Catalysis of the hydro-dechlorination of 4-chlorophenol by Pd(0)-modified MCM-48 mesoporous microspheres with an ultra-high surface area

Article abstract of DOI:10.1039/c5nj00617a

MCM-48 mesoporous microspheres (M48N) with a pore size of 3.77 nm, an ultra-high surface area of 1250.77 m² g^-1 and a super-large pore volume of 1.52 cm³ g^-1 were synthesized. The M48N microspheres were then modified with Pd, Pt and Ru nanoparticles using an incipient wetness impregnation technique. The modified M48N microspheres were investigated to determine their catalytic effect in the aqueous phase hydro-dechlorination (HDC) of 4-chlorophenol. The Pd-M48N, Pt-M48N and Ru-M48N catalysts were characterized by transmission electron microscopy, X-ray diffraction, nitrogen adsorption-desorption and X-ray photoelectron spectroscopy. The HDC experiments were performed using the fairly mild conditions of room temperature and an atmospheric pressure of H₂. Pd-M48N showed a higher catalytic performance than Pt-M48N and Ru-M48N and various other Pd-based catalysts. The high catalytic performance of Pd-M48N resulted from the ultra-high surface area, super-large pore volume and unique pore structure of the M48N, which improved the mass transfer and increased the rate of adsorption-desorption of the compounds. This method has potential applications in the treatment of industrial wastewaters contaminated with chlorinated organic compounds.

Full text of DOI:10.1039/c5nj00617a

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A study of Pd modified MCM-48 mesoporous microsphere with ultrahigh
surface area catalyzed hydrodechlorination of 4-Chlorophenol
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Yansheng Liu, Xinlin Li, Xuanduong Le,Wei Zhang, Hao Gu, Ruiwen Xue and Jiantai Ma*
Gansu Provincial Engineering Laboratory for Chemical Catalysis, College of Chemistry and
Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China.
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∗ Corresponding author, E-mail addresses: majiantai@lzu.edu.cn (Jiantai Ma).
Tel.: +86 0931 891 2577; Fax: +86 0931 891 2582
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Abstract
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MCMꢀ48 mesoporous microspheres (M48N) with pore size of 3.77 nm, ultrahigh
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surface area of 1250.77 m g and superꢀlarge pore volume of 1.52 cm g has been
synthesised. Pd, Pt and Ru nanoparticles (NPs) modified M48N were prepared by
incipient wetness impregnation and had been investigated for hydrodechlorination
(HDC) in aqueous phase using 4ꢀchlorophenol as target compound. The catalysts Pd,
Pt, Ru/M48N were characterized by transmission electron microscope, Xꢀray
diffraction, nitrogen adsorption–desorption and Xꢀray photoelectron spectroscopy
measurements. The HDC experiments were performed at room temperature and
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atmospheric H pressure, fairly mild conditions for a potential application to treat
industrial wastewater. By analysing the catalytic results, Pd/M48N exhibited higher
catalytic performance than Pt, Ru/M48N and some other kind of Pd based catalysts.
The high catalytic performance of Pd/M48N owing to ultrahigh surface area,
superꢀlarge pore volume and unique pore structure of the M48N which can improve
mass transfer and increase adsorption–desorption rate of compounds.
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Keywords: MCMꢀ48 mesoporous microsphere, metal nanoparticles, HDC reaction,
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4ꢀchlorophenol
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1. Introduction
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In recent years, environmental problems have attracted much attention. One of
these environmental issues is the pollution caused by synthetic industrial organic
waste. 4ꢀChlorophenol (4ꢀCP) as an important commercial product has been broadly
employed as the raw material for the preparation of herbicide, production of pesticides
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, 2
and disinfectors.
