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 N2, 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 N2: 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.

Synthesis of STW zeolites using imidazolium-based dications of varying length

“…In fact, partial reduction of the framework should be more likely for the bigger dications due to their higher reduction power and the increased difficulty of oxygen diffusion to the zeolite interior. 20 The 29 Si MAS NMR spectra of the silicogermanate zeolites ( Fig. S8 †) show multiple severely overlapped peaks in the À90 ppm to À120 ppm region, which are typical of germanosilicate zeolites.…”

“…As recently shown, calcination of organic-containing germanium zeolites can entail complex processes, including the (partial) reduction of the framework to GeO or metallic Ge0, reoxidation to GeO 2 at higher temperature, with the corresponding weight gain, and even GeO sublimation that would result in weight losses exceeding that of organics and uoride. 20 Although this is more likely to happen in small pore germanates, examples were also provided for germanosilicates with larger pores and, in fact, a small weight gaining step in the 800-825 C range can be clearly observed in the TGA trace of 4BDMI-Si,Ge-STW revealing reoxidation of some previously reduced GeO 2 . From a careful observation of the TG traces, we deem likely that weight gaining processes may occur in similar temperature ranges in all the Si, Ge zeolites shown in Fig.…”

Synthesis of a germanosilicate zeolite HPM-12 using a short imidazolium-based dication: structure-direction by charge-to-charge distance matching

“…The chemical compositions of several as-made HPM-12 zeolites are shown in Table 2. 26 The C/N molar ratios of the organics entrapped in the zeolites are a little higher than the theoretical values for the pristine 3BDMI, suggesting that the SDAs, while roughly intact in the as-made zeolites, may have suffered significant decomposition. This would not be very surprising, given previous reports of low hydrothermal stability of short dications based on propylene linkers.…”

Synthesis and Post‐Synthesis Transformation of Germanosilicate Zeolites

“…Respective issues and strategies for their overcoming are considered in the Supporting Information. [37] 3.1. High-Throughput Zeolite Synthesis…”