Cite this: DOI: 10.1039/c3ra41152d

Condensation reactions assisted by acidic hydrogenbonded hydroxyl groups in solid tin(II)hydroxychloride

Received 10th March 2013,Accepted 26th April 2013

DOI: 10.1039/c3ra41152d

www.rsc.org/advances

Vijaykumar S. Marakatti, Ganapati V. Shanbhag* and Anand B. Halgeri

Tin(II)hydroxychloride is reported as a heterogeneous Brønsted acid catalyst. Sn(OH)Cl was synthesized by a

precipitation method and characterized by XRD, FT-IR, 1H MAS NMR, SEM, TG-DTA and N2 sorption. The

acidity measurements of tin(II)hydroxychloride by FT-IR pyridine adsorption and 1H MAS NMR showed the

presence of Brønsted acidity. The Brønsted acidity can be attributed to strong hydrogen bonding between

the –OH and Cl groups. Sn(OH)Cl showed high activity for important condensation reactions such as the

Prins condensation, Claisen–Schmidt condensation and ketalization. The catalytic activity of Sn(OH)Cl was

compared with that of SnO, SnO2 and Sn2(OH)2O. The catalyst was recycled three times with a negligible

decrease in activity.

Introduction

There has been an increasing interest in recent years indeveloping novel solid acid catalysts and modify them for theselective synthesis of fine and speciality chemicals. The majortypes of heterogeneous acid catalysts include zeolites (ZSM-5,H-Beta), mesoporous materials (MCM-41 and SBA-15), clays(montmorillonite K-10), heteropoly acids (HPW and HSiW),anion modified oxides (WO3–ZrO2, SO4

22–ZrO2), functiona-lized resins (Amberlyst-15) and mixed oxides (SiO2–Al2O3). Theabove foremost catalysts and their modified forms have beenwell applied in a large number of organic transformations. Butstill there is a thirst for the chemist to design and develop newclasses of catalysts which are comparable to the leading ones.It is well known that metal chlorides and their hydroxides areapplied as acid and base catalysts respectively.1,2 However,metal hydroxychlorides, which fall in between metal chloridesand their hydroxides, are not well established as catalysts.Metal hydroxychlorides are well known as minerals found invarious parts of the world. One such example istin(II)hydroxychloride, a mineral known by the name abhur-ite.3 The crystallographic and mineralogical properties oftin(II)hydroxychloride are well understood but its applicationhas been overlooked so far. Its insolubility in water andorganic solvents stimulated our curiosity to study its proper-ties and application as a heterogeneous catalyst.

The Prins reaction is an important and well-known carbon–carbon bond forming reaction in organic synthesis. Thecondensation of an olefin with an aldehyde leads to theformation of 1,3-dioxanes, 1,3-diols and unsaturated alco-

hols.4 The Prins reaction of b-pinene with paraformaldehyderesults in the formation of nopol, an unsaturated primaryalcohol. Nopol finds wide applications in the agrochemicalindustry as a pesticide, and as a fragrance in the soap/detergent industries.5 Nopol was traditionally prepared usinghomogenous catalysts.6 However, these methods suffer fromdrastic reaction conditions, low yield and unwanted sideproducts. To overcome these problems several heterogeneouscatalysts like Sn–MCM-41,7a Sn–kenyaite,7b Sn–SBA-15,7c sul-fated zirconia,7d Fe–Zn metal cyanides,7e Zn–MCM-41,7f

mesoporous iron phosphate,5 ZnCl2 impregnated on mon-tmorillonite7g and Zr–SBA-157h were designed for use in thePrins reaction. Most of the above catalysts indicate therequirement of Lewis acidic sites for the selective synthesisof nopol. The use of a strong Brønsted acid catalyst usuallyresults in isomerized products.8

The condensation of ethyl acetoacetate with ethylene glycolto produce fructone is an example of ketalization.9a Fructonehas an apple scent and is used as a flavouring agent. Theketalization reaction is well catalyzed by acid catalysts likeZSM-5,9a WO3–ZrO2,9b USY9c and silica functionalized –SO3Hgroups.9d The Claisen–Schmidt condensation of 29-hydroxyacetophenone with benzaldehyde is another condensationreaction and results in the formation of flavanone. A largenumber of acid and base catalysts have been applied for thisreaction.10a–c The flavanoids are widely used in the pharma-ceutical industry for their anti-cancer, anti-inflammatory, anti-bacterial and anti-AIDS pharmacological activities.

