Mg-V/CO3 hydrotalcite: An efficient and reusable catalyst for one-pot synthesis of multisubstituted pyridines

Article abstract of DOI:10.1007/s11164-014-1890-4

Mg-V/CO₃ hydrotalicite (Mg-V/CO₃ HTlc) as an effective heterogeneous catalyst was synthesized by a simple technique. The catalyst was identified by PXRD, TEM, SEM and BET surface area analysis. The catalyst is fully recyclable and re

Full text of DOI:10.1007/s11164-014-1890-4

Res Chem Intermed
DOI 10.1007/s11164-014-1890-4
Mg–V/CO3 hydrotalcite: an efficient and reusable
catalyst for one-pot synthesis of multisubstituted
pyridines
•
Suresh Maddila Ramakanth Pagadala
•
•
•
Surjyakantha Rana Shravankumar Kankala
Sreekantha B. Jonnalagadda
Received: 25 August 2014 / Accepted: 4 December 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract Mg–V/CO3 hydrotalicite (Mg–V/CO3 HTlc) as an effective heterogeneous
catalyst was synthesized by a simple technique. The catalyst was identified by PXRD,
TEM, SEM and BET surface area analysis. The catalyst is fully recyclable and reusable
for several runs preserving its high activity. The catalytic activity of Mg–V/CO3 HTlc is
described by synthesis of a series of pyridines in good to excellent yields via a facile
one-pot multicomponent reaction with a shorter reaction time.
Keywords Heterogeneous catalyst Á Multicomponent reaction Á Mg–V/CO3 HTlc Á
Pyridines Á Reusability Á One-pot synthesis
Introduction
Pyridine-based heterocyclics form a major cluster in organic synthesis [1]. They are
constituents of many natural products and pharmaceuticals, which exhibit a wide
range of biological activity [2–4]. Within a long-term research program aimed at
developing pharmaceutically active materials, synthesss of substituted pyridines are
of special importance because of their varied biological activities [5], such as
anticancer [6], antimicrobial [7], antibacterial [8], antifungal [9], anti-inflammatory
[10], and antioxidant activity [11], cytotoxicity [12] and anti-HIV [13]. Thus, the
synthesis of poly substituted pyridine derivatives has fascinated many researchers,
and several approaches have been pursued using catalysts such as MgO [14], LaMnO3
Electronic supplementary material The online version of this article (doi:10.1007/s11164-014-1890-4)
contains supplementary material, which is available to authorized users.
S. Maddila Á R. Pagadala Á S. Rana Á S. Kankala Á S. B. Jonnalagadda (&)
School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Chiltern Hills,
Durban 4000, South Africa
e-mail: jonnalagaddas@ukzn.ac.za
123

S. Maddila et al.
[15], [bmIm]Br [16], [bmIm]OH [17], BF3/MCM-41 [18], K5CoW12O40Á3H2O [19]
and Co/SiO2 [20].
In recent years, multicomponent reactions (MCRs) have been developed as a
powerful strategy in the synthesis of organic [21], agro- [22], combinatorial [23],
bioactive medicinal [24] and heterocyclic molecules [25]. Advantages of MCRs are
wide, including simple protocols, inexpensive reactants and green principles. One-pot
MCRs are often feasible with shorter reaction times, while affording higher chemical
yields than multiple-step syntheses, resulting in conservation of energy and
manpower. Thus, such environmentally benign protocols are opportune and much
desired.
In this context, heterogeneous catalysts with potential for recyclability and
reusability have great scope and will be attractive in one-pot reactions involving
MCRs. For such a role, hydrotalcites and hydrotalcite-like compounds (HTlcs) are
good candidates which have proven records as catalysts and supports, and as ion-
exchangers. Thermal calcination of these compounds at 450 °C leads to the formation
of highly basic mixed metallic oxides with moderate surface area which are potential
catalysts for various base-catalyzed condensation reactions [26]. Relative to
conventional linear syntheses, this eco-friendly quick protocol has benefits such as
cost-effectiveness, atom economy and scope to engineer the surface properties.
In continuation of our research work in the domains of MCRs [27–30], we recently
reported the synthesis procedure for dihydropyridines, using acetanilide, ammonium
hydroxide and aromatic aldehydes in the presence of water at 60 °C and using Zn–V/
CO3 hydrotalcite as the catalyst with good yields [31]. Based on that success, in this
communication we report another reaction for multisubstituted pyridines involving
acetanilide, malononitrile and 11 different aldehydes, using ethanol as solvent, Mg–V/
CO3 hydrotalcite as catalyst and the reaction occurring at room temperature with
further improved yields. We believe that this is the first report of the use of a
heterogenous catalyst to promote a fast MCR protocol for the synthesis of
multisubstituted pyridines at RT with eco-friendly ethanol as solvent (Scheme 1).
Experimental
All chemicals and reagents required for the reaction were of analytical grade and
were used without any further purification. Melting points were determined in open
O
Ar
CN
C2H5
O
CH2(CN)2
O
N
H
Ar CHO
MgVCO3 HTlc
C2H5OH, 45 min, rt
N
H
CH2
H3C
N
O
3a-k
2a-k
1
Ar = C6H5 (3a), 2-OCH3C6H4 (3b), 4-OCH3C6H4 (3c), 2-ClC6H4 (3d),
2-NO2C6H4 (3e), 4-BrC6H4 (3f ),4-N(CH3)2C6H4 (3g),
3h
3i
3j
3k
2-BrC6H4 ( ), Furyl ( ), 4-FC6H4 ( ), 4-CF3C6H4 ( ).
Scheme 1 Synthesis of multifunctional pyridines (3a–k)
123

