Synthesis, characterization, and catalytic properties of clean and oxygen-modified tungsten carbides

The depth of our understanding in catalysis is governed by the information we have about the number of active sites and their molecular structure. The nature of an active center on the surface of a working heterogeneous catalyst is, however, extremely difficult to identify and precise quantification of active species is generally missing. In metathesis of propene over dispersed molybdenum oxide supported on silica, only 1.5% of all Mo atoms in the catalyst are captured to form the active centers. Here we combine infrared spectroscopy in operando with microcalorimetry and reactivity studies using isotopic labeling to monitor catalyst formation. We show that the active Mo(VI)–alkylidene moieties are generated in situ by surface reaction of grafted molybdenum oxide precursor species with the substrate molecule itself gaining insight into the pathways limiting the number of active centers on the surface of a heterogeneous catalyst. The active site formation involves sequential steps requiring multiple catalys...

In situ generation of active sites in olefin metathesis.

Publisher Summary This chapter focuses on the chemical design surfaces for active solid catalysts that can be regarded as a promising and scientific way to prepare well-defined active surfaces with excellent catalytic properties as well as to elucidate reaction mechanisms, including dynamic changes of active sites. These are new generation of catalysts resulting from scientific development at the boundary between homogeneous and heterogeneous chemistry. The use of such chemically and structurally controllable surface systems, rather than the more conventional supported catalysts with heterogeneous metal centers, can lead to novel information on the origins of catalysis. Once the essential processes of activity, selectivity, and specificity have been identified, it should be possible to incorporate this knowledge into the rational synthesis of practical catalysts that are deliberately tailored for specific reaction chemistries. Chemical surface design has strategic advantages in the synthesis of catalysts with novel and desired surface structures and compositions, and in the elucidation of heterogeneous catalysis on a molecular level.

Chemical Design Surfaces for Active Solid Catalysts

Mesoporous molecular sieves as advanced supports for olefin metathesis catalysts

Catalytic polymerizations in aqueous medium

Possible role of hydrido-metal complexes in metathesis, isomerization, dimerization, and polymerization of alkenes

Low molecular-weight products have been obtained during ring-opening polymerization of norborene catalysed by several transition-metal halides; their identities and other evidence show that hydridometal complexes are involved in both dimerization reactions and carbene initiation.

Mechanism of initiation of ring-opening polymerization of norbornene catalysed by transition-metal halides

A hydrogenation catalyst comprising finely divided metallic copper very stable to high temperature use is made by the continuous coprecipitation of copper compounds with hydrated silica from soluble silicate and copper salts with careful control of the pH in the range 4.5-8.0. Subsequent reduction wih hydrogen produces particularly copper (ca. 2-5 nm size) on a porous silica substrate.

Novel copper catalyst and process for making it

Base adsorption properties of the new high-silica zeolite HFu-1, studied by i.r. spectrophotometry, are such that Me3N and pyridine do not have access to the internal acid sites of this zeolite; the size constraint of about 0·5 nm is in marked contrast to that of ZSM-5 which will admit aromatic compounds.

Pore constraints in the novel high-silica zeolite HFu-1

Cycloheptatriene reacts readily in the presence of ethylene at >403 K on Re2O7–Al2O3 to give mainly benzene and a mixture of C1 to C4 products; the reaction provides good evidence for the participation of metallocycles and adsorbed methylenes in conversions of ethylene at these temperature on dismutation catalysts.

Allan Stewart

Papers

Possible role of hydrido-metal complexes in metathesis, isomerization, dimerization, and polymerization of alkenes

Mechanism of initiation of ring-opening polymerization of norbornene catalysed by transition-metal halides

Novel copper catalyst and process for making it

Pore constraints in the novel high-silica zeolite HFu-1

Conversion of cycloheptatriene into benzene on a Re2O7–Al2O3 dismutation catalyst. Formation and reactivity of adsorbed methylenes