Gas-phase epoxidation of propylene by molecular oxygen over Ag/BaCO3 catalysts: Effect of preparation conditions

Article abstract of DOI:10.1016/j.cattod.2016.01.015

Ag/BaCO₃ catalysts with different Ag crystallite sizes, prepared by the reduction-deposition method, were developed for gas-phase epoxidation of propylene to propylene oxide (PO) by molecular oxygen. Effects of preparation conditions, such as pretreatment of organic amines of BaCO₃ support, pretreatment way of ethylene diamine, reduction temperature of HCHO and calcination temperature, on the catalytic performance and Ag crystallite size of Ag/BaCO₃ catalyst were investigated. Ag/BaCO₃ ^* catalyst, with pretreatment of BaCO₃ support by ethylene diamine and prepared at the reduction temperature of HCHO of 10 °C and the calcination temperature of 250 °C, exhibited better catalytic performance and good durability, in which 12.5% of propylene conversion and 36.9% of PO selectivity were achieved under the reaction conditions of 20%C₃H₆–10%O₂–70%N₂, 200 °C, 0.1 MPa and GHSV of 3000 h^?1. Both lower reduction temperature of HCHO and lower calcination temperature can get smaller Ag crystallite sizes, which is more effective for epoxidation of propylene over Ag/BaCO₃ ^* catalyst. The catalytic reaction mechanism of Ag/BaCO₃ ^* catalyst for epoxidation of propylene is that propylene in the gas phase reacts with molecular oxygen species adsorbed on the catalyst surface to produce PO and follows Rideal–Eley mechanism.

Full text of DOI:10.1016/j.cattod.2016.01.015

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Contents lists available at ScienceDirect
Catalysis Today
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Gas-phase epoxidation of propylene by molecular oxygen over
Ag/BaCO3 catalysts: Effect of preparation conditions
Qing Zhang, Yanglong Guo∗, Wangcheng Zhan, Yun Guo, Li Wang, Yunsong Wang,
Guanzhong Lu∗
Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, PR
China
a r t i c l e i n f o
a b s t r a c t
Article history:
Ag/BaCO3 catalysts with different Ag crystallite sizes, prepared by the reduction-deposition method,
were developed for gas-phase epoxidation of propylene to propylene oxide (PO) by molecular oxygen.
Effects of preparation conditions, such as pretreatment of organic amines of BaCO3 support, pretreat-
ment way of ethylene diamine, reduction temperature of HCHO and calcination temperature, on the
Received 15 November 2015
Received in revised form
29 December 2015
Accepted 3 January 2016
Available online xxx
*
catalytic performance and Ag crystallite size of Ag/BaCO3 catalyst were investigated. Ag/BaCO3 catalyst,
with pretreatment of BaCO3 support by ethylene diamine and prepared at the reduction temperature of
HCHO of 10 ◦C and the calcination temperature of 250 ◦C, exhibited better catalytic performance and good
durability, in which 12.5% of propylene conversion and 36.9% of PO selectivity were achieved under the
reaction conditions of 20%C3H6–10%O2–70%N2, 200 ◦C, 0.1 MPa and GHSV of 3000 h−1. Both lower reduc-
tion temperature of HCHO and lower calcination temperature can get smaller Ag crystallite sizes, which is
more effective for epoxidation of propylene over Ag/BaCO3* catalyst. The catalytic reaction mechanism of
Ag/BaCO3* catalyst for epoxidation of propylene is that propylene in the gas phase reacts with molecular
oxygen species adsorbed on the catalyst surface to produce PO and follows Rideal–Eley mechanism.
© 2016 Elsevier B.V. All rights reserved.
Keywords:
Epoxidation of propylene
Propylene oxide
Molecular oxygen
Ag/BaCO3 catalyst
Ag crystallite size
1. Introduction
ciency is usually low. Gas-phase epoxidation of propylene to PO
by molecular oxygen is a one-step green chemical process, which
Epoxidation of propylene to propylene oxide (PO) is one of the
most important reactions in the chemical industry because PO is an
important bulk chemical and widely used for production of propy-
lene glycols, polyether polyols, polyurethane, surfactants and other
commercial products. PO is produced mainly by the chlorohydrin
process, the Halcon process and the hydroperoxide process, how-
ever, which have several shortcomings. The chlorohydrin process is
not environmentally friendly and produces a large number of chlo-
rinated by-products. The Halcon process generates the equimolar
coproducts which values depend on their demands in the market
[1]. The hydroperoxide process needs construction of large-scale
production equipments and its commercialization has been hin-
dered largely by the expense of H2O2 [2]. The Au-based catalysts
can produce PO from epoxidation of propylene by the in-situ pro-
duction of H2O2 from H2–O2 mixture with a high selectivity [3–10].
