Highly efficient sulfonated-polystyrene-Cu(II)@Cu3(BTC)2 core-shell microsphere catalysts for base-free aerobic oxidation of alcohols

Article abstract of DOI:10.1039/c4ta06046f

A novel catalyst consisting of a functional sulfonated-polystyrene (SPS) core, a porous Cu₃(BTC)₂ shell and an active Cu(ii) interface between the core and shell was developed via a facile step-by-step assembly method. The polystyrene core was sulfonated first to achieve functional -SO₃H groups on its surface. The main function of the -SO₃H groups was to graft Cu(ii) ions to generate an active Cu(ii) interface, and the excess -SO₃H could provide acid conditions for the catalytic reaction. The Cu(ii) interface along with the acid conditions and the co-catalyst 2,2,6,6-tetramethyl-piperidyl-1-oxy (TEMPO) enhanced the catalytic activity for the aerobic oxidation of alcohols to aldehydes by molecular oxygen under base-free conditions. A portion of Cu(ii) ions on the SPS surface was then coordinated with H₃BTC (1,3,5-benzenetricarboxylic acid) to form a porous Cu₃(BTC)₂ shell, which could protect the active metal from leaching as well as provide porous channels for mass transfer, resulting in high stability and recyclability in the catalysis procedure. The SPS-Cu(ii)@Cu₃(BTC)₂ catalyst could be recycled ten times without a significant loss in its activity and selectivity. Furthermore, the SPS-Cu(ii)@CuBDC (BDC = 1,4-benzenedicarboxylate) composite was also synthesized and showed high efficiency for catalyzing the aerobic oxidation of alcohols and aerobic homocoupling of arylboronic acids, suggesting that the unique nanostructure of SPS-Cu(ii)@MOFs can be easily extended to design complex catalysts with high efficiency and good stability for different catalytic reactions. This journal is

Full text of DOI:10.1039/c4ta06046f

Journal of
Materials Chemistry A
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Highly efficient sulfonated-polystyrene–Cu(II)
Cu (BTC) core–shell microsphere catalysts for
@
3
2
Cite this: DOI: 10.1039/c4ta06046f
base-free aerobic oxidation of alcohols†
Xiaowei Zhang, Wenjun Dong, Yi Luan, Mu Yang, Li Tan, Yangguang Guo, Hongyi Gao,
Yinhai Tang, Rui Dang, Jie Li and Ge Wang*
A novel catalyst consisting of a functional sulfonated-polystyrene (SPS) core, a porous Cu
an active Cu(II) interface between the core and shell was developed via a facile step-by-step assembly
method. The polystyrene core was sulfonated first to achieve functional –SO H groups on its surface.
The main function of the –SO H groups was to graft Cu(II) ions to generate an active Cu(II) interface, and
the excess –SO H could provide acid conditions for the catalytic reaction. The Cu(II) interface along
3 2
(BTC) shell and
3
3
3
with the acid conditions and the co-catalyst 2,2,6,6-tetramethyl-piperidyl-1-oxy (TEMPO) enhanced
the catalytic activity for the aerobic oxidation of alcohols to aldehydes by molecular oxygen under base-
free conditions. A portion of Cu(II) ions on the SPS surface was then coordinated with H
1,3,5-benzenetricarboxylic acid) to form a porous Cu (BTC) shell, which could protect the active metal
from leaching as well as provide porous channels for mass transfer, resulting in high stability and
recyclability in the catalysis procedure. The SPS–Cu(II)@Cu (BTC) catalyst could be recycled ten times
without a significant loss in its activity and selectivity. Furthermore, the SPS–Cu(II)@CuBDC (BDC ¼
,4-benzenedicarboxylate) composite was also synthesized and showed high efficiency for catalyzing the
3
BTC
(
3
2
3
2
Received 9th November 2014
Accepted 8th January 2015
1
aerobic oxidation of alcohols and aerobic homocoupling of arylboronic acids, suggesting that the unique
nanostructure of SPS–Cu(II)@MOFs can be easily extended to design complex catalysts with high
efficiency and good stability for different catalytic reactions.
DOI: 10.1039/c4ta06046f
www.rsc.org/MaterialsA
Recent investigations have conrmed that noble metals such
1
. Introduction
10
11,12
13
as palladium, ruthenium
and gold can efficiently and
Selective oxidation of alcohols to aldehydes is a vital and selectively catalyze the oxidation of alcohols with molecular
common transformation in the synthesis of important inter- oxygen. However, it is desirable to develop non-noble metal
1
–3
mediates and ne chemicals. Traditionally, stoichiometric catalysts to promote the catalytic oxidation reaction due to the
oxidants are required to achieve high catalytic efficiency, which high cost and complicated synthesis processes of noble metal
4
,5
14,15
are toxic and oen produce waste and harmful byproducts.
