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Fe-containing nickel phosphate molecular sieves as heterogeneous catalysts for phenol oxidation and hydroxylation with H2O2
Applied Catalysis B: Environmental 107 (2011) 197–204
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Fe-containing nickel phosphate molecular sieves as heterogeneous catalysts for phenol oxidation and hydroxylation with H2 O2 M.N. Timofeeva a,∗ , Zubair Hasan b , A.Yu. Orlov a,c , V.N. Panchenko a , Yu.A. Chesalov a , I.E. Soshnikov a , Sung Hwa Jhung b,∗∗ a
Boreskov Institute of Catalysis SB RAS, Prospekt Akad. Lavrentieva 5, 630090, Novosibirsk, Russian Federation Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Sankyuck-Dong, Buk-Ku, Daegu 702-701, South Korea c Novosibirsk State University, st. Pirogova 2, 630090, Novosibirsk, Russian Federation b
a r t i c l e
i n f o
Article history: Received 24 May 2011 Received in revised form 1 July 2011 Accepted 10 July 2011 Available online 20 July 2011 Keywords: Nickel phosphate molecular sieves VSB-5 Hydrogen peroxide Phenol Oxidation Hydroxylation
a b s t r a c t Fe-containing nickel phosphate molecular sieves (Fe-VSB-5) were hydrothermally synthesized in weak basic conditions under microwave irradiation and characterized by SEM, XRD, N2 -adsorption/desorption, DRS-UV–vis, and FT-IR spectroscopy using PhCN and CDCl3 as probe molecules. The catalytic activity of Fe-VSB-5 was tested for the phenol hydroxylation and wet phenol oxidation with H2 O2 . The increase in iron content in Fe-VSB-5 leds to an increase in the reaction rates. The increases in activity can be explained by the role of the Fe species, which increases the generation of radicals. The Fe-VSB-5 samples were stable against the leaching out of Fe ions. The catalytic activity of Fe-VSB-5 was compared to the catalytic activity of traditional Fe-containing materials. © 2011 Elsevier B.V. All rights reserved.
1. Introduction A novel class of microporous materials, nickel phosphate phases, has been discovered that represents a family of framework oxide molecular sieves synthesized without silica and alumina. Nickel phosphate molecular sieves, such as VSB-1 and VSB-5, are similar to zeolites in some properties and may find some uses as adsorbents for separations of molecular species and as catalysts or catalyst supports [1–4]. These materials have interesting catalytic properties suitable for reactions that do not require strong acidity. Thus, VSB-1 was used for the dehydrocyclodimerization of butadiene to ethylbenzene [4]. Moreover, these materials can be used as catalyst for oxidation reactions. For example, the catalytic properties of VSB-5 were demonstrated in the oxidation of alcohols to corresponding aldehydes or ketones under mild conditions with molecular oxygen [2] and for epoxidation of cyclic olefins, such as cyclohexene, cyclooctene [1] and cyclododecene [3] with H2 O2 . Isomorphous substitution of Ni2+ with bi- or trivalent transition-metal ions can, in principle, generate both new red–ox centers and Brönsted/or Lewis centers, that can improve catalytic properties. Moreover,
∗ Corresponding author. Tel.: +7 383 330 7284; fax: +7 383 330 8056. ∗∗ Corresponding author. Tel: +82 53 950 5341; fax: +82 53 950 6330. E-mail addresses: [email protected] (M.N. Timofeeva), [email protected] (S.H. Jhung). 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.07.014
metal-incorporated molecular sieves contain atomically dispersed metal species, and such sites may impart important red–ox and catalytic properties. Recently, transition-metal-incorporated nickel phosphate molecular sieves (TMI-VSB, M = Co2+ , V5+ , Fe3+ ) were synthesized by the isomorphous substitution of transition metal ions (TMI) in the VSB framework [5,6]. Recently, interest in the catalytic properties of TMI-VSB materials has increased. Thus, Co-VSB-5 exhibited promising catalytic activities for the oxidation of styrene with H2 O2 , in which styrene conversion and selectivity to benzaldehyde were 57% and 70%, respectively [7]. Jiang and Gao [8] demonstrated the high catalytic activity of Cu-VSB-5 in liquid phase catalytic hydroxylation of phenol. Fe-VSB-1 exhibited very good activity to oxidize ammonia into nitrate and nitrite ions under UV-irradiation [9]. The activity of Fe-VSB-1 was much better than that of pure FeCl2 as a photo-Fenton catalyst. It should be emphasized that various trivalent iron-substituted molecular sieves (silicates, aluminosilicates, aluminophosphates, etc.) have attracted considerable attention as heterogeneous Fenton type (Fe2+ /Fe3+ /H2 O2 ) catalysts for wet oxidation of organic pollutants in wastewaters. Phenol (PhOH) is one of the pollutants of industrial wastewaters. Various heterogeneous Fe-containing systems have been evaluated as catalysts for PhOH oxidation. Unfortunately, some of these systems possess certain disadvantages, such as low activity and stability. The maximal activity of many systems is observed at pH 3–4. However, the high acidity medium causes substantial iron
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M.N. Timofeeva et al. / Applied Catalysis B: Environmental 107 (2011) 197–204
leaching. According to Refs. [10,11], iron leaching may be negligible when Fe3+ is incorporated into the framework of molecular sieves, whereas small iron oxide clusters leach out during liquid-phase reactions. The relationship between chemical composition, textural data and catalytic activity of Fe,Al-pillared clays (Fe,Al-PILC) and Fe,Al-mesoporous silica was demonstrated at pH 6.2 [11,12]. Thus, 100% conversion of PhOH (0.001 M solution) was reached after 90 min with Fe,Al-PILC at 50 ◦ C. It was demonstrated that the nature of the OH-groups in mesoporous Fe,Al–SiO2 , which facilitate the sorption–desorption processes of reagents, had an effect on the reaction rate of phenol oxidation with H2 O2 [11]. Of note, the increase in the strength of basic sites for Fe-containing samples was accompanied by an increase of the H2 O2 degradation rate and therefore, an increase in the reaction rate [11]. According to Ref. [13], Fe-containing aluminophosphate molecular sieves (FeAlPO) possess significant catalytic activity and stability for liquid phase phenol oxidation with H2 O2 . The catalytic performance of Fe-AlPO-11 was similar to that of TS-1 [13]. It is interesting to note that the activity of Fe-AlPO depends on the Fe content in the framework of AlPO and on the acidity of the surrounding groups of iron sites. Although the details of the interaction between the reacting molecules and the solid catalysts were far from well understood, Wang and Otsuka [14] assumed that the catalytic activity of FeAlPO was related to the presence of the acid sites in the molecular sieve structure. In this work, we examined the acid–base properties of Fecontaining nickel phosphate molecular sieves (Fe-VSB-5). The catalytic properties of Fe-VSB-5 were assessed for phenol hydroxylation and full phenol oxidation with hydrogen peroxide. The relationships between the iron state, the nature of the surface functional groups, catalytic activity, and stability against leaching were established. Some preliminary kinetic studies were also done to get an insight into the reaction mechanism. The aim of this study was to determine the role of the surface-active acid and base sites in PhOH oxidation and H2 O2 degradation under moderate reaction conditions. 2. Experimental 2.1. Synthesis of Fe-VSB-5 materials The VSB-5 and Fe-VSB-5 materials were hydrothermally synthesized with microwave irradiation using NiCl2 ·6H2 O, FeCl2 ·4H2 O, and ortho-phosphoric acid (H3 PO4 , 85 wt%, Merck) as Ni, P, and Fe sources [6,15]. The designation of the samples and the reaction conditions of this synthesis are presented in Table 1. 2.2. Instrumental measurements The porous structure of the materials was determined from the N2 adsorption isotherms obtained at −196 ◦ C using a Micromeritics ASAP 2400 equipment. The X-ray diffraction patterns were measured on a X-ray diffractometer XRD (ThermoARL) with Cu K␣ ˚ radiation. The morphology of the Fe-VSB-5 samples ( = 1.5418 A)
was examined using a field emission scanning electron microscope (Hitachi, S-4300). Infrared spectra were recorded on a BOMEM-MB102 spectrometer in the 250–4000 cm−1 range with a resolution of 4 cm−1 using compressed pellets (2 mg of catalyst and 500 mg of KBr). The DR-UV–vis spectra were recorded on a UV-2501 PC Shimadzu spectrometer with an IRS-250A accessory in the 190–900 nm range with a resolution of 2 nm. BaSO4 was used as a standard. The chemical analyses were done by means of an inductively coupled plasma-atomic emission spectrometry (ICP-AES). 2.3. Acidity measurements For IR transmission measurements, the samples were pressed into self-supporting wafers (7–20 mg/cm2 ) and pre-treated within the IR cell by heating at 300 ◦ C under airflow for 1 h and under vacuum for 0.5 h at 300 ◦ C before the adsorption experiments. For the analysis of the basicity, the samples were exposed to saturated CDCl3 vapors for 3 min at room temperature. FT-IR spectra of the samples were obtained both before and after CDCl3 adsorption, and the difference was reported. The strength of the base sites was estimated from the shift of C–D using Eq. (1) [16]: log C−D = 0.0066PA − 4.36
(1)
where PA is the proton affinity. For the analysis of the Lewis surface acidity, the samples were exposed to saturated PhCN vapors at room temperature. FT-IR spectra of the adsorbed PhCN were recorded every 10 min up to saturation by PhCN. FT-IR spectra of adsorbed CDCl3 and PhCN were recorded on a Shimadzu FTIR-8300 spectrometer in the range of 400–6000 cm−1 with a resolution of 4 cm−1 . 2.4. Catalytic tests Phenol oxidation was carried out at 60–70 ◦ C in a glass thermostated vessel equipped with a stirrer and a reflux condenser. The reactor was charged with a mixture of H2 O2 /PhOH (30 wt.% in water) = (14–28)/1 mol/mol (20 ml of (1–10) × 10−3 mol/l phenol solution in water) and a catalyst (typically, 1.0–5.0 g/l). The phenol concentration in the solution was determined using a UV–vis equipment (Specord UV–vis M-40 instrument, = 273 nm, accuracy ±10%). The phenol hydroxylation was carried out at 60 ◦ C in the same vessel. A mixture of PhOH/H2 O2 at a 1:1.25 (mol/mol) ratio (5 ml of 0.4 M phenol solution in water) and a catalyst (3.0 g/l) was placed in the reactor. The identification and determination of the products (hydroquinone and catechol) were done using a Shimadzu Prominence HPLC system equipped with a Pathfinder 100 AS 2.5 UM column. 2.5. Adsorption tests PhOH adsorption on the VSB-5 and Fe-VSB-5 materials was examined in a glass thermostated reactor under stirring at 20 ◦ C. The reactor was loaded with 30 mg of sample and 5 ml of a PhOH
Table 1 Chemical composition of Fe-VSB-5 samples. Preparation gel (atomic%)
Fe-VSB-5 product Weight content (%)
1 2 3 a
VSB-5 3.1%Fe-VSB-5 6.5%Fe-VSB-5
Atomic ratio (%)
x
Fe Fe+Ni+P
Fe
Ni
P
Fe Fe+Ni+P
0 0.09 0.18
0 5.2 9.9
0 3.1 6.5
48.5 43.5 39.8
12.8 12.8 12.0
0 4.58 9.92
a
Molar composition of reaction mixture is xFeCl2 :0.63H3 PO4 :1.0NiCl2 :3.0NH3 :100H2 0.