An efficient and reusable catalyst for one-pot synthesis
capillaries on a Mel-Temp apparatus and are uncorrected. A Bruker AMX 400 MHz
1
NMR spectrometer was used to record the H NMR, 13C NMR and 15N NMR
(400 MHz) spectral values. The CDCl3/DMSO-d6 solution was utilized for this
while TMS served as the internal standard. TMS was further used as an internal
standard for reporting all the chemical shifts in d (ppm). The FT-IR spectrum for the
samples was established using a Perkin Elmer Precisely 100 FT-IR spectrometer at
the 400–4,000 cm-1 area. LCMS spectral data was recorded on a MASPEC low
resolution mass spectrometer operated at 70 eV. The purity of all the reaction
products was confirmed by TLC using aluminum plates coated with silica gel
(Merck Kieselgel 60 F254).
Preparation of catalyst
The double-layered hydroxide Mg–V/CO3 HTlc with a molar ratio of 2.0:1.6:0.4
was prepared by adapting the co-precipitation method described elsewhere [31]
using a constant pH. A mixed metal solution of magnesium and vanadium nitrates
with preferred concentrations was prepared and 0.30 M sodium hydroxide and
0.20 M sodium carbonate were added concurrently to a 500-ml beaker at the rate of
50 ml/h, with constant stirring at RT, at a pH of 11–12.5. The precipitate formation
occurred at 12 h at 65 °C, and the precipitate was separated immediately by
filtration. Next, the precipitate was washed repeatedly with deionized water to
remove the free nitrate ions. The precipitate was dried gradually at 100 °C over 12 h
to afford the Mg–V/CO3 HTlcs. This was further calcined at 450 °C for 4 h.
Characterization of catalysts
A Micromeritics Tristar-II porosity and surface area analyzer was used for estimation
of the pore size, pore volume and surface area of the catalysts. BJH adsorption–
desorption curves were obtained at -196 °C to assess the particulate properties of
the catalyst. All the catalyst materials were degassed by passing nitrogen overnight at
200 °C. A Bruker D8 advance instrument with a Cu K radiation source by
k = 1.5406 was used for the X-ray diffraction data for the catalyst.
A Jeol JEM-1010 electron microscope and JEOL JSM-6100 microscope were
used for TEM and SEM analysis, respectively. In TEM analysis for particles
distribution, the size of 40–60 particles were averaged with standard deviation. An
emission current (100 lA) by a Tungsten (W) filament with 12 kV accelerator
voltage was employed for EDX-analysis of the SEM images. Elemental compo-
sition of the catalyst materials was established by using an inductively coupled
plasma optical emission spectrometer (Optima 5300 DV).
General procedure for the synthesis of pyridines
A mixture of acetoacetanilide (1 mmol), benzaldehyde (1 mmol), and malononitrile
(1 mmol) were dissolved in ethanol (10 ml) and added to the Mg–V/CO3 HTlc
(20 mg) catalyst at RT and stirred continuously for 45 min. TLC was performed to
observe the complete consumption of the starting material in the reaction mixture. A
123

