Advanced
Synthesis of Quinoxaline Derivatives at Room Temperature Using Magnetic Material Separated from Coal Fly Ash
Synthesis of Quinoxaline Derivatives at Room Temperature Using Magnetic Material Separated from Coal Fly Ash
Journal of the Korean Chemical Society. 2013. Feb, 57(1): 73-80
Copyright © 2013, Korea Chemical Society
  • Received : November 06, 2012
  • Accepted : December 13, 2012
  • Published : February 20, 2013
Download
PDF
e-PUB
PubReader
PPT
Export by style
Article
Author
Metrics
Cited by
TagCloud
About the Authors
Aashish O. Dhokte
Mahadeo A. Sakhare
Machhindra K. Lande
Balasaheb R. Arbad

Abstract
An efficient synthesis of quinoxalines derivatives is described using magnetic material separated from coal fly ash. Coal fly ash is a waste material generated in huge amount by burning of coal for the generation of electricity in thermal power station. It contains SiO 2 , Al 2 O 3 and magnetic material in significant amounts, from which magnetic material was separated by using magnetic separation method. These separated magnetic material further characterized by XPS, XRD, EDS, FTIR, SEM, TEM and BET techniques. The merits of present method are mild reaction conditions, and also excellent yields and short reaction times.
Keywords
INTRODUCTION
The methodology for the synthesis of heterocyclic compounds represents a powerful approach for rapid building of molecular complexity from potentially simple starting materials. 1 Nitrogen containing heterocyclics are abundant in nature and exhibit diverse and important biological properties. 2 Quinoxaline derivatives are an important class of nitrogen-containing heterocyclics and known to exhibit a wide range of biological activities including anticancer, 3 antiviral 4 and antibacterial. 5 Also these moieties have been found an applications in dyes, 6 as building blocks in the synthesis of organic semiconductors, 7 chemically controllable switches 8 dehydroannulenes 9 anti-inflammatory, anti-protozoal and anti-HIV, 10,11 beside, these are also used in the agriculture field as fungicides, herbicides and insecticides. 12 A number of synthetic strategies have been reported for the synthesis of quinoxaline derivatives. 2,13 The most common method in which 1,2-dicarbonyl compound in acetic acid is refluxing for 2−12 h giving 34−85% yield 14 or in high boiling point solvent such as dimethylsulfoxide (DMSO) 15 in the presence of catalytic amounts of molecular iodine. Improved methods have been developed for the synthesis of quinoxaline derivatives including the Bi-catalyzed oxidative coupling of epoxides and ene-1,2-di-amines, 16 a microwave procedure, 17 MnO 2 , 18 Cerium ammonium nitrate 19 CuSO 4 .5H 2 O, 20 Montmorillonite K-10, 21 polyanilinesulphate salt. 22 However, some of these methods suffer from one or more drawbacks, such as long reaction times, low yields, harsh reaction conditions and tedious work up procedure, therefore, development of an efficient and versatile method is still required.
