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Ind. Eng. Chem. Res. 2002, 41, 2835-2841

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APPLIED CHEMISTRY Solvatochromic Characterization of Near-Critical Water as a Benign Reaction Medium Jie Lu,† James S. Brown,† Erica C. Boughner,† Charles L. Liotta,‡ and Charles A. Eckert*,† Schools of Chemical Engineering and Chemistry and Biochemistry, and Specialty Separations Center, Georgia Institute of Technology, Atlanta, Georgia 30332-0100

Near-critical water (liquid water between 200 and 374 °C) offers an environmentally benign alternative for the replacement of undesirable solvents and catalysts. This work characterizes the solvent strength of liquid water at saturation pressure from ambient temperature to 275 °C in terms of its Kamlet-Taft dipolarity/polarizability, hydrogen-bond-donating acidity, and hydrogen-bond-accepting basicity using in situ UV-vis spectroscopy. The results suggest that near-critical water exhibits a wide range of polarity and hydrogen-bond-donor ability for tailoring chemical reactions and separations. These Kamlet-Taft solvent parameters can be used to correlate kinetic properties for reactions in water. As model reactions, the temperature-dependent kinetics of the hydrolyses of two nitroaromatic compounds, 4-nitroaniline and N,N-dimethyl nitroaniline, were determined in NCW in the temperature range of 200-275 °C. The hydronium ion dissociated from water promotes the initial hydrolysis reaction without the addition of any acid. Solvent effects on the rate constant were correlated with Kamlet-Taft solvent parameters based on a linear solvation energy relationship (LSER). Introduction Near-critical water (NCW) at temperatures from 200 to 374 °C is an environmentally benign alternative to less desirable organic solvents and offers exciting opportunities for a variety of manufacturing processes spanning the chemical, petrochemical, pharmaceutical, and plastics industries. In contrast to supercritical water (Tc ) 374 °C, Pc ) 22.1 MPa), which is used primarily for degradation reactions, NCW is beneficial for a wider range of useful syntheses at substantially lower temperatures and pressures.1-3 The static dielectric constant () of liquid water decreases significantly as the temperature increases (e.g.,  ) 23.5 at 275 °C as compared to  ) 80.1 at 20 °C).4 As a result, NCW permits the simultaneous dissolution and reaction of both organic and ionic species.5,6 In contrast, at ordinary temperatures, these reaction systems are heterogeneous. The phase behaviors of numerous organic/water systems have been measured over wide temperature and pressure ranges, indicating that the mutual solubilities of organics and water are significantly enhanced under NCW conditions.7 For instance, the binary toluene/water system exhibits an upper critical solution temperature of 308 °C at 22 MPa, where the two components become totally miscible.7 After homogeneous aqueous/organic reaction is complete, the product mixture can be cooled, and the * Corresponding author. Tel.: 01-(404) 894-7070. Fax: 01(404) 894-9085. E-mail: [email protected]. † School of Chemical Engineering. ‡ School of Chemistry and Biochemistry.

organic products naturally separate from the aqueous phase, providing a facile downstream separation. More importantly, at temperatures of 200-300 °C, liquid water has a self-dissociation constant, Kw, 3 orders of magnitude greater than that of ambient water.8 This provides an increased source of hydronium and hydroxide ions, promoting acid/base-catalyzed reactions in a homogeneous reacting system. Because no added base or acid catalysts are needed, subsequent neutralization of the acid catalysts, which requires disposal of salt waste, can be avoided. Benefits also include elimination of the need for hazardous organic solvents, such as methylene chloride.9 A number of experiments have demonstrated that NCW can promote acid-catalyzed reactions.3,10-15 With little or no added mineral acids, Friedel-Crafts alkylations and acylations have been carried out in NCW.13-15 Friedel-Crafts acylation reactions are traditionally catalyzed by greater than stoichiometric equivalents of mineral acids or Lewis acids, such as H2SO4 and AlCl3,9 and the subsequent neutralization can give 5-10 lb of waste salt for each pound of product. For example, phenol and p-cresol have been alkylated with tert-butyl alcohol to synthesize a variety of sterically hindered phenols in NCW.13 Lesutis et al. also showed that substituted benzoic acid esters and anisole hydrolyses proceed without the addition of an acid or base catalyst.14 Additional advantages of NCW include enhanced kinetics and improved selectivity.16 Because of the tunable solvent properties of NCW, opportunities arise for better control of homogeneous reaction by manipulation of the temperature and pres-

