F-19 and Al-27 nmr spectroscopic study of the fluoro complexes of aluminum in aqueous solution and in zeolites: Dealumination of zeolites by fluoride ions

The addition of fluoride ions to aluminum(III) salts or to zeolites in aqueous solutions produces fluoro complexes of the general form [AlFn]((3-n)), which have been characterized by Al-27 and F-19 n.m.r. Each fluoroaluminate complex has distinct F-19 chemical shifts, although the Al-27 chemical shifts of all of the complexes are approximately the same. The Al-27 n.m.r. line width, however, changes as different complexes are formed. Zeolite reacts with aqueous fluoride ion solution at approximately neutral pH. n.m.r., FTi.r. and X-ray powder diffraction measurements suggest that the formation of fluoroaluminate species such as K2NaAlF6 at high fluoride ion concentration inside zeolite is an important dealumination mechanism.