Comparative study of molybdenum oxide in NaY zeolite prepared by conventional impregnation and vapor-phase deposition techniques

The interaction between Mo (2–30 wt.% expressed as MoO3) and NaY zeolite lattice by applying conventional impregnation with aqueous (NH4)6Mo7O24 · 4H2O (AHM) and vapor-phase deposition of volatile complex Mo(CO)6 has been compared. The samples were characterized by FTIR spectroscopy, X-ray diffraction and in situ FTIR of pyridine adsorption. Their pore structure and surface were analyzed with nitrogen adsorption measurements at 77 K. The results showed that Mo species derived from AHM interact with framework aluminum of NaY, and the interaction overlaps so strong with increasing Mo loading above an equivalent monolayer. A new Al2(MoO4)3 phase was developed due to continued dealumination of zeolite, leading to intense collapse of the crystalline structure and drastically reducing its adsorption capacity. Alternatively, a decarbonylation process owing to dissociative adsorption of Mo(CO)6 into dehydrated zeolite in vacuo at 353 K with concurrent evolution of CO gas was occurred. Therefore, weak structural features of adsorbed CO32- (1453 cm−1) and HCO3- (656 cm−1) species were observed. It was suggested that a molybdenum monomer subcarbonyl species get built into the zeolite via coordination by two or three oxygen atoms of zeolite framework. Subsequent exposure to air produced a high-dispersed tetrahedral monomeric MoOx (895 cm−1) species in zeolite. The dispersed state of MoOx species measured the highest specific surface area and unit cell parameter. These results were utilized to derive relationships between solid structures and acid properties. The MoOx surface species in NaY served in new formed Brönsted acid sites. These sites have higher acid strength than the original ones in the zeolite and those in analogous samples prepared by impregnating zeolite with AHM.