However, H2 must be used as a sacrificial coreactant and its effi-
has attracted much attention in recent years. Ag nanoparticles of
2–5 nm supported on WO3 nanorods could get 15.5% of propy-
lene conversion with 83% of PO selectivity [11]. Dissociation of O2
at the interface between Ag aggregates and Al2O3 support could
account for high catalytic activity for epoxidation of propylene over
Ag19/Al2O3 catalyst [12]. TiCuOx mixed oxide catalyst with highly
dispersed and stabilized active sites of Cu+ could get 69% of PO
selectivity [13]. A strong synergistic effect between CuOx and RuOx
was found to be responsible for PO formation over RuOx–CuOx/SiO2
catalyst [14]. SnO2–CuO–NaCl/SiO2 catalyst [15] and NaCl–Cu–Mn
mixed metal oxide catalyst [16] were also found to be effective for
epoxidation of propylene by molecular oxygen. Strong interaction
between Cs+ and CuOx nanoparticles favored the selective forma-
tion of PO over Cs+–CuOx/SiO2 catalyst [17] and CuI was thought to
be active sites for PO formation [17,18]. The light-induced reduction
of Cu catalysts could increase PO selectivity [19] and Cu2O rhom-
bic dodecahedra exposing (1 1 1) crystal planes were found to be
most effective for PO formation among Cu2O catalysts with differ-
ent morphologies [20]. The unsupported AgCu bimetallic catalyst
∗
Corresponding authors. Fax: +86 21 64252923.
E-mail addresses: ylguo@ecust.edu.cn (Y. Guo), gzhlu@ecust.edu.cn (G. Lu).
http://dx.doi.org/10.1016/j.cattod.2016.01.015
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: Q. Zhang, et al., Gas-phase epoxidation of propylene by molecular oxygen over Ag/BaCO3 catalysts:
Effect of preparation conditions, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.01.015

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with 15–20 nm or 30–50 nm Ag particle size for epoxidation of
propylene by molecular oxygen at high pressure. DFT calculations
showed that CO2 was the main product for epoxidation of propy-
lene by O2 or H2–O2 mixtures irrespective of the silver particle size
[35].
Ag(111)
Ag*/BaCO3
In this paper, unpromoted Ag/BaCO3 catalysts with low Ag
loading were prepared by the reduction-deposition method and
investigated for gas-phase epoxidation of propylene by molecular
oxygen at atmospheric pressure. Effects of preparation conditions,
such as pretreatment of organic amines of BaCO3 support, pre-
treatment way of ethylene diamine, calcination temperature and
reduction temperature of HCHO, on the catalytic performance and
Ag crystallite size of Ag/BaCO3 catalyst were investigated, and the
catalytic reaction mechanism for epoxidation of propylene was
proposed.
*
Ag/BaCO3
Ag/BaCO3
70
2. Experimental
10
20
30
40
50
60
80
2θ (o)
2.1. Preparation of catalysts
*
Fig. 1. XRD patterns of Ag/BaCO3, Ag/BaCO3 and Ag*/BaCO3 catalysts.
Ag/BaCO3 catalyst was prepared by the reduction-deposition
method. At room temperature, 2 g of formaldehyde (HCHO) was
dissolved in 40 mL of deionized water as the reductant, into which
8 g of BaCO3 support was added to form a slurry. After stirring for
20 min, 0.6 g of AgNO3 dissolved in 40 mL of deionized water was
added dropwise into the above slurry, then the solid was filtered
and washed with deionized water and ethanol for several times.
The solid was dried overnight at room temperature and then cal-
cined in N2 atmosphere at 450 ◦C for 4 h. The similar procedure was
used to prepare Ag*/BaCO3 catalyst except that AgNO3 was firstly
dissolved in deionized water and then ethylene diamine was added
N
N
CH2
CH2
NH2
NH2
+ ethylene diamine
+ HCHO
N
CH2
N
N
CH2
Ag
CH2
Ag+ + HCHO
calcination
+ AgNO3
Ag
N CH2
*
until the solution became clear. Ag/BaCO3 catalyst was prepared
by the similar method to Ag/BaCO3 catalyst except that BaCO3 sup-
port was firstly immersed into ethylene diamine aqueous solution
for 12 h and then dried under vacuum at 60 ◦C for 4.5 h.
*
Fig. 2. Schematic diagram of the preparation process of Ag/BaCO3 catalyst.
was more stable than unsupported Cu catalyst for epoxidation of
propylene [21].
