Vapor Phase Oxidation of Benzyl Alcohol over Nano Au/SBA-15 Catalysts: Effect of Preparation Methods

Article abstract of DOI:10.1007/s10562-015-1656-7

Au/SBA-15 nano catalysts were synthesized from four different methods, viz., homogeneous deposition-precipitation, micro-emulsion, impregnation and polyol, and their catalytic activities were investigated for the vapor phase oxidation of benzyl alcohol to

Full text of DOI:10.1007/s10562-015-1656-7

Catal Lett
DOI 10.1007/s10562-015-1656-7
Vapor Phase Oxidation of Benzyl Alcohol over Nano Au/SBA-15
Catalysts: Effect of Preparation Methods
Ashish Kumar1,2 Vanama Pavan Kumar2 Amirineni Srikanth2
•
•
•
Venkataraman Vishwanathan3 Komandur V. R. Chary2
•
Received: 28 April 2015 / Accepted: 9 November 2015
Ó Springer Science+Business Media New York 2015
Abstract Au/SBA-15 nano catalysts were synthesized
from four different methods, viz., homogeneous deposi-
tion–precipitation, micro-emulsion, impregnation and
polyol, and their catalytic activities were investigated for
the vapor phase oxidation of benzyl alcohol to benzalde-
hyde. The physico-chemical properties of the catalysts
were characterised by XRD, TEM, BET surface area, pore
size distribution, CO-chemisorption and XPS techniques.
The structural data of the catalysts along with their cat-
alytic studies indicate that the presence of very small
metallic Au0 species with particle size of 7–8 nm, was
responsible for the higher activity observed in the vapor
phase oxidation of benzyl alcohol reaction. The title reac-
tion, though industrially important, was used as a test
reaction to investigate firstly, the influence of different
preparation methods on the uniform dispersion of gold
particles on the support SBA-15 and to understand the
metal-support interaction in Au/SBA-15 catalysts, and
secondly, to study the catalytic performance of the catalyst
(Au/SBA-15) in terms of activity, selectivity and stability
over a period of reaction time. The conversion of benzyl
alcohol was found to increase with decrease in the size of
gold particles. Smaller gold particles with higher percent-
age of dispersion on the support SBA-15 had a beneficial
effect on the catalytic activity. Among the four methods
used for the preparation of gold on SBA-15 support, the
catalyst prepared by homogeneous deposition–precipitation
method showed the best performance in terms of conver-
sion, selectivity for benzaldehyde and longer catalyst life.
Graphical Abstract Vapor phase oxidation of benzyl
alcohol over nano Au/SBA-15 catalysts.
Keywords Gold nanoparticles Á SBA-15 Á Benzyl
alcohol Á Oxidation Á Benzaldehyde
1 Introduction
& Komandur V. R. Chary
kvrchary@iict.res.in
It was well established that the catalytic activity of sup-
ported gold nanoparticles depends on the particle size, the
nature of support and the preparation method. Among the
oxidation reactions, the vapor phase oxidation of benzyl
alcohol (PhCH2OH) to benzaldehyde (PhCHO) is com-
mercially of importance, since the process is environmen-
tally benign and needs less expensive additives.
1
School of Chemistry and Biochemistry, Thapar University,
Patiala 147004, India
2
Catalysis Laboratory, Inorganic & Physical Chemistry
Division, CSIR-Indian Institute of Chemical Technology,
Hyderabad 500007, India
3
Department of Chemistry, Sreyas Institute of Engineering
and Technology, Hyderabad 500068, India
123

