CWPO: An Environmental Solution for Pollutant Removal from

This study presents an evaluation of the catalytic performances of CuY-5 in wet hydrogen peroxide oxidation of phenol. The catalyst was prepared by io...
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Ind. Eng. Chem. Res. 2005, 44, 6110-6114

CWPO: An Environmental Solution for Pollutant Removal from Wastewater Stanka Zrncˇ evic´ * and Zoran Gomzi Faculty of Chemical Engineering and Technology, University of Zagreb, Zagreb, Croatia

This study presents an evaluation of the catalytic performances of CuY-5 in wet hydrogen peroxide oxidation of phenol. The catalyst was prepared by ionic exchange from the protonic form of the commercial HY-5 zeolite. The process was carried out within the temperature range from 323 to 353 K and at atmospheric total pressure. Other operating variables were stirrer speeds (200-800 min-1), hydrogen peroxide concentrations (0.008-0.254 mol dm-3), and catalyst loadings (0.05-0.4 g). The initial phenol concentration was 0.01 mol dm-3. The results show that the used catalyst entirely eliminated phenol and could be reused in successive runs, without significant loss of activity. Introduction Industrial, agricultural, and domestic wastes have contributed to the degradation of ground and surface waters. The increasingly stringent water quality regulations and demand for recycling of wastewater ask for efficient treatment of wastewater streams, which contain pollutants. Different strategies, such as “clean”, upgraded, or innovative technologies, have been developed to achieve the set requirements. Catalytic wet air oxidation (CWAO) with air or oxygen as an oxidant appears to be a very promising new technology for high conversion of organic pollutants (e.g., phenol and derivates) that are poorly biodegradable or even toxic to the microorganisms. Unfortunately, however, CWAO, requiring high operating pressure (1-10 MPa) and temperature (353-473 K), makes the investment rather costly. By contrast, hydrogen peroxide (a CWPO process) allows oxidation under or almost under the ambient conditions (atmospheric pressure and T e 323 K), which reduces the investment. Recently, Luck1 and Levec and Pintar2 have studied catalytic wet oxidation. A great variety of solid catalysts, including active carbon,3,4 supported and unsupported metal oxides,5-9 and metals supported on metal oxides,10-15 graphite,16 clays,17,18 and polymers,19 were tested during oxidation of phenol compounds. However, not many of them exhibited significant activity and stability in aqueous media. Also, a few studies on the use of zeolites containing copper or iron as active sites have been reported20-24 lately. Their aim was to improve the catalytic efficiency of the process by combining a porous support and an active phase in the adsorption of organic compounds to enhance H2O2 activation and to assist complete oxidation. The references to catalytic oxidation of phenol compounds given here indicate considerable variability in the activity and stability of different catalysts used. Because the economics of the catalytic oxidation technology is significantly influenced by the activity, selectivity, and stability of a catalyst, it is important that sufficiently comprehensive data on this area are available. The work presented here addresses this need. Moreover, it might also facilitate elucidation of the * To whom correspondence should be addressed. Tel.: ++3851-4597102. Fax: ++385-1-4577133. E-mail: [email protected].

source of activity, selectivity, and stability of such catalysts, help upgrade their design, and contribute to the process viability. Experimental Section Catalyst Preparation and Characterization. A CuY-5 sample was prepared by ionic exchange from the protonic form of the commercial HY-5 zeolite, supplied by Su¨d-Chemie AG, Mu¨nchen, Germany. Ionic exchange was carried out at 298 K in an aqueous solution of Cu(CH3OO)2 over 24 h and with 1.21-4.09 wt % of copper. After filtration and washing with redistilled water, the sample was dried overnight at 373 K. The specific surface area determined by nitrogen adsorption was 742 m2 g-1. The estimated pore volume of CuY-5 was 0.35 cm3 g-1, and the mean pore diameter was 1.92 nm. Prior to the catalytic test, the sample was washed thoroughly with redistilled boiling water to completely remove any adsorbed organic substances. Catalytic Reaction. Phenol oxidation was carried out in a 1 dm3 thermostated stirred batch reactor at atmospheric pressure and different stirrer speeds (200-800 rpm), temperatures (323-353 K), catalyst loadings (0.05-0.4 g dm-3), and hydrogen peroxide concentrations (0.008-0.254 mol dm-3) and a constant concentration of the catalytic active material (4.09 wt % Cu). The initial concentration of phenol was 0.01 mol dm-3. Throughout the experiments, the pH had been monitored and kept within 3.5-4. In a typical experiment, the fresh catalyst was charged into the reactor. Specified volumes of distilled water and phenol were added, and the stirrer was operated. When the reaction temperature was reached, hydrogen peroxide was added into the system and the reaction started. Liquid samples were taken at different time intervals in order to analyze the reaction mixture and to achieve phenol and hydrogen peroxide conversion. Phenol was detected and measured by UV absorbance at 254 nm wavelength by standard 4-aminoantipyrine colorimetric methods. Hydrogen peroxide was detected by titration with Na2SO3 with an excess of KI in the acidic medium. Stability Test. Leaching tests were carried out by two methods. In the first one, the stability of the CuY-5 catalyst to leaching of the active metal ingredient was

