The role of zeolite FeZSM-5 porous structure for heterogeneous Fenton catalyst actuvuty and stability

Microporous and Mesoporous Materials 189 (2014) 181–188 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso The role of zeolite Fe-ZSM-5 porous structure for heterogeneous Fenton catalyst activity and stability K.A Sashkina a, E.V Parkhomchuk a,b,⇑, N.A Rudina b, V.N Parmon a,b a b Novosibirsk State University, Pirogova st., Novosibirsk 630090, Russia Boreskov Institute of Catalysis SB RAS, Lavrentieva st., Novosibirsk 630090, Russia a r t i c l e i n f o Article history: Available online 27 November 2013 Dedicated to Dr Michael Stöcker on the occasion of his retirement as Editor-in-Chief of Microporous and Mesoporous Materials Keywords: Fe-ZSM-5 Hierarchical zeolite Nanozeolite Heterogeneous Fenton catalyst Catalyst stability a b s t r a c t Four types of iron containing materials have been synthesized: conventional zeolite Fe-ZSM-5 (conv), hierarchical zeolite Fe-ZSM-5 (hier), small crystals (d = 330 nm) of zeolite Fe-ZSM-5 (nano) and ferric oxide species supported on the amorphous silica Fe/SiO2 Samples were prepared by hydrothermal treatment, polystyrene spheres were used as a template for Fe-ZSM-5 (hier) and Fe/SiO2 The materials were characterized by different techniques Nature of iron-containing particles in the samples and stability of iron species in the reaction media were suggested by using thermodynamic considerations All solidphase Fe-containing samples as well as dissolved Fe(NO3)3 were tested in H2O2 decomposition reactions in absence or presence of iron-complexing agent Na2EDTA, which has been used to test the catalyst stability Catalytic activity of ferric species in hydrogen peroxide decomposition for small 330-nm crystals of Fe-ZSM-5 was 1.4 times higher than for large zeolite crystals, and significant decrease of the activity was observed for samples containing amorphous silica phase Experimental results showed that ferric sites in zeolite were stable due to the limited diffusion of Na2EDTA in zeolite phase Wet hydrogen peroxide oxidation of organic complexing agents by H2O2 using Fe-containing zeolites has a good potential for purification of nuclear waste water Ó 2013 Elsevier Inc All rights reserved Introduction Fe-ZSM-5 has been shown to be a promising heterogeneous solid-phase catalyst in total oxidation of a series of organic substrates with low molecular weight (MW) by hydrogen peroxide [1–4] Mineralization degree of phenol, 1,1-dimethylhydrazine and ethanol, as well as extent of H2O2 utilization is higher in such a heterogeneous system compared with the homogeneous Fenton system due to effective adsorption of organic substrate on zeolite surface [5] On the other hand conventional zeolitic material is ineffective in oxidation of high MW organics because of specific porous structure with pore size of 0.55 nm Mineralization degree of lignin is significantly lower in the Fe-ZSM-5/H2O2 system compared with homogeneous ones, such as Fe(NO3)3/H2O2 and H2O2/UV [6] This is due to excessive distance from catalytic sites where the hydroxyl radicals are formed inside of zeolite crystal to organic molecule adsorbed on the external surface of crystal particle These diffusion limitations result in prevalence of oxygen release reaction over organics oxidation processes in case of high MW substrates To expand zeolite use for wet peroxide oxidation of hard convertible macromolecules accessibility of catalytic sites for high MW substrates should be significantly increased In order to pre⇑ Corresponding author at: Boreskov Institute of Catalysis SB RAS, Lavrentieva st., Novosibirsk 630090, Russia Tel./fax: +7 (383)333 16 17 E-mail address: ekaterina@catalysis.ru (E.V Parkhomchuk) 1387-1811/$ - see front matter Ó 2013 Elsevier Inc All rights reserved http://dx.doi.org/10.1016/j.micromeso.2013.11.033 pare zeolites with additional meso or macroporosity a range of techniques may be used [7–11] Here, hierarchically ordered zeolitic material Fe-ZSM-5 (hier) has been prepared with the use of a template consisting of polystyrene (PS) spheres according to the method described by Stein in [12] Earlier hierarchical FeZSM-5 was tested in total oxidation of large organic molecules, Na2EDTA and lignin, used as large model compounds [13] As a result of a great increase in catalytic site accessibility performance of Fe-ZSM-5 (hier) in oxidation of large Na2EDTA molecule and high MW lignin appeared to be really