A. Kumar et al.
Conventionally, benzaldehyde was produced as a by-pro-
duct during the oxidation of toluene (PhCH3) to benzoic
acid (PhCOOH) or from the hydrolysis of benzyl dichlo-
ride. But, these reaction routes are known to promote a
large amount of hazardous by-products like organic chlo-
rine or benzoic acid. Hence, this limits the usage of ben-
zaldehyde in the cosmetics, pharmaceutical and flavouring
industries [1]. From the commercial view, air or molecular
oxygen is the favourite choice as the primary oxidant since
they produce water as the by-product [2]. However, the use
of air requires the development of newer and novel cata-
lysts in order to achieve higher catalytic activity under
ambient reaction conditions. Many studies have employed
in the recent past on the supported gold as catalysts in
several catalytic applications including selective aerobic
oxidation of alcohols such as benzyl alcohol [2–4]. In order
to achieve a high catalytic performance, these catalysts are
employed in the form of nano-composites where
nanoparticles of gold are loaded onto the support materials
like activated carbon, metal oxides and polymers. How-
ever, such batch reactions in liquid phase require a longer
period of time to reach the steady state and also require the
separation of catalysts from the products. From the view-
point of atom economy and green chemistry, the emphasis
has been laid more on the vapor phase catalytic oxidation
of benzyl alcohol to benzaldehyde [1, 4–8], since it is
solvent free continuous and provides a higher selectively
for benzaldehyde. Recently, Rossi et al. [3, 8] have
reported that gold catalysts to be more effective in the
vapor phase oxidation of volatile alcohols to form the
corresponding ketones and aldehydes.
The present investigation deals with the preparation of
Au/SBA-15 nano-catalysts from four different routes,
namely, homogeneous precipitation-deposition, ME, IMP
and POL. Here, we report the unique structural features of
SBA-15 materials as a support for preparing active catalyst
for producing benzaldehyde by vapor phase oxidation of
benzyl alcohol. Investigation of these catalysts in the oxi-
dation of benzyl alcohol under vapor phase conditions
resulted in a good stability in terms of conversion and
higher selectivity towards the formation of benzaldehyde.
The importance of vapor phase reaction is well known
since it can be performed continuously at moderate reac-
tion conditions with higher selectivity of the desired pro-
duct as compared to the liquid phase reactions.
The structural features of Au/SBA-15 catalysts were
investigated by XRD, TEM, BET surface area, PSD, CO-
chemisorption and XPS techniques. A comparative study
has been made with respect to their structural properties,
oxidation activity and selectivity for benzaldehyde.
2 Materials and Methods
2.1 Catalyst Preparation
2.1.1 Preparation of Mesoporous SBA-15 Support
Mesoporous SBA-15 was prepared by the procedure
described elsewhere [9–11]. Briefly, the procedure consist
of dissolving 2.0 g of triblock copolymer Pluronic P-123
template with stirring in a solution of 15 g of water fol-
lowed by adding 45 g of 2 M HCl at 40 °C. Further, about
5.9 g of tetraethylortho-silicate (TEOS) (Sigma-Aldrich,
99.8 %) was added dropwise in the homogeneous solution
with stirring. The molar ratios of TEOS:HCl:H2O:polymers
were maintained as 1.0:3.1:115:0.012, respectively. The
synthesis mixture was continuously stirred at 40 °C for
20 h, and finally hydrothermally treated at 98 °C for 24 h
in an oven. The as-prepared solid product was separated by
filtration, washed with deionized water and ethanol to
remove the excess of template and dried in air at room
temperature for 12 h. The organic template was removed
by calcination in air at 550 °C for 5 h.
Mesoporous SBA-15 materials exhibit several unique
characteristics such as high surface area, long range
ordering of mesoporous channels, larger pore volume and
their high pore wall thickness which provides good thermal
and hydrothermal stability as well as improved acid–base
tolerance [9–11].
In recent years, nano-sized gold particles have been
reported to show a higher catalytic activity [12–15]. We
have reported recently similar studies over Ru/SBA-15
catalysts synthesized from different methods viz., micro-
emulsion (ME), polyol (POL), impregnation (IMP) and
deposition–precipitation method [16]. The superior cat-
alytic behaviour is attributed to the formation of nanopar-
ticles of ruthenium over mesoporous SBA-15. The
nanostructure gold catalysts prepared in present work by
the homogeneous deposition–precipitation (HDP) using
urea as the precipitating agent has shown superior activity
for benzyl alcohol oxidation compared to the catalysts
derived from other methods. The HDP method has several
advantages such as simple, facile, better control of pore
structures and of higher dispersion of gold nanoparticles at
low concentration on the catalyst support [12].
2.1.2 Preparation of Gold Catalysts
The Au/SBA-15 catalysts were prepared by four different
preparation methods: HDP, ME, IMP and POL, using
HAuCl4Á3H2O (Sigma-Aldrich, 99.8 %) as metal precur-
sor. Prior to characterization and catalytic activity mea-
surements, all the catalysts were chemically reduced by
0.1 M, freshly prepared NaBH4 aqueous solution and cal-
cined at 400 °C for 3 h in N2 atmosphere. The EDAX-
123

