Palladium encapsulated within magnetic hollow mesoporous TiO2 spheres (Pd/Fe3O4@hTiO2) as a highly efficient and recyclable catalysts for hydrodechlorination of chlorophenols

Article abstract of DOI:10.1039/c5ra07765f

Herein, we introduced a method to encapsulate Pd(0) within TiO₂ hollow mesoporous nanostructures. TiO₂ hollow spheres offer a number of advantageous characteristics, including low cost, low toxicity, relatively high photocatalytic activity and high chemical stability. In addition, it can also effectively avoid the agglomeration and precipitation of noble metal nanoparticles in the catalytic reaction. The catalyst was characterized by TEM, XRD, XPS, ICP and VSM. We found that the catalyst presented a high activity for HDC of 4-CP under mild reaction conditions. Beyond that, the catalyst could be easily regained by an external magnet from the reaction mixture and recycled seven times without any significant loss in activity.

Full text of DOI:10.1039/c5ra07765f

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ARTICLE TYPE
Palladium encapsulated within magnetic hollow mesoporous TiO2
spheres(Pd/Fe O @hTiO ) as a highly efficient and recyclable catalysts
3
4
2
for hydrodechlorination of chlorophenols
a,b
b
b
b
b
b
Le Zhang , Xingchun Zhou , Hongfei Huo , Wenzhu li , Feng Li , Rong Li*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Herein, we introduced a method to encapsulate Pd(0) within TiO hollow mesoporous nanostructures.
2
TiO hollow spheres offer a number of advantageous characteristics, including low cost, low toxicity,
2
relatively high photocatalytic activity and high chemical stability. In addition, it can also effectively avoid
the agglomeration and precipitation of noble metal nanoparticles in the catalytic reaction. The catalyst
was characterized by TEM, XRD, XPS, ICP and VSM. We found that the catalyst presented a high
activity for HDC of 4ꢀCP under mild reaction conditions. Beyond that, the catalyst could be easily
regained by an external magnet from the reaction mixture and recycled seven times without any
significant loss in activity.
1
0
1
. Introduction
another important property for catalysts. As is wellꢀknown that
the traditional separation techniques, such as filtration and
Developing highꢀperformance and stable catalysts has become a
hot area of research recently.
centrifugation, are usually low efficiency and timeꢀconsuming,
1
,2,3
Noble metal nanoparticles
21,22
which is not adopted for recycling of catalysts.
To solve this
(NMNPs), especially Pd and Au, have been researched widely in
problem, magnetic nanoparticles, such as Fe O nanoparticles, are
3
4
recent years on account of their excellent catalytic performance in
many chemical reactions, such as hydrogenation reactions,
coupling reactions and oxidation reactions. It is generally
believed that their shape, size, crystallinity and the surface state
have large influence on the catalytic activity of NMNPs.
Unhappily, NMNPs are easy to aggregate during the process of
catalytic reaction, issuing in rapid decay of catalytic activity and
stability.
effectively synthesize and utilize NMNPs catalysts with
satisfying dimensions and improved catalytic performance is a
technologically significant and challenging issue.
utilized as this support materials because of their distinct physical
properties and easy separation from reaction mixture by using an
4
5
6
23,24,25
external magnet.
Based on above considerations, in this article, we described a
mild route to synthesize Pd/Fe O @hTiO catalyst with Pd/Fe O
4
7
3
4
2
3
nanoparticles resided inside the mesoporous spheres by using the
colloidal SiO spheres as the templates. The designed catalysts
8,9
2
Therefore, to develop varieties of strategies to
possess large magnetization, highly open and ordered mesopores,
which shown excellent activity for HDC of 4ꢀCP and could be
easily recycled multiple times without visible decrease in the
catalytic performance.
1
0,11,12
Many evidences represented that hollow space have attracted
considerable interest due to their high surface area, large void
volume and low density.
2. Experimental
1
3,14,15
Core−shell structured materials
with hollow space show a special type of complicated hybrids,
which affords an efficient solution to prevent the undesirable
aggregation, sintering, coalescence or corrosion/dissolution of
entrapped catalytic species, for example, we can use shell to
protect NMNPs. Meantime, diffusion and mass transfer of
reactants could be speeded by the porous voids endowed by
2.1 Materials and reagents
Ammonia (NH ·H O) solution (28 wt. %), tetraethoxysilane
3
2
(TEOS), ethanol, triethylene glycol (TEG), tetrabutyl
orthotitanate (TBOT), hydrochloric acid (HCl) (38 wt. %), N,Nꢀ
dimethylformamide (DMF), triethylamine ((C H ) N), ethyl
2
5 3
acetate (CH COOC H ) were purchased from Tianjing Chemical
16,17,18,19
3
2
5
hollow space.
