Ionic liquid-enhanced immobilization of biosynthesized Au nanoparticles on TS-1 toward efficient catalysts for propylene epoxidation

Article abstract of DOI:10.1016/j.jcat.2011.08.011

The direct vapor-phase epoxidation of propylene in the presence of hydrogen and oxygen was studied at a space velocity of 7000mLh^-1gcat.-1 over gold catalysts with varying gold and titanium contents prepared by ionic liquid-enhanced immobilization (ILEI) method in which biosynthesized gold nanoparticles (GNPs) were immobilized onto the titanium silicalite-1 (TS-1) supports through the assistance of a small amount of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]). The results showed that [BMIM] ⁺ specially adsorbed onto the support to increase its isoelectric point, leading to the enhanced immobilization. The propylene conversion of 14.6% and PO formation rate of 164.4g_poKgcat.-1h^-1 were higher than those in any other reports, probably attributing to enhanced interaction between the GNPs and TS-1. Furthermore, the excellent activity and high selectivity of the Au/TS-1 catalysts at a relatively high reaction temperature might be attributed to the existence of residual biomolecules on the catalysts.

Full text of DOI:10.1016/j.jcat.2011.08.011

Journal of Catalysis 283 (2011) 192–201
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Ionic liquid-enhanced immobilization of biosynthesized Au nanoparticles on TS-1
toward efficient catalysts for propylene epoxidation
a
a
b
a
a
a
a
Mingming Du , Guowu Zhan , Xin Yang , Huixuan Wang , Wenshuang Lin , Yao Zhou , Jing Zhu ,
a
a,
⇑
a
a
a,
⇑
Ling Lin , Jiale Huang , Daohua Sun , Lishan Jia , Qingbiao Li
a
Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, National Laboratory for Green Chemical Productions of Alcohols, Ethers
and Esters, Key Lab for Chemical Biology of Fujian Province, Xiamen University, Xiamen 361005, PR China
College of Chemical Engineering, Huaqiao University, Xiamen 361021, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 July 2011
Revised 22 August 2011
Accepted 23 August 2011
Available online 25 September 2011
The direct vapor-phase epoxidation of propylene in the presence of hydrogen and oxygen was studied at
ꢀ1
ꢀ1
a space velocity of 7000 mL h
g
over gold catalysts with varying gold and titanium contents prepared
cat:
by ionic liquid-enhanced immobilization (ILEI) method in which biosynthesized gold nanoparticles
GNPs) were immobilized onto the titanium silicalite-1 (TS-1) supports through the assistance of a small
amount of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF ]). The results showed that
(
4
+
[
BMIM] specially adsorbed onto the support to increase its isoelectric point, leading to the enhanced
immobilization. The propylene conversion of 14.6% and PO formation rate of 164:4 gpo K g
Keywords:
Gold
Biosynthesis
Ionic liquid
TS-1
ꢀ1
ꢀ1
h
were
cat:
higher than those in any other reports, probably attributing to enhanced interaction between the GNPs
and TS-1. Furthermore, the excellent activity and high selectivity of the Au/TS-1 catalysts at a relatively
high reaction temperature might be attributed to the existence of residual biomolecules on the catalysts.
Ó 2011 Elsevier Inc. All rights reserved.
Catalyst
Propylene epoxidation
Immobilization
Special adsorption
1
. Introduction
Propylene epoxide (PO) is an important bulk chemical widely
exhibited a high selectivity (>90%) toward PO at low propylene con-
version (ꢁ1%) using mixture of propylene, oxygen, and hydrogen
under ambient pressure. To date, it has been reported that the
gas-phase epoxidation of propylene has been performed over GNPs
used for the production of polyether polyols, polyurethane foams,
propene glycol, and propene glycol ethers [1]. PO is industrially
produced using chlorohydrin and Halcon (hydroperoxide) pro-
cesses [1,2]. The former process produces environmentally
unfriendly chlorinated by-products, while the later process pro-
duces large quantities of coproducts such as tert-butyl alcohol
and styrene, which pose economic problems when their demand
is lower than that of PO. Therefore, a great deal of effort has been
put into catalytic epoxidation of propylene since the late 1990s.
Clerici et al. utilized titanium silicalite (TS-1) catalyst for the epox-
idation of propylene in the liquid phase using hydrogen peroxide
supported on a number of Ti-containing materials like TiO
2 2
, TiO /
SiO , TS-1, Ti-MCM-41, Ti-MCM-48, three-dimensional Ti–Si meso-
2
porous materials (3D-Ti–Si), Ti-HMS, and so on [7–18]. The density
functional theory (DFT) of hydroperoxy (OOH) intermediates on
various model titanosilicalite Ti centers explained how microstruc-
tural aspects of Ti sites affect propylene epoxidation reactivity. It
also demonstrated that a well-defined tetrahedral titanium center
is essential for epoxidation [19]. These works have suggested that
the isolation of Ti in tetrahedral sites is a requirement for a stable
and active catalyst, motivating people to focus on TS-1 as the cata-
lyst support. Taylor et al. found that addition of carbon pearls dur-
ing the preparation of TS-1 would enhance the activity and stability
of the responding Au/TS-1 catalysts prepared by DP in propylene
epoxidation [16]. Though the DP method to prepare Au/TS-1 cata-
[
3,4]. However, the cost of H
2
O
2
is very high and also there are han-
[5].
dling problems when operating with H
In 1998, a significant demonstration of gas-phase propylene
epoxidation was reported by Hayashi et al. [6], who discovered that
2 2
O
Au/TiO
2
catalysts with very fine and highly dispersed gold nanopar-
2 2
lysts for propylene epoxidation with O and H mixture is very
ticles (GNPs) (2.0–5.0 nm) through deposition–precipitation (DP)
effective [10,13–18], the Au capture efficiency was always very
low. A study on the DP method for Au/TS-1 catalysts has shown that
only 1–3 wt.% of the gold available in solution was deposited on
conventional TS-1 supports at pH of 9–10 [17]. Even though depo-
sition of gold could be significantly boosted by pretreating TS-1
⇑
Corresponding authors. Fax: +86 592 2184822.
