Selective aerobic oxidation of alcohols over Au-Pd/sodium titanate nanotubes

Article abstract of DOI:10.1016/j.catcom.2014.09.018

The catalytic application of Au-Pd nanoparticles supported on sodium titanate nanotubes (NaTNTs) for liquid-phase aerobic oxidation of alcohols is reported, for the first time. This reaction occurs at 80-120 °C, 1 atm and solvent-/alkali-free conditions yielding the corresponding carbonyls in high selectivity. This catalyst was reusable and found to be more active/selective than the corresponding monometallic Au and Pd catalysts and Au-Pd/TiO₂. Higher dispersion, smaller particle size and higher amount of electron density at gold are the causes for the superior activity of Au-Pd/NaTNT catalyst.

Full text of DOI:10.1016/j.catcom.2014.09.018

Catalysis Communications 58 (2015) 149–153
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Selective aerobic oxidation of alcohols over Au–Pd/sodium
titanate nanotubes
⁎
Devadutta Nepak, Srinivas Darbha
Catalysis Division, CSIR-National Chemical Laboratory, Pune 411 008, India
Academy of Scientific and Innovative Research (AcSIR), New Delhi 110 001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 July 2014
Received in revised form 9 September 2014
Accepted 10 September 2014
Available online 19 September 2014
The catalytic application of Au–Pd nanoparticles supported on sodium titanate nanotubes (NaTNTs) for liquid-phase
aerobic oxidation of alcohols is reported, for the first time. This reaction occurs at 80–120 °C, 1 atm and
solvent-/alkali-free conditions yielding the corresponding carbonyls in high selectivity. This catalyst was
reusable and found to be more active/selective than the corresponding monometallic Au and Pd catalysts and
Au–Pd/TiO2. Higher dispersion, smaller particle size and higher amount of electron density at gold are the causes
for the superior activity of Au–Pd/NaTNT catalyst.
Keywords:
© 2014 Elsevier B.V. All rights reserved.
Bimetallic Au–Pd catalyst
Sodium titanate nanotubes
TiO2
Alcohol
Aerobic oxidation
1. Introduction
Gold supported on titanate nanotubes catalyzes water–gas shift [13]
and CO oxidation reactions [14]. We report here, for the first time,
Selective oxidation of alcohols is a commercially important organic
transformation as its products are intermediates in the manufacture of
pharmaceuticals, agrochemicals and perfumeries [1]. Conventionally,
this reaction is performed with stoichiometric reagents which are
expensive and associated with toxicity issues. Considerable focus has
been paid to aerobic oxidation over recyclable metal catalysts to make
the process green and eco-friendly [2–4]. Among them, gold changed
the era of research and development tremendously with the discovery
of supported gold nanoparticles (Au NPs) showing interesting catalytic
oxidation activity at mild to moderate temperatures and with air as
oxidant [5,6]. However, the instability of the catalyst due to the agglom-
eration of Au NPs at reaction conditions is an issue. Bimetallic Au–Pd
showed an improved activity and durability than the monometallic
counterparts due to geometric and electronic effects [7]. The choice of
support was also found to influence the performance of Au through
specific support–metal interactions [7]. The most commonly studied
metal oxide support is TiO2. Of late, a new family of 1D-titanates with
nanotube, nanoribbon or nanowire architecture has attracted attention
due to their unique physicochemical properties [8]. Among them,
titanate nanotubes possess high specific surface area and hollow
morphology. They are good ion-exchange materials [9]. They were
used as photocatalysts, adsorbents and solid base catalysts [10–12].
the application of Au–Pd/sodium titanate nanotubes (NaTNTs) for the
selective oxidation of alcohols under solvent and alkali-free conditions.
The influence of reaction conditions and composition of Au and Pd on
the oxidation activity and selectivity is studied.
2. Experimental section
2.1. Catalyst preparation
NaTNT was prepared by alkali treatment of anatase titania [9].
