Multimetallic catalysts of RuO2-CuO-Cs2O-TiO2/SiO2 for direct gas-phase epoxidation of propylene to propylene oxide

Article abstract of DOI:10.1039/c6ra12559j

RuO₂-CuO/SiO₂ catalysts doped with Cs₂O and TiO₂ were investigated for the direct gas phase epoxidation of propylene to propylene oxide (PO) using molecular oxygen under atmospheric pressure. The optimal catalyst was achieved at Ru/Cu/Cs/Ti = 8.3/4.2/0.6/0.8 by weight and total metal loading of 21 wt% on SiO₂ support. NH₃ and CO₂ temperature programmed desorption measurements of RuO₂-CuO/SiO₂ catalyst modified with Cs₂O showed that the surface's acidity decreased, resulting in enhanced PO selectivity. The addition of TiO₂ increased the PO formation rate by promoting the synergy effect between RuO₂ and CuO. Using the Box-Behnken design of experiments on the RuO₂-CuO-Cs₂O-TiO₂/SiO₂ catalyst, an extraordinarily high optimal PO formation rate of 3015 g_PO h^-1 kg_cat^-1 was obtained with a feed comprised of O₂/C₃H₆ at a volume ratio of 3.1 and (O₂ + C₃H₆)/He at a volume ratio of 0.26, all at 272 °C and 34 cm³ min^-1. To the knowledge of the authors, this is the highest PO formation rate ever reported for direct propylene epoxidation via O₂.

Full text of DOI:10.1039/c6ra12559j

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PAPER
Multimetallic catalysts of RuO2–CuO–Cs2O–TiO2/
SiO2 for direct gas-phase epoxidation of propylene
to propylene oxide†
Cite this: RSC Adv., 2016, 6, 56116
a
ab
P. Phon-in,a K. Charoen,a T. Witoon,a W. Donphai,ac
*
T. Chukeaw, A. Seubsai,
P. Parpainainar,a M. Chareonpanich,ac D. Noon,d B. Zohourd and S. Senkand
RuO2–CuO/SiO2 catalysts doped with Cs2O and TiO2 were investigated for the direct gas phase
epoxidation of propylene to propylene oxide (PO) using molecular oxygen under atmospheric pressure.
The optimal catalyst was achieved at Ru/Cu/Cs/Ti ¼ 8.3/4.2/0.6/0.8 by weight and total metal loading of
21 wt% on SiO2 support. NH3 and CO2 temperature programmed desorption measurements of RuO2–
CuO/SiO2 catalyst modified with Cs2O showed that the surface's acidity decreased, resulting in
enhanced PO selectivity. The addition of TiO2 increased the PO formation rate by promoting the synergy
effect between RuO2 and CuO. Using the Box–Behnken design of experiments on the RuO2–CuO–
Received 14th May 2016
Accepted 6th June 2016
Cs2O–TiO2/SiO2 catalyst, an extraordinarily high optimal PO formation rate of 3015 gPO hꢀ1 kgcat was
ꢀ1
obtained with a feed comprised of O2/C3H6 at a volume ratio of 3.1 and (O2 + C3H6)/He at a volume
ratio of 0.26, all at 272 ꢁC and 34 cm3 minꢀ1. To the knowledge of the authors, this is the highest PO
formation rate ever reported for direct propylene epoxidation via O2.
DOI: 10.1039/c6ra12559j
www.rsc.org/advances
catalysts (C3H6 + 1/2O2 / C3H6O). In the typical mechanism of
PO generation, O2 rst chemisorbs onto a solid catalyst's active
Introduction
center then dissociates to generate an adsorbed oxygen species
(Oa) which subsequently reacts with propylene to form relevant
intermediates such as allyl radicals, oxametallocycle, and other
intermediates. The key in this process is to use a catalyst suit-
able for the generation of the oxametallocycle that will, in turn,
form the PO product. The other intermediates mostly undergo
further combustion, i.e. producing CO2 and H2O.1,4 However,
the search for a catalyst capable of sustainable industrial-scale
PO production (i.e. >70% PO selectivity with 10% propylene
conversion)4 has been challenging due to the fact that the
current state-of-the-art catalysts have one or more of the
following shortcomings: high PO selectivity but low propylene
conversion or vice versa,5 low stability,6,7 and/or the need for
a costly additional co-feed (e.g. NOx, H2).8
Ag-based catalysts were rst investigated because they were
highly effective for the epoxidation of ethylene.1 However,
partial combustion preferentially takes place when applied to
propylene epoxidation due to the abstraction of an allylic
hydrogen from C3H6 by an adsorbed neighboring oxygen on the
Ag surface.1,9 Cu-based catalysts for the epoxidation of
propylene have been the focus of current research since Cu was
found to be a much more intrinsically selective epoxidation
catalyst for alkenes containing allylic hydrogens than Ag.10 This
is because the adsorbed oxygen atoms on Cu surfaces have low
basicity.11,12 So the adsorbed oxygen atoms favor interaction
with the pi-bond of propylene molecules and form oxame-
tallocycle to create PO molecules. Bi-, tri-, or multi-metallic
Propylene oxide (PO) is one of the most important feedstocks
used in the production of numerous commercial products.1
Global PO production is approximately 7.5 million tons per
year2 and is forecast to grow at a rate of about 4.2% per year
between 2015 and 2020.3 Currently, the chlorohydrin and
hydroperoxide processes are the two major industrial technol-
ogies used for PO manufacture. The latter can be divided into
two main routes: co-product and hydrogen peroxide to
propylene oxide (HPPO). All have major pitfalls. For example,
the chlorohydrin and co-product routes, respectively, produce
byproducts that are environmentally hazardous (e.g. 1,2-
dichloropropane) and of less economic value (e.g. styrene or
tert-butyl alcohol) compared to PO. For HPPO, the costly
production or acquisition of H2O2 is still a major drawback.1
Thus, in the past several years, research regarding PO synthesis
