Science & Technology Indonesia 
p-ISSN: 2580-4405 e-ISSN: 2580-4391 
Sci. Technol. Indonesia 1 (2016) 8-15 
 

Article http://sciencetechindonesia.com 

 

@2016 Published under the terms of the CC BY NC SA 4.0 license 

 
8 

THERMAL STABILITY OF POLYOXOMETALATE COMPOUND OF KEGGIN K8[2-SiW11O39]∙nH2O 
SUPPORTED WITH SiO2  
 

Yunita Sari M A1,* and Maria Danesti Situngkir1 
 
1Deoartment of Chemistry, Faculty of Mathematic and Natural Sciences, Sriwijaya University 

*Corresponding author e-mail: yuniaritonang15@yahoo.com 

 

ABSTRACT  

Synthesis through sol-gel method and characterization of polyoxometalate compound of K8[2-SiW11O39]∙nH2O supported with 
SiO2 have been done. The functional groups of polyoxometalate compound  was characterized by FT-IR spectrophotometer for the 

fungtional groups and the degree’s of crystalinity  using XRD. The acidity of K8[2-SiW11O39]∙nH2O/SiO2 was determined qualitative 
analysis using ammonia and pyridine adsorption and the quantitative analysis using potentiometric titration method. The results of 

FT-IR spectrum of K8[2-SiW11O39]∙nH2O appeared at  wavenumber 987.55 cm-1 (W=O), 864.11 cm-1 (W-Oe-W), 756.1 cm-1 (W-

Oc-W), 3425.58 cm-1 (O-H), respectively and spectrum of  K8[2-SiW11O39]SiO2 appeared at wavenumber  956.69 cm-1 (W=O), 

864.11 cm-1 (W-Oe-W), 3448.72 cm-1 (O-H), respectively. The diffraction of XRD pattern of K8[2-SiW11O39]∙nH2O and K8[2-

SiW11O39]∙nH2O/SiO2 compounds show high crystalinity. The acidic properties showed K8[2-SiW11O39]∙nH2O/SiO2 more acidic 

compared to K8[2-The SiW11O39]∙nH2O. The qualitative analysis showed pyridine compound adsorbed more of polyoxometalate 

compound of K8[2-SiW11O39]∙nH2O/SiO2. Analysis of stability showed that the K8[2-SiW11O39]∙nH2O/SiO2 at temperature 500°C 
has structural changes compare to 200-400oC which was indicated from vibration at wavenumber 800-1000 cm-1.  
 

Keywords : K8[2-SiW11O39]∙nH2O, polyoxometalate, SiO2. 
 

INTRODUCTION 
Polyoxometalate compound is the cluster compound of 

metal-oxygen which acid-base properties, it has various 
structural and oxidation rates, so it is very effective for both 
acid-base reaction and reduction oxidation reaction catalyst 
(Yamase dkk, 2002). Polyoxometalic compounds are effective as 
catalysts because they have higher acidity than sulfuric acid and 
not toxic (Okuhara et al, 1996). This compound has been applied 
as a catalyst in industrial processes in developed countries such 
as Japan (Nakagawa and Mizuno, 2007).  

The research of polyoxometalate compounds are primarily 
intended in terms of its superiority as a catalyst which can be 
performed either in homogeneous or heterogeneous systems 
depending on the medium are used. In a heterogeneous system, 
the polyoxometalate compound can be used repeatedly in 
catalytic reaction.   

The polyoxometalate compound has a low surface area and 
high solubility in a polar solvent (Kim et al, 2006).  The Catalyst 
which have a small surface area is suitable for homogeneous 
catalysts while the homogeneous catalysts can not be recycled. 
To designed the polyoxometalate compound as a heterogeneous 
catalyst, modification should carried out by embedding. 
Modification of polyoxometalate compounds can be embedding 
by inclusion using both metal oxide and non-oxide metals 
(Nerwman et al, 2006). The embedding is carried out to have a 
large surface area which can be used as heterogeneous catalysis 
and can increase the acidity of the compound, so that the  
 

Article  History 
Submitted: 2 August 2016 
Accepted: 15 September 2016 
 
DOI: 10.26554/sti.2016.1.1.8-15

catalytic properties increase. The catalytic activity is affected by 
the temperature, surface area, and acidity of the catalyst. The 
temperature affects collisions between molecules and certain 
chemical reactions require heating at high temperatures to 
obtain maximum results. As an example the hydroxylation 
reaction of n-hexane requires the temperatures above 400 ° C 
and requires the Bronsted acid side in the reaction to obtain a 
high percent conversion (Eid et al, 2013). 
 The synthesis of H4[γ-H2SiV2W10O40] has been carried out 
with various variations of embedding SiO2, TiO2, ZrOCl2 and 
TaCl5 by Karim (2014), which the product material has not been 
tested for its qualitative and quantitative acidity. Meanwhile 
Marci (et al 2013) has carried out the research by embedding 
Keggin H3PW12O40 type polyoxometalate compounds with 
various metal oxides such TiO2, SiO2, ZrO2, ZnO, and AlO2. 
From the many several metal oxides which has been used, 
polyoxometalate compounds which are embedded with SiO2 
have higher catalytic character which applied to the propene 
hydration reaction. 

