Characterization and catalytic activity of multivalent cation species occluded in high silica zeolites

Cation species occluded in the micropore space of zeolites offer unique catalytic properties. Specific examples are Ti-containing (hydrocarbon oxidation by peroxides), Fe-containing (direct oxidation of benzene to phenol by nitrous oxide), Zn- and Ga-containing (dehydrogenation and aromatization of alkanes) and Cu-, Fe-, and Co-containing zeolites (selective catalytic reduction of NOx). The study of such multivalent transition metal cations in high silica zeolite frameworks - where the aluminium occupied negatively charged oxygen tetrahedra are strongly spatially separated - is particularly interesting, since the mode of charge compensation remains elusive until now. We propose to address these issues for the multivalent ions Fe3+ and Ga3+ in narrow-pore ZSM5, mordenite and ferrierite zeolites. These materials offer opportunities for environmentally benign processes as selective oxidation of alkanes (Fe3+) and dehydrogenation of alkanes (Ga3+). The main target of the proposed project is the systematic investigation of the structural, chemical and catalytic properties of these multivalent cations in high silica zeolites. The key issues to be addressed are: (i) the siting of Fe3+ and Ga3+ in zeolites with a high Si/Al ratio in the framework, (ii) the compensation of the negative zeolite charge by these multivalent cationic species (mono- and binuclear oxo-ions) and (iii) insight into the chemical and catalytic properties of these ions in such an unusually low coordination. To this end, we will employ the strong mixed approach of experimental and computational studies as proven successful in earlier cooperations between the three groups: A. Since the structure of the cation species strongly depends on the lattice type, the Si/Al ratio, the method of cation introduction and further pretreatment, it is planned to carefully study various preparation methods. For characterization, we will employ a suite of characterization techniques (UV, fluorescence, ESR and DRIFT spectroscopy). Specifically we will use the following unique facilities available in the Russian groups: - DRIFT spectroscopy in combination with hydrogen and carbon monoxide molecular probes. The study of their perturbation allows detailed structural information as was already demonstrated for high silica zeolites modified with Zn cations (group Kazansky). - in situ ESR with paramagnetic molecular probes such as NO. Such probes allow detection of the catalytically active Fe2+ states which normally are not detected by ESR (group Zhidomirov). B. Quantum-chemical calculations give detailed information on the siting and structure of the cation species in order to differentiate between cations at cation positions, oxo-ions containing extra-lattice oxygen and channel- or cavity-stabilized neutral oxo clusters. First-principles studies are also valuable for quantitative interpretation of the spectral data. C. Catalytic studies (mainly carried out in Eindhoven) will include investigations of the partial oxidation of hydrocarbons by Fe zeolites and alkane dehydrogenation and aromatization on Ga zeolites. For the latter type of catalyst, isotopic exchange will provide insight into C-H activation by nanometric Ga-oxide particles.