Coal fly ash is waste product of coal combustion processes in a coal - fired thermal power stations. Large quantities of coal fly ash are produced in electric power plants throughout the world every year. The amount of coal fly ash formed is approximately 500 million tones per year and likely to increase. The global recycling rate of fly ash is only 15%. 23 It is being consumed in the production of constructions materials, in agriculture, metal recovery, in water and atmospheric pollution control, etc. 24 These applications could successful up to some extent to consume part of the huge amount of fly ash. Nevertheless, the search of new applications of the fly ash as either catalyst or as catalyst support material is still ongoing. Literature survey reveals that the fly ash is used as adsorption catalyst for the removal of dyes, 25 heavy metals, 26 etc. Major constituents of coal fly ash are SiO 2 , Al 2 O 3 and Fe 2 O 3 , Fe 3 O 4 . After high temperature combustion, these oxides are formed with high thermal stability. Utilization of fly ash for other industrial applications provides a cost effective and environmentally benign way of recycling this solid waste. Currently researchers have focused on how to improve the capability of fly ash through proper beneficiation techniques in order to increase its catalytic activity. Literature survey reports the catalytic role of activated or modified fly ash for different reactions such as oxidation, 27 dechlorination, 28 condensation and rearrangement reactions. 29 Fly ash is chemically activated by acid and used for esterification 30 etc. Separation of magnetic material from fly ash is carried out by using magnetic separatiom method. Magnetic nanoparticles represent a set of unique building blocks whose size and composition are tunable to meet the requirements for a range of applications including magnetic fluids, catalysis, data storage, biomedicine, and toxic waste remediation. 31 The most common methods used to prepare ferrite complex oxides are co-precipitation, sol-gel method, micro-emulsion, etc. However, major drawback of these required precursors is the high starting costs of the raw materials that results in high production cost and also traditional process. To overcome these difficulties, the best alternative source is the coal fly ash which is the waste product of coal combustion in thermal power station. In the present work, separated magnetic material is characterized and used as catalyst for a simple, selective and environmentally acceptable synthesis of quinoxaline derivatives via two component reaction 1,2-diamine and benzyl ( Scheme 1).
PPT Slide
Lager Image
Synthesis of quinoxaline derivatives using magnetic material as catalyst.
EXPERIMENTAL
- Materials
The coal fly ash was obtained from thermal power station, Parli-Vaijnath, District-Beed, Maharashtra state, India. Other chemicals used were of synthesis grade reagents (Merck) and used as such, without further purification.
- Isolation of Magnetic Material from Fly Ash
The coal fly ash slurry was prepared in clean 500 mL beaker by mixing coal fly ash with deionized water in 1:6 Wt/Vol ratio. The slurry was stirred magnetically using magnetic stir bar for 20−30 minutes, during stirring the magnetic material present in the slurry was attached on the surface of magnetic stir-bar, which was removed and collected several times till the magnetic material was separated completely, which was then dried in an oven at 120 °C for 2 h and used as catalytic material.
. 1 . shows Photographs of a) coal fly ash and b) isolated magnetic material from fly ash.
- Catalyst Characterization
The X-ray diffraction (XRD) patterns of catalysts were recorded on a Bruker D8 advance X-ray diffractometer using Cu-Kα radiation with a wavelength of 1.540 Å Infrared (FT-IR) spectra were recorded on a FT-IR spectrometer (JASCO, FT-IR, Japan) using dry KBr as a standard reference in the range of 500−4000 cm −1 . The scanning electron microscopic (SEM) analyses were carried out with a JEOL JSM-6330 LA operated at 20.0 kV and 1.0 nA. The elemental composition of the metal in the fresh fly ash and in magnetic material was estimated using an energy dispersive spectrophotometer (EDS). Brunnauer Emmett-Teller (BET) surface area was carried out on Quanta chrome CHEMBET 3000. X-ray photoelectron spectroscopy (XPS, ESCA 3000-VG, Uckfield, UK) was used to study the chemical composition of the sample. The morphology of material was also characterized with CM-200 PHILIPS transmission electron microscopy (TEM) operated at 200 kV and resolution, 0.23 nm. 1 H NMR spectra of quinoxaline derivatives were recorded on an 300 MHz FT-NMR spectrometer in CDCl 3 as a solvent and chemical shifts values δ (ppm) are recorded relative to tetramethylsilane (Me 4 Si) as an internal standard.
PPT Slide
Lager Image
Photographs of a) coal fly ash b) isolated magnetic material from fly ash.
- Reaction Procedure for Synthesis of Quinoxaline Derivatives
A mixture of 1,2-diamine (5 mmol), benzil (5 mmol) and catalytic amount of magnetic material (0.1 g) and ethanol (10 mL) was taken in round bottle flask and mechanically stirred for 3−5 min. The reaction progress was monitored using TLC (hexane/ethylacetate (7:3)). When the reaction was complete as indicated by TLC, the reaction mixture was poured on crushed ice. The obtained product was washed with deionized water dried and recrystallized from ethanol to afford pure product.
