Catalytic Wet Air Oxidation Reaction Research Articles

Improved phenol degradation by catalytic wet air oxidation on the Co3O4 spinel catalysts supported on SBA-15 modified with TiO2

The removal of phenol from aqueous industrial effluents by catalytic wet-air oxidation (CWAO) is a challenging process that requires novel catalysts resistant to leaching of the active phases during residence time in a batch reactor. In this sense, this work demonstrated that this can be archived with the catalyst having the spinel phase Co3O4 supported on mesoporous silica SBA-15 doped with an optimized amount of TiO2 (3 wt%). In the phenol CWAO reaction carried out at 160 °C and 10 bar O2, this best Co3O4/TiO2/SBA-15 catalyst showed an increase of phenol conversion from 49.0 % to 68.3 % and of CO2 selectivity from 87 % to 98 %. The extensive catalyst characterization and overall activity results indicated that this was related to the strength and nature of the acid sites and the oxidation states of the cobalt oxide phases. Combined factors influenced the final behaviour of the catalyst, such as its intrinsic activity, dispersion of the active phase, inhibition of CoSiO4 phase formation, high Lewis acidity and the large amount of active Co3O4 spinel phase. The presence of TiO2 nanoparticles (NPs) decorating the support surface improved the stability of the catalyst by inhibiting the leaching of cobalt species during the reaction. After three reaction cycles, the most efficient Co/3Ti/S15 catalyst showed high CO2 selectivity (∼92.7 %) and low selectivity towards carboxylic acid (∼5.3 %) and polyphenolic compounds (∼2%). It is proposed that surface oxidation is the rate-determining step in the CWAO reaction over TiO2/SBA-15-supported Co3O4 spinel catalysts.

Design composite oxide supported Pt catalyst for catalytic wet air oxidation of ammonia with a high concentration in chloride system

Massive discharge of ammonia nitrogen wastewater not only causes eutrophication of the water body but also has a toxic effect on humans and living things. How to deal with ammonia nitrogen wastewater is a crucial topic for researchers. Here, a novel catalyst of Pt@Ti–Si where platinum was supported on a composite oxide of titanium oxide (TiO2) and silicon oxide (SiO2) via a one-pot method was successfully synthesized for catalytic wet air oxidation (CWAO) of ammonia with a high concentration (more than 2000 ppm). Due to the improved specific surface area of SiO2 and the excellent acid-base resistance of TiO2, the prepared composite oxide-supported platinum catalyst has excellent catalytic performance and good stability for CWAO with a high concentration of ammonia. At 200 °C and the O2 pressure of 2 MPa for 2 h, the 1%Pt@Ti10–Si1 catalyst has a 96.32% conversion of 2470 ppm ammonia and 97.15% selectivity to N2 and has good catalytic performance even after five cycles. Under the same reaction conditions, when the chloride concentration in the system is 3000–10000 ppm, the CWAO reaction can be inhibited at an early stage and promote conversion and selectivity at a later stage. The results show that the catalyst has good tolerance to chloride ions, and the treated ammonia nitrogen wastewater can be used for subsequent biochemical processes. Therefore, the developed novel catalyst in this study is effective for the CWAO of highly concentrated ammonia and has potential industrial application value.

Catalytic wet air oxidation of toxic containments over highly dispersed Cu(II)/Cu(I)-N species in the framework of g-C3N4

Catalytic wet air oxidation (CWAO) is a harmless, cheap and effective technology for the degradation of toxic containments directly to CO2 and H2O. In this work, highly dispersed Cu(II)/Cu(I)-N that embedded in the framework of g-C3N4 (Cux-g-C3N4) were synthesized in a facile thermal polymerization method and used in the CWAO of phenols, antibiotics and vitamins. Characterization results confirmed that g-C3N4 formed in the prepared catalyst and copper was chemically coordinated with N in g-C3N4, which inhibited the aggregation of copper. Meanwhile, Cu(II) or Cu(I) in the framework of g-C3N4 was more effective for the degradation of phenol than Cu(0) and CuO, and more than 23 toxic containments could be degraded under mild conditions. The prominent performance of Cu0.1-g-C3N4 for CWAO reaction was discussed on the base of these experiments and it was disclosed that in-situ formed H2O2 might be contributed to the highly activity.

