Gregory L. Geoffroy

Carbon-supported FeMn and KFeMn clusters for the synthesis of C2C4 olefins from CO and H2: I. Chemisorption and catalytic behavior

Abstract Highly dispersed FeMn bimetallic particles were obtained on a high-surface-area amorphous carbon black support using FeMn and KFeMn carbonyl clusters. These catalysts were characterized by hydrogen adsorption and by CO chemisorption at 195 and 300 K, and their kinetic behavior for CO hydrogenation was studied at 1 atm in a differential, plug-flow microreactor. The K-promoted clusters with stoichiometries of KMnFe and KMnFe 2 gave 85–90 wt% ethylene, propylene, and butene with methane as the only other detectable hydrocarbon product. The KFe 3 cluster also had a high selectivity to olefins but showed somewhat more chain growth. The nonpromoted MnFe catalysts with Fe Mn = 2 , prepared from either stoichiometric mixed-metal carbonyl clusters or coimpregnation of the separate Fe and Mn carbonyl clusters, also had a high selectivity to light olefins; however, this selectivity was strongly dependent upon the pretreatment. The properties of the FeMn clusters without K is consistent with a proposed model in which a surface spinel, (Fe l - y Mn y ) 3 O 4 , plays a principal role in providing high selectivity to light olefins.

Mixed-Metal Clusters.

Publisher Summary This chapter discusses mixed-metal clusters, methods of characterization, reactivity, and dynamic nuclear magnetic resonance (NMR) studies. Transition-metal cluster compounds are currently under intensive scrutiny because of their potential catalytic applications, both as models for understanding catalytic metal surfaces and as catalysts in their own right. Noticeably, few metal clusters have been prepared by designed or rational synthetic procedures. Pyrolysis reactions generally involve heating together two or more stable compounds of different metals, presumably to give fragments that then combine to yield the mixed-metal clusters. Relatively few clusters have been prepared by the pyrolysis of two monomeric compounds, and only two examples are given. Metal–carbonyl dimers have proved to be useful reagents for the synthesis of mixed-metal clusters. The pyrolysis of clusters in the presence of monomers, dimers, or other clusters usually requires much more severe reaction conditions. These coordinatively unsaturated species are apparently the key intermediates that condense to give the cluster products. The reaction of carbonylmetalates with monomeric and dimeric carbonyls has yielded many mixed-metal clusters. The reaction of carbonylmetalates with trinuclear clusters provides, in many cases, a convenient synthesis of tetranuclear clusters. Carbonylmetalates displace a halide from a metal–halide complex to yield a metal–metal bonded species.

Characterization of supported silver catalysts: I. Adsorption of O2, H2, N2O, and the H2-titration of adsorbed oxygen on well-dispersed Ag on TiO2

Very small (3-nm) silver crystallites on TiO2 were prepared and characterized by XRD, TEM, chemisorption of O2, H2, and N2O, and the H2-titration of adsorbed oxygen. Similar monolayer coverages were obtained on these clean reduced Ag crystallites using O2 and N2O as oxygen precursors, with the latter producing only slightly lower coverages. In all cases, H2 was found to react stoichiometrically with the adsorbed oxygen layer at 170 °C, and this could be described by the reaction, where Ags is a surface atom: AgsOa + H2(g) → Ags + H2O(ads) because monolayer coverages of hydrogen were very low. This stoichiometric reaction between hydrogen and surface oxygen provides the following advantages for characterizing silver catalysts: (1) rapid equilibration, (2) increased sensitivity, (3) no oxygen contamination effects, and (4) distinguishes any irreversible oxygen uptake on support from that on silver.

