Catalytic Wet Air Oxidation: Are Monolithic Catalysts and Reactors

Raquel P. Rocha, Diogo F. M. Santos, Olívia Salomé M. P. Soares, Adrián M. T. Silva, Manuel Fernando R. Pereira, José L. Figueiredo. . Topics in Catal...
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Ind. Eng. Chem. Res. 2007, 46, 4007-4033

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Catalytic Wet Air Oxidation: Are Monolithic Catalysts and Reactors Feasible? Andrzej Cybulski* ZD CHEMIPAN, The Institute of Physical Chemistry of the Polish Academy of Sciences, Warsaw, Poland

The paper provides the literature data on heterogeneous catalysts for catalytic wet air oxidation (CWAO). A survey of investigations on process mechanism and kinetics of the CWAO is presented. The performance of various reactors types is analyzed using experimental data and results of simulations. Monolithic catalysts for the CWAO are discussed. The possibility of application of monolithic reactors for the CWAO process is analyzed. 1. Introduction The developments in world population, industrialization, agriculture, and urbanization contribute to a continuous rise of the amount of wastewaters produced. The main polluting branches of industry are refineries, coke ovens, organic compounds production plants, pharmaceuticals factories, pulp and paper mills, and textile and surface treatment plants. A significant proportion of emitters of pollutants to water are also agriculture enterprises such as slaughterhouses, milk production plants, poultry and pig farms, and units for disposal or recycling of animal carcasses and animal wastes. The proportion of urban sewage in total wastes is also significant. The main organic pollutants from industries are chloroalkanes and phenolic compounds.1 Regulations concerning disposing wastewaters would have to become more stringent, otherwise this would pose severe environmental, social, and political problems. Therefore, special attention has been paid by political and legislative authorities, leading to stringent regulations. The more stringent regulations stimulated an intense evolution of research and development (R&D) activities on new methods for wastewater treatment in recent years. The treatment of wastewaters before disposing typically includes a combination of methods classified as physical, chemical, and biological. Microbial degradation is effective in removal of most pollutants, and therefore, it is most commonly used. It is, however, a slow process and produces much sludge that must be landfilled. Moreover, it is unsuitable for wastes that are too toxic, too concentrated, hazardous, or nonbiodegradable. Microbials are vulnerable to (chemical) shocks, further limiting their use in the chemical industry. Nonmicrobial methods for wastewater treatment include conventional phaseseparation techniques (adsorption processes and stripping techniques) to increase the concentration of pollutants, if necessary or advantageous, and chemical methods to destroy the contaminants (chemical oxidation and/or reduction). Chemical destruction of components that are refractory to biotreatment relies mainly on oxidation. The very first commercial oxidation technique was incineration of wastewaters. In this process, pollutants are oxidized at temperatures above 1000 °C in the gaseous phase. The applicability of incineration is, in practice, limited to streams containing enough organics to make the process autothermal; otherwise, it requires much energy. Thus, incineration becomes practical for wastewaters with a chemical oxygen demand (COD) of at least 300 g/L.2 * To whom correspondence should be addressed. Tel.: +48 (0) 22 6324513. Fax: +48 (0) 22 6324513. E-mail: [email protected].

The potential of dioxin formation during incineration of Clcontaining compounds is a great concern. An alternative to the gas-phase oxidation is the liquid-phase oxidation. Two groups of liquid-phase oxidation processes can be distinguished: (1) wet oxidation (WAO) processes and (2) adVanced oxidation (AOP) processes. Molecular oxygen is used as the oxidant in WAO processes, while ozone, hydrogen peroxide, or species activated by special means such as radiation are the oxidants in AOP processes. The WAO processes can be subclassified in (1) thermal subcritical wet air oxidation (TWAO) and (2) supercritical wet air oxidation (SWAO) and catalytic wet air oxidation (CWAO). In all the processes, molecular oxygen is dissolved in the water phase and oxidizes organic pollutants present in this phase. TWAO processes are performed at high pressure and high temperature. High pressure increases solubility of oxygen in water and makes it more available for the reaction, while high temperature enhances reaction rate. Therefore, the process is typically run at 0.5-20 MPa and 400-700 K, with residence time from 10 to 120 min. Conversion of organics in these conditions ranges from 80 to 99%, and COD reduction is completed by ∼75-90%.3 The reactivity of various organic compounds in TWAO correlates well with the carbon content in the molecule, with minor exceptions. The oxidation proceeds through a radical mechanism, and two paths occur simultaneously: degradation to lower carboxylic acids (acetic acid is considered to be the most refractory to oxidation) and elimination of CO2 during the reaction.2 For nitrogen-containing compounds, the oxidation leads to ammonia, nitrate, dinitrogen gas, and nitrous oxide depending on the pollutant and reaction conditions. For most cyanide- and amine-containing compounds, ammonia is the end product of the TWAO.4 TWAO becomes energetically self-sustaining when the COD exceeds 20 g/dm3.5 The important drawback of TWAO is the high capital costs caused by operation at high temperature and pressure in a hostile environment (the oxygenated compounds formed are highly corrosive under these conditions, demanding the use of special alloys). The capital costs of TWAO are higher than those for incineration but the operating costs are lower, mainly because of the lower energy requirements of the process.4 However, TWAO is a proven technology with more than 200 full-scale installations in operation by 1996, with the first commercial unit for the treatment of sulfite liquors being started in the late 1950s.6 Maugans and Ellis7 presented an extensive review of industrial TWAO processes. Severe process conditions at WAO stimulated researchers to search for processes that occur at milder conditions. An alternative to conventional TWAO is SWAO, i.e., oxidizing

10.1021/ie060906z CCC: $37.00 © 2007 American Chemical Society Published on Web 01/09/2007

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organic pollutants in supercritical water at elevated temperatures and pressures, possibly in the presence of the catalyst.8-15 Under supercritical conditions (>374 °C and >253 atm), organic compounds become completely miscible with water, while inorganic salts become insoluble in water. Destruction efficiencies exceeding 99.99% in 1-10 min are typical. However, the operating costs for SWAO process are 2-4 times higher than those for the WAO process.3 Because of the very high cost, SWAO technique should only be used for very toxic and/or refractory pollutants. In CWAO, pollutants can efficiently be removed at much milder conditions in the presence of a suitable catalyst. This significantly cuts capital and operational costs compared with the established noncatalytic process. Both homogeneous and heterogeneous catalysts were applied. Base metal salts soluble in water were used for apparently cheaper homogeneous catalysis, with copper salts being the most active.2,6 However, the homogeneous catalysts require a thorough separation of the toxic compounds from after-CWAO effluents with a very high efficiency. This significantly increases capital costs. Therefore, heterogeneous catalysts are more promising. Many different catalysts have been elaborated, based upon base metals and noble metals. In the result of extensive studies, effective catalysts have been found which allow for relaxing of operating conditions for CWAO to aNiO > aMnO2 > aFe2O3 > aYO2 > aCd2O3 > aZnO > aTiO2 > aBi2O3 Heterogeneous copper catalysts studied were in a form of CuO, Cu salts, or CuO mixed with oxides of Co, Mo, Mn, Ni, Fe, Cr, Zn, also in a form of mixed oxides of base metals.45-74 The copper catalysts were unsupported or supported, usually on γ-Al2O3, SiO2, CeO2, and active carbon. Ceria and active carbon were also used as the catalysts. Both laboratory-made and commercial catalysts from Harshaw (Cu 0803 T), Engelhard (mostly Cu-0203 T), TopsøÄ e (LK-821), Chemetron (G-3A), and Su¨d Chemie (G 66 A and EX-1144.8) were tested. The activity of these catalysts was evaluated using batch or semibatch operated stirred-tank reactors with slurry (SR) or spinning basket reactors (SBR), and continuously operated fixed-bed reactors with upflow (UFBR) or downflow of reactants, with the latter usually operated in a trickle-flow regime (TBR). Copper catalysts and process conditions for testing them are listed in Table 1. Air was mostly used for oxidation that was performed at temperature range of 70-200 °C, preferentially below 150 °C, under oxygen pressure up to 3 MPa, usually below 1 MPa. Reaction time ranged between 10 min and several hours. Under these conditions, Cu catalysts exhibited high efficiency in removal of organics from wastewaters. For oxidation of phenol, the best efficiencies at the first period of operation were up to

