Kinetics of Wet Oxidation of Propionic and 3-Hydroxypropionic Acids

SI-1001 Ljubljana, Slovenia. Oxidation of aqueous solutions of 3-hydroxypropionic (3-HPA) and propionic acids (PA) was studied in a titanium high-pres...
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Ind. Eng. Chem. Res. 1999, 38, 2557-2563

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Kinetics of Wet Oxidation of Propionic and 3-Hydroxypropionic Acids Rajesh V. Shende and Janez Levec* Laboratory for Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, P.O. Box 30, SI-1001 Ljubljana, Slovenia, and Department of Chemical Engineering, University of Ljubljana, P.O. Box 53, SI-1001 Ljubljana, Slovenia

Oxidation of aqueous solutions of 3-hydroxypropionic (3-HPA) and propionic acids (PA) was studied in a titanium high-pressure reactor at 280-310 °C using oxygen partial pressures between 10 and 45 bar. Oxidation of both acids was found to obey first-order kinetic with respect to their concentrations as well as to their lumped TOC concentrations. Oxidation rate revealed a half order dependence with respect to oxygen for oxidation of both acids. In the case of 3-HPA oxidation, the activation energy was found to be 135 kJ/mol, and it was 140 kJ/mol when lumped concentration TOC was used. The activation energy for PA oxidation is 150 kJ/mol, and it is slightly higher, 158 kJ/mol, for TOC reduction. Almost complete conversion of 3-HPA was achieved at 300 °C after 1 h, whereas 95% conversion of PA acid was obtained at 310 °C after 3 h. During oxidation of 3-HPA, 3-oxopropionic and acetic acids were identified as intermediate products. Oxidation of PA yielded acetic and formic acids as intermediates; at oxygen partial pressures above 25 bar and 310 °C, the formation of acetic acid was appreciably reduced. In both cases, however, direct oxidation to carbon dioxide and water was found to be the main reaction route. Introduction Wet oxidation is an effective technique available to the process engineers to treat more complex industrial wastewater streams. During the oxidation of organic pollutants, several intermediates are produced before parent compounds eventually decompose to carbon dioxide and water. Some of the intermediates may have a short life, and some others may exhibit quite high stability. Low-molecular-mass carboxylic acids such as glyoxalic, oxalic, formic, propionic (PA), and acetic acids are known to be very refractory. Although acetic acid is the most refractory acid among these low-molecularmass mono- and dibasic acids, propionic acid is considered to be the second-most refractory compound. In a thorough review on wet oxidation, it has been discussed that oxidation studies of muconic, maleic, glyoxalic, and hydroxypropionic acids are needed because these acids are the earliest acid species generated that further undergo oxidation to the low-molecular-mass carboxylic acids.1 Oxidation kinetics of low-molecular-mass carboxylic acids have recently been reported,2,3 and some further exploration into the kinetics of wet oxidation of these acids has currently been conducted. From a proposed reaction pathway for phenol oxidation, one can observe that propionic and acetic acids are the end oxidation products of two different reaction routes.4 The first one involves hydrogenation across a C-C double bond in maleic acid yielding succinic acid that subsequently oxidizes into propionic acid. In the second route, oxidation of 3-hydroxypropionic acid (3HPA) produces malonic acid and finally acetic acid. Furthermore, propionic acid is also known to oxidize partly into acetic acid.5 These facts, however, revealed that propionic and 3-hydroxypropionic acids are the two * To whom correspondence should be addressed at the University of Ljubljana. E-mail: [email protected].

