Efficient Baeyer–Villiger Oxidation Catalysed by Silver Nanoparticles Stabilized on Modified Montmorillonite

Article abstract of DOI:10.1007/s10562-018-2566-2

Abstract: Silver nanoparticles supported on modified montmorillonite clay (Ag-NPs@mont), were utilized as catalyst for the Baeyer–Villiger oxidation of various ketones with hydrogen peroxide as an oxidant under solvent free condition at room temperature. The modification of Montmorillonite K10 clay was carried out with HCl under controlled conditions for generating a high surface area porous matrix which acts as support for the in situ generation of Silver nanoparticles. The synthesized nanocomposite material was characterized by UV–Visible spectroscopy, powder XRD, SEM-EDX, TEM and N₂ adsorption–desorption analysis. The catalyst can be recycled and reused several times without significant loss of their catalytic activity. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-018-2566-2

Catalysis Letters
https://doi.org/10.1007/s10562-018-2566-2
Efficient Baeyer–Villiger Oxidation Catalysed by Silver Nanoparticles
Stabilized on Modified Montmorillonite
Subrat Jyoti Borah1 · Diganta Kumar Das1
Received: 16 June 2018 / Accepted: 23 September 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Silver nanoparticles supported on modified montmorillonite clay (Ag-NPs@mont), were utilized as catalyst for the Baeyer–
Villiger oxidation of various ketones with hydrogen peroxide as an oxidant under solvent free condition at room tempera-
ture. The modification of Montmorillonite K10 clay was carried out with HCl under controlled conditions for generating
a high surface area porous matrix which acts as support for the in situ generation of Silver nanoparticles. The synthesized
nanocomposite material was characterized by UV–Visible spectroscopy, powder XRD, SEM-EDX, TEM and N2 adsorption–
desorption analysis. The catalyst can be recycled and reused several times without significant loss of their catalytic activity.
Graphical Abstract
Extended author information available on the last page of the article
Vol.:(0123456789)
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S. J. Borah, D. K. Das
Keywords Modified montmorillonite · Ag-NPs@mont · Baeyer–Villiger oxidation · Heterogeneous catalyst
Silver nanoparticles were used as active catalysts in many
organic reactions such as three-component coupling reac-
tion, carbon–carbon coupling reaction, dehydrogenation
reaction, oxidation reaction, Diels–Alder cycloaddition reac-
tion. In comparison with traditional catalysts, silver nanopar-
ticles, in particular, require only mild reaction conditions to
produce high yields of products in short reaction times and
can also be recycled [27, 28].
1 Introduction
Environmentally benign, efficient, economical and green
synthesis of metal nanocatalysts as well as green conditions
during catalysis has become more important to address
industrial and environmental concerns [1–4]. In this con-
nection, various organic transformations, multicomponent
reactions have been carried out for the synthesis of different
complex molecules [5, 6]. Baeyer–Villiger (BV) oxidation
is a very important class of organic reaction, where various
value-added lactones and esters in many fields such as the
production of antibiotics and steroids, can be generated in
one pot from their respective carbonyl compound in the pres-
ence of oxidants such as peroxides, per acids or persulfate
salts [7–9]. When organic per acids are used, the reaction
generates huge amount of hazardous by-products which is a
major problem often encountered in this reaction. Thus the
use of H2O2 as oxidant gets the advantages over other per-
acids because of its ease of use, being environment friendly,
produce water only as side product. Although both homoge-
neous and heterogeneous catalysts were used in this reaction,
heterogeneous catalysts have more advantages than homog-
enous ones, because of facile separation and reusability of
the catalyst and simple isolation of the product.
