Active Sites—Concluding Remarks

No one particular site or specie has been identified as the active site for the various reactions which occur on reduced molybdena catalysts. It may be that different sites are used for different types of reactions, e.g., hydrogenation and isomerization. Phase studies and measurement of Mo 

The picture here is even less clear than for reduced catalysts. In spite of the fine studies by Delmon and workers on the bulk mixed sulfides, and that of Schuit and De Beer and workers on various hybrid catalysts, it is not conclusively proved that bulk sulfides are the active ingredients for hydrodesulfurization reactions for the mildly sulfided catalysts employed in industry. 

Undoubtedly, under certain industrial conditions (e.g., coal liquefac- 

The author thanks Dr. Charles Kibby for helpful comments on the manuscript. 

Weisser, O., and Landa, S., Sulfide Catalysts, Their Properties and Applications. Pergamon, New York, 1973. 

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The inactive form GOin, which displays a typical Cu(II) EPR signal, yields upon one-electron oxidation the EPR silent active form GO0X. For many years the presence of a Cu(III) ion (ct,. S = 0) in the active site (121) of the fully oxidized state GO0X was assumed. The Whittakers (122) showed in 1990 that one-electron oxidation of the copper depleted apoenzyme of GO produced an EPR active, remarkably stable Tyr radical that was studied by UV-vis, EPR, and ENDOR spectroscopy. From these studies, they concluded that the thioether modified Tyr 272 was oxidized and, consequently, they proposed that GOcx contains a Tyr 272 radical coordinated to a Cu(II) ion. 

Based on their observations, they concluded that the ribosome is ribozyme and directs the catalytic properties of its all-RNA active site. The secondary structures of both 5S and 23S rRNA from H. marismortui are remarkably close to those deduced for them by phylogenetic comparison. 

The metathesis of propene on WOs/Si02 is speeded up by pretreatment of the catalyst with HCl (Pennella 1974 Aliev 1977) but the product but-2-ene undergoes considerable isomerization to but-l-ene (Aliev 1978). The inclusion of 1% cycloocta-1,5-diene (COD) in the propene stream also increases the rate of metathesis and reduces the break-in time from 20 min to less than 5 min at 500°C. The latter effect disappears when the additive is removed, so it is not due to reduction of the catalyst. The effects of both HCl and COD have been attributed to a favourable modification in the metal d-orbital levels as the result of the presence of new ligands (Pennella 1973, 1974). Pretreatment with hexamethyldisilazane (HMDS) at 250°C also has a remarkable effect on the activity, increasing it as much as 140 times for the metathesis of propene at 427°C. The same treatment of silica alone completely eliminates its capacity to isomerize rra/j5-but-2-ene at 427°C, so it is concluded that the Bronsted acidic hydroxyl groups, poisoned by HMDS, are not likely to be the precursors for the active sites in propene metathesis over W03/Si02 (van Roosmalen 1980a, 1982). 

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