Synthesis and Characterization ……….
CHAPTER-3
SYNTHESIS AND CHARACTERIZATION OF TITANIUM (IV)
MOLYBDOTUNGSTATE & TIN (IV) MOLYBDOTUNGSTATE
ABSTRACT
The present study focused on the structure elucidation of heteropolyacid salts
named Titanium (IV) molybdotungstate [TiMoW] & Tin [IV] molybdotungstate
[SnMoW] synthesised at pH 1.0, using sol-gel route. Instrumental techniques like IR,
XRD, TGA, SEM and EDS were used to elaborate the structural aspects. XRD
studies showed that prepared samples were having amorphous structure. Infrared
spectroscopy was used to investigate the functional groups and the active sites
supporting the acidic character of the synthesised samples. SEM and EDS
techniques described the elemental composition of the synthesised sample.
Thermogravimetric technique indicated the weight loss rate due to dehydration and
helps in ascertaining the number of water molecules. The structure derived on the
basis of above analytical techniques gave a picture of the defective lacunary
polyoxometallates structure embracing various functional groups, water molecules,
and acidic nature and ion exchange characteristics. Thus, TiMoW as well as
SnMoW both can also be used in electro-analytical analysis.
50
Synthesis and Characterization ……….
3.1 INTRODUCTION
Clearfield studied the inorganic ion exchangers, particularly heteropolyacid
compounds and gave a pioneer step towards characterization and their applications
creating a new beam of ray in the research world1-3. Matkovic et al investigation
demonstrated that the “ion exchange” methodology is suitable for the synthesis of
phosphotungstic and molybdic acids in a high yield4. These complex compounds are
composed of composite network of MO6 octahedra, the later having discrete fragments of
metal oxide structures5. J.F. Keggin was the first one who experimentally determined the
structure of α-Keggin heteropoly compounds with the help of X-ray diffraction
technology6. The heteropolyanion phase transfer chemistry created by M T Pope has
opened up one more new and broad field for heteropoly compound research7-8. Mittal et
al has some heteropolyacid salts of tin possessing good ion-exchange characteristics
which have been identified as electro-active materials and were applied effectively for
electro analytical studies9-13. Quereshi et al14 tin based amorphous heteropolyacid salt
named stannic tungstoarsenate having composition Sn:W:As::12:5:2 Siddiqi and Pathania
prepared titanium (IV) tungstophosphate and then applied for the separation of metal
ions15. Poojary and his coworkers resolved the crystal structure of sodium titanosilicate of
ideal composition Na2Ti2O3SiO4.2H2O using X-ray powder data16. A mixed cation
exchanger EDTA-stannic (IV) iodate has excellent performance up to the temperature of
500°C and retains 76.4% of ion exchange capacity17.
Besides the ion exchangers, heteropolyacids and their salts have been extensively
used as catalysts also for much industrial practices18-19 to replace environmentally
harmful liquid mineral catalysts20.
Work presented over here gives the particular account of the synthesis and