In addition, nanostructured TiO2 has
Reagent Co. Ferric acetylacetonate (Fe(acac) ), SnCl , palladium
3
2
attracted significant attention because of its beneficial properties
for actual catalytic applications. TiO2 hollow nanostructures
consisting of nanoscale porous shells are highly satisfying
because they possess high active surface area, reduced diffusion
resistance and improved accessibility, which offer many new
chloride (PdCl ), sodium formate (HCOONa), sodium acetate
2
(CH COONa), sodium carbonate (Na CO ), sodium hydroxide
3 2 3
(NaOH) were bought from Aladdin Chemical Co., Ltd. 4ꢀ
Chlorophenol (4ꢀCP), 2ꢀchlorophenol (2ꢀCP), 3ꢀchlorophenol (3ꢀ
CP), and 2,4ꢀdichlorophenol (2,4ꢀDCP) are purchased from
Lanzhou Aihua Chemical Company. All chemicals were of
analytical grade and were used as received without further
20
chances to the design of highly active nanostructured catalysts.
Except for good catalytic activity and high stability, recycling is
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2
9
purification.
.2 Preparation of catalysts
2.2.4 Synthesis of Pd/Fe
O @hTiO catalysts
3 4 2
Typically, 70 mg Pd/Fe O /SiO particles were dispersed in 100
mL ethanol and ultraphonic for 30 min. Then, 0.35 mL NH ·H O
3
4
2
2
3
2
was added to the suspension and continue to ultrasonic for 30min,
afterward, 0.75 mL of TBOT was added dropwise in 5 min, and
the reaction was allowed to proceed for 24 h at 45 ℃ under
vigorous magnetic stirring. After that, the products separated by
centrifugation and washed with distilled water and ethanol,
respectively. The final product was dried in vacuum at 50 ℃.
+
2
2
+
Sn
NH
3
·H
2
O
Fe
3
O
4
SnCl
2
TEOS
n
2
+
2
S
+
SiO
2
Fe
3
O
4
/SiO
2
Sn2+/Fe
O /SiO
3 4 2
Next, the Pd/Fe O @hTiO
was obtained by etching
Pd/Fe O /SiO @TiO spheres in 2M Na CO3 aqueous solution
3
4
2
Na
2
CO
3
3
4
2
2
2
Pd
TBOT
for 8 h and washed five times with distilled water and dried.
.3 Characterization of catalysts
The synthesized catalyst of Pd/Fe O @hTiO was confirmed by
2
Pd/Fe
3
O
4
/SiO
2
Pd/Fe
3
O
4
/SiO
2
@TiO
2
3 4 2
Pd/Fe O @hTiO
3
4
2
Scheme 1. Scheme of the synthetic procedure for the preparation of
Pd/Fe @hTiO
corresponding characterization means. The morphology and
microstructure of Pd/Fe O @hTiO were characterized by
3
O
4
2
3
4
2
The fabrication procedure involved six steps as shown in Scheme
. Firstly, SiO was prepared. Secondly, load Fe O nanoparticles
transmission electron microscopy (TEM). The TEM images were
obtained through Tecnai G2 F30 electron microscope operating at
300 kV. Xꢀray powder diffraction (XRD) measurements were
carried out at room temperature and performed on a Rigaku
D/maxꢀ2400 diffractometer using CuꢀKα radiation as the Xꢀray
source in the 2θ range of 10ꢀ90°. Xꢀray photoelectron
spectroscopy (XPS) was recorded on a PHIꢀ5702 and the C1S
line at 291.4 eV was used as the binding energy reference.
Specific surface areas were calculated by the Brunauer–Emmett–
Teller (BET) method and pore sizes by the Barrett–Joyner–
Halenda (BJH) methods using Brunauer–Emmett–Teller (Tristar
II 3020). Inductive coupled plasma atomic emission spectrometer
(ICPꢀAES) analysis was conducted with Perkin Elmer (Optimaꢀ
4300DV). Magnetic measurement of Pd/Fe O /SiO nanoreactor
1
2
3
4
2
+
on surface of SiO . Thirdly, deposit Sn onto the surface of SiO2
2
nanospheres with Fe O nanoparticles. Fourthly, load highly
3
4
dispersed Pd nanoparticles onto the surface of SiO nanospheres
2
with Fe O nanoparticles. Fifth, the Pd/Fe O /SiO @TiO core–
3
4
3
4
2
2
shell structures were prepared. Finally, the asꢀprepared
nanocomposites were etched by Na CO to remove the template.