improved compared with conventional zeolite [13] However the question on catalyst stability during oxidation of high MW organics when a range of different organic acids may be formed is still open This question is obviously related to the question on the nature of iron-containing sites in zeolites and amorphous silica phase While there are numerous available literature data on the nature of sites in zeolites, which are active in selective oxidation of benzene to phenol or methane to methanol by N2O [14–16], selective reduction of nitrogen oxides by hydrocarbons [17,18], and decomposition of N2O [19,20], the nature of active sites for heterogeneous Fenton reactions remains a disputable problem It is worth noting that hierarchical zeolite h-Fe-ZSM-5 represents a mixture of two phases: ZSM-5 nanocrystals and amorphous silica globules with a wide size distribution [21] Catalytic activity as well as stability of iron containing sites located in these two phases may be expected to be different 182 K.A Sashkina et al / Microporous and Mesoporous Materials 189 (2014) 181–188 To clarify separate roles of these two iron containing phases (zeolitic and silica) for wet peroxide oxidation catalysis we synthesized pure crystalline sample, consisting of small Fe-ZSM-5 crystals with the size of 330 nm, and pure amorphous sample, containing ferric species supported on the silica phase (labeled as Fe/SiO2) and compared them with conventional and hierarchical Fe-ZSM5 On the whole four types of iron containing materials were studied: conventional zeolite Fe-ZSM-5 (conv), hierarchical zeolite Fe-ZSM-5 (hier), small crystals of zeolite Fe-ZSM-5 (nano) and Fe/ SiO2 In order to reveal phase compositions and textural differences a range of techniques was used, including X-ray diffraction, low temperature N2 adsorption, scanning and high resolution transmission electron microscopies, UV–vis diffuse reflectance spectroscopy Samples were tested in hydrogen peroxide decomposition reaction to determine catalytic activity in formation of hydroxyl radical from H2O2 – the main source of oxidative activity of Fenton reagent in organics oxidation in acidic and neutral media Since a wide range of organic acids may be formed during Fenton reactions stability of catalytically active sites requires to be carefully studied The effect of iron-complexing agent Na2EDTA, having high stability constants with iron ions, on catalytic activity and stability of the samples was studied Unlike ferric oxide species supported on the amorphous SiO2 ferric sites in Fe-ZSM-5 appeared to be stable under the action of organic acids Experimental 2.1 Chemicals Styrene monomer, inhibited with 1% hydroquinone, was purchased from Ltd ‘‘Angara-reactive’’ It was washed times in separatory funnel with an equal volume of M aqueous solution of sodium hydroxide, followed by times distilled water to remove the inhibitor before polymerization Tetraethylorthosilicate (TEOS) was purchased from Ltd ‘‘Angara-reactive’’, hydrogen peroxide (30% aqueous solution) – from company ‘‘Baza No1 Khimreactivov’’, sulfuric acid (97%) – from Moscow Chemical company ‘‘Laverna’’, sodium hydroxide – from Ltd ‘‘Tellura’’, iron (III) nitrate nonahydrate were produced in Boreskov Institute of Catalysis, 95% ethanol EtOH of technical grade were obtained from Ltd ‘‘Pharmaceya’’ Silica fumed powder (99.8%), tetrapropylammonium hydroxide (TPAOH, 25% solution in water), tetrapropylammonium bromide (98%), potassium persulfate (99%) were purchased from Sigma Aldrich, Germany dried and calcined at 500 °C for h in air The sample is labeled Fe-ZSM-5 (conv-as) For catalyst pretreatment the powder of FeZSM-5 (conv-as) was suspended to the M aqueous solution of oxalic acid in concentration of 100 g L1 and stirred for h at 50 °C The catalyst was rinsed with distilled water to pH 7.0, dried in air and calcined at 500 °C for h The pretreated sample is labeled Fe-ZSM-5 (conv) 2.2.3 Synthesis of hierarchical zeolite Hierarchical zeolite Fe-ZSM-5 (hier) was synthesized using a PS template as a macropore generating agent and TPAOH as a SDA The sample Fe-ZSM-5 (hier) was produced using ferric nitrate with the SiO2:Fe2O3:TPAOH:H2O molar ratio of 1:0.015:0.7:17.5 Thereafter, PS template was impregnated with the gel with weight ratio SiO2:1 PS The mixture was subjected to hydrothermal synthesis at 110 °C for 40 h The product was washed with abundant amount of water, then dried at an ambient temperature overnight and finally