In the present work, we report, for the first time, thecatalytic properties of tin(II)hydroxychloride. To establish itscatalytic properties, important condensation reactions such asthe Prins condensation, Claisen–Schmidt condensation andketalization have been studied as model reactions (Scheme 1).The uniqueness of tin(II)hydroxychloride as an acid catalyst

Materials Science Division, Poornaprajna Institute of Scientific Research (PPISR),

Bidalur Post, Devanahalli, Bangalore-562110, India.

E-mail: shanbhag@poornaprajna.org; Fax: +9123611836; Tel: +9127408552

RSC Advances

PAPER

This journal is � The Royal Society of Chemistry 2013 RSC Adv.

Publ

ishe

d on

29

Apr

il 20

13. D

ownl

oade

d by

Kor

ea R

esea

rch

Inst

itute

of

Che

mic

al T

echn

olog

y (K

RIC

T)

on 0

2/06

/201

3 05

:09:

05.

View Article OnlineView Journal

was determined by establishing its physicochemical propertiesand comparing it with tin hydroxide and tin oxide.

Experimental

Materials

The chemicals SnCl2?2H2O, ethylene glycol, ethyl acetoacetate,paraformaldehyde, benzaldehyde and toluene were procuredfrom Merck, India. b-Pinene and 29-hydroxyacetophenonewere purchased from Sigma Aldrich, USA. All chemicals wereused directly without further purification.

Catalyst preparation

Tin(II)hydroxychloride [Sn(OH)Cl] was prepared using amethod reported elsewhere.11,12 In a typical catalyst prepara-tion, 25 g of tin(II)chloride dihydrate was dissolved inconcentrated HCl (20 ml) and water (10 ml). The solutionwas stirred until it was clear (step 1). To the solution, 1 : 1 (v/v)ammonia–water was added until the pH of the solutionreached 1.0 (step 2). Then 2 N ammonia solution was addeduntil the pH reached 2.5 (step 3). The precipitate obtained waswell stirred, filtered and washed successively with water,ethanol and diethyl ether. Finally the catalyst was dried at 120uC for 12 h, followed by drying for 1 h at 180 uC to obtainSn(OH)Cl. For the preparation of Sn2(OH)2O, the aboveprocedure was modified: in step 3 the precipitate was obtainedat pH 7.5. Due to the possibility of aerobic oxidation of tin(II),an inert atmosphere was maintained in all of the preparationsteps. The calcination of Sn(OH)Cl at 400 uC resulted in theformation of SnO2 [designated as SnO2-400]. The calcination ofSn2(OH)2O at 300 and 500 uC gave rise to SnO and SnO2

[designated as SnO2-500], respectively. Calcination was per-formed in a muffle furnace under static air for 2 h.

Catalyst characterization

The formation and phase purity of the tin based catalysts weredetermined by X-ray diffractometer instrument (Bruker-D2Phaser) with Cu-Ka source (l = 1.542 Å). The samples were

analyzed in the 2h range of 10 to 55 with a scanning rate of0.02 degrees per second. The FT-IR (ATR-technique–Bruker a-Tmodel) spectra of the tin based catalysts were obtained in therange of 4000 to 500 cm21. TGA and DTA curves were recordedsimultaneously using a Universal V 2.4F-TA analyzer byheating the sample at the rate of 10 uC min21 in flowingnitrogen gas. The 1H MAS NMR spectra of Sn(OH)Cl andSn2(OH)2O were measured using a Bruker DSX-300 wide-boresolid state spectrometer operating at a resonance frequency of300 MHz with a pulse length of 5 ms and a recycle time of 5 s.N2 sorption measurements were carried out at 77 K in a Nova1000 Quantachrome instrument and specific surface areaswere determined by the Brunauer–Emmett–Teller (BET)equation.

Catalytic activity

In a typical reaction procedure, b-pinene (10 mmol), paraf-ormaldehyde (20 mmol), catalyst (0.23 g) and solvent, toluene(5 ml) were added to a 100 ml glass reactor equipped with areflux condenser and a magnetic stirrer. The mixture wasstirred vigorously at atmospheric pressure at the desiredtemperature for a specific time. After the reaction, the productmixture was analysed by gas chromatography (Shimadzu-2041,FID detector) using a RTX-5 column. The products wereidentified by GC-MS using standards. Reaction products werequantified using a multi-point calibration curve through theexternal standard method. Similarly, Claisen–Schmidt andketalization reactions were carried out without solvent withthe required amount of reactants and catalyst as mentioned inTable 2.