2.2. Epoxidation of propylene
Epoxidation of ethylene by molecular oxygen over Ag-based
catalysts has been commercialized for several decades, in which
83–95% of selectivity of ethylene oxide is obtained [22–24]. Ag-
based catalysts have also been investigated for epoxidation of
propylene by molecular oxygen. However, PO selectivity is low
over unpromoted Ag catalysts, because propylene has the active
␣-H atoms and ␣-H atoms can easily react with oxygen to result
in combustion of propylene to CO2 and H2O. Therefore, various
attempts have been made to modify Ag catalysts for epoxidation of
propylene. NaCl, CuCl, Y2O3, K2O and MoO3 have been used as pro-
moters to enhance PO selectivity [25–30]. These Ag-based catalysts
were effective for epoxidation of propylene, but they were high
Ag loading or heavily promoted. AgCu bimetallic catalyst with low
loadings of Ag and Cu, could get PO selectivity of 55.1% at propylene
conversion of 3.6% [31].
Gas-phase epoxidation of propylene by molecular oxygen over
Ag-based catalysts is a structure-sensitive reaction, but opinions
are different. Zemichael et al. [27] found that Ag/CaCO3 catalyst
with Ag nanoparticles of 20–40 nm exhibited better catalytic per-
formance. Lu et al. [32] showed that large Ag nanoparticles favored
ethylene epoxidation by 3–5 fold at 473–493 K, whereas Ag parti-
cle size did not have a large effect on propylene epoxidation over
Ag/CaCO3 catalyst. Molina et al. [33] found that Ag/Al2O3 catalyst
with Ag nanoparticles of 23.3 nm was more selective towards PO
formation. Lei et al. [34] found that size-selected Ag3 clusters and Ag
nanoparticles of ∼3.5 nm on alumina supports could catalyze epox-
idation of propylene by molecular oxygen with high PO selectivity
at low temperature. Ghosh et al. [11] found that Ag/WO3 catalyst
with 2–5 nm Ag particle size was much more effective than those
Epoxidation of propylene was carried out in a fixed-bed quartz
reactor under the reaction conditions of 0.6 g catalyst, 200 ◦C,
0.1 MPa and GHSV of 3000 h−1. The feed gas consisted of 20% propy-
lene, 10% O2 and balance N2. Reaction products were analyzed
by two on-line gas chromatographs equipped with a two packed
columns (G.D.X-401 and Porapak Q) with FID and TCD detectors.
All lines between the reactor exit and the gas chromatographs
were heated to 120 ◦C to prevent condensation of the products. PO
selectivity and propylene conversion were calculated on a carbon
balance basis.
2.3. Characterization of catalysts
Elemental analysis was done by inductively coupled-plasma
atomic emission spectroscopy (ICP-AES) using a TJA IRIS ADVAN-
TAG 1000 instrument.
XRD patterns were performed on a Bruker AXS D8 Focus
diffractometer operated at 40 kV, 40 mA (Cu K␣ radiation,
ꢀ = 0.15406 nm).
TEM images were recorded on a JEM-1400 transmission elec-
tron microscope. The sample was ultrasonically suspended in the
ethanol solvent, and one drop of this slurry was deposited on a
copper grid. The liquid phase was evaporated before the grid was
loaded into the microscope.
XPS spectra were recorded on a Thermo ESCALAB 250 spec-
trometer with a monochromatized AlK␣ X-ray source (1486.6 eV),
and a passing energy of 20 eV. C1s (binding energy of 284.6 eV) of
adventitious carbon was used as the reference.
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3
*
Fig. 3. TEM images and the corresponding Ag particle size distribution histograms of Ag/BaCO3 (A) (D), Ag/BaCO3 (B) (E), Ag*/BaCO3 catalysts (C) (F).
*
Temperature programmed desorption of O2 (O2-TPD) and C3H6
(C3H6-TPD) and temperature programmed oxidation of O2 (O2-
TPO) were carried out on a Micromeritics AutoChem 2920 II
chemisorption analyzer with a Hiden HPR 20 mass spectrome-
ter. For O2-TPD and C3H6-TPD, 500 mg samples of 40–60 mesh
were loaded in a quartz reactor and pretreated in a He flow of
30 mL min−1 at 250 ◦C for 0.5 h. After it was cooled down to room
temperature, a gas flow of O2–He (3 vol. % O2) or a gas flow of
C3H6–Ar (1 vol. % C3H6) of 20 mL min−1 was introduced into the
reactor, the reactor temperature was raised to 200 ◦C at a rate of
10 ◦C min−1 and was kept at 200 ◦C for 0.5 h, then cooled down
to room temperature. After that, a He flow of 20 mL min−1 was
switched. When the baseline was stable, the reactor temperature
was raised to 600 ◦C at a heating rate of 10 ◦C min−1. The MS sig-
nals of O2 (m/z = 32) or C3H6 (m/z = 41) were recorded by the mass
spectrometer. For O2-TPO experiment, a gas flow of O2–He (3 vol. %
O2) of 10 mL min−1 was introduced into the quartz reactor loaded
with 500 mg samples of 40–60 mesh. The reactor temperature was
raised to 250 ◦C at a heating rate of 10 ◦C min−1. The MS signals of
CO2 (m/e = 44) and H2O (m/e = 18) were recorded.
