Synthesis, Characterization and Catalytic Activity of Ce
1Mg
xZr
1-xO
2(CMZO) Solid Heterogeneous Catalyst for the Synthesis of 5-Arylidine Barbituric acid Derivatives
Sandip B. Rathod, Anil B. Gambhire, Balasaheb R. Arbad, and Machhindra K. Lande*
Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad (M.S.), 431004, India
*E-mail: mkl_chem@yahoo.com
Received August 24, 2009, Accepted December 17, 2009
A series of Ce1MgxZr1-xO2 (CMZO) mixed metal oxide with different molar ratio were prepared by simple co-preci- pitation method. The prepared materials were tested for their catalytic activity performance using Knoevenagel condensation of various aromatic aldehydes with barbituric acid under solvent-free condition in microwave. The best catalytic activity was obtained with CMZO (1:0.6:0.4). The synthesized materials were characterized by using XRD, FT-IR, SEM-EDS techniques.
Key Words: Co-precipitation method, CMZO, Knoevenagel condensation, Solvent-free, Microwave
CMZO (1:0.6:0.4) 450 W, 3 - 5 min H
O
NH NH O
O
O N
H NH O
O O
1(a-j) 2 3(a-j)
R R
Scheme 1. Synthesis of 5-aryledine barbituric acid catalyzed by CMZO solid heterogeneous catalyst
Introduction
Barbituric acid and its derivatives are associated with a num- ber of biological activities such as antibacterial, hypotensive and sedative.1 They are also used as hypnotic and anesthetic agent.2 As a result of their importance from a pharmacological, industrial and synthetic point of view, there has been increasing intrest in the development of efficient methodologies for the synthesis of 5-arylidine barbituric acid via Knoevenagel con- densation reaction. Generally they are synthesized by condens- ing the barbituric acid with aldehydes under conventional reflux conditions in aqueous medium.3
Many synthetic methods4,6 for the preparation of these com- pounds have been reported and uses variety of catalysts such as aminosulphonic acid,7 NH4OH/AcOH, K-10 clay, silica gel, basic alumina, NaCl, KSF-Clay, KSF-NaCl,6 triethylbenzyl- ammonium chloride.8 Many of these methods involved expen- sive reagents, higher temperature and lower yields. However, the research continues to develop efficient and versatile method for the synthesis of 5-arylidine barbituric acid.
Organic synthesis in the absence of solvent is a powerful tool for the creation of structurally diverse molecules, due to their special selectivity, the easy set-up and work-up, is of great in- terest.9 Moreover, solvent free reactions sometimes are faster, taking just a contact with each other. This aspect, coupled with the lower overall cost of running a reaction without solvent and no specially needed equipment, could became a dicisive factor in industry. Recently, organic reactions under solvent free condition have attracted chemist’s interests, particularly from the view point of green chemistry.
The application of microwave irradiation to the combinatorial chemistry becomes a powerful tool in accelerating the pace of library synthesis.10-14 Domestic Microwave oven is most popu- larly used in the synthesis, because of its low cost and ready availability. However, specially fabricated mono-mode micro- wave reactors provide homogeneous heating, temperature con- trol and more importantly improved safety features. Major aim of this integrated technology is to exploit high degree of mole- cular diversity and high throughput organic synthesis to rapid
access greatly expanded drug-like compound collection without tedious or time-consuming processes.15,16
In recent years, the use of heterogeneous catalysts has receiv- ed considerable intrest in various disciplines including organic synthesis. Synthetic organic routes followed by using hetero- geneous catalysts have advantages over their counterparts in which, used catalyst can be easily recycled. Recently, mixed metal oxide as a solid heterogeneous catalyst would be an encouraging alternative owing to its ecofrindlyness and easy synthesis. The use of mixed metal oxide based catalyst offers several advantages, such as active over a wide range of tem- perature and more resistant to thermal excursions. A mixed metal oxide represents one of the most important and widely employed categories of solid catalyst, either as active phase or supports. A metal oxide and mixed metal oxides utilizes both acid-base and redox properties and constitutes the largest family of catalyst in heterogeneous catalysis.17-20 In the field of catalysis metal oxide and mixed metal oxide have been extensively used as a catalyst for various organic transformations reactions such as, oxidation reactions,21-23 dehydrogenation and condensation reaction,24,25 epoxidation reactions,26 photocatalytic reaction27,28 etc. Among the various metal oxide and bimetallic mixed metal oxides of CeO2, ZrO2 and MgO are widely used as catalyst and catalyst supports in some organic transformation reactions.29,30
In the present work, we have prepared a series of mixed metal oxides containing Cerium (IV), Mg (II), Zr (IV) metals with different molar ratio by simple co-precipitation method (Table 1) and decided to investigate, their catalytic activity in the synthesis of 5-arylidine barbituric acid derivatives.