In this research, the synthesis of Keggin K8[2-
SiW11O39]∙nH2O polyoxometalate compound which is 
embedded by SiO2 metal oxide. The metal oxide of SiO2 was 
obtained from reaction of tetraethyl orthosilical hydrolysis 
(TEOS) known as the sol gel method. The embedded result of 
synthesized polyoxometalate compounds were characterized by 
a Frourier Transform Infra Red (FT-IR) spectrophotometer and X-

Ray Difractometer (XRD). Polyoxometalate compounds K8[2-
SiW11O39]∙nH2O was soaked on the acid compounds before and 
after embeddded qualitatively and quantitatively. The thermal 

stability character of K8[2-SiW11O39]∙nH2O and K8[2-
SiW11O39]∙nH2O/SiO2 were tested by heating it at various 
temperature using furnace and the heating results were 



Sari et al. / Science and Technology Indonesia 1(1) 2016:8-15 

@2016 Published under the terms of the CC BY NC SA 4.0 license 9 

characterized by a FT-IR spectrophotometer. The acidity and 
thermal stability character of polyoxometalate compounds were 
tested perior to be used as catalysts in chemical reactions that 
require heating at high temperatures. 

 

EXPERIMENTAL SECTION 
The insturments which used in this research X-Ray 

diffractometer (XRD) Rigaku MiniFlex 600 and FT-IR 
Shimadzu Prestige-21 Spectrophotometer. The substances used 
in this research were sodium metasilicate (Na2O3Si), sodium 
tungstate (Na2WO4), hydrochloric acid (HCl), potassium 
chloride (KCl), potassium carbonate (K2CO3), tetraethyl 
orthosilika (TEOS), Sodium bis (2-Ethylhexyl) sulfosuccinate, 
cyclohexane, pyridine, ammonia (NH3), n-butylamine, 
acetonitrile and aquades (H2O). 
 

Synthesis of Polyoxometalate Compounds K8 [2-
SiW11O39]∙nH2O and Its Characterization 

The 11 g (50 mmol) Na2O3Si compound was dissolved in 
aquades (100 mL) and 4M HCl depleted slowly to a pH range of 
5-6 and stirred. The solvent was mixed with Na2WO4 of 182 g 
(0.55 mmol) which was dissolved with aquades (100 mL). The 
mixture was added with KCl of 80 g. With the addition of the 
mixture to be white and formed of sediment. After that, the 
mixture was filtered using filter paper, then the solid was dried 

to obtain the compound K8[2-SiW11O39]∙nH2O. The solid 
obtained was white solid. The polyoxometalate compound  

K8[2-SiW11O39]∙nH2O was characterized by a FT-IR 
spectrophotometer and X-Ray diffractometer (XRD). 
 

The`synthesis K8[2 SiW11O39]∙nH2O/SiO2 with Sol-gel 
Method and Its Characterization  

The synthesis of compound K8[2-SiW11O39] ∙nH2O/SiO2 
was modified from the procedure of (Kim et al 2006). Sodium 
bis (2-ethylhexyl) sulfosuccinate of 1.5 g was dissolved with 1 

mL of cyclohexane (solvent A). The compound K8[2-
SiW11O39]∙nH2O ∙ nH2O of 0.76 g was dissolved with slightly 
aquades (solvent B). Solvent B is added to solvent A while 
distirer. A total of 11.2 mL of tetraethyl orthosilicate (TEOS) 
was added dropwise. Stirred with stirer and heated at 60 oC for 

2 hours. The white solids formed are K8[2-
SiW11O39]∙nH2O/SiO2 and dried by vacuum. The compounds 

K8[2-SiW11O39]∙nH2O/SiO2  are characterized by a FT-IR 
spectrophotometer, and X-Ray diffractometer (XRD). 
 

The acidity test of compound K8[2-SiW11O39]∙nH2O 
and K8[γ-SiW10O36]∙nH2O/SiO2 is qualitatively  

A total of 0.5 g of each K8[2-SiW11O39]∙nH2O and K8[2-
SiW11O39] ∙nH2O/SiO2 were inserted into vials. A total of 10 mL 
of pyridine and 25% ammonia (NH3) were each fed into a beaker 
glass. A vial bottle was inserted into a beaker glass containing 
pyridine and ammonia and then sealed tightly with a kreb plastic. 
The compound was allowed for two days to adsorption between 
pyridine and ammonia with polyoxometalic compounds. 
Compounds that have been in contact with pyridine and 
ammonia were tested qualitatively by characterizing using a FT-
IR spectrophotometer. 
 