100% of phenol conversion, ca. 75-90% of TOC (total organic content) removal, and ca. 75-90% of COD (chemical oxygen demand) reduction. The difference between phenol conversion and TOC removal is caused by formation of some compounds that are refractory to oxidation under process conditions. Mineralization of phenol to CO2 and H2O occurs at 75-90%, while the remaining carbon atoms are in the effluent in a form of refractory organics. Phenol is easier to oxidize than substituted phenols, but all derivatives studied react quite easily with oxygen. The order of the copper catalyst activity in CWAO of phenol derivatives is

phenol > p-chlorophenol > p-nitrophenol62 The Cu catalysts deactivated relatively fast. The residual phenol conversion after some time-on-stream ranged from ca. 20% to aNaY-zeolite > aZrO2 > aTiO2 The order is not necessarily applicable for copper catalysts but is indicative. Pirkanniemi et al.36 reviewed CWAO catalysts supports. Metal oxides are the most commonly used. Among them, alumina and titania dominate; ceria and zirconia were used in several works. Ceramic materials such as cements containing alumina and/or silica were also studied. Silica support itself was found to be very sensitive to the acidic medium.59Alumina supports were reviewed by Kasprzyk-Hordern.77 Resistance of alumina in reaction conditions is a function of

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many factors since chemistry of alumina reactions in water is complex. However, no observable deterioration of alumina was reported under CWAO conditions. None of the oxides, alumina, titania, and zirconia, influenced the catalyst activity significantly. Ceria-containing catalytic systems are characterized by “stability under CWAO conditions, low catalyst leaching, the possibility of varying the composition over an almost unlimited range, and enhanced textural and redox properties at low temperature”.78 Therefore, ceria-based materials are promising for CWAO processes (see also the next sections). Active carbon (AC) was the promising support because of its high adsorptivity toward various organics. Moreover, carbons are resistant to acidic pH that is inherent to the CWAO process. For instance, active carbon was efficiently used as a catalyst support in CWAO, and leaching of copper is then significantly reduced.54 Moreover, it shows catalytic activity without the presence of any active metal. In terms of phenol removal, active carbon is superior to conventional oxidation copper catalysts. For instance, copper catalyst supported by active carbon performed better than the commercial G-66 A catalyst, and copper leaching from CuO/AC catalyst was lower than that from the commercial oxide catalysts (G-66).49 However, in typical CWAO conditions (140 °C and 0.9 MPa of oxygen partial pressure), there is a substantial loss of active carbon by combustion and, consequently, the catalyst deactivated for that reason. Partial combustion of active carbon was also observed even at temperatures as low as 120 °C.79 The combustion of active carbon can be significantly reduced (but not eliminated), e.g., by decreasing the oxygen partial pressure from 9 to 2 bar.54 Carbon materials for CWAO processes were reviewed by Stu¨ber et al.79 2.1.1.2. Copper Catalyst Deactivation: Fouling the Catalyst Surface. Polymeric substances are formed in condensation reactions between phenolic compounds and oxidation intermediates. According to Pintar and Levec,61 polymers are formed in two reactions taking place in the liquid phase: (a) stepwise addition polymerization of C-2 aldehyde (glyoxal), which is an intermediate in phenol oxidation, to phenol, and (b) polymerization of C-2 aldehyde. Benzoquinones were also considered to be precursors of polymeric deposits. Alvarez et al.49 suggested that organocupric polymers can be responsible for catalyst fouling. The formation of polymers is high when oxidation is performed in (semi)batchwise operated SRs. It is significantly suppressed in continuously operated FBRs.47,54,63 The probable reasons are that (a) residence time of the liquid phase in TBRs is much shorter than in SRs and (b) accessibility of the catalytic surface for intermediates that can either form polymers or be catalytically oxidized is better. The Cu-spinels were relatively stable in a TBR, but they lost their activity in SR (probably because of polymer formation). That was not observed for the Cu-Ni-spinel catalyst.47,48 No condensation products were found in the exited solution or deposited on the Cu catalysts in the TBR.54 Pintar and Levec61 have observed leaching of copper (phenol conversion dropped to ca. 40% and the Cu ion concentration in the effluent was 150 mg/L), but they claimed deactivation of the Cu catalyst was mainly due to formation of polymeric deposits. Kim and Ihm57 found carbonaceous deposits on the used transition metal catalysts for the wet oxidation of phenol. The amount and the nature of the carbonaceous deposits varied. Supported manganese oxide catalysts showed the highest amount of carbonaceous deposits. The Ce addition to the transition metal oxide catalysts increased the amount of carbonaceous deposits, leading at the same time to the enhancement of catalytic activity for phenol conversion. The carbonaceous

deposits must have their own micropores, which resulted in the decrease of the pore volume and the increase of the surface area. The nature of the carbonaceous deposits was mostly aromatic, and the aliphatic carbon appeared only for the Culoaded catalysts. Besides the aromatic nature of the carbonaceous deposits, NMR and Fourier transform infrared (FTIR) also confirmed the appearance of some oxygen-bearing groups such as carboxylic acids and alcohols. 2.1.2. Base Metal Catalysts. Catalysts with basic active sites, like Bi-, Co-, and Mn-catalysts, are more active in the oxidation of carboxylic acids than copper catalysts. Therefore, mixtures of oxides of these metals also with CuO were studied for phenol oxidation. Composite oxides containing CuO (Cu/Co, Cu/Co/ Bi, Cu/Bi/γ-Al2O3) exhibited relatively high activity.2 To avoid soluble copper in the catalyst, Bi/γ-Al2O3, Co/Bi, Co/Bi/γAl2O3, Sn/Bi, and ZnBi composite oxides were also tested in the oxidation of organics. The Co/Bi composite oxide catalyst appeared to be the most effective for the CWAO of lower carboxylic acids. The catalyst with the molar ratio Co/Bi ) 5:1 exhibited the highest activity, which was commensurable with that of homogeneous Cu salts2. Bismuth catalysts were probably that effective because of their high affinity to carboxylic acids. No information on Bi catalysts deactivation was given. CuO-MnO was found to be the most and sufficiently active from all combinations of CuO with oxides of Co, Fe, Mn, or Zn, but deactivation due to metal elution was fast.53 Duprez et al.80 confirmed that Mn/Ce (1:1) catalyst is soluble in an acidic medium. Moreover, tars (mostly of aromatic nature) were deposited on the surface, whereby the most massive depositions were observed for Mn-oxides.57 The Mn/Ce-oxides catalyst that was developed for oxidation of ammonia-containing solutions turned out to be active also in wet oxidation of various organic compounds.81 The Mn/Ce of the ratio 7:3 was found to be the most active for organics other than phenol.82 Chen et al.83 verified this conclusion in respect to phenol oxidation and found that the ratio Mn/Ce affected strongly the catalyst activity in this process. Their finding was that the most active catalyst was Mn/Ce with the ratio of 6:4. Maximum phenol conversion and TOC removal for this catalyst were ∼90% at 383 K, 0.5 MPa, and reaction time 10 min. Hamoudi et al.84 assessed Mn/Ce (6: 4) catalyst and found its remarkable activity. Phenol and TOC both went to zero in 10 min at optimum reaction conditions. Abecassis-Wolfovich et al.85 attempted to improve nonnoble metal catalysts for CWAO by synthesizing cerium-incorporated ordered manganese oxide OMS-2 (microporous manganese oxide octahedral molecular sieve) type catalysts containing cerium in the form of exchanged cations and/or pure CeO2 phase. The best performance in CWAO of phenol was only ca. 70% conversion. Stoyanova et al.86-88 and Christoskova and Stoyanova89 studied wet oxidation in the presence of Mn-Ni oxides that turned out to be extremely active catalysts under atmospheric pressure. Phenol conversion reached ca. 80% and 100% in 10 and 30 min, respectively, at 308 K andpH ) 6-786,87 and 100% conversion in 30-90 min in 288-338 min, respectively.89 p-Chlorophenol was fully oxidized at 298-318 K and pH ) 6-10. Nickel elution was negligible.88 Finally, iron oxide catalysts were evaluated in CWAO of phenol.90 Complete phenol conversion and 80% TOC removal were achieved at 400 K and 0.8 MPa in TBR containing Fe/active carbon. Again, short chain carboxylic acids remained intact. The results for Ni and Fe catalysts contradict findings of Kochektova et al.44 and require confirmation.