M.N. Timofeeva et al. / Applied Catalysis B: Environmental 107 (2011) 197–204 Table 2 Textural data of Fe-VSB-5 samples and Fe-containing samples. No. 1 2 3 4
VSB-5 3.1%Fe-VSB-5 6.5%Fe-VSB-5 1.8%Fe,Al-PILC
SBET (m2 /g)
Sext (m2 /g)
V (cm3 /g)
V (cm3 /g)
284 202 203 215
36 41 31 117
0.24 0.13 0.16 0.22
0.128 0.082 0.088 0.051
199
and morphology of VSB-5 did not change substantially with the incorporation of Fe ions. Diffuse reflectance UV–vis spectroscopy shows that the oxidation states of Fe ions in 3.1%Fe-VSB-5 and 6.5%Fe-VSB-5 were not the same (Fig. 1). In the spectrum for 3.1%Fe-VSB-5 low-intensity bands at 212 and 233 nm can be resolved in the second derivative of the spectrum (Fig. 1, inset). These bands indicate that iron was mainly in the form of Fe2+ ions. The second derivative of the 6.5%FeVSB-5 spectrum (Fig. 1, inset) revealed three new bands at 218, 227 and 243 nm indicating the appearance of Fe3+ ions in the sample. It should be noted that bands between 300 and 400 nm are usually assigned to small oligomeric species, while the bands between 200 and 300 nm are due to isolated Fe ions [17,18]. According to the data in this study, the majority of the Fe ions were in the form of isolated ions in the Fe-VSB-5 samples. However, a small amount of oligomeric species of iron oxide was also present in the samples. The incorporation of Fe ions into the VSB-5 framework was confirmed by the IR analysis. The IR spectra of the VSB-5 and Fe-VSB-5 materials are shown in Fig. 2. The IR-spectrum of VSB-5 exhibited absorption bands at s (PO4 ) 967 cm−1 , as (PO4 ) 1023 cm−1 , and ıas (PO4 ) 632 and 589 cm−1 . In the spectrum for the Fe-VSB-5 samples, the band at 1115 cm−1 (as (PO4 )) disappeared and new bands at 1128 cm−1 (as (PO4 )) and 1059 cm−1 (s (PO2 ) were observed (Fig. 2). Moreover, the insertion of Fe ions into the VSB-5 framework led to an IR-shift of the band from 1024 to 1028 cm−1 (as (PO4 )). Note that a shift of bands for (PO4 ) was also observed after the incorporation of Fe ions in the AlPO-5 framework [19].
aqueous solution ((1–10) × 10−4 mol/l). At regular time intervals, aliquots were taken and the phenol concentration was determined by UV–vis at = 273 nm. 2.6. Spin trap experiments Electron paramagnetic resonance (EPR) spin-trapping of radical species formed during the catalytic wet oxidation was done using a Bruker ER-200D spectrometer at room temperature using a flat quartz cell, which is typically used for EPR measurements in polar solvents. 2,2,6,6-Tetramethylpiperidine (TEMP) was obtained from Aldrich. Spin trap experiments were carried out by adding 0.58 mmol of H2 O2 to a mixture containing 0.2 mmol of PhOH, 40 mg of 6.5%Fe-VSB-5 and 0.4 mmol of the spin-trapping TEMP in 5 ml H2 O. 3. Results 3.1. Physicochemical properties of Fe-VSB-5
3.2. The nature of the surface functional groups of Fe-VSB-5 Microporous nickel phosphates VSB-5 and Fe-VSB-5, which are based on octahedral NiO6 units linked by tetrahedral PO4 forming one-dimensional 24-membered ring channel structures, were synthesized according to Refs. [5,6]. The chemical composition and textural data of VSB-5 and Fe-VSB-5 are shown in Tables 1 and 2. From Table 1, it can be seen that the atomic ratios of Fe/ (Fe + Ni + P) for the synthesized Fe-VSB-5 materials were close to the expected ratios. Both VSB-5 and Fe-VSB-5 are microporous materials exhibiting high nitrogen adsorption (Supporting information, Fig. 1). According to the XRD data and scanning electron microscopy (SEM) images (Supporting information, Figs. 2 and 3), the crystallinity
Even though the details of the interaction between reacting molecules and solid catalysts are far from understood, it is commonly accepted that catalytic activity depends on the nature and amount of acid sites in the molecular sieve structure. Both Brönsted and Lewis acid sites contribute to the acidity of molecular sieves. The aim of this work was to investigate the nature of the acid sites of VSB-5 modified by iron ions to understand the main role of these sites in catalysis. PhCN is a soft base that has a high propensity to Lewis acid sites and a low propensity to Brönsted acid sites [20,21]. Fig. 3 shows
8 3.1%Fe-VSB-5
444
210
233
227
4 406
F(R)
6.5%Fe-VSB-5
243
212
3.1%Fe-VSB-5
218
6.5% Fe-VSB-5
24 0
27 0
300
Wavelength, nm
VSB-5 0 300
400
500
600
Wavelength, nm Fig. 1. DR-UV–vis spectra of VSB-5 – (1), 3.1%Fe-VSB-5 – (2) and 6.5%Fe-VSB-5 – (3). The inset shows the second derivatives of the spectra.
1128 1115 1092
2252
1023
M.N. Timofeeva et al. / Applied Catalysis B: Environmental 107 (2011) 197–204
1.5
2262
(1/2)
VSB-5
0.5 2212
3
1.0 2254
4
2262
Absorbance/p
Absorbance
632
967
589
1047
200
2
6.5%Fe-VSB-5
1
1000
900
800
700
Wavenumber, cm
0.0 2150
600
2250
-1
Table 3 Characteristic frequencies of PhCN adsorbed on VSB-5 and 6.5%Fe-VSB-5. CN (cm−1 )
Assignment
SiO2 [22] Zr,Al –PILC [23]
2236–2245 2242 2265 2285
Interaction with OH-groups Interaction with OH-groups Interaction with Zr4+ Interaction with Al3+
Fe2 (MoO4 )3 [22] FePO4 [22] VSB-5
2275 2270 2229 2282 2288
Interaction with Fe3+ Interaction with Fe3+ Interaction with OH-groups Interaction with Ni2+ Interaction with Ni2+
6.5%Fe-VSB-5
2229 2268 2282
Interaction with OH-groups Interaction with Fe3+ Interaction with Ni2+
the spectra of PhCN adsorbed on VSB-5 and 6.5%Fe-VSB-5. The main positions of the bands observed are summarized in Table 3. Three bands at 2229, 2282 and 2288 cm−1 were observed in the IR-spectrum of VSB-5. The band at 2229 cm−1 can be attributed to
Fig. 4. FT-IR difference spectra of adsorbed CDCl3 on VSB-5 and 6.5%Fe-VSB-5.
the interaction of PhCN with the OH-groups, which is likely related to the bridging P–(OH)–Ni groups. These OH-groups are weaker than that of the Zr, Al-pillared layer clays (Table 3). Two bands at 2282 and 2288 cm−1 can be assigned to the interaction between the nonbonding electrons of PhCN and electron-deficient Ni ions (Lewis acid sites (LAS)) [22]. After the insertion of Fe ions into the VSB-5 network, the band at 2288 cm−1 disappeared and a new band at 2268 cm−1 appeared instead. These changes attest to the change in the coordination state of the Ni ions. The band at 2268 cm−1 can be attributed to the interaction between the nonbonding electrons of PhCN and the electron-deficient Fe ions [22]. It is well known that Ni–O–Ni and Ni–O–P bonds exist in the framework of the VSB5 molecular sieve and that Ni2+ ions in the structure of VSB-5 can be substituted by Fe ions [6]. These data show that the bands at 2282 and 2288 cm−1 were related to the electron-deficient Ni ions in Ni–O–P and Ni–O–Ni groups, respectively. The basicity of VSB-5 and 6.5%Fe-VSB-5 was analyzed by FTIR spectroscopy using CDCl3 as a probe molecule [11,16,23]. The spectra of CDCl3 adsorbed on VSB-5 and 6.5%Fe-VSB-5 are shown in Fig. 4. The spectrum of VSB-5 disclosed two overlapping bands at 2262 and 2252 cm−1 . It is assumed that these bands, which are both formed by the Ni ions in the skeleton, can be assigned to
VSB-5
6.5%Fe-VSB-5
30 min
2282
2268
2229
2282
6
2288
Absorbance/ρ
6
3
5 min
0 2200
2300
-1
Wavenumber, cm
Fig. 2. IR spectra of VSB-5 – (1), 3.1%Fe-VSB-5 – (2), 6.5%Fe-VSB-5 – (3) and 6.5%Fe–VSB-5 was used for 3 cycles - (4).