The acidity test of compound K8[2-SiW11O39]∙nH2O 

and K8[2-SiW11O39]∙nH2O/SiO2  

A total of 0.1 g of each of the K8[2-SiW11O39]∙nH2O and 

K8[2-SiW11O39]∙nH2O/SiO2 were dissolved in 8 mL acetonitrile 
and stirred for 3 hours with a magnetic stirrer. The suspension 
was titrated with n-butylamine 0.05 M which was monitored by 
glass electrode as a pH sensor. Each droplet per volume of 
titrant was recorded to be the potential generated and connected 
between the volume of the titrant and the resulting potential. 
The classification of forces from the acidity side is classified on 
a scale of: E> 100 mV (very acidic); 0> E> 100 mV (acid side); 
-100 <E <0 mV (weak acid side); And E <-100 mV (acid side is 
very weak).  
  

The Thermal Stability Test K8[2SiW11O39]∙nH2O/SiO2 
 The K8[γ-SiW10O36]∙nH2O/SiO2 was heated at 200 ° C, 300 
° C, 400 ° C and 500 ° C for 2 hours in the furnace. The heating 
compound was cooled and characterized by an FTIR 
spectrophotometer. 
 

RESULTS AND DISCUSSION 

Synthesis and Characterization of Keggin type 

Polyoxometalate Compounds K8[2-SiW11O39]∙nH2O 

The synthesis of compound K8[2-SiW11O39]∙nH2O was 
performed by addition of potassium chloride (KCl) to the 

compound [2-SiW11O39]∙nH2O acting as a K+ ion donator. At 
the end of the synthesis process there was obtained a white solid 

which was a compound K8[2-SiW11O39]∙nH2O. The result of 
synthesis in the form of white solid was then characterized using 
FT-IR spectrophotometer which aims to identify the functional 
group formed as shown in Figure 1. 
 

 
Figure 1. FTIR spectra of polyoxometalate compound K8[2-

SiW11O39]∙nH2O 
 

 The peaks of the functional groups of polyoxometalate 
compounds appearing at wave numbers 4000-300 cm-1 shows in 

Figure 1. The principal uptake of K8[2-SiW11O39]∙nH2O 
compounds shows the presence of vibration W = O appears in 
the 987.55  cm-1 region, the W-Oe-W vibration of 864.11 cm-1 in 
which oxygen is located on the edge of the compound 
Polyoxometalate and wave number 756.1 cm-1 for the vibration 
of the W-Oc-W group which is the oxygen atom located at the 
center of the polyoxometalate compound. According to 
Okuhara (et al  2001), the absorption of polyoxometalate 
compounds has W=O groups at wave number 980 cm-1, 



Sari et al. / Science and Technology Indonesia 1(1) 2016:8-15 

@2016 Published under the terms of the CC BY NC SA 4.0 license 10 

vibration of W-Oe-W group 878 cm-1, and vibration of W-Oc-

W 779 cm-1. The compound K8[2-SiW11O39]∙nH2O synthesis 
results in this study is in accordance with vibrations according 
to Okuhara (et al 2001). In addition to the 3425.58 cm-1 wave 
numbers indicating the vibration -OH which indicates the 

presence of H2O in the polyoxometalate compound K8[2-
SiW11O39]∙nH2O.  The presence of a hydrogen bonding effect is 
characterized by a widened peak in the FT-IR spectrum of Stuart 

(2004). K8[2-SiW11O39]∙nH2O.  

After the compound K8[2-SiW11O39]∙nH2O  was 
characterized using FT-IR spectrophotometer, then the poly-

oxometalic compound K8[2-SiW11O39]∙nH2O was analyzed by 

XRD. Diffractogram of K8[2-SiW11O39]∙nH2O polyoxo-
metalate compound ∙ nH2O is presented in Figure 2. 

 

 
Figure 2. XRD Diffraction Patterns Polyoxometalate 

Compound K8[2-SiW11O39]∙nH2O 
 

Figure 2 shows the diffraction for the polyoxometalate 

compounds K8[2-SiW11O39]∙nH2O with the highest intensity 
arising at 8°, 9°, 17°, 24° and 35° diffraction angles. According 
to Yang (et al 2011) the diffraction for polyoxometalic 
compounds is present in some diffraction regions, ie 6-10°, 15-
20°, 22-25°, and 35-40°. The diffraction that appears below 10° 
in region 2θ shows that the polyoxometalate compound has a 

very high crystallinity. Diffractogram of K8[2-SiW11O39]∙nH2O 
compounds exhibiting sharp diffraction peaks indicates 

polyoxometalate K8[2-SiW11O39]∙nH2O has very high 
crystalline properties in which the atoms of the polyoxometalate 

compound K8[2-SiW11O39]∙nH2O is arranged regularly based 
on the length and angle of regular bonding. 
 