Vapor Phase Oxidation of Benzyl Alcohol over Nano Au/SBA-15 Catalysts…
analysis showed that the presence of sodium was negligible
(\0.01 %).
(100)
Homogeneous Deposition–Precipitation (HDP) Method
The Au/SBA-15 catalyst was prepared by HDP method
using urea as the precipitating agent [12, 17]. The mixture
of an aqueous solution containing HAuCl4Á3H2O (Sigma-
Aldrich, 99.8 %, with desired gold loading) and urea was
stirred with gradual heating to a temperature up to 95 °C
for 6 h. On heating, the urea decomposes to give ammonia
and hence precipitation occurs in a homogeneous way in
the whole bulk solution as the pH shift towards basic
conditions (pH 6–8). Than after, the mesoporous SBA-15
was added to the above solution with continuous stirring.
The solid product was filtered, washed thoroughly with
deionized water until the filtrate contained no chloride ions
(confirmed with AgNO3 test) and subsequently dried in hot
air oven at 100 °C for 5 h.
(110)
(200)
(a)
(b)
(c)
(d)
(e)
1.0
1.5
2.0
2.5
3.0
3.5
4.0
2 θ (degree)
Micro-Emulsion (ME) Method The sample prepared by
this method was described briefly in the following method
[18, 19]. In container-A, 0.5 g of cetyltrimethyl ammonium
bromide (CTAB), 5 mL of n-butanol and 30 mL of
cyclohexane were added together and stirred until a clear
solution was obtained. In container-B, an aqueous solution
containing HAuCl4Á3H2O (0.086 g) precursor and 110 mL
of tetrahydrofuran (THF) were made. Now, the solution of
container-B was added to the container-A along with the
required amount of SBA-15 support. The solid product
formed was filtered and washed with THF to remove the
surfactant completely. The filtrate was further washed with
ethanol and subsequently dried in hot air oven at 100 °C
for 5 h.
Fig. 1 Low angle XRD patterns of a SBA-15 and Au/SBA-15
catalysts, b HDP, c ME, d IMP and e POL
Corporation, Japan). X-ray diffractometer using Ni filtered
Cu Ka radiation (k = 0.15406 nm) with a scan speed of 2°
min-1 and a scan range of 10°–80° for wide angle
diffraction at 30 kV and 15 mA. The crystallite size of gold
was calculated by using Debye–Scherrer equation and
phase identification with the help of the JCPDS files.
The CO-chemisorption measurements were carried out
on AutoChem 2910 (Micromeritics, USA) instrument.
About 100 mg of the catalyst was pre-treated with helium
gas for 1 h at 150 °C. The sample was subsequently cooled
to 50 °C in the same He gas stream. CO uptake was
determined by injecting pulses of 10 % CO/He from a
calibrated online sampling valve into the He gas stream
passing over the samples at 80 °C. Metal area, metal
Impregnation (IMP) Method The Au/SBA-15 catalyst
was prepared by impregnating of SBA-15 with an aqueous
solution of HAuCl4Á3H2O [20, 21]. The required amount of
HAuCl4Á3H2O (0.086 g) solution was added to the SBA-15
support and the mixture was then stirred for 4 h. The
mixture was finally dried at 50 °C for 4 h in a rotary
evaporator.
(111)
Au/SBA-15 Catalysts
Polyol (POL) Method The Au/SBA-15 catalyst was
prepared by using the following method [22]. Approxi-
mately 20 mL of ethylene glycol was placed in an ice bath
and required amount of SBA-15 support was added to it. In
another container, 0.086 g of HAuCl4Á3H2O was dissolved
in 10 mL of ethylene glycol separately. The above solution
was added drop wise to 20 mL ethylene glycol in an ice
bath with constant stirring for 20 min. The final mixture
was allowed to settle. The filtrate was washed with ethanol
and subsequently dried at 100 °C for 5 h.
(200)
(220)
(311)
(d)
(c)
(b)
(a)
35
40
45
50
55
60
65
70
75
80
2.2 Catalysts Characterization
2 θ (degree)
X-ray powder diffraction (XRD) patterns of the catalysts
were recorded on a Rigaku Miniflex (M/s. Rigaku
Fig. 2 Wide angle XRD patterns of Au/SBA-15 catalysts. a HDP,
b ME, c IMP and d POL
123

