Modified Zeolite with Transition Metals Cu and Fe for Removal of Methylene Blue from Aqueous Medium : Mass Spectrometry Study

Textile industries are one of the main sources of water pollution. Wastewater containing dyes present a serious environmental problem because of its high toxicity and possible accumulation in the environment. In this work were explored the characteristics of removal of methylene blue dye employing zeolites modified with transition metals (Cu, Fe). The zeolites with iron or copper were prepared by using NaY and Naβ zeolites as precursors, replacing part of ion sodium for copper or iron ions through the ion exchange method. All materials were characterized by several analytical techniques, in order to gain information about the structure and catalytic activity. Modified zeolites showed a remarkable activity in H2O2 decomposition and in the discoloration an organic dye in aqueous medium. ESI-MS studies of the methylene blue oxidation showed that the oxidation of the dye occurs via a Fenton type system in which *OH radicals are formed in situ and added to the ring structure of the organic substrate. In addition, modification of the zeolite with transition metal proved to be an interesting pathway to produce efficient catalysts for the oxidation of organic molecules, i.e. dyes in aqueous media. © 2015 BCREC UNDIP. All rights reserved


Introduction
Textile industries consume large volumes of water and chemicals for wet processing of textiles.There are more than 100,000 commercially available dyes with over 7x10 5 ton of dye produced annually [1].Wastewater containing dyes present a serious environmental problem because of its high toxicity and possible accumulation in the environment [2][3][4].The removal of textile dyes from wastewater is one of the most important environmental issues to be solved today [5][6][7].Several dyes used in the textile industry are particularly difficult to remove with conventional waste treatment methods since they are designed to be resistant to degradation [4,8,9].New and innovative methods are of great importance to devise technologies for dealing with environmental problems [10].The application of modified catalyst technology to solve environmental problems is one of these methods that have received considerable attention in recent years [11,12].The zeolites can be modified according to various strategic forms to improve their activity and catalytic selectivity [13,14].The zeolite modification by the introduction of transition metals causes the zeolite to present activity mechanisms comparable to those found in enzymes [15].
Acid solids are the most used catalytic material in the petrochemical industry.Among them, the zeolites have become extremely important for innumerable processes, such as oil refining, catalytic cracking of long hydrocarbons for high-octane gasoline, supports, gas separation, water treatment and production of fine chemical products [16,17].
The present work aims to investigate two zeolites supports, Y and β, both modified with iron or copper as catalyst for the degradation of the organic compound methylene blue dye in presence of hydrogen peroxide.

Modification of NaY and Naβ Zeolites
In the present experiment, NaY (Si/Al = 2.8) and Naβ (Si/Al = 20) zeolites were supplied by the Chemical Institute of Technology in Valence, Spain.Solids precursors were converted to active catalysts by ion exchange [18,19].Adsorbents NaY and Naβ were ion-exchanged using 5 g of each zeolite with 200 ml of copper nitrate 265 mmol.L -1 or solution of iron nitrate 278 mmol.L -1 .The mixture was stirred for 24 h at room temperature (25 ºC).The powders were filtered and washed until the filtrate was metallic ion free.Thereafter, the pH of the mixture was raised to 7.5 with ammonium hydroxide 3 mol.L -1 , leaving under stirring for a further 1 hour.The remaining material was filtered and dried at 110 ºC for 24 h and calcined in a muffle furnace using a heating rate 5 ºC min -1 at 500 ºC for 3 h in air followed by 2 h under H2 atmosphere.The prepared materials were labeled as Y-Fe, Y-Cu, β-Fe and β -Cu.

Characterization of the Materials
The textural properties of the Y-Fe, Y-Cu, β-Fe and β-Cu samples were determined by N2 adsorption and desorption isotherms at -196 ºC using a surface area measurement analyzer (Micromeritics ASAP-2000, Micromeritics Instrument Corporation, Norcross, Georgia, USA).The sample was degassed for 24 h at 110 ºC prior to the adsorption and desorption analysis.The BET surface area (SBET) and total pore volume (V) of the zeolites were obtained from The samples were analyzed also by X-ray diffractometry (Shimadzu 7000 diffractometer, Shimadzu Corporation, Tokyo, Japan) using the Bragg Brentano camera geometry and the Co-Kα incident radiation (λ = 1.5418A), 40 kV, 30 mA and acquisition rate of 2º min -1 between 10-60º (2θ) range.