Synthesis of Keggin Type Polyoxometalate Compound 

K8[2-SiW11O39]∙nH2O/SiO2 and Characterization 

 The polyoxometalate compound K8[2-SiW11O39]∙nH2O 
which has been obtained was further prepared by proportion to 
SiO2 obtained from TEOS hydrolysis. The embodiment was 
carried out by using a microemulsion and sol-gel method. 
Eriksson (et al 2004) describes that microemulsions are liquids 
derived from a mixture of water, hydrocarbons, and surfactants. 

Polyoxometalate compounds K8[2-SiW11O39]∙nH2O/SiO2 
synthesis and characterization by functional group analysis using 
FT-IR spectrophotometer with FT-IR spectra presented in 
Figure 3. The result of identification using FTIR 

spectrophotometer to compound K8[2-SiW11O39]∙nH2O/SiO2 
shows the specific vibration of polyoxometalic and SiO2 
compounds.  

Figure 3 shows the difference shown by the FT-IR spectrum 

of the polyoxometalate compound K8[2-SiW11O39]∙nH2O 
before being carried out with SiO2. According to Derrick (et al 
1999) the symmetric vibration of Si-O-Si is at the wave number 
1130-1000 cm-1. Smith (1999) reported that the vibration of Si-
O-Si symmetric stretching was stronger at 1085 cm-1. The FTIR 

spectra of compound K8[2-SiW11O39]∙nH2O/SiO2 undergo a 
shift of wave numbers for asymmetric Si-O-Si stretch vibration 
at 1103.28 cm-1. The shift of wave numbers occurs in vibration 
W=O. The W=O vibe before it is presented appears at the wave 
number 987.55 cm-1 and the vibration after carrying with SiO2 
appears at the wave number 956,69 cm-1. According to Stuart 
(2004) vibration -OH vibration in the presence of hydrogen 
bonding effect is in the range of 3500-2500 cm-1 wavelength 
characterized by a widened peak on the FT-IR spectra. Figure 3 
(B) experiences a shift in the number of waves at the peak of 
3448.72 cm-1 identifies the -OH vibration by the presence of 

H2O on the K8[2-SiW11O39]∙nH2O/SiO2 and the wave number 
at peak 3425.58 cm-1 identifies vibration -OH by the presence of 

H2O in the compound K8[2-SiW11O39]∙nH2O/SiO2. The 

primary wave numbers of K8[2-SiW11O39]∙nH2O and K8[2-
SiW11O39]∙nH2O/SiO2  are presented in Table 1.  
 

 
Figure 3. FTIR spectra of polyoxometalic compound K8[2-

SiW11O39]∙nH2O (A), FT-IR spectra of polyoxometalate 

compound K8 K8[2SiW11O39]∙nH2O/SiO2 (B) 
 

Table 1. Wave number K8[2-SiW11O39]∙nH2O and K8[2-
SiW11O39]∙nH2O/SiO2 

Vibration K8[2-
SiW11O39]∙nH2O 
(cm-1) 

Vibration  K8[2-                
SiW11O39]∙nH2O 
/SiO2 (cm-1) 

Type of 
Vibration 

987.55         956.69 W=O 
864.11         864.11 W-Oe-W 
756.1 
3425.58 
- 

        - 
        3448.72 
        1103.28 

W-Oc-W 
O-H 
Si-O-Si 

 

The polyoxometalate compound ∙ K8[2-SiW11O39]∙nH2O 
which was supported with SiO2 then characterized using XRD. 
The diffraction angle and crystallinity of polyoxometalate 

compounds K8[2-SiW11O39]∙nH2O/SiO2. Comparison of XRD 

of compound K8[2-SiW11O39]∙nH2O with K8[2-



Sari et al. / Science and Technology Indonesia 1(1) 2016:8-15 

@2016 Published under the terms of the CC BY NC SA 4.0 license 11 

SiW11O39]∙nH2O/SiO2 are presented in Figure 4. (A) K8[2-
SiW11O39]∙nH2O diffraction  emerging below 10° in region 2θ 

indicates that the polyoxometalate compound K8[2-
SiW11O39]∙nH2O has very high crystalline properties due to the 

atom of the  polyoxometalate compound K8[2-
SiW11O39]∙nH2O is arranged regularly based on the length and 
angle of bond formed. Figure 4 (B) shows the XRD pattern of 

K8[2-SiW11O39]∙nH2O/SiO2. The compound K8[2-
SiW11O39]∙nH2O/SiO2 has a high crystallinity with a diffraction 
angle of 2θ each at 8°, 18°, 27°, and 34° indicating the 

characteristics of the  polyoxometalate compound K8[2-
SiW11O39]∙nH2O/SiO2 in which the atoms of the  

polyoxometalate compound K8[2-SiW11O39]∙nH2O/SiO2 were 
arranged regularly on the basis and length of the regular 

bonding. Figure 4 (B) shows the pattern of XRD K8[2-
SiW11O39]∙nH2O/SiO2 indicating a change in the diffraction 
angle.  

