Fabrication of a Fe3O4@SiO2@mSiO2-HPG-COOH-Pd(0) supported catalyst and its performance in catalyzing the Suzuki cross-coupling reaction

Article abstract of DOI:10.1039/c4nj02407a

In this paper, magnetic Fe₃O₄@SiO₂@mSiO₂ microspheres with core-shell structure were chosen as a catalyst support, then hyperbranched polyglycerol (HPG) was successfully grafted onto the exterior surface and mesopore wall of this material under the catalysis of isopropanol aluminum. Subsequently, the terminal hydroxyl groups of HPG were successfully transformed into carboxyl groups after modification with succinic anhydride. Ultimately, palladium nanoparticles (Pd NPs) were successfully anchored onto the surface of the aforementioned magnetic Fe₃O₄@SiO₂@mSiO₂ microspheres with high density carboxylic HPG, nanocrystallization by the complexation between Pd²⁺ ions and carboxyl groups and the subsequent reduction, a novel Fe₃O₄@SiO₂@mSiO₂-HPG-COOH-Pd(0) supported catalyst was successfully obtained. This novel supported Pd NP catalyst is very conducive to the transference and exchange of each component in the reaction system for the orderly mesoporous opening structure. Furthermore, the introduction of a magnetism nucleus can provide convenient magnetic separation. More importantly, the numerous terminal carboxyl groups on the surface of the magnetic Fe₃O₄@SiO₂@mSiO₂ microspheres can provide plenty of sufficient binding sites for Pd NPs, and the unique hyperbranched structure is very conducive to capture uniformly dispersed nanosized palladium and can effectively enhance the catalytic activity and stability. Research indicates that this novel supported Pd NP catalyst not only possesses extremely high Pd NPs loading capacity but also shows remarkable catalytic activity to the Suzuki cross-coupling reaction between aryl halides and phenylboronic acid. Simultaneously, the catalytic activity of this supported catalyst did not show evident loss after being used at least eight times.

Full text of DOI:10.1039/c4nj02407a

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Page 1 of 24
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DOI: 10.1039/C4NJ02407A
1
Fabrication of Fe O @SiO @mSiO -HPG-COOH-Pd(0) Supported
3
4
2
2
Catalyst and Its Performance in Catalyzing Suzuki Cross-Coupling
Reaction
a,b
a,b
a,b
a,b
a,b
a,b
a,b
Wei Li, Yi Tian, Baoliang Zhang, Lei Tian, Xiangjie Li, Hepeng Zhang, Nisar Ali,
a,b
Qiuyu Zhang*,
a
Department of Applied Chemistry, College of Science, Northwestern Polytechnical University,
Xi’an 710072, China.
b
Key Laboratory of Space Physics and Chemistry, Ministry of Education, Northwestern
Polytechnical University, P. O. Box 624, Xi’an 710072, China.
3 4 2 2
Abstract: In this paper, magnetic Fe O @SiO @mSiO microspheres with coreꢀshell
structure were chosen as catalyst support, then the hyperbranched polyglycerol (HPG) was
successfully grafted onto the exterior surface and mesopore wall of this material under the
catalysis of isopropanol aluminum. Subsequently, the terminal hydroxyl groups of HPG were
successfully transformed into carboxyl groups after the modification with succinic anhydride.
Ultimately, palladium nanoparticles (Pd NPs) were successfully anchored onto the surface of
aforementioned magnetic Fe
3 4 2 2
O @SiO @mSiO microspheres with carboxylic HPG in the
2
+
form of high density and nanocrystallization by the complexation between Pd ions and
carboxyl
Fe @SiO
groups
and
the
following
reduction,
and
a
novel
3
O
4
2
@mSiO ꢀHPGꢀCOOHꢀPd(0) supported catalyst was successfully obtained. This
2
novel supported Pd NPs catalyst is very conducive to the transference and exchanges of each
component in the reaction system for the orderly mesoporous opening structure. Furthermore,
the introduction of magnetism nucleus can provide a convenient magnetic separation. More
1
Phone: +86ꢀ029ꢀ88431675, Fax: +86ꢀ029ꢀ88431675, Eꢀmail: qyzhang@nwpu.edu.cn (Qiuyu Zhang).
1

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importantly, the plenty of terminal carboxyl groups on the surface of magnetic
Fe O @SiO @mSiO microspheres can provide plenty of sufficient binding sites for Pd NPs,
3
4
2
2
and the unique hyperbranched structure is very conducive to the capture of nanoꢀsized
palladium in the form of good dispersion and can effectively enhance the catalytic activity
and stability. Research indicates that this novel suported Pd NPs catalyst not only possesses
extremely high Pd NPs loading capacity but also shows remarkable catalytic activity to
Suzuki crossꢀcoupling reaction between aryl halides and phenylboronic acid. Simultaneously,
the catalytic activity of this supported catalyst would not show evident loss after being used
at least eight times.
Keywords: magnetic response, mesoporous silica, polyglycerol, palladium, Suzuki coupling reaction
1
Itroduction
In the past decades, because mesoporous silica retains many advantages such as high specific
surface area and pore volume, tunable pore size, favorable biocompatibility, superior permeability,
fast mass transfer and easy of surface modification, etc, it had been widely applied in separation [1–
4
], catalysis [5–8], sensor [9–12], biomedicine [13–16] and imprinting [17–20], etc. Kros et al.[21]
prepared a folic acidꢀmodified mesoporous silica nanoparticles, then this material was successfully
applied in cellular and nuclear targeted drug delivery. Patel et al.[22] prepared the CoꢀB nanoparticle
catalysts supported on mesoporous silica and study the effect of support pore structure on the
2
catalytic properties in H production by hydrolysis of ammonia borane. Biesuz et al.[23] synthesized
a novel DFOꢀSAM on mesoporous silica for iron sensing by a new oneꢀpot synthesis, and the
kinetics of sorption reveals that the iron uptake is relatively fast, around 100 min at pH=2.5, from the
sorption profile of iron(III), the estimated capacity of the product obtained under optimized
ꢀ1
conditions was higher than 0.3 mmol·g . Connor et al.[24] synthesized a class of mesoporous silica
material (FDUꢀ12) with different pore sizes and amine functionalization, then they determine its
potential to differentially adsorb and separate dairy proteins. It was shown that the pore size of
unfunctionalised FDUꢀ12 is an important factor in the protein adsorption capacity. In this context, the
application of mesoporous silica materials is throughout the various fields.
