Study on the removal of phenol from aqueous solution by peroxide oxidation over pillar clays

Abstract. In this work, from Bent-CD, refined bentonite (Bent-TC) was prepared using the wet method and the pillared bentonites Bent-Al, Bent-Al/Fe and Bent-Al/Cu were synthesized by the intercalation of Al, Al/Fe and Al/Cu polycation pillars into the interlayer channels of bentonite solid, respectively. The catalytic activity of these pillared bentonites was studied in oxidation reaction of phenol with hydrogen peroxide (H2O2) in aqueous solution at 40◦C and atmospheric pressure. The obtained experimental results revealed that Bent-Al/Cu was the best catalyst. It can be interpreted that Bent-Al/Cu possesses large basal spacing (d001) and Cu likely is a good catalyst promoter for Al in order to generate more OH radials in that oxidation reaction.

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JOURNAL OF SCIENCE OF HNUE Chemical and Biological Sci., 2012, Vol. 57, No. 8, pp. 22-27 This paper is available online at STUDY ON THE REMOVAL OF PHENOL FROM AQUEOUS SOLUTION BY PEROXIDE OXIDATION OVER PILLAR CLAYS Le Minh Cam, Vu Thi Thiet, Nguyen Minh Tuan and Le Van Khu Faculty of Chemistry, Ha Noi National University of Education Abstract. In this work, from Bent-CD, refined bentonite (Bent-TC) was prepared using the wet method and the pillared bentonites Bent-Al, Bent-Al/Fe and Bent-Al/Cu were synthesized by the intercalation of Al, Al/Fe and Al/Cu polycation pillars into the interlayer channels of bentonite solid, respectively. The catalytic activity of these pillared bentonites was studied in oxidation reaction of phenol with hydrogen peroxide (H2O2) in aqueous solution at 40 ◦C and atmospheric pressure. The obtained experimental results revealed that Bent-Al/Cu was the best catalyst. It can be interpreted that Bent-Al/Cu possesses large basal spacing (d001) and Cu likely is a good catalyst promoter for Al in order to generate more OH radials in that oxidation reaction. Keywords: Al(Fe)-pillar clay, peroxide, phenol oxidation. 1. Introduction Water pollution is truly an urgently global problem. Among the numerous pollutants, phenols are a particularly serious problem because they are so widespread in the environment and because in an aqueous media they are highly toxicity to most organisms. Several technologies have been developed to remove organic contaminating compounds from waste-water, making use of adsorption, biological treatment and wet oxidation (WO). However, of these methods, only the WO processes show sufficient viability and efficiency for treating organic contaminants in dilute aqueous solution. Unfortunately, the cost of treatment in industrial applications is relatively high due to the need for high processing temperature and pressure. The use of an appropriate catalyst for WO (catalytic wet oxidation CWO) allows improvement of the oxidation ability of WO processes [1, 2]. In CWO, hydrogen peroxide is commonly used as the oxidizing agent because it can generate OH radicals which have the best oxidation power in the liquid oxidation reactions. Among the solid catalysts used for CWO reaction pillared clays seem Received June 4, 2012. Accepted September 25, 2012. Contact Le Minh Cam, e-mail address: leminhcamsp@yahoo.com 22 Study on the removal of phenol from aqueous solution by peroxide oxidation over pillar clays to stand out due to their versatility in the synthesis to obtain solids having adequate pore size and appropriate polycation pillars, i.e. to generate active and stable heterogeneous catalysts in aqueous media [3]. In this work, catalysts were synthesized from bentonite clays intercalated by Al, Al-Fe and Al-Cu polycation pillars. Over these catalysts the modified Fenton process can be performed to completely oxidize organic molecules under mild conditions and without considerable leaching of catalytically active cations. The remarkable performance of catalysts investigated in this work would highlight the importance of Al (Fe, Cu) pillared bentonites as efficient catalysts in oxidation reactions of organic pollutant. 