A. Kumar et al.
2.3 Catalytic Activity
fixed bed vertical down flow glass reactor
dispersion and metal average particle size were calculated
assuming the stoichiometric factor (CO/Au) as 1. Adsorp-
tion was deemed to be completed after three successive
runs showed similar peak area.
A
(length = 520 mm, i.d. = 12 mm) used for the vapor
phase oxidation of benzyl alcohol under ambient atmo-
spheric pressure. An amount of 1.0 g catalyst was diluted
with an equal amount of same size quartz grains was
packed in between two layers of quartz wool. The upper
portion of the reactor was filled with glass beads which
serve as a preheater for the reactant. Prior to the oxidation
reaction, each catalysts were activated at 250 °C for 2 h in
the presence of N2 flow (50 mL/min) to ensure a clean
surface of the catalysts. Thereafter, the benzyl alcohol
(Sigma-Aldrich, 99.8 %) was fed into the reactor in a
stoichiometric quantity (WHSV = 2.84 h-1) using a syr-
inge pump (Perfusor, B. Braun, Germany) by passing air
along with the reactant at 320 °C. The reaction products
were collected in an ice-cold trap at the bottom of the
reactor for every 1 h.
Gold content was determined by inductively coupled
plasma optical emission spectrometer (ICP-OES) on a
Varian 720-ES instrument. Solid samples were first
digested in a mixture of HF, HCl, and HNO3 in a micro-
wave oven for 2 h and further diluted with deionised water
to analyze the gold contents by ICP-OES. ICP analysis,
performed on the fresh samples of Au/SBA-15 catalysts.
The BET surface areas of the catalysts were obtained
from N2 adsorption–desorption isotherm (Autosorb
I/Quantachrome instruments, USA at -196 °C). The
samples were first out gassed at 300 °C to ensure a clean
surface prior to nitrogen adsorption. The Barrett–Joyner–
Halenda (BJH) method was used to calculate the pore-size
distribution from the desorption branch of the isotherm
(Autosorb I/Quantachrome, USA).
The reaction products were analyzed by a HP-6890 gas
chromatograph equipped with a HP-5 capillary column
having flame-ionization detector (FID) and the reaction
products were also identified by HP-5973 quadruple GC-
MSD system with a HP-1MS capillary column using He as
a carrier gas. However, the gaseous products were intro-
duced to a 1 cm3 gas sampler and analyzed by gas chro-
matography (SHIMADZU GC-2014) equipped with a
thermal conductivity detector (TCD) using Carboxen 1000
column, under an Helium as carrier gas.
Transmission electron microscopy (TEM) images of the
catalysts were obtained using a Technai-12, FEI, Nether-
lands at an accelerating voltage of 120 kV. The specimens
were prepared by dispersing the samples in methanol using
an ultrasonic bath and evaporating a drop of resultant
suspension onto the carbon coated copper grid.
Thermo gravimetric analysis (TGA) is used to measure
the heat flow and rate change as a function of temperature
or time of the catalysts. The TGA analysis of the catalyst
was measured on a Q500, TA instruments, USA.
X-ray photoelectron spectroscopy (XPS) is used to
study the chemical composition and oxidation state of
catalyst surfaces. The XPS spectra of the catalysts were
measured on a XPS spectrometer (KRATOS Axis 165,
Shimadzu, UK) with Mg Ka radiation 1253.6 eV at
75 W. The gold 4f core-level spectra were recorded and
the corresponding binding energies referenced to the C 1 s
line at 284.6 eV [accuracy within (0.2 eV)]. The back-
ground pressure during the data acquisition is kept below
10-9 torr.
3 Results and Discussion
3.1 Characterization of Catalysts
The low angle XRD pattern of the SBA-15 and different
Au/SBA-15 catalysts are shown in Fig. 1. As can be seen
from Fig. 1 the XRD peaks at 2h = 0.83°, 1.44°, 1.66°
corresponds to the planes of (100), (110), and (200) due to
Table 1 The physico-chemical properties of Au/SBA-15 catalysts
b
c
S. No. Au/SBA-15 (1 wt%) COirr (lmol/g) Au dispersion (%)a Am (m2/gcat
)
Am (m2/gAu
)
dAu (nm)d dAu (nm)e dAu (nm)f
1.
2.
3.
4.
a
HDP
ME
10.73
5.02
4.97
4.84
16.55
9.94
9.84
9.62
0.44
0.27
0.26
0.25
43.99
26.43
26.15
25.58
7.05
11.74
11.87
12.14
6.08
7.87
8.91
9.68
7.36
8.34
IMP
POL
9.44
10.38
Au dispersion
b
Metal area (catalyst)
Metal area (gold)
c
d
Au particle size determined from COirr uptake values
Au crystallite size determined from XRD
Au particle size determined from TEM
e
f
123

