NaCl-Promoted CuO-RuO2/SiO2 Catalysts for Propylene Epoxidation with O2 at Atmospheric Pressures: A Combinatorial Micro-reactor Study

Article abstract of DOI:10.1007/s10562-014-1454-7

A combinatorial approach is used to investigate several alkali metals promoted Cu-Ru binary oxide catalysts with improved catalytic performance in the propylene partial oxidation. 2 %Cu/5 %Ru/c-SiO₂ catalyst had the best yield with high propylene conversion and propylene oxide (PO) selectivity. Among the promoters screening in the study, NaCl promotion significantly increased the PO selectivity accompanied by some attenuation in the total propylene conversion. It was proposed that binary oxide catalysts revealed a greater number of exposed catalytically active adsorption sites as compared to monometallic oxide counterparts according to XPS and FTIR results. Besides NaCl addition alters the structure, yielding a significantly improved PO selectivity without any change in the particle size of Cu and Ru oxide according to XRD analysis. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-014-1454-7

Catal Lett
DOI 10.1007/s10562-014-1454-7
NaCl-Promoted CuO–RuO2/SiO2 Catalysts for Propylene
Epoxidation with O2 at Atmospheric Pressures:
A Combinatorial Micro-reactor Study
•
•
•
S¸ule Kalyoncu Derya Du¨zenli Isik Onal
•
Anusorn Seubsai Daniel Noon Selim Senkan
•
•
•
Zafer Say Evgeny I. Vovk Emrah Ozensoy
•
Received: 16 September 2014 / Accepted: 24 November 2014
Ó Springer Science+Business Media New York 2014
Abstract A combinatorial approach is used to investigate
several alkali metals promoted Cu–Ru binary oxide cata-
lysts with improved catalytic performance in the propylene
partial oxidation. 2 %Cu/5 %Ru/c–SiO2 catalyst had the
best yield with high propylene conversion and propylene
oxide (PO) selectivity. Among the promoters screening in
the study, NaCl promotion significantly increased the PO
selectivity accompanied by some attenuation in the total
propylene conversion. It was proposed that binary oxide
catalysts revealed a greater number of exposed catalytically
active adsorption sites as compared to monometallic oxide
counterparts according to XPS and FTIR results. Besides
NaCl addition alters the structure, yielding a significantly
improved PO selectivity without any change in the particle
size of Cu and Ru oxide according to XRD analysis.
1 Introduction
Propylene oxide (PO) is a very important chemical feed-
stock for the production of a wide variety of commodity
chemicals, such as polyether polyols and propylene glycol
[1]. Currently, chlorohydrin and organic hydroperoxide
processes are two of the commonly used industrial pro-
cesses for PO synthesis in the chemical industry. These
processes lead to the generation of a large amount of waste
water and organic byproducts. Thus, they are not preferable
due to their environmental and economical drawbacks [2].
Because of the deficiencies of the aforementioned PO
production processes, novel methods of producing PO have
been explored which included direct oxidation of propyl-
ene using various catalytic systems and proper oxidants,
such as H2O2 [3–5] O2–H2 gas mixture [6–9] and N2O [10,
11]. However, high costs of these oxidants restrict the
commercialization of these processes. Therefore, the direct
gas-phase epoxidation of propylene to PO by molecular
oxygen has been a focus of interest as an attractive alter-
native from both economical and environmental
standpoints.
Keywords Propylene Á Epoxidation Á Ru Á Cu Á SiO2
S¸. Kalyoncu Á D. Du¨zenli Á I. Onal (&)
Department of Chemical Engineering, Middle East Technical
University, Ankara 06800, Turkey
e-mail: ional@metu.edu.tr
The recent discovery of the highly active Au/TiO2 cat-
alysts in various catalytic reactions led to the use of Au in
conjunction with other reducible metal oxide support
materials [7–9, 12–15]. However, since highly selective Au
catalysts typically exhibit low propylene conversion and
require hydrogen co-feeding, such catalysts are industrially
less promising for PO production [8]. Successful results
obtained from modified Ag catalysts in the gas phase
epoxidation of ethylene by molecular oxygen led to many
studies on propylene epoxidation over different support
materials and modifiers [16–28]. However the conversion
and the selectivity of Ag-based catalysts in PO production
were lower than those for ethylene oxide production as a
A. Seubsai
Department of Chemical Engineering, Kasetsart University,
Bangkok 10900, Thailand
D. Noon Á S. Senkan
Department of Chemical and Biomolecular Engineering,
UCLA, Los Angeles, CA 90095-1592, USA
Z. Say Á E. I. Vovk Á E. Ozensoy
Department of Chemistry, Bilkent University, Ankara 06800,
Turkey
E. I. Vovk
Boreskov Institute of Catalysis, Novosibirsk 630090,
Russian Federation
123

S¸. Kalyoncu et al.
result of the existence of allylic hydrogen in propylene. The
allylic C–H bonds in propylene were reported to be much
more active towards oxidation than vinyl C–H bonds in
ethylene which is in line with the lower selectivity values
observed in propylene epoxidation catalysis [29]. Further-
more, surface chemistry studies on Ag and Cu single crystal
surfaces suggested that Cu is more selective than Ag for the
epoxidation of alkenes having allylic C–H bonds [30].
The reason behind the higher epoxidation activity of Cu
with respect to that of Ag was explained by the lower basicity
of oxygen adsorbed over Cu metal [31]. As a supporting
study, the effect of the oxygen basicity on PO selectivity was
investigated by Kızılkaya et al. [32]. In this DFT study over
Cu and Cu–Ru catalytic systems, Kızılkaya et al. proposed
that because of the higher basicity of atomic oxygen adsor-
bed over Cu–Ru, the scission of allylic hydrogen of pro-
pylene occurs with high probability (much lower activation
barrier) as compared to the formation of the PO intermediate.
Several studies in the literature focused on the active oxi-
dation state of Cu in epoxidation reactions. Vaughan et al.
proposed that Cu0 species in highly dispersed atomic-like
form are the active sites in epoxidation [33]. On the other
hand, Zhu et al. claimed that Cu? is the active form [34].