has been focused on the direct gas-phase epoxidation of
propylene with molecular oxygen by using heterogeneous
aDepartment of Chemical Engineering, Faculty of Engineering, Kasetsart University,
Bangkok, 10900, Thailand. E-mail: fengasn@ku.ac.th
bCenter for Advanced Studies in Industrial Technology and Faculty of Engineering,
Kasetsart University, Bangkok, 10900, Thailand
cNANOTEC Center for Nanoscale Materials Design for Green Nanotechnology,
Kasetsart University, Bangkok, 10900, Thailand
dDepartment of Chemical and Biomolecular Engineering, University of California Los
Angeles, CA, 90095, USA
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra12559j
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catalysts of Cu have been reported in the most recent studies ratio of the feed gases was O2/C3H6/He ¼ 2/1/97 at a total ow
due to the coexistence of distinct additional solid phase rate of 50 cm3 minꢀ1 (GHSV ¼ 848 hꢀ1) controlled by using
imparting synergistic effects. Examples include: Ag–Cu/ mass ow controllers (KOFLOC 3810 DSII). The reactor
BaCO3,13 RuO2–CuO–NaCl/SiO2,14,15 SnO2–CuO–NaCl/SiO2,16 temperature was set at 250 C under atmospheric pressure. In
ꢁ
Sb2O3–CuO–NaCl/SiO2,17 Cs+–CuOx/SiO2,18 Ti-modied Cu2O,19 the second step, the Box–Behnken design was applied to
etc.7,20–23 Crystalline CuOx was suggested to play the key role in determine optimal conditions for obtaining the maximized PO
the epoxidation of propylene while the co-component provides product. Four operating parameters were studied in this work:
a surface for dissociative O2 adsorption and subsequent surface reaction temperature (190–310 ꢁC), O2/C3H6 volume ratio
migration to CuOx for PO synthesis.24 Also, the addition of (0.4–20.0), (O2 + C3H6)/He volume ratio (0.03–0.33), and total
a combustion-inhibiting alkaline or alkali earth metal ion or feed gas ow rate (30–70 cm3 minꢀ1). Data analysis was con-
ionic compound as a promoter, such as K+,22 Cs+,18 NaCl,7 and ducted at a pseudo-steady condition (i.e. 0.5–1.0 h aer the
KAc,23 etc., has been found to improve PO selectivity and/or the reactor reached the target temperature) by on-line gas chro-
PO formation rate by: changing electronic properties of the matography (Varian CP-4900 Micro GC) with thermal conduc-
lattice oxygen to become electrophilic,20 reducing the acidity of tivity detector (TCD), Porapak U (10 m) and molecular sieve 5 A
˚
the active surface,18 or lowering the activation energy for the (10 m). The product selectivities and propylene conversions
overall consumption rate.25 In general, Cu-based catalysts gave were calculated on the basis of carbon balance,17 and propylene
19–58% PO selectivity and ꢂ1–20% propylene conver- oxide formation rates were calculated by grams of PO produced
sion.14,16,17,21,22,26 To date, the highest PO formation rate among per kilogram of catalyst in one hour. Note that, in this work, we
Cu-based catalysts in the epoxidation of propylene was obtained have only presented PO and CO2 products. The CO2 selectivities
from RuO2–CuO–NaCl/SiO2 at 40–50% of PO selectivities and and formation rates are not present in all gures. [The CO2
10–2ꢀ0%1 propylene conversions, representing 153 gPO hꢀ1 selectivity for an experiment can be calculated from 100%
ꢁ
kgcat , between 240 and 270 C at atmospheric pressure.
minus % PO selectivity.] The byproducts (acrolein, acetone,
In this work, we report an attempt to increase PO formation acetaldehyde, etc.) appeared in only trace amounts. The
rate of the previously discovered RuO2–CuO–NaCl/SiO2 catalyst. repeatability of all experiments was within ꢃ10%. Tests for the
The modication of the main active RuO2–CuO/SiO2 compo- degree of reusability of the catalyst was carried out under the
ꢁ
nent by adding Cs2O and TiO2 has been found to signicantly following conditions: reaction temperature of 272 C, O2/C3H6
enhance the epoxidation of propylene to PO by several-fold. The volume ratio of 3.1, (O2 + C3H6)/He volume ratio of 0.26, and
Box–Behnken design of experiments methodology was used to total feed gas ow rate of 34 cm3 minꢀ1. 6 runs of 30 min testing
ascertain the operating conditions-namely temperature, ow were done for the same catalyst, with and without treating with
rate, and feed composition-under which PO synthesis could be fumed HCl. For the testing without treating with fumed HCl,
maximized.
the catalyst was cooled down to room temperature and le in air
for 1 h before starting the new run. For the testing with treating
with fumed HCl, aer its previous run, the catalyst, still in the
quartz tube reactor, was fumed with HCl generated by heating
10 M of HCl at 80 ꢁC in a 2-way glassware connecting to N2
gas for 1 h. The ow rate of N2 gas containing HCl was 10
Experimental section
Catalysts preparation
All catalysts were prepared by co-impregnation. In a typical
synthesis, the catalysts presented in each gure or table were
prepared in parallel by mixing appropriate aqueous metal salt
precursor solutions of Ru [RuCl3$xH2O, Ru 38% min, Alfa
Aesar], Cu [Cu(NO3)2$3H2O, Ajax], Cs [CsNO3, Sigma-aldrich], Ti
[titanium plasma standard solution, Ti 1000 mg mlꢀ1, Alfa
Aesar], and/or Na [NaNO3, Alfa Aesar] with the SiO2 support [Alfa
Aesar, surface area of 89.59 m2 gꢀ1]. The precursor solution
volumes and support weights were varied to achieve catalysts
comprising a comprehensive set of weight metal ratios and
metal loadings. The mixture was stirred at room temperature
cm3 minꢀ1
.