Results and discussion

Sn(OH)Cl, Sn2(OH)2O, SnO and SnO2 prepared under differentpH and calcination conditions were characterized by powderXRD. The XRD patterns confirmed the formation of the above

Scheme 1 a) Prins condensation b) Ketalization and c) Claisen–Schmidtcondensation reactions.

Fig. 1 XRD patterns of tin based catalysts synthesized at different pH andcalcination temperatures.

RSC Adv. This journal is � The Royal Society of Chemistry 2013

Paper RSC Advances

Publ

ishe

d on

29

Apr

il 20

13. D

ownl

oade

d by

Kor

ea R

esea

rch

Inst

itute

of

Che

mic

al T

echn

olog

y (K

RIC

T)

on 0

2/06

/201

3 05

:09:

05.

View Article Online

tin compounds by identifying the respective pure crystallinephases.12 (Fig. 1)

The Prins reaction of b-pinene with paraformaldehyde wascarried out in the liquid phase at 90 uC, toluene was used asthe solvent. The catalytic activities of all the tin based catalystsare tabulated in Table 1. Sn(OH)Cl showed high catalyticperformance with 98% conversion of b-pinene and 99% nopolselectivity. On the other hand, SnO and SnO2 showed a lowconversion of y35%. The use of Sn2(OH)2O led to 3.8%conversion and 83% nopol selectivity, much lower comparedto Sn(OH)Cl. FT-IR and solid state 1H MAS NMR of thecatalysts were used to correlate the catalytic activity with theactive sites. An FT-IR pyridine adsorption study was used todetermine the type of acid sites, by the method reportedelsewhere.4 Initially, we assumed that Sn(OH)Cl possessedLewis acidity due to the presence of tin. However, the pyridineFT-IR spectra of Sn(OH)Cl clearly indicated the absence of apeak due to Lewis acidity at y1445 cm21 (Fig. 2). Instead, toour surprise, it showed a peak at 1532 cm21 due to Brønstedacidity. A similar observation was made in the case of

Sn2(OH)2O but with a less intense peak. On the other hand,SnO and SnO2 showed minor Lewis acidity.

These FT-IR results indicate that the active sites ofSn(OH)Cl and Sn2(OH)2O are Brønsted acidic, whereas fortin oxides they are Lewis acidic. The area of the peak at 1532cm21 indicated that Sn2(OH)2O possessed about half theamount of Brønsted acidity compared to Sn(OH)Cl. However,Sn2(OH)2O exhibited very low activity in the Prins reaction(3.8% conversion). This indicated that the strength of the acidsites might be different in Sn(OH)Cl and Sn2(OH)2O. Thepresence of these Brønsted acid sites could be due to thehydrogen bonded protons, which were confirmed by FT-IR and1H MAS NMR studies.

FT-IR spectra of Sn(OH)Cl and Sn2(OH)2O were used tounderstand the difference in the strength of the acid sites. TheFT-IR spectra of Sn(OH)Cl showed broad peaks in the region of3200 to 3600 cm21 due to hydrogen bonded –OH groups, asshown in Fig. 3. The peaks at 975 and 928 cm21 are assignedto the deformation of –OH groups, and the peak at 623 cm21 isdue to Sn–O stretching.13 The peaks at 3571 and 3409 cm21

indicate the presence of non- or weakly hydrogen bonded –OH

Table 1 Catalytic activity of tin catalysts in the Prins reactiona

Catalyst Calcination temperature Surface area Conversion of b-pinene Selectivity for nopol(uC) (m2 g21) (mol%) (mol%)

Blank — — 0 0Sn(OH)Cl 180 8 98 99SnO2-400 400 9 32 94Sn2(OH)2O 180 18 3.8 83SnO 300 13 35 96SnO2-500 500 16 32 92Sn(OH)Cl–R1b — — 91.4 96Sn(OH)Cl–R2b — — 91 97Sn(OH)Cl–R3b — — 88 94

a Reaction conditions: b-pinene = 10 mmol, paraformaldehyde = 20 mmol, solvent toluene = 5 ml, catalyst weight = 0.23 g, T = 90 uC, Time =12 h. b R = Catalyst recycle.

Fig. 2 Acidity measurements of tin based catalysts by pyridine FT-IR. Fig. 3 FT-IR spectra of tin catalysts.

This journal is � The Royal Society of Chemistry 2013 RSC Adv.

