v arguments discussed above and suggests that it is possible for a dangling hydroxyl to “walk” its way through the monolayer until it encounters another, whereupon they undergo condensation, thereby annihilating both defects. A control experiment was performed in which the deposition was performed for 5 min and then the sample was purged to remove all reaction byproducts and then characterized. This sample was then re-subjected to the standard SCCO2 conditions (7500 psi and 150 °C) and the same siloxane curing was observed as with those samples that were subjected to the standard SCCO2 treatment. Given the lack of water present during the curing phase of this control experiment, this result also supports the conclusion that it is pressure and not general acid catalysis that is responsible for the enhanced degree of cross-linking in these monolayers.
Langmuir, Vol. 17, No. 26, 2001 8177
In addition to the faster deposition chemistry, higher surface population density, higher quality monolayer coverage, and greater monolayer stability, other benefits of this SCCO2 protocol include the elimination of the extensive drying period postdeposition and the avoidance of flammable solvents. The waste stream inherent to the toluene preparation is also eliminated since the only byproducts of the SCCO2 process are CO2 and methanol, which are easily separated and recycled. Conclusions The ability to install functionalized monolayers in MCM41 is very important to the preparation of high-quality, selective sorbent materials. Specifically, thiol-terminated SAMMS have proven to be highly effective at scavenging mercury and other soft heavy metals. Utilization of SCCO2 as the reaction solvent for the preparation of thiol-SAMMS accelerates the silane deposition reaction considerably. This rate increase is thought to arise from a pressure enhancement of the deposition process resulting from a negative ∆vq and ∆v°, but acid-catalysis may play a minor role (carbonic acid would be present as a result of the water/CO2 equilibrium). In addition, the SCCO2 deposition provides higher quality monolayer coverage than standard solution phase methods. The reduced defect density is consistent with the ∆v arguments suggested for the rate enhancements. Acknowledgment. This work was supported by the Environmental Managed Science Program in the Office of Science, U.S. Department of Energy, under contract DE-ACO6-76RLO-1830 with Battelle Memorial Institute, which operates the Pacific Northwest National Laboratory for DOE. LA0107401
