Wet Peroxide Oxidation of Phenolic Solutions over Different Iron

A series of iron-containing zeolitic materials, prepared by different procedures, have been tested as heterogeneous catalysts for the oxidation of phe...
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Ind. Eng. Chem. Res. 2001, 40, 3921-3928

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Wet Peroxide Oxidation of Phenolic Solutions over Different Iron-Containing Zeolitic Materials Gabriel Ovejero,* Jose L. Sotelo, Fernando Martı´nez, Juan A. Melero, and Luis Gordo Chemical Engineering Department, Faculty of Chemistry, Complutense University, Madrid 28040, Spain

A series of iron-containing zeolitic materials, prepared by different procedures, have been tested as heterogeneous catalysts for the oxidation of phenolic solutions with hydrogen peroxide under mild conditions. Fe-TS-1 catalysts were synthesized through hydrothermal crystallization of wetness-impregnated Fe2O3-TiO2-SiO2 xerogels. The iron content within the raw xerogel was optimized with the aim of minimizing the degree of iron leaching into the solution. For the purpose of comparison, Fe-silicalite and Fe-ZSM-5 zeolites free of Ti species, synthesized through hydrothermal crystallization of basic hydrogels and Fe-NaY, Fe-USY, and Fe-ZSM-5 materials prepared by an ion-exchange procedure, were also tested. Heterogeneous catalytic systems showed a better catalytic performance than homogeneous systems based on dissolved Fe3+ salts. Moreover, Fe-TS-1 zeolite with a moderate Fe content (Si-Fe molar ratio of 76) showed the best results in terms of catalytic activity and loss of active species into the aqueous solutions. Finally, the stability of Fe species has been shown to be strongly dependent on the Fe environment into the zeolitic framework, the synthetic route, and the temperature of the treatment. 1. Introduction Many wastewater streams coming from industrial processes contain a high concentration of organic pollutants, which must be removed to comply with the environmental regulations. Phenol is one of the most important of these pollutants because of its high toxicity and poor biodegradability. During past decades, serious environmental concerns have led to extensive research into the removal of harmful organic compounds from industrial wastewater streams.1,2 Conventional treatments based on thermal destruction and chemical or biological methods have limitations in applicability, effectiveness, and cost in the treatment of wastewater streams. In this context, particularly attractive are oxidizing treatments through which the contaminant substance is converted into CO2 and water. These treatments involve ozonation,3 wet air oxidation (WAO),4,5 wet peroxide oxidation (WPO),6-8 and supercritical water oxidation (SCWO).9 Although each procedure has its own drawbacks, WPO processes using hydrogen peroxide as the oxidant have emerged as a viable alternative for the wastewater treatments of medium-high total organic carbon (TOC) concentrations.6 Hydrogen peroxide does not form any harmful byproducts, and it is a nontoxic and ecological reactant. Likewise, although hydrogen peroxide is a relatively costly reactant, the operating costs of WPO processes are overcome by the lower fixed costs as compared with ozonation, WAO, and oxidation in supercritical media. However, to enhance the decomposition of hydrogen peroxide to produce hydroxyl radicals and minimize their decomposition to water, the use of a catalytic system is desirable. These processes of catalytic wet oxidation (CWO) have been a significant subject of * To whom correspondence should be addressed. Phone: 3491-3944111. Fax: 34-91-3944114. E-mail: govejero@eucmos. sim.ucm.es.

research during the past decade.1,10-14 Furthermore, these kinds of processes are particularly attractive because of their lower energy requirements and higher selectivity toward biodegradable compounds as compared with those in the absence of catalyst. First attempts of these advanced oxidation processes were carried out under homogeneous catalytic systems. In this sense, Fenton-type reactions using metallic salts as catalysts have received special interest.15,16 The major weakness of these Fenton-type homogeneous catalytic systems is the tight pH control as well as the production of additional toxic wastes, which need to be treated. For these reasons, there has been a considerable interest in the development of heterogeneous catalysts for the oxidation of wastewater streams. Unlike the homogeneous systems, these solid catalysts could be recuperated by means of a simple separation operation and reused in the next runs. In this sense, one of the principal goals of these types of processes is the development of stable heterogeneous catalysts with minimal leaching of active species under the reaction conditions. The pioneering heterogeneous catalysts used for the abatement of water pollutants consisted of a variety of solids, including metal oxides (pure or mixtures) of Cu, Mn, Co, Cr, V, Ti, Bi, and Zn, as well as noble metals such as Ru, Pt, and Pd.1 Recently, interesting works have been published dealing with the use of zeolites containing active species for the oxidative treatment of phenolic aqueous solutions. For example, iron-containing zeolites have evidenced high catalytic activity in the presence of H2O2 for the removal of phenol17,18 and other refractory organic compounds.19,20 These studies have shown a significant dependence of the synthetic route on the activity and stability of the catalysts. In most studies described in the literature dealing with the use of zeolites containing active species for CWO processes, the metal has been supported onto the zeolite through current impregnation methods8 or by ion exchange of the protonated zeolite with the correspond-

10.1021/ie000896g CCC: $20.00 © 2001 American Chemical Society Published on Web 08/03/2001