calcined at 500 °C for h in air The control sample of Fe-containing hierarchical zeolite was pretreated by the oxalic acid as stated above, but this procedure did not change the catalytic activity, thus the sample described in the paper was not exposed to this procedure 2.2.4 Synthesis of small zeolite crystals Zeolite Fe-ZSM-5 (nano) with small crystal size was synthesized in hydrothermal conditions from the precursor gel containing TEOS, ferric nitrate, EtOH and TPAOH with the TEOS:Fe2O3:TPAOH:EtOH:H2O molar ratio of 1:0.01:0.275:4.8:12.3 Mixture of precursors were held in the autoclave at 115 °C for 24 h and then at 150 °C for 24 h The resulting suspension was characterized by laser diffraction to measure the size of particles Then the suspension was centrifuged, the precipitate was washed with abundant amount of water, dried at an ambient temperature overnight and finally calcined at 500 °C for h in air The sample is labeled FeZSM-5 (nano-as) The powder of Fe-ZSM-5 (nano-as)was suspended to the M aqueous solution of oxalic acid in concentration of 100 g L1 and stirred for 10 at 50 °C The pretreated catalyst was rinsed with distilled water to pH 7.0, dried in air and calcined at 500 °C for h The sample is labeled Fe-ZSM-5 (nano) 2.2.5 Synthesis of Fe-containing amorphous silica Fe-containing amorphous silica Fe/SiO2 was synthesized by the same way as h-Fe-ZSM-5 described above but without sustaining PS templates over the boiling water for h before hydrothermal synthesis 2.2 Catalyst preparation 2.3 Catalyst characterization 2.2.1 PS template preparation PS were synthesized using emulsifier-free emulsion polymerization technique as described elsewhere [22,23] Emulsion polymerization temperature was 90 °C PS spheres were packed by centrifugation at relative acceleration of 390g Obtained PS template was washed by ethanol and dried in air Before hydrothermal synthesis PS templates were put on the grid and sustained over boiling water for h The X-ray diffraction analysis was performed by a diffractometer HZG-4 with a Cu-Ka radiation in the angle range 2h from 5° to 40° The Fe content of the catalysts and iron concentration in the solutions after reactions were determined by the inductive coupled plasma optical emission spectroscopy (ICP–OES) Scanning electron microscopy (SEM) images were acquired using JSM-6460LV microscope at 15–20 kV accelerating voltage, high-resolution transmission microscopy (HRTEM) images of the samples were made on JEM-2010 microscope at 0.14 nm resolution and 200 kV accelerating voltage Particle size of sample n-Fe-ZSM-5 was measured using suspension dilution with ethanol by laser diffraction on the Mastersizer-2000 UV-vis diffuse reflection (DR) spectra were acquired at ambient temperature using a Shimadzu UV2501 PC at interval 11,000–54,000 sm1 Low-temperature nitrogen adsorption isotherms were measured at 196 °C on ASAP-2400 Prior to the measurements the samples were outgassed at 250 °C for h The specific surface area (SBET) was determined by applying Brunauer–Emmet–Teller (BET) equation 2.2.2 Synthesis of conventional zeolites The synthetic Fe-containing conventional zeolite Fe-ZSM-5 (conv) was produced hydrothermally from precursor gel containing silica powder, sodium hydroxide, ferric nitrate and TPABr as a structure-directing agent (SDA) The molar ratio in the SiO2:NaOH:TPABr:Fe2O3:H2O mixture was chosen as 1:0.2:0.11:0.028:25, respectively The mixture was placed to a Teflon-coated stainless steel autoclave and kept at 150 °C for 72 h Zeolite crystals were filtered, rinsed with distilled water, K.A Sashkina et al / Microporous and Mesoporous Materials 189 (2014) 181–188 from adsorption branches in the relative pressure range of 0.05– 0.3 The external surface area (SExt) and micropore volume (Vmic) were calculated by as-method The value of Vtot was single point total pore volume at P/P0 = 0.98 Hierarchy factor (HF) was calculated as SExt/SBET Vmic/Vtotal 2.4 Catalytic activity tests Iron-containing samples were tested in H2O2 decomposition reactions in absence and presence of g L1 sodium ethylenediaminetetraacetate (Na2EDTA) Hydrogen peroxide decomposition was carried out in magnetically stirred glass batch reactor with 50 mL of aqueous phase and 2956 mL of gaseous phase, both thermostated at 25 °C mM Fe(NO3)3 and 20 g L1 zeolites were used as catalysts for homogeneous and heterogeneous reactions, respectively The H2O2 decomposition rate W O2 at [H2O2]0 = 1.1 M was determined