Mesoporous silica has indeed shown its own unique superiorty in its applications, but its recycle is
2

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certainly a important issue to be resolved. Undoubtedly, it is one of the most effective means to fulfill
the convenient separation of mesoporous silica by introducing magnetic nucleus into mesoporous
silica microspheres, certainly, it has attracted many attentions of researchers [25-29]. Recently, it is a
hot subject in the research field to use magnetic mesoporous microspheres as catalyst carriers.
However, simple surface functionalization is very difficult to simultaneously achieve the
nanocrystallization of noble metal catalyst and super high density loads. In addition, precious metals
are particularly vulnerable during use, which would reduce the activity of catalyst to some extent and
is obviously considered wasteful. Consequently, it is particularly essential to perform a specific
surface modification on the surface of mesoporous microspheres to improve the load density, use
stability and catalytic activity of supported catalyst. Zhao et al.[30] reported the fabrication of
multifunctional microspheres which possess a core of nonporous silicaꢀprotected magnetite particles,
transition layer of active gold nanoparticles, and an outer shell of ordered mesoporous silica with
perpendicularly aligned pore channels. The unique nanostructure made the microsphere to be a novel
stable and approachable catalyst system for various catalytic industry processes. Cravotto et al.[31]
prepared a highly functionalized βꢀCyclodextrinꢀGrafted Silica by the microwave irradiation.
Bahnemann et al.[32] modified the mesoporous silica via the coꢀcondensation of
2
2
ꢀ(4ꢀchlorosulfonylphenyl)ethyltrimethoxysilane and tetraethyl orthosilicate, then the TiO particles
were immobilized on the mesopore channels. Ha et al.[25] synthesized a coreꢀshell magnetic
mesoporous silica hybrid nanoparticles, then the boronꢀspecificcisꢀdiol was produced on this material
surface by further modification with a glycidol reagent.
Based on our fundamental researches before, a good strategy to resolve aforementioned problems
had been proposed, which is to modify the surface of magnetic mesoporous silica microspheres with
hyperbranched structure. To our knowledge, it can introduce plenty of ligands to the material surface,
which can effectively capture noble metal ions and stabilize the noble metal nanocatalyst for the
special spatial conformation. Previously, the grafting of hyperbranched polyglycerol on the surface
3 4 2
of magnetic Fe O @SiO microspheres had been successively carried out by Xu, Neoh and Li et
al.[33-35], and all the results are satisfactory. Unexpectedly, it is not satisfied to graft hyperbranched
polyglycerol on the surface of magnetic mesoporous silica microspheres by the same method, which
is mainly because the presence of a large number of mesoporous opening structure in the
mesoporous silica coating reduces the network stability in the potassium methoxide system to some
3

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extent. Therefore, the mesoporous silica coating would be easily etched by the highly alkaline
potassium methoxide, and the complete coreꢀshell structure grafted with hyperbranched polyglycerol
can not be obtained. Through repeated experiments and tests, aluminium isopropoxide was chosen as
the catalyst, ultimately, the hyperbranched polyglycerol was successfully grafted chemically on the
surface of magnetic mesoporous silica microspheres but without significant influence to the
morphology of mesoporous silica coating. On this basis, the terminal hydroxyl groups of
hyperbranched polyglycerol were successfully transformed into carboxyl groups by the modification
2
+
of succinic anhydride, which can provides a large number of binding sites for capturing Pd ions.
Because of the effective combination between orderly mesoporous opening structure and
hyperbranched segment structure, this asprepared novel supported Pd NPs catalyst had displayed
more superior performance compared with conventional supported catalyst, and this may make its
due contribution to the effective reduction of useꢀcost.
2
Experimental
2
.1 Reagents and Materials. Ferric chloride hexahydrate (FeCl
3 2
·6H O), sodium acetate (NaOAc),
trisodium citrate, tetraethyl orthosilicate (TEOS), NH ·H O (28 wt%, A. R.),
3
2
hexadecyltrimethylammonium bromide (CTAB), aluminium isopropoxide (98 wt%), anhydrous
tetrahydrofuran (THF), anhydrous dioxane, glycidol (96 wt%), γꢀaminopropyltriethoxysilane
(
APTES), anhydrous ethanol, PdCl , sodium borohydride (NaBH ), and all of the chemical reagents
2
4
were purchased from SigmaꢀAldrich and were used without further treatment.
.2 Modification of Fe @SiO @mSiO with Amino-Group. The magnetic mesoporous
Fe @SiO @mSiO composite microspheres, which were prepared by optimizing our previous
method [36], were functionalized by APTES to introduce amonoꢀgroups on the material surface.
Primarily, 1.0 g of Fe @SiO @mSiO was uniformly dispersed in 80 mL of anhydrous ethanol
2
3
O
4
2
2
3
O
4
2
2
3
O
4
2
2
with the aid of ultrasound. Then, 4 mL of APTES was dropwise added into aforementioned
suspension, and the system was kept for 12 h under vigorous agitation at 45℃. After the reaction
finished, the aminoꢀfunctionalized composite microspheres were collected by applying an external
magnet and washed 3~5 times with ethanol. Finally, the asꢀprepared sample was dried for 24 h via
the vacuum freezeꢀdrying technology, then the dried aminoꢀfunctionalized magnetic mesoporous
4

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Fe
3 4 2 2
O @SiO @mSiO composite microspheres were obtained. (The product was expressed as
Fe O @SiO @mSiO ꢀNH .)
