CHEM. RES. CHINESE UNIVERSITIES 2011, 27(2), 177—180
1-Methyl-3-octylimidazolium Polyoxomolybdate Ionic Liquid with
Low Melting Point and High Stability:
Preparation and Photocatalytic Activity
DONG Tao, XU Yan-qing*, CHEN Fa-wang, CHI Ying-nan and HU Chang-wen*
State Key Laboratory of Explosion Science and Technology, Department of Chemistry,
Institute for Chemical Physics, Beijing Institute of Technology, Beijing 100081, P. R. China
Abstract The polyoxometalate(POM)-imidazole ionic liquid(IL) [C8mim]2[Mo6O19](C8mim=1-methyl-3-octylimidazolium) with a low melting point of 82.6 °C was successfully prepared and characterized by FTIR, XPS, NMR, TG
and so on. The polyoxomolybdate-based IL has high stability, and its decomposing temperature reaches 321 °C,
which is higher than that of 1-alkyl-3-methylimidazolium halides IL. Further photocatalytic performances of the IL
were measured via degrading dye rhodamine B(RB) in aqueous solution under the UV light irradiation. The experiments show that the conversion of RB reaches 80.5% after 90 min under UV-light and the degradation efficiency depends on the pH value of the solution, irradiation time and the dosage of the IL and so on.
Keywords Ionic liquid; Polyoxomolybdate; Low melting point; High stability; Photocatalytic activity
Article ID 1005-9040(2011)-02-177-04
1
Introduction
Ionic liquids(ILs) are a series of molten salts that are completely composed of ions and become liquid at low temperature(<100 °C). Ionic liquids have received considerable attention in different research fields due to their unique physicochemical properties, such as negligible vapor pressure, nonflammability, thermal and chemical stabilities. Recently, the
strategy of combination of polyoxometalate(POM) anions with
appropriate cations has been regarded as a feasible one to afford promising ionic liquids, for they might possess the advantages of both IL and POMs in the same material. So far, several
POM-based ILs have been prepared. A series of novel tetraalkylphosphonium polyoxometalate ILs has been synthesized in
Antonio’s laboratory[1,2], especially, the melting point of
[(C6H13)3P(C14H29)]2W6O19 is below room temperature.
However, the application of these tetraalkylphosphonium
ILs was blocked due to their inherent high viscosity and
low stability. In order to overcome these drawbacks, another
kind of POM-based imidazolium salts has received more
attention. For instance, [Cnmim]3PW12O40, [bmim]4[XM12O40],
[bmim]4[SiM12O40] and [bmim]4[S2M18O62](M=Mo, W; X=P,
Si; n=2,5; bmim=1-methy-3-butylimidazolium) have been
synthesized successfully that exhibit good electrochemical
properties of anion[3―8]. Unfortunately, the high melting
point in such systems can also block their application in
some fields.
It is known that the charge of anion and the symmetry of
cation are the two important factors governing the melting
point[1]. We purposely chose the lowest charge Lindqvist
of Mo6O192– to react with asymmetric [C8mim]Cl(C8mim=
1-methyl-3-octylimidazolium) and synthesized the novel
POM-imidazole IL [C8mim]2[Mo6O19], which shows a much
lower melting point than the analogue in the previous literature[5] as expected. Furthermore, it also shows good photocatalytic activity to degradate the dye RB under UV-light and acts
as a heterogeneous system photocatalyst insoluble in water,
which can be easily separated from the reaction mixture.
2
2.1
Experimental
Materials and Instruments
1-Methyl-3-octylimidazolium chloride was purchased
from Aldrich Chemical Co. and used without further purification. The water used in all the experiments was deionized to a
resistivity of 16―18 MΩ/cm. All other reagents purchased
from Beijing Chemical Company(China) were reagent-grade.
FTIR spectra of the samples were recorded on a Thermal
Nicolet 670 FTIR in a range of 4000―450 cm–1 via KBr pellet
technique at room temperature. NMR spectra were taken on a
Bruker Avance III(400 MHz). Differential scanning calorimetry(DSC) was performed on a TA Instruments model Q100
interfaced with a refrigerated cooling system. Thermogravimetric analysis(TGA) was carried out on a TA Instruments Q50 by
heating from 25 °C to a final temperature of 800 °C at a rate of
10 °C/min under N2 flow. The XPS spectra were obtained on a
Perker-Elemer PHI5300 X-ray Photoelectron Spectra.
