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Chemical Modification of Metal Oxide Surfaces in Supercritical CO2: The Interaction of Supercritical CO2 with the Adsorbed Water Layer and the Surface Hydroxyl Groups of a Silica Surface C. P. Tripp*,† and J. R. Combes*,‡ Department of Chemistry and Laboratory for Surface Science and Technology, University of Maine, Orono, Maine 04469, and Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2L1 Received May 13, 1998. In Final Form: October 24, 1998 Infrared spectroscopy was used to probe the interaction of CO2 under supercritical fluid (SCF) conditions with a fumed silica. The experimental difficulties associated with CO2 absorption of infrared light in the hydroxyl spectral region are overcome by using deuterated silica or by venting of the CO2 prior to obtaining a spectrum. It is shown that CO2 behaves quite differently from traditional nonaqueous solvents (i.e., carbon tetrachloride, toluene, and cyclohexane) with respect to interactions with the adsorbed layer of water on the surface. A dry silica easily extracts and adsorbs the residual water present in these nonaqueous solvents whereas, in contrast, a dry silica remains dry when placed in contact with the SCF CO2. Moreover, the CO2 extracts the adsorbed water from wet silica. The SCF solvent extracts more surface water into the fluid phase with increasing density, and repeated extraction cycles with SCF CO2 result in the removal of all water from the surface. The interaction of SCF CO2 with the hydroxyl groups was studied using deuterated silica. Using a dry or wet deuterated silica, the physisorption of CO2 with the isolated SiOD groups is shown to be weak in nature and of the same magnitude as that measured in CCl4. The SiOD band at 2762 cm-1 is completely shifted to 2710 cm-1 at relatively low pressures of CO2 (5 bar) and remains shifted with increasing amounts of CO2 up to the highest pressures studied (200 bar).
Introduction Supercritical fluids (SCFs) have a long history as effective solvent media for polymer modifications and analytical studies.1-4 It has also recently been shown that the chemical surface modification of metal oxide particles in supercritical CO2 offers advantages over conventional solution methods.5 Little or no solvent waste results with treatment in SCF CO2, and the solvent, reagent, and reaction byproducts are easily separated from the treated oxide product by venting. Furthermore, treatment in CO2 does not lead to “caking” of the powder.5 Obvious environmental and economic advantages would result in using a solvent that separates spontaneously and so completely from the desired product that traditional filtration and grinding operations are obviated. Clearly an understanding of the differences and advantages of SCF CO2 as a solvent for surface reagents in lieu of traditional nonaqueous liquid solvents in the chemical modification of oxide surfaces would aid development of this technology. This information is also important in areas such as supercritical fluid chromatography6-8 where silica gel is used as the stationary phase. Infrared spectroscopy is the most widely used technique † ‡
University of Maine. Xerox Research Centre of Canada.
(1) Condo, P. D.; Paul, D. R.; Johnston, K. P. Macromolecules 1994, 27, 365. (2) Condo, P. D.; Johnston, K. P. Macromolecules 1992, 25, 6730. (3) Jobling, M.; Howdle, S. M.; Poliakoff, M. J. Chem. Soc., Chem. Commun. 1990, 1762. (4) Howdle, S. M.; Healy, M. A.; Poliakoff, M. J. Am. Chem. Soc. 1990, 112, 4804. (5) Combes, J. R.; Mahabadi, H. K.; Tripp, C. P. U.S. Patent 5,725, 987, 10 Mar 1998. (6) Shafer, K. H.; Griffiths, P. R. Anal. Chem. 1983, 55, 1939. (7) Morin, P.; Pichard, H.; Richard, H.; Caude, M.; Rosset, R. J. Chromatogr. 1989, 464, 125. (8) Jordon, J. W.; Skelton, R. J.; Taylor, L. T. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1985, 30, 154.