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Table 2. Base Metal Catalysts for Oxidation of Phenol and Substituted Phenols catalytic species

process conditions

ref

CoMo/AC, Mo/AC, Mn/AC Mn-Ce-O composites MnO2/CeO2 Mn-Ce-oxides, cryptomelane Ni-Mn-oxides Ni-oxide system Ni-oxide system Fe/AC CeO2 CeO2 CeO2/γ-Al2O3, CeO2/TiO2, CeO2/SiO2, CeO2/AlPO4-5 CeO2/γ-Al2O3

SR, 473 K, 0.42 MPa (oxygen partial pressure) SR, 383 K, 0.5 MPa (oxygen partial pressure) SR, 353-403 K, 0.5 MPa (oxygen partial pressure) TBR, 373 K, 1.0 MPa (oxygen partial pressure) SR, 308 K, atm. pressure SR, 308 K, atm. pressure SR, 288-328 K, atm. pressure TBR, 373-400 K, 0.8 MPa SR, 413-453 K, 0.5-1.0 MPa (oxygen partial pressure) SR, 433-453 K, 0.5-1.0 MPa (oxygen partial pressure) SR, 453 K, 1.5 MPa (oxygen partial pressure) SR, 453 K, 0.5-2.0 MPa (oxygen partial pressure)

Cao et al.64 Chen et al.83 Hamoudi et al.84 Abecassis et al.85 Stoyanova et al.86,87 Stoyanova et al.88 Christoskova et al.89 Quintanilla et al.90 Lin et al.91 Lin et al.92 Chen et al.93 Chang et al.94

Table 3. Catalysts for Phenol Oxidation; Noble Metals catalytic species Pt, Pd, Ru on CeO2 Pt, Ru on AC and γ-Al2O3 Pt, Pd, Ru on CeO2 Pt, Ru on CBS, SM-1 (TiO2 and SiO2) Pt/TiO2 and Pt/γ-Al2O3 PtxAg1-x/MnO2-CeO2 Pt/graphite Ru/CeO2 Ru/AC, also promoted by CeO2 Pt/γ-Al2O3 Pt (4,45%)/TiO2 Pt (4,45%)/TiO2 Pt, Pd, Ru on CBC Pd on ZSM-5, MCM-41, ZrO2 and TiO2 Ru/TiO2 RuO2/γ-Al2O3 and RuO2-CeO2/γ-Al2O3 Ru (3%)/TiO2

process conditions

ref

SR, 393-503 K, 2.0 MPa (oxygen partial pressure) SR. 473 K, 0.42 MPa (oxygen partial pressure) SR, 443 K, 2.0 MPa (oxygen partial pressure) TBR, 393-473 K, 5-8 MPa (total pressure) BSR, 403-473 K, 2.5-4.3 MPa (total pressure) 353-403 K, 0.5 MPa (oxygen partial pressure) SR, 393-453 K, 1.8 MPa (total pressure), 0.01-0.8 MPa (oxygen partial pressure) SR, 473 K SR, 433 K, 2,0 MPa (oxygen partial pressure) SR, 353-448 K, 0.5-1.5 MPa (oxygen partial pressure) SR, 423-473 K, 3.4-8.2 MPa SR, 423-473 K TBR, 393-433 K, 5-8 MPa (total pressure) SR SR, 448-473 K, 0.34-1.38 MPa (oxygen partial pressure) SR, 423 K, 3 MPa (total pressure) SR, 373-413 K, 0.1 MPa (oxygen partial pressure)

Barbier et al.95 Cao et al.64 Duprez et al.80 Cybulski and Trawczyn˜ski 96 Gonzales et al.97 Hamoudi et al.98 Masende et al.99,100

Ceria, also admixed with γ-Al2O3, was also used as the CWAO catalyst for phenol removal. This might be a cheap alternative to the other CWAO catalysts. Ceria itself was a relatively active catalyst: phenol conversion of >90% was achieved.91,92 A search for the best CeO2/γ-Al2O3 catalysts was carried out.93 Conversion of phenol was 100% and COD was reduced by 80% after 2 h at 453 K and 1.5 MPa (oxygen pressure) for the optimum catalyst containing 20% of ceria. Although activity of this catalyst is somewhat lower than that for ceria itself, the much lower cost of the former makes it a feasible catalyst for the CWAO of phenol. Nearly total phenol conversion and 80-90% of TOC removal were reached at 453 K and 2 MPa in a slurry reactor for the optimum catalyst.94 Base metal catalysts and conditions for the catalysts that were tested are set in Table 2. 2.1.3. Precious Metal Catalysts. Catalyst containing noble metals exhibited usually higher activity than copper and other base metal catalyst, particularly in respect to carboxylic acids. Leaching of precious metals from CWAO catalysts has not been reported. They are, however, more sensitive to poisons such as halogen-, sulfur-, and phosphorus-containing compounds. The sensitivity of noble metals to these poisons can be reduced by the proper choice of metal oxide support. For instance, alkaliand alkaline-earth-metals are known to be appropriate for this purpose. Noble metals can lose the catalytic activity because of oxidation under CWAO conditions. Noble metal catalysts deactivate also because of deposition of polymers that are formed in the CWAO processes. Noble metal catalysts contained usually 0.1-5% of the metal supported on γ-Al2O3, SiO2, CeO2, TiO2, ZrO2, cement, or carbon. Laboratory-made catalysts were tested. Similar to base metal catalysts, the catalytic activity was evaluated using batch-