Recently, more sustainable reaction processes with molecular metal, is considered to be one of the most attractive catalysts for
catalysts. Copper species,
as a classical non-noble transition
6–8
oxygen as an ideal oxidant have attracted signicant atten- the aerobic oxidation of alcohols on account of its abundant
tion. However, due to the relatively weak oxidizing properties of resources, low cost, non-toxic properties and high catalytic
molecular oxygen, the direct oxidation of alcohols by dioxygen efficiency. In particular, copper(II) complexes combined with
16,17
is difficult, and appropriate catalysts are needed to complete the TEMPO
have been proved to have good catalytic perfor-
mance for alcohol oxidation. Until now, many efficient Cu(II)/
TEMPO catalytic systems such as Cu(II)-bipyridine/TEMPO,
copper(II)/2-N-arylpyrrolecarbaldimine/TEMPO and CuBr /N-
9
aerobic oxidation process.
18
19
2
20
benzylidene-N,N-dimethylethane-1,2-diamine/TEMPO
have
School of Materials Science and Engineering, University of Science and Technology
Beijing, Beijing 100083, China. E-mail: gewang@mater.ustb.edu.cn; Fax: +86-10-
been reported and favorable catalytic activities have been ach-
ieved. However, most Cu/TEMPO catalysts oen require a large
excess of base additives or alkaline ligands to neutralize acid
6
2327878; Tel: +86-10-62333765
†
Electronic supplementary information (ESI) available: Powder XRD patterns of
2
21,22
the Cu
microspheres before sulfonation. Powder XRD patterns and the FESEM
photograph of the SPS–Cu(II)@Cu (BTC) catalyst aer 10 runs of aerobic
oxidation of benzyl alcohol under base-free conditions. FESEM photograph of
the SPS–Cu(II)@Cu (BTC) catalyst aer 18 catalytic runs. Powder XRD patterns
3
(BTC)
2
and SPS–Cu(II)@Cu
3
(BTC)
2
nanoparticles. FESEM image of the PS products or deprotonate the alcohols,
which always causes
23,24
carboxylate product and basic waste disposal problems
as
3
2
25
well as separation difficulties for catalytic recycling. Interest-
ingly, other studies have revealed that some Cu(II) salts can
catalyze alcohol oxidation by using TEMPO as a co-catalyst
3
2
of the SPS–Cu(II)@CuBDC composite. See DOI: 10.1039/c4ta06046f
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without base additives or even under acid conditions. For
example, the CuCl/TEMPO catalytic system proposed by Sem-
melhack et al. has proven to be highly efficient for catalyzing
2
. Experimental section
2
6
2.1 Catalyst preparation
the selective aerobic oxidation of alcohols under base-free
Synthesis of the dumbbell-shaped sulfonated-polystyrene
(SPS) microspheres. Monodisperse polystyrene (PS) spheres
27
3 2 3 2
conditions. Cecchetto et al. found that a Mn(NO ) –Cu(NO ) /
TEMPO catalytic system allows the selective oxidation of alcohols with dumbbell morphology were synthesized via seeded emul-
to aldehydes and ketones under acid conditions. The formation sion polymerization technology according to the reported
40,41
of oxoammonium cations is believed to be the main mechanism procedures.
The sulfonated-polystyrene (SPS) template was
for the actual catalytic oxidation of alcohols in these catalytic prepared as follows: 1.0 g of monodisperse PS particles was
systems. Unfortunately, these copper salt catalysts are still dispersed in 100 mL concentrated sulfuric acid in a 250 mL
homogeneous and are difficult to separate and recycle from the three-necked ask equipped with a mechanical stirrer, and the
ꢀ
catalytic reaction.
mixed solution was heated to 55 C and stirred for 6 h. Then the
Recently, the proper design and preparation of a control- products were separated by centrifugation, washed with ethanol
lable active interface between the core and shell in a core–shell and deionized water, and subsequently dried by lyophilization.
structured catalyst has become a fascinating and signicant
3 2
Preparation of the SPS–Cu(II)@Cu (BTC) composite. The
2
8,29
object in nanoscience and nanotechnology.
