Novel zeolite framework LMU-2: unveiling of CHA-related structure with active sites by Co-complex structure-directing agent

While alkylammonium salts or organic amines are commonly used as structure-directing agents (SDAs) to synthesize zeolites, metal complexes can also serve this purpose. In this study, we present a novel interrupted framework, which can be regarded as a precursor of the CHA framework type with a pure-silica composition. The polycrystalline yellow powder was synthesized using a cobalt (III)-triazacyclononane complex [Co( tacn )₂]³⁺. Structure determination via 3D electron diffraction revealed details of the [Si–O⋅⋅⋅H⋅⋅⋅O–Si]^– hydrogen bonds, which are periodically ordered. They connect the D6R units of the CHA precursor-type structure. Upon calcination, the organic component is removed, yielding a blue material, resulting in the fully connected chabazite structure. The blue color is characteristic for tetrahedrally coordinated Co²⁺, which is most likely located in the chabazite cage and coordinated to the 6-rings of the framework. Temperature-dependent powder diffraction analysis was used to investigate the structural transformation and the discontinuous decrease in the constants a and c of the hexagonal lattice. Finally, spectroscopic and sorption measurements were conducted to characterize the properties of the material.