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The transmittance spectra were collected using an interferometric spectrometer (MB-102, ABB Bomem Inc., Quebec, Canada).KBr pellets were prepared by mixing 1 mg of bioglasses powders with 100 mg of KBr.FTIR spectra were acquired in the region 4000-400 cm -1 with spectral resolution of 4 cm -1 and 64 scans.
Temperature programmed reduction (TPR) technique was performed using a semiautomated analyzer (Chemisorption -Chem-BET ® 3000 TPR/TPD, Quantachrome Instruments, Boynton Beach, FL, USA).TPR experiments were performed using 20 mg of sample was loaded into a U-shaped quartz micro reactor (2 mm internal diameter) and then pretreated in a flow of He (40 ml.min -1 ) at 400 °C for 0.5 h.After the sample was cooled down to room temperature in He, the reduction of the sample was carried out from 30°C to 500°C in a flow of 5 % H2/N2 (27 ml.min -1 ) at a heating rate of 10 °C.min -1 .The consumption of H2 was monitored continuously by a thermal conductivity detector.The water produced during the reduction was trapped in a 5A molecular sieve column.Figure 1 shows a representative scheme summarizing all steps carried out in this work.

Oxidation Reaction
Two reactions were performed in the presence of the Y-Fe, Y-Cu, β-Fe and β -Cu samples: (i) the H2O2 decomposition to O2 in water and (ii) the oxidation of the dye methylene blue.For the first, a typical decomposition of hydrogen peroxide was carried out: 2 ml of H2O2 solution (30% v/v) with 5 ml of H2O and 30 mg of the composite magnetically stirred and monitored by measuring the formation of gaseous O2 in a volumetric glass system.The activity of the zeolites was tested for their effectiveness in discoloring the methylene blue dye in water (10 ml of a 100 mg.L -1 solution) by using 10 mg of the material in presence of hydrogen peroxide.The dye decomposition was measured at 665 nm with an UV/Vis spectrophotometer (UVPC 1600, Shimadzu Corporation, Tokyo, Japan).All reactions were performed at room temperature 25 ºC.

Studies by ESI-MS
In an attempt to identify the intermediate formation, the methylene blue decomposition was monitored with electrospray ionization mass spectrometry (ESI-MS) in the positive ion mode in an ion trap mass spectrometer (Agilent 6300 Ion Trap LCMS, Santa Clara, California, USA).The reaction samples were analyzed by introducing aliquots into the ESI source via a syringe pump at a flow rate of 5 μL•min -1 .The spectra were obtained as an average of 50 scans, each for 0.2 s.Typical ESI conditions were as follows: heated capillary temperature, 250 ºC; heath gas (N2) flow rate, 20 units (4 L.min -1 ); spray voltage 4 kV; capillary voltage 25 V; tube lens off set voltage, 25 V.