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ing salt.20 However, few works describe the use of zeolitic materials containing metal active species tetrahedrally coordinated into the zeolitic framework for catalytic abatement of water pollutants. A direct synthesis route would allow metal species to be more strongly bonded to the zeolite framework and possibly less readily subjected to extensive leaching during catalytic treatment. In this sense, Valange et al.21 have studied the preparation of MFI zeolitic materials containing copper by direct crystallization of silica gels admixed with the corresponding metallic salts and various mineralizing agents. These materials have shown a negligible leaching of active species into the aqueous phase for phenol removal at atmospheric pressure and 20 °C, although a low activity was clearly observed (78% phenol conversion). Fe-ZSM-5 zeolite has evidenced a remarkable catalytic activity for phenol elimination by H2O2 in an aqueous medium.17,18 Additionally, the incorporation of Ti atoms in framework positions of zeolites has received much attention in the past decade because the obtained materials have shown to be efficient selective oxidation catalysts.22 In this context, we have considered it interesting to study the performance of zeolites containing simultaneously Ti and Fe atoms tetrahedrally coordinated into the MFI zeolitic framework (denoted as Fe-TS-1) for the treatment of phenolic aqueous solutions. On the other hand, because the stability of the active sites in zeolites is extremely dependent on the synthesis procedure, the other objective of the present work was to study the catalytic WPO of phenolic solutions using different iron-containing zeolites under mild conditions. These materials were prepared through several synthetical procedures: metal ion exchange over zeolitic materials (Na-Y, USY, and ZSM-5 zeolites) and tetrahedral incorporation of the iron atoms into the zeolitic MFI framework through hydrothermal synthesis (Fesilicalite, Fe-TS-1, and Fe-ZSM-5). The catalytic performance of the different materials has been evaluated on the basis of the leaching degree of iron species into the aqueous solutions as well as their activity for phenol removal and TOC reduction. 2. Experimental Section 2.1. Catalyst Preparation. 2.1.1. Fe-TS-1 and TS-1 Samples. Fe-TS-1 samples were synthesized by hydrothermal crystallization of wetness impregnated Fe2O3-TiO2-SiO2 amorphous xerogels with a 20 wt % aqueous solution of tetrapropylammonium hydroxide (TPAOH; Alfa) at 170 °C for 24 h under autogenous pressure. Raw mixed oxides of Fe2O3-TiO2-SiO2 were prepared following a two-step sol-gel process described elsewhere23 using tetraethyl orthosilicate (TEOS; Alfa), titanium butoxide (TNBT; Alfa), and iron(III) sulfate pentahydrate (Fe2(SO4)3‚5H2O; Aldrich) as source materials. Amorphous xerogels were prepared with a SiO2TiO2 molar ratio of 40 and different SiO2-Fe2O3 molar ratios. The TS-1 sample was synthesized in a manner similar to that of Fe-TS-1 samples described above, starting from wetness-impregnated TiO2-SiO2 amorphous xerogels with a SiO2-TiO2 molar ratio of 40.24 As-synthesized materials were calcined for 5 h at 550 °C under an air atmosphere. Table 1 shows the chemical composition of the initial xerogels and Fe-TS-1 and TS-1 zeolites as well as their textural properties. 2.1.2. Fe-Silicalite and Fe-ZSM-5 Samples. Fesilicalite and Fe-ZSM-5 samples were synthesized by

Table 1. Physicochemical Properties of Fe-Containing Zeolitic Materials Synthesized through Direct Synthesis Ti/Fe Zeolites initial mixture zeolite composition composition type of catalyst Fe-TS-1 (1) (2) (3) TS-1

Si/Ti

Si/Fe

40 40 40 40

155 80 20 ∞

textural properties iron content SBET Vmicrop Si/Ti Si/Fe (wt %) (m2/g) (cm3/g) 60 62 43 49

141 76 19 ∞

0.64 1.18 4.43 0.00

518 497 443 510

0.18 0.17 0.15 0.19

Al/Fe Zeolites initial mixture zeolite composition composition type of catalyst

Si/Al

Si/Fe

Fe-silicalite Fe-ZSM-5

∞ 20

30 30

textural properties iron content SBET Vmicrop Si/Al Si/Fe (wt %) (m2/g) (cm3/g) ∞ 18

23 33

3.64 2.48

Table 2. Molar Composition of the Zeolitic Materials Synthesized by an Ion-Exchange Procedure exchanged zeolite type of catalyst

unexchanged zeolite Si/Al molar ratio

iron content (wt %)

efficiency of exchange (%)a

Fe-NaY Fe-USY Fe-ZSM-5

2.13 2.92 37.4

1.29 1.08 0.33

5.3 4.5 25.1

a

(Fe/Al molar ratio) × 100.