RSC Advances Paper

Publ

ishe

d on

29

Apr

il 20

13. D

ownl

oade

d by

Kor

ea R

esea

rch

Inst

itute

of

Che

mic

al T

echn

olog

y (K

RIC

T)

on 0

2/06

/201

3 05

:09:

05.

View Article Online

groups, whereas the band at 3296 cm21 is due to –OH groupsinvolved in strong hydrogen bonding. This difference in thestrength of hydrogen bonding might have weakened the –O–Hbond leading to Brønsted acidity in these catalysts. Similarhydrogen bonded –OH groups were also found in the FT-IRspectra of Sn2(OH)2O and therefore it was difficult todistinguish the difference in the strength of hydrogen-bondingbetween the two.

Hence, further differences in hydrogen bonded protonswere evaluated by 1H MAS NMR. Sn(OH)Cl exhibited threedifferent types of protons at 6.94, 5.43 and 0.12 ppm whereasSn2(OH)2O showed only two peaks at 6.35 and 5.0 ppm (Fig. 4).

The chemical shift region of 3–7 ppm is generally assignedto hydrogen bonded protons.14 The increase in chemical shift(from 6.35 to 6.94 and 5.0 to 5.43) for Sn(OH)Cl compared withSn2(OH)2O can be related to the stronger hydrogen bondingbetween the electronegative chlorine atom and the vicinal –OHgroups. Halogens directly bonded to metals are known to begood hydrogen bond acceptors.15 The strongly polarizedcharacter of the metal–halogen bond results in an enhancedpartial negative charge on the halogen. Such a halogen atomcan attract the hydrogen atom of an –OH group resulting inweakening of the O–H bond. The peak broadening in the 1HMAS NMR spectra of Sn(OH)Cl further confirms that thehydrogen atoms are loosely held due to strong hydrogenbonding. This hydrogen bonding between –OH and Cl couldbe responsible for generating the required amount of Brønstedacidity necessary to catalyze the Prins reaction with almostcomplete conversion.

To gain more insight into the physicochemical properties ofSn(OH)Cl, the thermal and morphological properties wereevaluated by TG-DTA, SEM and N2 sorption techniques. TheTG-DTA profile showed the weight loss in three steps as shownin Fig. 5. The initial weight loss below 210 uC is due todehydration, and the region between 200–400 uC can beattributed to a slow dehydroxylation. The substantial weightloss in the range of 400–550 uC is due to volatilization of tin asSnCl2 and the formation of oxide.12 It is evident that Sn(OH)Clis thermally stable up to around 200 uC and hence can be usedas a catalyst below this temperature. The SEM images of

Sn(OH)Cl showed the presence of thin plates arranged inspherical particles with an average size of 15 mm (Fig. 6). TheN2 adsorption measurements of all the tin based catalystsshowed a low specific BET surface area (,20 m2 g21) as shownin Table 1.

To establish general applicability, Sn(OH)Cl was alsostudied for the ketalization and Claisen–Schmidt condensa-tion reactions. The ketalization reaction of ethyl acetoacetate(EAA) with ethylene glycol was carried out in liquid phase at100 uC. Sn(OH)Cl showed 63% EAA conversion with 98%selectivity for fructone. Sn2(OH)2O and tin oxides were lessactive for this reaction compared to Sn(OH)Cl (Table 2). Thehigher activity of Sn(OH)Cl is due to the higher Brønstedacidity compared with the other Sn catalysts. Solvent freeClaisen–Schmidt condensation of 29-hydroxyacetophenone (2-HAP) with benzaldehyde was carried out at 140 uC under an N2

Fig. 4 1H MAS NMR spectra of a) Sn(OH)Cl and b)Sn2(OH)2O.

Fig. 5 Thermal analysis of Sn(OH)Cl.

Fig. 6 SEM images of Sn(OH)Cl.

RSC Adv. This journal is � The Royal Society of Chemistry 2013

Paper RSC Advances

Publ

ishe

d on

29

Apr

il 20

13. D

ownl

oade

d by

Kor

ea R

esea

rch

Inst

itute

of

Che

mic

al T

echn

olog

y (K

RIC

T)

on 0

2/06

/201

3 05

:09:

05.

