Silylation enhances the performance of Au/Ti-SiO2 catalysts in direct epoxidation of propene using H2 and O2

Article abstract of DOI:10.1016/j.jcat.2016.10.004

The effect of silylation on a series of Au/Ti-SiO₂ catalysts for the direct epoxidation of propylene in the presence of H₂ and O₂ was studied. It was found that silylation significantly improved catalyst performance: propylene conversion, propylene oxide (PO) selectivity, and H₂ efficiency increased. The extent of improvement depended on the Au and Ti content of the catalysts. The catalyst showing the best activity (Au(0.1)/Ti(1)-SiO₂) exhibited an average PO formation rate of 121 g_PO kg_cat^?1 h^?1 and a PO selectivity of 92% at 473 K, while the catalyst having the maximum Au and Ti loading (Au(1)/Ti(5)-SiO₂) showed the most significant improvement in performance with a 78% increase in the rate of PO formation upon silylation. The catalysts were characterized by contact angle measurements, FTIR, TGA, TEM, ICP-OES and these observations were used to elucidate the key factors governing the enhanced catalytic performance upon silylation. It was found that the silylated catalyst exhibited superior performance due to increased hydrophobicity, which aids product desorption, a decrease in acidic sites that are responsible for side-product formation, and a possible redistribution of the Au particles.

Full text of DOI:10.1016/j.jcat.2016.10.004

Journal of Catalysis 344 (2016) 434–444
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Silylation enhances the performance of Au/Ti–SiO2 catalysts in direct
epoxidation of propene using H2 and O2
S. Kanungo a, Kumer Saurav Keshri b, A.J.F. van Hoof c, M.F. Neira d’Angelo a, J.C. Schouten a, T.A. Nijhuis d,
E.J.M. Hensen c, B. Chowdhury b,
⇑
a Laboratory of Chemical Reactor Engineering, Schuit Institute of Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513,
5600 MB Eindhoven, The Netherlands
b Department of Applied Chemistry, Indian Institute of Technology (Indian School of Mines), Dhanbad 826004, India
c Laboratory of Inorganic Materials Chemistry, Schuit Institute of Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O.
Box 513, 5600 MB Eindhoven, The Netherlands
d Sabic, Geleen, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of silylation on a series of Au/Ti-SiO2 catalysts for the direct epoxidation of propylene in the
presence of H2 and O2 was studied. It was found that silylation significantly improved catalyst perfor-
mance: propylene conversion, propylene oxide (PO) selectivity, and H2 efficiency increased. The extent
Received 16 September 2016
Revised 2 October 2016
Accepted 3 October 2016
of improvement depended on the Au and Ti content of the catalysts. The catalyst showing the best activ-
ꢀ1
ity (Au(0.1)/Ti(1)–SiO2) exhibited an average PO formation rate of 121 gPO kg hꢀ1 and a PO selectivity of
cat
92% at 473 K, while the catalyst having the maximum Au and Ti loading (Au(1)/Ti(5)–SiO2) showed the
most significant improvement in performance with a 78% increase in the rate of PO formation upon sily-
lation. The catalysts were characterized by contact angle measurements, FTIR, TGA, TEM, ICP-OES and
these observations were used to elucidate the key factors governing the enhanced catalytic performance
upon silylation. It was found that the silylated catalyst exhibited superior performance due to increased
hydrophobicity, which aids product desorption, a decrease in acidic sites that are responsible for side-
product formation, and a possible redistribution of the Au particles.
Keywords:
Gold catalysis
Propene epoxidation
Silylation
Au–Ti catalysts
Au/Ti–SiO2
Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction
In an attempt to identify suitable catalysts for this desirable oxi-
dation reaction, gold has taken the center of attention since the
The development of green catalytic processes will contribute
greatly to the transition to a sustainable future. Particularly
promising in this context are nanoscale catalysts, the compositions
of which may be tailored to acquire desirable chemical properties.
A coveted green catalytic process is the direct epoxidation of pro-
pene to propene oxide (PO) [1]. The development of this one-step
green synthesis is important as traditional methods of PO produc-
tion are either environmentally stressful (the chlorohydrin
method) or have economic limitations (SM/PO and MTBE/PO both
produce equal amounts of co-products) [2]. The more recent pro-
cesses of PO production consist of multiple steps (the cumene
hydroperoxide process by Sumitomo Chemicals) and use expensive
oxidants (the HPPO process by Dow/BASF) [3], hence making the
direct synthesis route highly relevant.
discovery by Haruta et al. [4] that gold on titania can produce PO
in the presence of O2 and H2. Several groups have demonstrated
the possibility of propene epoxidation over catalysts comprising
Au nanoparticles dispersed on Ti-containing supports such as
TiO2, TiO2-SiO2, TS-1, Ti-MCM-41, Ti-MCM-48, Ti-SBA-15, and Ti-
containing hydrophobiꢀc1silsesquioxane [3,5]. The highestꢀi1nitial
PO rates (ꢁ300 gPO kgcat hꢀ1, 200 °C, GHSV = 14,000 mL gcat hꢀ1
)
have been reported by the group of Delgass on a Cs-promoted
Au/TS-1 catalyst. The major drawback of this system is rapid cata-
lyst deactivation [6]. Several attempts have been made to improve
the stability of Au/TS-1, such as using uncalcined TS-1 to ensure
that Au is deposited only on the external surface of the zeolite
[7,8]. Chen et al. [9] have reported another active catalyst system,
Au/Ti–SiO2, displaying stable PO reaction rates on the order of
120 gPO kg hꢀ1 (200 °C, GHSV = 10,000 mL g hꢀ1). Dispersing
ꢀ1
ꢀ1
cat
cat
Au nanoparticles on Ti-grafted silica resulted in stabler catalyst
performance with no significant deactivation on stream. Another
benefit of this catalytic system is that in the gold deposition step,
⇑
Corresponding author.
