Osmium clusters supported, activity

TViple-osmium clusters supported on polystyrene-divinylbenzene, silica gel and AI2O3 catalyze alkene isomerization [253]. IR spectra are indicative of the formation of a stable structure of (P)-(CH2)2-0-(CO)ioOsH-type. The authors suggest that these structures serve as a pool of catalytically active particles from which unsaturated coordination clusters form. 

Comparison of Activities of Supported Osmium Cluster Catalysts for Alkene Isomerization" (15)... 

The reactions of oxide-supported osmium clusters have been studied (108) at elevated CO and H2 pressures by in situ IR spectroscopy. Under 10 atm of a 1 4 CO -I- H2 mixture, Os3(CO)i2 is retained intact up to 573 K. However, at 523 K on AI2O3, Si02, or Ti02, it is transformed to H30s4(C0)i2 which is an active anchored species for catalyzing CO -I- H2 conversion to CH4. 

The structures of anionic, triple-nuclear osmium and iron cluster catalysts supported on copolymers of styrene and divinylbenzene were analyzed by means of IR spectroscopy. Their catalytic activity during 1-hexene hydroformylation [250] and C5H5NO2 carbonylation [251] were investigated. It was found that isomerization proceeds simultaneously in the presence conventional catalysts. In the absence of moisture, a triple-nuclear osmium complex could be removed from a polymeric support after reaction. This suggests catalytic activity for this complex, particularly in the fixed state. Furthermore, a definite correlation was found to exist between polynuclearity and selectivity of heptanol formation. For iron, however, the cluster structure altered during the course of the reaction. 

Zhou et al. [52] showed that strongly basic NaY zeolites containing entrapped osmium carbonyl clusters (described above) catalyzed CO hydrogenation and gave a non Schulz-Flory distribution of Ci-Q hydrocarbons with high alkene to alkane ratios. The catdyst was stable, operating at 300°C and 19 bar with a CO/H2 molar ratio of 1, for more than 20 days with only a small loss in activity and only a moderate loss in selectivity. However, the activity was markedly lower than that of conventional supported metal catalysts. 

EXAFS is a technique which has come into its own within the last five years [32-34] and has provided conclusive evidence which can be interpreted in terms of retention of the clustered state in active ruthenium catalysts derived from Ru3(CO)j 2 H4Ru4(C0)i2 active osmium catalysts derived from Os3(CO)32> H40s4(C0)] 2> 055(00) 3, supported on silica, alumina, and titania in our joint work with Hull University (Fig. 2) [35]. A particular advantage of EXAFS is that it is a technique which, in prospect, can be used iji situ to study working catalysts. 

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