hydrothermal crystallization of basic hydrogels containing Si, Fe, and Al species at 170 °C for 24 h under autogenous pressure following a procedure developed by Van Grieken et al.25 The initial mixtures of synthesis had the molar compositions of 1:60:20.9:685 Fe2O3SiO2-TPAOH-H2O and 1:40:1.5:31.4:1190 Fe2O3SiO2-Al2O3-TPAOH-H2O. The materials after synthesis were calcined for 5 h at 550 °C under an air atmosphere. Table 1 summarizes the chemical compositions of the initial hydrogel and final zeolite. 2.1.3. Samples Prepared by Ion Exchange with Metallic Salts. Fe-ZSM-5, Fe-NaY, and Fe-USY zeolites were prepared by ion exchange starting from the acidic-sodium form of the corresponding zeolites (Table 2). ZSM-5 and USY zeolites (3 g) were exchanged in 0.2 L of 0.002 M iron(III) chloride solutions at 70 °C during 24 and 2 h, respectively. However, NaY zeolite was exchanged at 25 °C for 2 h. The corresponding Feexchanged zeolites were washed with redistilled water three times and dried at 110 °C overnight. Table 2 summarizes the molar composition of the parent zeolite and the iron content incorporated after exchange treatment. 2.2. Characterization Techniques. Characterization of the samples was performed by different conventional techniques. Chemical analyses were performed by X-ray fluorescence (XRF) with a Philips PW 1480 spectrometer. X-ray diffraction (XRD) patterns were collected with a Philips X’PERT diffractometer with Cu KR radiation. Crystallinity was determined from the peak area at 2θ ) 22-25° using a highly crystalline TS-1 sample as the reference. Diffuse reflectance UVvis spectra (DR UV-vis) were obtained under ambient conditions on a Cary-1 spectrophotometer equipped with a diffuse reflectance accessory. 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of powdered samples were recorded at 78.14 MHz in a Varian VXR-300, with a spinning frequency of 4000 kHz and time intervals of 5 s between successive accumula-

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tions. Measurements were made at room temperature with [Al(H2O)6]3+ as the external standard reference with accumulations amounting to 400. Nitrogen adsorption-desorption isotherms were measured at 77 K using a Micromeritics ASAP 2000. The materials prior to the adsorption measurement were outgassed at 300 °C for 12 h. The surface area was estimated according to the BET method, and the micropore volume was calculated using the t-plot method. 2.3. Catalytic Tests. CWO experiments in the presence of hydrogen peroxide were carried out in a 200 cm3 Teflon-lined stainless steel batch reactor under continuous mechanical stirring (300 rpm) in contact with an air atmosphere. Such equipment eliminates the possible catalytic effect of the steel wall surfaces. All of the reactants, except hydrogen peroxide, and the catalysts were placed into the Teflon-lined reactor. Thereafter, the system was pressurized to 1 MPa with air and heated to the corresponding temperature with a heating rate of 3 °C/min. Once the temperature reached the set-point value, the oxidant was added completely. The evolution of the reaction was monitored along the time. Typically, 140 cm3 of a 1 g/L phenol aqueous solution (TOC ) 765 ppm; pH ) 5.6) and 0.6 g/L of catalyst were used with the stoichiometric amount of hydrogen peroxide (ca. 5 g/L) for complete mineralization of phenol according to the reaction

C6H5OH + 14H2O2 f 6CO2 + 17H2O Phenol removal was quantified by gas chromatography in a Varian Star 3400 CX chromatograph using a capillary column SPB-1 (60 m × 0.25 mm) and a flame ionization detector. Quantitative analysis of products coming from incomplete mineralization of phenol was performed by means of a high-performance liquid chromatography (HPLC) technique. HPLC chromatograph model Varian Prostar was equipped with a Spherisorb ODS column (250 × 4.6 mm) and an UV detector at 210 nm. A mixture of methanol (20% by volume) and water (80% by volume) buffered at a pH of 2.6 was used as the mobile phase. The TOC contents of the solutions before and after the reaction were analyzed using a combustion/nondispersive infrared gas analyzer model Rosemount Dohrmann DC-190. The peroxide concentration after reaction was evaluated by iodometric titration, except those solutions showing a brown color, where HPLC chromatography was used to estimate the oxidant conversion. 2.4. Leaching Tests. The aqueous solution after the reaction was cooled to 60 °C and filtered at this temperature to prevent the possible readsorption of active species. Thereafter, the content of the active metal in the filtered solution was measured by atomic emission spectroscopy with induced coupled plasma (ICP-AES). 2.5. Stability Tests. The stability of the catalysts was analyzed as follows: solid was recovered from the solution by hot filtration after the reaction, calcined at 550 °C for 5 h in order to remove any organic species remaining adsorbed, and finally tested again at the same reaction conditions. This procedure was repeated twice. 3. Results and Discussion 3.1. Preliminary Experiments. 3.1.1. Hydrogen Peroxide Conversion in the Absence of Phenol.

Figure 1. Blank experiments. Reaction conditions: initial TOC[Ph-OH] ) 765 ppm; temperature ) 100 °C; air pressure ) 1 MPa; stoichiometric amount of H2O2; catalyst concentration ) 0.6 g/L. (0) B-1, in the presence of air without catalyst and oxidant. (O) B-2, in the presence of oxidant without catalyst. (4) B-3, in the presence of catalyst without oxidant.

