Framework Reduction of GeO2 Zeolites during Calcination

Abstract

Calcination in air of GeO2 zeolites with AST topology causes reduction of the framework, hence structural destruction, in a notable extension. This is due to the impossibility for the organic cation occluded inside to react with ambient oxygen. As the temperature increases, reoxidation in air of the reduced framework causes episodes of weight-gain. When calcination is carried out in N-2, weight losses close to 70% imply a loss of Ge due to sublimation of germanium monoxide, GeO. The nature of the organic cation occluded determines the nature and oxidation state of the final residue after calcination in N-2: for tetramethylammonium the residue is GeO2 while trimethyl-terc-butylammonium, thanks to its larger C content, yields metallic Ge. Framework organoreduction is not unique to GeO2-AST zeolites but also occurs during the calcination in air of other germanium containing zeolites with larger pore openings.