physicochemical characterization of the novel heteropoly compounds [TiMoW] and
[SnMoW].
51
Synthesis and Characterization ……….
3.2 MATERIALS AND METHODS
3.2.1 CHEMICALS AND INSTRUMENTS
All the chemicals used during the experimental work were of analytical grade.
The essential chemicals like Tin chloride, sodium tungstate and sodium molybdate were
procured from CDH chemicals, India. Titanium chloride (TiCl4) was obtained from
Merck chemicals. Various other metal ion solutions were prepared by weighing AR grade
reagent and then standardization, wherever required. Balance Electronic Top Pan
(Endeavour) was used for all the weighing. Double Distilled water was prepared by
double distillation plant. Infrared studies of TiMoW were carried out with a Perkin, ABB
spectrophotometer (resolution better than 0.7 cm-1) using the KBr pellet technique
whereas for SnMoW an IR- EFFINITY-21 CE Shimadzu spectrophotometer (Spectral
resolution of 0.5 cm -1) was used. SEM and EDS studies of TiMoW were done with the
help of JEOL (JSM 6510LV) and SEM/EDS images of SnMoW were obtained by using
EDS advanced micro analytical solution AMETAK electron microscope. X-Ray powder
patterns of TiMoW were obtained with a Rigaku Dmax III C instrument with a Debye
Scherrer camera. X-ray spectrum of SnMoW was obtained with X-ray Diffractometer
(Powder Method) - Panalyticals X.Pert Procamera (Resolution = 12 arc-seconds and
absolute angular resolution= 0.0001degree). TGA analysis was based on the changes
observed on heating the materials on Mettler Toledo Star System.
3.2.2 PREPARATION OF TITANIUM [IV] MOLYBDOTUNGSTATE [TiMoW] AND
TIN [IV] MOLYBDOTUNGSTATE [SnMoW]
Titanium (IV) molybdotungstate was prepared by adding titanium (IV) chloride
(0.1 M) solution to a continuously stirred mixture of sodium molybdate (0.1 M) and
sodium tungstate (0.1 M) at 60ºC in a volume ratio of 2:1:1 respectively. Gelatinous
silvery light sky blue precipitates were obtained at pH 1, which was maintained by slow
and requisite addition of either HCl or NaOH solution. SnMoW was prepared by adding
tin (IV) chloride (0.1 M) to a mixture of sodium molybdate (0.1 M) and sodium tungstate
(0.1M) again by maintaining the same temperature and volume ratio as mentioned in case
of TiMoW. Gelatinous precipitates were obtained at pH 1. The precipitates were digested
52
Synthesis and Characterization ……….
for 3 hours to get the desired quality of the exchanger. After cooling, the precipitates were
filtered and washed with DDW to remove excess of halide ions. The dried gel product
broke down into small granules when immersed in DDW. The granular material was then
dried at 400C to retain the water of crystallisation. To convert the above material into
active ion exchanger i.e. H+ form, these were kept in HCl (0.1 M) solution overnight with
intermittent changing the acid. The activated form was washed with DDW in order to
remove excess acid and finally dried at 40ºC to retain the water of crystallisation. Similar
steps were followed for the SnMoW precipitates to get it in the activated form. Various