E-mail addresses: cola@xmu.edu.cn (J. Huang), kelqb@xmu.edu.cn (Q. Li).
0
021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2011.08.011

M. Du et al. / Journal of Catalysis 283 (2011) 192–201
193
supports with NH
4
NO
3
aqueous solution prior to DP [15], the cap-
Cheng Jie Chemical, Co. Ltd. Other chemical reagents mentioned
were purchased from Sinopharm Chemical Reagent Co. Ltd., China,
and were used as received.
ture efficiency of Au was still less than 20 wt.%. The sol-immobiliza-
tion (SI) technique is also an effective method for the preparation of
gold catalysts [20–23]. Compared with the DP method, the SI meth-
od has advantages because 100 wt.% of the gold in solution can be
immobilized on support materials. This method has been used to
prepare gold catalysts for benzyl alcohol oxidation [20,22], low-
temperature CO oxidation [21,23], and propylene epoxidation [24].
In recent years, room-temperature ionic liquids (ILs) have also
attracted much attention due to their special properties, such as
negligible vapor pressure, wide liquid temperature range, excellent
chemical stability, high thermal stability, and the strong solvent
power for a wide variety of organic, inorganic, and polymeric mol-
ecules. Many reactions have been carried out in ILs [25,26]. ILs are
also a good media to stabilize metal NPs [27,28] and have been
used to immobilize metal NPs onto solid supports to prepare sup-
ported catalysts. For example, Han and coworkers demonstrated
that Pd and Ru NPs in IL 1,1,3,3-tetramethylguanidinium lactate
2.2. Synthesis of TS-1 supports
TS-1 supports were synthesized using the method developed by
Khomane et al. [46]. In a typical synthesis, 12.7 mL of a 40 wt.% tet-
rapropylammonium hydroxide (TPAOH) solution and 2 g of poly-
oxyethylene 20-sorbitan monolaurate (Tween 20) were dissolved
in 35 mL deionized (DI) water. This solution was added dropwise
to 24.6 mL of tetraethylorthosilicate (TEOS) in a polypropylene
Erlenmeyer flask with vigorous stirring for 1 h. Then, 1.1 mL of tita-
nium(IV) butoxide (TBOT) was dissolved in 6.2 mL of isopropanol
under stirring. The solution was then added to the TEOS mixture
dropwise during vigorous stirring. Finally, another 4.6 mL of
TPAOH solution was added dropwise to the mixture under vigor-
ous stirring, leading to a molar composition of 1.000 Si:0.029
Ti:0.300 TPAOH for the resulting synthesis solution. Afterward,
the mixture was stirred for 1 h and then the mixture was trans-
ferred to a sealed Teflon-lined pressure vessel and heated at
170 °C for 24 h to facilitate the crystallization process. Next, the
solid was collected via centrifugation which after being washed
thoroughly with DI water, was dried in vacuum at 320 K for 24 h,
and finally calcined at 550 °C for 5 h. The TS-1 support with a Si/
Ti ratio of 35 was labeled as TS-1(35). Other TS-1(x) supports with
different Si/Ti ratios were synthesized in the same manner.
(
TMGL) immobilized onto supports, which were obtained through
reducing metal precursors by H , resulted in highly active hetero-
geneous catalysts [29–31] due to the excellent synergistic effects of
IL, support, and metal nanoparticles.
2
Although the use of such GNPs has a high potential for propyl-
ene epoxidation, CO oxidation reactions, and so on, novel, cost-
effective, and ‘‘greener’’ approaches for the preparation of gold
catalysts should be developed, because GNP production remains
expensive and/or involves hazardous chemicals. Biosynthesis of
GNPs as an emerging highlight of the intersection of nanotechnol-
ogy and biotechnology has received increasing attention due to a
growing need to develop environmentally benign technologies in
material in the last decade [32–37]. As an alternative to chemical
methods, biosynthesis methods can be used to synthesize NPs with
a narrow size distribution and a desired diameter without any aux-
iliary surfactant or capping agent, owing to the presence of plant
biomass that plays dual roles as both reductant and stabilizer
2.3. Preparation of Au/TS-1 catalysts
The gold sol was prepared by the biosynthetic method employ-
ing plant biomass extract. In order to obtain C. platycladi extract, the
carefully weighted biomass, 4.0 g, was added to 400 mL DI water in
a conical flask of 500 mL capacity. The mixture was thereafter sha-
ken at a rotation rate of 150 rpm for 2 h at 30 °C and filtrated to get
filtrate for further experiments. In a typical synthesis of GNPs, an
[
37–42]. However, the application of the biosynthesized GNPs to
catalytic system has been little reported. Vilchis-Nestor et al. pre-
pared Au(AgAu)/SiO –Al catalysts for the oxidation and hydro-
4
appropriate amount of aqueous HAuCl solution (48.56 mM) was
2
2
O
3
added to 25 mL extract in a conical flask to have a final concentra-
tion of Au(III) 0.5 mM, and the mixture was then vigorously stirred
for 1 h at room temperature. It should be noted that the solution
genation of CO [43], and our group has attempted to prepare Au/
TS-1 catalysts for propylene epoxidation by bio-reduction synthe-
sis method [44,45]. As mentioned above, the combination of bio-
synthesis and ILs may provide a new way to prepare highly
efficient Au catalysts. In the present work, we report an ionic li-
quid-enhanced immobilization (ILEI) method to prepare the Au/
TS-1 catalysts for the epoxidation of propylene. In this method,
biosynthesized GNPs were immobilized onto TS-1 by IL 1-butyl-
turned reddish-pink in a few seconds after HAuCl
4
was added into
the extract, indicating fast reduction of Au ions [37]. Afterward,
0.5 g TS-1 and 0.1 mL [BMIM][BF ] were added to the gold sols un-
3
+
4
der vigorous stirring. After 1 h, the suspension was filtered through
a cellulose filter membrane with the size of pores 0.45-mm, and the
retained solid was washed three times with DI water. Finally, the
obtained solid, after being dried at 50 °C in vacuum for 24 h, was
calcinated at 375 °C for 6 h. Thus, the obtained Au catalysts were
denoted as zAu/TS-1(x)-IL, where z was the calculated weight per-
cent of Au in the catalysts. The reference catalyst 0.5Au/TS-1(35)
was synthesized in the same manner without [BMIM][BF ], and
4
the catalyst 0.5Au/TS-1(35)-pH was prepared through acidifying
the colloid solution at pH 2 by sulfuric acid.