intermediates that yield acetic acid during deep oxidation of some aromatic compounds. Nevertheless, during oxidation of a monoazo dye such as Orange II, only acetic and formic acids were found as refractory acids.6 Because the oxidation of low-molecular-mass acids is the rate-controlling step, a thorough knowledge of the oxidation kinetics of these acids is a prerequisite for more efficient wet-reactor design. Besides being observed as intermediates during wet oxidation of many organic pollutants, 3-hydroxypropionic and propionic acids also have a wide range of applications in the industrial sector. 3-HPA (OHCH2CH2CO2H) is used as a cross-linking agent for polymer coatings, metal lubricants, antistatic agents for textiles, and so on, whereas propionic acid is largely used for the production of cellulose esters and plastic dispersions. The kinetic data on wet oxidation of propionic acid are very meager,5 and there is no study available in the open literature on the kinetics of 3-HPA oxidation. Kinetic study of 3-hydroxypropionic acid oxidation was also attempted with identification of intermediates generated during the oxidation. The oxidation of propionic acid occurs via two routes;5 therefore there is a possibility to choose the operating parameters in such a way that the formation of refractory acetic acid is minimized. In the oxidation of propionic acid, the attack of an R-carbon is considered the key step. However, under high oxygen pressure, the attack on β-carbon also seems to be possible.7 As a consequence of this attack, higher conversions to carbon dioxide are expected.8 In this work, particular attention was given to the effect of oxygen partial pressure on the yield of acetic acid. Comparison of the oxidation results for both acids further provides some insight into the reactivity of the two acids toward oxygen. It is known that the -COOH group increases the reactivity of C-H bonds in propionic acid;7 therefore, one can speculate that the presence of

10.1021/ie9900061 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/25/1999

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-COH group on the other side of 3-HPA may impart more reactivity to the molecule, which may consequently result in higher oxidation rates compared to those of propionic acid. The oxidation experiments were carried out in a titanium autoclave to avoid any potential catalytic effects that may occur because of the reactor wall surface. Earlier investigators, however, have used an SS-316 autoclave for the oxidation of propionic acid.5,9 In many cases, the oxidation of parent compounds was found to be a first-order reaction, whereas its rate with respect to the lumped parameter, e.g., total organic carbon (TOC), exhibited different kinetics.6 The oxidation kinetics of both acids were therefore studied with respect to the parent compound concentration, as well as with respect to their lumped concentration parameter TOC. Experimental Section Materials. 3-Hydroxypropionic acid as a 30% solution in water was obtained from Tokyo Kasei Kogyo Co., Ltd., Japan. Propionic acid was a product of Aldrich, U.S.A. Oxygen with a minimum purity of 99.5% was used for oxidation. Analysis. Concentrations of PA and 3-HPA were determined using a reverse-phase HPLC (HewlettPackard 1100/DAD) system. It was equipped with a Rheodyne 7725 injection valve having a 20 µL sample loop. Analysis was carried out using a 300 × 8 mm i.d. SS column which consists of sulfonated cross-linked styrene-divinylbenzene copolymer as a stationary phase (“Eurokat H” from Geratebau Saulentechnik Eurochrom, Germany). Temperatures of 75 °C for 3-HPA and 20 °C for PA, were employed across the column. A mobile phase of 0.01 N H2SO4 with a flow rate of 1 mL min-1 was used for the analysis. The detector (DAD) was set at the maximum absorption of 208 nm for 3-HPA and 210 nm for PA. Aliphatic acids that appeared as intermediate products during the oxidation experiments were quantitatively evaluated by means of the HPLC/DAD system. The total organic carbon (TOC) concentration of the samples collected during the oxidation experiments was measured using an advanced HTCO Rosemount/Dohrmann DC-190 TOC analyzer equipped with a nondispersive infrared CO2 detector. Total carbon (TC) was measured first followed by inorganic carbon (IC). TOC was determined by subtracting IC from TC. The relative standard deviation of three different measurements was never more than 1.4% for the range of TOC used. Identification of high-molecular-mass intermediate products (appeared during 3-HPA oxidation) was performed on a mass spectrometer LCQ (Finnigan, U.S.A.). Both quantitative and qualitative measurements of 3-HPA were carried out, but intermediates were analyzed qualitatively only. The following conditions were employed. For the pump, mobile phase was bidistilled water/methanol (1:1 v/v) with 1.0 (v %) formic acid; flow rate was 1.0 mL min-1; and loop volume was 20 µL and for the scanning, capillary temperature was 140 °C; vaporizer temperature was 400 °C; mode of scanning was negative; scanning range m/z was 80-400; multiplier voltage was -940.69 V; and lens offset was 22 V. Experimental Setup and Procedure. The experimental setup employed in this study is practically the same as that reported previously, except for the reactor.10 In the present study we used a 2-L titanium (grade