other conditions as given in the table 3.1 were also tried to get the exchanger of desirable
quality.
3.2.3 PHYSICOCHEMICAL CHARACTERIZATION
Ion Exchange Capacity: Ion-exchange capacity of the synthesized sample was deduced
by employing column operation methodology. It was determined by passing 400 mL of
NaCl (1 M) solution through glass column having 0.5g of the exchanger over a bed of
glass wool, at the rate of 8-9 drops/min. The eluted solution on titrating against NaOH
(0.01 M) gave the strength of the H+ ions given out by the exchanger, which in turn gave
the ion exchange capacity of the exchanger in meq g-1. TiMoW ion exchanger gave
0.98±0.02 meq g-1 ion exchange capacity whereas in case of SnMoW it was found to be
0.86±0.03 meq g-1.
Distribution Coefficient Studies: Distribution coefficients (Kd) for the various metal ions
were determined by keeping metal ion solutions with synthesised exchanger for a
sufficient time to equilibrate and saturate the activated sites of the exchanger. The
strength of the exchanged metal ion solution was obtained by titrating against 0.01 M
EDTA (standardized with PbNO3). Then the distribution coefficient was determined by
using the formulaKd =
I  F 20
X
I
0 .2
Where, I = Volume of EDTA (0.01 M) used to neutralize the metal ions initially.
F = Volume of EDTA (0.01 M) used to neutralize the metal ions after equilibrium.
Kd values for the various metal ions are compiled in the table 3.2.
53
Synthesis and Characterization ……….
Table 3.1: Preparation of TiMoW and SnMoW under different conditions
Sample Name
No.
of
the
respective
Ratio
Molar
Temp pH
Volume concentration
IEC
IEC
TiMoW SnMoW
constituents
1.
Titanium chloride/
2
1M
600C
1.0
Tin chloride
Sodium tungstate
1
1M
Sodium
1
1M
2
2M
0.982
0.864
meq/g
meq/g
0.664
0.554
meq/g
meq/g
0.845
0.80
meq/g
meq/g
0.878
0.734
meq/g
meq/g
0.975
0.790
meq/g
meq/g
molybdate
2.
Titanium chloride/
550C
1.8
Tin chloride
Sodium tungstate
1
1M
Sodium
1
1M
1.5
1M
molybdate
3.
Titanium chloride/
60oC
1.6
Tin chloride
Sodium tungstate
1
1M
Sodium
1
1M
2
2M
molybdate
4.
Titanium chloride
65oC
1.2
/Tin chloride
Sodium tungstate
1
1M
Sodium
1
1M
1
1M
molybdate
5.
Titanium chloride
/Tin chloride
Sodium tungstate
1
1M
Sodium
1
1M
molybdate
54
60oC
1.2
Synthesis and Characterization ……….
Table 3.2: The Kd values for various rare earth metal ions
S.No.
Metal ion
Kd (distribution coefficient)
TiMoW
SnMoW
1.
Gd(III)
16.7±0.2
13.7±0.2
2.
Pr(III)
17.7±0.3
19.1±0.3
3.
Er(III)
22.6±0.1
22.6±0.3
4.
Sm(III)
8.0±0.1
14.6±0.2
5.
La(III)
8.9±0.2
13.9±0.1
6.
Ce(III)
33.5±0.1
22.3±0.4
7.
Eu(III)
14.0±0.2
-
8.
Tb(III)
9.0±0.2
3.8±0.2
9.
Y(III)
37.8±0.1
31.0±0.2
3.2.4 STRUCTURAL CHARACTERIZATION
Infrared Spectra: Infra red (FTIR) spectroscopy is the technique that deals with
IR region of the electromagnetic spectrum. Fourier transform infrared (FTIR) spectra of
TiMoW and SnMoW samples were recorded using KBr pellet medium and were used to
ascertain various functional groups, characteristic of heteropolyacid salts. Four types of
metal-oxygen linkages exhibit characteristic vibrational bands for structure elucidation in
polyoxometalate chemistry.