3
4
-methylimidazolium tetrafluoroborate ([BMIM][BF ]) from their
solution. The Au/TS-1 catalysts showed an excellent performance
for the vapor-phase propylene epoxidation in the presence of
hydrogen and oxygen. Therefore, the ILEI method was effective
for preparation of the Au/TS-1 catalysts. The method was totally
different from the IL immobilization method previously reported
[
4
29–31]. In this work, the role of [BMIM][BF ] in the ILEI method
for the Au/TS-1 catalysts was investigated in detail. To the best
of our knowledge, this is the first report on enhanced immobiliza-
tion of biosynthesized GNPs onto solid support using IL, which will
be a breakthrough for the fabrication of supported Au catalysts.
2.4. Characterization of the catalysts
Surface area of the TS-1 supports was measured in a Micromer-
itics Tristar system (Tristar 3000). Before measurements, samples
were outgassed at 300 °C for 3 h. X-ray diffraction (XRD) measure-
ment was taken on an X’Pert Pro X-ray Diffractometer (PANalytical
BV, The Netherlands) operated at a voltage of 40 kV and a current
2
. Experimental
2.1. Materials
of 30 mA with CuKa radiation. The diffuse reflectance ultraviolet–
visible (DRUV–vis) spectra of the support were taken on a Varian
C. platycladi, purchased from Xiamen Jiuding Drugstore, China,
Cary-5000 spectrometer equipped with a diffuse-reflectance acces-
was first milled and screened by a 20-mesh sieve for further exper-
iments. The IL, [BMIM][BF ] (99%), was purchased from Shanghai
sory, using dehydrated BaSO
200–800 nm. UV–vis spectroscopy analyses of the liquid samples
4
as a reference in the range of
4

1
94
M. Du et al. / Journal of Catalysis 283 (2011) 192–201
were carried out at room temperature on an UNICAM UV-300 spec-
trophotometer (Thermo Spectronic) from 330 to 800 nm. FTIR
spectra were recorded on a Nicolet Avatar 660, where the samples
were ground with KBr and pressed into thin wafer. Transmission
electron microscopy (TEM) images of the samples were obtained
on a Tecnai F30 microscope. Size distribution of the resulting NPs
was estimated on the basis of TEM micrographs with the assistance
of SigmaScan Pro software (SPSS Inc., Version 4.01.003). The actual
Au loadings of the samples were determined by atomic absorption
spectroscopy (AAS). And the catalysts were characterized by X-ray
photoelectron spectroscopy (XPS) using a PHI Quantum-2000 spec-
trometer equipped with a hemispherical electron analyzer and a
a
b
c
d
AlKa (1486.6 eV) X-ray source. Zeta Plus for zeta potential analysis
was from Brookhaven Instruments Corporation. TG studies were
carried out on a Netzsch TG209F1 thermobalance under flowing
10
20
30
40
50
θ/0
60
70
80
90
ꢀ1
air atmosphere at a heating rate of 10 °C min
.
2
2.5. Catalytic activity measurements
Fig. 1. XRD patterns of TS-1 supports with different Si/Ti ratios. (a) TS-1(35), (b) TS-
(48), (c) TS-1(100), and (d) TS-1(200).
1
The catalytic tests were carried out in a half-inch-diameter ver-
tical fixed-bed stainless steel reactor using a feed concentration of
0/10/10/70 vol.% of propylene (99.999%), (99.999%), H
2
ꢀ1
supports (Fig. 2A) show that the absorption band at 960 cm was
observed in all the cases due to incorporation of titanium into the
1
O
2
(
99.999%), and N
2
(99.999%) and 150 mg of catalyst at a space veloc-
ꢀ1
ꢀ
1
ꢀ1
framework. Moreover, the absorption band at 960 cm decreased
with decreasing titanium content. The DRUV–vis spectra of the TS-
supports (Fig. 2B) with Si/Ti = 35, 48, 100, and 200 resulted in the
absorption bands around 210 nm, which were indicative of tetra-
hedral titanium species substitution into the zeolite framework.
There was no other absorption band in these samples implying
no extraframework species in the silicalite-1 framework [48]. Next,
the TS-1 supports were used for investigation of the ILEI method.
ity of 7000 mL h
g
. The temperature was measured by using a
cat:
glass tube covered Cr–Al thermocouple located in the center of
the catalyst bed. The gas leaving the reactor was heated at about
1
1
00 °C and analyzed online by two gas chromatographs. One of
the chromatograph was equipped with two thermal conductivity
detectors (TCD), using a MS 5A packed column (2 mm ꢂ 3 m) and
a Porapak Q packed column (2 mm ꢂ 3 m), respectively, and the
other equipped with a flame ionization detector (FID) was attached
to a Porapak T packed column (2 mm ꢂ 2 m). The MS 5A and Pora-
pak Q columns were used to detect hydrocarbons (e.g., propylene),
3.2. Characterizations of the Au/TS-1 catalysts
2 2 2 2 2
H , O , CO , N , and H O. The Porapak T column was used to detect
oxygenates (e.g., propylene, acetaldehyde, PO, acetone, propional-
dehyde, acrolein, and the like).
The Au/TS-1-IL catalysts were prepared by immobilizing GNPs
onto the TS-1 supports through the ILEI method. In this work,
the GNPs were biologically synthesized by the reduction of HAuCl
4
with C. platycladi extract leading to a reddish-pink sol within a few
3
. Results and discussion
seconds. Fig. 3A shows the typical TEM image of the GNPs synthe-
sized by reducing aqueous HAuCl
extract. The well-dispersed GNPs mostly ranging from 2.0 to
.0 nm with a mean diameter of 3.1 nm (the insert image in
4
(0.5 mM) with C. platycladi
3.1. Characterizations of TS-1 supports
5
Textural properties of the prepared TS-1 supports are shown in
Fig. 3A) are proper for preparation of the Au/TS-1 catalysts. The
representative TEM image of Au/TS-1(35)-IL in Fig. 3B clearly illus-
trates that the GNPs with a mean diameter around 3.6 nm (the in-
sert image in Fig. 3B) were well dispersed on the TS-1 surfaces.
