ARTICLE pubs.acs.org/IECR
Understanding Hydrogen in Bayer Process Emissions. 2. Hydrogen Production during the Degradation of Unsaturated Carboxylic Acids in Sodium Hydroxide Solutions Allan Costine,* Joanne S.C. Loh, and Greg Power CSIRO Process Science and Engineering (Parker CRC), CSIRO Light Metals National Research Flagship, P.O. Box 7229, Karawara WA 6152, Australia ABSTRACT: This is the second in a series of studies of the production of hydrogen from the degradation of organic compounds in sodium hydroxide solutions. Unsaturated carboxylates, which are intermediate products in the wet oxidation of the types of monoaromatic compounds typically present in Bayer process liquors, have been found to produce significant amounts of hydrogen during alkaline degradation. The alkaline degradation of nine unsaturated carboxylates was investigated under anaerobic conditions in an autoclave, and the effect of temperature (175275 °C) and NaOH concentration (06 M) on the degradation of acrylate, 2-butenoate, maleate, and 2-hexenoate was studied in detail for reaction times up to 120 min. All of the compounds investigated decompose to produce about 1 mol of hydrogen gas per mole of organic compound consumed, as well as a range of low molecular weight (LMW) carboxylates. The stoichiometries of the formation of hydrogen and LMW carboxylates from the unsaturated carboxylates observed here are consistent with the regioselective hydration of the CdC double bond to form the corresponding βhydroxycarboxylates, which then undergo further degradation by an ionic mechanism involving base-catalyzed oxidation by water. These findings significantly advance the understanding of the production of hydrogen in Bayer process digestion and have implications for the safe application of wet oxidation of Bayer process liquors. The results also advance the fundamental understanding of the alkaline degradation of α,β-unsaturated carbonyl compounds in general.
1. INTRODUCTION Hydrogen is a component of the noncondensable gases generated during bauxite digestion1,2 and the wet oxidation of Bayer process liquors.37 The production of hydrogen during wet oxidation processes is of particular concern due to its low-energy ignition, buoyancy, and wide flammability limits in comparison to most other gases.8 Knowledge of the degradation reactions of specific classes of organic compounds is important to an understanding of the origins and mechanisms of hydrogen production in the Bayer process. The breakdown of humates and lignins under Bayer digestion conditions9,10 produces a variety of stable monoaromatic compounds in the liquor.1114 The wet oxidation of aqueous solutions of monoaromatic compounds yields α,β-unsaturated carboxylic acids as intermediate products through decarboxylation and benzene ring-opening reactions.15 On this basis, it seems likely that the wet oxidation of Bayer process liquors could also produce α,β-unsaturated carboxylates. The few reports in the literature of the presence of unsaturated carboxylates in Bayer process liquors include aconitate,16 hexenedioate,17,18 9-octadecenedioate,19 and 9,12-octadecadienoate.20 In alkaline solutions, α,β-unsaturated carboxylates exist in equilibrium with their corresponding β-hydroxycarboxylates.21 We recently confirmed22 that the production of hydrogen from β-hydroxycarboxylates, such as 3-hydroxybutanoate and malate, can occur via an ionic degradation mechanism involving basecatalyzed oxidation by water.23,24 Above 200 °C, all of these compounds decompose to produce about 1 mol of hydrogen per mole of starting compound. The observed stoichiometry of the formation of hydrogen from the β-hydroxycarboxylates studied Published 2011 by the American Chemical Society
suggests that the hydration of their unsaturated counterparts, 2-butenoate and fumarate, respectively, is favorable under these conditions. Consistent with this, the base-catalyzed hydration of fumarate produces malate between 90 and 175 °C,25,26 and there is a patent which describes the near-quantitative conversion of maleate to malate by heating in alkaline solution between 140 and 190 °C.27 However, there is little information available on the hydration of other α,β-unsaturated carboxylates to their corresponding β-hydroxycarboxylates. It follows that, if the hydration of the CdC double bond yields β-hydroxycarboxylates, then the alkaline degradation of α,β-unsaturated carboxylates should also produce hydrogen gas. To test this, the alkaline degradation of a range of unsaturated carboxylates was studied to determine their hydrogen production capacities, define the reaction conditions that promote hydrogen production, provide quantitative information on the products of reaction, and interpret the results in terms of reaction mechanisms. This work is the first report detailing the production of hydrogen and low molecular weight (LMW) carboxylates during the alkaline degradation of unsaturated carboxylates.
