Removal of CO ligands from Pt anionic carbonyls in zeolites by heat treatment in vacuum or in a hydrogen atmosphere and following subsequent recarbonylation

Abstract

Pt tetrammine ions in sodium forms of X, Y, dealuminated Y and EMT were carbonylated to [Pt₃(CO)₆]_n²⁻, these primary complexes were decomposed under vacuum at different temperatures and recarbonylated. It appeared that the recarbonylation also yields multinuclear Pt carbonyls (secondary carbonyls); however, they differ from the parent primary complexes. Secondary carbonyls are characterized by a shift of both the linearly and bridge bonded CO ligands to higher wavenumbers, probably due to the formation of larger anionic or neutral complexes. The upward shift of IR wavenumbers concerns all Pt carbonyls in the above zeolites with Si/Al >2. The increasing basicity of zeolitic oxygens hinders these changes, so that in NaX a part of the secondary carbonyls preserves features of the primary carbonyls, and in KX zeolite only the primary carbonyls appear after recarbonylation. While the vacuum decomposition of Pt anionic carbonyls removes simultaneously both linearly and bridge bonded CO ligands, hydrogen reacts preferentially with the bridge bonded COs, as was previously found in the similarly oxidative treatment. Recarbonylation after hydrogen treatment at temperatures below 130°C results in Pt anionic complexes identical with the primary ones. Pt anionic carbonyls in MCM-41, NaM and NaBEA (prepared by carbonylation of these samples impregnated by Et₄NCl and H₂PtCl₆) cannot be recarbonylated to multinuclear Pt complexes after the decomposition neither under vacuum nor in hydrogen. Carbonylation, decarbonylation and recarbonylation processes were examined using IR spectroscopy. Mass spectrometric analysis of gases released during the vacuum decomposition of Pt–CO complexes was used to help in the characterization of the platinum carbonyls. The UV/Vis spectra of primary as well as of secondary carbonyls were also recorded.