As shown in Table 2, Ag loadings of Ag/BaCO3, Ag/BaCO3 and
Ag*/BaCO3 catalysts were at the similar values of 4.0 wt.%, 4.3 wt.%
and 4.3 wt.%, respectively. However, the catalytic performance of
these three catalysts varied considerably. When AgNO3 reacted
with ethylene diamine to form a complex and then reduced by
HCHO, the catalytic performance of Ag*/BaCO3 catalyst was not
improved compared with Ag/BaCO3 catalyst, in which propylene
conversion and PO selectivity were 1.0% and 24.2%, respectively.
3.2. XRD
*
Fig.
1
shows XRD patterns of Ag/BaCO3, Ag/BaCO3 and
Ag*/BaCO3 catalysts. As shown in Fig. 1, over these three catalysts,
only the characteristic diffraction peak of Ag (111) at 2ꢁ = 38.1◦
and the diffraction peaks of BaCO3 support were observed, which
was similar to Ag–Cu/BaCO3 catalyst [31]. Ag crystallite size was
determined by Scherrer’s equation with the full peak width at half
maximum height of the diffraction peak of Ag (1 1 1) from XRD
patterns. As shown in Table 2, Ag crystallite sizes of Ag/BaCO3,
Ag*/BaCO3 and Ag/BaCO3 catalysts were about 29 nm, 31 nm and
*
20 nm, respectively, which indicated that when BaCO3 support was
*
3. Results and discussion
pretreated with ethylene diamine, Ag/BaCO3 catalyst could get
much smaller Ag crystallite size. The schematic diagram of the
*
3.1. Catalytic performance of catalysts
preparation process of Ag/BaCO3 catalyst was proposed in Fig. 2.
The pretreatment of BaCO3 support with ethylene diamine made
BaCO3 support functionalized with amine groups. When BaCO3
support functionalized with amine groups was added into HCHO
aqueous solution, amine groups reacted with HCHO to form a Schiff
base (the N CH2 group) which had a strong complexation with
Ag+ due to metal to ligand back-bonding benefit for immobilization
and localization of Ag+ on the surface of BaCO3 support [36]. Then,
the excess HCHO reduced the complex to form Ag nanoparticles
deposited on the surface of BaCO3 support, leading to high dis-
persion of Ag nanoparticles with smaller Ag crystallite size. In this
preparation process, functionalization of BaCO3 support with ethy-
lene diamine was very important for anchoring Ag nanoparticles
due to the strong interaction between them. Yu et al. [36] prepared
the ultrasmall Ag nanoparticles based on the introduction of the
Table 1 shows effect of the pretreatment of different organic
amines of BaCO3 supports on the catalytic performance of
Ag/BaCO3 catalysts for gas-phase epoxidation of propylene by
*
molecular oxygen. As shown in Table 1, the pretreatment of organic
amines except for ethylene diamine of BaCO3 supports decreased
both propylene conversion and PO selectivity compared with
Ag/BaCO3 catalyst. When BaCO3 support was pretreated with ethy-
*
lene diamine, the catalytic performance of Ag/BaCO3 catalyst was
better than that of Ag/BaCO3 catalyst, in which propylene conver-
sion increased dramatically from 1.5% to 8.1%, and PO selectivity
increased obviously from 22.8% to 31.9%. Then, we investigated
effect of the pretreatment way of ethylene diamine on the catalytic
performance of Ag/BaCO3 catalysts, which is shown in Table 2.
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Table 1
*
Effect of the pretreatment of different organic amines of BaCO3 supports on the catalytic performance of Ag/BaCO3 catalysts.
Organic amines
Propylene conversion (%)
PO selectivity (%)
CO2 selectivity (%)
Blank
Diethylamine
Triethylamine
n-Butylamine
Ethylene diamine
1,6-Hexamethylene diamine
Triethylenetetramine
1.5
0.6
0.8
0.6
8.1
1.5
0.9
22.8
21.0
20.5
23.0
31.9
1.6
77.2
79.0
79.5
77.0
68.1
98.4
80.4
19.6
Reaction conditions: 0.6 g catalyst, 20%C3H6–10%O2–70%N2, 200 ◦C, 0.1 MPa, 3000 h−1
.
