Probing the basicity of lattice oxygen on H-form zeolites using CO₂

The basicity of lattice oxygen in H-form zeolite was characterized by the CO₂-probe IR method at room temperature and in the low-pressure region. The adsorbed CO₂ species on basic oxygens (>2350 cm⁻¹) were distinguished from hydrogen-bonded (2349 cm⁻¹) and physisorbed (2339 cm⁻¹) species. The pairing of a basic oxygen and an acidic OH group was evidenced by quantitative ammonia adsorption, where the amount of basic oxygen was decreased linearly with the amount of acidic OH consumed by ammonia adsorption. This clearly indicates the presence of basicity on H-form acidic zeolites. The number of lattice oxygens of H-form zeolites located around Al in (AlO₄)⁻ units was affected by the environment of the central Al atoms in the framework of the zeolites. Consequently, the number of base sites was dependent on zeolite topology. Zeolites diverse in the location of T sites represented by MFI showed plural IR absorption bands of adsorbed molecular CO₂ (2370–2350 cm⁻¹) due to the presence of various types of base sites, in contrast to zeolites with fewer types of T sites, such as CHA. While CO₂ adsorption onto basic metal oxides results in forming carbonate species through chemical reactions, IR observation of molecularly adsorbed CO₂ could be an appropriate method to detect weakly basic sites on acid catalysts.