12
2200
2229
1100
Absorbance/ρ
1200
2250
2300 -1
Wavenumber, cm
0 2200
2250
2300 -1
Wavenumber, cm
Fig. 3. FT-IR difference spectra of adsorbed PhCN on VSB-5 and 6.5%Fe-VSB-5.
M.N. Timofeeva et al. / Applied Catalysis B: Environmental 107 (2011) 197–204 Table 4 Spectral characteristics (C–D ) of OH-groups for VSB-5 and 6.5%-VSB-5 according to adsorption of CDCl3 . C–D (cm−1 )
C–D (cm−1 )
2268 2265 2260
– 3 8
– 733 797
1.8%Fe,Al–SiO2 [23]
2265 2260 2255
3 8 12
733 797 824
1.8%Fe,Al-PILC [23]
2265 2260 2252 2252
3 8 16 16
733 797 843 843
2254 2212
14 56
834 925
1 2 3 4 5 6 7 8 9 10 11 12
the weak interactions between CDCl3 and the base sites. A new band at 2212 cm−1 appeared in the spectrum of CDCl3 adsorbed on the 6.5%Fe-VSB-5 that is likely due to the Fe ions inserted into the VSB-5 framework. Unfortunately, no information about the nature of these sites is available; however, their strength was estimated using Eq. (1) [16]. The values for the proton affinity (PA) are presented in Table 4. The strengths of the strong basic sites (PA) were 843 and 925 kJ/mol for VSB-5 and 6.5%Fe-VSB-5, respectively. It is interesting to compare the strong strengths of the basic sites of VSB-5 and 6.5%Fe-VSB-5 with that of Fe-containing silica mesoporous materials and Fe-containing pillared layered clay (Fe,Al-PILC). According to the data presented in Table 4, the strength of the strong basic sites for VSB-5 is higher than that of Fe-containing silica mesoporous materials such as 1.8%Fe–SiO2 and 1.8%Fe,Al–SiO2 . However, the strength of the strong basic sites for VSB-5 is close to that of the strong basic sites for 1.8%Fe,Al-PILC. Of note, the strength of the strong basic sites for 6.5%Fe-VSB-5 was essentially higher than that of all Fe-containing materials. Therefore, the experimental evidence alludes to the fact that the insertion of Fe leads to an increase in the surface basicity of the 6.5% Fe-VSB-5. 3.3. Catalytic properties of Fe-VSB-5 The Fe-VSB-5 samples were tested for wet phenol oxidation with H2 O2 . pH of phenol solution after addition of Fe-VSB-5 changes from 5.6 to 8.2–8.3. Investigation on the activity of VSB-5
1.8%Fe,Al-PILC [11]
3
93 <0.1
C
-4.3 -4.4
25
-4.5 -4.6
40
0.005 mol/l 0.001 mol/l -6.0
20 0
H2O2/PhOH=14/1
50
4
150
-6.5
-7.0
5
-1 .9
- 1 .8
-1 .7
-1 .6
lg C H O 2 2
0
0
Time, min
76 <0.1 <0.1
0.011 mol/l
6.5%Fe-VSB-5, g/l
100
215
60
H2O2/PhOH=21/1
20 2
– 127 178 177
0.2
-4.7 1
qe d (109 mol m−2 )
and Fe-VSB-5 samples at pH 3–6 was not carried out due to their low stability in an acidic medium. According to the data from the catalytic tests, the reaction rate increased with increasing catalyst amounts, PhOH concentration, H2 O2 /PhOH ratio and reaction temperature (Fig. 5, Table 5). A kinetic investigation of the reaction using Van’t Hoff’s differential method [24] shows that the orders of reaction on PhOH and H2 O2 were 1.1 ± 0.1 and 1.0 ± 0.1, respectively. The activation energy for the reaction, which was determined by the Arrhenius equation k = Ae−Ea /RT , was 64.8 ± 8 kJ/mol (Supporting information, Fig. 5). The iron content in Fe-VSB-5 also affected the reaction rate (Table 5). The higher the iron contents in Fe-VSB-5, the higher the reaction rate. Since PhOH adsorption on the catalyst may contribute to the overall phenol consumption, the adsorption capacity (qe ) of 6.5%Fe-VSB-5 was measured (Table 5 and Supporting information, Fig. 4). The qe value (0.034 mmol/g) was much lower than the amount of phenol oxidized in the catalytic reaction (1.0 mmol/g).
0
PhOH conversion, %
40
30
τ1/2
– – <0.1 <0.1 (n.d., <0.1)e <0.1
75
50
50
>30 >30 9 7 (7, 7.5)e 2.5f >30a 10g >30a 2.5g 1.0h >30a 1.5h
H2O2/PhOH=28/1
60
, min
PhOH conversion, %
75
50
1.8%Fe,Al–SiO2 [11]
Fe leachingc (wt%)
B
2.5 g/l 2 g/l 1 g/l
0
1.8%Fe–SiO2 [11]
A
5 g/l
25
Without catalyst VSB-5 3.1%Fe-VSB-5 6.5%Fe-VSB-5
b TimePhOH 100% (h)
a Reaction conditions: PhOH 10−3 mol/l, H2 O2 /PhOH 28/1 mol/mol, catalyst 2 g/l, pH 8.2, 60 ◦ C. b Time of 100% conversion of PhOH. c The amount of iron leached from Fe-containing sample to solution (based on the initial iron content in the sample). d Phenol adsorption on Fe-containing sample at 25 ◦ C. e Catalyst was used in second and third cycles. f 70 ◦ C. g Reaction conditions: PhOH 10−3 mol/l, H2 O2 /PhOH 14/1 mol/mol, catalyst 1 g/l, pH 6.2, 60 ◦ C. h Reaction conditions: PhOH 10−3 mol/l, H2 O2 /PhOH 14/1 mol/mol, catalyst 1 g/l, pH 6.2, 50 ◦ C.
lg W
100
PhOH oxidationa
Sample
lg W
6.5%Fe-VSB-5
Run
PhOH conversion, %
VSB-5
Table 5 Phenol oxidation with H2 O2 in the presence of Fe-containing samples.
PA (kJ/mol)
CDCl3 1.8%Fe–SiO2 [23]
201
0
50
100
Time, min
150
0 0
50
100
150
-3.0
- 2 .5
-2 .0
lg C P h O H
Time, min
Fig. 5. Effect of 6.5%Fe-VSB-5 content (A); H2 O2 /PhOH molar ratio (B) and PhOH concentration (C) in reaction mixture on PhOH consumption. (Reaction conditions: (A) – PhOH 10−3 mol/l, H2 O2 /PhOH 28/1 mol/mol, pH 8.2, 70 ◦ C; (B) – PhOH 10−3 mol/l, 6.5%Fe-VSB-5 2 g/l, pH 8.2, 70 ◦ C; (C) - H2 O2 /PhOH 14/1 mol/mol, 6.5%Fe–VSB–5 1 g/l, pH 8.2, 65 ◦ C). Inset of figures: Plots of lg W0 versus lg CH2O2 (B) and lg W0 versus lg CPhOH (C), respectively.
202
M.N. Timofeeva et al. / Applied Catalysis B: Environmental 107 (2011) 197–204
100
pH
75
50 7.5 25
PhOH conversion, %
8.0
(1)
0
100
200
300
400
Fe3+ + HO + OH-
OH
OH H
+
. HO
(3)
Fe3+ +
. НО
(4)
Fe3+ + H2O2
(5)
Fe3+ + HO2/O2-
(2)
(6) 7.0
.
Fe2+ + H2O2
.
.
OH Fe3+
Fe2+ + OH-
OH
Fe2+
. .
Fe+2 + HO2/O2- + H+
. .
HO2 + H+
OH
.
Fe+2 + O2 + H+
O2-
Scheme 1. Reaction pathway of the generation of radical oxygen species for the phenol oxidation with H2 O2 over a iron-containing catalyst.
0 500
Time, min Fig. 6. The phenol consumption and changes in pH of solution in phenol oxidation over 6.5%Fe-VSB-5 (PhOH 10−3 mol/l, H2 O2 /PhOH 28/1 mol/mol, 6.5%Fe-VSB-5 2.5 g/l, pH 8.2, 65 ◦ C).
Fig. 6 shows the kinetic curves of the phenol and changes in pH of the phenol solution in the presence of 6.5%Fe-VSB-5. A drop in pH occurred at the beginning of the reaction, and this can be attributed to the formation of acidic intermediate products. Similar to all Fenton-like systems, the kinetic curves of PhOH oxidation with H2 O2 over Fe-VSB-5 had an induction period (Figs. 5–7). The increased Fe inserted into the VSB-5 network led to a decrease in the induction period. This phenomenon can be explained by the increase in the rate of radical formation. It is well known that radical oxygen species, such as HO2 • /O2 •− and HO• radicals are formed in the course of the decomposition of H2 O2 by Fe3+ (Scheme 1) [25]. It is generally believed [26] that the reaction of HO• with phenol leads to the formation of organic radicals (Scheme 1, Eq. (2)) and the Fe3+ /Fe2+ cycle is closed by the reaction (Scheme 1, Eqs. (3)–(5)). Radical generation can be confirmed by an in vivo spintrapping technique. Radical generation from phenol oxidation with H2 O2 over 6.5%Fe-VSB-5 was investigated by an EPR
g = 2.006 15
8.0x10
100
3300
3350
34 00
345 0
M ag netic field, G 15
50
4.0x10
25
2.0x10
Intensity
PhOH conversion, %
15
6.0x10
75
15
0 0
50
100
0.0 150
Time, min Fig. 7. Dependence of intensity of signals in EPR spectroscopy on the catalytic activity in phenol oxidation with H2 O2 over 6.5%Fe-VSB-5 and EPR spectrum of the TEMP radical adduct formed during reaction (0.2 mmol PhOH, 0.58 mmol H2 O2 , 6.5%FeVSB-5 40 mg, 0.4 mmol TEMP, 5 ml H2 O, pH 8.2, 70 ◦ C).
spectroscopy spin-trapping technique (using radical scavenger 2,2,6,6-tetramethylpiperidine). The transformation of TEMP to TEMPO is an indirect evidence of the formation of free radicals:
CH3 H3C
CH3 N
CH3
CH3 H3C
CH3 N
.
