ISSN 00231584, Kinetics and Catalysis, 2013, Vol. 54, No. 5, pp. 638–643. © Pleiades Publishing, Ltd., 2013. Original Russian Text © K.A. Sashkina, V.S. Semeikina, V.S. Labko, N.A. Rudina, E.V. Parkhomchuk, 2013, published in Kinetika i Kataliz, 2013, Vol. 54, No. 5, pp. 676–682.
“CATALYSIS: FROM SCIENCE TO INDUSTRY,” II RUSSIAN NATIONAL YOUNG SCIENTISTS’ CONFERENCE (TOMSK, OCTOBER 28–NOVEMBER 2, 2012)
Template Method for the Synthesis of a Heterogeneous Fenton Catalyst Based on the Hierarchical Zeolite FeZSM51 K. A. Sashkinaa *, V. S. Semeikinab, V. S. Labkoc, N. A. Rudinaa, and E. V. Parkhomchuka,b a
Boreskov Institute of Catalysis, Novosibirsk, 630090 Russia Novosibirsk State University, Novosibirsk, 630090 Russia c Joint Institute for Power and Nuclear Research—Sosny, Minsk, BY220109 Belarus *email:
[email protected] b
Received November 30, 2012
Abstract—Zeolite hFeZSM5 with a hierarchical micro/macropore system has been synthesized in the presence of a template based on the closepacked polystyrene (PS) spheres, and the conventional zeolite FeZSM5 has been obtained in the absence of a PS template. The zeolites have been characterized by Xray diffraction, scanning and highresolution transmission electron microscopy, and N2 sorption. The macropore walls of the hierarchical zeolite consist of ZSM5 nanocrystals and amorphous globules of silica. Compared to the conventional zeolite, the hierarchical one has a high BET and external surface areas of 245 and 472 m2/g, respectively, and a high pore volume of 0.6 cm3/g. The catalytic properties of the Fecontaining zeolites were studied in the H2O2 decomposition reaction in the absence and in the presence of EDTA ligands and in the oxidation of low and highmolecularweight organic compounds by hydrogen peroxide at 25°C. Hierarchical zeolite hFeZSM5 is highly efficient in the oxidation of large molecules. DOI: 10.1134/S0023158413050145 1
Fenton catalysts, which are aqueous solutions of fer ric chloride, nitrate, or sulfate containing hydrogen peroxide, are attractive for purification of water con taining organic contaminants at low concentrations. These catalysts combine high oxidative activity and environmental friendliness. Heterogeneous Fenton catalysts are more suitable for practical use, since they are ferric compounds supported on a solid phase (alu mina, silica, clays, zeolites, and others). The heteroge neous catalysts are readily separable from the reaction medium and work in a wider pH range than their homogeneous counterparts [1–3].
Ironcontaining zeolites are more promising among the heterogeneous Fenton catalysts [4, 5]. It was shown that ferric ions in zeolite active sites do not undergo complexing by oxidation intermediates; moreover, the degree of mineralization of lowmolecularweight (MW) substances, such as 1,1dimethylhydrazine and ethanol, was higher in the heterogeneous Fenton sys tem FeZSM5/H2O2 than in the homogeneous one [6, 7]. The high efficiency of the heterogeneous system FeZSM5/ H2O2 compared to the homogeneous Fen ton system resulted from substrate adsorption on the zeolite surface. This adsorption increased the rate of the interaction between the organic substrate and the OH radicals generated on Fecontaining sites located on the micropore surface. However, the degree of mineralization in highMW lignin oxidation was much lower with the FeZSM5/ H2O2 system than with a homogeneous one, such as 1
The article was translated by the authors.