Effective removal of naproxen from aqueous solutions by CWAO process using noble metals supported on carbon nanospheres catalysts

In this research the technical feasibility of catalytic wet air oxidation reactions (CWAO) with a novel ruthenium supported onto carbon nanospheres (CNS-Ru) catalyst for the degradation of a non-steroidal anti-inflammatory drug (NSAID), naproxen (NPR), has been tested. The operating conditions of the reaction, the different loads of ruthenium in the catalyst, the reuse of the material, the reaction kinetics and the degradation mechanism of the pollutant were studied. The complete degradation of the compound was reached in one cycle at the following conditions: 130 °C, 20 bar, [NPR]0 = 20 mg∙L−1, [CNS-Ru] = 0.75 g∙L−1, 2%wt. of ruthenium and an initial pH of 7.0. At these conditions, the efficiency for the pollutant degradation of a platinum-based catalyst (CNS-Pt) as well as its reusability were also tested. In this sense, better results regarding to the NPR degradation were obtained with the CNS-Ru material. Nevertheless, both catalysts could be considered stable after 4 consecutive runs. Furthermore, two potential kinetic models were investigated in order to simulate the degradation of NPR in both WAO and CWAO processes. Thus, the detection of 13 reaction compounds allowed to propose a degradation mechanism for NPR by CWAO. The efficiency of both catalysts was tested in a real hospital wastewater matrix; it was also proved that the NPR degradation decreased in comparison to that one found for the ultrapure water solutions, and that CNS-Ru catalyst achieved a higher pollutant degradation rather than that obtained for CNS-Pt catalyst.

Ti-Doped SBA-15 Catalysts Used in Phenol Oxidation Reactions.

Two Ti-SBA-15 catalysts are synthesized using techniques that should either deposit Ti atoms specifically at the SBA-15 surface or allow Ti-containing species to exist at both the surface and within the bulk of SBA-15. The materials have been characterized by Fourier transform infrared (FTIR), Raman and UV visible spectroscopies, transmission electron microscopy, scanning electron microscopy/energy-dispersive X-ray spectrometry microscopies, and N2 physisorption experiments. They have been applied in the total oxidation of phenol under catalytic wet air oxidation (CWAO) conditions and using photo- and plasma promotion. The materials retain the structure of SBA-15 following the doping in both cases and Ti incorporation is confirmed. The nature of the incorporated Ti remains unclear—with evidence for anatase TiO2 (from Raman and UV vis analysis) and evidence for atomically dispersed Ti from FTIR. In terms of reactivity, the presence of Ti in the in situ-prepared catalyst improves reactivity in the photopromoted reaction (increasing conversion from 28 to 60%), while both Ti catalysts improve reactivity in the CWAO reaction (by 7% over the in situ catalyst and by 25% over the grafted material). The presence of Ti has no beneficial effect on conversion in the plasma-promoted reaction. Here, however, Ti does affect the nature of the oxidized intermediates formed during the total phenol oxidation.

Process intensification in catalytic wet air oxidation of phenol via coupling gas‐liquid microdispersion with trickle bed reactors

AbstractIntensified trickle bed reactors were developed for catalytic wet air oxidation (CWAO) of phenol, by generating microbubbles before gas/liquid fluid flows along catalyst packed‐beds. By coupling gas‐liquid microdispersion module, phenol disappearance rate significantly increases under the same operating conditions and phenol conversion reaches 92% in 22 seconds at 160°C and 1000 kPa. Effects of operating conditions were studied systematically and results demonstrated characteristics of high‐interaction regime at low Reynolds number. Comparison of effects among mass transfer and reaction steps shows CWAO reaction is controlled by resistance to liquid‐solid mass transfer of phenol in most cases, while gas‐liquid mass‐transfer‐resistance is ignorable. A dimensionless correlation, Sh = 2.29φ−0.91(ReL + ReG)0.04, was established, by considering influences of liquid film thickness and fluid flow situations on liquid‐solid mass transfer coefficient in microbubble‐in‐liquid/catalyst‐particle system. The study would provide an effective method for intensifying CWAO and other liquid‐solid‐mass‐transfer‐resistance‐controlled gas/liquid/solid catalytic reactions and help understand mass transfer mechanism in new gas/liquid/solid system.