Vapor Phase Oxidation of Benzyl Alcohol over Nano Au/SBA-15 Catalysts…
Au/SBA-15 (HDP)
Fresh catalyst
Mean size
(7.36 nm)
6.0
6.5
7.0
7.5
8.0
8.5
9.0
Au particle size (nm)
Au/SBA-15 (ME)
Fresh catalyst
Mean size
(8.34 nm)
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
Au particle size (nm)
Au/SBA-15 (IMP)
Fresh catalyst
Mean size
(9.44 nm)
8.0
8.5
9.0
9.5
10.0
10.5
Au particle size (nm)
Au/SBA-15 (POL)
Fresh catalyst
Mean size
(10.38 nm)
9.5
10.0
10.5
11.0
11.5
Au particle size (nm)
Fig. 3 TEM images and histograms comparing the gold particle size distribution of different Au/SBA-15 catalysts
123

A. Kumar et al.
the characteristic peaks of mesoporous structure of SBA-15
[10]. The Braggs reflections, confirm the hexagonal sym-
metry (P6 mm) of the SBA-15 material [23]. The XRD
patterns of Au catalysts supported on SBA-15 reveals that
there were no significant changes observed upon Au
loading, indicating that the structure of the SBA-15 was
remain intact even after gold deposition. The decrease in
the intensity of XRD the peaks at 2h = 1.4° and 1.6° for
the synthesized catalysts are due to blockage of the pores of
SBA-15 by Au particles [24].
(a)
(e)
(d)
(c)
(b)
The wide-angle XRD patterns of 1 wt% loadings of Au
catalyst supported on SBA-15 are shown in the Fig. 2. This
figure clearly demonstrates that the broad diffraction peak
at 2h = 22.4° are observed for all the Au/SBA-15 catalysts
due to the amorphous silica framework of SBA-15. The
Au/SBA-15 catalysts exhibits four diffraction peaks at
2h = 38.2°, 44.3°, 64.3°, 77.4° which are indexed to (111),
(200), (220), and (311) reflections, respectively, for the
face-centred cubic (FCC) lattice structure of Au (JCPDS
file number: 04-0784) [24, 25]. However, the mesoporous
ordered structure of SBA-15 could be retained even after
complete gold deposition, as shown in the Fig. 2. In 1 wt%
Au/SBA-15 (HDP) the XRD reflections due to 38.2° and
44.3° are not resolved and appear due to presence of gold
nanoparticles (2–3 nm) in amorphous state. However, at
lower loadings we could not see the reflections due to
metallic gold indicating the presence of gold in a highly
dispersed form or the crystallite size could be very low
(\4 nm) which was beyond the detection limit of XRD.
The XRD patterns also indicate that there are no char-
acteristic peaks noticed due to the formation of mixed
phase due to interaction between Au and SBA-15 support.
This indicates the formation of Au0 phase. The peaks are
sharp and intense, indicating good crystallinity of metallic
Au and crystallite size for all the gold catalysts were cal-
culated from Scherrer’s equation and reported in Table 1.
(a)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Relative Pressure (P / P)
O
(b)
(e)
(d)
(c)
(b)
(a)
4.0
4.5
5.0
5.5
6.0
6.5
Pore diameter (nm)
Fig. 4 a N2 adsorption–desorption isotherms for a SBA-15 and Au/
SBA-15 catalysts, b HDP, c ME, d IMP, e POL. b BJH pore size
distribution for a SBA-15 and Au/SBA-15 catalysts, b HDP, c ME,
d IMP, e POL
Table 2 Textural properties of mesoporous SBA-15 and various Au/SBA-15 catalysts
S. No.
Au/SBA-15 (1 wt%)
Au content (wt%)a
Surface area (m2/g)b
Vt (cc/g)
DBJH (nm)
Binding energy (eV)
4f5/2
4f7/2
1.
2.
3.
4.
5.
SBA-15
HDP
ME
0.00
0.87
0.79
0.77
0.75
846
762
722
681
645
1.75
1.69
1.57
1.49
1.41
4.81
4.80
4.73
4.54
4.47
–
–
87.59
87.15
87.13
87.11
83.99
83.95
83.55
83.42
IMP
POL
Vt Total pore volume, DBJH average pore diameter calculated by BJH adsorption method
a
Gold content measured by ICP-OES
b
BET method
123

Vapor Phase Oxidation of Benzyl Alcohol over Nano Au/SBA-15 Catalysts…
Fig. 5 XPS spectrum of Au/
SBA-15 catalysts. a HDP,
b ME, c IMP and d POL
The physico–chemical properties of Au/SBA-15 cata-
lysts such as dispersion, metal area and particle size were
determined from CO-chemisorption and reported in
Table 1. From the Table 1, it is evident that the irreversible
CO uptake increase for Au/SBA-15 (HDP) catalyst and
decreases for others catalyst. It is observed that decrease in
CO uptake with increase in particle size as shown in
Table 1. This suggests to the agglomerization in turn leads
to the decrease in metal dispersion and increase in particle
size with decrease in catalytic reactivity (Table 1).
123