In this study, we have reported synthesis of silver nano-
particles (Ag-NPs) using modified montmorillonite clay as
stabilizer and its catalytic performance for the Baeyer–Vil-
liger oxidation of cyclic and aromatic ketones using aqueous
H2O2 as oxidant. Montmorillonite K10 clay is environmen-
tally benign, cheap, which is purchased from M/S Sigma
Aldrich, USA. The K10 clay was modified by treatment with
mineral acid (HCl) under controlled conditions to gener-
ate a matrix having high surface area and contain micro-
and mesopores. The modified montmorillonite clay acts as
a support for the in situ generation of silver nanoparticles
(Ag-NPs@mont). Ag-NPs@mont catalyses Baeyer–Villiger
oxidation of various ketones with excellent yield and 100%
selectivity under green reaction condition. The nanocatalysts
maintain their catalytic efficiency for several run and serve
as a potential reusable catalyst.
The nano-catalysts have high surface to volume ratio
which provide the advantages of high atom efficiency. Nano-
catalytic system also provides the advantage of simplified
isolation of products, easy recovery and recyclability of
the catalysts [10–12]. But due to their high surface energy,
metal nanoparticles have the tendency to aggregate during
catalytic reaction which decreases their catalytic efficiency
[13, 14]. Thus some stabilizers/supports are required to pre-
vent the aggregation of metal nanoparticles. Different types
of supports like mesoporous materials, activated carbons,
polymers, dendrimers, metal oxides, organic ligands, zeolite,
clay etc. have been employed to stabilize metal nanoparticles
[15, 16]. Some of the recently developed heterogeneous cat-
alytic systems for BV oxidation are: supported ionic liquids
[17], supported sulfonic acids [18], Sn-doped microporous
and mesoporous materials [19, 20], mesoporous magne-
sium–aluminum mixed oxide [7], modified Sn-Beta zeolite
[8], clay-supported Sn catalysts [21], stabilized Fe3O4 mag-
netic nanoparticles [22], Sn exchanged hydrotalcites [23]
etc. Moreover, due to stringent and growing environmental
regulations, the chemical industry needs the development
of ecofriendly and sustainable synthetic methods [1, 6].
In recent years, lot of researches has been focused on sil-
ver nanoparticles because of their important scientific and
technological applications in color filters, optical switching,
optical sensors, antibacterial activities and catalysis [24–26].
2 Experimental
2.1 Support Preparation
Montmorillonite K10 (6 g) was dispersed in 200 ml 3 M HCl
and refluxed for 2 h. After cooling, the supernatant liquid
was discarded and the activated montmorillonite was repeat-
edly washed with deionised water. The activated clay was
dried in air oven at 50 °C over for 12 h and the solid product
was designated as modified montmorillonite.
2.2 Catalyst Preparation
1 g of modified montmorillonite was impregnated with a
20 ml (0.6 mmol) aqueous solution of AgNO3 under vig-
orous stirring. The stirring was continued for another 8 h
followed by evaporated to dryness in a rotary evaporator.
0.3 g of the dry composite was dispersed in 20 ml of water
and 151.4 mg of NaBH4 (4 mmol) in 10 ml of distilled water
were added slowly over 10 min under constant stirring con-
ditions. The grey reaction product was recovered and washed
with deionised water several times and then dried in a desic-
cators for 15 h. The composite was designated as Ag-NPs@
mont.
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Efficient Baeyer–Villiger Oxidation Catalysed by Silver Nanoparticles Stabilized on Modified…
ratio) several times. Then small amount blank solution (2%
2.3 Typical Procedure for Oxidation of Ketone
HNO3) is added to the mixture and it was filtered off to a
100 ml volumetric flask. This solution is used for ICP analy-
sis. The blank solution used in the ICP experiment is 2%
HNO3.
Ketone (2 mmol), H2O2 (2.2 ml approx 20 mmol) and 25 mg
Ag-NPs@mont were taken in 10 ml round bottom flask and
the reaction mixture was stirred at room temperature for 4 h.
After completion of the reaction, the solid catalyst was recov-
ered by filtration. The product obtained contains water as a
byproduct was removed by using anhydrous sodium sulphate.
The conversion of the reaction was determined using GC.
