Ind. Eng. Chem. Res. 1999, 38, 1743-1753
1743
REVIEWS Catalytic and Noncatalytic Wet Oxidation Seiichiro Imamura* Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
The wet oxidation of organic compounds composed only of C, H, and O, dyes, amides, and watersoluble polymers is discussed to clarify the basic nature of this process. The reactivity of the organic compounds composed of C, H, and O, dyes, and some amides was correlated well with their carbon content in the molecule (C/MW) or carbon content in their skeletal structure (C/ MW′). This C/MW index can be applied to roughly estimate the behavior of wastewaters containing various organic compounds. Polymers are easily decomposed by the wet oxidation because they can undergo intramolecular hydrogen abstraction in the chain-propagation step. The biodegradability of polymers and amides was improved by wet oxidation treatment, indicating an asset of this process. The function of homogeneous copper salts and the effect of hydrogen peroxide are discussed. The action of Co/Bi composite oxide designed for the treatment of refractory carboxylic acids, Mn/Ce composite oxide for ammonia and other organic compounds, and Ru/CeO2 catalyst for the decomposition of PEG, formaldehyde, etc., is explained. The high potential of Ru and CeO2 as active catalyst components is emphasized. 1. Introduction
2. Noncatalytic Wet Oxidation
Wet oxidation is carried out under a high pressure of oxygen at elevated temperatures to decompose organic pollutants contained in wastewaters. This process has been applied for the treatment of wastewaters discharged from chemical industries which are toxic or refractory for biological treatment.1 Energy recovery is possible by this process;2 when a wastewater contains more than 30 g of COD/L, a spontaneous oxidation is maintained, while incineration by a kiln requires more than 300 g of COD/L to sustain self-combustion. Useful organic compounds are also recovered by the wet oxidation of biomass.3 Thus, this technique has a highly promising future from the standpoint of both environmental and energy-related issues. I began the investigation on the wet oxidation process nearly 2 decades ago. Although only a limited amount of data were available when I started this project, much literature is being published every year now, showing the increased interest in this process. As this process requires relatively severe reaction conditions, the development of active catalysts is one of the main research objectives. I present a clear, basic knowledge of wet oxidation as a consistent story. Thus, this paper confines its scope to my past work, which first deals with noncatalytic wet oxidation and then with the process of active catalyst development. As Mishra et al. and other researchers published excellent review papers on wet oxidation, interested readers should refer to them to understand more about wet oxidation and to see the recent activities in this field.4,5
The nature of noncatalytic wet oxidation must be assessed correctly in order to design active catalysts for this reaction. Therefore, a number of organic compounds were oxidized, the degradation of amides and poly(ethylene glycol) having been studied especially in detail. 2.1. Relationship between the Reactivity and the Oxygen Content in the Molecules. First, to avoid complication, compounds that contain only C, H, and O were employed as model pollutants.6 Although the rate of TOC decrease was scarcely affected by the oxygen partial pressure when supplied in an amount 3 or 4 times the theoretical oxygen demand,7 temperature had a profound effect. Scarce oxidation of benzyl alcohol took place at or below 180 °C; however, a temperature increment of only 20 °C resulted in drastic acceleration. Thus, temperature is the key factor controlling the reaction, and the development of the catalysts solely aims at decreasing the reaction temperature. Table 1 compares the reactivity of the compounds as determined by the decrease in TOC and COD after 2 h at 220 °C. Among the same series of compounds (e.g., alcohols and acids), it seems that the higher the molecular weight (MW), the higher the reactivity. However, there seems to be no such relationship covering all compounds. The reactivity is far lower for acids than for alcohols with the same molecular weight, e. g., acetic acid and isopropyl alcohol, tert-butyl alcohol and propionic acid, or tert-amyl alcohol and isobutyric acid. The result of the comparison between acids and phenols or acids and aldehydes is the same. However, when acids and alcohols are compared at different MW levels, the situation is different; methyl alcohol (MW: 32) and ethyl alcohol (MW: 46) had lower reactivities than isobutyric acid (MW: 88) or isovaleric acid (MW: 102). These phenom-
* Phone: +81-75-724-7534. Fax: +81-75-724-7580. E-mail:
[email protected].
10.1021/ie980576l CCC: $18.00 © 1999 American Chemical Society Published on Web 03/27/1999
1744 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 Table 1. Wet Oxidation of Organic Compounds Composed of C, H, and Oa compd
MW
O/MW, %
methyl alcohol ethyl alcohol isopropyl alcohol tert-butyl alcohol tert-amyl alcohol formic acid acetic acid propionic acid isobutyric acid isovaleric acid caproic acid oxalic acid succinic acid adipic acid formaldehyde propionaldehyde n-butyraldehyde methyl ethyl ketone phenol o-cresol benzyl alcohol propylene glycol dioxane acetylacetone diethylene glycol demethyl ether diethyl malonate PEG-200
32 46 60 74 88 46 60 74 88 102 116 90 118 146 30 58 72 72 94 108 108 76 88 100 134 160 200
50 35 27 22 18 70 53 43 36 31 28 71 54 44 53 28 22 22 17 15 15 42 36 32 36 40 36
% removed TOC COD 35 30 56 91 63 99 4 15 33 48 34 99 8 13 45 49 43 88 78 93 6 39 54 38 39 7
29 36 70 83 75 100 10 14 42 57 46 100 7 11 45 59 61 58 93 86 92 12 41 61 38 32 5
a [Organic compound]: 5000 ppm. P : 3 MPa. P : 3 MPa. O2 N2 Temperature: 220 °C. Reaction time: 2 h.
Figure 1. O/MW vs TOC and COD removal. [Organic compound]: 5000 ppm. PO2: 3 MPa. PN2: 3 MPa. Temperature: 220 °C. Reaction time: 2 h.