2. EXPERIMENTAL SECTION 2.1. Autoclave Facility. The thermal degradation experiments were carried out in a 2 L Inconel 600 autoclave (Parr Instruments Received: June 28, 2011 Accepted: October 1, 2011 Revised: September 26, 2011 Published: October 01, 2011 12334
dx.doi.org/10.1021/ie201388c | Ind. Eng. Chem. Res. 2011, 50, 12334–12342
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Unsaturated Compounds Used in the Current Work compound
refsa
structure unsaturated monocarboxylates
acrylate 2-butenoate 3-butenoate 2-pentenoate 2-hexenoate
CH2dCH 3 COh2
cellulose;30 phenol;31,32 pyridine;33 succinic acid34
CH3 3 CHdCH 3 COh2 CH2dCH 3 CH2 3 COh2
CH3 3 CH2 3 CHdCH 3 COh2 CH3 3 (CH2)2 3 CHdCH 3 COh2 unsaturated dicarboxylates
maleate (cis-butenedioate) fumarate
COh2 3 CHdCH 3 COh2
1,4-benzoquinone;35,36 catechol, hydroquinone;36 cellulose;30 2,6-dimethoxyphenol,
COh2 3 CHdCH 3 COh2
1,4-benzoquinone, hydroquinone;36 2,6-dimethoxyphenol, 2-methoxyphenol;37 phenol31,32,36,37,39,41,42
COh2 3 C(OH)dC(OH) 3 COh2 COh2 3 CH2 3 CHdCH 3 COh2
2,6-dimethoxyphenol, 2-methoxyphenol, phenol;37 pyridine33
2-methoxyphenol;37 1,4-hydroxybenzoic acid;35 phenol;31,32,3542 pyridine33
(trans-butenedioate) dihydroxyfumarate glutaconate a
Parent compounds that produce unsaturated compounds (in their acidic forms) as intermediate products in wet oxidation studies.
Co., IL, U.S.A.). One experiment was also performed in a 2 L Parr zirconium (Zr 705) autoclave to enable detection of any possible effect on product yields of the materials of construction of the Inconel autoclave. The production of hydrogen and LMW carboxylate products in both autoclaves were the same within experimental error. Duplicate experiments were within (0.05 mol % hydrogen. The oxygen and hydrogen concentrations in the headspace were measured using online electrochemical sensors. A detailed description of the autoclave facility is provided elsewhere.28 2.2. Organic Compounds. The range of organic compounds tested is shown in Table 1. All reagents were obtained in their acidic forms and used as received (AR grade, Sigma Aldrich). The initial concentration of the compounds was 40 mM. The group of unsaturated monocarboxylates included a series of α,β-unsaturated carboxylates ranging from acrylate to 2-hexenoate (C3 C6). 3-Butenoate was studied to determine whether the position of the CdC double bond has any effect on product yields. The group of unsaturated dicarboxylates included maleate and fumarate because a cis- to trans- isomerization reaction can occur under wet oxidation conditions.29 Dihydroxyfumarate has been included to test whether the presence of two hydroxy groups on the CdC double bond has any effect on the products formed. Of the nine compounds investigated, four have been specifically identified as intermediate products in the wet oxidation of aqueous solutions of monoaromatic compounds. The effects of various reaction parameters on the degradation of acrylate (99% acrylic acid inhibited with 200 ppm monoethyl ether hydroquinone), 2-butenoate (98% 2-butenoic acid), maleate (99% maleic acid) and 2-hexenoate (98% trans-2-hexenoic acid) were then studied in greater detail. 2.3. Solution Preparation. Sodium hydroxide solutions were prepared by dissolving the appropriate mass of NaOH pellets (>97%, 1% Na2CO3, Rowe Scientific) in deionized water. Solutions of each organic compound were made up by adding the required mass of organic compound to the sodium hydroxide solution. A solution volume of 1 L was used for each experiment. The autoclave was then sealed and inserted into the heating cavity. 2.4. System Preparation. The solutions were sparged with nitrogen until the concentration of residual oxygen in the headspace was below 0.01 mol %. The solutions were heated at a rate of approximately 5 °C/min, with stirring, to the designated
temperature. The time at which the required temperature was reached was taken as t = 0 min, the nominal starting point of the reaction. 2.5. Gas Analysis. The procedure for gas analysis was the same as previously described.22 For each analysis, it was necessary to stop the reaction by cooling the autoclave to enable the hydrogen content of the headspace gas to be measured. Each data point in Figures 26 therefore represents an individual autoclave run carried out for the nominated reaction time in the range 0 to 120 min. 2.6. Solution Analysis. Capillary electrophoresis and ion chromatography were used to measure the concentrations of the following LMW carboxylate products: formate, acetate, oxalate, malonate, butanoate, succinate, and lactate.43 The concentration of carbonate in the solutions was measured using an automated potentiometric titration system.44 Molar yields were calculated as the number of moles of product formed per mole of organic compound used.
3. RESULTS AND DISCUSSION 3.1. Screening Study. Figure 1 shows the production of hydrogen and LMW carboxylates during the anaerobic degradation of nine unsaturated compounds after holding at 250 °C for 15 min. Hydrogen and acetate are formed as common degradation products from all of the compounds studied. The production of oxalate as a major product was only observed for the group of C4 unsaturated dicarboxylates. The presence of carbonate in the reaction products was insignificant, except in the degradation of glutaconate. An explanation of the nature and amounts of products observed during the degradation of all the compounds studied is given in section 3.3, in relation to the proposed degradation mechanisms. Given that liquors generally contain a significant aromatic content,13,14 it is likely that the use of wet oxidation to remove organic compounds from liquor could result in the formation of unsaturated carboxylates as intermediate products through benzene ring-opening reactions (see Table 1). 3.2. Detailed Study of the Degradation of α,β-Unsaturated Carboxylates. Acrylate, 2-butenoate, maleate, and 2-hexenoate were selected for more detailed investigation in order to develop a kinetic and mechanistic understanding of their degradation reactions. Unsaturated carboxylates such as acrylate and maleate are commonly identified as intermediate products in the 12335
dx.doi.org/10.1021/ie201388c |Ind. Eng. Chem. Res. 2011, 50, 12334–12342
Industrial & Engineering Chemistry Research
ARTICLE
Figure 2. First-order plots calculated using eq 1 for the degradation of acrylate, 2-butenoate, maleate, and 2-hexenoate. Conditions: temperature = 250 °C, time at temperature = 0120 min, [NaOH] = 3.77 M. Figure 1. (A) Number of moles of H2 produced per mole of the organic compound and (B) distribution of LMW products. Conditions: temperature = 250 °C, time at temperature = 15 min, [NaOH] = 3.77 M. Legend: Ox = oxalate; For = formate; Ace = acetate; Lac = lactate; But = butanoate; Mal = malonate; Car = carbonate.
