The preparation of Fe/wood-based activated carbon catalyst for phenol hydroxylation from Fe2+ and Fe3+ precursors

Article abstract of DOI:10.1039/c4cy01705f

A series of Fe/wood-based activated carbon catalysts with 0.89-6.94 wt% Fe loading were prepared using an impregnation-co-precipitation method, and the catalysts were characterized by inductively coupled plasma-atomic emission spectrometry, N₂ adsorption-desorption, X-ray diffraction, X-ray photoelectron spectroscopy, magnetic measurements, ultraviolet-visible diffuse reflectance spectroscopy, Fourier transform infrared spectroscopy and high resolution transmission electron microscopy. The results indicated that iron oxide species including Fe²⁺ and Fe³⁺ forming the active sites were successfully loaded on wood-based activated carbon. With increasing concentration of iron ions in the preparation process, iron oxide species were saturated gradually on the support. These catalysts have been assessed for the hydroxylation of phenol to dihydroxybenzenes using H₂O₂ as oxidant. The results showed that the catalytic activity was improved with the increase in the Fe content. The active phase for phenol hydroxylation could be the cooperation of the two iron oxide species. Under the optimized conditions, the catalyst with an Fe content of 6.55 wt% gave a phenol conversion of 51.1% with 80.6% of selectivity to dihydroxybenzenes. Finally, the results of catalyst recycling illustrated that the catalyst could be reused with slight Fe leaching and slight loss of activity.

Full text of DOI:10.1039/c4cy01705f

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DOI: 10.1039/C4CY01705F
OH
Fe2+ + Fe3+
+
H
+
2 2
O
NH .
53 K
3
2
H O
Fe/wood-based
activated carbon
3
OH
OH
OH
Wood-based
activated carbon
Yield: 41.2%
OH
3
+
2+
The loading of Fe and Fe on wood-based activated carbon obtained Fe-based
catalyst with good catalytic performance for phenol hydroxylation.

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ARTICLE
The preparation of Fe/wood-based activated carbon
2
+
3+
catalyst for phenol hydroxylation from Fe and Fe
Cite this: DOI: 10.1039/x0xx00000x
precursors†
Ruiguang Yang, Guiying Li*, and Changwei Hu*
Received 00th January 2012,
Accepted 00th January 2012
A series of Fe/woodꢀbased activated carbon catalysts with 0.89ꢀ6.94 wt% Fe loading were
prepared using impregnationꢀcoꢀprecipitation method, and the catalysts were characterized by
DOI: 10.1039/x0xx00000x
inductively coupled plasma, N adsorptionꢀdesorption, Xꢀray diffraction, Xꢀray photoelectron
2
spectroscopy, magnetic measurements, ultravioletꢀvisible diffuse reflectance spectroscopy,
fourier transform infrared spectroscopy and high resolution transmission electron microscopy.
www.rsc.org/
2
+
3+
The results indicated that iron oxide species including Fe and Fe forming the active sites
were successfully loaded on woodꢀbased activated carbon. With increasing concentration of
iron ions in the preparation process, iron oxide species were saturated gradually on the support.
These catalysts have been assessed for the hydroxylation of phenol to dihydroxybenzenes
using H O as oxidant. The results showed that the catalytic activity was improved with the
2
2
increase of Fe content. The active phase for phenol hydroxylation could be the cooperation of
the two iron oxide species. Under the optimized conditions, the catalyst with an Fe content of
6
.55 wt% gave a phenol conversion of 51.1% with 80.6% of selectivity to dihydroxybenzenes.
Finally, the results of catalyst recycling illustrated that the catalyst could be reused with slight
Fe leaching and slight loss of activity.
1
Introduction
heterogeneous catalyst or catalytic support. Because of the presence
of surface oxygen functional groups, AC has been found useful in
catalysis. It is wellꢀknown that large surface area and high porosity
Hydroquinone (HQ) and catechol (CAT), as important chemical
materials and intermediates, have been widely used in photographic
developer, pharmaceutics, polymerization inhibitor, antioxidant, and
22
are helpful for the dispersion of active phases, which results in the
increase of catalytic activity. It means that AC may be an excellent
catalyst support. Jin et al. have applied Fe/AC as a catalyst to
directly convert phenol to DHB, and a phenol conversion of 41.3%
1ꢀ4
so on. The heterogeneous catalytic hydroxylation of phenol to
2
produce dihydroxybenzenes (DHB, that is, HQ and CAT) with H O2
as oxidant, has attracted much attention due to the favorable
2
with DHB yield of 36.0% was obtained. It was proposed that high
properties of solid catalysts. In recent years, there has been growing
research interest in finding suitable solid catalysts, especially those
containing transition metal ions, for the hydroxylation of phenol to
catalytic activity was mainly attributed to the efficient catalytic
component, Fe O , which formed by ferric species partly reduced to
ferrous species over AC at 673 K in air.
