Stability and deactivation of Fe-ZSM-5 zeolite catalyst for catalytic wet peroxide oxidation of phenol in a membrane reactor

Article abstract of DOI:10.1039/c5ra05039a

Stability and deactivation of Fe-ZSM-5 zeolite catalyst for catalytic wet peroxide oxidation (CWPO) of phenol were studied in a membrane reactor. Firstly, the Fe-ZSM-5 zeolite membrane catalyst was prepared by a paper-making/sintering process, secondary growth process and incipient wetness impregnation method. And the influence of residence time on the CWPO of phenol was evaluated by modifying the catalyst bed height. Then, stability of the Fe-ZSM-5 zeolite membrane catalyst was studied by the long-term experiment (40 hours). Finally, the deactivation mechanisms of the Fe-ZSM-5 zeolite membrane catalyst were investigated by N₂ adsorption-desorption, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), thermal gravimetric (TG) analysis, Fourier transform infrared (FT-IR) spectroscopy and Raman spectroscopy, respectively. The results of CWPO of phenol showed that complete phenol conversion with a high TOC conversion (about 60%) was obtained at the catalyst bed height of 4 cm. Meanwhile, good stability with low Fe leaching concentration (about 0.5 mg L^-1) and high phenol conversion (above 85%) were obtained after continuously running for 40 hours. Furthermore, the loss of active component, the partial phase change of Fe₂O₃, the crystallinity change of the ZSM-5 zeolite membrane and the coke formation on the surface of the catalyst were found to be responsible for the deactivation of the catalyst.

Full text of DOI:10.1039/c5ra05039a

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Stability and deactivation of Fe-ZSM-5 zeolite
catalyst for catalytic wet peroxide oxidation of
phenol in a membrane reactor†
Cite this: RSC Adv., 2015, 5, 41269
Songshan Jiang, Huiping Zhang, Ying Yan and Xinya Zhang*
Stability and deactivation of Fe-ZSM-5 zeolite catalyst for catalytic wet peroxide oxidation (CWPO) of
phenol were studied in a membrane reactor. Firstly, the Fe-ZSM-5 zeolite membrane catalyst was
prepared by a paper-making/sintering process, secondary growth process and incipient wetness
impregnation method. And the influence of residence time on the CWPO of phenol was evaluated by
modifying the catalyst bed height. Then, stability of the Fe-ZSM-5 zeolite membrane catalyst was studied
by the long-term experiment (40 hours). Finally, the deactivation mechanisms of the Fe-ZSM-5 zeolite
membrane catalyst were investigated by N
2
adsorption–desorption, X-ray diffraction (XRD), field
emission scanning electron microscopy (FE-SEM), thermal gravimetric (TG) analysis, Fourier transform
infrared (FT-IR) spectroscopy and Raman spectroscopy, respectively. The results of CWPO of phenol
showed that complete phenol conversion with a high TOC conversion (about 60%) was obtained at the
catalyst bed height of 4 cm. Meanwhile, good stability with low Fe leaching concentration (about
Received 22nd March 2015
Accepted 30th April 2015
ꢀ1
0
.5 mg L ) and high phenol conversion (above 85%) were obtained after continuously running for
4
2 3
0 hours. Furthermore, the loss of active component, the partial phase change of Fe O , the crystallinity
DOI: 10.1039/c5ra05039a
change of the ZSM-5 zeolite membrane and the coke formation on the surface of the catalyst were
found to be responsible for the deactivation of the catalyst.
www.rsc.org/advances
frequently used as active component for CWPO of phenol. The
support materials of catalyst have been always considered as a
1
. Introduction
The catalytic wet peroxide oxidation process is an attractive key factor to inuence the catalytic activity. And kinds of porous
14
6
12
13
catalytic approach aimed at deep oxidation of organic water materials such as ZSM-5, MCM-41, SBA-15, pillared clays,
15
16
pollutants with hydroxyl and hydroperoxyl radicals produced by activated carbon and graphite were used as catalyst support
decomposition of H
in the presence of a catalyst under mild for the CWPO of phenol aqueous solution.
conditions (0.1–0.5 MPa, T # 100 C). The heterogeneous
2 2
O
ꢁ
1,2
The stability and deactivation of a heterogeneous catalyst
CWPO process can overcome the disadvantages of homoge- has a signicant impact on the oxidation of phenol wastewater
neous CWPO processes such as the sensitivity to pH (always and may seriously compromise the success of industrial appli-
within the range of 2.3–3.5) and the continuous loss of the cation of CWPO technology. In this sense, a better under-
dissolved active component which moreover needs to be sepa- standing of deactivation mechanisms of the catalysts is needed,
3,4
rated from the effluent to avoid secondary contamination.
