A study of the chemistry of isomorphous substitution and characterization of Al-ZSM-5 and Sc-ZSM-5 synthesized in fluoride media
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Al-ZSM-5 and Sc-ZSM-5 samples have been synthesized using fluoride media at around neutral pH. The synthesis chemistry has been studied and it is shown how the lower tendency of scandium to undergo increased fluoro-complexation (compared with aluminum) coupled with its tendency to hydrolyze is conductive to its isomorphous substitution and framework incorporation into the MFI zeolite structure. Clear unit cell volume expansion, elongation of the c crystallographic axis with increased scandium content, and a strong positive linear correlation between the unit cell volume expansion and the calculated unit cell framework scandium content are shown. Chemical shifts are assigned to tetrahedral and octahedral scandium from 45Sc MAS NMR analysis. Shoulders and a peak at lower wavenumber on the main internal T-O asymmetric stretches in the FT-IR spectra indicate that an interaction exists between framework-incorporated scandium and the SiO4 tetrahedra. It is concluded from the experimental evidence that isomorphous substitution of scandium into the zeolite framework has been achieved.
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