ena suggest the possibility that the oxygen content in the molecule affects the reactivity. Therefore, the reactivity was plotted against the oxygen content in a molecule (O/MW) in Figure 1. We see that a considerably good relationship exists among all compounds except for formic acid and oxalic acid. Organic compounds suffer oxidative degradation during the reaction to produce lower carboxylic acids; my co-workers found that phenol degraded during wet oxidation to produce lower carboxylic acids, including acetic acid.8 The pH of the solution in which benzyl alcohol suffered wet oxidation first decreased during the reaction and was recovered in a later stage, indicating the formation of carboxylic acids as intermediates. Taking these facts into consideration, the phenomenon shown in Figure 1 is explained as follows. Organic compounds degrade to lower carboxylic acids, and they, especially acetic acid, are refractory to wet oxidation. If all TOC components were removed only after their degradation to lower carboxylic acids, the activity pattern shown in Table 1 would not have been obtained. If the elimination of
carbon dioxide occurs simultaneously during the degradation of organic compounds, we can expect the result shown in Figure 1. That is, the compounds with lower O/MW would lose more carbonaceous part through elimination of CO2 before lower carboxylic acids are formed, while those with larger O/MW are easily transformed to refractory acids due to their abundant oxygen and, consequently, protest against TOC removal. Although this explanation lacks theoretical basis for the mechanism of CO2 removal, the readers will further see that O/MW (later it was replaced by C/MW) can be a rough measure of the reactivity of the organic pollutants in the wet-oxidation treatment. Formic acid and oxalic acid do not follow the O/MW reactivity pattern. They are very reactive and are decomposed even without oxygen; the percentage decrease of TOC after 2 h at 220 °C under a nitrogen atmosphere was 93.6% (oxalic acid) and 45.5% (formic acid), compared with the value of 3.4% for acetic acid. Therefore, there surely exist compounds that do not follow the O/MW reactivity pattern; however, it is also true that the O/MW index can be applied to a wide range of compounds. Oxalic acid is produced by the partial oxidation of polyaromatic compounds.9 As oxalic acid (formic acid) is very reactive, elimination of CO2 may proceed through the formation of these fragile acids. However, details are not known. Acetic acid, one of the final degradation products, is especially refractory, and its decomposition is the key step in the wet oxidation treatment. Therefore, the choice of acetic acid as a model compound in developing wet oxidation catalysts has a reasonable basis.10,11 In the following section, we will see the nature of noncatalytic wet oxidation in more detail. 2.2. Wet Oxidation of Dyes. A total of 24 anionic and cationic dyes (azo, triarylmethane, xanthene, nitronaphthalene, indigoid, acridine, diphenylmethane, thiazine compounds) were oxidized.12 All dyes were very reactive, and higher than 80% decoloration was attained by treatment for only 30 min at 230 °C. As the dyes have various heteroatoms (N, S, Cl, etc.), it was difficult to relate their configuration to their reactivity. However, we found that the same idea as was applied for the organic compounds containing C, H, and O could be used. The important point here is that only C, H, and O, which are contained in the skeletal structure of the dyes, are taken into consideration, excluding azo, sulfonate groups, etc. This way of treatment is based upon the consideration that the reactivity depends on the stability of the skeletal structure of the molecule. However, we changed the index O/MW to C/MW (carbon content in a molecule) here. As a molecule with a small O/MW value has many carbon atoms (large C/MW value) to suffer CO2 elimination, C/MW can be the same and more general reactivity factor. In Figure 2, C/MW′ (carbon content in the skeletal structure of the dye) is plotted against the reactivity of the dyes, together with the plot of C/MW for the organic compounds composed only of C, H, and O. The reactivities of all compounds were normalized by that of propionaldehyde; propionaldehyde was oxidized under the same reaction conditions as in the oxidation of dyes, and the percentage decrease in its TOC was set to unity. The reactivity of dyes together with that of other organic compounds is well correlated with the carbon content in the molecule. Thus, the simultaneous elimination of CO2 also occurs in the wet oxidation of dyes during their degradation
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1745
Figure 2. Relationship between relative reactivity and C/MW or C/MW′. 2: Amides. 0: Dyes. O: Organic compounds composed of C, H, and O. b: Propionaldehyde. The value of C/MW′ is applied to dyes and amides.
Figure 3. Wet oxidation of amidessTOC and amide remained after 2 h. [Amides]: 5000 ppm. PO2: 3 MPa. PN2: 3 MPa. A: 235 °C. B: 275 °C.
to lower carboxylic acids. Thus, the C/MW index can be applied for various kinds of organic pollutants. 2.3. Wet Oxidation of Amides. Next the reactivity of the amides was investigated, which was found later not to necessarily follow the C/MW rule but, yet, to be partly related to it.13 Formamide (designated as F), N-methylformamide (MF), N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), acetamide (A), propionamide (P), n-butyramide (B), n-valeramide (V), N-methylacetamide (MA), N-ethylacetamide (EA), N,Ndimethylacetamide (DMA), N,N-diethylacetamide (DEA), and -caprolactam (-CL) were oxidized, and the results are shown in Figure 3. The pH of the solution was found to increase in most cases (both under oxygen and nitrogen atmospheres), suggesting that the nitrogen was transformed to ammonia or amine. The reactivity of F and N-substituted formamides (MF, DMF, DEF) is especially high. First, the reactivity of N-unsubstituted amides (A, P, B, V) was investigated at 235 °C (Figure 3A). As the molecular weight of these amides was increased, the stability of the amides increased; no
Figure 4. TOC removal in the wet oxidation of acetamide and valeramide. (s) Theoretical. (- - -) Observed.
reactivity difference was seen in the presence of oxygen and in its absence, and a TOC decrease did not occur. The formation of the corresponding acids was observed after the reaction, suggesting that the main reaction is the scission of C-N bond. The reaction temperature was increased to 275 °C (Figure 3B). In the nitrogen atmosphere, amides with larger molecular weights also have a higher stability. However, in this case, the degree of amide removal is higher in the presence of oxygen and the difference in the amount of remaining amide in the presence and absence of oxygen is larger for larger amides; TOC removal in oxygen is more remarkable for larger amides despite their higher thermal stability. This fact shows that the direct attack of oxygen to larger amides occurs simultaneously along with the occurrence of C-N bond scission. Therefore, in the case of small amides such as A (thermally unstable), thermal scission of the C-N bond occurs in the first step and, then, the resultant acids would be decomposed by oxygen. On the other hand, an attack of oxygen to the carbonaceous part would occur simultaneously with the C-N bond scission for amides with larger molecular weights (e.g., V), because more carbon atoms are available for oxygen attack. Kinetic analysis was carried out for A and V oxidation assuming that the first step of the reaction is the scission of the C-N bond, followed by the oxidative decomposition of the produced acids: O2
RCONH2 9 8 RCOOH 9 8 CO2v k k 1
2
(1)
In the case of the oxidation of A, the theoretical curve fits the observed one well, indicating that the thermal hydrolysis of acetamide occurs first and the resultant acetic acid is oxidized to give carbon dioxide. (Figure 4). As expected, both curves do not coincide for the oxidation of valeramide. The TOC decrease in the initial stage of the reaction is more remarkable for the observed curve, showing that the decomposition of this amide proceeds through the consecutive reaction (eq 1) and direct attack of oxygen to the amide occurs simultaneously (eq 2):
Although the thermal stability increases in the order A
1746 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999