Table 1. Series of Ce1MgxZr1-xO2 mixed metal oxide prepared by co- precipitation method
Entry Mixed
metal oxide Molar
ratio
1 ZrO2 1:0
2 CeO2:ZrO2 1:1
3 CeO2:MgO:ZrO2 1:0.2:0.8
4 CeO2:MgO:ZrO2 1:0.4:0.6
5 CeO2:MgO:ZrO2 1:0.6:0.4
6 CeO2:MgO:ZrO2 1:0.8:0.2
7 CeO2:MgO 1:1
8 MgO 1:0
Table 2. Knoevenagel condensation of 4-Cl-benzaldehyde and barbi- turic acid catalyzed by Ce1MgxZr1-xO2 at 450 W in microwave.
Entry Mixed
metal oxide Molar ratio
At 450 W in Microwave Yield (%)a Time (min)
1 ZrO2 1:0 35 10
2 CeO2:ZrO2 1:1 40 15
3 CeO2:MgO:ZrO2 1:0.2:0.8 45 15
4 CeO2:MgO:ZrO2 1:0.4:0.6 55 10
5 CeO2:MgO:ZrO2 1:0.6:0.4 94 5
6 CeO2:MgO:ZrO2 1:0.8:0.2 62 10
7 CeO2:MgO 1:1 50 15
8 MgO 1:0 60 10
aYield refer to isolated products.
Table 3. Knoevenagel condensation catalyzed by CMZO (1:0.6:0.4) solid heterogeneous catalyst at 450 W in microwave
Entry Ar Time
(min) Yielda,b (%)
M.P. (oC)
Found Lit.
3a C6H5 3 90 264 - 266 263 - 265[6]
3b 3-ClC6H4 3 94 272 - 274 274 - 278[7]
3c 4-ClC6H4 3 94 (94,94)c 298 - 300 304 - 308[7]
3d 3-NO2C6H4 4 92 252 - 254 248 - 250[7]
3e 4-OHC6H4 3 94 > 320 > 350[7]
3f 4-(CH3)2NC6H4 5 90 281 - 282 282 - 284[7]
3g 4-OCH3C6H4 3 92 306 - 308 307 - 309[7]
3h 4-CH3C6H4 3 90 281 - 283 282 - 284[7]
3i 2,4-Cl2C6H3 4 92 268 - 269 269 - 271[7]
3j 4-FC6H4 3 94 295 - 296 -
aYield refer to isolated products; bAll compounds are known and their phy- sical and spectroscopic data consistent with those of authentic samples;
cYield after consecutive cycles.
Experimental Section
Chemicals. All the chemicals used were of synthesis grade reagents (Merck) and obtained from commercial suppliers and used as such. All products are known compounds and their physical data; 1H NMR spectra were essentially identical with those of authentic samples.