Polymers, as core–shell structured SPS–Cu(II)@Cu (BTC) composite was
3 2
classic substrate materials, can construct a functional surface synthesized according to a facile one-pot procedure. First, the
for efficient heterogeneous catalysts with good dispersity, high as-prepared SPS spheres (0.1 g) were dispersed in an ethanolic
3
0
ꢁ1
stability and easy designability. Usually, the catalytically solution of Cu(CH COO) $H O (60 mmol L , 10 mL) under
3
2
2
active materials supported on polymer substrates show high magnetic stirring for 5 min at room temperature. The obtained
3
1–33
catalytic performance,
but they oen suffer from leaching SPS–Cu(II) sample was separated by centrifugation, washed
of active ingredients and poor recyclability during the catalytic several times with ethanol and then immersed into an ethanolic
3
4
reactions. To date, the preparation of a stable and efficient solution of 1,3,5-benzenetricarboxylic acid (H BTC) (20 mmol
3
ꢁ
1
heterogeneous catalyst with a controllable active interface is
L , 10 mL) at room temperature. Aer further stirring for about
still a challenging research target. It is well known that a 15 min, the product was collected and washed with ethanol, and
porous shell with appropriate mass-transfer channels would the SPS–Cu(II)@Cu (BTC) composite was nally obtained.
3
2
be very necessary for a stable catalyst. Metal–organic frame-
works (MOFs) are of a new class of porous materials con- under ambient pressure according to a previously reported
3 2
For catalysis comparison, pure Cu (BTC) was synthesized
3
5
42
sisting of metal/metal clusters and organic ligands. Cu
3
BTC
2
method and its phase purity was conrmed by powder XRD
(
also known as HKUST-1or CuBTC, BTC ¼ 1,3,5-benzene- (Fig. S1†). To extend the catalytic system, another novel heter-
tricarboxylate) as one of the most widely used MOF materials ogenous catalyst of SPS–Cu(II)@CuBDC with a different MOF
has received considerable attention in recent years due to its shell was also synthesized. The synthesis process was the same
high surface area, tunable pore structures and good chemical as that for the SPS–Cu(II)@Cu (BTC) nanocomposite, except
stability.
In this paper, a novel core–shell SPS–Cu(II)@Cu (BTC)
3
2
3
6–39
that 1,4-benzenedicarboxylic acid (H BDC) was added into the
2
3
2
reaction system instead of H BTC.
3
catalyst composed of a sulfonated-polystyrene (SPS) core, an
active Cu(II) interface and a microporous Cu (BTC) shell was
3
2
developed for catalyzing the aerobic oxidation of alcohols by
molecular oxygen under base-free conditions. In the synthetic
2.2 Catalytic reaction
procedure, Cu(II) ions were rst graed onto a sulfonated- The selective oxidation of alcohols was carried out in a 25 mL
polystyrene core to generate a well-dened active surface two-necked ask tted with a reux condenser. Typically,
decorated with uniform active Cu(II) sites. Part of the –SO H certain amounts of catalyst, alcohol (1 mmol) and TEMPO
3
graed copper ions to form the active surface and the rest of the (0.09 mmol) were added to 5 mL of acetonitrile. This system was
–
SO H provided acid conditions to produce a high catalytic purged three times with oxygen and then sealed with an O2
3
performance. Moreover, some of the Cu(II) ions linked with balloon. The reaction mixture was then heated to the desired
the sulfo group further coordinated with H BTC ligands to reaction temperature under stirring. Aer a required reaction
3
generate a microporous MOF shell. It is noteworthy that the time, the catalysts were removed from the solution by centri-
active copper(II) interface, the acid conditions provided by fugation and the liquid organic products were analyzed by GC-
–
SO
3
H groups on the SPS–Cu(II) surface, and the Cu
3
(BTC)
2
MS with an HP-5 capillary column by using 1 mmol of nitro-
shell with uniform porous channels made the catalyst highly benzene as the internal standard. Aer each cycle of catalytic
efficient for the aerobic oxidation of primary alcohols and it reaction, the catalysts were collected by centrifugation, dried at
could be recycled 10 times without a signicant loss in its room temperature and then reused in the next run to investigate
catalytic activity. Different SPS–Cu(II)@MOF catalysts such as their recyclability. For the leaching test, the catalytic reaction
SPS–Cu(II)@CuBDC were also prepared for catalyzing different was stopped aer 4 h and the solid catalyst was removed by
catalytic reactions.
centrifugation. The reaction solution without the solid catalysts
was then stirred for a further 4 h. The catalytic performance in
this reaction process was monitored by GC/MS.