< P < B < V, the TOC reduction is in the opposite order. Therefore, the direct attack of oxygen to the amides becomes more important as their molecular weight (content of C) increases. The reactivity of the produced acids also naturally contributes to the degree of TOC decrease. Next, the behavior of N-substituted acetamides and N-substituted formamides was discussed. The difference in the amount of remaining amides (N-substituted acetamides) in the presence of oxygen and in its absence is large compared with that of acetamide (Figure 3B). Therefore, the direct attack of oxygen occurs at the N-substituted alkyl groups. The TOC decrease in the presence of oxygen is more remarkable for the amides with two N-alkyl groups than with one alkyl group. This is in accordance with the result of Sager et al. that the methyl or the methylene group on the N atom of various amides is especially reactive in the oxidation.14 The reactivity of formamide and N-substituted formamides is remarkably high compared with other amides. This is because their thermal stability is very small and, as shown by the result of the reaction in nitrogen, TOC is removed by mere scission of the C-N bond, probably through the elimination of the formyl group. Comparison of the reactions in oxygen and in nitrogen suggests the occurrence of the attack of oxygen on the Nsubstituted alkyl groups. The thermal stability is higher for the amides with more alkyl groups on the N atom, and this is the cause for the smaller decrease of their TOC in oxygen despite their increased reactivity toward oxygen. In summary, the estimation of the reactivity of amides as expressed by their TOC decrease is complex; factors such as the thermal stability of amides, the reactivity of the acids formed by the scission of the C-N bond, and the reactivity of amides themselves toward oxygen all come into participation. The degree of the contribution of these factors determines the ratio of the thermal decomposition path (eq 1) to the direct oxygen attack to amides (eq 12). Here the relationship between C/MW′ and the reactivity (expressed by TOC decrease) was also investigated. As shown in Figure 2, a correlation was observed between C/MW′ (N was eliminated from MW) and the reactivity of A, P, B, V, MA, EA, DMA, DEA, and -CL (the reactivity was normalized to that of propionaldehyde). For F and N-substituted formamide, the C/MW′ index does not work and the tendency of C/MW′ vs reactivity is contrary. This is because these amides suffer thermal decomposition relatively easily and the reactivity is mainly controlled by the degree of C-N scission. The fact that they do not produce refractory acetic acid except for DEF is perhaps another reason. The relative reactivity of the amides is low (less than 0.5 relative to the reactivity of propionaldehyde) except for formamide and N-substituted formamides. Thus, wet oxidation is not a suitable process for the treatment of amides. Table 2 shows the improvement of biodegradability [BOD/COD × 100 (%)] of the amides after wet oxidation. Although DMF, DEF, MA, and DMA suffered scarce biodegradation, their biodegradability increased after wet oxidation. Therefore, if wet oxidation is combined with a biological treatment, it can be successfully applied to the detoxification of some amides. 2.4. Wet Oxidation of Poly(ethylene glycols) (PEG’s) and Other Water-Soluble Polymers. The compounds of this series (PEG’s) have almost the same
Table 2. Biodegradation of Amidesa BOD/COD × 100, % amide
before oxidn
after oxidnb
reaction temp, °C
F MF DMF DEF A B MA DMA
1 2 0 1 86 67 2 1
c 0 30 56 78 48 61 56
235 235 235 235 275 275 275 275
a [Amide]: 5000 ppm. P : 3 MPa. P : 3 MPa. b Wet oxidation O2 N2 was carried out for 2 h. c Data were not available owing to the complete removal of F.
Figure 5. Effect of the molecular weight in the wet oxidation of the PEG series. [TOC]: 2500 ppm. PO2: 1.96 Ma. PN2: 0.98 MPa. Temperature: 220 °C. (1) EG, (2) DEG, (3) TEG, (4) PEG-200, (5) PEG-400, (6) PEG-1000, (7) PEG-2000, (8) PEG-4000, (9) PEG6000, and (10) PEG-20000.
Figure 6. Co-oxidation of PEG-200 and PEG-20000. [TOC]: 2500 ppm. PO2: 1.96 MPa. PN2: 0.98 MPa. Temperature: 220 °C. The rate is expressed by TOC decrease. (1) Observed. (2) Theoretical.
value of C/MW, and it is interesting to see how they behave in the wet oxidation.15 The reactivity of PEG’s depends on the molecular weight (Figure 5). Ethylene glycol (DE) does not react at 220 °C, and the reactivities of diethylene glycol (DEG) and triethylene glycol (TEG) are low. The reactivity of PEG’s increases with an increase in the molecular weight. Although there is a maximum in the rate of TOC decrease with PEG 2000 (MW: 1800-2200), the PEG’s with higher molecular weight are generally more reactive compared with their low molecular weight analogues. The co-oxidation of PEG-200 and PEG-20000 was carried out to see what happens, and the result is shown in Figure 6. The theoretical line for the mixture of both PEG’s was obtained by taking the apparent reaction order with respect to PEG-20000 (1.0) and PEG-200 (0.75) into consideration. The observed line lies far above
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1747 Table 3. Biodegradability (%) of Polymersa biodegradability at reaction time, min polymer PEG-200 PEG-400 PEG-1000 PEG-2000 PEG-4000 PEG-6000 PEG-20000 PVA PPG PAM PAA
0 4 0 0 2 2 6 0 1 1 1 1
5 23 36 51 50 73 57 61 17 33 15 47
Table 4. Catalytic Activity of Transition-Metal Nitrates in the Wet Oxidation of Acetic Acida no catalyst Cu Fe Zn Mn Ni Cr Co
120 49 65 100 100 80 91 64 86 58 53 46
TOC removed,b % COD removed,b %
10 12
43 53
17 26
8 10
10 16
10 9 12 14
11 16
a [AcOH]: 5000 ppm. [Metal nitrate]: 5 mM. P : 2.9 MPa. P : O2 N2 2.9 MPa. Reaction temperature: 235 °C. b After 2 h.
a Biodegradability is expressed by (BOD/COD)100. Reaction temperature: 220 °C.
the theoretical one, which suggests that the reaction is radical in nature and active species produced from more fragile PEG-20000 attack PEG-200 and accelerate the reaction. Therefore, the presence of reactive compounds is desirable to decompose refractory pollutants in the wet-oxidation process. The change in the reactivity of PEG’s with an change in their MW can be explained on the basis of a radical mechanism. The reactivity of hydrocarbons in the autoxidation depends on the rate of the propagation step:
ROO‚ + RH f ROOH + R This step generally occurs intermolecularly. However, an intramolecular reaction is also possible, as in the case of the autoxidation of acyclic ethers.16 In the case of PEG’s, the intramolecular process would play an important role, especially when their concentration is low. Five, 6, and more than 14 membered cyclic transition states including peroxy oxygen atoms would be possible.17 PEG with a high molecular weight holds a long chain even after repeated chain scissions and continues intramolecular hydrogen abstraction. On the other hand, an intramolecular reaction would be difficult after several chain scissions for PEG’s with a low molecular weight such as PEG-200. This explanation clarifies the effect of MW on the reactivity of the PEG’s shown in Figure 5. However, it does not necessarily follow that the higher the MW, the higher the reactivity; the reactivity maximum is seen with PEG-2000. Therefore, both the intermolecular path and intramolecular one are working simultaneously, and the contribution from the intermolecular reaction would be suppressed by diffusion limitation when the MW of PEG is too high. It was found that other polymers showed the same molecular weight effect. Poly(vinyl alcohol) (PVA, MW 21 500), poly(propylene glycol) (PPG, MW 1000), poly(acryl amide) (PAM, MW 485 000), and poly(acrylic acid) (PAA, MW 38 000) all had almost the same reactivity as PEG-20000, which was much higher than those of their monomer model compounds. Table 3 shows the change in the biodegradability of these polymers after wet-oxidation treatment. All polymers become susceptible to biodegradation. Therefore, wet oxidation is an especially appropriate method for the treatment of water-soluble polymers. 3. Catalytic Wet Oxidation 3.1. Action of Homogeneous Copper Salts. To my best knowledge, copper ion is the only effective catalyst that works in the homogeneous state; it is used for the
Figure 7. Effect of the addition of O2 during the decomposition of AcOH under a N2 atmosphere. [AcOH]: 5000 ppm. PN2: 2.9 MPa. Temperature: 240 °C. O: [TOC]. 0: [Cu(II)]. ∆: [TN] (total nitrogen in the solution).