Huge amounts of applications of 4ꢀCP have resulted in
unꢀignorable environmental problems because of its persistent toxic and the
degradationꢀresistant. Therefore, how to deal with the pollution caused by 4ꢀCP has
become an important issue which has attracted a lot of researchers. Till now, many
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reported traditional methods including thermal combustion, reduction dechlorination,
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oxidation dechlorination, biodegradation and photocatalytic degradation, have been
employed to dispose the wastewater containing 4ꢀCP. Although there were some
advantages, it cannot be ignored that these kinds of methods cannot completely
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decompose 4ꢀCP into simple inorganic compound such as CO , H O and HCl. On the
contrary, toxic chlorinated chemicals such as dioxins, phosgene and chlorine can be
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formed in the process of these treatments of 4ꢀCP. These accompanied chemicals can
result in secondary pollution and especially did harm to the health of human beings
and atmospheric ecosystem. Compared with traditional treatments of 4ꢀCP mentioned
above, hydrodechlorination (HDC) had attracted much attention. By using efficient
catalysts, the reaction can be performed under a mild condition and energy
consumption can be reduced. In addition, it can reduce the formation of byꢀproduct
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and can be suitable for the treatment of a wide range of chlorinated compounds. In
the consideration of economic and environmental points, catalytic HDC had been
considered as an effective and promising method.
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The liquid phase catalytic HDC had been reported by many groups. The studies
in the literature mainly focus on the catalytic activities which affected by active phase,
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support, sources of hydrogen, or reaction media.
So far, a large number of
catalysts using metal NPs as active sites had been developed for HDC of 4ꢀCP, such
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as supported Pd
, Pt
, Rh and Ni. Among them, supported Pd catalysts
showed an excellent catalytic activity for the treatment of 4ꢀCP. According to the
experimental results which had been reported, it agreed that small NPs can provide
higher catalytic activity, which mostly contributed to surface atoms residing on the
edges of the crystallographic planes or on edge junctions. These atoms increased the
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, 19, 20
numbers of active sites which could be accessible to the 4ꢀCP molecules.
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According to studies of L. Calvo and F. J. Urbano1, it seems that HDC was
structureꢀsensitive reaction. So, the effects of supports in HDC reactions had been
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extensively studied. Many supports, such as activated carbon,
Al O
or
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zeolites
ChiaꢀMin Yang reported a synthesis of MCMꢀ48 mesoporous microsphere (M48N).
As support, M48N possessed some excellent characters such as pore size of 3.77 nm,
had been employed to prepare catalysts for HDC of 4ꢀCP. Recently,
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ultrahigh surface area of 1250.77 m g and superꢀlarge pore volume of 1.52 cm g .
The ultrahigh surface area can provide much external surface to load more metal NPs.
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The channel structure also can provide internal surface area and active sites. Besides,
ultrahigh surface area can make the loaded metal NPs disperse wellꢀdistributed. The
wellꢀdispersed metal NPs on support can make reactant molecules combine to active
sites more easily and make product molecules desorb from active sites more quickly
so that the reaction rate can be raised. These particular characters cannot be found in
some other catalysts with low surface areas and nonꢀchannel structure.
Based on the above considerations, in this study, we chose Pd, Pt and Ru NPs as
the active phase and M48N as support to prepare supported metal nanocatalysts, and
their catalytic performance for HDC of 4ꢀCP had also been studied.
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Scheme 1. Preparation of Pd/M48N nanocatalyst.
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2. Experimental
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2.1 Materials
Tetraethoxysilane (TOES), Pd(II) acetate, chloroplatinic acid , ruthenium chloride,
Benzylcetyldimethylammonium
chloride
(BCDAC)
and
(3ꢀaminopropyl)triethoxysilane (APTES) were purchased from Aladdin Chemical Co.,
Ltd. 2ꢀchlorophenol (2ꢀCP), 3ꢀchlorophenol (3ꢀCP), 4ꢀchlorophenol (4ꢀCP),
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2,4ꢀdichlorophenol (2,4ꢀDCP) and 2,4,6ꢀtrichlorophenol (2,4,6ꢀTCP) were purchased
from Lanzhou Aihua Chemical Company. NaBH was supplied by Sinophrm
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Chemical Reagent Co., Ltd. Organic solvents used were of analytical grade and did
not require further purification.
2.2 Preparation of Pd, Pt, Ru/M48N catalysts
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M48N was prepared according to the procedure reported by ChiaꢀMin Yang. In
this work, a simple and green method (Scheme 1) has been used to prepare the Pd, Pt,
Ru/M48N catalysts. Firstly, M48N was functionalized with APTES to obtain
M48NꢀNH nanoꢀcomposites. Secondly, 500 mg of M48NꢀNH nanocomposite were
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added in a 100 mL roundꢀbottom flask with 54 mg of Pd(OAc)
acetonitrile, then ultrasonically dispersed for 30 min and keep stirring for 12 h.