4) autoclave (Parr Instrument Co. Ltd., Illinois, U.S.A.), which was equipped with a magnetically driven turbine type impeller (titanium) and temperature, pressure, and speed of agitation control units. An upstream electronic mass-flow controller (Brooks 5850) and a downstream electronic back-pressure controller (Brooks 5866) were used to a maintain constant operating pressure and a constant delivery of oxygen (1.0 L min-1) when the apparatus was operated semicontinuously. A 200 mL liquid injection vessel (SS-316) was mounted on the top of the autoclave. In a typical experiment, 1.25 L of distilled water was charged into the autoclave and heated to the desired temperature. A continuous flow of nitrogen was maintained at 1.0 L min-1 throughout the heating period. In the meantime, a solution of 1.5 g of 30% 3-HPA solution (1.35 g of pure PA) was made to 100 mL in distilled water and placed into the injection vessel. Pressure in the injection vessel was set about 20% higher than that inside the autoclave before the injection was performed. After the injection, a drop of 6-7 °C in temperature was observed, and it required 8-10 min to reach the preset temperature. As soon as the temperature reattained the preset value, a sample was withdrawn before sparging oxygen, and it was referred as the “zero” time sample for the kinetic measurements. Two modes of the oxidation experiments were used: batch and semicontinuous. In the batch experiments, oxygen was introduced into the autoclave at high rate for a short time; the flow of oxygen was then stopped, and the autoclave pressure maintained constantly during the experiments. In the semicontinuous experiments, oxygen was sparged directly below the impeller at a flow rate of 1 L min-1 throughout the experiment. However, there was no difference in the results when the experiments were performed in the batch or semicontinuous mode of operation. The acid solution was found to be saturated with oxygen (at 8-10 L min-1) in 1-2 min. Liquid samples were collected periodically and analyzed. The oxidation experiments were carried out at 250310 °C using oxygen partial pressures between 10 and 45 bar (total pressure ) 46-122 bar). The initial mass concentration was 330 mg L-1 for 3-HPA and 1000 mg L-1 for propionic acid, respectively. Results and Discussion Wet oxidation is a gas-liquid reaction involving various transport processes that take place in series. Because oxygen has higher diffusivity in the gas phase and lower solubility in water, the gas-phase masstransfer resistance is negligible in the operating range of 250-310 °C. In preliminary experiments, the rate of oxidation was found to be independent of the impeller speed between 1000 and 1500 rpm, indicating the absence of the liquid-side mass-transfer resistance.11 All subsequent experiments were carried out at the impeller speed of 1000 rpm. Because the concentrations of 3-HPA and PA in aqueous solutions were very low, we have used the solubility of oxygen in water.12 Oxidation of 3-HPA. Reaction Pathway. Concentrations of 3-HPA measured by HPLC and HPLC-MS agreed within 1%. The HPLC-MS analysis of pure 3-HPA solution (30%) revealed the presence of an acid anhydride [O(CH2CH2COOH)2, molecular mass 162] in concentrations of less than 1%. The HPLC analysis of the treated solutions showed acetic acid (AA) as an

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Figure 1. Normalized concentration of intermediates observed during oxidation of 3-HPA at 280 °C using 10 bar of oxygen partial pressure (concentration of intermediate M ) 120 was estimated on the basis of relative peak heights observed in the mass spectral response).

intermediate product of 3-HPA oxidation, whereas the HPLC-MS analysis showed three additional intermediates corresponding to molecular mass 88 (87.1-89.4), 120 (119.1-122.2), and 160 (159.3-162.2). The intermediates with the molecular mass of 88 and 160 were identified as OHCCH2COOH (3-oxopropionic acid) and HOOCCH2CH2COCH2COOH (3-oxoadipic acid), respectively. It is believed that 3-oxoadipic acid (OAA) results from the acid anhydride oxidation. However, it is difficult to comprehend the presence of anhydride of 3-HPA at the reaction temperature. One can further speculate on the presence of 2-oxopropionic acid (ketone form) in addition to 3-oxopropionic acid (aldehyde form). It is, however, also possible that the compound with the molecular mass of 160 may be an esterified product of 2-oxopropionic acid. Deeper oxidation of 3-oxopropionic acid (3-OPA) may proceed via malonic acid and could probably produce tartronic acid (HOOCCHOHCOOH) with the molecular mass of 120. On the other hand, the presence of these compounds at 280 °C and above is rather doubtful as it is mentioned in the literature that the pure compounds decarboxylate around 200 °C.13,14 However, the normalized concentration profiles for the mother compound as well as for the intermediate products that appeared in a typical oxidation experiment is shown in Figure 1. The concentration of the 120 compound was estimated on the basis of the peak heights during the MS analysis. It can be seen from Figure 1 that the concentration profiles of the 120 compound, 3-OPA, and 3-OAA exhibit maxima, which are typical for the intermediates in a consecutive reaction scheme. The concentration of acetic acid slowly increases with time. The intermediates observed during oxidation seem to be responsible for the lower TOC reduction in the initial period at 280 °C (Figure 5). To elaborate the complete mechanism of 3-HPA oxidation, additional investigation may be needed because of the reaction complexity, as discussed above. However, on the basis of the intermediate products observed, one can propose oxidation pathway for wet oxidation of 3-HPA such as illustrated with eq 1. In all experiments, the acetic acid concentration was found below 5 mg L-1, and the concentration of 3-OPA was, for example at 280 °C and 10 bar of oxygen partial

