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, N2 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 δ(Mo–O) and δ(Mo–O–Mo), respectively.
The disappearance of the 236 cm−1 band as well as that at
344 cm−1 in favor of the 319cm−1 band, with the dealumination, was
related to the high dispersion of Mo species in the produced mesopore
surface assessed by theN2 adsorption at 77 K. No bands due to
bulk MoO3 were detected from the IR andXRDresults.Astrong 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 Mo6+, along with some tetrahedral Mo6+. New
spectral features as a consequence of dealumination events in the
far-IR range were evaluated and discussed. C |