for studying chemical modification on silica because of the ease of detecting the infrared bands of the surface hydroxyl groups9-11 and, more recently, low-frequency bands due to surface modes of adsorbed species.12,13 The development of in-situ infrared techniques has been instrumental to gaining an understanding of the surface chemistry in CO2 media. Recently Jin et al.14,15 reported infrared studies of the adsorption of several organic molecules (acetonitrile, tetrahydrofuran, diethyl ether, methanol, acetone, and triethylamine) onto silica from SCF CO2 solution. Although the hydroxyl spectral region is rendered opaque by CO2 absorption, the shift in frequency of the free SiOH band at 3747 cm-1 arising from the H-bonding with these organic molecules is sufficient (>300 cm-1) to be located in a transparent region of the spectrum. Their work showed a pressure dependence of the intensity of the H-bonded SiOH bands with a maximum occurring at low CO2 pressures. The decrease in intensity of the H-bonded SiOH band at high CO2 pressures was attributed to enhanced solubility of the organic molecules in the supercritical fluid and, second, to the competitive adsorption of CO2 solvent molecules onto the surface. While the work by Jin et al.14,15 shows how readily SCF solution and surface adsorption properties can be adjusted with relatively slight changes in pressure, it underlines the need to understand the (9) Kiselev, A. V.; Lygin, V. I. Infrared Spectra of Surface Compounds; John Wiley and Sons: New York, 1975. (10) Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Marcel Dekker: New York, 1967. (11) Morrow, B. A. In Spectroscopic Analysis of Heterogeneous Catalysts, Part A: Methods of Surface Analysis; Fierro, J. L. G., Ed.; Elsevier: Amsterdam, 1990. (12) Tripp, C. P.; Hair, M. L. Langmuir 1991, 7, 923. (13) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 1215. (14) Jin, D. W.; Nitta, T. J. Chem. Eng. Jpn. 1996, 29, 708. (15) Jin, D. W.; Onose, K.; Furukawa, H.; Nitta, T.; Ichimura, K. J. Chem. Eng. Jpn. 1996, 29, 139.
10.1021/la9805701 CCC: $15.00 © 1998 American Chemical Society Published on Web 12/04/1998
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Langmuir, Vol. 14, No. 26, 1998 7349
Figure 1. Schematic of experimental setup.
pressure dependent behavior of the interaction of CO2 with the surface groups on the silica and with the adsorbed water layer residing on the surface. Experimental Section Materials. The silica was Aerosil 380 obtained from Degussa A.G. and had a measured surface area (BET (N2)) of 375 m2 g-1. The silica (about 25 mg) was compacted into self-supporting 13 mm diameter disks using minimal pressure (5 bar). Carbon dioxide (Bone-dry) was obtained from Praxair and condensed as received into an ISCO Model 260D syringe pump with controller to generate pressures for the experiments. Spectroscopy. The transmission infrared cell used for experiments in supercritical CO2 was modeled after the design of Poliakoff et al.16 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 end caps using a Copps medium-viscosity amber epoxy. Each stainless steel end cap contained a 5 mm opening to allow for beam throughput. This particular cell design had a volume of 1.9 mL and allowed for safe operation over our operational pressure range of 10-4 to 200 bar. The cell containing the silica was heated to 50 °C and purged with dry air supplied from a Balston 76-60 air dryer. The amount of adsorbed water remaining on the silica before introduction of CO2 was approximately 0.5 H2O/nm2 of silica, as measured17 from the integrated intensity of the bending mode of water located at 1630 cm-1. Carbon dioxide was delivered to the cell through a series of three-way valves (High Pressure Equipment Co.) that is diagramed in Figure 1. Cell temperatures were maintained at 50 °C using an Omega CN4400 temperature controller with type K thermocouple monitoring. Deuteration of the silica (when required) was accomplished by passing a stream of D2O (Aldrich, 99%, as received)-saturated air over the silica disk. Deuteration was rapid and deemed complete by monitoring the disappearance of the SiOH bands (from 3747 to 3400 cm-1) and the appearance of SiOD bands (from 2762 to 2400 cm-1). Carbon dioxide was then slowly introduced from the syringe pump at a rate no higher than 0.2 bar/s to avoid breakage of the fragile silica disk. Spectra were recorded intermittently during this filling cycle (with all valves closed to avoid any hydraulic disturbance of the disk during spectroscopic measurement) as well as during the venting of the pressurized contents. In all cases, the spectra recorded were at equilibrium conditions. There were no spectral differences between a spectrum recorded immediately after reaching a given (16) Poliakoff, M.; Howdle, S. M.; Kazarian, S. G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1275. (17) Veregin, R. P. N.; Tripp, C. P.; McDougall, M. N. V.; Osmond, D. J. Imaging Sci. Technol. 1995, 39, 429.
Figure 2. Infrared spectra of (a) silica and (b) SCF CO2 at 25 °C and 200 bar recorded in the high-pressure infrared cell. Spectra are offset for clarity. pressure and a spectrum recorded 15 min later. Once the maximum pressure for the particular experiment was reached and a spectrum was recorded, the inlet valve was closed and an exit valve was slowly opened to, again, keep the rate of depressurization below 0.2 bar/s. This procedure of slowly filling with CO2 up to a maximum pressure and immersion of the silica and the subsequent venting of the contents constitutes one supercritical fluid extraction cycle. Infrared spectra were recorded on a Bomem 102 FTIR equipped with a CsI beam splitter and a DTGS detector at a resolution of 4 cm-1. Typical observation times were 5 min.