Table 2
Effect of the pretreatment way of ethylene diamine on the catalytic performance of Ag/BaCO3 catalysts.
Catalysts
Ag loading (wt.%)a
Propylene conversion (%)
PO selectivity (%)
Ag crystallite size (nm)b
Ag particle size (nm)c
Ag/BaCO3
Ag/BaCO3
4.0
4.3
4.3
1.5
8.1
1.0
22.8
31.9
24.2
29
20
31
30
20
32
*
Ag*/BaCO3
Reaction conditions: 0.6 g catalyst, 20%C3H6–10%O2–70%N2, 200 ◦C, 0.1 MPa, 3000 h−1
.
a
Measured by ICP-AES.
Determined from XRD patterns by Scherrer’s equation.
b
c
Determined from particle size distribution histograms.
functional monomer salicylaldimine Schiff base into a SiO2 matrix.
Yang et al. [37] prepared Pt–Ru nanoparticles dispersed uniformly
on the ethylene diamine-grafted CNTs. Wang et al. [38] developed
a two-step method including functionalization of the support with
amine groups for synthesis of Au–Ag alloy, which proved effective
in producing uniform bimetallic nanoparticles.
Ag*/BaCO3
*
Ag/BaCO3 catalyst with smaller Ag crystallite size got higher
propylene conversion and higher PO selectivity than Ag/BaCO3 cat-
alyst, which indicated that smaller Ag crystallite size was more
effective for epoxidation of propylene by molecular oxygen.
Ag/BaCO3*
3.3. TEM
Ag/BaCO3
378
Fig. 3 shows TEM images and Ag particle size distribution
histograms of Ag/BaCO3, Ag/BaCO3 and Ag*/BaCO3 catalysts. As
*
shown in Fig. 3, spherical-like Ag nanoparticles were supported
on the surface of bar-like BaCO3 supports. The number of Ag
nanoparticles on the surface of Ag/BaCO3* catalyst was greater than
those of Ag/BaCO3 and Ag*/BaCO3 catalysts, and Ag particle size
380
376
374
372
370
368
366
Binding Energy (eV)
*
*
Fig. 4. Ag3d XPS spectra of Ag/BaCO3, Ag/BaCO3 and Ag*/BaCO3 catalysts.
of Ag/BaCO3 catalyst was much smaller than those of Ag/BaCO3
and Ag*/BaCO3 catalysts. These three catalysts had a wide distribu-
tion of Ag particle size probably because of the preparation method
and the low surface area of BaCO3 support of 5 m2 g−1. The particle
size of most of Ag nanoparticles of Ag/BaCO3 and Ag*/BaCO3 cata-
lysts were from 30 nm to 32 nm, and the particle size of most of Ag
3.5. Stability of catalysts
*
Fig. 5 and Fig. 6 show the stability of Ag/BaCO3, Ag/BaCO3 and
Ag*/BaCO3 catalysts for epoxidation of propylene by molecular oxy-
gen at the reaction temperatures of 200 ◦C and 160 ◦C, respectively.
As shown in Fig. 5, for these three catalysts at the reaction temper-
ature of 200 ◦C, with an increase in time on stream, PO selectivity
kept nearly the same values, while propylene conversion changed
a little firstly and then remained steady, which implied that there
was an induction period for the epoxidation reaction. For Ag/BaCO3
and Ag*/BaCO3 catalysts, propylene conversion increased gradually
and then reached at 2.9% and 3.4%, respectively, after reaction for
*
nanoparticles of Ag/BaCO3 catalyst was about 20 nm, which were
consistent with the results of XRD characterization.
3.4. XPS
*
Fig. 4 shows Ag3d XPS spectra of Ag/BaCO3, Ag/BaCO3 and
Ag*/BaCO3 catalysts. The binding energies of Ag3d3/2 and Ag3d5/2
of Ag/BaCO3, Ag/BaCO3 and Ag*/BaCO3 catalysts were at the
*
*
8 h. For Ag/BaCO3 catalyst, propylene conversion increased from
same values of 374.3 eV and 368.3 eV, respectively, which were
attributed to Ag◦. This indicated the formation of metal state of
silver nanoparticles on the surface of BaCO3 support in consistent
with TEM images. At the same time, N element on the surface of
BaCO3 support was not detected in the XPS spectra, which implied
that ethylene diamine just played its role in the preparation pro-
cess of the catalyst and then decomposed during the calcination
process.
6.3% to 8.1% at the first hour and then decreased a little. Propy-
lene conversion of 7.2% and PO selectivity of 31% were achieved
*
after reaction for 8 h over Ag/BaCO3 catalyst. Therefore Ag/BaCO3,
Ag/BaCO3 and Ag*/BaCO3 catalysts exhibited good durability for
*
epoxidation of propylene at the reaction temperature of 200 ◦C.