10.1021/ie020160e CCC: $22.00 © 2002 American Chemical Society Published on Web 05/14/2002

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sure. In an effort to explore the thermodynamic and structural properties of water at elevated temperatures, researchers have applied a variety of experimental techniques ranging from visual phase behavior measurements7,17 to in situ spectroscopy (UV-vis,18,19 IR,20 Raman,20-22 NMR,23,24 X-ray absorption,6 and neutron scattering,25 etc.). Theoretical studies including molecular dynamics6,26-28 and computer simulations28,29 have also been considered important means for examining the microscopic structure of water up to the critical point, especially with regard to hydrogen-bonding interactions. The correlation of physicochemical processes in NCW is limited by the scarcity of available solvent parameters, which motivates our investigation. The KamletTaft solvent parameters [dipolarity/polarizability π*, hydrogen-bond-donating (HBD) acidity R, and hydrogenbond-accepting (HBA) basicity β] have been important indices for providing insight into the solvent strength of traditional liquid solvents and SCFs.30-39 The experimental measurements of π*, R, and β are based on the solvent-induced solvatochromism of certain spectroscopic indicators.40-44 Solvatochromism refers to the solvent-induced spectral shift of chromophores as a measure of solvent strength (e.g., polarity or hydrogen bonding). Moreover, these parameters can be considered molecular/microscopic properties because solvatochromism reflects the interaction of solvent molecules with the electronic transition of an indicator. The π*, R, and β values for more than 200 liquid solvents are available for convenient comparison.45,46 Linear solvation energy relationships (LSERs) are used to correlate π*, R, and β solvent parameters with a variety of configurational properties in solution, including solubility, partition coefficient, and reaction rate, among others. A vast number of publications on the applications of LSERs cover the range from conventional solutions to those under extreme conditions.47,48 One form for a generalized LSER is

where XYZ and (XYZ)0 denote the values of a solventdependent physicochemical property in a given solvent and in a reference solvent (gas or inert solvent), respectively. δ is a polarity correction parameter for aromatic (δ ) 1.0), polyhalogenated aliphatic (δ ) 0.5), and all other (δ ) 0.0) solvents. s, d, a, and b are solventindependent coefficients indicating the susceptibility of the property to the applied parameters. This multiparameter approach assumes additivity of three aspects of solvation: nonspecific van der Waals interactions, hydrogen-bond donation, and hydrogen-bond acceptance. We have previously reported preliminary results on the Kamlet-Taft solvent parameters of ambient to near-critical water.49 In this paper, we complete and apply the Kamlet-Taft parameters of liquid water at saturation pressure in the temperature range of 25275 °C. As a model reaction, the temperature-dependent kinetics of hydrolyses of two nitroaromatic compounds are determined in NCW and correlated with an LSER.

MPa. The path length of the cell was 10.5 mm, and the internal volume was 17.7 cm3 at room temperature. The path length change due to thermal expansion was calculated to be less than 0.1%. The cell was heated by heating cartridges connected to a temperature control unit (Omega Inc., CT), consisting of a microprocessor thermometer (model HH21) and a temperature controller (model CN76000). The temperature gradient throughout the cell was maintained to less than (1 °C in the temperature range of 25-275 °C. Nucleation of water vapor in the liquid region of the cell interfered with spectroscopic measurements, so the vapor headspace of the cell was held at a slightly higher temperature (the maximum temperature difference was less than 1 °C). Pressure was monitored by a pressure transducer and a pressure readout (Druck Inc., CT) with an uncertainty of 0.01% in the range of 0-20.7 MPa. A Teflon stir bar in the cell constantly stirred the solutions during the measurements. A Hewlett-Packard 8453 diode array UV-vis spectrophotometer (Agilent Technologies, Inc., CA; 1-nm resolution and (0.2-nm wavelength accuracy) was used for spectral measurements and processing. Products in the kinetic measurements were identified using GC-MS (EI mode) by comparison of the GC retention time and EI-MS results to those of commercially available standard compounds. Procedure. The cell was first filled with pure water using a syringe pump (Isco, Inc., NE, model 260D) and heated to the desired temperature. A sample loop of known volume (Valco Instruments Co. Inc., TX) was charged with a concentrated indicator solution by an HPLC pump (Eldex Lab., Inc., CA, model AA). After the cell temperature had equilibrated, the sample loop was flushed with a small amount of water to inject the indicator into the cell, immediately after which spectroscopic measurements were made to obtain the solvatochromic and kinetic information before the indicators decomposed completely. Although the material injected was at ambient temperature, the volume injected was sufficiently small to allow thermal equilibrium to be established in a few seconds. All sample spectra were corrected by subtraction of the spectrum of the pure solvent taken before indicator injection. The longest-wavelength absorbance bands attributed to the π f π* transitions of the indicators were used as the solvatochromic bands. Kinetic data were calculated from the change in intensity of the solvatochromic bands of the reactant. The cell was cleaned and then reloaded with pure water after each measurement. Materials. Water (Aldrich, HPLC-grade) was deoxygenated with nitrogen or helium (Air Products, highpurity grade) before being used. The solvatochromic indicators consist of 4-nitroanisole (Aldrich, 97%) for π*, a dichloro-substituted betaine dye [2,6-dichloro-4-(2,4,6triphenyl-1-pyridinio) phenolate, Fluka, HPLC-grade] for R, and 4-nitroaniline (Aldrich, >99%) and N,Ndimethyl-4-nitroaniline (Acros) for β. The organic solvents used were of the highest purity available from Aldrich. All chemicals were used without further purification.