- Spectroscopic Data of Compound
2,3-diphenylquinoxaline ( 3a ): IR (KBr) ν max /cm −1 3057, 1602, 1442, 1346, 1246, 852, 767, 698; 1 H NMR (CDCl 3 , 300 MHz): δ 8.18 (m, 2H), 7.78 (m, 2H), 7.48 (m, 4H), 7.34 (m, 6H).
6-nitro-2,3-diphenylquinoxaline ( 3b ): IR (KBr) ν max /cm −1 3439, 3063, 1614, 1521, 1446, 1340, 1246, 850, 767, 698; 1 H NMR (CDCl 3 , 300 MHz): δ 9.08 (s, 1H), 8.54 (d, 1H), 8.31 (d, 1H), 7.66 (m, 4H) 7.52 (m, 4H), 7.40 (m, 2H).
RESULTS AND DISCUSSION
- EDS Analysis
Fresh fly ash and magnetic material was analyzed qualitatively and quantitatively by EDS method are shown in ( 1 ). Magnetically isolated material contains an increased amount of iron (33.86%) as compared to fresh fly ash (4.72%).
- XRD Analysis
X-ray diffraction analysis was performed to understand the morphological nature of the fly ash and magnetic material. ( . 2a ) shows the XRD pattern for fresh fly ash. It is found that all the reflection peaks at 2θ = 16°, 20.8°, 23.2°, 26.3°, 30.5°, 33.1°, 36.1°, 39.2°, 40.7°, 42.3°, 45.6°, 50°, 51.9°, 54.5°, 57.4° and 59.8° corresponds to the (011), (−111), (−101), (021), (111), (−131), (030), (−230) (-231) (−222), (−240), (−124), (−233) (−250), (015) and (−311) planes and which indicates the crystalline monoclinic nature of fresh fly ash (JCPDS No. 860680) and lattice parameter a=5.0 b=8.6 c=8.2 Å Whereas ( . 2b ) shows the XRD pattern for magnetic material and It was found that the sharp reflection peaks at 2θ = 33.2°, 39.3°, 46.2°, 65.8°, 75.2°, 79.4°, 91.4°, 107.8°, 111.5° and 138.4° corresponds to the (104), (006), (007), (118), (217), (1110), (402), (407), (413) and (420) planes which indicates highly crystalline hexagonal structure of ferrite type material (JCPDS No. 860550) and a=b=5.035 c=13.74 Å.
Chemical composition of fresh fly ash and magnetic material
PPT Slide
Lager Image
Chemical composition of fresh fly ash and magnetic material
PPT Slide
Lager Image
X-ray diffraction pattern of a) fresh fly ash b) magnetic material.
- Crystallite Size Determination
The crystalline nature and the crystallite size of the sample was analyzed by X-ray diffeaction data. The particle size of the material plays an important role in determining the reactivity of fresh coal fly ash and isolated magnetic material. It was observed that the particles with smaller size exhibited higher reactivity due to availability of higher specific surface area. 32 Generally, the crystallite size was estimated by Debye-Scherrer equation ( T = 0.94λ / βcosθ), 33 where T is the particle size, λ is the wavelength, θ is the diffraction angle and β is (FWHM). The mean crystallites size of fresh fly ash is 50 nm and magnetic material is 10 nm.
- XPS Analysis
Although the XRD pattern of the samples ( . 2b ) clearly show the hexagonal structure, it is very difficult to exclude the possibility of the 𝛾- Fe 2 O 3 phase in the separated magnetic Fe 3 O 4 phase similarity. XPS is one of the most effective way to distinguish the two phases because it is very sensitive to Fe 2+ and Fe 3+ cations. In ( . 3c ) the levels of Fe 2p 3/2 and Fe 2p 1/2 have binding energies 711 and 720 eV respectively. It conform the presence of Fe 3 O 4 phase, 34,35 ( . 3b ) shows binding energy of Si (2p) at 102.9 eV, and ( . 3a and d ) shows absorption peaks of Al (2p) and O (1s) are at 74.84 and 531.9 eV respectively.
