Heterogeneous, Early Transition-Metal Catalysts

Supported Early Transition-Metal Complexes as Heterogeneous Hydrogenation Catalysts... 

Contrary to the late transition metals, in the last few years well-defined sihca supported early transition metal (Re, Mo, W) complexes have been found [27-29] as highly active heterogeneous olefin metathesis catalysts relative to their molecular equivalents. 

The reaction in Fig. 4.19 is catalyzed by compounds of high-valent, early transition metals such as Mo(VI), W(VI), V(V) and Ti(IV). Molybdenum compounds are particularly effective homogeneous catalysts and are used in the ARCO process in combination with TBHP or EBHP. In the Shell process, on the other hand, a heterogeneous Ti(IV)/Si02 catalyst is used with EBHP in a continuous,... 

The products of this reaction, an equimolar mixture of H2 and CO (called synthesis gas or syn gas some CO2 may be produced as a by-product), can be used with metallic heterogeneous catalysts in the synthesis of a variety of useful organic products. For example, the Fischer-Tropsch process, developed by German chemists in the early 1900s, uses transition metal catalysts to prepare hydrocarbons, alcohols, alkenes, and... 

Despite the early use of phosphonium salt melts as reaction media [12, 18, 25], the use of standard ionic liquids of type 1 and 2 as solvents for homogeneous transition metal catalysts was described for the first time in the case of chloroaluminate melts for the Ni-catalyzed dimerization of propene [5] and for the titanium-catalyzed polymerization of ethylene [6]. These inherently Lewis-acidic systems were also used for Friedel-Crafts chemistry with no added catalyst in homogeneous [7] as well as heterogeneous fashion [8], but ionic liquids which exhibit an enhanced stability toward hydrolysis, i. e., most non-chloroaluminate systems, have been shown to be of advantage in handling and for many homogeneously catalyzed reactions [la]. The Friedel-Crafts alkylation is possible in the latter media if Sc(OTf)3 is added as the catalyst [9]. 

Early transition metal carbides are catalyticaUy active for a variety of reactions. Since they are refractory and of high surface area, they could also be promising alternatives to oxides as supports for heterogeneous catalysts. Only a few investigations have used them for this purpose. And in these limited investigations, the metals were loaded onto intentionally passivated Mo C powders due to their high pyrophoricity. 

High-valent early transition metals like titanium(IV) and vanadium(V) have been shown to efficiently catalyze the epoxidation of alkenes. The preferred oxidants using these catalysts are various alkyl hydroperoxides, typically tert-butyl hydroperoxide (TBHP) or ethylbenzene hydroperoxide (EBH P). One of the routes for the industrial production of propylene oxide is based on a heterogeneous Ti ySi02 catalyst, which employs EBHP as the terminal oxidant [6]. 

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