2188
Langmuir 2003, 19, 2188-2192
Catalytic Hydrolysis of 4-Nitrophenyl Phosphate by Lanthanum(III)-Hectorite Steven T. Frey,*,† Benjamin M. Hutchins,† Brian J. Anderson,† Teresa K. Schreiber,† and Michael E. Hagerman‡ Department of Chemistry and Physics, Skidmore College, Saratoga Springs, New York 12866 and Department of Chemistry, Union College, Schenectady, New York 12308 Received August 5, 2002. In Final Form: November 21, 2002 The hydrolysis of 4-nitrophenyl phosphate (NPP), to produce 4-nitrophenol and inorganic phosphate, is catalyzed by an aqueous suspension of La3+ ion-bound hectorite clay (La3+-hectorite) at 50 °C. Sodium exchanged hectorite (Na+-hectorite) does not promote the hydrolysis of NPP under the same reaction conditions suggesting that La3+ ions are actively involved in the catalysis. Initial rates of the reaction are dependent upon the amount of clay present but independent of NPP concentrations in the range of 0.5-2.5 mM, indicating that these conditions produce a steady state. Reactions run over an extended period of time demonstrate that the rate drops off dramatically after a period of about 5 h, suggesting inhibition of the reaction by phosphate that is produced during hydrolysis. This inhibition is confirmed by experiments in which phosphate ion is present at the start of the reaction. Extended reaction times also indicate the catalytic nature of the reaction whereby 1.5 turnovers of substrate (relative to the theoretical number of moles of La3+ ions present) are accomplished after 9 days. A bell-shaped plot of pH versus initial rate demonstrates that the reaction is pH dependent. Initial rates rise from a pH of 5.5 to a maximum at pH 7.5 and then drop off again at pH values greater than 8.0. This plot fits to a theoretical model in which two La3+-bound water molecules are sequentially deprotonated, the first leading to the active catalyst, and the second producing an inactive species. A catalytic mechanism for the reaction is presented (for pH 7.5), on the basis of experimental results presented herein, in which hectorite-bound La3+ ions play an active role.
Introduction Numerous synthetic metal complexes have been studied as homogeneous reagents that promote the hydrolysis of amides, esters, nitriles, and phosphate ester bonds.1 This stems from the fact that metal ions are effective in facilitating hydrolysis by Lewis acid activation of substrate, providing a bound nucleophile, stabilizing the transition state, and/or assisting in departure of a leaving group (see Figure 1).1e,g Interest in these homogeneous systems is derived from their relationship to hydrolase enzymes and their potential use in biotechnological applications.1c,e,f Recently, attention has been focused on heterogeneous systems that facilitate the hydrolysis of environmental toxins. Applications of these materials include the detoxification2 and chemical detection1l of nerve gases. These reports support the need to identify and study other solid-state, metal-binding platforms as reagents for hydrolysis. New catalysts must be able to bind metal ions tightly yet allow them to retain vacant or solventcoordinated sites, provide locations for substrate and metal to interact, and be easily separated from a reaction mixture. * To whom correspondence should be addressed. † Skidmore College. ‡ Union College. (1) See, for example: (a) Frey, S. T.; Sun, H. J.; Murthy, N. N.; Karlin, K. D. Inorg. Chim. Acta 1996, 242, 329. (b) Frey, S. T.; Murthy, N. M.; Weintraub, S. T.; Thompson, L. K.; Karlin, K. D. Inorg. Chem. 1997, 36, 956. (c) Karlin, K. D. Science 1993, 261, 701. (d) Morrow, J. R.; Trogler, W. C. Inorg. Chem. 1988, 27, 3387. (e) Chin, J. Acc. Chem. Res. 1991, 24, 145. (f) Hegg, E. L.; Burstyn, J. N. Coord. Chem. Rev. 1998, 173, 133. (g) Tsubouchi, A.; Bruice, T. C. J. Am. Chem. Soc. 1995, 117, 7399. (h) Kimura, E. Prog. Inorg. Chem. 1994, 41, 443. (i) Chapman, W. H.; Breslow, R. J. Am. Chem. Soc. 1995, 117, 5462. (j) Gobel, M. W. Angew. Chem., Int. Ed. Engl. 1994, 33, 1141. (k) Kuo, L. Y.; Perera, N. M. Inorg. Chem. 2000, 39, 2103. (l) Sohn, H.; Letant, S.; Sailor, M. J.; Trogler, W. C. J. Am. Chem. Soc. 2000, 122, 5399. (2) LeJeune, K. E.; Wild, J. R.; Russell, A. J. Nature 1998, 395, 27.