A. Kumar et al.
[26]. The TEM results are found to be in good agreement
with results of the XRD and CO chemisorption
measurements.
PhCH2 OH Conversion (%)
PhCHO Selectivity (%)
100
80
60
40
20
0
The nitrogen adsorption/desorption isotherms of SBA-
15 and various Au/SBA-15 catalysts are shown in Fig. 4a.
All the samples exhibit Langmuir type IV isotherms [27],
which is a characteristic feature of mesoporous materials.
SBA-15 shows a hysteresis loop which typically features a
two-dimensional P6 mm structure formed by open cylin-
drical mesopores.
The isotherms of all Au/SBA-15 catalysts are identical
to the isotherm of SBA-15. The nitrogen adsorption iso-
therms of SBA-15 and Au/SBA-15 catalysts exhibit H1-
type hysteresis loop and features a sharp step in the P/P0
range of 0.65–0.95, the sharpness of this step is indicative
of the uniformity of the pore size [27]. However, the
hysteresis loops of all Au/SBA-15 catalysts become
smaller and position of the step shifts towards lower rela-
tive pressure, indicating that a smaller pore size is formed.
This could be due to the deposition of Au nanoparticles in
the pore of the SBA-15. Physical properties/structural
parameters of the samples derived from the nitrogen iso-
therms are reported in Table 2 and the pore size distribu-
tion calculated from desorption branch are shown in
Fig. 4b. The surface area and pore volume decreases as a
result of incorporation of Au nanoparticles into the meso-
porous channels of the support [28, 29].
HDP
ME
IMP
POL
Au/SBA-15 catalysts
Fig. 6 Effect on PhCH2OH conversion and selectivity for PhCHO over
various Au/SBA-15 catalysts, reaction conditions: weight of the
catalyst = 1.0 g; reaction temperature = 320 °C; WHSV = 2.84 h-1
TEM is a powerful technique to investigate the par-
ticle size of metal nanoparticles on catalytic support. The
gold nanoparticles are spherical in shape and confined to
the channels of the mesoporous SBA-15 as shown in
Fig. 3. The corresponding histogram (Fig. 3, inset)
shows the distribution of gold particles of Au/SBA-15
catalysts. The mean diameter of gold particles are found
to be *7–11 nm (TEM) while the average crystallite
size of gold particles were obtained to be *6–10 nm
from XRD results are reported in Table 1. However,
some of these Au nanoparticles were lying on the outside
surface wall of the pores and some of the particles lying
inside the pores of support which indicates the formation
of strong interactions between the Au and the support
ICP-OES analysis, performed on the fresh samples of
Au/SBA-15 catalysts and the results are shown in Table 2.
The prepared catalysts have shown about *75–85 % of the
expected Au loading.
In order to verify the oxidation states of gold element in
as synthesized Au/SBA-15 catalysts were investigated by
XPS are reported in Table 2. The XPS survey scan show
the peaks characteristic of C 1 s (285 eV), O 1 s (530 eV),
Si 2p (100 eV) and Au 4f (85 eV) elements. As can be seen
from Fig. 5 the high resolution XPS spectrum shows
binding energy of Au0 4f7/2 at 84.0 and Au0 4f5/2 at
87.7 eV, which are significantly different from Au? 4f7/2
(84.6 eV) and Au3? 4f7/2 (87.0 eV).
100
HDP
ME
90
80
70
60
50
40
IMP
POL
The results suggest that the gold species are in the
metallic state and these binding energy values correspond
to the metallic gold particles [30–32]. However the XPS
results confirmed the absence of any contamination from
sodium and chlorine species [33]. These results further
confirm that Au particles on the surface of SBA-15 support
are in zero valence state. The XPS spectra of the present
investigation did not show any peaks corresponding to the
binding energies at 84.6 eV (4f7/2) and 87.0 eV (4f7/2) due
to the cationic form of Au? and Au?3 oxidation states
respectively. This suggests that the formation of Au metal
nanoparticles takes place on the surface of SBA-15
support.
280
300
320
340
360
Reaction Temperature /oC
Fig. 7 Effect of temperature on PhCH2OH conversion over various
Au/SBA-15, reaction conditions: weight of the catalyst = 1.0 g;
WHSV = 2.84 h-1
123