Characterization of the Materials
The effects of acid activation on the zeolites surfaces are clearly seen in the scanning electron micrographs (SEM) of precursors and modified zeolites (supplementary materials, Figure SM-1).The analysis of the diffraction patterns for the pre cursors samples NaY and Naβ (see Figure 2a and 2b) indicated excellent fit with the standard Inorganic Crystal Structure Database (ICSD) Collection Code ICSD 34807 [20] and ICSD 97105 [21], corresponding typical structures of the sodium Y and sodium β zeolites, respectively [22].
Regarding to the modified Y zeolite the occurrence of specific diffraction peaks characteristic of crystalline phases associated with copper or iron in the Y-Cu and Y-Fe samples is unclear (Figure 2a).These results may be related to the small size of the formed particles and/or high dispersion of these on the surface and/or in the pore walls of zeolites, which would not be easily observed by XRD analysis.The corresponding XRD patterns of β-Cu and β-Fe samples (Figure 2b) showed subtle changes compared to the β zeolite which can be assigned to the Cu2O (ICSD 261853) [23] and Fe3O4 [24], respectively.
Fourier transform infrared (FTIR) spectra of zeolites are shown in Figure 3.The characteristic peaks of at ~3440 cm -1 and ~1640 cm -1 are corresponding to the OH-stretching vibration and the signal stretching and angular deformation of the hydroxyl molecules of water of hydration of the zeolites, respectively.The absorptions set in the region between 900 and 1300 cm -1 are related to the overlapping of sev-eral absorptions related to asymmetric stretching the Si-O in Si-O-Si and Si-O-M (M= Al, Fe or Cu) groups [25].Also in this spectral region are contained all absorptions regarding the Si-OH and Al-OH species [26].The region between 700 and 900 cm -1 exhibits a single and weak absorption at ~790 cm -1 for materials based on β zeolite and several small absorptions (more intense at ~790 and ~710 cm -1 ) for derived materials from Y zeolite.These spectral features are attributed to overlapping of Si-O bending mode in Si-O-Si and Si-O-M (M= Al, Fe or Cu) groups [27].Lower frequencies (400-700 cm -1 ) are localized the symmetric stretch vibrational modes of Si-O-Si and Si-O-Al groups where the Al is in octahedral coordination [26], stretching of Fe-O in magnetite crystal phase [27], stretching of Cu-O at ~525 cm -1 , and CuO nanostructures at ~580 cm -1 [28].
Figure 4 shows the profiles obtained by reduction of the temperature programmed reduction (TPR) to the zeolite before and after the process of ion exchange with Cu and Fe.TPR analysis for NaY zeolite precursor did not show any signal in the curve (Figure 4a), indicating the absence of species capable of undergoing reduction conditions employed in the analysis.In the temperature range studied, the TPR profile of Cu-Y shows a single reduction peak, broad and centered at 284 ºC.This signal is related to the reduction of Cu 2+ to Cu + from isolated species [29] and reduction of Cu 2+ to Cu 0 from CuO species [30] For Y-Fe zeolite was observed a complex reduction profile with three well-defined peaks: one centered at 424 ºC, a second peak more intense centered at 496 ºC and the third peak, the wider, with maximum at 593 ºC.These peaks are likely related to iron oxide formed onto the Y-Fe zeolite.These results are in agreement with TPR studies of iron oxidepillared clays in the literature which suggest that the reductions occurring at approximately 400 ºC are due to the partial reduction of Fe 3+ to produce Fe3O4 [31] and complete reduction for Fe 2+ [18], according to Equations (3-4), respectively: 3Fe2O3 + H2 → 2Fe3O4 + H2O (3) Fe3O4 + H2 → 3FeO + H2O (4) At higher temperatures, the FeO is reduced to Fe 0 [18], according to Equation ( 5): For zeolites Naβ was not observed the presence of any signal consumption of H2, nevertheless a negative signal was observed at 460 °C (Figure 4b).This signal may be attributed to support decomposition or desorption of H2 adsorbed inside the pores of zeolite, which would be someway trapping the H2 molecules.As the temperature is increased occurs the transfer of thermal energy to the molecules H2, reaching a threshold value, which promotes desorption and a negative sign in the TPR curve.For the β-Cu sample, it is possible to notice a region of H2 consumption very broad, centered at 169 and 296 ºC.These peaks reduction are associated with changes in the oxidation state of Cu 2+ to Cu + from isolated species [29] and Cu 2+ to Cu 0 from CuO species [30] (see Equations ( 1) and ( 2)).On the other hand, for the β-Fe desorption of H2 molecules within the pores of zeolites overshadowed signal reduction referring to Equations (3)(4), leaving only one reduction broad peak at high temperature (556 ºC) related formation of Fe 0 (see Equation ( 5)).TPR results show that the thermal treatment under a hydrogen atmosphere was not enough to produce a complete reduction of the iron or copper species, but only a partial reduction.
The N2-adsorption measurements of the NaY, Y-Fe, Y-Cu and Naβ, β-Fe and β-Cu samples are shown in Figure 5a and b, respectively.Table 1 summarizes the specific surface area BET and porosity values for NaY, Y-Fe, Y-Cu, Naβ, β-Fe and β-Cu samples.
In both supports, NaY and Naβ, the BET surface areas decreased after the sodium replacement within the host materials by Cu or Fe.There are two main factors that may affect the surface area: the size and the number of compensating cations [32].Copper and iron have different charge of sodium and the ionic radius of the Cu 2+ and Fe 2+/3+ are smaller than Na + .Thus, it would be expect to observe an effect inverse on the surface area behavior.However, the influence on surface area for multivalent cations is very complex.For these ions, not only the size, but also their configurations and their total number in the framework are parameters very important and must be considered [32].The Na + ion is larger than Cu 2+ or Fe 2+/3+ ions, thus more than one Cu or Fe are occupying each Na+ ion site.Strictly speaking, every Na + site is offset by cluster of Cu or Fe after ion exchange, assuming an exchange degree of 100%.It fills part of the pores of the pores reducing its vol-

