Novel Fe(III)-Al2O3 nanocomposite as Fenton-like catalyst for the CWHPO of phenolic wastewaters

CARLA DI LUCA; FERNANDO IVORRA; PAOLA MASSA; ROSA FENOGLIO

Barcelona

Congreso; 13th Mediterranean Congress of Chemical Engineering (13MCCE); 2014

SEQUI

Advanced Oxidation Processes (AOP?s) are aqueous phase oxidation methods based on the intermediacy of hydroxyl radicals, leading to the abatement of organic pollutants at mild reaction conditions. Among different AOP?s technologies, one interesting alternative is the Catalytic Wet Hydrogen Peroxide Oxidation (CWHPO). The so-called heterogeneous Fenton-like systems use a solid catalyst in the presence of hydrogen peroxide as oxidizing agent. These solid catalysts consist in transition metal species (primarily iron, but not exclusively) immobilized over different supports. One of the most relevant drawbacks for the practical implementation of these systems is the catalyst deactivation due to active species lixiviation into the acidic reaction medium. Among non-conventional methods of catalysts preparation, direct incorporation of active components during the synthesis of a porous matrix could provide a useful strategy to improve both active sites dispersion and metal-support interactions. In recent studies, it was demonstrated that direct inclusion of iron during the synthesis of SBA-15 by co-condensation route resulted in an enhancement of catalyst stability when compared with samples prepared by conventional impregnation [1]. The goal of the present work is to study the addition of ferric species during the synthesis of alumina through a sol?gel process by evaporation-induced self-assembly (EISA) and to investigate these catalytic materials in the CWHPO of phenol as model pollutant. The catalysts were synthesized from an adaptation of the experimental procedure reported by Morris et al. [2]. Aluminum isopropoxide and Fe(NO3)3.9H2O or FeCl3?6H2O were used as metallic precursors ([Fe+3]:[Al+3]=0.064) in a ethanolic solution of a triblock copolymer (Pluronic P123) using an acid catalyst (HNO3 or HCl, depending of the iron source nature). The molar ratios [Al+3]:[P123]:[EtOH]:[H+] in the final solution were fixed at 1:0.017: 34.4:2.5. The homogeneous sol was aging for three days at 40 ºC and then calcined at 400 ºC (1 ºC/min, 4 h). Post-calcinations at 700 ºC and 900 ºC were performed in a stepwise manner. After the thermal treatment at 400ºC, all organic residues from previous preparation steps were removed from samples (as confirmed by TGA). At this temperature, no crystalline phases were detected by XRD; however, the presence of γ-Al2O3 phase was registered for the samples calcined at 700 ºC and 900 ºC. The physisorption measurements revealed a high BET surface area (~ 300 m2/g) and TEM analysis showed the presence of a partially ordered structure with the formation of narrow channels in the nanometric range (~ 4 nm). XPS and Mössbauer measurements exposed the presence of Fe+3 ions inserted into the Al2O3 matrix, which is consistent with the methodology of preparation adopted. CWHPO of phenolic solutions (1 g/L) were performed in a batch reactor at 70 ºC for 6 h, using 1 g/L of catalyst and stoichiometric dosage of H2O2. The experimental set-up used was the same as reported elsewhere [3]. Preliminary reaction runs were carried out with powdered samples synthetized from ferric nitrate or ferric chloride, and calcined at different temperatures (400, 700 and 900 ºC). All Fe(III)-Al2O3 materials showed different time-evolution in the Phenol, Total Organic Carbon (TOC) and H2O2 conversion levels and pH. After 3 h, all the catalysts allowed the total abatement of phenol and at final time, the TOC6h resulted c.a. 80%, with a leached iron concentration below 6 ppm; except for the sample prepared from Fe(III)-nitrate and calcined at 900ºC (TOC6h = 57%; Felix = 12 ppm). The amount of leached iron in the final supernatant was directly related with the acidity of the reaction medium due to the accumulation of acids intermediates from phenol incomplete mineralization. The optimal catalytic properties were achieved with the catalyst prepared from ferric chloride and calcined at 700 ºC; the TOC3h resulted of 80% and the final leached iron was 1.9 ppm (~ 4% of the total initial Fe load), maintaining a constant pH ~ 3 during the last five hours of the reaction run. From these results, we concluded that Fe(III)-Al2O3 nanocomposites prepared by EISA methodology showed good structural properties and a promising catalytic performance (activity and stability) for the mineralization of organic wastewaters.