Onal et al. suggested that isolated ionic Cu2? species were
responsible for propylene epoxidation by O2 [35].
selective catalyst, a sequential combinatorial optimization
approach was followed starting with the exploration of the
optimum Ru and Cu relative loadings, followed by the
selection of the optimum silica support material and finally
the selection of the most effective promoter.
2 Experimental
2.1 Catalyst Preparation
SiO2-supported mono and bimetallic heterogeneous cata-
lysts were prepared using different synthesis methods.
Silica support materials were either synthesized by using a
template (t-SiO2) or a sol–gel method (s-SiO2) or they were
directly acquired commercially (c-SiO2, Alfa Aesar, sur-
face area 97 m2 g-1). The metal salts were also loaded on
SiO2 either by adding the metal salts to the synthesis
mixture during the support material preparation step or by
the incipient wetness impregnation of these salts onto the
silica support material.
Catalysts containing s-SiO2 were prepared using the
following precursors: tetraethyl orthosilicate (TEOS, 99 %
purity, Fluka), ethanol (EtOH), deionized water, 1 M
HNO3, 0.5 M NH4OH, copper nitrate (Cu(NO3)2, 99.99 %
purity, Aldrich) and ruthenium (III) chloride hydrate
(RuCl3ÁxH2O, 99.98 % purity, Aldrich). The corresponding
molar ratios of the chemicals used in the s-SiO2 synthesis
was TEOS:EtOH:HNO3:H2O:NH4OH = 1:20:1:13:0.5. Cu
and Ru metal loadings were also varied between 0 and 2
wt% and 0 and 5 wt%, respectively. In the s-SiO2 syn-
thesis, TEOS, HNO3, ethanol and water were mixed
together at room temperature and then heated at 80–85 °C
for 2 h. During heating, the solution was constantly stirred
by a magnetic stirrer and refluxed. Then, metal precursors
were added into the mixture and the mixture was stirred for
1 h under these conditions. In order to obtain the gel,
NH4OH was added to the synthesis mixture. After aging
the gel for 24 h under ambient conditions, catalysts were
dried at 120 °C and further calcined at 550 °C for 5 h. Pure
s-SiO2 material was prepared by skipping metal addition
steps for wet impregnation method.
The studies reported in the literature showed that
bimetallic or multimetallic catalytic systems for the pro-
pylene epoxidation reaction often show superior catalytic
properties in comparison to monometallic systems [35–37].
Onal et al. reported that PO yield is increased by several
folds for Ag–Cu bimetallic catalysts because of a syner-
gistic effect [35].In addition, Kahn et al. reported that the
PO formation rates of Cu–Mn bimetallic catalysts were
about five times higher than the corresponding monome-
tallic catalysts [36]. Although bimetallic catalysts have
been commonly employed in propylene epoxidation, most
of these catalytic systems suffered from low propylene
conversion. Seubsai and co-workers have reported a new
SiO2-supported trimetallic RuO2–CuOx–NaCl catalyst with
the highest PO yield (40–50 % PO selectivity at 10–20 %
conversion for the direct epoxidation of propylene by
molecular oxygen under atmospheric pressures [37]. Fur-
thermore, He et al. reported that Cs? modified CuOx/SiO2
catalyst under O2-rich atmosphere gave 2.6 % PO yield
(34 % selectivity @ 7.5 % conversion) which is close to
the performance of the aforementioned trimetallic catalyst
performance mentioned above [38].
Catalysts containing t-SiO2 were synthesized according
to the method given in the literature [39].The chemicals
used in synthesis of t-SiO2 were: TEOS, 15.8 M HNO3,
dodecylamine (DDA, 98 % purity, Fluka), EtOH and
water. The corresponding molar ratios of EtOH:HNO3:
H2O:DDA used in the synthesis were 6.54:0.02:36.3:0.27.
During the synthesis of the t-SiO2, DDA, HNO3 and
deionized water were mixed and stirred for 1 h. Then
TEOS and EtOH were added to the above mixture. The
mixture was further stirred for 4 h at room temperature.
Resulting mixture was aged under ambient conditions for
In this study, using a combinatorial approach, RuO2 and
CuO based monometallic and binary oxide catalytic systems
were prepared on various SiO2 support materials in the
presence of alkali promoters such as Na, K and Li for the
direct epoxidation of propylene to PO via molecular oxygen
at atmospheric pressures. In order to finda highly active and a
123

A Combinatorial Micro-reactor Study
18 h. Next, the product was filtered, dried at 70 °C for 24 h
and further calcined at 650 °C for 3 h. The synthesized
t-SiO2 material was used to prepare Ru–Cu bimetallic
oxides system by wet impregnation method.
recorded using a Hg–Cd–Te (MCT) detector. The samples
were mounted into the IR cell equipped with optically-
polished BaF2 windows. About 20 mg of finely ground
powder sample was pressed onto a high-transmittance,
lithographically-etched fine tungsten grid which was
mounted on a sample holder assembly, attached to a
ceramic vacuum feed through. A K-type thermocouple was
spot-welded to the surface of a thin tantalum plate attached
on the W-grid to monitor the sample temperature. The
sample temperature was controlled within 298–1,100 K via
a computer-controlled DC resistive heating system using
the voltage feedback from the thermocouple. After having
mounted the sample in the IR cell, sample was gradually
heated to 373 K in vacuum and kept at that temperature for
12 h before the experiments in order to ensure the removal
of water from the surface.
Wet impregnation method was also employed in order to
synthesize Ru–Cu catalysts using s-SiO2, t-SiO2 and
c-SiO2 support materials. The prepared sample was dried at
120 °C for 12 h and further calcined. s-SiO2 and c-SiO2
supported catalysts were calcined at 550 °C for 5 h while
t-SiO2 sample was calcined at 650 °C for 3 h. In the syn-
thesis of alkali-promoted catalysts, sodium nitrate
(NaNO3), lithium chloride (LiCl), potassium nitrate
(KNO3), sodium chloride (NaCl), and potassium acetate
(KAc) salts were used as promoters. The desired amount of
alkali salts was dissolved in distillated water and added to
the calcined catalyst. Next, the mixture was heated at 80 °C
with continuous stirring until all the water was evaporated.
Finally, the prepared sample was dried at 120 °C for 12 h
and further calcined at 350 °C for 3 h.