2
3
The amount of Pd in the obtained catalyst was found to be 2.26
wt. % based on ICP analysis.
2
6
2
.2.1 Synthesis of SiO nanospheres
2
In a typical synthesis, 4.5 mL of TEOS was rapidly added into a
mixture solution of 62 mL ethanol, 25 mL H O and 1.5 mL
2
NH ·H O. After stirring for 2 h, the SiO particles were collected
3
2
2
3
4
2
by centrifugation and washed with ethanol and deionized water.
This process was repeated three times before drying the SiO2
particles at 50 ℃.
and Pd/Fe O @hTiO were investigated with a Quantum Design
3 4 2
vibrating sample magnetometer (VSM) at room temperature in an
applied magnetic field sweeping from 15 to 15 kOe. The
conversions of the HDC reactions were estimated by gas
chromatographyꢀmass spectrometry GCꢀMS (P.E. AutoSystem
XL).
2
7
2
.2.2 Preparation of Fe O /SiO composite
3 4 2
Firstly, 250mg SiO particles and 60ml TEG were added into a
2
threeꢀnecked flask (100ml), and then the threeꢀnecked flask was
treated under ultrasonic to disperse uniformly. Secondly, 300mg
Fe(acac)3 was still added into the threeꢀnecked flask with
2
.4 Catalytic activity and recyclability
magnetic stirring under the protection of N at the same time. The
system was warmed till boiling and kept for 30min, after cooling
2
The HDC of 4ꢀCP reaction was carried out in a 50 mL twoꢀ
ꢀ
1
necked flask. In general, the catalyst (10mL, 2 mgꢁmL ), NaOH
solution (10 mL, 0.05 M) and CP (10 mL, 0.05 M) were mixed in
a 50 mL twoꢀnecked flask. The sample was collected at an
interval of 10 min. The filtrate was extracted by CH COOC H ,
down to room temperature. Fe O nanoparticles attached on the
3
4
surface of SiO nanospheres, were separated from the solution by
2
an external magnet and washed several times with ethanol and
distilled water until the pH value of the solution became
approximately 7 before ovenꢀdried at 50 ℃ for further use.
3
2
5
and then the reaction conversion of the HDC reaction was
estimated by GCꢀMS. To evaluate the reusability of the
Pd/Fe O @hTiO nanocatalyst in the HDC reaction, the
2
8
2
.2.3 Preparation of Pd/Fe O /SiO composite
3 4 2
3
4
2
2
00 mg Fe O /SiO sphere powders was added to 35 mL distilled
3 4 2
nanocatalyst was recovered by a magnet, washed with water, and
dried in vacuum at room temperature. In general, the catalyst was
redispersed in a new reaction system for subsequent catalytic
experiments under the same reaction conditions. Moreover, the
catalytic procedure was repeated seven times.
water and ultraphonic for 10 min as part A. 200 mg SnCl was
2
dissolved in 40 mL 0.02 M HCl solution as part B. Parts A and B
were mixed together under ultraphonic condition. 2 h later, the
precipitate was collected by centrifugation, followed by washing
with distilled water five times, and dispersed into 50 mL distilled
water. Then 50 mL 16 mg PdCl was added into it. 30 min later,
2
3
. Results and discussion
2
0 mL of 0.15 M HCOONa solution was added, followed by
3
.1 Characterizations of the catalyst Pd/Fe O @hTiO
3 4 2
stirring for 8 h. The suspension was separated from the solution
by centrifugation, after washing with distilled water and ethanol
for several times, the product dried in vacuum at 50 ℃ for further
use.
2
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3 4 2
Fig. 2 EDX spectrum of Pd/Fe O @hTiO
The elemental composition of the Pd/Fe O @hTiO samples was
3
4
2
determined by EDX analysis. The result shown in Fig. 2 revealed
that the asꢀprepared products contain Pd, Fe, Cu, Ti, C, Si ,Sn and
O. Among these elements, Cu and C are generally influenced by
the copper network support films and their degree of oxidation,
Pd, Fe and Ti signals result from the Pd/Fe O @hTiO . And from
3
4
2
the EDX result, the signal of Si element has not been found,
revealed that the SiO was totally removed.
