Highly dispersed palladium nanoparticles grafted onto nanocrystalline starch for the oxidation of alcohols using molecular oxygen as an oxidant

Article abstract of DOI:10.1039/c3dt51059j

Highly dispersed Pd-nanoparticles grafted onto amino-functionalized nanocrystalline starch were found to be excellent heterogeneous catalysts for the aerobic oxidation of a variety of alcohols to their corresponding carbonyl compounds in excellent yields. The prepared catalyst was found to be selective for the oxidation of primary alcohols to aldehydes without giving over-oxidation products and was recycled several times without any leaching of the metal into the solution.

Full text of DOI:10.1039/c3dt51059j

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Highly dispersed palladium nanoparticles grafted onto
nanocrystalline starch for the oxidation of alcohols
using molecular oxygen as an oxidant
Cite this: Dalton Trans., 2013, 42, 11522
Sanny Verma,a Deependra Tripathi,b Piyush Gupta,b Raghuvir Singh,b
Gajendra Mohan Bahuguna,b L. N. Shivakumar K,b R. K. Chauhan,b Sandeep Saranb
and Suman L. Jain*a
Highly dispersed Pd-nanoparticles grafted onto amino-functionalized nanocrystalline starch were found
to be excellent heterogeneous catalysts for the aerobic oxidation of a variety of alcohols to their corres-
ponding carbonyl compounds in excellent yields. The prepared catalyst was found to be selective for the
oxidation of primary alcohols to aldehydes without giving over-oxidation products and was recycled
several times without any leaching of the metal into the solution.
Received 22nd April 2013,
Accepted 17th June 2013
DOI: 10.1039/c3dt51059j
www.rsc.org/dalton
The development of nanobiocatalysis1 and nanostructured heterogeneous catalysts is desired as they offer the advantages
hybrid materials2 has attracted ever increasing attention of facile recovery and reusability of the expensive metal cata-
because of their potential applications in various fields includ- lyst.14 Recently, supported Pd nanoparticles have been widely
ing catalysis. Recently, starch nanocrystals owing to their desir- used for the selective oxidation of alcohols by molecular
able characteristics such as inexpensive, abundant, oxygen.15 Most of the supports used for the preparation of
biocompatible, biodegradable, and nontoxic have been receiv- these catalysts are limited to the use of activated carbon,
ing considerable interest.3 In addition, these nanocrystals alumina, silica and organic polymers.16
exhibit some unique properties, because of the nanometric
However, increasing attention has recently been paid to the
size effect, compared to their conventional microcomposite use of biopolymers for supporting catalytic metals as these
counterparts. Starch nanocrystals, prepared by hydrolysis of materials not only provide remarkable affinities for catalytic
native starch granules,4 have been extensively used in drug metals, but also their structure can stimulate the reactivity and
delivery systems;5 however, their use in the area of catalysis is selectivity of the catalyst.17 Among the different biopolymers
rather limited. Selective oxidation of alcohols to their corres- known, starch materials have been chosen as promising
ponding carbonyl compounds is a reaction of fundamental materials for supporting the metal catalysts via chemical modi-
importance in industrial organic chemistry.6 Conventionally fications.18 In this context, recently Clark et al.19 reported
the oxidation of alcohols is performed by using stoichiometric expanded corn starch as excellent support materials for Pd-
amounts of high-valent metal reagents, producing copious catalysts for organic transformations. However to the best of
amounts of metallic wastes, and therefore is undesirable from our knowledge, no literature exists on the use of nanocrystal-
both an environmental and an economic perspective.7 Thus, line starch materials as supports for developing heterogeneous
increasing attention is being paid to the development of cata- Pd-catalysts. In the present paper, we report for the first time
lytic oxidation methods by using environmentally friendly oxi- the synthesis of highly dispersed Pd(0) nanoparticles grafted
dants such as O2, H2O2 etc.8 In this regard, several methods onto chemically modified nanocrystalline starch, their charac-
exist that use molecular oxygen for the oxidation of alcohols by terization and we report on their use for the oxidation of alco-
using various homogeneous and heterogeneous Pd,9 Ru,10 hols using molecular oxygen as an oxidant (Scheme 1).