Therefore, the CWPO processes of organic wastewater over regeneration strategies. Catalyst deactivation can occur by
heterogeneous catalysts using H
as oxidant attracted much several mechanisms, such as sintering, poisoning of active
attention, and phenol was frequently chosen as a model sites, and metal leaching. P. Bautista et al. reported that the
in order to properly design stable catalysts and instruct the
2 2
O
17
18
pollutant in the studies, because it commonly exists in chemical growth of Fe
O particle size and partial blockage of active site
2 3
5
industries. Transition metal oxides and their complexes such by deposition of carbon-containing matter on the catalyst
6
7
8
9
10
11
as Fe, Cu and Mn, other metals such as Al, Ce and Au were surface could result in the deactivation of Fe-bearing catalysts.
19
J. A. Zazo et al. suggested that iron leaching and active sites
blockage by polymeric deposits were the main causes resulted
School of Chemistry and Chemical Engineering, South China University of Technology,
Guangzhou 510640, PR China. E-mail: cexyzh@scut.edu.cn; Fax: +86 2087112047; in the deactivation of Fe/active carbon catalyst. Most of the
Tel: +86 2087112047
researches about CWPO processes of phenol and the deactiva-
tion studies of the catalysts were carried out in batch opera-
tions. However, there were many shortcomings existed in the
batch reactor such as high concentration of complicated inter-
mediate products and long contact time which resulted in the
†
Electronic supplementary information (ESI) available: TGA proles of the
uncalcined Fe-ZSM-5 zeolite membrane catalyst (Fig. S1); HPLC pattern of
reaction by-products aer CWPO of phenol, (peak 1: phenol; peak 2: acetic acid;
peak 3: formic acid; peak 4: oxalic acid), (Fig. S2); the standard XRD patterns of
2 3 2 3
a-Fe O and g-Fe O from JCPDS les (Fig. S3). See DOI: 10.1039/c5ra05039a
This journal is © The Royal Society of Chemistry 2015
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20
decrease of longevity of the catalyst and these drawbacks
might be overcome by using a xed bed reactor. Fernando
21
Mart ´ı nez et al. reported a remarkable catalytic performance of
iron oxide over different silica supports (meso-structured SBA-
15 and non-ordered meso-porous silica), used for the CWPO
of phenolic aqueous solution in a xed bed reactor. They
pointed out that the chemical alteration of the crystalline iron
oxides could result in the partial deactivation of the catalyst.
However, there is no literature systematically focused on the
stability and deactivation study of heterogeneous catalyst over
CWPO process of phenol in a xed bed reactor, especially, in a
membrane reactor.
Fig. 1 Preparation procedure of Fe-ZSM-5 zeolite membrane catalyst.
2.2 Preparation of Fe-ZSM-5 zeolite membrane catalyst
In our previous work, novel Fe-ZSM-5 zeolite membrane
catalysts were prepared for catalytic wet peroxide oxidation of Details of the synthetic procedure of paper-like stainless sin-
22
phenol in a membrane reactor. ZSM-5 zeolite membrane tered steel bers (PSSFs) and ZSM-5 membrane/PSSFs were
23
(
about 6 mm) with a Si/Al ratio of 80 was successfully fabricated reported in our previous paper. The preparation procedures of
on the surface of PSSFs by a secondary growth method and the Fe-ZSM-5 zeolite membrane catalyst were concluded in Fig. 1.
three dimension structure and unique surface properties of the The Fe-ZSM-5 zeolite membrane was developed by the wet lay-
ZSM-5 zeolite membrane made it to be an excellent support up paper-making process, sintering process and secondary
candidate. Fe element in a form of Fe O was uniformly growth process. Firstly, stainless steel bers (6.5 mm in diameter,
2
3
dispersed on the surface of ZSM-5 zeolite membrane support 2–3 mm length) and cellulose with a mass ratio of 2 : 1 were added
and the Fe-ZSM-5 zeolite membrane catalysts showed excellent into 1 L water and stirred vigorously for 10 min to form a uniform
catalytic activity when used for CWPO of phenolic aqueous suspension. A circular precursor paper was prepared by ltering
solution in a membrane reactor. However, more meaningful the uniform suspension using a wet lay-up process, drying in air at
ꢁ
ꢁ
work should be done to study the stability of the Fe-ZSM-5 105 C for 24 h, and then sintered in N at 1050 C for 30 min to
2
zeolite membrane catalyst and the deactivation mechanisms destroy the cellulose and form a PSSFs support with three
of the catalyst should also be further investigated.
