T. Huang et al. / Journal of Alloys and Compounds 660 (2016) 17e22
19
characteristic peaks at 1468e1485 cmꢁ1 (pyridine ring), and
1144e1160 cmꢁ1 (pyridine HeCeC and HeCeN bending). The re-
sults confirm the successful assembly and existence of the organic
cation and Keggin unit in the compounds without depolymeriza-
tion or degradation.
3.2. XRD patterns
The phase and structure of the obtained materials are further
identified by using powder X-ray diffraction (XRD) in Fig. 2. The
peaks in the XRD patterns of these POM-ILs are apparently different
from that of the parent heteropoly acids. The broad diffraction
peaks at the region of 2
q
¼ 15e38ꢀ of the products indicate
smectic-state appearance of the samples, while their pure acids
have high crystallinity [29]. The phase change of the hybrid mo-
lecular compounds was caused by the replacement of protons in
HPA with PyPS and TEAPS cations [30].
Fig. 1. IR spectra of the gel-type POM-ILs. (a) [PyPS]5SiW11VO40; (b) [PyPS]7SiW9V3O40
(c) [TEAPS]5SiW11VO40 and (d) [TEAPS]7SiW9V3O40
;
.
In the small-angle region, the two monovanadium-substituted
POM-ILs display different Bragg diffraction peaks from the
trivanadium-substituted ones. [PyPS]5SiW11VO40 presents visible
peaks at 2
q
¼ 4.60ꢀ and 8.92ꢀ, which arises from the regular
nas (MeOd) > nas (SieOa) > nas (MeObeM) > nas (MeOceM),
(M ¼ W, Mo, V), and Oa, Ob, Oc and Od, each separately represent
inner oxygen, corner-sharing oxygen, edge-sharing oxygen and
arrangement of the molecules in layers with a d-spacing of 1.92 nm
[31]. Similarly, three Bragg diffraction peaks which appeared at
4.50ꢀ, 7.22ꢀand 8.36ꢀ in the XRD pattern of [TEAPS]5SiW11VO40 also
indicate a layered structure with a d-spacing of 1.96 nm. According
to the recent available paper, we can assume that such organized
layer-type structure is formed via self-assembly of the heteropoly
anions and organic cations through electrostatic interactions,
hydrogen bondings, and Van der Waals forces [32]. However,
terminal oxygen. There are characteristic bands of [SiW11VO40
the IR spectrum of [PyPS]5SiW11VO40 (curve a): 970 cmꢁ1
(MeOd); 915 cmꢁ1 nas (SieOa); 880 cmꢁ1
nas (MeObeM);
793 cmꢁ1
nas (MeOceM). The similarities of the spectra of
]
5ꢁ in
nas
,
,
,
,
[PyPS]7SiW9V3O40 (curve b), [TEAPS]5SiW11VO40 (curve c) and
[TEAPS]7SiW9V3O40 (curve d) with [PyPS]5SiW11VO40 strongly in-
dicates that they have the same structure as [SiM12O40]
4ꢁ, showing
[PyPS]7SiW9V3O40 shows only one peak at 8.25ꢀ, and [TEAPS]7
-
SiW9V3O40 shows the peaks at 8.52ꢀ and 9.86ꢀ, which attributed to
that they still maintain Keggin structure though the introduction of
vanadium atoms, which coincide with those reported in the liter-
ature for Keggin unit [25]. In comparison with the parent acids
H5SiW11VO40 and H7SiW9V3O40 according to the literature available
[23,26], the nas (MeOd) stretching vibration frequencies of these
polyoxometalate compounds have red shifts after protons in the
heteropoly acids were substituted by organic cations. The major
reason may be that the introduced cations have stronger in-
teractions to terminal oxygen atoms of the polyoxoanions,
impairing the MeOd bond, reducing the MeOd bond force constant
and leading to a decrease in the MeOd vibration frequency [27]. The
complete correlative assignments of the vibrational peaks of the
POM-IL gels are listed in Table 1. Bands corresponding to both the
organic cations and inorganic anions can be easily identified and
confirm the formations of the hybrid molecular compounds. The
bands at 1220e1236 cmꢁ1, which ascribed to S]O stretching vi-
brations, were detected to verify the existence of sulfonic groups in
these two series of products [28]. Besides, PyPS cation shows other
the characteristic peaks of Keggin structure at the region of
2q
¼ 7e10ꢀ [33].
3.3. TG and DTA curves
Fig. 3 indicates TG and DTA curves of these gel-type POM-ILs.
The initial minor weight loss below 150 ꢀC is attributed to the loss
of small amounts of absorbed water in the samples. The major
weight loss between 300 and 400 ꢀC is primarily due to the
decomposition of both organic and inorganic parts of the com-
pounds into their individual oxides. Generally, we take the tem-
perature of the exothermic peak of DTA curve as the sign of their
thermostability [34]. The decomposition temperature of [PyPS]5
-
SiW11VO40 (382 ꢀC) is much higher than that of [PyPS]7SiW9V3O40
(344 ꢀC). Similarly, [TEAPS]5SiW11VO40 is thermally stable up to
351 ꢀC, while the DTA curve of [TEAPS]7SiW9V3O40 shows that the
decomposition occurs at 312 ꢀC. The results demonstrate that the
Table 1
The complete correlative assignments of the vibrational peaks of the gel-type POM-ILs.
Wavenumber (cmꢁ1
[PyPS]5SiW11VO40
)
Vibrations
[PyPS]7SiW9V3O40
[TEAPS]5SiW11VO40
[TEAPS]7SiW9V3O40
3423
3070
2976
1635
1489
e
3435
3066
2976
1633
1490
e
34,235
e
3445
e
OeH stretching
pyridine ring CeH stretching
alkyl CeH stretching
HeOeH bending
eCH2 scissoring
eCH2 twisting
2990
1635
1482
1393
1220
1149
972
2990
1638
1480
1390
1220
1160
965
1236
1144
970
1230
1155
965
S¼O stretching
HeCeC & HeCeN bending
MeOd stretching
915
911
917
912
SieOa stretching
884
794
e
881
795
e
MeObeM stretching
MeOceM stretching
797
790