2. Content 2.1. Experiment 2.1.1. Pillared bentonite synthesis Refined bentonite (Bent-TC) was prepared from natural Co Dinh bentonite (Bent-CD) obtained in the Co Dinh region (Thanh Hoa Province, Vietnam) using the wet method [4]. Pillared bentonites were prepared as follows: * Bent-Al (Bentonite pillared by Al polycations) Bent-Al was prepared by the slow addition of 0.4M NaOH solution to 0.4M AlCl3 solution undergoing vigorous stirring until the molar ratio of OH-/Al3+ reached 2.0, followed by aging for 10 days at room temperature. Al polyoxocations were formed in the solution according to the equation: 13AlCl3 + 32NaOH→ Al13O4(OH)24Cl7 +32NaCl + 4H2O Four grams of Na-bentonite was placed in suspension in 100 ml of distilled water. The Al polyoxocation solution was added slowly to the Na-bentonite suspension, and the mixture was stirred for 2 hours at 70◦C in a water bath. The solid was filtered out of the suspension and thoroughly washed with distilled water. After drying at 50 - 60◦C for 12 hours, the resultant sample was calcined at 400◦C for 3 hours with a heat lamp at 3◦C /min to obtain the Bent-Al. * Bent-Al/Fe and Bent-Al/Cu (Bentonite pillared by Al-Fe and Al-Cu polycations respectively) Bent-Al/Fe and Bent-Al/Cu were prepared by adding the Fe (or Cu) polyoxocation solution to the Al-polyoxocation solution and stirring for 2 hours at room temperature to obtain a Fe (Cu)/Al ratio of 0.33. The resultant solution was aged in the same way as for the Al-pillaring solution. The bentonite pillared by Al-Fe polycations (Bent-Al/Fe) and Al-Cu polycations (Bent-Al/Cu) were subsequently prepared in the way similar to that of Bent-Al. * Bent-Al-Cu-imp ( Bent-Al impregnated with Cu2+ cation solution) Bent-Al-Cu-imp was separated using the impregnated method as follows: an 23 Le Minh Cam, Vu Thi Thiet, Nguyen Minh Tuan and Le Van Khu appropriate amount of Cu(NO3)2 was dissolved in 10 mL of distilled water. The Al-pillared bentonite was impregnated with freshly prepared solution. The resulting paste was then slowly evaporated till it was completely dried. The dried sample was powdered and calcined in open air for 6 hours at 400◦C to yield final catalyst: Bent-Al-Cu-imp. 2.1.2. Catalyst characterization Refined and pillared bentonite samples were characterized by the XRD technique, using a Philips Diffractometer, equipped with filtered Cu Kα radiation. The data were collected in the 2θ range of 2.5 - 8.5◦, with a step size of 0.02◦ and scan rate of 2◦/min. 2.1.3. Catalytic test Phenol oxidation in an aqueous solution was carried out in a 250 mL glass reactor, stirring at 40◦C and atmospheric pressure. In each test, the reactor was filled with 200 mL of 1 mg/L phenol solution and stirred for 30 min before adding 0.5 g of catalyst. Then, stirring continuous for another 5 min, a certain volume of hydrogen peroxide 30 % (wt) solution was continually added to the reactor with a flow rate of appropriately 0.6 mL/min. During the reaction process, after a certain period of time, liquid samples from the reactor were taken to determine phenol concentration. Conversion of phenol during reaction is calculated as follows α (%) = Cx100/C0 where α (%): conversion of phenol; C0: initial concentration of phenol (mg/L); C: concentration of phenol at time t (mg/L). Phenol concentration was determined using a LIUV-310S UV-Vis Spectrophotometer at λ = 270 nm. At the end of each reaction run, the amount of iron, copper and aluminium cations that leached from solid catalysts was also measured using the AAS method. 2.2. Result and discussion 2.2.1. Pillared bentonites Figure 1 presents the XRD patterns of the study samples Bent-TC, Bent-Al, Bent-Al/Cu and Bent-Al/Fe. It is well-known [3] that an XRD peak with 2θ < 100 is characteristic of a mesopore presence in solid layered materials and values of d001 indicate the dimension of channels (pores) between the aluminosilicate 2:1 layers in bentonite structure. From Figure 1, it is noted that due to pillaring Al, Al-Cu, and Cu-Fe polycations, the distance between interlayered channels of pillared bentonite clays increases from 13.97 A˚ (Bent- TC) to 20.17 A˚ (Bent-Al/Fe). This increase may provide favorable conditions for the mass-transfer of reactants in a heterogeneously catalytic reaction system. 24 Study on the removal of phenol from aqueous solution by peroxide oxidation over pillar clays Figure 1. XRD pattern of (a) Bent-TC, (b) Bent-Al, (c) Bent-Al/Cu and (c) Bent-Al/Fe 2.2.2. Phenol oxidation The catalytic activity of pillared bentonites was investigated in the phenol oxidation reaction with hydrogen peroxide (H2O2). The reaction occurs through the following equation C6H5OH + 14H2O2 → CO2+ 17H2O The obtained results are shown in Table 1. Table 1. Conversion (%) of phenol oxidation over different catalysts Time (minute) Samples 30 60 75 100 130 150 200 260 300 1400 Bent-TC 13.38 7.80 7.88 23.10 22.66 25.56 23.37 - 21.14 83.54 Bent-Al 16.83 18.38 - - 22.12 - - 23.86 - 97.10 Bent-Al/Fe - 17.94 19.44 - 21.18 - 21.77 - - 96.87 Bent-Al/Cu - 14.00 34.18 87.00 - 91.72∗ - - - 97.15 Bent-Al-Cu imp. 2.17 8.20∗∗ - - - 54.13∗ 90.16 92.34 - 96.26 Note. ∗: data at 160 min and ∗∗: data at 45 min. Table 1 shows that the phenol oxidation reaction with H2O2 under given conditions reached equilibrium at 1400 minutes (∼ 23 hours). In equilibrium, phenol conversion over all pillared bentonite catalysts is practically the same and equal to ∼ 97 % (Figure 2). It is demonstrated that Al, Al-Fe and Al-Cu polycation pillars dispersed on the internal or external surface of bentonite may catalyze phenol oxidation reaction. But, from data of Table 1 and Figure 3, it may be seen that the reaction rate of phenol oxidations over these catalysts in rather different. 