wet oxidation of monoaromatic compounds as shown in Table 1. The degradation reactions of 2-butenoate and maleate were compared with their corresponding β-hydroxycarboxylates, 3-hydroxybutanoate and malate, which were previously described.22 2-Hexenoate was selected for more detailed investigation because C6 unsaturated compounds such as 2,5-dioxo-3-hexenedioic and muconic acids are expected to be formed in the wet oxidation of monoaromatic compounds.29,31,37 The following sections show the effect of various reaction parameters on the thermal degradation of these unsaturated carboxylates. 3.2.1. Reaction Kinetics. After complete degradation, the number of moles of hydrogen produced is almost exactly equal to the initial number of moles of parent compound for all of the unsaturated carboxylates studied except dihydroxyfumarate. The concentration of the parent compounds (apart from dihydroxyfumarate) at any time during the reaction may be calculated from hydrogen measurements, as follows: ½At ¼ ½Ao nt =V
ð1Þ
where: [A]t = concentration of parent compound, A, in mol/L, at time t; [A]o = concentration of parent compound, A, in mol/L, at time t = 0; nt = number of moles of H2 produced in time t; V = volume of solution (L). The rate of degradation of each compound was calculated in this way as a function of time at 250 °C. Figure 2 shows that the dependence of [A] on time follows a first-order law for all of the compounds, although a degree of nonlinearity was observed at longer reaction times for 2-hexenoate. The molar ratio of compound A at time t to its initial concentration is defined as r ¼ ½At =½Ao From these results it follows that the reaction rates may be described by a first-order rate law of the form: d½A=dt ¼ k½A where k is the first-order rate constant. The order of reactivity was as follows: 2-butenoate > maleate >2-hexenoate > acrylate.
3.2.2. Evolution of Products. The evolution of the principal degradation products of the unsaturated carboxylates at 250 °C is shown in Figure 3. These results are presented in terms of the ratio of the number of moles of each degradation product to the initial number of moles of parent organic compound in each case. The solid lines represent the number of moles of hydrogen produced according to a first order law with the rate constants derived from Figure 2. For all of the organic compounds studied, these lines show an excellent fit to the experimental data provided that the starting time is adjusted to allow for a degree of reaction occurring during the heatup period. The best fits were found by offsetting the measurements by 59 min, indicating that the heatup period was equivalent to 59 min at temperature for the compounds studied. The dotted lines representing the concentrations of the parent unsaturated compounds as a function of time were calculated from the amount of hydrogen produced assuming the 1:1 molar ratio discussed above. In the degradation of acrylate (Figure 3A), the molar ratio of the sum of organic products to the hydrogen produced (and acrylate consumed) is slightly below 2:1. Acetate and formate are the main LMW carboxylates produced which account for 74% of the initial carbon after 120 min, with undegraded starting material accounting for the remainder. For 2-butenoate (Figure 3B), the only major product other than hydrogen is acetate, accompanied by small amounts of formate and oxalate. The molar sum of organic products to the hydrogen produced is slightly above 2:1. Acetate accounts for about 92% of the organic products, with about 4% formate and 4% oxalate. These organic products account for 100% of the initial carbon after 30 min. For maleate (Figure 3C), the molar sum of organic products is slightly more than twice the amount of hydrogen produced throughout the reaction period. Acetate and oxalate are the dominant products, followed by smaller amounts of formate. These LMW carboxylates account for almost 100% of the initial carbon after 120 min, indicating that the reaction is essentially complete. In the degradation of 2-hexenoate (Figure 3D), the molar ratio of the sum of organic products to the hydrogen produced is slightly below 2:1 at higher conversions of the parent compound. Acetate and butanoate are the main products with trace amounts of oxalate and formate detected. The carbon recovery from these products was 81% after 120 min. 12336
dx.doi.org/10.1021/ie201388c |Ind. Eng. Chem. Res. 2011, 50, 12334–12342
Industrial & Engineering Chemistry Research
ARTICLE
Figure 3. Number of moles of H2 and LMW carboxylate products produced per mole of (A) acrylate, (B) 2-butenoate, (C) maleate, and (D) 2-hexenoate. Conditions: temperature = 250 °C, time at temperature = 0120 min, [NaOH] = 3.77 M. Symbols: = total organic products; b = H2 (measured); 4 = acetate; O = oxalate; 0 = formate; ) = butanoate.