CH3
O
TEMPO
TEMP
TEMP has been used for the trapping of singlet molecular oxygen O2 with the formation of nitroxyl radical TEMPO [12,17]. Fig. 7 shows a typical EPR spectrum obtained upon the addition of TEMP to reaction mixtures containing 6.5%Fe-VSB-5 samples. The EPR signal at g = 2.006 and aN 17.9 G confirms the presence of TEMPO [17,27]. As can be seen from the experimental evidence (Fig. 7), the character of the change of intensity of a nitroxyl radical TEMPO correlated with that of the change of PhOH conversion during the reaction course. Phenol conversion and product selectivity in the hydroxylation of PhOH (PhOH:H2 O2 = 1:1.25 mol/mol) are shown in Table 6. The obtained results indicate conclusively that the selectivities of the products towards hydroquinone and catechol over 6.5%Fe-VSB-5 were close to those over 1.8%Fe–SiO2 [11]. The slightly high selectivity towards catechol, compared with those over 1.8%Fe,Al–SiO2 , is likely caused by the side-by-side adsorption of phenol and hydrogen peroxide on the surface of the 6.5%Fe-VSB-5 due to the basic properties of the surface sites. Note that the strong basic sites and Lewis sites of 6.5%Fe-VSB-5 favour a strong adsorption of reagents on the surface that can lead to a polymerization of hydrocarbons and therefore, decrease the total selectivity towards catechol and hydroquinone (Table 6). The effect of these sites on selectivity is also observed for the 1.8%Fe,Al–SiO2 [11]. Table 6 Phenol oxidation with H2 O2 in the presence of Fe-containing samples. PhOH conversion (mol%) 1.8%Fe–SiO2 b [11] 1.8%Fe,Al–SiO2 b [11] 6.5%Fe-VSB-5c a b c
6 22 25
Yielda (mol%) Catechol
Hydroquinone
25 5 29
45 58 50
Yield of catechol and hydroquinone based on PhOH consumed. Reaction conditions: PhOH, 0.1 M; H2 O2 , 0.2 M; catalyst 1 g/l, 40 ◦ C, 75 min. Reaction conditions: PhOH, 0.4 M; H2 O2 , 0.5 M; catalyst 3 g/l, 60 ◦ C, 4 h.
M.N. Timofeeva et al. / Applied Catalysis B: Environmental 107 (2011) 197–204
203
H O
O Fe3+
O Ni2+
P5+ O
O
. .
HO2/O2-
O
(I) H 2O 2
H+
H O
O Fe2+
. .
HO
+ HO2/O2-
O
OH
O Ni2+
P5+ O
O
(II)
OH
H 2O 2
.OH O H O
O Fe3+ O
P O
O
H+ H
O
O
O Fe2+
Ni2+
5+
O H
H 2O2
O
H
Ni2+
P5+ O
O
O
(III)
(IV) Scheme 2. Proposed mechanism of PhOH oxidation with H2 O2 over Fe-VSB-5.
Since the VSB-5 is not active in PhOH oxidation with H2 O2 , it is reasonable to suggest that the catalytic activity of Fe-VSB-5 is a result of the joint actions of Fe2+ and Ni2+ ions. Based on the investigation of acid–base properties (see Section 3.2) and the proposed structure of the active sites for Fe-AlPO [14], the structure of the active site for Fe-VSB-5 can be assumed to consist of Fe–O, P–O, and Ni–O units. The most plausible reaction mechanism may be composed of several steps (Scheme 2). At the very first moment, the reduction of Fe3+ to Fe2+ proceeds due to the action of H2 O2 (complexes I–II), and PhOH adsorbs onto Fe-VSB-5 because of the interactions with the basic OH− ions bound to the Ni2+ center (complex III). Reaction of H2 O2 with Fe2+ leads to the re-oxidation of the Fe3+ center and the generation of HO• radicals, which react with the phenol adsorbed on the Ni2+ center (complex IV). Finally, the next molecule of H2 O2 regenerates the Fe3+ center to Fe2+ and completes the catalytic cycle. Note that the radicals HO2 • /O2 •− can also reduce Fe3+ to Fe2+ (Scheme 1, Eqs. (5)–(6)). Of particular interest was a comparison of the reaction mechanism and catalytic activity of Fe-VSB-5 with the most active Fe-containing samples. It was found that the pH of the reaction mixtures, which is occasioned by catalyst, are different in the present of Fe–SiO2 , Fe,Al–SiO2 , Fe,Al-pillared layer clay (Fe,Al-PILC) and FeVSB-5 (Table 5). The pH of the reaction mixtures in the presence of Fe–SiO2 , Fe,Al–SiO2, and Fe,Al-PILC were 5.6 since these samples possess strong surface acid sites. On the other hand, the pH of the reaction mixtures in the presence of Fe-VSB-5 was 8.2 due to the strong surface basic sites of Fe-VSB-5 (see Section 3.2). According to the experimental data, Fe–SiO2 , Fe,Al–SiO2 and Fe,Al-PILC were not active at pH 8.2 even though these samples were active at pH 5.6. For this reason, slightly hard conditions for oxidation may be needed in the presence of Fe-VSB-5. Surface acidity plays an important role in PhOH oxidation with H2 O2 over these samples [28–30]. Lin and Gurol proposed that
Lewis acid sites, which are associated with iron, could facilitate the reduction of ferric ion by H2 O2 by attracting the electron density from the iron center and destabilizing the Fe3+ state [29]. Moreover, Brönsted acidity also affects the catalytic activity of Fe-containing materials [11,28,29]. Thus, the insertion of Al ions into a silica network increases the Brönsted surface acidity and therefore, catalytic activity of Fe,Al–SiO2 (Table 5). One of the reasons for this phenomenon may be the change in the adsorption capacity of Fe-containing materials towards the reagents, which facilitates the sorption–desorption processes of the reagents. As can be seen from Table 5, the increase in surface acidity favored a decrease in the PhOH adsorption capacity based on the specific surface area of the Fe-containing sample (qe ) and therefore, the reaction rate increased. Note that the qe for 6.5%FeVSB-5 was nearly twice as much as that for the 1.8%Fe,Al–SiO2 , and the adsorption capacity of 6.5%Fe-VSB-5 was close to that of the 1.8%Fe–SiO2 . However, compared with the 1.8%Fe–SiO2 , the 6.5%Fe-VSB-5 sample was more stable with respect to iron leaching (Table 5). In other words, after three reactions cycles of PhOH oxidation, iron leaching from 6.5%Fe-VSB-5 was negligible (about <0.1%) (Table 5). The low leaching of the Fe ions underlines the stability of the VSB-5 structure, which is in agreement with the IR spectroscopic data (Fig. 2). In turn, the leaching of Fe atoms from 1.8%Fe–SiO2 was higher and reached 0.2% in first cycle. In summary, we may conclude that Fe-containing nickel phosphate molecular sieves have the unique nature of having surface functional groups, which affect and adjust their catalytic properties. 4. Conclusion Fe-containing nickel phosphate molecular sieves (Fe-VSB-5) were hydrothermally synthesized in weak basic conditions (pH 7.3)
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under microwave irradiation using NiCl2 , FeCl2 and H3 PO4 as the Ni, P, and Fe sources. These materials were characterized by XRD, SEM, FT-IR, DR-UV–vis, and N2 -adsorption/desorption analysis. FTIR spectroscopy using PhCN as a probe molecule was used for the analysis of Lewis acid sites. Two types of Lewis acid sites were found in VSB-5. There are electron-deficient Ni ions in Ni–O–P and Ni–O–Ni groups. Two types of Lewis acid sites, formed by electrondeficient Ni ions in the Ni–O–Ni groups and by Fe ions, were found in Fe-VSB-5. FT-IR spectroscopy using CDCl3 as a probe molecule was used to investigate the surface basic sites. The results show that the insertion of Fe into the VSB-5 network led to an increase in the strength of the surface basic sites. Moreover, the basicity of 6.5%FeVSB-5 was stronger than that of 1.8%Fe,Al–SiO2 and 1.8%Fe-PILC. The Fe-VSB-5 samples were tested as catalysts for wet phenol oxidation with H2 O2 . The insertion of Fe into the VSB-5 network increased the catalytic activity of VSB-5. As the iron content in VSB-5 network increased, the activity of the FeVSB-5 increased. The increase in activity can be explained by the contribution of the Fe species in increasing the generation of radicals. The catalytic activity of Fe-VSB-5 was compared with that of traditional Fe-containing materials such as Fe–SiO2 , Fe,Al–SiO2 , and Fe,Al-pillared layer clay (Fe,Al-PILC). It was confirmed that the pH of the reaction mixtures in the presence of Fe-VSB-5 and Fe-containing materials were different from each other due to the different surface acid–base properties. The pH of the reaction mixtures in the presence of Fe-VSB-5 and Fecontaining materials were 8.2 and 5.6, respectively. For this reason PhOH oxidation with H2 O2 over Fe-VSB-5 needs slightly hard conditions. It was demonstrated that the adsorption capacity of Fe-containing materials towards PhOH affects the catalytic activity of these materials in PhOH oxidation with H2 O2 . The adsorption capacity for 6.5%Fe-VSB-5 was nearly twice as much as that for 0.8%Fe,Al–SiO2 and 1.8%Fe-PILC and was close to that of 1.8%Fe–SiO2 . It was demonstrated that the selectivity towards hydroquinone and catechol in the hydroxylation of phenol over 6.5%Fe-VSB-5 was similar to that over 1.8%Fe–SiO2 . However, different from the 1.8%Fe–SiO2 , the 6.5%Fe-VSB-5 was stable against leaching of iron species and could be used repeatedly in an oxidation reaction without suffering a loss of activity. Acknowledgement The authors thank K.F. Obzherina for FT-IR measurements. We also thank A.B. Ayupov for productive and beneficial discussion of our results. This work was supported partly by the Korea Sanhak Foundation.
