Ionic Liquid-Enhanced Photooxidation of Water Using the Polyoxometalate Anion [P₂W₁₈O₆₂]^6–as the Sensitizer

Abstract

Simple polyoxometalate anions are known to be photoreduced in molecular solvents in the presence of 2-propanol or benzyl alcohol. The use of ionic liquids (ILs) as the solvent is now reported to also allow the photooxidation of water to be achieved. In particular, the photochemistry of the classic Dawson polyoxometalate salt K(6)[P(2)W(18)O(62)] has been studied in detail when water is present in the aprotic IL, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF(4)]) and the protic IL, diethanolamine hydrogen sulfate (DEAS). In these and other ILs, irradiation with white light (wavelength 275-750 nm) or UV light (wavelength 275-320 nm) leads to overall reduction of the [P(2)W(18)O(62)](6-) anion to [P(2)W(18)O(62)](7-) and concomitant oxidation of water to dioxygen and protons. The modified structure of bulk water present in ILs appears to facilitate its oxidation. Analogous results were obtained in aqueous solutions containing the protic IL as an electrolyte. The photoproducts (reduced polyoxometalate anion, dioxygen, and protons) were identified by, respectively, voltammetry, a Clark electrode, and monitoring of pH. The formal reversible potentials E(0)(F) for [P(2)W(18)O(62)](6-/7-/8-/9-/10-) couples are much more positive than in molecular solvents. The [P(2)W(18)O(62)](8-) and more reduced anions, if formed as intermediates, would efficiently reduce photoproducts H(+) or dioxygen to produce [P(2)W(18)O(62)](7-), rather than reform to [P(2)W(18)O(62)](6-). Thus, under photoirradiation conditions [P(2)W(18)O(62)](7-) acts as a kinetic sink so that in principle indirect splitting of water to produce dioxygen and dihydrogen can be achieved. The equivalent form of photooxidation does not occur in liquid water or in molecular solvents such as MeCN and MeCN/CH(2)Cl(2) containing added water, but does occur for solid K(6)[P(2)W(18)O(62)] in contact with water vapor.

Photochemical oxidation of water and reduction of polyoxometalate anions at interfaces of water with ionic liquids or diethylether

“…1). In a previous study, it was shown that photoreduction of ½P 2 W 18 O 62 6− does not occur in either bulk water or bulk DEAS (8). In the present two-phase configuration, the upper phase was water, in which K 6 ½P 2 W 18 O 62 is highly soluble, and the lower phase was DEAS, in which the salt is essentially insoluble.…”

“…In recent studies, we have found that photoreduction of tetracyanoquinodimethane (TCNQ) to TCNQ − and ½P 2 W 18 O 62 6− to ½P 2 W 18 O 62 7− or more extensively reduced POMs occurs in "wet" ionic liquids (ILs) where water acts as an electron donor and is photo-oxidized to dioxygen (7,8 (Fig. S1) are summarized in Eqs.…”

“…However, while ½P 2 W 18 O 62 7− is the detected product it is probable that more extensively reduced POMs are generated (8) as intermediates in the overall water oxidation reaction [Eq. 3]:…”

“…However, while multistep two-and four-electron-proton-coupled reaction schemes have been postulated, full mechanistic details have yet to be established (8).…”

“…Both protic and aprotic ILs are often sparingly soluble or immiscible in water, so certain mixtures of ILs and water give rise to two phases. Furthermore, on examining ILs as a medium for photooxidation of water (7,8), it was noted that even if an IL is fully miscible with water, the kinetics of its dissolution are often sufficiently slow that two phases may persist for long periods of time. Consequently, studies on the photochemistry at both thermodynamically stable and unstable water-IL interfaces should be possible in a regime in which significant structural kinetic and thermodynamic differences occur relative to bulk solvents.…”