U.v.-vis-n.i.r. studies of Fe(II)-A zeolite

Transition-metal ions in zeolites: coordination and activation of oxygen

Zeolites containing transition-metal ions (TMIs) often show promising activity as heterogeneous catalysts in pollution abatement and selective oxidation reactions. In this paper, two aspects of research on the TMIs Cu, Co, and Fe in zeolites are discussed: (i) coordination to the lattice and (ii) activated oxygen species. At low loading, TMIs preferably occupy exchange sites in six-membered oxygen rings (6MR), where the TMIs preferentially coordinate with the O atoms of Al tetrahedra. High TMI loadings result in a variety of TMI species formed at the zeolite surface. Removal of the extralattice O atoms during high-temperature pretreatments can result in autoreduction. Oxidation of reduced TMI sites often results in the formation of highly reactive oxygen species. In Cu-ZSM-5, calcination with O(2) results in the formation of a species, which was found to be a crucial intermediate in both the direct decomposition of NO and N(2)O and the selective oxidation of methane into methanol. An activated oxygen species, called alpha-O, is formed in Fe-ZSM5 and reported to be the active site in the partial oxidation of methane and benzene into methanol and phenol, respectively. However, this reactive alpha-O can only be formed with N(2)O, not with O(2). O(2)-activated Co intermediates in faujasite (FAU) zeolites can selectively oxidize alpha-pinene and epoxidize styrene. In Co-FAU, Co(III) superoxo and peroxo complexes are suggested to be the active cores, whereas in Cu and Fe-ZSM-5, various monomeric and dimeric sites have been proposed, but no consensus has been obtained. Very recently, the active site in Cu-ZSM-5 was identified as a bent [Cu-O-Cu](2+) core (Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 18908-18913). Overall, O(2) activation depends on the interplay of structural factors such as the type of zeolite and sizes of the channels and cages and chemical factors such as the Si/Al ratio and the nature, charge, and distribution of the charge-balancing cations. The presence of several different TMI sites hinders the direct study of the spectroscopic features of the active site. Spectroscopic techniques capable of selectively probing these sites, even if they only constitute a minor fraction of the total amount of TMI sites, are thus required. Fundamental knowledge of the geometric and electronic structures of the reactive active site can help in the design of novel selective oxidation catalysts.

Study of the Influence of Water on Oxidative Properties of Fe3+ in ZSM-5 Zeolite Channels

The interactions of phenol and thiophene with Fe3+ ions have been studied in ZSM-5 zeolite channels in the presence of water. The Fe-ZSM-5 was prepared by the ion exchange of Fe3+ for Na+. The Fe3+ ions in zeolite channels have unsaturated co-ordination spheres and oxidize organic substrates (phenol, thiophene) at room temperature. The interaction of Fe3+ with phenol gives rise to the stabilized cation-radical PhOH•+ and thiophene forms low oligomers (2-6 monomeric units). The oligomers are present in the neutral as well as oxidized form as cation-radicals (polarons). The formation of dications (bipolarons) has not been observed.