Catalytic Wet Air Oxidation Of Phenol Research Articles

Developing Pt-M/C catalyst (M=Pb, Cu) for efficient catalytic wet air oxidation of phenol wastewater under mild conditions

Catalytic wet air oxidation (CWAO) is a promising process for degrading phenol in wastewater to CO2 and H2O. To achieve an active and stable Pt catalyst working at lower temperature, efficient catalysts are here developed by building a Pt-M (M=Pb, Cu) alloy structure where Pt is mainly in the form of Pt0 while M is mainly in the form of M2+. Firstly, carbon supported Pt-Pb/XC-72R and Pt-Pb/EC-300 catalysts are prepared for CWAO of phenol wastewater. It is found that Pt-Pb/XC-72R is highly active and stable for CWAO of phenol even at 100 °C. The initial TOC conversion is 91.1% and can be maintained higher than 75% in five consecutive cycles. Contrarily, Pt-Pb/EC-300 rapidly deactivates and TOC conversion decreased from 91.2% to 47.1% after five cycles. The characterization results indicate that two Pt-Pb alloy catalysts contained comparably high Pt0 concentration, which is responsible for the high activity. However, the Pb2+ concentration of Pt-Pb/XC-72R is much higher than that of Pt-Pb/EC-300, and higher Pb2+ concentration favors surface oxidation and keeps Pt0 from being oxidized during CWAO process, thus enhancing stability. For Pt-Pb/EC-300, both Pt0 and Pb0 are oxidized during the reaction, resulting in deactivation. To avoid using Pb in environmental considerations, the Pt-Pb is then replaced by Pt-Cu nanoparticles, where high catalytic activity and stability are maintained. More importantly, increasing Cu2+ can further enhance the catalytic performance. Tuning the second metal chemical state of Pt-based bimetallic alloy can be an effective approach to develop new catalysts for CWAO technology.

Enhancing electronic metal support interaction (EMSI) over Pt/TiO2 for efficient catalytic wet air oxidation of phenol in wastewater

Phenol is one of the major hazardous organic compounds in industrial wastewater. In this work, a highly active Pt/TiO2 catalyst for catalytic wet air oxidation (CWAO) of phenol was obtained by supporting pre-synthesized Pt on TiO2. During the followed hydrogen reduction, strong hydrogen spillover occurred without the migration of TiO2 onto Pt. The reduced support then enhanced the electron transfer from TiO2 to Pt, increasing the percentage of partially negative Pt (Ptδ-), which has been confirmed by XPS. The strong EMSI made the obtained catalyst far more active than Pt/TiO2 prepared by impregnation method. The electron-enriched Pt/TiO2 achieved total organic carbon (TOC) conversion of 88.8% and TOF 149 h−1 at 100 °C and 2 MPa O2, while conventional Pt/TiO2 gave TOC conversion of 39.5% and TOF 41 h−1 for CWAO of phenol. Our work indicates that the enhancement of EMSI between metal and support can be an effective approach to develop highly active catalysts for phenol treatment.

Catalytic wet air oxidation of phenol by cordierite honeycomb washcoated with Al/Zr pillared bentonite in a plug flow reactor

Cordierite monoliths coated with Al/Zr pillared bentonite clay powder were synthesized and tested for catalytic wet air oxidation (CWAO) of phenol aqueous solutions in a plug flow reactor. The catalysts were characterized by using various techniques (EDS, SEM, BET and XRD), confirming the effective deposition of Al/Zr pillared bentonite clay on the channel walls of cordierite monolith. The composite catalyst showed good mechanical stability after ultrasound treatment. The performance of the composite catalyst was evaluated in a catalytic plug flow reactor for continuous oxidation of phenol in terms of catalytic activity as well as their stability. The composite catalyst achieved high catalytic performance (100 % phenol conversion and 91 % total organic carbon reduction) with high stability after 140 h long experiment. During the CWAO of phenol reaction, aromatic ring intermediates (p-benzoquinone, catechol and hydroquinone) and short-chain carboxylic acids (oxalic acid, maleic acid, formic acid, acetic acid and malonic acid) were identified. Kinetic parameters of the model that describes the formation and oxidation of intermediate aromatic compounds and short-chain carboxylic acids were determined. The obtained experimental data results were described sufficiently with respect to phenol conversion. The proposed CWAO of the phenol kinetic model accounts for internal and external diffusion limitations, assuming the pseudo-first-order reaction. Therefore, synthesized monolithic catalysts have great potential for application in CWAO of phenol, since the materials used in the process are inexpensive, abundant and require minimal modifications.