However, as shown in Fig. 6, the stability of these three cat-
alysts at the reaction temperature of 160 ◦C was totally different
from the stability at the reaction temperature of 200 ◦C shown in
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Fig. 5. With an increase in time on stream, PO selectivity kept almost
the same values, but propylene conversion decreased significantly.
After reaction for several hours, propylene conversion decreased to
nearly 0 over Ag/BaCO3 and Ag*/BaCO3 catalysts and to 0.4% over
Ag/BaCO3* catalyst, which indicated that these three catalysts were
easily deactivated at low temperature due to coke deposition.
O2-TPO characterization was performed to demonstrate
whether coke deposition led to deactivation of Ag/BaCO3,
(A)
(B)
(C)
30
20
10
10
5
0
10
0
30
20
10
5
Ag/BaCO3 and Ag*/BaCO3 catalysts for epoxidation of propylene
*
at the reaction temperature of 160 ◦C, which is shown in Fig. 7.
The MS peaks of CO2 (m/e = 44) and H2O (m/e = 18) were observed
at the same temperatures, which demonstrated that carbonaceous
species on the surface of the deactivated catalysts were oxidized.
The MS peaks of CO2 and H2O of the deactivated Ag/BaCO3* catalyst
were observed at about 170 ◦C, while the peaks of CO2 and H2O of
the deactivated Ag/BaCO3 and Ag*/BaCO3 catalysts were observed
at about 190 ◦C. Carbonaceous species could be oxidized to CO2
and H2O at lower temperature over the deactivated Ag/BaCO3* cat-
alyst with smaller Ag crystallite size. Bhavani et al. [39] investigated
how the metal Ni particle size influenced the coke formation over
Ni-M/AlCeZrOx bimetallic catalysts for the oxidative CO2 reform-
ing of CH4, in which coke deposition was much easier to react with
oxygen over the catalyst with smaller Ni particle size.
0
10
0
30
20
10
0
5
0
0
1
2
3
4
5
6
7
8
Time on stream (h)
Fig. 5. The catalyst stability for epoxidation of propylene over Ag/BaCO3 catalyst
*
(A), Ag/BaCO3 catalyst (B) and Ag*/BaCO3 catalyst (C).
Reaction conditions: 0.6 g catalyst, 20%C3H6–10%O2–70%N2, 200 ◦C, 0.1 MPa,
3000 h−1
.
(A)
(B)
(C)
40
30
20
10
3
2
1
3.6. Effect of Ag crystallite size
3.6.1. Effect of calcination temperature
Smaller Ag crystallite sizes could get higher propylene conver-
0
3
0
*
sion and higher PO selectivity, comparing Ag/BaCO3, Ag/BaCO3
40
and Ag*/BaCO3 catalysts. In order to further optimize the catalytic
30
20
10
*
2
1
performance of Ag/BaCO3 catalyst for epoxidation of propylene,
experiments were performed to investigate effect of Ag crystallite
size by changing preparation parameters.
Thermal treatment is a simple and efficient way to control the
crystallite size of metal catalysts. Sintering and aggregation of Ag
nanoparticles happened more or less with the calcination tem-
perature over 200 ◦C [40]. Fig. 8 shows effect of the calcination
0
3
0
40
30
20
10
0
2
1
0
*
temperature on the catalytic performance of Ag/BaCO3 catalyst
for epoxidation of propylene. As shown in Fig. 8, the calcination
temperature had a significant effect on propylene conversion but
little effect on PO selectivity over Ag/BaCO3* catalyst. When the cal-
cination temperature increased from 250 ◦C to 650 ◦C, propylene
conversion decreased greatly from 9.5% to 2.7%, and PO selectivity
decreased gradually from 33.7% to 29.9%.
0
1
2
3
4
5
Time on stream ( h )
Fig. 6. The catalyst stability for epoxidation of propylene over Ag/BaCO3 catalyst
*
(A), Ag/BaCO3 catalyst (B), Ag*/BaCO3 catalyst (C).
Reaction conditions: 0.6 g catalyst, 20%C3H6–10%O2–70%N2, 160 ◦C, 0.1 MPa,
3000 h−1
.
*
Fig. 9 shows XRD patterns of Ag/BaCO3 catalysts with differ-
ent calcination temperatures. The diffraction peak of Ag (1 1 1) at
(B)
(A)
Ag/BaCO3
Ag/BaCO3
Ag/BaCO3*
Ag*/BaCO3
*
Ag/BaCO3
Ag*/BaCO3
125
100
150
175
200
225
250
100
125
150
175
200
225
250
Temperature (oC)
Temperature (oC)
Fig. 7. O2-TPO profiles of the deactivated catalysts for epoxidation of propylene at the reaction temperature of 160 ◦C: (A) CO2 MS signal (m/z = 44), (B) H2O MS signal
(m/z = 18).