X-O-M, long and weak bond (4 internal oxygens connecting X-M),

M-O-M (12 edge-sharing oxygen connecting M’s),

M-O-M (12 corner- sharing oxygen connecting M3O13 units) and

M-O, bond having almost double bond character (12 terminal metal-oxygen bonding).
55
Synthesis and Characterization ……….
FTIR spectra of the samples in the region 1200–600 cm−1 is of interest as it
explains the metal oxygen bonding in the heteropoly compounds21. The results of FTIR of
TiMoW and SnMoW are given in the figures 3.1 and 3.2 respectively.
Thermal Analysis: TGA was done by using Mettler Toledo Star System which can
detects the temperature deviations of ±0.25 K and the temperature precision is up to
±0.15K. The blank curve reproducibility of the instrument is better than ±10 micro gram
over the whole temperature range. A thermo-gravimetric curve was obtained between
50ºC-700ºC, with a heating rate of 10ºC per minute, under N2 at a flow rate of 50.0
mL/minute. The number and nature of water molecules embedded in the structure of the
HPA salts and thermal stability characteristics of the material can be derived on the basis
of resultant TGA curve. TGA curves of TiMoW and SnMoW are represented in the
figures 3.3 and 3.4 respectively.
X-Ray
Diffraction:
X–Ray
analysis
gives
the
information
about
the
crystalline/amorphous content and size/orientation of crystallites. X-ray diffraction
studies of both the salts were done by powder method and X-ray diffraction pattern of
TiMoW and SnMoW are shown in figures 3.5 and 3.6 respectively.
Scanning Electron Microscopy and Energy Dispersive Spectroscopy [SEM &EDS]:
Images generated via SEM reveal information about texture and orientation of the
sample. Electron diffraction spectra help in deriving the elemental composition of the
compounds. Scanning Electron Microscope images of the sample were resolved by
spraying the finally powdered synthesised material on a double faced conducting tape
locked on a brass support. Results of SEM images are shown in figures 3.7 and 3.8.
3.3 RESULTS AND DISCUSSION
Titanium (IV) molybdotungstate and Tin (IV) molybdotungstate prepared by solgel method exhibit an ion exchange capacity of 0.980.98±0.02 meq/g and 0.86±0.03
meq/g, respectively. Out of various samples prepared, using different volume ratio, molar
concentration, different pH and temperature conditions, sample no's 1 were picked up for
56
Synthesis and Characterization ……….
further studies, since these show maximum IEC. These are reproducible and expressed
maximum selectivity towards Y(III), as is evident from distribution coefficient studies.
165
160
155
150
145
140
%T
135
130
125
120
115
110
105
100
4000
3500
3000
2500
2000
1500
Wavenumbers (cm-1)
Figure 3.1: FTIR spectrum of Titanium (IV) molybdotungstate
Figure 3.2: FTIR spectrum of Tin (IV) molybdotungstate
57
1000
500
Synthesis and Characterization ……….
Figure 3.3: TGA curve of TiMoW
Figure 3.4: TGA curve of SnMoW
58
Synthesis and Characterization ……….
Figure 3.5: X–ray diffraction pattern of TiMoW
Counts
NS
400
300
200
100
0
10
20
30
Position [°2Theta] (Copper (Cu))
Figure 3.6: X–ray diffraction pattern of SnMoW
59
40
Synthesis and Characterization ……….
Figure 3.7: SEM image of a sample of TiMoW
Figure 3.8: SEM image of a sample of SnMoW
60
Synthesis and Characterization ……….
IR spectra of the titanium (IV) molybdotungstate show sharp and strong bands at
3625 and 2921 and band near 1740 and 1393 cm-1. Besides these there is a broad band at
985-675 cm-1.

Broad band at 3625 cm-1 -stretching of -OH groups of the interstitial water molecules.

A weak band at 2921 cm-1 -deformation vibration of the coordinated water molecules.

Sharp band at 1740 and 1393 cm-1 -deformation of interstitial water molecules and MOH, respectively.

A broad band at 985-675 cm-1 -wagging, twisting and rocking modes of metal oxygen
and aqua bonds.
IR spectra of the SnMoW represent the following sharp and strong bands at:

Broad band at 3256 cm-1 and 3043 cm-1 stretching of -OH groups of the interstitial
water molecules and M-OH (acidic) stretches.

A weak band at 2835 cm-1 -deformation vibration of the coordinated water molecules.

Sharp band at 1569 and 1312 cm-1 indicates deformation interstitial water molecules
and M-OH, respectively.

