Study of the copper leaching in the wet oxidation of phenol with CuO-based catalysts: Causes and effects

Catalytic wet oxidation of phenol as a model pollutant has been performed in a three phase fixed-bed reactor (FBR) by using a commercial catalyst based on copper oxide in order to analyze the variables affecting significantly the copper leaching. It has been found that temperature has an almost negligible influence in the range studied (70–160 °C). On the contrary, an important effect of the pH value was noticed. The copper leaching reduces when the pH of the solution fed to the reactor increases, being almost negligible at pH ≥ 5. Moreover, the composition of the reaction media also influences the leaching. Higher copper concentrations than those expected by the effect of the acid aqueous media have been measured in the reactor effluent when phenol, catechol, hydroquinone, p-benzoquinone and maleic acid are present in the reaction media. On the contrary, oxalic acid has a negative influence on the leaching, since it captures the copper in solution to form copper oxalate which precipitates on the catalyst surface. For a previously acidified medium, the acetic and formic acids do not have any other effect on the copper leaching. It has been also demonstrated that as copper in solution decreases, so does phenol conversion, because the homogeneous catalysis contributes significantly to the oxidation reactions even in fixed-bed reactors.     

Wet oxidation of phenol, cresols and nitrophenols catalyzed by activated carbon in acid and basic media

A commercial activated carbon, Industrial React FE01606A, was used as catalyst in the wet oxidation, in both acid and basic media, of phenolic pollutants, such as phenol, cresols and nitrophenols, currently found in industrial wastewaters. Reaction runs were carried out in a fixed-bed reactor (FBR) with concurrent upflow by feeding a 1000 mg L⁻¹ aqueous solution on each pollutant concurrently with a gas oxygen flow. Temperature and oxygen pressure of the reactor were set to 160 °C and 16 bar, respectively. The basic medium was maintained by using 500 ppm sodium bicarbonate as buffer reagent to keep the pH in the range 7–8. The initial pH 3.5 was set by adding sulphuric acid. Oxidation intermediates were identified and their distribution with respect to the pollutant oxidation progress was measured. Utilizing these results, oxidation routes for each phenolic compound were deduced. The intermediates produced were diverse in acid and basic media and their composition explains the evolution of the corresponding toxicity measured at the reactor effluent. While under acidic conditions hydroxybenzoic acids, dihydroxyl benzenes and quinones were obtained as primary products, these last two compounds (more toxic than the original pollutant) were not detected in basic conditions, and with such media lower toxicities at the reactor exit were obtained. Moreover, the catalyst was found to be stable in the time range studied (300 h).     

Influence of pH on the wet oxidation of phenol with copper catalyst

This work reports the influence of pH on the catalytic wet oxidation (CWO) of phenol performed with a commercial copper-based catalyst. The results obtained show that pH is a critical parameter able to modify the chemical stability of the catalyst, the significance of the oxidation reaction in the liquid phase, the reaction mechanism and, consequently, the oxidation route of phenol. Experiments have been carried out to study the mentioned aspects. Stirred basket and fixed bed reactors (FBRs) have been employed, at 140 °C and at 16 bar of oxygen pressure. Three initial pH values have been used: 6 (the pH of the phenol solution), 3.5 (adjusted by H₂SO₄) and 8 (by addition of Na₂CO₃). Furthermore, some phenol oxidation runs without solid catalyst but with different concentrations of copper in solution have been accomplish at pHo=3.5. At acid pH, important leaching of copper from the catalyst to the solution was achieved, finding this negligible at pH 8. It was found that the major contribution to the phenol conversion reached at acid pH by using the solid catalyst was due to the catalytic activity of the leached copper. Both oxidation mechanisms at acid and basic conditions have been elucidated to explain the differences in the type and distribution of the intermediates obtained. The catalytic phenol oxidation route found at pH=8 comprises intermediates less toxic than phenol while at acid pH the cyclic intermediates formed as first oxidation intermediates are far more toxic than phenol.     