Catalyst characterization
Powder X-ray diffraction (XRD) patterns were obtained on an
X-ray powder diffractometer (XRD: JEOL JDX-3530 and Philips
X-Pert) using Cu-Ka radiation, 45 kV and 40 mA to identify
crystalline phases. Specic surface area of the SiO2 support and
catalyst was characterized by N2 physisorption using a Quan-
tachrome Autosorp-1C instrument with BET method at
ꢀ196 ꢁC. A scanning electron microscope and an energy
dispersive X-ray spectrometer (FE-SEM/EDS, FE-SEM: JOEL
JSM-7600F) were used to image the catalysts' morphology and
elemental composition. X-ray photoelectron spectroscopic
studies (XPS, Kratos Axis Ultra DLD) were carried out using Al Ka
ꢁ
ꢁ
for 4 h, then stirred at 165 C until dry, and calcined at 480 C
for 8 h in air.
Catalytic performance evaluation
The propylene epoxidation performance of each catalyst was for the X-ray source to indentify electronic state of the mixing
examined in a traditional packed bed reactor. A prepared cata- elements. Continuous H2-temperature programmed reduction
lyst (1.5 mg) was packed in a quartz tube (0.5 cm in diameter) (H2-TPR) measurements were carried out in a continuous-ow
and sandwiched between two quartz wools. The reactant gases Inconel tube reactor held at 25–800 ꢁC with a heating rate of
were O2 (Praxair, 99.999%) and C3H6 (Linde, 99.5%), along with 5 C minꢀ1. The H2/Ar mixture gas (9.6% H2) was introduced
ꢁ
He (Praxair, 99.999%) as balance gas. In the rst step, a volume into the catalyst bed at total ow rate of 30 cm3 minꢀ1. The H2
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consumption was continuously monitored using
a
TCD- at about 2 wt% loading of Na (490 gPO hꢀ1 kgcatꢀ1, 35.4% PO
equipped GC (Shimadzu GC-2014). Ammonia-temperature selectivity and 6.0% propylene conversion). Under the same
programmed desorption (NH3-TPD, TPD/R/O Thermo Fin- testing conditions the optimum PO formation rate obtained
ꢀ1
nigan 1100) and CO2-programmed desorption (CO2-TPD, TPD/ from the Cs loading, as shown in Fig. 1b at 0.6 wt% (533 g
h
PO
R/O Thermo Finnigan 1100) techniques were used to analyze kgcatꢀ1, 22.6% PO selectivity and 10.7% propylene conversion),
the acidity and basicity of catalysts, respectively. For NH3-TPD, was higher than that of the Na loading, indicating that the Cs2O
the catalysts were pretreated under He ow at 400 ꢁC for 1 h and addition is clearly more effective than the NaCl, even though the
cooled down to 40 ꢁC before 10% NH3/He mixed gas was owed PO selectivity of the Cs2O addition was lower than that of the
over the catalysts for 30 min to adsorb on the acid sites. The NaCl addition, if these two promoters were compared in terms
ꢁ
excess ammonia was eradicated by owing N2 at 40 C for 20 of improvement of the catalytic activity for PO synthesis (i.e. PO
min. The catalysts were then heated to 800 ꢁC at a heating rate yield). This could be because, while NaCl is more effective than
of 20 ꢁC minꢀ1, while a ow of He passed over the catalysts at 20 Cs2O in reducing acidity of the lattice oxygen which inhibits the
cm3 minꢀ1. The TPD proles were detected by a TCD detector CO2 pathway, it is not as efficient as Cs2O in enhancing PO
and analyzed with a ChemiSo TPx soware. The CO2-TPD production by increasing active sites for PO synthesis (see
procedure was similar to the NH3-TPD procedure, except that N2 discussion in Table 1 and Fig. 6). Moreover, excess amounts of
was used for the inert gas and that pure CO2 was owed over the Cs2O, as with the NaCl loading, resulted in decreased PO
catalysts to adsorb on basic sites.
