FTIR study of the adsorption of chromium carbonyl complexes on silica

Abstract

IR spectra are reported of Cr(CO)₆, (C₆H₆)Cr(CO)₃ and (Me₄C₆H₂)Cr(CO)₃ adsorbed on silica which had been pretreated in vacuum at temperatures in the range 250–800 °C or in H₂O or D₂O vapour at 700 °C. Surface isolated silanol groups formed hydrogen bonds with arene rings in Cr complexes which also interacted with the surfaces via ligation involving carbonyl groups. The generation of active adsorption sites was promoted by increasing pretreatment temperature. Several types of site were involved and resulted from dehydroxylation of the silica surface. Adsorbed bridging carbonyl species resulted from interactions with active Si^δ+ O^δ– surface sites. There was negligible evidence for the loss of CO ligands from adsorbed molecules and the supported complexes were inactive for the polymerisation of ethene. However, some loss of benzene occurred for (C₆H₆)Cr(CO)₃ on silica pretreated at 800 °C. Pretreatment of silica in H₂O or D₂O at 700 °C promoted hydrothermal surface reorganisation of silica and destroyed a high proportion of the active adsorption sites generated at 700 °C on silica in vacuum.