Indian Journal of Vol. 54A, July 2015, pp. 843-850

Effective synthesis of quinoxalines over ceria based solid coated on honeycomb monoliths

Venkatesha, b, S Z Mohamed Shamshuddina, b, * & N M Mubarakc aChemistry Research Laboratory, HMS Institute of Technology, NH4, Kyathsandra, Tumkur, Karnataka, India bResearch and Development Center, Bharathiar University, Coimbatore, India cDepartment of Chemical Engineering, University of Malaya, Kuala Lumpur, Malaysia Email: [email protected]

Received 31 January 2015; revised and accepted 22 June 2015

Solid acids of ceria such as sulfated ceria, ceria-zirconia mixed and sulfated ceria-zirconia have been coated on honeycomb monoliths by slurry coating method. The catalyst materials are characterized by NH3-TPD for surface acidity, powder XRD for crystallinity and SEM for morphology. These materials are used as solid catalysts in the condensation reaction of various diamines and diketones to synthesize quinoxalines. In order to optimize the reaction conditions, the effect of reaction parameters such as the catalyst type, nature of the solvent, reaction temperature, nature of the catalyst and number of reaction cycles on the yield of the quinoxaline product has been studied. A correlation between the surface acidity and the catalytic activity is observed. The catalyst materials have also been prepared in their powder forms by impregnation method and their catalytic activity is compared with that of their honeycomb coated forms. The ceria based solid acids coated on honeycomb monoliths are economical, efficient and eco-friendly in quinoxaline synthesis.

Keywords: Catalysts, Solid acids, Quinoxalines, Honeycomb monoliths, Ceria, Sulfated ceria, Ceria-zirconia, Zirconia

Quinoxalines are a class of benzo-heterocycles enhanced after modification with suitable anions or having a wide range of biological activities, which cations17. Further, the physico-chemical properties, have made them important as pharmacologically structural and catalytic properties of metal active compounds1. Quinoxalines also play a vital role such as ceria can be modified by using a proper in the synthesis of many insecticides, fungicides, catalyst carrier by adopting various synthesis and herbicides, antibiotics, etc 2-9. Generally, quinoxalines post-synthesis routes. are synthesized by the condensation of a substituted Cordierite (Mg2Al4Si5O18) honeycomb monolith diamine (e.g. o-phenylene diamine) and substituted plays a vital role as catalyst carrier for heterogeneous dicarbonyl compound (e.g. benzil) in presence of catalysts. Monolith supports are uni-body structures homogenous acid catalysts such as sulfuric composed of interconnected repeating cells or acid, phosphoric acid, etc. However, the use of channels18. Catalysts (ceria based composites; homogeneous catalysts require neutralization and Ce1-xMxO2-δ) coated with honeycomb monoliths are separation from the final reaction mixture leading to a widely used in automotive applications which involve series of environmental problems related to the use of gas phase reactions such as combustion of volatile high amounts of solvents and energy as well. organic compounds, automotive applications, ozone Therefore, it is of interest to investigate the possibility abatement in aircrafts and selective reduction of 19 of replacing homogeneous catalysts by highly NOx, etc . Catalysts coated on HC monoliths have efficient heterogeneous solid acid catalysts in such advantages over powder catalysts due to high active industrially important reactions. surface area. A small amount of the catalyst loaded on Some of the solid acid catalysts employed for the the monoliths is highly effective and gives easy synthesis of quinoxalines include tungstophosphoric separation and complete recovery of the catalysts acid10, zeolites11, montmorillonite K-1012, MCM-4113, from the reaction mixture. The use of honeycomb heteropoly acid14, Amberlyst-1515, etc. Ceria is an monoliths as carriers especially for solid catalysts interesting material since it is amphoteric (having would be a very cost effective and eco-friendly both acidic and basic sites) and shows redox process and will also be an interesting initiative step properties16. The acidity or basicity of ceria could be in the field of liquid phase organic synthesis. 844 INDIAN J CHEM, SEC A, JULY 2015