3
4
2
2
2
2
.3 Grafting of Hyperbranched Polyglycerol on Fe
hyperbranched polyglycerol on the surface of coreꢀshell magnetic mesoporous Fe
composite microspheres was carried out by chemical grafting and polycondensation. Typically,
aforementioned Fe @SiO @mSiO ꢀNH were uniformly dispersed in 150 mL of anhydrous
3
O
4
@SiO
2
@mSiO
2
-NH
2
. The grafting of
3
O
4
@SiO @mSiO
2
2
3
O
4
2
2
2
dioxane with the aid of ultrasound, then 0.5 g of isopropylꢀaluminium was added into this suspension
when the temperature was rose to 80℃. At the same time, the mixed system need to been protected
under nitrogen atmosphere for 2 h to remove isopropanol generated in reaction. Subsequently, the
dioxane was removed via the vacuum distillation at 40℃, and the magnetic mesoporous composite
microspheres with a lot of active sites were successfully obtained. Then, the activated composite
microspheres were redispersed into 150 mL of anhydrous dioxane under ultrasound. Subsequently,
the temperature was adjusted to 80℃, and 20 mL of anhydrous dioxane containing 1 mL of glycidol
was slowly added into the mixed system during 7 h. Then, the system was kept for 24 h at 80℃
under the protection of nitrigen atmosphere. After the reaction finished, the asꢀprepared sample was
collected by applying an external magnet and quenched with ethanol. Then, the product was washed
several times with ethanol and distilled water to remove the unreacted glycidol and its oligomer
residue on the surface of magnetic mesoporous composite microspheres. Ultimately, the asꢀprepared
product was dried for 24 h via the vacuum freezeꢀdrying technology, then the hyperbranched product
was successfully obtained. (Here, it was expressed as Fe
3
O
4
@SiO
2
@mSiO
2
ꢀHPG.)
2 2
@nSiO @mSiO ꢀHPG
2
.4 Carboxyl Functionalization of the Fe @SiO @mSiO
3
O
4
2
2
-HPG. Fe
3
O
4
was successfully modified with succinic anhydride according to the method in literature [33].
Typically, 1.0 g of Fe @SiO @mSiO ꢀHPG was uniformly dispersed into 150 mL of pyridine
3
O
4
2
2
with the aid of ultrasound, then 0.6 g of succinic anhydride was added into aforementioned
suspension, and this mixed system was stirred vigorously for 8 h under argon atmosphere in the dark.
Subsequently, the product was collected by applying an external magnet and eluted several times
with methanol. Finally, the product was washed several times with ethanol and distilled water and
dried for 24 h via the vacuum freezeꢀdrying technology. (Here, it was expressed as
3 4 2 2
Fe O @SiO @mSiO ꢀHPGꢀCOOH.)
5

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2
.5 Preparaton of Fe
3 4 2 2
O @SiO @mSiO -HPG-COOH-Pd(0) supported catalyst. Palladium
nanoparticles were successfully anchored on the surface of Fe O @SiO @mSiO ꢀHPGꢀCOOH by
3
4
2
2
2
+
2+
the complexation between Pd ions and carboxyl groups and the following reduction of Pd ions.
Primarily, 0.8 g of Fe @SiO @mSiO ꢀHPGꢀCOOH and 70.0 mg of PdCl were uniformly
3
O
4
2
2
2
dispersed into 60 mL of anhydrous ethanol with the aid of ultrasound, then the mixed system was
stirred for 24 h under the speed of 260 rpm at 45℃. After the reaction finished, the product was
collected by applying an external magnet and washed 2ꢀ3 times with distilled water to remove
2
+
dissociative Pd ions. Subsequently, the product was redispersed into 60 mL of anhydrous ethanol,
then 100 mg of NaBH was added into this system, and this mixed system was stirred vigorously for
2 h at room temperature. After the reaction finished, the asꢀprepared product was collected by
applying an external magnet and washed several times with distilled water and dried for 24 h via the
vacuum freezeꢀdrying technology. (Here, it was represented as
Fe @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0).)
.6 Suzuki coupling reactions of aryl halides. The Suzuki crossꢀcoupling reactions were carried
out under the catalysis of Fe O @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0). The precursor solution was
4
1
3
O
4
2
2
2
3
4
2
2
composed of 1.2 mmol of phenylboronic acid, 1.0 mmol of aryl halides (chlorobenzene,
bromobenzene, iodobenzene, 4ꢀchloroacetophenone, 4ꢀchloronitrobenzene, 4ꢀchlorotoluene,
3
ꢀchlorotoluene and 2ꢀchlorotoluene), 0.2764 g of K
right amount of Fe @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0) (0.075 mol% Pd) was added into the
precursor solution, and the system with a cooling reflux device was vigorously stirred for 8, 10 and
2 h at 80℃, respectively. Finally, the reaction yields were obtained by high performance liquid
2 3
CO and 10 mL of isopropyl alcohol. Then,
3
O
4
2
2
1
chromatography (HPLC).
The yields of Suzuki coupling reactions were calculated by following formulas:
ꢀꢁ
/ꢂꢁ
ee = ꢀ
× 100%
(1)
ꢃꢄ/ꢂ
ꢃꢄ
W
p
is the mass of the produced biphenyl or its derivatives after the reaction, and Wah is the mass of
aryl halides before the reaction. M and Mah is the corresponding relative molecular mass of biphenyl
or its derivatives) and aryl halides, respectively.
p
(
W = C × V
ꢆ
(2)
ꢅ
ꢆ
6

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C
p
is the mass concentration of the produced biphenyl or its derivatives after the reaction, and V
p
is
the volume of the coupling reaction system after the reaction.
ꢇꢇꢇ
ꢀ
ꢊ ∙ꢋ
ꢌ
ꢊꢈ
ꢍꢈ
ꢈ
ꢈ
= ꢊ
= ꢊ = ꢍ
(3)
ꢇꢇꢇ
ꢀ
ꢈꢉ∙ꢋ
ꢈꢉ
ꢌ
ꢈꢉ
ꢈꢉ
C is the known mass concentration in the reference standard solution, and V is the volume of
pr
s
testing solution. A
p
and Apr is the corresponding peak areas of biphenyl or its derivatives in the
reaction solution and in the standard solution, respectively.