production. This can be explained in that, at low promoter
loading, the promoter species (Cs2O or NaCl) were expected to
be well dispersed, small, and incorporated into the solid
structures. They primarily occupy highly acidic sites on the
catalyst's surfaces, resulting in the CO2 suppression and the
lower propylene conversions. Aer all of the highly acidic sites
are capped (i.e. passing the optimal PO formation rates),
continuously increasing the promoter loadings means that the
remaining NaCl or Cs2O species start to form clusters and
segregate from the RuO2–CuO cluster, resulting in lowering PO
selectivities and slightly decreasing propylene conversions due
to the larger size of the promoter overlaying the active sites of
the RuO2–CuO clusters.24
The main goal of this work was to optimize PO formation
rate, therefore the optimal RuO2–CuO–Cs2O/SiO2 catalyst was
chosen for further study. Yang and coworkers have found that
TiOx modied on CuOx can promote PO yield for propylene
epoxidation because the surface of Cu–Ti mixed oxides is able to
anchor the oxametallocycle, a key intermediate in PO forma-
tion.19 Thus, an attempt to improve the PO production rate of
the RuO2–CuO–Cs2O/SiO2 catalyst by adding Ti (denoted as
TiO2) into RuO2–CuO–Cs2O/SiO2 was explored. As indicated in
Fig. 2a, increasing the Ti loading from 0.0 to 0.8 wt% resulted in
Results and discussion
We have previously shown that silica-supported multimetallic
RuO2–CuO–NaCl catalysts exhibit PO selectivities of 40–50%
and propylene conversions of 10–20% at 240–270 ꢁC and
atmospheric pressure with metal loadings of Ru/Cu/Na ¼ 7.16/
3.58/1.79 wt% on the SiO2 support providing the best perfor-
mance.14 Likewise, an early report on the modication of CuOx/
SiO2 by cesium was found to increase in PO selectivity, similar
to the behavior of NaCl in RuO2–CuO–NaCl/SiO2 system, by
weakening the acidity of the lattice oxygen.18 Hence, as shown in
Fig. 1, we rst assessed the catalytic performance of RuO2–CuO/
SiO2 doped with Cs instead of NaCl. The catalysts were prepared
by xing the weight ratio of Ru/Cu ¼ 7.16/3.58 on SiO2 with
various loadings of Na (Fig. 1a) and Cs (Fig. 1b) at 0.0–4.0 wt%
and 0.0–1.0 wt%, respectively. The weight percent range of Cs
employed was smaller than that of Na because the atomic size of
Cs is larger than Na. Note that Na and Cs will be denoted as
NaCl and Cs2O because of its nal form (see XPS spectrum in
Fig. 5). In Fig. 1a, the optimum PO formation rate and PO
selectivity of NaCl were consistent with the previous ndings,24
Fig. 1 Catalyst performance of (a) Na at 0.0–4.0 wt% and (b) Cs at 0.0–1.0 wt% loading on Ru–Cu/SiO2 catalysts (7.16 wt% Ru: 3.57 wt% Cu).
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Table 1 Catalyst performance of all combinations of RuO2, CuO, Cs2O, and/or TiO2 on SiO2. Each catalyst was prepared by fixing wt% of Ru, Cu,
Cs, and/or Ti on SiO2 ¼ 12.59, 6.29, 0.91, 1.21, respectively. The reaction temperature was 250 ꢁC. (AC ¼ acrolein)
Selectivity (%)
C3H6 conversion
(%)
PO yield
(%)
PO formationꢀr1ate
Catalyst no.
Catalyst
PO
AC
CO2
98.1
(gPO hꢀ1 kgcat
)
1
RuO2/SiO2
0.1
18.8
0
1.8
18.8
35.3
12.5
0.3
0.1
2.0
35.3
24.0
0
40.5
0.1
0.1
0.04
0.02
0
19
8
0
2
CuO/SiO2
62.4
64.7
87.5
97.8
99.6
97.8
64.7
46.0
100.0
79.8
91.4
97.6
100.0
82.5
3
Cs2O/SiO2
4
TiO2/SiO2
0
0.1
0
0
5
6
7
8
RuO2–CuO/SiO2
RuO2–Cs2O/SiO2
RuO2–TiO2/SiO2
CuO–Cs2O/SiO2
CuO–TiO2/SiO2
Cs2O–TiO2/SiO2
RuO2–CuO–Cs2O/SiO2
RuO2–CuO–TiO2/SiO2
RuO2–Cs2O–TiO2/SiO2
CuO–Cs2O–TiO2/SiO2
RuO2–CuO–Cs2O–TiO2/SiO2
1.9
0.3
0.2
0
30.0
0
42.5
32.4
34.6
0.2
0.1
0.1
6.2
15.4
2.6
0.2
21.0
0.80
0.09
0.05
0
0.03
0
1.21
1.19
0.03
0
347
41
22
0
14
0
520
509
14
0
9
10
11
12
13
14
15
19.7
7.7
1.2
0
0.5
0.9
1.2
0
16.8
0.7
3.49
801
increase of PO rate to the optimum (from 533 to 601 gPO hꢀ1 are shown in Fig. 3a and the XRD spectrum of each catalyst is
kgcatꢀ1), then decrease from the optimum at 0.8 wt% of Ti shown in Fig. 3b. Increasing the loading from 5 to 21 wt%
ꢀ1
loading to 587 g
h
kgcatꢀ1 at 1.0 wt% of Ti loading. The PO resulted in sharp increases in the PO formation rate, from 187
PO
ꢀ1
selectivities minimally decreased from 24.5 to 21.6%, while the to 801 g
h
kgcatꢀ1, and in propylene conversion, from 3.7 to
PO
propylene conversions slightly increased from 10.6 to 12.5% 20.9%. The PO selectivity gradually decreased from 22.7 to
with increasing Ti loading from 0.0 to 1.0 wt%. The reasons for 16.8%. Above 21 wt%, the PO rate and propylene selectivity
ꢀ1
this will become clear in the discussions of Fig. 6, 7 and Table 1. slightly decreased from 801 to 747 gPO hꢀ1 kgcat and 16.8–
Furthermore, the addition of Ti from 0.0 to 1.0 wt% into the 11.5%, respectively; however, the propylene conversion kept
optimal RuO2–CuO–NaCl/SiO2 catalyst was also investigated. As increasing to 29.2%. The analyses of the XRD spectra revealed
shown in Fig. 2b, as the Ti loading increased, the propylene the characteristic diffraction patterns of only RuO2 (2q ¼ 28.0,
conversion, PO formation rate, and PO selectivity consistently 35.7, 54.2) and CuO (2q ¼ 35.7, 39.0, 48.8). The characteristic
fell, indicating that TiO2 in the presence of NaCl did not diffraction patterns of Cs2O and TiO2 did not appear either
promote the active site. Therefore, the RuO2–CuO–NaCl/SiO2 because they could be amorphous or because they constituted
catalyst doped with TiO2 was not studied further. To further crystals too small to be detected (<2.0 nm). It can also be seen
optimize the propylene epoxidation performance of RuO2– that the peak intensities for both RuO2 and CuO crystals
CuO–Cs2O–TiO2/SiO2, the effects of varying the total metal increase as total metal loading increases and corresponding PO
loading from 5–29 wt%, while xing the metal ratio at Ru/Cu/ formation rates increase. This indicates that the existence of
Cs/Ti ¼ 8.3/4.2/0.6/0.8 by weight, was investigated. The results crystalline RuO2 and CuO is crucial for PO formation.24
Fig. 2 Catalyst performance of Ti at 0.0–1.0 wt% loading on (a) RuO2–CuO–Cs2O/SiO2 catalysts or (b) RuO2–CuO–NaCl/SiO2 (8.3 wt% Ru: 4.2
wt% Cu: 0.6 wt% Cs or 2.0 wt% Na) at testing temperature of 250 ꢁC.
