Synthesis of Palladium Nanoparticle by Bio-reduction Method and Its Effectiveness as Heterogeneous Catalyst Towards Selective Oxidation of Benzyl Alcohols in Aqueous Media

Article abstract of DOI:10.1007/s10562-016-1937-9

Abstract: Monodispersed palladium nanoparticles (PdNp) have been synthesized by a green rapid bio-reduction method under room temperature and pressure. The reducing agents mainly involved in these processes are the various water soluble plant metabolites, polyphenols as well as co-enzymes present in the leaf extract of Tulsi (Ocimum sanctum). An active and stable montmorillonite K-10 supported heterogeneous catalyst was prepared using these PdNp for oxidation of alcohols to their corresponding carbonyl compounds with efficient yield and having good reusability. These are characterized by TEM, XRD, BET surface area and UV–Vis absorption spectroscopy. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1937-9

Catal Lett
DOI 10.1007/s10562-016-1937-9
Synthesis of Palladium Nanoparticle by Bio-reduction Method
and Its Effectiveness as Heterogeneous Catalyst Towards Selective
Oxidation of Benzyl Alcohols in Aqueous Media
Nibedita Gogoi1 · Porismita Bordoloi1 · Geetika Borah1 · Pradip K. Gogoi1
Received: 5 August 2016 / Accepted: 27 November 2016
© Springer Science+Business Media New York 2016
Abstract Monodispersed
palladium
nanoparticles
1 Introduction
(PdNp) have been synthesized by a green rapid bio-reduc-
tion method under room temperature and pressure. The
reducing agents mainly involved in these processes are
the various water soluble plant metabolites, polyphenols
as well as co-enzymes present in the leaf extract of Tulsi
(Ocimum sanctum). An active and stable montmorillonite
K-10 supported heterogeneous catalyst was prepared using
these PdNp for oxidation of alcohols to their corresponding
carbonyl compounds with efficient yield and having good
reusability. These are characterized by TEM, XRD, BET
surface area and UV–Vis absorption spectroscopy.
The area of metal nanoparticle synthesis has received tre-
mendous boost during last two decades because of their
wide applications and unique characters. Among the vari-
ous metal nanostructures, those obtained from transition
metals are getting wide attention due to their unique phys-
icochemical properties as well as their practical utility in
catalysis and biochemistry [1], of which Pd based nano-
structures are well known for their excellent catalytic activ-
ity and selectivity [2]. There are various methods available
for synthesis of nanoparticles, but most of these involve
toxic chemicals and harsh conditions. Efforts are now being
directed to develop new green protocols for synthesis of
Pd-nanoparticles (PdNp). Use of biomaterials in this area
has emerged as a very promising green synthetic protocol.
There are a few such reported methods of preparing PdNps
using plant extracts of tea, coffee, C. Camphora etc. [3, 4].
The alcohol oxidation is one of the most frequently used
reactions in various organic syntheses involving manufac-
ture of drugs, agro-chemicals and fragrances. Traditionally,
alcohols are oxidized using oxidizing agents such as oxides
of manganese and chromium in presence of acid and harm-
ful organic solvents which produces toxic metal salts [5].
The alcohol oxidation by catalysts involving nanomaterials
is also found to be very effective for their high performance
and easy controllability. Several synthetic methodologies
have also been developed for alcohol oxidation involv-
ing the use of silver nanoparticles supported on hydrotal-
cites, gold nanoparticles supported on metal oxides and
PdNp supported on hydroxyapatite, mesoporous carbon,
silica–alumina, clay etc. [6–11]. But most of these methods
involve the use of large amount of additives as oxidant and
organic solvents contrary to green chemistry protocols.
Graphical Abstract
Keywords Palladium nanoparticles (PdNp) · Mont K-10 ·
Green synthesis · Alcohol oxidation · Heterogeneous
catalysis
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-016-1937-9) contains supplementary
material, which is available to authorized users.
