Vanadyl complex catalyst

Bazarova et al.so conclude from an i.r. study of V205/K2 S207 catalysts that a very complex system of at least four different vanadium compounds is formed and that the reaction conditions profoundly influence their relative amounts. Likhtenshtein et al.,sl using a high-temperature i.r. cell showed, for a similar system, that the vanadyl complexes are maintained above their melting point (370°C). 

A major effort was devoted to the characterization of these catalysts by vibrational and electronic spectroscopies [28i]. Use of IR and Raman spectroscopies confirmed that the monolayer consisted of vanadyl complexes, exhibiting both Lewis and Bronsted sites, and suppressing nucleophilic sites on the support. According to UV-VIS spectra, most of the vanadium in the calcined catalysts is in the Vv state, but part is easily reducible. Evidence for electronic interaction between vanadium and titanium ions was also presented. The monolayer is effective in suppressing Raman bands due to the anatase support, and an explanation for this effect has been offered [30]. 

The desire to convert benzene directly to phenol with 30% hydrogen peroxide was mentioned in Chap. 4. A polymer-supported salicylimine vanadyl complex (1 mol%) was used to catalyze this reaction. Phenol was obtained in 100% yield at 30% conversion.217 There was no leaching of the metal. The catalyst was recycled ten times after which it started to break up. Oxidation of ligands is often a problem with oxidation catalysts. Inorganic supports not subject to such oxidation need to be tried to extend the life of such catalytic agents. 

G. Centi, (ed), Forum on vanadyl pyrophosphate catalyst . Catalysis Today, 16(1), (1993). J. R. Ebner and J. T.GIeaves, The activation of oxygen by metal phosphorus oxides-the vanadium phosphorus system , Oxygen Complexes and Oxygen Activation by Transition Metals , (eds. A. E. Martell and D. T. Sawyer) Plenum Publishing Corp., New York, 273-292 (1988). 

Oxidation of allyl sulfides with aqueous H2O2 using the vanadyl complex (115) as a catalyst afforded chiral allyl sulfoxides with up to 97.3% ee and 57% yields. ... 

Vanadium compounds have been widely explored as catalysts for H O -based hydroxylation of benzene and other arenes [11, 77, 78]. A vanadyl complex grafted on to periodic mesopotous organosilicas, [VO(acac)j]/PMO, was used as recyclable heterogeneous catalyst for benzene hydroxylation, producing phenol with nearly 100% selectivity at 27% benzene conversion but with a low oxidant utilization efficiency [79]. 

Indeed, more complex catalysts are required for partial oxidation reactions. Although several catalytic systems have been studied in the last twenty years, a very limited number of catalysts have been reported for industrial or pre-industrial use. In fact, in addition to V-P-0 catalysts (based on vanadyl pyrophosphate), the unique catalyst used for an alkane oxidation industrialized process, only V-Sb- and MoVTe(Sb)NbO-based mixed-metal oxides have been proposed as sufficiently effective catalysts for the propane ammoxidation process. In both cases pilot plants using the latter catalysts have been announced on the bases of their catalytic results. 

Vanadium compounds have a complex chemistry because of the multiple oxidation states of vanadium. Among these, V2O5 is an important semiconducting material with potential applications as catalysts, chemical sensors, field effect transistors, and electrochemical and photochromism devices. Studies on the reactivity of hydrazinium chloride with NH4VO3 have led to the formation of the ammonium vanadyl complex (NH4)2VO (OH)2Cl2, which is a precursor to the formation of V2O5. 

The chiral vanadyl salen complex was anchored on mesoporous materials by a covalent grafting method. These heterogenized complex catalysts were evaluated as asymmetric catalysts for the asymmetric oxidation of sulfides to sulfoxides [92]. 

The vanadium(IV) complex of salen in zeolite was found to be an effective catalyst for the room temperature epoxidation of cyclohexene using t-butyl hydroperoxide as oxidant.88 Well-characterized vanadyl bis-bipyridine complexes encapsulated in Y zeolite were used as oxidation catalysts.101 Ligation of manganese ions in zeolites with 1,4,7-triazacyclononanes gives rise to a binu-clear complex stabilized by the zeolites but allows oxidation with excellent selectivity (Scheme 7.4). 

The dimer of the vanadyl silsesquioxane complex 148 was used by Mitsudo et al. to prepare catalysts with a characteristic pore structure and excellent activity toward the selective photoassisted catalytic oxidation of methane into methanal. " ... 

According to a different approach to tailoring a catalyst for the desired ionic character, the catalyst was modified by the attachment of an imidazolium ion. This approach was demonstrated for a vanadyl salen complex, VO(salen) IL (Scheme 14) (187). 

Complex vanadium-phosphorus-oxide catalysts are the most successful industrial catalysts for the selective oxidation of /i-butane to maleic anhydride (MA) with uses in tetrahydrofurans (THE) and polyurethane intermediates. A schematic diagram of the reaction is shown in figure 3.21(a). These catalysts have been studied extensively (e.g. Centi et al 1993, Bordes 1987). In the selective catalysation of a-butane to MA, the best active phase in the V-P-0 system is identified as the vanadyl pyrophosphate, (VO)2P207 (hereafter... 

Selective oxidation materials fall into two broad categories supported systems and bulk systems. The latter are of more practical relevance although one intermediary system, namely vanadia on titania [92,199-201], is of substantial technical relevance. This system is intermediary as titania may not be considered an inert support but rather as a co-catalysts [202] capable of, for example, delivering lattice oxygen to the surface. The bulk systems [100, 121, 135, 203] all consist of structurally complex oxides such as vanadyl phosphates, molybdates with main group components (BiMo), molybdo-vanadates, molybdo-ferrates and heteropolyacids based on Mo and W (sometimes with a broad variation of chemical composition). The reviews mentioned in Table 1.1 deal with many of these material classes. 

They next studied the asymmetric oxidative polymerization of achiral 2,3-dihydroxynaphthalene (Scheme 42). The polymerization of this monomer with CuCl2-(-)-sparteine complex resulted in a low yield and gave a low molecular weight oligomer, whereas the polymerization with CuCl-(S)-Phbox quantitatively gave a polymer with Mn of 10 600-15 300. The enantioselectiv-ity attained in this polymerization, however, was estimated to be low, with 43% ee from the model reaction [169]. When vanadyl sulfate (VOSO -Phbox complex was used instead of the copper catalyst system, the enantioselectivity was improved up to 80% ee [170]. Asymmetric cross-coupling polymerization of two kinds of naphthol derivatives was also reported [171,172]. 

Components of fluidized cracking catalysts (FCC), such as an aluminosilicate gel and a rare-earth (RE) exchanged zeolite Y, have been contaminated with vanadyl naphthenate and the V thus deposited passivated with organotin complexes. Luminescence, electron paramagnetic resonance (EPR) and Mossbauer spectroscopy have been used to monitor V-support interactions. Luminescence results have indicated that the naphthenate decomposes during calcination in air with generation of (V 0)+i ions. After steam-aging, V Og and REVO- formation occurred. In the presence of Sn, Tormation Of vanadium-tin oxide species enhance the zeolite stability in the presence of V-contaminants. 

FIGURE 29 Schematic representation of the dynamic processes that determine the formation of the active catalyst phase from vanadyl pyrophosphate. The dashed separation in solid-state and catalytic regimes is artificial and supports only the explanation of the complex interrelations. Reproduced with permission from Ref. (757). Copyright... 

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