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Langmuir 1999, 15, 7870-7875
Chemical Modification of Metal Oxide Surfaces in Supercritical CO2: In Situ Infrared Studies of the Adsorption and Reaction of Organosilanes on Silica J. R. Combes,*,† L. D. White,‡ and C. P. Tripp*,‡ Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2L1, and Department of Chemistry and Laboratory for Surface Science and Technology, University of Maine, Orono, Maine 04469 Received April 23, 1999. In Final Form: July 13, 1999 Infrared spectroscopy was used to probe the reaction of organosilanes with a fumed silica immersed in supercritical fluid CO2 (SCF CO2). Venting of the CO2 solvent eliminates the experimental difficulties associated with solvent absorption of the infrared radiation and enables repeated surface treatment cycles without disturbance to the amount of silica in the beam. This stability is requisite for detecting infrared bands due to adsorbed species in the spectral region containing the strong metal oxide bulk modes. SCF CO2 has been shown to extract water from silica, and we now exploit this feature for the silane treatment of silica particles. This utility of CO2 as a solvent for the reaction of organosilanes with silica is demonstrated with hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (OTS). The HMDS reaction in SCF CO2 proceeds according to the conventional gas-phase process even though the ammonia generated as a byproduct reacts with the CO2 to produce ammonium carbamate. The volatile ammonium carbamate is weakly physisorbed on the surface hydroxyl groups and is easily removed with evacuation or by purging. Moreover, carbamate formation can be completely avoided by performing the reaction at relatively low CO2 pressures. Physisorption of OTS from SCF CO2 does occur via a weak interaction with the surface hydroxyl groups. Although a small amount of OTS is hydrolyzed by the residual water present in the SCF CO2 and adsorbs on the silica, the amount hydrolyzed is much lower than that found with the use of traditional nonaqueous solvents.
Introduction The use of surface-modified silica particles in many important industrial applications is well established.1 Organosilanes of the form RnSiX4-n (where R is an organo group and X is usually a chloro or alkoxy group) are the most common materials used to chemically modify the surface of the silica particles. The preferred method of synthesis is to pass a gaseous stream of an organosilane at high temperatures (>300 °C) over the silica.2 The Cl or alkoxy group (X) reacts with the surface hydroxyl group on the metal oxide leaving the organo group extending from the surface (Sis denotes a surface silicon atom).
SisOH + X4-nSiRn f SisOSiRnX3-n + HX
(1)
The choice of organo group usually determines the behavior of the treated oxide powder.3 However, this gas-phase treatment is limited to volatile organosilanes. A more common approach is to disperse the oxide powder into a nonaqueous liquid solution containing the organosilane at temperatures below 100 °C. At these lower solution temperatures, there is no direct reaction of an alkyltrichlorosilane with the surface hydroxyl groups.4,5 Water (present either adsorbed upon the surface or in trace amounts in the nonaqueous liquid solvent) is needed to convert the chloro or alkoxy orga† ‡
Xerox Research Centre of Canada. University of Maine.
(1) Iler, R. K. The Chemistry of Silica; John Wiley and Sons: New York, 1979. (2) Tripp, C. P.; Hair, M. L. Langmuir 1991, 7, 923. (3) Veregin, R. P. N.; Tripp, C. P.; McDougall, M. N. V.; Osmond, D. J. Imaging Sci. Technol. 1995, 39, 429. (4) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120. (5) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1961.
