Zeolites carbonyl clusters

Extraction metal carbonyl clusters internal or external location of metal carbonyl cluster in zeolite Carbonyl cluster encaged in the zeolite cages cannot diffuse through the zeolite aperture and cannot be extracted out effective for anionic dusters but not effective for some neutral carbonyl clusters that are difficult to dissolve in solvent. 

Abstract This review is a summary of supported metal clusters with nearly molecular properties. These clusters are formed hy adsorption or sirnface-mediated synthesis of metal carbonyl clusters, some of which may he decarhonylated with the metal frame essentially intact. The decarhonylated 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. 

Supported metal carbonyl clusters are alternatively formed from mononuclear metal complexes by surface-mediated synthesis [5,13] examples are [HIr4(CO)ii] formed from Ir(CO)2(acac) on MgO and Rh CCOlie formed from Rh(CO)2(acac) on y-Al203 [5,12,13]. These syntheses are carried out in the presence of gas-phase CO and in the absence of solvents. Synthesis of metal carbonyl clusters on oxide supports apparently often involves hydroxyl groups or water on the support surface analogous chemistry occurs in solution [ 14]. A synthesis from a mononuclear metal complex precursor is usually characterized by a yield less than that attained as a result of simple adsorption of a preformed metal cluster, and consequently the latter precursors are preferred when the goal is a high yield of the cluster on the support an exception is made when the clusters do not fit into the pores of the support (e.g., a zeolite), and a smaller precursor is needed. 

Figure 1. Ship-in-a-bottle synthesis of metal carbonyl clusters in NaY zeolite.

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

FIGURE 9.3. FTIR spectrum of Pd carbonyl cluster in zeolite cages. 

Fe(CO)s], [Fe2(CO)g], [Co2(CO)8] and [Os3(CO)i2]) have been reacted with dicyanobenzene to form intrazeolite [M(Pc)] complexes [140]. Another class of materials prepared by the intrazeolite template synthesis method has been mixed ligand metal carbonyls and metal carbonyl clusters, frequently by reductive car-bonylation of metal ions in zeolite cages [175]. However, because these are frequently decomposed in situ to form, for example, nanoparticles, they are outside the scope of this chapter, and will be considered here only when they are used as precursors for metal complexes. 

Iridium carbonyl clusters of several nuclearities (2, 4 and 6) have been prepared by a controlled carbonylation of [lr(CO)2(acac)] complex adsorbed in the cages of a NaY zeolite. Then, decarbonylation of the clusters gave rise to lr2, lr4 and Ir frames. Studies of the dependence of the catalytic activity on the size of the iridium frames in NaY zeolites show that there is no simple explanation for the variation in catalytic performance in ethene hydrogenation with cluster size [208]. 

The hydrogenation of aromatics (benzene, toluene, a-methylstyrene) can be carried out under very low (1 12,000) catalyst substrate ratio, and mild conditions on Rh and Ni organometallic complexes anchored to USY zeolites.474 A Rh complex anchored to functionalized MCM-41 exhibits excellent performance in the hydrogenation of arenes (benzene, toluene, p-xylene, mesitylene) under mild conditions (45°C and 1 atm) 475 A uniquely selective hydrogenation of acenaphthene and acenaphthylene was performed by using a triruthenium carbonyl cluster 476... 

An alternative to this physical method of preparing structurally uniform metal clusters on supports involves chemistry by which molecular metal carbonyl clusters (e.g., [Rh6(CO)i6]) serve as precursors on the support. These precursors are decarbonylated with maintenance of the metal frame to give supported nanoclusters (e.g., Rh6). Advantages of this chemical preparation method are its applicability to many porous supports, such as zeolites (and not just planar surfaces) and the opportunities to use spectroscopic methods to follow the chemistry of synthesis of the precursor on the support and its subsequent decarbonylation. Zeolites, because their molecular-scale cages are part of a regular (crystalline) structure, offer the prospect of regular three-dimensional arrays of nanoclusters. 

For hydroformylation over cobalt and rhodium zeolites the active species have not been defined. However, in the case of RhNaY the in situ formation of a rhodium carbonyl cluster has been identified (226) by infrared spectroscopy. Interestingly, this cluster appears to be different from known compounds such as Rh4(CO)12 and Rh6(CO)16. This does suggest that alternative carbonyl clusters may possibly be formed in zeolites due to the spatial restrictions of the intracrystalline cavities. The mechanism of hydroformylation in these zeolites is probably similar to that known for homogeneous catalysis. 

Transition metal ions, within the zeolite framework, may undergo a reductive carbonylation to give mononuclear monovalent carbonyl coumpounds M(I)(CO) and ultimatly to give zerovalent polynuclear carbonyl clusters. The rhodium(I)and iridium(i)carbonyIs were identified using spectroscopic and volumetric methods, the zerovalent rhodium and Iridium clusters M (CO)j were also synthetized in the zeolite matrix and their structure investigated using IR, NMR and spin labelling methods. 

From the other directions, there have been a number of studies of the cluster species that are formed when mononuclear iridium precursors are deposited onto inorganic supports and then subjected to carbon monoxide pressures. Gates and coworkers have shown that Ir(CO)2(acac) will form higher nuclearity iridium carbonyl clusters, the exact nature of which depends on the substrate and the carbonylation conditions. For zeolite NaY, they have observed that Ir(CO)2(acac) will yield both It4(CO)i2 (45) and fr6(CO)i6... 

When calcination is carried out at 5(X)°C and reduction is at 2(X)°C, the original nuclearity n is 1. For room temperature and zeolite Y the final nuclearity an is 13. The migration and coalescence of the mobile primary Pd carbonyl clusters leading to the formation of PdnfCO) , clusters, with concomitant release of zeolite protons, are schematically illustrated in Fig. 8. 

Mo(CO)6 and W(CO)6 in zeolite display chemistry similar to that of Cr(CO)6 (244-247). Anchored tricarbonyl species are generally considered to be the active subcarbonyl catalysts for the stereoselective hydrogenations of simple dienes to c/j-2-olefins. [H20s(C0)4] was introduced from the vapor phase into NaN3-treated zeolite NaY. EXAFS and IR spectroscopic results suggest the formation of trisomium carbonyl clusters inside the su-... 

Zeolite-trapped clusters are stable toward oxidation-reduction cycles. A sample of [Rh6(CO),e]-NaY (A) was subjected to mild oxidation with dry O2 followed by heating from 293 to 473 K (to eliminate carbonyl ligands), and this treatment was followed by reduction with hydrogen at 473 and... 

Sachtler proposed 87) an explanation of the CO release and concomitant changes in the IR band characteristic of zeolite O—H vibrations involving chemical interaction of zeolite protons and Pd carbonyl clusters. 

In the presence of water, Fe2(CO)Q adsorbed on the external surfaces of zeolite NaY or NaX is readily converted to HFe3(CO)i2 at 297-333 K. It is proposed (145) that the active Fe(CO)4 radical species generated by decomposition of Fe2(CO)9 or Fe3(CO)i2 enters the zeolite framework to rebuild stable carbonyl cluster complexes such as [Fe3(CO)n] and [HFe3(CO)n]" as illustrated in the following reaction scheme ... 

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