dimensional network structure. Secondly, synthesis of ZSM-5
The aim of the present work is to (a) study the inuence of zeolite membranes on the PSSFs support by secondary growth
residence time on the CWPO of phenol in the membrane process. The silicalite-1 seeds were prepared by hydrothermal
reactor, (b) evaluate the stability of the Fe-ZSM-5 zeolite synthesis method using the synthesis solution with a molar ratio
membrane catalyst at the best operation conditions, and (c) 9TPAOH : 25TEOS : 500H O : 100C H OH. Then, the pretreated
2
2 5
investigate the deactivation mechanisms of the catalyst.
PSSFs support was dipped in a silicalite-1 seed solution (approx-
imately 2 wt% aq.) to adsorb silicate-1 seeds on the surface and
then air dried. The secondary synthesis solution with a molar
2 2
composition 1TEOS : 0.112TPAOH : 0.0125NaAlO : 111H O was
2
. Experimental
prepared and the seeded PSSFs support was dipped vertically in a
2.1 Material
ꢁ
2
00 mL Teon lined autoclave at 150 C for 48 hours. The
Stainless steel bers with average diameter of 6.5 mm were synthesized samples were washed, air dried and calcined at
ꢁ
obtained from Huitong advanced material company (China). 500 C for 4 hours to remove the TPAOH. Finally, Fe active
Tetrapropylammonium hydroxide (TPAOH, 25 wt% aqueous) component was loaded on the ZSM-5 membrane by means of
and cationic polymer (poly(dimethylamine-co-epichlorohy- incipient wetness impregnation. Fe ions concentrations were
drin), 50 wt%) were obtained from Sigma-Aldrich. Ethanol adjusted to obtain 25 wt% Fe loading on the catalyst. The sample
ꢁ ꢁ
2 5
(C H OH, >99.8%), sodium hydroxide (NaOH, 99%), sulfuric was air dried at 100 C for 12 h and calcined in air until 550 C
ꢁ
ꢀ1
acid (H
2
SO
4
, 95–98%), ammonia water (NH
, anhydrous) were all
3
, 25–28 wt% (6 h) using a slow ramp of temperature (1 C min ).
aqueous) and sodium aluminate (NaAlO
2
obtained from Sinopharm Chemical reagent Co., Ltd. Tetrae-
thoxysilane (TEOS, >99%) and phenol were obtained from
Guangzhou Chemical Reagent Factory. Hydrogen peroxide
2
.3 Catalytic wet peroxide oxidation of phenol in a
membrane reactor
2 2
(H O , 30 wt% aqueous) was obtained from Jiangsu Qiang- Catalytic wet hydrogen peroxide oxidation experiments were
sheng Chemical Co., Ltd. Iron nitrate nonahydrate carried out in a membrane reactor made of a stainless steel tube
ꢁ
(Fe(NO
3
)
3
$9H
2
O) was obtained from Tianjin Damao Chemical (20 mm i.d., 100 mm length), operating at constant 80 C of
Reagent Factory. Manganese dioxide was obtained from temperature under atmospheric pressure. The experimental set-
Shanghai Qiangshun Chemical Reagent Factory. Sodium thio- up used for the oxidation of phenol aqueous solution by means
sulfate and potassium iodide were obtained from Tianjin Bodi of CWPO is showed in Fig. 2. The concentration of reactants in
ꢀ1
chemical Co., Ltd. Deionized water was used in all synthesis the feed tank was a mixture of phenol solution, 1000 mg L of
ꢀ1
2 2
process. All of the chemical reagents used in this study were phenol and 5100 mg L of H O (stoichiometric amount for the
analytical grade.
total phenol oxidation according to reaction 1). The phenol
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concentrations of TOC in the inlet and outlet solution,
respectively.
2.4 Stability of the Fe-ZSM-5 zeolite membrane catalyst
The long-term activity of Fe-ZSM-5 zeolite membrane catalyst
was tested for the treatment of phenol aqueous solution. The
catalyst was used continuously up to 40 hours at the catalyst bed
ꢀ
1
height of 4.0 cm, the ow speed rate of 2 mL min and the
ꢁ
temperature of 80 C. Meanwhile, the conversions of phenol,
hydrogen peroxide and TOC as well as iron leaching concen-
tration in the solution were monitored.