Fe(NO3)3/H2O2 and H2O2/UV [8]. Perhaps, the low degree of mineralization of lignin in the heterogeneous system FeZSM5/H2O2 was due to the low accessibility of the zeolite porous area to highMW lignin. Despite the high surface area of ZSM5 zeolite (350–450 m2/g), the presence of 0.55nm micropores and the absence of wider pores reduces the accessibility of the active sur face to large molecules [9]. In the last decade, a great number of organic con taminants, including pharmacologically active ones, which are mostly highMW compounds, have been detected in the world water resources. The concentra tions of some antibiotics, hormones, and antidepres sants are low, ranging from nanograms to micrograms per liter, but such quantities make the water unsafe for consumers [10, 11]. To apply Fenton catalysts based on zeolites to water purification from macromolecular pol lutants, the accessibility of catalytic sites in zeolites to large molecules should be increased. There are different methods for producing hierarchical zeolites containing a meso/macroporous system along with micropores for increasing the accessibility of active sites [12, 13]. One of them is to use closepacked polymer nanospheres as templates in the hydrothermal crystallization of zeolites followed by removal of the template by calcination or extraction [14, 15]. This study focused on developing a method of syn thesis of hierarchical zeolite FeZSM5 using polysty rene (PS) nanospheres as a template. The main aim was to study the effect of texture characteristics of the zeolitic catalyst on the efficiency of oxidation of low and highMW substances by hydrogen peroxide.
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EXPERIMENTAL The following chemicals were used in this study. Inhibited styrene monomer (pure grade, Angarareak tiv, Russia), NaOH (analytical grade, Reakhim, Rus sia), K2S2O8 (98%, Aldrich, United States), tetrapropy lammonium hydroxide (TPAOH, 25% solution in water, Aldrich, United States), tetraethyl orthosilicate (TEOS, analytical grade, Angarareaktiv, Russia), Fe(NO3)3 ⋅ 9H2O (reagent grade, Institute of Catalysis, Russia), hydrogen peroxide (30% solution in water, Baza No. 1 khimreaktivov, Russia), phenol (special purity grade, komponent reaktiv, Russia), disodium ethylenediaminetetraacetate (Na2EDTA, Khimfarm produkt, Russia), lignin (Aldrich, United States), and HCl (special purity grade, Sigma tek, Russia). Polystyrene spheres were synthesized using the emulsifierfree emulsion polymerization technique as described in the literature [16]. The emulsion polymer ization temperature was varied between 60 and 90°C. The PS template was formed by centrifugation at a rel ative acceleration of 390 g for 12 h or by drying the sus pension in air. The PS template was washed with etha nol and was dried in air. Conventional zeolite FeZSM5 was synthesized under hydrothermal conditions using TPAOH as a molecular template at the following molar ratios of the reactants: 1.0 SiO2 : 0.02 TPAOH : 0.08 NaOH : 0.008 Fe(NO3)3 ⋅ 9H2O : 32 H2O. The reactants were stirred for 10 min and the resulting gel was placed in a Teflon lined stainless steel autoclave and was kept there at 150°C for 72 h. The resulting sample was filtered, rinsed with distilled water, dried, and calcined at 500°C for 5 h in air. For activation of FeZSM5, a 10g sample was treated with 100 mL of a 1 M aqueous solution of oxalic acid for 30 min at 50°C. The pretreated catalyst was washed with distilled water to pH 7.0, dried in air, and calcined at 500°C for 3 h [17]. Hierarchical zeolite hFeZSM5 was synthesized using a PS template at the following molar ratios of the reactants: 1.0 SiO2 : 0.015 Fe2O3 : 0.7 TPAOH : 17.5 H2O. Crystallization was carried out in the presence of PS at SiO2 : PS = 1 : 1 by weight at 110°C for 40 h. The product was washed with distilled water, dried, and cal cined at 500°C for 8 h in air. The reference sample of the Fecontaining hierarchical zeolite was pretreated with oxalic acid, but because no increase in catalytic activity was observed, the catalyst based on hFeZSM5 was not additionally activated. For determining the hydrothermal stability of the Fecontaining zeolites, their 2g samples suspended in 60 mL of deionized H2O were placed in a Teflonlined stainless steel autoclave and were kept there in an Ar atmosphere at 190°C and a pressure of 10 atm for 8 h. After hydrothermal treatment, the suspension was cen trifuged for 20 min and was dried at room temperature for 48 h in air and then at 140°C to constant weight. The samples subjected to hydrothermal treatment will be designated FeZSM5HT and hFeZSM5HT. KINETICS AND CATALYSIS
