Copper incorporated nanorod like mesoporous silica for one pot aerobic oxidative synthesis of pyridines

Article abstract of DOI:10.1016/j.catcom.2014.09.003

Copper incorporated nano-rod like mesoporous silica catalyst was synthesized, characterized by N₂ adsorption, HRTEM-EDX, XRD, AAS, XPS and TPD-NH₃ analyses and applied in the one-pot aerobic oxidative synthesis of highly substituted

Full text of DOI:10.1016/j.catcom.2014.09.003

Catalysis Communications 58 (2015) 97–102
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Catalysis Communications
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Short Communication
Copper incorporated nanorod like mesoporous silica for one pot aerobic
oxidative synthesis of pyridines
Suman Ray a, Biplab Banerjee b, Asim Bhaumik b, , Chhanda Mukhopadhyay
⁎
a,⁎⁎
a
Department of Chemistry, University of Calcutta, 92 APC Road, Kolkata 700009, India
Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 May 2014
Received in revised form 26 August 2014
Accepted 1 September 2014
Available online 16 September 2014
Copper incorporated nano-rod like mesoporous silica catalyst was synthesized, characterized by N2 adsorption,
HRTEM–EDX, XRD, AAS, XPS and TPD-NH3 analyses and applied in the one-pot aerobic oxidative synthesis of
highly substituted pyridines. Both the enhanced surface acidity of copper incorporated silica and the redox
property were essential for pyridine synthesis. Standard leaching experiment proved that the reaction was
heterogeneous.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Mesoporous silica nanorod
Copper incorporated silica
Redox catalyst
Pyridines
1. Introduction
stabilization of metal/metal oxide nanoparticles [9–11]. The nanoscale
porosity of such supports may prevent the formation of large and
Oxidation of organic compounds represents an important approach
towards the expansion of complex molecular structures. However, it
remains a tremendous challenge to develop a quasi-nature catalyst for
green and sustainable oxidation [1]. Oxidation by molecular oxygen as
stoichiometric oxidant offers one of the most environmentally benign
and ideal oxidation processes [2]. However, oxygen from air cannot be
readily utilized for oxidation purposes. This is because organic mole-
cules are in spin paired singlet state (S = 0) whereas oxygen molecule
is in spin free triplet state (S = 1) and the reaction between singlet and
triplet state is forbidden. All reactions of O2 require initial activation
from triplet to singlet state and thus the use of a catalyst becomes
inevitable. The traditional catalysts for catalytic oxidation can be
classified into the following categories: (a) supported precious metals
[3,4]; (b) supported metal oxides and non-precious metals [5]; and
(c) mixtures of metal oxides and precious metals [6]. Compared to
precious metal/metal oxides, transition metal oxides are more abundant
and less expensive [7]. However, the catalytic efficiency of bulk metal
oxide is seriously restricted because of its low surface area. One way
to circumvent this problem is to disperse the metal oxide particles
onto supports with high surface area. If the metal oxide nanoparticles
can be confined within a nanoporous host material, this may restrict
the size to which the metal oxide nanoparticles can grow [7,8]. Mesopo-
rous silicas have been considered as the most suitable hosts for the
catalytically inactive particles [12].
It has been noticed that silica and some of the support materials like
alumina and zirconia exhibit a significant amount of surface acidity in
the copper based catalysts [13]. Previously, silica supported copper has
been utilized as an acid catalyst in the Biginelli, Mannich, different
multicomponent reactions and catalytic transformation of benzyl alcohol
[13–16]. Besides the modification of acidity, the presence of multivalent
transition metal cations in the framework also creates isolated redox cen-
ters, which are suitable for their application as heterogeneous oxidation
catalysts. Magnificent results have been achieved in transition metal in-
corporated silica catalyzed oxidation of benzyl alcohol, selective oxidation
of cycloalkanes, oxidation of phenol and cyclohexanol, etc. [17,18].
In the present context, we have synthesized the copper incorporated
nanorod like mesoporous silica catalyst and exploited both its surface
acidity and the redox property in the one pot aerobic oxidative synthe-
sis of pyridines using molecular O2 as the stoichiometric oxidant
(Scheme 1).
2. Experimental
2.1. Materials and instrumentation (see supporting information)
2.1.1. Preparation of copper incorporated silica nanorod
390 mL water and 400 mL MeOH were taken together in a 1 L open
beaker fitted with a magnetic stirrer. Then 3.52 g CTAB was added at
room temperature (30–35 °C) and stirred for 30 min. After a clear
solution was obtained, tetraethylorthosilicate (TEOS) was added drop
⁎
Corresponding author.
⁎⁎ Corresponding author. Tel.: +91 33 23371104.
E-mail addresses: msab@iacs.res.in (A. Bhaumik), cmukhop@yahoo.co.in
(C. Mukhopadhyay).
http://dx.doi.org/10.1016/j.catcom.2014.09.003
1566-7367/© 2014 Elsevier B.V. All rights reserved.

