G. Zhan et al. / Catalysis Communications 12 (2011) 830–833
831
were recovered by centrifugation, washing, drying, and calcination at
23 K for 5 h. After obtaining the TS-1 supports, two specific modes
(Porapak Q column) and FID (Porapak T column) detectors. The Porapak
Q column (2 mm×3 m) was used to detect propylene and inorganic
8
have been provided herein for preparing Au/TS-1 catalysts, namely,
sol-immobilization (SI) method and adsorption–reduction (AR)
method. They are briefly described as follows.
2 2 2 2 2
substances (e.g., H , N , O , CO , and H O) while Porapak T column
(2 mm×2 m) was used to detect propylene and organic products (e.g.,
PO, acetone, ethanal, acrolein, etc.). Comparison of the catalyst
performance was summarized in terms of propylene conversion, PO
(
i) SI method: First of all, Au sol was biosynthesized through a
simple procedure by reducing HAuCl with CP extract which
2 2
selectivity, H efficiency, and PO formation rate. The H efficiency was
4
defined as the amount of PO produced divided by the amount of H
2
serves as both reducer and stabilizer. After 0.5 h of sol generation,
the Au sol (acidified at pH 2) was immobilized by adding TS-1
support under vigorous stirring conditions for 1.5 h. Then, the
suspension was filtered, subsequently washed thoroughly with
distilled water, dried at 323 K for 6 h, and finally activated by
calcination in air at 623 K for 2 h.
consumed.
3
. Results and discussion
3.1. Characterizations of bioreduction Au/TS-1 catalysts
(
ii) AR method: Firstly, as-prepared TS-1 support was immersed in an
aqueous solution of HAuCl (acidified at pH 2) for 1 h. Afterwards,
The essential difference between these two bioreduction methods
4
is that the SI method is designed via a biosynthesis of Au sol followed
by immobilization of the Au sol on TS-1 support, while the AR method
is designed via an adsorption of Au ions on the support followed by in
situ bioreduction of the Au ions. The key issue in each method is to
encourage the electrostatic interaction between TS-1 and Au (Au sol or
Au anionic species). Experimental results showed the isoelectric point
CP extract was added to the suspension. The Au precursors were
reduced in situ on the support surface by the added plant biomass
which serves dual roles as reducing and stabilizing agents. The
suspension was vigorously stirred for another 2 h and finally
filtrated. The solid then followed other post-treatments (e.g.,
washing, drying, and calcination), similar to the SI method.
(
IEP) of the biogenic Au sol to be about 1.2; therefore, the pH of the
In both procedures, the reduction of HAuCl
4
to Au nanoparticles
solution should be controlled to around 2 to make the TS-1 (IEP ~2.5)
positively charged and the Au sol negatively charged. Investigation of
the filtrate was conducted by atomic absorption spectrophotometer
(AAS) and UV–Vis spectrophotometer to check the complete loading
of Au on support [11]. In our experiment, no traces of Au were detected
in both analyses, indicating that the yield of Au deposition was 100%. In
addition, our experimental results revealed that incomplete Au load
would occur only in the case of high Au loading (≥5 wt.%).
was facilitated by CP extract; therefore, we refer to these original
catalysts as bioreduction catalysts. In addition, the bioreduction
catalysts are labeled according to their preparation method, Si/Ti
molar ratio, and Au loading, e.g., Au/TS-135/SI/1.0% corresponds to Au
catalyst prepared by the SI method with a Si/Ti molar ratio of 35 and
Au loading of 1.0 wt.%.
2
.2. Catalyst characterization
An overview of the obtained catalyst samples and their different
characteristics is given in Table 1. The pore structure parameters, such as
the BET surface, pore volume, and pore diameter of the pure TS-135
Transmission electron microscopy (TEM) was used to determine Au
particle size and its distribution, performing on a Philips Analytical FEI
Tecnai 30 electron microscope operated at an acceleration voltage of
2
−1
3
−1
(TS-150) support, were 406 (407) m g , 0.39 (0.35) cm g , and 6.1
(5.4) nm, respectively. As expected, catalyst BET surface and pore
volume were found to decrease with the increase of Au loading,
indicating Au particles partially occupy the mesopores of TS-1 support.
The trend of the average pore size indicates that Au nanoparticles play a
role in slightly increasing the catalyst pore size. Strikingly, from the AR
method, a considerably smaller Au size wasobtained compared with the
SI method (Fig. 1). This feature is attributed to the different pathways of
Au precursors loading on TS-1 support (SI method via the transference
of Au nanoparticles and the AR method via the mobility of Au ions). In
addition, TG and differential TG (DTG) analyses (in Fig. 2) indicated that
plant biomass weighed as 1.8 wt.% on uncalcined catalyst, against only
0.4 wt.% on calcined catalyst. We expect that the calcination treatment
benefits the removal of significant amount of plant biomass residual on
catalysts and thus helps to the exposure of active Au surface [12,13].
However, an outright elimination of the plant biomass may result in
sintering of Au particles. Hence, an appropriate calcination treatment
is recommended following by the results of TG and TEM measurements.
3
2
00 kV. N physisorption experiments were used to measure the surface
area, pore volume, and pore size of catalysts, performing at 77 K on a
Micromeritics TriStar 3000 porosimetry analyzer, using static adsorp-
tion procedures. Thermogravimetric (TG) analysis wasdoneto ascertain
the content of plant biomass on the bioreduction catalysts, which was
carried out on a Netzsch TG209F1 thermobalance under flowing air
−1
atmosphere at a heating rate of 10 K min from 300 to 1073 K.
2
.3. Catalyst evaluation
The catalytic performance of the catalysts samples for vapor phase
propylene epoxidation was carried out in a vertical flow reactor at 573 K
using 150 mg of Au/TS-1 catalyst at 1 atm. The feed gas compositions of
C
3
H
6
/O
2 2 2
/H /N were adjustedto10/10/10/70 (vol.%) and thefeed wasat
−1
−1
a gas space velocity of 4000 mL gcat
h
. Reactants and products were
measured online by two gas chromatographs, equipped with TCD
Table 1
Overview of the bioreduction Au/TS-1 catalyst samples.
Sample
Support
Prep. method
Au loading (wt.%)
Au size (nm)a
S
BET (m2 g−1
)
Pore volume (cm3 g−1)b
Pore size (nm)c
Au/TS-135/SI/0.5%
Au/TS-135/SI/1.0%
Au/TS-135/AR/0.5%
Au/TS-135/AR/1.0%
Au/TS-150/SI/0.5%
Au/TS-150/SI/1.0%
Au/TS-150/AR/0.5%
Au/TS-150/AR/1.0%
TS-135
TS-135
TS-135
TS-135
TS-150
TS-150
TS-150
TS-150
SI
SI
AR
AR
SI
SI
AR
AR
0.5
1.0
0.5
1.0
0.5
1.0
0.5
1.0
3.0± 0.4
4.2± 0.7
1.7± 0.3
2.6± 0.4
3.4± 0.4
4.6± 0.5
2.0± 0.3
2.7± 0.4
383
378
383
379
383
375
389
371
0.31
0.30
0.29
0.29
0.29
0.29
0.33
0.30
5.3
5.6
5.7
6.1
5.7
5.7
5.6
6.0
a
Determined by TEM observation.
Calculated from the volume adsorbed of P/P at 0.99.
0
Calculated by the BJH model (desorption).
b
c