* Pradip K. Gogoi
dr.pradip54@gmail.com
1
Department of Chemistry, Dibrugarh University, Dibrugarh,
Assam 786004, India
Vol.:(0123456789)
1 3

N. Gogoi et al.
In the present work, we report the development of an
efficient, reusable heterogeneous catalytic system involv-
ing mont K-10 supported PdNp, synthesized by using leaf
extracts of Tulsi (Ocimum sanctum) at room temperature
under aerobic condition in aqueous medium with excellent
results. This catalytic system was found to be very effective
for usually unreactive alcohol such as heteroatom contain-
ing alcohol, cinnamyl alcohol in aqueous media.
spectroscopy, XRD and inductively coupled plasma atomic
emission spectroscopy (ICP-AES) analysis.
2.1 TEM Study
The morphology, size and size distribution pattern of the
PdNps in colloidal solutions were investigated by TEM
(Fig. 2a) which indicate homogeneous particle size distri-
bution in the range of 1–13 nm with majority of particles
being in the range of 3–6 nm (Fig. 2a inset histogram).
The selective area electron diffraction (SAED) pattern
indicates the highly crystalline nature of PdNps (Fig. 2b,
inset). The spacing between two crystal planes is found to
be about 0.19 nm, which corresponds to the (200) plane of
the metal nanocrystals (Fig. 2b) [12]. The Fig. 3 depicts
the TEM image of PdNp@mont K-10 catalyst. Compar-
ing Figs. 2 and 3, it can be concluded that after using of
montmorillonite K-10 clay as supporting material, the par-
ticle size of PdNps were not changed and led to the uniform
deposition on the clay without any surface modification or
agglomeration.
2 Results and Discussion
When the leaf extract of 2 ml of Tulsi (O. sanctum) was
mixed in the 0.05 N (0.177 g PdCl2 in 20 ml ethanol) alco-
holic solution of palladium chloride, it started to change
colour (within 30 min) from light brown to black (Fig. 1b)
indicating the generation of PdNp. The various water solu-
ble plant metabolites, polyphenols as well as co-enzymes
present in the leaf extract act as reducing agent and thus
reduces Pd2+ ions. The formation of PdNp was character-
ized by TEM, BET surface area measurement, UV–Vis
Fig. 1 a change of colour after
adding leaf extract of tulsi
(20 s.). b Change of colour with
change of PdCl2 concentration
Fig. 2 a TEM image of PdNp,
b SAED pattern of PdNp
1 3

Synthesis of Palladium Nanoparticle by Bio-reduction Method and Its Effectiveness as…
The non appearance of any absorption peaks above
300 nm (Fig. 4b) is indicative of the complete reduction
of the Pd(II) ions. From ICP-AES analysis the Pd con-
tent in PdNp@mont K-10 was found to be 0.004 mol%
or 0.005 mg (0.21 ppm) per 20 mg of the solid catalyst
(0.2625 mg Pd per gram of the catalyst, PdNp@mont
K-10).
2.3 XRD
The XRD pattern of PdNp (Fig. 5a) exhibits diffraction
peaks at 2θ value of 40.1°, 46.3° and 67.8° corresponding
to (111), (200) and (220) planes of fcc structure of Pd(0)
nanoparticles [14]. On incorporation of PdNps to mont
K-10 reveals diffraction peaks with similar 2θ values with
comparatively weak intensity which is due to very small
amount of Pd(0) loading (0.21 ppm) in PdNp@mont
K-10. The appearance of an intense peak at 2 θ = 26.08°
is due to mont K-10 [15] (Fig. 5b).
Fig. 3 TEM image of PdNp@mont K-10
2.2 UV–Vis Spectroscopy and ICP-AES Analysis
The formation of PdNps was monitored by recording the
electronic spectrum (200–600 nm) of the colloidal PdNp
solution. Figure 4b shows the absorption spectrum of col-
loidal suspensions of PdNps after 48 h of bio-reduction
while Fig. 4a shows the absorption spectrum of PdCl2
solution. The weak bands appearing around 290 and
340 nm for PdCl2 solution could be due to the ligand to
metal charge transfer transition of the Pd(II) ions [13].
2.4 BET Surface Area Analysis
According to BET surface measurement, the surface
areas of the montmorillonite K-10 clay and PdNp@mont
K-10 were found to be 446 and 333 m2 g−1, respectively.