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To extend the catalytic system, the catalytic performance of
Journal of Materials Chemistry A
SPS–Cu(II)@CuBDC for aerobic oxidation of benzyl alcohol was
tested and the reaction conditions were the same as those of
SPS–Cu(II)@Cu (BTC) . Moreover, the catalytic activity of SPS–
3
2
Cu(II)@CuBDC for aerobic homocoupling of arylboronic acids
was also tested and the detailed reaction conditions were as
follows: 2 mol% of SPS–Cu(II)@CuBDC catalyst and 2 mmol
of phenylboronic acid were stirred in 2 mL of DMF at room
temperature for 12 h.
2.3 Characterization
Field-emission scanning electron microscopy (FESEM) images
of the products were taken on a ZEISS SUPRA55 instrument with
an acceleration voltage of 10 kV. The high-resolution trans-
mission electron microscopy (HRTEM) and high-angle annular
dark eld scanning transmission electron microscopy (HAADF-
STEM) images were recorded on a FEI Tecnai F20 electron
microscope operating at 200 kV. Fourier-transform infrared
spectra (FTIR) were analyzed using a Nicolet 6700 spectrometer.
Powder X-ray diffraction (XRD) patterns of the samples were
obtained using a Rigaku D/MAX-RB diffractometer using Cu Ka
Scheme
composite.
1
The synthesis process of the SPS–Cu(II)@Cu
3
(BTC)
2
FESEM and HRTEM images of the SPS–Cu(II)@Cu
3
(BTC)
2
microspheres with different magnications. All the SPS micro-
spheres are successfully coated with small Cu (BTC) nano-
crystals while the dumbbell shape and rough surface of the SPS
templates are well maintained (Fig. 1b). The Cu (BTC) nano-
crystals deposited on the surface of the SPS microsphere exhibit
a typical octahedron-like structure with a mean diameter of
3
2
˚
3
2
radiation (40 kV, 150 mA, l ¼ 1.5406 A). Copper contents in the
products were determined using inductively coupled plasma-
atomic emission spectrometry (ICP-AES). Nitrogen adsorption
measurements were performed at 77 K on a Quantachrome
Autosorb-1. The catalytic efficiency was calculated using a gas
chromatography-mass spectrum (Agilent 7890/5975C-GC/MSD).
8
0 nm (the inset in Fig. 1b). The HRTEM image in Fig. 1c further
reveals that the SPS–Cu(II)@Cu (BTC) particles show a core–
shell structure with a relatively thin shell composed of ne
Cu (BTC) nanoparticles and with a shell thickness of about 300
nm. The HAADF-STEM image of a SPS–Cu(II)@Cu (BTC)
3
2
3
2
3
2
3
. Results and discussion
microsphere (Fig. 1d) along with the corresponding elemental
mapping of S (Fig. 1e) and Cu (Fig. 1f) in the SPS–Cu(II)
3
.1 Synthesis and characterization
The typical procedure for the synthesis of the SPS–Cu(II)
Cu (BTC) composite is shown in Scheme 1. Initially, the
dumbbell-like PS particles were treated with concentrated sul-
phuric acid to generate a functional SO H-modied surface and
@
Cu (BTC) structure demonstrate that the surface of the
3 2
polystyrene microsphere is successfully modied with sulfonic
acid groups, and the SPS core is uniformly wrapped with the Cu
element. The obtained conguration is particularly important
@
3
2
3
the SPS template was formed. Subsequently, the SPS micro-
spheres were dispersed in an ethanol solution of cupric acetate
to attach the copper ions. The functional sulfonic groups
introduced into the SPS template had strong affinity for metal
2
+
cations, which graed Cu ions from the precursor ethanol
solution of cupric acetate to form an active Cu(II) interface (SPS–
3 2
Cu(II)). The Cu (BTC) shell, which stabilized and protected the
active Cu(II) ions, was generated when the 1,3,5-benzene-
tricarboxylic acid solution was added to interact with a fraction
2
+
of the attached Cu ions on the surface of the SPS core, and the
SPS–Cu(II)@Cu (BTC) composite was nally obtained.
