Reactive and non-reactive species formed during the methanolysis of NaBH4: a theoretical and experimental approach

Abstract

The formation, stability, and reactivity of the species generated by the reduction reaction of solid NaBH₄ with methanol in the liquid phase were investigated by experimental FTIR studies and density functional theory (DFT) calculations. A complete reaction system of NaBH₄ methanolysis considering solid–liquid–gas interfaces was outlined. The main species identified and differentiated through FTIR analysis of experimental tests correspond to [BH₄]⁻, alkoxyborohydride [BH_4−n(OCH₃)_n]⁻ (n = 1, 2, and 3), and tetramethoxyborate [B(OCH₃)₄]⁻. In correlation with the experimental results, the presence of reducing species in the liquid phase and in the equilibrium associated with Na⁺ in the solid phase is elucidated by DFT. Molecular modeling of the experimental results allows the establishment of the reaction mechanism involved in the generation of all the species and molecular hydrogen. The mechanism consists of four steps where the [BH₄]⁻ species initially interacts with methanol to generate hydrogen and the monosubstituted [BH₃(OCH₃)]⁻ species. Subsequently, this species reacts consecutively with methanol to generate the fully substituted species [B(OCH₃)₄]⁻.