Barbier et al.110 Oliviero101 Hamoudi84 Maugans and Akgerman102 Maugans and Akgerman103 Trawczyn˜ski104 Woo105 Vaidya106 Yang107 Kojima108

or semibatch-operated stirred-tank slurry reactors (SRs) and continuously operated trickle-bed reactors (TBRs). Air was mostly used for oxidation that was carried out at a temperature range of 80-200 °C, under oxygen pressure of 0.1-2.0 MPa. Reaction time ranged between 10 min and 3-4 h. Under these conditions, noble metal catalysts exhibited high efficiency in removal of phenol from wastewaters. 100% of phenol conversion and 85-98% reduction of TOC and COD was reached. Mineralization of phenol to CO2 and H2O was usually higher than that for metal base catalysts because of the higher activity in oxidation of carboxylic acids. Noble metal catalysts, which were used for phenol oxidation, are listed in Table 3. Okitsu et al.109 have studied activity of precious metals on alumina or titania in oxidation of p-chlorophenol at 423 K and 3 bar of oxygen partial pressure. They ranked activity of noble metals as follows:

aPt . aPd > aRu > aRh > aAg Cao et al.64 tested noble metal catalysts on AC support in oxidation of ammonia and phenol solutions and compared them with noble metals on metal oxides. Active carbon performed better as the support for Ru and Pt catalysts than TiO2 and γ-Al2O3. Phenol conversion was ca. 100% for the Pt/AC catalyst. Duprez et al.80 found phenol easily oxidizable at 403 K and 2.0 MPa (oxygen pressure) in the presence of Ru, Pt, and Rh. The Ru/AC was the most active, while Ru catalysts supported on TiO2 and ZrO2 exhibited lower activity. No leaching of ruthenium was detected. The Pt/γ-Al2O3 catalyst was moderately active in phenol oxidation, and carbonaceous deposits were formed on the catalyst surface.84 Barbier et al.95 compared activity of noble metals supported on ceria. The following activity order was established:

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aRu > aPd > aPt The initial oxidation rate was the highest for Pd catalyst, but rapid deactivation took place. A PtxAg1-xMnO2/CeO2 catalyst was very effective in phenol destruction but moderately selective to CO2 and H2O. Deactivating carbonaceous deposits were formed on this catalyst.98 Masende et al.99,100 oxidized phenol in the presence of 5% Pt/graphite and determined the operation window for a continuous stirred-tank reactor (CSTR). Complete oxidation to CO2 and H2O was achieved at 423 K, 1.5-2.0 MPa (total pressure) with oxygen loads 0.15-0.35 mol s-1 kgPt-1, and stoichiometric oxygen excess of 0-80%. High temperatures enhanced the catalyst activity. The activity remains high when the residual pressure was below 150 kPa; otherwise, deactivation of Pt occurred because of platinum oxidation that is reversible at reducing conditions. Formation of p-benzoquinone was favored at high oxygen pressure and low temperatures. Oxygen coverage on the Pt surface determines the reaction pathway and, consequently, selectivity to CO2 and H2O. Fully reduced Pt surface favors formation of refractory acetic and succinic acids, whereas free Pt surface is vulnerable to poisoning by carbonaceous deposits. Maugans and Akgerman investigated oxidation of phenol in the presence of Pt/TiO2 in SR102 and TBR.103 Complete oxidation of phenol and almost complete removal of TOC was reached. The catalyst deactivated in the TBR, whereby loss of activity was proportional to a cumulative amount of phenol passed over the reactor. Vaidya and Mahajani106 used Ru/TiO2 for oxidation of phenol at pH ≈ 6.5 and >12 (see Figure 2). Catalyst was effective at phenol and acetic acid removal at near-neutral conditions, while the amount of acetic acid at alkaline conditions was very high. Hydroquinone (free radical initiator) addition improves the CWAO rate, and tertbutyl alcohol (free radical scavenger) addition decreases the rate.106 Woo105 studied oxidation of phenol and other aromatic compounds, o-cresol and m-xylene, in the presence of Pd supported on ZSM-5, MCM-41, TiO2, and ZrO2. He found Pd/ TiO2 to be the most highly active in oxidation reactions. This catalyst efficiently suppressed formation of hardly degradable organic intermediates and polymers. Cybulski and Trawczyn´ski96 compared Pt and Ru supported on carbon black composites (CBC) and cement SM-1 prepared from SiO2 and TiO2. Nearly 100% phenol conversions were reached, whereby the platinum catalyst performed better. Both catalysts deactivated. Carbonaceous binder was likely to be combusted under reaction conditions, and components of SM-1 were eluted from the catalyst. Trawczyn´ski104 extended the study on Pd/CBC catalyst. All catalysts exhibited high activity, with the following order of activities:

Figure 2. Effect of pH on acetic acid formation (473 K, 2.27 MPa total pressure, 0.69 MPa oxygen pressure, initial COD 2 kg m-3, catalyst [Ru/ TiO2] loading 0.5 kg m-3): (]) without catalyst at pH 6.5; (9) with catalyst at pH 6.5; (2) without catalyst at pH > 12; (x) with catalyst at pH > 12. Reprinted with permission from Vaidya and Mahajani.106 Copyright 2002 Elsevier.

Figure 3. Oxidation of acetic acid ((4) initial pH ) 2.9) of an equimolar mixture of acetic acid and sodium acetate ((1) initial pH ) 4.7) and of sodium cetate ((0) initial pH ) 8.8) over Ru/CeO2 at 473 K. Reprinted with permission from Barbier et al.110 Copyright 1998 Elsevier.

conversion was high, and it was still improved by ceria. According to the authors, CeO2 improves Ru dispersion and promotes chemisorbed oxygen on the catalyst surface. The Ru/ Mn/Ce containing 3 wt % of Ru, with atomic ratio Mn/Ce of 1:9, was highly active in the CWAO of domestic wastewater.111 Qin et al.112 tested noble metal (1 wt %) and Mn (10 wt %) catalysts supported on AC, γ-Al2O3, and CeO2. The following activity order was found,

aPt > aPd > aRu . aCBC

aPt > aPd > aRu > aMn

Combustion of CBC was observed. Conversely to the Okitsu et al.109 ranking, Imamura2 found Ru to be the most effective among the noble metals catalysts. The function of CeO2 was found to bevery important. Precious metals behavior in the presence of CeO2 differs remarkably from that on other supports. This is also the finding of Barbier et al.,110 who attributed it to ceria’s ability to transfer oxygen. Oxidation rate was a strong function of pH; see Figure 3. Ruthenium catalysts supported on active carbon and/or ceria were investigated by Oliviero et al.101 The Ru/AC catalyst promoted by CeO2 was active at 433 K, but mineralization was far from total. Ceria was considered to be the oxygen donor. Ruthenium catalysts RuO2/γ-Al2O3 and RuO2/γ-Al2O3/CeO2 were studied in phenol oxidation by Yang et al.107 Phenol

for oxidation of p-chlorophenol. Strong leaching of Mn was observed. Pt was more active in oxidation of acids than Ru. Gonza´les-Velasco et al.97 compared activity of two lab-made catalysts: Pt/TiO2 and Pt/γ-Al2O3. Both catalysts exhibited a high activity, resulting in complete conversions of phenol after a reaction time of 5 h. Kojima et al.108 oxidized o-chlorophenol over 3% Ru/TiO2, varying the initial pH values. Complete decomposition and dechlorination of o-chlorophenol was attained at pH ) 9.8 in 1 h, while TOC removal of 85% was reached in 3 h. The catalyst performed worse at higher pH, probably because of a low pKa of carboxylic acids formed during the oxidation. 2.2. Oxidation of Carboxylic Acids. Copper and the other base metal catalysts have been found inefficient in removal of

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Table 4. Catalysts for Oxidation of Carboxylic Acids catalytic species

oxidized material

process conditions

Pt/AC Ru/AC and Ru/graphite Pt/C Ir/C Pt/C Pt/C Pt/Al2O3 Pt/ γ-Al2O3, sulfonated resins Pt and Ru on TiO2 and ZrO2 Au and Ru on TiO2 and ZrO2 Ru/TiO2

acetic and formic acids acetic acid succinic, acrylic and acetic acids acetic acid acrylic acid formic, maleic, acetic, oxalic and succinic acids; paper bleach effluent succinic, formic, oxalic, acryli, glycolic, and chloroacetic acids acetic, formic, oxalic, and maleic acids acetic acid acetic, propionic, and butyric acids butyric acids carboxylic acids (C2-C4) formic acid maleic acid maleic, oxalic and formic acids p-coumaric, acetic and succinic acids succinic acid succinic acid