Treatment of toxic industrial effluent containing nitrogenous organic compounds by integration of catalytic wet air oxidation at atmospheric pressure and biological processes

The treatment of non-biodegradable industrial effluent containing toxic pyridine compounds by integration of catalytic wet air oxidation (CWAO) and biological processes has been studied. The CWAO experiments were performed using ceria promoted Pt/Al2O3 catalyst. The catalyst was characterized by various techniques and the stability of the catalyst was studied. The research intended to achieve two main objectives: a) biodegradability enhancement of toxic industrial effluent by CWAO which is operated at mild conditions and b) complete mineralization of effluent obtained after CWAO treatment by biological processes. Therefore, the CWAO experiments were performed at atmospheric pressure and the effect of various operating parameters such as ceria loading, catalyst dosage and temperature on Chemical oxygen Demand (COD) removal of industrial effluent was optimized. The CWAO reaction followed a first order reaction kinetic model. The catalyst stability test was performed to determine its reusability and the catalyst was found active after 3 cycles run. The biodegradability of the industrial effluent was found to be improved from 0.07 to 0.56 and toxicity was completely removed after CWAO treatment. The effluent obtained after CWAO was further treated with biological aerobic treatment and a total COD removal of 98.4% was obtained after 10 days of aerobic treatment.

Enhanced activity and stability of copper oxide/γ-alumina catalyst in catalytic wet-air oxidation: Critical roles of cerium incorporation

By successive impregnation method, the Ce-modified Cu-O/γ-Al2O3 catalyst was prepared and characterized using nitrogen adsorption-desorption, scanning electron microscopy energy dispersive X-ray analysis (SEM-EDS), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman, and H2-Temperature programming reduction (H2-TPR). In catalytic wet-air oxidation (CWAO) process for the printing and dyeing wastewater (PDW), the effects of Ce addition on performance, mechanism and kinetics of the catalyst were investigated. The Ce addition increases the Brunauer-Emmett-Teller (BET) surface area and pore volume of the catalyst and makes the active components uniformly distributed on the catalyst surface. Formation of a stable CuAl2O4 solid solution by anchoring Cu onto the γ-Al2O3 crystal lattice leads to a significant decrease in metal leaching of the Ce-modified catalyst. The proportion of lattice oxygen in the catalyst substantially increases and the apparent activation energy of Cu-O/γ-Al2O3 catalyst decreases owing to Ce addition. Therefore, the catalytic activity and stability of the Ce-modified catalyst are considerably improved. The scavengers experiments identify the active species existed in the CWAO reaction system, with the order of reactivity: h+ > O2•− > H2O2 > HO•. This novel Cu-Ce-O/γ-Al2O3 catalyst has great potential in applications for treatment of concentrated organic wastewater due to its superior catalytic activity and improved stability.

Aqueous phase (catalytic) wet air oxidation of ammonia: Thermodynamic considerations

The equilibrium concentrations in ammonia and ammonium ion in the liquid and gas phases could be calculated in the temperature range 25–250°C, under typical operating conditions for the catalytic wet air oxidation of ammonia. A satisfactory agreement was obtained with the data collected upon blank experiments as well as in the presence of unreactive catalyst supports. Thermodynamic calculations appeared to be crucial to access a better comprehension of the reaction mixture composition under catalytic wet air oxidation reaction conditions, especially the ammonia partition and speciation among the different phases. New insights were gained in terms of ammonia reactivity.

