Transition-Metal Ions in Zeolites: Coordination and Activation of Oxygen

Abstract

Zeolites containing transition metal ions (TMI) often show promising activity as heterogeneous catalysts in pollution abatement and selective oxidation reactions. In this paper, two aspects of research on the TMI Cu, Co and Fe in zeolites are discussed: (i) coordination to the lattice and (ii) activated oxygen species. At low loading, TMI preferably occupy exchange sites in six-membered oxygen rings (6MR) where the TMI preferentially coordinate with the oxygen atoms of Al tetrahedra. High TMI loadings result in a variety of TMI species formed at the zeolite surface. Removal of the extra-lattice oxygens during high temperature pretreatments can result in auto-reduction. 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 α-oxygen, 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 α-oxygen can only be formed with N 2 O, not with O 2 . O 2 activated Co intermediates in Faujasite (FAU) zeolites can selectively oxidize α-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. USA 2009, 106, 18908-18913). Overall, O 2 activation depends on the interplay of structural factors such as type of zeolite, size of the channels and cages and chemical factors such as 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 structure of the reactive active site can help in the design of novel selective oxidation catalysts.

Methane Activation by Heterogeneous Catalysis

“…A second interesting low temperature methane oxidation pathway is that on zeolite catalysts containing transition metal ions [92], in particular Cu, Fe and Co. The extraframework transition metal ions decompose NO and N 2 O but also activate O 2 as oxidant.…”

Low temperature oxidation of linseed oil: a review

“…Transition metal complexes fulfil important roles in activating the molecular oxygen through the coordination and partial reduction of oxygen (Bakac 2010). Smeets et al investigated the coordination and activation of oxygen employing transition metal ions in zeolites (Smeets et al 2010). The metal and oxygen oxidation states undergo changes during the activation process.…”

Influence of lattice stability on hydrothermal deactivation of Cu-ZSM-5 and Cu-IM-5 zeolites for selective catalytic reduction of NOx by NH3

“…Cu-ZSM-5 and especially on the identity and possibility of dimer formation, which has been and is 644 still being discussed extensively [30,17,55,60,61,62]. For this reason the first paragraph will be 645 devoted to the subject of speciation followed by discussion of the deactivation mechanism wherein 646 an attempt to disentangle the contributions from dealumination and copper migration will be made.…”