10.1021/ie049182m CCC: $30.25 © 2005 American Chemical Society Published on Web 02/19/2005

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Figure 1. Conversion of phenol as a function of time: T ) 343 K, CPh ) 0.01 mol dm-3, CHP ) 0.03 mol dm-3, wcat ) 0.1 g dm-3, and YCu ) 4.09 wt %.

verified by atomic absorption of the filtered solution samples. The second method was aimed at establishing whether small amounts of dissolved copper were responsible for the recorded catalytic activity. Therefore, after wet oxidation with H2O2, the catalyst was filtered at the temperatures of the catalytic tests in order to prevent potential adsorption of the leached copper during cooling of the solution. Hydrogen peroxide and phenol were then added to the solution in the concentrations used before the catalytic tests. Phenol conversion was measured in the absence of the solid catalyst but at the temperatures as in the catalytic tests. The stability of the catalyst was analyzed by recovering CuY-5 by hot filtration from the solution after the catalytic test. The catalyst was washed with distilled water, dried overnight at 373 K, and then tested in phenol oxidation under equal reaction conditions. Results and Discussion Preliminary Experiments. A preliminary experiment showed that phenol oxidized with hydrogen peroxide without CuY-5, but its conversion after 3 h was only 10%. Uncatalyzed homogeneous phenol oxidation was characterized by an induction period followed by a faster reaction phase, typical for the reactions governed by a free-radical mechanism. The amount of copper leached during the test (after 3 h) having been 4.8% was minimal but not negligible. It should be pointed out that filtration of the solid, aimed at determining the amount of leached metal, was performed in the hot solution to avoid copper readsorption. To check whether the recorded conversion was exclusively due to the metal ions leached from the CuY-5 catalyst, the solution activity was tested after catalyst filtration and repeated addition of 0.01 mol dm-3 of the substrate. As shown in Figure 1, in the absence of the catalyst (filtered solution), phenol conversion was below 5% in 300 min as opposed to its conversion of around 80% in the presence of the solid catalyst. That showed that the fraction of copper leached from the catalyst was not capable of destroying the organic pollutant. Also, the activities of the reused and fresh catalysts were practically equal, indicating the absence of significant deactivation (Figure 1).

Figure 2. Effect of the catalyst loading on the rate of phenol oxidation: T ) 343 K, CPh ) 0.01 mol dm-3, CHP ) 0.03 mol dm-3, and YCu ) 4.09 wt %.

Figure 3. Influence of the concentration of the catalytic active material on the rate of phenol oxidation: T ) 343 K, CPh ) 0.01 mol dm-3, CHP ) 0.03 mol dm-3, and wcat ) 0.1 g dm-3.

In heterogeneous catalysis, intrinsic kinetics can be evaluated only if the external or internal mass-transfer resistances are minimized. Manipulating the agitation rate can eliminate the first-step resistance. In our case, the oxidation rate was increased irrespective of the agitation set to 400 rpm, which suggested the absence of external mass-transfer limitation on phenol destruction. Figures 2 and 3 show the effect of the mass, wk, and concentration of the catalytic active sites, YCu, on the specific rate of phenol oxidation. The proportionality between k1, wk, and YCu is apparent, evidencing that the internal mass transfer was not a limiting step. Phenol Oxidation. The operating temperature is known to be the important variable in wet peroxide oxidation of phenol. Figures 4 and 5 show normalized concentrations of the remaining phenol and hydrogen peroxide versus time in the experiments performed at different temperatures. As shown in Figures 4 and 5, an elevated temperature increased the rate and degree of phenol oxidation and hydrogen peroxide decomposition. The rate of phenol oxidation and peroxide decomposition was expressed as follows:

-dCPh/dt ) k1CnPhCHP

(1)

-dCHP/dt ) k2CHP + k1CnPhCHP

(2)

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Figure 6. Effect of the temperature on the rate constant in phenol removal: CPh ) 0.01 mol dm-3, CHP ) 0.03 mol dm-3, wcat ) 0.1 g dm-3, and YCu ) 4.09 wt %. Figure 4. Influence of the temperature on phenol removal: CPh ) 0.01 mol dm-3, CHP ) 0.03 mol dm-3, wcat ) 0.1 g dm-3, and YCu ) 4.09 wt %.