E-mail address: biswajit_chem2003@yahoo.com (B. Chowdhury).
http://dx.doi.org/10.1016/j.jcat.2016.10.004
0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

S. Kanungo et al. / Journal of Catalysis 344 (2016) 434–444
435
ꢁ100% gold is deposited on the support, which is not the case in
2. Experimental
the preparation of Au/TS-1 [8,10,11]. In the same work [9], it was
also found that although low Au and Ti loadings led to highly active
and stable Au/TiSiO2 catalysts, they also formed relatively large
amounts of propane, the formation of which could be suppressed
to some extent by optimizing the Ti grafting procedure. Another
report by the same authors described the addition of CO to the feed
to suppress propene hydrogenation [12,13].
The reaction mechanism for PO formation that is most widely
supported involves formation of a peroxy species, either in the
form of OOH or H2O2, on small Au nanoparticles. These peroxy spe-
cies spill over to preferably isolated tetrahedral Ti4+ species. The
resulting Ti-coordinated OOH/H2O2 intermediate then oxidizes
propene to PO and water [2,14,15]. Water is a by-product produced
in larger quantities than the stoichiometric amount formed during
epoxidation, which results from the direct combustion of H2. This
side reaction lowers the hydrogen efficiency of the process, which
along with low propene conversion and poor catalyst stability has
hindered commercialization of this process so far [1,2,16].
An attractive approach to improving the catalytic performance
is to increase the hydrophobicity of the catalyst surface by silyla-
tion, a strategy that has been explored well for various titania–sil-
ica catalysts for different reactions. In the silylation process, a
silanol group of the support reacts with the alkylsilyl group of
the silylating agent, thus rendering the material more hydropho-
bic. This in turn facilitates rapid desorption of polar products. Sily-
lation can be carried out both in gas and liquid phases using
different silylating agents [17–28]. Hydrophobization of Au–Ti cat-
alysts for propene epoxidation was reported in patents by Weis-
beck et al. [22], where hexamethyldisilazane was used as a
silylating agent, and by Hayashi et al. [23], where trimethyl-
methoxysilane was found to be an effective silylating agent. Fol-
lowing this, Uphade et al. [24] reported that silylation using
trimethylmethoxysilane (in the gas phase) led to increased PO
selectivity, H2 efficiency, and stability of the Au/Ti-MCM-48 cata-
lyst. In another study [25], silylation was carried out in the liquid
2.1. Catalyst synthesis
As support, we employed silica to which Ti was grafted accord-
ing to a known procedure [9]. In a typical synthesis, as-received sil-
ica (15 g, Davisil-643, 300 m2/g, average pore size 150 Å, pore
volume 1.15 cm3/g) was dried and suspended in 250 mL anhydrous
2-propanol (Aldrich, 99%) under N2 in a glove box. The resulting
suspension was stirred for 10 min, after which tetraethylorthoti-
tanate (TEOT, Aldrich, 97%) was added in such an amount that
the Ti coverage was 0.5, 1, or 5% of the monolayer coverage of
the silica. Based on the hydroxyl content of the silica, a loading
of 1 wt.% Ti corresponds to 5% monolayer coverage. After the slurry
had been stirred for 30 min, 2-propanol was slowly evaporated
using a rotary evaporator at 328 K and 100 mbar. Evaporation of
the solvent took around 4 h. The powder obtained was dried over-
night at 353 K, followed by calcination at 393 K (heating rate 5 K/
min, isothermal period 2 h) and 873 K (heating rate 10 K/min,
isothermal period 4 h). Gold was deposited on Ti–SiO2 using a de
position–precipitation method described by Chen et al. [9]. The
support (2 g) was dispersed in 100 ml ultrapure water
(18.2 M
X
cm at 25 °C). The pH was adjusted to ꢁ9.5 using a
2.5 wt.% ammonia solution. The desired amount of HAuCl4
(Aldrich, 30 wt.% in HCl) was diluted in 20 ml water, and was then
added dropwise using a burette in ca. 15 min. Thereafter, the slurry
was kept stirring for 1 h, while the pH was maintained at 9.4–9.5
by adding ammonia dropwise. The solid was collected by filtration
and washed three times using deionized water. The catalyst was
dried overnight at 353 K and calcined at 393 K (heating rate 5 K/
min, isothermal period 2 h) and 673 K (heating 10 K/min, isother-
mal period 4 h) in static air. The catalysts are denoted as Au(x)/Ti
(y)–SiO2, where x is the nominal Au loading (in wt.%) and y is the
percentage monolayer coverage.