A broad band at 837-542 cm-1 is due to wagging, twisting and rocking modes of metal
oxygen and aqua bonds.
Due to the coupling of outer valence electronic states with the vibrational states of the
molecules, the bands were either broadened or shifted from their normal modes. One
important inference drawn from the bands is that properties of HPA salts are largely
dependent on the presence of incorporated water.
Electron diffraction spectra of TiMoW contain Ti, Mo, W and O which were
characterized by their respective peaks in the spectrum. Atomic ratio of these elements
was Ti:Mo:W:O::19.64:18.75:27.10:34.52 respectively. Oxygen content shown in the
formula includes the molecular water, hydroxyl group and oxide. Based on the data, an
empirical formula of the synthesized exchanger can be represented as:
[(TiO2)1.047(H3MoO3) (H3WO4)1.445].nH2O
61
Synthesis and Characterization ……….
Elemental composition of the SnMoW exchanger contains the characteristics
peaks of elements Sn, Mo, W and O. Atomic ratio of characteristics elements was
Sn:Mo:W—3.62:5.27:78.13 respectively.
Thermo gravimetric study solves the dilemma of the external water molecules
which are consistent only up to 2000C.
TGA curve indicated that TiMoW experiences a weight loss of 8.5% up to a temperature
of 2000C. It is assumed that all the external water molecules are rooted up on heating the
hetropolyacid compounds above than the recommended temperature. The number of
external water molecules ‘n’ can be calculated by using Alberti formula22, which is given
as;
18 n = X (M+18 n) / 100 ………… (1)
Where, X is % age weight loss at 2000C, (M+18 n) is molecular weight of the material
and n represents the number of external water molecules. This gives the value of ‘n’ as
2.959≈3.
So,
the
formula
of
the
exchanger
can
be
written
as
[(TiO2)1.047(H3MoO3)(H3WO4)1.445].3H2O with a molecular weight 627.349. A further
weight loss of nearly 2.4% up to 5000C may be due to the dissipation/rearrangement of
co-ordinate water hydroxyl groups and other functional groups. These losses follow the
regular trend of inorganic ion exchangers. SnMoW experiences a weight loss of 10% up
to a temperature of 200ºC. By using the Alberti formula, the value of ‘n’ was found to be
33.53
≈
34.
So,
the
formula
of
the
exchanger
can
be
written
as
[(SnO2)(H3MoO3)1.4558(H3WO4)21.5828].34H2O with a molecular weight 6027.096 a.m.u.
A further weight loss of nearly 5.52% up to 500ºC may be attributed to the loss of
coordinated water and hydroxyl groups besides the rearrangement of functional groups.
X-ray powder diffraction studies revealed that there was no definite angle of
diffraction line to tell about the crystallinity. Hence, both the spectra indicated the
amorphous nature of the compounds.
62
Synthesis and Characterization ……….
SEM images of Titanium (IV) molybdotungstate and Tin (IV) molybdotungstate
explain that the particles were
(i)
Broad in size range
(ii)
Having an irregular shape
(iii)
No sign of crystalline structure and
(iv)
Lack of clarity
These observations strongly gave a favor to their amorphous nature. Pictorial
representation gave the evidence of linearly layered with little irregularity especially in
the structure of SnMoW.
3.4 APPLICATIONS AS ION SELECTIVE ELECTRODE
It was observed that Titanium (IV) molybdotungstate and Tin (IV)
molybdotungstate exhibited different selectivity towards different metal ions. However,
maximum response behavior was observed towards Y(III) ions over other rare earth metal
ions. Thus, both these electro-active materials have been used as electro-active
components in the preparation of heterogeneous solid membrane electrodes sensitive for
Y(III) ions. Potentiometric applications of these heteropolyacid salts as electro-active
materials towards the Y(III) ion have been elaborated in the 6th chapter.
3.5 CONCLUSION
Heteropolyacid salts named Titanium [IV] molybdotungstate [TiMoW] and Tin
[IV] molybdotungstate [SnMoW] have been synthesised and then structural aspects were