pressure, in the range of 9.75-2.7 mg L-1. At 300 °C, only traces of 3-OPA were found, but no significant increase of the acetic acid concentration was found. In the range of operating conditions, the oxidation rate of acetic acid is a very slow step, therefore all AA was found accumulated in the solution once formed from 3-OPA. One can further speculate that either the conversion of 3-HPA into 3-OPA is very slow or the oxidation of the latter into CO2 and H2O is a very fast reaction. Even at the mildest oxidation conditions employed, the total conversion of 3-HPA into 3-OPA and AA was less than 40%; thus, one can believe the major route of 3-HPA oxidation is its direct conversion into CO2 and H2O. Kinetics of 3-HPA Oxidation. 3-HPA was found to be stable at 250 °C and 10 bar of oxygen partial pressure, but its oxidation rate increases very rapidly as the temperature increases. For example, at 280 °C and 10 bar of oxygen partial pressure, a 84% reduction in its concentration was reached in 2 h, whereas at 300 °C only one 1 h was enough to attain 99.9% reduction. However, over the entire temperature range the reaction of 3-HPA oxidation was found to obey first-order kinetics. The first-order kinetic plot is shown in Figure 2, and the Arrhenius plot with the activation energy of 135 kJ/mol is depicted in Figure 3. The effect of oxygen partial pressure was studied over 10-30 bar at 280 °C. The order with respect to oxygen was found to be 0.5. This result suggests that only 1/6 of oxygen based on the complete oxidation stoichiometry (i.e., 3O2) has been utilized for oxidation, which indirectly confirms the production of intermediates via free radical formation. The experimental data of the 3-HPA disappearance rates can be well-correlated by the equation in the following form 0.5(0.02 1.0 -r3-HPA ) 1.73 × 109 exp(-16284/T)C3-HPA CO 2 (2)

A comparison of the experimental oxidation results with those calculated by eq 2 is presented in Figure 4 as a parity plot. As can be seen, all data points are found well inside (10% confidence limits. Because the oxidation of 3-HPA to carbon dioxide occurs via some intermediate products, it would be convenient for design purposes to present the kinetics also in terms of its lumped parameter, i.e., TOC. The temperature effect on the TOC reduction is depicted in Figure 5. One can observe lower TOC conversion in the initial period at 280 °C that may be attributed to the presence of various intermediates produced during the initial stage of 3-HPA oxidation. However, over the entire temperature range studied, the reaction was found to obey first-order kinetics with respect to TOC. The activation energy for lumped TOC conversion was estimated to be 140 kJ/mol (Figure 3). Slightly higher activation energy with respect to TOC as compared to

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Figure 2. First-order kinetic plot with 3-HPA concentration.

Figure 3. Arrhenius plot for 3-HPA oxidation.

Figure 5. Effect of temperature on TOC during oxidation of 3-HPA.

Figure 6. Comparison between experimental and calculated TOC conversion during 3-HPA oxidation.

kinetic expression 0.45(0.02 -rTOC ) 6.58 × 109 exp(-16847/T)C1.0 TOCCO2 (3)

Figure 4. Comparison between experimental and calculated concentration of 3-HPA.

that of the acid conversion might be due to oxidation of intermediates. The experimental data for the TOC oxidation rate can be well-described by the following