Fig. 4 UV–Visible spectra of a 0.1 N PdCl2 solution and b 0.05 N
PdNps
Fig. 5 XRD pattern of a PdNp and b PdNp@mont K-10
1 3

N. Gogoi et al.
The decrease in surface area of the catalyst can be attrib-
uted to the incorporation of PdNp into the clay.
noteworthy that the PdNp@mont K-10 catalyst shows
excellent performance even at room temperature in water
under aerobic condition (Table 1, entry 1). Solvent effect
on the activity of PdNp@mont K-10 was surveyed with dif-
ferent polar and nonpolar solvents although significant var-
iations in yields were noticed. The reaction was conducted
in toluene, acetonitrile, DMSO, DMF, iPrOH and methanol
(Table 1, entries 2–7) and best results was obtained when
water was used as solvent (entry 1). Generally, the presence
of water with high polarity index facilitates the solubil-
ity of the organic substrates, which in turn facilitates the
adsorption of reactants on the active site of the catalyst and
increases the effectiveness of the conversion and selectiv-
ity [16]. This may be the reason for low yield in toluene,
2.5 Use of PdNp@mont K-10 for Selective Alcohol
Oxidation Reaction
The prepared PdNp@mont K-10 catalyst was applied for
the selective aerobic oxidation of benzyl alcohols. For
this purpose, benzyl alcohol was chosen as model sub-
strate and initially the reaction was carried out in water at
room temperature as shown in Scheme 1.
The screening of different solvent, oxidant, amount
of catalyst and temperature is illustrated in Table 1. It is
i
acetonitrile, DMSO, DMF, PrOH and methanol (entries
2–7). When mixture of solvents was used, excellent yield of
products was obtained (entries 8–10). In solvent free condi-
tion, the yield of product was found to be of trace amount
(entry 11).When the loading of PdNp@mont K-10 was
decreased from 10 mg (0.002 mol%) to 5 mg (0.001 mol%),
the yield of product was slightly decreased (entry 12).
OH
Air, H2O, rt
O
PdNP@mont K-10(10 mg)
Scheme 1 Oxidation of benzyl alcohol
Table 1 Optimization of
reaction condition
OH
Air, H2O, rt
O
PdNp@mont K-10(10 mg)
Entry
Catalyst (mg)
Solvent
Time (h)
Yielda (%)
1
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
Mont K-10 clay
PdNp
H2O
5
6
5
5
7
6
5
5
5
5
5
5
5
5
5
5
5
98
2
acetonitrile
88
3
Toluene
85
4
DMSO
86
5
DMF
iPrOH
88
6
80
7
Methanol
H2O:iPrOH (1:1)
82
8
92
9
H2O:acetonitrile (1:1)
95
10
11
12
13
14
15
16
17
H2O:DMF (1:1)
90
–
Trace amount
94b
98c
H2O
H2O
H2O
H2O
H2O
H2O
Trace amount
97
–
Trace amount
98d
PdNp@mont K-10
Reaction condition: benzyl alcohol (1 mmol), PdNp@mont K-10 (10 mg), 3 ml H2O under aerobic condi-
tion at room temperature
aIsolated yield
bAmount of catalyst(5 mg)
cAmount of catalyst (15 mg)
d50 οC temperature used
1 3

Synthesis of Palladium Nanoparticle by Bio-reduction Method and Its Effectiveness as…
Similarly, increase of catalyst loading (15 mg, 0.003 mol%)
does not effect the product yield (entry 13). The reaction
was also performed in the presence of neat mont K-10 clay
and PdNp (entries 14, 15). But no significant increase in
product yield was observed in presence of neat mont K-10,
while in presence of only 10 mg of PdNp, yield was 97%,
indicating the prime role of PdNps in this reaction. Fur-
thermore, when the reaction was performed in absence of
catalyst no product was isolated (entry 16). Again, it was
noticed that increase of temperature beyond ambient condi-
tion has no significant effect in this current protocol (entry
17).
After investigating the effect of different parameter, we
next examined the catalytic activity of PdNp@mont K-10
catalyst in different substituted benzyl alcohol (Table 2).
