Abstract
V-SBA-15 ( n Si/V ≈ 10) mesoporous molecular sieves have been synthesized in different pH-value medias and characterized by XRD, N 2 adsorption, TEM, UV–vis, Raman, FT-IR, ESR, and NH 3-absorbed in situ FT-IR techniques. Results indicated that the pH value played an important role on the states of vanadium species in the prepared materials. As the pH value increased, the amount of the crystalline vanadium oxides decreased, the incorporated vanadium species with isolated tetrahedral (V 5+) and square pyramidal coordination (VO 2+) increased accordingly. At pH = 3.0, vanadium species were mainly in the silica framework and formed stronger acid sites, it seems to suggest that the linear configuration V 5+–ONH 3H +–O–V 4+ groups have been formed after ammonia was adsorbed. However, for the samples synthesized at the low pH values, the crystalline vanadium oxides existed and the amount of the dispersed tetrahedral vanadium species decreased, which caused the weaker acid sites.
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