Catalytic wet air oxidation of phenol: Review of the reaction mechanism, kinetics, and CFD modeling

Advanced oxidation processes, specifically catalytic wet air oxidation (CWAO), are considered as useful and robust methods for the treatment of refractory organic compounds such as phenol in wastewater. This paper reviews reaction mechanisms, kinetics and computational fluid dynamics (CFD) modeling of CWAO of phenol. Different reaction mechanisms have been proposed by various researchers to account for possible intermediates during phenol oxidation. These mechanisms can be either direct or indirect; with the indirect path resulting in the formation of intermediates, while with the direct mechanism, no intermediates are formed, the reaction proceeds straight to carbon dioxide and water. Acetic acid is considered as the most stable intermediate of phenol oxidation. Power law and Langmuir–Hinshelwood models were used to account for the adsorption/desorption of the reactants on the surface of the catalyst. The reviewed studies show that phenol conversion increases with increased temperature, pressure, gas velocity and decreases with increasing liquid space velocity. The formation of hot spots next to the walls of the reactor leads to safety issues and may cause catalyst deactivation and reactor thermal run away. This prompted a review of liquid mal-distribution and the formation of hot spots inside the reactor using the CFD modeling technique. In most cases, liquid channeling was observed, resulting in the formation of hot spots and catalyst deactivation.

Synthesis and characterization of nanomesoporous lanthanum aluminate (LaAlO3) for catalytic wet air oxidation of phenol

Nanomesoporous LaAlO3 synthesized by reverse microemulsion method were tested in the catalytic wet air oxidation (CWAO) of phenol in a fixed bed reactor. Synthesized LaAlO3 was characterized in depth by TG-DSC, XRD, FTIR, BET, TEM, TPD-CO2, TPD-NH3, and H2-TPR in order to relate its performances to the chemical and textural characteristics. Nanomesoporous LaAlO3 showed high activity towards phenol and in total organic carbon (TOC) removal with greater selectivity to non-organic compounds, a clear increase in the removal efficiency and selectivity relatively to non-catalytic wet air oxidation being observed. Effect of temperature, catalyst dosage, air flow, and phenol pumping rate in CWAO of phenol was investigated. LaAlO3 displayed high catalytic stability even after 6 h of continued reaction. Structural and textural stability of LaAlO3 was confirmed by XRD and BET surface area analysis of regenerated catalyst. Lewis basic sites and mesoporous nature of nano-LaAlO3 may play an important role in phenol degradation and TOC removal efficiency with high selectivity to non-organic compounds.

Effect of gold addition by the recharge method on silver supported catalysts in the catalytic wet air oxidation (CWAO) of phenol.

Catalysts Ag/ZrO2–CeO2 and Au/ZrO2–CeO2 were synthesized by a deposition–precipitation method and Ag–Au/ZrO2–CeO2 was prepared using a recharge method for the second metal (Au). The materials were characterized by physisorption of N2, XRD, ICP, UV-vis RDS, H2-TPR, XPS and TEM. The results obtained show that the specific areas for monometallic materials were 29–37 m2 g−1 and 27–74 m2 g−1 for bimetallics. The tetragonal crystal phase of ZrO2 stabilizes when CeO2 quantity increases. Using XPS an increment in Ce3+ species abundance was determined for bimetallic catalysts in contrast to the monometallic ones; according to the Ag 3d region, this metal oxidation was observed when augmenting the content of CeO2 in the materials, and with Au the opposite effect was produced. It was determined by TEM, that the average size of the metallic particles was smaller at bimetallic catalysts due the preparation method. Catalytic activity was evaluated by CWAO of phenol, the Ag–Au/ZrO2–CeO2 catalyst with 20% wt of cerium reached a degradation of 100% within an hour, being the most active catalyst. Maleic, formic and oxalic acid were identified as reaction intermediates; and at the end of the reaction acetic acid was identified as the main by-product, because it is the most refractory and the conditions for oxidation must be more severe.

A Novel Synthesis Method of Al/Cr Pillared Clay and its Application in the Catalytic Wet Air Oxidation of Phenol