2.3 Activity Tests
Catalyst performance tests were carried out using a com-
puter controlled array channel microreactor system which
is described elsewhere [40]. In this microreactor system, up
to 80 catalyst samples could be screened in a parallel
fashion. In the current set of experiments, 20 catalyst
candidates were tested in each screening experiment and a
performance data point was obtained for each catalyst in
about 3 min. In the array microreactors, reactant gases flow
over the flat surfaces of the powder catalyst samples which
are individually isolated within reactor channels where the
flow regime is similar to that of a monolithic reactor. All
experiments were performed using a 5 mg catalyst sample
under atmospheric pressure and at a gas hourly space
velocity (GHSV) of 20,000 h-1, representing a differential
reactor condition. Catalytic screening experiments were
performed at a C3H6/O2 molar ratio of 0.5 and at 300 °C.
Helium is used as a carrier gas. Products in the reactor
effluent streams were sampled and analyzed using a heated
capillary sampling probe and an on-line gas chromatograph
[Varian CP-4900 Micro GC with thermal conductivity
detector, Porapak Q (10 m) and Molecular sieve 13X
(10 m) columns]. The selectivity of PO is defined as the
moles of carbon in PO divided by the moles of carbon in all
of the carbon containing products. The selectivity of the
other C3 products, such as acrolein (AC), acetone (AT) and
acetaldehyde (AD) were also calculated in the same way.
2.2 Catalyst Characterization
The specific surface areas and total pore size of catalysts
were measured using a Nova 2200e Quantachrome gas
adsorption–desorption apparatus with nitrogen gas adsorp-
tion at 77 K. The Brunauer–Emmett–Teller (BET) method
was used to determine the specific surface areas and Barrett–
Joyner–Halenda (BJH) and Non-Local Density Functional
Theory (NL-DFT) methods were used to determine the pore
size distribution of the catalysts. NL-DFT method that pro-
vides a microscopic treatment of sorption phenomena in
micro and mesopores on a molecular level by statistical
mechanics was used to pore analysis of the support materials
used in this study beside BJH method.
Powder X-ray diffraction (XRD) analysis was per-
formed using Rigaku X-ray Diffractometer (Model, Mini-
˚
flex) with XuKa (30 kV, 15 mA, k = 1.54051 A).
XPS spectra were recorded using a SPECS spectrometer
with a PHOIBOS-DLD hemispherical energy analyzer and a
monochromatic Al Ka X-ray irradiation (hm = 1,486.74 eV,
400 W). Before XPS analysis, all samples were calcined at
753 K for 10 h under atmospheric conditions. The powder
samples were placed on Cu-based conductive sticky tape. An
e-beam flood gun was used for charge compensation during
the XPS analysis. The flood gun parameters were chosen to be
appropriate for compensating the binding energy (B.E.) shifts
and peak width broadening. Thus, e-beam flood gun was
operated using 5 eV electron energy and 70 lA emission
current. Allspectrawerecalibratedusingthe Si2psignalofthe
silica support material located at 103.3 eV.
3 Results and Discussion
3.1 Activity Tests
FTIR measurements were carried out in transmission
mode in a batch-type catalytic reactor coupled to an FTIR
spectrometer (Bruker Tensor 27). FTIR spectra were
In our preliminary combinatorial catalytic tests, several
catalysts containing mono-, binary and ternary oxides were
123

S¸. Kalyoncu et al.
Fig. 1 Catalytic performance
of mono and binary oxide Cu
and Ru catalysts supported on
15
12
9
13
s-SiO2 (GHSV = 20,000 h-1
,
Gas composition:
C3H6:O2 = 1:2, T = 300 °C)
10
7.6
7.2
6.9
6.9
6
5.5
5.5
6
5.3
4.9
4
3.7
2.3
3
1.2
1.1
1
0.7
0
screened to find a promising catalyst for direct oxidation of
propylene to propylene oxide. To achieve this goal, first Ag
and Cu salt precursors were loaded separately and together
into the TiO2, SiO2, c-Al2O3, and TiO2–SiO2 support
materials prepared by using single step sol–gel method and
into the a-Al2O3 by wet impregnation method [35]. Then
monometallic Cu oxide catalysts promoted with various
alkaline salt precursors were screened and observed rela-
tively poor propylene conversion values which were typi-
cally below 3 % [39]. On the other hand, in some of the
former reports in the literature such as the work of Seubsai
and coworkers [37], 40–50 % PO selectivity at 10–20 %
propylene conversion was reported over a new class of
silica-supported multimetallic RuO2–CuOx–NaCl catalysts.
Inspired by this work and the results obtained from our
previous studies, single and binary combination of Cu, Ru,
Mn and Ag oxide catalyst supported over silica material
were prepared by wet impregnation method and screened
into the high-throughput testing system. Among all the
catalysts the most promising one was determined as the
Cu–Ru oxide catalysts. Therefore in the current study, we
focused on NaCl promoted catalysts containing Ru–Cu
oxide which are prepared on different SiO2 support
materials.
and Ru precursor added into the support was calculated
based on the Cu and Ru metal weight ratio. Therefore the
symbolic designation of the catalysts was given according
to the metal amount and not the oxide form (CuO and
RuO2) added into the support material as determined by
XRD analysis. Propylene conversion and PO selectivity
results of these catalytic tests are presented in Fig. 1. It is
apparent in Fig. 1 that binary oxide catalysts typically
reveal higher propylene conversion and PO selectivity with
respect to that of their monometallic oxide counterparts
indicating a synergistic interaction between Cu and Ru
sites. Furthermore, Fig. 1 also points out that increasing the
Ru loading in the binary oxide system while keeping the
Cu metal content at a moderate level (e.g. 1 or 2 wt%)
increases the catalytic yield performance with increasing
propylene conversion. No significant change in PO selec-
tivity is observed. From these results and previous report
[37], through these experiments, 2 wt% Cu and 5 wt% Ru
loadings were chosen as the optimum metal oxide loadings
for the silica supported binary oxide catalysts for propylene
oxidation.