practical treatment of wastewaters discharged from petrochemical industries.18 Therefore, the action of copper ion was investigated in the oxidation of acetic acid as a model compound.7 The activity of various metal nitrates is shown in Table 4. Copper(II) nitrate had the highest activity, followed by iron(III) nitrate. Other metal ions were completely inactive. The rate of TOC decrease at 240 °C was expressed as
-d[TOC]/dt ) k[Cu]0.62[TOC]1.2
(3)
k ) 2.88 × 10-4 ppm-0.2 M-0.62 min-1 As the reactivity of formaldehyde, formic acid, oxalic acid, and methanol, which are assumed to be the degradation products of acetic acid, is much higher than that of acetic acid, eq 3 can be expressed also as
-d[AcOH]/dt ) k[Cu]0.62[AcOH]1.2
(3′)
The decomposition of acetic acid proceeded even in the absence of oxygen when copper(II) nitrate was used. However, the concentration of copper(II) ion and the total nitrogen in the solution (TN) decreased during the reaction. It was found that CuO was precipitated and NO and NO2 (in the vapor phase) were detected after the reaction. On addition of oxygen during the reaction under a nitrogen atmosphere, TN and copper(II) ion were recovered to some extent (Figure 7). From the analysis of the material balance, the following scheme was proposed for the reaction under nitrogen atmosphere in which the ligand, NO3-, works as an oxidizing reagent:
AcOH + Cu(NO3)2 f ROx (CO2, etc.) + CuO + NO + NO2 (4) where ROx are the oxidation products. An introduction of oxygen recovers Cu(NO3)2:
CuO + O2 + NO2 f Cu(NO3)2
(5)
1748 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999
Figure 8. Effect of the addition of HOOH with the change in the reaction temperature. [C6H5OH]: 1750 ppm. [HOOH]: 1000 ppm. PO2: 1 MPa. PN2: 3 MPa. Reaction time (min) ) (b) 30, (∆) 60, (0) 90, (O) 120. ∆TOC′ is the difference between the amount of TOC decreased in the presence and absence of HOOH (∆TOCHOOH ∆TOCno HOOH). (b) The stoichiometric amount of ∆TOC calculated according to the equation, C6H5OH + 14HOOH (1000 ppm) f 6CO2 + 17H2O.
Other copper salts such as CuCl2 and CuSO4 were fragile; some portion precipitated as Cu2O during the preheating period even in the presence of oxygen. An addition of LiNO3 helped maintain the stability of Cu(II) ion and increased the catalytic activity of CuSO4. Therefore, the counteranion plays an important role in the stability and, hence, the activity of Cu(II) ion. Copper(II) nitrate is recommended for high-temperature wet-oxidation treatment. 3.2. Effect of the Addition of Hydrogen Peroxide. The effect of the addition of hydrogen peroxide (HOOH) was investigated as an accelerating factor for the reaction.19 Phenol was employed as a typical fragile compound and acetic acid as a typical refractory pollutant. It was found that HOOH was very rapidly decomposed in the autoclave. A glass reaction tube was inserted in the autoclave, an agitator was coated with glass, and a glass sampling pipe was used to prevent the contact of HOOH with the metal portion of the autoclave as much as possible. However, HOOH was completely decomposed at 120 °C within 5 min, indicating that the metal part leading to the sampling valve catalyzed the decomposition of HOOH. This shows that, in the actual treatment at high temperatures using a steel reactor, HOOH is decomposed instantaneously on its addition. Thus, subsequent experiments were carried out in an autoclave without any glass lining or glass pipe; HOOH was decomposed immediately on its addition. The effect of the addition of HOOH (1000 ppm) on the oxidation of phenol (1750 ppm) is shown in Figure 8. The ordinate shows the difference in the amount of TOC decreased in the presence and absence of HOOH (∆TOC′ ) ∆TOCHOOH - ∆TOCno HOOH). At the reaction temperature of 100 °C, a stoichiometric reaction took place:
C6H5OH + 14HOOH f 6CO2 + 17H2O
(6)
At 130 and 150 °C, more than a stoichiometric amount of TOC was removed, showing that the radical chain reaction of phenol occurred, induced by the decomposition of HOOH. Phenol was found to be relatively stable at 100 °C. Thus, even if radicals are produced by the attack of •OH, they cannot continue the chain reaction
Figure 9. Effect of HOOH and Cu(NO3)2 on the wet oxidation of AcOH. [AcOH]: 5000 ppm. [HOOH]: 1000 ppm. [Cu(NO3)2]: 20 mM. PO2: 1 MPa. PN2: 3 MPa. Temperature: 220 °C. (1) No additive, (2) HOOH, (3) Cu(NO3)2, and (4) Cu(NO3)2 + HOOH.