Figure 1. Metal ion promotion of phosphate ester hydrolysis.
Because of their high surface area, crystallinity, and affinity for ions and organic molecules, layered or porous inorganic materials have been targeted as nanocomposite hosts,3a-c sequestering agents,3d mediums for molecular separation,3d,e chemical sensors,3d and catalysts.3b,f,g In addition to binding metal ions, clays possess high surface areas, are known to readily absorb and intercalate organic molecules, and in some cases form thin films3f that can be cast onto various other mediums such as glass. These characteristics make clay an attractive support for metal ions or metal complexes in the pursuit of potential heterogeneous catalysts.4 Indeed various clays have been shown to catalyze hydrolysis reactions, including those with pesticide substrates.5 In general, however, these studies have employed (3) (a) Vaia, R. A.; Hope, I.; Giannelis, E. P. Chem. Mater. 1993, 5, 1694. (b) Bergman, J. S.; Chen, H.; Giannelis, E. P.; Thomas, M. G.; Coates, G. W. Chem. Commun. 1999, 2179. (c) Carrado, K. A.; Xu, L. Chem. Mater. 1998, 10, 1440. (d) Mallouk, T. E.; Gavin, J. A. Acc. Chem. Res. 1998, 31, 209. (e) Moller, K.; Bein, T. Chem. Mater. 1993, 10, 2950. (f) Porter, T. L.; Eastman, M. P.; Hagerman, M. E.; Price, L. B.; Shand, R. F. J. Mol. Evol. 1998, 47, 373. (g) Pinnavaia, T. J. Science 1983, 220, 365. (4) Chibwe, M.; Ukrainczyk, L.; Boyd, S. A.; Pinnavai, T. J. J. Mol. Catal., A 1996, 113, 249.
10.1021/la020693s CCC: $25.00 © 2003 American Chemical Society Published on Web 02/04/2003
Catalytic Hydrolysis of 4-Nitrophenyl Phosphate
Langmuir, Vol. 19, No. 6, 2003 2189
a spectroscopic handle with which to follow the reaction. Herein, we report the catalytic hydrolysis of 4-nitrophenyl phosphate (NPP) by La3+ ion-exchanged hectorite and our initial findings with regard to the factors that control this reaction. Experimental Section
Figure 2. The structure of hectorite clay, Na0.66(Mg5.34Li0.66)(Si8O20)(OH)4.