3
4
DHB under mild reaction conditions, with clean oxidants like O and
2
Taking these factors into consideration, we loaded Fe on the
5ꢀ7
H O . Efforts are made to use various materials, such as zeolites
,
,
2+
2
2
woodꢀbased activated carbon (WAC) by the use of both Fe and
8
ꢀ11
12ꢀ14
mesoporous materials
,
hydrotaliciteꢀlike compounds
3+
Fe in the preparation process in an attempt to obtain high activity
1
5ꢀ17
nanoparticles
, and so on. Li et al. have prepared microporous
catalyst for the hydroxylation of phenol.
carbon molecular sieve for the titled reaction and obtained a phenol
1
8
conversion of 29.6% with selectivity of 85.1% to DHB. Among the 2 Experimental
transitional metals studied, Fe showed the best activity. Therefore,
much attention has been paid to the development of Feꢀcontaining 2.1 Materials used
catalysts. Adam et al. have hydrothermally synthesized Fe modified Commercially available WAC (40ꢀ60 mesh) was provided by
zeolite L nanocrystals, which gave a phenol conversion of 93.40% in Jiangsu Nantong Activated Carbon Cooperation (China). Prior
1
9
the presence of acetic acid in 30 min. Liang et al. have prepared to use, WAC was washed three times by deionized water and
mesoporous Feꢀincorporated materials, which gave phenol dried at 383 K for 24 h. Ferric nitrate (Fe(NO ) ·9H O) and
a
3
3
2
2
0
conversion of 47.3%.
ferrous sulfate (FeSO ·7H O) were used as the precursors of
4 2
Activated carbon (AC) is a general adsorbent of organic Fe. NH ·H O (1 mol/L) and glacial acetic acid were used to
3
2
compounds from aqueous solutions with huge surface area and adjust the pH value. Phenol and 30 wt% H O (Kelong,
2
2
2
1
abundant porosity. Apart from adsorbent, AC can also act as a Chengdu, China) were used as the materials for catalytic
This journal is © The Royal Society of Chemistry 2013
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activity
OH
test. eV). The binding energy scale was calibrated with respect to the
adventitious carbon (C 1s) at 284.6 eV. The data were analyzed with
the use of standard software. A Shirley background was subtracted
from all spectra.
OH
OH
OH
Fe/WAC
+
H O, H O
2
2
2
The magnetic property was studied by a vibrating sample
magnetometer (Lake Shore 7410) at room temperature.
Ultravioletꢀvisible diffuse reflectance (DR UVꢀvis) spectra were
obtained in the range from 200 to 800 nm using a UVꢀ3600
OH
Phenol
Catechol
Hydroquinone
Scheme 1 Catalytic phenol hydroxylation by H O
2
2
spectrometer equipped with a reflectance attachment. BaSO was
4
Table 1 The content of Fe in catalysts
a
used as the reference material.
Fourier transform infrared (FTIR) spectra were collected at 2 cm
Concentration of iron ions
mol/L)
Fe content of catalysts
ꢀ
1
(
(wt%)
0.89
2.76
4.82
6.55
6.94
resolution using a Nicolet Nexus 670 FTIR spectrometer equipped
with a mercury cadmium telluride detector.
0
0
0
0
0
.018
.054
.108
.162
.198
High resolution transmission electron microscopy (HRTEM)
2
0
images were obtained with an FEI Company Tecnai G2 Stwin
instrument operated at 200 kV. The samples were ultrasonically
dispersed in ethanol at room temperature for 30 min. The asꢀ
obtained solutions were dropped onto copper grids for TEM.
a
2+
3+
3+
2+
Iron ions included Fe and Fe , and the molar ratio of Fe to Fe
was 2:1.
2
.4 Phenol hydroxylation reaction
All the solvents and reagents were commercially available with
analytical purity (purity > 99%).
Phenol hydroxylation was catalyzed by Fe/WACs in water. The
analysis of the reaction mixture demonstrated that the main products
were CAT and HQ (Scheme 1). The main byꢀproducts were pꢀ
2
.2 Preparation of catalysts
The impregnationꢀcoꢀprecipitation method used for the preparation benzoquinone (BQ) and polymeric tars. In most cases, tars were not
of Fe/woodꢀbased activated carbons (Fe/WACs) was similar to that directly detectable, and the presence of tars in the reaction mixture
2
3ꢀ24
17
reported in literatures
. 5 g of WAC was impregnated with 50 ml was identified by mass balance.
of aqueous solution of Fe(NO ) ·9H O and FeSO ·7H O with
desired concentrations (the molar ratio of Fe to Fe being following procedure: the reaction was performed in a 50 ml twoꢀ
The investigation of the catalytic activity was carried out by the
3
3
2
4
2
2+
3
+
controlled to be 2:1) for 30 min under N atmosphere. The mixture necked roundꢀbottom flask, equipped with a magnetic stirrer, a
2
was heated to 353 K and 1 mol/L NH ·H O was added continuously reflux condenser and a temperature controllable waterꢀbath. In a
3
2
to adjust the pH to 9 under vigorous stirring. Then the sample was typical run, 0.48 g of phenol (5 mmol), 10.00 g of water, 0.26 g of
stirred and kept at 353 K for 1 h. The product was washed by acetic acid, and 0.05 g of catalyst were added to the reactor and the
deionized water (until the pH being 7), and dried at 353 K in a system was stirred at 313 K. To start the reaction, 0.50 g of 30 wt%
vacuum oven for 24 h. The obtained samples were sieved to obtain H O solution (5 mmol) was added at once. The reaction products
2
2
the 40ꢀ60 mesh catalysts and referred to Fe/WAC/x (x was the actual were collected, identified, and quantified by high performance liquid
Fe content of the catalysts from the ICPꢀAES results).
chromatography (HPLC, Comatex C18 column, 4.6 mm i.d. × 250
mm), using UV detector at 277 and 254 nm. The conversion of
2
.3 Characterization of the catalysts
phenol (X ), the selectivity to DHB (SDHB), the yield of products or
Ph
The actual Fe content loaded on the catalysts in the catalytic byꢀproducts (YDHB, YCAT, YHQ, YBQ and YOthers), and the effective
hydroxylation process was determined by inductively coupled
plasmaꢀatomic emission spectrometer (ICPꢀAES).