Structure, characterization, and dynamic performance of a wet air oxidation catalyst Cu–Fe–La/γ-Al2O3

A Cu–Fe–La/γ-Al2O3 (CFLA) catalyst was prepared by the excessive impregnation method and characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The results indicate that the catalyst contained mostly Cu2+, Fe3+, and La3+ and a small amount of Cu+, Fe2+, and La. The active components were uniformly distributed in the catalyst, and the particle size of the components was approximately 7.5nm. The CFLA catalyst was used for the treatment of methyl orange (MO) solution by catalytic wet air oxidation (CWAO), and it exhibited a high catalytic activity. The catalytic reaction involved variable valence states of metals and free-radical reaction mechanism. The CWAO reaction of MO solution was fitted by a segmented first-order dynamic model, and the rapid reaction apparent activation energy was 13.9kJ·mol−1.

Impact of cerium-based support oxides in catalytic wet air oxidation: Conflicting role of redox and acid–base properties

High surface area ceria is an excellent support of metals (Ru, Pt, etc.) for catalytic wet air oxidation reactions (CWAO). Due to the low solubility of oxygen in water, the rate of transfer of O2 from the gas phase toward the catalyst has a decisive impact on the whole oxidation process. The role of the ceria support would be to facilitate this oxygen transfer. Redox properties of ceria (generally measured by the oxygen storage capacity, OSC) have a great impact on this step. Attempts were made to modulate the redox properties of ceria by using mixed oxides: CeO2–ZrO2, CeO2–PrO2–ZrO2 or CeO2–TiO2. In doing so, acid–base properties of the supports are also modified. The role of the redox and acid–base properties on CWAO reactions (acetic acid, aniline and phenol) are reviewed. In most cases, acid–base properties can have a great impact on the catalyst stability (deactivation by carbonates or polymer deposit). Increasing the redox properties may have an adverse effect on the catalytic performances when acid–base properties bias the whole process. In some cases (phenol over CeO2–TiO2 supported catalysts), an adequate Lewis acidity is preferable to a high OSC of the catalyst.

Catalytic wet air oxidation of phenol with functionalized carbon materials as catalysts: Reaction mechanism and pathway

The development of highly active carbon material catalysts in catalytic wet air oxidation (CWAO) has attracted a great deal of attention. In this study different carbon material catalysts (multi-walled carbon nanotubes, carbon fibers and graphite) were developed to enhance the CWAO of phenol in aqueous solution. The functionalized carbon materials exhibited excellent catalytic activity in the CWAO of phenol. After 60min reaction, the removal of phenol was nearly 100% over the functionalized multi-walled carbon, while it was only 14% over the purified multi-walled carbon under the same reaction conditions. Carboxylic acid groups introduced on the surface of the functionalized carbon materials play an important role in the catalytic activity in CWAO. They can promote the production of free radicals, which act as strong oxidants in CWAO. Based on the analysis of the intermediates produced in the CWAO reactions, a new reaction pathway for the CWAO of phenol was proposed in this study. There are some differences between the proposed reaction pathway and that reported in the literature. First, maleic acid is transformed directly into malonic acid. Second, acetic acid is oxidized into an unknown intermediate, which is then oxidized into CO2 and H2O. Finally, formic acid and oxalic acid can mutually interconvert when conditions are favorable.

Perovskite hollow nanospheres for the catalytic wet air oxidation of lignin

A urea-assisted solvothermal-calcination method was used to prepare LaFe x Mn 1- x O 3 and La 0.9 Sr 0.1 MnO 3 hollow nanospheres, and their crystal phase, morphology, and specific surface area properties were analyzed by X-ray diffraction, transmission electron microscopy, and nitrogen adsorption-desorption isotherms. These materials were then evaluated as catalysts for the catalytic wet air oxidation (CWAO) of lignin, and showed much better catalytic performance than perovskite prepared by the traditional sol-gel method. The structure of the hollow nanospheres has been suggested as the reason of this higher catalytic activity. For the CWAO reaction at 120 °C and 0.2 MPa O 2 , the conversion of lignin was greater than 80% after 1 h. Analysis of the ions leaching from the material following the completion of the reaction showed that the catalysts were very stable under the reaction conditions because of their perovskite phase structure. Perovskite hollow nanospheres have been used as catalysts for the CWAO of lignin, where they exhibited good catalytic activity and excellent stability, revealing a new application in biomass conversion.