Natural bentonite clay was pillared with Al and Cr polycations (Al-PILCs and Cr-PILCs) as well as with both Al and Cr mixed polycations (Al/Cr-PILCs) at different Al/Cr molar ratios. The structural and textural properties of the pillared clay catalysts were characterized by various techniques, such as high resolution scanning electron microscopy with energy dispersive system, powder X-ray diffraction, adsorption–desorption of N2 at − 196 °C, Fourier transform infrared spectroscopy, thermogravimetric analysis, cation exchange capacity and zeta potential. The obtained results showed that the intercalation of polycations in the interlamellar space of clay led to changes in the clay properties, basal spacing and specific surface area of a mixed PILCs were higher compared to single PILCs. Moreover, diffractograms of the Cr-PILCs confirmed the presence of a Cr2O3 phase. The catalytic wet air oxidation (CWAO) of the phenol was studied in a batchwise slurry reactor, at the experimental conditions of 100 °C temperature, 10 bar oxygen (O2) partial pressure, at pH 3.0, 100 mg/L concentration and 0.4 g of catalyst. All catalysts were analysed to determine the conversion of phenol and removal of total organic carbon (TOC). The results showed that, Al-PILCs displayed higher removal of phenol and TOC conversion than Cr-PILCs. In the case of mixed Al/Cr-PILCs, maximum removal values of phenol and TOC conversion were achieved with Al/Cr molar ratio of 1:1. Moreover, aromatic intermediate products as catechol, hydroquinone and benzoquinones which are identified in the beginning of the phenol oxidation oxidized into short-chain carboxylic acids such as oxalic acid, maleic acid, formic acid, acetic acid and malonic acid. The obtained results demonstrate the potential of Al/Cr-PILCs catalyst as robust and cheap catalyst for CWAO of phenol under mild reaction conditions.

Synthesis of highly active and stable Al/Zr pillared clay as catalyst for catalytic wet oxidation of phenol

Natural bentonite clay was used in the synthesis of single and mixed oxide Al/Zr pillared clay catalysts by means of ultrasonic treatment during the aging and intercalation steps. The raw bentonite and pillared clay catalysts were characterized using high resolution scanning electron microscopy with energy dispersive system (HRSEM-EDS), powder X-ray diffraction (PXRD), N2 adsorption/desorption, Fourier transform infrared spectroscopy, thermogravimetric analysis, cation exchange capacity, zeta potential and all the catalysts evaluated in the catalytic wet air oxidation (CWAO) of phenol. Calcination of the synthesized Al/Zr-PILCs at 500 °C for 120 min yielded optimum physical properties with BET surface area of 230 m2/g, pore volume of 0.115 cm3/g and basal spacing of 1.92 nm and with good thermal stability. Complete removal of phenol was obtained after 120 min and 88% TOC was removed after 180 min in aqueous solution under mild reaction conditions of 100 °C and 10 bar after. Catalyst reusability studies showed that the Al/Zr-PILCs catalyst demonstrated excellent chemical and structural stability with insignificant leaching of Zr4+ after six consecutive catalytic runs. The Al/Zr-PILCs catalyst therefore demonstrated excellent catalytic properties in the CWAO of phenol in aqueous solution.

Catalytic wet air oxidation of phenol over carbon nanotubes: Synergistic effect of carboxyl groups and edge carbons

It has been widely documented that the oxygen-containing functionalities, especially carboxyl groups, on carbon surfaces play crucial roles in the catalytic wet air oxidation (CWAO) reactions. Herein, the CWAO of phenol with molecular O2 as terminal oxidant using cylindrical and herringbone carbon nanotubes (CNTs) underwent different treatments as catalysts were conducted in batch mode. It was uncovered that the catalytic function of carboxyl groups strongly depended on the structure of CNTs. Parallel CNTs (p-CNTs) with cylindrical graphitic walls displayed the highest catalytic activity after nitric acid oxidation, due to the carboxylated carbon debris (CDs) coating on surfaces. For herringbone CNTs (h-CNTs), carboxyl groups are mostly linked to its sidewalls, where abundant edge carbons were exposed, providing an enhanced CWAO activity. The positively synergistic effect of carboxyl groups and adjacent edge carbons was responsible for the activation of O2, leading to the improved CWAO activity. This study provides a new insight into the structural effect of carbon materials on the CWAO of phenol.

Synthesis and characterization of an efficient and stable Al/Fe pillared clay catalyst for the catalytic wet air oxidation of phenol.

A facile method for synthesizing Al/Fe pillared clays (PILCs) from natural bentonite clay, by using ultrasonic treatment during the aging and intercalation steps, has been established. Single metal (Fe-PILCs and Al-PILCs) and mixed metal (Al/Fe-PILCs, with varying compositions of the pillaring precursors) pillared clays were prepared and characterized with a combination of chemical and instrumental methods. The pillared clays were evaluated as catalysts in the catalytic wet air oxidation (CWAO) of phenol and the mixed metal pillared clay catalysts, especially with an Al/Fe ratio of 3 : 1, were found to be highly active and stable, with superior properties such as surface areas, basal spacing, high porosity and thermal stability. The catalysts could also be reused several times without significant loss of activity. The results of the study show that Al/Fe-pillared clay catalysts are effective catalysts for the oxidation of phenol and removal of TOC in aqueous solutions. The main intermediate products in the CWAO of phenol were hydroquinone, pyrocatechol, and benzoquinone and oxalic, formic, malonic, oxalic, malonic and maleic acids.