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Xray diffraction analysis was performed on an HZG4 diffractometer (SeifertFPM, Germany) using CuKα radiation in the 2θ range from 5° to 40°. The elemental composition of the Fecontaining catalysts was determined by inductively coupled plasma optical emission spectroscopy (PerkinElmer, United States). The samples had the following elemental com position: 2.82 wt % Fe, 0.06 wt % Al, and 0.03 wt % Na for FeZSM5 before activation; 2.74 wt % Fe, 0.06 wt % Al, and 0.04 wt % Na for FeZSM5 after activation; 2.7 wt % Fe, 0.08 wt % Al, and 0.65 wt % Na for hFeZSM5. Scanning electron microscopy (SEM) images were acquired using a JSM6460LV (JEOL, Japan) micro scope at an accelerating voltage of 15–20 kV. Highres olution transmission electron microscopy (HRTEM) images were obtained on a JEM2010 (JEOL) micro scope at 0.14 nm resolution and an accelerating voltage of 200 kV. Lowtemperature nitrogen adsorption isotherms were measured at –196°C on an ASAP2400 (Micromeritics, United States). The external surface area SExt was calculated as the difference between the specific surface area (SBET) and the micropore area (Smic). Thus the external surface area includes the values of the specific meso and macropore areas and also the specific “geometric” surface area of the powder. Phenol and lignin were adsorbed from aqueous solu tions with dissolved total organic carbon (TOC) con tent from 0 to 1800 mg/L at 25°C in a batch reactor for 2 h at an adsorbent mass (g) to solution volume (mL) ratio of 1 : 50. The acidity of the FeZSM5 and FeZSM5 suspensions was pH 4 and pH 7, respectively. The concentrations of Fe(NO3)3 and Fezeolites in catalytic experiments were 5 mmol/L and 20 g/L, respectively. The ironcontaining zeolites were tested in H2O2 decomposition in the absence and presence of 1 g/L Na2EDTA at 25°C in a temperaturecontrolled, magnetically stirred, 2.956L batch reactor. The H2O2 decomposition rate (W O 2 ) at [H2O2]0 = 0.56 mol/L was determined as the oxygen release rate measured baro metrically in Pa/s. The catalytic oxidation of phenol, Na2EDTA, and lignin with hydrogen peroxide was conducted at 25°C in a 100mL temperaturecontrolled glass batch reactor agitated with a magnetic stirrer. The initial hydrogen peroxide concentration was 0.25 to 3 mol/L, and the initial phenol, Na2EDTA, and lignin concentrations were 450, 1000, and 340 mg/L, respectively. The reac tion time for phenol, Na2EDTA, and lignin was 1.5, 16, and 5 h, respectively. The degree of mineralization of an organic compound was determined as the ratio of the difference between the initial and final dissolved TOC contents to its initial amount of TOC in the solution. The dissolved organic carbon was measured with a TOCV analyzer (Shimadzu, Japan). Hydrogen perox ide was removed from the samples before TOC analysis by adding 0.1 mL of 10 M NaOH and 0.1 mL of 0.1 M
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0.5 μm
(а)
65
70
75
80 85 90 95 Temperature, °С
Fig. 1. Polystyrene spheres obtained by PS suspension centrifugation: (a) SEM image of the PS template obtained at 90°C; (b) dependence of the PS particle size on the emulsion polymerization temperature.
(a)
(b) FeZSM5
400 200 0 600
FeZSM5HT
400 200 0
10
15
20 25 2θ, deg
30
35
40
Intensity, arb. units
Intensity, arb. units
600
300 200 100 0 300
hFeZSM5
hFeZSM5HT
200 100 0
10
15
20 25 2θ, deg
30
35
40
Fig. 2. Xray diffraction patterns of the initial and hydrothermally treated samples: (a) FeZSM5 and (b) hFeZSM5.