———————————
*Corresponding author. E-mail: [email protected]; [email protected]
Received January 25, 2010; accepted March 29, 2010.
Supported by the National Natural Science Foundation of China(Nos.20671011, 20731002, 20801004, 10876002, 20801005)
and the Specialized Research Fund for the Doctoral Program of Higher Education of China(No.200800070015).
178
CHEM. RES. CHINESE UNIVERSITIES
2.2 Preparation of POMs-imidazole Ionic Liquid
[C8mim]2[Mo6O19]
The POM-imidazole ionic liquid was prepared and characterized as detailed in Ref.[9]. Typically, Na2MoO4·2H2O
(10.3 mmol) in 10 mL of water was acidified with 2.9 mL of 6
mol/L HCl in a flask with a magnetic bar over one minute at
room temperature. A solution containing 3.75 mmol [C8mim]Cl
was added to the above solution with vigorous stirring, forming
a white precipitate and then heated at 70 °C for 45 min. It was
washed with 20 mL of water for three times. The crude product
was crystallized by hot acetone.
2.3
Further XPS measurement was performed to identify elemental composition of the IL. The IL exhibits peaks corresponding to C1s(Eb=286.5 eV), N1s(Eb=402.0 eV), O1s (Eb=533.8
eV), Mo3d5/2(Eb=236.6 eV) and Mo3d3/2(Eb=239.8 eV)(Fig.2).
The N1s is attributed to the nitrogen atom of the N―C bond in
the imidazole ring. The C1s signal can be assigned to the carbon
atoms in [C8mim]+. The O1s is ascribed to the oxygen atoms in
Mo6O19. Those XPS results further confirm the existence of the
Mo6O19 and imidazolium in the POM-based ionic liquid that is
conformable to the results of FTIR spectra.
Photocatalytic Test
The photoreactor was designed with a cylindrical quartz
cell and an internal light source surrounded by a quartz jacket,
where the suspension of catalyst and RB aqueous completely
surrounded the light source. The temperature of suspension was
maintained at (30±2) °C by circulation of water through an
external cooling coil. The light source was a 500 W high pressure mercury lamp. The photocatalytic procedure was carried
out as follows: first the catalyst was suspended in a fresh
aqueous solution of RB. The suspension(25 mL) was ultrasonicated for 10 min and stirred in the dark for 30 min to obtain a
good dispersion. The lamp was inserted into the suspension
after its intensity became stable. The suspension was vigorously stirred in an open system during the process.
3
Vol.27
Results and Discussion
3.1 Structure of Ionic Liquid
The structure of the POM-imidazole ionic liquid was investigated by FTIR and XPS. The FTIR spectra were used to
identify the structure of hexamolybdate ionic liquid shown in
Fig.1. The characteristic peaks at 3150, 3104, 1574, 1470, 1170
and 622 cm–1 are attributed to imidazole ring ν(C―H), imidazole ν(ring), imidazole H―C―C and H―C―N bending,
imidazole C2―N1―C5 bending, respectively. In the region of
700―1100 cm-1, four characteristic peaks at 961, 908, 801 and
741 cm–1 are ascribed to ν(Mo―Oa), ν(Mo―Ob―Mo) and
ν(Mo―Oc―Mo) of the [Mo6O19]2– polyoxoanion[6]. The results
indicate that [C8mim]+ and [Mo6O19]2– retaining the original
structure exist in the POM-based IL.
