Aluminum Hydride Separation Using N-Alkylmorpholine

Abstract

We describe experimental and theoretical studies of several amine·alane adducts for alane separation. First, N-alkylmorpholine·alane adducts (NMM·AlH3 and NEM·AlH3; NMM = N-methylmorpholine, NEM = N-ethylmorpholine) were synthesized and characterized by NMR, IR, and XRD studies. Because of the bifunctionality (or dual coordination mode) of N-alkylmorpholine, NMM·AlH3 and NEM·AlH3 exhibit significantly improved thermal stability compared with the related amine·alane adducts. In the solid state, both NMM·AlH3 and NEM·AlH3 are polymers, which readily dissociate into monomers in donor solvents, as suggested by IR spectroscopy. In addition, the cost- and energy-effective transamination of (amine)2·AlH3 with NMM (or NEM) has been achieved. Because of the fast reaction kinetics, the transamination reaction could be combined with hydrogenation of Al metal to prepare NMM·AlH3 in a single step, further improving the efficiency of the process. Moreover, the thermal decomposition pathways of NMM·AlH3 and NEM·AlH3 have been elucidated. While NMM·AlH3 decomposes to Al metal directly, NEM·AlH3 can be selectively decomposed to give AlH3 under certain conditions. The dramatically different thermal properties of N-alkylmorpholine·AlH3 could be attributed to the different steric hindrance and basicity of N-alkylmorpholine compounds. Compared with the Et3N/Et3N·AlH3 process, our new approach using N-alkylmorpholine significantly improves the kinetics, selectivity, yields, and energy efficiency of AlH3 recovery. Lastly, theoretical calculations of molecular geometries, absolute free energies, Al–H vibrational frequencies, and thermodynamics of amine·alane adducts with different structures are in good agreement with experimental observations and provide further information for the interactions between amines and AlH3.