Albert Theolier

Surface supported metal cluster carbonyls. Chemisorption decomposition and reactivity of Rh4(CO)12 supported on silica and alumina

Abstract Chemisorption of Rh 4 (CO) 12 on to a highly divided silica (Aerosil “0” from Degussa), Leads to the transformation: 3 Rh 4 (CO) 12 → 2 Rh 6 (CO) 16 + 4 CO. Such an easy rearrangement of the cluster cage implies mobility of zerovalent rhodium carbonyl fragments on the surface. Carbon monoxide is a very efficient inhibitor of this reaction, and Rh 4 (CO) 12 is stable as such on silica under a CO atmosphere. Both Rh 4 (CO) 12 and Rh 6 (CO) 16 are easily decomposed to small metal particles of higher nuclearity under a water atmosphere and to rhodium(I) dicarbonyl species under oxygen. From the Rh I (CO) 2 species it is possible to regenate first Rh 4 (CO) 12 and then Rh 6 (CO) 16 by treatment with CO ( P co ⩾ 200 mm Hg) and H 2 O ( P H 2 O ⩾ 18 mm Hg). The reduction of Rh I (CO) 2 surface species by water requires a nucleophilic attack to produce an hypothetical [Rh(CO) n ] m species which can polymerize to small Rh 4 or Rh 6 clusters in the presence of CO but which in the absence of CO lead to metal particles of higher nuclearity. Similar results are obtained on alumina.

Surface supported metal cluster carbonyls. Chemisorption, reactivity, and decomposition of Ru3(CO)12 on silica

Abstract Ru3(CO)12 supported on silica is oxidised by surface SiOH groups in presence of water and/or dioxygen to form oxidised species, probably incorporated into the silica surface, such as RuII(CO)n(OSi )2 (n = 2, 3). The presence of air (involving both water and dioxygen) greatly accelerates this oxidative process. When supported in total absence of dioxygen, Ru3(CO)12 reacts with surface silanol groups to produce the grafted cluster HRu3(CO)10(OSi ), which has been characterized by chemical methods and by infrared and Raman spectroscopies. The grafted cluster is not very stable; it is an intermediate in the formation, by controlled thermal decomposition, of both small metallic particles and some RuII(CO)n(OSi )2 (n = 2, 3) carbonyl surface species (this oxidation by surface protons is also evidenced by the formation of molecular hydrogen). The formation of metallic ruthenium accounts for the production of hydrocarbons during the thermal decomposition.

The characterization and thermal stability of a cluster grafted on silica surface

Abstract Ru 3 (CO) 12 , supported on silica in the absence of oxygen, reacts with silanol groups of the surface to produce a grafted cluster , which has been characterized by IR and Raman spectroscopy; the molecular formula of this cluster is in agreement with the stoichiometric balance of CO evolved during its formation from Ru 3 (CO) 12 . The grafted cluster is an intermediate step to produce by thermal decomposition small metallic ruthenium particles of 14 A together with some Ru(II) carbonyl species encapsulated in the silica surface.

The water gas shift reaction catalyzed by Rh6(CO)16 adsorbed on alumina or [Rh(CO)2Cl]2 adsorbed on alumina, Na-Y zeolite and H-Y zeolite

Abstract Rh 6 (CO) 16 chemisorbed on η-alumina or [Rh(CO) 2 Cl] 2 chemisorbed on η-alumina, Na-Y zeolite and H-Y zeolite catalyze the water gas shift reaction to various degrees. The following order of activity was observed: alumina > Na-Y > H-Y zeolite. With alumina the reaction occurs between 25 and 100°C. Turnover numbers as high as 255/Rh atom/h are obtained at 50°C and under 50 atm. These turnover numbers are the same whether the precursor complex is Rh 6 (CO) 16 or [Rh(CO) 2 Cl] 2 . The mechanism of such a reaction has been deduced from infrared studies, mass balance and labeling experiments. It involves three steps: electrophilic attack by surface protons on the metallic frame with formation of Rh I (CO) 2 (OAl )(HOAl ) and possibly Rh III (H)(H)(OAl )HOAl ); reductive elimination of H 2 from Rh(H)(H)(OAl )(HOAl ) under CO pressure; and nucleophilic attack by water on CO coordinated to rhodium(I) with formation of CO 2 , H + and regeneration of Rh 6 (CO) 16 . If the CO pressure is too low or if liquid water is used, aging of the catalyst occurs which seems to be due to the formation of metallic rhodium. The intermediacy of [Rh(CO) 4 ] − is also suspected in the step of metal-metal bond formation.