as the oxygen release rate measured barometrically in Pa s1 Results and discussion Morphologies of Fe-containing materials synthesized in given work are shown in Figs and Conventional zeolite Fe-ZSM-5 (conv) was found to have large polycrystals of 2–5 lm in diameter (Fig 1a) Zeolite Fe-ZSM-5 (nano) contains uniform small crystals (Fig 1b) According to laser diffraction analysis Fe-ZSM-5 (nano) crystals have the mean size of 330 nm and narrow size distribution (Fig 1, inset) It can be also seen in the HRTEM images of the sample (Fig 2a) The hierarchical zeolite Fe-ZSM-5 (hier) has interconnected macroporous system, obtained macropores being PS template replica (Fig 1c) In our previous work walls of macropores were shown to contain small zeolite ZSM-5 crystals stuck together with amorphous silica, both zeolite crystals and SiO2 globules having wide particle size distribution [21] The last sample Fe/SiO2 consists of amorphous silica globules with wide particle size distribution (Figs 1d and 2b) XRD patterns of materials obtained are given in Fig XRD reflexes for Fe-ZSM-5 (conv), Fe-ZSM-5 (nano) and Fe-ZSM-5 (hier) samples correspond to MFI structure [24], Fe/SiO2 sample is amorphous It should be emphasized that Fe-ZSM-5 (conv) and Fe-ZSM-5 (nano) have high crystallinity (Table 1) The crystallinity of Fe-ZSM-5 (hier) is only 58% due to the presence of amorphous phase Textural properties of materials are shown in Table 1, low temperature adsorption isotherms are shown in Fig One can see that all samples have high values of total surface area (416–838 m2/g) measured by BET method Hierarchical zeolites and amorphous samples are characterized also by high external surface area and total pore volume resulting from the microporosity of particles (Fig 5), which in turn have a wide size distribution with a large proportion of fine particles particularly in case of Fe/SiO2 (Fig 2b) Hierarchy factor [HF = (Vmic/Vtotal)  (SExt/SBET)] was calculated for all obtained samples, the amorphous sample having the highest value of HF = 0.11 (Table 1) In our previous work we have found that the presence of amorphous phase resulted to reduction of the micropore volume and therefore decreasing of the HF value, but this is not always the case Amorphous sample Fe/SiO2 has both high micropore volume and external surface area due to presence of TPAOH and PS template during the synthesis The pore size distribution confirms the micropores generation in the amorphous sample, micropores are likely to be formed due to the presence of TPAOH during the hydrothermal synthesis N2 adsorption isotherms for hierarchical zeolite and amorphous sample have a large hysteresis loop indicating the presence of mesopores and wide pore distribution (Fig 4) A horizontal hysteresis loop is observed for highly crystallized samples Fe-ZSM-5 (conv) and Fe-ZSM-5 (nano), indicating inkbottle-type mesopores A a b Volume, % 20 Mean size, 330 nm 15 10 0,1 10 100 Particle size, 10-6 m c 183 1000 d Fig SEM images of Fe-containing samples: (a) Fe-ZSM-5 (conv), (b) Fe-ZSM-5 (nano), (c) Fe-ZSM-5 (hier) and (d) Fe/SiO2 The particle size distribution of the sample FeZSM-5(nano), determined by laser diffraction, is shown in the inset 184 K.A Sashkina et al / Microporous and Mesoporous Materials 189 (2014) 181–188 Fig HRTEM images of (a) Fe-ZSM-5 (nano) and (b) Fe/SiO2 sloped hysteresis loop in case of samples containing amorphous silica phase indicates a presence of cylindrical mesopores As we will see later in this work the type of mesopores will play a key role for iron containing catalytic site stability in Fenton reactions The state of iron species in the obtained samples were studied by HRTEM analyses and UV–vis DR spectroscopy Fig shows the UV–vis DR spectra of materials For all samples two strong bands at 46,500 b 41,500 cm1 can be ascribed to t1 ? t2 and t1 ? e transitions due to the metal–oxygen charge transfer The spectrum for Fe/SiO2 with the band at 20,000 cm1 indicates that iron presents here in large oxide aggregates [25] For nonactivated white zeolite samples Fe-ZSM-5 (conv-as) and Fe-ZSM-5 (nano-as) absorption band edge at the 37,500 cm1 was observed and it was typical for ZSM-5 zeolites Extremely weak bands, referred to forbidden d–d transitions of the zeolitic Fe3+ in tetrahedral oxygen coordination, shown in the Fig 6b, may be clearly distinguished in the Fe-ZSM-5 (conv-as) and barely seen in the Fe-ZSM-5 (nano-as) According to Tanabe–Sugano diagram the band at 22,700 cm1 is referred to transition A1  T1 ðGÞ, at 24,600 cm1 – to A1  T2 ðGÞ and the band at 26,800 cm1 corresponds to sum of transitions A1  A2 and A1  EðGÞ [26] UV–vis DR spec- 800 400 Fe-ZSM-5 (conv) Intensity, r u 800 400 Fe-ZSM-5 (nano) 800 400 h-FeZSM-5 800 400 Fe/SiO2 10 15 20 25 30 35 40 2Θ (degree) Fig XRD patterns of the Fe-containing samples Table Iron content, crystallinity and textural properties of iron-containing samples Sample Fe (wt.