Catalytic wet air oxidation of pentachlorophenol over Ru/ZrO2 and Ru/ZrSiO2 catalysts

ZrSiO2 and ZrO2 supports loaded with active component Ru were unprecedentedly evaluated in catalytic wet air oxidation reaction of pentachlorophenol (PCP). The study results showed that the specific surface area of ZrSiO2 which was prepared by high temperature aging method was much larger than that of ZrO2, and it performed a better catalytic activity in catalytic wet air oxidation after being loaded with Ru. When the initial concentration of PCP was 125mgL−1, PCP conversion reached 92% after 90min under the low temperature and pressure conditions of 180°C and 0.1MPa oxygen partial pressure. PCP conversion was increased by raising reaction temperature, increasing oxygen partial pressure, reducing the initial concentration of PCP and prolonging reaction time. The kinetics study of PCP in catalytic wet air oxidation showed that the catalytic wet air oxidation reaction of PCP obeyed first order kinetic model and its Arrhenius activation energy was 96.98kJmol−1. The main intermediates of PCP in CWAO were acetic acid, oxalic acid and chlorine ion.

Deactivation phenomena during catalytic wet air oxidation (CWAO) of phenol over platinum catalysts supported on ceria and ceria–zirconia mixed oxides

Catalytic wet air oxidation (CWAO) of aqueous solution of phenol was carried out with pure oxygen at 160°C in a stirred batch reactor on platinum supported oxide catalysts (Pt/CeO2c calcined at 650 and 800°C and Pt/CexZr1−xO2 with x=0.90, 0.75 and 0.50). The catalysts were characterized before (BET, FT-IR spectroscopy, hydrogen chemisorptions, oxygen storage capacity (OSC)) and after reaction (TPO, elementary analysis, GC–MS and DTA–TGA). The results demonstrate a poisoning of the catalysts during CWAO reaction due to the formation of different forms of carbon deposit on the materials: carbonates and polymeric carbon species. This poisoning phenomenon is limited by the introduction of 50% of zirconium into ceria lattice for the catalysts presenting the lowest surface area. Polymeric deposits play a major role in the catalyst deactivation.

Mechanism of stearic acid oxidation over nanocrystalline [formula omitted] (A′ = Sr, Ce; B = Co, Mn): The role of oxygen mobility

Catalytic wet air oxidation reaction (CWAO) of stearic acid is carried out in a batch reactor over a series of transition metal-based perovskite samples synthesized by reactive grinding. It is observed that the LaCoO3 sample presents the highest initial activity for this reaction. For comparison, pure and substituted LaMnO3 samples show largely smaller activity in spite of similar specific surface areas. The results show that the accessibility of the low temperature active oxygen (or the reducibility of the transition metal) on the surface of the catalyst conditioned the initial activity of the catalyst. Then, a mechanism involving the reaction of stearic acid molecules with adsorbed oxygen surface site (Co3+O2−) is proposed on the basis of the experimental results. Stearic acid oxidation proceeds via a recurrent decarboxylation process that results in the reduction of cobalt surface sites and formation of surface carbonate species (CO3−). In opposition to what was observed for gas phase oxidation reactions, lanthanum (and Ce or Sr) is found to be massively carbonated during the reaction, leading to the destruction of the perovskite structure and loss of the catalytic activity.