The following formula can be obtained by joining three equations of above:
ꢍ ꢊ ꢋ ꢂ
ꢈ
ꢈꢉ
ꢈ
ꢎꢏ
ee = ꢍ
(4)
ꢂ ꢀ
ꢎꢏ
ꢈꢉ
ꢈ
TEOS
CTAB, TEOS
NH OH
thermal treatment
NH OH
4
4
Fe O
Fe O @SiO
Fe O @SiO @mSiO /CTAB
Fe
3
4 2 2
O @SiO @mSiO
3
4
3
4
2
3
4
2
2
n
io
t
la
y
x
o
r
d
S
y
E
OO
h
T
O
O
O
O
e
P
OO
O
O
r
O
A
O
O
O
O
O
O
H
2
N
NH
2
NH
2
N
HN
2
H
O
O
O
O
O
O
OO
O
H
2
N
2
2
NH
O
H
N
2
N
N
H
H
2
NH
NH
NH
NH
NH
NH
2
O
2
H
2
N
O
O
O
2
O
H
2
N
2
OO
O
O
O
O
OO
O
H
2
N
2
O
O
O
H
2
N
2
H
2
N
2
O
O
O
OO
O
O
H
2
N
NHN2H
2
O
O
O O
H
2
N
O
O
O
O O
H
2
N
NH
2
2
O
H
2
N
NH
2
NH
O
O
O
O
O
O
O
O O
O
O
Fe O @SiO @mSiO ꢀNH
3
4
2
2
2
Fe O @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0)
3
4
2
2
e
OH
id
x
o
p
o
O
r
p
o
is
m
CO OH
u
C
OOH
C
OO
O
H
in
COO
O
H
HO OC
O
H
OO
C
m
HOO
O
C
O
O
O
O
H
O
HO
lu
O
COO
H
O
C
OO
H
HO OC
O
O
O
CO OH
O
O
O
O
O
O
CO OH
COO
HO
a
HO OC
H
OO
C
O
HO
O
H
O
H
O
O
O
O
O
CO OH
O
H
O
O
O
O
H
O
H
O
O
O
O
O
O
O
O
H
O
O
OH
H
CO OH
C
OO
O
H
O
O
O
O
C
OO
O
H
O
O
O
O
O
H
O
O
O
HO OC
O
H
O
O
O
HO
HO
O
H
HO OC
O
O
COO
H
OO
O
C
CO OH
O
HO
OH
OH
O
O
HO OC
O
O
O
O
O
H
OO
C
O
CO OH
O
O
O
H
O
OH
O
CO OH
O
O
HOO
C
CO OH
CO OH
HOO
C
O
O
O
C OO H
HO
O
OH
HO OC
O
O
O
CO OH
O
O HO OC
O
O
O
O
H
OOC
O
O
O
O
COO
H
HO
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OH
O
O
O
O
O
O
O
O
CO OH
O
O
O
O
O
HO
HO
O
H
OO
C
O
O
O
O
O
HO OC
O
O
O
O
O
COO H
O
O
O
O
O
O
O
O
O
OH
O
H
OH
O
O
O
O
O
O
O
O
O
O
O
O
O
O
H
O
O
O
O
O
O
COO
H
O
O
CO OH
OH
OH
OH
HOO
C
O
O
O
O
O
O
HO OC
O
O
O
O
O
O
HO
OH
OH
O
O
O
O
O
O
O
O
HO OC
O
O
O
PdCl
2
O
O
O
O
O
O
O
O
O
CO OH
H
OO
C
O
O
O
O
O
O
H
C
OO H
OH
O
O
O
OH
O
H
O
O
O
O
O
O
O
O
O
O
O
H
OO
C
O
O
O
O
O
HO
O
O
O
COO
C OO OH
COO
H
H
HO OC
C
OOH
O
CO OH
CO OH
O
O
H
H
OO
C
CO OH
C
OO
HO OC
O
COO H
OO
H
O
OH
HO
HO
O
HO
O
HO
O
O
O
O
O
O
O
O
C
H
O
O
H
O
O
HOO
HOO
C
O
O
O
O
O
O
O
O
C
OO
H
HO OC
HO OC
O
O
O
O
O
O
O
HO
O
O
O
O
H
HO
OH
O
H
O
O
O
O
O
O
O
CO OH
C
O
O
O
O
O
HO
O
O
O
O
O
O
O
O
O
HOO
C
O
O
O
O
O
O
O
O
CO OH
CO OH
O
O
O
O
O
O
HO
O
O
H
O
O
O
O
CO OH
O
O
HO OC
O
O
O
O
COO H
O
HO
O
O
O
COO
HOO
H
O
O
HO
O
O
H
O
O
O
OH
O
O
O
O
O
O
O
O
O
O
O
O
O
HO OC
O
O
O
O
O
O
O
COO H
H
O
HO
OH
O
O
O
O
O
O
O
O
O
O
C
O
O
O
O
H
O
H
H
O
OH
OH
H
OO
C
O
O
O
HO OC
O
O
O
O
O
O
O
O
H
HO
HO OC HO OC
CO OH
H
O
O
COO
H
O
HO OC
OO
C
C
OO
H
HOO
C
HOO
C
H
OO
C
COO
H
HO OC
CO OH
O
CO OH
COO
H
CO OH
H
OO C
HO OC
3
Fe O
4 2 2
@SiO @mSiO ꢀHPG
Fe
3
O
4
@SiO
2
@mSiO
2
ꢀHPGꢀCOOH
Fe
3
O
4
@SiO
2
@mSiO
2
ꢀHPGꢀCOOHꢀPd(II)
Scheme 1. Preparation process of the Fe
3
O
4
@SiO
2
@mSiO
2
ꢀHPGꢀCOOHꢀPd(0).
2
.7 Characterization. Fourier transformꢀinfrared (FTꢀIR) spectra of samples were recorded on a
PerkinꢀElmer 580BIR spectrophotometer using the KBr pellet technique. Xꢀray powder diffraction
(XRD) analysis was performed on a Bruker AXS D8ꢀadvance Xꢀray diffractometer with Cu Kα
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2
radiation. N adsorption/desorption isotherms were obtained on a TriStar II 20 apparatus. The pore
size distribution was calculated from the adsorption branch of the N adsorption/desorption isotherm
2
and the BarretꢀJoynerꢀHalenda (BJH) method. The BrunauerꢀEmmettꢀTeller (BET) surface areas
were determined by using the datas between 0.05 and 0.35 just before capillary condensation, and the
pore volume was obtained by the tꢀplot method. The magnetic moment was recorded at 300 K on a
JPMSꢀ9HꢀEVERCOOL (QD) vibratingꢀsample magnetometer (VSM). Transmission electron
microscopy (TEM), energy dispersive Xꢀray (EDS) spectra and transmission electron diffraction
patterns were performed on a JEOL JEL2010 electron microscope at 200 kV. Scanning electron
microscope (SEM) were performed on a FEISIRION 200 fieldꢀemission microscope. Xꢀray
photoelectron spectroscopy (XPS) datas were collected to examine the chemical states of the
multiꢀcomponent photocatalyst with an Axis Ultra instrument (Kratos Analytical, Manchester, U.K.)