harsh reaction conditions including high temperatures, long reaction times, acidic pH, and photolysis. Moreover, reactions have typically been carried out with substrateadsorbed or intercalated clays (solid-state reactions in the absence of a liquid phase) to simulate the interaction of pesticides with soil particles. Most importantly, lanthanide(III) (Ln3+) ion-exchanged clays have never been examined for their ability to promote hydrolysis reactions. This is significant in light of recent reports of Ln3+ ion complexes which display efficient, catalytic hydrolysis of phosphate esters.1g,6 Ln3+ ions are particularly attractive metals for hydrolysis since they are excellent Lewis acids and form geometrically flexible, labile complexes (promoting rapid catalytic turnover).7 With that in mind, we have begun to study lanthanum(III) (La3+) ion-exchanged hectorite clay (hereafter La3+-hectorite) as a medium for the metal ion-mediated hydrolysis of phosphate ester substrates. Hectorite clay, a smectite clay, is a 2:1 layer silicate in which two silicate, tetrahedral sheets sandwich an octahedral sheet of Mg2+ cations (see Figure 2). The interlayer charge of hectorite, approximately -0.33 per formula unit,8 results from the substitution of some Mg2+ ions by Li+ ions. This charge is balanced by the presence of Na+ ions, found naturally within the intergallery region of the clay. These sodium ions are readily exchangeable for other metal cations including transition and lanthanide metals. The intergallery cations are partially exposed on the surface of the clay, at step edges or faults, and may also be available to substrate through tiny micropore defects or substrate intercalation.3f For the present study, we have chosen the substrate 4-nitrophenyl phosphate (NPP) as a less toxic analogue of the phosphate ester family of pesticides and chemical warfare agents. This substrate produces the brightly colored 4-nitrophenolate ion upon hydrolysis, providing (5) See, for example: (a) Mingelgrin, U.; Saltzman, S.; Yaron, B. Soil Sci. Soc. Am. Proc. 1977, 41, 519. (b) Skipper, H. D.; Volk, V. V.; Mortland, M. M.; Raman, K. V. Weed Sci. 1978, 26, 46. (c) Fusi, P.; Ristori, G. G.; Franci, M. Clays Clay Miner. 1982, 30, 306. (d) Bastide, J.; Coste, C. M.; Sabadie, J. Weed Res. 1984, 24, 1. (e) Herrmann, M.; Kotzias, D.; Korte, F. Chemosphere 1985, 14, 3. (f) Nguyen, T. T. Clays Clay Miner. 1986, 34, 521. (g) Pusino, A.; Gessa, C.; Kozlowski, H. Pestic. Sci. 1988, 24, 1. (h) Rengasamy, S.; Parmar, B. S. J. Agric. Food Chem. 1989, 37, 430. (i) Camazano, M. S.; Martin, M. J. S. Soil Sci. 1983, 136, 89. (6) See, for example: (a) Takasaki, B. K.; Chin, J. J. Am. Chem. Soc. 1995, 117, 8582. (b) Schneider, H.-J.; Rammo, J.; Hettich, R. Angew. Chem., Int. Ed. Engl. 1993, 32, 1716. (c) Chappell, L. L.; Voss, D. A.; Horrocks, W. D., Jr.; Morrow, J. R. Inorg. Chem. 1998, 37, 3989. (d) Welch, J. T.; Sirish, M.; Lindstrom, K. M.; Franklin, S. J. Inorg. Chem. 2001, 40, 1982. (7) Moeller, T.; Martin, D. F.; Thompson, L. C. Chem. Rev. 1965, 65, 1. (8) Moore, D. M.; Reynolds, R. C., Jr. X-ray Diffraction and the Identification and Analysis of Clay Minerals; Oxford University Press: New York, 1997; p 156.