Oxidation of phenol in aqueous solution with copper catalysts

The oxidation of phenol in aqueous phase over four different catalysts based on copper has been studied in a basket stirred tank reactor. Runs have been carried out at 140°C and 16 bar of oxygen pressure, with a catalyst loading of 60 g/l and at an initial acidic pH. Phenol, total organic carbon and some intermediates have been measured with the reaction time. The improvement achieved with the catalyst is established by comparison with a blank (reaction without catalyst). The commercial catalyst Engelhard Cu-0203T (CuO, 67–77%, Cu chromite, 20–30%, graphite sint., 1–3%) was found to be the most active catalyst with an acceptable mechanical and chemical stability. Copper leaches from the catalyst are higher when the mineralization of the acid intermediates does not occur (lower values of pH are obtained).     

Catalytic wet peroxide oxidation of phenol with a Fe/active carbon catalyst

A Fe on activated carbon catalyst has been prepared and tested for phenol oxidation with H₂O₂ in aqueous solution at low concentration (100 mg/L). Working at 50 °C, initial pH 3 and a dose of H₂O₂ corresponding to the stoichiometric amount (500 mg/L) complete removal of phenol and a high TOC reduction (around 85%) has been reached. Oxidation of phenol gives rise to highly toxic aromatic intermediates which finally disappear completely evolving to short-chain organic acids. Some of these last showed to be fairly resistant to oxidation being responsible for the residual TOC. In long-term continuous experiments the catalyst undergoes a significant loss of activity in a relatively short term (20–25 h) due to Fe leaching, this being related with the amount of oxalic acid produced. Deactivation may also be caused by active sites blockage due to polymeric deposits on whose formation some evidences were found. Washing with 1N NaOH solution allows to recover the activity although complete restoration was not achieved.     

Characterization and activity of copper and nickel catalysts for the oxidation of phenol aqueous solutions

Catalysts based on CuO/γ-alumina, CuAl₂O₄/γ-alumina, NiO/γ-alumina, NiAl₂O₄/γ-alumina and bulk CuAl₂O₄ have been structurally characterized by BET, porosimetry, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Their catalytic behaviors have also been tested for the oxidation of 5 g/l phenol aqueous solutions using a triphasic tubular reactor working in a trickle-bed regime and air with an oxygen partial pressure of 0.9 MPa at a temperature of 413 K. The copper and nickel catalysts supported on γ-alumina have surface areas of the same order as the support γ-alumina of ca. 190 m²/g and high active phase dispersions which were also confirmed by SEM, whereas the bulk copper aluminate spinel has a surface area of ca. 30 m²/g. XRD detects the phases present and shows a continuous loss of CuO by elution and the formation of a copper oxalate phase on the surface of the copper catalysts which also elutes with time. The NiO was also eluted but less than the copper catalysts. Only the copper and nickel spinel catalysts were stable throughout the reaction. Phenol conversion vs. time shows a continuous overall decrease in activity for the CuO/γ-alumina and NiO/γ-alumina catalysts. In turn, the copper and nickel spinel catalysts reach steady activity plateaus of 40 and 10%, respectively, of phenol conversion. The bulk copper aluminate spinel shows an activity plateau of 20% of the conversion which is lower than that from the copper aluminate/γ-alumina catalyst due to its lower surface area. Nickel catalysts always have lower activities than the copper catalysts for the phenol oxidation reaction. The copper catalysts drive a mechanism of partial phenol oxidation to carboxylic acids and quinone-related products with very high specific rates, and the nickel catalysts mainly drive a mechanism of CO₂ formation with lower conversion but with a potential higher catalyst life. The triphasic tubular reactor using trickle-bed regime largely avoids the mechanism of polymer formation as a catalyst deactivation process.     

Kinetic model of wet oxidation of phenol at basic pH using a copper catalyst

Catalytic wet oxidation of phenol employing a commercial copper catalyst has been studied. The pH of the media was maintained in the range 7-8 by using 500 mg/L of sodium bicarbonate as buffer solution. The use of this chemical avoids the copper leaching from the solid catalyst and presents another additional advantage since the organic oxidation intermediates obtained at these basic conditions are far less toxic than phenol. Thus, detoxification of phenolic wastewater is directly linked to the phenol conversion achieved. The kinetic model of the phenol disappearance rate has been discriminated by fitting experimental data. These data have been obtained by feeding a phenol solution of 1000 mg/L to a three phase fixed bed reactor at different catalyst weights to liquid flow rates ratios. Temperature and oxygen pressure were changed in the range from to and from 8 to 16 bar, respectively. The kinetic model proposed is based on a heterogeneous free radical mechanism which takes into account the scavenger effect of the bicarbonate on the phenoxy radicals formed on the catalyst surface by reduction of the active copper sites. A slight positive influence of the temperature, but no effect of the oxygen pressure has been found on the kinetic equation for phenol oxidation, at least in the ranges studied. These facts can be explained by the free radical mechanism proposed. Besides, a linear relationship was found between phenol and TOC disappearance that allows quite an accurate prediction of the mineralization achieved.     

