formation of metal acac surface species covalently bonded to Chemical modification of a silica surface with chromium acetylthe support surface (2,6). Finally, chemical reaction of the acetonate (Cr(acac) 3 , acac Γ
C 5 H 7 O 2 ) was carried out from the M n/ (acac) n molecule with coordinatively unsaturated ions gas phase at 190ΠC. This results in the formation of chromium acac on the support surface may take place (8, 9).
species which interact with the silica surface because of hydrogen
The mechanism of the metal acetylacetonates interaction bonding between the surface hydroxyl groups and the quasi pwith the support surface depends on the properties of the electron system of two acac ligands of Cr(acac) 3 . Ligand substitusupport used (8, 10) as well as on the shape and stability tion in the chromium acac species on the silica surface pretreated of M n/ (acac) n (1, 5). For example, Cu acetylacetonate, at different temperatures was studied by IR spectroscopy, thermowhich has square-planar symmetry, interacts with the silica gravimetry, temperature-programmed oxidation, and chemical analysis. Water molecules and surface hydroxyl groups can replace surface due to hydrogen bonding between the surface hyacac ligands in the chromium acac surface species at 230 and droxyl groups and acetylacetonate ligands (5). In the case 270ΠC, respectively. The oxidation of the acac ligands in air occurs of the unstable octahedral Fe and Mn acetylacetonate moleat 340ΠC and is accompanied by conversion of Cr 3/ to Cr 6/ . α§ 1997 cules, acac ligands are replaced by the surface hydroxyl Academic Press groups under mild conditions (8, 10). In contrast, the highly Key Words: silica; chemical modification; chromium acetylacetostable and coordinatively saturated Cr(acac) 3 molecule has a nate; hydrogen bonding; ligands substitution. low affinity toward the silica surface in the same conditions. Kenvin et al. (5) showed that about 90% of the adsorbed Cr(acac) 3 was removed from the Cab-O-Sil surface by 1 To whom correspondence should be addressed.
Very little is also known about the role of different active 2 Current address: University of Antwerp (U.I.A.) Department of Chemiscenters on the silica surface (i.e., isolated and H-bonded try, Laboratory of Inorganic Chemistry, Universiteitsplein 1, 2610 Wilrijk, surface hydroxyl groups, adsorbed water molecules, siloxane Belgium. bridges) in the chemisorption of Cr(acac) 3 , whose reactivity