For silylation, vapors of methoxytrimethylsilane were passed
through the catalyst bed, which was maintained at 423 K for
15–30 min. This was done by bubbling Ar through a saturator
containing methoxytrimethylsilane, which was kept at 298 K. This
was followed by flushing with Ar at 473 K for 5 h [28]. The silylated
catalysts are denoted by the suffix ‘‘Sil.”
phase
using
N-methyl-N-(trimethylsilyl)trifluoroacetamide
(MSTFA) on Ti-MCM, and the resulting catalyst was found to have
high PO selectivity even at higher operating temperatures,
although there was no significant increase in the overall PO yield.
There was also no improvement in terms of stability. Although
there have been a few more studies [26–28], the effect of silylation
has not yet been investigated for the most recent and more active
generation of Au–Ti catalysts for direct propene epoxidation. Fur-
thermore, the influence of silylation on factors other than surface
hydrophobicity has not been elucidated yet. A final aspect, which
warrants further investigation, is the effect of silylation on Au–Ti
synergy.
In this work, we investigate in more detail the influence of sily-
lation on the catalytic performance of Au/Ti–SiO2 in direct propene
epoxidation using H2 and O2. Au/Ti–SiO2 was chosen because of the
excellent stability it exhibits, good shelf life, and high gold uptake
in homogeneous deposition–precipitation. It is also one of the most
active Au–Ti catalystsꢀr1eported to date, with a stable rate of PO
formation >100 gPO kgcat hꢀ1 at 473 K [9,16]. Earlier, it was found
that prolonged grafting of Ti resulted in decreased propane
side-product formation. Here, we show that longer grafting can
completely suppress propene hydrogenation. We use this grafting
procedure to synthesize supports with different Ti coverage of the
silica surface (0.5, 1, and 5% monolayer Ti), followed by Au deposi-
tion (0.05, 0.1, and 1 wt.%). Thereafter, all the catalysts were
subjected to the silylation treatment in order to study its effect
on the propene epoxidation performance. The Ti coordination in
the Ti-SiO2 was investigated by DR-UV–vis spectroscopy. Charac-
terization of optimum catalysts further included contact-angle
measurements, TGA, FTIR, and TEM techniques.
2.2. Catalyst characterization
The coordination state of Ti was determined by diffuse reflec-
tance UV–vis (DR-UV–vis) spectra using a Shimadzu UV-2401PC
spectrometer with BaSO4 serving as the reference.
The Ti and Au content of the catalysts was analyzed by induc-
tively coupled plasma optical emission spectroscopy (ICP-OES) in
a SpectroCiros CCD spectrometer. First aqua regia was added to
the samples, and then this mixture was heated under stirring for
30 min. The solutions were cooled and then dilute HF (1:15 by vol-
ume in water) was added and the solution was swirled until clear.
Transmission electron microscopy (TEM) was used to deter-
mine the size of the Au nanoparticles. Sample preparation involved
sonication of the samples in pure ethanol and application of a few
droplets of the suspension to a 200 mesh Cu TEM grid with a holey
carbon support film. TEM micrographs were acquired on an FEI
Tecnai 20 transmission electron microscope at an acceleration
voltage of 200 kV with a LaB6 filament. TEM images were recorded
at different magnifications using a Gatan 1 k ꢂ 1 k CCD camera.
Particle size and distribution were obtained by measuring at least
250 nanoparticles for each sample.
The hydrophobicity of the silylated catalyst was measured
using a goniometer (Dataphysics OCA 30). The catalyst sample
was pressed into a wafer and a drop of water (ꢁ8
lL) was placed

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S. Kanungo et al. / Journal of Catalysis 344 (2016) 434–444
on its surface using a needle. Images clearly showing the liquid–
solid interface were collected and left and right contact angles
were measured using the Dataphysics software. The TG-DTG data
profiles were measured using Mettler Toledo TGA/DSC.
Infrared spectra were recorded on a Bruker Vertex 70v appara-
tus. The samples were pressed into self-supporting discs (ꢁ10 mg
weight) and placed in the IR cell. The samples were degassed at
423 K for 1 h and spectra were recorded at 323 K. For pyridine FTIR
measurements, the same setup was used and pyridine vapors were
passed over the catalyst wafer at 423 K for 1 h. Spectra were
recorded at 423 K following desorption for 1 h at 473 K and 623 K.
2.3. Catalytic activity measurements
Catalytic activity measurements were performed in a flow setup
equipped with an Interscience gas chromatography system (analy-
sis time ꢁ5 min) containing two analysis channels equipped with
Porabond Q and Molsieve 5A columns and thermal conductivity
detectors. The catalyst was loaded in a quartz reactor tube, which
was placed in a tubular oven. A typical reaction cycle was 5 h fol-
lowed by a regeneration step at 573 K with 10 vol% O2 in He for 1 h.
In a typical test, 150 mg of catalyst was loaded into the reactor, andꢀth1e
total flow was adjusted to 25 ml/min (GHSV = 10,000 ml gꢀca1t h ).
Reaction cycles were carried out typically in the temperature range
423–493 K. The rate of each catalytic cycle was calculated as the
average of the rates between 150 and 300 min on stream.