Evidently, the GNPs nearly retained their size after immobilization
on TS-1 supports through the ILEI method.
Table 1. Little change in the surface properties with increasing tita-
nium content in the supports indicated that Ti atoms were either
well dispersed on the walls of the silica matrix or present inside
the framework without pore blocking effect. XPS analysis of TS-1
indicated that surface composition was approximate to theoretical
value. XRD patterns of the TS-1 supports with different Si/Ti ratios
FT-IR spectroscopy was employed to determine the presence of
(
Fig. 1) showed that all the supports are highly crystalline TS-1
the [BMIM][BF
[BMIM][BF ] (Fig. 4a) shows that the C–H stretching region be-
tween 2700 and 3200 cm possessed the strongest absorption.
The absorption band at 1570 cm could be assigned to the C@N
stretching vibration of IL [49]. The FT-IR spectrum of the catalyst
0.5Au/TS-1(35)-IL before being pretreated under air (Fig. 4c) shows
4
] on the Au/TS-1 catalysts. The FT-IR spectrum of
with the MFI structure [47,48]. As evidenced by the disappearance
of diffraction lines splitting at 2h = 24.5° and 29.5° (marked with
arrows), the conversion of the monoclinic structure of silicalite-1
to an orthorhombic structure indicated the incorporation of Ti(IV)
into the framework. The typical FTIR absorption spectra of the TS-1
4
ꢀ1
ꢀ1
Table 1
Characterization of the TS-1 supports.
BET (m2
g
ꢀ1
)
Pore diametera (nm)
Pore vol.b (cm3
g )
ꢀ1
Sample
Normal Si/Ti
(Si/Ti)xps
DTS-1 (nm)
S
TS-1(35)
TS-1(48)
TS-1(100)
TS-1(200)
35
48
100
200
40.6
50.2
111.2
195.3
270 ± 22
253 ± 23
244 ± 21
234 ± 26
444
474
446
427
5.6
4.7
5.0
4.9
0.35
0.35
0.32
0.33
a
Calculated from the desorption branch of nitrogen isotherm by using the BJH model.
Calculated from the volume adsorbed of P/P at 0.99.
0
b

M. Du et al. / Journal of Catalysis 283 (2011) 192–201
195
a
(A)
(B)
b
c
d
a
b
c
d
1800 1600 1400 1200 1000 800 600 400
200
300
400
500
600
700
800
-
1
Wavenumber (cm )
Wavelength/nm
Fig. 2. FT-IR (A) and DRUV–vis (B) of TS-1 supports with different Si/Ti ratios. (a) TS-1(35), (b) TS-1(48), (c) TS-1(100), and (d) TS-1(200).
Fig. 3. TEM images and corresponding histogram of size distribution of GNPs (A) and the catalyst 0.5Au/TS-1(35)-IL (B) by C. platycladi extract; [HAuCl
4
] = 0.5 mmol L 1; [C.
ꢀ
ꢀ
1
platycladi] = 10.0 g L ; reaction temperature is 30 °C.
+
[
BMIM] ions cannot be removed by washing. No absorption bands
+
of [BMIM] ions were presented in the FTIR spectrum (Fig. 4d) of
the catalyst after pretreatment under air at 375 °C for 6 h meaning
that the IL had been decomposed.
a
b
c
Nitrogen physisorption measurements were also taken for the
Au/TS-1 catalysts through the ILEI method with different gold load-
ings. The pore structure parameters, such as the specific area (SBET),
pore volume, and pore diameter of the TS-1 and Au/TS-1 catalysts,
are summarized in Table 2. After Au immobilization, the surface
area of the supports slightly decreased, while the pore diameter
slightly increased, in comparison with those of the pristine TS-1
support (Table 1). The isotherms of type IV, typical of mesopore
materials are still observed in Fig. 5A. The pore size distribution
is shown in Fig. 5B indicating that over Au/TS-1 catalysts, some
small mesopores were created with a shoulder peak of pore size
at about 4.1 nm, which was in agreement with the previous study
d
4
000
3500
3000
2500
2000
1500
1
1000
500
-
Wavenumber(cm )
[
13,45].
Fig. 4. FT-IR spectra of [BMIM][BF4] (a), TS-1(35) (b), 0.5Au/TS-1(35)-IL before
calcination (c) and 0.5Au/TS-1(35)-IL after calcination (d).
3.3. Propylene epoxidation over the Au/TS-1 catalysts
ꢀ
ꢀ
1
the CH stretching region between 2700 and 3200 cm and the
3.3.1. Influence of IL on the catalytic performance of the Au/TS-1
catalysts
1
absorption band of C@N stretching vibration at 1570 cm , besides
the typical absorption bands of the TS-1(Fig. 4b), suggesting that
the [BMIM] ions adsorbed on the TS-1 surface, and these adsorbed
The test results of the 0.5Au/TS-1(35)-IL catalyst through the
ILEI method, the 0.5Au/TS-1(35) catalyst without the IL assistance
+

1
96
M. Du et al. / Journal of Catalysis 283 (2011) 192–201
Table 2
Characterization of the Au/TS-1 catalysts.
BET (m2
g
ꢀ1
)
Pore diametera
nm)
Pore vol.
b
Actual Au loading
(wt.%)
Sample
Normal Si/Ti
DTS-1 (nm)
S
(cm3
g
ꢀ1
)
(
0.25Au/TS-1(35)-IL
0.5Au/TS-1(35)-IL
1.0Au/TS-1(35)-IL
0.5Au/TS-1(48)-IL
0.5Au/TS-1(100)-IL
0.5Au/TS-1(200)-IL
35
35
35
48
100
200
270 ± 22
270 ± 22
270 ± 22
253 ± 23
244 ± 21
234 ± 26
409
403
385
410
339
354
5.8
6.0
8.5
6.7
4.6
6.8
0.34
0.34
0.32
0.36
0.21
0.31
0.25
0.5
1.0
0.5
0.5
0.5
a
Calculated from the desorption branch of nitrogen isotherm by using the BJH model.
b
Calculated from the volume adsorbed of P/P
0
at 0.99.