2
Fig. 1 TEM images of (a) SiO
Inset pictures: size distribution histograms image of the Pd nanoparticles
crystal structure in detail on Pd/Fe /SiO ), (e) Pd/Fe /SiO @TiO , (f)
Pd/Fe @hTiO
2 3 4 2 3 4 2
, (b) and (c) Fe O /SiO , (d) Pd/Fe O /SiO
(
3
O
4
2
3
O
4
2
2
3
O
4
2
The morphologies and structures of the products at different
synthetic steps were observed by TEM. Fig. 1a shows the TEM
image of SiO2 nanospheres, in which well dispersed particles
with average diameter about 200 nm could be observed clearly.
Both fig. 1b and 1c show the TEM images of the Fe O /SiO
2
3
4
composite, and the Fe O particles with an average size of about
3
4
1
0 nm are uniformly distributed on the surface of the SiO2
nanospheres. Fig. 1d displays the TEM image of Pd/Fe O /SiO
2
3
4
nanospheres with Pd and Fe O nanoparticles well dispersed on
3
4
the outer surfaces of SiO2 nanospheres. The sizes of Pd
nanoparticles are estimated to be 5 nm. Fig. 1e shows the TEM
images
of
Pd/Fe O /SiO @TiO
nanospheres.
The
3
4
2
2
3 4 2 3 4 2
Fig. 3 XRD patterns of (a) Pd/Fe O /SiO , (b) Pd/Fe O @hTiO
Pd/Fe O /SiO @TiO nanospheres with the diameter of ~320nm
3
4
2
2
have relatively smooth surfaces and better dispersion. The SiO2
core was removed using concentrated Na CO aqueous solution,
(● represents Pd nanoparticles; ■ represents Fe O nanoparticles)
3
4
2
3
Fig. 3 shows the XRD patterns between 10 and 90° of the
samples. Fig. 3a shows the wide angle XRD pattern of
Pd/Fe O /SiO composites. Five diffraction peaks appear at
leaving only Pd and Fe O nanoparticles inside the hollow TiO ,
3
4
2
resulting in the formation of Pd/Fe O /SiO @hTiO hollow
3
4
2
2
spheres. The TEM images of Pd/Fe O @hTiO shown in Fig. 1f.
3
4
2
3
4
2
2
θ=30.6°, 35.8°, 43.4°, 57.5° and 63.2°, corresponding to the
It clearly displays the hollow mesoporous structures of TiO with
2
reflections of (220), (311), (400), (511) and (440) crystal planes
of Fe O , respectively. Fig. 3b shows the wide angle XRD pattern
a shell thickness of ~40 nm.
3
4
of Pd/Fe O @hTiO composites. Four diffraction peaks appear at
3
4
2
2
θ=40.1°, 46.5°, 68.0° and 82.6°, corresponding to the reflections
of (111), (200), (220) and (311) crystal planes of Pd, respectively.
Meanwhile, the result indicating the crystalline structure of the
support was well maintained in the Pd samples. The average
crysallite size of Pd nanoparticles in Pd/Fe O @hTiO were
3
4
2
calculated using Scherrer’s equation, is about 5 nm.
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Fig. 4 (A)XPS spectrum of the elemental survey scan of
Pd/Fe @hTiO and (B) highꢀresolution spectrum of Pd 3d
3
O
4
2
Fig.
4
shows the XPS spectrum of the synthesized
Pd/Fe O @hTiO nanoreactor. Peaks corresponding to O, C, Fe,
3
4
2
Pd, Ti and Sn are observed. However, typical peaks of Sn, Fe and
Pd elements are not obviously detected, which indicates that Sn,
Fig. 6 Room temperature magnetization curves of (a) Pd/Fe
3 4 2
O /SiO , (b)
Pd/Fe @hTiO
3
O
4
2
Fe and Pd particles have been coated by TiO , since the analysis
2
Magnetic measurements were carried out by employing VSM at
room temperature. The magnetization curves measured for
depth of XPS is only several nanometers. To ascertain the
oxidation state of the Pd, XPS studies are carried out. The XPS
signature of Pd 3d doublet for the Pd nanoparticles is given in the
fig.4(B). The Pd 3d3/2 and Pd 3d5/2 peaks appeared at 342.6 and
Pd/Fe O /SiO and Pd/Fe O @hTiO are compared in Fig. 6.