The propene conversion (X) and selectivity to different products
(Sx, where x denotes the product) were calculated for the reaction
nC3H6 ! mCx and expressed as
Fig. 1. DR–UV–vis spectra of the three different supports having Ti monolayer
coverage of 0.5% (solid line), 1% (dashed–dotted line), and 5% (dashed line).
on the other hand, also contains Ti species in higher coordination,
in isolated form and as agglomerated species [9,29,30].
P
P
mn Px
mn Px
3.2. Effect of silylation on activity
X ð%Þ ¼
ꢂ 100 ¼
mn Px
ꢂ 100;
ð1Þ
P
PoCut
þ
Pin
3H6
C3H6
Gold was deposited at different loadings on these Ti-modified
supports by homogeneous deposition–precipitation. After calcina-
tion, an aliquot of each sample was silylated. Before discussing in
more detail the effect of silylation on the various Au–Ti catalysts,
we first report its influence on the catalytic performance for Au
(0.1)/Ti(1)–SiO2. Fig. 2 shows the reaction rates of PO and water
for the parent and the silylated sample at 473 K. The absence of
deactivation with time on stream is in line with earlier reported
data for propene oxidation on similar Au–Ti catalysts [5,9,16].
The silylated catalyst displays stable performance with time on
stream and shows a higher PO reaction rate than that on the unsi-
lylated one, as evident from Fig. 2a. A smaller amount of water pro-
duced after silylation, as shown in Fig. 2b, results in higher H2
efficiency of the silylated catalyst.
mn Px
P
Sx ð%Þ ¼
ꢂ 100;
ð2Þ
mn Px
where Px is the partial pressure of the carbon-containing products
obtained during the epoxidation of propene (PO, acrolein, acetone,
propanal, acetaldehyde, and CO2) and the terms Pin
and PoCut
C3H6
3H6
denote the propene concentrations at the inlet and outlet of the
reactor respectively; and
rPO
rPO þ rH2O
gH2 ð%Þ ¼
ꢂ 100;
ð3Þ
where rPO and rH2O are the rates of PO and water formation,
respectively.
The absence of mass transfer limitations was verified using the
Weisz–Prater criterion (CWP < 1) and the Mears criterion
(CM < 0.15) for internal and external mass transfer limitations,
respectively. CWP was found to be on the order of 10ꢀ6 (ꢃ1) and
CM on the order of 10ꢀ6 (ꢃ0.15), thus ensuring that the system
was free of mass transfer limitations (see Supporting Information).
We observe that the silylated sample shows activation behavior
during subsequent activity–regeneration cycles. This feature was
not observed for the parent (nonsilylated) sample. This is high-
lighted in Fig. 3 for Au(0.1)/Ti(1)–SiO2–Sil, in which reaction rates
for a period of 5 h are presented at temperatures of 473–453–493–
473 K, with intermittent oxidative regeneration at 573 K. In the
first cycle (473 K), the amount of PO formed increases with time.
In the fourth cycle (473 K), the PO formation is higher and nearly
stable. Subsequent cycles under similar conditions lead to nearly
similar stable activity (not shown). The initial low activity of a sily-
lated catalyst has also been reported in a previous study [24]. To
gain insight into the cause of this activation behavior, we studied
various catalysts with CO-FTIR. It is found that calcination of the
silylated sample leads to an increase of a CO band due to interac-
tion with Ti4+. We speculate that silylation of Lewis acid Ti sites,
essential in the mechanism of PO oxidation involving OOH/H2O2
intermediates, is blocked during silylation. Calcination slowly
removes the species that block these active sites, so that the
increased activity can be explained as a function of reaction–regen-
eration (Supporting Information).
3. Results and discussion
3.1. Support characterization
Grafting of TEOT onto the silica surface resulted in three differ-
ent Ti–SiO2 supports. Elemental analysis showed that the Ti load-
ings (percentile monolayer coverage between brackets) were
0.1 wt.% (0.5% ML), 0.21 wt.% (1% ML), and 0.88 wt.% (5% ML) Ti
for Ti(0.5)–SiO2, Ti(1)–SiO2, and Ti(5)–SiO2, respectively. Fig. 1
shows the diffuse reflectance UV–vis spectra of the three supports
used in this study. The relatively narrow band around 210 nm for
Ti(0.5)–SiO2–0.5 and Ti(1)–SiO2 points to the high dispersion of
Ti on the silica surface as isolated tetrahedral species. Ti(5)–SiO2,

S. Kanungo et al. / Journal of Catalysis 344 (2016) 434–444
437
Fig. 2. Time-on-stream formation rate of (a) PO and (b) water during a 5 h catalytic test over Au(0.1)/Ti(1)–SiO2 (black squares) and Au(0.1)/Ti(1)–SiO2–Sil (red circles)
ꢀ1
catalysts at 473 K, H2/O2/C3H6/He = 1:1:1:7, GHSV = 10,000 mL g hꢀ1
.
cat
Fig. 4. TG profiles of Au(0.1)/Ti(1)–SiO2 (solid line) and Au(0.1)/Ti(1)–SiO2–Sil
(dashed–dotted line) from 323 to 1023 K.