As the second stage of the optimization approach,
influence of the type of the silica support material on the
catalytic performance was explored. Along these lines, we
have prepared binary oxide catalysts on t-SiO2, s-SiO2 and
c-SiO2, which were loaded with 2 wt% Cu and 5 wt% Ru
via wet impregnation (Fig. 2). The catalytic performance
results presented in Fig. 2 suggests that the binary oxide
catalyst supported on s-SiO2 reveals the highest propylene
conversion with a very poor PO selectivity (0.79 % PO
yield), while the bimetallic catalyst supported on t-SiO2
The first part of the catalytic performance tests was
devoted to the optimization of the relative Ru and Cu
loadings on a given silica support material (i.e. s-SiO2). In
order to achieve this goal, different monometallic and
binary oxide were prepared in the absence of NaCl, by
varying the Cu and Ru loadings within 0–2 wt% and 0–4
wt%, respectively via sol–gel method. The amount of Cu
123

A Combinatorial Micro-reactor Study
40
35
30
25
20
15
10
5
40
35
30
35.6
22.6
25
20
15
10
5
19.8
0
11.9
NaNO3
11.97
LiCl
0
KNO3
19.30
NaCl
35.60
KAc
8.10
9.4
PO selecꢀvity
7.1
Propylene Conversion
PO Yield
1.19
0.14
0.56
0
0.66
0.13
9.40
3.35
1.00
0.08
3.7
3.5
0
2Cu/5Ru/t-SiO2 2Cu/5Ru/s-SiO2 2Cu/5Ru/c-SiO2 2Cu/5Ru/NaCl/c-SiO2
Fig. 3 Catalytic performance of 2 %Cu/5 %Ru catalyst modified by
different alkaline metal salts (GHSV = 20,000 h-1, Gas composi-
tion: C3H6:O2 = 1:2, T = 300 °C)
Fig. 2 Catalytic performance of 2 %Cu/5 %Ru catalyst supported on
t-SiO2, s-SiO2 and c-SiO2 and NaCl-promoted 2 %Cu/5 %Ru catalyst
supported on c-SiO2 (GHSV = 20,000 h-1
C3H6:O2 = 1:2, T = 300 °C)
, Gas composition:
NaCl enhanced the PO yield of Ru-Cu catalyst by
increasing both overall conversion and selectivity more
significantly than other promoters. The addition of KNO3
also promoted PO selectivity but conversion remained at a
very low level (0.7 %).The same situation has happened
for KAc promoted catalyst with lower selectivity. The
other sodium salt, NaNO3, slightly increased PO selectivity
as compared with Cu–Ru (7.1 % selectivity @ 19.8 %
conversion) catalyst but a drastic decrease was observed in
propylene conversion. Lithium chloride completely sup-
pressed PO formation during reaction. Therefore, in the
current text, we will particularly focus on the binary oxide
catalysts promoted with the most effective promoter (i.e.
NaCl). Corresponding catalytic conversion and selectivity
results of the 2 %Cu/5 %Ru/1.75 %NaCl/c-SiO2 catalyst
in comparison with the 2 %Cu/5 %Ru/c-SiO2 catalyst is
shown again in Fig. 2. It can be seen in Fig. 2 that NaCl
promotion has a significantly positive influence on the PO
selectivity, while the increase in the PO selectivity is
accompanied by some attenuation in the total propylene
conversion.
Table 1 The specific surface area, pore size and pore volume of
different silica support materials with and without Ru and Cu loading
Catalyst
Specific
surface
Pore
size (A) size
Pore
Pore
volume
(NLDFT) (cc/g)
˚
area (m2/g) (BJH)
c-SiO2
96
82
34
34
34
30
37
34
51
70
35
22
52
38
0.15
0.23
0.57
0.49
0.31
0.27
2 %Cu/5 %Ru/c-SiO2
t-SiO2
936
799
718
605
2 %Cu/5 %Ru/t-SiO2
s-SiO2
2 %Cu/5 %Ru/s-SiO2
presented the highest PO selectivity with a relatively poor
propylene conversion (0.44 % PO yield). Preference of the
s-SiO2 supported material towards total combustion over
selective oxidation may be attributed to the extensive mi-
cropores and thus high surface area of this material as
given in Table 1. The residence time of the partial oxida-
tion products in the micropores is increased facilitating the
total oxidation process. On the other hand, the corre-
sponding binary oxides catalyst supported on c-SiO2 yiel-
ded a comparable propylene conversion to that of the
s-SiO2 catalyst without a drastic compromise in PO
selectivity. Thus, in the light of these results, 2 %Cu/
5 %Ru/c-SiO2 was chosen in the second stage of the
optimization approach as the preferred catalyst revealing
both reasonable propylene conversion and PO selectivity
gave the highest PO yield as 1.4 %.
Having determined the optimum ternary oxide catalyst
(i.e. 2 %Cu/5 %Ru/1.75 %NaCl/c-SiO2) using the combi-
natorial approach discussed above, a detailed product dis-
tribution analysis can be made for the relevant catalysts
investigated in the current work. Figure 4 shows relative
selectivity values of 2 wt% Cu and 5 wt% Ru monometallic
oxide catalysts on c-SiO2 as well as their NaCl promoted
counterparts. For comparison, product distribution analysis
of the 2 %Cu/5 %Ru/c-SiO2 and 2 %Cu/5 %/Ru/1.75 %
NaCl/c-SiO2 catalysts are also given. The major products
detected in the catalytic performance tests were CO2, ace-
tone (AT), acetaldehyde (AD), acrolein (AC) and propylene
oxide (PO). As seen in Fig. 4, monometallic oxide 2 %Cu/c-
SiO2catalyst reveals a high selectivity towards AC while
yielding a very low selectivity towards PO. This catalyst also
leads to the highest selectivity towards AT among all of the
other catalysts that were currently tested. On the other hand,
As the final stage of the optimization approach, influ-
ence of a promoter on the 2 %Cu/5 %Ru/c-SiO2 catalyst
was investigated. For this purpose, we have used various
alkali salt precursors such as NaCl, NaNO3, KNO3,
KAc and LiCl as promoters. Figure 3 shows the catalytic
performance of the promoted catalyst. As seen in Fig. 3,
123

S¸. Kalyoncu et al.
CO2
AT
AD
AC
PO
3.2 Characterization
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
The most effective catalyst in the direct oxidation of pro-
pylene to PO synthesis was determined by the selection of
the optimum metal–metal ratio, silica support material and
loading of the promoter by combinatorial screening
method. 5 %Ru–2 %Cu-NaCl/c-SiO2 catalyst showed the
highest performance among the screened catalysts. The
second stage of the study was followed by characterization
of this catalyst with several methods to clarify the effect of
each parameter on the physicochemical and electronic
structure of the catalyst.