2
2
2
1
1
1
1
1
40
20
00
80
60
40
20
00
(
A)
(B)
80
0.0
0.2
0.4
0.6
0.8
1.0
1
10
100
Relative pressure /P/Po
Pore diameter /nm
Fig. 5. Nitrogen adsorption–desorption isotherms (A) and the corresponding pore size distributions (B) for different Au loadings; j: TS-1(35), N:0.25Au/TS-1(35)-IL, .:
.5Au/TS-1(35)-IL, d: 1.0Au/TS-1(35)-IL.
0
and the 0.5Au/TS-1(35)-pH catalyst through acidifying the colloid
solution are presented in Table 3 and Fig. 6. Very low C conver-
sion of 0.8% and H efficiency of 4.9% were obtained with the cata-
lyst 0.5Au/TS-1(35), resulting in a very low PO formation rate of
attracting the NPs with negative charges. Grunwaldt et al. [21,23]
reported the preparation of supported Au catalysts for low-tem-
perature CO oxidation via this method. Hutchings and coworkers
3
H
6
2
2
[20,22] reported that Au–Pd NPs were supported on TiO and car-
ꢀ
1
ꢀ1
9
:2 gpo K g
h
. The catalyst 0.5Au/TS-1(35)-pH only gave propyl-
bon prepared via SI technique for solvent-free oxidation of benzyl
alcohol. The real surface of the biosynthesized GNPs was covered
by anions that were associated with the negatively charged bio-
molecules with carboxyl groups, protecting the GNPs from aggre-
gating [37]. The IEP value of the TS-1 support is about 2–3 [50].
Here, the catalyst 0.5Au/TS-1(35)-pH was prepared at the pH value
of 2 below the IEP of TS-1 support; the surface of TS-1 support was
covered by positive charges, which attract the GNPs. As a conse-
quence, the GNPs can be completely adsorbed on the TS-1 support.
The catalyst 0.5Au/TS-1(35) was prepared at the pH value of 3.3
above the IEP of TS-1 support, so the surface of TS-1 support was
covered by negative charges, which repulse the GNPs and prevent
their adsorption. As a result, the GNPs cannot be completely ad-
sorbed on the TS-1 support. On the contrary, the GNPs can be fully
adsorbed on the TS-1(35) support with IL at the same conditions.
cat:
ꢀ1
ꢀ1
ene conversion of 5.0 and PO formation rate of 60:4 gpo K g
h
. In
conversion was markedly enhanced to 10.1% with
.5Au/TS-1(35)-IL, leading to high PO formation rate of
. Therefore, the catalytic performance of the
cat:
3 6
contrast, C H
0
1
a
ꢀ1
ꢀ1
20:2 gpo K g
h
cat:
Au/TS-1 catalysts could be enhanced by adopting the ILEI method
and using IL [BMIM][BF ].
We are interested in understanding the role of [BMIM][BF
4
4
] in
the ILEI method. It is generally recognized that UV–vis spectros-
copy could be used to examine the surface plasmon resonance
(
SPR) of NPs in aqueous suspensions. The UV–vis spectrum of the
filtrate of the three catalysts is shown in Fig. 7. The UV–vis spectra
spectrum b and c) of the pale yellow filtrate (photographs d and e)
(
of 0.5Au/TS-1(35)-IL and 0.5Au/TS-1(35)-pH did not show any SPR
band at about 540 nm, indicating that GNPs were fully immobi-
lized on the TS-1 support. In contrast, the UV–vis spectrum (spec-
trum a) for the reddish-pink filtrate (photograph f) of 0.5Au/TS-
4
In order to further investigate the role of [BMIM][BF ] in
promoting adsorption of the GNPs on the TS-1 support with IL,
1
(35) clearly exhibited a SPR band at about 540 nm, suggesting
the zeta potentials (f) of the TS-1 supports under different
that a certain amount of the GNPs remained in the filtrate. The
AAS analysis results of the Au concentration in the filtrate showed
that only about 70% of gold in the precursor solution was immobi-
lized onto the TS-1. The GNPs could be completely immobilized
onto the TS-1 by sulfuric acid and IL; however, the catalytic perfor-
mance of the catalyst 0.5Au/TS-1(35)-IL was superior to that of the
catalyst 0.5Au/TS-1(35)-pH.
4
[BMIM][BF ] concentrations were obtained. f characterizes the po-
tential difference between the slipping plane of the solid particles
and bulk solution, which could reflect the state of the ions in the
electrical double layer [51]. As shown in Fig. 8, the initial f of the
TS-1(35) support was ꢀ35.78 mV, revealing that anions dominated
on the surface of the TS-1 support according to the electrical dou-
4
ble-layer theory [51]. When a moderate amount of [BMIM][BF ]
NPs are immobilized on support materials by the electrostatic
attractive forces between support materials and NPs during the
process of the SI method. Usually the colloid solution is acidified,
so that its pH value is below the isoelectric point (IEP) of support
materials, which results in positive charges on the support surface,
was added to the TS-1 hydrosol, the absolute value of the negative
f decreased, indicating that [BMIM] squashed into the stern layer
+
and neutralized part of the negative charges. In such a process, the
van der Waals forces as well as the electrostatic attractive forces
+
between [BMIM] ions and the negative charges in the stern layer

M. Du et al. / Journal of Catalysis 283 (2011) 192–201
197
Table 3
Catalytic performance for propylene epoxidation over Au/TS-1 catalysts.