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S. Ray et al. / Catalysis Communications 58 (2015) 97–102
O
Ar1
O
20 mg
Cu/SiO2
O2
R1
R1
Ar1
H Ar2
NH4OAc
EtOH
4h reflux
N
Ar2
O
O
O
R1 = H, Me
5
4
1
2
3
(1 mmol) (1 mmol) (1 mmol)
(1 mmol)
Scheme 1. Oxidative synthesis of pyridines.
0.010
0.008
0.006
0.004
0.002
0.000
Adsorption
Desorption
(a)
160
140
120
100
80
2.57 nm
(b)
NLDFT Model
Vp = 0.2482 cc/
g
2
Surface area=339.4 m /g
60
0
100
200
300
0.0 0.2 0.4 0.6 0.8 1.0
Relative pressure P/P
Pore width ( )
Å
(
)
Fig. 1. N2 adsorption isotherm and pore size distribution of Cu/SiO2.
wise from a dropping funnel under stirring condition. Next, 142.6 mg
Cu(OAc)2 was added and the stirring was continued for another
5 min. Then 10 mL 0.4 N NaOH solution was added drop wise taking
the time period of 1 h. The stirring was continued for the next 8 h at
room temperature and then aged overnight (12–14 h) at room
temperature. It was filtered, washed thoroughly with deionized
Fig. 2. (a–c) HRTEM images and (d) EDX spectrum Cu/SiO2.

S. Ray et al. / Catalysis Communications 58 (2015) 97–102
99
Fig. 3. Dark-field STEM image of a) Cu/SiO2 and elemental mapping of b) Si, c) O, and d) Cu.
water and dried at 35–40 °C for 5 days. The dry powder was calcined
at 550 °C for 6–8 h under static air.
All compounds were synthesized and characterized by IR, NMR and
CHN analyses. Spectral data of the compounds are given in the
supporting information.
2.1.2. Preparation of pyridines
3. Results and discussions
The reactions were carried out in a three necked round-bottomed
flask equipped with a flow of oxygen (10 mL/min). A mixture of ketone
(1 mmol), 1,3-diketoneone (1 mmol), aldehyde (1 mmol) and ammo-
nium acetate (1 mmol) in ethanol (4 mL) was refluxed for 4 h using
20 mg of the Cu/SiO2 catalyst under a steady flow of O2. After comple-
tion of the reaction, the ethanol was evaporated and the crude product
was taken up in dichloromethane (5 mL) and filtered to separate the
products as filtrate from the catalyst (residue). The solvent (dichloro-
methane) was evaporated in rotary evaporator and the crude product
was further purified by silica gel column chromatography [10% EtOAc/
90% petroleum ether (60–80 °C)].
3.1. Characterization of catalyst
3.1.1. Nitrogen adsorption analysis
The isotherm shown in Fig. 1 (panel a) is a typical type IV
isotherm with the presence of hysteresis loop, which indicates the
presence of mesopores in the material [19]. The BET surface area
calculated from this isotherm is found to be 339 m2 g−1. The
estimated pore volume is 0.248 ccg−1. Corresponding pore size
distribution employing the non-local density functional theory
(NLDFT) model is shown in panel b. Estimated pore dimension for
0.10
(a)
(b)
0.08
0.06
0.04
0.02
0.00
-0.02
50 100 150 200 250 300 350 400 450 500
Temperature (°C)
Fig. 4. (a) Powder XRD pattern and (b) TPD-NH3 profile of Cu/SiO2.