Keeping in view the industrial importance of solution was coated on a wash coated HC by the quinoxalines and ceria based solid acid catalysts dipping and drying process using a muffle furnace coated on honeycomb (HC) carriers, in the present maintained at 400 °C. Dipping and drying steps were article the synthesis of quinoxalines over solid carried out 5-7 times such that ~0.05 g of the active 2- acids such as CeO2, SO4 /CeO2, CeO2-ZrO2 and catalyst was coated on the honeycomb. A higher 2- SO4 /CeO2-ZrO2 coated on HCs is reported. The amount of catalyst cannot be coated on a honeycomb solid acids were coated on HCs by slurry coating since it may lead to the formation of multilayer method and characterized for their surface acidity catalyst coating. Further, due to increase in the by NH3-TPD, crystallinity by PXRD and morphology thickness of the inner walls of the HC, the reactant by SEM techniques. Quinoxalines were synthesized molecules may be blocked from entering into the by the condensation of various 1,2-dimaines and holes of the HC. A lower amount is compensated 1,2-ketones over these solid acids. The ceria based by its higher efficiency due to good mass-transfer solid acids were also synthesized in their powder characteristics18. forms and their catalytic activity has been compared Ceria-zirconia mixed oxide was prepared in a with that of their HC coated forms. In order to obtain similar way by using ceric ammonium nitrate and the highest possible yield of the quinoxaline product, zirconyl nitrate. the reaction conditions were optimized by varying the 2- 2- Preparation of (SO4 /CeO2) and (SO4 /CeO2-ZrO2) catalyst type, the solvent, reaction temperature and A known amount of previously prepared hydrated nature of the catalyst. The effect of reactivation ceria (3.0 g) was taken in a beaker to which and reusability of ceria solid acid on the yield of 6 M H2SO4 (2 mL) was added and the resulting quinoxaline derivatives was also investigated. mixture was made into a fine slurry. The slurry was then coated on a wash coated HC by dipping Materials and Methods and drying method by using a muffle furnace Cordierite honeycomb monoliths having dimensions maintained at 400 oC. (height = 1.20 cm, diameter = 2.50 cm and hole = 0.2 cm) Sulfated ceria-zirconia was prepared in a similar were supplied by Shreya ceramics, Baroda, India. way by using previously prepared ceria-zirconia Ceric ammonium nitrate, zirconyl nitrate and sulfuric (3.0 g) mixed oxide and 6 M H SO (2 mL). acid were supplied by Loba Chemie India Ltd., India. 2 4 All the catalytic materials were also prepared in Diamines and diketones were from either Merck or their powder forms by impregnation method. The Sigma Aldrich India Limited. catalytic materials (both HC coated and powder Preparation and characterisation of catalytic materials forms) were calcined at 550 oC for 5 h before their The HCs were wash-coated with metal oxides such use as solid acid catalysts. The catalytic materials as alumina or zirconia before coating the active are abbreviated as CeO2 (C), CeO2-ZrO2 (CZ), 2- 2- catalysts. It was reported that before coating an active SO4 /CeO2 (SC) and SO4 /CeO2-ZrO2 (SCZ). catalyst, if the bare honeycomb is wash coated with The catalytic materials (both HC coated and simple metal oxides such as alumina or zirconia, the powder forms) were characterized by NH3-TPD active catalyst can form an adherent coating on the method for the measurement of surface acidity, honeycomb surface20. Wash coating with zirconia was Powder XRD (PXRD) patterns were recorded on a preferred as it increases the surface area and gives the X-ray diffractometer (Philips X’pert) using CuKα HC support material a better interaction with the (λ = 1.5418 Å) and SEM images were recorded on active catalysts when compared to other metal JEOL-2010. oxides20. For wash coating with zirconia on a bare Catalytic activity HC, 1.5 g of zirconyl nitrate was dissolved in 50 mL All the solid acids were evaluated for their catalytic of deionized water. The resulting solution was coated activity in the condensation reaction of 1,2-diamine, on a HC by dipping and drying in a furnace preheated o 1,2-diketone in presence of a suitable solvent to 400 C 10-12 times till ~0.02 g of zirconia was (Scheme 1). coated on the HC. In a typical reaction, a mixture of 1,2-diamine, Preparation of ceria (CeO2) and ceria-zirconia (CeO2-ZrO2) 1,2-diketone in 1:1 molar ratio, solvent (20 mL) and Known amounts of ceric ammonium nitrate and 0.05 g of the solid acid catalyst (either in HC coated deionised water were taken in a beaker and this or powder form) was taken in a 50 mL specially VENKATESH et al.: SYNTHESIS OF QUINOXALINES OVER CeO2 SOLID ACIDS COATED ON HC MONOLITHS 845

