Characterization of zeolitic intraframework molybdenum sites

• Molecular structures for molybdenum T-site environment in SOD zeolite and BEA zeolite are presented.
• Density functional theory calculation revealed the stability of Mo(III) and Mo(VI) T-site as a function of hydration rate.
• Interpretation of the corresponding IR spectra and acid base properties are performed.
• Where possible the theoretical results were compared with experimental ones.

An integrated experimental and computational study was carried out on molybdenum loaded BEA zeolite in order to scrutinize the most stable state of the molybdenum site at various oxidation and hydration states. After a two-step post-synthetic process, the incorporation of Mo ions into the vacant T-atom sites of the SiBEA framework zeolite as mononuclear MoVI centers has been evidenced by combined use of XRD, FTIR and diffuse reflectance UV–vis spectroscopy. The consumption of OH groups upon Mo loading has been monitored by FTIR and adsorbed pyridine is used as probe molecules for characterization of Brønsted and Lewis acidity in SiBEA and Mo3.0SiBEA. Periodic DFT calculations in sodalite (SOD) framework indicate a trigonal-bipyramidal conformer at the MoVI site with molybdenum being incorporated into the zeolite framework by four SiO– bonds and a MoO functionality under anhydrous conditions. Water splitting formally to O2− and 2H+ is computed to be feasible in SOD cage, whereas it is strongly favored in MoBEA framework at the T1 position. Depending on the hydration state, it is shown that the Mo sites formed can exhibit two MoO functionalities in a trigonal-bipyramidal and a pseudo-octahedral arrangement.

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