Co,11 bimetallic Au-Pd12 and Cu13 based catalysts. Owing to
the limitations such as difficult recovery and recycling of
homogeneous catalysts, the oxidation of alcohols using
aChemical Sciences Division, CSIR-Indian Institute of Petroleum, Mohkampur,
Dehradun-248005, India. E-mail: suman@iip.res.in; Fax: +91-135-2660202;
Tel: +91-135-2525788 (O)
bAnalytical Sciences Division, CSIR-Indian Institute of Petroleum, Mohkampur,
Dehradun-248005, India
Scheme 1 [AmP-SNC/Pd(0)] catalyzed oxidation of alcohols.
11522 | Dalton Trans., 2013, 42, 11522–11527
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Synthesis and characterization of amino-
functionalized nanocrystalline starch grafted
Pd(0) nanoparticles {AmP-SNC/Pd(0)} 6
A schematic representation of the synthesis of amino-functio-
nalized nanocrystalline starch grafted Pd nanoparticles 6 is
shown in Scheme 2. Nanocrystalline starch was readily syn-
thesized by the acid hydrolysis of native corn starch material
by following a literature procedure.4
Fig. 2 TGA pattern of (a) nanostarch 4 and (b) [AmP-SNC/Pd(0)] 6.
Starch nanocrystals possess a reactive surface covered with
hydroxyl groups, which provides the possibility of modification
via a chemical reaction strategy. Synthesized nanocrystals of around 2θ = 16.84 and 2θ = 22.68. The XRD pattern of amino-
starch were treated with 3-aminopropyltrimethoxysilane propyl modified nanocrystalline starch (AmP-SNC) grafted
(APTMS) to obtain amino-functionalized starch nanocrystals
(SNC-NH2). The chemical modification of starch nanocrystals material is of almost amorphous nature, which is revealed by
Pd(0) nanoparticles 6 indicates that the developed catalytic
(SNC) was confirmed by FTIR analysis.
very weak peaks at 2θ = 39. This peak confirms the grafting of
The presence of a large number of hydroxyl and amine
Pd(0) nanoparticles on the nanocrystalline starch support.
groups in the starch nanocrystals explains the high affinity of Inductively coupled plasma (ICP-AES) analysis indicated
the support for metal ions. The chemical nature and function- 6.25 wt% of Pd on the nanocrystalline starch.
alities present on starch nanocrystals were revealed by FTIR.20
The thermal stability of the as-synthesized materials 4 and
The presence of bands at 3309 cm−1 in the SNC-NH2 sample 6 was determined by TGA analysis22 (Fig. 2). It can be seen
assigned to the N–H stretching vibrations and at 2931 and that nanocrystals of starch 3 show 26.21% weight loss near
2870 cm−1 corresponding to C–H vibrations confirmed the
100–150 °C, evidently owing to evaporation of the water mole-
presence of aminopropyl functionalities on the starch nano- cules which are held in the material. The second significant
crystals. The crystalline nature of the material was confirmed weight loss of about 70% is observed in the range of
by XRD21 as shown in Fig. 1. The XRD pattern of nanocrystal-
300–350 °C, due to thermal decomposition of chemical func-
line starch (SNC) 3 indicates that the developed catalytic tionalities present on the starch nanocrystals. Similarly, in
material is of almost crystalline nature with very small fraction, material 6, an initial weight loss is observed near 100 °C,
which is revealed by very weak and broad diffraction peaks at
because of the evaporation of water. The second significant
weight loss of about 26.98% is observed at 200–250 °C, due to
the decomposition of organic functionalized moieties. With the
temperature enhanced continuously, the material was found
to be decomposed gradually, and the weight loss became more
and more obvious. The decomposition of the starch grafted
palladium was complete when the temperature was raised to
370 °C with the formation of palladium oxide as a residue.
The transmission electron microscope (TEM) image shown
in Fig. 3 clearly indicates that the Pd nanoparticles produced
are well dispersed with a narrow size distribution in the range
of 2–5 nm in the synthesized material 6.
In order to ascertain the complete reduction and oxidation
states of the palladium, the prepared [AmP-SNC/Pd(0)] catalyst
was characterized by XPS analysis (Fig. 4). The XPS spectrum
shows two peaks with binding energies of 335.7 and 341.1 eV,
Scheme 2 Synthesis of [AmP-SNC/Pd(0)] 6.
Fig. 1 XRD pattern of (a) nanostarch and (b) [AmP-SNC/Pd(0)] catalyst.
Fig. 3 TEM image of the [AmP-SNC/Pd(0)].