3.2.3. Effect of Temperature. The effect of temperature on the product yields from the unsaturated carboxylates is shown in Figure 4. Acrylate is the most stable of the compounds studied, with little reaction occurring below 225 °C (Figure 4A). Hydrogen, acetate, and formate are the major products of acrylate degradation, with the ratio of the molar yields of these products being close to 1:1:1. Only trace amounts of oxalate were produced. The absence of lactate in the products indicates that the hydration of the CdC double bond to form this α-hydroxycarboxylate did not occur. The observed LMW products at 275 °C accounted for 70% of the initial carbon. In contrast, the thermal decomposition of acrylic acid in aqueous solutions is negligible up to 280 °C.29 The complete decomposition of acrylic acid in aqueous solutions at lower temperatures requires the use of catalytic wet oxidation.45 For 2-butenoate degradation, little reaction occurs below 200 °C (Figure 4B). Acetate and hydrogen are the dominant products. The ratio of the molar yields of acetate to hydrogen is about 2:1 over the entire temperature range, indicating that these products are formed through a common degradation pathway. Only small amounts of formate and oxalate were produced. The degradation of 2-butenoate is complete after 15 min at 275 °C as confirmed by a full carbon recovery in the products. Similarly, for maleate degradation, little reaction occurs below 200 °C (Figure 4C). Hydrogen, acetate, and oxalate are the main products with the ratio of the molar yields of these products being about 1:1:1. The amount of formate produced was small and constant up to 275 °C. At this temperature, the observed LMW products accounted for almost all of the initial carbon.
Additional tests (not shown) on the alkaline degradation of fumarate under identical conditions gave similar product distributions. In aqueous solutions, maleic and fumaric acids are completely degraded under anaerobic conditions after 5 min at 280 °C.29 Formic acid was a major product from both isomers in the absence of oxygen, whereas acetic acid was formed only in the presence of oxygen. Likewise, for 2-hexenoate degradation, the extent of reaction below 200 °C is insignificant. Hydrogen, acetate, and butanoate are the dominant products. After 15 min at 275 °C, the observed LMW products accounted for 78% of the initial carbon. The rate constants at different temperatures can be estimated from measurements of the extent of reaction at a fixed time provided that the rate constant has been measured for at least one temperature. This was achieved using Polymath version 6.10 software to create a model calibrated with the rate constants measured at 250 °C (Figure 2). The results are shown in Figure 5 in the form of an Arrhenius plot, from which the activation energies have been estimated. The activation energies for the reactions of acrylate, 2-butenoate, maleate, and 2-hexenoate were found to be 173, 158, 173, and 176 kJ/mol respectively, indicating a common rate-determining step for the alkaline degradation of the unsaturated carboxylates. 3.2.4. Effect of NaOH Concentration. The production of hydrogen from the unsaturated carboxylates studied was found to be significantly influenced by the NaOH concentration. A basecatalyzed mechanism has previously been shown to apply in the degradation of malonate.46 In that study, the hydroxide concentration appears as a factor in the first-order rate constant as 12337
dx.doi.org/10.1021/ie201388c |Ind. Eng. Chem. Res. 2011, 50, 12334–12342
Industrial & Engineering Chemistry Research
ARTICLE
Figure 4. Number of moles of H2 and LMW carboxylate products produced per mole of (A) acrylate, (B) 2-butenoate, (C) maleate, and (D) 2-hexenoate. Conditions: temperature = 175275 °C, time at temperature = 15 min, [NaOH] = 3.77 M.
Figure 5. Arrhenius plot for the degradation of acrylate, 2-butenoate, maleate, and 2-hexenoate. Conditions: temperature = 200275 °C, time at temperature = 15 min, [NaOH] = 3.77 M.
follows: k ¼ ko ½OH Using the modeling approach described in section 3.2.3, the rate constant determined under one set of conditions can be used to calibrate a model which is used to predict the extent of reaction at any given time under any chosen set of conditions, assuming that the molar ratios of the organic products formed to the hydrogen produced are constant. This method has been used to compare the first-order model with measurements of the extent of reaction after 15 min at 275 °C for the unsaturated carboxylates as
Figure 6. Number of moles of H2 produced per mole of the organic compound. The points show the experimental data and the lines show the first-order degradation models. Conditions: temperature = 275 °C, time at temperature = 15 min, [NaOH] = 06 M.