The oxidizing properties of hydrogen peroxide are wellknown for the catalytic or not wet oxidation of phenol.26 These oxidation processes in the liquid phase are promoted by the generation of different radical species such as organic hydroperoxyl (ROO•), hydroxyl (OH•), and hydroperoxyl (HO2•), whose formation rate is significantly enhanced in the presence of metallic species.12,26 To investigate this issue, two experiments were carried out to observe the H2O2 decomposition in the presence or absence of catalyst. In the first experiment 30 mg of an Fe-TS-1 zeolite (Si-Fe molar ratio of 76) and 50 mL of redistilled water free of phenol were charged into the Teflon-lined reactor at 1 MPa of air pressure. Once the temperature reached up to 100 °C, the oxidant was fed into the reactor (H2O2-catalyst mass ratio of 8.5). After just 20 min of reaction, an oxidant conversion of ca. 90% was measured. The second experiment performed under similar conditions but in the absence of catalyst led to an oxidant conversion of ca. 15% after 30 min. These preliminary experiments indicate the effectiveness of Fe species to promote H2O2 conversion for the formation of free radicals in contrast with the slower thermal decomposition favored in the absence of a catalytic system. 3.1.2. Blank Reactions. A first kinetic run was performed in the presence of an aqueous solution of phenol (TOC ) 765 ppm, pH ) 5.6) and an air atmosphere but in the absence of either catalyst or H2O2. The results after 2 h of reaction showed a low phenol conversion and a negligible TOC removal (B-1 in Figure 1). An additional kinetic experiment was carried out under experimental conditions similar to those used in run B-1 but in the presence of H2O2 (B-2 in Figure 1). The data demonstrate the high oxidizing power of hydrogen peroxide even in the absence of a catalytic system. Furthermore, these results also indicate the higher oxidizing activity of H2O2 in comparison

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Table 3. Relative Standard Deviation of Reaction Parameters for WPO of Phenolic Solutions over Fe-TS-1(2) runa

XTOC (%)b

XPhOH (%)c

Fe leached (ppm)d

pHe

A B C average value

66.0 66.4 63.8 65.4 ( 1.1

100 100 100 100 ( 0.0

1.30 1.42 1.27 1.33 ( 0.06

2.76 2.64 2.81 2.73 ( 0.07

a Reaction conditions: initial TOC [Ph-OH] ) 765 ppm; stoichiometric amount of H2O2 for phenol mineralization; catalyst concentration ) 0.6 g/L; temperature ) 100 °C; air pressure ) 1 MPa; reaction time ) 120 min. b TOC conversion defined as (initial TOC - final TOC)/initial TOC × 100. c Phenol conversion defined as (initial phenol - final phenol)/initial phenol × 100. d ppm of Fe leached into the aqueous solution after 2 h. e Final pH after reaction.

to oxygen from the air. Finally, an oxidation experiment was performed in the presence of a catalysts (Fe-TS1(2)) in an air atmosphere but in the absence of H2O2 (B-3 in Figure 1), with the rest of the experimental conditions being analogous to previous experiments. From the results displayed in Figure 1, it can be clearly inferred that the presence of Fe does not promote the formation of radical species in the presence of oxygen. However, the low iron concentration into the aqueous solution under this particular condition after 2 h of reaction must be pointed out (0.1 ppm). The pH of the aqueous solutions after the reaction was around 4 for B-1 and B-3 experiments and 3 for B-2. These results show a higher contribution of carboxylic acids into the aqueous solution for the run carried out in the presence of H2O2, which is in fairly good agreement with its catalytic performance. 3.1.3. Repeatability of Experiments. To check the repeatability of the results obtained with our reaction system, a similar run has been carried out three times over Fe-TS-1(2) zeolite. The reaction parameters obtained (TOC, phenol conversion, and pH) for each run are shown in Table 3. A low relative standard deviation for the different reaction parameters can be seen, evidencing the accuracy of the results shown in this contribution. 3.2. Catalytic Properties of the Fe-TS-1 Zeolitic Material for the WPO of Phenolic Solutions. Three Ti-containing zeolitic materials with MFI structure and different contents of Fe species have been synthesized as described in the Experimental Section and tested on the CWO of phenolic solutions in the presence of H2O2. These zeolites were tested with the objective of finding high active materials in terms of phenol removal and TOC reduction accompanied with a minimum leaching of iron species into the solution. Figure 2 illustrates the XRD spectra of the calcined Fe-TS-1 zeolitic materials. The three samples show the typical pattern corresponding to a MFI structure. However, a slight loss of crystallinity is observed in the final materials with an increase of the iron content in the initial xerogel, which might be attributed to the presence of amorphous phases. The chemical analyses of these samples are depicted in Table 1. These data show that the presence of Fe in the initial xerogel lowers the incorporation of Ti in the zeolitic framework, whereas Fe species seems to be completely incorporated independent of the initial amount into the xerogel. This trend has also been observed in previous works dealing with the synthesis of Al- and Ti-containing materials through wetness-impregnated mixed oxides.27

Figure 2. XRD and DR UV-vis spectra of the zeolitic materials: (a) TS-1; (b) Fe-TS-1(1); (c) Fe-TS-1(2); (d) Fe-TS-1(3); (e) Fesilicalite; (f) Fe-ZSM-5.

