Synthesis and Properties of a Series of Pure Inorganic Ionic Liquids Based on Rare Earth Cations and Polyoxometalates

doi:10.6023/A21040130

Abstract

Ionic liquids (ILs) usually refer to ionic compounds that are liquid at a temperature of 100 ℃ and below. As salts, ionic liquids are composed of ions. So by selecting the corresponding cations, anions or ion combinations, they can be designed for specific applications. Various new types of ionic liquids have been prepared and discovered. Polyoxometalates (POMs) are a kind of metal-oxygen cluster compounds with rich composition and diverse structure. Compared with other common inorganic anions (such as PO₄^3-, SO₄^2-, CO₃^2-,etc.), polyoxometalate anions have the characteristics of large ion radius and low charge density, which are a good anion choice for constructing ionic liquids. Most of the currently reported polyoxometalate ionic liquids are composed of organic cations and polyoxometalate anions, while pure inorganic ionic liquids composed of inorganic cations and polyoxometalate anions are rarely reported. In this work, we report a series of lanthanide- containing compounds based on a Ti-substituted Keggin-type polyoxometalate prepared by ion exchange, KLnH₃PTi₂W₁₀O₄₀•xH₂O [Ln=La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Er (7), Yb (8)]. They were characterized by inductively coupled plasma emission spectrometry (ICP), thermal gravimetric analysis (TG), Fourier transform infrared (FT-IR) spectroscopy, powder X-ray diffraction (PXRD). Surprisingly, these inorganic ionic compounds behave as liquid with good fluidity in the temperature range of 4~65 ℃, and their conductivity are all higher than 10 mS•cm^-1, showing ionic liquid behavior. Water is an indispensable component in their composition. After losing water, they become transparent and colored solids with amorphous structure. The study found that the conductivity of these ionic liquids change significantly with temperature. For example, when the temperature rises from room temperature to 65 ℃, the conductivity of Ce-containing ionic liquid gradually increases from 13.3 mS•cm^-1 to 22.6 mS•cm^-1. When the temperature rises from 65 ℃ to 90 ℃, the conductivity drops significantly, as low as 1.22 mS•cm^-1 at 90 ℃. As far as we know, this kind of pure inorganic ionic liquid is very rare. The synthesis method of series ionic liquids is simple, easy to operate, and environmentally friendly. These properties make the series of ionic liquids have good machinability and potential applications in the fields of catalysis and phase separation.

Key words: Ti-substituted Keggin-type polyoxometalate, rare earth, inorganic ionic liquids, conductivity, upper critical solution temperature (UCST)

Cite this article

He-Nan Wang, An-Ge Zhang, Zhong Zhang, Hong-Rui Tian, Qian Yue, Xue Zhao, Ying Lu, Shu-Xia Liu. Synthesis and Properties of a Series of Pure Inorganic Ionic Liquids Based on Rare Earth Cations and Polyoxometalates[J]. Acta Chimica Sinica, 2021, 79(7): 920-924.

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Fig. & Tab. 7

Figure 1. (a) Serious of ionic liquids (1~8); (b) The solids (1s~8s) obtained by drying the series of ionic liquids at 50 ℃ for 48 h

Figure 2. IR spectra of (a) as-synthesized K7PTi2W10O40, (b) ionic liquid 2, (c) solid 2s and (d) oil 2o

Figure 3. PXRD spectra of (a) as-synthesized K7PTi2W10O40, (b) simulated K7PTi2W10O40, (c) ionic liquid 2, (d) solid 2s and (e) oil 2o

Ionic liquid	H₂O (w)	x	σ/(mS•cm^-1)	ECW/V
1	20.69	40.38	10.87	1.6
2	18.66	35.52	13.30	1.9
3	19.02	36.38	12.65	1.7
4	15.93	29.39	13.96	1.7
5	14.76	26.91	14.24	1.8
6	17.49	32.96	13.25	1.6
7	17.98	34.28	12.04	1.8
8	20.1	39.42	10.63	1.8

Table 1. Properties of the series of ionic liquids: water content, the value of x, conductivity at 25 ℃, electrochemical windows

Figure 4. Conductivity of ionic liquid 2 at different temperatures. Inset: Visual representation of temperature-dependent viscosity change of the ionic liquid 2

Figure 5. Cyclic voltammograms of ionic liquid 2. Inset: The dependence of cathodic peak and anodic peak currents on scan rates of ionic liquid 2

Figure 6. Liquid-liquid equilibrium phase diagram of the ionic liquid 2/H2O binary mixtures. Inset: Visual representation of temperature- dependent miscibility of the ionic liquid 2 with water (ionic liquid 2, w=60%)

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