2.8% Ru/TiO2 (Engelhard) Ru/CeO2 Ru-CeO2 on HSA5 and HSA5M Ru/CeO2 Ru, Pt, Ir and Pd on CeO2

succinic acid, acetic acid, cyclohexanol acrylic, succinic and acetic acids maleic acid acetic acid stearic acid

Ag/Ce, Co/Ce, Mn/Ce, CeO, MnO Ti-Ce Co-Bi complex oxides CeO2-ZrO2 doped withCuO or MnOx Bi/γ-Al2O3, Co/Bi, Co/Bi/γ-Al2O3, Sn/Bi, Zn/Bi Bi/γ-Al2O3, Co/Bi, Co/Bi/γ-Al2O3, Sn/Bi, Zn/Bi, Ni/Bi, Cu/Co/Bi Mn/Ce Ru/TiO2, Ru/ZrO2

acrylic acid acetic acid acetic acid acetic acid

SR, 473 K, 1.0 MPa (oxygen partial pressure) SR, 473 K, 2.0 MPa (oxygen partial pressure) SR, 473 K, 2.0 MPa (oxygen partial pressure) SR, 473 K, 2.0 MPa (oxygen partial pressure) SR, 433 K, 2.0 MPa (oxygen partial pressure) SR and TBR, 473-703 K, 0.74-5.5 MPa (oxygen partial pressure) SR and TBR, 326-473 K1.5-5.5 MPa (total pressure) SBR, 293-463 K, 0.1-1.5 MPa (total pressure) SBR, 448-473 K, 10 MPa (total pressure) SR, 473 K, 0.69 MPa (oxygen partial pressure) SBR, 473 K, 0.69 MPa of oxygen pressure SR, 473 K, 0.69 MPa (oxygen partial pressure) SR, 282-293 K; 0.6 MPa (total pressure) 415-478 K, 0.4-1.4 MPa (oxygen partial pressure) SR, 1.15 MPa and atm. pressure SR, 413-463 K, 5.0 MPa (oxygen partial pressure) SR, 463 K, 5.0 MPa (total pressure) SR, 180-200 °C, 0.3-1.8 MPa (oxygen partial pressure) TBR, 423-473K, 5 MPa (total pressure) SR, 433 K, 2.0 MPa (oxygen partial pressure) SR433-473 K, 2.0 MPa (oxygen partial pressure) SR, 443-463 K, 1.5 MPa (oxygen partial pressure) SR, 433-503 K, 0.1-2.0 MPa (oxygen partial pressure) SR, 473 K, 1.5 MPa (oxygen partial pressure) SR, 503 K, 2-2.5 MPa (oxygen partial pressure) SR SR, 463 K, 1.2 MPa (oxygen partial pressure)

acetic, formic and oxalic acids

SR, 521 K, 1.0 MPa (oxygen partial pressure)

Imamura et al.136

formic, acetic, propionic, butyric, valeric, hexanoic, adipic, succinic, oxalic, and glutamic acids acetic acid succinic and p-hydroxybenzoic acids

SR, 385-521 K, 1.0 MPa (oxygen partial pressure)

Imamura et al.137

SR, 520 K, 1.0 MPa (oxygen partial pressure) SR and TBR, 398-463 K, 4.6-10.4 (oxygen partial pressure)

Imamura et al.138 Besson et al.139

Ru/Ce, Rh/Ce, Pt/Ce, Ir/Ce, Pd/Ce Ru, Rh, Pt on AC and TiO2 Pt, Pd and Ru on CeO2 Ru/TiO2 , Ru/Al2O3, Ru/CeO2 Ru/AC also promoted by CeO2 Pt and Ru on AC, TiO2 and ZrO2 Pt/C, Ru/C, Ru/TiO2, Ru/ZrO2

the most refractory, short-chain carboxylic acids that are intermediates in the CWAO of phenol. Therefore, the search for the catalyst that might be active in the oxidation of carboxylic acids was limited to noble metals catalysts. These catalysts were tested in reactors of formerly mentioned types and under similar operating conditions (see Table 4). Noble metals supported on carbon or in base metal oxides were evaluated. Besson et al.113 oxidized formic, maleic, and succinic acids in the presence of Pt and Ru on active carbon and TiO2 and ZrO2. The Pt/AC catalyst was found to be active in moderate conditions (low temperatures), while Ru/TiO2 and Ru/ZrO2 required higher temperatures. Kuster et al.121 studied Pt/AC catalyst in a CSTR at pH from 1.1 to 13.0. The catalyst exhibited high activity over the whole range of operation conditions. Gallezot et al.115 found Pt/AC catalysts to be active in the oxidation of carboxylic acids. Total conversion of formic and oxalic acids at 326 K and 0.1 MPa and maleic acid at 326 K and 1.5 MPa (total pressure) was reached. Their Pt/AC catalyst was, however, inactive for the oxidation of acetic acid. This suggests that degradation of the former acids did not occur via acetic acid. Gallezot et al.116 developed the Ru/C catalyst using both active carbon and graphite as a support. Oxidation tests showed that graphite-supported catalysts were more active than AC-supported ones, whereby total conversion to CO2 was observed. No ruthenium was detected in effluents from CWAO. The Ru/AC promoted by ceria was, however, of low activity in the oxidation of acrylic acid at 433 K and 2.0 MPa.101 Gomes and co-workers117,120 used Pt/AC catalysts for the oxidation of carboxylic acids C2-C4. Conversions of 60-75% was reached

ref Imamura et al.76 Duprez et al.80 Barbier et al. 95 Barbier et al.110 Oliviero et al.101 Besson et al.113 Besson et al.114 Gallezot et al.115 Gallezot et al.116 Gomes et al.117 Gomes et al.118,119 Gomes et al.120 Kuster et al.121 Rivas et al.122 Lee and Kim123 Perkas et al.124 Besson et al.125 Be´ziat et al.126 Be´ziat et al.127 Oliviero et al.128 Oliviero et al.129 Hosokawa et al.130 Renard et al.131 Silva et al.132 Jiang et al.133 Jiang et al.134 Leitenburg135

after 2 h, while selectivity to CO2 and H2O was 100%. Iridium on active carbon appeared to be a less active catalyst. Conversions of butyric and i-butyric acids in oxidations at 473 K and 0.69 MPa (oxygen pressure) in an SR reached up to ca. 53% after 2 h.118,119 γ-Al2O3, CeO2, TiO2, and ZrO2 were also used as supports of noble metals. Rivas et al.122 assessed activity of 0.5% Pt/γAl2O3 in the oxidation of maleic acid. COD removal was strongly dependent on oxygen concentration and its excess. Lee and Kim123 tested Pt/γ-Al2O3 and sulfonated resins in the oxidation of maleic, formic, and oxalic acids under elevated and atmospheric pressures. Oxalic and formic acids readily oxidized into CO2 and H2O at 353 K and atmospheric pressure. Maleic acid could be oxidized under high pressure only. Sulfonated resins reduced the pressure under which maleic acid can be oxidized. Besson et al.113 oxidized formic, maleic, and succinic acids in SR and TBR in the presence of Ru supported on TiO2 and ZrO2. Complete oxidation was reached for both catalysts that were stable for a prolonged operation. No traces of Ru and Ti in the outlet streams were detected. Duprez et al.80 found Ru/TiO2 and Ru/ZrO2 catalysts to be moderately active in the oxidation of acetic acid in SR. No leaching of noble metals was observed. Perkas et al.124 used Pt and Ru supported on TiO2 and ZrO2 for the oxidation of succinic acid. The Pt catalysts were the most active and stable in the removal of the acid. Pt and Ru supported on TiO2 and ZrO2 were studied by Besson et al.114 in the oxidation of succinic, formic, oxalic, acrylic, glycolic, and chloroacetic acids. Complete degradation of acids was attained at 323-503 K and 0.74-5.5 MPa for