designed reactor and heated over magnetic stirrer at Further, it is well cited in the literature that the ~45 °C. After a stipulated time, the reaction mixture sulfation of pure metal oxides increases the acidity to was cooled and mixed with 10 mL of and a certain extent23. A similar finding has been observed filtered to separate the solid acid catalyst. The filtrate in the present study, i.e., both SC and SCZ were was rota-vapourised to remove the solvents and the found to be highly acidic when compared to their pure solid product was recrystallized from ethanol. forms. An increase in the surface acidity in the case of The product analysis was carried out by measuring SCZ, may be due to the existence of different types of their melting point and by NMR spectroscopic sulphate phases such as CeOSO4 and Zr(SO4)2 as methods. The melting point of the isolated products reported by Putlasudarshan et al.24 Further, it was was recorded in an open capillary tube. 1HNMR observed that pure ceria or pure zirconia consisted and 13CNMR spectra were obtained at 400 MHz NMR of only ‘weak’ and ‘moderate’ acid sites. Whereas, spectrometer (Bruker). CZ consisted of mostly ‘moderate’ acid sites and SC or SCZ consisted of ‘moderate’ as well as ‘strong’ Results and Discussion acid sites.

Characterization of catalytic materials Powder X-ray diffraction (PXRD)

Total surface acidity (TSA) PXRD patterns of solid acids (C, CZ, SC, SCZ) The TSA values of solid acid catalytic materials coated on HC monoliths are given in Fig. 1. (HC coated form) used in the present study are given Pure ceria shows the characteristic reflection at in Table 1. TSA values were found to follow the 2θ (deg.) = 28.5, 33.4, 47.3, 56.2, which corresponds 25 order: SCZ > SC > CZ > Z > C. Among the solid to the fluorite structure of ceria . In the case of acids, pure ceria was found to be least acidic when CZ mixed oxide, reflections due to both fluorite compared to its modified forms. It has been reported structure of ceria and reflections due to tetragonal that the incorporation of ceria into zirconia or vice- phase of zirconia (2θ (deg.) = 30.2, 35.1, 50.4, 60.0) 26 versa increases the acidity of the resulting mixed could be observed . However, reflections due to the monoclinic phase of zirconia were absent. This oxide (i.e., CeO2-ZrO2) to a reasonable extent. A similar observation has been reported by Pengpaniche indicates the structure stabilizing property of ceria on et al.21 and Reddy et al.22 A possible explanation zirconia wherein the tetragonal phase of zirconia is for the enhance acidity may be as follows: The stabilized by ceria. incorporation of zirconium cation in to the ceria As can be seen in the Fig. 1, the PXRD pattern of unit cell or vice versa modifies the surface acid-base CZ and SCZ differ very much indicating a strong sites, as the exposed Ce4+ and Zr4+ ions act as Lewis influence of sulfate ions on ceria-zirconia mixed acid sites. Both CeO2 and ZrO2 exhibit the same metal-oxygen stoichiometry but possess different ionic characters. Ceria is considered to be more ionic than zirconia. The of the mixed oxide varies depending on the charge-to-radius ratio of the cation as well. The Zr4+ ion has an ionic radius of 0.84 Å which is smaller than that of Ce4+ (0.97 Å) and is expected to generate a larger number of acid sites in their mixed form.

Table 1—Acid site distribution and TSA values of solid acid catalysts Catalyst Acid site distribution (mmol/g) TSA (mmols/g) Weak Moderate Strong C 0.11 0.18 0.00 0.29 Z 0.13 0.21 0.00 0.34 CZ 0.13 0.56 0.02 0.71

SC 0.14 0.37 0.22 0.73 Fig. 1—PXRD patterns of HCs coated with solid acids. [# = ceria; SCZ 0.11 0.39 0.29 0.79 o = tetragonal zirconia; v = Zr(SO4)2; × = Zr(OH)2SO4]. 846 INDIAN J CHEM, SEC A, JULY 2015