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S. Ray et al. / Catalysis Communications 58 (2015) 97–102
3.1.2. HRTEM–EDX analysis and EDX mapping
Representative TEM images show that the sample possesses porous
nanorod like particle morphology. The dimension of the pores is 2–4 nm
as seen throughout the specimen grid (Fig. 2, panels a–c). These data
match well with the pore dimension obtained from N2 sorption analysis.
Chemical characterization of the material was carried out by EDX
analysis. EDX analysis reveals that three elements, \O, Si and Cu, exist
in the material (Fig. 2, panel d). Furthermore almost homogeneous
distribution of copper on silica was confirmed by EDX-mapping
(Fig. 3) [21].
3.1.3. Powder XRD
The formation of copper oxide within the silica matrix was
confirmed by the powder XRD pattern of the material (Fig. 4, panel a).
Several diffraction peaks can be indexed to the diffraction planes
(110), (002), (111), (202), (020), (202), (113), (311), and (113) for
CuO in the monoclinic phase [22].
3.1.4. TPD-NH3 result
Surface acidity of the Cu/SiO2 catalyst was determined by tempera-
ture programmed desorption of ammonia (TPD-NH3). TPD-NH3 profile
(Fig. 4, panel b) indicates the presence of two major peaks centered at
68 °C and 107 °C, corresponding to two weak acid sites in the material.
However, the total acidity calculated from this TPD profile is consider-
ably high and amounts to 2.73 mmol g−1. The high surface acidity
could be responsible for high catalytic activity of this material [23].
Fig. 5. XPS binding energy profile for Cu2p in Cu/SiO2.
the sample is found to be 2.57 nm, which is in line with metal grafted
mesoporous silica based materials synthesized by using CTAB as the
template [20].
Cl
OMe
NO2
NO2
O
O
O
O
H
H
H
H
N
N
N
N
5d (86%)
5c (87%)
5a (92%)
5b (95%)
S
OMe
NO2
Cl
Br
Br
NO2
O
O
O
O
H
H
H
H
N
N
N
N
5f (95%)
5g (86%)
5h (92%)
5e (94%)
S
NO2
Cl
Br
CN
NO2
Cl
66
O
O
O
O
CH3
CH3
CH3
H
N
N
N
N
5j (85%)
5l (90%)
5i (83%)
5k (86%)
OMe
Br
O2N
O
H
N
N
5m (80%)
5n (87%)
OMe
Scheme 2. Structures of synthesized pyridines.

