Heterogeneous POM catalysts

However, styrene and cyclohexene gave complex product mixtures, and 1-octene did not react under the same reaction conditions. Thus, the activity of this catalyst is intrinsically low. Jacobs and co-workers [159,160] applied Veturello s catalyst [PO WCKOj ]3- (tethered on a commercial nitrate-form resin with alkylammonium cations) to the epoxidation of allylic alcohols and terpenes. The regio- and diastereoselectivity of the parent homogeneous catalysts were preserved in the supported catalyst. For bulky alkenes, the reactivity of the POM catalyst was superior to that of Ti-based catalysts with large pore sizes such as Ti-p and Ti-MCM-48. The catalytic activity of the recycled catalyst was completely maintained after several cycles and the filtrate was catalytically inactive, indicating that the observed catalysis is truly heterogeneous in nature. 

Catalyst filtration experiments performed at the reaction temperature showed that a-pinene conversion in the fdtrate stopped after separation of the catalyst in the case of a-pinene autoxidation (Figure 7a), which corroborates substantially a heterogeneous nature of the oxidation catalysis. Yet, no leaching was found by the elemental analysis after 5 cycles of a-pinene autoxidation over the supported Co-POM catalysts. In contrast, further a-pinene transformation occurred in the fdtrate in the case of a-pinene and IBA co-oxidation (Figure 7b), the reaction rate being significantly higher than that in the blank experiment. 

The examples of dative binding of catalytically active POMs to acquire heterogeneous POM-based catalysts can be found in the literature quite rarely compared to the examples of electrostatic attachment which were discussed in the previous section. 

Reaction of the sandwich-type POM [(Fc(0H2)2)j(A-a-PW9034)2 9 with a colloidal suspension of silica/alumina nanopartides ((Si/A102)Cl) resulted in the production of a novel supported POM catalyst [146-148]. In this case, about 58 POM molecules per cationic silica/alumina nanoparticle were electrostatically stabilized on the surface. The aerobic oxidation of 2-chloroethyl ethyl sulfide (mustard simulant) to the corresponding harmless sulfoxide proceeded efficiently in the presence of the heterogeneous catalyst and the catalytic activity of the heterogeneous catalyst was much higher than that of the parent POM. In addition, this catalytic activity was much enhanced when binary cupric triflate and nitrate [Cu(OTf)2/Cu(N03)2 = 1.5] were also present [148],... 

As mentioned in Sect. 2, many efficient homogeneous and heterogeneous oxidation systems based on POMs have been developed. Among them, some POM catalysts with multimetallic active sites show remarkable activity and selectivity in comparison with the conventional monometalhc complexes. According to the proposed reaction mechanisms, the oxidation catalyses by multimetallic active sites of POMs are classified into the following four categories (1) cooperative activation of oxidants, (2) simultaneous activation of oxidants and substrates, (3) stabihzation of reaction intermediates, and (4) multielectron transfer (Fig. 1). In Sect. 3, we focus on the selective oxidations by multimetallic active sites of POM-based molecular catalysts. 

Various types of POMs are effective catalysts for the H202- and 02-based environment-friendly oxidations. Most of these oxidations are carried out in homogeneous systems and share common drawbacks, that is, catalyst/product separation and catalyst recycling are very difficult. The heterogenization of POMs can improve the catalyst recovery and recycling. This chapter focuses on the development of (1) homogeneous catalysts with POMs and (2) the heterogenization for liquid phase-oxidations. 

Similarly, heterogeneous catalyst prepared by immobilization of POMs on chemically modified hydrophobic Si02 has been applied to the selective epoxidation of various alkenes with 15% aqueous H202 without organic solvents [168],... 

In view of the numerous advantages of POMs the development of strategies for converting them to solid catalysts is of primaiy interest. First, catalytically active POMs can be heterogenized in the form of insoluble salts using Cs, Ag, K, NH/ and some organic cations [37,49, 58-64]. Such salts possess micro/mesoporous structure and their smface area is typically in the range of 10-150 mVg. 

Before closing this section devoted to the eleetrostatie attaehment of POMs to supports, we would like to come back to the simple siliea- and earbon-supported POM eatalysts. As was mentioned in the Introduetion, a wide majority of these eatalysts are not stable to leaehing and, in effect, behave as homogeneous rather than heterogeneous eatalysts. However, in few cases, a eareful eontrol of the POM loading, the solvent used for impregnation as well as the textural and surfaee properties of the support allowed the preparation of solid catalysts which were relatively stable to leaching, provided the reaetion mixture polarity was also under eontrol [105,127-129]. 

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