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Fe-containing nickel phosphate molecular sieves as heterogeneous catalysts for phenol oxidation and hydroxylation with H2 O2 M.N. Timofeeva a,∗ , Zubair Hasan b , A.Yu. Orlov a,c , V.N. Panchenko a , Yu.A. Chesalov a , I.E. Soshnikov a , Sung Hwa Jhung b,∗∗ a
Boreskov Institute of Catalysis SB RAS, Prospekt Akad. Lavrentieva 5, 630090, Novosibirsk, Russian Federation Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Sankyuck-Dong, Buk-Ku, Daegu 702-701, South Korea c Novosibirsk State University, st. Pirogova 2, 630090, Novosibirsk, Russian Federation b
a r t i c l e
i n f o
Article history: Received 24 May 2011 Received in revised form 1 July 2011 Accepted 10 July 2011 Available online 20 July 2011 Keywords: Nickel phosphate molecular sieves VSB-5 Hydrogen peroxide Phenol Oxidation Hydroxylation
a b s t r a c t Fe-containing nickel phosphate molecular sieves (Fe-VSB-5) were hydrothermally synthesized in weak basic conditions under microwave irradiation and characterized by SEM, XRD, N2 -adsorption/desorption, DRS-UV–vis, and FT-IR spectroscopy using PhCN and CDCl3 as probe molecules. The catalytic activity of Fe-VSB-5 was tested for the phenol hydroxylation and wet phenol oxidation with H2 O2 . The increase in iron content in Fe-VSB-5 leds to an increase in the reaction rates. The increases in activity can be explained by the role of the Fe species, which increases the generation of radicals. The Fe-VSB-5 samples were stable against the leaching out of Fe ions. The catalytic activity of Fe-VSB-5 was compared to the catalytic activity of traditional Fe-containing materials. © 2011 Elsevier B.V. All rights reserved.
1. Introduction A novel class of microporous materials, nickel phosphate phases, has been discovered that represents a family of framework oxide molecular sieves synthesized without silica and alumina. Nickel phosphate molecular sieves, such as VSB-1 and VSB-5, are similar to zeolites in some properties and may find some uses as adsorbents for separations of molecular species and as catalysts or catalyst supports [1–4]. These materials have interesting catalytic properties suitable for reactions that do not require strong acidity. Thus, VSB-1 was used for the dehydrocyclodimerization of butadiene to ethylbenzene [4]. Moreover, these materials can be used as catalyst for oxidation reactions. For example, the catalytic properties of VSB-5 were demonstrated in the oxidation of alcohols to corresponding aldehydes or ketones under mild conditions with molecular oxygen [2] and for epoxidation of cyclic olefins, such as cyclohexene, cyclooctene [1] and cyclododecene [3] with H2 O2 . Isomorphous substitution of Ni2+ with bi- or trivalent transition-metal ions can, in principle, generate both new red–ox centers and Brönsted/or Lewis centers, that can improve catalytic properties. Moreover,
∗ Corresponding author. Tel.: +7 383 330 7284; fax: +7 383 330 8056. ∗∗ Corresponding author. Tel: +82 53 950 5341; fax: +82 53 950 6330. E-mail addresses: [email protected] (M.N. Timofeeva), [email protected] (S.H. Jhung). 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.07.014
metal-incorporated molecular sieves contain atomically dispersed metal species, and such sites may impart important red–ox and catalytic properties. Recently, transition-metal-incorporated nickel phosphate molecular sieves (TMI-VSB, M = Co2+ , V5+ , Fe3+ ) were synthesized by the isomorphous substitution of transition metal ions (TMI) in the VSB framework [5,6]. Recently, interest in the catalytic properties of TMI-VSB materials has increased. Thus, Co-VSB-5 exhibited promising catalytic activities for the oxidation of styrene with H2 O2 , in which styrene conversion and selectivity to benzaldehyde were 57% and 70%, respectively [7]. Jiang and Gao [8] demonstrated the high catalytic activity of Cu-VSB-5 in liquid phase catalytic hydroxylation of phenol. Fe-VSB-1 exhibited very good activity to oxidize ammonia into nitrate and nitrite ions under UV-irradiation [9]. The activity of Fe-VSB-1 was much better than that of pure FeCl2 as a photo-Fenton catalyst. It should be emphasized that various trivalent iron-substituted molecular sieves (silicates, aluminosilicates, aluminophosphates, etc.) have attracted considerable attention as heterogeneous Fenton type (Fe2+ /Fe3+ /H2 O2 ) catalysts for wet oxidation of organic pollutants in wastewaters. Phenol (PhOH) is one of the pollutants of industrial wastewaters. Various heterogeneous Fe-containing systems have been evaluated as catalysts for PhOH oxidation. Unfortunately, some of these systems possess certain disadvantages, such as low activity and stability. The maximal activity of many systems is observed at pH 3–4. However, the high acidity medium causes substantial iron
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leaching. According to Refs. [10,11], iron leaching may be negligible when Fe3+ is incorporated into the framework of molecular sieves, whereas small iron oxide clusters leach out during liquid-phase reactions. The relationship between chemical composition, textural data and catalytic activity of Fe,Al-pillared clays (Fe,Al-PILC) and Fe,Al-mesoporous silica was demonstrated at pH 6.2 [11,12]. Thus, 100% conversion of PhOH (0.001 M solution) was reached after 90 min with Fe,Al-PILC at 50 ◦ C. It was demonstrated that the nature of the OH-groups in mesoporous Fe,Al–SiO2 , which facilitate the sorption–desorption processes of reagents, had an effect on the reaction rate of phenol oxidation with H2 O2 [11]. Of note, the increase in the strength of basic sites for Fe-containing samples was accompanied by an increase of the H2 O2 degradation rate and therefore, an increase in the reaction rate [11]. According to Ref. [13], Fe-containing aluminophosphate molecular sieves (FeAlPO) possess significant catalytic activity and stability for liquid phase phenol oxidation with H2 O2 . The catalytic performance of Fe-AlPO-11 was similar to that of TS-1 [13]. It is interesting to note that the activity of Fe-AlPO depends on the Fe content in the framework of AlPO and on the acidity of the surrounding groups of iron sites. Although the details of the interaction between the reacting molecules and the solid catalysts were far from well understood, Wang and Otsuka [14] assumed that the catalytic activity of FeAlPO was related to the presence of the acid sites in the molecular sieve structure. In this work, we examined the acid–base properties of Fecontaining nickel phosphate molecular sieves (Fe-VSB-5). The catalytic properties of Fe-VSB-5 were assessed for phenol hydroxylation and full phenol oxidation with hydrogen peroxide. The relationships between the iron state, the nature of the surface functional groups, catalytic activity, and stability against leaching were established. Some preliminary kinetic studies were also done to get an insight into the reaction mechanism. The aim of this study was to determine the role of the surface-active acid and base sites in PhOH oxidation and H2 O2 degradation under moderate reaction conditions. 2. Experimental 2.1. Synthesis of Fe-VSB-5 materials The VSB-5 and Fe-VSB-5 materials were hydrothermally synthesized with microwave irradiation using NiCl2 ·6H2 O, FeCl2 ·4H2 O, and ortho-phosphoric acid (H3 PO4 , 85 wt%, Merck) as Ni, P, and Fe sources [6,15]. The designation of the samples and the reaction conditions of this synthesis are presented in Table 1. 2.2. Instrumental measurements The porous structure of the materials was determined from the N2 adsorption isotherms obtained at −196 ◦ C using a Micromeritics ASAP 2400 equipment. The X-ray diffraction patterns were measured on a X-ray diffractometer XRD (ThermoARL) with Cu K␣ ˚ radiation. The morphology of the Fe-VSB-5 samples ( = 1.5418 A)
was examined using a field emission scanning electron microscope (Hitachi, S-4300). Infrared spectra were recorded on a BOMEM-MB102 spectrometer in the 250–4000 cm−1 range with a resolution of 4 cm−1 using compressed pellets (2 mg of catalyst and 500 mg of KBr). The DR-UV–vis spectra were recorded on a UV-2501 PC Shimadzu spectrometer with an IRS-250A accessory in the 190–900 nm range with a resolution of 2 nm. BaSO4 was used as a standard. The chemical analyses were done by means of an inductively coupled plasma-atomic emission spectrometry (ICP-AES). 2.3. Acidity measurements For IR transmission measurements, the samples were pressed into self-supporting wafers (7–20 mg/cm2 ) and pre-treated within the IR cell by heating at 300 ◦ C under airflow for 1 h and under vacuum for 0.5 h at 300 ◦ C before the adsorption experiments. For the analysis of the basicity, the samples were exposed to saturated CDCl3 vapors for 3 min at room temperature. FT-IR spectra of the samples were obtained both before and after CDCl3 adsorption, and the difference was reported. The strength of the base sites was estimated from the shift of C–D using Eq. (1) [16]: log C−D = 0.0066PA − 4.36
(1)
where PA is the proton affinity. For the analysis of the Lewis surface acidity, the samples were exposed to saturated PhCN vapors at room temperature. FT-IR spectra of the adsorbed PhCN were recorded every 10 min up to saturation by PhCN. FT-IR spectra of adsorbed CDCl3 and PhCN were recorded on a Shimadzu FTIR-8300 spectrometer in the range of 400–6000 cm−1 with a resolution of 4 cm−1 . 2.4. Catalytic tests Phenol oxidation was carried out at 60–70 ◦ C in a glass thermostated vessel equipped with a stirrer and a reflux condenser. The reactor was charged with a mixture of H2 O2 /PhOH (30 wt.% in water) = (14–28)/1 mol/mol (20 ml of (1–10) × 10−3 mol/l phenol solution in water) and a catalyst (typically, 1.0–5.0 g/l). The phenol concentration in the solution was determined using a UV–vis equipment (Specord UV–vis M-40 instrument, = 273 nm, accuracy ±10%). The phenol hydroxylation was carried out at 60 ◦ C in the same vessel. A mixture of PhOH/H2 O2 at a 1:1.25 (mol/mol) ratio (5 ml of 0.4 M phenol solution in water) and a catalyst (3.0 g/l) was placed in the reactor. The identification and determination of the products (hydroquinone and catechol) were done using a Shimadzu Prominence HPLC system equipped with a Pathfinder 100 AS 2.5 UM column. 2.5. Adsorption tests PhOH adsorption on the VSB-5 and Fe-VSB-5 materials was examined in a glass thermostated reactor under stirring at 20 ◦ C. The reactor was loaded with 30 mg of sample and 5 ml of a PhOH
Table 1 Chemical composition of Fe-VSB-5 samples. Preparation gel (atomic%)
Fe-VSB-5 product Weight content (%)
1 2 3 a
VSB-5 3.1%Fe-VSB-5 6.5%Fe-VSB-5
Atomic ratio (%)
x
Fe Fe+Ni+P
Fe
Ni
P
Fe Fe+Ni+P
0 0.09 0.18
0 5.2 9.9
0 3.1 6.5
48.5 43.5 39.8
12.8 12.8 12.0
0 4.58 9.92
a
Molar composition of reaction mixture is xFeCl2 :0.63H3 PO4 :1.0NiCl2 :3.0NH3 :100H2 0.