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oxygen atmosphere, the reaction did not take place even in a
prolonged reaction time of 12 h.
Next, the scope of the developed catalytic system was
studied by performing the oxidation of a variety of primary
and secondary alcohols under the described reaction con-
ditions. The results are presented in Table 2. The catalytic
system demonstrated an excellent reactivity and selectivity for
the oxidation of benzyl alcohols to corresponding benz-
aldehydes without any evidence for the formation of over-
oxidation products, i.e. carboxylic acids. From the results for
the benzylic substrates with different substituents (Table 2,
entries 8–12), we found that electron-donating groups such as
Me, OMe in the para-position enhanced the reaction more
significantly than the electron-withdrawing groups. Similarly a
variety of secondary alcohols including benzoins and 1,2-diols
could be oxidized to the corresponding ketones in good to
excellent yields under similar reaction conditions (Table 2,
entries 1–7). The developed catalytic system could also be
applied successfully for the oxidation of heterocyclic alcohols
such as 2-thiophenemethanol (Table 2, entry 13). This is an
advantage over several transition metal based catalysts, where
the strong coordination of the heteroatom to the metal center
is well known to deactivate the catalyst.
Next we tested the recycling of the catalyst for the oxidation
of benzhydrol under the reaction conditions described above
for the aerobic oxidation of alcohols. After the completion of
the reaction, the catalyst could be easily recovered by centrifu-
gation followed by washing with water and reused for sub-
sequent runs. The recovered catalyst showed an efficient
recycling ability without giving any change in the reaction
time and the yield of the product (Fig. 5).
The hot filtration test was performed in the oxidation of 1a
to confirm that no leaching had occurred. The catalyst was
refluxed in toluene for 3 h and then was separated by filtration
of the hot reaction mixture. The solvent was then charged with
the substrate and subjected to oxidation under an oxygen atmos-
phere for 12 h under identical reaction conditions. GC analysis
of the catalyst-free reaction indicated that no oxidation of the
substrate had occurred under these conditions. The absence of
palladium in the filtrate solution was further confirmed by
ICP-AES analysis, which indicated no detectable content of Pd in
the filtrate. These results confirmed the heterogeneous nature of
the developed catalytic system for the oxidation of alcohols.
Fig. 4 XPS spectra of the [AmP-SNC/Pd(0)].
which are characteristic peaks of Pd 3d5/2 and Pd 3d3/2. These
are the typical valence state peaks of Pd(0) and are comparable
with the literature values.23
Catalytic activity of the nanostarch grafted
Pd catalyst 6
The catalytic activity of the synthesized catalyst 6 was checked
in the aerobic oxidation of alcohols under molecular oxygen.
Benzhydrol was used as a model substrate to study the effects
of the catalyst, the reaction temperature and the solvent. The
results of these experiments are summarized in Table 1.
Initially the reaction was tested in different solvents by using
different organic solvents such as acetonitrile, DMF, toluene
and p-xylene. Among the various solvents studied, aprotic
polar solvents such as acetonitrile and DMF were found to be
least effective and provided poor conversion of benzhydrol to
benzophenone under an oxygen atmosphere (Table 1, entries 1
and 2). This is probably due to the coordination of the solvent
molecules to the palladium metal, resulting in poisoning and
deactivation of the catalyst. The reaction proceeded with mode-
rate yield when water was used as a solvent (Table 1, entry 3).
Aromatic solvents such as toluene and p-xylene were found to
be the most promising solvents for this reaction (Table 1,
entries 3 and 4). Based on these findings we have selected
toluene as the reaction media for further studies. The reaction
was found to be very slow at room temperature; however at
90 °C, the reaction was found to be faster and provided 94%
isolated yield of the benzophenone in 1 h under an oxygen
atmosphere. Similarly in the absence of a catalyst under an
Conclusions
Table 1 Results of screening experimentsa
In conclusion, we have demonstrated the synthesis of a highly
dispersed Pd nanocatalyst grafted on nanocrystalline starch for
the first time. The developed nanocatalyst was found to be a
highly efficient heterogeneous catalyst for the oxidation of alco-
hols by using molecular oxygen as the terminal oxidant. Primary
aliphatic and benzylic alcohols were oxidized to their corres-
ponding carbonyl compounds in high to excellent yields. The
catalyst was found to be highly stable and was recycled several
times without any leaching of the metal into the solution.