%) Crystallinity (%) SBET (m2/g) SExt (m2/g) Vtotal (cm3/g) Vmic (cm3/g) Hierarchy factor Fe-ZSM-5 (conv) Fe-ZSM-5 (nano) Fe-ZSM-5 (hier) Fe/SiO2 2.74 1.73 1.29 1.89 97 100 58 416 543 454 838 27 77 397 475 0.22 0.54 0.70 0.79 0.19 0.19 0.07 0.16 0.06 0.05 0.09 0.11 a b 0.10 dV/dW, cm3/g/nm Volume, cm3/g, STP 700 600 500 400 Fe/SiO2 200 ier) (h SMZ e F Fe-ZSM-5 (nano) 100 FeZSM-5 (conv) 300 0.0 0.2 0.4 0.6 P/P0 0.8 0.08 0.06 Fe-ZSM-5 (conv) 0.04 Fe-ZSM-5 (nano) Fe/SiO2 0.02 FeZSM-5 (hier) 1.0 0.00 10 15 20 Pore width, nm Fig N2 adsorption (solid symbols) and desorption (open symbols) isotherms at 77 K (a) and pore size distribution according to DFT method (b) for different Fe-containing samples K.A Sashkina et al / Microporous and Mesoporous Materials 189 (2014) 181–188 185 Fig HRTEM images of the (a) Fe-ZSM-5 (hier) and (b) Fe/SiO2 Fig UV–vis DR spectra of the Fe-containing samples: (1) Fe/SiO2, (2) Fe-ZSM-5 (conv-as), (3) Fe-ZSM-5 (conv), (4) Fe-ZSM-5 (hier), (5) Fe-ZSM-5 (nano), (6) Fe-ZSM-5 (nano-as) troscopy data and white color of nonactivated samples Fe-ZSM-5 (conv-as) and Fe-ZSM-5 (nano-as) indicated that isolated Fe3+ ions mainly occupy the tetrahedral framework positions Zeolite samples Fe-ZSM-5 (conv-as) and Fe-ZSM-5 (nano-as) were activated by oxalic acid treatment followed by drying and calcination Less than wt.% of Fe was leached during zeolite activation according ICP–OES Activated samples became tan indicating the formation of small iron oxide or hydroxide clusters This fact was also confirmed by the UV–vis DR spectra – intensive band at 35,000 cm1 for treated samples can be seen (Fig 6) This band may be ascribed to the metal–oxygen charge transfer in the clusters of iron in octahedral oxygen coordination The sample FeZSM-5 (nano-as) also contains such clusters but in the fewer quantity than in the treated Fe-ZSM-5 (nano) Crystallization of zeolite Fe-ZSM-5 (hier) in the presence of PS templates resulted in significant modifications of UV–vis DR spectra, which can be described by the superposition of absorption of ferric ions in small oxide clusters and iron in large oxide aggregates on the surface of amorphous silica [27] According to HRTEM analyses all active samples contain uniformly distributed iron oxide clusters of 2–3 nm (Fig 7) Elemental EDX analyses showed that 35 atomic% of iron in Fe-ZSM-5 (hier) was located in zeolite crystals; the rest one was in amorphous silica phase Experimental determination of the phase composition of ironcontaining particles is complicated by the fact that their relative content in zeolite is small and they have a very small size The nature of iron-containing particles in the samples can be assumed theoretically from thermodynamic probability of the existence of known iron containing oxide phases, they may be hematite aFe2O3, magnetite Fe3O4, crystal FeOOH or amorphous Fe(OH)3 In presence of M hydrogen peroxide equilibrium existence of magnetite is not probable: free standard Gibbs energy of the reaction 2Fe3 O4solid þ H2 O2solute () 3a  Fe2 O3solid þ H2 Oliquid is 312.73 kJ mol1 Because of negative standard entropy change of the reaction (69.85 J mol1 K1) phase transformation of hematite to magnetite is less probable during thermal treatment of the samples Due to large negative standard free energy of transformation of amorphous Fe(OH)3 to hematite, presence of the first form may be taken out of the consideration Among crystal forms of FeOOH there are goethite a-FeOOH, akaganit b-FeOOH and lepidocrocite c-FeOOH According to literature data c-FeOOH, being produced at low concentrations of ferric aqua ions from ferric hydroxide particles with a small molecular weight, and akaganit b-FeOOH, being formed in the presence of chloride ions, easily transform to goethite or hematite [28] During the aging of ferric hydroxide polymers in alkaline medium goethite a-FeOOH preferentially is produced, while hematite is formed in acidic medium Thus the samples just after the hydrothermal treatment most probably contain