Binding of chromium acetylacetonate on a silica support

The binding of Cr(acac)(3) vapor on silica pre-heated at 200, 560, and 820 degrees C was studied between 160 and 240 degrees C in a fixed bed flow reactor by determining the Cr and carbon concentrations, recording FTIR spectra and analyzing the thermal stability of the supported Cr complexes (thermogravimetric analysis and temperature programmed desorption). Saturation of silica with the Cr complex was observed at all reaction temperatures. The saturation density varied from 0.3 to 0.8 Cr nm(-2) with variation in the reaction temperature and the pre-treatment temperature of silica (200, 560 or 800 degrees C), evidently determined by the number of available sites on silica and by the size of the Cr complex. At 160 degrees C, both associatively adsorbed and reacted Cr(acac)(3), were concluded to exist. The amount of reacted Cr(acac)(3), increased with increase in reaction temperature, and at 200 degrees C the samples appeared to contain mainly SiO-Cr(acac)(2) species formed in the reaction with isolated OH groups. Samples prepared at slightly higher temperature of 220-240 degrees C also contained partially decomposed acac ligands, denoted Cr acetate species. The ligands of the surface complex could be removed by air treatment with concurrent oxidation of Cr3+ to Cr6+ surface species. The oxidation was almost complete in the samples prepared at 200-240 degrees C, whereas clustering of Cr3+ to Cr2O3 occurred in the samples prepared at 160 degrees C.