Novel metal modified diatomite, zeolite and carbon xerogel catalysts for mild conditions wet air oxidation of phenol: Characterization, efficiency and reaction pathway

Abstract Metal (Co, Ce, Ni, Fe, Zn, Mn and Cu) containing Diatomite (D), Zeolite (Z) and Carbon Xerogel (CX) catalysts were prepared, characterized (SEM-EDX, TEM, FTIR and XRD) and tested as new catalysts in Catalytic Wet Air Oxidation (CWAO) of phenol in aqueous solution. Maximum values of Chemical Oxygen Demand (COD) or Total Organic Carbon (TOC) efficiencies were obtained in the case of Fe-D, Cu-Z, and Co-CX catalysts. Reaction pathway and kinetics for CWAO of phenol revealed a pseudo first-order catalytic process and a kcat between 0.0473 × 10−3 and 1.4183 × 10−3 L/min g. (Di)carboxylic acids, acetic and oxalic were identified as oxidation organic products.

Improvement of (transition metal-modified) activated carbon regeneration by H2O2-promoted catalytic wet air oxidation

ABSTRACTOxidative regeneration of activated carbon (AC) exhausted with phenolic compounds is still a challenging issue due to the frequent porosity loss. Addition of low H2O2 amount is investigated as a way to promote catalytic wet air oxidation (CWAO) of adsorbed pollutants and thereby to recover absorbent properties. Commercial AC and transition metal (iron or copper)-modified counterparts are tested in repeated adsorption/batch peroxide-promoted CWAO of phenol. Cycles are operated in both fixed bed and slurry reactors to vary the initial pollutant distribution in between the two phases. Influence of metal location is also studied by adding iron salt to the pollutant solution prior to perform peroxide-promoted oxidation on bare carbon. Regeneration results are analyzed through a detailed analysis of both the solid and the liquid phases during the oxidative treatment. It is proved that a convenient H2O2 dosage can increase the lifetime of adsorbent in adsorption–oxidation cycles, but coupling with (un)supported metal oxide does not provide significant gain.

Wastewater treatment by catalytic wet air oxidation process over Al-Fe pillared clays synthesized using microwave irradiation

Microwave irradiation has been used to prepare Al, Fe-pillared clays from a natural Tunisian smectite from the El Hicha deposit (province of Gabes). Chemical analysis, XRD spectra and surface properties evidenced the success of pillaring process. The obtained solids present higher surface area and pore volume than conventionally prepared Al-Fe pillared clays. The main advantages of the microwave methodology are the considerable reduction of the synthesis time and the consumption of water. The microwave-derived Al-Fe pillared clays have been tested for catalytic wet air oxidation (CWAO) of phenol in a stirred tank at 160°C and 20 bar of pure oxygen pressure. These materials are efficient for CWAO of phenol and are highly stable despite the severe operating conditions (acidic media, high pressure, high temperature). The catalyst deactivation was also significantly hindered when compared to conventionally prepared clays. Al-Fe pillared clays prepared by microwave methodology are promising as catalysts for CWAO industrial water treatment.

Optimal design and operation of an industrial three phase reactor for the oxidation of phenol

Abstract Among several treatment methods catalytic wet air oxidation (CWAO) treatment is considered as a useful and powerful method for removing phenol from waste waters. In this work, mathematical model of a trickle bed reactor (TBR) undergoing CWAO of phenol is developed and the best kinetic parameters of the relevant reaction are estimated based on experimental data (from the literature) using parameter estimation technique. The validated model is then utilized for further simulation and optimization of the process. Finally, the TBR is scaled up to predict the behavior of CWAO of phenol in industrial reactors. The optimal operating conditions based on maximum conversion and minimum cost in addition to the optimal distribution of the catalyst bed is considered in scaling up and the optimal ratio of the reactor length to reactor diameter is calculated with taking into account the hydrodynamic factors (radial and axial concentration and temperature distribution).

Catalytic wet air oxidation of phenol over metal catalyst (Ru,Pt) supported on TiO2–CeO2 oxides

The catalytic performances of Ru and Pt supported on TiO2–xwt% CeO2 were studied in catalytic wet air oxidation (CWAO) of phenol at 160°C and 20bar of pure oxygen pressure. Contrary to expectations, improved oxygen storage capacities of the materials prove to be detrimental to catalytic performances since they favor the formation of polymers in solution and the accumulation of adsorbed species. On the opposite, the presence of Lewis acid sites promotes the phenol total oxidation. They would favor the activation of the hydroxyl function, thus promoting the ortho-oxidation of phenol and finally the formation of CO2. On the other hand, platinum appears to be more efficient than ruthenium for CWAO of phenol.