2.5 Deactivate mechanism of the Fe-ZSM-5 zeolite
membrane catalyst
Fig. 2 Flowchart of the experimental set-up.
The crystallographic structures of the fresh and used Fe-ZSM-5
zeolite membrane catalyst samples were studied by X-ray
diffraction patterns carried out on a PANalytical X'Pert Pro
X-ray diffractometer using Cu Ka radiation, with a xed power
source (40 kV, 40 mA) and 2q range from 5 to 90 . The textural
and morphological information of the samples were charac-
aqueous solution was fed to the reactor in up-ow operation by
a peristaltic pump. The membrane reactor was heated by a
thermal bath.
ꢁ
terized using eld emission scanning electronic microscopy on
C
6
5
H OH + 14H
2
O
2
/ 6CO
2
2
+ 17H O
(1)
a Zeiss Merlin FE-SEM. N adsorption–desorption isotherms of
2
catalyst samples were tested using a Micromeritics Tristar-
IISurface Area and Porosity (Micromeritics Instrument Co.,
USA) at 77 K. The sample was out-gassed on a Micromeritics
Vacrep O61 Sample Degas System at 523 K for 6 hours before
measurement. Thermal gravimetric analysis of the fresh and
used Fe-ZSM-5 zeolite membrane catalysts were investigated
using thermo gravimetric analysis in nitrogen on a SDT Q600
Samples were hourly withdrawn from the treated effluent
and diluted 10 times. H O concentration was rstly deter-
mined by iodometric titration using a 0.02 mol L sodium
thiosulfate solution. For the purpose of eliminating residual
H O from the diluted samples, all the diluted solutions were
2 2
mixed with 0.1 g manganese dioxide at least 15 min before
measuring the phenol concentration, total organic carbon
2
2
ꢀ
1
ꢀ1
(
TA instrument Co., USA). Nitrogen ow rate was 100 mL min
ꢁ
ꢀ1
with a heating rate of 10 C min , from room temperature to
8
(
TOC) concentration, and iron leaching concentration.
ꢁ
00 C. Fourier transform infrared spectra of the fresh and used
Phenol concentration was measured by an HPLC Chromato-
graph (Agilent 1100) equipped with a Agilent HC-C18(2)
column and a UV detector adjusted at 210 nm. The meth-
anol and ultrapure water (v/v ¼ 40 : 60) was chosen as the
mobile phase. TOC concentration of the samples was
measured by a Liqui TOC (Elementar, Germany). Iron leach-
ing concentration in the treated effluent was tested by an
atomic absorption spectrophotometer (AA240FS, Varian Co.,
USA). The H O conversion (X O2, %), the phenol conversion
Fe-ZSM-5 zeolite membrane catalysts were obtained from a
ꢀ1
Perkin-Elmer Spectroscopy 100 with a resolution of 4 cm in
ꢀ
1
the range from 400 to 4000 cm at room temperature. A typical
pellet containing 1 wt% of sample was prepared by mixing 1 mg
sample with 100 mg KBr. Raman spectra were recorded on a Lab
RAM Aramis Raman Spectrometer (HORIBA JOBIN YVON,
ꢀ
1
France) with the spectrum range from 200 to 1600 cm , using
32.81 nm laser as the spectrum source.
6
2
2
H
2
(
Xphenol, %) and TOC conversion (XTOC, %) are dened as
follows:
3
. Results and discussion
C
H O ðinÞ ꢀ CH O ðoutÞ
2
2
2
2
3.1 Effects of residence time on the CWPO of phenol in a
membrane reactor
X
H O
2
¼
¼
¼
ꢂ 100%
ꢂ 100%
ꢂ 100%
(2)
(3)
(4)
2
C
H O ðinÞ
2
2
The residence time of owing material in the xed bed reactor is
dened in ref. 24. The catalytic volume (catalyst bed height) or
volumetric ow rate (feed ow rate) can be adjusted to study the
inuence of residence time on the CWPO of phenol in the xed
bed reactor.