3
2
The morphology and structure of the SPS and SPS–Cu(II)
Cu (BTC) composite were identied by FESEM and HRTEM
@
3
2
techniques (Fig. 1). Fig. 1a shows the FESEM image of the as-
prepared SPS templates (ꢂ6.3 mm length and ꢂ4.1 mm diam-
Fig. 1 FESEM images of (a) the SPS template and (b) SPS–Cu(II)
eter) with a uniform dumbbell shape and a pleated surface. @Cu
3
(BTC)
the Cu (BTC)
c) HRTEM image of the SPS–Cu(II)@Cu
tively low-magnification HRTEM image of one SPS–Cu(II)@Cu
particle); (d) HAADF-STEM image of an SPS–Cu(II)@Cu (BTC) particle
and the corresponding elemental mapping of S (e) and Cu (f) in the
2
composite (inset: a high-magnification FESEM image of
nanoparticles on the surface of the SPS–Cu(II) core);
(BTC) particle (inset: a rela-
(BTC)
Compared to the PS microspheres with a smooth surface
Fig. S2†), the wrinkles on the surface of the SPS particles
3
2
(
3
2
(
43
3
2
formed during the sulfonation process increase the surface
area of the polystyrene microspheres, which facilitates the
3
2
3 2
attachment of the active copper ions. Fig. 1b and c show the SPS–Cu(II)@Cu (BTC) structure.
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parameters of Cu
further analysed in the ESI (Fig. S1†).
The porosity of the SPS–Cu(II)@Cu (BTC) microspheres is
3 2 3 2
(BTC) and SPS–Cu(II)@Cu (BTC) has been
3
2
conrmed by nitrogen adsorption–desorption isotherms
Fig. 3). When Cu (BTC) nanocrystals are generated on the
surface of the SPS-Cu(II) core, the Brunauer–Emmett–Teller
BET) surface area of the SPS–Cu(II)@Cu (BTC) composite
increases from 3.88 m g to 227.62 m g , indicating that the
porous Cu (BTC) shell has been successfully obtained.
However, the BET surface area (227.62 m g ) and total pore
(
3
2
(
3
2
2
ꢁ1
2
ꢁ1
Fig. 2 (a) FTIR spectra of the PS and SPS templates, (b) powder XRD
patterns of the SPS template and SPS–Cu(II)@Cu (BTC) microspheres.
3 2
3
2
2
ꢁ1
3
ꢁ1
volume (0.37 cm g ) of the SPS–Cu(II)@Cu (BTC) composite
3
2
are much lower than those of the synthesized parent Cu (BTC)
3
2
to achieve a highly efficient catalyst with multiple functions.
2
ꢁ1
3
ꢁ1
2
+
sample (1112 cm g and 0.51 cm g ), which may be attrib-
The free Cu ions graed with the sulfo groups on the surface
of the SPS particles aim to gain a robust catalytic activity for the
aerobic oxidation of alcohols, and the compact Cu (BTC) shell
50
utable to the existence of the nonporous SPS–Cu(II) core or the
51
low synthesis temperature and the short reaction time. The
pore diameter of the Cu (BTC) calculated by the H–K method is
.608 nm (the Horv ´a th–Kawazoe (H–K) method was developed
for slit pores and has corrections for different pore geometries
cylinder and sphere), and it is a common method for analyzing
3
2
3
2
with a porous network structure offers a stable protection layer
for the active copper ions as well as appropriate reaction
channels to interact with the substrates and reagents.
0
(
The successful functionalization of the sulfonic groups is
conrmed by FTIR spectroscopy (Fig. 2a). The absorptions at
micropores (<2 nm)). With such a porous shell composed of
Cu (BTC) nanocrystals, the active Cu(II) interface is well pro-
ꢁ1
ꢁ1
ꢁ1
3
2
1
038 cm , 620 cm and 1181 cm are attributable to the
3 2
tected and the stability of the SPS–Cu(II)@Cu (BTC) catalyst
was enhanced.
S]O symmetric and asymmetric stretching vibration of the
44,45
ꢁ1
–
SO H group.
The characteristic absorption at 1127 cm is
3
46
attributable to a sulfonate anion attached to a phenyl ring,
indicating strong bonding interactions between the sulfonic
groups and the polystyrene. The crystal structure and chemical The selective oxidation of benzyl alcohol to benzaldehyde was
composition of the SPS template and SPS–Cu(II)@Cu (BTC)
chosen as a model reaction to characterize the activity of the
composite are further conrmed by powder XRD (Fig. 2b). The SPS–Cu(II)@Cu (BTC) catalyst under base-free conditions. The
3.2 Catalytic properties
3
2
3
2
ꢀ
ꢀ
broad diffraction peak of the SPS template around 2q ¼ 20 is catalytic reaction was carried out at 75 C using 50 mg of SPS–
47
attributable to the amorphous PS polymer. For the SPS–Cu(II) Cu(II)@Cu (BTC) (2 mol% Cu, determined by ICP) as the
3
2
@
Cu (BTC) microspheres (Fig. 2b), the characteristic diffrac- heterogenous catalyst, 0.09 mmol of TEMPO as the co-catalyst, 1
3 2
ꢀ
ꢀ
ꢀ
ꢀ
tion peaks at 2q values of about 9.52 , 11.68 , 13.46 , 14.61 , atm of molecular oxygen as the oxidant and 5 mL of acetonitrile
ꢀ
ꢀ
ꢀ
ꢀ
1
(
5.01 , 16.49 , 17.50 and 19.08 are assigned to the (220), (222), as the solvent. The yield of benzaldehyde versus reaction time is
400), (331), (420), (422), (333) and (440) planes of Cu (BTC)
shown in Fig. 4. An increase in the yield of benzaldehyde can be
crystals, which is in good agreement with the previous observed as the reaction proceeds. Upon reaction for 8 h, the
3
2
48,49
results.