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Fig. 3 (a) Various total metal loadings of RuO2–CuO–Cs2O–TiO2 on SiO2 from 5–30 wt%, Ru/Cu/Cs/Ti ¼ 8.3/4.2/0.6/0.8, and (b) their XRD
spectra.
However, the rate of increase in the crystallite sizes could mono-metallic catalysts no. 1, 3, and 4 were inactive for
eventually rise to the point at which active sites agglomerate propylene reactions. RuO2/SiO2 (1) typically exhibits a high
with each other, creating a net decrease in the external surface propylene conversion but a complete combustion is dominant,
area available to interact with gases as the loading increases. indicating the absorption of O2 onto the RuO2 surface is pref-
This plausibly could account for the ultimate fall in the PO erential.24 CuO/SiO2 (2) is catalytically active for PO synthesis
formation rate above 21 wt% loading as seen in Fig. 3a.
but the propylene conversion is small, consistent with other
Fig. 4 shows SEM images and element distributions (Ru, Cu, reports.10,24 Bi-metallic catalysts no. 6–8 and 10 produced trace
Cs, and Ti) of the catalysts prepared at different total metal amounts of PO at best, indicating that combinations of RuO2–
loading. Each metal was uniformly dispersed on the SiO2 Cs2O, RuO2–TiO2, CuO–Cs2O and Cs2O–TiO2 exhibit no
support. Increasing the total metal loading le the particle synergy. However, RuO2–CuO/SiO2 (5) gave a relatively high PO
sizes, approximately 30–50 nm, virtually unchanged. The BET yield and PO formation rate compared to the other bi-metallics,
surface area of the optimal catalyst (i.e. 21 wt% loading) was conrming the synergy between RuO2 and CuO reported
found to be 76.21 m2 gꢀ1 compared to 89.59 m2 gꢀ1 for the earlier.24 This suggests that an O2 molecule rst adsorbs onto
unloaded-metal SiO2 support. The reason for this is that, aer the RuO2 surface and dissociates into two surface O atoms. The
the impregnation, the active components were loaded into the O atoms then migrate across the surface to a neighboring CuO
SiO2 support's pores, thus the pore volume decreased, i.e. the site forming CuO–O. Gas phase propylene then interacts with
surface area decreased.
the CuO–O, ultimately forming the PO via the oxametallocycle.
Fig. 5 shows XPS scanning spectra of Ru, Cu, Cs, and Ti Compared to CuO/SiO2 (2), the CuO–TiO2/SiO2 (9) catalysts
species. The XPS peaks of Ru 3d (Fig. 5a) were Ru 3d3/2 ¼ 285.1 presented relatively high PO and AC selectivity but lower CO2
eV and Ru 3d5/2 ¼ 281.1 eV, indicating that the resolved binding selectivity and unchanged propylene conversion. This implies
energy of ruthenium represents the value of RuO2.27 Note that that the CO2 formation route is inhibited by; (1) anchoring the
the binding energy of C 1s (284.6 eV) also appeared in the Ru oxametallocycle, thus favoring the generation of PO mole-
region. The binding energies of Cu 2p (Fig. 5b) were Cu 2p1/2
¼
cules19 and/or (2) changing the acidity of the CuO surface (see
953.2 eV and Cu 2p3/2 ¼ 933.7 eV, indicating that Cu existed as additional discussion in Fig. 6). Tri-metallic catalyst no. 11
CuO.28 The characteristic XPS peaks of Cs 3d appeared at Cs 3d3/2 showed the most promising PO selectivity and PO formation
¼ 739.2 eV and Cs 3d5/2 ¼ 725.2 eV (Fig. 5c), indicating that Cs rate compared to the other tri-metallics, suggesting that the
predominately presents itself in the form of Cs2O.29 Further- addition of Cs2O into RuO2–CuO can enhance propylene
more, the binding energies of Ti 2p appeared at Ti 2p1/2 ¼ 464.1 epoxidation to PO by reducing the strong acidity (see discus-
eV and Ti 2p3/2 ¼ 458.7 eV (Fig. 5d), conrming that Ti appears sion in Fig. 6) and increasing the surface active sites for PO
in the catalyst as TiO2.30 The XPS peak of Ru 3p3/2 also showed in formation. The addition of TiO2 into RuO2–CuO (catalyst no.
this region at 463.2 eV. These analyses imply that all four 12) also improved either PO rate or PO selectivity but was not as
materials are distinct and immiscible, suggesting they effective as adding Cs at this weight ratio. The tri-metallic
contribute some participatory role in the active site for epoxi- catalysts no. 13 and 14, without combination of RuO2 and
dation when in close proximity.
