Indian Journal of Engineering & Materials SciencesVol. 5, April 1998, pp. 77-82

Transport properties of organic derivatives of zirconium molybdate

Beena Pandit, B Beena & Uma Chudasama*

Department of Chemistry, Faculty of Science, M S University of Baroda, Baroda 390 002, India

Received 25 February 1997; accepted 9 September 1997

An amorphous inorganic ion exchanger zirconium molybdate (ZM) and its organic derivatives havebeen prepared by anchoring ortho and para chlorophenol onto it, termed as ZMoCP and ZMpCPrespectively. The materials have been assessed for chemical stability in several acidic, basic and organicmedia. Further they have been characterised for elemental analysis, thermal analysis and FTIR.spectroscopy. Protonic conductivity in these materials were determined at various temperatures usingSOLARTON SI 1260 Impedance analyser. The protonic conductance values obtained have been discussedand compared for ZMoCP and ZMpCP.

Frequency dependent measurements of the acconductivity have been recognized as an importanttool for the study of ionic properties of the solidsl

-

3. In recent years there has been intense researchaimed at discovering new proton conductors andthe mechanism of conductance in solids. Thisrenewed activity has been driven by the potentialuse of such compounds in fuel cells, sensors, waterelectrolysis units and other electrochemicalproperties.

The underlying science of fast conduction ofelectrical charge through solids by proton (H+),migration is approaching maturity and applicationof proton conducting materials in devices such asfuel cells and electronic displays is being seriouslyinvestigated". Protonic conduction is often presentin systems in which it may not be anticipated suchas-smectite clays conduct protons due to theacidity of hydrated interlayer cations, the operationof the glass membrane (PH) electrode depends onprotonic conduction in soda glass arid protonicconduction is even found in some oxide ceramics.Proton conduction and devices employing it can beconsidered in three temperature ranges--{a) nearambient temperature, (b) medium temperature(150-300°C) and (c) high temperature (>600°C).

Protonic conductors at near ambienttemperatures are hydrated (with H+ migration byprotons hopping between H20 and H30+ or OIrand include materials as diverse as H-forminorganic ion exchangers, heteropoly acids and

*For correspondence

NAFIONS. In inorganic systems, conduction viathe surfaces of particles often dominatesconduction through a sample.

Studies in the medium temperature ranges haveestablished protonic conduction in H30+exchanged and phosphoric acid bonded forms ofknown alkali ion conductors such as NASI CON(Nal_x Zr2 Six P3-x0\2,0 < X < 3) and in anhydroussalts such as M203 doped (M=Y, rare earth)BaCe03 ceramics with a perovskite structure.Protonic conductivity in such materials is a resultof reactions with moisture and/or H2. It isreported" that the glasses in the lithiumoxyflourophosphate system [LiF Li20AI(P03)3] show unusually high conductivity atelevated temperatures.

Inorganic ion exchangers of the type of acidsalts of tetravalent metals have the general formulaM(IV) (HX04)2 nH20, where M = Zr, Ti, Ce, Th,ete: and X = P, Mo, W, As, Sb, etc. They possessstructural hydroxyl groups which are responsiblefor their ion exchange behaviour. These materialshave been extensively studied in the field of

. . 89 d I· 10 Hseparation science" an cata y~IS. owever,attempts have also been made to study thetransport properties of these materialsll

,12. Theycan behave as protonic conductors because of thepresence of structural hydroxyl groups. Whenthese -OH groups are hydrated, the protons caneasily move on the surface, accounting for theirconductivities which depend strongly on relativehumidity, surface area and degree of

78 INDIAN J. ENG. MATER. scr., APRIL 1998

crystallinity 13 • The conductivity in layered a-zirconiumphos-phates has been studied I

2 in detail.Alberti et al.1 have shown that the surfaceconducts protons thousand times faster than theinterior.

A new class of ion exchange materials hasemerged'" having the general formula Zr(03P-R)rnS (R is organic radical and S is solvent molecule)formed due to intercalation of the organic moietywithin the layers of the inorganic matrix such as a-zirconium phosphate (a-ZrP). These compoundshave been in general termed as inorgano-organicion exchangers. Because of the large variety oforganic groups that can be attached to theinorganic backbone, there are more possibilities ofOwing to their protonic conductivity, amorphousor microcrystalline powders of a-ZrP wereinitially mixed to organic binders to prepareh . . b 1516 Aeterogeneous morgano-orgamc mem ranes . .literature survey shows that very fewinvestigations of the protonic conductivity of thesecompounds have been investigated'T'", Protonicconduction of amorphous inorganic ion exchangershas been investigated in our laboratory". Howeverthere are no reports on the protonic conduction ofamorphous inorgano-organic materials.