Kinetic runs of different Fe-TS-1 samples for the catalytic wet hydrogen peroxide oxidation of phenol in air at 100 °C are shown in Figure 3a. The results indicate that the activity of the different catalysts is closely related to the Fe content incorporated in the material. However, after 2 h of reaction, the three catalysts display similar TOC reduction with a complete absence of phenolic species and the presence of carboxylic acids such as maleic, acetic, formic, and oxalic as the main reaction byproducts. Furthermore, these results are achieved using just the stoichiometric amount of H2O2, in contrast with other works described in the literature, which used an excess of oxidant.14,17 Figure 3b illustrates the chemical composition of the reaction mixture after 12 min. It can be seen that the content of aromatic compounds decreases rapidly with the Fe

Ind. Eng. Chem. Res., Vol. 40, No. 18, 2001 3925 Table 4. Catalytic Properties of Different Fe-Containing Materials for WPO of Phenolic Solutionsa catalyst

XTOC XH2O2 (%)b (%)c

leaching test pHd [Fe]Catalyste [Fe]Dissolvedf % Fe

Fe-TS-1(1) Fe-TS-1(2) Fe-TS-1(3) Fe-silicalite Fe-ZSM-5

64.1 66.0 70.7 79.1 68.2

Direct Synthesis 100 2.89 3.8 100 2.76 7.1 100 3.08 26.2 100 3.21 21.8 3.11 14.8

0.6 1.3 11.1 9.4 14.1

15.7 18.3 42.4 43.1 95.3

Fe-NaY Fe-USY Fe-ZSM-5

78.5 67.3 54.5

Ion Exchange 95.7 4.74 7.6 4.67 6.4 2.3

2.1 3.7 1.7

27.6 57.8 73.9

a Reaction conditions: initial TOC [Ph-OH] ) 765 ppm; stoichiometric amount of H2O2 for phenol mineralization; catalyst concentration ) 0.6 g/L; temperature ) 100 °C; air pressure ) 1 MPa; reaction time ) 120 min. b TOC conversion defined as (initial TOC - final TOC)/initial TOC × 100. c Hydrogen peroxide conversion defined as (initial oxidant - final oxidant)/initial oxidant × 100. d Final pH after reaction. e Milligrams of Fe in the catalyst per liter of dissolution. f Milligrams of Fe leached after the reaction per liter of dissolution.

Figure 3. WPO of phenolic solutions over zeolitic materials. Reaction conditions: initial TOC[Ph-OH] ) 765 ppm; temperature ) 100 °C; air pressure ) 1 MPa; stoichiometric amount of H2O2; catalyst concentration ) 0.6 g/L. (a) TOC conversion versus reaction time: (]) B-2; (0) TS-1; (O) Fe-TS-1(1); (4) Fe-TS-1(2); (3) Fe-TS-1(3). (b) Chemical composition of the reaction mixture and peroxide conversion after 12 min. Aromatics include phenol, catechol, and hydroquinone. Carboxylic acids include acetic, formic, and oxalic acids. Others include oxygenated products such as alcohols and aldehydes.

content during the first minutes of the reaction, whereas the proportion of carboxylic acids and oxygenated products such as alcohols and aldehydes increases significantly. Figure 3b also evidences the enhancement of peroxide decomposition with the increase of Fe species within the zeolitic materials. These aforementioned results are in fairly good agreement with the influence of the Fe content on the catalytic efficiency of the different materials outlined in terms of TOC conversion in Figure 3a.

For the purpose of comparison, a catalytic test performed in the presence of hydrogen peroxide but in the absence of a catalytic system (B-2 in Figure 1) has also been included in Figure 3. It is readily observed that the presence of a catalytic system enhances significantly the TOC conversion and peroxide decomposition, confirming the important role of the metallic species in the catalytic abatement of aqueous solutions of phenol. Additionally, an experiment using a TS-1 zeolite was carried out in order to assess the effectiveness of Ti species in the oxidation process. From the results, depicted in Figure 3, it can be inferred that titanium species show a TOC conversion similar to that shown by the blank experiment (B-2) after 2 h of reaction. However, the rate of TOC reduction and aromatics removal during the first minutes of reaction is higher, as compared with that of run B-2. Analogously, the TS-1 catalyst shows a lower activity in terms of TOC conversion than Fe-TS-1 materials. The low rate of peroxide decomposition and high percentage of aromatic compounds during the first minutes of the reaction also confirm this fact (Figure 3b). The amount of Fe species leached into the aqueous solution after 2 h of reaction have been measured for the different Fe-TS-1 materials (Table 4). It is readily observed that the stability of Fe species decreases as their contents increase, which suggests the coexistence of different Fe environments in the synthesized materials. DR UV-vis spectroscopy has been applied to assess the Fe atoms coordination in the zeolitic framework. From the studies described in the literature,28 two bands centered at 215 and 240 nm can be assigned to Fe species in the [FeO4] tetrahedral groups. However, Fe species in octahedral complexes or present in small clusters show strong and broad adsorptions centered at 280 and 333 nm, respectively.28 Analogously, Ti atoms occupying tetrahedral positions on the zeolitic framework show a narrow band centered on 220 nm, whereas the presence of extraframework TiO2 can be detected by the observance of an adsorption band at 330 nm.29 Figure 2 illustrates DR UV-vis spectra of the different Fe-TS-1 and TS-1 samples. TS-1 and Fe-TS-1 with low and medium Fe contents (samples b and c) evidence a narrow band centered at 210 nm, which is accompanied with a slight shoulder at 240 nm for the Fe-

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Figure 4. Influence of the temperature of WPO of phenolic solutions over Fe-TS-1 zeolitic materials. Reaction conditions: initial TOC[Ph-OH] ) 765 ppm; air pressure ) 1 MPa; stoichiometric amount of H2O2; catalyst concentration ) 0.6 g/L; catalyst ) FeTS-1(2).