Materials. Hectorite clay, Na0.66(Mg5.34Li0.66)(Si8O20)(OH)4, was obtained from Rheox, Inc and used without further purification. The La3+ ion exchange capacity of this clay is 29 mmol/100 g clay. The chemicals used in this study were of reagent grade and were obtained from commercial sources. The water used was distilled and deionized. Preparation of La3+ Ion-Exchanged Hectorite. In a typical preparation, 500 mg of native Na+-hectorite was added to 50 mL of 1 M aqueous solution of lanthanum(III) chloride. The mixture was capped and allowed to stir at room temperature for 24 h. The clay was then separated from the solution by centrifugation and washed repeatedly with deionized water until the supernatant tested negative for the presence of chloride ion (by AgNO3 test). The clay was then stored as a mixture in deionized water to maintain maximum hydration. The concentration of clay was determined by stirring the mixture and transferring 1 mL aliquots into three preweighed vials, heating the mixtures to dryness in an oven at 140 °C for 24 h, and reweighing the vials to obtain an average dry weight of the clay. Typical concentrations were about 10 mg dry clay per 1 mL of solution. Reactions of La3+-Hectorite with NPP. For a typical reaction, 30-50 mL of 50 mM TRIS buffer at pH 8.0 was added to a large test tube along with a stir bar. An aliquot of the clay stock mixture was added to the buffer solution to give a final concentration in the range of 0.2-2 mg clay per mL of buffer. The tube was sealed with a rubber septum, vented with a syringe needle, and placed in a water bath on top of a stir plate. The mixture was then stirred and equilibrated at 50 °C. Following equilibration, an aliquot of substrate stock solution (10 mM NPP) was added to give a final concentration of substrate in the range of 0.05-5.0 mM. A 3-mL sample was immediately removed from the reaction mixture and centrifuged at 14 000 rpm for 2 min to separate the clay. The supernatant was filtered through a 0.2µm nylon syringe filter and placed in a cuvette. The absorbance of this sample at 400 nm, relative to the buffer solution, was recorded using an HP 8453 UV-vis spectrophotometer. This initial point was recorded to give a baseline value for absorbance of the samples at 400 nm. Thereafter, 3-mL samples were removed at various time points (typically every 10 min) and analyzed by the same procedure. Absorbance at 400 nm for each of the time points was corrected for background by subtracting the absorbance of the initial (time ) 0) sample. Absorbance values were then converted into concentration of 4-nitrophenolate ion using an extinction coefficient of 18 700 M-1cm-1 1d and correcting for the degree of ionization of 4-nitrophenol at pH 8.0 (determined experimentally to be 85%, see Dependence of the Reactions on pH below). Product Analysis. A reaction mixture was prepared with 30 mL of 50 mM TRIS, pH 8.0, 30 mg La3+-hectorite, and enough NPP to give a final concentration of 0.5 mM substrate. This mixture was stirred and heated at 50 °C for a period of 19 h. After centrifugation to separate the clay, the supernatant was acidified to pH 1.8 with 10% HCl and extracted four times with an equivalent amount of diethyl ether. The extract was dried over anhydrous sodium sulfate, filtered, and evaporated to dryness using a rotary evaporator. A small amount of the residue was taken up in methylene chloride for TLC analysis. This solution was spotted on an alumina plate along with solutions of 4-nitrophenol and NPP. The plate was eluted with 10% methanol in methylene chloride. A spot corresponding to 4-nitrophenol (Rf ) 0.62) was visualized for the reaction mixture with no other apparent products. A small amount of the residue from the reaction was also taken up in D2O and analyzed using a Varian Galaxy 200 MHz 1H NMR relative to 4-nitrophenol and NPP. Only 4-nitrophenol was observed in the spectrum (6.8 ppm (d) and 8.0 ppm (d)). Aside from solvent impurities, no other bands were observed in this spectrum. Inhibition Reactions. Reactions were carried out as described above in Reactions of La3+-Hectorite with NPP, except
2190
Langmuir, Vol. 19, No. 6, 2003
Frey et al.
Figure 3. Dependence of the initial rate of NPP hydrolysis on the amount of La3+-hectorite or Na+-hectorite present in the mixture. Reactions were run in 50 mM TRIS, pH 8.0, and 50 °C, with an NPP concentration of 1 mM.
Figure 4. Production of 4-nitrophenol versus time for the reaction of 1 mM NPP with 0.5 mg/mL La3+-hectorite in 50 mM TRIS, pH 8.0, and 50 °C.