owing to the short radical chain, leading only to the stoichiometric TOC decrease. On the other hand, as phenol has an enough reactivity at higher temperatures of 180 and 200 °C, they can be readily decomposed without HOOH and the addition of HOOH has almost no effect. Therefore, HOOH exhibits its effect best at the reaction temperature where phenol has a moderate reactivity. Refractory acetic acid was not decomposed at low temperatures (e.g., 150 °C), and the addition of HOOH at that temperature had almost no effect. Therefore, reaction was carried out at 220 °C where acetic acid began to be decomposed. As Figure 9 shows, addition of HOOH has an accelerating effect. However, the effect was not large. Although active radicals from HOOH attack acetic acid, the reaction soon stops due to the low reactivity of acetic acid, and HOOH exhibits its effect only at the instance of its addition. There may be an optimum temperature in this case also as in the reaction of phenol. However, we cannot say that the addition of HOOH is effective even for phenol oxidation because of the low improvement in the TOC decrease. The conclusion here is that the use of HOOH is not practical for the actual wet-oxidation process, considering its low efficiency and high reagent (HOOH) cost. A recent report shows the effect of HOOH plus Fe-ZSM-5 in the wet oxidation of phenol.20 However, the reaction temperature employed was 70 °C, and the reaction proceeded in a stoichiometric way. Thus, their system is not a wet oxidation but should be categorized as Fenton’s reagent. Shown also in Figure 9 is the result of the oxidation of acetic acid in the presence of Cu(NO3)2 and Cu(NO3)2 plus HOOH. Cu(NO3)2 naturally is more effective than HOOH. When HOOH was combined with Cu(NO3)2, the reaction proceeded more rapidly. However, the degree of acceleration by HOOH is indifferent to the presence or absence of Cu(NO3)2; HOOH is decomposed spontaneously at this temperature. The most notable function of the metal ion in the liquid-phase oxidation is its redox action to form active radicals (RO•, ROO•) from the hydroperoxide produced:
Mn+ + ROOH f RO• + OH- + Mn+1
(7)
Mn+1 + ROOH f ROO• + H+ + Mn+
(8)
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1749
However, in the case of the high-temperature wet oxidation, a function of this kind is not required for the catalysts because hydroperoxides, if produced, are easily decomposed under the severe reaction conditions and through the contact with the steel wall of the reactor. This fact is important in designing active catalysts, and I think that the sole function of the catalysts is to directly or indirectly (via, for example, the activation of oxygen) interact with the substrate to produce active radicals in the first step of the reaction. This point will be further addressed later. 3.3. Design of Heterogeneous Catalysts. 3.3.1. Co/ Bi Catalyst. Although homogeneous copper salts are efficient and are used practically, recovery of copper ion is necessary after the treatment to prevent the contamination of the receiving water by toxic copper ion. First we tried to develop active catalysts for the wet oxidation of refractory acetic acid.21,22 The activity of various transition-metal oxides and mixed oxides was checked, and we placed our focus on the catalysts in which bithmus (Bi) was combined with the transition metals. The reason for our choice of Bi is that Bi-transitionmetal composite oxides are reported to have oxidation activity,23 and we tried to take advantage of the basic nature of Bi to endow the catalysts with the affinity toward acetic acid. Although composite oxides containing Cu (Cu/Co, Cu/ Co/Bi, Cu/Mn/Bi, Cu/Bi/γ-Al2O3) exhibited relatively high activity, Cu was eluted from these catalysts. Bi/ γ-Al2O3, Co/Bi, Co/Bi/γ-Al2O3, Sn/Bi, and Zn/Bi were found to be active. The variation in the method of preparation, however, did not result in the improvement of the activity of these composite catalysts except for Co/Bi. The Co/Bi catalyst with 16.0 mol % Bi (Co/Bi ) 5/1) exhibited the highest activity, which was comparable to that of homogeneous copper salt. The measurement of the adsorption isotherm of acetic acid revealed that Coi/Bi(5/1) (surface area: 61.6 m2/g) had almost the same ability of adsorbing acetic acid as γ-Al2O3 (surface area: 330 m2/g) despite its smaller surface area. This shows that the basic site induced by Bi plays an important role in the reaction. Supporting this idea is the fact that the initial rate of TOC removal (ppm TOC/ min) decreased with an increase in the pH of the reaction solution, i.e., 69.1 (pH ) 3.4), 47.7 (pH ) 8.0), and 42.2 (pH ) 10.0) in the reaction at 248 °C. At high pH, acetic acid is present as AcO- and its interaction with the Co/Bi becomes weak. This situation is also shown in Figure 10. Organic compounds including carboxylic acids were oxidized in the presence and absence of Co/Bi(5/1). The abscissa is the initial rate of TOC decrease in the absence of Co/Bi(5/1) (Rno cat) and indicates the inherent reactivity of the compounds themselves. The ordinate is the ratio of the initial rate of reaction in the presence of the catalyst to that in its absence (Rcat/Rno cat) and, hence, is the measure of the effectiveness of the catalysts. For the reactive compounds such as phenol and formic acid, Co/Bi had a scarce effect because they are decomposed easily at 248 °C without catalysts. As the reactivity of the compounds decreases, the effect of Co/Bi(5/1) naturally increases. However, an increment in the effect of this catalyst is clearly much more remarkable for carboxylic acids than for other compounds. The high efficiency of this catalyst toward acetamide is due to the formation of acetic acid produced through the C-N bond scission of acetamide.
Figure 10. Wet oxidation of organic compounds. [Organic compound]: 5000 ppm. [Co/Bi(5/1) calcined at 550 °C]: 20 mM (total metal concentration). PO2: 1 MPa. PN2: 3 MPa. Temperature: 248 °C. O: Carboxylic acids. ∆: Compounds other than carboxylic acid. Rno cat: Initial rate of TOC decrease in the absence the catalyst. Rcat/Rno cat: The ratio of the initial rate of TOC decrease in the presence of Co/Bi(5/1) to that in its absence. 1: Glutamic acid. 2: Succinic acid. 3: Adipic acid. 4: Acetic acid. 5: Valeric acid. 6: Butyric acid. 7: Hexanoic acid. 8: Propionic acid. 9: Phenol. 10: Acetaldehyde. 11: Methanol. 12: Propionamide. 13: Valeramide. 14: Acetamide.
Thus, the interaction of the basic site of the Co/Bi with acidic compounds is very important. It was found that Co/Bi(5/1) exhibited a higher activity in decomposing HOOH than Cu(NO3)2, indicating that this catalyst has a considerably high redox property. Thus, the high activity of Co/Bi composite oxide toward carboxylic acids comes partly from its affinity toward them and partly from its redox property. This Co/Bi composite catalyst was also effective for the vapor-phase combustion of acetic acid.24 3.3.2. Mn/Ce Catalyst. Next we tried to develop an active catalyst for the oxidation of ammonia, which ultimately led to Mn/Ce composite oxide catalyst with a universal activity not only for ammonia detoxification.25 The nitrogen component contained in various organic compounds is converted to ammonia by wet oxidation, and its further oxidation is quite difficult.26 As ammonia is one of the potent pollutants that causes eutrophication of the receiving water, the second treatment to remove ammonia is necessary even after the wet oxidation is applied. First, the choice of Co/Bi mixed oxide was the starting point; however, an addition of a small amount of Bi [Co/ Bi(97/3)] only had a slight accelerating effect, and a further increase in Bi remarkably retarded the reaction. The basic nature of Bi decreased the affinity of this catalyst toward ammonia, which was ascertained by the measurement of the adsorption isotherm of ammonia. Ammonia exists as NH3 in the alkaline region and as NH4+ in the acidic region, the pKa of NH4+ being 9.27. The reaction did not proceed in the acidic region both in the presence and absence of catalysts, while the TN removal increased remarkably at pH above 9, indicating that NH3 is more reactive than NH4+. Various lanthanide elements (La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, and Yb) were combined with Co/Bi(97/3) to improve the oxidation ability of the catalyst,27 and it was found that Gd, Ho, Yb, and, especially, Ce exhibited an effect. Thus, the combination of Ce with the transition metals of the first series was reinvestigated, excluding Bi, and, finally, Mn/Ce composite oxides were developed. Figure 11 shows the effect of the composition of Mn/ Ce composite oxides calcined at 350 °C. The removal of
1750 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 Table 5. Wet Oxidation Catalyzed by Cu(NO3)2, Co/Bi(5/ 1), and Mn/Ce(7/3)a reactant acetic acid
Figure 11. Effect of the composition of Mn/Ce catalyst. [TN]: 1000 ppm. [Mn/Ce]: 20 mM (total metal concentration). PO2: 1 MPa. PN2: 3 MPa. Temperature: 263 °C. Reaction time: 1 h. O: ∆TN. b: ∆TN per unit surface area of the catalyst. ∆: BET surface area. Mn/Ce was calcined at 350 °C in air for 3 h.