iron in the form of goethite However as a result of subsequent heat treatment of the sample, phase transformation of goethite to hematite may occur The known temperature of goethite to hematite transformation is 136 °C, but if particle size is several nanometers it may reach 700 °C Thus a stabilization of iron oxide species in the form of goethite in all samples is most probable, but the existence of hematite cannot be denied Assuming that the most iron containing particles is goethite or hematite, it is possible to estimate the possible amount of iron particles with diameter of nm in the zeolite particle of microns The ideal cell of calcined ZSM-5 (silicalite-1) is described by the formula Si96xFexO192 [29] At the iron content in the zeolite of wt.%, two atoms are located in a single cell: x = If the mean size of a zeolite particle is lm then the volume of this particle is 1.4  1011 cm3 (assuming a spherical form of the particle), whereas the volume of a cell is 5.4  1021 cm3 [29] Then zeolite particle contains 2.6  109 cells and  109 iron atoms If we assume that the volume of an iron containing particle with the size of nm is 1.4  1020 cm3, and its density is 4–5 g cm3, then each iron particle contains 300–400 iron atoms Thus there may be 186 K.A Sashkina et al / Microporous and Mesoporous Materials 189 (2014) 181–188 Fig HRTEM images of (a) Fe-ZSM-5 (conv), (b) Fe-ZSM-5 (nano), (c) Fe-ZSM-5 (hier) and (d) Fe/SiO2 1.4  107 of nanosized iron-containing particles in primary zeolite particle with a size of lm (if all iron in zeolite is in oxidic clusters, but really it is ca 30 at.%, the rest iron is in the form of isolated ferric ions in tetrahedral oxygen coordination) Note that iron containing particles in samples occupy from 0.1% to 4% of total pore volume of amorphous Fe/SiO2 and Fe-ZSM-5 (conv), respectively Samples obtained were tested in hydrogen peroxide decomposition as since this reaction is the main source of oxidative activity of Fenton reagent in organic substrates oxidation Comparative catalytic experiments were carried out in presence of EDTA anions, which form strong complexes with iron ions, to test catalytic stability of iron species supported on zeolite and amorphous samples One can see H2O2 decomposition kinetic curves and values of initial hydrogen peroxide decomposition rates in Fig and Table 2, respectively Catalytic activity of zeolites Fe-ZSM-5 (conv) and Fe-ZSM-5 (nano) is lower than that of homogeneous Fenton system, the fact was also observed in [3] For samples, containing Fig H2O2 decomposition kinetics at [H2O2]0 = 1.1 M in absence (open symbols) and presence (solid symbols) of Na2EDTA in Fe-containing systems The catalyst concentration was mM and 20 g L1 for homogeneous and heterogeneous systems, respectively amorphous silica phase, Fe/SiO2 and hierarchical Fe-ZSM-5 (hier), catalytic activity is significantly lower compared with highly crystallized zeolites This fact seems to be a result of wide size distribution of iron containing particles unlike that one’s with specified size of 2–4 nm in the zeolite However specific structure of these particles inside zeolite mesopores also should be taken into account and requires an additional study Values of catalytic activity in decomposition of 1.1 M H2O2 at 25 °C for Fe(NO3)3, Fe-ZSM-5 (conv), Fe-ZSM-5 (nano), Fe-ZSM-5 (hier) and Fe/SiO2 are 17, 6.4, 9.0, 0.6 and 0.2 mmol H2O2 min1 g (Fe)1 It is worth noting that pure zeolites show also the remarkable stability in presence of EDTA anions as no induction period indicating complex formation is observed for Fe-ZSM-5 (conv) and Fe-ZSM-5 (nano) samples despite of high EDTA tendency to form complexes with iron: logarithm of the stability constant is 14.3 and 25.1 for 1:1 complex of EDTA with Fe(II) and Fe(III), respectively Initial H2O2 decomposition rate for Fe/SiO2 in presence of EDTA is observed to be higher than without it After 75 the reaction accelerates and H2O2 decomposition rate becomes close to the one in the homogeneous system This means that iron species supported on the amorphous silica are not stable and form complexes with EDTA which are subsequently leached When EDTA is oxidized by H2O2 iron in form of hydroxides adsorbs on the surface of the catalyst since pH = This may explain why iron concentration in solutions after the reactions was less than  105 M according to ICP–OES The kinetic curve behavior for hierarchical zeolite is similar to the amorphous sample, however longer