- TEM Analysis
The TEM images ( . 4 ) shows the presence of small spherical particles. From TEM images, measured diameter of particles is ~10 nm, which is consistent with the results of XRD analysis. Electron diffraction pattern of magnetic material ( . 4d ) reveals that the sample is polycrystalline, which can be indexed to the hexagonal structure of magnetic material and accord with the XRD result. TEM image also show that the magnetic material is roughly spherical in shape. Usually, spherical shapes are formed because the nucleation rate, per unit area is isotopic at the interface between the Fe 3 O 4 magnetic nanoparticles. 34 Materials are magnetic in nature as well as nano-sized particles are known to have very large surface areas hence, catalytic material will also have high surface energy. Consequently, these fine spherical particles have coated with aluminum silicate network and form aggregated nano-particles. The dark iron center and white aluminum silicate surface of particles are visible.
PPT Slide
Lager Image
High-resolution XPS spectra of magnetic material contains (a) Al (2p) XPS spectra, (b) Si (2p) XPS spectra, (c) Fe(2p) XPS spectra, and (d) O1s XPS spectra.
PPT Slide
Lager Image
TEM images (a, b, c) of magnetic material and (d) Diffraction pattern of magnetic material.
- SEM Analysis
. 5a SEM image of fresh fly ash shows hollow cenospheres, irregularly shaped, mineral aggregates and agglomerated particles. Similar particles were also observed in other reported micrographs. 31 SEM image of magnetic material ( . 5b ) shows sub-angular and spherical particles, increase in spheriodal nature of the magnetic material due to the magnetic separation method.
PPT Slide
Lager Image
SEM images of a) fresh fly ash b) magnetic material.
- BET Analysis
The specific surface area of fresh fly ash and magnetic material are 1.6233 m 2 /g and 105 m 2 /g respectively. Thus magnetic material provided surface for reactant to adsorbed and decrease the time to convert reactant to desired product.
- FT-IR Analysis
The FT-IR spectrum of fresh fly ash in ( . 6a ) shows a broad band at 1060 cm −1 is attributed to Si−O−Si stretching vibrations. ( . 6b ) shows the IR spectrum of magnetic material. The bands absorptions at 1078, 784, 560 cm −1 , 3437 cm −1 and 1634 cm −1 . The 1078 cm −1 is due to the asymmetric stretching of Si−O−Si bands of the SiO 4 tetrahedron. The 784 cm −1 band is composed of the contributions from Si−O−H and Si−O−Fe vibrations, and the band at 560 cm −1 is related with the Fe−O stretching. 36,37 The IR spectrum of magnetic material also shows a broad intense band at 3457 cm −1 due to hydroxyl groups on the catalyst surface and peak at 1634 Cm −1 is attributed to bending mode (δ O−H ). 38 It is assumed that during the process of burning of coal at high temperature there is conversion of Fe 2 O 3 into Fe 3 O 4 which may be coated around aluminosilicate framework. However, these peaks are absent in the FT-IR spectrum of the fresh fly ash ( . 6a ) which do not posses any catalytic activity.
PPT Slide
Lager Image
The FT-IR spectra of a) fresh fly ash b) magnetic material.
- Study of Pyridine Adsorption
The probe of our catalyst acidity and basicity was briefly elucidate by pyridine adsorption, which was carried out taking small amount of catalyst and evacuation was done at room temperature for 24 h and second at 150 °C for 2 h. FTIR spectrum of catalyst obtained with evacuation at room temperature ( . 7a ) shows intensive bands at 1536 and 1444 cm −1 are indicating the presence of Brönsted and Lewis acid sites, respectively on catalyst surface, with which pyridine forms coordination bonds. 39,40 However, a third band at 1482 cm −1 is widely believed to be the result of a combined contribution of Brönsted and Lewis acid sites. 41 The spectrum also shows bands 3517, 3610 and 3710 cm −1 was due to present of bronsted acidic sites or hydroxyl group on catalyst surface. 42,43 However spectrum ( . 7b ) obtained after evacuation at 150 °C for 2 h the band 1536 cm −1 was disappeared and band 1631 cm −1 appear due to the presence of strong proton centers on the catalyst surface, with which pyridine molecules can interact with a creation of Py H + . 3941 The spectrum also shows bands in region 3500−3700 cm −1 was due to present of bronsted acidic sites or hydroxyl group on catalyst surface. Pyridine adsorption study shows that catalyst posses both Lewis acidic sites and Bronsted acidic sites.
