More insight on the isothermal spreading of solid MoO3 into ZSM-5 zeolite

Abstract

Pure MoO3 undergoes dehydration under helium stream between room temperature and 220 °C before being transformed into MoO2.87 and lost by evaporation at 650 °C. However, in the presence of NH4+–ZSM-5 zeolite (Mo/Al = 1), the dehydration of the mixture and the deammonization of the zeolite take place below 440 °C. In this water–rich atmosphere, a fraction of MoO3 is rehydrated and transformed into gaseous MoO2(OH)2 at 220–440 °C instead of being reduced. Despite the significant Mo weight loss, MoO2(OH)2 (g) reacts with ZSM-5 zeolite and is transformed into polymolybdate species upon 4 h of isothermal exchange at 500 °C. However, when the reaction time is prolonged from 4 to 8 h, the residual MoO3 undergoes an agglomeration before being transformed into mono- and dimolybdate species following 12 h of exchange. The determination of band gap energy by means of the diffuse reflectance spectroscopy allowed the identification of Mo species and the quantification of residual MoO3 (55% of the total Mo), which occupied the zeolite surface (46% of residual MoO3) and the internal cavities (9% of residual MoO3).