Single site spectroscopy of transition metal ions and reactive oxygen complexes in zeolites

Transition metal ions (TMIs) are commonly introduced in zeolites in the form of, or as part of, cations that form a coordination complex with the zeolite framework. Their introduction enables the use of zeolites as redox catalysts. Of particular interest are TMI containing reactive oxygen complexes that may be used for mild selective partial oxidations. Various spectroscopic techniques are suited to study TMIs, but for TMIs in zeolites the interpretation of the spectroscopic data is particularly complicated because of their heterogeneous occurrence. The TMIs may simultaneously occur in different oxidation states (Cu 2+ , Cu + , Fe 2+ , Fe 3+ , …). They may agglomerate into di- and trinuclear clusters, or nanoparticles of the type [M n x+ O y ] (xn − 2y) . Moreover various extraframework ligands (NO, O, OH, …) can be part of the first coordination sphere in various binding modes besides the zeolite framework. Disentangling spectroscopic data obtained on heterogeneous TMI zeolites requires spectroscopic features that can unambiguously be linked to specific TMI species in the zeolite, or ‘site selective’ spectroscopic data. In addition, individual spectroscopic techniques rarely allow access to sufficient chemical properties to achieve a full model of the TMI species and a combination of techniques is required. In this article the background on zeolite-TMI interactions is given in combination with the so far successfully determined TMI sites in zeolites to demonstrate how site selective spectroscopy is achieved. Reactive surface oxygen sites that are part of, or derived from, these sites are given extra attention. The treatment of spectroscopically identified reactive oxygen is expanded with examples on non-transition metal ions (such as Ga and Zn) in zeolites. These examples demonstrate the use of site selective spectroscopy and spectroscopic handles to study reactions site selectively in the last section.