- Catalytic Activity Results
The methodology of synthesis of quinoxaline derivatives is reported using very low amount of magnetic material for the reaction of 1:1 mole ratio of benzil and aromatic 1,2-diamine in ethanol and mechanically stirred at room temperature ( 1 ), and quantitative yield of the product was obtained in 3−5 min. The results of this synthetic method are found to be inspiring. In the similar manner, a variety of substituted quinoxaline derivatives were synthesized using various substituted benzil and aromatic 1,2-diamines ( 3 ) to confirm the role of catalyst; however when the same reaction was carried out in the absence of catalyst, the desired product was not appeared even after 1 h. It means that the catalyst plays a crucial role in the success of the reaction in terms of the rate and yield.
Industrial point of view, the role of catalyst is very important therefore we have studied the quinoxaline reaction at various mili mole of benzil and 1,2 diamine and observed that the yield of the desired product is in the range (94 to 90%) ( . 8 ). While performing the laboratory scale reaction of quinoxaline synthesis using variable amount of catalyst, the reaction time and yield of the product remained nearly same, this indicates that if the reaction is to be carried out in large scale it is necessary to use the amount of the catalyst in proportion to the initial amount of the reactants so as to complete the reaction in this stipulated time with the same yield. In order to present the merits of the present work in comparison with previously reported catalysts ( 2 ), it is observed that magnetic material promotes the reaction more effectively than other catalysts. The same reaction in the presence of other catalysts required longer reaction times for completion or the reaction works at higher temperature.
PPT Slide
Lager Image
FT-IR spectrum of pyridine adsorbed on the magnetic material a) at RT and b) at 150 °C.
Comparison of synthesis of quinoxaline with different catalysts
PPT Slide
Lager Image
Comparison of synthesis of quinoxaline with different catalysts
Synthesis of quinoxaline derivatives catalyzed by magnetic materiala
PPT Slide
Lager Image
aReaction conditions: 1 (5 mmol), 2 (5 mmol), 0.1 g magnetic material, 10 mL EtOH mechanically stirred; bIsolated yield
- Reusability of the Catalyst
Catalyst reusability was tested using the o-phenylene-diamine (5 mmol) and benzil (5 mmol) as model reaction. The catalyst was separated by simple filtration during the recrystallization, washed with n -hexane, dried at 60 °C and activated at 120 °C for 1 h and reused for the next run. The results are summarized in ( 4 ), which reveals that, the catalyst could be used at least four times without significant loss in catalytic activity.
PPT Slide
Lager Image
% yield of reaction on various mili mole of benzyl and O-phenyldiamine.
Reusability of magnetic material for quinoxaline reactiona
PPT Slide
Lager Image
aReaction conditions 1 (5 mmol), 2 (5 mmol), 0.1 g magnetic material, 10 mL EtOH mechanically stirred; bIsolated yield
CONCLUSIONS
The present method describes a simple, efficient and eco-friendly method for the synthesis of various quinoxaline derivatives using magnetic material catalyst. Easy synthesis of catalyst, good stability at working temperature, simple handling, convenient work-up procedure, mild reaction conditions, versatility, recyclability inexpensive and eco-friendly nature of the catalyst, made this method a valid contribution to the existing methodologies. The catalytic material is easily obtained from the waste of power plants and proved to be efficient catalyst on the basis of conversion of reactants to products, this observation can be used to understand the cost effectiveness of the reaction.