The commercial silica support material (c-SiO2) with
(2 %Cu–5 %Ru) and without metal loading revealed a type
IIb adsorption isotherm while corresponding t-SiO2 and
s-SiO2 samples showed type IV adsorption iso-
therms(Fig. 5).The adsorption/desorption hysteresis loop
corresponding to commercial silica (c-SiO2) is close to H4
type in terms of IUPAC classification [41]. A broad hys-
teresis loop is observed for the silica synthesized with sol–
gel method (s-SiO2), showing a desorption branch which is
much steeper than the adsorption branch suggesting the
filling and evacuation of the mesopores by capillary con-
densation. The relatively reversible adsorption isotherm of
the t-SiO2 sample indicates a type IVc isotherm which is
associated with the reversible filling/evacuation of uniform
cylindrical-like pores [41]. Addition of Ru and Cu metals
into the support materials (Table 1) caused a decrease in
the specific surface areas, pore sizes and volume adsorbed
of the t-SiO2 and s-SiO2 materials most probably due to the
partial plugging of the pores with metal particles while
there is an increase in the pore volume of the c-SiO2 at the
constant pore size. NL-DFT analysis provides a much
better interpretation about pore size analysis.
Fig. 4 Selectivity of the propylene oxidation reaction towards
various products such as CO2, acetone (AT), acetaldehyde (AD),
acrolein (AC), propylene oxide (PO) over currently investigated
catalysts (GHSV = 20,000 h-1, gas composition: C3H6:O2 = 1:2,
T = 300 °C)
although the promotion of this catalyst with NaCl results in a
noticeable increase in the PO selectivity at the expense of AC
selectivity, overall PO selectivity is still considerably small.
Meanwhile, monometallic Ru oxide catalyst (i.e.
5 %Ru/c-SiO2) presents a quite different behavior from its
Cu counterpart. Figure 4 shows that 5 %Ru/c-SiO2 catalyst
leads to almost complete total oxidation of propylene to
CO2, with relatively minor amounts of AT, AC and PO
production. Although NaCl promotion of the 2 %Cu/
c-SiO2 catalyst leads to an increase in the PO selectivity, an
opposite trend is observed for the 5 %Ru/1.75 %NaCl/
c-SiO2 catalyst, where the NaCl promotion drastically
decreases the partial oxidation capability of the catalyst by
significantly facilitating the total oxidation.
700
600
500
400
300
200
100
0
c-SiO2
s-SiO2
t-SiO2
When the product distribution of the binary oxide
2 %Cu/5 %Ru/c-SiO2 system is investigated, it is seen
that the PO selectivity is comparable to that of the
2 %Cu/1.75 %NaCl/c-SiO2 catalyst. On the other hand it
is visible that the AC selectivity of the 2 %Cu/5 %Ru/c-
SiO2 system resembles to that of the 5 %Ru/1.75 %NaCl/
c-SiO2 catalyst, indicating an increasing tendency
towards total oxidation to CO2. Thus, it is apparent that
NaCl promotion suppresses the AC production pathways
to a great extent for both of the 2 %Cu/c-SiO2 and
5 %Ru/c-SiO2 monometallic systems. Finally, the NaCl
promotion of the 2 %Cu/5 %Ru/c-SiO2 system yields the
highest PO selectivity among all of the currently inves-
tigated samples.
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
P/Po
Fig. 5 N2 adsorption/desorption isotherms of c-SiO2, s-SiO2 and
t-SiO2 support materials
123

A Combinatorial Micro-reactor Study
In addition to the specific surface area analysis, the
micropore analysis of the samples was made by Non-Local
Density Functional Theory (NL-DFT) model.DFT analysis
of samples showed that the ratio of cumulative surface area
higher BE shoulder at 465–466 eV can be attributed to
hydrated RuO2 (RuO2ÁxH2O) [42, 43].The extra feature in
the spectrum of 5 %Ru–2 % Cu-NaCl/c-SiO2 sample (a) at
498 eV is associated with the Auger signal of Na.
˚
of s-SiO2 material up to a pore width of 20 A is 27 % while
The Cu2p XP spectra of 5 %Ru–2 %Cu-NaCl/c-SiO2,
2 %Cu/c-SiO2, and 5 %Ru-2 %Cu/c-SiO2 samples are
presented in Fig. 8. Cu2p XP spectrum of the 5 %Ru-
2 %Cu-NaCl/c-SiO2 sample yields a Cu2p3/2 peak at
933 eV, which is consistent with the presence of CuO
species [44, 45]. On the other hand, Cu2p XP spectra of the
(NaCl-free) 2 % Cu/c-SiO2, and 5 %Ru–2 %Cu/c-SiO2
samples reveal a strong differential charging behavior,
yielding a significant Cu2p3/2 BE shift of c.a. ?5 eV. This
detrimentally large differential charging behavior, which is
extremely sensitive to the flood gun charge compensation
parameters, is observed only in the absence of NaCl and is
most likely due to the poor electrical conductivity at the
Cu/SiO2 or Cu/Ru/SiO2 interfaces. As a result of this
drastic differential charging, Cu 2p BE values of the NaCl-
free samples are shifted out of the regular Cu 2p BE
window. The changing of the flood gun electron beam
energy leads to a moderate Cu2p3/2 peak shift (up to 3 eV),
while BE values of the Ru and Si signals are not altered by
this change. These results clearly demonstrate the complex
differential charging behavior of the NaCl-free samples
and the strong dependence of the Cu2p XPS signal to the
charge compensation parameters. Interestingly, the pre-
sence of the NaCl promoter drastically facilitates the
charge compensation efficiency of the flood gun, shifting
this value is zero for t-SiO2 and c-SiO2 materials. This
result is in line with the poor PO selectivity over s-SiO2
supported catalysts which can be explained by the limited
diffusion of the oxygenated products out to the bulk gas
phase, favoring total oxidation inside the micropores.