The textural characterization of the catalyst was performed by N2
physisorption at liquid nitrogen temperature (77 K) using a
Micromeretics Tristar 3020 automatic analyzer. Prior to the
U
H2O2
utilization of H O (
) were defined as follows:
2
2
0
0
X Ph = (n − n )/n ×100%
Ph
Ph
Ph
0
Ph
SDHB = (nCAT + nHQ )/(n − nPh )×100%
0
Ph
YDHB = (nCAT + nHQ)/n ×100%
0
ph
measurements, samples were degassed at 573 K for 4 h under YCAT = nCAT / n ×100%
vacuum. The surface area (SBET) was determined by BET method,
and the specific surface area of the micropores (Smic) and micropore
volume (Vmic) were calculated by the tꢀplot method. The pore size
distributions were calculated by DFT method.
0
Ph
YHQ = nHQ / n ×100%
0
YBQ = nBQ / n ×100%
Ph
0
Ph
0
Ph
YOthers = (n − nPh − nCAT − nHQ − nBQ )/ n ×100%
Xꢀray diffraction (XRD) patterns were obtained on
a
0
H
U H2O2 = (n
+ nHQ + 2nBQ )/ n
×100%
CAT
2 2
O
diffractometer (LTD DXꢀ1000 CSC) using Cu K (λ = 1.54056 Å)
0
α
n
nPh
Where Ph and
denoted the initial and final amounts
0
radiation. The apparatus was operated at 25 mA and 40 kV. The data
o
n
H2O2
were collected with a step of 0.0544 , using continuous scanning (moles) of phenol, respectively, and
denoted the initial
o
n
nHQ
nBQ
mode in the range of 10 ≤ 2 θ ≤ 80 .
amount (moles) of hydrogen peroxide, while CAT,
and
Xꢀray photoelectron spectroscopy (XPS) was taken with an AXIS
denoted the produced amounts (moles) of CAT, HQ and BQ,
Ultra DLD (KRATOS) spectrometer with Al K radiation (1486.6
α
2
| J. Name., 2012, 00, 1-3
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ARTICLE
Fig. 2 The Xꢀray diffraction patterns of samples: (a) WAC, (b)
Fe/WAC/0.89, (c) Fe/WAC/2.76, (d) Fe/WAC/4.82, (e)
Fe/WAC/6.55, (f) Fe/WAC/6.94.
However, when the concentration of iron ions exceeded 0.162
mol/L, the Fe content of the catalyst changed slightly. Visually, there
were a lot of black isolated granules belonged to iron oxides apart
from the WAC after the coꢀprecipitation process. The results
illustrated that with high concentration of iron ions, a lot of isolated
iron oxides were not loaded on WAC after the saturated adsorption
2+
3+
of Fe and Fe on WAC. It implied that the Fe loading depended
on the nature of WAC under the present preparation conditions.
3
.1.2 The N adsorption-desorption isotherms
2
Fig. 1 N2 adsorptionꢀdesorption isotherms (A) and pore size
distributions (B) of catalysts: (a) WAC, (b) Fe/WAC/0.89, (c)
Fe/WAC/2.76, (d) Fe/WAC/4.82, (e) Fe/WAC/6.55, (f)
Fe/WAC/6.94.
Fig. 1 showed the N adsorptionꢀdesorption isotherms and pore size
distributions of WAC and Fe/WACs. WAC exhibited type I eminent
characteristic of the microporous material. It was clear that
2
Fe/WACs had similar N adsorptionꢀdesorption isotherms to that of
2
WAC. Obviously, Fig. 1 (B) showed that all samples possessed
heterogeneous micropores, and some small micropores disappeared
with increasing Fe content. The structural properties of Fe/WAC
samples were listed in Table 2. The SBET, Smic, V , and V slightly
Table 2 The structural properties of Fe/WAC samples
S
S
V
V
Vmic/Vt
(%)
BET
2
mic
2
t
3
mic
3
Catalyst
(
m /g)
922
912
896
873
844
838
(m /g)
769
759
745
721
690
683
(cm /g)
0.51
(cm /g)
0.30
t
mic
WAC
58.8
61.2
60.4
60.9
58.7
60.0
decreased with increasing Fe content, but the Vmic/V nearly kept
t
Fe/WAC/0.89
Fe/WAC/2.76
Fe/WAC/4.82
Fe/WAC/6.55
Fe/WAC/6.94
0.49
0.48
0.46
0.46
0.30
0.29
0.28
0.27
unchanged. Thus, the textural parameters of the Fe/WACs were not
significantly different from WAC. Because of the low Fe content, it
was assumed that Fe loading did not influence significantly the
textural parameters of the support in the preparation process of
catalyst.
0.45
0.27
respectively. The subscript “Others” denoted the sum of tars
formed.
3
.1.3 The XRD patterns
The XRD patterns of all the samples were presented in Fig. 2.