a function of [OH]. This is shown in Figure 6, in which the curves were calculated using the pseudo-first-order model and the points are the experimental results. These results suggest a common rate-determining step involving a unimolecular reaction with OH in each case. 3.3. Proposed Degradation Mechanisms. An ionic degradation mechanism, consistent with the formation and distribution of the dominant reaction products of acrylate (1), 2-butenoate (2), maleate (3), and 2-hexenoate (4), is illustrated in Scheme 1. 12338
dx.doi.org/10.1021/ie201388c |Ind. Eng. Chem. Res. 2011, 50, 12334–12342
Industrial & Engineering Chemistry Research
ARTICLE
Scheme 1. Proposed Degradation Mechanism for Acrylate (1), 2-Butenoate (2), Maleate (3), and 2-Hexenoate (4) in the Explanation of the Results Presented in Figures 3 and 4
The reversible hydration of the unsaturated carboxylates yields their corresponding β-hydroxycarboxylates (58). Cleavage of the β-hydroxycarboxylates can be initiated by the deprotonation of the β-hydroxy group. In pathway A, which is common to all of the compounds studied, an alkene species (9) reacts with water to produce acetate (10) and regenerates a hydroxyl ion. The initial cleavage reaction also produces various aldehyde intermediates (1114), the nature of which depends on the starting compound (pathway B). Subsequent nucleophilic attack by a hydroxyl ion on the carbonyl carbon results in an intermediate species (1518). These intermediate species undergo base-catalyzed oxidation by water,23,24 involving a hydride transfer to a water molecule, to produce aliphatic carboxylates (formate (19), acetate (10), oxalate (20), or butanoate (21)), hydrogen gas, and hydroxyl ion. It is worthwhile to review the product distributions of the other five unsaturated compounds investigated (Figure 1), in relation to the degradation mechanism proposed in Scheme 1. For 3-butenoate, the only major product other than hydrogen is acetate and the ratio of the molar yields of acetate to hydrogen is about 2:1 (not shown). These results are almost identical to the product distributions observed for 2-butenoate (2). This indicates that both of these compounds hydrate to form 3-hydroxybutanoate (6), which further highlights the high reactivity of the β-carbon position in the parent compounds. For 2-pentenoate, the main products expected are acetate, propanoate, and hydrogen. However, the presence of propanoate in the reaction products could not be confirmed with the analytical methods used in the current work. For fumarate, the distribution of degradation products is comparable to its cis isomer maleate. The degradation mechanism shown in Scheme 1 is consistent with the nature and amounts of products observed from all of the compounds investigated, except dihydroxyfumarate and glutaconate. Dihydroxyfumarate was not expected to degrade according to the mechanism shown in Scheme 1 because the presence of the two hydroxyl groups precludes the ready formation of a β-hydroxycarboxylate in alkaline solutions. If glutaconate (22) degrades according to this mechanism, then the main products expected would be acetate, malonate, and hydrogen. The presence of carbonate in the products, rather than malonate, is indicative of a decarboxylation mechanism as illustrated in Scheme 2. The decarboxylation of 3-hydroxyglutarate (23) can produce 3-hydroxybutanoate (6), which subsequently reacts to form acetate
(10) and hydrogen. The direct decarboxylation of glutaconate (22) could also occur, either at its saturated or unsaturated end, to form 2- or 3-butenoate, respectively. As noted above, both of these compounds degrade to produce acetate and hydrogen as the dominant products. These mechanistic interpretations are consistent with the observed product distributions for glutaconate in Figure 1. The observed stoichiometry of the formation of hydrogen and LMW carboxylates from the unsaturated carboxylates studied is consistent with the regioselective hydration of the CdC double bond to form their corresponding β-hydroxycarboxylates (Scheme 1). The nature and amounts of products observed are the same when either the unsaturated carboxylates or their corresponding β-hydroxycarboxylates are used as the starting materials. For example, 2-butenoate (2) and 3-hydroxybutanoate (6) degrade to produce similar products; maleate (3) and malate (7) degrade to produce similar products.22 Machold et al.26,47 have reported acetate and oxalate as the major products when either fumarate or malate are used as the starting materials. The production of minor products during the alkaline degradation of 2-butenoate (formate and oxalate) and maleate (formate) can be explained by the mechanism shown in Schemes 2 and 3 in the work of Costine et al.22 for their corresponding β-hydroxycarboxylates. Only trace quantities of minor products were observed during the degradation of acrylate and 2-hexenoate. A degree of nonlinearity was observed in the first-order plot for 2-hexenoate at longer reaction times, as shown in Figure 2. This could result from secondary reactions involving the butanal (14) cleavage product, which may account for the slightly lower butanoate yields at longer reaction times. 3.4. Relevance to the Alkaline Degradation of α,β-Unsaturated Carbonyl Compounds. The current work confirms that the production of hydrogen from the α,β-unsaturated carboxylates studied can occur via an ionic degradation mechanism involving base-catalyzed oxidation by water. It is likely that these findings are also applicable to the alkaline degradation of α,βunsaturated carbonyl compounds in general. This is illustrated in Scheme 3 for the degradation of 4-phenyl-3-buten-2-one (24). In alkaline solutions, this compound exists in equilibrium with 4-phenyl-4-hydroxy-2-butanone (25),48 and it was suggested that the observed cleavage products, acetone (27) and benzaldehyde (28), are formed from the degradation of the ketene (24) rather than the ketol (25). The presence of benzoate (30) in the 12339
dx.doi.org/10.1021/ie201388c |Ind. Eng. Chem. Res. 2011, 50, 12334–12342
Industrial & Engineering Chemistry Research
ARTICLE
Scheme 2. Proposed Degradation Mechanism for Glutaconate (22) Involving the Decarboxylation of 3-Hydroxyglutarate (23) in the Explanation of the Results Presented in Figure 1
Scheme 3. Proposed Degradation Mechanism for 4-Phenyl-3-buten-2-one (24) to Produce Acetone (27) and Benzaldehyde (28) Based on the Results of Noyce and Reed.48
reaction products was not reported and the production of hydrogen was not measured. At the higher temperatures used in the current work, 24 would probably degrade according to the mechanism shown in Scheme 3. This mechanism is consistent with an earlier study in which hydrogen was observed as a product of the alkaline degradation of benzaldehyde.49 We also confirmed the production of hydrogen from benzaldehyde under the alkaline conditions reported in Figure 1 (not shown). The presence of organic compounds in the liquor with structural similarities to 24 could result from the breakdown of lignins under Bayer digestion conditions. As the lignin macromolecule is thought to be formed by the polymerization of cinnamyl alcohols,50 it is probable that unsaturated side-chains are retained as end groups (—CHdCH—CH2OH). Scheme 3 shows that basecatalyzed oxidation by water can account for the formation of monoaromatic carboxylates in Bayer liquors and also the presence of volatile organic compounds and hydrogen in refinery emissions, as oxidation and reduction products, respectively.13,51 The patent literature also describes the reactions of α,βunsaturated carbonyl compounds in alkaline solutions to produce aldehydes as cleavage products.52 For example, 3-decen-2one reacts to produce heptanal and acetone; 5-methyl-3-hexen-2one reacts to produce isobutyraldehyde and acetone; and cinnamaldehyde reacts to produce benzaldehyde and acetaldehyde. Product yields exceeding 70% were obtained when either pure parent compounds or natural sources of α,β-unsaturated
carbonyl compounds were used as the starting materials. The potential for hydrogen production during these reactions was not noted by the authors, although this is very likely under the conditions used in the current work.