25 Le Minh Cam, Vu Thi Thiet, Nguyen Minh Tuan and Le Van Khu Figure 2a. Catalytic activity of modified Figure 2b. Phenol conversion (%) clays in the phenol oxidation reaction at 1400 minutes Figure 3. Phenol conversion over catalysts Figure 4. The effect of Cu from 0 to 300 minutes in catalytic activity of modified clays Figure 4 indicates that after about 200 minutes, phenol conversion over a Bent-Al/Cu sample can reach 97% and over Bent-Al-Cu-imp it is up to 90%. This means that Bent-Al/Cu and Bent-Al-Cu-imp samples possess more active sites for reaction. H2O2 → OH Clearly, the more OH radicals are formed, the more phenol conversion occurs. Thus, in this case Cu is probably a good catalytic promoter for Al to improve catalytic activity of Al pillared bentonite catalysts. Based on phenol conversion (%) at 200 minutes (Figures 3 and 4), we can predict propensity for catalytic activity as follows: Bent-Al/Cu > Bent-Al-Cu-Imp > Bent-Al Bent-Al/Fe > Bent-Tc Note that Bent-Al/Fe has the largest d001 (20.10 A˚, from Figure 1). However, the catalytic activity of the Bent-Al/Fe sample is not higher than that of the Bent-Al sample. So, in this reaction the role and the nature of catalytic sites seem to be more important than the dimension of the pores (channels) of the material. Indeed, the phenol molecule (∼6 A˚) is smaller than d001 (14-20 A˚), that of pillared bentonites, and consequently the transfer of H2O2 and C6H5OH molecules within catalyst micropore is not difficult. 26 Study on the removal of phenol from aqueous solution by peroxide oxidation over pillar clays From Table 1 and Figures 2a and 2b we can see that the Bent-TC sample also catalyzes phenol oxidation by H2O2 although the amount of conversion is insignificant. This can be interpreted as an influence of metal (Cu, Fe, Ti) impurities in Bent-TC and a possible role that they play regarding catalytic sites for that reaction. From Figures 3 and 4, it is noted that the catalytic activity of the Bent-Al-Cu-imp sample is lower than that of Bent-Al/Cu. The reason for this is that with the impregnation method, Cu2+ ions do not make contact with Al3+ ions as well as they do with the pillaring method. Indeed, the impregnation method can disperse some Cu2+ ions at the external surface of bentonite causing there to be fewer catalytic sites (Al-Cu) than there would be with the pillaring method. 3. Conclusion This work has been a preliminary investigation into the catalytic behavior of pillared bentonite clays when acted upon by Al, Al-Fe and Al-Cu polycations (polyoxides) causing a phenol oxidation reaction by hydrogen peroxide. Some conclusions obtained from this work are as follows: 1-Refined bentonite (Bent-TC) from Co Dinh (Thanh Hoa Province, Vietnam) was used in synthesizing four pillared bentonite catalysts by the intercatalation of single and mixed oxide pillars of aluminium and iron (or copper): Bent-Al, Bent-Al/Fe , Bent-Al/Cu and Bent-Al-Cu-imp. The basal spacing dimention of pillared beutonites was increased revealing the success of catalyst synthesis as intended. 2-Phenol oxidation by H2O2 at low temperature was performed in five samples: Bent-TC, Bent-Al, Bent-Al/Fe, Bent-Al/Cu and Bent-Al-Cu-imp. The results showed that Cu is a good promoter for Al as a Bent-Al catalyst. Bent-Al/Cu is the best catalyst to effect the oxidation of phenol by H2O2 under the conditions given in this work. The relative strength of catalytic activity of the samples is as follows: Bent-Al/Cu > Bent-Al/Cu-imp > Bent-Al ∼ Bent-Al/Fe > Bent -TC 3-From the obtained results it can be suggested that pillared bentonite catalysts containing Al-Cu pillars could be promising catalysts for phenol oxidation by H2O2. REFERENCES [1] R. M. Liou, S. H. Chen, J. Hazar., 2009. Mater. 172, 498. [2] M. E. S. Ojeda, A. Fabregat, F. Stuber, 2007. Chem. Eng. J. 132, 105. [3] J. Carriazo, E. Guelou, J. Barrault, J. M. Tatibouet, R. Molina, S. Moreno,. 2005. Water Res. 39, 3981. [4] James O.O et al., 2008. Journal of North Carolina Academy of Science 124(4), 154. 27
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