Experimental Section

Results and Discussion

Apparatus. UV-vis spectra were measured in a high-pressure titanium cell with two 6.4-mm-thick sapphire windows. The windows were sealed with gold gaskets (Aldrich, 99.99% purity) capable of withstanding temperatures exceeding 300 °C and pressures up to 25

I. Solvatochromic Parameters of Ambient to Near-Critical Water. Dipolarity/Polarizability. The Kamlet-Taft π* parameter provides a comprehensive measure of the ability of a solvent to stabilize a solute molecule based on the dielectric effects.41 It is a quan-

XYZ ) (XYZ)0 + s(π* + dδ) + aR + bβ

(1)

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Figure 1. Dipolarity/polarizability π* and density of saturated liquid water as functions of temperature.

Figure 2. ET(30) parameters of saturated liquid water at different temperatures.

titative index of solvent dipolarity and polarizability. The longest-wavelength band due to the π f π* electronic transition of 4-nitroanisole was measured to determine π* for a given solvent.41 For this chromophore, the dipolarity/polarizability of the solvent is the primary contributor to the solvatochromic shift observed. Cyclohexane (π* ) 0.0) and dimethyl sulfoxide (π* ) 1.0) are two reference solvents used to normalize the π* scale. Quantitatively, one can use eq 2 to calculate π*, in which ν is the frequency (cm-1) of the solvatochromic band maximum and s refers to the solvent under measurement.

π* )

ν(s) - ν(cyclohexane) ν(DMSO) - ν(cyclohexane)

(2)

The π* value and density of saturated liquid water at different temperatures from 25 to 275 °C are shown in Figure 1, where density data were calculated using DIPPR 801 Thermophysical Properties Database (2001 Public Release). The dipolarity/polarizability for liquid water continuously decreases as the temperature increases, as indicated by the π* value of 0.69 at 275 °C in comparison with 1.08 at 25 °C. NCW at 275 °C has a dipolarity/polarizability comparable to that of ambient acetic acid. Interestingly, the observed decline in π* for water is consistent with the change of density, which might be due to a reduction of the dielectric constant with decreased density at higher temperatures.4,45 Hydrogen-Bonding Acidity/Basicity. The extent of hydrogen bonding decreases as the temperature increases from ambient to near-critical conditions according to the findings obtained by a variety of approaches.21,24,28,50 The Kamlet-Taft R and β parameters offer quantitative measures of the hydrogen-bonding ability of a solvent from the two perspectives of donating acidity and accepting basicity. The HBD acidity R for a solvent is indicative of its ability to donate a proton in a solvent-to-solute hydrogen bond.43 Our estimation for R is based on the experimental determinations of π* and ET(30). The ET(30) scale, developed by Reichardt et al.,39,44 indicates a solvent strength by combining polarity and HBD acidity, which itself serves as a useful solvent parameter for physicochemical correlations in a wide range of solvents including SCFs.44,51-53 The parameter ET(30) refers to the energy of the intramolecular charge-transfer π f π* transition of pyridinium-N-phenoxide betaine dye in a given solvent and can be calculated according to

