Supported metals carrier selection

Another important and well studied paramagnetic ion in the lattice of oxide semiconductors is Zr3+ in Zr02. Zirconia dioxide is widely used both as a catalyst of different chemical processes, and as a carrier for constructing supported metal-complex catalysts. In the last years, sulfated zirconia attracted significant interest as an active and selective catalyst in skeletal isomerization of normal alkanes at low temperatures, cracking of paraffins, alkylation and acylation of aromatics [42, 53 and Refs therein]. The appropriate experimental data are collected in the following Table 8.2. 

Higher activity catalyst can be achieved by increasing the metal content up to the limit of the support capacity, although the molybdenum efficiency decreases. Consequently, we have worked on the different steps of a catalyst preparation (carrier selection and shaping, Co/Mo ratio, molybdenum and cobalt introduction methods, promotor, thermal and hydrothermal treatments...) and examined the activity of the resulting catalyst at each step. 

On the other hand, the preparation of a supported catalyst involves selecting precursors of the active components and any necessary promoters, and mixing them in a solvent. Then an inert carrier is coated with this mixture and the active metal or precursor is dispersed on the carrier. The product is dried, mixed with a binder then ground, pelletized, extruded, or otherwise shaped. Finally, the material is calcined and activated by oxidation, reduction, or other means. 

The oxidation of ethylene to ethylene oxide over silver was first published in a patent to Lefort in 1931 (S,9). Since that time many studies of the reaction have been made, and important industrial processes have been developed. Much private research has not been published. Many patents have been issued. Recently a number of new publications have appeared, mainly from academic and government laboratories. In the available information there is much that is conflicting or dubious. In many experiments it is likely that unsuspected impurities played a major role, for silver catalysts have low surface areas and are often significantly moderated by minor amounts of impurities, either from the preparation or from the gaseous reactants. Nevertheless, the main facts are clear. The catalyst is metallic silver and its surface should be moderated with a very small amount of a halogen or similar electronegative material for optimum selectivity. The support or carrier plays a small role it should be inert and of rather low surface area. 

For the selective nitrogenation of benzene with ammonia, Hodlerich et al. investigated the catalytic effect of a series of Group Vlll metals (e.g., Ru, Rh, Pd, Pt, Cu, and Ni) as the catalyst supported on carriers such as alumina, silica and zeolite in a plug slow reactor or in a continuously stirred tank reactor. Oxygen or carbon monoxide was employed respectively to shift the reaction equilibrium towards aniline formation [69]. Durante et al. developed a process for catalytic oxidative amination of aromatic hydrocarbons in which aromatic feedstock contacts with oxygen under suitable reaction conditions in the presence of a catalyst... 

The catalysts designed for hydrotreating of the selected type of feedstock should contain the supported metals only in the form of the active sites, arranged in the pores with optimal size. The preparation of the carriers with required average pore diameter is easy-to-implement task. The catalysts containing Co and Mo only in the form of the active sites can be prepared using bimetallic complexes [3]. The selective synthesis of the active sites inside of the pores with the required size is much more difficult. 

Most industrial catalysts are heterogeneous catalysts consisting of solid active components dispersed on the internal surface of an inorganic porous support. The active phases may consist of metals or oxides, and the support (also denoted the carrier) is typically composed of small oxidic structures with a surface area ranging from a few to several hundred m2/g. Catalysts for fixed bed reactors are typically produced as shaped pellets of mm to cm size or as monoliths with mm large gas channels. A catalyst may be useful for its activity referring to the rate at which it causes the reaction to approach chemical equilibrium, and for its selectivity which is a measure of the extent to which it accelerates the reaction to form the desired product when multiple products are possible [1],... 

A different direction in ion-selective electrode research is based on experiments with antibiotics that uncouple oxidative phosphorylation in mitochondria [59]. These substances act as ion carriers (ionophores) and produce ion-specific potentials at bilayer lipid membranes [72]. This function led Stefanac and Simon to obtain a new type of ion-selective electrode for alkali metal ions [92] and is also important in supporting the chemi-osmotic theory of oxidative phosphorylation [69]. The range of ionophores, in view of their selectivity for other ions, was broadened by new synthetic substances [1,61]. 

Other applications of supported liquid membranes have been related to metal speciation. For example, recently a system for chromium speciation has been developed based on the selective extraction and enrichment of anionic Cr(VI) and cationic Cr(III) species in two SLM units connected in series. Aliquat 336 and DEHPA were used respectively as carriers for the two species and graphite furnace atomic absorption spectrometry used for final metal determination. With this process, it was possible to determine chromium in its different oxidation states [103]. 

Belkhouche, N.E., Didi, M.A., Romero, R., Jonsson, J.A. and Villemin, D. (2006) Study of new organophosphorus derivates carriers on the selective recovery of M(II) and M(III) metals, using supported liquid membrane extraction. Journal of Membrane Science, 284, 398. Schlosser, S. (1997) Method and equipment for mass and heat transfer among several liquids (in Slovak), Slovak pat. No. 278547. 

Catalysts were some of the first nanostructured materials applied in industry, and many of the most important catalysts used today are nanomaterials. These are usually dispersed on the surfaces of supports (carriers), which are often nearly inert platforms for the catalytically active structures. These structures include metal complexes as well as clusters, particles, or layers of metal, metal oxide, or metal sulfide. The solid supports usually incorporate nanopores and a large number of catalytic nanoparticles per unit volume on a high-area internal surface (typically hundreds of square meters per cubic centimeter). A benefit of the high dispersion of a catalyst is that it is used effectively, because a large part of it is at a surface and accessible to reactants. There are other potential benefits of high dispersion as well— nanostructured catalysts have properties different from those of the bulk material, possibly including unique catalytic activities and selectivities. 

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