nosilane to an organosilanol which, in turn, reacts with surface silanol groups.6
RnSiCl4-n + (4 - n)H2O f RnSi(OH)4-n + (4 - n)HCl (2) SisOH + RnSi(OH)4-n f SisO-Si(OH)3-nRn + H2O (3) Variability in film quality is the main drawback in using organosilane-treated surfaces. This variability is attributed to the self-polymerization of the organosilane giving rise to an ill-defined, polymerized silane coating.7 The competition between a surface reaction and selfpolymerization is dictated by several factors including the nature of the organosilane,3,8 the choice of solvent,9 reaction temperature,10 and, in particular, the level of water adsorbed on the surface.11-15 In principle, a direct reaction of an organosilane with surface SisOH groups is possible at room temperature in the absence of water through alternate strategies involving amine-catalyzed or amino-functional silanes.16-18 How(6) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (7) Trau, M.; Murray, B. S.; Grant, K.; Grieser, F. J. Colloid Interface Sci. 1992, 148, 182. (8) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 149. (9) McGovern, M. E.; Kallury, K. M. R.; Thompson, M. Langmuir 1994, 10, 3607. (10) Brzoska, J. B.; Shahidzadeh, N.; Rondelez, R. Nature 1992, 360, 719. (11) Parikh, A. N.; Allara, D. L.; Rondelez, F. Nature 1994. (12) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236. (13) Silberzan, P.; Le´ger, L.; Ausserre´, D.; Benattar, J. J. Langmuir 1991, 7, 1647. (14) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 1215. (15) Le Grange, J. D.; Markham, J. L.; Kurkjian, C. R. Langmuir 1993, 9, 1749. (16) Hertl, W.; Hair, M. L. J. Phys. Chem. 1971, 75, 2181.
10.1021/la990495+ CCC: $18.00 © 1999 American Chemical Society Published on Web 08/11/1999
Chemical Modification of Metal Oxide Surfaces
ever, in practice, performing these reactions in the complete absence of water is extremely difficult. Removal of adsorbed water from silica is straightforward and easily accomplished by evacuation of the silica at room temperature.19 However, when this dried silica is exposed again to atmospheric conditions, water vapor immediately readsorbs. Similarly, when a dried silica is immersed in conventional nonaqueous solutions, the silica (being a drying agent) readily adsorbs any adventitious water found in the solvent.14 Silica’s sensitivity to moisture leads to a day-to-day variance in the amount of water adsorbed and, hence, to a variable rate of hydrolysis of silane products on the surface.15 Most surface treatment regimens start with a fully hydrated silica in order to minimize this day-to-day variance in the level of adsorbed water.11,12,15 The water level on a saturated surface is less susceptible to drastic changes than when using a procedure that attempts to preclude any water from the experiment. Nevertheless, a reproducible level of water only ensures a reproducible level of the ill-defined surface polymerized layer. It has been recently shown that the chemical modification of metal oxide particles with organosilanes in supercritical carbon dioxide (SCF CO2) offers advantages over conventional solution methods.20 CO2-based processes are relatively environmentally friendly (no or minimal solvent waste) and can be used with a diverse range of organosilane reagents. Furthermore, the solvent and reagent are easily removed from the treated oxide product by venting. Conventional technologies employ filtration methods, which are tedious and expensive. Furthermore, solvent removal in CO2 systems does not lead to “caking” of the powder product, and hence, the product remains amenable to conventional solids handling methods such as air entrainment. More important, it has been shown that SCF CO2 behaves differently from traditional nonaqueous solvents with respect to the adsorbed water on the surface of silica.21 A dry silica extracts and adsorbs the trace amounts of water present in nonaqueous solvents, whereas in SCF CO2 the opposite occurs. SCF CO2 extracts water from silica, giving rise to a silica surface that is completely dry. Thus, the use of SCF CO2 as a solvent leads to the possibility of eliminating adsorbed water while conducting silane reactions on silica. This report focuses on the synthesis of alkylsilanetreated silica immersed in SCF CO2 and examines the potential use of this novel solvent medium as a viable alternative to conventional liquid solution methodologies for metal oxide surface treatment. Specifically, comparisons between conventional surface treatment regimens in the gas phase and in liquid solution with those in SCF CO2 are undertaken using volatile and nonvolatile organosilane reagents. Infrared spectroscopy is used to assess the quality of the samples prepared from SCF CO2 solution with respect to samples obtained using the conventional methodologies. Experimental Section The silica was Aerosil A380 from DeGussa AG and had a measured surface area (BET N2) of 375 m2 g-1. Hexamethyldisilazane (HMDS), octadecyltrichlorosilane (OTS), urea, and (17) Tripp, C. P.; Hair, M. L. J. Phys. Chem. 1993, 97, 5693. (18) Tripp, C. P.; Kazmaier, P.; Hair, M. L. Langmuir 1996, 12, 6407. (19) Morrow, B. A. In Spectroscopic Analysis of Heterogeneous Catalysts, Part A: Methods of Surface Analysis; Fierro, J. L. G., Ed.; Elsevier: Amsterdam, 1990. (20) Combes, J. R.; Mahabadi, H. K.; Tripp, C. P. US Patent 5725987, 10 mar 1998. (21) Tripp, C. P.; Combes, J. R. Langmuir 1998, 14, 7348.