Subsequently, the fresh NaBH solution (0.2 M, 20 mL) was added dropwise into the
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and 50 mL of
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abovementioned suspension. The product was isolated by centrifugation and washed
several times with deionized water and ethanol, and then dried in a vacuum overnight.
The Pt/M48N and Ru/M48N were prepared through the same method.
The metal loading of Pd, Pt and Ru were 5.01%, 4.72 and 5.05% which were
measured by ICP measurement.
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2.3 General procedure for the HDC of 4-CP.
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HDC experiments were performed in a 50 mL threeꢀnecked jacketed glass reactor
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equipped with H supplied. 20 mg of catalyst was placed into mixed solution of 30
mL solvent, 1 mmol of CPs and a certain amount of base under H
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continuously

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passed at 30 mL min . The reaction maintained for 2 h under vigorous stirring at 20
°C. The results of the experiments were analysed by Gas Chromatography (GC).
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2.4 General methods
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Transmission electron microscopy (TEM) images were obtained on a Tecnai G2
F30, FEI, USA. The Brunauer–Emmett–Teller (BET) surface area and poreꢀsize
distribution were obtained by measuring N adsorption isotherms at 77 K using a
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TriStar 3020 (Micromeritics). Powder xꢀray diffraction (XRD) spectra were obtained
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by a Rigaku D/maxꢀ2400 diffractometer using CuꢀKα radiation in the 2θ range of
0.5°ꢀ80°. Xꢀray photoelectron spectroscopy (XPS) was recorded on a PHIꢀ5702 and
the C1S peak at 284.6 eV was used as the binding energy reference. The reaction
conversion was estimated using GC (P.E. AutoSystem XL).
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3. Results and Discussion
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In this study, the catalyst activity of M/M48N was tested using CPs as target
compound under different condition and glass flask supplied with H
2
was used as
reaction reactor.
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3.1 Characterization
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Figure1. FTꢀIR spectra of M48N and M48N ꢀNH .
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Figure 1 shows the FTꢀIR spectra of M48N and M48NꢀNH . The adsorption peaks at
−1
−1
1066 cm and 804 cm corresponding to the antisymmetric and symmetric stretching
vibrations of Si–O–Si bond in oxygenꢀsilica tetrahedron, respectively. The peak at
−
1
459 corresponds to Si–O stretching. The strong peak at 3430 cm shows a large
number of Si–OH groups proved to be advantageous to the modification of APTES on
−1
the M48N surface by hydrogen bonds. The adsorption peak at 2925 cm corresponds
−1
to ꢀCH stretching. In the FTꢀIR spectrum of M48NꢀNH , the peak around 3430 cm
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represents the adsorption of –OH and –NH groups. The nitrogen, hydrogen, and
carbon contents of M48N were 0.0%, 0.70%, and 0.267% and the nitrogen, hydrogen,
and carbon contents of M48NꢀNH were 2.37%, 9.02 %, and 2.452%, measured by
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the elementary analysis, respectively. The FTꢀIR spectra and elementary analysis
result reveal that the APTES is successfully grafted on the M48N surface, thus
enabling them to act as robust anchors for metal NPs.
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Figure 2. SEM image of M48N(a) and EDX Spectroscopy and TEM images of M48N (b),
Pd/M48N (c, f2), Pt/M48N (d, f2), Ru/M48N (e, f3).
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TEM images of M48N and Pd, Pt, Ru/M48N were shown in Figure 2. M48N
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particles with diameter of ~200 nm and sphere structure were exhibited in Figure 2a
and Figure 2b. From TEM image of M48N, it indicated that all channels in M48N
seemed connected. EDX Spectroscopies of asꢀprepared samples were exhibited in
Figure f which suggested the metal NPs were loaded in M48N. The TEM images of
Pd, Pt and Ru loaded on M48N were displayed in Figure 2c, 2d and 2e, respectively.
