engleski

ACTIVITY OF Cu/ZSM5 CATALYSTS FOR THE OXIDATION OF PHENOL WITH HYDROGEN PEROXIDE

Introduction. Organic compounds, e.g. phenol and its derivatives which are extremely toxic and refractory in nature, are common pollutants in many industrial wastewaters. In the last years, the treatment of phenols in aqueous solutions was generally carried out by oxidation with molecular oxygen or air, at elevated pressure (1-10 MPa) and temperature (373 – 573 K) in the presence or absence of catalysts thus increasing the treatment cost. An alternative process which allows performing oxidation reaction at ambient or close to ambient conditions, limiting the investment costs is the use of hydrogen peroxide in so-called catalytic wet peroxide oxidation (CWPO). Great number of different catalysts, as example different metals supported on metal oxides, clays, graphite, polymers, and carbon, supported and unsupported metal oxides and so on. The main objective of this work is to evaluate the activity and stability of copper species incorporated within zeolite support. The catalytic performance has been monitored in terms of phenol and TOC conversions. Experimental. The kinetic experiments and activity tests were carried out in stainless steel Parr reactor in batch operation mode at temperature between 50 0C and 80 0C and atmospheric pressure. The mass ratio of the active metal component on the zeolite was in the range of 1.62-3.24 wt. %, and the initial concentration of phenol and hydrogen peroxide was 0.01mol dm-3 and 0.1mol dm-3, respectively. The catalyst samples (Cu/ZSM-5) were prepared by direct hydrothermal synthesis. Characterization of the catalysts extends to X-ray diffraction (XRD) and scanning electron microscopy (SEM), while the adsorption techniques were used for the measurement of the specific surface area. Results and discussion. Preliminary experiment showed that phenol oxidized with hydrogen peroxide without catalyst, but its conversion after 3 h was only 10%  1 . No catalyzed homogeneous phenol oxidation was characterized by an induction period followed by a faster reaction phase, typical for the reactions governed by free-radical mechanisms. Fig.1. Influence of copper loading on TOC, and phenol and hydrogen peroxide conversion Fig. 1 shows the influence of copper loading on phenol removal, hydrogen peroxide consumption, and TOC reduction after 180 min of reaction. It is well known that CWPO of phenol is very complex process consisting of a set of parallel and series reaction in which many types of intermediates and final products are involved. These consisted of aromatic compounds, mainly benzoquinone, hydroquinone and cathecol, carboxylic acids (acetic, maleic, oxalic and fumaric acids) and other oxygenated compounds such as aldehydes and ketones. Looking at phenol conversion presented in Fig. 1, all catalyst samples were able to oxidize the parent pollutant, phenol to an extent of 92 -97 % depending upon the copper loading on zeolite. Meanwhile, the TOC reduction is in all cases, significantly lower that phenol conversion indicating formation of different intermediates and final results is far from complete mineralization of phenol. It can be also evidenced that the TOC removal enhanced with the increase in the copper loading on zeolite. Deeper removal of the organic load was impeded, by the accumulation of refractory organic byproduct acids that were stable in the reaction medium because they were already highly oxidized molecules. The low pH found after 180 min of reaction confirm the presence of organic carboxylic acids as by products of phenol oxidation. Conclusions. The experimental results indicate that in the space of 180 min, the use of these catalysts allows almost total elimination of phenol and significant removal of chemical oxygen demand, without significant leaching of Cu ions from zeolite. The best candidate was Cu/ZSM5-DHS3 catalyst containing the highest copper loading. References 1. Zrnčević, S., Gomzi, Z., Ind. Eng. Chem. Res. 44 (2005) 6110. 2. Maduna Valkaj, K., Katović, A., Zrnčević, S., J.Hazard. Mat. 144 (2007) 663.

Wastewater treatment; CWPO; Cu/ZSM5; Phenol oxidation; Kinetics;

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