Structure of alkaline aqueous NaBH4 solutions by X-ray scattering and empirical potential structure refinement

Abstract

Structure of alkaline aqueous NaBH4 solutions at NaBH4 concentrations of 1.0 to 9.1 mol·dm−3 (almost saturated) as well as a 1.3 mol·dm−3 NaOH solution was studied by X-ray diffraction and Empirical Potential Structure Refinement (EPSR) modelling. In the 1.0 mol dm−3 alkaline NaBH4 solution, Na+ is surrounded by around six water molecules with octahedral geometry. With an increase in NaBH4 concentration, the hydration number of Na+ decreases from 4.8 ± 1.1 at 2.25 mol·dm−3 solution to 2.9 ± 1.3 at 9.1 mol·dm−3 with an almost constant Na+-O (W: H2O) distance of 2.34 Å. The decrease in hydration number for Na+ is compensated by the formation of ion pairs between Na+ and BH4− to ensure an octahedral hydrated geometry for Na+. About 6.0 ± 1.6 water molecules are likely to bond to BH4−via tetrahedral edges or tetrahedral corners without a very specific hydration geometry, giving rise to each hydrogen atom of BH4− bound to 2.3 ± 1.0 water molecules through dihydrogen bonds. The hydration number around BH4− decreases with increasing concentration. BH4− tends to form contact ion pairs with Na+ at the Na-B distances of 3.24 Å and 2.82 Å in tetrahedral-corner-shared and tetrahedral-edge-shared fashions, respectively; the number of Na-B interactions increases from 0.3 ± 0.5 in 1.0 mol·dm−3 (almost negligible) to 2.1 ± 0.9 in 9.0 mol·dm−3. Tetrahedral-edge-shared bidentate ion pair is likely to be the preferred and dominant ion cluster over the wide concentration range. Ion association between the Na+ and BH4− was crosschecked by density function theory (DFT) calculations on [NaBH4(H2O)6] clusters.