Langmuir, Vol. 15, No. 22, 1999 7871 ammonium carbamate (all from Aldrich) were used as received. The experiments using CCl4 as a solvent are detailed elsewhere.4,14 Carbon dioxide (Bone-dry) was obtained from Praxair and condensed as received into an ISCO model 260D motorized syringe pump. The transmission infrared cell used for our experiments was modeled after the design of Poliakoff et al.22 and enabled in situ spectral characterization of silica over the entire pressure range of our experiments (from 10-4 bar up to 200 bar). The silica disks were mounted in a holder and placed inside the cell. Zinc selenide infrared windows (15 mm diameter by 10 mm thick, Spectral Systems, Inc.) provided a lower spectral cutoff of 650 cm-1. The windows were sealed into the endcaps using a Copps medium viscosity amber epoxy. Each stainless steel endcap contained a 5 mm opening to allow for beam throughput. This particular cell design has a volume of 1.9 mL. For recording infrared spectra of silica in the 4000-1300 cm-1 region, the silica (20-25 mg/cm2) was compacted into selfsupporting disks using minimal pressure (50 psi). This pellet was then mounted in a holder and inserted into the cell. The cell containing the silica was heated to 50 °C and purged with dry air supplied from a Balston 76-60 air-dryer. Typically, the amount of adsorbed water remaining on the silica before introduction of CO2 was 0.5 H2O/nm2 of silica, as measured from the integrated intensity of the bending mode of water located at 1630 cm-1.3 A thin film technique was used for recording spectra below 1300 cm-1. In this case, a small amount (ca. 1 mg) was smeared onto a KBr pellet and then mounted into the cell in a manner similar to the pressed disk. A detailed procedure for preparing thin silica films is described elsewhere.2 The cell with the assembly of valves employed to deliver the carbon dioxide and reagent to the silica is diagrammed in Figure 1. This setup is similar to that employed previously21 with a notable change in the reagent delivery system. In our previous setup,21 delivery of the reagent was through a six-port Valco GC switching valve with a 50 µL sample loop. However, the hydraulic shock caused by exposing thin film disks directly to undissolved 50 µL “plugs” of liquid produced small changes in the amount of silica probed by the IR beam. As discussed previously,2 these minute sample changes prevent spectral analysis in the low wavenumber region (below 1300 cm-1). In the present setup the Valco switching valve has been replaced with a 10 mL mixing cell. The mixing cell allows for effective mixing and dissolution of the added silane into SCF CO2 prior to contacting the silica. Typically 50 µL of silane reagent was injected via a syringe into the mixing cell (under a dry air purge) before pressurization. This modification ensured delivery of a homogeneous SCF solution to the silica. Cell and disk temperatures were maintained at 50 °C with the aid of an Omega CN4400 controller with type K thermocouple monitoring. Removal of the adsorbed water on the silica was then accomplished by pressurization of the cell via a slow addition of CO2 (
