Heteropolyoxometallate catalysts

The tendency in the past decades has been to replace them with solid acids (Figure 13.1). These solid acids could present important advantages, decreasing reactor and plant corrosion problems (with simpler and safer maintenance), and favoring catalyst regeneration and environmentally safe disposal. This is the case of the use of zeolites, amorphous sihco-aluminas, or more recently, the so-called superacid solids, that is, sulfated metal oxides, heteropolyoxometalates, or nation (Figure 13.1). It is clear that the well-known carbocation chemistry that occurs in liquid-acid processes also occurs on the sohd-acid catalysts (similar mechanisms have been proposed in both catalyst types) and the same process variables that control liquid-acid reactions also affect the solid catalyst processes. 

Several metal oxides could be used as acid catalysts, although zeolites and zeo-types are mainly preferred as an alternative to liquid acids (Figure 13.1). This is a consequence of the possibility of tuning the acidity of microporous materials as well as the shape selectivity observed with zeolites that have favored their use in new catalytic processes. However, a solid with similar or higher acid strength than 100% sulfuric acid (the so-called superacid materials) could be preferred in some processes. From these solid catalysts, nation, heteropolyoxometalates, or sulfated metal oxides have been extensively studied in the last ten years (Figure 13.2). Their so-called superacid character has favored their use in a large number of acid reactions alkane isomerization, alkylation of isobutene, or aromatic hydrocarbons with olefins, acylation, nitrations, and so forth. 

Isopolyoxometalates and heteropolyoxometalates such as [W6019]2-, [W10O32]4-and [PWi2O40]3- could act as efficient, stable, selective photooxidation catalysts... 

Cysteic acid is obtained in nearly quantitative yield from cysteine with aqueous hydrogen peroxide in the presence of iron(II) ions.397 Molybdates and tungstates have also been used as effective catalysts for similar transformations.398 An excellent route for the oxidation of 2-thioethanol to isothionic acid has been developed.399 Heteropolyoxometallates supported on alumina400 can also be used to oxidize a range of organo-sulfur compounds. For example, alkyl monosulfides to sulfoxides and sulfones, and thiols to sulfonic acids are a few possibilities (Figure 3.98). 

It is known [41] that partial oxidation reactions in heterogeneous catalysis involves redox properties of the solid catalysts, allowing the well known Mars-van Krevelen mechanism [42] to occur, or at least to be facilitated. Acid-base properties are also an important feature, as they play a determining role in the activation of the reactants and in the desorption of the intermediate compounds, for instance, an acid surface will favor desorption of acid products, thus avoiding further over-oxidation, while a basic surface will favor desorption of basic products as olefins. It follows that heteropolyoxometallate compounds, in particular TMSP, appear as potential... 

The results on the effect of temperature, contact time and methanol to toluene ratio on the isomer composition of xylenes on K2.5 salt are given in Table 3. It is seen that selectivity of p-xylene decreases with increase in the temperature whereas the selectivity of m-xylene increases, obviously, due to the isomerization. As contact time increases p-xylene selectivity increases. It is also found that the p-xylene selectivity increases with increasing methanol to toluene ratio. As methanol to toluene ratio increases the catalyst surface will be saturated with more of alkylating species which offers hindrance to the approach of the aromatic substrate and thereby resulting in the preferential alkylation at para position. In conclusion,it may be suggested that high Bronsted acidity is responsible for high para selectivity found in heteropolyoxometallates. 

It is increasing interested in the utilization of heteropoly compounds as catalysts for the oxidation of various organic compounds. Recently, we have found that the mixed addenda heteropolyoxometallates such as (NH4)5H4PV0Mo0O4o 6H2O (PV0MO0) was efficient catalysts for the aerobic oxidation of olefinic compounds in the presence of isobutyraldehyde. We now present here the direct oxidation of amines and hydrocarbons with molecular oxygen by PV0MO0 catalyst. 

The acetalization of ethyl acrylate with ethanol is effected by the use of mixed addenda heteropolyoxometallates such as molybdovanadophosphate (NPMoV) with a catalyst system consisting of Pd(OAc)2-hydroquinone and (Scheme 27). Recently, the use of supercritical carbon dioxide (ieC02) as a solvent in Pd(II)-catalyzed acetalization of methyl acrylate with MeOH (Scheme 28) has been reported. ... 

Heteropolyoxometalates of the Keggin structural class, including their transition-metal-substituted derivatives, are effective homogeneous catalysts for selective oxidations of organic and inorganic substrates by a variety of oxidants. ... 

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