NaTNT-supported Au and Au–Pd catalysts were prepared by a deposi-
tion–precipitation method using 2 mM aqueous HAuCl4·3H2O and
Pd(OAc)2 as precursor solutions for gold and palladium, respective-
ly, and by maintaining the pH at 7 to 8 (Supplementary data, S1).
2.2. Characterization techniques
Au and Pd contents in the catalysts were determined by an
inductively-coupled plasma-optical emission spectrometer (ICP-OES;
Spectro Arcos). X-ray powder diffractograms (XRD) were recorded on
a PANalytical X'Pert PRO diffractometer (Ni-filtered Cu Kα radiation;
2θ = 5–80°). High-resolution transmission electron microscopic
(HRTEM) images were measured on a FEI Technai F30 instrument
(300 kV field emission gun). Specific surface area (SBET) was determined
from N2 physisorption studies (−196 °C; NOVA 1200 Quanta Chrome).
Diffuse reflectance UV–visible (DRUV–vis) spectra were recorded on a
⁎
Corresponding author at: Catalysis Division, CSIR-National Chemical Laboratory,
Pune 411 008, India. Tel.: +91 20 25902018; fax: +91 20 25902633.
E-mail address: d.srinivas@ncl.res.in (S. Darbha).
http://dx.doi.org/10.1016/j.catcom.2014.09.018
1566-7367/© 2014 Elsevier B.V. All rights reserved.

150
D. Nepak, S. Darbha / Catalysis Communications 58 (2015) 149–153
Table 1
Composition and textural properties of Au/NaTNT and Au–Pd/NaTNT.
Catalyst
Metal content (output, wt.%; ICP-OES)
Textural properties (N2 physisorption)
Au
Pd
Na
S
BET (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
Au(1 wt.%)/NaTNT
Au(2 wt.%)/NaTNT
Au–Pd(2 wt.%, 3:1)/NaTNT
Au–Pd(2 wt.%, 1:1)/NaTNT
Au–Pd(2 wt.%, 1:3)/NaTNT
Au–Pd(2 wt.%, 1:1)/TiO2
0.80
1.54
1.10
0.65
0.35
0.67
–
6.90
5.31
5.10
5.20
6.60
–
180
149
200
181
153
23
0.60
0.47
0.70
0.64
0.55
0.08
6.0
6.3
6.4
6.6
5.9
7.0
–
0.48
0.70
1.20
0.95
Shimadzu UV-2550 spectrophotometer. X-ray photoelectron spectra
(XPS; VG Microtech Multilab ESCA 3000) were acquired with Al Kα
radiation. The peak corresponding to carbon 1s (285 eV) was taken as
reference.
reactor was raised to 120 °C and the reaction was conducted for a specific
time while stirring the contents. It was then cooled to 25 °C. The reaction
mixture was centrifuged and the catalyst was separated out. To 0.5 ml of
the liquid portion, 1 ml of mesitylene was added and analyzed by gas
chromatography (Varian 3800; CP-8907 — 15 m × 0.25 mm × 0.25 μm
column and flame ionization detector). Products were identified by
GC–MS (Varian CP-3800; CP-Sil8CB — 30 m × 0.25 mm × 0.25 μm
capillary column). In catalyst recycling studies, at the end of each run
the reaction mixture was centrifuged and the catalyst was separated. It
was then washed with ethanol and water, dried at 110 °C for 12 h and
reused in the next recycle without pre-reduction with hydrogen.