S. Maddila et al.
crude product was afforded upon filtering the reaction mass and subsequent
evaporation under reduced pressure. This crude was then recrystallized by an
EtOAc:Hexane mixture to obtain the pure product (3a–k). The filtered catalyst was
washed with ethanol and dried, being reused for six successive runs.
Results and discussion
With the aim of maximizing the yield and reducing reaction times, in order to
establish the optimum reaction conditions, the reaction was investigated out under
varied conditions of catalysts and solvents. In a pilot experiment, acetoacetanilide
(1), benzaldehyde (2a), and malononitrile were conducted in ethanol without any
catalyst at room temperature (RT), but with no reaction. Even after 48 h under
reflux conditions, no reaction was able to obtain any product (Table 1, entries 1 and
2). A trace amount of the anticipated product was achieved when K2CO3 and NaOH
were used as catalysts in the ethanol media (Table 1, entries 3 and 4), but no
reaction was observed in the presence of strongly basic catalysts, such as Cs/Al2O3
Table 1 Optimization condition for the synthesis of pyridines by Mg–V/CO3 HTlc
O
CHO
Catalyst
CN
C2H5
O
O
N
H
CH2(CN)2
C2H5OH, 45 min, rt
N
H
CH3
H3C
N
O
2a
Catalyst
3a
1
Entry
Solvent
Yield (%)a
1
2b
No catalyst
No catalyst
K2CO3
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
THF
No reaction
No reaction
3
08
4
NaOH
12
5
Cs/Al2O3
V/Al2O3
MgO
No reaction
6
No reaction
7
37
48
54
94
80
78
85
82
8
Mg–Al HTlc
9
Mg–V/CO3 HTlc Uncal
Mg–V/CO3 HTlc
Mg–V/CO3 HTlc
Mg–V/CO3 HTlc
Mg–V/CO3 HTlc
Mg–V/CO3 HTlc
10
11
12
13
14
CH3CN
DCM
DMF
All products were characterized by NMR and mass spectral analysis
a
Isolated yields
b
Under reflux for up to 48 h
123

An efficient and reusable catalyst for one-pot synthesis
and V/Al2O3 with the ethanol solvent (Table 1, entries 5 and 6). With MgO, simple
HTlc (Mg–Al) and uncalcined Mg–V/CO3 HTlc as catalysts in ethanol, 37, 48 and
54 % yields were obtained, respectively (Table 1, entries 7 and 8). Next, we tried to
synthesize the heterocyclic framework using Mg–V/CO3 HTlc as the catalyst with
ethanol as the solvent. To our delight, in just 45 min at RT, a multicomponent
reaction was accomplished successfully and produced the expected substituted
pyridine (3a) selectively and in good yield (94 %) (Table 1, entry 10). The identity
of the product was confirmed by spectral and analytical data (ESI). The HTlc
catalysed reaction was found to proceed almost instantaneously, although about
45 min was taken for completion of the reaction. In the proposed MCR, Mg–V/CO3
HTlc displayed greater efficiency compared to the other catalysts investigated due to
the synergetic effect of the metal. Normally, the basic nature of hydrotalcite is the
crucial factor in its catalytic activity. The surface properties of HTlcs can be
modified by loading the cations of varied properties. Inclusion of V3? into HTlc in
place of Al3? provides acidity and acidic sites on the catalyst surface. An optimal
distribution of the acidic and basic sites due to loading of Mg–V/CO3 mixed oxides
possibly contributed to its enhanced catalytic efficiency, which is evident from the
high yield, selectivity and speed of the reaction achieved in the title reaction.
Taking advantage of the proven performance of Mg–V/CO3 as the catalyst for the
MCR, we then focused on simplifying the protocol for the reaction. Firstly, we
performed screening tests varying the HTlc used. Results of the effect of Mg–V/
CO3 HTlc concentration on the reaction yields of 3a were also presented (see ESI).
With the amount of Mg–V/CO3 HTlc increased from 10 mg to 20 mg, the yield
increased from 75 to 94 % (Table S1, entry 2). However, further enhancement in the
amount of catalyst had only a marginal effect on product yield (ESI, Table S1,
entries 3 and 4), but without the Mg–V/CO3 HTlc, there was no reaction (Table 2,
entry 1). Furthermore, the ethanol as solvent (Table 1, entry 10) displayed
superiority to the other solvents examined THF (Table 1, entry 11), CH3CN
(Table 1, entry 12), DCM (Table 1, entry 13) and DMF (Table 1, entry 14).
The scope of the catalyzed MCR was further explored. Choosing the optimal
conditions for the synthesis of 3a, i.e. 20 mg of Mg–V/CO3 HTlc at RT and ethanol
as solvent (Scheme 1), we employed variious structurally different aldehydes (1b–
k) for the MCR. To our delight, most of the reactions afforded the desired pyridine
derivatives (3b–k) in good yields (82–94 %) with good selectivity and with no by-
products. The results are depicted in Table 2. The MCRs with substrates bearing
electron-donating or electron-withdrawing groups on the aromatic ring proceeded
smoothly and formed the corresponding multisubstituted pyridines in good yields
under the chosen conditions. The structure of the substituted pyridines (3a–k) was
established and confirmed on the basis of elemental analysis and spectral data [1H
NMR, 13C NMR, 15N NMR (GHSQC) and mass]. The details of the product
characterization are presented in the ESI.
Powder X-ray diffractogram (XRD)
The sample crystallinity was examined using PXRD analysis. The PXRD pattern of
calcined MgVCO3 HTlc is shown in Fig. 1. The diffractogram of the catalyst is in
123