CoCl2 to an aliquot (3 mL) of the solution to be ana lyzed. The aliquot was left standing until oxygen evolu tion ceased. Next, the precipitate was separated by cen trifugation and the solution was adjusted to pH 5 by adding a few drops of 1 M hydrochloric acid. RESULTS AND DISCUSSION Monodisperse PS spheres ranging from 280 to 1150 nm were obtained by varying emulsion polymer ization temperature from 90 to 60°C. The dependence of the particles size on the temperature was nonlinear (Fig. 1). Ordered arrays of particles in the template were formed by centrifugation or drying in air. The drying produced a hexagonal close packing of spherical parti cles, while the centrifugation yielded a mixture of hex agonal and cubic packings of nanospheres. The tem plate surface produced by closepacked monodisperse PS spheres with a particle size in the range of visible light wavelengths had properties of a diffraction grating and iridescent shining. According to Xray diffraction data (Fig. 2), all sam ples had a crystalline phase identical to zeolite ZSM5 [18]. In the Xray diffraction pattern of the hierarchical
sample, the reflections were less intense and broader compared to the reflections observed for the conven tional sample because of the smaller coherent scattering domain and the presence of amorphous phase (Fig. 2). The broadening of the diffraction lines could be caused by a decrease in the crystallite size and by structural imperfection. The SEM images of the synthesized Fecontaining samples are shown in Fig. 3. According to the SEM data, the zeolite sample obtained in the absence of a PS template contains singlecrystal particles with a size of about 1–2 µm. An ordered interconnected macro porous structure was observed in the sample obtained in the presence of the PS template (Fig. 3b). HRTEM data confirmed that the walls of the hierarchical sample consisted of ZSM5 crystals with a size of about 200 nm and amorphous globules of silica of various sizes and textures. It is worth noting that the synthesis of a highcrystal linity ZSM5 sample is a prerequisite for obtaining an active heterogeneous Fenton catalyst. The precursors of active sites, which are Fe(III) ions isomorphically sub stituted for silicon atoms in the zeolite lattice, form active ~3nm ferric oxohydroxo complexes on being KINETICS AND CATALYSIS
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1 µm (b)
(а)
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1 µm
Fig. 3. SEM images of (a) FeZSM5 and (b) hFeZSM5.
10 nm
(а)
(b)
1 µm
Fig. 4. HRTEM images of (a) hFeZSM5 and (b) hFeZSM5HT.
treated with oxalic acid [19]. In the case of amorphous silica gel or lowcrystallinity zeolite sample, large Fe2O3 particles with a low activity are generated on the zeolite surface [19]. Probably, the active sites of the Fecontaining sam ples are 3 to 4nm iron oxide particles located in zeo lite mesopores or on the zeolite surface, which are visu alized by HRTEM (Fig. 4a). In the case of traditional zeolite FeZSM5 obtained in the absence of the PS template, it is these particles that display the greatest activity in peroxidation [19, 20]. The N2 adsorption–desorption data for the synthe sized samples are presented in Table 1. The hierarchical zeolite had high BET and external surface areas and a high pore volume compared to those of the conven tional zeolite. The external surface area of the conven tional zeolite was not larger than 10% of the BET sur face area, while the SExt of the hierarchical sample reached 50%. Hydrothermal treatment at 190°C and 10 atm resulted in a decrease in the surface area of hFeZSM5; however the crystallinity of hFeZSM5 was unchanged by the hydrothermal treatment (Fig. 2b). The decrease in SBET was due to the agglom KINETICS AND CATALYSIS
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eration of small microporous particles of the amor phous silicate phase with the formation of 1µm nonpo rous globules upon hydrothermal treatment (Fig. 4b). The catalytic activity of the zeolites obtained was initially determined in hydrogen peroxide decomposi tion since the catalytic oxidation of organics by H2O2 occurs only if the catalyst has sufficient activity in H2O2 decomposition producing an oxidant, specifically the hydroxyl radical. Table 2 lists the reaction rates in the homogeneous Fenton system and in the heterogeneous systems FeZSM5/H2O2 and hFeZSM5/H2O2. The activity of FeZSM5 was no more than 30% lower than the activity of hydrated ferric ions. However, the activity of the hierarchical zeolite with almost the same iron content was 30 times lower, possibly due to the presence of an amorphous phase on which ferric particles with a lower activity formed. In the homogeneous Fenton system in the presence of disodium EDTA, which produced stable complexes with hydrated ferric ions, a long induction period was observed (Table 2). By contrast, in the zeolitecontain ing systems, no induction period was observed, indicat ing the heterogeneous nature of H2O2 decomposition.