3 Results and Discussion
3.1 Characterization of Support and Catalyst
2.4 Sample Preparation Procedure for ICP‑AES
Analysis
The modified montmorillonite was characterized thoroughly
with the help of different sophisticated analytical instru-
ments like Powder-XRD, N2 adsorption–desorption etc.
The parent montmorillonite K10 exhibited an intense basal
The sample used for metal analysis (Ag-NPs@mont) was
digested with aqua regia (conc. HNO3 and HCl in the 1:3
Fig. 1 a Powder XRD pattern of parent and modified mont, b powder
XRD pattern of Ag-NPs@mont, c SEM image and d EDX pattern of
Ag-NPs@mont, e UV–Visible spectrum of Ag-NPs@mont, f TEM
image Ag-NPs@mont, g N2 adsorption/desorption isotherm and h
BJH pore size distribution curve of modified mont and Ag-NPs@
mont
1 3

S. J. Borah, D. K. Das
reflection at 7.10° 2θ corresponding to a basal spacing of
12.7 Å (Fig. 1a). The reflection intensity decreased with
time and no basal reflection was seen after 2 h acid activa-
tion. The modified montmorillonite contained mesopores
(>2 nm), a high specific surface area of 368 m2/g and a large
specific pore volume of ~0.67 cm3/g (Table 1).
throughout the surface of modified montmorillonite
(Fig. 2). TEM image (Fig. 1f) showed the silver nano-
particles are dispersed on modified montmorillonite with
particle sizes of 1–10 nm. From surface area study, it
is observed that there is an appreciable decrease of the
specific surface area and the specific pore volume after
supporting Ag-NPs (Table 1), but it exhibits a similar
type of isotherm and hysteresis loop to that of modified
montmorillonite (Fig. 1g, h). This might be due to clog-
ging of some pores by silver nanoparticles. The Ag con-
tents in Ag-NPs@mont as analyzed by ICP-AES, reveals
the presence of 6.421 mg Ag in 100 mg Ag-NPs@mont
catalyst (0.595 mmol/g).
The preliminary investigation of the formation of the sil-
ver nanoparticles (Ag-NPs) was carried out by UV–Visible
spectroscopy. The appearance of surface plasmon resonance
band at around ~400 nm of Ag-NPs@mont substantiated the
formation of silver nanoparticles (Fig. 1e).
The powder XRD suggests face centred cubic lattice
(fcc) lattice type arrangement of Ag-NPs (Fig. 1b). The
four reflections were assigned to the (111), (200), (220)
and (311) diffraction of a fcc lattice of silver. The SEM-
EDX analysis also shows the presence of silver nanopar-
ticles on the surface of modified mont (Fig. 1c, d). The
elemental dot mapping shows the distribution of silver
3.2 Catalytic Activity
The synthesized Ag-NPs@mont was utilized as catalyst
precursor for the well-known Baeyer–Villiger oxidation of
Fig. 1 (continued)
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Efficient Baeyer–Villiger Oxidation Catalysed by Silver Nanoparticles Stabilized on Modified…
different cyclic and aromatic ketones using H2O2 as oxidant
at room temperature. A series of oxidation reactions were
carried out using cyclohexanone as the model substrate to
determine the best reaction condition (i.e., solvent, reac-
tion time etc.). The reaction with ethanol as solvent was
the first choice, where about 86% conversion of the desired
product was obtained after stirring upto 6 h at room tem-
perature (RT). While the use of acetonitrile as a solvent at
RT decreased the yield of the desired product to 65% in
6 h. The use of 1,2-dichloroethane as solvent increased the
conversion surprisingly to 99% in 3 h. But when the same
reaction was carried under solvent free condition, it also
gave 99% conversion in 4 h (Table 2). Thus all the reactions
were carried out under solvent free condition with H2O2 as
oxidant by considering the concept of green chemistry. A
control reaction was carried out to test the requirement of a
catalyst by stirring the model substrate at RT in the absence
of the catalyst. No conversion was observed in this case.
