Multimetallic catalysis based on heterometallic complexes and clusters.

HRTEM and molecular modeling of the MoS2–Co9S8 interface: understanding the promotion effect in bulk HDS catalysts

Metal-sulfur centers play an important role in the activity of metalloproteins in enzymatic catalysis and the activity of metal sulfides as heterogeneous catalysts The systematic search for M[BOND]S model compounds led to the discovery of an interesting and novel structural chemistry, which stems from the numerous coordination possibilities of sulfur ligands The intention of this review article is to present and outline new approaches to sulfur chemistry from the organometallic point of view Reactive cyclopentadienyl-transition metal fragments incorporate elemental sulfur to give polynuclear sulfur-rich complexes, which can contain either mono-, di- or polysulfido ligands or several such ligands in combined form The versatile structural chemistry of the complexes formed and their reactivity towards organic, inorganic and organometallic compounds are discussed, and examples of some simple and rational procedures for their synthesis starting from cyclopentadienylcarbonyl- and cyclopentadienylhydrido-complexes are outlined Their reactivity is manifested in numerous metal- and ligandcentered reactions Finally the, albeit far less extensive, complex chemistry of the other chalcogens (O, Se, Te) is also considered for comparison, thus providing a more detailed survey of particular aspects of this area of chemistry

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Synthesis, Structure and Reactivity of Sulfur‐Rich Cyclopentadienyl‐Transition Metal Complexes: Sulfur Chemistry from an Organometallic Point of View

The kinetics of the reaction of a series of aromatic and aliphatic thiols with cluster 1 were determined. These reactions form cluster 2 and the arene or alkane corresponding to the thiol:  Cp‘2Mo2Co2S3(CO)4 (1) + RSH → Cp‘2Mo2Co2S4(CO)2 (2) + RH + 2CO. These reactions are first order in thiol and first order in cluster 1 with appreciable negative entropies of activation. These data suggest that the rate determining step of the desulfurization reaction is the initial association of the thiol to the cluster. The more nucleophilic thiolate anions react with 1 at −40 °C to form an adduct in which the thiolate anion is bound η1 to the Co atom. At −25 °C, the initial adduct rearranges to a fluxional μ2, η1-bound thiolate. The fluxional process is proposed to involve a concerted “walking” of the thiolate and a μ2-bound sulfide ligand on the surface of the cluster. Near 35 °C, the thiolate−cluster adduct undergoes C−S bond homolysis to give the paramagnetic anion of cluster 1 and the phenyl or alkyl radical. The...

Homolytic C−S Bond Scission in the Desulfurization of Aromatic and Aliphatic Thiols Mediated by a Mo/Co/S Cluster: Mechanistic Aspects Relevant to HDS Catalysis

Transition metal cluster chalcogenides with cubane-type structures are relevant to our understanding of several industrial and biological catalytic processes. Recent advances in cluster chemistry owe much to the development of rational synthetic approaches, an aspect that is emphasised in this review which focuses on cuboidal clusters of molybdenum and tungsten (M) with M2M′2S4 and M3M′Q4 (Q = S, Se) central units that incorporate a first-row transition metal atom (M′). Special attention has been paid to the non-aqueous chemistry of these compounds. The structural features of these complexes are discussed in relation to their electronic structures within the framework of molecular orbital theory, spectroscopic, magnetic and electrochemical experimental data. Finally, the importance of these clusters in catalysis, nonlinear optics and in the formation of supramolecular adducts is discussed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Heterodimetallic Chalcogen‐Bridged Cubane‐Type Clusters of Molybdenum and Tungsten Containing First‐Row Transition Metals

https://deepblue.lib.umich.edu/bitstream/handle/2027.42/27613/0000657.pdf;sequence=1

Syngas and HDS catalysts derived from sulphido bimetallic clusters

Discrete, organometallic clusters containing S, Mo, and Fe or Co have been deposited intact on metal oxide supports. These supported clusters are subjected to temperature programmed decomposition (TPDE) which causes loss of the organic ligands and produces catalysts active for CO hydrogenation and thiophene hydrodesulfurization (HDS). In-situ Mossbauer spectra reveal that the surface is remarkably uniform and that high spin ferrous iron is the predominate species (ca. 90%) under catalytic reaction conditions. EXAFS and XANES spectra show that the clusters are irreversibly oxidized by the surface at ca. 400 K under H2.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400574946

Characterization of HDS and Syngas Catalysts Derived from Heterobimetallic Clusters

Environmental concerns have led to an increased interest in hydrotreating catalysts, i.e. catalysts for hydrodesulfurization (HDS) or hydrodenitrogenation (HDN). Although HDS may be catalyzed by bulk or supported MoS/sub 2/ and related sulfides, the most effective catalysts are those derived from ''sulfided cobalt molybdate'' supported on Al/sub 2/O/sub 3/. These catalysts are prepared conventionally by impregnating the alumina support with solutions of molybdate ions and cobaltous ions. The impregnated support is then calcined to convert the adsorbed species to their respective oxides. The supported metal oxides are then converted to sulfides (''sulfided'') with a mixture of H/sub 2/ and H/sub 2/S (or the feed stream itself) at temperatures in the range 300-400/sup 0/C. Alternate promoters, e.g. Ni and Fe, may be used in the place of Co, but the resulting catalysts usually show a lower activity. Despite intensive study, the exact nature of the active ''CoMoS'' phase is still uncertain, and hence the role of the Co promoter and the exact mechanism of HDS remain obscure. XPS and Mossbauer Emission Spectroscopy has been used to show that a ''CoMoS'' phase, distinct from Co/sub 9/S/sub 8/ or MoS/sub 2/, is present and that the activity of the catalyst parallels the amountmore » of this CoMoS phase present. More recently, EXAFS has been used to study these cobalt molybdate catalysts in both the oxidized and sulfided states.« less

O. Baralt

Papers

Syngas and HDS catalysts derived from sulphido bimetallic clusters

Characterization of HDS and Syngas Catalysts Derived from Heterobimetallic Clusters

Methanation and hds catalysts based on sulfided, bimetallic clusters.

Syngas and HDS Catalysts Derived from Sulfido Bimetallic Clusters