Fig.2 X-Ray photoelectron spectra of C(A), N(B),
O(C) and Mo(D) in the IL
The 1H NMR spectrum was used to identify the purity of
[C8mim]2[Mo6O19] IL in dimethylsulfoxide(Fig.3). The chemical shifts for the HNMR of [C8mim]2[Mo6O19] appear as
follow[400 MHz, (CD3)2SO]: δ 0.86[t, 6H, ―(CH2)7CH3],
1.25[m, 20H, ―(CH2)2(CH2)5CH3], 1.78[m, 4H,
―CH2―CH2―(CH2)5CH3], 3.85(s, 6H, CH3―N), 4.15[t, 4H,
―CH2―CH2―(CH2)5CH3], 7.69(t, 2H, N―CH=CH―N),
7.76(t, 2H, N―CH=CH―N), 9.09(s, 2H, N―CH=N). The
results indicate that the IL is a pure material.
Fig.3
1
H NMR spectrum of [C8mim]2[Mo6O19]
3.2 Differential Scanning Calorimetry(DSC) and
Thermogravimetric Analysis(TG)
Fig.1
FTIR spectra of 1-methyl-3-octylimidazolium
chloride(a) and hexamolybdate ionic liquid(b)
As shown in Fig.4(A), DSC curve of [C8mim]2[Mo6O19]
IL shows only a single endothermic transition，as indicates its
melting point at 82.6 °C. Compared with the literatures[3―8]
reported on POM-based imidazolium salts, it decreased from
over 340 °C to below 100 °C, as proves that our synthesized
compound belongs to the IL species. Fig.4(B) depicts the
No.2
DONG Tao et al.
results of TG studies of hexamolybdate IL(curve a) and
1-methyl-3-octylimidazolium chloride(curve b). The mass
loss(ca. 32%) for hexamolybdate IL occurs at 321―368 °C,
which corresponds to the removal of organic portion [C8mim]+
(calcd. 32%). It also confirms the stoichiometric ratio for
[C8mim]+/[Mo6O19]2– is 2 in hexamolybdate IL. For comparison with the hexamolybdate IL, the TG curve of [C8mim]Cl is
shown in Fig.4(B) curve b. The decomposition of [C8mim]Cl
occurs at 186 °C. Analysis indicates that the hexamolybdate IL
has higher thermal stability than that of 1-alkyl-3-methylimidazolium halides IL under the same condition[10].
Fig.4
3.3
Differential scanning calorimetry heating scans
for hexamolybdate ionic liquid(scan rate:
10 °C/min)(A) and TG curves(B) of [C8mim]2·
[Mo6O19](a) and [C8mim]Cl(b)
Photocatalytic Test
The photocatalytic activity of [C8mim]2[Mo6O19] was
tested by the model reaction of degrading the dyes of RB under
UV irradiation. At first, the experiment showed no concentration change of RB in the dark for 90 min in the absence of the
catalyst. Then we exposed RB to UV-light for 90 min without
the catalyst, the conversion of RB was 12.6%. Finally, when
the catalyst was added into the system under the UV radiation
for 90 min, we found the conversion of RB increased rapidly to
80.5%. Moreover, the investigations on the effect of pH value
and the dosage of catalyst on RB degradation were detailed as
shown in Fig.5. The results indicate that when pH value is
larger than 1 and the dosage is 20 mg/L, the title catalyst shows
the optimal photocatalytic degradation efficiency on RB with
the increase of the irradiation time.
179
The photocatalytic degradation shown in Fig.6 follows an
apparent first order process, in agreement with a generally observed Langmuir-Hinshelwood kinetics model[11]:
dc
kKρ
(1)
r=
=
d t 1 + Kρ
where r is the degradation rate of the reactant(mg·L–1·min–1), ρ
is the concentration of the reactant(mg/L), t is the illumination
time, k is the reaction rate constant(mg·L–1·min–1), and K is the
adsorption equilibrium reactant(L/mg). When the chemical
concentration ρ0 is micromolar, Kρ<<1, the above model can be
expressed as Eq.(2)[12]:
lnr=ρ0/ρt=kKt=kappt
(2)
Fig.6
Relationship between ln(ρ0/ρt) and UV
irradiation time
pH=1.04, ρ(cat.)=0.4 g/L; ρ0/(mg·L–1): a. 10; b. 20;
c. 30; d. 40.
