Coordination of Co²⁺ cations inside cavities of zeolite MFI with lattice oxygen and adsorbed ligands

The redox chemistry of Co ions in cavities of zeolite MFI has been studied in materials prepared by solid-state ion exchange and displaying Co/Al ratios varying from 0.4 to 1.0. Fourier Transform Infrared (FTIR) spectroscopy, electron spin resonance (ESR), and diffuse reflectance UV-visible spectroscopy were used to identify the oxidation state and the coordination of the Co ions. In the dehydrated blue material, Co²⁺ ions in cationic exchange sites strongly interact with the zeolite framework; their coordination symmetry is tetrahedral. ESR reveals that the ions are in their high-spin state, detectable only below 60 K; no ESR signal was detected at 77 K. However, adsorption of ammonia ligating to the Co²⁺ cations gives rise to an ESR signal at 77 K, indicating a strong reconfiguration of the electronic states. After coadsorption of oxygen and ammonia, ESR reveals the presence of two kinds of cobalt: Co²⁺ in a high-spin state at g = 5.1, detected at 77 K, and low-spin (Co³⁺L_xO₂^-)²⁺ adducts inside the zeolite cavities at g -2.0, where L = NH₃ and probably x ≤ 4. The (Co³⁺L_xO₂^-)²⁺ complexes are thought to be located inside the main channel intersections where a large number of ammonia ligands can be attached to a Co²⁺ ion. The high-spin Co²⁺ ions at g = 5.1 are assumed to be located in small cavities with five- and six-membered rings, and carry a smaller number of ammonia ligands.