Fig. 3. Rate of formation of PO on Au(0.1)/Ti(1)–SiO2–Sil at different reaction
temperatures at conditions H2/O2/C3H6/He = 1:1:1:7 and
GHSV = 10,000 mL gꢀ1
ꢀ1
h
.
cat
shows that silanol groups have been converted. This will be con-
firmed by FTIR spectroscopy.
We also investigated by IR spectroscopy the changes brought
about by silylation and activation of the catalyst in oxygen at
523 and 573 K. For this purpose, the parent and silylated Au(0.1)/
Ti(1)–SiO2 samples were evacuated in the environmental cell at
423 K for 1 h. IR spectra were recorded at 323 K. Fig. 5a shows that
these spectra contain bands at 3743 cmꢀ1 due to isolated silanol
groups and a broad band centered around 3555 cmꢀ1 correspond-
ing to hydrogen-bonded silanols and associated water [20,25]. The
difference spectra before and after silylation (Fig. 5b) clearly show
the decreased intensity of isolated silanol groups. Comparatively,
the changes in hydrogen-bonded silanol groups and associated
water are less pronounced. The peaks at 2964 and 2905 cmꢀ1
(see inset of Fig. 5a) are due to trimethylsilyl groups on the silica
surface [41,21,42]. All samples including the nonsilylated one con-
tain additional C–H vibrations, which, in case of the unsilylated
sample (peak at 2928 cmꢀ1), can be attributed to residual ethoxy
groups from the TEOT precursor used during Ti grafting. On the
other hand, the additional peak at 2857 cmꢀ1 observed in the sily-
lated sample can be assigned to methoxy groups, which may be
formed due to the reaction of the methanol (the by-product of sily-
lation) with silanols or siloxane bridges. A similar observation was
made when trimethylethoxysilane was used as a silylating agent
on Ti-MCM-41 and additional bands due to ethoxy groups were
observed [21]. The reaction of the silylating agent trimethyl-
3.3. Characterization of silylated catalyst
We then investigated the influence of the silylation treatment
on the catalytic surface. It is expected that silylation will render
the surface more hydrophobic. It should be noted that silylation
was carried out on the Au-containing samples, as previous work
[24] showed that this was more beneficial than loading gold onto
a silylated sample. This is reasonable, as the absence of hydroxyl
groups would otherwise lower the gold dispersion. The high
hydrophilicity of the surface of Au(0.1)/Ti(1)–SiO2 follows from
the contact angle of ꢁ5°. After silylation, the contact angle
increased to ꢁ31°, showing that silylation strongly increased the
hydrophobicity of the surface (Supporting Information).
Fig. 4 shows TGA curves for the parent and the silylated Au(0.1)/
Ti(1)–SiO2. Before heating, the samples were kept in saturated
water vapor for 1 h at room temperature. The weight loss up to
430 K is mainly due to associated water [41], and is observed to
be lower in the silylated sample. Hence, the TGA curves demon-
strate that silylation increases the hydrophobicity of the sample,
which is in agreement with the contact angle measurements
reported above. Another significant difference, indicated by TGA,
is the absence of the feature due to dehydroxylation (condensation
of silanol groups) around 623 K in the silylated sample, which

438
S. Kanungo et al. / Journal of Catalysis 344 (2016) 434–444
Fig. 5. (a) IR Spectra of Au(0.1)/Ti(1)–SiO2 (i) unsilylated, (ii) silylated, (iii) silylated and treated in O2 for 30 min at 523 K followed by (iv) treatment at 573 K in O2 for 30 min.
Inset: features between 3000 and 2800 cmꢀ1. (b) Difference spectrum (ii)–(i) showing the changes after silylation. (c) Difference spectrum (iv)–(ii) showing the changes of
silylated sample before and after treatment at 573 K in O2.
methoxysilane (TMMS) with silanols and siloxane bridges is shown
in Scheme 1, along with possible side reactions of methanol from
surface silylation.
(3743 cmꢀ1). These changes can be attributed to the dehydroxyla-
tion of the hydrogen-bonded silanols, yielding siloxane bridges and
silanols as shown in Scheme 2.
Calcination of the silylated samples at 523 and 573 K in oxygen
did not profoundly affect the C–H bands in the silylated samples, as
seen in the difference spectra, Fig. 5c, except for the erosion of the
2928 cmꢀ1 band and the decrease in the 2857 cmꢀ1 band. This sug-
gests that the oxidation treatment results in further conversion of
residual alkoxy groups. Fig. 5c also shows that this treatment led to
significant decrease of the broad band around 3555 cmꢀ1, with a
simultaneous increase in the number of isolated silanols
3.4. Catalytic performance: Influence of Au and Ti loading
We then investigated the influence of Au and Ti loading on cat-
alytic performance before and after silylation. The resulting perfor-
mance data are provided in Table 1 for comparison. The variation
in Au and Ti loading has a considerable effect on the performance
of the parent (nonsilylated) catalysts. First we observe that these

S. Kanungo et al. / Journal of Catalysis 344 (2016) 434–444
439
Scheme 1. Reaction of silylating agent trimethylmethoxysilane (TMMS) with (i) silanols and (ii) siloxane bridges, and probable reactions of methanol side product with (iii)
silanols and (iv) siloxane bridges.