induction period is observed, perhaps due to presence of different iron species supported on the amorphous silica and enclosed in zeolite pores Such low catalyst stability can be related with the location of iron species on the external surface of amorphous particles or in cylindrical mesopores which not limit the diffusion of EDTA On the contrary iron species included in the zeolite are likely to be mainly distributed inside crystals in the inkbottle mesopores forming the ‘‘ship in a bottle’’ catalytic sites This type of a structure results in spatial inaccessibility of catalytic sites for EDTA anions Thus the zeolite microporous structure encourages high activity and stability of catalytic sites during Fenton reaction 187 K.A Sashkina et al / Microporous and Mesoporous Materials 189 (2014) 181–188 Table Initial hydrogen peroxide decomposition rate, Pa s1in absence and presence of iron complexing agent for Fenton-type systems at 25 °C, [H2O2]0 = 1.1 M W O2 , [Na2EDTA] = W O2 , [Na2EDTA] = g/L Fe(NO3)3 Fe-ZSM-5 (conv-as) Fe-ZSM-5 (conv) Fe-ZSM-5 (nano-as) Fe-ZSM-5 (nano) Fe-ZSM-5 (hier) Fe/SiO2 14.4 0.16 (11.3 after 90 min) 0.63 – 10.77 9.69 6.11 5.75 9.39 6.66 0.46 0.34 (1.7 after 115 min) 0.26 0.41 (15.3 after 75 min) Again theoretical thermodynamics estimation may support this version Let us consider the dissolution reaction of iron containing particles without any supporting silicate matrix in pure water and in presence of hydrogen peroxide Dissolution reactions for goethite and hematite are as follows: FeOOHsolid ỵ 3Hỵaqua () Fe3ỵ aqua ỵ 2H2 Oliquid ; Fe2 O3solid ỵ 6Hỵaqua () 2Fe3ỵ aqua ỵ 3H2 Oliquid : The standard Gibbs energy change for these reactions with massive solid phases is 0.2 and 0.4 kJ mol1, respectively [30] In case of nanometer spherical particle dissolving DG0 decreases on the value  solid 2rV ; r  – is a molar where r – is a surface tension, r – is a particle radius, V volume of the solid phase If assuming that r  0.6 J m2 and r = 1.5 nm, than this reduction is 16.7 and 24.4 kJ mol1 for goethite and hematite particles, respectively Thus DG0 is 16.5 and 24 kJ mol1 for dissolution of dispersed goethite and hematite phases, respectively From the equation of isotherm of the reactions: a 3ỵ Dr G ỵ ln Fe3 0; RT aHỵ a 3ỵ Dr G ỵ ln Fe6 0: RT aHỵ follows that at 25 °C the equilibrium activity of aqua iron ions depends on the acidity as follows: lgaFe3ỵ 2:9  3pH; lgaFe3ỵ 2:1  3pH: This means that in pure water only at pH < the iron leaching from the particles to the solution should be expected But the situation changes dramatically when the dissolution occurs in presence of hydrogen peroxide Let us assume that iron-containing particles exist in the form of Fe2O3 and consider the reaction: Fe2 O3solid ỵ 2H2 O2soluted ỵ 4Hỵqaua () 2Fe2ỵ aqua ỵ 3H2 Oliquid þ 2HO2aqua : The standard Gibbs energy change for this reaction is 39.5 kJ mol1 In the case of nanosized particle dissolving the Gibbs energy of the reaction will be less on 24.4 kJ mol1 as described above and it is 63.9 kJ mol1 If one takes that aH2 O2 = M and aHO2 = 108 M then lgaFe2ỵ ¼ 13:6  2pH: Thus, the estimation means that at pH < the total reductive dissolution of nanosized iron containing particles must occur at experimental conditions used in the work Undoubtedly the iron ion hydrolysis reaction should be taken into account together with the reductive dissolution reaction, but if there are iron complexing agents in the solution, the iron leaching from the solid particles should be expected to prevail Nevertheless no any significant iron leaching and no any consecutive deactivation are observed for zeolite samples unlike Fe/SiO2 and Fe-ZSM-5 (hier), containing amorphous silica phase This fact may indicate a key role of microporous structure of zeolite on the high activity and stability of the heterogeneous Fenton catalyst Catalytic site protection by zeolitic matrix could be potentially explored for development of technology for purification of waste water from nuclear power plants The problem is in large amount of wastes containing radionuclides, for example 60Co, which are enclosed in soluble complexes including EDTA ones that pass through filters and cannot be separated from the solution without a special pretreatment Wet hydrogen peroxide oxidation of complexing agents by H2O2 using Fe-containing zeolites has a good potential for this application Conclusions Four types of iron containing materials have been synthesized and studied: conventional zeolite Fe-ZSM-5 (conv), hierarchical zeolite Fe-ZSM-5 (hier), small crystals (d = 330 nm) of zeolite FeZSM-5 (nano) and ferric oxide species supported on the amorphous silica Catalytic activity of ferric species in hydrogen peroxide decomposition for small 330-nm crystals of Fe-ZSM-5 was 1.4 times higher than for large zeolite crystals, and significant decrease of the activity was observed for samples containing amorphous silica phase The experimental results show that ferric sites composed of zeolite are stable due to the limited diffusion of Na2EDTA in microporous zeolitic phase Ferric species supported on the amorphous silica are extremely unstable during wet peroxide oxidation reactions and undergo leaching to the solution Acknowledgements The authors thank A.B Ayupov, S.V Bogdanov, E.Yu Gerasimov for their help in the catalyst characterization Financial supports by Department of Science and Education (projects Nos 14.512.12.0005 and 8440), Russian Federation President Grant for the Leading Scientific Schools #NSh 524.2012.3, integration projects Nos 35 and 24 are gratefully acknowledged References [1] M.B Kasiri, H Aleboyeh, A Aleboyeh, Appl Catal B Environ 84 (1–2) (2008) 9– 15 [2] A Zecchina, M Rivallan, G Berlier, C Lamberti, G Ricchiardi, Phys Chem Chem Phys (27) (2007) 3483–3499 [3] E.V Kuznetsova, E.N Savinov, L.A Vostrikova, V.N Parmon, Appl Catal B Environ 51 (3) (2004) 165–170 [4] L.F Liotta, M Gruttadauria, G.Di Carlo, G Perrini, V Librando, J Hazard Mater 162 (2009) 588–606 [5] O.A Makhotkina, E.V Kuznetsova, L.G Matvienko, V.N Parmon, Catal Ind (2006) 30–37 [6] O.A Makhotkina, S.V Preis, E.V Parkhomchuk, Appl Catal B Environ 84 (2008) 821–826 [7] G Soler-Illi, C Sanchez, B Lebeau, J Patarin, Chem Rev 102 (2002) 4093– 4138 [8] A Corma, Chem Rev 97 (1997) 2373–2419 [9] Y Tao, H Kanoh, L Abrams, K Kaneko, Chem Rev 106 (2006) 896–910 [10] C.S Cundy, P.A Cox, Microporous Mesoporous Mater 82 (2005) 1–78 [11] J Perez-Ramırez, C.H Christensen, K Egeblad, Ch.H Christensen, Chem Soc Rev 37 (2008) 2530–2542 [12] A Stein, Microporous Mesoporous Mater 44–45 (2001) 227–239 188 K.A Sashkina et al / Microporous and Mesoporous Materials 189 (2014) 181–188 [13] K.A Sashkina, V.S Labko, N.A Rudina, V.N Parmon, E.V Parkhomchuk, J Catal 299 (2013) 44–52 [14] G.I Panov, A.K Uriarte, M.A Rodkin, V.I Sobolev, Catal Today 41 (1998) 365– 385 [15] A.J.J Koekkoek, H Xin, Q Yang, C Li, E.J.M Hensen, Microporous Mesoporous Mater 145 (1–3) (2011) 172–181 [16] El.-M El-Malki, R.A Santen, W.M.H Sachtler, J Catal 196 (2000) 212–223 [17] P Sazama, N.K Sathu, E Tabor, B Wichterlová, Š Sklenák, Z Sobalík, J Catal 299 (2013) 188–203 [18] A Ates, Ch Hardacre, A Goguet, Appl Catal A 441–442 (2012) 30–41 [19] A.R Overweg, M.W.J Crajé, A.M Kraan, I.W.C.E Arends, A Ribera, R.A Sheldon, J Catal 223 (2004) 262–270 [20] J Pérez-Ramírez, F Kapteijn, G Mul, J.A Moulijn, Catal Commun (2002) 19– 23 [21] E.V Parkhomchuk, K.A Sashkina, N.A Rudina, N.A Kulikovskaya, V.N Parmon, Catal Ind (2012) 23–32 [22] D Zou, S Ma, R Guan, M Park, L Sun, J.J Aklonis, R.J Salovey, Polym Sci A 30 (1992) 137–144 [23] B.T Holland, C.F Blanford, A Stein, Science 281 (1998) 538–540 [24] M.M.J Treacy, J.B Higgins, R Ballmoos, Collection of Simulated XRD Powder Patterns for Zeolites, third ed., The Commission of the International Zeolite Association, 1996 [25] P Ratnasami, R Kumar, Catal Today (4) (1991) 357 [26] E.J.M Hensen, Q Zhu, M.M.R.M Hendrix, A.R Overweg, P.J Kooyman, M.V Sychev, R.A Santen, J Catal 221 (2004) 560 [27] A Ribera, I.W.C.E Arends, J Catal 195 (2000) 287 [28] Ch Flynn, Chem Rev 84 (1984) 31–41 [29] Ch Baerlocher, W.M Meier, D.H Olson, Atlas of Zeolite Framework Types, Elsevier, 2001 [30] I Barin, Thermochemical Data of Pure Substances, third ed., VCH Verlagsgesellschaft mbH, D-6945 Weinheim (Federal Republic of Germany), 1995  ... structure of zeolite on the high activity and stability of the heterogeneous Fenton catalyst Catalytic site protection by zeolitic matrix could be potentially explored for development of technology for. .. transformation of amorphous Fe(OH)3 to hematite, presence of the first form may be taken out of the consideration Among crystal forms of FeOOH there are goethite a-FeOOH, akaganit b-FeOOH and lepidocrocite... diameter of nm in the zeolite particle of microns The ideal cell of calcined ZSM-5 (silicalite-1) is described by the formula Si96xFexO192 [29] At the iron content in the zeolite of wt.%, two atoms