Interestingly, as Fe loading increased from 2.76 to 6.94 wt%, only a
broad peak appeared with increased intensity at 35.6 . Although
3
Results and discussion
o
Fe O was designed to form on WAC in the preparation process of
3
4
3
.1 Characterizations of the catalysts
.1.1 The ICP-AES analysis
The Fe content of the catalysts before reaction was shown in Table 1.
o
the catalyst, it was difficult to assert the formation of Fe O (18.3 ,
3
4
o
3
o
o
o
o
o
o
o
3
5
0.1 , 35.5 , 43.1 , 62.6 ) or Fe O (23.2 , 32.9 , 35.7 , 49.4 ,
2 3
3.3 ) . Anyway, the peak should be attributed to the existence of
o 24
3+
When the concentration of iron ions (total concentrations of Fe and
iron oxide species. The XRD pattern of low Fe loading sample
Fe/WAC/0.89) was similar to that of WAC. The diffused character
2
+
Fe ) increased from 0.018 to 0.162 mol/L, the Fe content of the
catalysts monotonically increased from 0.89 to 6.55 wt%
accordingly.
(
of lowꢀintensity diffraction peak was indicative of the high
dispersion of iron oxide species, which might be caused by the
interaction of the surface groups of WAC with iron ions. The huge
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Fig. 3 Fe2p spectra of catalysts: (a) Fe/WAC/0.89, (b) Fe/WAC/2.76, Fig. 4 C1s spectra of catalysts: (a) WAC, (b) Fe/WAC/0.89, (c)
(c) Fe/WAC/4.82, (d) Fe/WAC/6.55, (e) Fe/WAC/6.94.
Fe/WAC/2.76, (d) Fe/WAC/4.82, (e) Fe/WAC/6.55, (f)
Fe/WAC/6.94.
Table 3 Surface compositions of samples determined by XPS
3
+
2+
a
b
Fe /Fe
Fe
O
Fe/O
Catalyst
(molar ratio) (atom.%) (atom.%) (%)
Fe/WAC/0.89
Fe/WAC/2.76
Fe/WAC/4.82
Fe/WAC/6.55
Fe/WAC/6.94
0.63
0.61
0.52
0.52
0.50
4.71
7.88
11.80
12.86
13.79
16.85
26.14
38.47
41.05
43.30
0.28
0.30
0.31
0.31
0.32
a
Fe meant the whole iron present on the surface.
O meant the whole oxygen present on the surface.
b
surface of WAC and the interaction between iron ions and surface
groups of WAC might prevent the formation of crystal iron oxides.
This might be the reason for the saturation of Fe loading.
Fig. 5 O1s spectra of catalysts: (a) WAC, (b) Fe/WAC/0.89, (c)
Fe/WAC/2.76, (d) Fe/WAC/4.82, (e) Fe/WAC/6.55, (f)
Fe/WAC/6.94.
3
.1.4 The XPS analysis
Fig. 3 showed the Fe 2p spectra of the Fe/WACs samples. Surface
3
+
2+
analysis by XPS confirmed the presence of Fe and Fe on
Fe/WACs, on the basis of the Fe 2p peak position and the energy
phenol carbon, and carboxylic carbon according to the binding
energy of 284.6, 286.0, and 288.6 eV, respectively, after
difference between the Fe 2p excited state and the Fe 2p3/2 ground
1
/2
28
deconvolution. The surface oxygen species of WAC should be
25
state. After deconvolution, the peaks at 710.7 and 712.7 eV could
respectively be assigned to Fe and Fe 2p3/2, while those at 724.5
and 727.4 eV could respectively be assigned to Fe and Fe 2p1/2
assigned to carbonyl oxygen, phenol oxygen, and carboxylic oxygen
according to the binding energy of 531.0, 532.4 and 533.5 eV,
2
+
3+
2
+
3+
,
27
respectively. It was notable that the O1s peaks (529.5 eV) in
and those at 718.7 and 732.9 eV could respectively be assigned to
Fe/WACs spectra appeared and shifted to lower binding energy with
broadening with respect to that of WAC, which was characteristic of
2
+
3+
26
2+
Fe and Fe shakeꢀup satellites. It was illustrated that Fe and
3
+
3+
2+
Fe coꢀexisted on the surface of WAC. The Fe /Fe molar ratios
on the surface of Fe/WACs were presented in Table 3. With
29
FeꢀO in iron oxide species. The peaks at 288.6 and 533.5 eV had a
declined trend with the increase of Fe content. It implied that iron
ions might combine with carboxylic groups in the preparation
3
+
2+
increasing Fe content, the Fe /Fe molar ratios on the surface
decreased from 0.63 to 0.50 gradually, although the Fe /Fe molar
ratio was controlled to be 2/1 in the preparation process. It implied
that with increasing Fe content, the distribution of Fe on the
surface was enhanced. As similar XRD patterns were attained with
different Fe content, the above data indicated that the formation of
iron oxide species on all Fe/WACs was similar.
3
+
2+
3
0
process. The results were consistent with that of Zhang et al.
3+
2+
Additionally, Fe and Fe might interact strongly with surface
oxygen functional groups, which could inhibit the formation of
crystal Fe O . The chemical compositions of the samples in atomic
2+
3
4
percentage were shown in Table 3. As seen, the Fe and O atomic
percentage increased with increasing Fe content, but the Fe/O atomic
ratio kept nearly invariable. Based on the above data, we could not
confirm the formation of any kind of crystal iron oxide species.
