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: conven
Trang 1The role of zeolite Fe-ZSM-5 porous structure for heterogeneous Fenton
catalyst activity and stability
K.A Sashkinaa, E.V Parkhomchuka,b,⇑, N.A Rudinab, V.N Parmona,b
a
Novosibirsk State University, 2 Pirogova st., Novosibirsk 630090, Russia
b
Boreskov Institute of Catalysis SB RAS, 5 Lavrentieva st., Novosibirsk 630090, Russia
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 treat-ment, 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 solid-phase Fe-containing samples as well as dissolved Fe(NO3)3were tested in H2O2decomposition reactions
in absence or presence of iron-complexing agent Na2EDTA, which has been used to test the catalyst sta-bility 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 H2O2using Fe-containing zeolites has a good potential for purification of nuclear waste water
Ó 2013 Elsevier Inc All rights reserved
1 Introduction
Fe-ZSM-5 has been shown to be a promising heterogeneous
so-lid-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
etha-nol, as well as extent of H2O2utilization is higher in such a
heter-ogeneous system compared with the homheter-ogeneous 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
struc-ture 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/H2O2and 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
ad-sorbed 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
con-vertible macromolecules accessibility of catalytic sites for high
MW substrates should be significantly increased In order to
pre-pare zeolites with additional meso or macroporosity a range of techniques may be used[7–11] Here, hierarchically ordered zeo-litic 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 Fe-ZSM-5 was tested in total oxidation of large organic molecules,
Na2EDTA and lignin, used as large model compounds[13] As a re-sult 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 conven-tional zeolite[13] However the question on catalyst stability dur-ing oxidation of high MW organics when a range of different organic acids may be formed is still open This question is obvi-ously 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 nanocrys-tals 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
1387-1811/$ - see front matter Ó 2013 Elsevier Inc All rights reserved.
⇑ Corresponding author at: Boreskov Institute of Catalysis SB RAS, 5 Lavrentieva
st., Novosibirsk 630090, Russia Tel./fax: +7 (383)333 16 17.
E-mail address: ekaterina@catalysis.ru (E.V Parkhomchuk).
Contents lists available atScienceDirect
Microporous and Mesoporous Materials
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / m i c r o m e s o
Trang 2To clarify separate roles of these two iron containing phases
(zeolitic and silica) for wet peroxide oxidation catalysis we
synthe-sized 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-ZSM-5 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 N2adsorption, scanning and high resolution
transmis-sion electron microscopies, UV–vis diffuse reflectance
spectros-copy 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 SiO2ferric sites in Fe-ZSM-5 appeared to be stable
under the action of organic acids
2 Experimental
2.1 Chemicals
Styrene monomer, inhibited with 1% hydroquinone, was
pur-chased from Ltd ‘‘Angara-reactive’’ It was washed 4 times in
separ-atory funnel with an equal volume of 1 M aqueous solution of
sodium hydroxide, followed by 4 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
nonahy-drate 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
2.2 Catalyst preparation
2.2.1 PS template preparation
PS were synthesized using emulsifier-free emulsion
polymeri-zation 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
tem-plate was washed by ethanol and dried in air Before hydrothermal
synthesis PS templates were put on the grid and sustained over
boiling water for 1 h
2.2.2 Synthesis of conventional zeolites
The synthetic Fe-containing conventional zeolite Fe-ZSM-5
(conv) was produced hydrothermally from precursor gel
contain-ing 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,
dried and calcined at 500 °C for 5 h in air The sample is labeled ZSM-5 (conv-as) For catalyst pretreatment the powder of Fe-ZSM-5 (conv-as) was suspended to the 1 M aqueous solution of oxalic acid in concentration of 100 g L1 and stirred for 1 h at
50 °C The catalyst was rinsed with distilled water to pH 7.0, dried
in air and calcined at 500 °C for 3 h The pretreated sample is la-beled 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 There-after, PS template was impregnated with the gel with weight ratio
1 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 fi-nally calcined at 500 °C for 8 h in air The control sample of Fe-con-taining hierarchical zeolite was pretreated by the oxalic acid as stated above, but this procedure did not change the catalytic activ-ity, 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 suspen-sion was centrifuged, the precipitate was washed with abundant amount of water, dried at an ambient temperature overnight and finally calcined at 500 °C for 5 h in air The sample is labeled Fe-ZSM-5 (nano-as) The powder of Fe-Fe-ZSM-5 (nano-as)was sus-pended to the 1 M aqueous solution of oxalic acid in concentration
of 100 g L1and stirred for 10 min at 50 °C The pretreated catalyst was rinsed with distilled water to pH 7.0, dried in air and calcined
at 500 °C for 3 h The sample is labeled Fe-ZSM-5 (nano)
2.2.5 Synthesis of Fe-containing amorphous silica Fe-containing amorphous silica Fe/SiO2was synthesized by the same way as h-Fe-ZSM-5 described above but without sustaining
PS templates over the boiling water for 1 h before hydrothermal synthesis