Structure of the Cu or Ru oxide containing monome-
tallic catalysts, Ru-Cu binary oxidecatalysts as well as
NaCl promoted Ru–Cu trimetallic catalysts were also
investigated with XRD (Fig. 6). These samples revealed
characteristic diffraction signals of RuO2(2h = 28.49,
35.59, 55.65, 67.30°), CuO (2h = 35.70, 38.95°) and NaCl
(2h = 31.81, 45.54°) phases. Thus, XRD results suggested
that Ru and Cu existed in oxidic forms on the freshly
prepared catalysts, while Na was found to form crystalline
NaCl. The particle sizes of CuO and RuO2 remained almost
constant (20 nm for RuO2 and 28 nm for CuO) according
to Scherrer equation after modification with NaCl. NaCl
has no effect on the size and distribution of the oxide over
support surface.
The structure of the 5 %Ru/c-SiO2, 2 %Cu/c-SiO2,
5 %Ru-2 %Cu/c-SiO2, and 5 %Ru–2 %Cu-NaCl/c-SiO2
catalysts were also analyzed by XPS (Figs. 7, 8). The Ru3d
(i.e. the main XPS signal of Ru) signal in XPS overlaps
with the intense C1s signal. Therefore, determination of the
Ru oxidation states using the Ru3d region, particularly for
low Ru loadings, is non-trivial. Thus, for the currently
analyzed samples, Ru3p region was investigated (Fig. 7).
Figure 7 shows that a poor and an asymmetric Ru3p3/2
feature is observed at a BE of *463 eV which is attributed
to the RuO2 species in agreement with the XRD data. The
Ru3p
Ru3p3/2
463.0
Ru3p1/2
c
A –RuO2
A
B –CuO
C –NaCl
A
Na KLL
c-SiO2
2Cu/c-SiO2
2Ru/c-SiO2
2Cu/5Ru/c-SiO2
2Cu/5Ru/1.75NaCl/c-SiO2
A
C
C
B
A
A
B
b
B
B
a
20
30
40
50
60
70
80
90
500 490 480 470 460 450 440
2θ, deg.
Binding energy (eV)
Fig. 6 XRD profiles of c-SiO2, 2 %Cu/c-SiO2, 2 %Ru/c-SiO2, 2 %Cu/
5 %Ru/c-SiO2 and 2 %Cu/5 %Ru/1.75 %NaCl/c-SiO2 samples
Fig. 7 Ru3p XP spectra of the calcined a 2 %Cu/5 %Ru/1.75 %NaCl/
c-SiO2, b 2 %Cu/5 %Ru/c-SiO2 and c 5 %Ru/c-SiO2 samples
123

S¸. Kalyoncu et al.
The un-treated catalysts (where Ru and Cu sites exit in
oxide form) showed negligible CO adsorption signal in FT-
IR suggesting a relatively weak CO adsorption capabil-
ity (data not shown here).
Cu2p
Cu2p3/2
938.0
933.0
However, an interesting result was obtained for H2-trea-
ted samples. Figure 9 presents the FT-IR spectra obtained
after CO(g) adsorption on 2 %Cu/c-SiO2, 5 %Ru/c-SiO2,
2 %Cu–5 %Ru/c-SiO2, and 2 %Cu–5 %Ru/1.75 %NaCl/
c-SiO2 samples at 323 K. Prior to CO adsorption, samples
were treated with 10 Torr of H2(g) at 473 K for 15 min. For
the 2 %Cu/c-SiO2 sample, the major vibrational feature is
the signal at 2,124 cm-1which is attributed to Cu?–CO
species. It was observed that for the H2-treated samples
(Fig. 9) CO probe molecule adsorbed strongly on the Cu?
sites [38, 46].The Cu2?–CO species which are also detect-
able are associated with the minor vibrational feature at
2,240 cm-1 [46, 47].
Cu2p1/2
c
b
a
The CO adsorption on the 5 %Ru/c-SiO2 sample (Fig. 8,
spectra b–d) leads to the appearance of some characteristic
vibrational bands such as the shoulder around
2,120–2,140 cm-1 (Fig. 8b) associated with CO chemi-
sorbed over reduced Ru0 sites [48]. The band at 2,067 cm-1
has been previously assigned to Ru(CO)3 species on metallic
Ru centers in the literature [49]. The broad feature at about
1,987 cm-1 has been assigned to CO adsorbed on coordin-
atively unsaturated Ru sites (i.e. Ru defect sites) and/or to
960
950
940
930
Binding energy (eV)
Fig. 8 Cu2p XP spectra of the calcined 2 %Cu/5 %Ru/
1.75 %NaCl/c-SiO2, b 2 %Cu/c-SiO2 and c 2 %Cu/5 %Ru/c-SiO2
a
samples
the Cu2p BE back to the regular Cu2p energy window.
Apparently, NaCl functions as a promoter enabling the
charge transfer at the Cu/SiO2 and/or Cu/Ru/SiO2 interface
preventing the charge build up on the surface during the
XPS analysis.
In other words, although RuO2 phase does not seem to
reveal any indication of severe differential charging (i.e.
charge build-up), Cu2p signal for NaCl-free samples dis-
close a severe and a remarkable amount of charge build-up
on the CuO sites or at the CuO/SiO2 interface. These two
strikingly different differential charging behaviors of Ru
and Cu sites on the same binary oxide catalyst surface
suggest that in the absence of NaCl, Ru and Cu sites on the
binary oxide catalyst are very well dispersed but probably
not in a direct physical contact. Unlike the behavior
described above, on the NaCl-promoted samples, CuO sites
seem to be readily charge-compensated suggesting that
NaCl functions as a structural promoter preventing the
charge build-up at the Cu-SiO2 and Cu-/Ru-SiO2
interfaces.
H2O
CO2
d
c
b
a
Nature of the transition metal adsorption sites on the
catalytic systems were also analyzed by FT-IR spectra
obtained after CO(g) adsorption and saturation at 323 K over
the catalyst surfaces. FT-IR analysis was performed for H2-
treated and un-treated 2 %Cu/c-SiO2, 5 %Ru/c-SiO2,
2 %Cu–5 %Ru/c-SiO2, and 2 %Cu–5 %Ru/1.75 %NaCl/
c-SiO2 catalyst samples at 323 K.
