This review article explores three major aspects of the wet oxidation (WO) and catalytic wet oxidation (CWO) processes (with the major focus being on WO and CWO, using air or oxygen as an oxidant). These aspects are (a) the fundamental chemistry of WO/CWO; (b) important aspects of catalysts, with regard to the CWO process; and (c) engineering aspects of the WO/CWO process. The applications of WO and CWO technology to treat industrial wastewater clearly illustrate the potential of these processes for treating wastewater from a wide number of industries. WO/CWO is a fertile area of research with significant scope for further research and innovation, particularly in the areas of catalyst development, the understanding of catalytic mechanisms, and the fundamental chemistry that occurs during WO/CWO. Selected examples of findings to date are discussed in this review.

Wet Oxidation and Catalytic Wet Oxidation

Catalytic wet-air oxidation processes: A review

OMS-2 nanorods with tunable K+ concentration were prepared by a facile hydrothermal redox reaction of MnSO4, (NH4)2S2O8, and (NH4)2SO4 at 120 °C by adding KNO3 at different KNO3/MnSO4 molar ratios. The OMS-2 nanorod catalysts are characterized by X-ray diffraction, transmission electron microscopy, N2 adsorption and desorption, inductively coupled plasma, and X-ray photoelectron spectrometry. The effect of K+ concentration on the lattice oxygen activity of OMS-2 is theoretically and experimentally studied by density functional theory calculations and CO temperature-programmed reduction. The results show that increasing the K+ concentration leads to a considerable enhancement of the lattice oxygen activity in OMS-2 nanorods. An enormous decrease (ΔT50 = 89 °C; ΔT90 > 160 °C) in reaction temperatures T50 and T90 (corresponding to 50 and 90% benzene conversion, respectively) for benzene oxidation has been achieved by increasing the K+ concentration in the K+-doped OMS-2 nanorods due to the considerable enhan...

Tuning the K+ Concentration in the Tunnel of OMS-2 Nanorods Leads to a Significant Enhancement of the Catalytic Activity for Benzene Oxidation

The use of supercritical fluids as solvents for the formation of micro- and nano-particles has shown tremendous over the past few years due to increased demands from industry for materials with high performance specifications and high or unique functionality. Supercritical fluids allow synthesis of many types of particles since the solvent's chemical and physical properties can be varied with temperature or pressure, both of which can affect the degree of supersaturation and nucleation. In this review, we describe methods for the formation of fine particles using supercritical fluids CO2 and water. Basic principles and specific features of each solvent are described, and the advantages and limitations are discussed.

Fine particle formation using supercritical fluids

A series of manganese−cerium oxide composites with Mn concentrations in the range of 1−20 mol % in ceria was prepared by the solution combustion technique using urea as fuel. The nature, type, and oxidation state of Mn species in ceria were investigated by X-ray diffraction (XRD), diffuse reflectance UV−visible spectroscopy, electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy, and temperature-programmed reduction techniques. The study reveals that the method of preparation significantly influences the type of manganese species in ceria. Wet-impregnation, coprecipitation, and solid-state synthesis techniques lead to clustered MnOx-like species in the ceria matrix, while the present method of preparation (solution combustion route) yields a highly dispersed form of Mn species. In the reported series of samples, Mn is present mainly in +2 and +3 oxidation states and there is no evidence for the presence of Mn4+ species. Powder X-ray diffraction studies at variable temperatures (298−1323 K...

Nature of Manganese Species in Ce1-xMnxO2-δ Solid Solutions Synthesized by the Solution Combustion Route

Cryptomelane type manganese oxide OMS-2 material was synthesized by redox reaction between potassium permanganate and manganese sulphate in acidic medium under reflux or hydrothermal crystallization conditions. Ce was added by ion-exchange, impregnation or directly at crystallization stage. The chemical composition, structure, texture, morphology and thermal stability of the materials were measured by EDX spectroscopy, XRD, N 2 -adsorption, SEM and TGA. Results of the catalytic wet oxidation of phenol at 100 °C indicate best performance of the well-crystallized octahedral molecular sieve with cryptomelane structure where all the accessible potassium ions were exchanged for cerium cations. OMS-2 materials containing pure CeO 2 phase and excess of active oxygen species had lower stability, activity and capacity for oxidative reactive adsorption of phenol. Implementation of Ce-exchanged crystalline OMS-2 catalyst improves the wastewater treatment capacity by a factor of 1.5–2 and 3 compared with co-precipitated Mn–Ce-oxide and activated carbon, respectively.