 

 
Figure 4. The diffraction pattern of K8 K8[2-SiW11O39]∙nH2O 

(A), diffraction pattern of K8[2-SiW11O39]∙nH2O/SiO2 (B) 
 

The acidity test of compound K8[2-SiW11O39]∙nH2O 

and K8[2-SiW11O39]∙nH2O/SiO2 in qualitatively 
 

The Acidity test of K8[2SiW11O39]∙nH2O 
The acidity measurement of the  polyoxometalate 

compound ∙ K8[2-SiW11O39]∙nH2O is carried out both 
qualitatively and quantitatively. The FT-IR spectrophotometer 

method was performed for qualitatively, where K8[2-
SiW11O39]∙nH2O polyoxometalate compound was saturated 
with ammonia and with pyridine for 2 days resulting in 

adsorption on the surface of K8[2-SiW11O39]∙nH2O.. The 
saturation result were compared before or after being saturated 
with ammonia or pyridine. The saturation result was measured 

by a FTIR spectrophotometer . The FTIR spectra K8[2-
SiW11O39]∙nH2O saturation result shown in Figure 5. 

Polyoxometalate compound K8[2-SiW11O39]∙nH2O of the 
FT-IR spectra of Figure 5 shows no absorption bands at 1400-

1440 cm-1 wavelengths on a K8[2-SiW11O39]∙nH2O saturated 
polyoxometalate compound with or without saturation Pyridine. 
According to Dines (et al, 1991) ammonia forms ammonium 
ions (NH4+) with the observed wave numbers at 1400-1440 cm-

1, but in the  polyoxometalate compound∙ K8[2-
SiW11O39]∙nH2O not exhibit ammonium ion vibrations (NH4+) 
at wave number 1400-1440 cm-1. Ammonia can be adsorbed on 
the acid side of the heteropoly compound and in the metal 

cation (Seo et al, 1988). Ammonia was not adsorbed on the  

polyoxometalate compound K8[2-SiW11O39]∙nH2O. It was 

possible that the polyoxometalate compound K8[2-
SiW11O39]∙nH2O does not exhibit ammonium ion vibration 
(NH4+) at wave numbers 1400-1440 cm-1. Figure 5 (B) shows a 
vibration of -OH at a wave number of 3448.72 cm-1 which 
identifies the presence of H2O in a polyoxometalate compound 

K8[2-SiW11O39]∙nH2O. According to Stuart (2004), vibration -
OH vibration, with the effect of hydrogen bonds in the range of 
3500-2500 cm-1 wavelengths characterized by wider peaks in the 
FT-IR spectra. 
 

 
Figure 5. FTIR spectrum of polyoxometalate compound K8[2-

SiW11O39]∙nH2O  (A), FTIR spectra of polyoxometalate 

compound K8[2-SiW11O39]∙nH2O. Saturation with ammonia 

(B), FT-IR spectra of  polyoxometalate compound K8[2-
SiW11O39]∙nH2O saturation with pyridine (C) 

 

Polyoxometalate compound K8[2-SiW11O39]∙nH2O 
compounds with saturation with pyridine in Figure 5 (C) exhibit 
an –OH shift vibration appearing at a 3425.58 cm-1  wave 

number identifying H2O in a polyoxometalate compound K8[2-
SiW11O39]∙nH2O. According to Khalifah and Prasetyo (2008) 
pyridine molecules bound to Lewis acid sites absorbed at wave 
numbers 1400-1700 cm-1.  Figure 5 (C) FT-IR spectra does not 
show the wave number. In this case, pyridine was unsorbed in 

the polyoxometalate compound K8[2-SiW11O39]∙nH2O. FT-IR 

digital specimen data of K8[2-SiW11O39]∙nH2O compounds 
with saturation with ammonia and pyridine. 
 

The Acidity Test of K8[2-SiW11O39]∙nH2O/SiO2 

 The compound K8[2-SiW11O39]∙nH2O/SiO2 was saturated 
using ammonia and pyridine. Then the material was 
characterized by an FT-IR spectrophotometer. The saturation 

result was measured by an FTIR. The FT-IR spectra of K8[2-
SiW11O39]∙nH2O/SiO2 of the saturation results are shown in 
Figure 6. 

Figure 6 (A) shows the FTIR spectra of a polyoxometalate 

compound K8[2-SiW11O39]∙nH2O/SiO2 before being saturated. 
Figure 6 (B) shows the FTIR spectra of a polyoxometalate 

compound K8[2-SiW11O39]∙nH2O/SiO2 with saturation using 



Sari et al. / Science and Technology Indonesia 1(1) 2016:8-15 

@2016 Published under the terms of the CC BY NC SA 4.0 license 12 

ammonia. Dennis (et al 1991) states that ammonia forms an 
ammonium ion (NH4+) with an observed wave number at 1400-
1440 cm-1. Seo (et al 1988) states that ammonia can be adsorbed 
on the acid side of the heteropoly compound and in the metal 

cation. The compound K8[2-SiW11O39]∙nH2O/SiO2 exhibits 
the vibration of the NH4+ ammonium ion appearing at the wave 
number 1404.18 cm-1. 
 