Interaction of Ketenes with Organometaliic Compounds: Ketene, Ketenyl, and Ketenylidene Complexes

Publisher Summary This chapter discusses the interaction of ketenes with organometallic compounds and four separate but related aspects of the interaction of ketenes with metals; the reactions of ketenes with organometallic complexes; the preparation, structures, and reactions of stable ketene complexes; the chemistry of ketenyl complexes that have a metal as one of the ketene substituents; and the chemistry of ketenyfidene complexes in which the ketene carbon has only metals as substituents. Also discussed are reactions in which ketene complexes are suspected as intermediates, but proof of their structure and formulations is lacking. There are numerous examples of reactions of ketenes with coordinated ligands with little or no direct interaction of the ketene with the metal. A number of organometallic complexes have been prepared that contain ketene and substituted ketene ligands, but these have a variety of different ketene coordination modes. A variety of synthetic procedures have been used to prepare ketene complexes, with the most useful involving direct addition of ketenes to unsaturated organometallic complexes, carbonylation of carbene complexes, and deprotonation of acyl complexes. A number of ketene complexes have been prepared by the addition of carbon monoxide to preformed metal–carbene complexes. Similar proton abstraction from bridging acetyl ligands would give μ-ketene complexes. A synthetic method that has often been used for preparing new ketene complexes involves the modification of the coordination sphere of existing ketene complexes.

Carbon-supported FeMn and KFeMn clusters for the synthesis of C2C4 olefins from CO and H2: II. Activity and selectivity maintenance and regenerability

Abstract Highly dispersed carbon-supported MnFe and KMnFe catalysts were prepared which showed activity stabilization after a loss of approximately 50% of the initial activity during 24–110 h on-stream. The deactivation was attributed to carbon deposition, rather than sintering, and could be reversed by a treatment in hydrogen at reaction temperatures. Precursors with Fe Mn = 2 optimized the olefin selectivity, and mixed-metal clusters of this type gave higher selectivities than their coimpregnated counterparts. A low reduction temperature (473 K) for the unpromoted NEt 4 [Fe 2 Mn(CO) 13 ]catalyst gave a high selectivity to olefins which remained stable during the 24-h period the catalyst was maintained under reaction conditions. The particularly high C 2 C 4 olefin yields obtained with the K[Fe 2 Mn(CO) 13 ] catalyst were sustained throughout the 26-h activity maintenance run.

Characterization of supported silver catalysts: III. Effects of support, pretreatment, and gaseous environment on the dispersion of Ag

Small Ag crystallites at low loadings on certain supports appear to be quite resistant to sintering under a variety of gas environments. The metal fraction exposed (dispersion) of such crystallites on η-Al2O3, SiO2, and TiO2 was monitored using oxygen chemisorption and H2 titration methods before and after exposure to He, H2, and O2 (3 and 10%) at 673 K as a function of time, and effects of various pretreatments on adsorption behavior were examined. Selected samples were also characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Exposures to O2 up to 16 hr at 673 K caused only small decreases (20–30%) in the dispersion of η-A12O3-supported silver, virtually no change for the TiO2-supported silver, while a substantial increase in Ag surface area occurred for SiO2-supported Ag. The Ag surface area of the 1.89% Agη-Al2O3 catalyst treated in H2 for 16 hr at 673 K showed no change when monitored by the H2 titration method, whereas heating in He under the same experimental conditions decreased the silver dispersion by approximately 15%. The 1.81% AgTiO2 showed virtually no change in Ag metal surface area after similar treatments in either H2 or He. Although heating in H2 did not cause any change in dispersion, some reduction of the TiO2 support occurred. Because of irreversible adsorption of oxygen by the reduced TiO2 support and complications attributed to “subsurface” or “bulk” oxygen formation in some sintered samples, the H2 titration of the chemisorbed surface oxygen provides a more accurate method than the O2 chemisorption method alone for monitoring the dispersion of these catalysts.

Sol-gel processing of 0.91Pb(Zn13Nb23)O3-0.09PbTiO3: Stabilization of the perovskite phase

Abstract 0.91Pb(Zn 1 3 Nb 2 3 )O 3 -0.09PbTiO 3 was prepared by a sol-gel route in which Zn and Nb were prereacted to form “Zn[Nb(OEt) 6 ] 2 ” in situ. Calcination conditions were varied to optimize the percentage of perovskite phase stabilized. The maximum amount of perovskite phase stabilized was 75%, which occurred at a calcination temperature of 1000°C.

