Hydrothermal synthesis of Ga-substituted MFI zeolites via a mechanochemical process and their catalytic activity for methane transformation

The isomorphous substitution of zeolite framework is a useful means of improving the catalytic performance of zeolites. Such materials are also known to sometimes show catalytic behaviors that typical aluminosilicate zeolites do not exhibit. In the work reported herein, a two-step synthesis method comprising mechanochemical and hydrothermal treatments was used to synthesize Ga-substituted MFI-type zeolites ([Ga]-MFI) with various Si/Ga ratios. The initial planetary ball milling of SiO₂ and β-Ga₂O₃ was central to this synthetic method as it induced mechanochemical reaction between these oxides, which resulted in the formation of amorphous composite containing Si–O–Ga linkages. A subsequent hydrothermal synthesis employing this composite as a source of both Si and Ga gave MFI zeolites incorporating tetrahedral Ga species in their frameworks. A [Ga]-MFI synthesized in this manner with a Si/Ga ratio of 50 selectively transformed methane into CO in the presence of oxygen. In contrast, [Ga]-MFI prepared viaa conventional one-pot hydrothermal synthesis accelerated the complete oxidation to produce CO₂.