ET/(kJ mol-1) ) hcν˜ NA

(3)

Figure 3. HBD acidity R of saturated liquid water as a function of temperature.

where h is Planck’s constant, c is the speed of light, ν˜ is the wavenumber of the solvatochromic absorption maximum, and NA is Avogadro’s number. When subjected to the pH of water at 25-275 °C, the ET(30) indicator (pKa ) 8.6) undergoes protonation, and the characteristic charge-transfer solvotochromic band thus disappears. Therefore, we chose a less basic dichlorosubstituted betaine dye [also called the ET(33) indicator, pKa ) 4.8], which remains deprotonated under NCW conditions.54 In the same manner, ET(33) is calculated from solvatochromic measurements according to eq 3. Then, the ET(30) can be obtained using the correlation established between ET(30) and ET(33).54 The ET(30) value of water decreases with increasing temperature (Figure 2). The literature data of 15 alcohols as well as water at ambient conditions46 have been regressed to correlate R with ET(30) and π* (r ) 0.9869), giving

R ) -1.47 - 0.532π* + 0.0508ET(30)

(4)

Figure 3 shows that the R value for liquid water decreases from 1.16 at 25 °C to 0.84 at 275 °C. The increase in temperature shifts the hydrogen-bonding equilibrium toward free monomers because hydrogenbond formation is exothermic. Bennett and Johnston observed, however, that, at temperatures as high as 380 °C, hydrogen bonding between water and acetone is still well-established at 0.5Fc and changes little with increased density.19 We demonstrate that NCW exhibits considerable HBD acidity, comparable to that of ambient ethanol. The HBA basicity parameter β is an index of the solvent’s ability to accept a proton in a solute-to-solvent

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Figure 5. Disappearance of 4-nitroaniline in water at 225 °C. Figure 4. HBA basicity β of saturated liquid water as a function of temperature.

hydrogen bond. 4-Nitroaniline and N,N-dimethyl-4nitroaniline were used to obtain the basicity β based on a comparison method.44 4-Nitroaniline is a primary indicator of medium effects on the solvatochromic absorption bands, including both polarity and hydrogen bonding. N,N-Dimethyl-4-nitroaniline is not believed to act as a hydrogen-bond donor to solvents. Thus, subtraction of the two solvatochromic shifts eliminates the polarity effect; the remaining shift is due solely to the hydrogen-bonding basicity of the solvent. Two reference solvents at room temperature were chosen to normalize the β scale from 0.0 for cyclohexane to 1.0 for hexamethylphosphoric triamide. The β values of water are shown in Figure 4. Various values of β are reported in the literature.46,53 The result at room temperature in this work is consistent with that of Taft et al.53 In contrast to π* and R of water, β actually increases slightly with increasing temperature. The slight increase of β might be brought about by the breakdown of the hydrogen-bond network of water at elevated temperature, which makes the oxygen atom of the water molecule more accessible as a hydrogen-bond acceptor.21 II. Kinetics and Mechanism. Product Analysis. The hydrolyses of two nitroaromatic compounds (4-nitroaniline and N,N-dimethyl-4-nitroaniline) in the absence of oxygen were measured from 200 to 275 °C in NCW as model reactions for the application of LSERs and Kamlet-Taft solvent parameters. The initial concentrations of 4-nitroaniline and N,N-dimethyl-4-nitroaniline were 3 × 10-5 and 4 × 10-5 mol/L, respectively. We speculate that the hydrolysis reaction proceeds by an acid-catalyzed mechanism. The correlation of an LSER can provide insight into the role of water in these reactions. Additionally, these two compounds have different HBD acidities and provide a relative measure of the hydrogen-bonding interactions with water. Aniline and N,N-dimethyl aniline, respectively, are the products of the hydrolyses as determined by GC-MS analysis of samples taken from our titanium optical cell. A similar literature study reported that aniline and 4-aminoaniline are the hydrolysis products of 4-nitroaniline at 300 °C when stainless steel reactors are used.55 Initial Reaction Rate. Figure 5 shows the natural logarithm of peak absorption A as a function of time for 4-nitroaniline at 225 °C in water as an example. At early times, the majority of the hydronium ions present are produced from the autoionization of water, and the concentration of hydronium ion remains essentially constant. However, at longer times, the amount of nitric

Figure 6. Arrhenius plots of the hydrolysis for 4-nitroaniline (4) and N,N-dimethyl-4-nitroaniline (9) in NCW.