 
Figure 6. FTIR spectra of  polyoxometalate compounds K8[2-
SiW11O39]∙nH2O/SiO2 (A), FT-IR spectra of polyoxometalate 

compounds K8[2-SiW11O39]∙nH2O/SiO2. Saturation with 
ammonia (B), FT-IR spectra of  polyhydroxide compound K8 

[2- K8[2-SiW11O39]∙nH2O/SiO2 saturation with pyridine (C) 
 

 Polyoxometalate compounds K8[2-SiW11O39]∙nH2O/SiO2 
saturated with ammonia absorbance bands not found at 
wavelength 1400-1440 cm-1. In this case it is possible that the 
ammonia was not adsorbed on the polyoxometalate compound 

K8[2-SiW11O39]∙nH2O/SiO2. Stuart (2004) states that vibration 
-OH vibration in the presence of hydrogen bonding effect is in 
the range of 3500-2500 cm-1 wavelengths characterized by a 
widened peak on the FTIR spectra. Polyoxometalate 

compounds K8[2-SiW11O39]∙nH2O/SiO2 exhibit a -OH loop 
vibration at 3425.58 cm-1 wave numbers identifying the presence 

of H2O in the polyoxometalate compound K8[2-
SiW11O39]∙nH2O/SiO2.  

Figure 6 (C) shows the spectra of K8[2-
SiW11O39]∙nH2O/SiO2 compounds with saturation using 
ammonia also exhibiting a -OH looping vibration at a 3425.58 
cm-1 wave number that identifies the presence of H2O in the 

polyoxometalate compound K8[2-SiW11O39]∙nH2O/SiO2. The 
Caliph and Prasetyoko (2008) explain that pyridine molecules 
bound to Lewis acid sites are absorbed at wave numbers 1400-
1700 cm-1.  Picture no. 10 (C) shows that the pyridine molecule 
has been adsorbed by the polymoxometalate compound K8 [2- 

K8[2-SiW11O39]∙nH2O/SiO2 shown in the wave number 
1481.33 cm-1. This shows that the polyoxometalic compound 

K8[2-SiW11O39]∙nH2O/SiO2 has Lewis acid character. 
Based on the FTIR spectrum of Figure 5 and 6, it can be 

seen that the wave number 1404.18 cm-1 is a vibration of the 

ammonium ion NH4+. The compound K8[2-
SiW11O39]∙nH2O/SiO2 saturated with ammonia (Figure 6 B) 
does not show the uptake of ammonia molecules at the 1404.18 

cm-1 wave number as well as the compound K8[2-
SiW11O39]∙nH2O (Figure 5  B). In this case the ammonia is not 

adsorbed on the compound K8[2-SiW11O39]∙nH2O and the 

compound K8[2-SiW11O39]∙nH2O/SiO2. The FT-IR spectra at 
Figure 5 (C) does not show the uptake of pyridine molecules at 
the 1404.18 cm-1 wave number while the FT-IR spectra of 

Figure 6 (C) of K8[2-SiW11O39]∙nH2O/SiO2 see the wave 
number 1481,33 cm-1  but does not show sharp spectra. It can 
be concluded that qualitatively pyridine adsorbed more on the 

polyoxometalate compound K8[2-SiW11O39]∙nH2O/SiO2. 
 

The acidity test of compound K8[2-SiW11O39]∙nH2O 

and K8[2-SiW11O39]∙nH2O/SiO2 in Quantitative 
 

The acidity measurements of the K8[2-SiW11O39]∙nH2O and 

K8[2-SiW11O39]∙nH2O/SiO2 compounds were quantitatively 
measured by titration of potentiometric potassium using n-
butylamine as titrant and acetonitrile as solvent. Acetonitrile is 

an aprotic solvent as a solvent on the compound K8[2-

SiW11O39]∙nH2O and K8[2-SiW11O39]∙nH2O/SiO2 so that only 

the acidity of K8[2-SiW11O39]∙nH2O and K8[2-
SiW11O39]∙nH2O/SiO2. According to Pecchi (et al 1985) using 
benzene solvent, acetonitrile and iso-octane as solvents in 
potentiometric titration and selected acetonitrile as a polar 
solvent to avoid the adsorption of n-butylamine and acetonitrile 
as an inert solvent  

The measurement by potentiometric method can determine 
the total acidity and acidity strengths of a polyoxometalate 
compound. The initial potential value (Ei) identifies acidity 
strength from the surface side and classifies the acidity strength 
based on the range that classifies in scale: : Ei > 100 mV (acidity 
is very strong), 0 < Ei < 100 mV (strong acidity), -100 < Ei < 0 
mV (weak acidity), Ei < -100 mV (acidity is very weak) 
(Romanelli et al, 2004). The first derivative curve and the second 
derivative are made to be able to see where the titration 
equivalent point is shown in Figure. 11 The equivalent point was 
performed to see the condition in which the base of n-

butylamine is added precisely reacts with the acidic K8[2-
SiW11O39]∙nH2O nH2O which was titrated.  In addition, an 
equivalence point is performed to determine the amount of base 

volume of n-butylamine required to neutralize K8[2-
SiW11O39]∙nH2O/SiO2 acid.  