oxide. In the case of SCZ, in addition to reflections could be observed, which indicates the formation due to fluorite structure of ceria and tetragonal phase of a homogenous solid solution between ceria and of zirconia, reflections due to different types of zirconia. surface zirconium sulfates such as zirconium sulfate Catalytic activity hydroxide [Zr(OH)2SO4] and zircosulfate [Zr(SO4)2] 22 Synthesis of quinoxalines was carried out to were observed . No reflections pertaining to cerium evaluate the activity of the catalytic materials sulphate or bare HC were seen. The PXRD patterns of (Scheme 1). To begin with, the condensation reaction solid acids in their HC coated forms as well as in their between the diamine (o-phenylene diamine; OPDA) powder forms were found to be similar. and diketone (diphenyl ketone; DPK) was carried Scanning electron microscopy (SEM) out in the absence of any catalyst and no product SEM images of HCs coated with solid acids as well was observed up to 4 h. However, about 12% of as bare HC (uncoated) are shown in Fig. 2. These product was observed after refluxing the reaction images show the strong adherence of the active mixture for 24 h. catalyst on the surface of the HC. Also, the coating of When the reactions were carried out in presence of the catalyst is considerably uniform. This information solid acid catalysts (C, CZ, SC, SCZ), the expected obtained from SEM also indicates that the method quinoxaline product (2,3-diphenylquinoxaline) was used for coating the catalytic material was suitable to obtained in higher yield in shorter reaction period. obtain adherent and homogeneous coating of the This study indicates that the condensation reaction catalytic material. In the case of the SEM images of between a diamine and a diketone is a catalytic either C or CZ or SCZ, no distinguishable features reaction.

Fig. 2—SEM images of HCs coated with (a) bare HC, (b) C, (c) CZ and (d) SCZ. (Magnification = 2000x; 10 m).

VENKATESH et al.: SYNTHESIS OF QUINOXALINES OVER CeO2 SOLID ACIDS COATED ON HC MONOLITHS 847

Effect of catalyst 100% selective towards the formation of desired Quinoxaline synthesis was carried out over quinoxaline products. Even though pure C and Z OPDA and DPK in presence of EtOH-water solid acids were 100% selective towards the (3:2) over different HC catalysts and the results formation of quinoxalines, the yield of quinoxalines are given in Table 2. The catalytic activity of the was very poor as they consisted of very few solid acid catalysts used for the present work followed ‘moderate’ acid sites. the order similar to the order of their TSA, i.e., Over CZ catalyst no by-products were formed SCZ > SC > CZ > Z > C. This indicates that the TSA and no discoloration of the catalyst was observed. values of the solid acid catalyst and yield (%) of This can be attributed to the absence of ‘strong acid’ quinoxaline derivative (2,3-diphenylquinoxaline) sites. Since CZ consists of a reasonably high are co-relative. In the case of sulphated oxides such number of ‘moderate’ acid sites, it showed good as SC and SCZ, the yield of the desired quinoxaline activity in quinoxaline synthesis with 100% product is 94% and 98% respectively, which is higher selectivity. Hence, CZ was selected as the solid acid than the yield of quinoxaline product over CZ catalyst catalyst for further optimization studies. (i.e., 89%). However, the selectivity of the desired product (quinoxaline) depends on the strength of the Effect of solvent In order to study the effect of solvent on the yield acid sites on the catalyst material. It has been reported of quinoxalines, the reactions were carried out with that the formation of a quinoxaline product required 27 OPDA and DPK in presence of polar-protic solvents ‘moderate’ acid sites . The same results are reflected such as methanol, ethanol, n-propanol and mixed in the present study. Since sulphated catalysts consist solvents such as ethanol-water, methanol-water and of both ‘moderate” and “strong’ acid sites, in addition n-propanol-water over HC coated with CZ catalyst. to the desired quinoxaline product by-products were It has been reported that the yield of quinoxaline also formed to an extent of 2–6%. Formation of (2,3-diphenylquinoxaline) could be higher in presence by-products other than quinoxaline can be attributed of polar-protic solvents when compared to polar- to the presence of ‘strong’ acid sites in sulphated ceria aprotic or non-polar solvents26. The yield of and sulphated-ceria-zirconia. Moreover, the color of quinoxalines over the solvents such as methanol, the catalyst as well as the color of the reaction ethanol, n-propanol, water, and mixed solvents mixture turned grayish during the course of the such as methanol-water, ethanol-water and reaction over SC or SCZ. The formation of n-propanol-water were 64%, 65%, 69%, 71%, 75%, by-products may be due to the decomposition of 88% and 72% respectively. The yield of quinoxaline either the reactants or reactive intermediates or the was highest (88%) when ethanol-water was used products in to by-products by side reaction. as a solvent. Therefore, sulphated oxides such as sulphated ceria Further, the reactions were carried out by varying and sulphated ceria-zirconia were less selective the ratio of ethanol to water. The order of yield was towards the formation of the desired quinoxaline found to be (i.e, EtOH:water = 1:4,(72%) 2:3,(76%) product when compared to non-sulphated oxides 1:1,(80%) 3:2(89%) and 4:1(91)). The highest yield such as zirconia, ceria, ceria-zirconia which are (%) of quinoxaline was obtained with the EtOH:water ratio of 3:2. However, not much increase in the yield Table 2—Yield (%) and selectivity (%) of quinoxaline derivative over different catalyst materials used in the present study. of quinoxalines was observed when the concentration [React. cond.: React. temp. = ~40 °C; wt. of catalyst = 0.05 g of EtOH in EtOH:water mixture was increased beyond of solid acid; solvent = EtOH:water (3: 2); molar ratio of 3:2. Therefore, the solvent mixture comprising of DPK: OPDA = 1: 1] EtOH:water in the ratio 3:2 was found to be suitable Catalyst Yield of Selectivity of for obtaining highest yield of quinoxaline.