TN (∆TN) reached more than 70% after 1 h at a Ce content region from 20 to 50 mol %. It was found that the ∆TN using Cu(NO3)2 under the same reaction condition ([Cu]: 20 mM) was 54.7%, indicating the superiority of the Mn/Ce catalyst. In this case, an addition of Ce increased the surface area of the catalysts, and the specific activity decreased monotonically with an increase in the Ce content. However, the apparent activity, as judged from ∆TN, shows that the Mn/Ce catalysts are much more useful in the actual treatment of NH3 than Mn catalyst. An X-ray diffraction analysis revealed that the form of Mn was Mn2O3 and CeO2 was in a considerably amorphous state. It was found that no NO and a trace amount of NO2 were detected in the vapor phase and only a small amount of NO3- (about 1.5% based upon ∆TN) was present in the solution, indicating that most of NH3 was converted into N2. The Mn/Ce catalyst was found to be effective for the wet oxidation of various compounds.28,29 Table 5 shows the activity of the Mn/Ce catalyst in comparison with those of Cu(NO3)2 and Co/Bi(5/1). Judging from ∆TOC and the initial reaction rate (Ri), the Mn/Ce catalyst exhibits a higher activity than the other two for almost all compounds. When the composition of the Mn/Ce catalysts was varied, the tendency in the change of the activity in the wet oxidation and that in the decomposition of HOOH roughly coincided. Therefore, we can safely deduce that the redox property plays an important role in the catalyst activity for wet oxidation. The rate of the TOC decrease in the wet oxidation of PEG-200 catalyzed by Mn/Ce(4/6) was expressed by
-d[TOC]/dt ) k[Cat]0.62[TOC]0.75
(9)
The observed reaction order with respect to a catalyst concentration of less than unity (0.61) suggests that a radical chain reaction (eqs 10-13) occurs at least partly. Cat
RH 98 R
ki
R• + O2 f ROO• ROO• + RH f ROOH + R•
(10) k2
(11) k3
(12)
The situation is the same for copper catalyst (eq 3).
catalyst
none Cu(NO3)2 Co/Bi(5/1) Mn/Ce(7/3) n-butylamine none Cu(NO3)2 Co/Bi(5/1) Mn/Ce(7/3) PEG-200 none Cu(NO3)2 Co/Bi(5/1) Mn/Ce(7/3) pyridine none Cu(NO3)2 Co/Bi(5/1) Mn/Ce(7/3) ammonia none Cu(NO3)2 Co/Bi(5/1) Mn/Ce(7/3)
temp, °C ∆TOC,b % Ri,c ppm/min 247 247 247 247 220 220 220 220 220 220 220 220 270 270 270 270 263 263 263 263
42.0 87.2 95.5 99.5 3.5 16.6 5.3 35.4 4.6 30.7 62.2 59.4 10.7 16.3 17.1 22.1 7.7d 54.7d 13.0d 69.9d
7.0 63.7 50.0 90.0 0.3 2.0 0.8 9.0 0.7 6.7 20.3 20.3 1.8 2.0 1.0 2.0
a [TOC], [TN]: 2000 ppm. [Cat]: 20 mM (total metal concentration). PO2: 1 MPa. PN2: 2.5 MPa. Sw (m2/g): 49.8 [Co/Bi(5/1)], 94.6 [Mn/Ce(7/3)]. b After 1 h. c Initial rate of TOC decrease. d Percentage decrease in TN after 1 h. Co/Bi(5/1) and Mn/Ce(7/3) were calcined at 350 °C in air for 3 h.
2ROO• f inactive products
k4
(13)
Different from the usual liquid-phase oxidation in organic solvents, the radical chain cannot be long enough to sustain an efficient autoxidation mode in protic media like water, especially when the concentration of the organic pollutant is small. This is the most serious problem in the wet-oxidation process, and therefore, the role of the catalysts is very important. As described before, the action of the catalysts is never to decompose hydroperoxide (ROOH) produced during the reaction (eqs 7 and 8). The only function of the catalysts is to produce active radicals in the first step of the reaction (eq 10). The interaction of CeO2 with Mn2O3 is not wellknown. However, we found that Ce helps maintain the valence state of Mn of higher than +3 when Mn/Ce was calcined below 500 °C.30 This kind of action of Ce to increase the valence of Mn (that is, to provide oxygen to Mn) is probably related to the well-known oxygenstorage function of CeO2.31 Although details are not known, the action of Ce may be to increase the valence of Mn and increase its oxidation ability to effectively initiate the reaction (eq 10). 3.3.3. Ru/Ce Catalyst. Finally, the action of precious metals was investigated.32 Table 6 shows the result of the oxidation of PEG-200 over precious metals supported on CeO2, together with the activities of Cu(NO3)2 and Mn/Ce(1/1). The activities of Ru, Pt, and Rh were much higher than those of Cu(NO3)2 and Mn/Ce(1/1). As Ru supported on CeO2 exhibited the highest activity among the three precious metals (∆TOC at 45 min: 99.1% for Ru, 95.7% for Pt, and 82.8% for Rh), the action of Ru was further investigated. Examination on the effects of supports (CeO2, γ-Al2O3, NaY-zeolite, ZrO2, and TiO2) revealed that CeO2 was the best. The Ru was deduced to be in the form of RuO2 on the basis of ESCA and XRD analyses. Table 7 shows the comparison of the activities of Ru/Ce and Cu(NO3)2. Ru/Ce exhibited almost the same or even a higher activity than Cu(NO3)2 for the oxidation of the compounds listed in the table.
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1751 Table 6. Wet Oxidation of PEG-200a catalystb
∆TOC,c %
catalystb
∆TOC,c %
None Ru/Ce Rh/Ce Pt/Ce
9.4 100 100 100
Ir/Ce Pd/Ce Cu(NO3)2 Mn/Ce(1/1)
74.8 49.7 12.3 43.8
a [TOC]: 2000 ppm. [Cat]: 12 mM (total metal concentration). PO2: 1 MPa. PN2: 2 MPa. Temperature: 200 °C. b Precious metal loaded is 5 wt %. c After 1 h.
Table 7. Wet Oxidation Catalyzed by Ru/Ce and Cu(NO3)2 at 200 °Ca ∆TOC,b % reactant
Ru/Cec
Cu(NO3)2
n-propyl alcohol n-butyl alcohol phenol acetamide PPG-1000f acetic acid
47.2 27.8 94.8 51.6 54.3 44.5d
28.3 40.1 93.5 18.1 29.5 32.6e
a [TOC]: 2000 ppm. [Cat]: 12 mM (total metal concentration). PO2: 1 MPa. PN2: 2 MPa. b After 1 h. c Ru: 5 wt %. d pH: 2.7. e pH: 2.5. f Poly(propylene glycol).
Table 8. Wet Oxidation Catalyzed by Ru(5 wt %)/Ce and Cu(NO3)2 at 150 °Ca ∆TOC,b % reactant
Ru/Cec
Cu(NO3)2
PEG-200 ethylene glycol formic acid formaldehyde
48.3 98.0 100 96.4
9.1 6.2 64.7 24.1
a [TOC]: 2000 ppm. [Cat]: 12 mM (total metal concentration). PO2: 1 MPa. PN2: 2 MPa. b After 1 h. c Ru: 5 wt %.