Fig. 4 and 5 exhibited the C 1s and O 1s spectra of the six samples,
respectively. Both the C 1s and O 1s spectra of all samples were
asymmetric, indicating the convolution of different surface oxygen
2
7
and carbon species. The surface carbon species of all samples
should be assigned to graphite carbon and contaminant carbon,
4
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ARTICLE
a
Table 4 Effect of Fe content of the catalysts for the hydroxylation of phenol
Yield of product (%)
UH
2
O
2
TOF
(hꢀ1)
Catalyst
XPh (%)
SDHB (%)
YDHB (%)
b
CAT
0
2.7
HQ
0
BQ
0
Others
0
(%)
ꢀ
5.5
23.8
35.4
38.2
44.6
43.7
Blank
WAC
0
4.6
0
0
3.5
ꢀ
ꢀ
76.1
67.5
76.6
78.5
80.6
80.3
0.8
7.9
12.7
15.1
19.2
19.6
1.1
3.3
3.3
2.1
1.7
1.1
0
Fe/WAC/0.89
Fe/WAC/2.76
Fe/WAC/4.82
Fe/WAC/6.55
Fe/WAC/6.94
25.5
37.6
43.3
51.1
51.7
17.2
28.8
34.0
41.2
41.5
9.3
5.0
5.5
7.2
8.2
9.1
245.5
116.7
77.0
66.9
63.8
16.1
18.9
22.0
21.9
a
Reaction condition: 0.05 g of catalyst, 0.48 g of phenol (5 mmol), 0.50 g of 30 wt% aqueous H O (5 mmol), 10 ml water, reaction
2
2
b
temperature 313 K, reaction time 40 min. Turnover frequency (TOF) = moles of phenol converted per mole of Fe in the catalyst per hour.
a
Table 5 Control experiments for the hydroxylation of phenol
Time
SDHB
(%)
YDHB
(%)
Yield of product (%)
Catalyst
XPh (%)
(
min)
5
CAT
7.5
7.0
21.5
11.5
HQ
5.3
BQ
3.3
4.3
1.6
2.2
Others
6.0
6.8
b
Fe/WAC/6.55
Leachate
Solid catalyst
Leachate
22.1
23.0
50.5
35.4
57.9
51.7
71.3
56.5
12.8
11.9
40.0
20.0
b
60
40
40
4.9
c
18.5
8.5
12.9
13.2
c
a
Reaction condition: 0.05 g of Fe/WAC/6.55, 0.48 g of phenol (5 mmol), 0.50 g of 30 wt% aqueous H O (5 mmol), 10 ml of
2
2
b
c
water, reaction temperature 313 K. from the first control experiment. from the second control experiment.
broad peak should be attributed to iron oxide species. Liang et al.
proposed that the peaks at about 333 and 427 nm were assigned to
3
+
octahedral complex of iron and Fe in small oligomeric Fe O
x
y
20
clusters.
FTIR spectra were shown in Fig. S2
at 1300ꢀ900 cm corresponded to CꢀO stretching was weakened
with increasing Fe content. Similarly, the intensity of the bands at
†
. The intensity of the bands
ꢀ
1
33
ꢀ
1
ꢀ
34
1
384 cm corresponded to COO stretching was weakened with
increasing Fe content. The results were consistent with that of XPS.
It implied that iron species could combine with carboxylic groups.
3
.1.7 The HRTEM analysis
The HRTEM characterizations have been carried out, and the
results were shown in Fig. S3†. HRTEM profiles clearly
showed how the iron dispersion varied, which was in the
Fig. 6 Magnetic hysteresis loop of the Fe/WAC/6.55.
35
sense: Fe/WAC/0.89 > Fe/WAC/6.55. With increasing Fe
content, the catalyst exhibited a slightly worse Fe dispersion.
The results were consistent with those obtained by XRD. On
the other hand, no obvious crystal iron oxide species were
found in the images of C and D. It implied that the iron oxide
species was amorphous on the catalysts. Correspondingly, the
peak at about 35.6° was a broad peak in XRD patterns. Thus,
the increase of diffraction peak intensity with increasing Fe
content was related to the formation of more iron oxides.
3
.1.5 Magnetic properties
To detect if Fe O was formed, magnetic property of Fe/WAC/6.55
was measured. As shown in Fig. 6, the average saturation
magnetization of Fe/WAC/6.55 was 0.09 emu/g. Compared to the
values reported, that is, 0.53 emu/g for Fe/WACꢀ400 by Jin et al. ,
.73 emu/g for CꢀFeꢀ750ꢀ1.0 by Zhang et al. and 63.5 emu/g for
3
4
2
3
1
5
32
Fe O nanoparticles , it was too weak to confirm the formation of
3
4
Fe O species. It could be caused by the fact that Fe content of the
3
4
catalysts used in the present work was comparatively low to that of
Fe/WACꢀ400 and Fe O nanoparticles.
3
4
3
.2 Catalytic phenol hydroxylation
.2.1 Effect of Fe content of the catalyst
Liquid hydroxylation of phenol catalyzed by Fe/WACs using H O
3
3
.1.6 The DR UV-vis and FTIR analysis
2
2
DR UVꢀvis was used to characterize the catalysts (Fig. S1
†
). The
as oxidant had been studied. The catalytic performances of various
catalysts in the hydroxylation of phenol were presented in Table 4.
The result of blank experiment exhibited that H O alone was unable
spectra of Fe/WAC/0.89 and 2.76 showed little variation compared
to that of WAC. With increase of Fe content, the intensity of a broad
peak at the scope of 300ꢀ500 nm became strong. It implied that the
2
2
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to oxidize phenol to a significant extent in the absence of the catalyst,
1
especially at low temperature (313 K). Moreover, X and SDHB
Ph
were very low on pure WAC. After Fe/WACs were added, a
remarkable increase in the XPh was observed.
