Zeolites with encapsulated transition metal species are extensively
applied in the chemical industry as heterogenous catalysts for favorable
kinetic pathways. To elucidate the energetic insights into formation of
subnano-sized molybdenum trioxide (MoO)
encapsulated/confined in zeolite Y (FAU) from constituent oxides, we
performed a systematic experimental thermodynamic study using high
temperature oxide melt solution calorimetry as the major tool.
Specifically, the formation enthalpy of each MoO/FAU is
less endothermic than corresponding zeolite Y, suggesting enhanced
thermodynamic stability. As Si/Al ratio increases, the enthalpies of
formation of MoO/FAU with identical loading (5 Mo-wt%)
tend to be less endothermic, ranging from 61.1 ± 1.8 (Si/Al = 2.9) to
32.8 ± 1.4 kJ/mol TO (Si/Al = 45.6). Coupled with
spectroscopic, structural and morphological characterizations, we
revealed intricate energetics of MoO – zeolite Y guest
– host interactions likely determined by the subtle redox and/or phase
evolutions of encapsulated MoO.