2.3. Reaction procedure
50 mg of catalyst and 25 mmol of alcohol were charged into a triple-
necked, glass, round-bottom flask (25 ml) placed in a temperature-
controlled oil bath. The reactor was connected with a water-cooled
reflux condenser and an air-filled rubber balloon. Temperature of the
3. Results and discussion
3.1. Catalyst characterization
(200)
Na/Ti mole ratio of NaTNT determined by ICP-OES (0.65) was nearly
the same as the theoretical value (0.67) for Na2Ti3O7. For supported
metal catalysts, depending on their composition, 65–80% of input Au
and 70–96% of Pd remained in their final composition (Table 1). XRD
showed typical peaks at 10.1, 24.3, 28.4 and 48.5° corresponding to re-
flections from (200), (110), (211) and (020) planes, respectively, of
NaTNT with Na2Ti3O7 composition (JCPDS files: 31-1329 and 41-0192;
Fig. 1). The peak at 10.1° is a signature for the nanotube morphology
[15]. Metal deposition didn't alter the support structure. No additional
peaks due to metals (JCPDS files: 04-0784 and 05-068) were detected
for Au/NaTNT and Au–Pd/NaTNT indicating that Au and Pd particles
were, perhaps, below the X-ray detection limit. HRTEM (Fig. 2) con-
firmed the tubular morphology with outer and inner diameters being
9.8 and 4.8 nm, respectively (Supplementary data, S2). It pointed out
the complete conversion of TiO2 into Na2Ti3O7. Pd reduced the average
particle size of Au from 2.3 nm (Au(2 wt.%)/NaTNT) to 1.2 nm (Au–
Pd(2 wt.%, 1:1)/NaTNT) (Supplementary data, S3). Metal particles
were evenly distributed on the surface of NaTNT in Au–Pd(2 wt.%,
(110)
(211)
(020)
Spent Au-Pd(2 wt%, 1:1)/NaTNT
Au-Pd(2 wt%, 1:1)/NaTNT
Au(2 wt%)/NaTNT
NaTNT
10
20
30
40
50
60
70
80
2
θ
(degrees)
Fig. 1. XRD profiles of NaTNT, Au(2 wt.%)/NaTNT, Au–Pd(2 wt.%, 1:1)/NaTNT and spent
Au–Pd(2 wt.%, 1:1)/NaTNT.
Fig. 2. HRTEM images: (a) Au(2 wt.%)/NaTNT and (b) Au–Pd(2 wt.%, 1:1)/NaTNT.

D. Nepak, S. Darbha / Catalysis Communications 58 (2015) 149–153
151
nanotubes of NaTNT, their position at lower BE suggests a higher amount
of electron density (Auδ−) as a consequence of electron transfer from
NaTNT to Au. Reduction in particle size is an alternative reason for a
lower BE value of Au on NaTNT compared to that on TiO2. The lines for
Pd0 in Au–Pd(2 wt.%, 1:1)/TiO2 appeared at 335.4 (3d5/2) and 340.7 eV
(3d3/2) and in Au–Pd(2 wt.%, 1:1)/NaTNT at 335.3 and 340.4 eV,
respectively.