To further verify the heterogeneous character of the catalyst, two
control experiments were operated. In the first control experiment,
the reaction was carried out over Fe/WAC/6.55 under the same
reaction conditions. After 5 min, the catalyst was removed, but the
reaction kept on going until 60 min. The results were presented in
Table 5. It indicated that after removal of catalyst, the hydroxylation
of phenol could not continue to be conducted. In the second control
experiment, only 0.05 g of Fe/WAC/6.55 and solvent (H O and
2
o
acetic acid) were used to carry out the reaction at 40 C for 40 min.
Then the mixture was taken out and filtered. The solid catalyst and
leachate were obtained and used to catalyze the phenol
hydroxylation under the same conditions, respectively. As shown in
Table 5, the activity of the solid catalyst was almost the same as the
a a
Fig. 7 Effect of catalyst amount for the hydroxylation of phenol
Reaction condition: Fe/WAC/6.55, 0.48 g of phenol (5 mmol), 0.50
g of 30 wt% aqueous H O (5 mmol), 10 ml of water, reaction
2
2
temperature 313 K, reaction time 40 min.
fresh catalyst (X : 50.5% vs 51.1%), whereas the leachate showed
Ph
much smaller activity than the fresh catalyst (X : 35.4% vs 51.1%).
Ph
The results above demonstrated that the reaction was mainly
catalyzed by iron oxide particles on the surface of the heterogeneous
catalyst. An additional experiment was carried out to identify the
iron species leached. Small amount of aqueous solution of KSCN
was added into the leachate from the second control experiment, and
3+
the solution turned to be light red. It indicated that tiny Fe existed
in the leachate. Then the pH of leachate was adjusted to 5,
centrifuged, and bromine water was added into the supernatant.
Small amount of aqueous solution of KSCN was added, no red color
3
+
appeared. These results indicated that Fe was inclined to leach.
To work out what is the true active phase for the hydroxylation of
phenol, five catalysts were prepared with different molar ratios of
3
+
2+
Fe to Fe (0/1, 1/1, 3/1, 4/1, 1/0) but identical total iron content
6.55 wt%) through the same preparation process. Under the
optimized conditions, the catalytic activity was tested, the results
were shown in Table S1 . The results showed that the changes of
a
Fig. 8 Effect of reaction temperature for phenol hydroxylation
(
a
Reaction condition: 0.05 g of Fe/WAC/6.55, 0.48 g of phenol (5
mmol), 0.50 g of 30 wt% aqueous H O (5 mmol), 10 ml of water,
2
2
†
2+
reaction time 40 min.
3+
molar ratio of Fe to Fe had little effect on the activity of titled
3
+
2+
reaction. But Fe or Fe alone on the catalysts exhibited only low
catalytic activity. It implied that the true active phase for phenol
hydroxylation was the cooperation of the two Fe species, nothing
with the difference of Fe or Fe amount. It was found that Fe
species was easier to leach than Fe species above all. This led us to
suspect that the binding of Fe species with WAC surface was
stronger and the attachment of Fe was prior to Fe on WAC
considering the saturation of Fe species on WAC surface. There
would be enough Fe interacted with WAC forming the catalyst
which satisfied the demand of phenol hydroxylation in the series of
control experiments, even though the proportion of Fe was relative
3
+
2+
3+
2
+
2
+
2
+
3+
2
+
2
+
3
+
2+
low. That is to say, the variation of Fe /Fe ratios within the
experiments did not make significant changes of the surface Fe
species on WAC. This was why no relation was observed between Fig. 9 Effect of reaction time for the hydroxylation of phenol
a
2+
3+
a
the amount of Fe and that of Fe in the impregnation solution.
Reaction condition: 0.05 g of Fe/WAC/6.55, 0.48 g of phenol (5
The XPh and SDHB increased with increase of Fe content from 0.89 to mmol), 0.50 g of 30 wt% aqueous H
.55 wt% respectively and reached a maximum (51.1% and 80.6%) reaction temperature 313 K.
at 6.55 wt%. When the Fe content increased further from 6.55 to
.94 wt%, the XPh and SDHB kept almost invariant, which was likely
attributed to the saturation of Fe content of catalysts. Due to the fact
O (5 mmol), 10 ml of water,
2
2
6
6
6
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a
Table 6 Effect of phenol/H O molar ratio for phenol hydroxylation
2
2
Phenol/H O
Yield of product (%)
UH
O
2 2
2
2
XPh (%)
SDHB (%)
YDHB (%)
(
molar ratio)
:2
:1.5
:1
CAT
19.9
20.6
22.0
15.5
8.8
HQ
16.4
17.8
19.2
13.5
7.5
BQ
1.6
1.6
1.7
1.7
1.6
Others
42.3
25.5
8.2
9.7
12.6
(%)
19.8
27.7
44.6
49.1
59.1
1
1
1
80.2
65.5
51.1
40.4
30.5
45.3
58.6
80.6
71.8
53.4
36.3
38.4
41.2
29.0
16.3
1
1
:0.66
:0.33
a
Reaction condition: 0.05 g of Fe/WAC/6.55, 0.48 g of phenol (5 mmol), 10 ml of water, reaction temperature 313 K, reaction time 40 min.
a
Table 7 Effect of catalyst recycling on the hydroxylation of phenol
Yield of product (%)
Fe content
(wt%)
6.55
Catalyst
XPh (%)
SDHB (%)
YDHB (%)
CAT
22.0
21.5
19.4
18.1
HQ
19.2
18.5
17.4
17
BQ
1.7
1.6
1.8
2.1
Others
8.2
12.9
12.8
12.7
b
BR
51.1
50.5
47.4
45.8
80.6
71.3
69.2
67.7
41.2
40.0
36.8
35.1
c
AR1
6.04
5.47
4.83
AR2
AR3
a
Reaction condition: 0.05 g of Fe/WAC/6.55, 0.48 g of phenol (5 mmol), 0.50 g of 30 wt% aqueous H O (5 mmol), 10 ml of water, reaction
2
2
b
c
temperature 313 K, reaction time 40 min. BR meant before reaction. AR1 meant after once run, ARN means after N times run.
that iron oxide species loaded on Fe/WACs were in a similar form, phenol and H O , and hence the X and SDHB kept almost invariant.