Figure 5. Stability of Fe-TS-1 zeolitic materials. Reaction conditions: initial TOC[Ph-OH] ) 765 ppm; temperature ) 100 °C; air pressure ) 1 MPa; stoichiometric amount of H2O2; catalyst concentration ) 0.6 g/L; catalyst ) Fe-TS-1(2).

TS-1 samples. These spectroscopic results indicate the presence of Ti atoms in tetrahedral positions into the zeolitic framework, whereas the signal located at 240 nm suggests the presence of iron atoms also incorporated in a tetrahedral environment.29 The Fe-TS-1 sample with the highest Fe content (sample d) shows a wide absorption band. This band could indicate the presence of extraframework Ti and Fe species as octahedral complexes or small clusters.28,29 These different Fe environments detected by DR UVvis are closely related with the leaching of active species. Assuming that nontetrahedrally coordinated Fe species are more labile than those tetrahedrally coordinated, the high amount of Fe leached for the Fe-TS-1(3) sample can be easily explained. However, when Fe species are mostly tetrahedrally coordinated into the zeolitic framework, the leaching is clearly reduced. Furthermore, the evidence of extraframework oxides of Ti and Fe detected by DR UV-vis for the Fe-TS-1 sample with higher Fe content correlates fairly well with the textural properties determined by means of N2 adsorption tests (Table 1). Fe-TS-1 samples with low and medium Fe contents show BET surface areas and micropore volumes corresponding to a pure MFI zeolitic structure. In contrast, the Fe-TS-1 sample with a high Fe content evidences a significant decrease of the BET surface area and micropore volume. These adsorptive data confirm the existence of extraframework Ti and Fe species for this sample that might be located within the zeolite pores in agreement with the DR UV-vis and XRD results. The influence of operating temperature has been studied for WPO of phenol in the presence of Fe-TS-1 zeolitic material (molar ratio: 76). In this way, two additional experiments have been performed at 80 and 60 °C, respectively. Figure 4 illustrates TOC conversion and leaching results after 2 h of reaction. The experiment at 80 °C allows retention of the catalytic activity with enhancement of Fe species stability as compared

with the reaction performed at 100 °C. Furthermore, if the temperature is diminished to 60 °C, the leaching of Fe species is significantly minimized without an appreciable change in phenol removal. However, a remarkable reduction of TOC conversion must be pointed out. A further research was focused on the stability of the Fe-TS-1 zeolitic materials. Fe-TS-1(2) was reused twice at 100 °C, with the results being depicted in Figure 5. The catalytic activity in terms of TOC conversion is slightly modified after 2 h of reaction for the fresh and reused catalysts. However, a similar leaching of Fe species for the fresh and reused catalysts is readily evidenced. This fact could be related with the possible readsorption of iron species onto the catalyst surface even although the solution was recovered by hot filtration after reaction. 3.3. Comparison of Different Fe-Containing Zeolitic Materials. Because the nature of active sites in zeolites is extremely dependent on the synthesis procedure, it was our particular interest to see how the activity and stability of Fe species are affected by different procedures of synthesis. Table 4 reports the results for the catalytic wet hydrogen peroxide oxidation of phenol over different Fe-containing zeolitic materials after 2 h of reaction. The different catalysts were prepared according to different procedures: hydrothermal crystallization of either xerogels (Fe-TS-1 samples) or basic hydrogels (Fe-ZSM-5 and Fe-silicalite) and ion exchange (Fe-NaY, Fe-USY, and Fe-ZSM-5). Catalytic oxidation results over Fe-containing zeolitic materials prepared by ion exchange evidences that FeNaY zeolite displays a higher stability than exchanged Fe-USY and Fe-ZSM-5 zeolites. It must be noteworthy for these exchanged materials that Fe species incorporation as well as the strength of their interaction with the zeolite framework are markedly dependent on the nature of the zeolitic counterion, strength, and concentration of acidic sites and the conditions of exchange