View Article Online

atmosphere. This reaction is known to be catalyzed by bothacidic and basic sites, and with hydrogen bonded protons.16

The conversion of 2-HAP and selectivity for flavanone werealmost the same for both Sn(OH)Cl and Sn2(OH)2O after a 10 hreaction (Fig. 7). However, Sn2(OH)2O showed a high initialactivity of 86% after 2 h, whereas Sn(OH)Cl exhibited only 10%conversion after 2 h which gradually increased to 93% at 10 h.This could be due to the presence of the basic –OH groups inSn2(OH)2O which catalyze the Claisen–Schmidt reaction moreeffectively compared with Sn(OH)Cl. SnO gave 68% 2-HAPconversion with 75% flavanone selectivity, whereas SnO2

showed very low activity (11% 2-HAP conversion with 20%flavanone selectivity) (Table 2).

For the leaching test, the Prins reaction was carried outunder the conditions mentioned in Table 1. The reaction wasstopped after 1 h (25% conversion and 99% selectivity) and thehot reaction mixture containing the catalyst was filtered. Tothe filtrate, the required amount of paraformaldehyde wasadded and the reaction was continued up to 10 h. The

conversion remained the same (25%) even after 10 h ofreaction time, indicating that there was no leaching of activesites into the reaction media. This confirmed that theSn(OH)Cl catalyst is truly heterogeneous in nature.

A recyclability study was carried out to check the efficiencyof the catalyst. The catalyst was separated by filtration after thereaction, washed with acetone and dried at 120 uC for 12 hfollowed by activation at 180 uC for 1 h. The dried catalyst wasreused for the next cycle. Sn(OH)Cl exhibited good recyclabilityfor the Prins reaction with minimal decrease in activity after 3recycles. The XRD pattern of Sn(OH)Cl after 3 recycles matchedthat of the fresh catalyst indicating the structural integrity ofthe catalyst (Fig. 8).

Investigations into the application of the Sn(OH)Cl catalystfor several other acid catalyzed organic transformations, andmodifications to improve the catalyst’s properties are inprogress. The evaluation of other metal hydroxychlorides likeZn(OH)Cl, Pb(OH)Cl and Cu(OH)Cl for suitable organictransformations are also under investigation.

Fig. 7 Effect of reaction time on the Claisen–Schmidt condensation reaction. Fig. 8 XRD pattern of Sn(OH)Cl, a) fresh catalyst and b) catalyst after 3 recycles.

Table 2 Catalytic activity of tin catalysts in ketalization and Claisen–Schmidt condensation reactions

Ketalizationa Claisen–Schmidt condensationb

Entry CatalystConversion ofethyl acetoacetate

Selectivity forfructone

Conversion of 29-hydroxyacetophenone

Selectivity forflavanone

Selectivity forchalcone

(mol%) (mol%) (mol%) (mol%) (mol%)

1 Blank 4 98 2 65 352 Sn(OH)Cl 63 98 93 77 233 SnO2-400 5.5 80 13 25 754 Sn2(OH)2O 20 86.5 94 74 265 SnO 6.3 80 68 75 256 SnO2-500 5.2 78 11 20 807 Sn(OH)Cl–R1c 63 96 89 74 268 Sn(OH)Cl–R2c 62 97 87 73 279 Sn(OH)Cl–R3c 63 94 86.3 70 30

a Reaction conditions: ethyl acetoacetate = 12 mmol, ethylene glycol = 24 mmol, catalyst weight = 0.15 g, T = 100 uC, time = 2 h. b Reactionconditions: 29-hydroxyacetopheneone = 5 mmol, benzaldehyde = 10 mmol, catalyst weight = 0.09 g, T = 140 uC, Time = 10 h. c R = Catalystrecycle.

This journal is � The Royal Society of Chemistry 2013 RSC Adv.

RSC Advances Paper

Publ

ishe

d on

29

Apr

il 20

13. D

ownl

oade

d by

Kor

ea R

esea

rch

Inst

itute

of

Che

mic

al T

echn

olog

y (K

RIC

T)

on 0

2/06

/201

3 05

:09:

05.

View Article Online

Conclusions

The present study describes the synthesis, characterizationand application of Sn(OH)Cl as a heterogeneous catalyst. Thecharacterization by FT-IR pyridine adsorption and 1H MASNMR showed the presence of Brønsted acidity in the catalyst.This Brønsted acidity in Sn(OH)Cl is attributed to a stronghydrogen bonding between the –OH and Cl groups. Sn(OH)Clis a novel and highly efficient acid catalyst for the selectivesynthesis of nopol, flavanone and fructone by appropriatecondensation reactions. The higher activity of Sn(OH)Clcompared with Sn2(OH)2O, SnO and SnO2 is due to thepresence of Brønsted acidity. The catalyst is truly hetero-geneous and can be used in up to 3 recycles with minimaldecrease in activity.