Scheme 2. Dehydroxylation of hydrogen-bonded silanols to form siloxane bridges and silanols.
samples do not produce propane, which was the case in a previous
study [9]. This could be achieved by prolonging the Ti grafting
time, which resulted in a decrease of propene hydrogenation to
propane (Supporting Information). The data also reveal that the
samples with predominantly isolated tetrahedral Ti atoms (Au
(0.05)/Ti(0.5)–SiO2 and Au(0.05)/Ti(1)–SiO2) show much higher
activity than the sample containing agglomerated Ti (Au(0.05)/Ti
(5)–SiO2. The lower activity of Au(1)/Ti(5)–SiO2 is likely due to
the decreased number of isolated tetrahedral Ti atoms, which low-
ers the selectivity toward PO and hence the total PO formation
[9,34,36]. The highest PO formation rate is obꢀse1rved for the catalyst
Au(0.1)/Ti(1)–SiO2 with rPO ꢁ 107 gPO hꢀ1 kgcat, a PO selectivity of
90%, and a H2 efficiency of 12%. The overall trends observed are
in keeping with insights from the literature [9].
CO2 selectivity decreases in all cases. This is reasonable, as the
more hydrophobic reaction environment should decrease the resi-
dence time of PO on the surface and, accordingly, its propensity
toward decomposition. For most of the catalysts, we also observe
an increase in the PO selectivity, except for Au(1)/Ti(0.5)–SiO2. In
this case, silylation leads to decreased PO selectivity (from 80% to
72%) and increased acrolein selectivity (from 7% to 11%). The best
performance upon silylꢀat1ion is obtained for Au(0.1)/Ti(1)–SiO2–
Sil with rPO = 121 gPO kgcat hꢀ1 at a PO selectivity of 92% and a H2
efficiency of 14%. The greatest improvement in activity upon sily-
lation is observed for Au(1)/Ti(5)–SiO2, where the rate of PO pro-
duction increases from 41 to 73 gPO kg hꢀ1 and the selectivity
ꢀ1
cat
to PO increases from 74 to 82%, while the conversion increases
from 3.4 to 5.3%. From Table 1, it is also observed that the perfor-
mance enhancement upon silylation is not the same at all Ti and
Au loadings.
Comparing these performances with data for the silylated cata-
lysts, we observe that the propene conversion increases and the

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S. Kanungo et al. / Journal of Catalysis 344 (2016) 434–444
Table 1
Performance of silylated and unsilylated catalysts.a
Catalyst
Au loading, Ti loading, Conversion Selectivity (%)
H2 efficiency, rPO
ꢀ1 ꢀ1
wt.%
Nominal
wt.%
Nominal
(%)
gH2 (%)c
(gPO kg
h
)
cat
PO
Acetaldehyde Acetone Propanal Acrolein CO2
(Realb)
(Realb)
Au(0.05)/Ti(0.5)–SiO2
Au(0.05)/Ti(0.5)–SiO2–Sil
0.05 (0.07)
1.0 (0.76)
0.1 (0.1)
1.8
2.6
87.6 1.1
92.5 1.2
4.0
2.4
2.3
1.7
4.9
2.2
0.08
0.06 12
8
43
66
d
Au(1)/Ti(0.5)–SiO2
1.7
1.9
79.9 2.8
71.9 4.2
5.7
5.8
2.5
3.2
6.7
10.6
2.4
1.1
3
3
38
38
Au(1)/Ti(0.5)–SiO2–-Sild
Au(0.05)/Ti(1)–SiO2
Au(0.05)/Ti(1)–SiO2–Sil
0.05 (0.07)
0.1 (0.11)
1.0 (0.75)
0.2 (0.21)
2.3
3.2
84.6 0.2
6.2
2.8
5.6
3.6
2.9
3.0
0.3
0.2
11
17
49
71
90.3
0
Au(0.1)/Ti(1)–SiO2
Au(0.1)/Ti(1)–SiO2–Sil
4.5
4.8
89.5 1.3
91.9 1.3
3.4
2.4
2.2
2.4
2.2
1.3
1.4
0.8
12
14
107
121
d
Au(1)/Ti(1)–SiO2
4.4
4.6
82
0.2
5.3
5.6
0.1
0.1
7.0
7.5
5.4
5.2
4
4
82
84
Au(1)/Ti(1)–SiO2–Sild
81.4 0.2
Au(0.05)/Ti(5)–SiO2
Au(0.05)/Ti(5)–SiO2–Sil
0.05 (0.07)
1.0 (0.5)
1.0 (0.88)
0.9
1.0
65.9 1.7
86.3 1.2
12.0
2.0
13.6
6.3
6.8
4.2
0
0
9
17
16
25
e
Au(1)/Ti(5)–SiO2
3.4
5.3
74.0 1.8
82.2 2.0
6.8
3.4
8.4
5.2
3.9
3.6
5.0
3.7
7
10
41
73
Au(1)/Ti(5)–SiO2–Sile
a
b
c
ꢀ1
Reaction conditions: H2/O2/C3H6/He = 1:1:1:7, GHSV = 10,000 mL g hꢀ1 at 473 K unless mentioned otherwise.
cat
As found by ICP-OES.
rPO
H2 Efficiency ð%Þ ¼
ꢂ 100.
rPOþrH2O
d
e
Reaction performed at 423 K.