Figure 7 shows the results of measurement of the compound 

K8[2-SiW11O39]∙nH2O  has an initial potential value of 54.4 mV. 
Based on the potential value range of 0 < Ei < 100 the 

polyoxometalate compound  K8[2-SiW11O39]∙nH2O has a 
strong acid side. The titration equivalent point is at 0.2 mL of n-
butylamine volume reinforced by the first derivative curve and 
the second derivative of potentiometric titration. The titration 
equivalent point can be observed with sharp potential changes 
(Mulja and Suharman, 1995). Figure 8 and 9 show the first 
derivative curves and the second derivative curves of the 

polyoxometalate K8[2-SiW11O39]∙nH2O.  
The measurement of the acidity level of the  polyoxometalate 

compound K8[2-SiW11O39]∙nH2O/SiO2 is also carried out  
through potentiometric titration. From the titration curve 
presented in Figure 10, the titration equivalent point was 
obtained at the time of titration volume of 0.4 mL n-butyamin. 
Based on the data of the equivalence point it is found that the 



Sari et al. / Science and Technology Indonesia 1(1) 2016:8-15 

@2016 Published under the terms of the CC BY NC SA 4.0 license 13 

polyoxometalate K8[2-SiW11O39]∙nH2O/SiO2 compound 
requires more base volume of n-butylamine to neutralize the  

polyoxometalate compound K8[2-SiW11O39]∙nH2O/SiO2. This 

indicates that K8[2-SiW11O39]∙nH2O/SiO2 is more acidic than a 

polyoxometalate compound K8[2-SiW11O39]∙nH2O. This is also 
supported by looking at the potential initial value comparison. 

The initial potential value of K8[2-SiW11O39]∙nH2O/SiO2 ions 

is 76.6 mV whereas the initial potential value of K8[2-

SiW11O39]∙nH2O compounds is 54.4 mV. The compound K8[2-
SiW11O39]∙nH2O/SiO2 is included in the acidic acid classification 
strong based on the potential value range of acid strength. 

Increased density of polyoxometalate compounds K8[2-

SiW11O39]∙nH2O/SiO2 because the compound K8[2-
SiW11O39]∙nH2O interacts with the carrier SiO2. 
 

 
Figure 7. Potentiometric titration curve of compound K8[2-

SiW11O39]∙nH2O 
 

 
Figure 8. The first titular titration curve of potentiometric 

titration of compound K8[2SiW11O39]∙nH2O 
 

 
Figure 9. The second titular titration curve of potentiometric 

titration of compound K8[2-SiW11O39]∙nH2O 
 

 
Figure 10. Potentiometric potentiometric curve of compound 

K8[2-SiW11O39]∙nH2O/SiO2 
 

 
Figure 11. The first derivative titration curve, potentiometric 

titration of compound K8[2-SiW11O39]∙nH2O/SiO2 
 

 

 
Figure 12. The second derivative titration curve, potentiometric 

titration of compound K8[2-SiW11O39]∙nH2O/SiO2 
 

Potentiometric titration method is an analytical technique 
based on the potential measurement of a sensor or electrode. 
The electrodes used are glass-containing glassed electrode, the 
liquid having the potential difference properties between the 
membrane and the electrolyte in contact with the membrane is 
determined by the activity of the particular ion. The membrane 
electrode used is a glass electrode. These glass electrodes are said 
to be ion-selective because they are specific only to H+ ions. 

Potential measurements of polyoxometalate compound K8[2-
SiW11O39]∙nH2O/SiO2 can be performed with potentiometric 

titration because the compound K8[2-SiW11O39]∙nH2O/SiO2 

has H+ ions. Potentiometric titration curve of K8[2-
SiW11O39]∙nH2O/SiO2 can be seen in Figure 10.  
 

The thermal Stability Compound of K8[2-
SiW11O39]∙nH2O/SiO2  

The compound K8[2-SiW11O39]∙nH2O/SiO2 of results the 
preparation was heated at various temperatures to see the 

thermal stability of K8[2-SiW11O39]∙nH2O/SiO2. The heating 
results at various temperatures were characterized by FTIR 



Sari et al. / Science and Technology Indonesia 1(1) 2016:8-15 

@2016 Published under the terms of the CC BY NC SA 4.0 license 14 

spectrophotometers. Figure 13 shows the FT-IR spectra of the 

wavelengths that appear on the K8[2-SiW11O39]∙nH2O and 

K8[2-SiW11O39]∙nH2O/SiO2 unheated and heated at various 
temperatures from 200-500°C. 

