Solubility thermodynamics of amine boranes in polar solvents

Abstract

Amine boranes are solid hydrogen storage materials and are difficult to transport within systems unless dissolved in solvents. The solubility and free energy of solution were experimentally determined for ammonia borane, methylamine borane, dimethylamine borane, trimethylamine borane, and tert-butylamine borane in methanol, ethanol, 1-propanol, 2-propanol, acetonitrile, dimethyl sulfoxide, and tetrahydrofuran. The solubilities of ammonia borane/methylamine borane mixtures in methanol were also measured. The enthalpy of solution was experimentally determined for ammonia borane, methylamine borane, dimethylamine borane, and 30/70 wt% ammonia borane/methylamine borane eutectic in four different solvents: methanol, ethanol, acetonitrile, and dimethyl sulfoxide. For all solutes, dimethyl sulfoxide solutions were the most exothermic or least endothermic. Increased methyl-substitution on the amine led to more endothermic processes. For methanol, ethanol, and acetonitrile, the entropy of solution increased as solubility increased. The entropy of solution for ammonia borane dissolving in dimethyl sulfoxide is negative. COSMO-RS solubility calculations using density functional theory optimized geometries agree relatively well with experimental values for amine borane materials dissolved in polar solvents.