Synthesis and hydrogen release properties of alkyl-substituted amine-boranes

Abstract

Three organic amine-boranes—diethylenetriamine-borane (C₄H₁₃N₃·3BH₃, DETAB), triethylenetetramine-borane (C₆H₁₈N₄·4BH₃, TETAB) and tetraethylenepentamine-borane (C₈H₂₃N₅·5BH₃, TEPAB)—are synthesized via the liquid-phase reaction of diethylenetriamine (C₄H₁₃N₃, DETA), triethylenetetramine (C₆H₁₈N₄, TETA) and tetraethylenepentamine (C₈H₂₃N₅, TEPA), respectively, with BH₃–THF solution. By using high-resolution synchrotron powder X-ray diffraction (HR-XRD), Fourier transform infrared (FTIR), elemental analysis and solid-state ¹¹B nuclear magnetic resonance (NMR) measurements, the structural properties of the three compounds are characterized. Hydrogen desorption properties of these compounds are measured by temperature-programmed desorption (TPD) and thermogravimetry (TG) over a temperature range from 50 to 250 °C, in which 5.5, 6.6 and 6.9 equivalents hydrogen are released in two steps based on the combination of protic (N–H) and hydridic (B–H) hydrogens. It is confirmed by mass spectrometry (MS) results that only H₂ is liberated during the thermal decomposition of the three compounds. The dynamics are investigated by isothermal dehydrogenation at various temperatures. Compared with ammonia borane (NH₃BH₃, AB), these compounds show a faster dehydrogenation rate. A regeneration study shows that DETAB can be regenerated by treating its dehydrogenated products with lithium aluminium hydride (LiAlH₄) and ammonium chloride (NH₄Cl) at room temperature.