Figure 13 shows the difference shown by the FT-IR spectra 

of the K8[2-SiW11O39]∙nH2O compound before being carried 
out with SiO2 or after heating. Based on the FTIR spectrum the 
warming of vibrations emerging undergoes a shift in the number 
of waves. Figure 13 (A) and (B) show the wave numbers 3425.58 
cm-1 and 3448,72 cm-1 while in Figure 13 (C), (D), (E) and (F) 
indicate wave numbers 3433,29 cm-1, 3441,01 cm-1, 3425,58 cm-
1, 3402,43 cm-1 are identified as -OH groups in the presence of 
H2O with a slight amount seen from percent transmittance. 
Figure 13 (F) shows excellent thermal stability properties in the 
presence of small amount of H2O which was characterized by a 
shift in the wavelength number of the -OH group. The vibration 

of polyoxometalate compound K8[2-SiW11O39]∙nH2O/SiO2 in 
the 800-1000 cm-1 wave range at 500°C shows slight differences 
due to vibration W=O overlapping with vibrations W-Oe-W and 
W-Oc-W. This suggests that on increasing the heating 
temperature may cause changes in the structure of 
polyoxometalate compounds. Table 2 shows the vibrations of 

the K8[2-SiW11O39]∙nH2O , K8[2-SiW11O39]∙nH2O/SiO2 
without heating  and after heating at a temperature of 200°C - 
500°C. 

 

 
Figure 13. FT-IR spectra of polyoxometalic compound K8[2-

SiW11O39]∙nH2O (A), FT-IR spectra of polymoxyalate 

compound K8[2-SiW11O39]∙nH2O/SiO2 (B), FT-IR spectra of  

polyoxometalate compound K8[2-SiW11O39]∙nH2O/SiO2 of 
heating at 200°C (C), 300°C (D), 400°C (E), 500°C ( F). 

 

Table 2. Wave number of compound K8[2-SiW11O39]∙nH2O, 

K8[2-SiW11O39]∙nH2O/SiO2 without heating and heating at a 
temperature of 200°C-500°C 

K8[2-
SiW11
O39]∙n
H2O 

K8[2-
SiW11O

39]∙nH2
O/SiO2 

K8[2-
SiW11O39]∙nH2O/SiO2 

  
Vibrati
on 

 200°
C 

300°
C 

400°
C 

500°
C 

  

987.55 
864.11 
756.1 
3425.5
8 

956.69 
864.11 
- 
3448.72 
 

- 
887.
2 
794.
6 
3433
.2 

- 
887.
2 
740.
6 
3441
.0 

987.
5 
864.
1 
732.
9 
3425
.5 

979.
8 
858.
3 
748.
3 
3402
.4 

W   
W=O 
W  W-
Oe-W 
  WW-
Oc-W 
Si-  O-
H 

 

CONCLUSION  

  Polyoxometalate compounds K8[2-SiW11O39].nH2O and 

K8[2-SiW11O39]∙nH2O/SiO2 were synthesized. The main 

vibration of the IR spectra of K8[2-SiW11O39]∙nH2O shows the 
presence of vibration W=O appears in the area of 987.55cm-1. 
W-Oe-W appears in the area of  864.11cm-1, W-Oc-W appears in 
the area of  756.1cm-1, O-H, OH appears in the area of  of  

3425.58 cm-1 and for the compound K8[2-
SiW11O39]∙nH2O/SiO2vibration W=O appears in the region 
956.69cm-1, W-Oe-W appears At 864.11cm-1 and -OH area 
appears at 3448.72 cm-1. The XRD diffraction  pattern for 

K8[2-SiW11O39]∙nH2O differs at the diffraction angles of 8°, 9°, 

17°, 24°, 35° and for the compound K8[2-
SiW11O39]∙nH2O/SiO2 appears at the angle of diffraction of 2θ 

each of 8°, 18°, 27°, and 34°. Polyoxometalate compound K8[2-
SiW11O39]∙nH2O/SiO2 with a potential value of 76,6 mV has a 

higher acidity value than the compound K8[2-SiW11O39]∙nH2O 
having a potential value of 54.3 mVV. Qualitative analysis by 

using ammonia and pyridine to compound K8[2-

SiW11O39]∙nH2O and K8[2-SiW11O39]∙nH2O/SiO2 it was found 
that the pyridine compound adsorbed more on the  

polyoxometalate compound K8[2-SiW11O39]∙nH2O/SiO2. The 

thermal stability test of the compound K8[2-
SiW11O39]∙nH2O/SiO2 shows at a temperature of 500oC of 

K8[2-SiW11O39]∙nH2O/SiO2 compound slightly altered the 
structure of the polyoxometalic compound characterized by 
overlapping of vibrations in the range of 800-1000cm-1. 
 

ACKNOWLEDGEMENT 
The author would like to thank you to Prof. Aldes Lesbani, 

Ph.D and Dr.rer.nat. Risfidian Mohadi who have a rol  in helping 
and guiding the author in this study 
 

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