When the model reaction was carried out using only modi-
fied montmorillonite clay as catalyst, no conversion was
observed. Thus the catalytic reaction takes place in presence
of Ag-NPs. The effect of various catalyst amounts was also
studied to optimize Ag content on the catalytic activity. The
effect of catalyst amount was studied with 10, 15, 20, 25 and
30 mg of the catalyst in the model reaction (Table 2). With
increased in the amount of catalyst, the yield of the reaction
increased. But, almost same yields were observed for 25 and
30 mg catalyst which was maximum for the model reaction.
Thus, all the reactions were performed using 25 mg catalyst
(contains 0.0148 mmol Ag).
Table 1 Surface properties of Ag-NPs@mont and modified montmo-
rillonite
Specific
Specific pore
surface
Volume (cm3/g)
Area (m2/g)
Modified montmorillonite
Ag-NPs@mont (fresh)
Ag-NPs@mont (after 1st run)
Ag-NPs@mont (after 2nd run)
Ag-NPs@mont (after 3rd run)
368
264
238
216
198
0.67
0.53
0.48
0.42
0.36
The results of the reactions are represented in Table 3.
The high conversion of the product (Maximum 99%), excel-
lent selectivity and less reaction time (4 h) for the reaction
indicate the efficacy of the catalyst at RT (Table 3). In this
work, we used different cyclic and aromatic ketones and
Fig. 2 Electron image and elemental dot mapping of Si, O and Ag on the surface of Ag-NPs@mont
1 3

S. J. Borah, D. K. Das
Table 2 Effect of solvent and
Sl no
Solvent
Catalyst
Catalyst amount
(mg)
Time (h)
Conver-
sion
catalyst
(%)
Effect of solvent
1
2
3
4
Ethanol
Acetonitrile
1,2-dichloroethane
Solvent free
Ag-NPs@mont
Ag-NPs@mont
Ag-NPs@mont
Ag-NPs@mont
25
25
25
25
6
6
3
4
86
65
99
99
Ag-NPs@mont
Ag-NPs@mont
Ag-NPs@mont
Ag-NPs@mont
Ag-NPs@mont
Modified mont
10
15
20
25
30
25
4
4
4
4
4
4
72
79
88
99
99
–
Reaction conditions: Cyclohexanone (2 mmol), H2O2 (20 mmol), Catalyst (25 mg, contains 0.0148 mmol
Ag), Room Temp
Table 3 Ag-NPs@mont
catalysed Baeyer–Villiger
oxidation of cyclic and aromatic
ketones
Entry
Substrate
Product
Conversiona (%)
S
elecꢀvity (%)
O
90
100
S1.
O
O
99
98 (2nd run)
96 (3rd run)
93 (4th run)
O
100
O
S2.
S3.
O
O
O
90
96
94
O
O
89
87 (2nd run)
85 (3rd run)
82 (4th run)
O
O
O
S4.
S5.
O
O
84
99
98
96
O
O
S6.
79
O
91
O
O
89 (2nd run)
87 (3rd run)
86 (4th run)
S7.
S8.
S9.
O
O
O
O
88
93
90
O
O
O
Cl
86
Cl
aFrom GC analysis; Reaction conditions: Substrate (2 mmol), H2O2 (20 mmol), Catalyst (25 mg, contain
contains 0.0148 mmol Ag), solvent free, room temperature, Time: 4 h
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Efficient Baeyer–Villiger Oxidation Catalysed by Silver Nanoparticles Stabilized on Modified…
Table 4 Reproducibility of Ag-NPs@mont catalysed BV oxidation
Substrate
Product
Conversion
(%)
Selectivity (%)
99
100
99
98
99
all the ketones yield the desired product with a very good
conversion. The model reaction gives maximum 99% con-
version with 100% selectivity when Ag-NPs@mont is used
as catalyst (entry S2 in Table 3). In this study, it is observed
that cyclic ketones showed a slightly higher conversion than
aromatic ketones. This may be due to higher stability of
aromatic ketones than cyclic ketones which decreases their
reactivity. Again substituted ketones showed slightly lower
conversion than unsubstituted one (Table 3). To check the
reproducibility, we have carried out the same experiment
(BV oxidation) of substrate S2 i.e., cyclohexanone four
times using fresh Ag-NPs@mont catalyst in each experiment
under controlled room temperature and the results obtained
are listed in Table 4. First two experiments give 99% con-
version, third experiment gives 98% conversion and the 4th
experiment gives again 99% conversion.