2400 2200 2000 1800 1600 1400
Wavenumber, cm-1
Fig. 9 In-situ FTIR spectra acquired after CO adsorption on
a 2 %Cu/c-SiO2, b 5 %Ru/c-SiO2, c 2 %Cu/5 %Ru/c-SiO2 and
d 2 %Cu/5 %Ru/1.75 %NaCl/c-SiO2 samples (see text for details)
123

A Combinatorial Micro-reactor Study
isolated Ru0–CO species [50–52]. The band at 2,175 cm-1
can be attributed to Run?–CO where n [ 2 [49]. The IR
feature at 1,783 cm-1 can be assigned to bridging carbonyls
(i.e. Ru02–CO) [50].
propylene conversion while increasing the PO selectivity
(35.6 % selectivity @ 9.4 % conversion in Fig. 2).
As a conclusion, the reason behind the positive effect of
NaCl addition in the Ru-Cu catalytic performance can be
further elaborated in the light of the current XPS and in situ
FT-IR experiments. XPS experiments (Figs. 7, 8) clearly
demonstrate the drastic alterations in the structure of the
2 %Cu/5 %Ru/c-SiO2 surface upon NaCl addition, as
expected from an promoter. On the other hand, current
in situ FTIR data (Fig. 9, spectrum d) for H2-treated cat-
alysts also point to the fact that NaCl promotion decreases
the total number of CO-adsorption sites (i.e. Cu? and Ru?x
surface species). Thus, it is clear that although NaCl
functions as a structural promoter facilitating the partial
oxidation pathways over total oxidation (i.e. CO2 produc-
tion), NaCl also decreases the overall propylene conversion
by decreasing the number of Cu? and Ru?x sites generated
during the reaction (Fig. 2).
A comparison of the spectra (a), (b) and (c) in Fig. 9
suggests that going from monometallic (i.e. 2 %Cu/c-SiO2
and 5 %Ru/c-SiO2) to a binary oxidecatalyst (2 %Cu–
5 %Ru/c-SiO2) increases not only the total CO adsorption
on the overall catalyst surface but it also increases the CO
uptake of the individual Ru and Cu sites. This observation
points to the fact that Cu–Ru binary oxide catalyst reveals a
synergistic behavior leading to the formation Cu? and
Rux? sites even after H2-treatment.
Finally, comparison of the spectra (c) and (d) also pro-
vides an important insight regarding the influence of the
NaCl promoter on the 2 %Cu–5 %Ru/c-SiO2, structure. It
is apparent that the presence of NaCl decreases the inten-
sities of all of the CO adsorption signals in the FT-IR data.
This can be attributed to a strong interaction between NaCl
and the active sites on the catalyst surface, in line with the
current XPS results discussed above. CO adsorption
experiments via FT-IR clearly demonstrate that binary
oxide systems reveal a synergistic behavior by retarding
the complete reduction of the metal oxide sites to metal
sites and by forming a greater number of exposed
adsorption sites. Furthermore, NaCl promoter directly
interacts with the Cu and Ru active sites and modifies their
adsorption properties towards CO.
4 Conclusions
In this study, a combinatorial approach was used in order to
investigate an alkali salt-promoted binary oxide catalyst
having an improved catalytic performance in the propylene
partial oxidation. In order to find the optimum catalyst
formulation, first the relative Cu (0–2 wt%) and Ru (0–5
wt%) loadings of the prepared catalysts were varied overs-
SiO2 support material, where it was found that 2 %Cu/
5 %Ru/s-SiO2 system revealed the best performance
among the synthesized catalyst set in the micro reactor-
based activity/selectivity tests. As the next step, influence
of the nature of the silica support material on the catalytic
performance was investigated by comparing the catalytic
performances of 2 %Cu/5 %Ru/t-SiO2, 2 %Cu/5 %Ru/s-
SiO2 and 2 %Cu/5 %Ru/c-SiO2 catalysts. These experi-
ments suggested that 2 %Cu/5 %Ru/c-SiO2 catalyst yiel-
ded the most preferable performance with a high propylene
conversion and a high PO selectivity. As the final step of
this screening tests, several alkali salts (1.75 wt%) were
incorporated into the 2 %Cu/5 %Ru/c-SiO2 catalyst. It was
observed that NaCl promotion significantly increased the
PO selectivity which was accompanied by some attenua-
tion in the total propylene conversion. These findings were
explained in the light of the current XPS and in situ FTIR
data. Along these lines, it was proposed that the 2 %Cu/
5 %Ru/c-SiO2 binary oxide system revealed a greater
number of exposed active Cu? and Ru?x surface sites with
respect to that of the monometallic systems, combined (i.e.
2 %Cu/c-SiO2 and 5 %Ru/c-SiO2). Although NaCl addi-
tion is likely to decrease the total number of exposed Cu?
and Ru?x active surface sites, promotional effect of NaCl
leads to an enhanced PO selectivity. Beside it was found
The effect of the alkali promotion on the chemical states
of copper in CuOx/SiO2 catalyst was investigated by He
and co-workers under reaction condition through in situ
XRD and CO-adsorbed FT-IR studies. They reported that
Cu? species forms during reactions over modified and un-
modified catalysts. They also reported that the presence of
the alkali metal ions retards the reduction of the CuO as a
result of the H2-TPR analysis. The pulse-reaction experi-
ments in this study showed that Cu2?-containing catalysts
yielded a low propylene conversion and a low PO selec-
tivity while the presence of a low concentration of Cu?
surface sites enhances the PO selectivity. On the other
hand, it was also reported that for high Cu? to Cu2? ratios,
PO selectivity started to decrease while propylene con-
version increased [38].
In the light of this study [38] and our current FT-IR
results, it can be argued that on for 2 %Cu–5 %Ru/c-SiO2
catalyst surface, Cu? and Ru?x species are readily gener-
ated under reaction conditions (i.e. in the presence of C3H6
and O2). As discussed previously, such a catalyst yielded a
low PO selectivity with a high propylene conversion
(7.1 % selectivity @ 19.8 % conversion). However when
NaCl is added to the bimetallic oxides catalytic system,
accessible number of Cu? and Ru?x surface sites decreases
as compared to the bimetallic oxides system decreasing the
123

S¸. Kalyoncu et al.
