Active Functionalities on Alumina Supports

Furthermore, the removal of these groups by thermal treatment (dehydroxyl-ation) yields coordinatively unsaturated oxygens and anions in which coordina-tively unsaturated aluminum atoms are exposed (Lewis acid sites). In general, the total concentration of OH groups on a alumina support ranges between 10 and 

In contrast with the behavior of [Cr(CO),s], [Mo(CO)5(OH)] has not been observed, but in separate experiments the [Mo2( X2-H)(CO)io] species has been isolated from the reaction of [Mo(CO)6] and sodium tetraborohydride. 

Furthermore, chemical oxidation of [Mo(CO)6] with organic molecules containing acidic hydrogen gives rise to molybdenum species in the +2, -i-3 and +A oxidation states. Higher oxidation states (r-5 and -i-6) have been observed when halogens directly oxidize [Mo(CO)6], A Mo tetranuclear complex has been obtained when hydrochloric acid reacts with the tri-p-hydroxy-dimolybdenum complex, as shown in Equation 9.2  

This short analysis of the molecular reactivity of [Mo(CO)6] shows that one or more CO ligands can be displaced quite easily, and that various oxidation states can be reached. Though such chemical behavior carmot be directly applied to the reactivity of surface functionalities with [Mo(CO)6], it might provide useful suggestions about possible similarities. 

Two different approaches have been used to graft molybdenum on alumina, namely, either a two-step process involving gas-phase impregnation and further decomposition at high temperature (GPID) or the direct contact of [Mo(CO),5] vapor with the alumina support placed in a hot zone so as to achieve its decomposition. All of the relevant studies point to the existence of a close relationship between the OH group density on the support and the amount of deposited molybdenum as well as the chemical nature of such deposits. Hence, we successively deal with three types of alumina highly, partially and fully dehydroxylated surfaces. 

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