2,3-diphenylquinoxaline (%) quinoxaline (%) Effect of reaction temperature HC coated Powder form The condensation reaction between OPDA and C 41 29 100 DPK was carried out over HC coated with CZ catalyst Z 48 30 100 in presence of EtOH:water (3:2) solvent mixture CZ 89 45 100 in the reaction temperature ranging from 25 °C to SC 94 51 94 60 °C. Although the formation of quinoxaline product SCZ 98 57 98 began at room temperature, the yield (%) of the 848 INDIAN J CHEM, SEC A, JULY 2015

quinoxaline was found to increase with the reaction Table 3—Yield (%) of quinoxaline derivatives over HC coated temperature. The maximum yield of quinoxaline was with CZ solid acid. [React. cond.: React. temp. = 40 °C; wt. of observed when the temperature was increased from the catalyst = 0.05 g of CZ coated on a HC; solvent = EtOH: water z= 3:2; molar ratio of diketone: diammine = 1:1] 30 °C to 40 °C (there was 30% increase in the yield from 69% to 89%). However, when the temperature was No. R1, R2, R3 Reaction Quinoxaline deriv. Yield time (min) (%) increased from either 40 °C to 50 °C or 50 °C to 60 °C, the increase in the yield was only 9% (from 89% to 98%). 1 H, Ph, Ph 10 89 N Comparative catalytic activity of HC coated and powder forms The catalytic activity of solid acids in their HC N coated forms and powder forms were compared in the synthesis of quinoxalines (OPDA+DPK) (Table 2). 2 Cl, Ph, Ph 15 83 When the activity of these two types of catalysts was Cl N observed, the solid acid catalysts coated on HCs were found to be more efficient when compared to their N powder form. Even though the same amount of solid acid 3 NO2, Ph, Ph 75 82 O N N catalyst (~0.05 g) was used either in its HC 2 coated form or in its powder form, approximately N 1.7 fold increase in the yield (%) of quinoxaline (2,3-diphenylquinoxaline) was observed over HC 4 CH3, Ph, Ph 10 85 CH N coated catalysts. This difference in the activity of HC 3 coated catalyst and the powder form of the same catalyst can be explained as follows: In the case of N solid acids coated on HC monolith, the number of Cl channels, their diameter and wall thickness determines 5 H, C6H5Cl, 50 85 the wall density, expressed as cells per square inch C6H5Cl N

(cpsi). This in turn allows the calculation of the N geometric surface area; the sum of the areas of all the channel walls upon which the solid acid catalyst is Cl 6 H, 15 CH3 80 deposited. This leads to one of the most important O C6H5OCH3, N advantages of the HC in that it has a large open frontal C6H5OCH3 area. The lower catalyst loading in case of HC catalyst N

