Langmuir 1990,6, 1784-1792
Chemistry of Organochromium Complexes on Inorganic Oxide Supports. 2. Interactions of Carbon Oxides with Chromocene on Silica Catalysts Shi-Liang Fu and Jack H. Lunsford' Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received April 2, 1990. In Final Form: June 14, 1990
The interactions of carbon monoxide and carbon dioxide with chromocene supported on silica were studied by infrared spectroscopy. Chemisorption of CO resulted in the formation of various carbonyl complexes. The nature of these complexes was dependent upon CO pressure, Cr loading, adsorption time, and dehydration temperature of the silica. Monocarbonyl complexes were the major species formed at a low pressure (0.02 Torr). Upon an increase in the CO pressure, the predominant complexes became dicarbonyls which converted to bridging carbonyls after prolonged adsorption times. Except for the dicarbony1 complexes, the CO ligands were strongly bonded to Cr and were not removed by evacuation at room temperature. The desorption of CO could be initiated by addition of oxygen, which subsequently oxidized the Cr complexes on the surface. The chemisorption of CO suggests that severaltypes of carbonyl complexes are formed in at least two slightly different environments on the silica. The interaction of CO ligands with neighboring species, such as Cr complexes and hydroxyl groups, resulted in large shifts of the C-0 stretching bands to lower frequencies. Carbon dioxide was associatively adsorbed on Cr to form surface carboxylates. No carbonate or bicarbonate species were observed upon CO or COP adsorption.
Introduction Chromocene (bis(cyclopentadienyl)chromium, Cp2Cr) reacts on the surface of partially dehydrated silica to form one of the most effective catalysts for ethylene polymerization.'" Chromium is thought to be anchored to the silica by reaction with hydroxyl groups and subsequent release of one or two cyclopentadienyl (Cp) ligand as cyclopentadiene.' The catalytic activity has been attributed to a cyclopentadienyl chromium complex (CpCr) on the silica surface. In contrast to other supported chromium catalysts that are used in commercial processes, the chromocenederived catalyst produces polyethylene with a relatively narrow molecular weight distribution. Another unique feature of this catalyst is its high chain-transfer response t o hydrogen, which allows the molecular weight of polyethylene to be tailored. Although fundamental experimental and theoretical studies have been carried out on chromocene-derived catalysts regarding structure and kinetics, little spectroscopic information is available as to the nature of the Infrared spectroscopic studies of adsorbed probe molecules can provide valuable information regarding the formal oxidation state, the degree of coordinative unsaturation, and the symmetry of the surface species.12 Carbon monoxide is a particularly suitable probe molecule for such (1) Karol, F. J.; Karapinka, G. L.; Wu, C.; Dow, A. W.; Johnson, R. N.; Carrick, W. L. J. Polym. Sci., Polym. Chem. Ed. 1972, 10, 2621. (2) Karol, F. J.; Wu, C. J. Polym. Sci., Polym. Chem. Ed. 1974,12,1549. (3) Karol, F. J.; Johnson, R. N. J.Polym. Sci., Polym. Chem. Ed. 1975, 13. 1607. (4) Freeman, J. W.; Wilson, D. R.; Ernst, R. D.; Smith, P. D.; Klendworth, D. D.; McDaniel, M. P. J . Polym. Sci., Polym. Chem. Ed. 1987, 25. 2063. - ., - - -.
(5) Karol, F. J.; Brown, G. L.; Davison, J. M. J. Polym. Sci., Polym. Chem. Ed. 1973, 11,413. (6) Karol, F.J.; Wu, C.; Reichle, W. T.; Maraschin, N. J. J . Catal. 1979,
60, 68. (7) Kozorezov, Y. I.; Seikho, A. Zh. Prik. Khim. 1983,56, 1614. (8) Amstrong, D. R.; Fortune, R.; Perkin, P. C. J. Catal. 1976,42,435. (9) McKenna, W. P.; Bandyopadhyay, S.; Eyring, E. M. J. Mol. Catal. 1984,38, 834. (10)Rebenstorf, B.; Larsson, R. J. Mol.Catal. 1981, 11,247. (11) Zecchina, A,; Spoto, G.; Bordiga, S. Faraday Discuss. Chem. Soc. 1989, 87, 149. (12)Kung, M. C.; Kung, H. H. Catal. Reu.-Sci.Eng. 1985, 27, 425.
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adsorption studies, and, in fact, CO adsorbed on chromocene-derived catalyst has been previously studied by Rebenstorf and LarssodO and more recently by Zecchina and co-workers." As the formation of chromocenederived catalysts and their catalytic activities are affected by Cr loading and the dehydration temperature of silica, the chemisorption of CO on these catalysts and t h e resulting adsorption spectra are expected to depend upon these preparation variables. However, the importance of these variables in the chemisorption of CO has not been addressed in the previous studies."" In the present study, these effects are explored in detail. In addition, the stability of the adsorbed CO under various conditions has been determined. A study of COz chemisorption is also included for comparison. The results are compared to the Phillips ethylene polymerization catalyst (Cr/SiOz), which also has been extensively in~estigated.'~Information from the present study is integrated with related catalytic and spectroscopic experiments in order to better understand the nature of the chromocene-derived catalysts.
Experimental Section Materials. As described in the first paper of this series, the preceding paper in this issue,I4Davison 952MS silica gel was used as the support. Chromocene (Strem Chemicals, sublimed) was purified by sublimation at 40 O C in vacuo (