acid produced as a byproduct of the hydrolysis becomes significant, and the reaction becomes autocatalytic. With the assumption that the hydronium concentration at early time remains constant, the initial reaction rate can be considered to be pseudo-first-order in the reactant. The initial rate constants (k′) are calculated from the initial slope of the plot of ln A versus time, assuming that the concentration of hydronium ions from the autoionization of water is not significantly affected by nitric acid byproduct. The activation energies of the hydrolysis reactions (Figure 6) were calculated to be 142 ( 6 kJ/mol for 4-nitroanisole and 76 ( 3 kJ/mol for N,N-dimethyl-4nitroaniline. These results are consistent with an activation energy of 159 ( 25 kJ/mol for 4-nitroaniline in SCW in the temperature range of 385-420 °C.56 Mechanism. For the autocatalytic reaction57

A+RfB+R+R

(5)

an autocatlysis function f(x) is defined as

[

ln

M + xA

M(1 - xA)

]

) CA0(M + 1)kt

M)

CR0 CA0

(6)

(7)

where x is the mole fraction and C0 is the initial concentration. As shown in Figure 7, the autocatalysis function f(x) vs time for each 4-nitroaniline and N,Ndimethyl-4-nitroaniline at 200 °C is a straight line with a slope of CA0(M + 1)k, indicating an autocatalytic mechanism in both cases.

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Figure 7. Autocatalysis function vs time for 4-nitroaniline and N,N-dimethyl-4-nitroaniline at 200 °C.

Figure 8. LSER correlation of initial rate constant with KamletTaft solvent parameters.

Scheme 1. Acid-Catalyzed Mechanism for the Hydrolysis of 4-Nitroaniline in NCW

Figure 9. Correlation of initial rate contant of the hydrolysis with ET(30) of water. Table 1. Coefficents of LSER Equation for Initial Rate Constants of Hydrolysis in NCW

Judging from the kinetic analysis, we propose that the hydrolysis of 4-nitroaniline in NCW occurs according to an acid-catalyzed mechanism (Scheme 1). At the beginning of the reaction, NCW provides a sufficient concentration of hydronium ion to make hydrolysis occur. NCW acts as both the acid catalyst and the solvent. Kinetic analyses indicate that aniline is formed from nitroaniline via an acid-catalyzed mechanism that involves a protonated nitroaniline as an intermediate. III. LSER Correlation. An LSER can be applied for kinetic correlations and written as

ln k ) (ln k)0 + sπ* + aR + bβ

(8)

where k is the rate constant in a given solvent and k0 is the rate constant in a reference solvent. When the concentration of hydronium ion ([H+]) remains constant, the reaction rate can be described by

v ) k[H+][R] ) k′[R]

(9)

in which the pseudo-first-order rate constant is k′ ) k[H+]. k is a function of temperature only, whereas k′ depends on both temperature and [H+]. For the hydrolysis reaction, we define cyclohexane at ambient temperature as the reference solvent, which leads to a zero value of (ln k′)0. The HBA term bβ is negligible because water, as a hydrogen-bond donor, has comparable interactions with the reactant and with the intermediate. Therefore, water contributes little to the kinetics observed. Thus, an applicable LSER for the

ln k′ ) ln[H+] + sπ* + aR + bβ compound

s

a

b

4-nitroaniline N,N-dimethyl-4-nitroaniline

-29.1 -23.1

35.0 28.1

0 0

Table 2. Correlation of ET(30) with Initial Rate Constants of Hydrolysis in NCW ln k′ ) ln[H+] + mET(30) + n compound

m

n

4-nitroaniline N,N-dimethyl-4-nitroaniline

-1.37 -1.03

81.1 61.7

hydrolysis reaction can be expressed as

ln k′ ) ln[H+] + sπ* + aR + bβ

(10)

with b ) 0. [H+] was calculated from the ionization constant of liquid water at the corresponding temperature.8 Values of the susceptibility coefficients in eq 10 were obtained by multiple regression of k′ and the Kamlet-Taft parameters (Table 1). Figure 8 shows the consistency between the experimental and correlated k′ values (x axis). The correlation for N,N-dimethyl-4nitroaniline (non-HBD) appears to be better than that for 4-nitroaniline. Similarly, k′ can be correlated in terms of ET(30)

ln k′ ) ln[H+] + mET(30) + n

(11)