ꢀ6
under ultrahigh vacuum condition (<10 Pa) and using a monochromatic Al Kα Xꢀray source
1486.6 eV). HPLC analysis was performed on a HITACHI Lꢀ2400. The ICPꢀAES was finished on
an ICAP6300.
(
3
Results and discussion
3
.1 Characterization of the Morphology of Materials
a
b
c
d
e
f
g
4000 3500 3000 2500 2000 1500 1000 500
ꢀ1
Wavenumbers (cm )
Fig.1 FTIR spectra of Fe
O
3 4
3 4 2 3 4 2 2 3 4 2 2
(a), Fe O @SiO (b), Fe O @SiO @mSiO /CTAB (c), Fe O @SiO @mSiO (d),
Fe O @SiO @mSiO ꢀNH (e), Fe O @SiO @mSiO ꢀHPG (f) and Fe O @SiO @mSiO ꢀHPGꢀCOOH (g).
3
4
2
2
2
3
4
2
2
3
4
2
2
The preparation process of Fe
3
O
4
@SiO
2 2
@mSiO ꢀHPGꢀCOOHꢀPd(0) supported catalyst was
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shown as scheme 1. At first, FTIR was applied to characterize the samples collected in each step (as
shown in Fig.1aꢀg). It shows that all seven spectra have the adsorption peak corresponding to the
ꢀ
1
characteristic absorption of FeꢀO bond at 580 cm . After being coated with silica, the characteristic
absorption peaks corresponding to the antisymmetric and symmetric stretching vibration of SiꢀOꢀSi
ꢀ1
bond in oxygenꢀsilica tetrahedron can be clearly observed at 1088 and 799 cm (as Fig.1bꢀg). The
ꢀ1
characteristic absorption peaks at 2923 and 2852 cm in Fig.1c can be attributed to the symmetric
and antisymmetric stretching vibrations of νCꢀH in methyl and methylene of CTAB which exists in the
outer shell. In comparison, the characteristic absorption peaks of νCꢀH disappeard after the
highꢀtemperature calcination, which indicates that the porogenic agent was removed completely after
the highꢀtemperature calcination (as Fig.1d). Subsequently, the magnetic mesoporous
Fe
absorption peak appeared at 900 cm (Fig.1e), which must be attributed to the stretching vibration of
NꢀH. Besides, the adsorption peaks correspongding to the symmetric and antisymmetric stretching
3 4 2 2
O @SiO @mSiO composite microspheres were functionalized with aminosilane, and a new
ꢀ1
γ
vibrations of νCꢀH in methylene can be observed again, which indicates that the
aminoꢀfunctionalization had been successfully achieved. Furthermore, HPG was grafted on the
3 4 2 2
surface of the magnetic mesoporous Fe O @SiO @mSiO composite microspheres by the
ringꢀopening reaction and the following polycondensation, which can introduce plenty of hydroxyl
groups to the surface of this composite microsphere. Compared with Fig.1e, the characteristic
ꢀ1
absorption peaks at 2923 and 2852 cm had been increased (as Fig.1f), and the adsorption peak of
ꢀ1
ν
OꢀH at 3437 cm had also increased to some extent, both of which indicates that the HPG had been
successfully grafted on the surface of magnetic mesoporous Fe @SiO @mSiO composite
3
O
4
2
2
microspheres. Finally, the terminal hydroxyl groups of HPG were transformed into carboxyl groups
by succinic anhydride. As shown as Fig.1g, a new characteristic absorption peak corresponding to
ꢀ1
the stretching vibration of νC=O can be observed clearly at 1737 cm , which indicates that the
terminal hydroxyl groups had been successfully transformed into carboxyl groups, and the
Fe
3
O
4
@SiO
2
@mSiO
2
ꢀHPGꢀCOOH had been successfully obtained.
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110
105
a
100
b
9
5
0
5
0
c
9
8
8
0
100
200
300
400
500
600
700
800
900
Tem (℃ )
Fig.2 TG analysis of Fe
3
4 2 2 3 4 2 2
O @SiO @mSiO (a), Fe O @SiO @mSiO ꢀHPG (b) and
Fe O @SiO @mSiO ꢀHPGꢀCOOH (c).
3
4
2
2
To further confirm the successful implementation of the HPG grafting and carboxyl modification
on the surface of magnetic mesoporous Fe @SiO @mSiO composite microspheres,
3
O
4
2
2
thermogravimetric analysis (TG) was necessary to carry out. As shown as Fig.2, the trace amount of
weight loss within the range of 100—200℃ is caused by the trace amount of water vapour adsorped
by mesopore channels. Compared with Fig.2a, both of Fe
3 4 2 2
O @SiO @mSiO ꢀHPG and
Fe @SiO @mSiO ꢀHPGꢀCOOH has obvious weight loss within the range of 230—560℃, and the
3
O
4
2
2
latter (~7.4%) is more than the former(~6.2%) (as Fig.2b and Fig.2c), which demonstrates that the
HPG grafting and the carboxyl modification had been successfully achieved. Furthermore, both of
carboxyl
Fe @SiO
group
content
on
the
surface
of
Fe
3
O
4
@SiO
2
ꢀHPGꢀCOOH
and
3
O
4
2
@mSiO ꢀHPGꢀCOOH was measured by the inverse titration method on the EasyPlus
2
Titration of METTLER TOLEDO, and it is respectively 1.04 mmol/g and 2.13 mmol/g. Evidently,
the latter is much more than the former. Which indicates that some small HPG segments were
successfully introduced into the mesopore channels during the HPG grafting process, so the content
of carboxyl groups on Fe O @SiO @mSiO ꢀHPGꢀCOOH is much larger than that of
3
4
2
2
3 4 2
Fe O @SiO ꢀHPGꢀCOOH.
