Abstract
Abstract A series of Keggin heteropolytungstate salts (M1.5PW12O40, M=Cu, Co, Zn and Fe) were prepared and characterized utilizing inductively coupled plasma spectrometry (ICP), Fourier transform infrared (FTIR) spectra, and ultraviolet-visible (UV-Vis) light spectroscopy. The as-prepared catalysts were tested for the oxidation of ethylbenzene by using carbon dioxide/hydrogen peroxide (CO2/H2O2) as the oxidizing agent system under solvent-free conditions. The results indicated that the heteropolytungstates catalyzed the side chain oxidation of ethylbenzene leading to acetophenone as a major product. The effect of various reaction parameters on ethylbenzene oxidation over the best catalyst of the series, namely Co1.5PW12O40 loaded on activated carbon (AC), was investigated. It was found that the selectivity depends strongly on the reaction temperature. Higher reaction temperatures reduce the conversion due to the decomposition of H2O2. Oxidation by a large amount of H2O2 decreases the conversion owing to a decrease of the solubility of ethylbenzene in an aqueous medium, and favors the oxidation of the reaction products, which are more soluble in an aqueous medium. The increase of the CO2 pressure improves both the conversion and the selectivity of acetophenone due to the involvement of the percarbonate species (HCO4−) responsible for oxidation by oxygen transfer.
Highlights
The conversion of alkylbenzenes into carbonyl compounds is one of the most important processes in petrochemistry
These results are in agreement with those reported in the literature for Keggin heteropolyanions [22, 23]
The increase of acetophenone selectivity may be due to acidic sites on the surface of the functionalized activated carbon (AC), which is favorable to oxidation reactions
Summary
The conversion of alkylbenzenes into carbonyl compounds is one of the most important processes in petrochemistry. Regarding the oxidation of alkyl benzenes, that of ethylbenzene has been the subject of considerable interest. When used alone, CO2 favors cracking and dehydrogenation reactions rather than oxidation reactions, whereas when used along with an oxygen donor source, the oxydehydrogenation reaction occurs. This synergistic effect is useful for the creation of carbonyl groups in hydrocarbons. It is worth noting that several homogeneous heteropolyanion based catalysts have proven their performance in oxidation reactions owing to their redox and acid–base properties that can be adjusted by varying the heteroatom, counter anion, and addenda atoms [16,17,18]. It is worth noting that several heteropolyanion based catalysts have proven their performance in
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