Comparison of the calculated total TOC conversions (eq 3) with those experimentally measured is shown in Figure 6. It can be seen that only a few points at 280 °C (using 10 bar oxygen partial pressure) are lying outside the confidence limits. Oxidation of Propionic Acid. Reaction Pathway. During oxidation of propionic acid, acetic acid was observed as a major intermediate product; formic acid was found in traces only. For instance, at 300 °C using 30 bar oxygen partial pressure, the concentration of acetic acid rose to 75 mg L-1 in 2 h. The total carbon balance made of acetic acid and nonconverted propionic acid in all experiments agreed within 6% of the measured TOC in the solution. Because a part of this difference is attributed to formic acid, one can realize that other intermediates, if any, were below the separation/detection limits of the analytical methods employed. Although the production of acetic acid in-

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Figure 7. Acetic acid formation during oxidation of propionic acid.

creases with temperature (Figure 7), the converted PA found in the form of acetic acid was practically the same for the experiments at 290 and 300 °C (30 bar of oxygen). At 310 °C, acetic acid already oxidizes with a measurable rate; therefore, its concentration in the aqueous phase was reduced. The effect of oxygen partial pressure on the acetic acid formation was found to be crucial. At 300 °C, as oxygen partial pressure was increased from 25 to 45 bar, the conversion of propionic acid increased from 28 to 72%, but its conversion into acetic acid decreased from 45 to 17.5%. It should be recalled at this point that the conversion of acetic acid in a titanium autoclave at 300 °C and 55 bar of oxygen partial pressure was experimentally found negligible. The above facts undoubtedly imply that higher fraction of propionic acid was oxidized directly to CO2 and water (and some gaseous products as reported by earlier investigator5). The radical attack of R-carbon is the main route of propionic acid oxidation, which leads to the formation of acetic acid as well as to carbon dioxide.1,5 Because the experiment performed at highest oxygen partial pressure (45 bar) yielded less acetic acid, one can speculate that at high oxygen pressure the radical attack on β-carbon might have occurred as suggested in the literature.7 As a consequence of the β-rupture under high oxygen pressure, larger amounts of peroxide radicals (ROO•) are formed and hence, more CO2 is produced.8 In general, oxygen pressure has an important role in the formation of acetic acid during the wet oxidation. Because no other compounds but acetic and formic acids were observed during the course of propionic acid oxidation, the following simplified reaction scheme reflects the experimental findings of this work

Acetaldehyde appearance at 227 °C is known from the literature,5 but it was not observed in this study. Therefore one can consider acetaldehyde oxidation into acetic acid at 290 °C and above as a very rapid reaction.

Figure 8. First-order kinetic plot with concentration of propionic acid.

Figure 9. First-order kinetic plot for propionic acid oxidation based on TOC conversion.

As discussed above, PA mainly disappears by its direct oxidation to CO2; thus, the oxidation rate expression given below concerns primarily this route. Kinetics of PA Oxidation. The conversion of propionic acid increases with temperature: it doubles from 290 (44%) to 310 °C (88%). The experimental data obtained below 25 bar of oxygen partial pressure do not obey first-order dependence with respect to the concentration of PA, whereas at higher pressures (30 bar and above) the data can be well-fitted by the first-order kinetics. Higher order (>1) with respect to PA concentration was, however, reported before by other investigators.5 A first-order kinetic plot in terms of the acid concentration is depicted in Figure 8, and the one obtained with the lumped parameter TOC is shown in Figure 9. The order with respect to oxygen was found to be 0.5 when the concentration-dependent term is based on the PA concentration. When TOC is used, the order increased to the value of 0.61 (Figure 10). The activation energy for two different types of concentration measurement is shown in Figure 11: as can be seen, the lumped concentration TOC gives it for about a 5% higher value; this is due to the oxidation of formic acid and partly due to acetic acid conversion. Thus, the

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The activation energies reported in the literature on PA oxidation are much lower (134 and 139 kJ/mol, respectively)5,9 than the value obtained in this work. Its value with respect to COD reduction is also reported to be lower (142.4 kJ/mol).9 It is believed that in their cases the reactor wall surface (SS-316) probably acted as a catalyst and promoted the oxidation reaction as discussed by some investigators in the case of other organics.15,16 Because in the present case the reactor was made of titanium, which is supposed to be an inert material, it is not surprising that higher temperatures were needed for the propionic acid oxidation, and consequently, a much higher value for the activation energy was found. However, one may assume that the results presented here are not affected by the reactor material and account for homogeneous oxidation only. Conclusion Figure 10. Order with respect to oxygen for propionic acid oxidation.