Although acetic acid was not decomposed completely by both catalysts, it was found that its complete removal was attained by Mn/Ce(1/1) under the same reaction conditions. These facts may suggest that Ru/Ce, different from Mn/Ce, has selectivity toward specific substances. PEG-200 and its degradation products were oxidized over Ru/Ce and Cu(NO3)2 (Table 8). We see that the former is especially effective for those oxygencontaining substances; especially stable ethylene glycol can be readily decomposed. The Ru/Ce is effective also for the vapor-phase combustion of toxic formaldehyde.33 The improvement of the performance of Mn/Ce and Ru/Ce catalysts was tried by combining these three elements.34 As an attempt to apply the wet-oxidation process for the treatment of domestic wastewater treatment, we used a model wastewater that contains dextrin (91.8 ppm), peptone (196.2 ppm), yeast extract (196.2 ppm), beef extract (223.8 ppm), NaCl (20.1 ppm), MgSO4 (12.1 ppm), KH2PO4 (55.8 ppm), and KCl (40.2 ppm). Figure 12 compares the activity of Ru(3 wt %)/Mn/Ce and Mn/Ce by varying the Mn/Ce ratio. Although an addition of Ru to Mn/Ce improves the performance of the latter, the effect is very small for Mn/Ce with a low Ce content. The effect of Ru is more remarkable at the higher Ce content region. Thus, the interaction of Ru with Mn is not important, and we could not succeed in utilizing the asset of the Mn/Ce by the combination with Ru. The result suggests the importance of CeO2 in the future catalyst development. 4. Summary The reactivity of various organic compounds was correlated well with the carbon content in the molecule,
Figure 12. Wet oxidation of a model domestic wastewater over Ru(3 wt %)/Ce/Mn and Mn/Ce. [TOC]: 315 ppm. [Catalyst]: 20 mM (total metal concentration). PO2: 1 MPa. PN2: 2.5 MPa. Temperature: 200 °C. Reaction time: 3 h. O: Mn/Ce. b: Ru/Mn/Ce. Catalysts were calcined at 500 °C in air for 3 h.
although there are exceptions. This phenomenon was explained on the basis of the assumption that two reaction paths occur simultaneously; one is the degradation to lower carboxylic acids, and the other is the elimination of CO2 during the reaction. The reaction proceeded through a radical mechanism, although the radical chain is rather short. Thus, the presence of reactive substances is advantageous to decompose refractory compounds. Improvement of biodegradability of the compound’s resistant to biological treatment is one asset of the wet-oxidation process. As lower carboxylic acids (final degradation products), especially acetic, are refractory against oxidation, their removal is a key step in the wet oxidation. Therefore, acetic acid can be used as a model compound in developing active wet-oxidation catalysts. The Co/Bi composite oxide catalyst, which has a basic nature derived by Bi, was effective for the wet oxidation of refractory lower carboxylic acids. Thus, the affinity of the catalysts toward pollutants is important. The design of hydrophobic catalysts, if possible, would be one strategy because such catalysts will have an affinity toward almost all organic compounds. The Mn/Ce composite catalyst exhibited high performance for the oxidation of various organic compounds, and Ru/CeO2 was effective for specific oxygen-containing compounds such as PEG and formaldehyde. The action of the catalyst is solely to produce active radicals via an interaction with the pollutants in the first step of the reaction. Although the details of this step are not known, it is surely one important strategy to design catalysts that have a high redox ability to carry out an efficient electron transfer with the pollutants. However, the action of copper ion (the only effective homogeneous catalyst) poses some ambiguity. Its redox potential is not higher than those of Mn3+ and Fe3+ (redox potential: 0.17 V for Cu2+, 0.771 V for Fe3+, and 1.488 V for Mn3+ in one-electron reduction).35 This fact may imply the possibility of the involvement of other factors. If catalysts that can activate oxygen are designed, it will be an enormous progress. Clear evidence for the activation of oxygen in the usual liquid-phase oxidation in organic solvents has not been attained during the past few decades. However, a work by Okamura et al. seems to shed light on the peculiar action of copper ion in the wet oxidation.36 They oxidized benzene to phenol with molecular oxygen in aqueous acetic acid at room temperature. Cu-loaded MCM-41 (mesoporous silicate) was
1752 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999
the catalyst, and ascorbic acid was needed to reduce Cu2+. Hydrogen peroxide was produced during the reaction, and it was assumed to be the active species to oxidize benzene. This suggests that the reduced state of Cu activates oxygen to form HOOH. In the highly severe wet-oxidation condition in the presence of organic pollutant, Cu2+ can be more readily reduced and may activate oxygen. Thus, the activation of oxygen can be considered as one important factor to design efficient wet-oxidation catalysts. Ru seems to be effective.11,37 Although the reason for the high performance of Ru in the water-phase oxidation is not clear, I also think that Ru is surely one promising element. The function of CeO2 is also important. When combined with precious metals or other elements, it works in various ways, not only in the purification of vehicle exhausts but also in the reactions such as detoxification of N2O, methanol decomposition, combustion of formaldehyde, etc.31,33,38,39 Precious metals are remarkably activated and behave quite differently on CeO2 compared with their action on other supports. Thus, I think at present that CeO2 and Ru are highly promising components.40 Further improvement of catalyst performance, however, is needed for the wide use of the wet-oxidation process. For example, the reaction condition employed here is too severe for the treatment of domestic wastewaters. If catalysts are developed that work effectively under the temperature below 150 °C, wet oxidation can be a potential process for more general water purification. Acknowledgment I sincerely thank Mr. K. Utani of Kyoto Institute of Technology for his kind cooperation and continuous encouragement. Literature Cited (1) Keckler, K. P.; Brandenburg, B. L.; Momont, J. A.; Lehman, R. W. Treatment of Wastewater from Acrylonitrile Manufacturing Plant. U.S. Patent 5192453, March 1993; Chem. Abstr. 1993, 118, 197432. (2) Versar, Inc. A Study of Industrial Waste Streams for Wet Oxidation. Report DOE/ID10368-1, Vol. 2, 1989; Chem. Abstr. 1991, 114, 68424. (3) McGinnis, G. D.; Wilson, W. W.; Prince, S. E.; Chen, C. C. Conversion of Biomass into Chemicals with High-Temperature Wet Oxidation. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 633. (4) Mishra, V. S.; Mahajani, V. V.; Joshi, B. Wet Air Oxidation. Ind. Eng. Chem. Res. 1995, 34, 2. (5) Matatov-Meytal, Y. I.; Sheintuch, M. Catalytic Abatement of Water Pollutants. Ind. Eng. Chem. Res. 1998, 37, 309. (6) Imamura, S.; Tonomura, Y.; Terada, M.; Kitao, T. Wet Oxidation of Organic Substrates Containing only C, H, and O as the Constituent Atoms. Mizushori Gijutsu 1979, 20, 317. (7) Imamura, S.; Sakai, T.; Ikuyama, T. Wet Oxidation of Acetic Acid Catalyzed by Copper Salts. Sekiyu Gakkaishi 1982, 25, 74. (8) Iwai, S.; Kitao, T.; Sugawara, M. Terada. M. Oxidation of Wastewater Containing Organic CompoundssConsideration on the Mechanism of Phenol Oxidation. Gesuido Kyokaishi 1974, 11, 49. (9) Kitao, T.; Terada, M. Oxidation of Wastewater Containing Organic CompoundssConsideration on the Mechanism of the Oxidation of Poly-aromatic compounds. Gesuido Kyokaishi 1976, 13, 25. (10) de Leitenburg. C.; Goi, d.; Primavera, A.; Trovarelli, A.; Dolcetti. G. Wet Oxidation of Acetic Acid Catalyzed by Doped Ceria. Appl. Catal. B 1996, 11, L29. (11) Gallezot, P.; Chaumet, S.; Perrard, A.; Isnard, P. Catalytic Wet Air Oxidation of Acetic Acid on Carbon-Supported Ruthenium Catalysts. J. Catal. 1997, 168, 104.