Catalytic Tests
The catalytic activity of the zeolites was studied using two reactions: (i) the H2O2 decomposition to O2 (H2O2 → H2O + 0.5O2); (ii) the oxidation of the model contaminant the methylene blue dye with H2O2 in aqueous medium.Figure 7a shows the performance of materials in the H2O2 decomposition.
Kinetics study of the oxidation reaction of hydrogen peroxide in the presence of NaY, Naβ, and Y-Fe zeolites showed that the H2O2 concentration remains constant, indicating that these materials are not able to catalyze the decomposition of peroxide in the studied conditions.Conversely, kinetics study in the presence of β-Fe, β-Cu, and Y-Cu reveal a linear dependence between the decreases of H2O2 concentration with the increase of reaction time.
This behavior for β-Fe, β-Cu, and Y-Cu zeolites show that although the H2O2 decomposition is a complex reaction, the linear dependence of the decomposition curves between 5 and 20 min suggests that the process can be approximate to pseudo-zero order kinetics.The quick decomposition of H2O2 in the first 5 minutes of reaction for β-Fe, β-Cu, and Y-Cu zeolites reveals a high catalytic activity for the H2O2 oxidation that is very interesting in Fenton-type reactions.Furthermore, Y-Cu showed a catalytic activity slightly higher than that observed for the β-Cu or β-Fe samples.The distinct behavior exhibited by β-Cu and Y-Cu in the H2O2 decomposition reaction is related to reduced copper species present in the phases formed in the modified zeolite.In addition, the reduction of Cu(II) by H2O2 (4.6 x 10 2 M -1 .s - ) occurs more easily than that of Fe(III), and the Cu(I)-H2O2 system, which possesses a higher reaction rate (1 x 10 4 M -1 .s 1 ) than the Fe(II)-H2O2 system (76 M -1 .s 1 ), can efficiently produce *OH [33].

Bulletin of Chemical Reaction Engineering & Catalysis, 10 (3), 2015, 242
Copyright © 2015, BCREC, ISSN 1978-2993 Vtotal 0.95 cm 3 .g - Vmicro cm 3 .g - Vmeso cm The heterogeneous Fenton reactions were carried out using the dye methylene blue as probe molecule.The reaction kinetics was investigated by UV/Vis discoloration measurements (Figure 7b) to form non-colored intermediates and the oxidation efficiency was measured by ESI-MS as showed in Figure 8.
In the control experiment (only methylene blue and H2O2 but no zeolite) there is no significant discoloration even after 120 min. of reaction time (not shown here).On the other hand, in the presence of the zeolite supports Naβ and NaY a slight discoloration was observed, which it was exclusively related to adsorption process (absence of *OH -Figure 7a).Furthermore, the results showed that the Naβ has a higher adsorption capacity compared to NaY.This result is interesting since the surface area of the NaY is three times higher than the Naβ (Table 1), suggesting that the cavity size in the NaY prevents the entry of methylene blue molecules within the pores.In fact, Naβ features a majority mesoporous structure (VMesopores = 0.25 cm 3 .g - ), while a microporous structure is predominant in the NaY (VMesopores = 0.01 cm 3 .g - ).
The presence of Fe and Cu species in the modified zeolites increased considerably the velocity of removal of the methylene blue dye (Figure 7b).This effect is clearly observed for the catalysts β-Cu or β-Fe that showed a remarkable increase in the discoloration compared to Y-Cu and Y-Fe zeolites.According to the kinetics study for H2O2 decomposition, would be expected a higher activity for Y-Cu and β-Cu samples, owing to formation of very reactive *OH specie which is extremely effi-cient for oxidation of several organic substrate.Apparent discrepancy between H2O2 decomposition and oxidation kinetics results should addressed based on the mechanism and structure of zeolites.Although, Y-Cu was efficient for H2O2 degradation and *OH radical production, the methylene blue molecules cannot access the micropores.Taking into account that the *OH life-time is very short (high reactivity), there is not enough time to travel all the way toward the  outside of the pores and attack the methylene blue molecules in solution.Thus, only the *OH radical produced on the surface and inside the pores in the macropores are effective in the methylene blue oxidation.On the contrary, β-Cu or β-Fe samples exhibit a mesoporous structure allowing the entry of methylene blue onto cavities, consequently, the dye can be readily oxidized.