Au catalysts
Actual Au loading
wt.%)
Conversion of
(%)
Efficiency of
(%)
Selectivity (%)
PO formation rate
(
C
3
H
6
H
2
ꢀ1
ꢀ1Þ
ðgpo K g
h
PO
Et
8.9
Pr
An
Ac
CO
2
cat:
0
0
0
.5Au/TS-1(35)
.5Au/TS-1(35)-pH
.5Au/TS-1(35)-IL
0.35
0.5
0.5
0.8
5.0
10.1
4.9
16.9
27.6
70.9
73.2
72.1
0.0
2.0
1.5
5.1
6.4
4.3
10.3
7.3
8.4
4.7
2.5
2.7
9.2
60.4
120.2
8.6
10.9
ꢀ1
ꢀ1
Reaction conditions: Catalyst, 0.15 g; temperature, 300 °C; feed gas, C
3
H
6
/O
2
/H
/N
2 2
= 1/1/1/7(vol.%); space velocity, 7000 mL h
g
; all the data were taken under a steady
cat:
state after 2-h duration. Propylene oxide, PO; ethanol, Et; propanol, Pr; acrolein, An; acetone, Ac; pretreated under air at 375 °C for 6 h.
1
00
180
1
1
1
8
5
2
9
6
3
0
40
1
50
20
80
1
30
6
4
2
0
0
0
0
9
0
0
2
1
0
0
6
0
30
0
0
1
2
3
4
5
6
7
8
9
10
0
1
2
3
4
5
6
7
8
9
10
Time on stream /h
Time on stream /h
Fig. 6. Epoxidation of propylene over Au/TS-1; j, h: 0.5Au/TS-1(35)-IL, N, 4: 0.5Au/TS-1(35)-pH, ., O: 0.5Au/TS-1(35); reaction conditions: Catalyst, 0.15 g; temperature,
ꢀ
1
ꢀ1
3
00 °C; feed gas, C
3
6
H /O
2
/H
/N
2 2
= 1/1/1/7(vol.%); space velocity, 7000 mL h
g
.
cat:
2.5
2.0
1.5
1.0
0.5
0.0
8
6
4
2
0
0
0
0
0
a
d
e
f
b
-
-
20
40
c
400
500
600
700
800
0
5
10
15
20
25
Wavelength (nm)
[
BMIM] [BF ] concentration /mM
4
Fig. 7. UV–vis spectra of the filtrate of Au/TS-1 catalysts; (a) 0.5Au/TS-1(35)
4
Fig. 8. Zeta potential of the TS-1(35) versus [BMIM][BF ] concentrations.
(
(
photograph f), (b) 0.5Au/TS-1(35)-IL (photograph d), and (c) 0.5Au/TS-1(35)-pH
photograph e).
indicating saturated adsorption of [BMIM]+ on the TS-1 support,
and a balance was thus established between the van der Waals
attractive forces, the electrostatic repulsive forces, and steric hin-
drance effects. Fig. 9 shows the zeta potential behavior and IEP of
TS-1(35) with or without IL in the suspension. The IEP of TS-
1(35) with IL had a pH value of 8.0 which was shifted from the
pH value of 2.8 for the TS-1(35) because of the special adsorption
played a positive role. When the concentration of [BMIM][BF
4
] was
+
1
.1 mM, f became positive, suggesting that [BMIM] could still en-
ter into the stern layer. In this circumstance, the electrostatic
+
forces between the [BMIM] ions inside and outside the stern layer
became repulsive and had a negative effect on continual access of
+
the [BMIM] ions to the stern layer; However, it is well known that
+
in general the specific adsorption of an ion is enhanced by larger
size, which is a function of ion size [51]. Hence, driven by strong
of [BMIM] ions. Therefore, the GNPs with negative charges can
+
be fully adsorbed on the TS-1 support with [BMIM] ions. And
+
van der Waals forces, at this stage, [BMIM] ions could still be ad-
the interaction between GNPs and TS-1 supports by the ILEI meth-
od was stronger than that by acidifying the colloid solution. The
enhancement of catalytic performance could be attributed to
stronger interaction between GNPs and TS-1 supports caused by IL.
sorbed into the stern layer and f kept growing when concentration
of [BMIM][BF
tion of [BMIM][BF
4
] increased from 1.1 to 6.5 mM. For the concentra-
] from 6.5 to 21.6 mM, f changed little (Fig. 8),
4

1
98
M. Du et al. / Journal of Catalysis 283 (2011) 192–201
Uphade et al. [10] proposed that hydrogen peroxide formed
from H and O over the gold surface was first converted into a
hydroperoxo-like species on the Ti –SiO
reacted with propylene preadsorbed on the support surface to pro-
duce PO. The influence of Au loadings and Si/Ti ratio on the cata-
lytic performance of Au/TS-1 catalysts might be explained as
9
6
3
0
0
0
0
2
2
4
+
2
surface, which then
follows. The H
increased. Then, H
produce more H
ing rise to the increase in the propylene conversion. However, the
2
conversion increased if Au loadings of the catalyst
reacted with O over the surface of GNPs to
and form more Ti–OOH at active Ti sites, giv-
2
2
4
+
2 2
O
4
+
-
30
60
2 2
active Ti sites were limited, and the effective capture of H O by
4
+
active Ti sites to form Ti–OOH became very important for C
epoxidation. Otherwise, excessive H will be directly decom-
posed to produce H O and did not contribute to PO synthesis lead-
ing to lower H utilization efficiency. Hence, the H utilization
3 6
H
2 2
O
-
2
2
2
2
4
6
8
10
efficiency will increase with decreasing Si/Ti ratio according to
the analysis above.
pH
Fig. 9. Zeta potential curves of TS-1(35) (j) and TS-1(35)-IL (N) versus pH values.