Primary benzyl alcohols with electron donating and with-
drawing groups such as p-Cl, p-NO2, p-MeO, p-Me in the
benzene ring gave the desired products in 79–98% yield
(Table 2, entries 1–5). Primary benzyl alcohols bearing hal-
ogen group as substituent was also successfully oxidized to
benzaldehyde with satisfactory yields (entry 2).
Table 2 Aerobic oxidation of different substituted benzyl alcohol
In order to expand the scope of the use of our catalyst for
secondary alcohol, initially we have conducted the screen-
ing reaction with 1-phenyl ethanol as model substrate using
the same optimum condition used for benzyl alcohol. Our
result suggests that air is not sufficient for complete con-
version of secondary alcohol to their corresponding prod-
uct. Therefore we have used hydrogen peroxide (H2O2) as
external oxidant, which is considered as a stoichiometric
and environmentally acceptable oxidant [17, 18] (Table 3).
However, search for an alternative reaction condition
revealed that changing the solvent with toluene, acetoni-
trile and DMSO, increasing the temperature and increasing
the catalyst loading to 20 mg does not affect in conversion
Air, H2O, rt
OH
O
PdNp@mont K-10(10 mg)
R
R
Entry Substrate (R) Time (h) Yield (%)a TON
TOF (h−1
)
1
2
3
4
5
4-OCH3
4-Cl
5
7
7
7
5
95
79
92
88
98
47,500 9500
39,500 5642
46,000 6571
44,000 6285
49,000 9800
4-CH3
4-NO2
H
Reaction condition: alcohol (1 mmol), PdNp@mont K-10 (10 mg),
H2O (3 ml) in air at room temperature
aIsolated yield
TON=Mole of product/Pd mole; TOF=TON/time in h
Table 3 Optimization of
reaction condition for secondary
alcohol
OH
O
Air, Solvent, rt
PdNp@mont K-10(10 mg)
Entry
Catalyst (mg)
Solvent
Time (h)
Yield (%)a
1
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
PdNp@mont K-10
H2O
5
6
5
5
5
6
4
4
4
4
4
82
2
acetonitrile
Toluene
DMSO
72
3
75
4
70
5
H2O:acetonitrile(1:1)
H2O:DMF (1:1)
H2O
75
6
75
7
95b
95c
88d
95e
95f
8
H2O
9
H2O
10
11
H2O
H2O
Reaction condition: 1-phenyl ethanol (1 mmol), PdNp@mont K-10 (10 mg), solvent (3 ml) in air at room
temperature
aIsolated yield
b0.1 ml H2O2 used as oxidant
c20 mg catalyst used
d5 mg catalyst used
e50 °C temperature used
f0.2 ml H2O2 used
1 3

N. Gogoi et al.
Table 4 Oxidation of different secondary alcohola
O
H2O2 (0.1ml), H2O, rt
OH
PdNp@mont K-10(10 mg)
R1
R2
R1
R2
aReaction condition: alcohol (1 mmol), PdNp@mont K-10 (10 mg), H2O (3 ml), H2O2 (0.1 ml) at room temperature
bIsolated yield
TON=Mole of product/Pd mole; TOF=TON/time in h
of alcohol to corresponding carbonyl compound (Table 3,
entries 1–6).
be attributed to the presence of delocalization. Simi-
larly, alcohols with electron donating substituent were
found to slow down the oxidation process whereas in
case of electron withdrawing substituent, it is acceler-
ated. The PdNp@mont K-10 catalyst comfortably cata-
lyzed the usually unreactive cinnamyl alcohol as well as
cyclic alcohol (Table 4, entries 8, 10). Furfuryl alcohol
After optimization of reaction conditions various alco-
hols were oxidized in the presence of 10 mg catalyst and
H2O2 as an oxidant at room temperature in water (entry
7). The results are summarized in the Table 4. Alcohols
with an aromatic substituent were found to be more reac-
tive than aliphatic alcohols (Table 4, entry 9), which can
1 3

Synthesis of Palladium Nanoparticle by Bio-reduction Method and Its Effectiveness as…
Table 5 Reusability of Pd catalyst on alcohol oxidation of 1-phenyl
ethanol
Matthey Limited. Alcohol substrates were purchased from
Sigma-Aldrich and Tokyo Chemical Industry and were
used as received without further treatment.
