Cluster-derived catalysts

A MgO-supported W—Pt catalyst has been prepared from IWsPttCOIotNCPh) (i -C5H5)2l (Fig. 70), reduced under a Hs stream at 400 C, and characterized by IR, EXAFS, TEM and chemisorption of Hs, CO, and O2. Activity in toluene hydrogenation at 1 atm and 60 C was more than an order of magnitude less for the bimetallic cluster-derived catalyst, than for a catalyst prepared from the two monometallic precursors. 

Ru, RuCo, and Co carbonyl cluster-derived catalysts, 38 362-363 structural model, 38 366-367 synthesis gas conversion, 38 364-365 ZSM-5-supported Pd catalysts, 39 207-208... 

C. Mihut, B. D. Chandler, and M. D. Amiridis, BimetalUc R—Au cluster-derived catalysts for the selective catalytic reduction of NO by propylene, Catal. Commun. 3, 91—97 (2002). 

Figure 5. Model for the alumina-supported, Cp2M2lr2(CO)io and CpMh 3-(CO)n cluster-derived catalysts.

In summary, cluster-derived catalysts have been widely used in various types of CO-based reactions such as Fischer-Tropsch synthesis, methanol synthesis, hydroformylation, carbonylation, and water-gas shift reactions. The catalytic performances of cluster-derived species are evaluated in terms of higher activities and selectivities for lower olefins and oxygenates in CO hydrogenation, compared with those of metal complexes in solution and conventional metal catalysts (Table XIII). 

A. Studies of Mixed-Metal Cluster-Derived Catalysts... 

Hydroformylation of Propylene on SiO -Supported Rh, RhFe, and Fe Carbonyl Cluster-Derived Catalysts"... 

This set of results suggests that bimetallic carbonyl clusters derived from mixed Fe/Rh cluster complexes yield discrete RhFe atom sites that are active for olefin hydroformylation as well as for successive hydrogenation of these products to alcohols. Enhancement of hydroformylation over simple hydrogenation of olefins on the Fe-containing Rh cluster-derived catalysts results in an appreciable improvement in oxygenate selectivity. 

The structure and metal compositions of RhFe and PdFe bimetal carbonyl cluster-derived catalysts have been studied by means of EXAFS, Mossbauer, and Fourier transform IR of CO chemisorption. The Fourier transform spectra of Rh-K and Fe-K EXAFS on the H2-reduced catalysts prepared from Si02-supported [Rh4Fe2(CO) s][TMBA]2 (4 wt% Rh) suggest that RhFe bimetal ensembles are formed in high dispersion (10 A in size), having direct Rh—Fe—O bonding [Rh—Rh CN = 5.6, r = 2.70 A Rh—Fe CN = 1.8, r = 2.54 A (137) as shown in Table XVIII. 

Because the cluster-derived catalysts on Si02 were prepared in higher metal dispersions regardless of the precursors used, such marked enhancement of CO conversion and higher selectivities toward alcohols on Fe-containing Ir catalysts are likely to be associated with the generation of IrFe adjacent sites, possibly located at the cluster-oxide support interface, active not only for... 

CO dissociation but also for CO insertion. A similar promotion mechanism was previously proposed for RhFe and PdFe carbonyl cluster-derived catalysts which exhibited substantial improvement in C1-C2 alcohol production in CO hydrogenation. The conventional IrFe catalysts prepared from coimpregnation of IrCl4 and FeCls provided poor yield and selectivity toward alcohols versus the corresponding cluster-derived catalysts. The bimetallic IrFe cluster precursors offer the advantages of higher metal dispersion and more uniform distribution of active IrFe sites over the conventional salt-derived catalyst preparation. 

Catalytic Performances (f Ru, RuCo. and Co Carbonyl Cluster-Derived Catalysts in CO Hydrogenation"... 

Fig. 25. Carbon efficiencies of oxygenated products in the CO + H2 reaction and ethanol selectivities on bimetallic RhCo cluster-derived catalysts impregnated on Zr02 (0.2 wt% metal) (CO/H2 ratio 20/45 cmHg, 200°C). O.C. represents oxygenated products on a carbon basis.

Fig. 26. Proposed structural model for two-site CO activation to promote oxygenate formation in olefin hydroformylation and CO hydrogenation reactions on RhFe, PdFe, and IrFe bimetal cluster-derived catalysts.

Data for both hydrogen and carbon monoxide chemisorption for the series of reduced [Rhe Ir,]/NaY (x = 0,2,4, and 6) are presented in Table XXV. It was found that, in particular, the CO/M values on Rh-rich cluster-derived catalysts were higher than those for Ir-rich clusters. The trend of CO chemisorption amounts is in good agreement with the decreasing order of electron deficiency on the clusters inside zeolites which has been estimated from XE NMR chemical shifts (SS). 

In addition to practical applications, metal cluster-derived catalysts, particularly intrazeolite metal cluster compounds, may aid in the identification of catalytically important bonding and structural patterns and thereby further our molecular understanding of surface science and heterogeneous catalysis. The ship-in-bottle technique for the synthesis of bulky metal-mixed metal cluster compounds inside zeolites and/or interlayered minerals has gained growing attention for the purpose of obtaining catalytic precursors surrounded by the interior constraint, imposing molecular shape selectivity. Such approaches may pave the way to offer the molecular architecture of hybrid (multifunctional) tailored catalysts to achieve the desired selectivity and stability for industrial processes. 

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