(a) Au(2 wt%)/NaTNT
(b) Au-Pd(2 wt%, 1:1)/NaTNT
3.2. Catalytic activity
(b)
(a)
Au/NaTNT and Au–Pd/NaTNT catalyzed the oxidation of benzyl alco-
hol with aerial oxygen at 120 °C under solvent- and alkali-free condi-
tions (Table 2). Benzaldehyde is the major product (70.2–98.5 wt.%
selectivity) with benzoic acid, benzyl benzoate, benzene and toluene
as minor products. While benzaldehyde and benzoic acid formed via
oxidation reaction, benzene occurred by decarbonylation and toluene
by the disproportionation of benzyl alcohol [18]. Benzyl benzoate is pro-
duced by the esterification of benzoic acid with benzyl alcohol [12]. At
our experimental conditions (air = 1 atm, reaction temperature =
120 °C, reaction time = 10 h), benzyl alcohol conversion was very little
(1.5 wt.%) in the absence of a metal catalyst. The supported gold
[Au(1 wt.%)/NaTNT] catalyzed this reaction yielding benzyl alcohol con-
version of 15.2 wt.% and benzaldehyde selectivity of 98.5 wt.%. This
value is comparable to the reports by others using erstwhile supports
[19,20]. With increasing Au content from 1 to 2 wt.%, the conversion
of benzyl alcohol doubled (from 15.2 to 30.0 wt.%) but the selectivity
for benzaldehyde decreased (from 98.5 to 94.3 wt.%); the selectivity
for benzyl benzoate and benzene + toluene increased from 1.5 to
4 wt.% and 0 to 1.7 wt.%, respectively (Table 2; run nos. 1 and 2). Upon
promotion with Pd, the conversion of benzyl alcohol raised from 15.2
to 63.0 wt.% and TOF increased from 187 to 318 h−1; benzaldehyde
selectivity decreased from 98.5 to 82.7% as more and more benzaldehyde
converted to benzoic acid and benzyl benzoate (compare run nos. 1
and 4). The presence of Pd also enhanced the selectivity of undesired
products, although its overall content was low — benzene (from 0 to
6.4 wt.%) and toluene (from 0 to 1.1 wt.%). The activity of Au–Pd/
NaTNT (TOF = 318 h−1 based on total metal content and 516 h−1
based on surface metal atoms) was superior to Au–Pd/TiO2 (TOF =
240 h−1 based on total metal content and 470 h−1 based on surface
metal atoms). Selectivity for benzaldehyde was lower and toluene forma-
tion was higher on the latter (compare run nos. 4 and 6).
570
540
200
300
400
600
700
800
Wavele5n0g0th (nm)
Fig. 3. DRUV–vis spectra: (a) Au(2 wt.%)/NaTNT and (b) Au–Pd(2 wt.%, 1:1)/NaTNT.
1:1)/NaTNT (Fig. 2(b)); only the lattice fringes corresponding to (111)
plane of Au with an interplanar distance of 0.24 nm [16] were detected
indicating that it is the most exposed plane (Supplementary data, S2).
Pd is expected to form a monolayer on the surface of Au at certain con-
centrations and thereby control the size of Au particles. Also based on
electrode potentials, Pd leads to the reduction of Au which in turn hin-
ders the growth of the nanoparticle. These materials were mesoporous
showing type IV nitrogen-isotherms with H2-hysteresis loop (Supple-
mentary data, S4). SBET of the catalysts was in the range of 149–
200 m2/g and pore diameter was between 5.9 and 7.0 nm.
Au(2 wt.%)/NaTNT showed a characteristic localized surface plasmon
resonance at 540 nm (Fig. 3). In the case of Au–Pd(2 wt.%, 1:1)/NaTNT,
this band broadened and shifted to 570 nm because of changes in the
band structure of Au in the presence of Pd forming bimetallic nanocom-
posite and reduction in Au particle size (Fig. 3). Au–Pd(2 wt.%, 1:1)/TiO2
showed XPS lines at 83.4 and 87.1 eV arising from the core levels of Au
4f7/2 and 4f5/2, respectively (Fig. 4). Metallic gold shows 4f7/2 line at
84.0 eV [17]. For Au–Pd(2 wt.%, 1:1)/NaTNT, these spectral lines were
weak and appeared at still lower BE values (82.8 and 86.5 eV, respective-
ly). While weak intensity is indicative of the location of Au inside the
The composition of Au–Pd affected benzyl alcohol conversion and
benzaldehyde selectivity. A composition with 1:1 wt/wt of Au and Pd
Pd
Au
Au- Pd(2 wt%, 1:1)/NaTNT
Au- Pd(2 wt%, 1:1)/NaTNT
4f7/2
4f5/2
3d5/2
3d3/2
Au- Pd(2 wt%, 1:1)/TiO2
Au- Pd(2 wt%, 1:1)/TiO2
328
92
332
336
340
344
348
80
82
84
86
88
90
Binding energy (eV)
Binding energy (eV)
Fig. 4. XPS of Au–Pd(2 wt.%, 1:1)/TiO2 and Au–Pd(2 wt.%, 1:1)/NaTNT.