4. CONCLUSIONS This is the second in a series of related studies aimed at developing a fundamental understanding of the production of hydrogen from specific classes of organic compounds in sodium hydroxide solutions. The main findings from this study are that, in the alkaline degradation of a range of C3C6 unsaturated organic compounds: • α,β-Unsaturated carboxylates, which are commonly identified as intermediate products in wet oxidation studies, were found to be key hydrogen producers. Above 200 °C, these compounds decompose to produce 1 mol of H2 per mole of the parent compound as follows: OH
R 3 CHdCH 3 CO2 sf R 3 CO2 þ CH3 3 CO2 þ H2 where R is a hydrogen atom, an alkyl group, or a carboxylate group. 12340
dx.doi.org/10.1021/ie201388c |Ind. Eng. Chem. Res. 2011, 50, 12334–12342
Industrial & Engineering Chemistry Research • The alkaline degradation of the α,β-unsaturated carboxylates studied follows pseudo-first-order reaction kinetics in which the rate constant contains [OH]. The order of reactivity is: 2-butenoate > maleate >2-hexenoate > acrylate. • Quantitative analysis of the products of reaction of the unsaturated carboxylates studied indicates that the hydration of the CdC double bond yields the corresponding βhydroxycarboxylates, which then degrade via an ionic mechanism involving base-catalyzed oxidation by water. This mechanism accounts for the observed stoichiometry of formation of the main LMW carboxylate products and hydrogen gas. • These findings show that the degradation of monoaromatic compounds in Bayer process liquors may produce significant hydrogen via unsaturated carboxylate intermediates and also advance the fundamental understanding of the alkaline degradation of α,β-unsaturated carbonyl compounds in general.
’ AUTHOR INFORMATION Corresponding Author
*E-mail address:
[email protected]. Tel.: +61 8 9334 8031. Fax: +61 8 9334 8001.
’ ACKNOWLEDGMENT The authors thank Milan Chovancek for conducting the analyses of the LMW degradation products. The financial support of the Parker CRC for Integrated Hydrometallurgy Solutions (established and supported under the Australian Government’s Cooperative Research Centres Program) and CSIRO Light Metals National Research Flagship is gratefully acknowledged. ’ REFERENCES (1) Costine, A.; Schibeci, M.; Loh, J.; Power, G.; McDonald, R. Hydrogen production in Bayer process digestion. Travaux 2010, 35 (39), 369–375. (2) Graham, G.; Capil, R.; Davies, R. Odour destruction for digestion vent gases. 6th International Alumina Quality Workshop; AQW Inc: Brisbane, Australia, September 813, 2002; pp 316319. (3) Anderson, J.; Armstrong, L.; Thomas, D. Wet oxidation in QAL [Queensland Aluminum Limited] high temperature digestion for plant productivity improvement. Light Met. 2001, 121–126. (4) Arnswald, W.; Kaltenberg, H. G.; Guhl, E. Removal of organic carbon from Bayer liquor by wet oxidation in tube digesters. Light Met. 1991, 23–27. (5) Baker, C. L.; Lehmann, R. W.; Maugans, C. B.; Burnet, S.; Wellisch, K. F.; Lewi, C. Method of hydrothermal treatment of organic contaminants of alkaline Bayer process solutions. Patent no. AU2008229997, 2008. (6) Brown, N. Kinetics of copper-catalysed oxidation of Bayer liquor organics. Light Met. 1989, 121–130. (7) The, P. J.; Williams, F. S.; Guthrie, J. D. Mass transfer studies on wet oxidation of Bayer liquor. Pilot plant evaluation. Light Met. 1985, 103–116. (8) H€aussinger, P.; Lohm€uller, R.; Watson, A. M. Hydrogen; WileyVCH Verlag GmbH & Co. KGaA: Weinheim, 2007. (9) Ellis, A. V.; Wilson, M. A.; Forster, P. Bayer poisons: Degradation of klason lignin in sodium hydroxide at 145 degrees C. Ind. Eng. Chem. Res. 2002, 41 (25), 6493–6502. (10) H€anninen, K.; Niemel€a, K. Alkaline-degradation of peat humic acids. 2. Identification of hydrophilic products. Acta Chem. Scand. 1992, 46 (5), 459–463.