S. Maddila et al.
Table 2 Synthesis of multisubstituted pyridines by Mg–V/CO3 HTlc (3a–k)
O
Ar
2a-k
Ar CHO (
CH2(CN)2
)
CN
C2H5
O
O
N
H
Mg-V/CO3 HTlc
C2H5OH, 45 min, rt
N
H
CH3
H3C
N
O
3a-k
1
Entry
Ar (2a–l)
Product
Yield (%)a
1b
2
C6H5 (2a)
3a
3a
3b
3c
3d
3e
3f
No reaction
2a
94
92
90
92
89
90
91
90
92
92
93
3
2-OCH3C6H4 (2b)
4-OCH3C6H4 (2c)
2-ClC6H4 (2d)
2-NO2C6H4 (2e)
4-BrC6H4 (2f)
4-N(CH3)2C6H4 (2g)
2-BrC6H4 (2h)
Furyl (2i)
4
5
6
7
8
3g
3h
3i
9
10
11
12
4-FC6H4 (2j)
4-CF3C6H4 (2k)
3j
3k
All products were characterized by NMR and mass spectral analysis
a
Isolated yields
b
Without catalyst
Fig. 1 XRD spectra of Mg–V/
CO3 hydrotalcite
10
20
30
40
50
60
Degree (2θ)
good agreement in the standard hydrotalcite peaks, the indexes of which correlate
with the reported hydrotalcites [32]. The value of the calculated c-axis corresponds
to three times the thickness of the ‘sandwich’ formed between the two consecutive
brucite-like structures and the interlayer where carbonate ions exist together. The
thickness calculated from the position of the diffraction peak corresponding to the
123

An efficient and reusable catalyst for one-pot synthesis
˚
(0 0 3) plane was recorded in the range of 7.3–7.6 A for carbonate precursors. Other
main peaks corresponding to the plane (0 0 6) were recorded in the range of
˚
3.5–3.8 A for carbonate precursors. The two sharp intense peaks at low diffraction
angles of 23.2° and 26.9° are ascribed to the diffraction by basal planes (006) and
(102) [33]. Furthermore, broad, less intense peaks at higher angles around 31.4,
36.5, 41.4, 45.8 and 60° and indexed to (105), (108), (110), (113) and (200) planes
also confirm the carbonate-containing hydrotalcite structure [34]. Powder diffrac-
tograms and the d-spacing values of mixed metal oxide precursors for MgVCO3
correlate with the ICDD file No. 50-532.
SEM, TEM and ICP–OES analysis
The SEM micrograph (Fig. 2) shows that agglomeration of the metal oxide
particles, which is caused by the calcination of the HTlc precursor and these
aggregates, range from 0.85 to 2.00 lm. EDS analysis of this catalyst showed that
Mg and V are homogenously distributed in the catalyst, and the metal ratio (Mg:V)
is also in agreement with the ICP elemental analysis. The catalyst morphologies as
indicated by the SEM image clearly indicate the homogeneity in shape for the
sample and the high crystallinity. ICP–OES analysis showed that the magnesium to
vanadium metal ratio is 1:3 which a stoichiometric metal ratio of the hydrotalcite.
The TEM micrograph in Fig. 3 shows that the calcined catalyst has a cubic-like
structure, which is a typical vanadate structure. Thee sixe of these cubic planar
structures is 24 3 nm. The selected area diffractions showed that the catalyst is
polycrystalline in nature which is further observed by the XRD-diffractogram.
Surface area analysis
The N2 isotherms of type-IV with a H2 hysteresis loop were used for the catalyst
sample (Fig. 4) [35]. This was symptomatic of their mesoporous structure. A quite
Fig. 2 SEM spectra of Mg–V/CO3 hydrotalcite
123

