Crystalline aluminium hydroxy fluorides—Suitable reference compounds for 19F chemical shift trend analysis of related amorphous solids

On the basis of MAS NMR-data for crystalline AlFx(OH)3−x·H2O samples in the pyrochlore structure, 19F chemical shifts correlate with the average chemical composition of the octahedral environment, given by AlFxO6−x in these compounds.The attribution of local structures in sol–gel derived amorphous AlFx(OX)3−x·XOH (X = H, R (alkyl)) compounds is of special interest as these or consecutively prepared solids exhibit remarkable features, for example, a high surface (HS) area accompanied by a high Lewis acidity.By transferring this scale of a 19F chemical shift trend analysis to such compounds a prediction of the chemical nature of the average Al coordination becomes possible.A new synthetic approach to crystalline aluminium hydroxy fluorides involving a sol gel fluorination as the first reaction step and an aluminium alkoxide as precursor compound is presented. Varying the amount of HF leads to different F–OH-ratios in the AlFx(OH)3−x compounds.

Well defined crystalline aluminium hydroxy fluorides AlFx(OH)3−x were prepared with varying Al/F ratios through the sol–gel route followed by hydrolysis. Correlations between experimental 19F MAS NMR-data and the average composition of AlFxO6−x species allowed a 19F chemical shift trend analysis to be developed. The prediction of the average chemical surrounding of aluminium in related amorphous compounds is now possible, as there exist several similarities between crystalline AlFx(OH)3−x·H2O and amorphous phases AlFx(OX)3−x·XOH (X = H, R (alkyl)).Figure optionsDownload as PowerPoint slide