ARTICLE
(11) Niemel€a, K.; Grocott, S. C. Determination of carboxylic acids in sodium aluminate liquor (Bayer liquor). International Conference on Biological Mass Spectrometry, Kyoto, Japan, September 2024, 1992; pp 480181. (12) Picard, F.; Audet, D.; Boily, H.; Larocque, J. Identification of hydrate active organics (HAO) present in spent Bayer liquors by stateof-the-art analytical methods. 6th International Alumina Quality Workshop; AQW Inc: Brisbane, Australia, September 813, 2002; pp 4653. (13) Power, G.; Loh, J. Organic compounds in the processing of lateritic bauxites to alumina: Part 1: Origins and chemistry of organics in the Bayer process. Hydrometallurgy 2010, 105 (12), 1–29. (14) Power, G.; Loh, J. S. C.; Niemel€a, K. Organic compounds in the processing of lateritic bauxites to alumina: Addendum to Part 1: Origins and chemistry of organics in the Bayer process. Hydrometallurgy 2011, 108 (12), 149–151. (15) Mishra, V. S.; Mahajani, V. V.; Joshi, J. B. Wet air oxidation. Ind. Eng. Chem. Res. 1995, 34 (1), 2–48. (16) Xiao, J. B.; Ren, F. L. Rapid separation and determination of organic acids in bauxite by RP-HPLC after solid-phase extraction. Rev. Chim. 2007, 58 (1), 79–83. (17) Chen, Q. Y.; Xiao, J. B.; Chen, X. Q. Rapid determination of organic acids in Bayer liquors by high-performance liquid chromatography after solid-phase extraction. Miner. Eng. 2006, 19 (14), 1446–1451. (18) Xiao, J. B.; Chen, X. Q.; Jiang, X. Y.; Wu, S. D. Rapid separation and analysis of six organic acids in Bayer liquors by RP-HPLC after solidphase extraction. Ann. Chim. 2006, 96 (56), 347–354. (19) Brunow, G.; Stick, R. V.; Syrjanen, K.; Tilbrook, D. M. G.; Williams, S. J. A Synthesis of (Z)-Octadec-9-Enedioic Acid. Aust. J. Chem. 1995, 48 (11), 1893–1897. (20) Xiao, J. B. Identification of organic acids and quantification of dicarboxylic acids in Bayer process liquors by GC-MS. Chromatographia 2007, 65 (34), 185–190. (21) Miltenberger, K., Hydroxycarboxylic acids, aliphatic. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2005. (22) Costine, A.; Loh, J. S.C.; Power, G.; Schibeci, M.; McDonald, R. G. Understanding hydrogen in Bayer process emissions: 1. Hydrogen production during the degradation of hydroxycarboxylic acids in sodium hydroxide solutions. Ind. Eng. Chem. Res. 2011, 50, doi: 10.1021\ie201387x. (23) Loh, J.; Brodie, G.; Power, G. P.; Vernon, C. Mechanisms of degradation of hydrate yield inhibitors by wet oxidation. 8th International Alumina Quality Workshop; AQW Inc: Darwin, Australia, September 712, 2008; pp 199205. (24) Loh, J. S. C.; Brodie, G. M.; Power, G.; Vernon, C. F. Wet oxidation of precipitation yield inhibitors in sodium aluminate solutions: Effects and proposed degradation mechanisms. Hydrometallurgy 2010, 104 (2), 278–289. (25) Erickson, L. E.; Alberty, R. A. Kinetics and mechanism of the base-catalysed hydration of fumarate to malate. J. Phys. Chem. 1958, 63, 705–709. (26) Machold, T.; Laird, D. W.; Rowen, C. C.; May, P. M.; Hefter, G. T. Decomposition of Bayer process organics: Phenolates, polyalcohols, and additional carboxylates. Hydrometallurgy 2011, 107 (34), 68–73. (27) Connor, D. S.; Kretschmar, H. C. Process for making malate salts and thereby, amlic acid or 2,20 -oxodisuccinates. US patent no. 5,298,634, 1994. (28) Costine, A.; Loh, J.; McDonald, R.; Power, G. Unique hightemperature facility for studying organic reactions in the Bayer process. Light Met. 2010, 47–52. (29) Shende, R. V.; Levec, J. Subcritical aqueous-phase oxidation kinetics of acrylic, maleic, fumaric, and muconic acids. Ind. Eng. Chem. Res. 2000, 39 (1), 40–47. (30) Jin, F. M.; Zhou, Z. Y.; Moriya, T.; Kishida, H.; Higashijima, H.; Enomoto, H. Controlling hydrothermal reaction pathways to improve acetic acid production from carbohydrate biomass. Environ. Sci. Technol. 2005, 39 (6), 1893–1902. (31) Devlin, H. R.; Harris, I. J. Mechanism of the oxidation of aqueous phenol with dissolved oxygen. Ind. Eng. Chem. Fundam. 1984, 23 (4), 387–92. 12341
dx.doi.org/10.1021/ie201388c |Ind. Eng. Chem. Res. 2011, 50, 12334–12342
Industrial & Engineering Chemistry Research
ARTICLE