´
22. Lu J, Bravo-Suarez JJ, Takahashi A, Haruta M, Oyama ST (2005)
that an addition of 1.75 wt% NaCl has no effect on the
particle size of the Cu and Ru oxides according to XRD
analysis.
J Catal 232:85
23. Lu J, Bravo-Suarez JJ, Haruta M, Oyama ST (2006) Appl Catal A
302:283
´
24. Yao W, Guo Y, Liu X, Guo Y, Wang Y, Wang Y, Zhang Z, Lu G
(2007) Catal Lett 119:185
25. Lei Y, Mehmood F, Lee S, Greeley J, Lee B, Seifert S, Winans
¨
˙
Acknowledgments This research was supported in part by TUBI-
TAK through MAG Project No: 108T378. High throughput testing
facilities at UCLA were provided by Prof. Selim Senkan. E.O.
acknowledges support from Turkish Academy of Sciences (TUBA)
through the ‘‘Outstanding Young Investigator’’ Grant. E.V.
acknowledges RFBR (Russia) #12-03-91373-CT_a, for financial
support.
¨
RE, Elam JW, Meyer RJ, Redfern PC, Teschner D, Schlogl R,
Pellin MJ, Curtiss LA, Vajda S (2010) Science 328:224
26. Suo Z, Jin M, Lu J, Wei Z, Li C (2008) J Nat Gas Chem 17:184
27. Wang R, Guo X, Wang X, Hao J (2003) Catal Lett 90:57
28. Wang R, Hao J, Guo X, Wang X, Liu X (2004) Stud Surf Sci
Catal 154:2632
29. Wang Y, Chu H, Zhu W, Zhang Q (2008) Catal Today 131:496
30. Lambert RM, Williams FJ, Cropley RL, Palermo A (2005) J Mol
Catal A 228:27
References
31. Torres D, Lopez N, Illas F, Lambert RM (2007) Angew Chem Int
Edit 46:2055–2058
32. Kizilkaya AC, Senkan S, Onal I (2010) J Mol Catal A 330:107
33. Vaughan OPH, Kyriakou G, Macleod N, Tikhov M, Lambert RM
(2005) J Catal 236:401
34. Zhu W, Zhang Q, Wang Y (2008) J Phys Chem C112:7731
35. Onal I, Du¨zenli D, Seubsai A, Kahn M, Seker E, Senkan S (2010)
Top Catal 53:92
36. Kahn M, Seubsai A, Onal I, Senkan S (2010) Comb Chem High
Throughput Screen 13:67
37. Seubsai A, Kahn M, Senkan S (2011) Chem Cat Chem 3:174
38. He J, Zhai Q, Zhang Q, Deng W, Wang Y (2013) J Catal 299:53
39. Duzenli D, Seker E, Senkan S, Onal I (2012) Catal Lett 142:1234
40. Senkan S (2001) Angew Chem Int Ed 40:312
41. Rouquerol F, Rouquerol J, Sing K (1999) Adsorption by powders
and porous solids principles, methodology and applications.
Academic Press, San Diego
42. Chetty R, Xia W, Kundu S, Bron M, Reinecke T, Schuhmann W,
Muhler M (2009) Langmuir 25:3853
43. Zeng Q, Zheng L, Zeng J, Liao S (2012) J Power Sources
205:201
44. Peebles DE, Peebles HC, Ohlhausen JA (1998) Colloids Surf A
144:89
45. Lee S, Song D, Kim D, Lee J, Kim S, Park IY, Choi YD (2004)
Mater Lett 58:342
¨
46. Hadjiivanov K, Knozinger H (2001) Phys Chem Chem Phys
3:1132
47. Busca G (1987) J Mol Catal 43:225
48. Yokomizo GH, Louis C, Bell AT (1989) J Catal 120:1
49. Sayan S, Kantcheva M, Suzer S, Uner DO (1999) J Mol Struct
480–481:241
1. Monnier JR (2001) Appl Catal 221:73
2. Nijhuis TA, Makkee M, Moulijn JA, Weckhuysen BM (2006) Ind
Eng Chem Res 4:3447
3. Clerici MG, Bellussi G, Romano U (1991) J Catal 129:159
4. Zuwei X, Ning Z, Yu S, Kunlan L (2001) Science 292:1139
5. Kamata K, Yonehara K, Sumida Y, Yamaguchi K, Hikichi S,
Mizuno N (2003) Science 300:964
6. Wang R, Guo X, Wang X, Hao J, Li G, Xiu J (2004) J Appl Catal
A 261:7
7. Hayashi T, Tanaka K, Haruta M (1988) J Catal 178:566
8. Sinha AK, Seelan S, Tsubota S, Haruta M (2004) Angew Chem
Int Ed 43:1546
9. Chowdhury B, Bravo-Sarez JJ, Date M, Tsubota S, Haruta M
(2006) Angew Chem Int Ed 45:412
10. Ananieva E, Reitzmann A (2004) Chem Eng Sci 59:5509
11. Wang Y, Yang W, Yang LJ, Wang XX, Zhang QH (2006) Catal
Today 117:156
12. Sacaliuc E, Beale AM, Weckhuysen BM, Nijhuis TA (2007) J
Catal 248:235
13. Stangland EE, Stavens KB, Andres RP, Delgass WN (2000) J
Catal 191:332
14. Qi C, Akita T, Okumura M, Haruta M (2001) Appl Catat A
218:81
15. Zwijnenburg A, Makkee M, Moulijn JA (2004) Appl Catal
270:49
16. Lu GZ, Zuo XB (1999) Catal Lett 58:67
17. Lu J, Luo M, Lei H, Li C (2002) Appl Catal A 237:11
18. Palermo A, Husain A, Tikhov M, Lamber RM (2002) J Catal
207:331
19. Zemicheael F, Palermo A, Tikhov M, Lambert RM (2002) Catal
Lett 80:93
50. Kantcheva M, Sayan S (1999) Catal Lett 60:27
51. Guglielminotti E, Bond GC (1990) J Chem Soc Faraday Trans
86:979
20. Jin G, Lu G, Guo Y, Guo Y, Wang J, Liu X (2003) Catal Lett
87:249
21. Takahashi A, Hamakawa N, Nakamura I, Fujitani T (2005) Appl
Catal A 294:34
52. Robbins JL (1989) J Catal 115:120
123