Catalyst preparation. A series of Ce1MgxZr1-xO2 mixed oxide were prepared by simple co-precipitation method. An aqueous solution containing the requisite quantities of ammonium ceric nitrate, magnesium nitrate and zirconyl nitrate, were prepared separately by using deionized water and mixed together with constant stirring followed by the addition of 20 mL 5% poly- ethylene glycol (PEG-400) as structure directing agent. This solution was hydrolyzed with 1:1 aqueous ammonia with vigo- rous stirring until the solution reached to (pH = 9). A yellowish precipitate was formed and the precipitate was allowed to settle down in an electric oven at 60 oC for 24 h. The resulting preci- pitate was filtered and washed with deionized water and dried at 120 oC for 12 h. Finally the dried powders were calcined at 500 oC for 1 h in air atmosphere. All pure single and mixed oxides were prepared by following same procedure.
Catalyst characterization. The prepared mixed metal oxides were characterized by XRD, FT-IR, SEM-EDS techniques. The X-ray powder diffraction patterns of catalyst were recorded on Bruker 8D advance X-ray diffractometer using CuKα radiation of wavelength = 1.54056 Ao. The IR spectra were recorded on FT-IR spectrometer (JASCO-FT-IR/4100) Japan, using dry KBr as standard reference in the range of 4000 - 500 cm-1. To study the morphology of CMZO (1:0.6:0.4) the SEM analyses were carried out with JEOL; JSM-6330 LA operated at 20.0 kV and 1.0000 nA. The elemental compositions of metal in CMZO (1:0.6:0.4) catalyst were examined using energy dispersive spectrophotometer (EDS).
Microwave method: General procedure for the synthesis of 5-arylidine barbituric acid derivative. All chemicals used in this study were commercially available and used without further puri- fication. In a typical reaction procedure an aromatic aldehyde (1 mmol) and barbituric acid (1 mmol) and 0.2 gm of CMZO (1:0.6:0.4) catalyst were taken in a 50 mL of beaker and well mixed with the help of glass rod and irradiated at 450 wt in the domestic microwave oven for 3 - 5 min. The progress of reaction was monitored by TLC [n-hexane:ethylacetate (7:3)], after com-
pletion of reaction, the reaction mixture washed with n-hexane followed by cold water to remove excess of aldehydes and any unreacted barbituric acid. The mixture was then placed in ethanol and heated to dissolve the product. After filtration of the catalyst, the solvent was evaporated. A solid was obtained which was recrystallized from ethanol to give the desired product in excel- lent yield. The authenticity of the products was established by comparing their melting points and 1H NMR spectra with data in the literature.
Conventional method: General procedure for the synthesis of 5-arylidine barbituric acid derivative. In a typical reaction reaction procedure and aromatic aldehyde (1 mmol) and barbi- turic acid (1 mmol) and 0.2 gm of CMZO (1:0.6:0.4) and 10 mL ethanol were taken in 100 mL round bottom flask. The reaction mixture was reflux at 60 - 70 oC for appropriate time mentioned in Table 3. The progress of reaction was monitored by TLC [n-hexane:ethylacetate (7:3)]. At the end of reaction the reaction mixture was cooled at room temperature and filtered to recover the catalyst. Filtrate was poured on ice-cold water; the solid product was obtained, filtered the product and recrystallized from ethanol. The authenticity of the products was established
a b c d e f g h
20 30 40 50 60 70 80
2θ h
g f
e d
c b a
Figure 1. XRD patterns of series of, (a) pure ZrO2 (b) CeO2-ZrO2
(1:1), (c) CeO2-MgO-ZrO2 (1:0.2:0.8), (d) CeO2-MgO-ZrO2 (1:0.4:0.6), (e) CeO2-MgO-ZrO2 (1:0.6:0.4), (f) CeO2-MgO-ZrO2 (1:0.8:0.2), (g) CeO2-MgO (1:1), (h) pure MgO calcined at 500 oC.