Reaction performed at 453 K.
Fig. 6. TEM images of (a) Au(0.1)/Ti(1)–SiO2 and (b) Au(0.1)/Ti(1)–SiO2–1–Sil along with the corresponding particle size distributions; also shown is the distribution for the
corresponding spent sample.

S. Kanungo et al. / Journal of Catalysis 344 (2016) 434–444
441
Fig. 7. TEM images of (a) Au(1)/Ti(5)–SiO2 and (b) Au(1)/Ti(5)–SiO2–Sil, along with the corresponding particle size distributions.
3.5. Effect of silylation on particle size and acidity
and silylated Au(1)/Ti(5)–SiO2 samples. We investigated this sam-
ple because silylation affected its performance the most among all
catalysts. Fig. 8 shows that silylation results in a decrease of the
amount of Brønsted acid sites as followed from the lower intensity
of the 1637, 1545, and 1490 cmꢀ1 bands [32,33]. Brønsted acidity
in these samples is assumed to originate from Ti–O–Si bridges,
with Ti being in penta- or hexacoordination as in TiOx aggregates
rather than in tetrahedral coordination [31,34]. These sites are
To explain the activity trends observed upon silylation, we char-
acterized selected silylated and unsilylated catalysts in more
detail. Fig. 6 shows representative TEM images of the parent Au
(0.1)/Ti(1)–SiO2 catalyst and its silylated counterpart along with
the corresponding gold particle size distributions. The parent sam-
ple contains a range of gold nanoparticles ranging from just below
1 nm to about 3–5 nm. The average particle size is 1.6 0.9 nm.
Upon silylation, the particle size distribution changed, in the sense
that particles larger than 2.5 nm were nearly absent. The average
particle size of the silylated catalyst is 1.1 0.4 nm, slightly lower
than that in the fresh catalyst. This may indicate that silylation of
the Au/Ti–SiO2 sample results in redistribution of gold over the
surface. We also analyzed the spent catalyst in the same way. Typ-
ically, it is seen that the average particle size increased only
slightly and, in particular, no particles larger than 3 nm were
observed in the silylated catalysts, which contrasted with the par-
ticles seen for the fresh catalysts.
The particle redistribution argument was further investigated
for Au(1)/Ti(5)–SiO2, as this sample showed the greatest improve-
ment upon silylation. The results are shown in Fig. 7 and further
support the finding that gold particles appear to redisperse during
the silylation treatment. The nonsilylated catalyst contains parti-
cles larger than 6 nm, while these are absent in the silylated cata-
lyst. Moreover, the particle size distribution is narrower after
silylation. This, in turn, also leads to a decrease in the average par-
ticle size, as indicated in Fig. 7.
Fig. 8. Pyridine FTIR of Au(1)/Ti(5)–SiO2 (black line) and Au(1)/Ti(5)–SiO2–Sil (red
line). L: peak assigned to Lewis acid sites; B: peak assigned to Brønsted acid sites.
To investigate whether silylation led to any change in the acidic
properties of the catalyst, pyridine FTIR was carried out on parent

442
S. Kanungo et al. / Journal of Catalysis 344 (2016) 434–444
Scheme 3. Schematic diagram showing epoxidation on (a) unsilylated catalyst and (b) silylated catalyst. Au: gold nanoparticles; Tiep: Ti sites active for epoxidation; Tia:
acidic Ti sites. PO forms at the Au–Tiep interface; the water formation along with PO is not shown, for simplicity.
likely responsible for the formation of bidentate propoxy species
from PO, which gives rise to such undesired side-products as pro-
panal and acetone [34–37]. Silylation converts these acidic groups
into (inactive) Si–O–Ti (CH3)3 moieties, explaining the decreased
Brønsted acidity. This leads to an increase in the selectivity to
PO, as shown above.
tion included IR spectroscopy, where it was found that there were
fewer isolated silanols in the silylated sample, along with the pres-
ence of signature peaks of trimethylsilyl groups, indicating suc-
cessful silylation. The retention of these peaks, when subjected to
high temperatures and O2 flow, indicated that the silyl groups were
not lost under reaction and regeneration conditions.
Further characterization revealed that silylation also led to a
decrease in the number of Brønsted acid sites of the catalyst (pyr-
idine FTIR) and also hinted at possible redistribution of large Au
particles into smaller ones (TEM), the mechanism of which needs
further systematic investigation. So in summary, the effect of sily-
lation on the catalyst was found to be threefold: an increase of sur-
face hydrophobicity, a decrease in the number of Brønsted acid
sites, and an increase in Au dispersion. These three factors will
be used next to explain the catalytic activity trends observed for
the silylated catalysts at different Au and Ti content.
3.6. General discussion
Au/Ti–SiO2 is reported to be an efficient catalyst for the hydro-
epoxidation of propene [9,12,13,16,40]. Its ease of preparation,
high activity, and stability on stream, along with facile regenera-
tion, make this a promising catalyst for future application in indus-
try. Major drawbacks are undesired propane formation, low
propene conversion, and low H2 efficiency [9]. In this study, an
attempt was made to address these shortcomings. While support
preparation was optimized to suppress propene hydrogenation,
silylation was employed to further enhance catalyst performance.