Vapor Phase Oxidation of Benzyl Alcohol over Nano Au/SBA-15 Catalysts…
Scheme 1 Reaction pathway for the catalytic oxidation of benzyl alcohol over Au/SBA-15 catalysts
was employed to examine the catalytic performance of the
Au/SBA-15 catalysts. In the absence of catalysts, only
5.7 % of benzyl alcohol was converted due to non-catalytic
oxidation. Also, the pure SBA-15 support by itself showed
a poor catalytic activity under the similar reaction
conditions.
3.2.1 Effect of Preparation Methods
The catalytic activity in terms of PhCH2OH conversion and
selectivity for PhCHO over various Au/SBA-15 catalysts
are shown in Fig. 6. The effect of catalysts preparation
method was investigated over oxidation of benzyl alcohol.
The Au/SBA-15 catalyst from HDP method showed a
higher activity as well as selectivity for PhCHO compared
to the catalysts prepared by other methods. However, the
catalytic properties reveal that negligible amounts of ben-
zoic acid (PhCOOH), benzyl benzoate (PhCH2O2CPh),
benzene (PhH) and toluene (PhCH3) are formed as by-
products with catalysts prepared by ME, IMP and POL
methods. The decrease in the conversion of benzyl alcohol
is probably due to lesser number of active gold sites on the
mesoporous SBA-15 surface was available. This further
suggests the promotion of agglomerization of gold
nanoparticles and confirmed from XRD and TEM results.
The enhanced catalytic activity was probably attributed to
high dispersion of gold particles of smaller size on the
mesoporous SBA-15 support.
3.2.2 Effect of Reaction Temperature
The effect of temperature on the vapor phase oxidation of
PhCH2OH was carried out at different temperatures in the
range 280–360 °C over Au/SBA-15 catalysts as shown in
Fig. 7. The results suggest that below 280 °C no oxidation
reaction of PhCH2OH takes place and above 360 °C, the
product undergoes faster thermal degradation along with
agglomerization of gold nanoparticles [1]. However, at
320 °C, the Au/SBA-15 (HDP) catalyst shows a maximum
conversion of PhCH2OH up to 90 % as compared to the
other three supported gold catalysts (Fig. 7). It is known
that high reaction temperature favours complete oxidation
of PhCH2OH and leads to the formation of benzoic acid
Scheme 2 Proposed reaction mechanism for the catalytic oxidation
of benzyl alcohol over Au/SBA-15 catalysts
3.2 Catalytic Performance for Vapor Phase
Oxidation of Benzyl Alcohol
The selective vapor phase oxidation of benzyl alcohol
(PhCH2OH) to benzaldehyde (PhCHO) with air as oxidant
123

A. Kumar et al.
Au/SBA-15 (HDP)
Spent catalyst
Mean size
(9.39 nm)
6
7
8
9
10
11
12
Au particle size (nm)
Au/SBA-15 (IMP)
Spent catalyst
Mean size
(17.21 nm)
10
20
30
40
Au particle size (nm)
Fig. 8 TEM images of spent Au/SBA-15 catalysts
(PhCOOH) and trace amount of benzene (PhH) and toluene
(PhCH3) as a by-product [4, 34]. Therefore, the reaction
temperature (320 °C) was considered as the optimum
reaction temperature for the title reaction and also referred
as trade-off between PhCHO selectivity and PhCH2OH
conversion. It was interesting to note that benzyl benzoate
is the only other product formed apart from PhCHO and the
formation of benzoic acid (PhCOOH) was not at all
detected in GC and GC-MS analysis [5]. The absence of
benzoic acid in the reaction product indicates that, as soon
as PhCHO was formed, it reacts immediately with
PhCH2OH, which was available in much higher concen-
tration, forming benzyl benzoate as the by-product and the
following reaction mechanism [34] is represented in the
Scheme 1. Here the following esterification reaction was
generally an acid catalyzed reaction. The mesoporous
SBA-15 support is mildly acidic [10] and hence it favors to
undergo corresponding esterification reaction.
PhCH2OH and the product selectivity were monitored at
different reaction intervals. In order to estimate the catalyst
life in the present study the time on stream analysis was
carried out at optimum reaction temperature of 320 °C.
The gas analysis suggests that CO, CO2 were not detected
under the reaction condition (in the range of reaction
temperature up to 320 °C). The benzyl alcohol conversion
showed an optimum conversion after 2 h over Au/SBA-15
(HDP) catalyst as compared to other three Au/SBA-15
catalysts prepared by ME, IMP and POL methods. Further
increase in the reaction time, showed a decreased PhCH2-
OH conversion. The supported gold catalyst needs induc-
tion period (at least 2 h) to reach its optimal catalytic
activity. The presence of induction period indicates that the
metallic gold nanoparticles on the catalyst surface are
required to be activated at the initial stage and the oxygen
molecules adsorbed on the metallic gold. Further it was
dissociated to create an oxidized gold surface for the oxi-
dation of benzyl alcohol and a possible mechanism was
proposed as shown in Scheme 2. The proposed mechanism
for the oxidation of benzyl alcohol over the Au/SBA-15
catalysts with slight modification done from the earlier
reported [35, 36]. Based on the reaction route, to increase
the conversion of PhCH2OH, the reaction time needs to be
3.2.3 Time-On-Stream (TOS) Analysis
The time on stream analysis on Au/SBA-15 catalysts were
investigated to understand the stability of catalysts during
benzyl alcohol oxidation reaction. The conversion of
123