O
OH
H2O2 (0.1ml), H2O, rt
4.2 Synthesis of PdNp [19]
PdNp@mont K-10(10 mg)
No. of runs
1st run 2nd run 3rd run 4th run 5th run
95 95 94 94
To synthesize PdNp, 2 gm wet leaves of Tulsi (O. sanc-
tum) were collected and washed thoroughly with distilled
water and crushed in a mortar. The leaf extract obtained
after boiling in 20 ml water for 5 min was centrifuged and
the filtrate was collected for further experimental purpose.
Two milliliters of leaf extract was added to 20 ml of 0.05 N
alcoholic PdCl2 (0.177 g PdCl2 in 20 ml ethanol) solutions
and kept for 5 min at room temperature followed by stirring
for 5 h. The mixture so obtained was again centrifuged and
the nanoparticles were collected for characterization.
Isolated yield (%) 95
containing the heteroatom and aliphatic octanol afforded
the products in good yield (entries 9, 11) (Table 4).
To know the leaching of Pd metal in the solution, we
have performed ICP-AES analysis of the filtrate after 5th
run. Detection of negligible amount (below detection limit)
of palladium in the solution suggests the robust nature of
the catalyst. Thus it confirms the heterogeneity of the cat-
alyst [16]. After completion of first cycle, catalyst can be
separated from the reaction mixture by simple centrifu-
gation process. Upon being isolated, 1-phenyl ethanol as
model substrate, the catalyst is fully active upto fifth cycle
giving excellent yield (Table 5). When the catalyst was
removed from the reaction mixture by centrifugation after
about 50% conversion of 1-phenyl ethanol, the reaction
ceased completely, indicating heterogeneous nature of the
catalyst.
4.3 Procedure for Preparation of PdNp@mont K-10
Catalyst [20]
Two grams of mont K-10 clay was added to a PdNp suspen-
sion and continuously stirred for 6 days. After that the mix-
ture was filtered and washed several times with water and
dried in air for 24 h to get the PdNp@mont K-10 catalyst.
4.4 Characterization of PdNp and PdNp@mont K-10
Catalyst
Prepared PdNp and PdNp@mont K-10 were character-
ized by high resolution TEM (HR-TEM), X-ray diffrac-
tion (XRD) and UV–Vis spectroscopy. TEM images were
recorded on a JEOL/JEM- 2100 operating at 200 kV.
XRD study was conducted on a Bruker AXS D8 advance
diffractometer with Cu-Kα (λ=1.541 A˚) radiation.
The surface area of the catalyst was determined using
Brunauer–Emmett–Teller (BET) surface area analysis with
nitrogen gas adsorption method (Quantachrome Instru-
ments, Boynton Beach, FL). Amount of Pd on PdNp@mont
K-10 catalyst was analyzed by ICP-AES on a thermo elec-
tron IRIS intrepid II XSP DUO. All products were charac-
3 Conclusion
Synthesis of PdNp was done by adopting green methodol-
ogy. The active heterogeneous catalyst PdNp@mont K-10
catalyst was prepared with mont K-10 as support which
was found to be an excellent catalyst for oxidation of both
primary and secondary alcohols. This protocol was also
effective for usually unreactive substrates such as cinnamyl
alcohol, cyclohexanol, aliphatic alcohol as well as alcohol
containing heteroatom like fufuryl alcohol. The advanta-
geous features of the present catalytic system include the
use of aqueous solvent, air as oxidant, no ligands and reus-
ability of catalyst upto fifth cycle.
1
terized using H NMR, 13C NMR and mass spectroscopy.
1H NMR and 13C NMR spectra were recorded at room tem-
perature with a Bruker Avance 400 MHz instrument.
4.5 General Procedure for Aerobic Alcohol Oxidation
4 Experimental
4.1 Material
The alcohol oxidation reaction was performed by taking
10 mg PdNp@mont K-10 catalyst, 1 mmol alcohol and
3 ml water in a 50 ml round bottom flask under aerobic
condition. The mixture was stirred at room temperature
up to the required time. The progress of the reaction was
monitored by taking thin layer chromatography (TLC).