S. Ray et al. / Catalysis Communications 58 (2015) 97–102
101
Ar1
O
O
O
Acid (Cu/SiO2)
R1
R1
Ar2
H
catalyzed formation of
Ar1
(NH4)2CO3
O
Ar2
NH2
enaminoketone
and chalcone
O
H
O
6
7
1
2
3
4
Acid catalyzed
Michael addition
reaction
Ar1
Ar1
O
O
Ar1
O
Acid catalyzed
H
R1
R1
cyclization
-H2O
R1
Ar2
Ar2
N
H
Ar2
N
H
NH
2O
OH
9
10
8
Cu/SiO2
O2
O
Ar1
R1
Ar2
N
5
Scheme 3. Mechanism for the synthesis of pyridines.
3.1.5. AAS and XPS analysis
The acidity of Cu/SiO2 was crucial for the cyclization but not
sufficient to affect acid sensitive moieties like heteroaryl ketones and
methoxy-substituted aryl aldehydes (Scheme 2). Sterically bulky
2-acetylfluorene reacted quantitatively under mild condition. Despite
steric hindrance and ring tension, cyclic ketones also reacted smoothly.
For precursors 2 and 3 bearing either electron-donating or electron-
withdrawing substituents on the aromatic ring, all the reactions
proceeded very efficiently to provide the corresponding pyridines.
Atomic absorption spectroscopic analysis of the catalyst provides the
accurate bulk content of Cu in the sample. Estimated Cu content was
found to be 19.12 wt.%, suggesting very high loading of CuO in silica
matrix. The XPS profile (Fig. 5) of the sample suggests the presence of
Cu2p3/2, Cu2p1/2 and their corresponding satellite peaks, which are the
characteristics of Cu(II) species bound at the silica surface [24].
3.1.6. Catalysis
3.1.7. Plausible mechanism
The reaction was first tried with different homogeneous acids like
HCl, H2SO4, HClO4, TfOH, PTS and AcOH in the presence of an additional
oxidation catalyst (copper acetate) under a flow of oxygen (tables given
in supporting information). Good to excellent conversion was achieved
with these acids; however, they required repeated work-up, neutraliza-
tion of strong acids and extensive chromatographic purification.
Ultimately the isolated yields were not satisfactory. Different heteroge-
neous acid catalysts like TiO2, SiO2, and CeO2 were also used. However,
their acidity was not sufficient for good yields of the product. Compared
to these catalysts copper incorporated silica (Cu/SiO2) catalyst showed
best isolated yield of pyridine. Therefore, Cu/SiO2 was chosen both as
an acid catalyst and as an oxidation catalyst for obtaining best yield of
pyridine.
It is well known that introduction of copper within silica network
produces enhanced surface acidity in comparison to pure silica [13].
The surface acidity of silica material is crucial for the condensation
reactions producing 1,4-dihydropyridine and the redox property of
the catalyst is responsible for oxidation of 1,4-dihydropyridine to
pyridine. It is thought that the reaction proceeds via a cascade of
condensation reactions (Scheme 3). At first, dimedone undergoes an
acid catalyzed condensation with ammonia to form enaminoketone
(6). Ketone also condenses with aldehyde to form chalcone (7). These
two intermediates then undergo an acid catalyzed Michael addition
type reaction to afford 8. Subsequent acid catalyzed cyclization and
dehydration give the 1,4-dihydropyridine (10). The cyclization of the
intermediate 8 is initiated through protonation of its carbonyl oxygen
O
Ar1
O
Ar1
O
20 mg
SiO2
H
H
O2
EtOH
4h reflux
Ar1
CH3
NH4OAc
Ar2
N
H
Ar2
N
Ar2
O
O
H
O
5
10
4
1
2
3
Trace amount
(1 mmol) (1 mmol) (1 mmol)
Major product
(1 mmol)
(10a) Ar1 = 4-Br, Ar2 = 4-NO2 (67%)
(10b) Ar1 = 3-NO2, Ar2 = 4-Cl (65%)
Scheme 4. Synthesis of 1,4-dihydropyridines (10).