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both Ru/TiO2 and Ru/ZrO2. The catalysts were physically and chemically stable in hot acidic oxidizing solutions. The extension of the Besson and Gallezot139 investigations on behavior of Ru/ TiO2 and Ru/ZrO2 catalysts in the oxidation of succinic and p-hydroxybenzoic acids in SR and TBR showed high conversions of acids. Moreover, supports were stable to leaching, sintering, and fouling. Ruthenium deactivated as exposed to air (especially in the case of very small clusters), whereby the loss of activity due to this phenomenon was reversible. Besson and Gallezot125 found the activity of Ru/TiO2 and Au/ZrO2 catalysts in the oxidation of succinic acid to be strongly dependent on metal particle size. High conversions of succinic acid were reported by Be´ziat et al.126 in oxidations. Inorganic salts present in the solutions only slightly decreased the oxidation rate. Substitution of acetic acid with Cl, OH, or NH2 gave an increase in the oxidation rates. No ions of Ru and Ti in effluents were found. Activity of the catalyst becomes stable after a steadystate coverage of the Ru particle by oxygen is achieved. Be´ziat et al.127 extended their studies on 2.8% Ru/TiO2 catalyst in the oxidation of succinic, acrylic, and acetic acids and cyclohexanol. High conversions were achieved, and the catalyst was stable. Oliviero et al.128 studied the role of ceria as a support of ruthenium in oxidation of acrylic, succinic, and acetic acids. Transmission electron microscopy (TEM) and diffraction measurements favored the hypothesis that ceria plays a specific role in oxygen transfer from the gas phase to metal sites. The number of contact points between Ru particles and ceria crystallites constitutes a key parameter in oxidations of carboxylic acids (acrylic, succinic, and acetic).101 The study of Oliveiro et al.129 on the oxidation of maleic acid in the presence of 5% Ru/CeO2/ HSA5 and 5% Ru/CeO2/HSA5M showed that maleic acid converted easily to short-chain carboxylic acids. Of them, acetic acid was the major compound formed and was relatively refractory at reaction conditions. Barbier at al.110 oxidized acetic acid in the presence of various metals supported on γ-Al2O3, CeO2, and TiO2. The following activity order was found:

aRu > aIr > aPd ≈ aFe ≈ aCu > aAg ≈ aNi ≈ aCo ≈ aCr The activity of unpromoted ceria was similar to that of chromium. Oxidation rate was found to decrease significantly with the rise of pH. Barbier et al.95 compared catalytic activity of Pt, Pd, and Ru supported on ceria. The Ru/CeO2 catalyst performed the best. Conversion of 100% for acetic, succinic, and acrylic acids was attained in 90 min, while only ca. 40% was reached in the case of Pt/CeO2 and 90% for Cu and Mn doped catalysts), although surface areas were approximately the same for all catalysts. The catalyst leaching during reaction was very low. Imamura et al.136 studied the oxidation of acetic acid catalyzed by Co-Bi oxides. A maximum in catalytic activity was found at the ratio Co/Bi of 5:1. This catalyst was even more active than Co/Bi catalysts with CuO, which was considered to be the most active among base metal catalysts. ∆TOC60min for this catalyst at 521 K and 1.0 MPa of oxygen partial pressure was 100%. The catalyst was highly reactive toward formic and oxalic acids. Durability of this catalyst was tested by several injections of acetic acid to the reaction mixture every 60 min and by recording changes in ∆TOC. The decrease in ∆TOC after the fifth run was only 5% compared with the initial ∆TOC. It was assumed that acetic acid is adsorbed on the basic sites of Co/Bi (5:1) catalyst in the first step of the reaction, followed by a redox reaction between the catalyst and the adsorbed acetic acid to induce its decomposition. The Co/Bi (5:1) catalyst was also the subject of another investigation of Imamura et al.137 Acetic acid appeared to be much more refractory to oxidation than C2+ carboxylic acids and formic acid. ∆TOC20min for acetic acid was 25.1%, while for hexanoic acid it amounted to 80.6%. Jiang et al.134 used Co and Bi oxides and Co-Bi complex oxides for the oxidation of acetic acid. Two mixed oxides of optimum Co/ Bi ratio (5:1) performed differently: COD reduction was 25.8% and 84.1% for A and B, respectively. It was found that, in catalyst A, interaction between cobalt and bismuth oxides, which is caused by a special structure, may activate an adsorbed oxygen and play an important role in the oxidation of acetic acid. In catalyst B, such interaction is weak. Many active sites provided by cobalt oxide are covered by large particles of bismuth oxide aggregating on the surface of cobalt oxide, and bismuth oxide contributes only a little to the oxidation of acetic acid. Both catalysts were prepared by the impregnation method, but a little soluble polymer was added to the solution for catalyst B 2.3. Nitrogen-Containing Compounds. Oliviero et al.34 recently presented an extensive review on CWAO for treating water solutions of ammonia and nitrogen-containing compounds. Therefore, hereunder only some papers are discussed with attention paid mainly on industrial wastewaters containing N-compounds. According to literature sources cited by Oliviero et al.,34 pollution of wastewaters by ammonia comes mostly from agriculture. In great Britain, agriculture contributes 85%

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of the ammonia emissions. One of the negative effects of ammonia is the eutrophication of rivers, even though nitrates are mainly responsible for this problem. Inoue et al.140 showed an excellent efficiency of the Co(II) oxide in the ammonia oxidation. The process was also very selective in molecular nitrogen. Aguilar et al. studied oxidation of aqueous ammonia141 and mono- and dimethylamines142 in the presence of active carbon. The N-compounds were selectively oxidized to nitrogen, water, and carbon dioxide at 468 K and 1.6 MPa, whereby conversion of methylamines ranged between 25 and 45% after 2 h. Barbier et al.143 oxidized ammonia and aniline in the presence of Pt, Pd, and Ru on ceria using SR at 423-23 K and 0.05-2.0 MPa (oxygen pressure). Aniline was fully eliminated while ∆TOC ) 85-96% and ∆COD ) 94-100% for 30 min operation at 473-503 K. Selectivity toward N2 exceeded 90%; the oxidation proceeds via hydroxylamine. Nitrites and nitrates appear in the solution when temperature is increased. Barbier et al.95 extended investigations of CWAO over noble metal catalysts. The process was run at 433-473 K and 2.0 MPa. ∆COD at 3 h was 68%, 60%, and 49% at 433 K and 94%, 84%, and 70% at 473 K for Ru/CeO2, Pd/CeO2, and Pt/CeO2, respectively. The most active catalyst, Ru/CeO2, was very selective to CO2 but not selective to N2. Many metals, Pt, Ru, Cu, Co-Mo, Mo, and Mn, supported on active carbon and ceria, titania, alumina, and MCM-41 were tested in the oxidation of aqueous ammonia and highly polluted landfill leachate.64 Platinum on active carbon was the best in terms of ammonia removal. Conversion of ammonia was 52% and 88% at 473 K and at initial pH of 5.6 and 12, respectively. The composite Mn/Ce (7:3) oxide catalyst was used to oxidize ammonia.81 Conversion of 70% was reached in 1 h. The adsorption capacity of ammonia was considered to be the determining factor for the catalytic activity. Accordingly, catalysts of a great affinity for ammonia would be the most active in liquid-phase oxidation. Chakchouk et al.144 investigated the CWAO of ammonia in the presence of the same Mn/Ce (7:3) catalyst. They confirmed that a temperature of at least 260 °C is necessary to reach a significant conversion (90%) of ammonia. The final product of ammonia oxidation in the presence of this catalyst is dinitrogen. Qin and Aika145 used the RuO2/Al2O3 catalyst to oxidize ammonia at 503 K and 1.5 MPa (total), adding aq. NaOH to maintain a pH of 12. At least 99% ammonia was decomposed, and N2 was produced almost exclusively. Quantities of nitrates were very small. The reaction proceeded quickly in the high pH region, suggesting that ammonia is more reactive than ammonium ion. The Ru catalyst remains active after four cycles of oxidation at 453 K and 2.7 MPa with ammonia conversion of 99%. Conversely, nitrates and nitrites in large amounts were present in effluents when oxidizing ammonia in the presence of Cu/La/Ce composite catalyst in TBR at 503 K and 2.0 MPa (oxygen pressure).146 Ammonia removal was the highest at pH ≈ 12, but nitrite selectivity was also high at these conditions. 3% Ru/TiO2 catalyzes oxidation of ammonia to nitrous acid.147 Noble metals, such as Pt, Pd, Ru, and Ir, supported on graphite, active carbon, titania, and zirconia were tested in SR at 424453 K and 1.5 MPa for ammonia oxidation.148 Pt appeared to be more active but less selective than Pd and Ru. N2 was mainly produced, while N2O was the second major product. The Pt/ TiO2 and Pt/ZrO2 catalysts were less active than the Pt/graphite one because of the lower platinum dispersion. The Ir/TiO2 catalyst was more active and selective to N2 than Pt/TiO2. The activity of noble metals strongly depends on the nature of the support. Takayama et al.149 tested 3 wt % Pd on AC, TiO2,