A plot of ln(ρ0/ρt) versus time exhibits a straight line and
the slope of which upon linear regression equals to the apparent
first-order rate constant kapp. The value of kapp decreased in the
order of 0.01829>0.01003>0.0031> 0.0027 with the increase of
RB concentration. Therefore, the reaction of the photocatalytic
degradation of RB by [C8mim]2[Mo6O19] is apparently
first-order kinetics of a Langmuir-Hinshelwood model, whose
apparent relationship is due to the low concentration of RB
chosen. Although some POMs anions can act as strong oxidant
upon illumination with near Vis and UV light, it has been established that photodecomposition mainly takes place via OH
radicals[13―16]. There were two possible radicals(O2–, ·OH) for
the RB degradation in our experiments. The formation process
of the two radicals could be described by Eqs.(3)―(5):
POM*→e–+h+
(3)
e +O2→O2–
(4)
OH–+h+→·OH
(5)
After irradiation, the powder of the catalyst was separated
from the dye solution and recovered by rinsing with a lot of
deionized water and dried in vacuum. When the catalyst was
recycled for three times, the conversions of RB were found to
be 78.5%, 78.2% and 77.6%, respectively.
POM
–
4 Conclusions
Fig.5
Photocatalytic degradation conversions of
RB solution at different pH values(A) and
in different dosages of catalyst(B)
(A) 10 mg/L RB; 0.4 g/L cat.; (B) 10 mg/L RB; pH=1.03.
In summary, 1-methyl-3-octylimidazolium polyoxometalate IL with low melting point and high thermal stability was
successfully designed and synthesized. It shows good photocatalytic activity for degrading RB with a conversion of 80.5%
under UV-light for 90 min. Together with its high thermal
stability and good catalyst efficiency, it may provide a novel
180
CHEM. RES. CHINESE UNIVERSITIES
type of material with latent application in the field of photoelectrochemical cells and fuel cells.
Vol.27
[8] Rao G. R., Rajkumar T., Varghese B., Solid State Sci., 2009, 11, 36
[9] Fournier M.; Ed. by Ginsberg A. P., Inorganic Synthesis, John Wiley,
New York, 1990, 77
References
[1] Rickert P. G., Antonio M. R., Firestone M. A., Kubatko K. A.,
Szreder T., Wishart J. F., Dietz M. L., J. Phys. Chem. B, 2007, 111,
4685
[2] Rickert P. G., Antonio M. R., Firestone M. A., Kubatko K. A.,
Szreder T., Wishart J. F., Dietz M. L., Dalton Trans., 2007, 529
[3] Chang M. H., Dzielawa J. A., Dietz M. L., Antonio M. R., J.
Electroanal. Chem., 2004, 567, 77
[4] Zhang J., Bond A. M., MacFarlane D. R., Forsyth S. A., Pringle J. M.,
Inorg. Chem., 2005, 44, 5123
[5] Mariotti A. W. A., Xie J. L., Abrahams B. F., Bond A. M., Wedd A.
G., Inorg. Chem., 2007, 46, 2530
[6] Rajkumar T., Rao G. R., Materials Letters, 2008, 62, 4134
[7] Rajkumar T., Rao G. R., Mater. Chem. Phys., 2008, 112, 853
[10] Huddleston J. G., Visser A. E., Reichert W. M., Heather D. W.,
Broker G. A., Rogers R. D., Green Chem., 2001, 3, 156
[11] Konstantinou I. K., Albanis T. A., Appl. Catal. B: Environ., 2004, 49,
1
[12] Yang Y., Guo Y. H., Hu C. W., Wang Y. H., Wang E. B., Appl. Catal.
A: Gen., 2004, 273, 201
[13] Guo Y. H., Li D. F., Hu C. W., Wang Y. H., Wang E. B., Zhou Y. C.,
Feng S. H., Appl. Catal. B, 2001, 30, 337
[14] Guo Y. H., Hu C. W., Jiang S., Guo C. X., Yang Y., Wang E. B., Appl.
Catal. B, 2002, 36, 9
[15] Stattari D., Hill C. L., J. Am. Chem. Soc., 1993, 115, 4649
[16] Mylonas A., Hiskia A., Androulaki E., Dimotikali D., Papaconstantinou E., Phys. Chem. Chem. Phys., 1999, 1, 437