Activation and functionalisation of the C–H bonds of methane and higher alkanes by a silica-supported tantalum hydride complex

Silica-supported tantalum hydride activates at low temperature the C–H and the C–D bonds of cyclooctane and CD4, respectively, to form the corresponding cyclooctyl and perdeuteriomethyl-tantalum surface complexes; these complexes are transformed under molecular oxygen into the corresponding tantalum-alkoxy derivatives which with acetic acid give rise to the corresponding alkylacetates.

Chemical grafting of tin alkyl complexes on the external surface of mordenite: A method for controlling the size of the pore entrances of zeolites

Abstract The tetra(alkyl)tin complexes SnR 4 (R = Me, Et, i -Pr, Ph or cyclohexyl) and hydridotris(butyl)tin (Bu 3 SnH) react with the OH groups of the external surface of H-mordenite under the same experimental conditions as silica. For R = Bu, the formation of3 SiOSnBu 3 as the major surface complex could be deduced from the 13 C and 119 Sn magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra. The adsorption properties of these modified mordenites are considerably affected by the presence of the grafted complexes, in a way which is dependent on the nature of the alkyl ligands around Sn. When modified by grafted SnMe 3 [2.3% (w/w) Sn], mordenite adsorbs n -hexane, 2-methylpentane at a lower rate, small amounts of 2,3-dimethylbutane (after 16 h) and no significant amounts of 2,2,4-trimethylpentane. All these results can be explained by the restriction of the size of the pore entrances by the grafted organometallic fragments, allowing, in some cases, the separation of mono- and dibranched hydrocarbons.

Surface organometallic chemistry: formation of the grafted dianionic cluster [Rh6(CO)15]2− upon adsorption of Rh6(CO)16 on a partially-hydroxylated magnesia

Abstract The adsorption of the cluster Rh 6 (CO) 16 on the surface of partially hydroxylated magnesia leads mainly to the dianionic cluster [Mg] 2+ [Rh 6 (CO) 15 ] 2− , which has been characterized by in situ infrared spectroscopy, extraction by surface anion exchange, and analysis of gases evolved. It is suggested that Rh 6 (CO) 16 undergoes on the surface a nucleophilic attack at coordinated CO, leading to a [HRh 6 (CO) 15 ] − intermediate, which would undergo a deprotonation by the surface OH − groups of magnesia.

IR studies and hydrogenation catalytic activity of heterogeneous catalysts prepared by thermal treatments of a face-bridged triruthenium carbonyl cluster supported on silica and alumina

Abstract The interaction of the cluster complex [Ru3 (μ-H) (μ3, ν2-ampy) (CO)9] (1) (ampy = 2-amino-6-methylpyridinate) with silica and γ-alumina at different temperatures (under vacuum and under nitrogen) has been studied by IR spectroscopy. These studies have shown that the presence of the face-bridging ligand ampy in complex 1 does not prevent cluster fragmentation on the support surface, since thermal treatments of these systems just above room temperature lead to mononuclear ruthenium surface species. The solids obtained by different thermal treatments of the 1-SiO2 and 1-Al2O3 systems were tested as catalyst precursors for the hydrogenation of phenylacetylene, resulting in: (i) the silica-based solids are better catalyst precursors than the alumina-based ones, (ii) the solids with mononuclear surface species are more active than those with cluster surface species, (iii) all the supported systems are worst catalysts than complex 1 under homogeneous conditions.

Separation properties of an H-Mordenite modified by anchored tris(methyl)-tin and -germanium complexes

The sorption properties of an H-Mordenite, modified by controlled chemical reaction with tetramethyl-tin, SnMe 4 , and -germanium, GeMe 4 , were determined for linear and branched hydrocarbons. Their variations are correlated with the amount of anchored complex; the influence of the size of the alkyl ligands was determined. The separation of linear and monobranched C 6 hydrocarbons from their dimethyl isomers could be achieved with some of these modified mordenites.

Supported Metal Carbonyl Clusters: Decomposition to Metals and Reactivity

The growing interest in the last 15 years for organometallic chemistry and homogeneous catalysis with organometallic catalysts supported experimental and theoretical evidence for strong analogies between organometallic chemistry or homogeneous catalysis and surface catalysis. In Table 1 some of the relevant contributions, dealing with this particular subject, are chronologically evidenced. Some logical and often evident similarities between the processes of coordination and chemisorption have been first pointed out; they have produced some new ideas on the electronic origin of the catalytic effects and on the mechanisms of surface catalysis. However for a long period the homogeneous reactions were only molecular processes in which one or two metal centers were involved in the catalytic activation, whilst it was quite obvious even at that time that on metal surfaces aggregates of few metals were involved in many catalytic processes.