In this research, the inuence of the residence time on the
CWPO of phenol in the xed bed reactor was studied by
modifying the catalyst bed height for 2.0 cm, 3.0 cm, and 4.0 cm
while other operation parameters were kept as constants
C
phenolðinÞ ꢀ CphenolðoutÞ
X
phenol
C
phenolðinÞ
C
TOCðinÞ ꢀ CTOCðoutÞ
X
TOC
C
TOCðinÞ
where CH2O2(in) (mg L 1) and CH
concentrations of H O in the inlet and outlet of the solution,
2 2
respectively, Cphenol(in) (mg L ) and Cphenol(out) (mg L ) are the
concentrations of phenol in the inlet and outlet solution,
ꢀ
(mg L ) are the
2
O (out)
ꢀ1
2
ꢀ
1
ꢀ1
ꢀ
1
(namely, the feed ow rate of 2.0 mL min , and the tempera-
ꢀ1
ꢀ1
respectively, and CTOC(in) (mg L ) and CTOC(out) (mg L ) are the
ꢁ
ture of 80 C). The conversions of phenol, H
2
O
2
and TOC, as well
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as the Fe leaching concentrations in the treated effluent were
measured.
The experimental results are shown in Fig. 3. As can be seen
in Fig. 3(a), phenol conversion increased with the catalyst bed
height increased. Phenol conversion of 100% was obtained
when the catalyst bed height reached 4.0 cm. Meanwhile, H O
2 2
conversion depicted in Fig. 3(b) kept the same variation trend as
phenol conversion, increased obviously with the catalyst bed
2 2
height increased. The H O conversion reached above 99.0%
and kept steady trend when phenol conversion reached 100% at
the catalyst bed height of 4.0 cm. However, the H O conversion
2
2
decreased to 80.0% and 60.0%, but the phenol conversion still
kept high conversion above 92.0% when the catalyst bed height
decreased to 3 cm and 2 cm. These results showed that the
catalytic reaction was not according to the stoichiometric
amount for the total phenol oxidation. The phenol was partially
oxidized and generated some intermediate products and low-
25
molecular-weight organic acids. The residual TOC was
contributed to these low-molecular-weight organic acids (which
were more resistant to oxidation) evolved by the primary prod-
ucts from phenol ring opening and continue oxidation of
20
aromatics intermediates. As can be seen in Fig. 3(c), TOC
conversion increased from 40.0% to 60.0% when the catalyst
bed height increased from 2 cm to 4 cm. Phenol conversion of
100% and H
2 2
O conversion about 99.0% did not attribute to the
1
00% TOC conversion because the H O was partly decomposed
2
2
into oxygen which was not capable to oxidize phenol under mild
condition and generated some carboxylic acids such as formic,
acetic and oxalic acids which were detected by HPLC analysis as
shown in Fig. S2.† All of above showed that the higher the
catalyst bed height, which leaded to longer residence time, so
2 2
the higher phenol conversion, H O conversion and TOC
conversion can be obtained.
The Fe leaching concentration in the treated effluent was
hourly recorded and the results are showed in Fig. 3(d). The Fe
leaching concentration increased with the catalyst bed height
ꢀ
1
ꢀ1
ꢀ1
increased: about 5 mg L , 15 mg L and 50 mg L at the
catalyst bed height of 2 cm, 3 cm and 4 cm, respectively. This is
due to the use of higher catalyst bed height (longer residence
time) could produce a higher loss of Fe ions in the liquid
solution. Similar results were found by Fernando Mart ´ı nez
using Fe O /SBA-15 as catalyst in CWPO of phenolic
2
3
2
1
effluents.
3.2 Stability of the Fe-ZSM-5 zeolite membrane catalyst
The long-term experiment of CWPO of phenol aqueous solution
over the Fe-ZSM-5 zeolite membrane catalyst was carried out in
a membrane reactor in order to test the stability of the catalyst.
The catalyst was used in continuous operation up to 40 hours at
the catalyst bed height of 4.0 cm, the ow speed rate of 2 mL
ꢀ
1
ꢁ
min and the temperature of 80 C. Meanwhile, the conver-
sions of phenol, hydrogen peroxide and TOC as well as iron Fig. 3 Effects of the catalyst bed height (residence time) on the
2 2
catalytic performance: (a) phenol conversion (Xphenol), (b) H O
leaching concentration in the treated effluent were monitored
and the results were depicted in Fig. 4. It can be found in
Fig. 4(a)–(c), phenol conversion decreased from 100% to 85%,
conversion (XH2O2), (c) TOC conversion (XTOC), and (d) Fe leaching
concentration (CFe), respectively.
2 2
H O conversion decreased from 99.0% to 65.0%, and TOC
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conversion decreased from 60.0% to 49.0% aer used 40 hours,
respectively. As can be seen in Fig. 4(d), iron leaching concen-
tration in the treated effluent showed a maximum concentra-
ꢀ
1
tion (about 50 mg L ) between 1 and 8 hour, and the
ꢀ
1
concentration decreased to 0.5 mg L aer used 40 hours.