No other impurity peaks can be observed. A conversion of benzyl alcohol was complete with >99% selectivity
comparison of the powder diffraction patterns and the unit cell (Table 1, entry 1).
Fig. 4 Yield of benzaldehyde versus reaction time catalyzed by the
Fig. 3 Nitrogen adsorption–desorption isotherms (inset: pore size SPS–Cu(II)@Cu (BTC) nanocomposite. Reaction conditions: benzyl
3
2
distribution) of the SPS–Cu(II)@Cu
at 77 K.
3
(BTC)
2
composite measured alcohol (1 mmol), SPS–Cu(II)@Cu
3
(BTC)
2
(2 mol%), TEMPO (0.09 mmol),
ꢀ
acetonitrile (5 mL), 75 C, 1 atm O
2
and 1 mmol nitrobenzene.
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Journal of Materials Chemistry A
a
Table 1 Aerobic oxidation of benzyl alcohol with various catalysts
Entry
Catalysts
Time (h)
Yield (%)
1
2
3
SPS–Cu(II)@Cu
—
SPS
3
(BTC)
2
8
8
8
8
8
8
8
8
>99
0
0
4
5
>99
79
>99
b
4
5
Cu
3
(BTC)
2
b
SPS + Cu
3
(BTC)
2
6
7
SPS–Cu(II)
SPS–Cu(II)
SPS–Cu(II)@CuBDC
Fig. 5 Recycling results for selective oxidation of benzyl alcohol to
c
benzaldehyde. Reaction conditions: benzyl alcohol (1 mmol), catalyst
8
ꢀ
(
2 mol%), TEMPO (0.09 mmol), acetonitrile (5 mL), 75 C, 8 h, 1 atm O
2
a
and 1 mmol nitrobenzene.
Reaction conditions: benzyl alcohol (1 mmol), catalyst (2 mol%),
ꢀ
TEMPO (0.09 mmol), acetonitrile (5 mL), 75 C, 1 atm O
nitrobenzene. 50 mg of SPS particles. 0.5 mmol of Na
added.
2
and 1 mmol
CO was
b
c
2
3
recycles, and the yield of benzaldehyde was 99.1% aer ten
runs. The powder XRD patterns (Fig. S3†) and FESEM images
(
Fig. S4†) of the catalyst aer 10 cycles show that the overall
structure of the material remains intact aer the catalytic
experiments, indicating that the SPS–Cu(II)@Cu (BTC) catalyst
is quite stable under the base-free reaction conditions. Aer 11
cycles, the conversion of benzyl alcohol decreased from 99.1%
To investigate the catalytically active components of the
SPS–Cu(II)@Cu (BTC) composite, a series of controlled exper-
3
2
3
2
iments were carried out. The results are summarized in Table 1.
Without catalysts (Table 1, entry 2) or just using the SPS
template (Table 1, entry 3) as a catalyst, no benzyl alcohol is
(aer 10 cycles) to 86.4% (aer 11 cycles) with the selectivity of
converted to benzaldehyde. Pure Cu
3 2
(BTC) results in a very low
benzaldehyde more than 99% and the structure of the catalyst
were well maintained. Aer 18 cycles, the structure of the
SPS–Cu(II)@Cu (BTC) composite was broken (Fig. S5†). In
3 2
contrast, in the case of the SPS–Cu(II) sample (Fig. 5), the
benzaldehyde yield decreases signicantly to 68% aer only
three cycles.