152
D. Nepak, S. Darbha / Catalysis Communications 58 (2015) 149–153
Table 2
Catalytic activity data for the aerial oxidation of benzyl alcohol over NaTNT- and TiO2-supported Au and Au–Pd catalysts.a
Run no.
Catalyst
Conversion (wt.%)
TOF (h−1
)
Product selectivity (wt.%)
Benzaldehyde
Benzoic acid
Benzyl benzoate
Benzene
Toluene
1
2
3
4
5
6
7
Au(1 wt.%)/NaTNT
Au(2 wt.%)/NaTNT
15.2
30.0
50.5
63.0
53.9
57.0
50.2
187 (264)
192 (283)
250 (419)
318 (516)
206
98.5
94.3
81.6
82.7
70.2
75.6
78.8
0
0
1.5
4.0
11.1
9.2
20.1
2.6
10.7
0
0
1.2
5.9
6.4
7.2
1.1
5.3
0.5
0.6
1.1
1.5
15
Au–Pd(2 wt.%, 3:1)/NaTNT
Au–Pd(2 wt.%, 1:1)/NaTNT
Au–Pd(2 wt.%, 1:3)/NaTNT
Au–Pd(2 wt.%, 1:1)/TiO2
Pd(2 wt.%)/NaTNT
0.8
0.6
1.0
5.7
2.2
240 (470)
205
3.0
a
Reaction conditions: Catalyst = 50 mg, benzyl alcohol = 25 mmol, p(air) = 1 atm, reaction temperature = 120 °C, and reaction time = 10 h, Turnover frequency (TOF) = moles
of benzyl alcohol converted per mole of total metal in the catalyst (ICP-OES) per hour. TOF values in parentheses are those calculated based on mole of benzyl alcohol converted per mole of
surface metal atoms (estimated from TEM) per hour.
was found to be highly active and selective. Such enhancement in cata-
lytic activity is already known for Au–Pd on other supports [19,20].
Higher activity of the present catalyst is due to the uniform dispersion
and smaller size of metal particles (1.2 instead of 2.3 nm; HRTEM) as
well as synergistic electronic effects between the metal and support.
Au atoms draw electron density away from Pd and NaTNT. Enache
et al. [21] and Chen et al. [22] have also made such observation in related
systems. The smaller the particle size, the higher would be the availabil-
ity of surface active sites and interactions with the support which then
can lead to a higher activity of the catalysts.
Benzyl alcohol conversion over Au–Pd(2 wt.%, 1:1)/TiO2 and Au–
Pd(2 wt.%, 1:1)/NaTNT increased with increasing reaction time (Sup-
plementary data, S5). However, benzaldehyde selectivity decreased at
higher conversion levels due to the consecutive reaction of ester forma-
tion. At similar conversion levels, Au–Pd(2 wt.%, 1:1)/NaTNT showed a
higher selectivity for benzaldehyde than Au(2 wt.%)/NaTNT and Au–
Pd(2 wt.%, 1:1)/TiO2 (Fig. 5). TOF at the end of 1 h over TiO2- and
NaTNT-supported Au–Pd(2 wt.%, 1:1) was 964 and 1635 h−1 (based
on total metal content), respectively. Metal particles on TiO2 have an av-
erage diameter of 1.5 nm while those on NaTNT are of 1.2 nm (Supple-
mentary data, S6). The difference in metal particle sizes is the possible
cause for difference in the catalytic activity of these two systems. This
reaction occurred even at a temperature as low as 80 °C over Au–
Pd(2 wt.%, 1:1) catalyst and benzyl alcohol conversion of 50.0% and
benzaldehyde selectivity of 93% were obtained (Supplementary data,
S7). With a view to optimize the temperature, reactions were conducted
at 80, 100 and 120 °C. Benzyl alcohol conversion increased with increas-
ing reaction temperature, but a loss in selectivity for benzaldehyde was