These results showed that good stability of the Fe-ZSM-5 zeolite
membrane catalyst was observed although the activity of the
catalyst decreased aer used 40 hours.
3.3 Deactivation study of Fe-ZSM-5 zeolite membrane
catalyst
To study the deactivate mechanism of Fe-ZSM-5 zeolite
membrane catalyst, the fresh and used catalyst was character-
ized by N
and Raman spectra.
.3.1 N adsorption–desorption isotherms. Specic surface
2
adsorption–desorption, XRD, FE-SEM, TGA, FT-IR
3
2
areas and porosity properties of fresh and used Fe-ZSM-5 zeolite
membrane catalysts were characterized by N2 adsorption–
desorption. The N adsorption–desorption isotherms of the
2
catalysts were displayed in Fig. 5, and the BET surface areas and
pore properties were summarized in Table 1. The specic
surface areas (SBET) were calculated from adsorption branches
in the relative pressure range of 0.06–0.3 using the Brunauer–
Emmett–Teller method. The total pore volume (V ) was esti-
t
mated by analysis the N adsorption–desorption isotherms, the
2
micropore volume (Vmicro) and mesopore volume (Vmeso) were
calculated using the t-plot method and the Barrett–Joyner–
Halenda (BJH) method, respectively. Fig. 5 shows that the
volume adsorbed increases with increasing relative pressures
for all isotherms which is due to the volume lling of micro-
pores in ZSM-5 zeolite membrane. The volume adsorbed
increases continually when the relative pressures increase,
which should be caused by the multilayer adsorption. Both of
the catalysts show a similar hysteresis loop at a high relative
pressure t for type IV of adsorption isotherms which are given
26
by many mesoporous adsorbents. The hysteresis loop is
Fig. 4 Stability of the Fe-ZSM-5 zeolite membrane catalyst: (a) phenol
2 2
conversion (Xphenol), (b) H O conversion (XH2O2), (c) TOC conversion
(X
TOC), and (d) Fe leaching concentration (CFe), respectively.
Fig. 5
2
N adsorption–desorption isotherms of the catalysts at 77 K: (a)
fresh Fe-ZSM-5 zeolite membrane catalyst, and (b) used Fe-ZSM-5
zeolite membrane catalyst, respectively.
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Table 1 Physiochemical properties of the catalysts
zeolite was still well crystallized aer using 40 hours. Two
ꢁ
ꢁ
peaks of iron oxide at 2q ¼ 33.0 and 2q ¼ 35.6 were detected
but its peaks intensity decreased compared with the fresh
catalyst. This could be attributed to the loss of Fe O compo-
S
BET
V
t
V
micro
V
meso
(cm g )
2
ꢀ1
3
ꢀ1
3
ꢀ1
3
ꢀ1
Sample
(m g
)
(cm g
)
(cm g
)
2
3
a
Fresh Fe-ZSM-5b
Used Fe-ZSM-5
177.4
126.0
0.1280
0.09481
0.05651
0.03872
0.06309
0.05556
nent which distributed out of the zeolite membrane surface. It
can be seen from Fig. 6(b) that no other changes happened but
the peak intensities of the used catalyst decreased which indi-
cated the loss of Fe species aer used 8 hours and this is in
agreement with the Fe leaching concentration depicted in
a
b
Relative to fresh Fe-ZSM-5 zeolite membrane catalyst. Relative to
used Fe-ZSM-5 zeolite membrane catalyst.
ꢁ
Fig. 4(d). Meanwhile, two new peaks of carbon around 45 and
ꢁ
7
5 were observed in the Fe-ZSM-5 zeolite membrane catalyst
associated with capillary condensation taking place in meso-
pores, and the limiting uptake over a range of high relative
pressure. As listed in Table 1, the BET specic surface area and
total pore volume of used Fe-ZSM-5 zeolite membrane catalyst
aer used 40 hours, as shown in Fig. 6(c). The existence of
carbon may lead to a partial blockage of active sites. P. Bautista
18
et al. concluded that the presence of carbon-containing matter
adsorbed onto the catalyst had a negative effect on the catalytic
activity.