3.3.3. Possible reasons for excellent performance of the Au/TS-1
catalysts under high reaction temperature
3
.3.2. Influence of Au loadings and Si/Ti ratio over the catalytic
The reaction temperature is one of the vital parameters that are
related to the activation of reactants and the efficiency of hydro-
peroxide species [52,53]. The catalytic performance of the catalyst
0.5Au/TS-1(48)-IL at 220, 240, 260, 280, and 300 °C is presented in
Fig. 12. It can be seen that when the reaction temperature was
220 °C, the conversion of propylene was only 2.7% albeit with H2
efficiency and medium PO selectivity as high as 54.3% and 81.7%,
respectively. When the reaction was conducted at 300 °C, the pro-
pylene conversion was promoted to 8.8%, whereas the PO selectiv-
ity decreased to 73.8% and H2 efficiency was 21.0%. Based on the
reaction temperature investigations, the Arrhenius plots for the
performance of the Au/TS-1 catalysts
The epoxidation performance of Au/TS-1 catalysts with Au load-
ings of 0.25–1.0 wt.% at 300 °C was also tested. The catalytic per-
formance of these catalysts is presented in Fig. 10. It can be seen
that the catalyst 0.25Au/TS-1(35)-IL with the lowest Au loading
of 0.25% gave the propylene conversion of 5.1% and PO formation
ꢀ
1
ꢀ1
rate of 60:7 gpo K g
h . The catalyst 0.5Au/TS-1(35)-IL with
cat:
intermediate Au loading gave intermediate propylene conversion
and PO selectivity. The catalyst 1.0Au/TS-1(35)-IL with the highest
Au loadings of 1.0% gave the highest propylene conversion of 14.6%
ꢀ
1
ꢀ1
and PO formation rate of 164:4 gpo K g
h
, which are the highest
formation of PO, ethanol, and CO gave apparent activation ener-
cat:
2
1
ꢀ
in data reported so far. From the above analysis, we can see that
the catalytic performance of Au/TS-1 catalysts was strongly af-
fected by Au loading.
Si:Ti ratio is another vital parameter influencing the catalytic
performance of Au/TS-1 catalysts. The influence of Si/Ti ratio upon
the catalytic performance with the catalysts 0.5Au/TS-1(35)-IL,
gies of 30, 57, and 51 kJ mol , respectively. These activation
energy values are close to those reported in literatures [15,16],
suggesting that the reactions in our experimental system occur
via similar mechanisms.
In the previous studies on propylene epoxidation using gold
catalysts [14,16–18,54,55], reaction temperatures were usually
less than 200 °C. Above this temperature, the PO selectivity was
reported to drop significantly [9,56]. This easily leads to confusions
about the high activity and PO selectivity (above 70%) of our cata-
lyst at reaction temperature as high as 300 °C in the presence of H2
0
.5Au/TS-1(48)-IL, 0.5Au/TS-1(100)-IL, and 0.5Au/TS-1(200)-IL
was studied. The catalytic performance of these catalysts is pre-
sented in Fig. 11, indicating that decreasing titanium contents
resulted in the increase in PO rate for the catalysts. Moreover,
the H
in Si/Ti ratio from 200 to 35. We also found that the catalysts
.5Au/TS-1(35)-IL and 0.5Au/TS-1(48)-IL with lower Si/Ti ratio
showed high stability and the catalysts 0.5Au/TS-1(100)-IL and
.5Au/TS-1(200)-IL with higher Si/Ti ratio suffered deactivation.
2
efficiency increased from 5.9% to 27.6% with the decrease
and O . As a matter of fact, Haruta et al., who is a well-known
authority in the field of propylene epoxidation, have reported high
yield of propylene oxide (PO) with its selectivity above 80% using
silylated Au catalysts at a temperature of 250 °C, about 100 °C
higher than the optimum temperature for non-silylated Au
2
0
0
2
2
1
1
5
0
5
0
5
0
8
6
4
2
0
0
0
0
0
40
3
2
1
1
00
40
80
20
30
20
10
0
60
0
0
1
2
3
4
5
6
7
8
9
10
0
1
2
3
4
5
6
7
8
9
10
Time on stream /h
Time on stream /h
Fig. 10. Epoxidation of propylene over Au/TS-1 with different Au loadings; j, h: 0.25Au/TS-1(35)-IL, N, 4: 0.5Au/TS-1(35)-IL, ., O: 1.0Au/TS-1(35)-IL; reaction conditions:
ꢀ
1
ꢀ1
Catalyst, 0.15 g; temperature, 300 °C; feed gas, C
3
H
6
/O
/H
2 2
/N
2
= 1/1/1/7(vol.%); space velocity, 7000 mL h
g
.
cat:

M. Du et al. / Journal of Catalysis 283 (2011) 192–201
199
1
1
1
60
40
20
1
1
1
8
5
2
9
6
3
0
8
6
4
2
0
0
0
0
0
30
20
10
0
100
80
60
40
20
0
0
1
2
3
4
5
6
7
8
9
10
0
1
2
3
4
5
6
7
8
9
10
Time on stream /h
Time on stream /h
Fig. 11. Epoxidation of propylene over Au/TS-1 with different Si/Ti ratios; j, h: 0.5Au/TS-1(35), N, 4: 0.5Au/TS-1(48)-IL, .,O: 0.5Au/TS-1(100)-IL, d, s: 0.5Au/TS-1(200)-IL;
ꢀ1
ꢀ1
reaction conditions: Catalyst, 0.15 g; temperature, 300 °C; feed gas, C
3
H
6
/O
2
/H
/N
2 2
= 1/1/1/7 (vol.%); space velocity, 7000 mL h
g
.
cat:
9
8
7
6
0
120
9
6
3
0
50
40
30
20
0
0
0
90
60
30
0
50
220
240
260
280
300
220
240
260
280
300
o
o
Reaction Temperature/ C
Reaction Temperature/ C
Fig. 12. C
feed gas, C
3
H
6
conversion, PO selectivity, H
2
efficiency, PO formation rate as a function of reaction temperature over 0.5Au/TS-1(48)-IL; reaction conditions: Catalyst, 0.15 g;
ꢀ
1
ꢀ1
3
H
6
/O /H /N = 1/1/1/7(vol.%); space velocity, 7000 mL h
2
2
2
g
.
cat:
that there are a large number of known or unknown complicated
biomolecules in C. platycladi extract [38,44,45]. According to the
TG profile, the main weight loss was between 200 and 500 °C, and
the main DTG peaks centering at about 230 and 450 °C. The weight
loss at 230 °C was attributed to the loss of some volatile
compounds, and the weight loss at 450 °C was due to crack and
vaporization of the organic molecules. Therefore, there remains
biomolecules on the surface of the Au/TS-1 catalyst after calcination
at 375 °C. Such residual biomolecules might be even larger than the
1
00
0.0
80
60
40
20
0
-
-
-
-
0.6
1.2
1.8
2.4
SiMe
3
groups, leading to larger steric hindrance on the transfer of
the active intermediate H
2 2
O
from the gold to the Ti sites. This
might explain why the optimal reaction temperature in our case
was even higher than that of the silylation [57].