Acknowledgements
The authors are grateful to UGC New Delhi for financial assistancs through SAP scheme and the Head, Department of chemistry, Dr.Babasaheb Ambedkar marathwada university, Aurangabad for providing the laboratory facility
References
Islami M. R. , Hassani Z. 2008 ARKIVOC 15 280 -
Ghosh P. , Mandal A. 2011 Advances in Applied Science Research 2 255 -
Lindsley C. W. 2005 Bio org. Med. chem. Lett. 15 761 -    DOI : 10.1016/j.bmcl.2004.11.011
Loriga M. , piras S. , Sanna P. , Paglietti G. 1997 Farmaco Societa Chimica Italiana 52 157 -
Seitz L. E. , Suling W. J. , Reynolds R. C. 2002 J. Med. Chem. 45 5604 -    DOI : 10.1021/jm020310n
Jaung J.-Y. 2006 Dyes and Pigments 71 245 -    DOI : 10.1016/j.dyepig.2005.07.008
Dailey S. , Feast W. J. , Peace R. J. , Sage L. C. , Till S. , Wood E. L. 2001 J. Mater. Chem. 11 2238 -    DOI : 10.1039/b104674h
Crossley M. J. , Johnston L. A. 2002 Chem. Commun. 1122 -
Sascha O. , Rudiger F. 2004 Synlett 1509 -
Yb K. , Yh K. , Jy P. , SK K. 2004 Bio Org. Med. Chem. Lett. 14 541 -    DOI : 10.1016/j.bmcl.2003.09.086
Hui X. , Desrivot J. , Bories C. , Loiseau P. M. , Franck X. , Hocquemiller R. , Findadere B. 2006 Bio Org. Med. Chem. Lett. 16 815 -    DOI : 10.1016/j.bmcl.2005.11.025
Mohammad R. I. , Zahra H. O. 2008 ARKIVOC 15 280 -
Woo G. H. C. , Snyder J. K. , Wan Z. K. 2002 Heterocycl. Chem. 14 279 -
Brown D. J. 2004 Quinoxalines: Supplement II. In The chemistry of Heterocyclic Compound; Taylor, E. C.; Wipf, P., Ed. Wiley New Jersey
Bhosale R. S. , Sarda S. R. , Ardhapure S. S. , Jadhav W. N. , Bhusare S. R. , Pawar R. P. 2005 Tetrahedron Lett 45 7183 -
Antoniotti S. , Dunach E. 2002 Tetrahedron Lett 43 3971 -    DOI : 10.1016/S0040-4039(02)00715-3
Zhao Z. , Wisnoski D. D. , Wolkenberg S. E. , Leister W. H. , Wang Y. , Lindsley C. W. 2004 Tetrahedron Lett 45 4783 -
Raw S. A. , Wilfred C. D. , Taylor R. J. K. 2004 Org. Biomol. Chem. 2 788 -    DOI : 10.1039/b315689c
More S. V. , Sastry M. N. V. , Yao C. F. 2006 Green Chem 8 91 -    DOI : 10.1039/b510677j
Heravi M. M. , Taheri S. , Bakhtiari K. , Oskooie H. A. 2007 Catal. Commun. 8 211 -    DOI : 10.1016/j.catcom.2006.06.013
Huang T. , wang R. , Shi L. 2008 Catal. Commun. 9 1143 -    DOI : 10.1016/j.catcom.2007.10.024
Srinivas C. , kumar C. N. S. S. P. , Rao V. J. , Srinivasan P. J. 2007 Mol. Catal. A: Chem. 265 227 -    DOI : 10.1016/j.molcata.2006.10.018
Belardi G. , Massimilla S. , Massimilla L. P. 1998 Resour. Conserv. Cycle. 24 167 -    DOI : 10.1016/S0921-3449(98)00016-0
Blanco F. , Garcia M. P. , Ayala J. 2006 Fuel 85 2018 -    DOI : 10.1016/j.fuel.2006.03.031
