Solid–state ion exchange of molybdenum (VI) acetylacetonate into ZSM-5 zeolite

Abstract

The thermal decomposition of Mo(VI) acetylacetonate salt (Mo Acac) has been performed under helium stream between room temperature and 700°C (2Kmin−1) and monitored by thermogravimetric analysis coupled to mass spectrometry. The viscous MoO2(C5H7O2)2 salt evolves two acetylacetonato ligands between room temperature and 325°C which decompose into methane and acetic acid. Molybdenum dioxide MoO2(s) is the final product of decomposition as confirmed by the thermal behavior of pure MoO3(s) salt and thermodynamic calculations. In the presence of NH4+–ZSM-5 zeolite (6wt.% of Mo), the departure of the two ligands was accelerated and the salt transformed into MoO2(s) at 285°C. However, the oxidation of CH4 by molecular oxygen was inhibited at 245°C by the simultaneous formation of MoO3(s) and MoO2(OH)2(g). The solid–state exchange of the volatile MoO2(OH)2 specie and the protonated zeolite occurs between 320 and 570°C. Nevertheless, the exchange process overlaps with the partial dehydroxylation of the zeolite at 520°C.