Niobium-based catalyst systems

Nb-based catalysts are among the investigated systems in total removal of -butanol due to the capacity of niobium to adopt variable oxidation states. Thus, Nb-doped hierarchically micro(meso) macroporous Ti02 (anatase phase) (Figure 17.10) synthesized via a self-formation procedure showed close performances with catalysts in which the same materials served as supports for noble metals [46]. The efficiency of these catalytic systems in the total oxidation of butan-l-ol was enhanced by the improved diffusion through the intrinsic macro-porous network. An effect of the niobium content was evidenced as welL... 

Hydroamination is an atom-economical process for the synthesis of industrially and pharmaceutically valuable amines. The hydroamination reaction has been studied intensively, including asymmetric reactions, and a variety of catalytic systems based on early and late transition metals as well as main-group metals have been developed." However, Group 5 metal-catalysed hydroaminations of alkenes had not been reported until Hultzsch s work in 2011. Hultzsch discovered that 3,3 -silylated binaphtho-late niobium complex 69 was an efficient catalyst for the enantioselective hydroaminoalkylation of iV-methyl amine derivatives 70 with simple alkenes 71, giving enantioselectivities up to 80% (Scheme 9.30). Enantiomerically pure (l )-binaphtholate niobium amido complex 69 was readily prepared at room temperature in 5 min via rapid amine elimination reactions between Nb(NMe2)5 and l,l-binaphthyl-2-ol possessing bullqr 3,3 -silyl substituents. Since the complex prepared in situ showed reactivity and selectivity identical... 

Overview A large number of catalysts based on vanadium [35-37], titanium [38 0], zirconium [41], hafnium [42], lanthanides (in particular neodymium, samarium, and ytterbium) [43], cobalt [44, 45], niobium [46], chromium [47], nickel [48], and palladium [49] provide well-defined polyolefins. Many of these systems are able to meet the requirements for living polymerizations by suppressing P-hydride and P-methyl eliminations, as well as chain transfer to cocatalysts, such as alkyl aluminums or methylaluminoxane (MAO). Since MAO is usually obtained as a liquid solution with residual trimethylalumi-num, drying MAO to a white powder and removing residual trimethylaluminum can help minimize chain transfer to cocatlysts. 

---