Heterogeneous process reductive activation

Metal hydrides and acyl-like CO insertion products are two types of species likely to be present in any homogeneous or heterogeneous process for the catalytic reduction of carbon monoxide. The discovery and understanding of new types of reactivity patterns between such species are of fundamental interest. As discussed elsewhere (11,22,54-57), bis(pentamethylcyclo-pentadienyl) actinide hydrides (58) are highly active catalysts for olefin hydrogenation as well as H-H and C-H activation. 

To approach truly living conditions in free radical polymerization, the extent of the termination reactions has to be reduced as much as possible. The natural way to establish such conditions is the reduction of the concentration of active chains, which, in turn, results in the decrease of polymer productivity. To contrast such decrease, the segregation of the radical chains, which is present in heterogeneous processes such as emulsion polymerization, can be exploited. This allows reducing terminations while preserving the overall concentration of radical chains. However, the appUcation of RAFT mediated polymerizations to emulsion systems has also been rather problematic, mainly with respect to the transport of RAFT agent through the aqueous phase. Attempts to carry out RAFT polymerizations in ab-initio emulsion processes... 

What was not known at the time when these first PHIP experiments with heterogeneous catalysts were performed was the fact that metal clusters and particles can produce PHIP effects as well. In fact, it was widely believed that the mechanism of catalytic hydrogenation on metal surfaces was incompatible with the requirement of the pairwise H2 addition to a substrate. Therefore, the possibility cannot be excluded that the immobilized Rh complexes used in the early studies were precatalysts rather than the actual catalysts, especially in some of the gas - solid hydrogenations. Extended catalyst activation periods may have resulted in the reduction of supported metal complexes and the production of nanoparticulate Rh catalysts. However, the main conclusion that PHIP effects can be produced in heterogeneous processes is still valid. 

Another example of heterogeneous catalysis by oxo-ions is the one-step oxidation of benzene to phenol with nitrous oxide, N2O. Fe/MFI catalysts have, again been found to be very active. This catalysis was discovered by Iwamoto and has been extensively studied by the group of G. Panov in Novosibirsk. " Preparations of Fe/MFI which appear highly active for this reaction display poor activity for NOj reduction and those which are optimum for that process, are poor for benzene oxidation. This shows that different sites are used. Work by Jia et al. revealed that the active sites for benzene oxidation appear to be Fe-oxo-ions containing only one Fe ion. This does not necessarily mean that the sites are mononuclear. A recent work by Zhu et al. has rather suggested that the site consists of one Fe and one Al + ion, the latter ion having left the zeolite framework. ... 

Industrial heterogeneous catalysts and laboratory-scale model catalysts are commonly prepared by first impregnating a support with simple transition metal complexes. Catalytically active metal nanoparticles (NPs) are subsequently prepared through a series of high temperature calcination and / or reduction steps. These methods are relatively inexpensive and can be readily applied to numerous metals and supports however, the NPs are prepared in-situ on the support via processes that are not necessarily well understood. These inherent problems with standard catalyst preparation techniques are considerable drawbacks to studying and understanding complex organic reaction mechanisms over supported catalysts. (4)... 

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