Decarbonylation, supported metal

Although decarbonylation of supported metal carbonyl clusters sometimes occurs almost without changes in the metal frames, the chemistry is complex and only partially understood. When decarbonylation takes place at elevated temperatures (depending on the support), migration and aggregation of the metal inevitably occur, and these processes are less well understood than the decarbonylation with near retention of the metal frame. 

The decarbonylation of oxide-supported metal carbonyls yields gaseous products including not just CO, but also CO2, H2, and hydrocarbons [20]. The chemistry evidently involves the support surface and breaking of C - O bonds and has been thought to possibly leave C on the clusters [21]. The chemistry has been compared with that occurring in Fischer-Tropsch catalysis on metal surfaces [20] support hydroxyl groups are probably involved in the chemistry. 

These comparisons prompted the Rosch group [32,33] to conclude that some Ugands remained on the supported clusters after decarbonylation this conclusion may be quite general—supported metal clusters are highly reactive, and typical oxide and zeoUte supports are not unreactive. Thus, a representation of supported clusters such as tetrairidium on 7-AI2O3 as 4/7-AI2O3 is a simplification that fails to account for the ligands on the cluster. 

Synthesis methods such as those described earlier for monometallics have been applied with metal carbonyls incorporating two metals. The resultant supported species may be small supported metal clusters [41,42], and, as for monometallics, the usual products are supported species that are nonuniform in both composition and structure [42]. There are several examples of well-defined metal carbonyl clusters in this category but hardly any examples of well-defined decarbonylated bimetalhcs on supports. 

The appropriate decarbonylation of surface carbonyl species can lead to the retention of the metal frame. When the original metal frame is preserved under reaction conditions, and it is properly characterized, a direct structure-catalytic properties relationship can be established, and the catalytic behavior of supported metaUic clusters can be differentiated from that of the supported metallic particles. 

Miessner (1994) showed that partial decarbonylation of this supported complex by treatment in H2 at temperatures of 200 to 250°C leads to complexes that are so highly reactive that they combine with N2 to give well-defined supported complexes with dinitrogen ligands. This remarkable reactivity suggests possibilities for new catalytic properties of these and related supported metal complexes. 

Abstract This review is a summary of supported metal clusters with nearly molecular properties. These clusters are formed by adsorption or sirnface-mediated synthesis of metal carbonyl clusters, some of which may be decarbonylated with the metal frame essentially intact. The decarbonylated clusters are bonded to oxide or zeolite supports by metal-oxygen bonds, typically with distances of 2.1-2.2 A they are typically not free of ligands other than the support, and on oxide surfaces they are preferentially bonded at defect sites. The catalytic activities of supported metal clusters incorporating only a few atoms are distinct from those of larger particles that may approximate bulk metals. 

Metal carbonyl clusters on supports are important to the subject reviewed here because they are the best known precursors of structurally simple supported metal clusters, which are formed by decarbonylation of the precursors. The routes for preparation of molecularly or ionically dispersed metal carbonyl clusters on zeolite and metal oxide supports include syntheses from mononuclear precursors on the support surface [5,9]. Ship-in-a-bottle syntheses of this type take place when the clusters formed in zeolite cages are trapped there because they are too large to fit through the apertures. Syntheses in the nearly neutral NaY zeolite are similar to those occurring on the nearly neutral y-Al203 and in nearly neutral solutions. Examples are the syntheses of [Ir4(CO)i2] [8] and of [Ir6(CO)i6] [10] from [Ir(CO)2(acac)] in the presence of CO. Syntheses in the more basic NaX zeolite are similar to those occurring in basic solutions and on the basic surface of MgO, e g., those of [HIr4(CO)ii]-and [Ir6(CO)i5]2- [11]... 

The preparation of supported metal clusters by decarbonylation of supported metal carbonyl clusters is exemplified by the removal of the CO ligands from the metal frame of [Ir4(CO)i2] dispersed in NaY zeolite by treatment in hydrogen at 300°C [12] Decarbonylation of [Ir6(CO)i6] in NaY zeolite cages occurs similarly [10,12]. 

MgO-supported Rh6 and Irg clusters were also investigated as catalysts for ethene hydrogenation. Decarbonylation of hexanuclear metal carbonyl cluster precursors on MgO was used to prepare the catalysts. EXAFS data support octahedral clusters as the catalytically active species. Rh5/MgO is 1-2 orders of magnitude more active than Ir /MgO, as is the activity ratio for the conventionally prepared supported metals on silica. 

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