Acknowledgements

Vijaykumar S.M acknowledges Admar Mutt EducationFoundation (AMEF) for a fellowship. The authors are thankfulto NMR Centre IISc, Bangalore for NMR analysis and BIT,Bangalore for the surface area measurements. VSM is thankfulto the Manipal University for permitting this research as a partof the Ph.D programme.

References

1 L. Peng, L. Lin, J. Zhang, J. Zhuang, B. Zhang and Y. Gong,Molecules, 2010, 15, 5258.

2 H. Hattori, Appl. Catal., A, 2001, 222, 247.3 R. Edwards, R. D. Gillard and P. A. Williams, Mineral. Mag.,

1992, 56, 221.4 V. S. Marakatti, G. V. Shanbhag and A. B. Halgeri, Appl.

Catal., A, 2013, 451, 71.5 U. R. Pillai and E. S. Demessie, Chem. Commun., 2004, 826.

6 J. P. Bain, J. Am. Chem. Soc., 1946, 68, 638.7 (a) E. A. Alarcon, L. Correa, C. Montes and A. L. Villa,

Microporous Mesoporous Mater., 2010, 136, 59; (b) A. L. Villade P, E. Alarco‘n and C. Montes de C, Catal. Today, 2005,107, 942; (c) M. Selvaraj and Y. Choe, Appl. Catal., A, 2010,373, 186; (d) S. V. Jadhav, K. M. Jinka and H. C. Bajaj, Appl.Catal., A, 2010, 390, 158; (e) M. V. Patil, M. K. Yadav and R.V. Jasra, J. Mol. Catal. A: Chem., 2007, 273, 39; (f) E.A. Alarcon, A. L. Villa and C. M. Correa, MicroporousMesoporous Mater., 2009, 122, 208; (g) M. K. Yadav and R.V. Jasra, Catal. Commun., 2006, 7, 889; (h) D. M. Do,S. Jaenicke and G. Chuah, Catal. Sci. Technol., 2012, 2,1417.

8 J. Wang, S. Jaenicke, G. K. Chuah, W. Hua and Z. Gao,Catal. Commun., 2011, 12, 1131.

9 (a) M. J. Climent, A. Corma, A. Velty and M. Susartey, J.Catal., 2000, 196, 345; (b) J. Justus, A. Vinu, B. M. Devassy,V. V. Balasubramanian, W. Bohringer, J. Fletcher and S.B. Halligudi, Catal. Commun., 2008, 9, 1671; (c) F. Zhang,C. Yuan, J. Wang, Y. Kong, H. Zhub and C. Wang, J. Mol.Catal. A: Chem., 2006, 247, 130; (d) S. Shylesh, S. Sharma, S.P. Mirajkar and A. P. Singh, J. Mol. Catal. A: Chem., 2004,212, 219.

10 (a) M. T. Drexler and M. D. Amiridis, J. Catal., 2003, 214,136; (b) E. Sujandhi, E. A. Prasetyanto and S. E. Park, Appl.Catal., A, 2008, 350, 244; (c) S. V. Jadhav, K. M. Jinka and H.C. Bajaj, Catal. Today, 2012, 198, 98.

11 J. D. Donaldson, W. Moser and W. B. Simpson, J. Chem.Soc., 1963, 1727.

12 S. Ichiba and M. Takeshita, Bull. Chem. Soc. Jpn., 1984, 57,1087.

13 S. Jouen, B. Lefez, M. T. Sougrati and B. Hannoyer, Mater.Chem. Phys., 2007, 105, 189.

14 M. Muller, G. Harvey and R. Prins, Microporous MesoporousMater., 2000, 34, 281.

15 A. Kovacs and Z. Varga, Coord. Chem. Rev., 2006, 250, 710.16 F. Niu, C. Liu, Z. Cui, J. Zhai, L. Jiang and W. Song, Chem.

Commun., 2008, 2803.

RSC Adv. This journal is � The Royal Society of Chemistry 2013

Paper RSC Advances

Publ

ishe

d on

29

Apr

il 20

13. D

ownl

oade

d by

Kor

ea R

esea

rch

Inst

itute

of

Che

mic

al T

echn

olog

y (K

RIC

T)

on 0

2/06

/201

3 05

:09:

05.

View Article Online