MgO, CeO2, Sm2O3, and La2O3. All supports lead to a selectivity > 90% in dinitrogen, except with La2O3, which favors the formation of nitrates. The Pd/AC catalyst was the most active, even more active than the RuO2/Al2O3 catalyst of Qin and Aika.145 Aniline and its derivatives are components of many dyes. Therefore, oxidation of this compound was also extensively studied. A Cu/MCM-1 catalyst was used to oxidize aniline in SR at 473 K and 0.69 MPa (oxygen partial pressure).150 Conversion of aniline after 2 h was 96% and selectivity toward CO2 formation was 76%. Some leaching of copper was observed, but the catalyst exhibited quite a good performance and stability after reusing it at least three consecutive times. Aniline as well as p-nitrophenol and nitrobenzene were oxidized in the presence of active carbon in TBR at 413 K and 1.3 MPa (total pressure).151 Conversions less than 5% and almost no TOC abatement and COD reduction were recorded. A complex mixture of dimethylformamid, carbamide, acetonitrile, and aliphatic alcohols was oxidized in SR at 433-523 K and 0.55.0 MPa in the presence of Co3O4, Fe2O3, Mn2O3, Zn-FeMn-Al-O, Pt, and Ru supported on alumina, ceria, and active carbon.152 The optimum conditions for the best catalyst (Ru/ AC) are as follows: 473-513 K and 1.0 MPa. Residual content of toxic compounds in the effluent was then 95% at 438 K when using ceria as a catalyst. Durability of both catalysts was considered to be fair. Arslan and Balcioglu165 used polyoxytungstates for oxidation of acid dye Orange II. TOC removal depended strongly on the catalyst concentration. The process was also carried out in the presence of OH- scavengers such as isopropyl alcohol and bromide. The catalytic process was relatively insensitive to the scavengers, suggesting a freeradical-type reaction mechanism is not involved in the heterogeneous oxidation mechanism. Pt on multiwalled carbon nanotubes (MWNTs) was used for oxidation of aniline, monoazo-, diazo-, and triazo dye.166 Color was completely removed, and TOC abatement was 51.2%. Selectivity of the catalyst vs dyes was ranked as follows: diazo > monoazo > triazo. Zhu et al.167 investigated oxidation of H-acid (1-amino-8-naphthol3,6-disulfonic acid) in the presence of catalysts containing Cu, Ce, Cd, and Co-Bi prepared by codeposition. The Ce-Cu (3: 1) catalyst was found to be the best one. At 473 K, 3.0 MPa, and pH of 12, after 30 min, removal of COD was >90%. All the H-acid was decomposed in 5 min and oxidized into NH4+, SO2-, formic and acetic acids, etc. 2.4. Industrial Wastewaters. p-Coumaric acid was chosen by some groups of scientists as a representative model compound of the biologically recalcitrant polyphenolic fraction in olive processing and wine distilleries wastewaters. It belongs to a group of phenolic components of these streams comprising gallic, caffeic, and vanillic acids that are known to inhibit biological treatment of wastewaters of agricultural origin. Poly(ethylene glycol)s (PEGs) are an important group of nonionic water-soluble polymers that are commonly used in the production of surfactants, lubricants, pharmaceuticals, etc. The rate of their biodegradation substantially decreases with the increasing of molecular weight. The CWAOs of PEGs and p-coumaric were often studied together. Catalysts used for CWAO to remove these and other model compounds as well as pollutants in real industrial wastewaters together with process conditions to test the catalysts are set in Table 5. Imamura et al.76 compared the activities of various precious metals (5 wt % metal) or metal oxide supported on γ-Al2O3, CeO2, TiO2, ZrO2, and NaY-zeolite in a process of oxidation of PEG-200. The activity on TOC removal was as follows:

aRu ) aRh ) aPt > aIr > aPd > aMnO The activity of Ru on ceria was the most effective among the catalysts investigated and was even better than that of homogeneous catalyst Cu(NO3)2. The reactivity of intermediates such

Figure 4. TOC removal during the oxidation of PEG 10000 at various temperatures with Pt/Al2O3 in slurry and without catalyst: -0- Pt/Al2O3, 383 K; -4- uncatalyzed, 383 K; -]- Pt/Al2O3, 393 K; -9- uncatalyzed, 423 K; -+- uncatalyzed, 463 K; -O- Pt/Al2O3, 403 K; -bPt/Al2O3, 423 K; -x- Pt/Al2O3, 463 K. Reprinted with permission from Mantzavinos et al.169 Copyright 1996 Elsevier.