3
4
2
3
4
2
There is no hysteresis in the magnetization for the two tested
nanoparticles. It can be seen that the values of the saturation
3
37.2 eV respectively are the characteristic peaks for the metallic
ꢀ
1
magnetization are 3.22 emu·g for Pd/Fe O /SiO and 1.68
Pd(0). We can see that there is no oxidation state of Pd species in
the catalyst. In the experiment, the reducing agent is excessive so
that the Pd(II) could be reduced into Pd(0) completely, which is
beneficial for the reaction.
3
4
2
ꢀ
1
emu·g for Pd/Fe O @hTiO respectively. The decrease of the
3
4
2
saturation magnetization suggests the presence of some Pd
particles inside the uniformly pore of hollow mesoporous
spheres. Even with this reduction in the saturation magnetization,
the catalyst still can be efficiently separated easily from the
solution with the help of an external magnetic force.
3
.2 Catalytic activity for HDC of 4ꢀCP
The catalytic activity of the Pd/Fe O @hTiO catalyst was
3
4
2
established by the HDC of 4ꢀCP under a mild condition. The
HDC of 4ꢀCP was negligible without catalyst or in the presence
of pure hollow magnetic mesoporous TiO spheres at the same
2
conditions, which shows that the presence of NMNPs are
indispensable for high catalytic activity. As Diaz et al.
Fig. 5 (A) N
2
adsorption/desorption isotherms of the Pd/Fe
the corresponding pore diameter distribution
3
O
4
@hTiO
2
(B)
30
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
Fig. 5 shows nitrogen adsorption/desorption isotherms and the
corresponding pore size distribution of the Pd/Fe O @hTiO . The
3
4
2
2
ꢀ1
BET surface area and single point total pore volume is 180 m ·g
3
ꢀ1
and 0.13 cm ·g , respectively, which are considerably large
values because the solid core and Pd nanoparticles have been
included in the calculations. The average BJH (Barrette Joynere
Halenda) pore diameter calculated from the adsorption branch of
the isotherm and the desorption branch of the isotherm are 4.5
nm. On the basis of above analysis, it can be concluded that these
data further identified the hollow mesoporous structure of the
synthesized Pd/Fe O @hTiO catalyst.
3
1
low selectivity of HDC of 4ꢀCP performed in fixed bed reactor
30
or under a high H pressure condition, such as: Pd/C catalyzed
2
HDC of 4ꢀCP with the conversion of 93% and the selectivity of
85% were reported by J. Rodriguez, which formic acid as
32
hydrogen source in continuous stirredꢀtank reactor. Pd/Al
O
as
HDC catalyst was also studied by J. Juan , in fixed bed reactor
with the conversion was 87% and the selectivity 62%,
3
4
2
2
3
31
respectively. Compared with reports above mentioned, in our
reaction, a batch stirred tank reactor was used and only H was
2
flowing phase. The main production was phenol and less than
0
.1% CYC was detected after 2 h 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
4
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which phenol as major product was detected.
Table 1 Effect of different solvents and different bases
3 4 2
Table 2 Catalytic HDC of different substrates over the Pd/Fe O @hTiO
a
nanocatalyst
a
Entry
Reactants
Cl
Main products
OH
Yield
Pd/HMMS, H (1atm)
2
HO
Cl
OH
+ HCl
Base, Solvent, 25℃
Entry
Base
Solvent
OH
Yield
12.2%
22.8%
4.1%
1
2
99.9%
OH
1
2
3
4
5
6
7
NaOH
2 5
C H
Cl
NaOH
NaOH
NaOH
CH
3
COOC
DMF
2 5
H
99.9%
68.6%
OH
OH
OH
H
2
H
2
H
2
H
2
O
O
O
O
99.9%
72.9%
97.2%
93.9%
Cl
Na
CH COONa
(C
2
CO
3
b
3
OH
Cl
3
2
H
5
)
3
N
a
Conditions: chlorophenol (0.5 mmol), base (1 equiv.), solvent (30 mL),
catalyst (20 mg), H (30 mL·min ), 25 ℃, 2 h. 2equiv. of NaOH.
2
ꢀ1
b
a
Conditions: 4ꢀCP (0.5 mmol, 64.3 mg), base (0.5 mmol), solvent
30mL), catalyst (20 mg), H balloon (30 mL·min ), 25 ℃, 2 h.
