G Model
CATTOD-9833; No. of Pages9
ARTICLE IN PRESS
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J. Tuteja et al. / Catalysis Today xxx (2015) xxx–xxx
OH
OH
Pd standards (ICP grade) were obtained from Wako Pure
HO
HO
Chemical Industries, Ltd. Kanto Chemical Co., Inc. supplied
sodium hydroxide (NaOH), potassium chloride (KCl), and sulfuric
acid (H2SO4). N,N-Dimethyldodecylamine N-oxide (DDAO), 1,2-
hexanediol, 2-hydroxyhexanoic acid, 1,7-heptanediol, pimelic acid,
1,8-octanediol, 8-hydroxy octanoic acid and suberic acid were
purchased from Sigma–Aldrich, Co. LLC. Acros Organics provided 6-
hydroxycaproic acid (HCA) and poly(N-vinyl-2-pyrrolidone) (PVP,
K29-32, Mw; 58,000). Melatonin and hydrotalcite (HT, Mg/Al = 5.4)
was obtained from TCI Chemicals Pvt. Ltd. and Tomita Pharmaceut-
icals Co. Ltd., respectively.
n
n
O
Scheme 1. Selective oxidation of aliphatic diol to mono-hydroxycarboxylic acid.
+
N,N-dimethyldodecylamine N-oxide (DDAO)
2.2. Catalyst preparation
Poly(N-vinyl-2-pyrrolidone) (PVP)
Capped AuPd-X/HT catalysts have been synthesized as reported
in the literatures [32,39,40] with some modifications. In a typ-
ical synthesis, an aqueous solution (50 mL) of HAuCl4·4H2O
(0.04 mmol) and PdCl2 (0.06 mmol) including KCl (0.18 mmol) were
mixed with DDAO, PVP or PVA and stirred for 5 min at room temper-
ature. Thereafter, EG (50 mL) was added into the aqueous mixture
and again stirred for 5 min at room temperature, then the obtained
mixture was refluxed for 2 h at 413 K, followed by addition of HT
(1.0 g) into the formed colloidal dispersion to stabilize the formed
AuPd-X NPs onto the surface of HT. The resultant mixture was
stirred again for 1 h at 413 K. The obtained precipitates were cooled,
filtered, washed and dried in vacuo overnight.
Fig. 1. Structure of capping agents used in this study.
selectivity of the product [31,32]. The development of colloidal syn-
thesis has the ability to precisely tailor the structural characteristics
(size, shape and distribution) usually by varying the amount of cap-
ping agent [33–35]. On the other hand, the choice of capping agent
would significantly affect the electronic structures of bimetallic
NPs along with the alterations in size- and/or shape-dependent
properties of the NPs. Multiple research groups have explained
the size-dependent properties of metal NPs but only few of them
have discussed the influences from stabilizers [31,32,36–38]. Over-
for studying heterogeneous catalysis.
2.3. Catalytic testing
Fig. 1)-capped AuPd bimetallic NPs supported on hydrotalcite (HT)
surface as reusable heterogeneous catalyst for selective oxidation of
1,6-hexanediol (HDO) to 6-hydroxycaproic acid (HCA) using aque-
ous H2O2 under high-pH conditions (Scheme 2) [32]. The results
suggested that the synergetic interactions between Au and Pd or
Au–Pd nanoalloy center played crucial role in the excellent catal-
ysis of AuPd-DDAO/HT for the selective oxidation of HDO toward
HCA.
Our previous communication with supported AuPd-DDAO
bimetallic NPs raises some additional interesting questions on the
role of DDAO as capping agent. In this paper, in order to explore
the influence of capping agent on the oxidation of HDO to HCA,
the catalytic activity of DDAO-capped AuPd bimetallic catalyst
was compared with those of AuPd capped with the two most
widely used capping agents; poly(N-vinyl-2-pyrrolidone) (PVP)
and poly(vinyl alcohol) (PVA) (Fig. 1).
The detailed characterizations of the catalysts have been carried
out using XRD, TEM, XPS, XAS and STEM-HAADF-EDS techniques,
and then the relationship between electronic structure and activ-
ity of AuPd nanostructures has been suggested. A suitable reaction
mechanism and a role of capping agent are also proposed based
on the experimental results. Finally, the catalysis of reusable and
highly selective AuPd-DDAO/HT is verified for the selective oxi-
dation of other long chain aliphatic diols like 1,7-heptanediol,
1,8-octanediol and 1,2-hexanediol.
All experiments to test the catalytic activity were performed in
a Schlenk tube (50 mL vol.) attached to a condenser. The catalytic
activity was evaluated for HDO oxidation in basic aqueous media
with H2O2 as oxidant to obtain HCA. In a typical reaction proce-
dure, aliphatic diol (0.5 mmol) and catalyst (25 mg) were weighed
and dispersed in deionized water (3.5 mL) in a Schenk tube. 30%
H2O2 (0.75 mL) and 0.5 M NaOH (0.75 mL) were added to the above
mixture, and then the Schlenk tube was mounted on a preheated
oil bath at 353 K. The mixture was allowed to react for various
time intervals with continuous magnetic stirring (500 rpm). After
the reaction, a part of the resultant solution was diluted 20 times
with an aqueous H2SO4 (10 mM) solution, and the catalyst was fil-
tered off using a 0.20 m filter (Milex®-LG). The obtained filtrate
was analyzed by high performance liquid chromatography (HPLC,
WATERS 600) using an Aminex HPX-87H column (Bio-Rad Labo-
ratories, Inc.) attached to a refractive index detector. An aqueous
10 mM H2SO4 solution (eluent) was run through the column (main-
tained at 323 K) at a flow rate of 0.5 mL min−1. The conversion and
yield(s) were determined with a calibration curve method using
commercial products.
2.4. Product isolation
The catalyst was separated from the reaction mixture via
centrifugation followed by filtration using a 0.20 m filter (Milex®-
LG). The obtained mixture was acidified till pH = 1–2 by adding
aq. H2SO4 drop wise. The compound was extracted with CHCl3
(four times). The combined organic layers were condensed under
reduced pressure to obtain product. The product was dissolved
in CDCl3 and subjected to NMR spectroscopy (Bruker BioSpin Inc.
AVANCE III) for identification of product.
2. Experimental
2.1. Chemicals
Hydrogen tetrachloroaurate (III) (HAuCl4·4H2O), palladium
chloride (PdCl2), ethylene glycol (EG), adipic acid (AA), 1,6-
hexanediol (HDO), 30% hydrogen peroxide (H2O2), 1-hexanol, hex-
anoic acid, potassium iodide (KI), chloroform (CHCl3), poly(vinyl
alcohol) (partially hydrolyzed) (PVA, Mw; 3500) and Au and
2.5. Characterization
Crystal structure was analyzed by powder X-ray diffraction
(XRD) with a SmartLab (Rigaku Co.) using a Cu K˛ radiation
( = 0.154 nm) at 40 kV and 30 mA in the range of 2ꢀ = 4–80◦.
Please cite this article in press as: J. Tuteja, et al., Change in reactivity of differently capped AuPd bimetallic nanoparticle
catalysts for selective oxidation of aliphatic diols to hydroxycarboxylic acids in basic aqueous solution, Catal. Today (2015),