as ethylene glycol, formic acid, and formaldehyde toward this catalyst was also investigated by Imamura et al.81 Again, the activity of Ru/CeO2 catalyst was higher than that of copper nitrate. The lower the pH, the higher was the TOC removal. Imamura et al.82 studied the decomposition of PEG 200 in the presence of Mn/Ce (7:3) and Co/Bi (5:1) composite oxides. Both catalysts were fairly active in oxidation, with ∼60% in TOC removal in 60 min. Mantzavinos et al.168 studied CWAO for the destruction of p-coumaric acid. A Co/Bi composite oxide was the most effective for the degradation of p-coumaric acid with almost total conversion and 75% TOC removal in 10 min at 403 K. The possible leaching of cobalt, a toxic metal, excludes application of this catalyst, since it could inhibit the microorganisms in the after-CWAO biological treatment. CuO-ZnO/Al2O3 and CuO-CoO-ZnO/Al2O3 catalysts performed similarly (∆TOC ≈ 80% in 90 min), but leaching of Cu and Zn occurred at acidic conditions, while at alkaline conditions Al was dissolved. The same authors169 also oxidized PEG 10 000 in the presence of base and noble metal catalysts. TOC removal was the highest (97% after 90 min) for the Pt catalyst; Pd performed slightly worse. Metal oxides catalysts efficiency was much lower. Pelleted noble metal catalysts were also very active. Pd, Ru, and Pt (max ∆TOC ≈ 80% in 60 min) were the best, while Rh and Re were much less active. Exemplary temperature dependence of oxidation rate is shown in Figure 4. Noble metals were more active than metal oxides. Stability of the Pd/Al2O3 catalyst was studied. Deactivation occurred via metal sintering and blockage, while leaching was not observed. Mantzavinos et al.170 also investigated the activity of noble metals on various metal oxides in the oxidation of p-coumaric acid and poly(ethylene glycol). Lower carboxylic acids were easily attainable even under mild operating conditions, but total oxidation was difficult. Leaching of noble metals was not reported. Mantzavinos et al.171 investigated the activity of metal oxides in the oxidation of p-coumaric acid and poly(ethylene glycol) of molecular weight from 32 000 to 35 000. The CuO‚ZnO-Al2O3 catalyst was found to be effective for the oxidation, but leaching of metal into the solution was observed. Ethylene glycol and acetic acid were found to be intermediates that are difficult to oxidize. Catalytic oxidation of ethylene glycol was a subject of study by Silva et al.172 They used commercial catalysts, CuOZnO/Al2O3, Pt/Al2O3, CuOXY-Z, and CuO-X, as well as labmade catalysts, Mn-Ce-O, Ag-Ce-O, and Ce-O. The MnCe-O catalyst exhibited the highest activity: >99% conversion of glycol and TOC removal. Formic acid was identified as the only intermediate, and pH evolution was related to its formation

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and oxidation. Formaldehyde and industrial effluent containing it were oxidized in the presence of commercial CuO-ZnO/ Al2O3 catalyst and lab-made Mn/Ce, Co/Ce, and Ag/Ce catalysts.173 TOC removal was 99.4%, 71.4%, 54.2%, and 78.7% for Mn/Ce, Ag/Ce, Co/Ce, and CuO-ZnO/Al2O3, respectively. The best catalyst, Mn/Ce, removed formaldehyde from 800 ppm in the industrial wastewater to 0.1 ppm, and TOC removal was 91.7%. Perkas et al.124 used Pt and Ru supported on TiO2 and ZrO2 for the oxidation of p-coumaric acid. Pt catalysts were the most active and stable in the removal of the acid and intermediates of its oxidation. Minh et al.174 examined Pt and Ru on TiO2 and ZrO2, in the oxidation of p-coumaric acid. Mineralization proceeds through different aromatic compounds, further to aliphatic intermediates (mainly acids). Important mineralization was achieved at 413 K and 5.0 MPa (total pressure). Oxidation of p-coumaric acid was also investigated in the presence of FeCeO2 and Zn-CeO2 catalysts.175 The Fe-CeO2 catalysts containing 20-50% iron were more effective than unpromoted and Zn-promoted ones. Addition of iron modifies structural and redox properties of ceria catalysts. Belkacemi and co-workers176,177 attempted to apply the CWAO process for the removal of organics from high-strength distillery waste liquors of initial TOC > 10 g/L and COD ) 26 g/L. 1% Pt/Al2O3, Mn/Ce oxides, and Cu(II)-exchanged NaY zeolites were used as the catalysts. The Mn/Ce (7:3) catalyst performed best, and ∆TOC was ∼60%. Activity of this catalyst decreased for prolonged exposures, and gradual fouling was observed. Many papers were dedicated to CWAO for effluents from pulp and paper mills. Besson et al.113 oxidized paper bleach effluent in the presence of Pt and Ru on AC, TiO2, and ZrO2. TOC was effectively removed from the effluent for a long time in the presence of both catalysts. No Ru and Ti were found in the outlet streams. Besson and Gallezot139 investigated the stability of ruthenium catalysts supported on titania and zirconia in the oxidation of p-hydroxybenzoic and succinic acids (model compounds for the paper-pulp industry). They found supports stable to leaching, sintering, and fouling. Besson et al.114 used Ru/TiO2 catalyst for the oxidation of paper-pulp effluents of the initial TOC 8 690 mg/L. Rapid initial oxidation was observed; overall TOC removal exceeded 90%. Pintar et al.178 used also Ru/TiO2 catalyst (3% of Ru) for treating kraft-pulp bleaching streams that included Cl2O and NaOH. Tests were conducted at 463 K in TBR and batch-recycle FBR. Effluents were completely biodegradable, and mostly mineralization took place. The same catalyst and TiO2 itself were tested in SR in the oxidation of acidic and alkaline bleach plants effluents.179 At 463 K and 5.5 MPa, TOC was removed at 87% for the metallic catalyst. No leaching of Ti or Ru was detected. Similar results were achieved using Ru/TiO2 and TiO2.180 The catalysts were tested in SR, TBR, and FBR operated in batch-recycle mode. Initial TOC was 1 138 and 1 331 mg/L for the acidic and alkaline streams, respectively. Moderate abatement of organics was reached by using TiO2 only. TOC removal in SR and TBR was 46% and 26%, respectively. ∆TOC for the Ru/TiO2 catalyst was 79-89%, and it was enhanced to 95-98% when operating in the batch-recycle mode. Ti and Zr oxides and Ru supported on them were evaluated in the oxidation of acidic and alkaline kraft bleach plant effluents of TOC 665 and 1 380 mg/L, respectively.181 The presence of metal on oxides enhanced their activity sharply. TOC removal amounted to 79-88%. Palladium and platinum catalysts were also extensively studied in CWAO for paper and pulp waste liquors. Pd-Pt on

Al2O3 catalysts were tested in SR.182-184 The initial fast step was followed by a slow reaction step. Addition of ceria promoted the catalytic activity. In the presence of the Pd-PtCe/Al2O3, TOC was removed at 65% and color was reduced at 99% after 3 h at 443 K. A Pd/Al2O3 catalyst was used for the oxidation of acidic effluent in SR. High TOC removal was reached. No leaching of platinum and cerium was observed, but palladium and aluminum were detected in the effluent after a 3 h run at 443 K. An et al.185 tested Pd and Pd-Pt eggshell catalysts and uniform supported catalysts. Pd was highly active toward TOC and color removal (up to 75% and up to 90%, respectively). Eggshell catalyst with a Pd loading of 0.2% was very promising. The density of active sites was high in shell and the diffusion path was shorter in eggshell catalysts. No apparent deactivation was observed after 40 h of operation. Akolekar et al.186 developed many catalysts for CWAO: Pd, Cu, Mn, Pd/Cu, Pd/Mn, and Cu/Mn. The liquor was oxidized in SR at pH from 11 to 14. Single transition metals (Cu and Mn) and Pd catalysts were appreciably active in the removal of organics. Bimetallic catalysts exhibited even higher activity for TOC removal (>84%), whereby activity of the catalysts was ranked as follows

aCu/Pd > aMn/Pd > aCu/Mn Garg et al.187 compared activities of homogeneous catalyst (CuSO4) and heterogeneous catalysts (perovskite based oxides, zeolites, and bagasse flyash) in the oxidation of pulp and paper mill effluent under atmospheric pressure. An optimum heterogeneous catalyst, La0.3Ce0.7CoO3, performed only marginally worse than the homogeneous catalyst that is considered to be one of the most active ones in CWAO. Abu-Hassan et al.188 found that application of mixed copperzinc oxides and noble metals (Pt, Pd, and Ru) improved efficiency of CWAO in the oxidation of sodium dodecylbenzene sulfonate (DBS). Biodegradability of after-CWAO effluent was, however, not improved compared with nonoxidized streams. Noble metal catalysts exhibited a lower activity but were more stable. Suarez-Ojeda et al.151 used active carbon for the oxidation of DBS and various model organic compounds. Conversions were rather low: 30-35% for phenolic compounds and