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3 4 3 4 2 3 4 2 2
Fig.3 The TEM images of Fe O (a), Fe O @SiO (b) and Fe O @SiO @mSiO (c).
Afterwards, the morphology of the microparticles obtained in every stage was characterized by
TEM. As shown as Fig.3a, the magnetic Fe particles prepared by solvothermal method exhibit
3
O
4
cauliflowerꢀlike morphology. The particle size is very uniform, which is about 200 nm. After being
coated with a thin layer of dense silica, the coreꢀshell structure can be clearly observed, and the
thickness of the silica coating is about 60 nm (as Fig.3b). After a layer of mesoporous silica coating
was coated on the outer surface of Fe O @SiO , the subsequent doubleꢀshell structure and mesopore
3
4
2
structure appeared, and the thickness of the outer shell is about 80 nm (as Fig.3c).
3 4 2 2 3
Fig.4 The TEM images of Fe O @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0) prepared in CH OK technique (a and b), in
11

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isopropanol aluminum technique (c). (d) is the corresponding electron diffraction patterns of c and the insert is the
magnified TEM image of c.
3
To the best of our knowledge, CH OK is the common catalyst for the HPG grafting on the surface
of silica, so the HPG grafting on the surface of Fe
3 4 2 2
O @SiO @mSiO was also carried out by us, then
this material was used to anchor Pd NPs catalyst. As shown as Fig.4a and Fig.4b, the doubleꢀshell
structure was destroyed basically after this process, and the palladium aggregations were clearly
observed near the supports. Which is because that the mesoporous silica coating had been etched for
3
the strong alkaline of CH OK in the HPG grafting process, so that this material was unable to anchor
the Pd NPs catalyst, and the etched material and palladium aggregations were obtained. Based on
this problem, after a lot of tests, aluminium isopropylate was found to be a facile catalyst which can
successfully catalyze the HPG grafting on the surface of mesoporous silica coating, so the
Fe O @SiO @mSiO ꢀHPGꢀCOOH was also successfully obtained after the modification with
3
4
2
2
succinic anhydride. Subsequently, Fe
NPs catalyst, and Fig.4c gives the TEM image of Fe
supported catalyst. Obviously, the Pd NPs with fcc structure (as Fig.4d), which partice size is about
~10 nm, were successfully anchored on the surface of the Fe @SiO @mSiO ꢀHPGꢀCOOH, and
3 4 2 2
O @SiO @mSiO ꢀHPGꢀCOOH was applied to anchor the Pd
3
4 2 2
O @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0)
5
3
O
4
2
2
there was no any palladium aggregation to be observed. Which indicates that aluminium isopropylate
is indeed a facile catalyst for the HPG grafting on the surface of mesoporous silica. Simultaneously,
the unique threeꢀdimensional conformation of the carboxylated HPG segments is very beneficial to
the uniform dispersion of Pd NPs on the carriers, which can significantly increase the Pd NPs
loading capacity, and the effective contact area between Pd NPs and reactants can also be increased
for the uniform dispersion of Pd NPs. Therefore, it will be very conducive to the enhancement of the
stability and catalytic activity of this novel supported Pd NPs catalyst. To further demonstrate the
successful immobilization of Pd NPs, the EDX was necessary to carried out. Fig.5 shows that
palladium indeed exists in the ultimate sample except iron, oxygen and silicon, which indicates the
successful synthesis of the novel Fe
addition, the palladium loading capacity of this supported catalyst was examined by ICPꢀAES and
was about 5.13 wt%, it was much larger than that of the previous Fe @nSiO @mSiO ꢀNH ꢀPd(0)
3 4 2 2
O @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0) supported catalyst. In
3
O
4
2
2
2
supported catalyst [25], which demonstrates that the HPG segments indeed played an important role
in anchoring Pd NPs catalyst on the surface of mesoporous silica.
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4000
3000
2000
1000
0
O
Si
Si
C
Pd
Pd
Pd
Pd
Fe
Fe
0
2
4
6
8
10
Energy (keV)
Fig.5 EDS spectrum of Fe O @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0) catalyst.
3
4
2
2
The XPS characterization provides further insight into the assessment of the surface composition
of Fe @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0) catalyst. The XPS spectrum in Fig.6a reveals that the
3
O
4
2
2
surface of this supported catalyst was consisted with O, C, Pd and Si, and it indicates that the Pd
catalyst should have been anchored on the surface of the coreꢀshell magnetic mesoporous
3 4 2 2
Fe O @SiO @mSiO microspheres. Two peaks at 339.1 eV and 334.0 eV in Fig.6b were
0
0
respectively attributed to Pd 3d3/2 and Bi 3d5/2, which indicates the existence of Pd(0) on the surface
of the supports. It well demonstrates that the Pd NPs catalyst had been successfully anchored on the
surface of the coreꢀshell magnetic mesoporous Fe
3
4
O @SiO
2 2
@mSiO microspheres.
O1s
0
a
b
0
Pd 3d
5
/2
Pd 3d
3/2
C1s
Pd3d
Si2p
800
600
400
200
0
342
340
338
336
334
332
330
Binding Energy (eV)
Binding Energy (eV)
Fig.6 XPS spectra of Fe
3
O
4
@SiO
2
2
@mSiO ꢀHPGꢀCOOHꢀPd(0) catalyst: (a) survey and (b) Pd3d.
Furthermore, Fe O @SiO , Fe O @SiO @mSiO and Fe O @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0)
3
4
2
3
4
2
2
3
4
2
2
2 2
were characterized by N adsorption/desorption, and the N adsorption/desorption isotherms were
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shown in Fig.7. The N
2
adsorption/desorption isotherms of coreꢀshell magnetic Fe
3
O @SiO
4 2
2
composite microspheres (Fig.7a) show a smooth curve, and the BET surface area is about 71.85 m /g.