0
100
200
300
400
500
600
700
800
Although the reaction temperature is relatively high compared
with conventional catalysts, our catalysts are highlighted by their
excellent catalytic stability and high selectivity (above 70%) at a
relatively high reaction temperature. This might also be attributed
to the existence of residual biomolecules on the catalysts which
were speculated to play dual roles as both catalyst surface modifier
and GNP stabilizer. In the first place, modification of the catalyst by
the residual biomolecules, similar to silylation [57], might change
the surface property of the support from hydrophilicity to hydro-
phobicity; the surface acidic sites of the support, which are
assumed to be active for ring-opening reactions [58], were also
passivated, hence favoring easy desorption of PO from the support
and thus inhibiting further oxidation of PO [11,57]. In addition, we
also studied the performance of the 0.5Au/TS-1(35)-IL catalyst by
bio-reduction with the IL assistance that has high extraframework
o
Temperature/ C
Fig. 13. Simultaneous TG and DTG analysis profiles for the biomass of C. platycladi
leaf. The biomass was obtained by removing the water of C. platycladi extract.
catalysts [57]. It has been speculated that the SiMe
3
groups on the
catalyst might inhibit the transfer of the active intermediate H
from the gold to the Ti sites, thus leading to poor activity at low
temperature [57].
Herein, to figure out the possible reasons for the high activity
and selectivity of the 0.5Au/TS-1(48)-IL catalyst, TG and differential
TG (DTG) analysis of the extract of C. platycladi was conducted as
shown in Fig. 13. The extract was dried at 50 °C under vacuum for
2 2
O
1
0 h to obtain the powder plant biomass. It is widely acknowledged

2
00
M. Du et al. / Journal of Catalysis 283 (2011) 192–201
1.0
100
1
1
1
4
2
0
8
6
4
2
0
(
A)
(B)
0
0
0
0
0
.8
.6
.4
.2
.0
80
60
40
2
0
0
0
1
2
3
4
5
6
7
8
9
10
200
300
400
500
600
700
800
Time on stream /h
Wavelength/nm
Fig. 14. Epoxidation of propylene over 0.5Au/TS-1(35)-IL catalyst with high extraframework Ti content (A) and UV–vis spectra (B) of TS-1(35); reaction conditions: Catalyst,
ꢀ
1
ꢀ1
0
.15 g; temperature, 300 °C; feed gas, C
3
H
6
/O
/H
2 2
/N
2
= 1/1/1/7(vol.%); space velocity, 7000 mL h
g
.
cat:
Ti content, as illustrated by the DRUV–vis spectroscopy in Fig. 14B.
It is generally believed that excess of anatase and extraframework
Ti in Au–Ti catalysts could accelerate PO oligomerization, thus
blocking the active sites and deactivating the catalyst [59]. None-
theless, different from the conventionally prepared catalysts [17],
the catalyst 0.5Au/TS-1(35)-IL showed high catalytic stability
the biosynthesis using C. platycladi extract. Therefore, the GNPs
with negative charges can be fully adsorbed on the TS-1 support
by the electrostatic attractive forces. And the interaction between
GNPs and TS-1 supports by the ILEI method was stronger than that
by acidifying the colloid solution. The enhancement of catalytic
performance could be attributed to stronger interaction between
GNPs and TS-1 supports caused by IL. The excellent activity and
high selectivity of the Au/TS-1 catalysts at a relatively high reaction
temperature might be attributed to the existence of residual bio-
molecules on the catalysts. They were supposed to prevent the
GNPs from agglomeration and change the surface property of the
support, thus promoting desorption of PO from the catalysts. We
believe that the ILEI method can be extended to fabrication of other
supported noble metal nanocatalysts, and the resulting materials
could be used as efficient nanocatalysts.
(Fig. 14A). Such a result is consistent with the previous speculation
that existence of the residual biomolecules might be able to
enhance the desorption of PO from the support, thus dramatically
decreasing the chances of PO oligomerization. Also, it has been
proved that biomolecules could work as both reductants of the
metal precursors and stabilizing agents of GNPs [37,44,45,60].
And compared with the citrate-protected GNPs, biosynthesized
GNPs under the protection of unknown biomolecules possess
stronger stability toward salt-induced aggregation [60]. Identically,
in the 0.5Au/TS-1(35)-IL catalyst, the existence of the residual bio-
molecules on the surface of GNPs could prevent GNPs from aggre-
gation even at a relatively high temperature [44,45,60]. As
evidenced by TEM image (Fig. 3B), the GNPs on the support main-
tained the same particle size (3.1 nm) without agglomeration after
calcination at 375 °C.
Acknowledgments
This work was supported by the National Natural Science Foun-
dation of China (Nos. 21036004, 20776120, and 20976146), the
Fundamental Research Funds for the Central Universities (No.
Above is a plausible explanation for the excellent activity and
high selectivity of the Au/TS-1 catalysts at a relatively high reaction
temperature. As a matter of fact, prior to this work, our group has
already reported active Au/TS-1 catalysts by bio-reduction method
without the addition of ionic liquid for propylene epoxidation with
similar selectivity at 300 °C [44,45]. As well, according to Vilchis-
Nestor et al., biosynthesized catalysts demonstrated much better
activity in oxidation and hydrogenation of CO than the convention-
ally prepared catalysts at high temperature [43]. Such results, in
addition to this work, clearly indicate the undeniable advantages
and great potentials of the bio-reduction method in the prepara-
tion of precious metal catalysts. Admittedly, due to the complicate
composition of the aqueous extract of the plant biomass, the exact
roles of the biomolecules in the biosynthesis of catalyst and their
catalytic performance remain unknown, and our efforts are direc-
ted toward achieving this goal in the nearest future.
2
010121051), the Research Fund for the Doctoral Program of Higher
Education of China (No. 20100121110032), and the Natural Science
Foundation of Fujian Province of China (Grant Nos. 2010J05032,
2
010J01052, and 2008J0169). Prof. Mingshu Chen is gratefully
acknowledged for his helpful discussion.
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