2
2
Ph
the increase of activity might be mainly caused by the increase of It also indicated that excess amount of catalyst gave no more
active iron oxide species. Under the same conditions, Fe/WAC/6.55 benefits to promote the phenol hydroxylation. For cost saving, 0.05 g
gave the best activity, with 51.1% of X , 80.6% of SDHB, 41.2% of was chosen as the optimal catalyst amount.
Ph
U H
O
2
2
3.2.3 Effect of reaction temperature
yield to DHB and 44.6% of
, which were higher than those
2
,8
The effect of reaction temperature was presented in Fig. 8. From 303
to 343 K, the XPh increased from 37.5% to 54.7% correspondingly.
While the SDHB increased firstly, reached the maximum (80.6%) at
reported in literatures . On the other hand, the method used for the
calculation of TOF value was based on the total Fe content, whereas
only part of Fe was exposed and accessible for reactants. Thus, the
calculated of TOF value was smaller than the actual value. Although
the moles of phenol converted increased with increasing total Fe
content in the catalyst, but the moles of total Fe increased was more
than that of phenol converted in the same time. So the TOF value
3
13 K, and then decreased. It was suggested that the increase of
reaction temperature was beneficial to enhance the X , but
Ph
1
disadvantageous to improve the SDHB. Moreover, according to the
Arrhenius law, an increase in reaction temperature should result in
an increase of reaction rate. On the other hand, an increase in
reaction temperature would accelerate the decomposition of H O ,
2 2
which was disadvantageous to the target reaction. Therefore, the
insignificant changes in the XPh detected above 313 K could be
ꢀ
1
calculated decreased. Especially, 66.9 h of TOF on Fe/WACs was
much higher than the values (13.0 h or smaller) reported in
ꢀ
1
2,36,37
literatures
.
It was found that the selectivity of CAT was higher than HQ.
There were many similar reports about the same phenomenon
1
1,2,8
attributed to the decomposition of H O . Similar observation had
.
2
2
4
0
38
been reported for other iron containing catalysts. Thus, 313 K was
the optimal temperature for the hydroxylation of phenol over
Fe/WAC/6.55. The same trend was also reported by Choi et al.
Mugo et al. suggested that the predominance of CAT was expected
for metalꢀmediated hydroxylation occurring via a pathway involving
initial weak coordination of both phenol and H O to the active site.
The anchoring of the two reactants in close proximity to each other
led predominantly to a cis arrangement, which in turn resulted in
orthoꢀsubstitution of the phenol. On the other hand, our previous
studies found that the aromatic compounds had predominant
4
0
2
2
2
over FeꢀMCMꢀ41 catalyst, other than that reported by Jin et al. over
Fe/AC catalyst. Furthermore, from the energy consumption and
safety perspective, the optimized reaction temperature (313 K) in
this work was more favorable than those (343 K or higher) reported
8, 10
39
in literatures.
selectivity to orthoꢀhydroxylated products over Fe/AC catalyst.
3
.2.4 Effect of reaction time
3
.2.2 Effect of catalyst amount
The dependence of catalytic activity on the reaction time was shown
in Fig. 9. A pronounced induction period was not observed at the
beginning of the reaction. From 5 to 40 min, the XPh and SDHB were
increased from 22.1% to 51.1% and from 57.9% to 80.6%,
respectively. From 40 to 120 min, the XPh slightly increased, but the
SDHB slightly decreased. The result was consistent with that reported
The effect of the catalyst amount on the variation of activity was
shown in Fig. 7. It was observed that from 0.015 to 0.05 g, the XPh
and SDHB enhanced. From 0.05 to 0.1 g, the XPh and SDHB kept nearly
invariant. When the amount of catalyst was 0.015 g, iron oxide
species, which acted as active center, were not enough to
simultaneously activate phenol and H O effectively, resulting in
2
2
1
by Shi et al.
low activity. When the amount of catalyst was greater than or equal
to 0.05 g, iron oxide species were enough to simultaneously activate
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The decomposition behavior of H O was investigated from 0 to were detected by ICPꢀAES, and listed in Table 7. It was observed
2
2
1
50 min and the results were shown in Fig. S4†. The results showed that there was low level leaching occurred in every reaction, which
that the decomposition of H O on WAC was slower than that on could be the partial reason for catalyst deactivation. On the other
2
2
Fe/WAC/6.55. It implied that the decomposition of H O2 was hand, tars, which was found during the tests performed, successively
2
2
0
enhanced with loading Fe species.
As shown in Fig. S4 , about 30% of H O was decomposed at 40
min in blank experiment. The invariant character of the XPh should the results were shown in Table S2
be related to partly degradation of H O and the blocking of active XPh and Fe content decreased from 25.5% to 5.0% and from 0.89 to
deposited on the surface and blocked active sites.