Vapor Phase Oxidation of Benzyl Alcohol over Nano Au/SBA-15 Catalysts…
Fig. 9 TGA analysis of spent Au/SBA-15 (HDP) catalysts
prolonged. However, the PhCHO selectivity decreases due
to subsequent reaction of benzaldehyde to benzyl benzoate.
There was no significant difference in the PhCH2OH con-
version with increase of the reaction time beyond 2 h.
Whereas the PhCHO selectivity decreased continuously.
The PhCHO selectivity of 80 % was attained at 12 h, while
that of benzyl benzoate (a by-product) increased to 20 %.
After 2 h, the conversion reaches a constant value of more
than 70 %.
analysis further confirmed that the particle size of gold
becomes larger during the course of the reaction and there
is no coke deposition over Au/SBA-15 catalysts
respectively.
The PhCH2OH activity results suggest that Au/SBA-15
catalyst prepared by HDP method is highly an efficient
method for the vapour phase oxidation of benzyl alcohol to
benzaldehyde. The reasons for faster deactivation of the
catalyst may be attributed to the lower surface area and
lower dispersion of larger size nano particles formed dur-
ing the reaction time. The catalytic activity was correlated
with the particle size of gold on Au/SBA-15 catalysts. The
Au/SBA-15 catalyst prepared by ME, IMP and POL
method exhibits low catalytic activity compared to Au/
SBA-15 catalyst prepared by HDP method and this may be
attributed to the decrease in the number of active sites of
gold on SBA-15 due to agglomeration of gold nanoparti-
cles as evidenced from TEM results.
To understand the deactivation of the catalyst, TEM and
TGA analysis was carried out over spent catalysts, as
shown in Figs. 8 and 9 respectively. These results suggest
that either the particle size of gold becomes larger during
the course of the reaction causing the deactivation to occur
or due to carbon deposition over Au/SBA-15 catalysts. To
confirm this, an experiment was carried out on the spent
catalyst (after the reaction run 360 °C) which was recal-
cined at 400 °C for 4 h in air. On this calcined sample, a
fresh experiment was carried under similar reaction con-
ditions (WHSV = 2.84 h-1 and T = 320 °C). Interest-
ingly, it was observed that the activity of this catalyst was
much lower than the earlier reaction carried out on the
fresh catalyst under identical conditions at 320 °C. This
suggests that the decrease in oxidation activity is not due to
coke deposition but due to agglomeration of smaller gold
nano particles to larger size gold particles. TEM and TGA
4 Conclusions
Au nanoparticles supported on SBA-15 were prepared by
HDP method shows a higher catalytic performance in
terms of benzyl alcohol activity and selectivity for ben-
zaldehyde as compared to the other three Au/SBA-15
123

A. Kumar et al.
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Nanosci Nanotechnol 14:3137–3146
17. Kumar A, Kumar VP, Vishwanathan V, Chary KVR (2014)
Mater Res Bull 61:105–112
catalysts prepared by ME, IMP IMP and POL method.
XRD and pore size distribution confirm that the pore
structure remains intact even after the insertion of gold
nanoparticles into the SBA-15 mesoporous support. XPS
results reveal that the nanoparticles of gold are present in
the metallic state (Au0), which is further supported by TEM
and CO-chemisorption measurements. The high perfor-
mance of Au/SBA-15 catalyst makes it a promising cat-
alytic material for the oxidation of benzyl alcohol.
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Acknowledgments Ashish Kumar thanks University Grants Com-
mission (UGC), New Delhi, India, for the award of Senior Research
Fellowship (SRF).
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