Scheme 1. Hydrolysis of 4-nitrophenyl Phosphate by Hydroxide Ion to Produce Hydrogen Phosphate and 4-nitrophenol
Table 1. Initial Rates for the Hydrolysis of 4-Nitrophenyl Phosphate (NPP) by La3+ Ion-Exchanged Hectorite Clay in 50 MM TRIS, pH 8.0, and 50 °C
that the clay was allowed to incubate with various amounts of sodium phosphate for 10-15 min at 50 °C, prior to the addition of NPP. Reaction mixtures were prepared in 25-mL volumes with final concentrations of 0.5 mg/mL clay, 0-0.14 mM sodium phosphate. Time points were recorded every 10 min for 1 h using the procedure described above. Slopes of the plots of concentration of 4-nitrophenol versus time were determined to give the initial rates of the reaction in the presence of various amounts of sodium phosphate. Dependence of the Reactions on pH. Buffered solutions at 0.5 pH unit increments were prepared between pH 5.5 and 10.0 using 50 mM MES (pH 5.5-6.5), 50 mM HEPES (7.0 and 7.5), 50 mM TRIS (pH 8.0), and 50 mM CHES (pH 8.5-10.0). These buffer solutions were used to carry out reactions with NPP as described above in Reactions of La3+-Hectorite with NPP. Reaction mixtures at each pH were prepared with concentrations of 0.5 mg/mL clay and 1.0 mM NPP and were run for 1 h at 50 °C. The concentration of 4-nitrophenol produced at various time points was corrected for the degree of ionization of 4-nitrophenol at a given pH. The percent ionization at each pH was determined by recording the absorbance of a known concentration of 4-nitrophenol at 400 nm, determining an apparent extinction coefficient at each pH, and comparing this extinction coefficient to that of fully ionized 4-nitrophenol (18 700 M-1cm-1).1d Slopes of the plots of concentration of 4-nitrophenol versus time were determined to give initial rates for each reaction.
Results and Discussion Reaction of La3+-Hectorite with NPP. The phosphate ester substrate NPP is highly soluble in aqueous solution and produces 4-nitrophenol upon hydrolysis (see Scheme 1) which is readily detected by its strong absorbance at 400 nm as the 4-nitrophenolate ion (at pH 8.0, 4-nitrophenol is 85% ionized). Using visible absorption spectroscopy, one can readily quantitate the amount of 4-nitrophenolate that is produced during a reaction, and this value can be converted to the amount of 4-nitrophenol produced by adjusting for the degree of ionization at a given pH. Figure 3 shows the initial production of 4-nitrophenol (less than 5% consumption of NPP) versus time for reactions of varying amounts of La3+-hectorite with NPP,
clay, mg/mL
[NPP], M
initial rate, Ms-1
0.50 0.50 0.50 0.25 1.00
0.5 1.0 2.5 1.0 1.0
6.5 × 10-9 6.6 × 10-9 6.4 × 10-9 2.6 × 10-9 1.5 × 10-8
in aqueous solution at pH 8.0 and 50 °C. Product analysis by TLC and 1H NMR confirms that 4-nitrophenol is produced cleanly by these reactions. Initial rates for these reactions, given by the slopes of the plots, are presented in Table 1 along with the initial rates of reactions with varying substrate concentrations. These rates are comparable to those that have been reported previously for the hydrolysis of NPP by homogeneous catalysts. For example, the hydrolysis of 1 mM NPP by 5 mM [Cu(2,2′bipyridine)]2+ (pH 8.0 and 75 °C) proceeds with an initial rate of 1.5 × 10-8 Ms-1.1d Likewise, a series of 0.25 mM dimeric zinc(II) complexes catalyze the hydrolysis of 0.5 mM NPP (pH 8.36, 55 °C) with initial rates about 1 × 10-9 Ms-1.1i Clearly La3+-hectorite is an effective catalyst of NPP hydrolysis, under the conditions employed, while Na+-hectorite, plotted for the purpose of comparison in Figure 3, does not promote NPP hydrolysis in this time frame. This suggests that clay-bound La3+ ions have a specific function in the catalytic mechanism. The data also demonstrate that the rates of these reactions are dependent on the amount of clay present in the mixture. However, the initial rates are independent of the substrate concentrations employed as judged by reactions of 0.5 mg/mL La3+-hectorite with varying amounts of NPP (see Table 1). This indicates that catalytic sites are initially saturated with substrate and that binding occurs rapidly in comparison to hydrolysis at the start of the reaction. Figure 4 shows the production of 4-nitrophenol over an extended period of time for the reaction of 0.5 mg/mL La3+-hectorite with 1.0 mM NPP. The initial rate of this reaction is 6.6 × 10-9 Ms-1 (determined from points within the first hour of reaction when substrate is less than 5% reacted). The slope of the plot changes dramatically after approximately 5 h of reaction time. This behavior suggests that NPP is hydrolyzed rapidly at the start of the reaction, releasing 4-nitrophenol into the reaction solution where it is measured, but that phosphate remains bound to the catalytic sites. We have confirmed that phosphate does indeed inhibit the reaction (see Inhibition by Phosphate Ion below). Our presumption is that once the catalytic
Catalytic Hydrolysis of 4-Nitrophenyl Phosphate
Figure 5. Dependence of the initial rate of NPP hydrolysis on the amount of phosphate ion present in the reaction mixture. Reactions were run in 50 mM TRIS, pH 8.0, and 50 °C, with an NPP concentration of 1 mM and 0.5 mg/mL La3+-hectorite present.