S. Maddila et al.
Fig. 3 TEM spectra of Mg–V/CO3 hydrotalcite
Fig. 4 N2 adsorption and desorption spectra and pore size distribution of Mg–V/CO3 HTlc
narrow and mono-modal pore size distribution was obtained for the MgV-doped
carbonate mesoporous HTlcs. The relative pressure (P/P0) range of 0.3–0.7 of the
hysteresis loops indicates the fairly small pore size and good homogeneity of the
catalyst [36]. This further indicated that the average pore size was increased with
increasing the loading amount of the carbonate with a specific surface area
2
3
˚
(16.2 m /g), pore size (0.014 cm /g) and total pore volume (71 A). The large
surface areas were obtained for the catalyst sample with high pore volumes due to
the narrow pore size distribution of the catalyst.
123

An efficient and reusable catalyst for one-pot synthesis
100
80
60
40
20
0
1
2
3
4
5
6
Catalytic Runs
Fig. 5 Recyclability of Mg–V/CO3 hydrotalcite catalyst
Reusability of Mg–V/CO3 HTlcs
The facile heterogeneous character of the catalyst was shown by a recycling
experiment using the model reaction. After completion of the reaction, the catalyst
was recovered by filtration, washed with ethanol and dried under vacuum. The
recovered catalyst was reused for six times with a slight loss in catalytic activity
(Fig. 5). The minor loss observed in the catalytic activity after the 6th run could be
due to temporary poisoning by organic impurities or due to minor changes in the
structure and morphology of the catalyst under the operating conditions.
Conclusion
In conclusion, we report an efficient and environmentally benign one-pot multicom-
ponent protocol for the synthesis of multisubstituted pyridines with good atom
efficacy. We have developed a simple, highly efficient and recyclable heterogeneous
catalyst, Mg–V/CO3 HTlc, for MCR protocol at RT, which can be put into practice for
the selective synthesis of multisubstituted pyridines using eco-friendly ethanol. The
main objectives of mild reaction conditions, use of a recyclable catalyst, cost-
effectiveness and an eco-friendly simple method are achieved.
Acknowledgments The authors are grateful to the National Research Foundation of South Africa for
financial support and the University of KwaZulu-Natal for financial support and research facilities.
References
1. L. Racane, S.K. Pavelic, I. Ratkaj, V. Steoanic, K. Pavelic, V. Tralic-Kulenovic, G. Karminski-
Zamola, Eur. J. Med. Chem. 55, 108 (2012)
123

