Abstract
By comparing direct synthesis and post-modification (ion exchange and galliation) of MFI samples, this study provides a comprehensive picture of how the presence of gallium in MFI zeolites alters the materials’ properties. Directly synthesized [Ga]MFI samples have evenly distributed gallium according to STEM/EDX and the framework Si/Ga ratio quantified by 29Si MAS NMR gives a reliable, indirect picture of the coordination of gallium species. Complementing the method by EXAFS fitting and 71Ga MAS NMR provides a detailed picture of the material bulk properties. Acidity measurements show that with increasing gallium content both BAS and LAS densities increase. Tetrahedral framework gallium can form LAS, while the formed BAS are of weaker strength than those in aluminum samples. Post modifications generated significant amounts of extra-framework gallium. Galliation of the silicalite-1 sample generated BAS at low density. 1H{27Al} and 1H{71Ga} TRAPDOR show that no gallium-based BAS are generated in [Al]MFI samples. BAS originate exclusively from aluminum in the form of Si(OH)Al groups and show the respective higher acid site strength. No external BAS were found after triphenylphosphine (TPP) loading on the herein synthesized samples. Loading with TMPO provided only qualitative evidence of BAS and LAS as a result of its limited access to the MFI pores. Ammonia coordinated at Na+ was identified by a peak at δ1H = 0.9 ppm. With the systematic comparisons in this study a better understanding and targeted syntheses will be possible.