Studies of cations in zeolites: adsorption of carbon monoxide; formation of Ni ions and Na 3+ 4 centres

Abstract

Zeolite cations lying on the intracrystalline pore surface of the Linde X and Y molecular sieves are linked to only three oxide ions and consequently are not well shielded electrically. They therefore create very large electrostatic fields, extending into the main zeolitic cavities, causing carboniogenic catalytic activity. This activity follows in magnitude the changes in the field, and is independent of the presence of OH groups; no change in activity is observed even after 99 % of OH protons present after ordinary activation in vacuum at 500°C are removed. Bivalent cations exposed on the surface of the main intracrystalline cavities adsorb carbon monoxide with infra-red frequencies specific to cation; the adsorption follows Langmuir isotherms, suggesting that a single carbon monoxide molecule can be independently attached to every surface cation. Certain transition-metal zeolites adsorb carbon monoxide much more tenaciously than alkaline earth cations, indicating a highly significant difference in their ability to form co-ordination bonds. Univalent nickel ions can be prepared both on the intracrystalline surface and at fully co-ordinated positions by heating Ni^IIY with alkali metal vapour. The surface Ni^I ions are chemically very reactive but thermally unstable; those at fully co-ordinated positions are thermally stable even at 400°C, and are inert to H₂, NH₃, and CO although they react with oxygen to form O^– ₂. The alkali-metal X and Y zeolites react with alkali metal vapour to form coloured products of non-stoichiometric compositions containing the paramagnetic centres Na^{5 +} ₆ and Na^{3 +} ₄. The Na^{3 +} ₄ centres are stable up to 500°C and they react reversibly with gases; with oxygen they form O^– ₂ radicals.