Binuclear transition metal ion centers in zeolites: Their preparation, characterization, and catalytic properties

Abstract

Transition metal ion (TMI)-exchanged zeolites have shown excellent activity and selectivity in the oxidation of methane (CH4) to methanol (CH3OH) at room temperature. Therefore, a detailed understanding of the structure-activity-stability relationship for their use in the reaction is required. In this thesis, newly developed (reported for the first time) TMI-based Fe(II)-, Co(II)-, Ni(II)-, and Mn(II)-ferrierite (FER) along with Fe(II)-mordenite (MOR) zeolite catalysts are presented for molecular oxygen (O2) and nitrous oxide (N2O) splitting for the selective oxidation of CH4 towards CH3OH. In addition, experimental and theoretical methods were applied to investigate the O2 and N2O activation over the presented metallozeolites. This research has confirmed the formation of highly reactive species called alpha oxygen (?-O) and their reactivity in CH4 oxidation. The theoretical study of O2 activation was also extended to Fe(II)-beta (*BEA) and Fe(II)-Linde Type A (LTA) zeolites. The first part of this thesis focuses on explaining the methodology, which was based on a combination of several techniques, such as in-situ Fourier-transform infrared spectroscopy (FTIR), X-ray absorption near-edge structure, Mössbauer, ultraviolet-visible spectroscopy, mass spectrometry, FTIR analysis in the gas phase, and density functional theory calculations used to study the formation, stabilization, evolution, and reactivity of ?-O from O2 cleavage over Fe-FER, and following treatment with CH4. In the study, special attention was dedicated to analyzing the stabilization, siting, and location of bare divalent cations of Fe, Co, Ni, and Mn in the extra-framework cationic positions in the FER matrix. In addition, the unique type of active (binuclear) centers was investigated in the TMI-based FERs, which is the crucial point for subsequent treatment with O2 or N2O and CH4. Also, the potential of zeolites (MOR, *BEA, and LTA) other than the FER matrix was evaluated in order to find an arrangement suitable for the formation of an optimal distance between two divalent metal (M(II)) ions (binuclear center formation), which could cooperate in O2 dissociation. Additionally, the aluminum (Al) distribution was studied in MOR zeolite. The following sections are devoted to the investigation of the potential for O2 activation and the following CH4 selective oxidation over Co(II), Ni(II), and Mn(II) in the FER matrix. The formation of ?-O, obtained by N2O decomposition and its activity toward CH4 to CH3OH oxidation, was studied over the binuclear centers of Fe(II), Co(II), and Ni(II)-FERs. Finally, the influence of zeolite topology on the catalytic performance in catalytic reduction (CRN2O) and selective catalytic reduction (SCRN2O) reactions over Fe(II)-FER and Fe(II)-MOR with similar Fe(II) loadings was investigated.