Fig. 4 TEM image of recovered Ag-NPs@mont after 4th run
important factor for practical application. The recyclability
of the catalyst for Baeyer–Villiger oxidation of ketones was
investigated with the substrate of entry S2, entry S4 and
entry S7 (Table 3). The catalyst was recovered by simple
filtration technique after each experiment. The recovered
catalyst was washed with acetone, dried in a desiccator and
reused directly for another run without further purification
for the desired BV oxidation. The recovered catalyst was
used upto the 4th run and showed only a slight decrease
in activity (Table 3). The recovered catalyst was further
A
13C NMR spectrum for product of entry S2 is repre-
sented in Fig. 3. The recyclability of the catalyst is a very
Fig. 3 13C NMR spectrum for product of entry S2
1 3

S. J. Borah, D. K. Das
: Ag-NPs@mont
O
Substrate
H2O2
O
Ag-NPs@mont
O
Regeneration of
Catalyst
O
H2O +
H
O
Product
H
O
O
-
OOH
OH
Criegee intermediate
Scheme 1 Tentative mechanism for Baeyer–Villiger oxidation
investigated through N2 adsorption–desorption, HRTEM
analysis. The specific surface areas of the recovered cata-
lysts decrease compared to the fresh catalyst with surface
area of 264 m2 g−1(Table 1) which may be due to blockage
of the pores by the reactant molecules after each reaction. In
the TEM analysis of the recovered catalyst (after 4th run), it
was found the sizes of Ag-NPs are still below 10 nm (Fig. 4).
A filtration test was carried out to investigate whether
any leaching of silver occurs during the course of the reac-
tion. Here, the BV oxidation of cyclohexanone was carried
under solvent free condition using H2O2 as oxidizing agent
for 2 h using our Ag-NPs@mont catalyst. At this stage, the
conversion of the desired product is 68%. The Ag-NPs@
mont catalyst was then filtered off and with the filtrate
the reaction was continued for another 2 h. But no further
increase in the conversion of the product was observed.
Again, the silver content in Ag-NPs@mmt before the cata-
lytic oxidation was 6.421 mg Ag in 100 mg Ag-NPs@mont
catalyst (0.595 mmol/g). After 4th run, silver content in
the catalyst was found to be 6.395 mg per 100 mg catalyst
(0.592 mmol/g) which was marginally decreased. Thus, the
catalyst maintains its heterogeneous nature and no leaching
of silver occurs during the course of the reaction.
(Scheme 1) and our experimental result. Initially, the sub-
strate is adsorbed on the catalyst surface. Hydrogen perox-
ide then attacks at the more electrophilic carbonyl carbon
forming the Criegee intermediate. After rearrangement, a
new product (cyclic ester) is obtained with a theoretical
byproduct H2O.
4 Conclusion
In conclusion, we have synthesized silver nanoparticles
(Ag-NPs) with particle size below 10 nm into the nano-
pores of modified montmorillonite clay. The silver nano-
particles demonstrated high catalytic activity for the cata-
lytic BV oxidation of various cyclic and aromatic ketones
with H2O2 as oxidant under solvent free condition. The
Ag-NPs@mont nanocatalyst was reused for new batch of
reactions (upto 4th run) without significant loss in activ-
ity under the same conditions. The simple procedure for
catalyst preparation, easy recovery, mild reaction condi-
tion and reusability of the catalyst make its utilization for
the large scale synthesis of cyclic ester.