2
ꢀ1
(
In order to investigated the application of the Pd/Fe O @hTiO
2
3
4
nanocatalyst for HDC of chlorophenols, we also studied the HDC
of 2ꢀCP, 3ꢀCP and 2,4ꢀdichlorophenol (2,4ꢀDCP) catalyzed by
the Pd/Fe O @hTiO nanocatalyst and the corresponding results
The HDC reaction mechanism was described below: H2
adsorbed on the active site of the Pd/Fe O @hTiO catalyst was
3
4
2
3
4
2
activated into two hydrogen atoms which combined with 4ꢀCP
also adsorbed on the surface of the catalyst. The CꢀCl bond of 4ꢀ
were listed in Table 2. The values listed in Table 2 revealed that
3ꢀCP and 2ꢀCP were also almost completely transformed into
phenol within 2 h. Except, the low yields of phenol obtained from
33
CP was attacked by the active hydrogen atoms to from phenol.
The abovementioned results clearly indicated that the
Pd/Fe O @hTiO nanocatalysts exhibited the favorable catalytic
37,38
2
,4ꢀDCP.
The reason was ascribed probably to the electronic
3
4
2
and steric hindrance of 2,4ꢀDCP. Compared with the
performance toward HDC of 4ꢀCP. The catalytic properties and
recyclability were evaluated by performing HDC of 4ꢀCP.
Pd/Fe O @hTiO was used as a probe catalyst to optimize the
33,39
literature
,
this synthesized catalyst Pd/Fe O @hTiO
3 4 2
exhibited comparable or much better catalytic activity for HDC.
3
4
2
reaction conditions and to catch the best reaction conditions.
Therefore, a series of reactions was performed using several
bases and solvents to obtain the best possible combination (Table
1
). In order to prevent the poisoning of the catalysts, hydrochloric
acid (HCl) produced during the HDC process was neutralized by
3
4
the bases added to the reaction system. Therefore, NaOH was
first used as a neutralizer to test different solvents (Table 1,
entries 1–4). The yields of phenol in C H OH, CH COOC H ,
2
5
3
2
5
DMF, and H O were 12.2, 22.8, 4.1, and 99.9%, respectively.
2
These low yields of phenol obtained in the abovementioned
organic solvents were mainly ascribed to the low solubility of
NaOH that leads to the poisoning of the nanocatalyst. As a result,
based on the aboveꢀmentioned results, H O was considered to be
2
the best solvent for the HDC of 4ꢀCP. To further optimize the
reaction conditions, different bases were tested in the aqueous
solution (Table 1, entries 4–7). The results indicated that the yield
of phenol in the HDC reaction was significantly affected by the
basicity of the neutralizer for HCl as reported by Zahara M. de
Fig. 7 The recycling experiments of the catalyst Pd/Fe
3 4 2
O @hTiO .
The recyclability of Pd/Fe O @hTiO catalyst was further
3
4
2
35,36
Pedro.
achieved when the process was carried out in strong base (NaOH)
solution. In comparison, when weak base (Na CO ,
CH COONa, NH ·H O, or (C H ) N) was employed, a low yield
The highest reaction rate and phenol yield were
investigated. After completion of the reaction, the catalyst was
recovered from the reaction mixture by using an external magnet.
The recovered catalyst was washed with ethanol for several
times, and dried at room temperature. As illustrated in Fig. 7, the
catalyst was reused for at least seven times without a significant
loss of activity.
a
2
3
3
3
2
2
5 3
of phenol was obtained under the same reaction conditions (Table
). Hence, NaOH was considered as the optimized base for HDC
reaction. Thus, the catalytic properties of Pd/Fe O @hTiO in the
1
3
4
2
HDC of 4ꢀCP were investigated using NaOH and H O as the
optimized base and solvent, respectively.
2
4. Conclusions
In conclusion, we describe a method to encapsulate Pd(0) within
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DOI: 10.1039/C5RA07765F
TiO2 hollow mesoporous nanostructures. The designed
Pd/Fe O @hTiO nanoreactors possess hollow core/mesoporous
21. V. Polshettiwar, R. Luque, A. Fihri, H. Zhu, M. Bouhrara and J.ꢀM.
Basset, Chemical Reviews, 2011, 111, 3036ꢀ3075.
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4
2
2
2
2
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Green Chemistry, 2009, 11, 1337ꢀ1342.
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shell structures, high catalytic activity and magnetic recyclability
properties. The designed catalyst exhibits high activity for HDC
of 4ꢀCP under mild conditions and could be recovered in a facile
manner from the reaction mixture and recycled seven times
without any loss in activity. Furthermore, the method is also
extended to introduce other NMNPs including Au, Ag, Pt, and
their bimetallic nanoparticles inside TiO2 hollow mesoporous
nanostructures to prepare other new catalysts.
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Acknowledgements
The authors are grateful to Projects in Gansu Province Science an
d Technology Pillar Program.
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