Entry
Solvent
Temp. (°C)
Time (h)
Yieldb (%)
1
2
3
4
5
CH3CN
DMF
H2O
p-Xylene
Toluene
Reflux
90
Reflux
90
8
8
3.5
1.0
1.0
10
5
65
89
94
90
a Reaction conditions: benzhydrol (10 mmol), 6 (0.2 g), and solvent
(15 mL) under an oxygen atmosphere. b Isolated yields.
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Table 2 Pd-catalyzed oxidation of alcoholsa
Entry
1
Alcohol
Product
Time (h)
1.5
Yieldb (%)
97
TOF (h−1
)
64.5
2
1.0
95
95.0
3
4
0.5
0.5
98
98
196
196
5
6
7
8
0.5
2.5
3.5
1.0
96
94
92
95
192
37.6
26.3
95.0
9
1.25
2.0
96
89
76.8
44.5
10
11
12
1.0
3.5
92
86
92.0
24.6
13
3.0
88
29.3
a Reaction conditions: substrate (10 mmol), 6 (1 mol%), and toluene (15 mL) at 90 °C under an oxygen atmosphere. b Isolated yields.
performed using a thermal analyzer TA-SDT Q-600. All samples
were analyzed in the temperature range of 30–650 °C at a
heating rate of 10 °C min−1 under nitrogen flow. Loading of
palladium was determined using an inductively coupled
plasma atomic emission spectrometer (ICP-AES, PS-3000UV,
Leeman Labs). 10 mg of each sample was dissolved in 1 mL of
concentrated HNO3, heated and diluted to 10 mL. The result-
ing solutions were filtered and subjected to an analysis for Pd
content by ICP-AES analysis. Further, a nanostructural feature
of the prepared catalyst was examined using a transmission
electron microscope (TEM, JEOL 3010) at an accelerating
voltage of 300 kV.
Fig. 5 Recycling experiments.
Experimental section
Synthesis of nanocrystalline starch (SNC) 3
The nanostarch and the final catalyst powder were each mixed Nanocrystalline starch was prepared by acid hydrolysis of
with potassium bromide (KBr) powder to form pellets in order native starch material by following the literature procedure
to record the FTIR spectra. Thermogravimetric analyses (TGA) given by Angellier et al.4a In a typical procedure, starch powder
and differential thermal analyses (DTA) of these samples were (36 g) was mixed with a sulfuric acid (250 mL; 2.87 mol L−1
)
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solution. The suspension was placed at 45 °C to hydrolyze for
7 days under stirring; the resulting suspension was centrifuged
and washed successively with deionized water until neutrality.
An ultrasonic treatment was then employed to ensure better
dispersion of starch nanocrystals.
Notes and references
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Chemical modification of 3 by (3-aminopropyl)trimethoxysilane
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Chemical modification of corn starch nanocrystals was carried
out using 3-aminopropyltrimethoxysilane as a grafting agent.
In a typical experiment, starch nanocrystals (1 g) were mixed
with toluene (20 mL) and 3-aminopropyltriethoxysilane
(20 mmol, 4.5 g). The resulting mixture was then refluxed for
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The yield was 1.8 g.
Preparation of Pd(0)/AmP-SNC 6
The chemically modified 4 (1.0 g) was suspended in a pH-
adjusted deionized water solution by using 0.1 N LiOH (pH 8;
25 mL) and was allowed to stir at room temperature for
15 min. Li2PdCl4 was prepared by mixing LiCl (0.13904 g,
3.28 mmol) and PdCl2 (0.29082 mg, 1.64 mmol) in water
(30 mL), and the suspension was allowed to stir at 80 °C until
a homogeneous solution was obtained. The resulting dark red
solution was cooled and filtered, pH-adjusted (pH 8), and
then added to a dispersion of AmP-SNC in water, and the
resulting suspension was left to stir overnight. The suspension
was then centrifuged, and the separated brown solid was
washed with water (5 times). The brown solid was then re-
suspended in water (30 mL), followed by slow addition of
NaBH4 (0.6204 mg, 16.4 mmol) suspended in water (15 mL),
and the mixture was allowed to stir for 2 h. The resulting
suspension was then centrifuged to separate the desired solid
material which was thoroughly washed with ethanol and dried
under vacuum for 5 h.
General procedure for the oxidation
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