M.N. Timofeeva et al. / Applied Catalysis B: Environmental 107 (2011) 197–204 Table 2 Textural data of Fe-VSB-5 samples and Fe-containing samples. No. 1 2 3 4
VSB-5 3.1%Fe-VSB-5 6.5%Fe-VSB-5 1.8%Fe,Al-PILC
SBET (m2 /g)
Sext (m2 /g)
V (cm3 /g)
V (cm3 /g)
284 202 203 215
36 41 31 117
0.24 0.13 0.16 0.22
0.128 0.082 0.088 0.051
199
and morphology of VSB-5 did not change substantially with the incorporation of Fe ions. Diffuse reflectance UV–vis spectroscopy shows that the oxidation states of Fe ions in 3.1%Fe-VSB-5 and 6.5%Fe-VSB-5 were not the same (Fig. 1). In the spectrum for 3.1%Fe-VSB-5 low-intensity bands at 212 and 233 nm can be resolved in the second derivative of the spectrum (Fig. 1, inset). These bands indicate that iron was mainly in the form of Fe2+ ions. The second derivative of the 6.5%FeVSB-5 spectrum (Fig. 1, inset) revealed three new bands at 218, 227 and 243 nm indicating the appearance of Fe3+ ions in the sample. It should be noted that bands between 300 and 400 nm are usually assigned to small oligomeric species, while the bands between 200 and 300 nm are due to isolated Fe ions [17,18]. According to the data in this study, the majority of the Fe ions were in the form of isolated ions in the Fe-VSB-5 samples. However, a small amount of oligomeric species of iron oxide was also present in the samples. The incorporation of Fe ions into the VSB-5 framework was confirmed by the IR analysis. The IR spectra of the VSB-5 and Fe-VSB-5 materials are shown in Fig. 2. The IR-spectrum of VSB-5 exhibited absorption bands at s (PO4 ) 967 cm−1 , as (PO4 ) 1023 cm−1 , and ıas (PO4 ) 632 and 589 cm−1 . In the spectrum for the Fe-VSB-5 samples, the band at 1115 cm−1 (as (PO4 )) disappeared and new bands at 1128 cm−1 (as (PO4 )) and 1059 cm−1 (s (PO2 ) were observed (Fig. 2). Moreover, the insertion of Fe ions into the VSB-5 framework led to an IR-shift of the band from 1024 to 1028 cm−1 (as (PO4 )). Note that a shift of bands for (PO4 ) was also observed after the incorporation of Fe ions in the AlPO-5 framework [19].
aqueous solution ((1–10) × 10−4 mol/l). At regular time intervals, aliquots were taken and the phenol concentration was determined by UV–vis at = 273 nm. 2.6. Spin trap experiments Electron paramagnetic resonance (EPR) spin-trapping of radical species formed during the catalytic wet oxidation was done using a Bruker ER-200D spectrometer at room temperature using a flat quartz cell, which is typically used for EPR measurements in polar solvents. 2,2,6,6-Tetramethylpiperidine (TEMP) was obtained from Aldrich. Spin trap experiments were carried out by adding 0.58 mmol of H2 O2 to a mixture containing 0.2 mmol of PhOH, 40 mg of 6.5%Fe-VSB-5 and 0.4 mmol of the spin-trapping TEMP in 5 ml H2 O. 3. Results 3.1. Physicochemical properties of Fe-VSB-5
3.2. The nature of the surface functional groups of Fe-VSB-5 Microporous nickel phosphates VSB-5 and Fe-VSB-5, which are based on octahedral NiO6 units linked by tetrahedral PO4 forming one-dimensional 24-membered ring channel structures, were synthesized according to Refs. [5,6]. The chemical composition and textural data of VSB-5 and Fe-VSB-5 are shown in Tables 1 and 2. From Table 1, it can be seen that the atomic ratios of Fe/ (Fe + Ni + P) for the synthesized Fe-VSB-5 materials were close to the expected ratios. Both VSB-5 and Fe-VSB-5 are microporous materials exhibiting high nitrogen adsorption (Supporting information, Fig. 1). According to the XRD data and scanning electron microscopy (SEM) images (Supporting information, Figs. 2 and 3), the crystallinity
Even though the details of the interaction between reacting molecules and solid catalysts are far from understood, it is commonly accepted that catalytic activity depends on the nature and amount of acid sites in the molecular sieve structure. Both Brönsted and Lewis acid sites contribute to the acidity of molecular sieves. The aim of this work was to investigate the nature of the acid sites of VSB-5 modified by iron ions to understand the main role of these sites in catalysis. PhCN is a soft base that has a high propensity to Lewis acid sites and a low propensity to Brönsted acid sites [20,21]. Fig. 3 shows
8 3.1%Fe-VSB-5
444
210
233
227
4 406
F(R)
6.5%Fe-VSB-5
243
212
3.1%Fe-VSB-5
218
6.5% Fe-VSB-5
24 0
27 0
300
Wavelength, nm
VSB-5 0 300
400
500
600
Wavelength, nm Fig. 1. DR-UV–vis spectra of VSB-5 – (1), 3.1%Fe-VSB-5 – (2) and 6.5%Fe-VSB-5 – (3). The inset shows the second derivatives of the spectra.
1128 1115 1092
2252
1023
M.N. Timofeeva et al. / Applied Catalysis B: Environmental 107 (2011) 197–204
1.5
2262
(1/2)
VSB-5
0.5 2212
3
1.0 2254
4
2262
Absorbance/p
Absorbance
632
967
589
1047
200
2
6.5%Fe-VSB-5
1
1000
900
800
700
Wavenumber, cm
0.0 2150
600
2250
-1
Table 3 Characteristic frequencies of PhCN adsorbed on VSB-5 and 6.5%Fe-VSB-5. CN (cm−1 )
Assignment
SiO2 [22] Zr,Al –PILC [23]
2236–2245 2242 2265 2285
Interaction with OH-groups Interaction with OH-groups Interaction with Zr4+ Interaction with Al3+
Fe2 (MoO4 )3 [22] FePO4 [22] VSB-5
2275 2270 2229 2282 2288
Interaction with Fe3+ Interaction with Fe3+ Interaction with OH-groups Interaction with Ni2+ Interaction with Ni2+
6.5%Fe-VSB-5
2229 2268 2282
Interaction with OH-groups Interaction with Fe3+ Interaction with Ni2+
the spectra of PhCN adsorbed on VSB-5 and 6.5%Fe-VSB-5. The main positions of the bands observed are summarized in Table 3. Three bands at 2229, 2282 and 2288 cm−1 were observed in the IR-spectrum of VSB-5. The band at 2229 cm−1 can be attributed to
Fig. 4. FT-IR difference spectra of adsorbed CDCl3 on VSB-5 and 6.5%Fe-VSB-5.
the interaction of PhCN with the OH-groups, which is likely related to the bridging P–(OH)–Ni groups. These OH-groups are weaker than that of the Zr, Al-pillared layer clays (Table 3). Two bands at 2282 and 2288 cm−1 can be assigned to the interaction between the nonbonding electrons of PhCN and electron-deficient Ni ions (Lewis acid sites (LAS)) [22]. After the insertion of Fe ions into the VSB-5 network, the band at 2288 cm−1 disappeared and a new band at 2268 cm−1 appeared instead. These changes attest to the change in the coordination state of the Ni ions. The band at 2268 cm−1 can be attributed to the interaction between the nonbonding electrons of PhCN and the electron-deficient Fe ions [22]. It is well known that Ni–O–Ni and Ni–O–P bonds exist in the framework of the VSB5 molecular sieve and that Ni2+ ions in the structure of VSB-5 can be substituted by Fe ions [6]. These data show that the bands at 2282 and 2288 cm−1 were related to the electron-deficient Ni ions in Ni–O–P and Ni–O–Ni groups, respectively. The basicity of VSB-5 and 6.5%Fe-VSB-5 was analyzed by FTIR spectroscopy using CDCl3 as a probe molecule [11,16,23]. The spectra of CDCl3 adsorbed on VSB-5 and 6.5%Fe-VSB-5 are shown in Fig. 4. The spectrum of VSB-5 disclosed two overlapping bands at 2262 and 2252 cm−1 . It is assumed that these bands, which are both formed by the Ni ions in the skeleton, can be assigned to
VSB-5
6.5%Fe-VSB-5
30 min
2282
2268
2229
2282
6
2288
Absorbance/ρ
6
3
5 min
0 2200
2300
-1
Wavenumber, cm
Fig. 2. IR spectra of VSB-5 – (1), 3.1%Fe-VSB-5 – (2), 6.5%Fe-VSB-5 – (3) and 6.5%Fe–VSB-5 was used for 3 cycles - (4).