deduced on the basis of instrumental techniques like FTIR, XRD, TGA, SEM and EDS
analysis. On the basis of above analysis, the empirical formula of the amorphous TiMoW
product was derived and it is given as [(TiO2)1.047(H3MoO3)(H3WO4)1.445].3H2O with a
molecular weight 627.349 a.m.u. Whereas the empirical formula of the SnMoW was
formulated as [(SnO2)(H3MoO3)1.4558(H3WO4)21.5828].34H2O with a molecular weight
6027.096 a.m.u. Thus, TiMoW and SnMoW both have been identified polyoxometallates
having defected lacunary geometry and physicochemical description of these compounds
characterized these as electro active materials in potentiometric analysis of Y(III) ion.
63
Synthesis and Characterization ……….
REFERENCES
[1]
A. Clearfield, G.H. Nancollas, R.H. Blessing, J.A. Marinsky, in Y. Marcus (Ed.),
Ion exchanger and Solvent Extraction, Marcel Dekker, New York, 5 (1973).
[2]
A. Clearfield, “Role of ion exchange in solid-state chemistry”, Chem. Rev., 88
(1988) 125.
[3]
A. Clearfield, “Inorganic Ion Exchangers, Past, Present, And Future”, Solvent
Extr. Ion Exc., 18 (2000) 655-678.
[4]
S.R. Matkovic, G.M. Valle, L.E.
Briand, “Optimization of the operative
conditions for heteropolyacid synthesis through ion exchange”, Mater. Res., 8
(2005) 351-355.
[5]
M.T. Pope, Heteropoly and Isopoly Oxometalates, Verlag, Berlin (1983).
[6]
J.F. Keggins, “The Structure and Formula of 12-Phosphotungstic Acid”, Proc.
Roy. Soc. A, 144 (1934) 75-100.
[7]
M.T. Pope, A. Mullar,
“Polyoxometalate
Chemistry: An Old Field with New
Dimensions in Several Disciplines”, Angew. Chem. Int. Ed. Engl., 30 (1991) 3438.
[8]
D.E. Katsoulis, M.T. Pope, “New Chemistry for Heteropolyanions in
Anhydrous non-polar solvent”, J. Chem. Soc., Dalton Trans., 8 (1989)1483.
[9]
R.S. Sandhu, S.K. Mittal, P.I. Singh, “Solid membrane of tin(iv) arsenoantimonate
in araldite as Pb(II) selective electrode”, J. Electrochem. Soc., 38 (1989) 221-223..
[10]
S.K. Mittal, P.P. Singh, “Synthesis, ion exchange properties and applications of tin (IV)
antimonoarsenate”, React. Func. Polym., 40 (1999) 231-240.
[11]
S.K. Mittal, “Solid membranes of tin(iv) based ion exchangers as Pb(II) sensitive
electrodes”, Indian J. Tech., 29 (1991) 283-286.
[12]
S.K. Mittal, P. S. Thind, “Electrochemical properties of tin(iv) antimonophosphate
membrane”, Bull. Electrochem. (India), 9 (1992) 427-429.
[13]
S.K. Mittal, H.K. Sharma, S.K.A. Kumar, “Synthesis, characterization and
analytical application of zirconium (IV) antimonoarsenate as a potentiometric
sensor”, React. Funct. Polym., 66 (2006) 1174.
64
Synthesis and Characterization ……….
[14]
M. Qureshi, R. Kumar, V. Sharina, T. Khan, “Synthesis and ion exchange
properties of Tin (IV) tungstoarsenate”, J. Chromatogr., 118 (1976) 175.
[15]
Z.M. Siddiqi, D. Pathania, “Titanium(IV) tungstosilicate and titanium(IV)
tungstophosphate: two new inorganic ion exchangers”, J. Chromatogr. A., 147
(2003) 987.
[16]
D.M. Poojary, R.A. Cahill, A. Clearfield, “Synthesis crystal-structure, and ion
exchange properties of novel porous Titanosilicate”, Chem. Mater., 6 (1994)
2364.
[17]
S.A. Nabi, A.H. Shalla, “Synthesis and characterization of a new cation
exchanger-zirconium(IV)iodotungstate”, J. Por. Mater., 16 (2009) 587-597.
[18]
C.L. Hill,
“Introduction:
Polyoxometalates
Multicomponent Molecular
Vehicles To Probe Fundamental Issues and Practical Problems”, Chem. Rev., 98
(1998) 1.
[19]
U. Filek, E. Bielanska, R.P. Socha, A. Bielanski, “Reduced copper salt of WellsDawson type heteropolyacid as a bifunctional catalyst”, Catal. Today, 169 (2011)
150-155.
[20]
G.P. Romanelli, J.C. Autino, “Recent applications of heteropolyacids and related
compounds in heterocycles synthesis”, Mini-Rev. Org. Chem., 6 (2009) 359-366.
[21]
C.N.R. Rao, “Chemical Applications of Infrared Spectroscopy”, Academic Press,
New York, (1963) 353.
[22]
G. Alberti, P.C. Galli, U. Costantino, E. Torracca, “Crystalline insoluble salts of
polybasic metals—I Ion-exchange properties of crystalline titanium phosphate”, J.
Inorg. Nucl. Chem., 29 (1967) 571-578.
65