The catalytic stability of Fe/WAC/0.89 was also investigated, and
. Under the same conditions, the
†
2
2
†
2
2
sites by products or tars. The decrease of SDHB from 40 to 120 min 0.51 wt%, respectively. A small amount of leaching of Fe did not
might be resulted by the fact that the anticipated products were deep significantly affect its catalytic performance in the second run. The
oxidized, and the YHQ and YCAT decreased within overꢀlong reaction decrease of activity of AR2 and 3 (Table S2) should be mainly
time. Consequently, the optimal reaction time was 40 min, which attributed to the deposition of tars on the catalyst surface, which
was shorter than that over most of the catalysts available for the blocked active sites. In brief, the leaching of a small amount of Fe
4
1
production of CAT and HQ (about 6 h).
might not be the main reason for the reduction of the catalytic
performance. Although the presence of carbon support excluded
regeneration by calcination, the high activity and simple preparation
3
.2.5 Effect of phenol/H O molar ratio
2
2
The reaction was carried out with varying amount of H O to a fixed process made Fe/WACs potential for phenol hydroxylation. It was
2
2
amount of phenol from 1:2 to 1:0.33 (phenol/H O molar ratios). It interesting to study the regeneration of the catalyst by other methods
2
2
was observed from Table 6 that the XPh was higher at a lower instead of calcination.
phenol/H O molar ratio, because the higher dose of H O meant
2
2
2
2
1
greater generation of hydroxyl radicals. With the variation of 4 Discussions
phenol/H O molar ratio from 1:2 to 1:0.33, the SDHB increased
2
2
Because Fe O on AC was found to be active for phenol
firstly, achieved the highest value (80.6%) at 1:1, and then decreased.
At the molar ratio of 1:2, the amount of H O was excessive,
3
4
2
hydroxylation, we envisaged to prepare supported Fe O species by
3
4
2
2
impregnationꢀcoꢀprecipitation method. However, the results did not
support the formation of crystal Fe O on the surface of WAC by
undesirable H O selfꢀdecomposition and deep oxidation of DHB
producing tars might be serious, consequently the SDHB was very low.
At the molar ratio of 1:1, the production of reactive species from
2
2
3
4
2
+
XRD and magnetic measurement. Only the coꢀexistence of Fe and
3+
Fe on the surface of WAC was confirmed. The control experiments
implied that the cooperation of two iron oxide species were the true
active phase for titled reaction. Although the average saturation
magnetization of Fe/WAC/6.55 was smaller than those of Fe/ACꢀ
both phenol and H O were proper, so the highest S was obtained.
DHB
2
2
At the molar ratio of 1:0.33, the amount of H O was too small to
2
2
produce enough reactive species to combine with that from phenol;
hence linkꢀcoupled compounds were produced leading to numerous
tar generation, whereas the yield of others increased, leading to the
decrease of the SDHB. The results were consistent with those reported
4
00 and CꢀFeꢀ750ꢀ1.0 respectively, Fe/WAC/6.55 gave higher
activity under similar conditions. The possible reasons were the
followings. First, the desirable Fe O species might not be generated,
2,20
in literatures . It should be noted that this variation tendency was
3
4
1
8,42
while the phenol hydroxylation reaction required only the coꢀ
different from other reports
. The inconsistency might stem from
2+
3+
1
8,42
existence of iron oxide species (Fe and Fe ) with a proper
the fact that in different reports , the actual concentration of H O2
2
2
+
3+
U H2O2
arrangement. In the present work, the arrangement of Fe and Fe
in the solution was different. Additionally, the
gradually
on WAC satisfied the demand of phenol hydroxylation. Second, the
Fe O species might be generated. However, it was difficult to be
increased with the molar ratio of phenol/H O varied from 1:2 to
2
2
3
4
1
:0.33. It was caused by the fact that a part of H O decomposed into
2 2
detected by the magnetic measurement because of the high
dispersion. In brief, few iron oxide species on the surface of catalyst
offered favorable activity of phenol hydroxylation.
water and oxygen at high concentrations of H O . Therefore, it was
2
2
conservatively concluded that the optimal molar ratio of
phenol/H O was 1:1.
2
2
5
Conclusions
3
.2.6 Catalyst recycling
Considering the economic benefits, the reusability of the catalyst
was investigated. The favorable Fe/WAC/6.55 was used 4 runs for
the target reaction. After every reaction, the catalyst was separated
by filtration, washed with deionized water and acetonitrile several
times, dried at ambient temperature, and reused for fresh reaction
mixture. All the 4 runs were carried out under the same conditions,
however, the XPh and SDHB decreased partly from 51.1% to 45.8%
and from 80.6% to 67.7%, respectively, as presented in Table 7.
It was wellꢀknown that the main limitation of using Fe as the
active metal was the leaching of active species from solid to
Fe/WACs were successfully prepared by impregnationꢀcoꢀ
precipitation method for phenol hydroxylation with H O as oxidant.
2
2
2+
3+
It was confirmed that Fe and Fe with a proper arrangement coꢀ
existed on WAC. The cooperation of two species could be the active
phase for phenol hydroxylation. With the increase of concentration
of iron ions in the preparation process, iron oxide species were
saturated gradually on WAC. The thusꢀobtained Fe/WACs satisfied
the demand on phenol hydroxylation. Among all the investigated
catalysts, Fe/WAC/6.55 showed the highest catalytic activity with
5
1.1% of conversion of phenol and 80.6% of selectivity to DHB.
21
solution. The Fe content of Fe/WAC/6.55 before and after reaction
The yield of DHB obtained was 41.2%. Fe/WACs acting as efficient
8
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ARTICLE
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