Graphene oxide as an effective catalyst for wet air oxidation of phenol

The graphene oxide (GO) and chemically reduced graphene oxides, used as catalysts in absence of any metals, were investigated in the catalytic wet air oxidation (CWAO) of phenol in a batch reactor. The characterization of the materials was measured with scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman, fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The carbon materials exhibited good phenol and total organic compounds (TOC) removals in the CWAO of phenol. The GO had the highest catalytic activity, total phenol removal was achieved after 40min, and ca. 84% TOC removal was obtained after 120min at reaction temperature of 155°C, total pressure of 2.5MPa and catalyst loading of 0.2gL−1.

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.

Catalytic performance of heteroatom-modified carbon nanotubes in advanced oxidation processes

Multi-walled carbon nanotubes (CNTs) were submitted to chemical and thermal treatments in order to incorporate different heteroatoms on the surface. O-, S- and N-containing groups were successfully introduced onto the CNTs without significant changes of the textural properties. The catalytic activity of these heteroatom-modified CNTs was studied in two liquid phase oxidation processes: catalytic ozonation and catalytic wet air oxidation (CWAO), using oxalic acid and phenol as model compounds. In both cases, the presence of strongly acidic O-containing groups was found to decrease the catalytic activity of the CNTs. On the other hand, the introduction of S species (mainly sulfonic acids) enhanced the removal rate of the model compounds, particularly in the CWAO of phenol. Additional experiments were performed with a radical scavenger and sodium persulfate, in order to clarify the reaction mechanism. Nitrogen functionalities improve the catalytic performance of the original CNTs, regardless of the process or of the pollutant.

Catalytic wet air oxidation of phenol, nitrobenzene and aniline over the multi-walled carbon nanotubes (MWCNTs) as catalysts

Wet air oxidation (WAO) is one of effective technologies to eliminate hazardous, toxic and highly concentrated organic compounds in the wastewater. In the paper, multi-walled carbon nanotubes (MWCNTs), functionalized by O3, were used as catalysts in the absence of any metals to investigate the catalytic activity in the catalytic wet air oxidation (CWAO) of phenol, nitrobenzene (NB) and aniline at the mild operating conditions (reaction temperature of 155°C and total pressure of 2.5 MPa) in a batch reactor. The MWCNTs were characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), gas adsorption measurements (BET), fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). The functionalized MWCNTs showed good catalytic performance. In the CWAO of phenol over the functionalized MWCNTs, total phenol removal was obtained after 90 min run, and the reaction apparent activation energy was ca. 40 kJ·mol−1. The NB was not removed in the CWAO of single NB, while ca. 97% NB removal was obtained and 40% NB removal was attributed to the catalytic activity after 180 min run in the presence of phenol. Ca. 49% aniline conversion was achieved after 120 min run in the CWAO of aniline.

Preparation, characterization and catalytic properties of carbon nanofiber-supported Pt, Pd, Ru monometallic particles in aqueous-phase reactions

Carbon nanofibers (CNF) synthesized by catalytic chemical vapor deposition (CVD) method were used to prepare supported platinum, palladium and ruthenium monometallic (2.0wt.%) catalysts by means of incipient-wetness impregnation method. The CNF support and catalysts were characterized by X-ray powder diffraction (XRD), nitrogen adsorption/desorption isotherms, volumetric chemisorption of hydrogen, temperature-programmed reduction (H2-TPR) and scanning electron microscopy (SEM). Solids were tested in catalytic wet-air oxidation (CWAO) of phenol aqueous solution (180–240°C and 10.0bar of oxygen partial pressure) carried out in a continuous-flow trickle-bed reactor. Trends of phenol and total organic carbon (TOC) conversion demonstrate that the CNF support and CNF-Pt catalyst did not exhibit constant activity for CWAO of phenol. A decrease of catalyst activity, detection of carbon dioxide in the off-gas stream while examining catalyst stability and significant textural changes observed, provide an evidence that under net oxidizing reaction conditions gasification of the CNF support occurs. The prepared catalysts were also tested in liquid-phase thermal decarboxylation of formic acid in inert atmosphere (60–220°C). Among solids examined, the CNF-Pd exhibited the highest activity. At the employed conditions, no decomposition of the CNF support was observed during the thermal decarboxylation of formic acid.