CuO, exhibited relatively low to no PO. The most outstanding
Table 1 shows the performance of all uni-, bi- and tri- PO formation rate was achieved from catalyst no. 15. All of
metallic variants of RuO2, CuO, Cs2O, and TiO2, as well as these results suggested that RuO2–CuO/SiO2 is the main active
the quaternary RuO2–CuO–Cs2O–TiO2/SiO2 (15) as the refer- site for PO generation. Cs2O and TiO2 act as promoters to
ence catalyst, and reveals the function of each metal. The enhance PO formation.
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Fig. 4 SEM/EDS of catalysts at 5, 9, 13, 17, 21, 25 and 29 wt% total metal loading on SiO2, Ru : Cu : Cs : Ti ¼ 8.3/4.2/0.6/0.8. Each scale bar is 200 nm.
Altering the acidity18,31 and basicity32 of surfaces has been medium, and strong acidic sites appeared at approximately 150,
ꢁ
reported to serve as a useful tool for tuning selectivity. The 400–500, and 800 C, respectively. However, the integral peak
acidic and basic properties of the RuO2–CuO/SiO2, RuO2–CuO– areas of each site differed slightly among the materials. The
Cs2O/SiO2, and RuO2–CuO–Cs2O–TiO2/SiO2 catalysts at the addition of Cs2O to RuO2–CuO/SiO2 decreases the peak areas
optimal weight ratio were assessed using the temperature pro- with the medium and strong acidic sites relative to the weak
grammed desorption (TPD) of NH3 (Fig. 6a) and CO2 (Fig. 6b), acidic sites. The catalytic activity shown in Fig. 7 indicates that
respectively. The re-plots of the performance of RuO2–CuO/SiO2 the PO formation rate increased with dramatically decreasing
(5), RuO2–CuO–Cs2O/SiO2 (11), and RuO2–CuO–Cs2O–TiO2/SiO2 propylene conversion and increasing PO selectivity. This
(15) catalysts from Table 1 with the acidity and basicity suggests that Cs2O lessens the presence of high acidity surfaces,
strengths are also presented in Fig. 7. As shown in Fig. 6a, all thereby inhibiting CO2 formation in a manner similar to NaCl.7
three catalysts exhibited a similar prole, in which the weak, This nding is in agreement with He and coworkers' study on
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Fig. 5 XPS spectra of RuO2–CuO–Cs2O–TiO2/SiO2 catalyst showing
(a) Ru, (b) Cu, (c) Cs, and (d) Ti species; weight ratio of Ru/Cu/Cs/Ti/
SiO2 ¼ 12.59/6.29/0.91/1.21/79, total metal loading on SiO2 ¼ 21 wt%.
Unidentified peaks are satellite or plasmon peaks.
the modication of Cs+ on CuOx/SiO2.18 They found that the Cs+
inhibited (1) the isomerization of PO to CO2 because of the
weakened acidity of CuOx, thus contributing to the increase in
PO selectivity and (2) the reactivity of the lattice oxygen to
promote PO production by suppressing the allylic oxidation
route of propylene to acrolein and subsequently CO2.
The addition of TiO2 to RuO2–CuO–Cs2O/SiO2 increases the
peak area of the strong acidic sites relative to those of the weak
and medium sites. This shows that the total number of strong
acidic sites has increased again, potentially enhancing CO2
synthesis. However, as indicated in Fig. 7, doping RuO2–CuO–
Cs2O/SiO2 with TiO2 signicantly improved the propylene
Fig. 6 (a) NH3 and (b) CO2 TPD profiles for RuO2–CuO/SiO2, RuO2–
CuO–Cs2O/SiO2, RuO2–CuO–Cs2O–TiO2/SiO2 catalyst. Weight ratio
of Ru/Cu/Cs/Ti/SiO2 ¼ 12.59/6.29/0.91/1.21/79, total metal loading on
conversion and the PO formation rate while leaving the PO SiO2 ¼ 21 wt%.
selectivity scarcely changed, suggesting the strong acidic site
may equally enhance both PO and CO2 formation. This may be
because TiO2 itself possesses acidity. Thus, when doping the
catalyst with TiO2 the lattice oxygen becomes more electro-
philic, increasing the efficiency of the epoxidation of propylene
to PO and thereby increasing PO production. Nevertheless, the
total oxidation of the generated PO molecules is also likely to
take place as the overall acidity increases, thereby increasing the
amount of CO2. From all the NH3-TPD results, the overall acidity
of the catalysts can be ordered as follow: RuO2–CuO/SiO2
RuO2–CuO–Cs2O–TiO2/SiO2 > RuO2–CuO–Cs2O/SiO2.
>
The CO2-TPD results of Fig. 6b and each basic strength
related to the performance of each catalyst of Fig. 7 are assessed
in a similar manner. The peaks of the weak, medium, and
strong basic sites of the catalysts prepared appeared around 110
ꢁC, 590 C, and 800 C, respectively. The addition of Cs2O into
the RuO2–CuO/SiO2 catalyst increased the total basicity of the
catalyst, particularly that of the medium basic site. The strong
basic site almost disappeared. Then the addition of TiO2 into
ꢁ
ꢁ
Fig. 7 Relationship between the catalytic performance and acidity/
basicity of RuO2–CuO/SiO2, RuO2–CuO–Cs2O/SiO2, RuO2–CuO–
Cs2O–TiO2/SiO2 catalysts.