2
ꢀ1
3
ꢀ1
were 126.0 m g and 0.09481 cm g , respectively. The fresh
Fe-ZSM-5 zeolite membrane catalyst exhibited higher BET
2
ꢀ1
3.3.3 Field emission scanning electron microscopy anal-
ysis. Morphology and structure of the fresh and used Fe-ZSM-5
zeolite catalysts were investigated by FE-SEM. Fig. 7 shows the
FE-SEM images of fresh and used Fe-ZSM-5 zeolite membrane
catalysts. Fig. 7(a) and (c) clearly show that the junctures of
stainless steel bers were sintered together to form a three
specic surface area (177.4 m
g
) and total pore volume
3
ꢀ1
(
0.1280 cm g ) than the used one. Meanwhile, the volumes of
3
ꢀ1
total pores and micropores decreased from 0.1280 cm g to
3
ꢀ1
3
ꢀ1
3
ꢀ1
0
.09481 cm g and from 0.05651 cm g to 0.03872 cm g ,
respectively. The decreases of volume may result from the
blockage of the Fe-ZSM-5 zeolite membrane catalyst. It can be
concluded that the decreases of BET surface area and volume
blockage have resulted in the deactivation of the Fe-ZSM-5
zeolite membrane catalyst. These results indicated that the
surface characteristics and pore structures of Fe-ZSM-5 zeolite
membrane catalyst were changed aer continuously using
2 3
dimension network structure and Fe O active component was
uniformly distributed on the surface of ZSM-5 zeolite
membrane. Microstructure and morphological information of
the used Fe-ZSM-5 zeolite membrane catalyst are shown in
Fig. 7(b) and (d) respectively. As can be seen in Fig. 7(b), the
space of ZSM-5 zeolite membrane was blocked and thus resul-
ted in the loss of three dimension structure of PSSFs.
Morphological information of the used Fe-ZSM-5 zeolite
40 hours.
3.3.2 X-ray diffraction (XRD) analysis. The crystallographic
structures of the fresh Fe-ZSM-5 zeolite membrane catalyst and
used Fe-ZSM-5 zeolite membrane catalyst (8 hours) and
2 3
membrane catalyst shows that the active Fe O component
could not uniformly disperse on the surfaces of ZSM-5 zeolite
membrane. As can be seen in Fig. 7(d), some cracks are detected
on the zeolite membrane, and this could inuence the activity
of the catalyst. The XRD pattern with low peaks intensity dis-
cussed before can be explained.
(
40 hours) were studied by X-ray diffraction patterns, as shown
in Fig. 6. All of the XRD patterns showed the diffraction peaks at
ꢁ
ꢁ
the ranges of 2q ¼ 7–9 and 2q ¼ 23–25 , matching well with the
27
standard phase of ZSM-5 zeolite, indicating that the ZSM-5
Fig. 7 FE-SEM images of the catalyst samples: (a) fresh Fe-ZSM-5
Fig. 6 XRD patterns of the samples: (a) fresh Fe-ZSM-5 zeolite zeolite membrane catalyst, (b) used Fe-ZSM-5 zeolite membrane
membrane catalyst (b) used Fe-ZSM-5 zeolite membrane catalyst (8 catalyst, (c) morphological information of the fresh Fe-ZSM-5 zeolite
hours) and (c) used Fe-ZSM-5 zeolite membrane catalyst (40 hours), membrane catalyst, and (d) cross-sectional SEM image of the used Fe-
respectively.
ZSM-5 zeolite membrane catalyst, respectively.
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ꢁ
ꢁ
3
.3.4 Thermal gravimetric analysis. TGA and DTA experi- respectively. Meanwhile, two peaks at 202 C and 286 C were
30
ments of the used and fresh Fe-ZSM-5 zeolite membrane cata- observed by T. Barakat et al., and they concluded that the rst
lysts were conducted, and the results were shown in Fig. 8 and 9. peak was attributed to the removal of lightly adsorbed aromatic
As can be seen in Fig. 8(a), slight weight loss (about 2%) below molecules and the second peak was attributed to the removal of
ꢁ
1
10 C was observed, which could be assigned to the evapora- highly adsorbed aromatic molecules formed by the degradation
tion of adsorbed molecular water, and no weight loss occurred of toluene. So, the two peaks of the used Fe-ZSM-5 zeolite
ꢁ
from 110 to 800 C. Meanwhile, two weight losses (about 1% membrane catalyst could be attributed to the removal of light
ꢁ
and 3%) were observed from 110 to 800 C in the used catalyst coke (phenol etc.) and hard coke (polyaromatic compounds).