CH3 is compensated by the higher efficiency due to the O 18 good mass-transfer characterization . This may also be 7 H, C6H5Br, 10 75 attributed to the availability of more active sites of the C6H5Br solid acid catalyst on the surface of HC due to homogenous dispersion of the catalyst, which is not possible when the catalyst is used in its powder form. 8 CH , 12 OMe 73 Synthesis of quinoxaline derivatives over CZ catalyst coated on 3 C H OCH , M Ne HC under optimized conditions 6 5 3 C6H5OCH3 After obtaining the optimized reaction conditions N for the synthesis of 2,3-diphenylquinoxaline (Entry 1), OMe the reactions were extended towards the synthesis 9 NO2, 15 72 of different quinoxaline derivatives (Table 3). The C6H5OCH3, reactions were carried out over 0.05 g CZ coated C6H5OCH3 on HC as a catalyst at a reaction temperature of o 40 C and in presence of EtOH: water = 3:2 as 10 H, C6H5F, 10/RT 77 solvent. The data related to the melting point and C6H5F NMR of the quinoxaline derivatives synthesized for the present work are given as Supplementary Data. VENKATESH et al.: SYNTHESIS OF QUINOXALINES OVER CeO2 SOLID ACIDS COATED ON HC MONOLITHS 849

Reactivation and reusability of catalyst materials separation or washing or filtration process. Loss of An ideal catalytic material not only displays active catalyst from the surface of a honeycomb high catalytic activity but also offers inherent is not possible because of strong adherence of the advantages, such as high stability, low deactivation active catalyst on the surface of the wash coated rate, long life and easy regeneration. The reusability honeycomb. of solid acids was investigated. This study reveals that the catalyst coated on HCs The used solid acid catalyst (HC coated with CZ) are more stable and more reusable than the solid acid recovered from the reaction mixture was washed catalyst in their powder forms. with acetone, dried at 120 °C for 2 h and reused for the second reaction cycle in the synthesis Mechanism of acid catalyzed condensation of a diketone and diammine of quinoxaline (2,3-diphenylquinoxaline) under The mechanism involves a solid acid acting as an optimized reaction conditions. Such reactivation and acid catalyst in the protonation of a diketone, and reusability cycles were repeated for 4 times. The also playing a role in promoting the dehydration to reusability of the catalyst in its powder form was also give a carbocationic intermediate (Scheme 2). The reactivated and reused in the similar manner and the comparative results are shown in Fig. 3. As can be seen from the figure that the yield (%) of quinoxaline (6-phenyl-2,3-diphenylquinoxaline) over HC catalyst decreased very slightly when reused for 4 reaction cycles. After the 4th reaction cycle, the HC catalyst coated with CZ was washed with acetone, dried at 120 °C in an oven and calcined at 550 °C for 1 h in a muffle furnace. The thermally reactivated HC catalyst was used for the next 5th and 6th reaction cycles which interestingly showed similar yield (%) of quinoxaline derivative as that of the fresh catalyst. Further, when reusability of CZ in its HC coated form was compared with that of its powder form, the rate of decrease in the activity of CZ catalyst was more in case of its powder form of the catalyst Fig. 3—Reusability of CZ in HC coated and powder forms on the yield (%) of 6-phenyl-2,3-diphenylquinoxaline. [React. cond.: when reused for six reaction cycle. This decrease in o React. temp. = 40 C; wt. of the catalyst = 0.05 g of CZ coated on the activity of powder form of the catalyst can be a HC; solvent = EtOH:water (3:2); molar ratio of DPK:OPDA = attributed to the loss of the powder catalyst during 1:1. Curve 1, HC catalyst; Curve 2, powder catalyst].