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Table 1. Textural properties of the conventional and hierar chical Fecontaining ZSM5 zeolites according to low temperature nitrogen adsorption data Sample
SBET, SExt, m2/g m2/g
FeZSM5 FeZSM5HT hFeZSM5 hFeZSM5HT
349 436 472 137
53.7 82.0 245 96.1
ϕ, %
Vtotal, cm3/g
Vmicro, cm3/g
15.4 18.8 51.8 70.1
0.23 0.26 0.59 0.24
0.14 0.16 0.11 0.02
Table 2. Initial H2O2 decomposition rate in the absence and in the presence of Na2EDTA for three Fentontype sys tems at 25°C and [H2O2]0 = 0.56 mol/L
WO2 , Pa/s
Fenton system
[Na2EDTA] = 0 [Na2EDTA] = 1 g/L Fe(NO3)3/H2O2 FeZSM5/H2O2 hFeZSM5/H2O2
7.18 5.26 0.23
0.08* 4.74 0.17
* The rate reached its maximum value of 5.55 Pa/s 90 min after the beginning of the reaction.
The synthesized samples were tested in the liquid phase catalytic peroxide oxidation of organic contami nants having similar chemical nature but different molecular sizes. One of them was phenol, with a molec ular diameter of ~0.8 nm, and the other one was lignin, for which the mean diameter of a polymeric particle can be assumed to be 20 nm. For determining the contribu tion from adsorption to the water decontamination process, the phenol and lignin adsorption properties of the samples were studied. The isotherms of phenol adsorption on the FeZSM5 and hFeZSM5 zeolites from aqueous suspensions were curves indicating satu ration, which were approximated by Langmuir’s model. The Langmuir equation was used to determine the phe nol adsorption constants and phenol monolayer capac ities: K = 907 M–1 and a∞ = 0.40 mmol/gcat for FeZSM5 and K = 197 M–1 and a∞ = 0.33 mmol/gcat for hFeZSM5. The adsorption constants and phenol monolayer capacities of the hierarchical zeolite materi als were intermediate values between the adsorption characteristics of the zeolite phase and amorphous sil ica. The presence of the latter led to a decrease in the adsorption parameters of the sample. To compare the efficiencies of the homogeneous and heterogeneous Fenton systems, the degree of min eralization of the organic substrates was determined. The degree of mineralization means the degree of total oxidation of organic compounds yielding inorganic compounds, namely, CO2 and H2O in the case of phe nol and lignin oxidation. The degree of mineralization in the hFeZSM5/H2O2 system was barely 17% at 25°C and [H2O2]0 = 1 mol/L after 1.5 h and reached 70% only after 16 h. The maximum degree of mineral
ization in the homogeneous Fenton system was 70% at [H2O2]0 = 0.5 mol/L after 1.5 h; a subsequent increase in the hydrogen peroxide concentration did not increase the degree of mineralization (Table 3). Virtu ally complete oxidation was observed in the heteroge neous Fenton system FeZSM5/H2O2 at [H2O2]0 = 1 mol/L after the 1.5hlong reaction. Thus, the high phenol adsorption capacity of the zeolite surface allows the efficiency of oxidant utilization in the mineraliza tion of lowMW compounds to be increased over the efficiency afforded by the homogeneous Fenton system. In the case of adsorption and peroxide oxidation of highMW lignin, a completely different behavior of the Fenton systems was observed. Lignin was adsorbed multilayerly on FeZSM5 from the aqueous suspen sions, and the maximum amount adsorbed was 35 mg/gcat at an equilibrium lignin concentration of 1.1 g/L and pH 4. Under the assumption that the area occupied by one lignin molecule is about 400 nm2, the lignin monolayer capacity of the external surface area of 53.7 m2/g should be ca. 3.6 mg/gcat. The adsorption value of 35 mg/gcat indicated the formation of lignin multilayers on the surface, which may be explained by the tendency of lignin molecules to coagulate at pH < 4.0–4.5. The maximum value of lignin adsorption on the hierarchical zeolite hFeZSM5 was only 1.3 mg/gcat at pH 7, which corresponds to partial monolayer adsorption of lignin on the external zeolite surface. Earlier, we showed that the heterogeneous FeZSM 5/H2O2 system based on the conventional zeolite was much less efficient in the oxidation of