Also size distribution of M/M48N has been made, and the main size of Pd, Pt and Ru
NPs range between 3ꢀ6 nm. From each M/M48N, it can be seen that the noble metal
NPs loaded on both surface and pore surface of M48N and this caused the difference
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of TEM images of M48N and M/M48N.
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Table 1. The surface area, pore volume, pore size of M48N and Pd, Pt, Ru/M48N.
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Samples
M48N
Surface Area (m /g)
Pore Volume (cm /g)
Pore Size (nm)
3.772
1250.77
1126.65
1230.33
1219.27
1.523
0.977
1.225
1.123
Pd/M48N
Pt/M48N
Ru/M48N
3.036
3.201
2.776
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Figure 3. Nitrogen adsorption–desorption isotherms and pore size distribution (inset)
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of M48N and Pd, Pt, Ru/M48N.
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N
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adsorptionꢀdesorption isotherms for the M48N and asꢀprepared samples were
given in Figure 3. According to the IUPAC classification, the curves of M48N and all
samples were type IV isotherms with a very sharp capillary condensation step at P/P
= 0.25–0.40 and H ꢀtype hysteresis loop characterizing smallꢀpore mesoporous
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materials with cylindrical channels. The pore size of M48N derived from the BJH
analysis on the desorption branch was 3.77 nm. The calculated BET surface area and
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pore volume of M48N were 1250.77 m g and 1.52 cm g , respectively. Compared
with M48N, the pore size of all samples reduced from 3.77 nm to ~3.0 nm and the
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pore volume values reduced from 1.52 cm g to ~1.1 cm g which owing to the
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channels dispersed by metal NPs. Besides, surface area of asꢀprepared samples
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maintained at the level of ~1200 m g (Table 1).
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Figure 4. Smallꢀangle (a) and Wideꢀangle (b) XRD patterns of M48N and Pd, Pt, Ru/M48N.
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The smallꢀangle and wideꢀangle XRD patterns of M48N and the asꢀprepared
samples were given in Figure 4. Frome Figure 4, there were two peaks belong to
M48N which correspond to plane (211) and (220) can be observed. One major
diffraction peak of metal NPs modified M48N was observed. By comparing with
M48N, this difference may be caused by the NPs which load in pore of M48N and
resulted in the structure degeneration of M48N. Combined with TEM images and
nitrogen adsorption–desorption isotherms, it indicated the mesopore structure of
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M48N. From the Figure 4b, Pd/M48N and Pt/M48N possessed three peaks
corresponding to planes (111), (200), (220) of Pd and Pt NPs. Ru/M48N possessed
one peak corresponds to plane (101) which belonged to Ru NPs.
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Figure 5. XPS spectra of Pd/M48N (inset image: high resolution spectrum of Pd 3d).
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The electronic state of the Pd species on the M48N was measured with the XPS. The
pattern of Pd/M48N catalyst was exhibited in Figure 5. The Pd 3d peaks in XPS spectra
showed that the binding energy values at 334.7 eV and 340.3 eV, which were attributed
to the 3d5/2 and 3d3/2 peaks of Pd 3d. In addition, no evidence proved the presence of
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palladium oxide which indicates Pd(OAc) was completely reduced to Pd NPs.
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3.2 HDC of 4-CP
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Scheme 2. Schematic of the HDC pathway for CPs.
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The catalytic activity of the M48N nanocatalyst was established by the HDC of 4ꢀCP
under a mild condition. The HDC of 4ꢀCP was negligible without catalyst or in the
presence of pure M48N at the same conditions, which shows that the presence of
noble metal NPs were indispensable for high catalytic activity. The HDC pathway is
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shown in scheme 2. As Diaz et al. explained in a previous work the route of 4ꢀCP
HDC proceeded through a set of seriesꢀparallel reactions where 4ꢀCP gives rise to
phenol and cyclohexanone (CYC) being this last also produced from phenol
hydrogenation. Compared with the reactant 4ꢀCP, the two products detected are low
toxic and useful as intermediates in production of high valueꢀadded chemicals. In this
work, the high selectivity product phenol was observed. The low selectivity of HDC
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of 4ꢀCP performed in fixed bed reactor
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or under a high H pressure condition,
such as: Pd/C catalyzed HDC of 4ꢀCP with the conversion of 93% and the selectivity
of 85% were reported by L. Calvo which formic acid as hydrogen source in
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continuous stirredꢀtank reactor. Pd/Al O as HDC catalyst were also studied by E.