Overall rate of aqueous-phase catalytic oxidation of phenol: pH and catalyst loading influences

The catalytic oxidation of phenol in aqueous-phase has been investigated using a commercial catalyst CuO–2(CuO)·Cr₂O₃–CrO₄Ba–Al₂O₃ at 400 K and an oxygen pressure of 8 atm, changing the pH and catalyst concentration. The phenol conversion, TOC consumption, the intermediates and the pH variation with time have been determined. An irreversible bonding of phenol on the catalyst surface has been found as a previous step to the phenol oxidation reaction. An overall pseudo-first order kinetic constant has been calculated for phenol conversion and values obtained at basic pH are lower than those calculated for acid pH media. An important homogeneous contribution for the phenol oxidation rate was found. However, the intermediates mineralization is notably influenced by the catalyst load.     

Reaction pathway of the catalytic wet air oxidation of phenol with a Fe/activated carbon catalyst

Catalytic wet air oxidation (CWAO) of phenol with molecular oxygen using a home-made Fe/activated carbon catalyst at mild operating conditions (100–127 °C; 8 atm) has been studied in a trickle-bed reactor. Ring compounds (hydroquinone, p-benzoquinone and p-hydroxybenzoic acid) and short-chain organic acids (maleic, malonic, oxalic, acetic and formic) have been identified as intermediate oxidation products. CWAO experiments using each one of these intermediates as starting compound have been carried out (at 127 °C and 8 atm) in order to elucidate the reaction pathway. It was found that phenol is oxidized through two different ways. It can be either para-hydroxylated to hydroquinone, which is instantaneously oxidized to p-benzoquinone or para-carboxylated to p-hydroxybenzoic acid. p-Benzoquinone is majorly mineralized to CO₂ and H₂O through oxalic acid formation whereas p-hydroxybenzoic acid gives rise to short-chain acids. Only acetic acid showed to be refractory to CWAO under the operating conditions used in this work. The catalyst avoids the presence of ring-condensation products in the reactor effluent which were formed in absence of it. This is an additional important feature because of the ecotoxicity of such compounds.     

Effect of calcination temperature on the low-temperature oxidation of CO over CoOx/TiO2 catalysts

A 5 wt% CoO_x/TiO₂ catalyst has been used to study the effect of calcination temperature on the activity of this catalyst for CO oxidation at 100 °C under a net oxidizing condition in a continuous flow type fixed-bed reactor system, and the catalyst samples have been characterized using TPD, XPS and XRD measurements. The catalyst after calcination at 450 °C gave highest activity for this low-temperature CO oxidation, and XPS measurements yielded that a 780.2-eV Co 2p_3/2 main peak appeared with this catalyst sample and this binding energy was similar to that measured with pure Co₃O₄. After calcination at 570 °C, the catalyst, which had possessed practically no activity in the oxidation reaction, gave a Co 2p_3/2 main structure peak at 781.3 eV which was very similar to those obtained for synthesized Co_nTiO_n+2 compounds (CoTiO₃ and Co₂TiO₄), and this catalyst sample had relatively negligible CO chemisorption as observed by TPD spectra. XRD peaks indicating only the formation of Co₃O₄ particles on titania surface were developed in the catalyst samples after calcination at temperatures ≥350 °C. Based on these characterization results, five types of Co species could be modeled to exist with the catalyst calcined at different temperatures. Among these surface Co species, the Type A clean Co₃O₄ particles were predominant on a sample of the catalyst after calcination at 450 °C and highly active for CO oxidation at 100 °C, and the calcination at 570 °C gave the Type B Co₃O₄ particles with complete Co_nTiO_n+2 overlayers inactive for this oxidation reaction.     

