Effect of mordenite dealumination on the structure of encapsulated molybdenum catalysts.

A series of dealuminated mordenites treated under various conditions of acid leaching was impregnated in an aqueous solution of ammonium heptamolybdate to achieve a loading of 12 wt% Mo. These samples were characterized by XRD, UV-DRS, N(2) adsorption, TGA, and FTIR techniques. Special attention was given to the far-IR measurements and IR study of surface hydroxyl groups before and after dealumination. A polymolybdate species was recognized by the appearance of bands at 344, 319, and 236 (229) cm(-1) due to the vibrational modes of delta(Mo-O) and delta(Mo-O-Mo), respectively. The disappearance of the 236 cm(-1) band as well as that at 344 cm(-1) in favor of the 319 cm(-1) band, with the dealumination, was related to the high dispersion of Mo species in the produced mesopore surface assessed by the N(2) adsorption at 77 K. No bands due to bulk MoO(3) were detected from the IR and XRD results. A strong interaction between Mo species and dealuminated mordenite surfaces (OH groups) was recognized by a decrease in intensity and a marked shift of the band at 3745 to 3727 cm(-1) as well as the appearance of a new band at 3668 cm(-1). The latter band was produced by the interaction of the framework Al-OH with Mo species. The BET surface areas of Mo-dealuminated mordenite samples were higher than the corresponding Mo-free ones. The diffuse reflectance measurements suggested that Mo cations are predominantly present as an octahedrally coordinated Mo(6+), along with some tetrahedral Mo(6+). New spectral features as a consequence of dealumination events in the far-IR range were evaluated and discussed.