After completion of the reaction, stirring was stopped and
The chemicals were obtained commercially and used as
received without further drying or purification. The sol-
vents such as DMSO, acetonitrile, ethyl acetate, methanol
and toluene and oxidant such as H2O2 were purchased from
MERCK. Palladium(II) chloride was purchased from Arora
1 3

N. Gogoi et al.
3. Nadagouda MN, Varma RS (2008) Green Chem 10:859
4. Huang J, Li Q, Sun D, Lu Y, Su Y, Yang X, Wang H, Wang
Y, Shao W, He N, Hong J, Chen C (2007) Nanotechnology
18:105104
5. Polshettiwar V, Varma RS (2009) Org Biomol Chem 7:37
6. Mori K, Hara T, Mizugaki T, Ebitani K, Kaneda KJ (2004) J Am
Chem Soc 126:10657
7. Lu AH, Li WC, Hou Z, Schuth F (2007) Chem Commun 43:1038
8. Chen J, Zhang Q, Wang Y, Wan H (2008) Adv Synth Catal
350:453
the catalyst was subjected to centrifugation and the residual
solid after filtration was washed with water (4 ml) three
times. To this filtrate 10 ml ether was added and extracted
the product with ether. The resultant organic phases was
dried over anhydrous Na2SO4, filtered and evaporated
under reduced pressure. The residue was subjected to silica
gel column chromatography (ethyl acetate–hexane, 2:8) to
obtain the desired products.
9. Enache DI, Edwards JK, Landon P, Espriu BS, Carley AF, Her-
zing AA, Watanabe M, Kiely CJ, Knight DW, Hutchings GJ
(2006) Science 311:362
4.6 Recycling Test
10. Kotani M, Koike T, Yamaguchi K, Mizuno N (2006) Green
Chem 8:735
11. Wang H, Deng SX, Shen ZR, Wang JG, Ding DT, Chen TH
(2009) Green Chem 11:1499
12. Iranpoor N, Firouzabadi H, Montevallo S, Talebi M (2012) J
Organomet Chem 118:708
13. Seok S, Hussain MA, Park KJ, Kim JW, Kim DH (2016) Ultra-
son Sonochemistry 28:178
Catalyst was isolated from the reaction mixture by sim-
ple centrifugation process after complete conversion and
washed with ethyl acetate and water to remove the organic
contaminants. After getting the solid part of catalyst, it was
drying at 110οC in an oven for whole night. Then catalyst
was used for fresh oxidation reaction of 1-phenyl ethanol.
Reaction conditions were kept similar to the other oxida-
tion reactions.
14. Yang X, Li Q, Wang H, Huang J, Lin L, Wang W, Sun D, Su Y,
Opiyo JB, Hong L, Wang Y, He N, Jia L (2010) J Nanopart Res
12:1589
15. Bharadwaj SK, Boruah PK, Gogoi PK (2014) Catal Comm
57:124
16. Gogoi N, Begum T, Bora U, Gogoi PK (2015) RSC Adv 5:95344
17. Noyori R, Aoki M, Sato K (2013) Chem Commun 16:1983
18. Mahanta A, Adhikari P, Bora U, Thakur AJ (2015) Tetrahedron
Lett 56:1780
Acknowledgements We gratefully acknowledge SAIC-Tezpur Uni-
versity for recording XRD and STIC-Cochin for HR-TEM and ICP-
AES analysis. N. Gogoi also thanks to UGC, New Delhi for financial
assistance in the form of UGC-BSR (RFSMS) fellowship.
19. Kim SW, Park J, Jang Y, Chung Y, Hwang S, Hyeon T (2003)
Nano Lett 3:1289
20. Gogoi PK, Begum T, Borthakur B, Das G, Bora U, Kumar A
(2015) Natl Acad Sci Lett 33:231
References
1. Narayanan R, El-Sayed MA (2005) J Phys Chem B 109:12663
2. Mittal AK, Chisti Y, Banerjee UC (2013) Biotechnol Adv 31:346
1 3