12
2200
2229
1100
Absorbance/ρ
1200
2250
2300 -1
Wavenumber, cm
0 2200
2250
2300 -1
Wavenumber, cm
Fig. 3. FT-IR difference spectra of adsorbed PhCN on VSB-5 and 6.5%Fe-VSB-5.
M.N. Timofeeva et al. / Applied Catalysis B: Environmental 107 (2011) 197–204 Table 4 Spectral characteristics (C–D ) of OH-groups for VSB-5 and 6.5%-VSB-5 according to adsorption of CDCl3 . C–D (cm−1 )
C–D (cm−1 )
2268 2265 2260
– 3 8
– 733 797
1.8%Fe,Al–SiO2 [23]
2265 2260 2255
3 8 12
733 797 824
1.8%Fe,Al-PILC [23]
2265 2260 2252 2252
3 8 16 16
733 797 843 843
2254 2212
14 56
834 925
1 2 3 4 5 6 7 8 9 10 11 12
the weak interactions between CDCl3 and the base sites. A new band at 2212 cm−1 appeared in the spectrum of CDCl3 adsorbed on the 6.5%Fe-VSB-5 that is likely due to the Fe ions inserted into the VSB-5 framework. Unfortunately, no information about the nature of these sites is available; however, their strength was estimated using Eq. (1) [16]. The values for the proton affinity (PA) are presented in Table 4. The strengths of the strong basic sites (PA) were 843 and 925 kJ/mol for VSB-5 and 6.5%Fe-VSB-5, respectively. It is interesting to compare the strong strengths of the basic sites of VSB-5 and 6.5%Fe-VSB-5 with that of Fe-containing silica mesoporous materials and Fe-containing pillared layered clay (Fe,Al-PILC). According to the data presented in Table 4, the strength of the strong basic sites for VSB-5 is higher than that of Fe-containing silica mesoporous materials such as 1.8%Fe–SiO2 and 1.8%Fe,Al–SiO2 . However, the strength of the strong basic sites for VSB-5 is close to that of the strong basic sites for 1.8%Fe,Al-PILC. Of note, the strength of the strong basic sites for 6.5%Fe-VSB-5 was essentially higher than that of all Fe-containing materials. Therefore, the experimental evidence alludes to the fact that the insertion of Fe leads to an increase in the surface basicity of the 6.5% Fe-VSB-5. 3.3. Catalytic properties of Fe-VSB-5 The Fe-VSB-5 samples were tested for wet phenol oxidation with H2 O2 . pH of phenol solution after addition of Fe-VSB-5 changes from 5.6 to 8.2–8.3. Investigation on the activity of VSB-5
1.8%Fe,Al-PILC [11]
3
93 <0.1
C
-4.3 -4.4
25
-4.5 -4.6
40
0.005 mol/l 0.001 mol/l -6.0
20 0
H2O2/PhOH=14/1
50
4
150
-6.5
-7.0
5
-1 .9
- 1 .8
-1 .7
-1 .6
lg C H O 2 2
0
0
Time, min
76 <0.1 <0.1
0.011 mol/l
6.5%Fe-VSB-5, g/l
100
215
60
H2O2/PhOH=21/1
20 2
– 127 178 177
0.2
-4.7 1
qe d (109 mol m−2 )
and Fe-VSB-5 samples at pH 3–6 was not carried out due to their low stability in an acidic medium. According to the data from the catalytic tests, the reaction rate increased with increasing catalyst amounts, PhOH concentration, H2 O2 /PhOH ratio and reaction temperature (Fig. 5, Table 5). A kinetic investigation of the reaction using Van’t Hoff’s differential method [24] shows that the orders of reaction on PhOH and H2 O2 were 1.1 ± 0.1 and 1.0 ± 0.1, respectively. The activation energy for the reaction, which was determined by the Arrhenius equation k = Ae−Ea /RT , was 64.8 ± 8 kJ/mol (Supporting information, Fig. 5). The iron content in Fe-VSB-5 also affected the reaction rate (Table 5). The higher the iron contents in Fe-VSB-5, the higher the reaction rate. Since PhOH adsorption on the catalyst may contribute to the overall phenol consumption, the adsorption capacity (qe ) of 6.5%Fe-VSB-5 was measured (Table 5 and Supporting information, Fig. 4). The qe value (0.034 mmol/g) was much lower than the amount of phenol oxidized in the catalytic reaction (1.0 mmol/g).
0
PhOH conversion, %
40
30
τ1/2
– – <0.1 <0.1 (n.d., <0.1)e <0.1
75
50
50
>30 >30 9 7 (7, 7.5)e 2.5f >30a 10g >30a 2.5g 1.0h >30a 1.5h
H2O2/PhOH=28/1
60
, min
PhOH conversion, %
75
50
1.8%Fe,Al–SiO2 [11]
Fe leachingc (wt%)
B
2.5 g/l 2 g/l 1 g/l
0
1.8%Fe–SiO2 [11]
A
5 g/l
25
Without catalyst VSB-5 3.1%Fe-VSB-5 6.5%Fe-VSB-5
b TimePhOH 100% (h)
a Reaction conditions: PhOH 10−3 mol/l, H2 O2 /PhOH 28/1 mol/mol, catalyst 2 g/l, pH 8.2, 60 ◦ C. b Time of 100% conversion of PhOH. c The amount of iron leached from Fe-containing sample to solution (based on the initial iron content in the sample). d Phenol adsorption on Fe-containing sample at 25 ◦ C. e Catalyst was used in second and third cycles. f 70 ◦ C. g Reaction conditions: PhOH 10−3 mol/l, H2 O2 /PhOH 14/1 mol/mol, catalyst 1 g/l, pH 6.2, 60 ◦ C. h Reaction conditions: PhOH 10−3 mol/l, H2 O2 /PhOH 14/1 mol/mol, catalyst 1 g/l, pH 6.2, 50 ◦ C.
lg W
100
PhOH oxidationa
Sample
lg W
6.5%Fe-VSB-5
Run
PhOH conversion, %
VSB-5
Table 5 Phenol oxidation with H2 O2 in the presence of Fe-containing samples.
PA (kJ/mol)
CDCl3 1.8%Fe–SiO2 [23]
201
0
50
100
Time, min
150
0 0
50
100
150
-3.0
- 2 .5
-2 .0
lg C P h O H
Time, min
Fig. 5. Effect of 6.5%Fe-VSB-5 content (A); H2 O2 /PhOH molar ratio (B) and PhOH concentration (C) in reaction mixture on PhOH consumption. (Reaction conditions: (A) – PhOH 10−3 mol/l, H2 O2 /PhOH 28/1 mol/mol, pH 8.2, 70 ◦ C; (B) – PhOH 10−3 mol/l, 6.5%Fe-VSB-5 2 g/l, pH 8.2, 70 ◦ C; (C) - H2 O2 /PhOH 14/1 mol/mol, 6.5%Fe–VSB–5 1 g/l, pH 8.2, 65 ◦ C). Inset of figures: Plots of lg W0 versus lg CH2O2 (B) and lg W0 versus lg CPhOH (C), respectively.
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100
pH
75
50 7.5 25
PhOH conversion, %
8.0
(1)
0
100
200
300
400
Fe3+ + HO + OH-
OH
OH H
+
. HO
(3)
Fe3+ +
. НО
(4)
Fe3+ + H2O2
(5)
Fe3+ + HO2/O2-
(2)
(6) 7.0
.
Fe2+ + H2O2
.
.
OH Fe3+
Fe2+ + OH-
OH
Fe2+
. .
Fe+2 + HO2/O2- + H+
. .
HO2 + H+
OH
.
Fe+2 + O2 + H+
O2-
Scheme 1. Reaction pathway of the generation of radical oxygen species for the phenol oxidation with H2 O2 over a iron-containing catalyst.
0 500
Time, min Fig. 6. The phenol consumption and changes in pH of solution in phenol oxidation over 6.5%Fe-VSB-5 (PhOH 10−3 mol/l, H2 O2 /PhOH 28/1 mol/mol, 6.5%Fe-VSB-5 2.5 g/l, pH 8.2, 65 ◦ C).
Fig. 6 shows the kinetic curves of the phenol and changes in pH of the phenol solution in the presence of 6.5%Fe-VSB-5. A drop in pH occurred at the beginning of the reaction, and this can be attributed to the formation of acidic intermediate products. Similar to all Fenton-like systems, the kinetic curves of PhOH oxidation with H2 O2 over Fe-VSB-5 had an induction period (Figs. 5–7). The increased Fe inserted into the VSB-5 network led to a decrease in the induction period. This phenomenon can be explained by the increase in the rate of radical formation. It is well known that radical oxygen species, such as HO2 • /O2 •− and HO• radicals are formed in the course of the decomposition of H2 O2 by Fe3+ (Scheme 1) [25]. It is generally believed [26] that the reaction of HO• with phenol leads to the formation of organic radicals (Scheme 1, Eq. (2)) and the Fe3+ /Fe2+ cycle is closed by the reaction (Scheme 1, Eqs. (3)–(5)). Radical generation can be confirmed by an in vivo spintrapping technique. Radical generation from phenol oxidation with H2 O2 over 6.5%Fe-VSB-5 was investigated by an EPR
g = 2.006 15
8.0x10
100
3300
3350
34 00
345 0
M ag netic field, G 15
50
4.0x10
25
2.0x10
Intensity
PhOH conversion, %
15
6.0x10
75
15
0 0
50
100
0.0 150
Time, min Fig. 7. Dependence of intensity of signals in EPR spectroscopy on the catalytic activity in phenol oxidation with H2 O2 over 6.5%Fe-VSB-5 and EPR spectrum of the TEMP radical adduct formed during reaction (0.2 mmol PhOH, 0.58 mmol H2 O2 , 6.5%FeVSB-5 40 mg, 0.4 mmol TEMP, 5 ml H2 O, pH 8.2, 70 ◦ C).
spectroscopy spin-trapping technique (using radical scavenger 2,2,6,6-tetramethylpiperidine). The transformation of TEMP to TEMPO is an indirect evidence of the formation of free radicals:
CH3 H3C
CH3 N
CH3
CH3 H3C
CH3 N
.