In the light of the recent advances in the field ofprotonic conduction, the present endeavour dealswith the synthesis, characterization and study ofthe transport properties of an amorphous inorganicion exchanger zirconium molybdate (ZM) and oforganic molecules, i.e., ortho chlorophenol andpara chlorophenol anchored onto ZM, termed asZMoCP and ZMpCP respectively. ZM, ZMoCPand ZMpCP have been assessed for their chemicalstability in various acidic, basic and organic media.They have been characterised for elementalanalysis, TGA and FTIR spectroscopy. Furtherprotonic conductivity in these materials have beendetermined at various temperatures and compared.

Experimental ProcedurePreparation of the materials-Zirconium

molybdate was prepared by the method reportedearlier24

. ZMoCP and ZMpCP were prepared byheating under reflux ZM (I g) with 0-

chlorophenollp-chlorophenol each for 2h. Theexcess of the phenols were decanted and theexchanger was washed several times with ethanoland dried at 40°C, at a relative humidity of 80%.The decanted phenols were retained in order to

estimate the amount of phenols anchored onto thematrix. This has been determined on the basis ofthe difference between the residual and initialconcentration of the reagent by the brominationmethod",

Chemical stability- The chemical stability ofthe materials was assessed by soaking theexchangers in several mineral acids, bases andorganic solvents.

Elemental analysis-The samples were analysedfor zirconium and molybdenum gravimetrically aszirconium oxide by cup ferron method andmolybdenum oxide by the a-benzoin oximemethod respectively. Carbon and hydrogen contentwere determined by Coleman analyser model 33.

Thermal analysis-The thermogravimetricanalysis of the samples were performed on aShimadzu DT-30 thermal analyser at a heating rateof 10°C/min.

Fourier Transform Infrared Spectroscopy-FTIR spectra of the samples were performed usinga KBr wafer on a Perkin Elmer model 1720X withEpson Hi 80 printer/plotter.

Conductivity measurements-The electricalconductivity of the solids were measured on pelletsof 10 mm diameter and 1,5-2 mm thickness, with arelative humidity of 80%. The opposite sides of thepellets were coated with conducting silver paste toensure good electrical contact. Impedancemeasurements were taken using Solarton SI 1260Impedance analyser, over a frequency range 10Hz- 32 MHz at a signal level below 1Y, interfaced toa mini computer for data collection. In all cases,since the impedance plots of the materials consistof single depressed impedance semi-circle, thepellet conductivity was calculated by arcextrapolation to the real axis, taking into accountthe geometrial size of the pellets. Themeasurement was made jn the temperature range2S-120°C at a relative humidity of 80%.Results and Discussion

It is observed that the colour of ZM changedfrom pure white to brownish yellow when treatedwith o-chlorophenollp-chlorophenol, which re-sembles the colour of organic resins. This gives anindication that the organic moiety is anchored onto the inorganic matrix.

ZM was found to be stable in mineral acids suchas HCI (2N), H2S04 (4N), HN03 (2N), bases suchas NaOH (2N) and KOH (O.SN).ZMoCP andZMpCP were found to be stable in HCl (2N),

PANDIT et al.: ORGANIC DERIVATIVES OF ZIRCONIUM MOLYBDATE 79

10090 (a) 1008070 ~ 90:I:

C)LJJ 80 .....•.•---.~ -- ...•....

;!70

20

10O~~~~-L __ -L__~ __~ __ij

4000 3200 24002000 1600 1200 800 400ern"

10o...--------------=,-------.,90 (b)807060

~ 50~ 40

3020104g'="00~32:l:0-:-0-:-2400~-±20=:;00:-.:--:llb~-00~12:l:0-:-0-:8::!0-:-0-:4~00

cm'100r----------::,-------,90 (c)

8070

30ZO10o

4 00ZO:-:3::-::2:-k0"0~24~00~2oo=0-;;:16~0"0""'12~00""8~0"0-'4:-:100cm'

Fig.l-TGA of (a) ZM, (b) ZMoCP, (c) ZMpCP

H2S04 (4N), HN03 (2N), bases, NaOH (0.5N) andKOH (0.5N). ZM, ZMoCP and ZMpCP werefurther found to be stable in ethanol, acetone anddiethyl ether.