It has been proposed that certain TiOx species contribute to pro-
pane formation in Ti–SiO2 supported gold catalysts [9]. The time of
Ti grafting affected the dispersion and coordination state of Ti.
Careful optimization of the Ti grafting procedure led to highly dis-
persed Ti on silica and complete suppression of propane formation.
Further optimization of Au and Ti content was carried out and Au
(0.1)/Ti(1)–SiO2 was identified to be the bꢀes1t-performing catalyst,
with a rate of PO formation of 107 gPO kgcat hꢀ1,ꢀP1O selectivity of
1. From the catalytic activity results, we observed that most of the
catalysts showed activation behavior during the reaction before
reaching their optimum performance. CO FTIR measurements
show that this is due to blocking of active Au–Ti sites by the
silylating agent, which was slowly removed in situ during the
reaction. After several reaction–regeneration cycles, the cata-
lysts attained their final and stable activity.
2. Performance results showed that the influence of silylation was
highly dependent on Au and Ti loading. The maximum rate of
PO production was obtained for Au(0.1)/Ti(1)–SiO2–Sil, while
the maximum improvement due to silylation was observed
for Au(1)/Ti(5)–SiO2. It was observed that there was a signifi-
cant improvement in performance at lower Au loadings for all
the supports. This was reflected by increases in propene conver-
sion, PO selectivity, rate of formation of PO, and H2 efficiency.
On the other hand, at higher Au loadings, the trend was some-
what different. For the catalyst with lowest Ti loading (Au(1)/Ti
(0.5)–SiO2–Sil), there was, in fact, a decrease in PO rate and
selectivity; at intermediate Ti loadings (Au(1)/Ti(1)–SiO2–Sil)
there was only a slight improvement in performance; for the
90%, and g
H2 = 12% at 473 K (GHSV = 10,000 mL gcat hꢀ1). The effect
of gas phase silylation using TMMS was then studied on this cata-
lyst and a further enhancement of performance was observed
(Table 1). This encouraged us to study the effect of silylation on a
series of Au–Ti catalysts with the aim of gaining insight into the
main effect of silylation on the catalyst.
First, we verified the effectiveness of silylation by contact angle
and TGA measurements. Contact angle measurements showed that
silylation increased the hydrophobicity of the catalyst. This was
also evident from TGA, where the weight loss due to water was
found to be smaller for the silylated sample. Further characteriza-

S. Kanungo et al. / Journal of Catalysis 344 (2016) 434–444
443
155 gPO kg hꢀ1 at 493 K, stꢀil1l maintaining a high selectivity of
ꢀ1
highest Ti-containing catalyst (Au(1)/Ti(5)–SiO2–Sil), the
improvement is most significant, as mentioned earlier. We
explain these trends as follows:
cat
88% (GHSV = 10,000 mL gꢀca1t h ). This performance is comparable
to the best activities reported to date for catalysts without
promoters.
ꢄ
At low Ti content and sufficiently high Au loading (Au(1)/Ti
(0.5)–SiO2), the catalyst contains significant numbers of Au
nanoparticles in contact with pure silica. Au/SiO2 exhibits
negligible PO formation, as opposed to Au in contact with
Ti sites during propene epoxidation. Instead, it has been
found that Au/SiO2 produces mainly acrolein and acetalde-
hyde during propene oxidation [38,39]. In this particular cat-
alyst, the redispersion of Au has a negative effect, as it will
lead to an increasing fraction of Au nanoparticles in contact
with silica rather than with the few isolated Ti sites. This
seems to be in line with the increased selectivity toward
acrolein and acetaldehyde, as observed in the performance
data.
Acknowledgments
The authors acknowledge the Netherlands Organization for Sci-
entific Research (NWO) for financial support for this research, Pro-
ject (10017587). We also thank Dr. N. Kosinov for TG analysis, B.
Mezari for helping with IR measurements, and Dr. I. Jiménez Pardo
for assisting in contact angle measurements. BC acknowledges DST
for funding in an India–Netherland bilateral project (DST/INT/NL/
FM/P-2/2013). KSK would like acknowledge Indian School of Mines
for research fellowship.
ꢄ
At high Ti content (5% ML), the surface is characterized by
the presence of agglomerated forms of titania as well as
Brønsted acid sites, as evident from pyridine FTIR measure-
ments. Earlier studies have shown that PO adsorbs strongly
on both Si–OH and Ti–OH sites [34–37]. The resulting biden-
tate propoxy groups can react further to propanol and ace-
tone, which are side-products of propene epoxidation [34].
The main effect of silylation is to deactivate these acidic
sites. Although observed for most of the catalysts, increased
PO selectivity is most pronounced for the catalysts with the
highest Ti loading (Au(0.05)/Ti(5)–SiO2–Sil and Au(1)/Ti(5)–
SiO2–Sil). This is likely because this catalyst contains the lar-
gest number of penta/hexa-coordinated Ti sites, which were
successfully deactivated by silylation.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2016.10.004.
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