Yamada K. , Haraguchi K. , Gacho C. C. , Wongsiri B.P. , Pena M. L. Removal of Dyes from Aqueous Solution by Sorption with Coal Fly Ash In Proceedings of the International Ash Utilisation Symposium 22 -
ohen H. , Lederman E. , Werner M. , Pelly I. , Polat M. Synergetic Effect of Coal Fly Ash as a Scrubber to Acidic Wastes of the Phosphate Industry In Proceedings of the International Ash Utilisation Symposium 20 -
Zhang A. L. , Deng F. F. , Zhou J. T. , Jin R. F. , Ling L. L. , Zhang G. L. 2009 Huan Jing Ke Xue 7 1942 -
Ghaffar A. , Tabata M. 2009 Catal. Lett. 97 35 -    DOI : 10.1007/s11144-009-0020-6
Gopalakrishnan M. , Sureshkumar P. , Kanagarajan V. , Thanusu J. , Govindaraju R. A. 2006 ARKIVOC 13 130 -
Khatri C. , Rani A. 2008 Fuel 87 2886 -    DOI : 10.1016/j.fuel.2008.04.011
Dai Q. , Lam M. , Swanson S. , Rachel Yu R.-H. , Delia J. , Milliron, Topuria T. , Jubert P.-O. , Nelson A. 2010 Langmuir 26 17546 -    DOI : 10.1021/la103042q
Kimura T. 2005 Micropor. Mesopor. Mater. 77 97 -    DOI : 10.1016/j.micromeso.2004.08.023
Henry N. F. M. , Lipson J. , Wooster W. A. 1951 The Interpretation of x-ray Diffraction Photographs Macmillan London
Wensheng L. , Shen Y. , Xie A. , Zhang W. 2010 J. Magnetism and Magnetic Mater. 322 1828 -    DOI : 10.1016/j.jmmm.2009.12.035
Deng X. , Lee J. , Matranga C. 2010 Surface. Sci. 604 627 -    DOI : 10.1016/j.susc.2010.01.006
Cristina F. , Diniz A. P. P. , Viana, N. Mohallem D. S. 2005 J. Sol-Gel Sci. Tech. 35 115 -    DOI : 10.1007/s10971-005-1378-1
Zhao L. , Yang H. , Cui Y. , Zhao X. , Feng S. 2007 J. Mater. Sci. 42 4110 -    DOI : 10.1007/s10853-006-0876-z
Mishra B. G. , Rnga Rao G. 2002 Bull. Mater. Sci. 25 155 -    DOI : 10.1007/BF02706236
Bezrodna T. , Puchkovska G. , Shimanovska V. , Chashechnikova I. , Khalyavka, T. Baran J. 2003 Appl. Surf. Sci. 214 222 -    DOI : 10.1016/S0169-4332(03)00346-5
Akcay M. 2004 J. Mol. Struct. 694 21 -    DOI : 10.1016/j.molstruc.2004.01.010
Seddigi Z. S. 2001 React. Kinet. Catal. Lett. 73 63 -    DOI : 10.1023/A:1013968719834
Herrero J. , Pajares J. , A Blanco C. 1991 Clays and Clay Minerals 39 651 -    DOI : 10.1346/CCMN.1991.0390611
Ravichandran J. , Sivasankar B. 1997 Clays and Clay Minerals 45 854 -    DOI : 10.1346/CCMN.1997.0450609
Hasaninejad A. , Zare A. , Mohammadizadeh M. R. , Karami Z. 2009 J. Iran. Chem. Soc. 6 153 -    DOI : 10.1007/BF03246514
Hossein A. , Oskooie, Majid M. , Heravi K. , Bakhtiari, Taheri S. 2007 Monatshefte Fur Chemie 138 875 -    DOI : 10.1007/s00706-007-0694-2
Niknam K. , Saberi D. , Mohagheghnejad M. 2009 Molecule 14 1915 -    DOI : 10.3390/molecules14051915
Heravi M. M. , Tahrani M. H. , Oskooie H. A. 2006 ARKIVOC 14 16 -
More S. V. , Sastry M. N. V. , Yao C. F. 2006 Green Chem 91 -