144 1065
11572
3423 1012
14251672
3424
h
1044
1573
3422 111114211571
3427
g f
1442 1118
14201572
3396
e d
1011
14241572 1120
14271574
3423
c
14221576
3433
b
3389
a
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1) h
a b c d e f g
Figure 2. FT-IR spectrum of series of (a) pure ZrO2, (b) CeO2-ZrO2
(1:1), (c) CeO2-MgO-ZrO2 (1:0.2:0.8), (d) CeO2-MgO-ZrO2 (1:0.4:0.6), (e) CeO2-MgO-ZrO2 (1:0.6:0.4), (f) CeO2-MgO-ZrO2 (1:0.8:0.2), (g) CeO2-MgO (1:1), (h) pure MgO calcined at 500 oC.
by comparing their melting points and 1H NMR spectra with data in the literature.
3c: 1H NMR (DMSO-d6, 400 MHz) δ 7.54 (d, J = 8.6 Hz, 2H), 8.08 (d, J = 8.6 Hz, 2H), 8.25 (s, 1H), 11.27 (s, 1H), 11.41(s, 1H).
3d: 1H NMR (DMSO-d6, 400 MHz) δ 7.64 ~ 8.32 (m, 4H), 8.90 (s, 1H), 11.36 (s, 1H), 11.15 (s,1H). 3e: 1H NMR (DMSO-d6, 400 MHz) δ 6.88 (d, J = 8.1 Hz, 2H), 8.21 (s, 1H), 8.33 (d, J = 8.1 Hz, 2H), 10.82 (s, 1H), 11.12 (s,1H), 11.25 (s, 1H). 3f: 1H NMR (DMSO-d6, 400 MHz) δ 3.12 (s, 6H), 6.80 (d, J = 9.2 Hz, 2H), 8.15 (s, 1H), 8.42 (d, J = 9.5 Hz, 2H), 10.90 (s, 1H) 11.03 (s, 1H) 3g: 1H NMR (DMSO-d6, 400 MHz) δ 3.86 (s, 3H), 7.15 (d, J = 8.3 Hz, 2H), 8.43 (d, J = 8.3 Hz, 2H), 8.24 (s, 1H), 11.04 (s, 1H), 11.17 (s, 1H).
Results and Discussion
We report for the first time, synthesis of a series of mixed metal oxides of CMZO with different molar ratio using simple co-precipitation method (Table 1) and planed to investigate their catalytic activity. The structural and morphological appearances of these materials were examined by XRD, FT-IR, SEM-EDS etc techniques.
XRD analysis. In order to understand the phase symmetry of the prepared powdered materials a systematic study on the XRD was undertaken. Fig. 1(a) shows XRD pattern for pure ZrO2 powder, calcined at 500 oC for 1 h in air, sharp peaks were obtained at (2θ = 30o, 35o, 50o, 54o, 60o, 62o) corresponding to the tetragonal, structure of ZrO2. Fig. 1(h) shows XRD pattern for pure MgO powder, calcined at 500 oC for 1 h in air, sharp peaks were obtained at (2θ = 36o, 42o, 62o, 74o, 78o) corres- ponding to the planes (111), (200), (220), (311), (222) indicates the cubic structure of MgO and found to be highly crystalline
in nature. Fig. 1(b-f) shows the XRD patterns of series of mixed oxides with different molar ratio for Ce1MgxZr1-xO2 system.
The peaks were apparently broad, due to small particle size and poor crystallinity. The highly intense and sharp peaks were obtained at (2θ = 28o, 33o, 47o, 51o, 57o, 59o, 77o) corresponding to the planes of (111), (200), (220), (221), (311), (222), (322), (331) indicates the formations of cubic solid solutions.31 The XRD phases identified are in good agreement with Alifants and Kim etals.31,32
FT-IR analysis. The FT-IR spectra of series of Ce1MgxZr1-xO2
calcined at 500 oC for 1 h in air are shown in Fig. 2. From the IR spectrum it was observed that in each sample (a-h) the peak in the range 3200 - 3500 cm-1 appeared due to the hydroxyl group adsorbed on the surface of mixed metal oxide and which helps to generate the Bronsted acidic sites. Similarly in all the cases the peaks in the range of 1500 - 1600 and 1410 - 1446 cm-1 due to the deformation mode of surface hydroxyl group.33 As can be noted from the IR spectrum of each sample, the peaks in the range of 1010 - 1079 cm-1 is assigned due to the M-O-M bonding (M = Ce, Mg, Zr).