Acknowledgements The authors gratefully acknowledge SAIF, NEHU
A plausible mechanism for the BV oxidation with H2O2
as oxidant is proposed on the basis of available literature
for the TEM analysis.
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Efficient Baeyer–Villiger Oxidation Catalysed by Silver Nanoparticles Stabilized on Modified…
15. Rogatis LD, Cargnello M, Gombac V, Lorenzut B, Montini T,
References
Fornasiero P (2010) ChemSusChem 3:24
16. Kaneda K, Mitsudome T, Mizugaki T, Jitsukawa K (2010) Mol-
ecules 15:8988
1. Narayanan R (2012) Green Chem Lett Rev 5:707
2. Chen WW, Nguyen RV, Li CJ (2009) Tetrahedron Lett 50:2895
3. Zeng T, Chen WW, Cirtiu CM, Moores A, Song G, Li CJ (2010)
Green Chem 12:570
17. Chrobok A, Baj S, Pudlo W, Jarzebski A (2009) Appl Catal A
366:22–28
18. Piscopo CG, Loebbecke S, Maggi R, Sartori G (2010) Adv Synth
Catal 352:1625–1629
4. Zhou X, Lu Y, Zhai LL, Zhao Y, Liu Q, Sun WY (2013) RSC Adv
3:1732
19. Corma A, Nemeth MT, Renz M, Valencia S (2001) Nature
412:423–425
5. Karimi B, Gholinejad M, Khorasani M (2012) Chem Commun
48:8961
20. Corma A, Navarro MT, Renz M (2003) J Catal 219:242–246
21. Hara T, Hatakeyama M, Kim A, Ichikuni N, Shimazu S (2012)
Green Chem 14:771
6. Borah SJ, Das DK (2016) Catal Lett 146:656–665
7. Paul M, Pal N, Mondal J, Sasidharan M, Bhaumik A (2012) Chem
Eng Sci 71:564–572
22. Saikia P, Sarmah PP, Borah BJ, Saikia L, Saikia K, Dutta DK
(2016) Green Chem 18:2843–2850
8. Otomo R, Kosugi R, Kamiya Y, Tatsumia T, Yokoi T (2016) Catal
Sci Technol 6:2787
23. Pillai UR, Demessie ES (2003) J Mol Catal A 191:93–100
24. Saito Y, Wang JJ, Smith DA, Batchelder DN (2002) Langmuir
18:2959
9. Ten Brink GJ, Arends IW, Sheldon RA (2004) Chem Rev
104:4105–4123
10. Campelo JM, Luna D, Luque R, Marinas JM, Romero AA (2009)
ChemSusChem 2:18–45
25. Lee D, Cohen RE, Rubner MF (2005) Langmuir 21:9651
26. Khan MAM, Kumar S, Ahamed M, Alrokayan SA, AlSalhi MS
(2011) Nanoscale Res Lett 6:434
11. Templeton AC, Wuelfing WP, Murray RW (2000) Acc Chem Res
33:27–36
27. Kushal DB, Pawan JT, Kishor PD, Bhalchandra MB (2010) Catal
Commun 11:1233
12. Rao CNR, Kulkarni GU, Thomas PJ, Edwards PP (2000) Chem
Soc Rev 29:27–35
28. Safaei Ghomi J, Ghasemzadeh M (2013) J Chem Sci 125:1003
13. Borah BJ, Dutta D, Dutta DK (2010) Appl Clay Sci 49:317–323
14. Schröder F, Esken D, Cokoja M, Van den Berg MWE, Lebedev
OI, Tendeloo GV, Walaszek B, Buntkowsky G, Limbach HH,
Chaudret B, Fischer RA (2008) J Am Chem Soc 130:6119–6130
Affiliations
Subrat Jyoti Borah1 · Diganta Kumar Das1
1
* Subrat Jyoti Borah
Department of Chemistry, Gauhati University, Guwahati,
subratjyotiborah@yahoo.in
Assam 781 014, India
Diganta Kumar Das
digkdas@yahoo.com
1 3