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treatment. Consequently, the nature of the Fe-zeolite framework interaction set clearly the leaching of Fe species into the aqueous solution. Likewise, when exchanged zeolitic materials (Fe-NaY and Fe-USY) are compared with Fe-containing materials prepared through a direct synthesis procedure with similar Fe content (Fe-TS-1(2)), the leaching of Fe species for the latter is clearly reduced. These results confirm that the direct synthesis procedure with a moderate Fe content in the synthesis mixture allows active species to be incorporated into the zeolitic framework more strongly bonded than by ion exchange. Finally, a zeolitic material with MFI structure synthesized through direct synthesis of basic hydrogels containing Si and Fe atoms but free of Ti species (Fesilicalite) has been tested on the oxidation process (Table 4). This material shows a poor stability with a high leaching of Fe species (ca. 40%), which agrees with the high presence of extraframework Fe species detected by DR UV-vis (Figure 2, sample e). Moreover, when this material is synthesized in the presence of Al atoms (FeZSM-5), almost a complete leaching of Fe species is detected. 27Al MAS NMR of this sample (figure not shown) displays clearly just a signal centered at 50 ppm associated with Al atoms in a tetrahedral environment into the framework with a complete absence of a peak at 0 ppm corresponding to the presence of an Al extraframework.30 In contrast, DR UV-vis of this material (Figure 2, sample f) shows a wide signal from 280 to 330 nm, which is a strong indication of the presence of extraframework Fe species.28 Both characterization results suggest that the incorporation of Fe species into the MFI zeolitic framework is dramatically influenced by the presence of Al atoms when this synthetic route is used. This high content of Fe species not incorporated effectively into the zeolitic framework explains the high percentage of leaching evidenced for this material. Figure 6 illustrates the TOC conversion along the reaction time for Fe-TS-1(2) and exchanged Fe-NaY zeolites. For the purpose of comparison, two homogeneous Fe catalysts based on iron(III) sulfate and iron(II) chloride with an iron content in the aqueous solution equivalent to that present in the above-mentioned catalysts have also been included. These homogeneous experiments demonstrated a higher efficiency of Fe2+ ions as compared with that shown by Fe3+ species (Figure 6a). From the catalytic results, it can be clearly inferred that the presence of Fe3+ species effectively incorporated into a zeolitic framework (Fe-TS-1(2)) exhibits a higher activity than those directly dissolved in an aqueous medium. This fact confirms that the activity of heterogeneous systems in not due to the presence of Fe species leached off to the aqueous solution during the oxidation process. The catalytic behavior shown by the Fe-NaY zeolite is also remarkable, where an initial induction time with low activity is observed before reaching the highest TOC reduction of the tested catalytic systems. The catalytic activity observed in terms of aromatics removal and transformation into refractory compounds, such as carboxylic acids, after 12 min, depicted in Figure 6b, is in fairly good agreement with the TOC conversion values. Fe-TS-1(2) shows a better catalytic performance for the degradation of aromatic compounds than homogeneous catalysts based on Fe3+ species and exchanged Fe-NaY zeolite.

Figure 6. WPO of phenolic solutions over different Fe catalytic systems. Reaction conditions: initial TOC[Ph-OH] ) 765 ppm; temperature ) 100 °C; air pressure ) 1 MPa; stoichiometric amount of H2O2; catalyst concentration ) 0.6 g/L. (a) TOC conversion versus reaction time: (]) B-2; (0) Fe-TS-1(2); (O) FeNaY; (4) homogeneous FeIII; (3) homogeneous FeII. (b) Chemical composition of the reaction mixture and peroxide conversion after 12 min. Aromatics include phenol, catechol, and hydroquinone. Carboxylic acids include acetic, formic, and oxalic acids. Others include oxygenated products such as alcohols and aldehydes.

Finally, it must also be pointed out that all of the heterogeneous catalytic systems seem to tend toward a maximum value for TOC conversion after 2 h of reaction. This upper limit is probably set for the total oxidation of phenolic species and the appearance of refractory carboxylic acids, which are not suitable to be mineralized under the reaction conditions.

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4. Conclusions WPO of phenolic solutions over iron-containing TS-1 (Fe-TS-1) with a moderate Fe content is shown as a promising technology for the abatement of water pollutants. This catalytic system, in the presence of a stoichiometric amount of hydrogen peroxide, has evidenced an outstanding activity and stability for the direct mineralization of phenol in comparison with homogeneous catalysts and ion-exchanged zeolitic materials. Several factors have been shown to be crucial for the leaching of active species into the aqueous solution: the environment of Fe species into the zeolitic framework, the synthetic route for the preparation of the catalytic solids, and the temperature of the treatment. Acknowledgment This work has been funded by the “Comisio´n Interministerial de Ciencia y Tecnologı´a” from Spain (Project CICYT AMB-97-500/96). Literature Cited (1) Matatov-Meytal, Y. I.; Sheintuch, M. Catalytic Abatement of Water Pollutants. Ind. Eng. Chem. Res. 1998, 37, 309. (2) Lin, S. H.; Ho, S. J. Treatment of High Strength Industrial Wastewater by Wet Air Oxidationsa Case Study. Waste Manage. 1997, 17 (1), 71. (3) Luo, D. Treatment of High Concentration of Organic Industrial Wastewater by Ozone Catalytic Oxidation. Water Treat. 1989, 4, 73. (4) Luck, F. Wet Air Oxidation: Past, Present and Future. Catal. Today 1999, 53, 81. (5) Mishra, V. S.; Mahajani, V. V.; Joshi, J. B. Wet Air Oxidation. Ind. Eng. Chem. Res. 1995, 34, 2. (6) Plant, L.; Jeff, M. Hydrogen Peroxide: A Potent Force to Destroy Organics in Wastewaters. Chem. Eng. 1994, 16, 16. (7) Borup, M. B.; Ashcroft, C. T. An Advanced Oxidation Process Using Hydrogen Peroxide and Heterogeneous Catalysts. Proc. Ind. Waste. Conf. 1992, 47, 301. (8) Al Hayek, N.; Dore´, M. Oxidation of Phenols in Water by Hydrogen Peroxide on Alumina Supported Iron. Water Res. 1990, 24, 973. (9) Ding, Z. Y.; Frish, M. A.; Li, L.; Gloyna, F. Catalytic Oxidation in Supercritical Water. Ind. Eng. Chem. Res. 1996, 35, 3257. (10) Barrault, J.; Bouchoule, C.; Echachoui, K.; Frini-Srasra, N.; Trabelsi, M.; Bergaya, F. Catalytic Peroxide Oxidation of Phenol over Mixed (Al-Cu)-Pillared Clays. Appl. Catal. B 1998, 15, 269. (11) Hamoudi, S.; Larachi, F.; Sayari, A. Wet Oxidation of Phenolic Solutions over Heterogeneous Catalysts: Degradation Profile and Catalyst Behavior. J. Catal. 1998, 177, 247. (12) Luck, F. A Review of Industrial Catalytic Wet Air Oxidation Processes. Catal. Today 1996, 27, 195. (13) Levec, J.; Pintar, A. Catalytic Oxidation of Aqueous Solutions of Organics: An Effective Method for Removal of Toxic Pollutants from Wastewaters. Catal. Today 1995, 24, 51. (14) Sanger, A. R.; Lee, T. T. K.; Chuang, K. T. Catalytic Wet Air Oxidation in the Presence of Hydrogen Peroxide. Progress in