SEM-EDS analysis. To evaluate morphology of CMZO (1:
0.6:0.4) catalyst as shown in Fig. 3. The SEM micrograph of CMZO catalyst shows homogeneous agglomeration of particles and was irregular in shape and agglomerated, with an average primary particle size > 1 µm. It was also observed that there is a generation of some porosity, which may be due to addition of 5% PEG as structure directing agent, which certainly allows the alteration in the particle size and morphology. As can be noted from the micrograph of CMZO, all the three metal oxides are strongly interacted and highly dispersed on their surfaces.
The elemental composition of CMZO (1:0.6:0.4) catalyst are shown in Fig. 3 represents the elemental distribution of Ce,
H O
+ NH
HN O O
O H H M+
H O
NH HN
O O O
H
M+ O H
O
NH HN
O O
HO H2O
NH NH O
O O
+ H2O+
O
M+ O
M+ O R
R
R
R H
M = Metal
Scheme 2. Plausible/probable mechanism catalyzed by CMZO solid heterogeneous catalyst
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
KeV
2400 2200 2000 1800 1600 1400 1200 1000 800 600 400 200 0
Counts
Figure 3. SEM and EDS patterns of CMZO (1:0.6:0.4) calcined at 500 oC.
Mg, Zr, O as 60.10 wt % (22.53 a%), 1.27 wt % (2.75 a%), 19.25 wt % (11.09 a%), and 19.38 wt % (63.63 a%).
After understanding the suitability and formation of prepared mixed metal oxides, their catalytic activity studies were under- taken. The catalytic performance was examined using Knoeve- nagel condensation reaction. Initially we performed a reaction of benzaldehyde and barbituric acid using pure and mixed metal oxides under solvent-free conditions using microwave (450 W) with respect to the time and yield of the products. The obtained results were summarized in Table 2. Catalytic activity result reveals that, the pure ZrO2 and CeO2-ZrO2 mixed oxides exhi- bited negligible activity for the synthesis of 5-arylidine barbi- turic acid derivatives. The pure MgO was also employed for the same reaction, but obtained results were not found satisfactory.
Insertion of MgO additive in the mixed oxides increases the catalytic activity of catalyst to result in excellent yield of the products, probably due to involvement of strong acidic and basic sites.34 Interestingly, among these different molar ratio of mixed oxide of Ce1MgxZr1-x O2 (1:0.6:0.4), catalytic material shows very good catalytic activity for the synthesis of various 5-aryl- idine barbituric acids (Scheme 1). Therefore it was further used as a catalyst to prepare the various derivatives of 5-arylidine barbituric acids (Table 3). A variety of different substituted aro-
matic aldehydes possessing an electron donating (-CH3, -OCH3, -OH) and electron withdrawing groups (NO2) offered good yield (90 - 94%) and reactions were completed within 3 - 5 min.
(Table 3). Mechanically, we believe that the CMZO facilitates both acidic and basic sites (M+____ O‒), (M = Metal cation), due to this reason the rate of reaction may enhances and giving rise to excellent yields. The plausible/probable mechanism is ex- plained in Scheme 2.
Inspiring from these results as MgO is the best additive in the mixed metal oxide and shows excellent catalytic activity.
Therefore, we have introduced CaO, SrO and BaO as an other additives in the mixed metal oxide and investigated their cataly- tic potency for the synthesis of 5-arylidene barbituric acid.