The adsorption of the material to nitrogen is not obvious, which further indicates that the silica shell
of coreꢀshell magnetic Fe
is basically nonexistent. The N2 adsorption/desorption isotherms of magnetic mesoporous
Fe @SiO @mSiO composite microspheres is shown in Fig.7b, which displays a typical type
3 4 2
O @SiO composite microspheres is indeed dense and the porous structure
3
O
4
2
2
Ⅳ
isotherms with two inflection points in the relative pressure of 0.4 and 0.8, simultaneously, the
hysteresis loops can be observed obviously. All of which indicates the existence of the mosopore
2
3
structure. The BET surface area, BJH pore volume and pore size is ~203.98 m /g, ~0.2491 cm /g and
—10 nm, respectively. The appropriate surface area and pore volume can provide a favorable space
for anchoring Pd NPs. The adsorption/desorption isotherms of
Fe O @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0) supported catalyst is shown in Fig.7c. Compared with the
3
N
2
3
4
2
2
3 4 2 2 2
that of magnetic mesoporous Fe O @SiO @mSiO composite microspheres, the N adsorption
capacity on this material reduced to some extent, and three inflection points in the relative pressure
of 0.4, 0.8 and 0.9 can be observed clearly. Substantially, the main reason is that the magnetic
3 4 2 2
mesoporous Fe O @SiO @mSiO composite microspheres had changed more or less during the
HPG grafting process, so some smaller Pd NPs might be introduced into the mesopore channels.
Which ultimately leaded to the increase of some mesopores and the decrease of others. In addition,
2
the measured BET surface area, BJH pore volume and pore size is respectively ~117.27 m /g,
3
~
0.1809 cm /g and 3—13 nm, which is basically conformed to the aforementioned inference.
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1
80
60
40
20
00
1
b
c
1
1
1
80
6
0
a
40
20
0
0
.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P )
0
Fig.7 N adsorption/desorption isotherms of Fe O @SiO (a), Fe O @SiO @mSiO (b) and
2
3
4
2
3
4
2
2
Fe
O
3 4
@SiO
2
2
@mSiO ꢀHPGꢀCOOHꢀPd(0) (c), and the insert is the corresponding pore size distribution.
Afterwards,
Fe @SiO
characteristic diffraction peaks (2θ = 30.2°, 35.6°, 43.3°, 53.8°, 57.3° and 63.0°) corresponding to
220), (311), (400), (422), (511) and (440) reflections of inverse spinel Fe can be observed clearly
the
crystalline
structures
of
Fe
3
O
4
,
3
Fe O
4
@SiO
2
@mSiO
2
and
3
O
4
2 2
@mSiO ꢀHPGꢀCOOHꢀPd(0) were determined by XRD. As displayed in Fig.8, six
(
3 4
O
in all three curves. By comparing with Fig.8a, a broad peak around 2θ = 24° corresponding to the
characteristic diffraction peak of amorphous silica can be observed (Fig.8b and Fig.8c). After
anchoring Pd NPs catalyst, four characteristic diffraction peaks (2θ = 34°, 40°, 46° and 52°)
corresponding to (311), (111), (200) and (422) reflections of palladium can be observed (Fig.8c),
which also indicates the successful immobilization of Pd NPs with faceꢀcentered cubic structure. The
amount of Pd NPs is very low relative to the carriers so that the diffraction peaks is very weak.
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a
b
c
20
30
40
50
60
70
80
2θ
(deg.)
Fig.8 XRD pattern of Fe O (a), Fe O @SiO @mSiO (b) and Fe O @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0) (c).
3
4
3
4
2
2
3
4
2
2
To investigate the magnetic performance, Fe O , Fe O @SiO , Fe O @SiO @mSiO and
3
4
3
4
2
3
4
2
2
3 4 2 2
Fe O @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0) were characterized by VSM. Fig.9 shows that the
magnetic saturation of aforementioned materials is about 80, 63, 35 and 30 emu/g, respectively, and
the magnetization curves of four magnetic materials were confirmed a typical feature of
superparamagnetism as no hysteresis is observed in low fields, which indicates that any remanence
can not be observed after removing the external magnetic field. In summary, it indicates that all these
magnetic materials can fulfill the simple separation requirment with the aid of external magnetic
field. In addition, the inserted digital photos in Fig.9 show that this novel supported Pd NPs catalyst
can be redispersed inot the original system after removing the external magnetic field, which can
meet the application requirement well.
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100
a
b
c
d
8
0
60
40
20
0
ꢀ
20
40
60
80
100
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ30000 ꢀ20000 ꢀ10000
0
10000 20000 30000
Applied magnetic field(Oe)
Fig.9 Magnetization curve of Fe (a), Fe @SiO (b), Fe @SiO @mSiO (c) and
3
O
4
3
O
4
2
O
3 4
2 2
Fe
O
3 4
@SiO
2
@mSiO
2
ꢀHPGꢀCOOHꢀPd(0) (d) measured at room temperature. And the insert is the corresponding
magnetic seperation digital photos.
3
.2 The Invesigation of the Catalytic Activity of the Novel Supported Catalyst
First of all, to investigate the influence of reaction time to the yields of Suzuki coupling reaction, 8
h, 10 h and 12 h were respectively chosen as the reaction time, and the corresponding reactions were
finished at 80℃. Table 1 shows that the yield would increase to some extent as time goes on, then it
would remain unchanged after reaching the maximum. For instance, the reaction of iodobenzene can
be finished in 8 h, and the corresponding yield is about 99%. But the reactions of bromobenzene,
4
ꢀchloroacetophenone and 4ꢀnitrochlorobenzene can be finished in 10 h, and the corresponding yield
is about 91%, 96% and 92%, respectively. What is more, the reactions of chlorobenzene and its
methyl substituded derivatives would require 12 h or even longer, which is mainly because that the
electronegativity of the halogen substituent in aryl halide is weaker, the higher the reaction rate is,
and the shorter the reaction time is. The electronegativity of Cl, Br and I from strong to weak is Cl＞
Br＞I, so the corresponding reaction time of left would be longer than that of right.
Furthermore, the catalytic activity of this novel supported catalyst was also investigated by
catalyzing a series of Suzuki coupling reactions for 10 h at three different temperatures. Table 1
shows that almost all the reaction yields reached the maximum at 80℃, and they were basically the
same at 80℃ and 90℃, but it is relatively low at 70℃. The main reason is that both the breaking
and formation of the chemical bond need enough energy, theoretically, when the reaction is carried
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out at lower temperature, the energy supply is not enough to generate enough activated molecules to
participate in the reaction, but when the reaction is carried out at higher temperature, it would
generate more activated molecules for the sufficient energy supply in the system, so the reaction can
be carried out effectively. It should be noted that excessive temperature may result in the formation
of byꢀproducts, so we chose the appropriate temperature for this reaction, and here the reaction
temperature was chosen as 80℃.
