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Ind. Eng. Chem. Res. 2000, 39, 40-47
Subcritical Aqueous-Phase Oxidation Kinetics of Acrylic, Maleic, Fumaric, and Muconic 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
Unsaturated carboxylic acids such as acrylic, maleic, fumaric, and muconic acids have been observed as intermediate products in wet oxidation reactions of phenols. Oxidation kinetics of these compounds dissolved in water were studied in a titanium autoclave at a temperature range of 180 and 280 °C and oxygen partial pressures between 10 and 55 bar. The acid-decay data are well-represented by a three-half power-law kinetics for all acids. The disappearance rate of acrylic and fumaric acids is proportional to a square root of the oxygen partial pressure; for maleic acid the order takes a very low value (0.12). Thermal decomposition of acrylic acid can be neglected at temperatures employed, but it was found highly significant for maleic, fumaric, and muconic acids. These three acids completely decomposed at 280 °C with formic acid as a major intermediate product, whereas acetic acid was formed only in the presence of oxygen. The conversion of acrylic acid into CO2 and other acid intermediates at 290 °C was found to be about 72%. The lumped TOC (total organic carbon) concentration in solutions reduced by a first-order process for all acids investigated, but the order with respect to oxygen was between 0.3 and 1.0. The activation energy for the thermal decomposition of maleic and fumaric acids was 56.6 and 71 kJ/mol, respectively, whereas for the total decomposition (oxidative and thermal) of acrylic, maleic, and fumaric acids it was 94.3, 99.2, and 83.6 kJ/mol, respectively. The data for TOC reduction of acrylic and muconic acids gave the activation energy of 102 and 87.4 kJ/mol, respectively, whereas for maleic and fumaric acids its values were found to be 104.7 and 84.2 kJ/mol for the first step and 83.4 and 54.4 kJ/mol for the second step, respectively. Introduction Wet oxidation of an aqueous stream of aromatic compounds produces several intermediates, among them unsaturated carboxylic acids such as acrylic, maleic, fumaric, and muconic acids. The unsaturated acids are the earliest formed acid species that further undergo oxidation to more refractory low molecular mass carboxylic acids.1 The unsaturated acids are generally formed by the decarboxylation and benzene-ring opening reactions, which occur during oxidation of aromatic compounds. Fumaric acid is a trans isomer of maleic acid and can also be produced during oxidation of an aromatic pollutant such as phenol.2 Among these compounds acrylic acid undergoes oxygen addition to form 3-hydroxypropionic acid, which further yields acetic acid. The oxidation kinetics of low molecular mass mono- and dibasic acids were recently thoroughly investigated.3-6 Although most of the unsaturated acids are thermally unstable, the rate of their chemical oxidation is substantially faster than that of the thermal degradation.7 In fact, maleic acid can be considered as the main unsaturated acid formed during oxidation of phenolsit further oxidizes to 3-hydroxypropionic acid via acrylic acid and also produces propionic acid via * To whom correspondence should be addressed at the University of Ljubljana. E-mail:
[email protected]. † National Institute of Chemistry. ‡ University of Ljubljana.
succinic acid formation under limited oxygen conditions. Both these reaction pathways probably result in acetic acid.5 The ratio of organic substrate and oxygen was known to crucially influence the concentration profile of acids (unsaturated and aliphatic) that are generated during the oxidation processes. However, under oxygen conditions that are either nearly stoichiometric or in excess, the unsaturated acids were observed as intermediates in the main reaction pathway of phenol oxidation.7,8 Besides being products of wet oxidation, they are also very important industrial commodities. For instance, a petrochemical industry manufacturing methyl tetrabutyl ether (MTBE), methyl ethyl ketone (MEK), and secbutyl alcohol (SBA) produces wastewaters, which usually contain acids with CdC (e.g., maleic, fumaric, and acrylic). On one hand, the oxidation kinetics of such a wastewater (mainly, a mixture of unsaturated acids) with respect to its lumped concentration parameter (COD) was found to be a first-order two-step process with acetic acid as a major product.9 On the other hand, in the oxidation of a mixture of low molecular mass acids (glyoxalic, oxalic, formic, propionic, and acetic), the order with respect to the lumped TOC concentration was observed to be a second-order process; this behavior was ascribed to the slower rates of oxidation of more refractory acids such as formic, propionic, and acetic present in the mixture.6 During oxidation of organics, the concentration of mother compounds reduces much faster than its TOC
10.1021/ie990385y CCC: $19.00 © 2000 American Chemical Society Published on Web 12/03/1999
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or COD value. The initial rapid conversion of starting compounds produces a mixture of acid intermediates (unsaturated and aliphatic), which exhibit a relatively high resistance toward further oxidation before the entire content of organic material is converted into the most refractory phase of low molecular mass mono- and dibasic acids. The kinetics with respect to the lumped concentration parameter (TOC/COD) usually exhibits different behavior than that for the mother compounds, as was recently demonstrated with an azo dye oxidation.10 Thus, the oxidation kinetics based on the lumped concentration parameter which accounts for all oxidizable species present in aqueous solutions is much more valuable for practical purposes. So far, no information on muconic acid oxidation has been provided in the literature. Because maleic acid formation is possible only via muconic acid and/or 2,5hexenedioic acid, basic oxidation kinetic data on muconic and maleic acids should be very appreciated. Furthermore, no data have been reported on the reactivity of isomers such as maleic and fumaric acids toward oxygen. The experimental