Surprisingly, we have got excellent results in terms of product yield and slight reduction in reaction time (Table 4). From the observed results, it was proved that, the catalytic activity follows the order CaO < SrO < BaO, that might be due to increase in the basic strength of alkaline earth metal oxides.35 From this investigation we conclude that, not only MgO is the best addi- tive but other II-A group metal oxides also serves as a best additives in the mixed metal oxides and shows good catalytic activity for the synthesis of 5-arylidene barbituric acid within a very short reaction times under solvent-free condition in microwave.
For the comparison purpose the same reaction were per- formed by conventional method (Table 5) under reflux condi- tions at 50 - 60 oC in ethanol medium. It was observed that, longer reaction time and poor yields were found for conven- tional methods. It consumes more energy in comparison with microwave method. In addition to that, microwave method con- sumes less energy provided high to excellent yields (90 - 94%) within extremely short reaction time. Therefore the microwave method is the best methods for the preparation of 5-arylidene barbituric acids derivatives using CMZO solid heterogeneous catalyst. The reusability of the catalyst is important for industrial point of view. Therefore, recovery and reusability of the catalyst was examined. The catalyst was separated, washed it with n- hexane, dried at 60 oC and activated at 120 oC for 1 h before catalytic run. The reusability of catalyst was investigated in the
Table 4. Knoevenagel condensation of 4-Cl-benzaldehyde and barbi- turic acid catalyzed by Ce1MxZr1-xO2 at 450W in microwave (M = Ca2+, Sr2+ and Ba2+)
Entry Mixed
metal oxide Molar ratio At 450 W in microwave Yield (%)a Time (min)
1 CeO2:CaO:ZrO2 1:0.6:0.4 94 2.5
2 CeO2:SrO:ZrO2 1:0.6:0.4 95 2
3 CeO2:BaO:ZrO2 1:0.6:0.4 95 1.5
aYield refer to isolated products.
Table 5. The comparative study for the Knoevenagel condensation of 3a, 3c, 3d, 3h by conventional and microwave methods
Entry Conventional method Microwave method Time (min) Yielda,b (%) Time (min) Yielda,b (%)
3a 55 84 3 90
3c 60 85 3 94
3d 60 84 4 92
3h 60 83 3 92
aYield refer to isolated products; bAll compounds are known and their physical and Spectroscopic data consistent with those of authentic samples.
reaction of 4-chlorobenzaldehyde with barbituric acid for three cycles with almost consistent activity (Table 3, entry 3c).
Conclusion
We have prepared for the first time a series of mixed metal oxides Ce1MgxZr1-xO2 catalytic material by simple co-preci- pitation method and explore its potential uses in the synthesis of 5-arylidine barbituric acid derivatives as a heterogeneous catalyst. Catalytic activity results revealed that, the Ce1Mgx
Zr1-xO2 (1:0.6:0.4) catalyst exhibits excellent catalytic activity for the condensation of various aromatic aldehydes and barbi- turic acid. Most importantly this catalyst facilitates the reaction under solvent-free conditions providing solid supports, with enhancing reaction rate and thereby the excellent yields of the products. We have also compaired the yield obtained using microwave and conventional methods using CMZO solid hetero- geneous catalyst. A conventional method gives poor yield in comparison with microwave methods. Therefore, the microwave method is the best method for the preparation of 5-arylidene barbiturates. Also, not only MgO is the best additive but also other alkaline earth metal oxides are acts as best additive in the mixed metal oxide and exhibits excellent catalytic activity in the order of CaO < SrO < BaO. A simple procedure combined with low toxicity and reusability of the catalysts, makes this method an economic and waste-free chemical process for the synthesis of 5-arylidine barbituric acid derivatives.
Acknowledgments. The authors are grateful to the Head, Department of Chemistry, Dr. Babasaheb Ambedkar Marath-
wada University, Aurangabad for providing the laboratory facility. One of the authors (SBR) is thankful to the university authority for awarding Golden Jubilee Research Fellowship.
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