The metal leaching tests were also carried out to further
verify the stability of the SPS–Cu(II) and SPS–Cu(II)@Cu (BTC)
3 2
catalysts, in which the solid catalyst was centrifuged and
removed aer 4 h and the reaction continued. The results
showed that aer the removal of the SPS–Cu(II) catalyst, the
yield of benzaldehyde increased from 23% to 65% during the
next 4 h aer centrifugation, indicating obvious copper leach-
ing in the catalytic reaction. However, in the case of the SPS–
3 2
Cu(II)@Cu (BTC) catalyst, the yield of benzaldehyde was 21%
in the rst 4 h, and no further conversion of benzyl alcohol
could be observed when the reaction continued for another 4 h
benzaldehyde yield of about 4% (Table 1, entry 4) in the absence
52
of base additives, which agrees with the previous studies.
Trace benzaldehyde is also obtained when using a simple
mixture of SPS and Cu (BTC) as the catalyst (Table 1, entry 5),
3
2
suggesting that these two ingredients or the mutual inuence
between them have almost no catalytic activity for the aerobic
oxidation of alcohols under such base-free reaction conditions.
When graing Cu(II) ions onto the SPS surface, the obtained
SPS–Cu(II) (Table 1, entry 6) exhibits a benzaldehyde yield of
more than 99% aer a 8 h reaction time, which is the same as
that of the SPS–Cu(II)@Cu
Cu(II) interface is the main catalytically active component of
the SPS–Cu(II)@Cu (BTC) catalyst. Interestingly, aer adding
.5 mmol of Na CO into the SPS–Cu(II)/TEMPO catalytic system
3 2
(BTC) catalyst, indicating that the
3
2
0
2
3
(Table 1, entry 7), the yield of benzaldehyde is reduced to 79%,
suggesting that the acid conditions are more favorable for the
aerobic oxidation of benzyl alcohol in this catalytic system.
These results are in conformity with the previously reported
without the solid catalyst, suggesting that the porous Cu (BTC)2
3
shell efficiently protected the heterogenous catalyst from metal
leaching as well as provided mass transfer channels during the
catalytic reaction.
53–56
catalytic mechanism,
in which the oxoammonium cations
formed from the synergy of the Cu(II) interface and TEMPO
played a key role in the oxidation of alcohols.
3 2
To sum up, the SPS–Cu(II)@Cu (BTC) catalyst with such a
In order to verify the effect of the Cu (BTC) shell on stabi-
3
2
well-designed structure exhibited excellent catalytic activity,
high stability and good recyclability. It is also important to
emphasize that the sulfonation treatment of the SPS micro-
lizing the active Cu(II) interface, the recyclability of the SPS–
Cu(II) and SPS–Cu(II)@Cu (BTC) catalysts was examined for the
3
2
selective oxidation of benzyl alcohol. Aer each reaction cycle,
the catalysts were separated by centrifugation, dried at room
temperature, and then reused for a consecutive run under the
same reaction conditions. Fig. 5 shows that the SPS–Cu(II)
2+
spheres was designed to gra Cu ions to form the active Cu(II)
interface as well as provide acid conditions for high catalytic
2+
efficiency. The attached Cu ions also played a dual role in
3 2
the formation of the SPS–Cu(II)@Cu (BTC) core–shell struc-
tures. Firstly, the copper ions as the active sites produced high
@
3 2
Cu (BTC) catalyst remains substantially active for over 10
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Table 2 Selective oxidation of various alcohol substrates using the
a
3 2
SPS–Cu(II)@Cu (BTC) catalyst with molecular oxygen
Time
(h)
Yield
(%)
Selectivity
(%)
Entry
1
Substrates
Products
8
>99
>99
>99
Fig. 6 FESEM images of the SPS–Cu(II)@CuBDC composite under
different magnifications.
2
10
>99
show 100% conversion to their corresponding aldehydes aer a
required reaction time. Aerobic oxidation of heterocyclic
alcohol such as pyridin-2-yl-methanol (Table 2, entry 4) exhibits
excellent pyridine-2-carboxaldehyde yield (>99%) as well. The
allylic alcohol reactions also conrm that both cinnamyl
alcohol (Table 2, entry 5) and 3-methyl-2-buten-1-ol (Table 2,
entry 6) result in complete conversion with >99% selectivity of
the corresponding aldehydes without disturbing their C]C
double bonds. However, secondary alcohols such as 1-phenyl-
ethanol, 2-cyclohexen-1-ol and cyclopentanol (Table 2, entries
3
4
9
82
>99
>99
12
>99
5
6
9
>99
>99
>99
>99
7
–9) are difficult to oxidize to their corresponding ketones in the
11
1,7
present protocol, which is probably due to steric interactions.