Wet air oxidation of acetic acid over platinum catalysts supported on cerium-based materials: Influence of metal and oxide crystallite size

Platinum catalysts (2.5 wt%) prepared by impregnation on pure commercial ceria and on Zr 0.1(Ce 0.75Pr 0.25) 0.9O 2 sol–gel mixed oxide were synthesized and used for catalytic wet air oxidation (CWAO) of acetic acid. The influence of the platinum sintering (reducing treatment) was studied for both supports to modify the oxygen transfer from the gas phase onto the metallic active site of the catalyst and also to determine the effect of this treatment on the catalytic activity and stability. The catalysts were characterized by BET, TEM, XRD, FT-IR and hydrogen chemisorption before and after CWAO reaction, and oxygen storage capacity (OSC) was measured. The nature of the support [CeO 2 and Zr 0.1(Ce 0.75Pr 0.25) 0.9O 2 compounds] and the preparation conditions led to samples displaying a Pt dispersion ranging from 0.4 to 41%, and BET surface area varying from 4 to 45 m 2 g −1. The results demonstrate poisoning of the catalysts during the CWAO experiments due to the formation of carbonate species. This limits the performance of the mixed oxide-based catalysts displaying the highest OSC values. A correlation was established between the OSC of the catalysts and the formation of carbonate species. Nevertheless, there is no correlation between OSC and catalytic activity, with the ceria-based samples showing the highest reaction rate. An optimal platinum crystallite size is required to achieve the highest conversion of acetic acid in terms of turnover frequency values.

Characterizations of platinum catalysts supported on Ce, Zr, Pr-oxides and formation of carbonate species in catalytic wet air oxidation of acetic acid

Catalytic wet air oxidation (CWAO) of aqueous solution of acetic acid (78 mmol L −1) was carried out with pure oxygen (2 MPa) at 200 °C in a stirred batch reactor on platinum supported oxide catalysts (Pt/oxide, oxide = CeO 2, Zr 0.1Ce 0.9O 2, Zr 0.1(Ce 0.75Pr 0.25) 0.9O 2 and ZrO 2). Platinum was loaded on oxides by impregnation (5 wt%), and then the catalysts were reduced under H 2. Homogenous dispersions of 2–3 nm metal crystallites were obtained. The catalytic activity depended on the ability of the support to resist to the formation of carbonates. Ce(CO 3)OH species, determined by FT-IR and XRD, were rapidly formed during the CWAO reaction especially on mixed oxides. These carbonates were responsible to a drastic drop in catalytic performances. Amounts of carbonate species increase with the ability of the catalyst to transfer oxygen.

Catalytic wet-air oxidation of carboxylic acids on carbon-supported platinum catalysts

Catalytic wet-air oxidation (CWAO) of aqueous solutions (5 g · l−1) of car☐ylic acids (formic, oxalic, and maleic) was carried out with air at 293–463 K on carbon-supported platinum catalysts. Platinum was loaded on active charcoal by cationic exchange, then reduced under H2. Homogeneous dispersions of 1–2 nm metal particles were obtained. CWAO reactions were performed at 1 or 15 bar air pressure in stirred, batch reactors. Total conversion of formic and oxalic acids into carbon dioxide was obtained under very mild conditions (air at atmospheric pressure, 326 K). The Pt/C catalyst was almost inactive for the oxidation of acetic acid but maleic acid was oxidized under moderate conditions (15 bar of air pressure, 405 K) which indicates that the degradation of this acid does not occur via acetic acid.

Catalytic wet-air oxidation of carboxylic acids on carbon-supported platinum catalysts

Catalytic wet-air oxidation (CWAO) of aqueous solutions (5 g · l −1) of car☐ylic acids (formic, oxalic, and maleic) was carried out with air at 293–463 K on carbon-supported platinum catalysts. Platinum was loaded on active charcoal by cationic exchange, then reduced under H 2. Homogeneous dispersions of 1–2 nm metal particles were obtained. CWAO reactions were performed at 1 or 15 bar air pressure in stirred, batch reactors. Total conversion of formic and oxalic acids into carbon dioxide was obtained under very mild conditions (air at atmospheric pressure, 326 K). The Pt/C catalyst was almost inactive for the oxidation of acetic acid but maleic acid was oxidized under moderate conditions (15 bar of air pressure, 405 K) which indicates that the degradation of this acid does not occur via acetic acid.