sites are saturated with phosphate, the reaction proceeds at a much slower rate that is controlled by the slow release of this product. Similar kinetics have been observed for the hydrolysis of 4-nitrophenyl actetate by the enzyme chymotrypsin whereby an initial burst phase is followed by a slower rate of reaction, dictated by the release of acetate ion.9 The catalytic nature of the reaction is also confirmed by the data in Figure 4. The initial turnover rate is 0.17 h-1 on the basis of the theoretical number of moles of La3+ ions present in the clay (29 mmol/100 g clay, 3.93% by mass). Since the rate decreases after 5 h, one turnover takes approximately 48 h under these conditions, and the reaction produces 1.5 turnovers after approximately 9 days (218 h). These turnover estimates are conservative since it is very likely that only a fraction of the La3+ ions bound to the clay are available to substrate (i.e., provide a site for catalysis). These ions are expected to locate themselves in the intergallery spaces of the clay and, because of the negative charge of the surface of this region, it is unlikely that NPP (negatively charged at pH 8.0) would access these spaces. Hence, reactions probably take place on the surface of the clay where La3+ ions have been exposed by step edges, faults, or micropore defects in the clay.3f Inhibition by Phosphate Ion. To confirm that phosphate ion remains bound to the catalytic sites after hydrolysis, we have carried out reactions in which the clay is incubated with various amounts of phosphate ion prior to the addition of substrate. Figure 5 shows plots of the production of 4-nitrophenol versus time as a function of initial phosphate ion concentration. Phosphate ion concentrations were chosen to correspond to 0.25, 0.5, 0.75, and 1 molar equivalent additions with respect to the amount of La3+ ions present in the reaction mixture. These plots clearly indicate that phosphate ion inhibits the reaction with a one molar equivalent addition of phosphate nearly shutting down the reaction completely. The fact that phosphate ion inhibits the reaction is not surprising since lanthanide ions bind negatively charged oxygen donating ligands in aqueous solution, particularly those that can bind in a bidentate fashion.7 Dependence of the Reaction Rate on pH. The initial rates of a series of reactions conducted with 0.5 mg/mL La3+-hectorite and 1mM NPP demonstrate a strong dependence on pH (see Figure 6). The plot of initial rate (9) Nelson, D. L.; Cox, M. M. Lehninger Principles of Biochemistry; Worth Publishers: New York, 2000; p 275.
Langmuir, Vol. 19, No. 6, 2003 2191
Figure 6. Dependence of the initial rate of NPP hydrolysis on pH. Reactions were run in 50 mM MES (pH 5.5, 6.0, and 6.5), HEPES (pH 7.0 and 7.5), TRIS (pH 8.0 and 8.5), or CHES (pH 9.0 and 9.5) at 50 °C, with an NPP concentration of 1 mM and 0.5 mg/mL La3+-hectorite present. The solid line represents a least-squares fit of the data using eq 1 where k ) 9.5 × 10-9 M/s, K1 ) 1.0 × 10-6, and K2 ) 1.0 × 10-9.
