Zeolite-supported transition metal catalysts

W. M. H. Sachtler and Z. Zhang, Zeolite supported transition metal catalysts, Adv. Catal. 39, 129-220(1993). 

Metal clusters in zeolites an intriguing class of catalysts Zeolite-supported transition metal catalysts Stoichiometric and catalytic reactivity of organometallic fragments supported on inorganic oxides Silver clusters and chemistry in zeolites Structure and reactivity of surface species obtained by interaction of organometallic compounds with oxidic surfaces infra-red studies... 

The present model deals with a supported transition metal cation which is highly dispersed, at the molecular scale, on an oxide, or exchanged in a zeolite. In the case of zeolite-supported cations, the formation of different metal species in metal/zeolite catalysts (metal oxides, metal oxocations, besides cationic species) has been considered by different authors who have suggested these species to play key roles in SCR catalysis [14,15], This supported cation can also be considered as located at a metal oxide/support interface. 

As can be seen, the catalytic process over a zeolite-supported cation, or an oxide-supported cation, can be considered as a supported homogeneous catalysis, as far as adsorbed reactants and products behave like reactive ligands. The model developed for lean DcNO. catalysts over supported cations (function 3), as well as this supported homogeneous catalysis approach, is also suitable for stoichiometric mixture (TWC) comprising CO and H2 as reductants over supported transition metal cations [20-22],... 

Supported non-framework elements, as well as substituted or doped framework atoms, have been important for zeolite catalyst regeneration. By incorporating metal atoms into a microporous crystalline framework, a local transition state selectivity can be built into the active site of a catalytic process that is not readily attainable in homogeneous catalysis. The use of zeolites for carrying out catalysis with supported transition metal atoms as active sites is just beginning. The local environment of transition metal elements as a function of reaction parameters is being defined by in situ Mossbauer spectroscopy, electron spin echo measurements, EXAFS, and other novel spectroscopic techniques. This research is described in the second part of this text. 

Various strategies have been pursued in order to immobilise chiral epoxidation catalysts and these encompass covalent attachment to solid supports,[41] steric occlusion in nanosized cages of zeolites,[42 44] entrapment in a polydimethylsiloxane membrane145,461 and fluorous biphasic systems.1471 However, these approaches frequently require tedious ligand modifications and often lead to a marked decrease in both selectivity and activity of the transition metal catalyst. 

A promising and cleaner route was opened by the discovery of titanium silica-lite-1 (TS-1) [1,2]. Its successful application in the hydroxylation of phenol started a surge of studies on related catalysts. Since then, and mostly in recent years, the preparation of several other zeolites, with different transition metals in their lattice and of different structure, has been claimed [3]. Few of them have been tested for the hydroxylation of benzene and substituted benzenes with hydrogen peroxide. Ongoing research on suppoi ted metals and metal oxides has continued simultaneously. As a result, knowledge in the field of aromatic hydroxylation has experienced major advances in recent years. For the sake of simplicity, the subject matter will be ordered according to four classes of catalyst medium-pore titanium zeolites, large-pore titanium zeolites, other transition metal-substituted molecular sieves, and supported metals and mixed oxides. 

The need to produce liquid fuels from low-quality hydrocarbon feedstocks has stimulated numerous investigations over the supported and non-supported transition metal sulfides (TMS) used as catalysts in hydroprocessing (ref. 1). Co-Mo and Ni-Mo sulfides supported on alumina are widely used as hydrogenation (HD) or hydrodesulfurization (HDS) catalysts. Application of zeolites as the supports for these sulfides seems to be promising due to the dual functionality of such systems (ref.2)... 

An integrated, two-zone layout, incorporating two, multi-component, transition-metal catalyst compositions on zeolite, or oxide supports (10). 

Whereas the Mobil process starts with syn gas based methyl alcohol, Olah s studies were an extension of the previously discussed electrophilic functionalization of methane and does not involve any zeolite-type catalysts. It was found that bifunctional acidic-basic catalysts such as tungsten oxide on alumina or related supported transition metal oxides or oxyfluorides such as alumina or related supported transition metal oxides or oxyfluorides such as tantalum or zirconium oxyfluoride are capable of condensing methyl chloride, methyl alcohol (dimethyl ether), methyl mercaptan (dimethyl sulfide), primarily to ethylene (and propylene) (equation 65) . 

Catalytic oxidation processes are usually connected with transfer of electrons and changes of structure and valence state of active catalyst components. This chapter presents methods that are especially suitable for monitoring these kinds of changes (UV-vis-DRS, EPR, X-ray scattering, XPS, XAS, TPO, TPR, TPRS, TAP and SSITKA). After a short section on basic principles and experimental details, the potential of each technique is illustrated by selected application examples that include a wide variety of oxidation catalysts such as mixed metal oxides and oxynitrides, zeolites containing transition metal ions, heteropoly acids and supported noble metals. 

The high costs of transition metal catalysts along with the toxic effects associated with many transition metals have led to an increased interest in immobilizing catalysts onto a solid support. Various supported (like silica, zeolites, polymer, etc.) Cu(I) catalyzed triazole synthesis has been reported in rfie literature. 

Partial oxidation reactions are usually carried out over transition metal oxides capable of changing their valent state during their interaction with reacting molecules. Naturally, zeolites with their alumina-silicate composition did not prove themselves as good oxidation catalysts. They failed also to serve as efScient catalyst supporters, since transition metals being introduced into the zeolite matrix lose their ability to activate dioxygen [3,4],... 

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