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X. Huang et al. / Catalysis Communications 11 (2010) 487–492
hydroxyls, and can be easily obtained from the extraction of plants.
According to the chemical structure of tannins, they are classified
into hydrolyzable tannins and condensed tannins. The general
characteristics of tannins are that they are able to chelate with
many kinds of metal ions through their dense ortho-phenolic
hydroxyls [19–21], and have no swelling or deformation in polar
solvent. Additionally, tannins can be covalently grafted onto –
NH2 containing inorganic supports through the Mannich reaction.
All these properties suggest that tannins are ideal polymers for
the preparation of organic–inorganic hybrid support. In this study,
we developed a route for the grafting of bayberry tannin (BT, a kind
of typical condensed tannin) onto MTS, and then anchored Ru
complexes onto MTS–BT to prepare heterogeneous Ru catalysts
(MTS–BT–Ru). The objective of the present work is therefore to
investigate the catalytic activity of MTS–BT–Ru for the liquid phase
hydrogenation of olefins, as well as its stability and reusability.
2.4. Preparation of MTS–BT–Ru
MTS–BT (1.0 g) was added into 20 mL of RuCl3 solution at pH
2.0, where the concentration of Ru(III) was 500.0 mg/L. The reac-
tion was conducted at 303 K with constant stirring for 24 h. Then,
MTS–BT–Ru was obtained after fully washed with deionized water
and dried in vacuum at 303 K for 24 h. Based on the measurements
of inductively coupled plasma atomic emission spectroscopy (ICP-
AES, Perkin Elmer Optima 2100 DV), the Ru loading on MTS–BT–Ru
was determined to be 0.96%
ꢀ
ꢁ
Initial amount of RuðIIIÞðgÞ ꢂ Residual amount of RuðIIIÞðgÞ
Amount of MTSðgÞ
calculated as :
ꢁ 100%
:
2.5. Characterization of catalyst
Fourier Transform Infrared Spectroscopy (FTIR) of samples was
analyzed using FTIR-7600 instrument. Proton Nuclear Magnetic
Resonance (H NMR) spectrum of the samples was measured by
Bruker DPX400 NMR instrument using DMSO-d6 as solvent.
Wide-angle X-ray diffraction (XRD) patterns of the catalysts were
recorded by an X0Pert PRO MPD diffractometer (PW3040/60) with
2. Experimental
2.1. Reagents
Cyclohexane, n-hexanol, tetraethyl orthosilicate (TEOS), 3-ami-
nopropyl-triethoxysilane (APES), glutaraldehyde, ruthenium chlo-
ride (RuCl3ꢀnH2O) and other chemicals were all analytic reagents.
BT was obtained from the barks of myrica esculenta by extraction
with an acetone–water solution (1:1, v/v) and then spray dried.
The tannin content of the extract was determined to be 76.3%
according to Hide Powder Method, a national standard method of
China (Code: GB2615–81).
Cu Ka radiation. Transmission electron microscopy (TEM) images
of the catalysts were obtained in a FEI-Tecnai G2. The specific sur-
face area of MTS–BT–Ru was analyzed by N2 adsorption/desorption
using Surface Area and Porosity Analyzer.
2.6. Catalytic test
The liquid phase hydrogenation of allyl alcohol was conducted
in an autoclave type reactor at 1.0 MPa H2 pressure, at a tempera-
ture of 30 °C and using 0.05 g of catalyst. In each test, 25.0 mL of
methanol was used as solvent, and the obtained products were ana-
lyzed by gas chromatography (Shimadzu, GC-2010). Then, the used
catalyst was recovered by filtration, thoroughly washed with meth-
anol, and then reused. The turnover frequency (TOF) of the catalysts
was calculated as:½Substrate hydrogenatedðmolÞꢃ=½RuðmolÞotðhÞꢃ;
and the turnover number (TON) of the catalysts was calculated
as: ½Substrate hydrogenatedꢃ=½Ruꢃ . As control, the hydrogenation
was also carried out using homogenous RuCl3 complexes as the cat-
alysts. To evaluate the universal application of MTS–BT–Ru in liquid
phase hydrogenation, the hydrogenations of 2-methyl-3-buten-2ol,
2.2. Preparation of MTS
MTS was prepared following a methodology similar to that de-
scribed by Cheng et al. [22]. Briefly, 1 mL of n-hexanol, 1 mL of Tri-
ton X-100 and 4 mL of cyclohexane were added into 500 mL
deionized water with vigorous stirring to obtain emulsion solution.
Then, 8 mL of TEOS (silica source) and 2 mL of APES (aminating
agent) were added into the emulsion solution, followed by vigor-
ous stirring at 303 K for 2 h. A proper amount of ammonia was
drop-wise added into the solution to promote the hydrolysis of sil-
ica precursor, and then the mixture was further stirred at 303 K for
another 2 h. Afterwards, 2 mL of acetone, used as the emulsion
breaker, was added into the emulsion. When the breaking of the
emulsion was completed, the MTS was collected by filtrating,
washed thoroughly with deionized water and dried in vacuum at
353 K for 24 h.
acrylic acid, a-methacrylic acid, styrene and cyclohexene were also
carried out. As control experiments, all the olefin hydrogenations
were also carried out using MTS without the immobilization of Ru
3. Results and discussion
2.3. Preparation of MTS–BT
3.1. Preparation of characterization of the catalysts
BT (0.1 g) was dissolved in 50 mL deionized water, and then
mixed with 1.0 g of MTS prepared above, followed by constant stir-
ring at room temperature for 2 h. About 2 mL of glutaraldehyde
(50%, w/w) was drop-wise added into the mixture under constant
stirring in order to graft BT onto MTS. After reaction for 12 h at
313 K, MTS–BT were collected by filtration, fully washed with
deionized water and dried in vacuum at 303 K for 24 h. The con-
centration of BT in solutions was analyzed by ultraviolet–visible
spectrum (UV–Vis, TU-1901), and the grafting degree of BT on
MTS–BT was defined as:
The molecular structure of BT is shown in Scheme 1. It can be
seen that there are a large number of adjacent phenolic hydroxyls
at the B-rings of BT, which can form chelate rings with many kinds
of metal ions, and the partly attached galloyl groups at the C-rings
of BT can enhance such chelating ability. Each phenolic hydroxyl at
the B-rings of BT has lone electron pair, thus playing a role of
strong donor to center metal ions with empty electron orbits. For
this reason, the adjacent two phenolic hydroxyls of B-rings are very
likely act as a bidentate ligand to bond with center metal ion form-
ing five-membered chelate ring, which has been proved in our pre-
vious work using H NMR technique [23,24]. Since transition-metal
ions have empty orbits in their electron configurations, they should
be very reactive towards BT. Our research group demonstrated that
BT has high affinity towards many transition-metal ions including
Pd(II), Pt(IV) and Au(III) [25,26]. Considering that Ru(III) is transi-
Amount of BTðgÞgrafted onto MTSðgÞ
ꢁ 100%:
Amount of MTSðgÞ
As a result, the grafting degree of BT on the MTS–BT was approxi-
mately 8.7%.