versus pH in Figure 6 rises from pH 5.5 to a maximum at about pH 7.5 and then falls again at values above pH 8.0. The bell-shaped profile of this pH versus rate curve has been observed previously in the hydrolysis of phosphate esters by numerous other metallic systems1g,1i,10 and provides evidence for the participation of the La3+ ions in hydrolysis. The La3+ ions, which commonly have coordination numbers of 8 or 9, undoubtedly bind water molecules in addition to coordinating atoms that are provided by the clay. The rise and fall in the curve is consistent with the sequential titration of La3+ ion-bound water molecules. Our presumption is that the initial rise in the curve corresponds to the deprotonation of one water molecule to produce a bound hydroxide ion that acts as the nucleophile for the reaction. The fall in the curve probably represents the deprotonation of a second bound water molecule, producing a second, tightly bound hydroxide ion that prevents or disrupts the coordination of substrate (i.e., substitution of a bound water molecule by the substrate). We have fit the data in this curve to the following equation
kobsd )
k K2 [H ] 1+ + K1 [H +]
(
+
)
(1)
where kobsd is the experimentally determined initial rate for a particular pH value, k is the theoretical initial rate for the reaction catalyzed by the active catalytic species, and K1 and K2 are acid dissociation constants. This equation is based on a model in which the active catalytic species is generated by a single deprotonation step and is consumed by a second. A least-squares fit of the data to this equation, shown in Figure 6, provides pK1 and pK2 values of 6 and 9, respectively. Active catalysis occurs in a pH range where NPP is expected to be fully deprotonated.11 Therefore, the behavior must be attributed to the catalyst. Deprotonation of La3+ bound water molecules is likely in this pH range according to the pKa values of small La3+ complexes.1g (10) See, for example: (a) Young, M. J.; Chin, J. J. Am. Chem. Soc. 1995, 117, 10577. (b) Molenveld, P.; Engbersen, J. F. J.; Kooijman, H.; Spek, A. L.; Reinhoudt, D. N. J. Am. Chem. Soc. 1998, 120, 6726. (11) Sturtevant, J. M. J. Am. Chem. Soc. 1955, 77, 255.
2192
Langmuir, Vol. 19, No. 6, 2003
Frey et al.
Scheme 2. Proposed Catalytic Mechanism for the Hydrolysis of 4-nitrophenyl Phosphate by La3+-hectorite Clay. The Solid Spheres Represent La3+ Ions Bound to the Intergallery Space of the Clay
Conclusions On the basis of the above observations, we suggest a catalytic mechanism (at pH values around 7.5) in which hectorite-bound La3+ ions, exposed or partially exposed to the surface of the clay particles, are initially coordinated to a hydroxide ion and at least one water molecule (see Scheme 2). In the first step, substrate comes in and binds to a La3+ ion, displacing a water molecule. The Lewis acidity of the metal activates the substrate toward nucleophilic attack and brings it in close proximity to the hydroxide ion. Hydrolysis then takes place and 4-nitrophenolate ion is released into the solution while phosphate ion remains bound to the La3+ ion. Slow release of the phosphate ion is then followed by regeneration of the active catalyst. We are currently probing further aspects of this mechanism including the location of catalytically active La3+ ions within the clay framework and the interaction of La3+ ions with substrate and phosphate ions. For
example, we are investigating Laponite, a synthetic hectorite with a smaller aspect ratio, that provides increased surface area and therefore a greater availability of edge sites. In addition to this clay, we will also examine clays of varying cation exchange capacity. We also intend to study clays bound with other lanthanide ions, including luminescent Ce3+ and Eu3+, that will provide a direct measure of the interaction of the metal ions with chemical species involved in the reaction mechanism. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the ACS, for partial support of this research. S. T. F and B. M. H. also wish to acknowledge the Skidmore College Collaborative Research Program, funded in part by the W. M. Keck Foundation. M. E. H. would like to thank the Camille and Henry Dreyfus Foundation for their support. LA020693S