Results and Discussion Infrared Studies in SCF CO2. An important criterion for a spectroscopic study of metal oxide surfaces is that the solvent used for surface modification, in this case CO2, should be transparent in the spectral region of interest. Parts a and b of Figure 2 show the infrared spectra of silica and of SCF CO2 (25 °C, 200 bar), respectively. While CO2 is transparent over much of the infrared region (This transparency has been exploited to identify organic molecules in SCF chromatography6,7,18), it is opaque in the important spectral region containing the bands due to the surface hydroxyl groups of silica. Two techniques (18) Ikushima, Y.; Saito, N.; Hatakeda, K.; Ito, S.; Arai, M.; Arai, K. Ind. Eng. Chem. Res. 1992, 31, 568.
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Figure 3. Infrared spectra of silica (a) recorded in air and (b) after one SCF CO2 extraction cycle. (c) Difference spectrum b - a. Spectra are offset for clarity.
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Figure 4. Changes in the water bending mode on silica recorded (a) in air and (b-h) after addition of CO2 at increasing pressures (bar) of (b) 13, (c) 53, (d) 107, (e) 93, (f) 113, (g) 133, and (h) 200. The underlying silica bands in this region have been subtracted from the spectra.
are employed to circumvent this problem. In-situ studies of interactions with the free silanols (band at 3747 cm-1) can be inferred by deuterating the surface. The SiOD band is located at 2762 cm-1 and is in a transparent region for SCF CO2. However, a more simple and convenient approach to alleviate the opacity of CO2 in the hydroxyl region is to vent the solvent and record spectra at 1 atm or after a vacuum evacuation (10-4 bar). The ready solvent-product separation intrinsic to a solid dispersed in CO2 enables repeated extraction and solvent removal cycles with infrared analysis at each step. The venting of the CO2 is of particular advantage in studying chemisorbed products on oxides.19 Interaction with the Surface-Adsorbed Water. Parts a and b of Figure 3 are the infrared spectra of silica recorded in air (1 atm) prior to and following one extraction cycle with SCF CO2 (200 bar). This extraction was accomplished via slow pressurization up to 200 bar, followed by a slow depressurization via venting of the cell contents. The difference spectrum (Figure 3c) shows the reduction in water bands at 3400 and 1630 cm-1 accompanied by an increase in the number of free surface silanols (3747 cm-1). In addition, a weak band at 2338 cm-1 appears, which is due to residual adsorbed CO2 on the silica surface.14,15 These spectral differences in Figure 3c show that SCF CO2 extracts water from the surface of the silica, exposing additional isolated silanol groups in the process. This result is consistent with gas-phase evacuation of water from silica at ambient temperature.20 The first extraction cycle with SCF CO2 removed about 90% of the adsorbed water (as determined from the measured integrated absorbance of the 1630 cm-1 band). The second extraction cycle removed the remaining water, giving rise to a silica that was free of any adsorbed water. Additional SCF extraction cycles did not result in further spectral changes. In a separate experiment, the amount of water extracted from the silica was measured as a function of CO2 pressure (at a constant 50 °C) by monitoring the changes in the bending mode of adsorbed water at 1630 cm-1. The results are shown in Figure 4.21 The 1630 cm-1 peak decreases slightly in intensity at pressures below 60 bar. Above 60
bar, however, the 1630 cm-1 band decreases rapidly in intensity and is accompanied by the appearance of a new band at 1610 cm-1. The band at 1610 cm-1 is due to the bending mode of water dissolved in SCF CO2.22 Clearly, the reduction in intensity of the 1630 cm-1 band on silica accompanied by the appearance of the band at 1610 cm-1 in Figure 4 is due to adsorbed water extracted from silica and into the SCF CO2 solution. During the vent cycle, with the concomitant pressure reduction, the band at 1610 cm-1 decreases in intensity whereas the band due to adsorbed water at 1630 cm-1 does not change. This result shows that there is no readsorption of water on the surface during the vent cycle. Complete elimination of the 1610 cm-1 peak occurs at a system pressure of about 67 bar, near the critical pressure (74 bar) of CO2. This pressure lies in the region of maximum compressibility for the fluid solution, and this is the point at which the solubility of H2O in CO2 is rapidly diminishing. Below 67 bar the band for adsorbed water at 1630 cm-1 remains unchanged with further venting. The above experiment is one subset of several experiments in which SCF CO2 extraction cycles have been conducted using various commercial fumed silicas. The highest pressure employed, and hence the highest solution density for silica immersion, was 200 bar. For extraction cycles where the maximum pressure was 67 bar, there were very little change in the amount of adsorbed water. Water is extracted from the silica only with the use of higher pressures, and the amount extracted increases with increasing system density. Thus, adjusting the CO2 pressure offers a means to control the distribution of water in solution and adsorbed on the surface. This behavior with the adsorbed water layer is in contradistinction to that observed for the more traditional nonaqueous solvents. Dry silica extracts and adsorbs residual water present in nonaqueous solvents. For this reason, siliceous materials are often used as drying agents. As a result, performing reactions in nonaqueous solvents
(19) Tripp, C. P.; Combes, J. R. Manuscript in preparation. (20) Tripp, C. P.; Hair, M. L. Langmuir 1993, 9, 3523. (21) The band shown in Figure 4 is solely due to water. The underlying bands due to silica in the region containing the water bending mode (for example, see Figure 3b) have been removed from the spectra by subtracting a spectrum of dried silica.