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20
40
Ag(111)
650oC
550oC
450oC
15
10
5
30
20
10
0
350oC
250oC
0
200
300
400
500
600
700
10
20
30
40
2
50
60
70
80
Calcination temperature ( oC )
o
θ
( )
Fig. 8. Effect of the calciantion temperature on the catalytic performance of
Fig. 9. XRD patterns of Ag/BaCO3* catalysts with different calcination temperatures.
*
Ag/BaCO3 catalyst for epoxidation of propylene.
Reaction conditions: 0.6 g catalyst, 20%C3H6–10%O2–70%N2, 200 ◦C, 0.1 MPa,
3000 h−1
.
Fig. 10 shows effect of the reduction temperature of HCHO
on the catalytic performance of Ag/BaCO3 catalyst for epoxida-
2ꢁ = 38.1◦ became much sharper with increasing the calcination
temperature. Ag crystallite size, determined by Scherrer’s equation,
grew from about 20 nm at the calcination temperature of 250 ◦C
to about 34 nm at the calcination temperature of 650 ◦C. Metal
catalysts usually sinter and aggregate seriously at high temper-
*
tion of propylene. These four catalysts with different reduction
temperature of HCHO were calcined at 250 ◦C in N2 atmosphere.
As shown in Fig. 10, the reduction temperature of HCHO had a
*
great effect on the catalytic performance of Ag/BaCO3 catalyst.
*
atures [41,42], however, Ag nanoparticles of Ag/BaCO3 catalyst
With the reduction temperature of HCHO increasing from 10 ◦C to
50 ◦C, propylene conversion decreased significantly from 12.5% to
1.6%, and PO selectivity decreased greatly from 36.9% to 16.5%.
*
didn’t grow much, which indicated that Ag/BaCO3 catalyst had
a good thermostability. Therefore both propylene conversion and
PO selectivity decreased with the growth of Ag crystallite size of
*
Fig. 11 shows XRD patterns of Ag/BaCO3 catalysts with dif-
*
Ag/BaCO3 catalyst by increasing the calcination temperature.
ferent reduction temperatures of HCHO. The diffraction peak of
Ag (1 1 1) at 2ꢁ = 38.1◦ became much sharper with increasing the
reduction temperature of HCHO. Ag crystallite size, determined
by Scherrer’s equation, grew from about 18 nm at the reduction
temperature of HCHO of 10 ◦C to about 40 nm at the reduction
temperature of HCHO of 50 ◦C. The reasons for the growth of Ag
crystallite size were as follows. With increasing the reduction tem-
3.6.2. Effect of reduction temperature of HCHO
*
Ag/BaCO3 catalyst with smaller Ag crystallite sizes at lower
calcination temperature could get better catalytic performance for
epoxidation of propylene, therefore we chose 250 ◦C as the calci-
nation temperature in the following researches.
20
15
10
5
40
30
20
10
0
0
10
20
30
40
50
HCHO reduction temperature(oC)
*
Fig. 10. Effect of reduction temperature of HCHO on the catalytic performance of Ag/BaCO3 catalyst for epoxidation of propylene.
Reaction conditions: 0.6 g catalyst, 20%C3H6–10%O2–70%N2, 200 ◦C, 0.1 MPa, 3000 h−1
.
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Effect of preparation conditions, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.01.015

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Ag(111)
50oC
35oC
20oC
10oC
0
100
200
300
400
500
600
-100
10
20
30
40
50
60
70
80
Temperature (oC)
o
2 ( )
θ
*
Fig. 12. C3H6-TPD profile of Ag/BaCO3 catalyst at the reduction temperature of
Fig. 11. XRD patterns of Ag/BaCO3* catalysts with different reduction temperatures
of HCHO.
HCHO of 10 ◦C.
50oC
35oC
perature of HCHO, the reducing power of HCHO became stronger
and then led to faster reduction of Ag cations to larger Ag nanopar-
ticles. Moreover, at higher reduction temperature of HCHO, Ag
nanoparticles aggregated faster. Therefore both propylene conver-
sion and PO selectivity decreased with the growth of Ag crystallite
size of Ag/BaCO3* catalyst by increasing the reduction temperature
of HCHO.