observed at higher temperatures (Supplementary data, S6). Upon reuse,
Au–Pd(2 wt.%, 1:1)/NaTNT showed a marginal loss (by 5 wt.%) in ben-
zyl alcohol conversion but no further loss in the activity was detected in
subsequent recycles (Supplementary data, S8). The structure of the cat-
alyst was intact even after the 5th recycle (XRD, Fig. 1). ICP-OES analysis
revealed a marginal loss in the metal content (500 ppm for Au and
200 ppm for Pd). Zhang and co-workers [23,24] reported the application
of silica-supported Au–Cu and Au–Ag alloy nanoparticles for the selec-
tive oxidation of alcohols at 80 °C. Benzyl alcohol conversion of 96%
and benzaldehyde selectivity of N99% were obtained. But 50 times
higher amount of the catalyst compared to that used in the present
study was employed to achieve the reported conversion. Moreover,
the catalyst needed reduction in between the recycles to maintain the
conversion, which is not the case with the catalysts of the present
study. The activity of Pd(2 wt.%)/NaTNT was higher (50.2 wt.%,
TOF = 205 h−1) than that of Au(2 wt.%)/NaTNT (30.0 wt.%; TOF =
192 h−1 based on total metal content). However, the selectivity for al-
dehyde on the former was lower (78.8 wt.%) than on the latter
(94.3 wt.%). Bimetallic Au–Pd/NaTNT showed an enhanced activity
(63.0 wt.%; TOF = 318 h−1) than either of the monometallic catalysts.
It was proposed [25,16] that in the case of Pd the rate determining
step is the transfer of H-atom from the β-carbon of the adsorbed alkox-
ide forming the aldehyde and a Pd-hydride species. For Au, it is the H-
abstraction by an Au-superoxide species. Since Au draws electron den-
sity from Pd, it can activate molecular oxygen more easily than without
Pd, forming a superoxide like active oxygen species which in turn initiates
the reaction. At the same time, the electron depleted Pd (in the presence
of Au) easily eliminates hydride species from the β-carbon of alkoxide.
Thus, the co-presence of Pd with Au facilitates the oxidation rate. The
scope of Au–Pd(2 wt.%, 1:1)/NaTNT for a range of structurally different
alcohols was investigated (Table 3). The structure and substituent have
100
95
90
Table 3
Aerobic oxidation alcohols over Au–Pd(2 wt.%, 1:1)/NaTNT catalyst.a
85
Run no.
Alcohol
Alcohol
conversion (wt.%)
Aldehyde/ketone
selectivity (wt.%)
80
1
2
3
4
5
6
7
8
3-Methoxybenzyl alcohol
4-Methoxybenzyl alcohol
4-Methy benzyl alcohol
4-Chlorobenzyl alcohol
4-Nitrobenzyl alcohol
1-Phenylethanol
2-Phenylethanol
Cinnamyl alcohol
Crotyl alcohol
Furfuryl alcohol
53.7
41.5
40.0
10.0
10.0
84.0
10.0
60.5
75.6
18.8
11.5
84.3
85.0
86.0
82.0
89.0
86.0
100
75.5
70.0
65.0
100
Au(2 wt%)/NaTNT
75
70
Au-Pd(2 wt%, 1:1)/NaTNT
Au-Pd(2 wt%, 1:1)/TiO2
0
10
20
30
40
50
60
70
9
10
11
Benzyl alcohol conversion (wt%)
Cyclohexanol
Fig. 5. Oxidation of benzyl alcohol over supported Au and Au–Pd NPs: Conversion versus
selectivity plot.
a
Reaction conditions: Same as in Table 2.

D. Nepak, S. Darbha / Catalysis Communications 58 (2015) 149–153
153
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