Chemical analysis indicate the composition ofZr:Mo to be 1:1. Based on this and the TGA data,the molecular formula of ZM is proposed to bezrOz. Mo04.15H20. The number of watermolecules was determined using Alberti'sformula26

. The carbon and hydrogen content inZMoCP and ZMpCP were found to be %C =4.908; %H == 2.480 and %C == 5.316; %H == 2.371respectively.

The thermogram of ZM (Fig. 1 a) indicates an8% weight loss within the temperature range of 80-180aC corresponding to the loss of external water

(a)

90100 200 300 400 500TEMPERATURE,oC

70;!-

90100 200 300 400 500TEMPERATURE, °C

Fig.2-FTIR of (a) ZM, (b) ZMoCP, (c) ZMpCP

molecules after which a slow change in weight lossis observed till about 600aC. This may be due tothe condensation of structural hydroxyl groups.

The TGA of ZMoCP and ZMpCP (Fig.1 b&c)indicate a ~ 10% weight loss at a temperature of~60ac which could be attributed to the loss ofadsorbed moisture and traces of solvents used forwashing the exchanger after the reflux period. Agradual decrease in weight occurs upto 500aCindicative of the dissociation and decomposition ofthe organic part from the inorganic support and tothe condensation of the structural hydroxyl groups.

The FTIR spectrum of ZM (Fig.2 a) showsbroad bands in ~3400 em" region characteristic ofthe asymmetric and symmetric hydroxo -OH andaquo -OH stretches. A medium band at ~ 1620 ern', is characteristic of the H-O-H bending frequency.A shoulder at ~935 cm-' is observed characteristicof the Zr-O stretching frequency.

•

80 INDIAN J. ENG. MATER. SCL, APRIL 1998

'I 4'00 (a)~ 3·00"01a 2·00EN lOO

o ·OO·~.--JL-----'----'----'---'----"''''''0·00 1'00 2-00 3'00 4·00 5·00 9'00

Zrel x 10-4

(b)" 4·00I01a.~ 2·00N

00·0 '--_---' __ ___'___ --L--'~_.J

0·00 2·00 4·00 6·00zrel x 10-(.

8·00

<DI

~ 1·5

0·20 1·00 1'50 2·00 2'50 3'00Zre. II 10-1 •

0-80 (d)

•.. 0·60I

go 0'40ENO·2

+

00.0 L----.J~___'__--.-J.. _ __'__~=__:~0.00 0·20 0·40 0'60 080 1·00 1·20

Zrpl x 10-6

1·00••. (e)o 0.75Igo 0·50EN 0·25

O·OOIL--L----.JL.----'::-:--...,..-L-::--:~,_l....J0·00 0·25 0·50 0'7!> 2·00 2' 25 0'50

Zrel x 10

Fig.3- Temperature - frequency dependant impedance plotsfor ZMpCP(a) T= 25°C, (b) T= 30°C, (c) T= 60°C, (d) T= 90°C and (e)T= I20°C

The FTIR spectra of ZMoCP and ZMpCP (Fig.2b&c) show additional bands in the 1580-1460 cm'!region characteristic of the C = C skeletal inplanevibrations. Bands in the 1390-1330 cm'! region aredue to the interactions of the OH bending and C -Hstretching vibrations. Weak bands in the 1410-1310 ern" region and at 1200 cm'! are due to theC-O stretching frequency.

Table I-Variation of specific conductance (0) withtemperature

0, (0 em)"!Temperature°C ZM

1.I2xIO,51.08xlO,5I.23x I0,65.20xlO,73.39xlO,8

ZMoCP

6.77xIO,75.86xlO,73.03xIO,72.64xlO,71.8IxIO,7

ZMpCP

3.98xIO,63. 17x 10'62.I2xIo,7l.57xlO,89.25xlO,9

25306090120

It is seen that the value of specific conductivity(0') ofZM at 25°C is 1.12x10,5(0 cmr\vhereas forZMoCP and ZMpCP the value of 0' is 6.77xlO'7COcm)"' and 3.98xlO-6 COern)" respectively.