20oC
Both lower calcination temperature and lower reduction tem-
perature of HCHO can get smaller Ag crystallite sizes, which is more
effective for epoxidation of propylene by molecular oxygen over
*
*
Ag/BaCO3 catalyst. Ag/BaCO3 catalyst with the Ag crystallite size
of about 18 nm, prepared at the reduction temperature of HCHO
of 10 ◦C and the calcination temperature of 250 ◦C in N2 atmo-
sphere, achieved 12.5% of propylene conversion and 36.9% of PO
selectivity at the reaction temperature of 200 ◦C. Although Ag/WO3
catalyst could achieve 15.5% of propylene conversion and 83% of
PO selectivity for epoxidation of propylene by molecular oxygen
at high reaction pressure of 2 MPa and 250 ◦C, Ag/WO3 catalyst
only achieved 3% of propylene conversion and 27% of PO selec-
tivity at 0.1 MPa and 250 ◦C [11], which is much worse than the
10oC
50
100 150 200 250 300 350 400 450 500 550 600
Temperature (oC)
*
Fig. 13. O2-TPD profiles of Ag/BaCO3 catalysts with different reduction tempera-
tures of HCHO.
*
catalytic performance of Ag/BaCO3 catalyst in this work. Unfor-
*
tunately, Ag/BaCO3 catalyst with Ag crystallite size smaller than
tion temperatures of HCHO. As shown in Fig. 13, the desorption
peak of O2 was observed from 100 ◦C to 250 ◦C, which indicated
that O2 could adsorb on the catalyst surface at the reaction temper-
ature of 200 ◦C and there was strong interaction between O2 and
Ag nanoparticles. Moreover, the O2 uptakes decreased significantly
with an increase in the reduction temperature of HCHO, which indi-
cated that larger Ag nanoparticles adsorbed fewer oxygen species
because of the fewer active sites. As shown in Fig. 13, there were
three types of the overlapping peaks of O2-TPD from 50 ◦C to 600 ◦C.
Based on O2-TPD profiles of an electrolytic silver catalyst reported
in the literature [43], these three peaks of O2-TPD were attributed
to desorption of three types of oxygen species from the catalyst
surface. The peak of O2-TPD at 150 ◦C was attributed to desorption
of a weakly held oxygen species, identified as a state of molec-
ularly chemisorbed oxygen species (nominally superoxo species).
The peak of O2-TPD at 200–220 ◦C was attributed to desorption of
weakly chemisorbed surface atomic oxygen species. The peak of
O2-TPD at 350–400 ◦C arose from segregation and desorption of
bulk-dissolved (subsurface) oxygen species. Because the reaction
temperature was 200 ◦C, we just discussed the former two oxy-
gen species here. The desorption peak of molecular oxygen species
gradually disappeared and the desorption peak of atom oxygen
18 nm couldn’t be prepared in this work because of the low surface
area of BaCO3 support of 5 m2 g−1. Therefore smaller Ag crystallite
size is more effective for epoxidation of propylene by molecular
oxygen over Ag/BaCO3 catalyst with Ag crystallite size between
18 nm and 40 nm.
*
3.7. Catalytic reaction mechanism for epoxidation of propylene
To get a knowledge of how Ag crystallite size affected the
*
catalytic performance of Ag/BaCO3 catalyst, the temperature-
*
programmed desorption and kinetic experiments of Ag/BaCO3
catalysts with different reduction temperatures of HCHO were
performed. Fig. 12 and Fig. 13 show temperature-programmed des-
*
orption of C3H6 and O2 over Ag/BaCO3 catalyst, respectively. As
shown in Fig. 12, only a desorption peak of C3H6 from −100 ◦C to
0 ◦C was observed over Ag/BaCO3 catalyst (prepared at the reduc-
*
tion temperature of HCHO of 10 ◦C and the calcination temperature
of 250 ◦C), which indicated that C3H6 could not adsorb on the cat-
alyst surface at the reaction temperature of 200 ◦C and there was
very weak interaction between C3H6 and Ag nanoparticles. Fig. 13
shows O2-TPD profiles of Ag/BaCO3* catalysts with different reduc-
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9
(B)
(A)
9
8
8
7
6
5
7
y=4.3+0.91x
y=6.49+0.18x
3.5
6
5
1.5
2.0
2.5
3.0
3.5
4.0
4.5
2.0
2.5
3.0
4.0
ln PC3H6 (KPa)
ln PO2 (KPa)
Fig. 14. Dependence of the reaction rates of C3H6 on the partial pressure of C3H6 (A) or the partial pressure of O2 (B).
species moved to lower temperature with increasing the reduc-
tion temperature of HCHO. Therefore molecular oxygen species
were probably responsible for epoxidation of propylene to PO by
molecular oxygen.
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Please cite this article in press as: Q. Zhang, et al., Gas-phase epoxidation of propylene by molecular oxygen over Ag/BaCO3 catalysts:
Effect of preparation conditions, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.01.015