Table 1 The Suzuki coupling reactions of aryl halide and phenylboronic acid catalyzed by
Fe O @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0) under different conditions.
3
4
2
2
R
R
Cat
X
B(OH)2
K CO
2
3
a
Yield (%)
h (80℃) 10 h (80℃) 12 h (80℃) 10 h (70℃) 10 h (90℃)
entry
Aryl halide
Product
8
1
2
3
99
80
34
99
91
53
99
91
62
88
75
14
99
90
54
I
Br
Cl
O
O
4
Cl
89
96
96
67
92
5
6
O N
Cl
80
17
92
26
92
29
73
1
96
26
2
2
O N
H
3
C
Cl
3
H C
Cl
7
8
28
30
44
50
46
53
14
21
44
50
H
3
C
3
H C
Cl
CH
3
CH3
a
HPLC yield based aryl halide.
Beyond that, substituent groups also have a big impact on the yield of reaction. By investigating
the Suzuki coupling reaction of chlorobenzene and its derivations, it is not hard to find that the
existence of electronꢀwithdrawing in aryl halides is very conducive to the reaction and the existence
of electronꢀdonating group in aryl halides would lead to lower reaction yield. As shown in Table 1,
the reaction yield of chlorobenzene is about 62%, and it is respectively about 96% and 92% to
4
ꢀchloroacetophenone and 4ꢀnitrochlorobenzene, but it is only about 26% to 4ꢀchlorotoluene. To the
best of our knowledge, the difference of substitutional position also has an obvious impact on the
reaction yield. By comparing the reaction yields of 4ꢀchlorotoluene, 3ꢀchlorotoluene and
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2
ꢀchlorotoluene, it can be found that 2ꢀchlorotoluene have the most high reaction yield (53%),
ꢀchlorotoluene is the second one (44%), and 4ꢀchlorotoluene is the lowest one (26%). This is
3
mainly because that methyl is orthoꢀpara directing group, thus higher reaction yield would be
obtained for methyl orthoꢀsubstituted chlorobenzene, which is basically matched with our previous
study [35].
100
8
0
0
0
0
0
6
4
2
a
b
c
1
2
3
4
5
6
7
8
Cycle times
Fig.10 Investigation of the stability of Fe O @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0) in catalyzing Suzuki coupling
3
4
2
2
reaction of iodobenzene (a), 4ꢀchloroacetophenone (b) and chlorobenzene (c), respectively. The reaction time is 8
h.
Fig.11 TEM image of the recovered Fe O @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0) supported catalyst after being used
3
4
2
2
several times.
To further investigate whether the carboxylated HPG segments are conducive to the enhancement
of the stability of this novel supported catalyst, it was regenerated and reused for several times.
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Fig.10 displays the corresponding yields of the reused supported catalyst in the reaction time of 8 h,
which indicates that the catalytic activity of this supported catalyst would not significantly decreased
after being used eight times, and the yield can still respectively reach up to 97%, 88% and 32% for
iodobenzene, 4ꢀchloroacetophenone and chlorobenzene after being used eight times. Furthermore,
the residual ratio to the original palladium content was 88.11% after the supported catalyst being
used eight times by ICPꢀAES, and the TEM image of this novel supported catalyst after being used
eight times had been also given. As shown as Fig.11, the palladium nanoparticles on the surface of
the HPG grafted magnetic mesoporous Fe
ultrasmall nanoparticle morphology and good dispersion, and very small amounts of Pd aggragations
can be observed. Which indicates that this novel Fe @SiO @mSiO ꢀHPGꢀCOOHꢀPd(0)
3 4 2 2
O @SiO @mSiO microspheres still exhibited the
3
O
4
2
2
supported catalyst shows very robust stability. All of which demonstrates that the HPG segments on
the surface of Fe O @SiO @mSiO can effectively stabilize Pd NPs through both the steric
3
4
2
2
2
+
hindrance effect of HPG segments and complexation between Pd ions and terminal carboxyl
groups. Beyond that, the mesopore channels can also play an important role in stabilizing Pd NPs for
some Pd NPs may enter the mesopore channel.
4
Conclusion
This paper mainly resolves the problem about the HPG grafting on the surface of
mesoporous silica coating by choosing isopropanol aluminum as a facile catalyst, then the
terminal hydroxyl groups of HPG were successfully transformed into carboxyl groups after
the modification with succinic anhydride. Subsequently, Pd NPs were successfully anchored
2
+
on the surface of aforementioned material through the complexation between Pd ions and
carboxyl groups and the following reduction, thus Fe @SiO @mSiO ꢀHPGꢀCOOHꢀPd (0)
3
O
4
2
2
supported catalyst was successfully obtained. This novel supported Pd NPs catalyst is very
conducive to the transference and exchanges of each component in the reaction system for
the orderly mesoporous opening structure. Furthermore, the introduction of magnetism
nucleus can provide a convenient magnetic separation. More importantly, the plenty of
3 4 2 2
terminal carboxyl groups on the surface of magnetic Fe O @SiO @mSiO microspheres can
provide plenty of sufficient binding sites for Pd NPs, and the unique hyperbranched structure
is very conducive to the capture of nanoꢀsized palladium in the form of good dispersion and
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DOI: 10.1039/C4NJ02407A
can effectively enhance the catalytic activity and stability. Research indicates that this novel
suported Pd NPs catalyst not only possesses extremely high Pd NPs loading capacity but also
shows remarkable catalytic activity to Suzuki crossꢀcoupling reaction between aryl halides
and phenylboronic acid. Simultaneously, the catalytic activity of this supported catalyst
would not show evident loss after being used at least eight times.
Acknowledgment
The authors are grateful for the financial support provided by the National Natural Science
Foundation of China (No. 51173146 and NO. 21201140), basic research fund of Northwestern
polytechnical university (3102014JCQ01094, 3102014ZD).
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