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the RuO2–CuO–Cs2O/SiO2 catalyst was found to decrease the parameters could be performed to further maximize the PO
catalyst surface's overall basicity, though its basicity remained production rate and to predict the best operating conditions for
higher than that of the RuO2–CuO/SiO2 catalyst. Thus, the total the RuO2–CuO–Cs2O–TiO2/SiO2 catalyst. But since many thou-
basicity of the catalysts can be ranked inversely relative to the sands of experiments would be needed to do so, the Box–
NH3-TPD results as follows: RuO2–CuO–Cs2O/SiO2 > RuO2– Behnken design, a frequently employed optimization tool, was
CuO–Cs2O–TiO2/SiO2 > RuO2–CuO/SiO2. These NH3- and CO2- used in our study. Box–Behnken designs allow efficient esti-
TPD results suggest that an excellent catalyst in the propylene mation of the best conditions for complex, multi-variable
epoxidation should provide not too high acidity or not too high experiments by manipulating a limited number of data points
basicity, in other words, intermediate basicity is the key in the throughout a range of options.36 A subset of the effects Box–
search for propylene epoxidation catalysts.12
Behnken predicts these four operating parameters should have
Fig. 8 represents the H2-TPR spectra of the prepared cata- on PO formation rate is illustrated in Fig. 9 (see detailed results
lysts. The CuO/SiO2 catalyst showed a single peak around in Table S1;† the PO selectivity and propylene conversion are
290 ꢁC, representing the reduction of bulk CuO consistent with shown in Fig. S1 and S2,† respectively). The full ranges of
previous reports.24,33 The single RuO2/SiO2 catalyst showed two conditions were: 190–310 ꢁC, 0.4–20.0 for O2/C3H6 volume ratio,
peaks. The main peak (ꢂ175 ꢁC) was attributed to the complete 0.03–0.33 for (O2 + C3H6)/He volume ratio, and 30–70 cm3 minꢀ1
reduction of Ru4+ to Ru0, and the lower temperature peak for the total feed ow rate.
(ꢂ135 ꢁC) was associated with ruthenium species interacting
The images displayed in Fig. 9a–c show the effects of reac-
with the support.34 The Cs2O and TiO2 on SiO2 (not shown here) tion temperature varied with O2/C3H6 volume ratio, (O2 + C3H6)/
had no reduction peak observed in this range of temperatures.35 He volume ratio, and total feed gas ow rate, respectively.
All combinations of RuO2 and CuO appeared as a single sharp Fig. 9a and b indicate that the PO formation rate was optimized
ꢁ
peak around 170–180 C, similar to the reduction peaks of the at reaction temperatures around 250–290 ꢁC when the O2/C3H6
RuO2/SiO2 catalyst. Interestingly, the reduction peak of CuO was volume ratio was above ꢂ16 or below ꢂ3.5 and when the (O2 +
not observed. This is because of the rapid reduction of CuO C3H6)/He volume ratio was above 0.20. Fig. 9c indicates that the
induced by a H2 spillover.24 In addition, the H2 consumption total feed gas ow rate was optimized around 30–40 cm3 minꢀ1
.
spectra of all materials that include at least RuO2 and CuO Moreover, increasing the total feed gas ow rate results in
together were larger relative to the RuO2/SiO2 spectrum, indi- a lower PO formation rate (Fig. 9c) due to a reduction in contact
cating the additional H2 consumption of CuO. Also, when the time with the catalyst. The images in Fig. 9d–f display the PO
CuO or other metal species were added to the catalyst, the formation rates at a reaction temperature of 272 ꢁC when
shoulder disappeared, either because the peaks were convo- varying (O2 + C3H6)/He volume ratio vs. O2/C3H6 volume ratio,
luted or because the ruthenium species disappeared. Hence, the total feed gas ow rate vs. O2/C3H6 volume ratio, and total feed
reduction of RuO2 and CuO occurred simultaneously because gas ow rate vs. (O2 + C3H6)/He feed volume ratio, respectively.
their nanoparticles were in close contact with each other,24 The most impactful variables on the PO formation rate are the
consistent with the understood synergy between the two O2/C3H6 volume ratio and (O2 + C3H6)/He volume ratio.
responsible for PO synthesis.
Changing the total feed gas ow rate at the same (O2 + C3H6)/He
The catalytic performance of these propylene epoxidation volume ratio (Fig. 9f) had less effect on PO formation than did
catalysts is most heavily inuenced by reaction temperature, O2/ changing the reaction temperature and O2/C3H6 rꢀat1io. The
ꢀ1
C3H6 volume ratio, (O2 + C3H6)/He volume ratio, and total feed highest predicted PO formation rate was >3000 gPO h kgcat
gas ow rate. Investigations of each of these operating at O2/C3H6 volume ratio of above ꢂ16 or below ꢂ3.5, (O2
+
C3H6)/He volume ratio of above ꢂ0.2, total feed gas ow rate of
30–40 cm3 minꢀ1, and the reaction temperature of 272 C.