(12) Imamura, S.; Shimai, J.; Kitao, T. Wet Oxidation of Dyes. Mizushori Gijutsu 1980, 21, 109. (13) Imamura, S.; Fukuhara, M.; Kitao, T. Wet Oxidation of Amides. Nihon kagaku Kaishi 1980, 270. (14) Sagar, B. F. Autoxidation of N-Alkylamides II. N-Alkylamide Hydroperoxides and Di-N-Alkylamide Peroxides. J. Chem. Soc. B 1966, 690. (15) Imamura, S.; Tonomura, Y.; Kawabata, N.; Kitao, T. Wet Oxidation of Water-Soluble Polymers. Bull. Chem. Soc. Jpn. 1981, 54, 1548. (16) Howard, J. A.; Ingold, K. U. Absolute Rate Constants for Hydrocarbon Autoxidation XVIII Oxidation of Some Acyclic Ethers. Can. J. Chem. 1970, 48, 873. (17) Sisido, M. Statistical Treatment of the Intramolecular Reaction Between Two Functional Groups Connected by a Polymethylene Chain. Macromolecules 1971, 4, 737. (18) Akitsune, K. Catalytic Wet Oxidation of Wastewater from Acrylonitrile Manufacturing Plant. Nikkakyo Geppo 1976, 29, 9. (19) Imamura, S.; Okuda, K. Effect of Additives on the Wet Oxidation of Phenol and Acetic Acid. Mizushori Gijutsu 1981, 22, 9. (20) Fajerwerg, K.; Debellefontaine, H. Wet Oxidation of Phenol by Hydrogen Peroxide using Heterogeneous Catalysis. Fe-ZSM5: A Promising Catalyst. Appl. Catal. B 1996, 10, L229. (21) Imamura, S.; Hirano, A.; Kawabata, N. Wet Oxidation of Acetic Acid Catalyzed by Co-Bi Complex Oxides. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 570. (22) Imamura, S.; Kinunaka, H.; Kawabata, N. The Wet Oxidation of Organic Compounds Catalyzed by Co-Bi Complex Oxide. Bull. Chem. Soc. Jpn. 1982, 55, 3679. (23) Scott, W. W. Catalyzers for the Oxidation of Ammonia. Ind. Eng. Chem. 1924, 16, 74. (24) Imamura, S.; Matsushige, H.; Kawabata, N.; Inui, T.; Takegami, Y. Oxidation of Acetic Acid on Co-Bi Composite Oxide Catalysts. J. Catal. 1982, 78, 217. (25) Imamura, S.; Doi, A.; Ishida, S. Wet Oxidation of Ammonia Catalyzed by Cerium- Based Composite Oxides. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 75. (26) Ottengraf, S. P.; Lotens, J. P. Heat Treatment and LowPressure Oxidation of Aqueous Sewage Sludge. Water Res. 1978, 12, 171. (27) Gallagher, P. K.; Johnson, D. W., Jr.; Vogel, E. M. Preparation, Structure, and Selected Catalytic Properties of the System LaMn1-xCux3-y. J. Am. Ceram. Soc. 1977, 60, 28. (28) Imamura, S.; Nakamura, M.; Kawabata, N.; Yoshida, J.; Ishida, S. Wet Oxidation of Poly(ethylene glycol) Catalyzed by Manganese-Cerium Composite Oxide. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 34. (29) Imamura, S.; Nishimura, H.; Ishida, S. Preparation of Mn/ Ce Composite Oxide Catalysts for the Wet Oxidation of Acetic Acid and Their Catalytic Activities. Sekiyu Gakkaishi 1987, 30, 199. (30) Imamura, S.; Shono, M.; Okamoto, N.; Hamada, R.; Ishida, S. Effect of Cerium on the Mobility of Oxygen on Manganese Oxides. Appl. Catal. A 1996, 142, 279. (31) Trovarelli, A. Catalytic Properties of Ceria and CeO2Containing Materials. Catal. Rev. 1996, 38, 439. (32) Imamura, S.; Fukuda, I.; Ishida, S. Wet Oxidation Catalyzed by Ruthenium Supported on Cerium(IV) Oxides. Ind. Eng. Chem. Res. 1988, 27, 718. (33) Imamura, S.; Uematsu, Y.; Utani, K.; Ito, T. Combustion of Formaldehyde on Ruthenium/Cerium(IV) Oxide Catalyst. Ind. Eng. Chem. Res. 1991, 29, 18. (34) Imamura, S.; Okumura, Y.; Nishio, T.; Utani, K.; Matsumura, Y. Wet Oxidation of a Model Domestic Wastewater on a Ru/Mn/Ce Composite Catalyst. Ind. Eng. Chem. Res. 1998, 37, 1136 (35) Dean, J. A., Ed. Lange’s Handbook of Chemistry; McGrawHill: New York, 1985; 6-2. (36) Okamura, J.; Nishiyama, S.; Tsuruya, S.; Masai, M. Formation of Cu-supported Mesoporous Silicates and Aluminosilicates and Liquid-Phase Oxidation of Benzene Catalyzed by the Cu-Mesoporous Silicates and Aluminosilicates. J. Mol. Catal. A: Chem. 1998, 135, 133. (37) Qin, J.; Aika, K. Catalytic Wet Air Oxidation of Ammonia over Alumina Supported Metals. Appl. Catal. B 1998, 16, 261. (38) Imamura, S.; Okamoto, N.; Saito, Y.; Ito, T.; Jindai, H. Decomposition of Nitrous Oxide on Rhodium-Ceria Composite Catalyst. Sekiyu Gakkaishi 1996, 39, 350.
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1753 (39) Imamura, S.; Yamashita, T.; Hamada, R.; Saito, Y.; Nakao, Y.; Tsuda, N.; Kaito, C. Strong Interaction between Rhodium and Ceria. J. Mol. Catal. A 1998, 129, 249. (40) Barbier J., Jr.; Delanoe, F.; Jabouille, F.; Duprez, D.; Blanchard, G.; Isnard, P. Total Oxidation of Acetic Acid in Aqueous Solutions over Noble Metal Catalysts. J. Catal. 1998, 177, 378.
Received for review September 8, 1998 Revised manuscript received January 15, 1999 Accepted February 10, 1999 IE980576L