Full phenol peroxide oxidation over Fe-MMM-2 catalysts with enhanced hydrothermal stability

Iron-containing mesoporous mesophase materials Fe-MMM-2 have been synthesized by a sol–mesophase route under mild acidic conditions and characterized by DRS-UV–vis, XRD, and N₂ adsorption measurements. It was found that pH of the synthesis solution and iron content in the samples affect both the textural characteristics and the state of iron atoms. Isolated iron species predominate in silica framework under Fe < 2 wt% and pH  1.0 or Fe  1 wt% and pH < 2.0. These species are stable to leaching and highly active in full H₂O₂-based phenol oxidation. The increase in iron loading and pH of the synthesis solution lead to the agglomeration and formation of oligomeric iron species, which, in turn, results in the reduction of the catalytic activity of Fe-MMM-2 and the increase of iron leaching.     

CuO/PSSF复合催化剂的制备及其在苯酚降解中的应用

以纸状不锈钢微纤材料（paper-like sintered stainless steel fibers,PSSF）为载体,采用化学气相沉积法（chemical vapor depositon,CVD）制备CuO/PSSF复合催化剂并在固定床反应器上进行苯酚湿式催化氧化降解研究。采用SEM、XRD、XPS等技术对催化剂的表面形态、物相结构、元素价态进行分析,改变流量及床层高度考察停留时间对湿式催化氧化降解苯酚过程中苯酚转化率、H₂O₂转化率、TOC转化率及Cu²⁺浸出浓度的影响规律。催化剂表征结果表明,活性组分CuO成功负载在PSSF微纤材料上;活性评价测试结果表明,低流量条件对各活性指标变化影响十分明显;随床层高度增加各活性指标均显著提高,4cm床层高度下达到最高苯酚转化率和TOC转化率,分别为96.5%和47.4%,同时没有高毒副产物产生。本文初步探究了复合催化剂与固定床反应器结合的工艺可行性,旨在为工业化含酚废水的降解提供一些思路。     

Three-phase reactors for environmental remediation: catalytic wet oxidation of phenol using active carbon

Wet oxidation of phenol aqueous solutions was carried out in a fixed bed reactor operating in trickle flow regime. Mild conditions of temperature (140°C) and oxygen partial pressure (1–9 bar) were used. Three active carbons and one commercially available supported copper catalyst were tested as catalytic material. Previous studies demonstrated that active carbon gives higher phenol conversion than conventional oxidation catalysts, although significant loss of active carbon due to combustion was also found. In the present study, the combustion of the active carbon during the process is highly reduced by lowering the oxygen partial pressure from 9 to 2 bar, maintaining an acceptable phenol conversion. The comparison of the performance of three different active carbons shows that their physical and chemical characteristics largely influence on the phenol conversion achieved.     

Hydrodechlorination of alachlor in water using Pd, Ni and Cu catalysts supported on activated carbon

The hydrodechlorination of alachlor with hydrogen in aqueous phase was studied in a trickle bed reactor using different activated carbon-supported catalysts. The reactor was continuously fed with a 50 mg/L solution of alachlor in water and a H₂/N₂ gas stream. The variables studied were space-time (44.8–448.3 kg_cat h/mol), H₂:N₂ volumetric ratio in the gas phase (1:1–1:4), temperature (308–373 K) and pressure (0.24–0.6 MPa). The results of the hydrodechlorination experiments were evaluated in terms of alachlor conversion and ecotoxicity of the exit stream. High conversion values and important reductions of ecotoxicity were obtained working under mild conditions of temperature (323–348 K) and pressure (0.24 MPa). Palladium catalysts supported on activated carbon were found as the most active in the hydrodechlorination of alachlor, although copper and nickel catalysts led also to high conversions in the 80–93% range. The hydrodechlorination of alachlor was performed successfully with metal loads between 0.5 and 2.5 wt.% on the catalysts. A significant metal leaching was observed from the nickel and copper catalysts, which negatively affected the ecotoxicity of the final effluents. Oxidative treatment of the activated carbon supports with nitric acid previous to the impregnation with the metal precursor improved the anchorage of the active phase and reduced leaching dramatically. Likewise, the activity was not influenced by the oxidation of the supports and reductions of ecotoxicity by more than 90% were observed.     