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Diaz. in fixed bed reactor with the conversion was 87% and the selectivity 62%,
respectively. Compared with reports above mentioned, in our reaction, a batch stirred
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tank reactor was used and only H was flowing phase. The main production was
phenol and less than 0.1% CYC was detected after 120 min reaction time. These
differences may be caused by the using of different reaction systems. This result was
agreed with the study using same reaction system which phenol as major product was
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detected.
Table 2. The yield of HDC of 4ꢀCP catalyzed by Pd, Pt, Ru/M48N in different reaction conditions.
catalyst
OH
OH
Cl
Reactant
Catalyst
Base
Na CO
NaAc
Solvent
Yield %
83.6%
64.2%
47.4%
1.4%
a
a
a
a
a
a
4
4
4
4
4
4
4
4
2
3
ꢀCP
ꢀCP
ꢀCP
ꢀCP
ꢀCP
ꢀCP
ꢀCP
ꢀCP
ꢀCP
ꢀCP
Pd/M48N
Pd/M48N
Pd/M48N
Pd/M48N
Pd/M48N
Pd/M48N
2
3
2
H O
H O
2
triethyl amine
NaOH
2
H O
ethyl acetate
ethyl alcohol
NaOH
0.6%
NaOH
H
2
2
O
O
99.1%
39.2%
0.4%
a
Pt/M48N
NaOH
H
a
Ru/M48N
NaOH
H O
2
a
a
Pd/M48N(2ꢀCP)
Pd/M48N(3ꢀCP)
NaOH
H O
2
33%
NaOH
H
2
2
O
O
65%
b
2
,4ꢀDCP
Pd/M48N(2,4ꢀDCP)
NaOH
H
14%
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Reaction conditions: catalyst (20 mg), solvent (30 mL), CPs (1 mmol), H (30 mL min ), reaction
a
b
time (2 h); 1 mmol base, 2 mmol base.
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The catalytic properties of the Pd/M48N catalysts were investigated using the HDC
of 4ꢀCP as a model reaction, which has been proved to be an effective way to evaluate
the removal efficiency of noble metal nanoꢀcatalysts. Contrast HDC experiments of
2ꢀCP, 3ꢀCP and 2,4ꢀDCP were also tested and reaction results were exhibited in Table
2. According to the result of HDC of 4ꢀCP in Table 2, it can be easily concluded that
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the optimum condition was NaOH as base and H
2
O as solvent. The HDC reaction
pathway was described below: H adsorbed on the active site of the M48N
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nanoꢀcatalyst was activated into two hydrogen atoms which combined with 4ꢀCP also
adsorbed on the surface of the M48N nanoꢀcatalyst. The CꢀCl bond of 4ꢀCP was
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attacked by the active hydrogen atoms to from phenol. Simultaneously, in the
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process HDC of 4ꢀCP, HCl was formed as byꢀproduct which can poison catalysts.
In order to restrain the catalyst poising which caused by HCl, NaOH, Na CO3, NaAc
and triethyl amine were tested as base respectively. Compared with yield value, it can
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be summarized that NaOH was the best choice. In the reaction process, NaCl was
generated by the neutralization of HCl and NaOH. In organic solvent, low solubility
of NaCl can deposits on the surface of Pd/M48N blocked the pores of M48N support,
thereby inhibiting adsorption and activation which resulting in a decrease of reaction
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rate.
According to the research of Diaz et al. size of metal NPs was an important factor
on the catalytic activity. The results of the study indicated that the optimum metal size
ranged from 2ꢀ4 nm. The agglomeration of smaller metal particles will cause activity
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reducing. In addition, many studies suggested that edges and corners were the best
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place for support to load metal NPs in HDC. Metal NPs loaded on such place can
provide more surface area of active phase and exposed lattice plane can affect the
selectivity of HDC. In this study, the Pd NPs possessed higher opportunity to load on
the edges and corners of M48N because of unique pore structure and superꢀlarge pore
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volume of M48N.
