Abstract
The reactions of imidazolium-based ionic liquids having different substituent groups on the ring with hydrated electrons (e −aq ), hydroxyl radicals (•OH), and sulfate anion radicals (SO •−4 ) were investigated using nanosecond pulse radiolysis techniques. The spectra of these ionic liquids on reaction with eaq − all exhibited a similar peak at about 320 nm, and a typical peak for e −aq in aqueous ionic liquid solutions. The reaction rate constants for 1,3-disubstituted imidazolium-based ionic liquid hexafluorophosphates (BMIPF6) with eaq − were deduced to be 1010 L mol−1 s−1, however the values were lower for trisubstituted ionic liquids. For example, the rate constant for 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (BMMIBF4) was 5.5×109 L mol−1 s−1. Imidazolium-based ionic liquids reacted with hydroxyl radicals via adducts to produce a mixture of isomeric OH adducts, and the pk a value of the OH adducts was deduced to be 8.4±0.4 for 1-butyl-3-methylimdazolium tetrafluoroborate (BMIBF4). Moreover, imidazolium cations were also oxidized by SO4 •− to produce bivalent cation radicals, which exhibit a peak at 320 nm, and these rate constants are of the same order of magnitude, i.e., 109 L mol−1 s−1, except that for 1-(2-hydroxyethyl)-3-methylimdazolium tetrafluoroborate with SO4 •− (k=2.8×108 L mol−1 s−1). Theoretical calculations were carried out to estimate the structures of the products of reduction by eaq −1 and the results were related to the experimental data.
Highlights
The reactions of imidazolium-based ionic liquids having different substituent groups on the ring with hydrated electrons (e−aq), hydroxyl radicals ( OH), and sulfate anion radicals (SO4 −) were investigated using nanosecond pulse radiolysis techniques
To achieve reactions of hydrated electrons, the solutions were pre-saturated with N2 in the presence of 1.0 mol L−1 tert-butyl alcohol (t-BuOH), which can be converted into the relatively inert t-BuOH radicals by scavenging hydroxyl radicals and hydrogen radicals (see eqs. (3) and (4)) [17]
The transient absorption spectrum was obtained under pulse radiolysis for an aqueous solution containing BMIBF4 (0.1 mmol L−1) and t-BuOH (1.0 mol L−1), deoxygenated with N2 (Figure 1)
Summary
All ionic liquids were provided by the Chengjie Chemical Company (Shanghai, China), and were of the highest purity available commercially. The ionic liquids were treated with activated charcoal for at least 48 h and filtered twice by passing through a Celite column. The purified ionic liquids were stored in an airproof desiccator. Tert-butyl alcohols and K2S2O8 were purchased from Sigma Company (Shanghai, China). All sample solutions were freshly prepared in triply distilled water and deaerated with high purity N2 (99.99%) or N2O, for different purposes, by bubbling for at least 20 min immediately before the experiments. The imidazolium-based ionic liquids containing the anions BF4− or NO3− are soluble in water, whereas those with NTF2− and PF6− are less soluble. At 20°C, the solubility of BMINTF2 in water is about 1.4% and that of BMIPF6 is 2% [15]
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