shown in Fig. 8(b). Fig. 9(a) and (b) showed the DTA curves of The deposition of carbonaceous compounds on the active sites
the fresh and used catalyst, respectively. Two new exothermic could result in the deactivation of Fe-ZSM-5 zeolite membrane
ꢁ
ꢁ
peaks at 186 C and 376 C were observed in Fig. 9(b). Caerio catalyst, and this could also be veried by XRD and SEM
28
et al. observed that the presence of two types of coke molecular analysis.
which denoted as “light coke” (monoaromatic polysubstituted
3.3.5 Fourier transform infrared spectra analysis. FT-IR
compounds) and “hard coke” (polyaromatic compounds). spectra of the fresh and used Fe-ZSM-5 zeolite membrane
2
9
ꢁ
Hosseini et al. observed two exothermic peaks at 325 C and catalyst showed all the bands characterizing skeletal vibrations
ꢁ
5
25 C which they related to the light and hard coke, of MFI zeolite structure. As can be seen in Fig. 10, the band
ꢀ
1
around 543 cm was typically due to the pentasil framework
ꢀ1
variation and the ratio of band intensities at 550 and 450 cm
was oen used as an indication of zeolite crystallinity. The
31
ꢀ1
absorption band around 796 cm
was due to Si–O–Si
symmetric stretching, and absorption band around 1099 and
ꢀ
1
32
1
230 cm were assigned to asymmetric stretching of Si–O–Si.
ꢀ1
The weak absorption band around 3499 cm
existence of hydroxyl groups. The relative intensity of used Fe-
ZSM-5 zeolite membrane catalyst decreased and two new weak
absorption bands around 1359 and 1315 cm were observed in
the catalyst used for 40 hours. The absorption bands around
300–1700 cm (bands of C–H deformation and C–C stretching
implies the
33
ꢀ1
ꢀ1
1
28
vibration) were investigated by G. Caeiro et al. and they
concluded that the integrated surface in the 1300–1700 cm
ꢀ
1
region is proportional to the amount of carbon in the zeolite. So,
the newly observed absorption bands around 1359 and
ꢀ
1
1
315 cm could be due to the carbon coked on the surface of
the catalyst. FT-IR patterns of the catalyst aer used 8 hours
Fig. 8 TGA profiles of the catalyst samples: (a) fresh Fe-ZSM-5 zeolite
membrane catalyst, and (b) used Fe-ZSM-5 zeolite membrane catalyst,
respectively.
ꢀ1
showed that no absorption band around 1359 cm
was
Fig. 10 FT-IR spectra of the catalyst samples: (a) fresh Fe-ZSM-5
Fig. 9 DTA profiles of the catalyst samples: (a) fresh Fe-ZSM-5 zeolite zeolite membrane catalyst (b) used Fe-ZSM-5 zeolite membrane
membrane catalyst, and (b) used Fe-ZSM-5 zeolite membrane catalyst, catalyst (8 hours) and (c) used Fe-ZSM-5 zeolite membrane catalyst
respectively.
(40 hours), respectively.
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catalyst. The complete phenol conversion, high TOC conversion
about 60.0% and the H O conversion above 99.0% were ach-
2
2
ieved at the optimized condition, namely, at the catalyst bed
height of 4.0 cm (the longest residence time), the temperature of
ꢁ
ꢀ1
8
0 C, and H
2 2
O concentration of 5100 mg L , respectively.
Meanwhile, good stability with low Fe leaching concentration
ꢀ
1
(
about 0.5 mg L ) and high phenol conversion (above 85%)
were obtained aer successive ran for 40 hours. Furthermore,
the loss of active component in the rst 8 hours, partial phase
change from a-Fe O to g-Fe O , the crystalline change of the
2
3
2 3
ZSM-5 zeolite membrane support and the coke formations on
the surface of the catalyst were found to be responsible for the
deactivation of the catalysts. Further research should focus on
the regeneration strategies of the Fe-ZSM-5 zeolite membrane
catalyst.
Fig. 11 Raman spectra of catalyst samples: (a) fresh Fe-ZSM-5 zeolite
membrane catalyst, and (b) used Fe-ZSM-5 zeolite membrane catalyst,
respectively.
Acknowledgements
We gratefully acknowledge the nancial support of the National
Natural Science Foundation of China (Grant no. 21176086 and
2
1376101).
observed and this indicated that no carbon deposition
happened in the rst 8 hours, and this is in agreement with the
XRD results discussed before. It can be concluded that the
initial deactivation of the catalyst is caused by the leaching of Fe
and the deactivation occurred later is partly caused by carbon
deposition. The decreased intensity of used catalyst revealed
that the crystalline property of the ZSM-5 membrane was
changed.
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