Catalysts, heterogeneous/homogeneous

Keywords Catalysis, polymer-supported oxidovanadium (IV) complex, Solid supported catalysts, heterogenized homogeneous catalysts... 

Decade First Commer- cialized Process Reaction Catalyst Heterogeneous/ homogeneous (H/h)... 

Keywords Immobilized metal catalysts Heterogenized homogeneous catalysts Solid supported catalysts Industrial catalysis ... 

The reaction of iodobenzene with acrylate is a good synthetic method for the cinnamate 17[7]. In the competitive reaction of acrylate with a mixture of 0-and /i-iodoanisoles (18 and 19), the o-methoxycinnamate 20 was obtained selectively owing to the molecular recognition by interlamellar montmorillonite ethylsilyldiphenylphosphine (L) as a heterogenized homogeneous catalyst used as a ligand[28]. 

Increasing efforts to heterogenize homogeneous catalysts for LPO are apparent (2,206—209). Significant advantages in product recovery, catalyst use, and catalyst recovery are recognized. In some instances, however, the active catalyst is reported to be material dissolved from the sotid catalyst (210). 

The situation has now changed and currently an area of considerable research interest is in heterogenizing homogeneous catalysts. One such instance is to be found in the ethylene based manufacture of vinyl acetate (11). A homogeneous catalytic process based on palladium and copper salts was first devised, but corrosion problems were made much less serious in a heterogeneous system based on the same chemical principles. 

Many catalysts, both homogeneous and heterogeneous, have been used for this reaction. Although there are several examples of the former, the most common... 

HE = heterogeneous catalyst, HO = homogeneous catalyst, E = enzyme, Y = yeast. 

Hydrogenation of aromatic nitro compounds [8,18,29] and hydrogenation of benzene derivatives [2,9,21] have been generally accepted as model reactions to check the heterogeneous nature of catalyst, because homogeneous species are not believed to be active. But at least two well-studied examples show that molecular catalysts can hydrogenate benzene [36,37]. 

Catalytic transformations can be divided on the basis of the catalyst-type - homogeneous, heterogeneous or enzymatic - or the type of conversion. We have opted for a compromise a division based partly on type of conversion (reduction, oxidation and C-C bond formation, and partly on catalyst type (solid acids and bases, and biocatalysts). Finally, enantioselective catalysis is a recurring theme in fine chemicals manufacture, e.g. in the production of pharmaceutical intermediates, and a separate section is devoted to this topic. 

An alternative that has received a great deal of attention in recent years is the immobilisation of a chiral catalyst on a nonsoluble support (polystyrene resins, silica gel, zeolites, etc.), thereby creating a chiral heterogeneous catalyst. Unlike homogeneous catalysts, these supported complexes can be recovered from the... 

The majority of heterogeneous chemical and physical-chemical processes lead to formation of the intermediate particles - free atoms and radicals as well as electron- and oscillation-excited molecules. These particles are formed on the surface of solids. Their lifetime in the adsorbed state Ta is determined by the properties of the environment, adsorbed layer, and temperature. In many cases Ta of different particles essentially affects the rate and selectivity of heterogeneous and heterogeneous-homogeneous physical and chemical processes. Therefore, it is highly informative to detect active particles deposited on surface, determine their properties and their concentration on the surface of different catalysts and adsorbents. 

In general, there are heterogeneous, homogeneous, and biological catalysts. This is a somewhat arbitrary division, but serves the purpose of condensing the range of kinds of catalysts. This range is shown in the Fig. 1.1. 

There are two kinds of catalysts. Heterogeneous catalysts are insoluble in the medium in which the reaction is taking place so that reactions of gaseous or liquid reagents occur at the surface, whilst homogeneous catalysts are dissolved in the reaction medium and hence all catalytic sites are available for reaction. Some of the properties of catalysts are collected in Table 1.1, where heterogeneous and homogeneous catalysts are compared. 

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