(22) The assignment of the band at 1610 cm-1 was based on a separate control experiment in which spectra of CO2 seeded with water (no silica disk present) were recorded as a function of pressure. In this case, the bending mode of water dissolved in SCF CO2 appears at 1610 cm-1 and does not change in position or intensity as a function of the system pressure.
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without the presence of some level of adsorbed water on the surface is difficult.13 In water sensitive reactions, such as silation, the lack of reproducibility in surface modification has been attributed to the presence of residual water.23-25 The desiccation of silica by CO2 under SCF conditions opens potential synthetic methodologies not previously available. Interaction of CO2 with Surface Hydroxyl Groups. The versatility of SCF CO2 as a medium for performing surface reactions with silica depends, among other factors, on the interaction of the solvent molecules with the surface silanols. The above experiments on wet silica samples show that repeated extraction cycles remove the adsorbed water from the surface. Once all water was removed, further SCF CO2 extraction cycles did not produce changes in the infrared spectrum. Thus the CO2 is, at most, only physisorbed with the silanols on the surface of the silica. The relative strength of the physisorption of CO2 with the silanols can be estimated from the shift in the 3747 cm-1 peak upon immersion in CO2. For common organic solvents, this physisorption is weak and the shift in frequency is relatively small. For example, the 3747 cm-1 band is shifted to 3711 cm-1 in hexane,26 3690 cm-1 in carbon tetrachloride,25 and 3590 cm-1 in toluene.27 The strength of this physisorption is important, as the displacement of solvent molecules by the reactant molecule must first occur before physisorption or chemisorption of a surface-treating reagent. The importance of the strength of physisorption is demonstrated with the adsorption of polystyrene from the solvents carbon tetrachloride and toluene. Polystyrene will adsorb onto silica from carbon tetrachloride solution20 (the adsorption of the styrenemers shifts the silanol band to 3590 cm-1) but does not adsorb from toluene solution. As shown by Jin et al.,14,15 the strong H-bonding between organic molecules and silanols shifts the 3747 cm-1 band in excess of 300 cm-1 and can be observed directly in SCF CO2. However, direct measurement of the shift in the 3747 cm-1 peak due to CO2 interaction is not possible, as this interaction is relatively weak and, thus, the shifted peak lies in the spectral region obscured by CO2 bands. An estimate of the strength of the interaction of CO2 with the silanols can be obtained by using a deuterated silica. On a dry deuterated silica, the isolated SiOD band located at 2762 cm-1 shifts to 2710 cm-1 with exposure to CO2 vapor at 50 °C (see Figure 5). When a deuterated silica is dispersed in CCl4, the SiOD band is also shifted to 2710 cm-1. Thus, the physisorption of CO2 with the hydroxyl groups on silica is relatively weak and of the same magnitude as that of CCl4. In reaching this conclusion, we assume that the interactions of CO2 with the SiOD are representative of the interactions with SiOH groups. While measurement of the shift in the SiOH band is not possible in carbon dioxide, the SiOH band at 3747 cm-1 (which is located on the high-frequency edge of the strong CO2 band and therefore lies in a transparent region at low pressures of CO2) disappears to the same extent at low CO2 pressures (
