RBS and PIXE study of gallium depth profiling in ZSM-5 Gallo-Aluminosilicate zeolites

Gallium concentration depth profiling in various as-synthesised and post-synthesis thermally treated Ga- and (Ga,Al)-ZSM-5 zeolites was quantitatively evaluated by RBS. This technique proved for the first time that Ga-ZSM-5 synthesised in the presence of methylamine involves a homogeneous Ga framework distribution. When both Al and Ga are present in the gel precursor, they form mixed complexes with methylamine and incorporate the zeolite lattice at different rates while some unreacted Ga-amino complex overcoats the outer rim of the crystals. Gallosilicates synthesised in the fluoride medium show a fairly homogeneous Ga incorporation with an increased Ga concentration on surface due to its overcoating by residual Ga fluoro complexes. Post-synthesis thermal treatments result in a partial degalliation of the framework that could be quantified by RBS. The nature, mobility and location of the extra framework Ga species markedly depend on the calcination conditions. A rapid calcination in dry conditions leads to the formation of extra framework Ga oxides that migrate towards the crystal core. Under milder heating, these species stay homogeneously partitioned within the crystal channels. Under a humid atmosphere, the extra framework Ga species migrate towards the crystal surface, the migration being enhanced by a partial reduction of Ga during non-oxidative treatments. Humid air treatment maintains the homogeneous distribution of both Ga2O3 and residual Ga3+ framework ions.