Photoinduced formation of phosphido-bridged clusters derived from Ru3(CO)9(PPh2H)3. Crystal and molecular structure of Ru3(μ-H)(μ-PPh2)3(CO)7

Abstract The new complex Ru 3 (CO) 9 (PPh 2 H) 3 (I) was prepared by the direct thermal reaction of Ru 3 (CO) 12 with PPh 2 H and was spectroscopically characterized. Irradiation of I with λ ≥ 300 nm leads to the formation of Ru 2 (μ-PPh 2 ) 2 (CO) 6 (II) and three new phosphido-bridged complexes, Ru 3 (μ-H) 2 (μ-PPh 2 ) 2 (CO) 8 (III), Ru 3 (μ-H) 2 (μ-PPh 2 ) 2 (CO) 7 (PPh 2 H) (IV) and Ru 3 (μ-H)(μ-PPh 2 ) 3 (CO) 7 (V). These complexes have been characterized spectroscopically and Ru 3 (μ-H)(μ-PPh 2 ) 3 (CO) 7 by a complete single crystal X-ray structure determination. It crystallizes in the space group P 2 1 / n with a 20.256(3), b 22.418(6), c 20.433(5) A, β 112.64(2)°, V 8564(4) A 3 , and Z = 8. Diffraction data were collected on a Syntex P2 1 automated diffractometer using graphite-monochromatized Mo-K α radiation, and the structure was refined to R F 4.76% and R w F 5.25% for the 8,847 independent reflections with F 0 > 6σ(F 0 ). The structure consists of a triangular array of Ru atoms with seven terminal carbonyl ligands, three bridging diphenylphosphido ligands which bridge each of the RuRu bonds, and the hydride ligand which bridges one RuRu bond. Complex IV was also shown to give V upon photolysis and is thus an intermediate in the photoinduced formation of V from I.

Production of hydrogen by ultraviolet irradiation of Mo/sub 2/(SO/sub 4/)/sub 4//sup 4 -/ in aqueous sulfuric acid. Electronic absorption spectrum of K/sub 3/MO/sub 2/(SO/sub 4/)/sub 4/. 3. 5H/sub 2/O at 15 K. [15/sup 0/K]

Ultraviolet irradiation (lambda 254 nm) of Mo/sub 2/(SO/sub 4/)/sub 4//sup 4 -/ in 5 M H/sub 2/SO/sub 4/ produces H/sub 2/ and the one-electron oxidation product Mo/sub 2/(SO/sub 4/)/sub 4//sup 3 -/. The disappearance quantum yield of Mo/sub 2/(SO/sub 4/)/sub 4//sup 4 -/ is 0.17 at 254 nm. The spectrum of K/sub 3/Mo/sub 2/(SO/sub 4/)/sub 4/ . 3.5H/sub 2/O exhibits an absorption band at 1405 nm epsilon 143); this band shows a vibrational progression in a 350-cm/sup -1/ fundamental even in 5 M D/sub 2/SO/sub 4/ solution at room temperature. At 15 K additional vibronic structure is resolved. This band is assigned to the N ..-->.. E type transition delta ..-->.. delta* (/sup 2/B/sub 2g/ ..-->.. /sup 2/B/sub 1u/).

Photoassisted synthesis of mixed-metal clusters: [PPN] [CoOs3(CO)13], H2RuOs3(CO)13, and H2FeOs3(CO)13

Abstract The utility of photochemical methods for the directed synthesis of mixed-metal metal clusters has been explored. The 366 nm photolysis of a solution containing [PPN] [Co(CO)4] (PPN = (Ph3P)2N+) and Os3(CO)12 gives the new cluster [PPN][CoOs3(CO)13] in 33% yield. Irradiation of a mixture of Fe(CO)5 and H2Os3(CO)10 yields H2FeOs3(CO)13 in 95% yield, and photolysis of Ru3(CO)12 in the presence of H2Os3(CO)10 gives the new cluster H2RuOs3(CO)13. Details of these syntheses, their probable mechanisms, and the characterization of the new compounds are discussed.