850 INDIAN J CHEM, SEC A, JULY 2015

following steps are involved in the mechanism: 3 Hui X J, Desrivot C, Bories P M , Loiseau X, Franck R, (i) Co-ordination of a 1,2- dicarbonyl onto acid site of Hocquemiller B & Figadere, Bioorg Med Chem Lett, 16 (2006) 815. the solid acid, followed by (ii) the nucleophillic attack 4 Shivaji V M , Sastry M N V, Wang C C & Ching-Fa Y, on the carbonyl carbon producing an intermediate (I), Tetrahedron Lett, 46 (2005) 6345. (iii) which on dehydration gives a carbocation 5 Brown D J & Talyor E C, The Chemistry of Heterocyclic intermediate and on elimination of a proton gives the Compounds, (John Wiley & Sons, New Jersey), edited by quinoxaline product. E C Taylor & P Wipf (2004). 6 Seitz L E, Suling W J & Reynolds R C, J Med Chem, 45 (2002) 5604. Conclusions 7 Lee J, Murray W V & Rivero R A, J Org Chem, 62 (1997) Ceria based solid acid catalysts coated on 3874. honeycomb monoliths were found to be efficient 8 Gazit A, Appe H, McMohan G, Chen J A, Levitzki F & catalytic materials in the synthesis of quinoxaline Bohmer D, J Med Chem, 39 (1996) 2170. derivatives. A correlation between the total surface 9 Renault J, Baron M, Mailliet P, Giorgirenault S, Paoletti C & acidity and the catalytic activity of the catalysts was Cros S, Eur J Med Chem, 16 (1981) 545. 10 Kunkuma V, Bethala L A P, Bhongini Y, Rachapudi B N P observed. The solid acid, CeO2-ZrO2 consisting of ‘moderate’ acid sites was found to be highly active in & Patharaju S S P, Eur J Chem, 2-4 (2011) 495. Chinese Chem Lett quinoxaline synthesis with 100% selectivity. 11 Fan L-Y, Wei L, Hua W-J & Li X-X, , 25 2- (2014) 1203. However, over the solid acids such as SO4 /CeO2 and 12 Huang T K, Wang R, Shi L & Lu X X, Catal Commun, 2- SO4 /CeO2-ZrO2 which comprise ‘strong’ acid sites, 9 (2008) 1143. by-products were also formed in addition to the 13 Heravi M M, Hosseini M, Oskooie H A & Baghernejad B, quinoxalines. The solid acid catalysts coated on J Korean Chem Soc, 55 (2011) 235. honeycomb monoliths are very efficient materials 14 Heravi M M, Bakhtiari K, Bamoharram F F & Tehrani M H, as they are easy to prepare, easy to use, easily Monatsh Chem, 138 (2007) 465. removable, reactivable, reusable as compared to their 15 Chari M A, Tetrahedron Lett, 52 (2011) 6108. 16 Lermontov S A, Malkova A N, Yurkova L L, Baranchikov Y E powder forms. & Ivanov V K, Nano Sys Phy Chem Mat, 4-5 (2013) 690. 17 Tanabe K, Solid Acids and Bases (Academic Press, Supplementary Data New York) 1970. Supplementary data associated with this article are 18 Nijhuis T A, Kreutzer M T, Romijn A C J & Kapteijn F J A available in the electron form at http://www.niscair.res.in/ Moulijn, Chem Engg Sci, 56 (2001) 823. jinfo/ijca/IJCA_54A(07)843-850_SupplData.pdf. 19 Heck R M, Gulati S & Farrauto R J, Chem Engg J, 82 (2001) 149. 20 Patil K C, Hegde M S, Rattan T & Aruna H T, Chemistry of Acknowledgement Nano Crystalline Oxide Materials, Combustion Synthesis, The authors are grateful for the financial support Properties and Applications (World Scientific Publishing given by Vision Group of Science and Technology, Pvt. Ltd, Singapore) 2008. Government of Karnataka (GRD-375/2014-15). The 21 Pengpaniche S, Meeyoo V, Ricksomboon T & Bunyakiat K, authors acknowledge SAIF, Indian Institute of Appl Catal A Gen, 234 (2002) 221. Science, Bangalore and St. Joseph’s College, Bangalore 22 Reddy B M, Sreekanth P M, Lakshmanan P & Khan A, for H1NMR and PXRD analysis respectively. The J Mol Catal A, 244 (2006) 1. 23 Sugunan S & Kumaree Seena C R, Indian J Chem, 38A authors are thankful to Indian Institute of Technology (1999) 1123. Madras, Chennai for TPD analysis. The authors also 24 Sudarsana P, Mallesham B, Reddy P S & Reddy B M, thank the authorities of University of Malaya, Kuala J Chem Sci Technol, 3 (2013) 161. Lumpur for SEM analysis. 25 Sunaja Devi K R & Jayashree S, Reaction Kinet Mech Catal, 108 (2013) 183. References 26 Yadav G D & Nair J J, Micropor Mesopor Mater, 33 (1999) 1. 1 Sakata G & Makino K, Heterocycles, 27 (1998) 2481. 27 Rekha M, PhD Thesis, Manganese Oxide/porous Support 2 Sato N, Katritzky A R, Rees C W & EFV Scriven, Catalysts for Organic Transformations - Investigation of Comprehensive Heterocyclic Chemistry II, edited by Their Structure Activity Relationship, (Manipal University, (Pergamon Press, Oxford, UK) 6 (1996) 233. Manipal) August 2013.