Choosing the best molecular precursor to prepare Li4Ti5O12 by the sol-gel method using H-1 NMR: evidence of [Ti-3(OEt)(13)](-) in solution

摘要

H-1 NMR spectroscopy at 400 MHz in toluene-d(8) of evaporated mixtures of lithium ethoxide and titanium(IV) isopropoxide in ethanol, used to prepare the spinel Li4Ti5O12 by the sol-gel method, may help clarify why the atomic ratio 5Li : 5Ti and not 4Li : 5Ti is the right choice to obtain the pure phase when performing hydrolysis at room temperature. The mixtures xLiOEt/yTi(OPri)(4) in ethanol undergo alcohol exchange at room temperature, and the evaporated residues contain double lithium-titanium ethoxide [LiTi3(OEt)(13)] rather than simple mixtures of single metal alkoxides; this is of great relevance to truly understanding the chemistry and structural changes in the sol-gel process. Detailed inspection of the H-1 and C-13 VT NMR spectra of mixtures with different Li/Ti atomic ratios unequivocally shows the formation of [LiTi3(OEt)(13)] in a solution at low temperature. The methylene signals of free lithium ethoxide and Li[Ti-3(OEt)(13)] coalesce at 20 degrees C when the atomic ratio is 5 : 5; however, the same coalescence is only observed above 60 degrees C when the atomic ratio is 4 : 5. We suggest that the highest chemical equivalence observed by H-1 NMR spectroscopy achieved through chemical exchange of ethoxide groups involves the highest microscopic structural homogeneity of the sol precursor and will lead to the best gel after hydrolysis. Variable temperature H-1 NMR spectra at 400 MHz of variable molar ratios of LiOEt/Ti(OPri)(4) are discussed to understand the structural features of the sol precursor. While the precursor with the atomic ratio 5Li : 5Ti shows no signal of free LiOEt at 20 degrees C, both 4Li : 5Ti and 7Li : 5Ti show free LiOEt at 20 degrees C in their H-1 NMR spectra, indicating that the molar ratio 5Li : 5Ti gives the maximum rate of chemical exchange. DFT calculations have been performed to support the structure of the anion [Ti-3(OEt)(13)](-) at room temperature.