2.3 Catalyst characterization The X-ray diffraction analysis was performed by a diffractome-ter HZG-4 with a Cu-Karadiation 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 elec-tron 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 mea-sured 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
UV-2501 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 8 h The specific surface area (SBET) was determined by applying Brunauer–Emmet–Teller (BET) equation
Trang 3from adsorption branches in the relative pressure range of 0.05–
0.3 The external surface area (SExt) and micropore volume (Vmic)
were calculated byas-method The value of Vtotwas single point
total pore volume at P/P0= 0.98 Hierarchy factor (HF) was
calcu-lated as SExt/SBETVmic/Vtotal
2.4 Catalytic activity tests
Iron-containing samples were tested in H2O2 decomposition
reactions in absence and presence of 1 g L1sodium
ethylenedi-aminetetraacetate (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 5 mM Fe(NO3)3and 20 g L1zeolites were
used as catalysts for homogeneous and heterogeneous reactions,
respectively The H2O2decomposition rate WO2at [H2O2]0= 1.1 M
was determined as the oxygen release rate measured
barometri-cally in Pa s1
3 Results and discussion
Morphologies of Fe-containing materials synthesized in given
work are shown inFigs 1 and 2 Conventional zeolite Fe-ZSM-5
(conv) was found to have large polycrystals of 2–5lm 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
sam-ple (Fig 2a) The hierarchical zeolite Fe-ZSM-5 (hier) has
intercon-nected 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
SiO2globules having wide particle size distribution[21] The last sample Fe/SiO2 consists of amorphous silica globules with wide particle size distribution (Figs.1d and2b)
XRD patterns of materials obtained are given inFig 3 XRD re-flexes 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 inTable 1, low tem-perature adsorption isotherms are shown inFig 4 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 cal-culated 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/SiO2has 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 sam-ple have a large hysteresis loop indicating the presence of mesop-ores 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
0,1 1 10 100 1000 0
5 10 15
Particle size, 10 -6 m
Fig 1 SEM images of Fe-containing samples: (a) Fe-ZSM-5 (conv), (b) Fe-ZSM-5 (nano), (c) Fe-ZSM-5 (hier) and (d) Fe/SiO 2 The particle size distribution of the sample
Trang 4Fe-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 6 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/SiO2with the band at 20,000 cm1indicates 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 cm1was observed and it was typical for ZSM-5 zeolites Extremely weak bands, referred to for-bidden d–d transitions of the zeolitic Fe3+ in tetrahedral oxygen coordination, shown in theFig 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 cm1is referred to transition6A14T1ðGÞ, at 24,600 cm1
– to 6A14T2ðGÞ and the band at 26,800 cm1 corresponds to sum of transitions6A14A2and6A14EðGÞ[26] UV–vis DR
spec-Fig 2 HRTEM images of (a) Fe-ZSM-5 (nano) and (b) Fe/SiO 2
0
400
800
2Θ (degree)
0
400
800
0
400
800
Fe-ZSM-5 (nano) Fe-ZSM-5 (conv)
h-FeZSM-5
Fe/SiO2
0
400
800
Fig 3 XRD patterns of the Fe-containing samples.
Table 1
Iron content, crystallinity and textural properties of iron-containing samples.
Sample Fe (wt.%) Crystallinity (%) S BET (m 2 /g) S Ext (m 2 /g) V total (cm 3 /g) V mic (cm 3 /g) Hierarchy factor
a
0.0 0.2 0.4 0.6 0.8 1.0 100
200 300 400 500 600 700
Fe/SiO 2
Fe-Z SM -5 (h ier)
Fe-ZSM-5 (nano)
3 /g
FeZSM-5 (conv)
P/P0
b
0.00 0.02 0.04 0.06 0.08 0.10
Fe-ZSM-5 (conv) Fe-ZSM-5 (nano)
FeZSM-5 (hier)
3 /g /n
Pore width, nm Fe/SiO2
Fig 4 N 2 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
Trang 5troscopy 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
sam-ples Fe-ZSM-5 (conv-as) and Fe-ZSM-5 (nano-as) were activated
by oxalic acid treatment followed by drying and calcination Less
than 3 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
con-firmed 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
clus-ters of iron in octahedral oxygen coordination The sample
Fe-ZSM-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
clus-ters 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
iron-containing 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 a
-Fe2O3, magnetite Fe3O4, crystal FeOOH or amorphous Fe(OH)3 In
presence of 1 M hydrogen peroxide equilibrium existence of
magnetite is not probable: free standard Gibbs energy of the
reaction
is 312.73 kJ mol1 Because of negative standard entropy change
of the reaction (69.85 J mol1K1) phase transformation of hema-tite to magnehema-tite is less probable during thermal treatment of the samples Due to large negative standard free energy of transforma-tion of amorphous Fe(OH)3to hematite, presence of the first form may be taken out of the consideration Among crystal forms of
FeO-OH there are goethitea-FeOOH, akaganit b-FeOOH and lepidocro-cite 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 goethitea-FeOOH prefer-entially 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 par-ticles with diameter of 3 nm in the zeolite particle of 3 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
2 wt.%, two atoms are located in a single cell: x = 2 If the mean size
of a zeolite particle is 3lm then the volume of this particle is 1.4 1011cm3 (assuming a spherical form of the particle), whereas the volume of a cell is 5.4 1021cm3[29] Then zeolite particle contains 2.6 109cells and 5 109iron atoms If we as-sume that the volume of an iron containing particle with the size
of 3 nm is 1.4 1020cm3, and its density is 4–5 g cm3, then each iron particle contains 300–400 iron atoms Thus there may be
Fig 5 HRTEM images of the (a) Fe-ZSM-5 (hier) and (b) Fe/SiO 2
Fig 6 UV–vis DR spectra of the Fe-containing samples: (1) Fe/SiO 2 , (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).