(32) Oliviero, L.; Barbier-Jr, J.; Duprez, D.; Guerrero-Ruiz, A.; Bachiller-Baeza, B.; Rodriguez-Ramos, I. Catalytic wet air oxidation of phenol and acrylic acid over Ru/C and Ru-CeO2/C catalysts. Appl. Catal., B 2000, 25 (4), 267–275. (33) Crain, N.; Tebbal, S.; Li, L. X.; Gloyna, E. F. Kinetics and reaction pathways of pyridine oxidation in supercritical water. Ind. Eng. Chem. Res. 1993, 32 (10), 2259–2268. (34) Beziat, J. C.; Besson, M.; Gallezot, P.; Durecu, S. Catalytic wet air oxidation on a Ru/TiO2 catalyst in a trickle-bed reactor. Ind. Eng. Chem. Res. 1999, 38 (4), 1310–1315. (35) Quintanilla, A.; Casas, J. A.; Mohedano, A. F.; Rodriguez, J. J. Reaction pathway of the catalytic wet air oxidation of phenol with a Fe/ activated carbon catalyst. Appl. Catal., B 2006, 67 (34), 206–216. (36) Santos, A.; Yustos, P.; Quintanilla, A.; Rodriguez, S.; Garcia-Ochoa, F. Route of the catalytic oxidation of phenol in aqueous phase. Appl. Catal., B 2002, 39 (2), 97–113. (37) Suzuki, H.; Cao, J.; Jin, F.; Kishita, A.; Enomoto, H.; Moriya, T. Wet oxidation of lignin model compounds and acetic acid production. J. Mater. Sci. 2006, 41 (5), 1591–1597. (38) Dobrynkin, N. M.; Batygina, M. V.; Noskov, A. S.; Tsyrulnikov, P. G.; Shlyapin, D. A.; Schegolev, V. V.; Astrova, D. A.; Laskin, B. M. Catalysts Ru-CoCeO2/Sibunit for catalytic wet air oxidation of aniline and phenol. Top. Catal. 2005, 33 (14), 69–76. (39) Fortuny, A.; Bengoa, C.; Font, J.; Castells, F.; Fabregat, A. Water pollution abatement by catalytic wet air oxidation in a trickle bed reactor. Catal. Today 1999, 53 (1), 107–114. (40) Santos, A.; Yustos, P.; Gomis, S.; Ruiz, G.; Garcia-Ochoa, F. Reaction network and kinetic modeling of wet oxidation of phenol catalyzed by activated carbon. Chem. Eng. Sci. 2006, 61 (8), 2457–2467. (41) Vogel, F.; Harf, J.; Hug, A.; von Rohr, P. R. Promoted oxidation of phenol in aqueous solution using molecular oxygen at mild conditions. Environ. Prog. 1999, 18 (1), 7–13. (42) Yang, S. X.; Li, X.; Zhu, W. P.; Wang, J. B.; Descorme, C. Catalytic activity, stability and structure of multi-walled carbon nanotubes in the wet air oxidation of phenol. Carbon 2008, 46 (3), 445–452. (43) Power, G.; Loh, J. S. C.; Wajon, J. E.; Busetti, F.; Joll, C. A review of the determination of organic compounds in Bayer process liquors. Anal. Chim. Acta 2011, 689 (1), 8–21. (44) Connop, W. L. A new procedure for the determination of alumina, caustic, and carbonate in Bayer liquors. 4th International Alumina Quality Workshop; AQW Inc: Darwin, Australia, June 27, 1996; pp 321329. (45) Silva, A. M. T.; Marques, R. R. N.; Quinta-Ferreira, R. M. Catalysts based in cerium oxide for wet oxidation of acrylic acid in the prevention of environmental risks. Appl. Catal., B 2004, 47 (4), 269–279. (46) Dong, J.; Power, G.; Loh, J.; Tardio, J.; Vernon, C.; Bhargava, S. Fundamentals of wet oxidation of Bayer process Liquor: Reactivity of malonates. Ind. Eng. Chem. Res. 2010, 49 (11), 5347–5352. (47) Machold, T.; Macedi, E.; Laird, D. W.; May, P. M.; Hefter, G. T. Decomposition of Bayer process organics: Low-molecular-weight carboxylates. Hydrometallurgy 2009, 99 (12), 51–57. (48) Noyce, D. S.; Reed, W. L. Carbonyl reactions.9. The rate and mechanism of the base-catalyzed condensation of benzaldehyde and acetone - Factors influencing the structural course of condensation of unsymmetrical ketones. J. Am. Chem. Soc. 1959, 81 (3), 624–628. (49) Ashby, E. C.; Doctorovich, F.; Liotta, C. L.; Neumann, H. M.; Barefield, E. K.; Konda, A.; Zhang, K.; Hurley, J.; Siemer, D. D. Concerning the formation of hydrogen in nuclear waste - quantitative generation of hydrogen via a Cannizzaro intermediate. J. Am. Chem. Soc. 1993, 115 (3), 1171–1173. (50) Martinez, C.; Rivera, J. L.; Herrera, R.; Rico, J. L.; Flores, N.; Rutiaga, J. G.; Lopez, P. Evaluation of the chemical reactivity in lignin precursors using the Fukui function. J. Mol. Model. 2008, 14 (2), 77–81. (51) Galbally, I. Wagerup air quality review, Report C/0936, http:// www.epa.wa.gov.au/docs/1215/Response/AppC.pdf. (52) Buck, K. T.; Boeing, A. J.; Dolfini, J. E.; Glinka, J. Method of catalytically hydrolyzing alpha, beta-unsaturated carbonyl compounds. US patent no. 4,766,249, 1988. 12342
dx.doi.org/10.1021/ie201388c |Ind. Eng. Chem. Res. 2011, 50, 12334–12342
