Catalytic oxidation of cresols using molecular ozone
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Date
2017
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University of Zululand
Abstract
Catalytic oxidation of cresols using molecular ozone. The environment, both in general and specifically our habitable segment is currently under threat from surfacing chemical pollutants that are generated mostly from pharmaceutical and industrial sectors. The extended use of cresols (meta-, ortho-, and para-cresol) in these sectors makes them potential pollutants and yet they are of hazardous nature to human, plants and animals. Therefore, the necessity to efficiently degrade these compounds is heightened and one of the reliable tools would be chemical oxidation, mainly advanced oxidation processes, which in this work refers to heterogeneously catalyzed oxidation of cresols using molecular ozone. The main objective of this study was to catalytically oxidize various cresol isomers using ozone. All oxidation reactions were studied as a function of time.
Metal (Ni, Fe, Mn, V) loaded γ-Al2O3, SiO2 and V2O5 catalysts were synthesized by wet impregnation method. These catalysts were further calcined (300 oC) in air and characterized using powder X-ray diffraction (XRD), Fourier Transform-Infrared (FT-IR), inductively coupled plasma-optical emission spectroscopy (ICP-OES), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, transmission electron microscopy (TEM) and Brunauer-Emmet-Teller (BET) surface area analyzer. These analytical techniques provided essential information about the catalysts, ranging from solid phase composition and purity, catalyst surface morphology, concentration of a loaded metal on a support to the specific surface area of the catalyst material. Prior to the application of these calcined catalysts in the oxidation of meta-, ortho- and para-cresol, a brief study of column mounted oxidation of cresols was done.
A volume of 3.0 mL for each cresol isomer was adsorbed on pure γ-Al2O3 and SiO2, mounted on a vertical glass column and oxidized with O3 (0.123 mg/L) for 1, 3 and 5 h intervals. After each oxidation reaction, the samples were collected and dissolved in absolute ethanol. The resultant oxidation products were characterized using FT-IR and gas chromatography coupled with mass spectrometer (GC-MS), which separately provided data on functional groups that were introduced in the reaction and both percentage conversions & selectivities. Specific products that were identified include m-tolyl acetate (m-TA) and 2,3-dihydroxy toluene (2,3-DT) from m-cresol; o-tolyl acetate (o-TA) and 2,5-dihydroxy toluene (2,5-DT) from o-cresol; p-tolyl acetate (p-TA) and 3,4-dihydroxy toluene (3,4-DT) from p-cresol. Diethyl maleate (DM) and diethyl oxalate (DO) products were commonly found in all cresols oxidation. Column and reactor reactions yielded the same products of the respective cresol (m-, o-, p-cresol). Of all the products that were obtained, dihydroxy toluenes are the direct oxidation products of cresol whereas the rest are ethanol derivatised oxidation products. i.e. DM and DO were esterification products of maleic acid and oxalic acid, respectively. However, all products are termed „oxidation products‟ in this thesis for simplicity. Detection of these oxidation products laid preliminary grounds for the more traditional catalytic oxidation reactions. In a glass reactor (impinger), 25.0 mL of each cresol was ozonated in absence of a catalyst for 24 h and the obtained percentage conversions and selectivity results were used as reference data in catalyzed oxidations. The economically friendly pure activated charcoal was initially used as a catalyst (1% w/v charged) followed by pure supports viz. γ-Al2O3, SiO2 and V2O5 separately. Activated charcoal showed lowest efficiency than the pure supports and thus the attention was put on improving their activity by loading different metals on their surfaces. Catalytic activity of pure supports assumed the order of SiO2 > V2O5 > γ-Al2O3, with silica as the most active support.
Moreover, the metal loaded catalysts were utilized in the oxidative degradation of cresols and 1% w/v of catalyst was used in all catalytic work. Of all γ-Al2O3 supported catalysts, Mn(2.5%)/γ-Al2O3 yielded highest conversion of m-cresol (70%) due to Mn species present on surface γ-Al2O3, whereas Ni(2.5%)/γ-Al2O3 showed highest percentage conversions of both o- (66%) and p-cresols (81%). Amongst metal loaded SiO2 catalysts, Fe(2.5%)/SiO2 was more active in the degradation of m- (81%) and o-cresol (70%) due to its relatively large surface area and presence of Fe in a form of catalytically active Fe2O3, while Ni(2.5%)/SiO2 degraded p-cresol (85%) the most. Mn(2.5%)/V2O5 catalyst converted most of m- (71%) and o-cresol (67%) to oxidation products whereas Fe(7.5%)/V2O5 relatively converted most of p-cresol (64%) to products which was promoted by Fe3O4 presence on V2O5 surface. The efficiency of metal loaded catalysts was decreased from SiO2 to V2O5 support. When supports (γ-Al2O3, SiO2, V2O5) were used as catalysts, there was an apparent improvement in product selectivity compared to uncatalyzed ozonation reactions. However, the use of metal (Ni, Fe, Mn, V) loaded supports as catalysts resulted in the decrease in product selectivities but significantly improved percentage conversions.
Description
A dissertation submitted to the Faculty of Science and Agriculture in fulfillment of the requirements for the Degree of Master of Science in the Department of Chemistry at the University of Zululand, 2017.
Keywords
Chemical pollutants --cresols --chemical oxidation