Cerium incorporated ordered manganese oxide OMS-2 materials: Improved catalysts for wet oxidation of phenol compounds

Low-temperature combustion of 2,4,6-trichlorophenol in catalytic wet oxidation with nanocasted Mn-Ce-oxide catalyst

Catalytic wet oxidation of phenol solutions at low temperatures of 80−130 °C and space velocities of 1−100 h-1 using Mn−Ce catalysts was studied with an emphasis on the reactive adsorption mechanism and total organic carbon (TOC) removal. Eight catalysts (Mn/Ce = 6:4) were activated under different conditions and promoted with alkali metals (K, Cs) or noble metals (Pt, Ru). The compositions and physical properties of all catalysts were measured. Preliminary runs were conducted in a batch reactor, but most experiments were carried out in a continuous-flow trickle-bed reactor. Catalysts containing mixed Mn3O4−CeO2 phases pure and promoted with alkali metals displayed a higher activity and a higher adsorption of organic deposits on their surface. Noble metals had little effect on process performance. The adsorption capacity of the catalysts was found to be considerably higher than that reported for activated carbon. Furthermore, complete regeneration of a catalyst in three consecutive tests was demonstrated ...

Catalytic Wet Oxidation of Phenol with Mn−Ce-Based Oxide Catalysts: Impact of Reactive Adsorption on TOC Removal

Mesoporous chromia aerogels with a surface area of 484–735 m 2  g −1 , a pore volume of 0.4–0.9 cm 3  g −1 and a pore diameter of 3–9 nm were prepared by urea-assisted homogeneous precipitation from an aqueous Cr(NO 3 ) 3 solution, followed by continuous supercritical extraction with CO 2 under different conditions (pressure and time) after replacement of the water with a hexane/2-butanol mixture. The texture and chemistry of the aerogels transformed by heating in air or an inert atmosphere and the structure of the nanoparticles were characterized by means of N 2 -adsorption isotherms, AA, HRTEM, FTIR, a variety of thermoanalytical methods (TPD, DSC, TGA, TPO–TPK) and X-ray diffraction in combination with structure modeling. At the CO 2 extraction stage, a pressure of about 400 bars was critical for production of aerogels with surface areas >700 m 2  g −1 . The fresh chromia aerogels consisted of closely packed almost globular, 3- to 5-nm nanoparticles with a structure analogous to that of monoclinic α-CrOOH, in which half of the O atoms and OH groups were replaced with coordinately bonded water molecules. After dehydration at 550–600 K, the materials retained their texture, being converted to faceted 3- to 5-nm nanoparticles, consisting of two-dimensional fragments (clusters) of α-CrOOH crystals built on [Cr(OH) 3 O 3 ] octahedra without bonding along the Z -axis. The texture of dehydrated chromia aerogels was stable at temperatures up to 650 K in air and up to 773 K in an inert atmosphere. At higher temperatures, the material underwent a glow transition, yielding microcrystalline 50-nm particles with the well-defined structure of α-Cr 2 O 3 and a surface area 2  g −1 .

Texture and nanostructure of chromia aerogels prepared by urea-assisted homogeneous precipitation and low-temperature supercritical drying

A process which comprises providing a porous matrix, mixing said porous matrix with a precursor solution containing manganese and cerium ions dissolved therein, triggering the formation of a non-fluid phase which contains manganese and cerium in the interior of said porous matrix, separating said porous matrix from a liquid phase containing soluble metals, completing the formation of said non-fluid phase which contains manganese and cerium in the interior of said porous matrix, heat treating said porous matrix to convert said non-fluid phase containing manganese and cerium placed therein into a manganese oxide-cerium oxide composite, and isolating said composite from said porous matrix. The resulting composite, and its use as a catalyst, are also provided.

M. Abecassis-Wolfovich

Papers

Cerium incorporated ordered manganese oxide OMS-2 materials: Improved catalysts for wet oxidation of phenol compounds

Low-temperature combustion of 2,4,6-trichlorophenol in catalytic wet oxidation with nanocasted Mn-Ce-oxide catalyst

Catalytic Wet Oxidation of Phenol with Mn−Ce-Based Oxide Catalysts: Impact of Reactive Adsorption on TOC Removal

Texture and nanostructure of chromia aerogels prepared by urea-assisted homogeneous precipitation and low-temperature supercritical drying

Manganese Oxide-Cerium Oxide Composite