Trang 61.4 107of nanosized iron-containing particles in primary zeolite
particle with a size of 3lm (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
fer-ric ions in tetrahedral oxygen coordination) Note that iron
con-taining particles in samples occupy from 0.1% to 4% of total pore
volume of amorphous Fe/SiO2and Fe-ZSM-5 (conv), respectively
Samples obtained were tested in hydrogen peroxide
decompo-sition as since this reaction is the main source of oxidative activity
of Fenton reagent in organic substrates oxidation Comparative
cat-alytic 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
sam-ples One can see H2O2decomposition kinetic curves and values of
initial hydrogen peroxide decomposition rates inFig 8andTable 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
amorphous silica phase, Fe/SiO2and hierarchical Fe-ZSM-5 (hier), catalytic activity is significantly lower compared with highly crys-tallized zeolites This fact seems to be a result of wide size distribu-tion 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 activ-ity in decomposition of 1.1 M H2O2at 25 °C for Fe(NO3)3, Fe-ZSM-5 (conv), Fe-ZSM-5 (nano), Fe-ZSM-5 (hier) and Fe/SiO2are 17, 6.4, 9.0, 0.6 and 0.2 mmol H2O2min1g (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: loga-rithm 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 decomposi-tion rate for Fe/SiO2in presence of EDTA is observed to be higher than without it After 75 min 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 sub-sequently leached When EDTA is oxidized by H2O2iron in form of hydroxides adsorbs on the surface of the catalyst since pH = 7 This may explain why iron concentration in solutions after the reactions was less than 5 105M according to ICP–OES The kinetic curve behavior for hierarchical zeolite is similar to the amorphous sam-ple, however longer induction period is observed, perhaps due to presence of different iron species supported on the amorphous sil-ica 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 do 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 ink-bottle mesopores forming the ‘‘ship in a ink-bottle’’ catalytic sites This type of a structure results in spatial inaccessibility of catalytic sites for EDTA anions Thus the zeolite microporous structure encour-ages high activity and stability of catalytic sites during Fenton reaction
Fig 7 HRTEM images of (a) Fe-ZSM-5 (conv), (b) Fe-ZSM-5 (nano), (c) Fe-ZSM-5 (hier) and (d) Fe/SiO 2
Fig 8 H 2 O 2 decomposition kinetics at [H 2 O 2 ] 0 = 1.1 M in absence (open symbols)
and presence (solid symbols) of Na 2 EDTA in Fe-containing systems The catalyst
concentration was 5 mM and 20 g L 1 for homogeneous and heterogeneous
Trang 7Again 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
goe-thite and hematite are as follows:
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 dissolvingDG0decreases on the value
wherer– is a surface tension, r – is a particle radius, V – is a molar
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 mol1for 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:
DrG0
H þ
DrG0
Fe 3þ
H þ
follows that at 25 °C the equilibrium activity of aqua iron ions
depends on the acidity as follows:
This means that in pure water only at pH < 1 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 Fe2O3and consider
the reaction:
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
aH 2 O 2= 1 M and aHO
2= 108M then
Thus, the estimation means that at pH < 8 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 zeo-lite samples unlike Fe/SiO2and Fe-ZSM-5 (hier), containing amor-phous silica phase This fact may indicate a key role of microporous structure of zeolite on the high activity and stability of the hetero-geneous 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 prob-lem is in large amount of wastes containing radionuclides, for example60Co, 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 perox-ide oxidation of complexing agents by H2O2using Fe-containing zeolites has a good potential for this application
4 Conclusions Four types of iron containing materials have been synthesized and studied: conventional zeolite Fe-ZSM-5 (conv), hierarchical zeolite ZSM-5 (hier), small crystals (d = 330 nm) of zeolite Fe-ZSM-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 amor-phous silica phase The experimental results show that ferric sites composed of zeolite are stable due to the limited diffusion of Na
2-EDTA 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
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