Catalysis; Elsevier Science Publishers BV: Amsterdam, The Netherlands, 1992; pp 197-201. (15) Hocking, M. B.; Intibar, D. J. Oxidation of Phenol by Aqueous Hydrogen Peroxide Catalyzed by Ferric Iron-Catechol complexes. J. Chem. Technol. Biotechnol. 1985, 35A, 365. (16) De Laat, J.; Gallard, H. Catalytic Decomposition of Hydrogen Peroxide by Fe(III) in Homogeneous Aqueous Solution: Mechanism and Kinetic Modeling. Environ. Sci. Technol. 1999, 33, 2726. (17) Fajerwerg, K.; Foussard, J. N.; Perrard, A.; Debellefontaine, H. Wet Oxidation of Phenol by Hydrogen Peroxide: the Key Role of pH on the Catalytic Behavior of Fe-ZSM-5. Water Sci. Technol. 1997, 35 (4), 103. (18) Fajerwerg, K.; Debellefontaine, H. Wet Oxidation of Phenol by Hydrogen Peroxide using Heterogeneous Catalysis. Fe-ZSM5: A Promising Catalyst. Appl. Catal. B 1996, 10, 229. (19) Centi, G.; Perathoner, S.; Torre, T.; Verduna, M. G. Catalytic Wet Oxidation with H2O2 of Carboxylic Acids on Homogeneous and Heterogeneous Fenton-type Catalysts. Catal. Today 2000, 55, 61. (20) Larachi, F.; Le´vesque, S.; Sayari, A. Wet Oxidation of Acetic Acid by H2O2 Catalyzed by Transition Metal-Exchanged NaY Zeolites. J. Chem. Technol. Biotechnol. 1998, 73, 127. (21) Valange, S.; Gabelica, Z.; Abdellaoui, M.; Clacens, J. M.; Barrault J. Synthesis of Copper Bearing MFI zeolites and their Activity in Wet Peroxide Oxidation of Phenol. Microporous Mesoporous Mater. 1999, 30, 177. (22) Bellussi, G.; Rigutto, M. S. Advanced Zeolite Science and Application; Elsevier Science Publishers BV: Amsterdam, The Netherlands, 1994. (23) Ovejero, G.; Van Grieken, R.; Melero, J. A. Verified Synthesis of Zeolites. Microporous Mesoporous Mater. 1998, 22, 638. (24) Uguina, M. A.; Ovejero, G.; Van Grieken, R.; Serrano, D. P.; Camacho, M. Synthesis of Titanium Silicalite-1 from an SiO2TiO2 Co-gel Using a Wetness Impregnation Method. J. Chem. Soc., Chem. Commun. 1994, 27. (25) Van Grieken, R.; Sotelo, J. L.; Menendez, J. M.; Melero, J. A. Anomalous Crystallization Mechanism in the Synthesis of Nanocrystalline ZSM-5. Microporous Mesoporous Mater. 2000, 39, 135. (26) Rivas, F. J.; Kolaczkowski, S. T.; Beltran, F. J.; McLurgh, D. B. Hydrogen Peroxide Promoted Wet Air Oxidation of Phenol: Influence of Operating Conditions and Homogeneous Metal Catalysts. J. Chem. Technol. Biotechnol. 1999, 74, 390. (27) Ovejero, G.; Van Grieken, R.; Uguina, M. A.; Serrano, D. P.; Melero, J. A. Study on the Ti and Al co-incorporation into the MFI Zeolitic Structure. J. Mater. Chem. 1998, 8, 2269. (28) Bordiga, S.; Buzzoni, R.; Geobaldo, F.; Lamberti, C.; Giamello, E.; Zecchina, A.; Leofanti, G.; Petrini, G.; Tozzola, G.; Vlaic, G. Structure and Reactivity of Framework and Extraframework Iron in Fe-Silicalite as Investigated by Spectroscopic and Physicochemical Methods. J. Catal. 1996, 158, 486. (29) Geobaldo, F.; Bordiga, S.; Zecchina, A.; Giamello, E.; Leofanti, G.; Petrini, G. DRS UV-Vis and EPR Spectroscopy of hydroperoxo and superoxo complexes in Titanium Silicalite. Catal. Lett. 1992, 16, 109. (30) Engelhardt, G.; Michel, D. High-Resolution Solid State NMR of Silicates and Zeolites; Wiley: New York, 1987.

Received for review October 16, 2000 Revised manuscript received March 12, 2001 Accepted May 30, 2001 IE000896G