highmolecular lignin than the homogeneous systems, such as Fe(NO3)3/H2O2 and H2O2/UV. The maximum degree of mineralization of lignin was 35, 82, and 78% for the FeZSM5/H2O2, Fe(NO3)3/H2O2, and H2O2/UV sys tems, respectively [8]. As can be seen in Table 3, the hierarchical zeolite showed significantly improved per formance in the oxidation of highMW lignin than the conventional zeolite: the degree of mineralization of lignin with the former zeolite was 54% at 25°C. Table 3 lists the degrees of mineralization of Na2EDTA in the FeZSM5/H2O2, Fe(NO3)3/H2O2, and hFeZSM5/H2O2 systems. The degree of Na2EDTA mineralization was significantly higher in the presence of the hierarchical zeolite than in the pres ence of the conventional one. The same maximum val ues of the degree of mineralization in the Fe(NO3)3/H2O2 and hFeZSM5/H2O2 systems were reached at different reaction times. In the heteroge neous Fenton system, the maximum rate of Na2EDTA oxidation was observed in the first 5 min of the reaction; in the homogeneous one, oxidation began after a long induction period of 70 min. Thus a new heterogeneous Fenton catalyst was designed for highMW organics oxidation with hydrogen peroxide. The size of PS nanospheres in the 280–1100 nm range depends nonlinearly on the emulsion polymer KINETICS AND CATALYSIS
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TEMPLATE METHOD FOR THE SYNTHESIS OF A HETEROGENEOUS FENTON Table 3. Degrees of mineralization of phenol, lignin, and Na2EDTA in 20 g/L FeZSM5/H2O2 and 20 g/L hFeZSM5/H2O2 systems at [phenol]0 = 0.45 g/L, [lignin]0 = 0.34 g/L, [Na2EDTA]0 = 1 g/L, T = 25°C, and different initial hydrogen peroxide concentrations Substrate Phenol
Catalyst FeZSM5
hFeZSM5
Fe(NO3)3
Lignin
FeZSM5
hFeZSM5
Na2EDTA FeZSM5 hFeZSM5 Fe(NO3)3
[H2O2]0, mol/L
Degree of mineral ization, %
0.5 1.0 2.0 0.5 1.0 2.0 0.5 1.0 2.0 0.4 0.9 1.9 0.5 1.0 2.0 1.1 1.1 1.1
82.5 93 95 16 17 15 72 72 68 16 30 25 33 48 54 38 51 58
ACKNOWLEDGMENTS We thank our colleagues from the Boreskov Institute of Catalysis for characterization of catalysts: T.Ya. Ephimenko (lowtemperature nitrogen adsorp tion), S.V. Bogdanov (Xray diffraction analysis) and Vol. 54
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E. Yu. Gerasimov (highresolution transmission elec tron microscopy). This work was carried out as part of integration project no. 35 between the Siberian, Ural, and Far East ern Branches of the Rusian Academy of Sciences and integration project no. 24 between the Natioanal Acad emy of Sciences of Belarus and the Siberian Branch of the Russian Academy of Sciences. Financial support from the Ministry of Science and Education (Federal Target Program “Scientific and Educational Person nel”, contract 8440 for 2009–2013), Russian Federa tion President Grant for the Leading Scientific Schools no. SS 524.2012.3, Russian Foundation for Basic Research (grant no. 120393116NCRL), and a BP grant is gratefully acknowledged. REFERENCES
ization temperature between 90 and 60°C. The zeolite sample hFeZSM5 with hierarchical micro/ macropore system was synthesized using a PS template. The walls of the macroporous structure consisted of ZSM5 nanocrystals and amorphous globules of silica. The hierarchical zeolites had high BET and external surface areas of 245 and 472 m2/g, respectively, and a high pore volume of 0.6 cm3/g. The Fecontaining hierarchical ZSM5 zeolites are less active in the decomposition of hydrogen peroxide and in the oxidation of lowMW organic molecules than the conventional zeolite FeZSM5, but they are more effective in the mineralization of macromolecular compounds. The catalytic sites of the Fecontaining zeolites did not form complexes with EDTA ligands, unlike hydrated ferric ions. In the oxidation of macromolecular substances, the heterogeneous Fenton system based on the hierarchical FeZSM5 shows an activity similar to the activity of the homogeneous Fenton system, but they are superior to the latter in terms of ease of handling and stability.
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