The scope of the aerobic oxidation can be extended by simply
adjusting the copper component or just changing the reaction
7
8
8
8
3
3
>99
>99
conditions. The SPS–Cu(II)@Cu
3 2
(BTC) catalyst performance is
26,27
well consistent with that of copper salt catalysts,
further
indicating that Cu(II) is the main active component of the SPS–
Cu(II)@Cu (BTC) catalyst.
3
2
9
8
4
>99
Different MOF shells can be easily prepared to form a variety
of SPS–Cu(II)@MOF catalysts. For example, a novel dumbbell-
shaped SPS–Cu(II)@CuBDC composite with a laminated CuBDC
shell (Fig. 6) was synthesized. The characteristics of CuBDC
were conrmed by powder XRD (Fig. S6†). The catalytic activity
of SPS–Cu(II)@CuBDC (Table 1, entry 8) for the aerobic oxida-
tion of benzyl alcohol under the same conditions is comparable
to that of the SPS–Cu(II)@Cu (BTC) composite, owing to the
existence of the effective mass transfer channels provided by the
porous structure of CuBDC and the interstices among the
adjacent CuBDC nanosheets. The results also conrmed that
the Cu(II) interface between the SPS core and the MOF shell was
the active component of the SPS–Cu(II)@MOF catalyst. To
extend the catalytic system, the obtained SPS–Cu(II)@CuBDC
composite was used to catalyse aerobic homocoupling of phe-
nylboronic acid to biphenyl at room temperature, and high
catalytic activity (85% conversion, 100% selectivity) was ach-
ieved aer reaction for 12 h, indicating that the multifunctional
structure of the SPS–Cu(II)@MOF catalyst can be applied for
different catalytic reactions with high efficiency.
a
Reaction conditions: alcohol (1 mmol), SPS–Cu(II)@Cu
3
(BTC)
2
(2
2
ꢀ
mol% Cu), TEMPO (0.09 mmol), acetonitrile (5 mL), 75 C, 1 atm O
and 1 mmol nitrobenzene.
3
2
catalytic efficiency. Furthermore, as one of the most important
starting materials, the Cu ions initiated the nucleation of the
Cu (BTC) nanocrystals. The MOF shells stabilized the linkage
between the copper ions and the sulfo groups on the surface of
the SPS template, which enhanced the catalytic recyclability.
3 2
Moreover, the porous Cu (BTC) shell provided uniform pore
channels to accelerate the mass transfer in the catalytic reaction
process and further improved the catalytic performance of the
SPS–Cu(II)@Cu (BTC) catalyst.
To explore the scope of the novel catalytic system, the cata-
lytic activities of the SPS–Cu(II)@Cu (BTC) composite assisted
by TEMPO as the co-catalyst for the selective oxidation of
various alcohol substrates were also tested under the same
conditions. The catalytic results are summarized in Table 2. It
can be observed that the corresponding carbonyl compounds
are the sole detectable products under such base-free reaction
2
+
3
2
3
2
3
2
4. Conclusions
conditions. Specically, p-substituted benzylic alcohols such as In summary, a novel heterogeneous core–shell SPS–Cu(II)
p-methylbenzyl (Table 2, entry 1), p-methoxy benzyl alcohols @Cu (BTC) catalyst with a controllable active Cu(II) interface
Table 2, entry 2) and 4-uorobenzyl alcohol (Table 2, entry 3) all was fabricated via a facile method under mild synthesis
3
2
(
conditions. The active Cu(II) interface generated by attaching
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2
+
Cu ions with –SO
3
H groups on the surface of the SPS cores was 13 H. L. Liu, Y. L. Liu, Y. W. Li, Z. Y. Tang and H. F. Jiang, J.
the main active component of the SPS–Cu(II)@Cu (BTC) cata-
Phys. Chem. C, 2010, 114, 13362–13369.
3
2
lyst. The excess –SO H groups provided acid conditions to 14 S. E. Allen, R. R. Walvoord, R. P. Salinas and M. C. Kozlowski,
3
enhance the catalytic efficiency. The porous Cu (BTC)2 shell
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formed by the coordination between a portion of Cu(II) ions and 15 S. Pande, A. Saha, S. Jana, S. Sarkar, M. Basu, M. Pradhan,
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3
H
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2
@
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Acknowledgements
We would like to thank the National High Technology Research
and the Development Program of China (863 Program) (no. 24 H. Miyamura, R. Matsubara, Y. Miyazaki and S. Kobayashi,
2013AA031702) and the Co-building Special Project of Beijing
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