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3
4
Figure 6. The yield curves of HDC of 4ꢀCP catalyzed by Pd, Pt, Ru/M48N.
5
6
7
8
9
By comparing the catalytic activity of Pd/M48N, Pt/M48N and Ru/M48N,
Pd/M48N showed a higher catalytic performance than Ru/M48N and Pt/M48N
(Figure 6). The experimental result was in agreement with the previously reported
works in which the Pd based catalysts have been found more active for HDC of
18, 41, 42
4ꢀCP.
1
1
0
1
Figure 7. Fitted Kinetic Rate Constants of the HDC of 4ꢀChlorophenol catalyzed by Pd/M48N.
1
1
2
3
2
The concentration of H can be considered constant since all the reactions were
1
1
4
5
carried out with a high excess of that reactant. Therefore, the reaction followed
pseudoꢀfirstꢀorder reaction kinetics. According to the concentration value related to
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2
3
4
reaction time, through plotted the logarithm of 4ꢀCP concentration versus reaction
time, the linear fittings were drew in Figure 7. The squared correlation coefficient of
the line reached up to 0.9939 which indicated that the linear fittings much better
conformed to firstꢀorder kinetics equations:
C
C
ꢀ
kt
=
e
or ln
=ꢀkt
C0
C0
5
6
7
8
9
0
1
2
3
The first order kinetic reaction rate constant had been calculated which was 0.0429
ꢀ1
′
min . The reaction rate constant per unit mass k = k/M was calculated to be 42.9
Pd
−
1
−1
2 3
min g . The kinetic reaction rate of HDC catalyzed by Pd/Al O , Pd/pillared clays
−
1
−1
−1
−1
−1
−1
and Pd/KCC catalyst was 3.33 min g , 7.6 min
g
and 21.15 min g ,
Compared with some other Pd based catalysts which had been
reported, the catalyst which Pd loaded on M48N performed higher catalytic activity.
1
6, 30 32
respectively.
1
1
1
1
2
ꢀ1
2 3
The lower catalytic activity may be due to low surface areas of Al O with 92 m g
and unordered pore structure of active carbon. This indicated the great catalytic
activity of Pd/M48N nanocatalyst.
1
1
4
5
Figure 8. HDC turnover rates of 4ꢀCP HDC over recycled catalyst.
1
6
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2
3
4
5
6
7
8
9
0
1
2
The stability experiments is performed under optimal condition using Pd as catalyst
in a centrifuge tube with H supplied. The catalyst was recovered by centrifugation
2
and simple decantation of liquid products. The catalyst was then washed with
deionized water and used directly for the next cycle of the reaction without further
purification. The recoverability and reusability were investigated by the HDC reaction
of 4ꢀCP and the results are summarized in Figure 8. After 5 recycling times, the metal
loading of catalyst of 4.43% is measured instead of 5.01% and the catalytic activity of
Pd/MSNs shows a slight weakness. This result confirmed the high rate of recyclability
of the Pd/DMSNs nanocatalyst and indicated that metal loss is an influence factor for
catalytic activity decreasing. In addition, by comparing TEM images (Figure 2 and
Figure s1) of fresh and used catalyst there were no visible differences between the
fresh and used catalyst.
1
1
1
13
4. Conclusion
14
15
16
17
18
19
20
Initially, M48N with ultrahigh surface areas, superꢀlarge pore volume and unique
pore structure was synthesized. Pd, Pt and Ru NPs were successfully loaded on the
interior surface of M48N through a method of incipient wetness impregnation. TEM,
XRD, BET and XPS experiments were used for analysing M48N and asꢀprepared
samples. Via analysing the result of hydrodechlorination reaction catalyzed by
asꢀprepared samples, Pd/M48N showed a great catalytic performance. During 120
′
min reaction time, the reaction rate constant per unit mass k = k/MPd was calculated
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−
1
−1
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
to be 42.9 min g which higher than Pt, Ru/M48N and some other kind of Pd based
catalysts. The good catalytic performance was due to ultrahigh surface area,
superꢀlarge pore volume and unique pore structure of M48N.
1
1
1
1
1
1
1
1
1
1
2
2
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