Figure 7. Effect of the temperature on the rate constant in hydrogen peroxide decomposition: CPh ) 0.01 mol dm-3, CHP ) 0.03 mol dm-3, wcat ) 0.1 g dm-3, and YCu ) 4.09 wt %.

Figure 5. Influence of the temperature on hydrogen peroxide decomposition: CPh ) 0.01 mol dm-3, CHP ) 0.03 mol dm-3, wcat ) 0.1 g dm-3, and YCu ) 4.09 wt %.

The kinetic parameters in eqs 1 and 2 were estimated using the Nelder-Mead method of nonlinear regression. The residual sum of squares calculated from the difference between the experimental and predicted concentrations was minimized in the regression. In nearly all cases, the reaction order with respect to phenol was almost similar and slightly lower than unity and, therefore, taken as l. The results of the model analysis are shown as solid lines in Figures 4, 5, and 8. The agreement is fairly good. The temperature dependence of the rate constants in phenol oxidation and peroxide decomposition is shown in Figures 6 and 7, respectively. From the slope of the plot presented in Figure 6, EA in phenol oxidation was 90.32 kJ mol-1. EA values from the literature25-28 varied from 55 to 175 kJ mol-1. The lower ones were more characteristic of total oxidation to CO2, whereas the higher ones probably resulted from polymerization to tars rather than from true oxidation. Our results, thus, fall within the range of total oxidation, as suggested by Pruden and Le.25 The apparent activation energy for hydrogen peroxide decomposition was 96.77 kJ mol-1 from the slope in Figure 7. That was comparable with the value reported by PerezBenito.29

Figure 8. Influence of the hydrogen peroxide loading on phenol oxidation: T ) 343 K, wcat ) 0.1 g dm-3, CPh ) 0.01 mol dm-3, and YCu ) 4.09 wt %.

The effect of the hydrogen peroxide concentration on phenol oxidation was investigated within 0.008-0.254 mol cm-3 ranges. Figure 8 shows typical results of the influence of oxidant loading on phenol decomposition. If complete mineralization of phenol by hydrogen peroxide occurs in the reaction

C6H5OH + 14H2O2 f 6CO2 + 17H2O

(3)

it is possible to estimate the stoichiometric amount of

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hydrogen peroxide for phenol oxidation. Thus, 14 mol of hydrogen peroxide is required to completely oxidize 1 mol of phenol. Figure 8 shows that complete phenol conversion was achieved in 150 min when the oxidant supplied for the reaction was close to the stoichiometric amount for phenol oxidation. When the oxidant supplied for the reaction was almost twice the stoichiometric amount, complete phenol conversion was achieved in only 40 min. As expected, lower conversion and a shorter induction period (at a peroxide concentration of 0.008 mol dm-3) were recorded in the reaction carried out with a substoichiometric amount of hydrogen peroxide (Figure 8). It is also important to mention that when using hydrogen peroxide rather than oxygen as the oxidant, complete phenol oxidation can be performed at atmospheric pressure and at 343 K; i.e., the reaction can be performed under mild conditions. During oxidation, the starting phenol is known to tend to convert to hydroquinone and catechol, which then readily oxidize to o- and p-benzoquinone. The latter hypothesis was supported during the reaction progress by a color change of our reaction mixture, from clear to brownish. The brownish color was due to the formation of p-benzoquinone as an intermediate product, which agreed with others.5,16,25 According to the reaction pathway proposed by Masende et al.,16 oxidation of phenol generated p-benzoquinone, which further reacted to maleic acid (C-4), then to carboxylic acid (C-2), and finally to the end product. It is a known fact that polymerization is likely to occur once p-benzoquinone is formed. In our case, however, the reused catalyst exhibited practically equal activity, meaning that p-benzoquinone probably reacted to maleic acid. Conclusion The present work describes wet oxidation of aqueous solutions of phenol with hydrogen peroxide using heterogeneous catalysts under mild conditions. At a peroxide concentration of almost twice the stoichiometric amount for phenol oxidation, the CuY-5 catalyst enabled total phenol oxidation in only 40 min. In addition, CuY-5 could be reused in successive runs, without significant loss of activity. A more detailed study of phenol oxidation is being performed to detect intermediate products and to improve the operating conditions by process optimization. This will be presented in the next paper. Notation CHP ) hydrogen peroxide concentration (mol dm-3) CPh ) phenol concentration (mol dm-3) EA ) activation energy (kJ mol-1) k1 ) reaction rate constant for phenol oxidation (dm3 mol-1 min-1) k2 ) reaction rate constant for hydrogen peroxide decomposition (min-1) n ) reaction order with respect to phenol t ) time (min) T ) temperature (K) wcat. ) mass of catalyst (g) XA ) phenol conversion YCu ) concentration of catalytic active sites (wt %)

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Received for review September 1, 2004 Revised manuscript received January 12, 2005 Accepted January 14, 2005 IE049182M