3-HPA was found to be refractory below 250 °C using 10 bar of oxygen partial pressure, whereas the oxidation of propionic acid becomes reasonable fast at 290 °C and 30 bar of oxygen pressure. Because both functional groups, i.e -COOH and -COH, are known to be readily attacked by a free radical species, the higher oxidation rates of 3-hydroxypropionic acid compared to propionic acid are expected. Oxidation of propionic acid seems to be more difficult because -CH3 is an electron donor to the carbon having dO and -OH groups and, therefore, stabilizes the whole molecule. In the case of propionic acid oxidation, it was possible to reduce acetic acid formation by employing higher oxygen partial pressures (>25 bar). Both acids were found to oxidize mainly via routes leading directly to carbon dioxide and water. The oxidation kinetics of propionic acids do not differ whether given in the form of acid concentration or lumped TOC concentration, which further supports the above conclusion. Acknowledgment

Figure 11. Arrhenius plot for propionic acid oxidation.

The authors acknowledge financial support from Slovenian Ministry of Science and Technology under the Grant J2-0783. R.V.S. also thanks the National Institute of Chemistry, Slovenia, for fellowship aid.

kinetic expression for the disappearance rate of propionic acid reads

Nomenclature

0.50(0.01 -rPA ) 2.65 × 1010 exp(-18115/T)C1.0 PACO2

(5)

Earlier investigation on propionic acid oxidation reported an order of 1.43 with respect to the PA concentration and an oxygen order of 0.39.5 On the other hand, the oxidation rate linearly proportional to the PA concentration and independent of the oxygen concentration is also mentioned in the literature.9 The disappearance rate of TOC can be written as 0.61 (6) -rTOC ) 7.57 × 1010 exp(-19073/T)C1.0 TOCCO2

Slightly increased order with respect to oxygen concentration is contributed by the oxidation of formic acid. The same dependence with chemical oxygen demand (COD) but the rate independent of oxygen has also been reported.9

AA ) acetic acid CA ) concentration of acid, mg L-1 Co2 ) oxygen concentration, mol L-1 COD ) chemical oxygen demand CTOC ) TOC concentration, mg L-1 E ) activation energy, kJ/mol HPA ) hydroxypropionic acid k ) rate constant, L mol-1 s-1 OAA ) oxoadipic acid OPA ) oxopropionic acid R ) gas constant, 8.314 J/mol K -r ) rate of oxidation mg L-1 s-1 t ) time, s T ) temperature, K TOC ) total organic carbon

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2563 Registry Numbers: OHCH2CH2COOH, 503-66-2; OHCCH2COOH, 926-61-4; HOOCCH2CH2COCH2COOH, 689-316.

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(9) Merchant, K. P. Studies in Heterogeneous Reactions. Ph.D. Thesis, Department of Chemical Technology, University of Bombay, Bombay, India, 1992. (10) Pintar, A.; Levec, J. Catalytic Oxidation of Organics in Aqueous Solutions. J Catal. 1992, 135, 345-357. (11) Doraiswamy, L. K.; Sharma, M. M. Heterogeneous Reactions: Analysis, Examples and Reactor Design; John Wiley and Sons: New York, 1984; Vol. 2. (12) Crammer, S. D. The Solubility of Oxygen in Brines from 0 to 300 °C. Ind. Eng. Process Des. Dev. 1980, 19, 300-305. (13) Cornils, B.; Lappe, P. Dicarboxylic Acids, Aliphatic. In Ullmann’s Encyclopedia of Industrial Chemistry; VCH: Weinheim, Germany, 1989; Vol. A8. (14) Miltenberger, K. Hydroxycarboxylic Acids, Aliphatic. In Ullmann’s Encyclopedia of Industrial Chemistry; VCH: Weinheim, Germany, 1993; Vol. A13. (15) Bjerre, A. B.; Sorensen, E. Thermal Decomposition of Dilute Aqueous Formic Acid Solutions. Ind. Eng. Chem. Res. 1992, 31, 1574-1577. (16) Yu, J.; Savage, P. E. Decomposition of Formic Acid under Hydrothermal Conditions. Ind. Eng. Chem. Res. 1998, 37, 2-10.

Received for review January 6, 1999 Revised manuscript received March 22, 1999 Accepted April 4, 1999 IE9900061