The nature of the graphs in ZMoCP and ZMpCPare the same. Temperature and frequencydependent complex impedance plots for aparticular case ZMpCP are exhibited in Fig. 3. Forall the three materials, specific conductivitydecreases with increasing temperature (Table I).This is attributed to the loss of water of hydrationas well as the condensation of structural -OHgroups. This suggests the mechanism oftransportation as a Grotthus type27 where theconductivity depends on ability of the waterlocated on the surface to rotate and participate in aGrotthus type transport. It is also required that thehydronium ion must reorient to the proper positionfor the proton transfer to occur. This fact is also inkeeping with the suggestion that protons are notable to diffuse along an anhydrous surface wherethe spacing of the -OR groups is high". Moreoverit is reported that in case of amorphous zirconiumphosphate, swelling occurs in water andhydronium ions may form'" , allowing for aGrotthus transport mechanism. Besides the factthat the loss of proton resulting from the hydroxylcondensation causes a considerable decrease inconductivity, indicates that the conduction isprotonic also.

The ac conductivity experiments of differentcompounds reported earlier revealed that the acconductivity of phases obtained from intercalationand thermal de-intercalataion of 1,4 amino butanolin a-ZrP at 150°C was 3xlO,6S cm'! which is abouttwo orders of magnitude higher than that ofanhydrous a_ZrP30

• Suitable guest molecules whenintercalated in the interlayer region of a-ZrCHP04)2' H20 affect the proton transport, actingas bridges between the phosphate ionogenicgroups. High activation energy and low

PANDIT et al.: ORGANIC DERIVATIVES OF ZIRCONIUM MOLYBDATE 81

conductivity were observed for intercalationcompounds containing as guests, relatively strongBronsted bases probably due to the protonlocalization, whereas a significant improvement inconductivity was obtained after intercalation ofhydrazine'". Ac conductivity in the intercalationcompounds with imidazole and pyrazole show aremarkable increase. Similar values were obtainedfor compounds containing pyrazine, pyrimidineand pyridazine. Lowest conductivity was observedin case of the compounds containing 3,4 diaminopyridine because of a high proton localization onthe carboxy n-alkyl phosphonates (wheren=number of carbons in the alkyl group). Witha-layered structure, for n= 1,3,5 the conductancevalues are 1.96xlO-9S ern" at 200°C. These resultsshow the presence of an odd even effect on the acconductivity, pointing to different arrangements ofcarboxyl groups for even and odd number ofcarbon atoms.

As observed in the present study, specific 2conductivities for ZMoCP and ZMpCP are lessas compared to that of ZM at room temperature. 3This can be explained on the basis of the fact thatsince the inorganic host ZM is amorphous and non 4layered" the organic guests get anchored only onthe surface of the inorganic matrix. Thus interlayerprotonic conduction is not possible as has been 5observed for intercalation compounds reported 6earlier22. Further, as reported by Alberti et al.inspite of the very high concentration of acid 7groups in layered materials, the bulk contribution 8to the conductivity is negligible because the mainpart of the measured conductivity is related to thesurface P-OH groups. The very low bulk 9conductivity at room temperature could be due tolack of orientation and polarization of theP-OHgroups present in the interlayer region ofa-zirconium phosphate34

• On the other hand, theP-OH groups present on the surface have muchmore freedom to rotate than the internal ones andtransport is assisted by water molecules forming abridge between the acid groups'",

In the present case, however the transport of thesurface protons of ZM is sterically blocked due tothe presence of organic moieties on the surface. As 16a result, the orientation and polarization of the M-OH groups on the ZM surface is disturbed leading 17to a decrease in the specific conductance forZMoCP and ZMpCP as compared to that of ZM.Further, it is also observed that conduction in case

of the ortho derivative ZMoCP is less as comparedto that of the para compound, ZMpCP. This maybe due to the location of the phenolic -OH at theortho position in ZMoCP which causes moresterics hindrance as compared to that in the paraderivative ZMpCP.

AcknowledgementThanks are due to the Head, Department of

Chemistry, for providing necessary laboratoryfacilities. The help rendered by Dr A R Kulkarni.IIT, Bombay for impedance measurements is grate-fully acknowledged. The authors (BP) and (BB)are thankful to the MS University of Baroda and tothe UGC, New Delhi, respectively for financialassistance.

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