CH3
O
TEMPO
TEMP
TEMP has been used for the trapping of singlet molecular oxygen O2 with the formation of nitroxyl radical TEMPO [12,17]. Fig. 7 shows a typical EPR spectrum obtained upon the addition of TEMP to reaction mixtures containing 6.5%Fe-VSB-5 samples. The EPR signal at g = 2.006 and aN 17.9 G confirms the presence of TEMPO [17,27]. As can be seen from the experimental evidence (Fig. 7), the character of the change of intensity of a nitroxyl radical TEMPO correlated with that of the change of PhOH conversion during the reaction course. Phenol conversion and product selectivity in the hydroxylation of PhOH (PhOH:H2 O2 = 1:1.25 mol/mol) are shown in Table 6. The obtained results indicate conclusively that the selectivities of the products towards hydroquinone and catechol over 6.5%Fe-VSB-5 were close to those over 1.8%Fe–SiO2 [11]. The slightly high selectivity towards catechol, compared with those over 1.8%Fe,Al–SiO2 , is likely caused by the side-by-side adsorption of phenol and hydrogen peroxide on the surface of the 6.5%Fe-VSB-5 due to the basic properties of the surface sites. Note that the strong basic sites and Lewis sites of 6.5%Fe-VSB-5 favour a strong adsorption of reagents on the surface that can lead to a polymerization of hydrocarbons and therefore, decrease the total selectivity towards catechol and hydroquinone (Table 6). The effect of these sites on selectivity is also observed for the 1.8%Fe,Al–SiO2 [11]. Table 6 Phenol oxidation with H2 O2 in the presence of Fe-containing samples. PhOH conversion (mol%) 1.8%Fe–SiO2 b [11] 1.8%Fe,Al–SiO2 b [11] 6.5%Fe-VSB-5c a b c
6 22 25
Yielda (mol%) Catechol
Hydroquinone
25 5 29
45 58 50
Yield of catechol and hydroquinone based on PhOH consumed. Reaction conditions: PhOH, 0.1 M; H2 O2 , 0.2 M; catalyst 1 g/l, 40 ◦ C, 75 min. Reaction conditions: PhOH, 0.4 M; H2 O2 , 0.5 M; catalyst 3 g/l, 60 ◦ C, 4 h.
M.N. Timofeeva et al. / Applied Catalysis B: Environmental 107 (2011) 197–204
203
H O
O Fe3+
O Ni2+
P5+ O
O
. .
HO2/O2-
O
(I) H 2O 2
H+
H O
O Fe2+
. .
HO
+ HO2/O2-
O
OH
O Ni2+
P5+ O
O
(II)
OH
H 2O 2
.OH O H O
O Fe3+ O
P O
O
H+ H
O
O
O Fe2+
Ni2+
5+
O H
H 2O2
O
H
Ni2+
P5+ O
O
O
(III)
(IV) Scheme 2. Proposed mechanism of PhOH oxidation with H2 O2 over Fe-VSB-5.
Since the VSB-5 is not active in PhOH oxidation with H2 O2 , it is reasonable to suggest that the catalytic activity of Fe-VSB-5 is a result of the joint actions of Fe2+ and Ni2+ ions. Based on the investigation of acid–base properties (see Section 3.2) and the proposed structure of the active sites for Fe-AlPO [14], the structure of the active site for Fe-VSB-5 can be assumed to consist of Fe–O, P–O, and Ni–O units. The most plausible reaction mechanism may be composed of several steps (Scheme 2). At the very first moment, the reduction of Fe3+ to Fe2+ proceeds due to the action of H2 O2 (complexes I–II), and PhOH adsorbs onto Fe-VSB-5 because of the interactions with the basic OH− ions bound to the Ni2+ center (complex III). Reaction of H2 O2 with Fe2+ leads to the re-oxidation of the Fe3+ center and the generation of HO• radicals, which react with the phenol adsorbed on the Ni2+ center (complex IV). Finally, the next molecule of H2 O2 regenerates the Fe3+ center to Fe2+ and completes the catalytic cycle. Note that the radicals HO2 • /O2 •− can also reduce Fe3+ to Fe2+ (Scheme 1, Eqs. (5)–(6)). Of particular interest was a comparison of the reaction mechanism and catalytic activity of Fe-VSB-5 with the most active Fe-containing samples. It was found that the pH of the reaction mixtures, which is occasioned by catalyst, are different in the present of Fe–SiO2 , Fe,Al–SiO2 , Fe,Al-pillared layer clay (Fe,Al-PILC) and FeVSB-5 (Table 5). The pH of the reaction mixtures in the presence of Fe–SiO2 , Fe,Al–SiO2, and Fe,Al-PILC were 5.6 since these samples possess strong surface acid sites. On the other hand, the pH of the reaction mixtures in the presence of Fe-VSB-5 was 8.2 due to the strong surface basic sites of Fe-VSB-5 (see Section 3.2). According to the experimental data, Fe–SiO2 , Fe,Al–SiO2 and Fe,Al-PILC were not active at pH 8.2 even though these samples were active at pH 5.6. For this reason, slightly hard conditions for oxidation may be needed in the presence of Fe-VSB-5. Surface acidity plays an important role in PhOH oxidation with H2 O2 over these samples [28–30]. Lin and Gurol proposed that
Lewis acid sites, which are associated with iron, could facilitate the reduction of ferric ion by H2 O2 by attracting the electron density from the iron center and destabilizing the Fe3+ state [29]. Moreover, Brönsted acidity also affects the catalytic activity of Fe-containing materials [11,28,29]. Thus, the insertion of Al ions into a silica network increases the Brönsted surface acidity and therefore, catalytic activity of Fe,Al–SiO2 (Table 5). One of the reasons for this phenomenon may be the change in the adsorption capacity of Fe-containing materials towards the reagents, which facilitates the sorption–desorption processes of the reagents. As can be seen from Table 5, the increase in surface acidity favored a decrease in the PhOH adsorption capacity based on the specific surface area of the Fe-containing sample (qe ) and therefore, the reaction rate increased. Note that the qe for 6.5%FeVSB-5 was nearly twice as much as that for the 1.8%Fe,Al–SiO2 , and the adsorption capacity of 6.5%Fe-VSB-5 was close to that of the 1.8%Fe–SiO2 . However, compared with the 1.8%Fe–SiO2 , the 6.5%Fe-VSB-5 sample was more stable with respect to iron leaching (Table 5). In other words, after three reactions cycles of PhOH oxidation, iron leaching from 6.5%Fe-VSB-5 was negligible (about <0.1%) (Table 5). The low leaching of the Fe ions underlines the stability of the VSB-5 structure, which is in agreement with the IR spectroscopic data (Fig. 2). In turn, the leaching of Fe atoms from 1.8%Fe–SiO2 was higher and reached 0.2% in first cycle. In summary, we may conclude that Fe-containing nickel phosphate molecular sieves have the unique nature of having surface functional groups, which affect and adjust their catalytic properties. 4. Conclusion Fe-containing nickel phosphate molecular sieves (Fe-VSB-5) were hydrothermally synthesized in weak basic conditions (pH 7.3)
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M.N. Timofeeva et al. / Applied Catalysis B: Environmental 107 (2011) 197–204
under microwave irradiation using NiCl2 , FeCl2 and H3 PO4 as the Ni, P, and Fe sources. These materials were characterized by XRD, SEM, FT-IR, DR-UV–vis, and N2 -adsorption/desorption analysis. FTIR spectroscopy using PhCN as a probe molecule was used for the analysis of Lewis acid sites. Two types of Lewis acid sites were found in VSB-5. There are electron-deficient Ni ions in Ni–O–P and Ni–O–Ni groups. Two types of Lewis acid sites, formed by electrondeficient Ni ions in the Ni–O–Ni groups and by Fe ions, were found in Fe-VSB-5. FT-IR spectroscopy using CDCl3 as a probe molecule was used to investigate the surface basic sites. The results show that the insertion of Fe into the VSB-5 network led to an increase in the strength of the surface basic sites. Moreover, the basicity of 6.5%FeVSB-5 was stronger than that of 1.8%Fe,Al–SiO2 and 1.8%Fe-PILC. The Fe-VSB-5 samples were tested as catalysts for wet phenol oxidation with H2 O2 . The insertion of Fe into the VSB-5 network increased the catalytic activity of VSB-5. As the iron content in VSB-5 network increased, the activity of the FeVSB-5 increased. The increase in activity can be explained by the contribution of the Fe species in increasing the generation of radicals. The catalytic activity of Fe-VSB-5 was compared with that of traditional Fe-containing materials such as Fe–SiO2 , Fe,Al–SiO2 , and Fe,Al-pillared layer clay (Fe,Al-PILC). It was confirmed that the pH of the reaction mixtures in the presence of Fe-VSB-5 and Fe-containing materials were different from each other due to the different surface acid–base properties. The pH of the reaction mixtures in the presence of Fe-VSB-5 and Fecontaining materials were 8.2 and 5.6, respectively. For this reason PhOH oxidation with H2 O2 over Fe-VSB-5 needs slightly hard conditions. It was demonstrated that the adsorption capacity of Fe-containing materials towards PhOH affects the catalytic activity of these materials in PhOH oxidation with H2 O2 . The adsorption capacity for 6.5%Fe-VSB-5 was nearly twice as much as that for 0.8%Fe,Al–SiO2 and 1.8%Fe-PILC and was close to that of 1.8%Fe–SiO2 . It was demonstrated that the selectivity towards hydroquinone and catechol in the hydroxylation of phenol over 6.5%Fe-VSB-5 was similar to that over 1.8%Fe–SiO2 . However, different from the 1.8%Fe–SiO2 , the 6.5%Fe-VSB-5 was stable against leaching of iron species and could be used repeatedly in an oxidation reaction without suffering a loss of activity. Acknowledgement The authors thank K.F. Obzherina for FT-IR measurements. We also thank A.B. Ayupov for productive and beneficial discussion of our results. This work was supported partly by the Korea Sanhak Foundation.
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