Heterogeneous photo-Fenton degradation of phenolic aqueous solutions over iron-containing SBA-15 catalyst

A novel iron-containing mesostructured material has been successfully tested for the heterogeneous photo-Fenton degradation of phenolic aqueous solutions using near UV–vis irradiation (higher than 313 nm) at room temperature and close to neutral pH. This catalyst is a composite material that contains crystalline hematite particles embedded into the mesostructured SBA-15 matrix in a wide distribution of size (30–300 nm) and well dispersed ionic iron species within the siliceous framework. The outstanding physico-chemical properties make this material a promising photocatalyst leading to better activity than other unsupported iron oxides. An experimental design model has been applied to assign the weight of catalyst and hydrogen peroxide concentrations in the photo-Fenton processes over this particular material. The catalytic performance has been monitored in terms of aromatics and total organic carbon (TOC) conversions, whereas the catalyst stability was evaluated according to the metal leached into the aqueous solution. Hydrogen peroxide concentration plays an important role in the stability of the iron species, preventing their leaching out into the solution, in contrast to the effect shown in typical dark-Fenton reaction. The homogeneous leached iron species result in very little contribution to the overall photocatalytic process. Catalyst loadings of 0.5 g/L and concentration of hydrogen peroxide close to the stoichiometric amount have yielded a total abatement of phenol and a remarkable organic mineralization.     

Wet air oxidation of phenol using active carbon as catalyst

Catalytic wet air oxidation is a promising alternative for the treatment of phenolic waste water which cannot be treated in conventional sewage plants. Catalytic wet air oxidation of an aqueous phenol solution was conducted in a fixed bed reactor operating in trickle flow regime. Either active carbon or a commercial copper oxide supported over γ-alumina was used as catalyst. The performance of both materials was compared in terms of phenol conversion in 240 h tests. The results showed that the active carbon, without any active metal supported, gives the highest phenol conversion. The supported copper catalyst undergoes a rapid deactivation due to the dissolution of the metal active species in the acidic medium in which the reaction takes place. On the other hand, the active carbon maintains a higher activity throughout the test, although a decrease of the phenol conversion was also observed due to both the loss of active carbon by combustion and the reduction of its surface area. The phenol oxidation was proved to occur through a first order mechanism with respect to phenol. After the ten-day run, the catalytic activity of the active carbon was found to be about eight times higher than that of the commercial catalyst, also showing high selectivity to the production of carbon dioxide.     

Highly stable Fe/γ‐Al2O3 catalyst for catalytic wet peroxide oxidation

BACKGROUND: A highly stable Fe/γ‐Al₂O₃ catalyst for catalytic wet peroxide oxidation has been studied using phenol as target pollutant. The catalyst was prepared by incipient wetness impregnation of γ‐Al₂O₃ with an aqueous solution of Fe(NO₃)₃· 9H₂O. The influence of pH, temperature, catalyst and H₂O₂ doses, as well as the initial phenol concentration has been analyzed. RESULTS: The reaction temperature and initial pH significantly affect both phenol conversion and total organic carbon removal. Working at 50 °C, an initial pH of 3, 100 mg L^?1 of phenol, a dose of H₂O₂ corresponding to the stoichiometric amount and 1250 mg L^?1 of catalyst, complete phenol conversion and a total organic carbon removal efficiency close to 80% were achieved. When the initial phenol concentration was increased to 1500 mg L^?1, a decreased efficiency in total organic carbon removal was observed with increased leaching of iron that can be related to a higher concentration of oxalic acid, as by‐product from catalytic wet peroxide oxidation of phenol. CONCLUSION: A laboratory synthesized γ‐Al₂O₃ supported Fe has shown potential application in catalytic wet peroxide oxidation of phenolic wastewaters. The catalyst showed remarkable stability in long‐term continuous experiments with limited Fe leaching, < 3% of the initial loading. Copyright © 2010 Society of Chemical Industry     