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Figure 8 shows the results obtained from the mass spectrometry analyzes, applying the ESI for the screening of the oxidized species formed during the oxidation reaction of methylene blue in the presence of the zeolites and H2O2.The ESI-MS spectrum obtained for the methylene blue dye solution shows only a strong signal at m/z = 284 (Figure 8a), related to the methylene blue ion.After 15 min of reaction with the Y-Cu and hydrogen peroxide, new signals appear at m/z = 300 and m/z = 316, as shown in Figure 8b, but, the signal of the dye (m/z = 284) is still intense.Figure 8c show the ESI-MS spectrum of the reaction after 15 min for the β-Cu catalyst.As can be observed the signal of the m/z = 284 was reduced below 20 % and a new signal with m/z = 130 appears in the spectrum which is related to the presence of intermediates of the methylene blue oxidation, suggesting that the structural ring of dye was somehow broken apart.Only a very small signal associated with m/z = 130 was noticed in the ESI-MS spectrum for dye solution in Y-Fe even after 60 minutes whereas the m/z = 284 remains close to 80% (Figure 8e).This result confirms that for this material the adsorption process is the main mechanism for removal dye.Conversely, for β-Fe strong signal with m/z = 130 about 90 % is clearly noticed while the signal related to methylene blue (m/z = 284) was reduced below 10 % after 10 minutes of reaction (Figure 8f).
Figure 9 shows the proposed intermediate structures of the complete oxidation reaction pathway from methylene blue to CO2 and H2O.Based on the results from Figure 8 we could propose that the reaction initiated by the activation of H2O2 by zeolites was followed by the transference of *OH radical to organic dye as showed by the intense fragments.The proposed mechanism was based on the activation of H2O2 via a heterogeneous Fenton mechanism to form a radical *OH as suggested by ESI-MS data [2,34].
It is worth to point out that the solutions after the heterogeneous Fenton reactions were submitted to elemental analysis using atomic absorption spectrometry (AAS) in order to investigate the possibility of homogeneous reac-tions owing to Cu and Fe ions lixiviation from modified zeolites.The results have not revealed the presence of Cu or Fe species in the solutions confirming the heterogeneous oxidation process.

Conclusion
In the presented study, we report a study of catalytic activity of the Y and β zeolites modified with iron or copper employing the ion exchange based preparation method in two reactions:  showed that the oxidation of the dye occurs via a Fenton type system in which *OH radicals are formed in situ and added to the ring structure, suggesting that it is possible to reach the total mineralization of the organic substrate; not only its discoloration.

Figure 1 .
Figure 1.Representative scheme summarizing all steps carried out in this work.

Figure 6 .
Figure 6.Schematic representation of the formation of copper and iron clusters within the pores of zeolites.

Figure 8 .
Figure 8. ESI mass spectra in the positive ion mode for monitoring the oxidation of methylene blue dye in water: (a) methylene blue dye water solution (100 ppm and m/z = 284) (without zeolite), (b) Y-Cu and (c) b-Cu in the presence of H2O2 for 15 min of reaction, (d) methylene blue dye water solution (10 ppm and m/z = 284) (without zeolite), (e) Y-Fe in the presence of H2O2 for 12 hours of reaction, and (f) b-Fe in the presence of H2O2 for 10 min of reaction.