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S. Ray et al. / Catalysis Communications 58 (2015) 97–102
by the Brönsted acidic sites of the Cu/SiO2 catalyst or coordination with
the Lewis acidic sites of Cu/SiO2. The electrophilicity of the corresponding
carbonyl carbon is thereby increased. Concomitant intramolecular
nucleophilic attack by the nearby NH2 group leads to cyclization product
(9) which undergoes dehydration to give 1,4-dihydropyridine (10). This
1,4-dihydropyridine after being oxidized by molecular O2 in the presence
of the Cu/SiO2 catalyst produces the target pyridine molecule (5).
In order to fully establish the role of the catalyst, the above reac-
tion was carried out without any catalyst in ethanol under reflux.
Practically no product was formed in that case. Again, 65–67% yield
of 1,4-dihydropyridine was obtained along with trace amount pyri-
dine when the reaction was carried out with pure silica without in-
corporation of Cu metal in the presence of oxygen (Scheme 4). The
trace amount of pyridine probably resulted from the aerial oxidation
of 1,4-dihydropyridine. Interestingly, when the reaction was repeated
with the Cu/SiO2 but in the argon atmosphere, no pyridine was formed
but the 1,4-dihydropyridine was formed in excellent yield. Therefore,
both the enhanced surface acidity and the redox behavior of the catalyst
were necessary for the formation of pyridine in good yield.
Acknowledgment
We acknowledge CSIR for fellowship (SR).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.09.003.
References
[1] J.A. Jiang, C. Chen, J.G. Huang, H.W. Liu, S. Cao, Y. Fei Ji, Green Chem. 16 (2014)
1248–1254.
[2] P. Hu, Q. Wang, Y. Yan, S. Zhang, B. Zhang, Z. Wang, Org. Biomol. Chem. 11 (2013)
4304–4307.
[3] Z. Qu, S. Shen, D. Chen, Y. Wang, J. Mol. Catal. A Chem. 356 (2012) 171–177.
[4] C.B. Zhang, H. He, K. Tanaka, Appl. Catal. B Environ. 65 (2006) 37–43.
[5] X.F. Tang, Y.G. Li, X.M. Huang, Y.D. Xu, H.Q. Zhu, J.G. Wang, W.J. Shen, Appl. Catal. B
Environ. 62 (2006) 265–273.
[6] C.Y. Li, Y.N. Shen, M.L. Jia, S.S. Sheng, M.F. Adebajo, H.Y. Zhu, Catal. Commun. 9
(2008) 355–361.
[7] J. Taghavimoghaddam, G.P. Knowles, A.L. Chaffee, J. Mol. Catal. A Chem. 358 (2012)
79–88.
[8] G. Laugel, J. Arichi, M. Molière, A. Kiennemann, F. Garin, B. Louis, Catal. Today 138
(2008) 38–42.
3.1.8. Leaching test and recycling
[9] A. Martinez, C. López, F. Márquez, I. Diaz, J. Catal. 220 (2003) 486–499.
[10] D. Yin, W. Li, W. Yang, H. Xiang, Y. Sun, B. Zhong, S. Peng, Microporous Mesoporous
Mater. 47 (2001) 15–24.
[11] Y. Ohtsuka, Y. Takahashi, M. Noguchi, T. Arai, S. Takasaki, N. Tsubouchi, Y. Wang,
Catal. Today 89 (2004) 419–429.
[12] A.Y. Khodakov, R. Bechara, A. Griboval-Constant, Appl. Catal. A Gen. 254 (2003)
273–288.
[13] M. Dixita, M. Mishraa, P.A. Joshia, D.O. Shah, Procedia Eng. 51 (2013) 467–472.
[14] G. Kour, M. Gupta, S. Paul, R., V.K. Gupta, J. Mol. Catal. A Chem. 392 (2014)
260–269.
[15] Z. Li, X. Ma, J. Liu, X. Feng, G. Tian, A. Zhu, J. Mol. Catal. A Chem. 272 (2007) 132–135.
[16] S.M. Inamdar, V.K. More, S.K. Mandal, Tetrahedron Lett. 54 (2013) 579–583.
[17] J. Andas, F. Adam, I. Ab. Rahman, Appl. Surf. Sci. (2014) http://dx.doi.org/10.1016/j.
apsusc.2014.07.118.
[18] S. Samanta, N.K. Mal, A. Bhaumik, J. Mol. Catal. A Chem. 236 (2005) 7–11.
[19] M. Nandi, J. Mondal, K. Sarkar, Y. Yamauchi, A. Bhaumik, Chem. Commun. 47 (2011)
6677–6679.
In order to prove that the reaction was heterogeneous, a stan-
dard leaching experiment was conducted. A mixture of dimedone,
4-nitroacetophenone, 4-bromobenzaldehyde, and NH4OAc in ethanol
was allowed to react for 30 min under reflux in the presence of Cu/SiO2.
After 30 minute period the reaction mixture was filtered (hot). The
filtered reaction mixture was then refluxed without the catalyst for
the next 4 h; no further formation of the corresponding product was
observed, indicating that no homogeneous catalyst was involved. XPS
analysis of the filtrate showed that copper was not leached in the
solution. The recycled catalyst could be used at least five times. Only a
slight decrease in yield was due to loss of some amount of catalyst
during filtration (recycling table given in supporting information).
[20] D. Chandra, A. Bhaumik, Ind. Eng. Chem. Res. 45 (2006) 4879–4883.
[21] A.K. Patra, A. Dutta, A. Bhaumik, Chem. Eur. J. 19 (2013) 12388–12395.
[22] D.P. Volanti, D. Keyson, L.S. Cavalcante, A.Z. Simões, M.R. Joya, E. Longo, J.A. Varela,
P.S. Pizani, A.G. Souza, J. Alloys Compd. 459 (2008) 537–542.
[23] S.K. Das, M.K. Bhunia, A.K. Sinha, A. Bhaumik, ACS Catal. 1 (2011) 493–501.
[24] J.P. Espinos, J. Morales, A. Barranco, A. Caballero, J.P. Holgado, A.R. Gonzalez-Elipe, J.
Phys. Chem. B 106 (2002) 6921–6929.
4. Conclusion
Therefore, this protocol highlights the combined effects of both the
enhanced surface acidity and the redox property of copper incorporated
silica catalyst for the development of new eco-compatible strategy for
heterocycle synthesis.