FTIR STUDY OF THE ADSORPTION OF CHROMIUM CARBONYL-COMPLEXES ON SILICA

IR spectra are reported of Cr(CO)(6), (C6H6)Cr(CO)(3) and (Me(4)C(6)H( 2))Cr(CO)(3) adsorbed on silica which had been pretreated in vacuum at temperatures in the range 250-800 degrees C or in H2O or D2O vapour a t 700 degrees C. Surface isolated silanol groups formed hydrogen bonds with arene rings in Cr complexes which also interacted with the surfa ces via ligation involving carbonyl groups. The generation of active a dsorption sites was promoted by increasing pretreatment temperature. S everal types of site were involved and resulted from dehydroxylation o f the silica surface. Adsorbed bridging carbonyl species resulted from interactions with active (SiOdelta-)-O-delta+ surface sites. There wa s negligible evidence for the loss of CO ligands from adsorbed molecul es 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 degrees C. Pretreatment of silica in H2O or D2O at 700 degrees C promoted hydrothermal surface reorganisation of s ilica and destroyed a high proportion of the active adsorption sites g enerated at 700 degrees C on silica in vacuum.