Investigation of LiAlH 4-THF Formation by Direct Hydrogenation of Catalyzed Al and LiH
Chabal, Yves J.
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The formation of LiAlH 4-THF by direct hydrogenation of Al and LiH in tetrahydrofuran (THF) was investigated using spectroscopic and computational methods. The molecular structures and free energies of the various possible adducts (THF-AlH 3, THF-LiH and THF-LiAlH 4) present in a LiAlH 4/THF solution were calculated and the dominant species were determined to be contact ion pairs where three THF molecules coordinate the lithium. Raman and X-ray absorption spectroscopy were used to investigate the effect of different Ti precursors on the formation of Al-H species and LiAlH 4-THF and determine the optimal reaction conditions. A unique sample stage was developed from a microfluidic cell to evaluate the catalysts in situ. The effectiveness of two types of catalysts, titanium chloride (TiCl 3) and titanium butoxide (Ti(C 4H 9O) 4), and the catalyst concentration were evaluated under similar reaction conditions. Both catalysts were effective at facilitating hydrogenation, although TiCl 3 was more effective over the first few cycles with the greatest kinetic enhancement achieved with a low concentration of around 0.15 mol%. These results were qualitatively supported by infrared spectroscopy, which indicated that although a small amount of Ti is necessary for disassociating H 2, excess surface Ti (>0.1 ML) hinders the formation of Al-H species. © the Owner Societies 2012.