S. Maddila et al.
2. H.C. Kolb, K.B. Sharpless, Drug Discov. Today 8, 1128 (2003)
3. M. Movassaghi, M.D. Hill, J. Am. Chem. Soc. 128, 4592 (2006)
4. R. Leon, J. Marco-Contelles, A.G. Garcia, M. Villarroya, Bioorg. Med. Chem. 13, 1167 (2005)
5. F. Manna, F. Chimenti, A. Bolasco, B. Bizzarri, W. Filippelli, A. Filippelli, L. Gagliardi, Eur. J. Med.
Chem. 34, 245 (1999)
6. S. Zheng, Q. Zhong, M. Mottamal, Q. Zhang, C. Zhang, E. Lemelle, H. McFerrin, G. Wang, J. Med.
Chem. 24, 3369 (2014)
7. G. Jose, T.H.S. Kumara, G. Nagendrappa, H.B. Sowmya, J.P. Jasinski, S.P. Millikan, N. Chandrika,
S.S. More, B.G. Harish, Eur. J. Med. Chem. 22, 288 (2014)
8. H. Foks, D. Pancechowska-Ksepko, A. Ke˛dzia, Z. Zwolska, M. Janowiec, E. Augustynowicz-Kopec,
Il Farmaco 60, 513 (2005)
9. K. Nakamoto, I. Tsukada, K. Tanaka, M. Matsukura, T. Haneda, S. Inoue, N. Murai, S. Abe, N. Ueda,
M. Miyazaki, N. Watanabe, M. Asada, K. Yoshimatsu, K. Hata, Bioorg. Med. Chem. Lett. 20, 4624
(2010)
10. H. Liu, Y. Li, W. Xiang-Ying, W. Bo, H. Hai-Yun, L. Ji-Yan, X. Ming-Li, H. Jun, W. Xiao-Hua, Y.
Li, Bioorg. Med. Chem. Lett. 23, 2349 (2013)
11. Y. Hu, J. Zhang, C. Yu, Q. Li, F. Dong, G. Wang, Z. Guo, Int. J. Biol. Macromol. 70C, 44 (2014)
12. K. Radha, T. Pritam, Y.Y. Han, K.M. Tara, P. Pil-Hoon, N. Youngwha, L. Eunyoung, J. Kyung-Hwa,
C. Won-Jea, C. Heesung, K. Youngjoo, L. Eung-Seok, Eur. J. Med. Chem. 49, 219 (2012)
13. S.E. Metobo, H. Jin, M. Tsiang, C.U. Ki, Bioorg. Med. Chem. Lett. 16, 3985 (2006)
14. R. Murugan, K. Ramamoorthy, S. Sundarrajan, S. Ramakrishna, Tetrahedron 68, 7196 (2012)
15. T. Sanaeishoar, H. Tavakkoli, F. Mohave, Appl. Catal. A Gen. 470, 56 (2014)
16. B.C. Ranu, R. Jana, S. Sowmiah, J. Org. Chem. 72, 3152 (2007)
17. P.J. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. 39, 3772 (2000)
18. M. Abdollahi-Alibeik, A. Rezaeipoor-Anari, Catal. Sci. Technol. 4, 1151 (2014)
19. K. Srinivas, M.V. Chary, V.N.V. Srinivasu, Tetrahedron 63, 13024 (2007)
20. T. Shamim, M. Gupta, S. Paul, J. Mol. Catal. A: Chem. 302, 15 (2009)
21. R.V.A. Orru, M. de Greef, Synthesis 10, 1471 (2003)
22. D.B. Ramachary, S. Jain, Org. Biomol. Chem. 9, 1277 (2011)
23. H. Bienayme, K. Bouzid, Angew. Chem. Int. Ed. 37, 2234 (1998)
24. A. Domling, W. Wang, K. Wang, Chem. Rev. 112, 3083 (2012)
25. B. Jiang, T. Rajale, W. Wever, S.J. Tu, G. Li, Chem. Asian J. 5, 2318 (2010)
26. K. Yamaguchi, K. Ebitani, T. Yoshida, H. Yoshida, K. Kaneda, J. Am. Chem. Soc. 121, 4526 (1999)
27. R. Pagadala, S. Maddila, V. Moodley, W.E. van Zyl, S.B. Jonnalagadda, Tetra. Lett. 55, 4006 (2014)
28. R. Pagadala, S. Maddila, S. Rana, S.B. Jonnalagadda, RSC Adv. 4, 6602 (2014)
29. S. Maddila, S.B. Jonnalagadda, Bull. Chem. Soc. Ethiop. 26, 467 (2012)
30. S. Maddila, P. Lavanya, S.B. Jonnalagadda, Arab. J. Chem. (2013). doi:10.1016/j.arabjc.2013.09.030
31. R. Pagadala, S. Maddila, V.D.B.C. Dasireddy, S.B. Jonnalagadda, Catal. Commun. 45, 148 (2014)
32. N.A. Comelli, M.L. Ruiz, N.A. Merino, I.D. Lick, E. Rodriguez-Castellon, A. Jimenez-Lopez, M.I.
Ponzi, Appl. Clay Sci. 80–81, 426 (2013)
33. F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11, 173 (1991)
34. JCPDS: Joint Committee on Powder Diffraction Standards, International Centre For Diffraction Data,
Swarthmore, PA, 1977
35. S. Maddila, V.D.B.C. Dasireddy, S.B. Jonnalagadda, Appl. Catal. B Environ. 150–151, 305 (2014)
36. V.S.R.R. Pullabhotla, S.B. Jonnalagadda, J. Adv. Oxid. Technol. 11, 445 (2008)
123