ꢁ
To conrm the predicted value of the optimal PO formation
rate from the Box–Behnken design experiment, the previously
ascertained process conditions were experimentally employed
in catalytic performance testing (see Table S2†). Remarkably,
under the selected testing condition (the reaction temperature
ꢁ
of 272 C, the O2/C3H6 volume ratio of 3.1, the (O2 + C3H6)/He
volume ratio of 0.26, and the total feed gas ow rate of 34
cm3 minꢀ1) the hꢀig1hest experimental PO formation rate was
3015 gPO hꢀ1 kgcat (7.1% PO selectivity and 40.1% propylene
conversion). To the best of our knowledge, this PO formation
rate is the highest ever reported for the direct gas-phase epox-
idation of propylene to PO under atmospheric pressure using
only O2 (see Fig. S3 and Table S3†), about 8 times higher than
the best catalyst reported in the literature. The maximum PO
selectivity was also ascertained using a similar procedure. The
predicted conditions included a reaction temperature of 219 ꢁC,
O2/C3H6 volume ratio of 4.1, the (O2 + C3H6)/He volume ratio of
Fig. 8 H2-TPR profiles of all combinations of RuO2, CuO, Cs2O, and
TiO2 on SiO2. Each catalyst has wt% of Ru, Cu, Cs and/or Ti on SiO2 of
12.59%, 6.29%, 0.91%, and 1.21%, respectively.
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Fig. 9 (a)–(f) Contour plots showing PO formation rate (gPO
h
ꢀ1 kgcatꢀ1) from all combinations of 4 operating parameters; (a) A vs. B, (b) A vs. C,
(c) A vs. D, (d) B vs. C, (e) B vs. D, (f) C vs. D; where A ¼ reaction temperature (190–310 ꢁC), B ¼ O2/C3H6 volume ratio (0.4–20.0), C ¼ (O2 + C3H6)/
He volume ratio (0.03–0.33), and D ¼ total feed gas flow rate (30–70 cm3 minꢀ1 by using He as balance gas). When two parameters were studied,
A, B, C and/or D were fixed at 272 ꢁC, 3.1, 0.26, and/or 34 cm3 minꢀ1, respectively. Weight ratio of Ru/Cu/Cs/Ti/SiO2 ¼ 12.59/6.29/0.91/1.21/79,
total metal loading on SiO2 ¼ 21 wt%.
0.32, and total feed gas ow rate of 70 cm3 minꢀ1. The PO used catalyst was treated with fumed HCl. As seen in Fig. 10, the
selectivity was 38.4% (1.3% propylene conversion and 573 gPO catalyst treated with fumed HCl aer every run showed activity
hꢀ1 kgcatꢀ1 for the PO formation rate). All of the results obtained remarkably close to that of the fresh catalyst. Even aer 6 runs,
from the experiments were in good agreement with the pre- the activity for PO production was virtually unchanged,
dicted values from the design experiment, i.e. less than ꢃ3%
error.
Since catalyst reusability is essential, a multiple test of the
optimal catalyst with the optimal operating condition for PO
formation rate was performed. Fig. 10 (also see Table S4†)
charts PO selectivities and propylene conversions with PO
formation rates of the optimal catalyst with and without treat-
ing with fumed HCl for 6 runs. Note that the data were collected
30 min into each run under the optimal condition. Aer the 6
times of using the catalyst, the activity for PO production
decreased, particularly the PO formation rates and the
ꢀ1
propylene conversions, from 3015 g
h
kgcatꢀ1 with 40.1% of
PO
propylene conversion to 732 gPO hꢀ1 kgcat
with 5.8%
ꢀ1
propylene conversion, indicating that the catalyst had a deacti-
vation problem. This behavior was similar to the RuO2–CuO–
NaCl/SiO2 catalysts previously reported in which the loss of Cl
from the catalysts' surface resulted in deactivation.15 It should
be noted that Cl remaining on the surface comes from the RuCl3
precursor. An investigation using SEM-EDS, comparing the
fresh catalyst with the same catalyst aer 6 runs, conrmed that
Fig. 10 Multiple test runs of the optimal RuO2–CuO–Cs2O–TiO2/
SiO2 catalyst with (w/) and without (w/o) treating with fumed HCl
the overall Cl content decreased (see Fig. S4†). Therefore, the under the optimal operating condition for PO formation rate.
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indicating that the treatment with fumed HCl restores catalytic
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Conclusion
Catalysts based on RuO2–CuO/SiO2 were modied with Cs2O
and TiO2 for the direct gas-phase epoxidation of propylene to
PO using only O2 under atmospheric pressure. Catalytic
performance was rst optimized by varying the weight
percentage of Cs2O in the RuO2–CuO/SiO2 catalyst and by
varying the weight percentage of TiO2 added to the best of those
RuO2–CuO–Cs2O/SiO2 catalysts. The multi-metallic catalyst
performed best at weight ratios of Ru/Cu/Cs/Ti ¼ 8.3/4.2/0.6/0.8
at a total metal loading of 21 wt%. Further optimization of the
PO formation rate was pursued using the Box–Behnken design
of experiments, varying the reaction temperature, O2/C3H6
volume ratio, (O2 + C3H6)/He volume ratio, and total feed gas
ow rate simultaneously. The highest PO formation rate and PO
selectivity over the RuO2–Cꢀu1O–Cs2O–TiO2 catalyst were ach-
ieved at 3015 gPO hꢀ1 kgcat and 38.4%, respectively, repre-
senting the highest PO formation rate ever reported for the title
reaction. The characterizations of the optimal catalyst using
XRD, XPS, NH3-TPD, CO2-TPD, SEM, and H2-TPR technique
revealed that the main active site for PO formation was the close
proximity between crystalline RuO2 and CuO where the synergy
effect takes place. Cs2O and TiO2 acted as promoters by
modulating the acidity or the basicity of RuO2–CuO/SiO2
surfaces. The catalyst exhibited a deactivation due to the loss of
Cl. However, it can be recovered by treating with fumed HCl.
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Acknowledgements
This research is supported in part by the Graduate Program 11 L. J. Yang, J. L. He, Q. H. Zhang and Y. Wang, J. Catal., 2010,
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