The result is shown in Figure 9, and the coefficients m and n are given in Table 2. This correlation appears to

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give reasonably consistent results. Thus, ET(30) is also a useful solvent parameter for NCW. Conclusion The Kamlet-Taft solvent parameters π*, R, and β of liquid water at saturation pressure were determined for temperatures from 25 to 275 °C. π* and R are highly tunable and decrease significantly with increasing temperature. However, β remains low and relatively constant throughout the entire temperature range. Rapid and precise measurements were achieved without the complication of sampling by using in situ UV-vis spectroscopy. Because of the high sensitivity of UVvis spectroscopy, very dilute components could be monitored with negligible perturbation of the bulk properties. The temperature-dependent thermodynamics and kinetics of the hydrolyses of 4-nitroaniline and N,Ndimethyl-4-nitroaniline were investigated in NCW as model reactions. Both a Kamlet-Taft linear solvation energy relationship (LSER) and the ET(30) parameter were used to correlate the kinetic data. These methods can provide a useful tool for describing other chemical reactions in this novel medium. Moreover, the kinetic analysis suggests an acid-catalyzed mechanism for the hydrolysis reactions, which again demonstrates the efficacy of NCW as a neutral acid catalyst. Acknowledgment The authors are grateful for the financial support of the National Science Foundation (CTS-9613063) and the Environmental Protection Agency (R-825325 and R-82813001-0). Literature Cited (1) Katritzky, S. M. Reactivity of Organic Compounds in Hot Water: Geochemical and Technological Implications. Science 1991, 254, 231. (2) Savage, P. E. Organic Chemical Reactions in Supercritical Water. Chem. Rev. 1999, 99, 603. (3) Bro¨ll, D.; Kaul, C.; Kra¨mer, A.; Krammer, P.; Richter, T.; Jung, M.; Vogel, H.; Zehner, P. Chemistry in Supercritical Water. Angew. Chem., Int. Ed. 1999, 38, 2998. (4) Uematsu, M.; Franck, E. U. Static Dielectric Constant of Water and Steam. J. Phys. Chem. Ref. Data 1980, 9, 1291. (5) Ryan, E. T.; Xiang, T.; Johnston, K. P.; Fox, M. A. ExcitedState Proton-Transfer Reactions in Subcritical and Supercritical Water. J. Phys. Chem. 1996, 100, 9395. (6) Wallen, S. L.; Palmer, B. J.; Pfund, D. M.; Fulton, J. L.; Newville, M.; Ma, Y.; Stern, E. A. Hydration of Bromide Ion in Supercritical Water: An X-ray Absorption Fine Structure and Molecular Dynamics Study. J. Phys. Chem. A 1997, 101, 9632. (7) Connolly, J. Solubility of Hydrocarbons in Water Near the Critical Solution Temperature. J. Chem. Eng. Data 1966, 11, 13. (8) Marshall, W. L.; Franck, E. U. Ion Product of Water Substance. J. Phys. Chem. Ref. Data 1981, 10, 295. (9) Harrington, P. J.; Lodewijk, E. Twenty Years of Naproxen Technology. Org. Process Res. Dev. 1997, 1, 72. (10) Ramayya, S.; Brittain, A.; DeAlmeida, C.; Mok, W.; Antal, M. J., Jr. Acid-Catalysed Dehydration of Alcohols in Supercritical Water. Fuel 1987, 66, 1364. (11) Xu, X.; Antal, M. J., Jr.; Anderson, D. G. M. Mechanism and Temperature-Dependent Kinetics of the Dehydration of tertButyl Alcohol in Hot Compressed Liquid Water. Ind. Eng. Chem. Res. 1997, 36, 23. (12) Antal, M. J. J.; Carlsson, M.; Xu, X.; Anderson, D. G. M. Mechanism and Kinetics of the Acid-Catalyzed Dehydration of 1and 2-Propanol in Hot Compressed Liquid Water. Ind. Eng. Chem. Res. 1998, 37, 3820. (13) Chandler, K.; Deng, F.; Dillow, A. K.; Liotta, C. L.; Eckert, C. A. Alkylation Reactions in Near-Critical Water in the Absence of Acid Catalysts. Ind. Eng. Chem. Res. 1997, 36, 5175.

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Received for review February 26, 2002 Revised manuscript received April 15, 2002 Accepted April 16, 2002 IE020160E