oxidation of this work was analyzed in terms of acid disappearance rates as well as their lumped TOC reduction rates and thus represents a thorough kinetic study on thermal and oxidative decomposition of these unsaturated acids. However, knowledge of this kind is a prerequisite for the prediction of TOC/COD reduction in wastewaters containing organic acids with CdC bonds. Experimental Section Materials. Maleic, fumaric, acrylic, and muconic acids were obtained from Aldrich and used as supplied. Oxygen with a minimum purity of 99.5% was used for oxidation and to check for thermal effects nitrogen of 99.99% was used. Experimental Setup and Procedure. Experiments were performed in a titanium autoclave (Parr Instrument Co., IL). However, the experimental setup and the procedure are described in detail elsewhere.5 Analysis. Concentrations of the acids were determined using a reverse-phase HPLC (Hewlett-Packard 1100/DAD) system integrated with an autosampler device (Marathon) equipped with a Rheodyne 7010-080 injection valve having a 10-µL sample loop and it was mounted on the top of the autosampler. Analysis was carried out using an anion retention SS column of 300 × 8 mm i.d. that consisted of sulfonated cross-linked styrene-divinylbenzene copolymer as a stationary phase (“Eurokat H” from Geratebau Saulentechnik Eurochrom, Germany). For the determination of maleic acid a temperature of 75 °C was employed across the column. A gradient elution method was used with two mobile phases for the analysis, namely, (a) 0.01 N H2SO4 and (b) 0.006 N H2SO4. Initially, about 10% of the total time of elution (12 min), mobile phase (a) was used with a flow rate of 1 mL/min followed by mobile phase (b) with the same flow rate. Mobile phase (b) was used until 70% of the total elution time elapsed and then mobile phase (a) was introduced again (2 mL/min). A temperature of 20 °C was used for acrylic and fumaric acid analysis, whereas 75 °C was applied for muconic acid analysis using mobile phase (a) alone. Low molecular mass aliphatic carboxylic acids that appeared as intermediate products during the oxidation experiments were also quantitatively determined. The detector (DAD)
was set for maximum absorption at 202 nm and the concentration was determined from the calibration for each individual acids at this selected wavelength. A set of acids solutions of known concentrations was run through the HPLC to establish a quantitative correlation between the peak area and acid concentration. In the range of concentrations used, a linear dependence between these two variables was found for all acids. 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) and TOC was determined by subtracting IC from TC. Identification of other intermediate products that appeared during acid oxidation was performed on a mass spectrometer LCQ (Finnigan, USA). These intermediates were analyzed qualitatively only. The following conditions were employed. For the pump, the mobile phase was bidistilled water/methanol (1:1 v/v) with 1.0 (vol %) formic acid, the flow rate was 1.0 mL min-1, and the loop volume was 20 µL; for scanning, the capillary temperature was 140 °C, the vaporizer temperature was 400 °C, the mode of scanning was negative, the scanning range m/z was 80-400, the multiplier voltage was -940.69 V, and the lens offset was 22 V. The initial mass concentrations for acrylic, maleic, fumaric, and muconic acids were 0.5, 0.3, 0.3, and 0.2 g/L, respectively. Acrylic and maleic acids are readily soluble in water, whereas fumaric acid is soluble in hot water only. Because muconic acid has a limited solubility in water, its solution in water can only be prepared in boiling water. To prevent acid precipitation at room temperature, a hot solution was injected directly into the autoclave at the operating temperature. During sampling and analysis no precipitation was observed. Because the concentrations of acids in aqueous phase were very low, the oxygen concentration in the liquid phase was assumed equal to the solubility in water.11 Results and Discussion Reaction Pathways. The unsaturated acids (acrylic, maleic, fumaric, and muconic) undergo reactions that are characteristic for both reactive centers present in these compounds, namely, CdC bond and -COOH group. The β-carbon in acrylic acid is highly reactive and polarized by the carbonyl group, which behaves like an electrophile and favors the addition of nucleophile (oxygen-rich species). In addition, the CdC bond undergoes radical-initiated addition reactions, Diels-Alder reactions, and polymerization reactions, which may also be heat-catalyzed. Maleic and fumaric acids are extremely reactive because of the -COOH group and Cd C bond and undergo decarboxylation easily. On heating, they form respective anhydrides and cis-trans molecular rearrangements may also occur. The addition of water across the double bond in maleic acid gives malic acid (C4H6O5). Muconic acid is reported as a very unstable unsaturated acid under wet oxidation conditions and readily forms maleic and oxalic acids, even at room temperature.7 Because the unsaturated acids are thermally unstable under wet oxidation conditions, they decompose by both thermal and oxidative routes. It is therefore needed to
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Figure 1. Normalized concentration of intermediates observed during muconic acid thermal decomposition.
undertake systematic studies on their decomposition in the absence and in the presence of oxygen. Except acrylic acid, which was found thermally stable up to 280 °C, maleic, fumaric, and muconic acids easily thermally decompose. For example, maleic and fumaric acids were found to be completely decomposed after 5 min at 280 °C, while complete muconic acid decomposition was observed after 30 min at 170 °C. Thermal decomposition of muconic acid yielded glyoxalic, oxalic, formic, fumaric, and maleic acids as major intermediates (Figure 1), whereas thermal decomposition of maleic and fumaric acids resulted in glyoxalic, oxalic, and formic acids. The detail reaction pathways of their interconversions (GA f OA f FA) can be found elsewhere.6 It was also observed that during heating a small fraction (