Phenol oxidation over CuO/Al2O3 in supercritical water

Phenol was oxidized in supercritical water at 380–450°C and 219–300 atm, using CuO/Al₂O₃ as a catalyst in a packed-bed flow reactor. The CuO catalyst has the desired effects of accelerating the phenol disappearance and CO₂ formation rates relative to non-catalytic supercritical water oxidation (SCWO). It also simultaneously reduced the yield of undesired phenol dimers at a given phenol conversion. The rates of phenol disappearance and CO₂ formation are sensitive to the phenol and O₂ concentrations, but insensitive to the water density. A dual-site Langmuir–Hinshelwood–Hougen–Watson rate law used previously for catalytic SCWO of phenol over other transition metal oxides and the Mars–van Krevelen rate law can correlate the catalytic kinetics for phenol disappearance over CuO. The supported CuO catalyst exhibited a higher activity, on a mass of catalyst basis, for phenol disappearance and CO₂ formation than did bulk MnO₂ or bulk TiO₂. The CuO catalyst had the lowest activity, however, when expressed on the basis of fresh catalyst surface area. The CuO catalyst exhibited some initial deactivation, but otherwise maintained its activity throughout 100 h of continuous use. Both Cu and Al were detected in the reactor effluent, however, which indicates the dissolution or erosion of the catalyst at reaction conditions.     

Activated carbon as catalyst in wet oxidation of phenol: Effect of the oxidation reaction on the catalyst properties and stability

Catalytic wet oxidation (CWO) of phenol has been carried out in a continuous three-phase reactor by using a commercial activated carbon (AC) as catalyst, feeding oxygen as gas phase and an aqueous solution 1000 ppm in phenol to the reactor. A stable catalyst under operation conditions is one of the main difficulties to pass up in the catalytic wet oxidation process, so the stability of the activated carbon with the time on stream (TOS) was investigated. To do this the phenol conversion change was analyzed with TOS and results were contrasted to the change of the physicochemical properties of the AC with the TOS. Gas adsorption/desorption, TPD, XPS and SEM measurements were applied to the AC taken from the reactor after several TOS values. A significant reduction of the micro-pore volume and BET surface area of the catalyst was observed with TOS. However, as reaction proceeded the external surface area and the total amount of oxygen surface group increased. Moreover, regeneration of the initial catalyst properties was done by washing with water saturated in oxygen, at the reaction conditions or by heating in N₂ atmosphere at 450, 700 and 900 °C. The total micro-pore volume and internal surface area of the catalyst were not recovered by the regeneration process, probably due to blockage of the narrow micropores by pyrolytic carbon produced during the first step of the wet oxidation process.     

Fe-exchanged Y zeolite as catalyst for wet peroxide oxidation of reactive azo dye Procion Marine H-EXL

This paper evaluates the degradation of a reactive azo dye, Procion Marine H-EXL, by catalytic wet hydrogen peroxide oxidation (CWHPO). The catalyst was prepared by ion-exchange, starting from a commercially available ultrastable Y zeolite. All experiments were performed on a laboratory scale set-up. The effects of different reaction parameters such as initial pH, catalyst and hydrogen peroxide concentrations on the oxidation of the dye aqueous solution were assessed. Apart from the conventional parameters, the toxic potential of the dye’s degradation products was investigated using the bioluminescence test. HPICE analysis was also performed to obtain detailed information on the resulting oxidation products (organic and inorganic anions). The results indicate that after only 10 min at 50 °C, 20 mmol/l H₂O₂ and 1g/l FeY_11.5 the color removal was as high as of 97% at pH=3 and 53% at pH=5. More than 96% removal of the dye could be attained in 30 min at pH=5, t=50 °C, 20 mmol/l H₂O₂ and 1 g/l FeY_11.5 which corresponds to about 76% reduction of the initial COD and 37% removal of the initial TOC. A preliminary study of catalytic oxidation with hydrogen peroxide of the synthetic textile wastewater containing the specific dye is also presented. Leaching tests indicate that the activity of the catalyst is not due to leached iron ions, although an amount of 0.1–4.0 ppm of iron ions was found in aqueous solution. The catalyst allows almost total elimination of the dye and a significant removal of COD and TOC without the significant leaching of Fe ions. It was also observed that by using this catalyst, it is possible to extend the range of pH values for which Fenton-type oxidation can occur and no iron hydroxide sludge is formed.