A series of mesoporous siliceous MCM-41 (SiMCM-41) molecular sieves with si
licon partially substituted by Al, Ti, and Zr were synthesized hydrothermal
ly. A combination of electron spin resonance (ESR), diffuse reflectance UV-
vis spectroscopy (UV-vis), Raman spectroscopy, and temperature-programmed r
eduction (TPR) was used successfully to characterize the chemical environme
nt of chromium in Cr/MeMCM-41 (Me = Al, Ti, and Zr). UV-vis, ESR, and Raman
spectroscopy show that Cr2O3 is present in SiMCM-41, ZrMCM-41, and AlMCM-4
1. With increasing Al content in AlMCM-41, the amount of Cr2O3 increases, w
hich contrasts with Cr/SiO2. Al2O3. Cr2O3 is reduced in Cr/TiMCM-41 with a
Si/Ti ratio of less than 25 compared to higher ratios. The results reveal a
strong interaction between chromium and surface titanium centers, which im
mobilizes the chromium species and reduces the formation of Cr2O3 UV-vis an
d Raman spectra show that Cr/MeMCM-41 materials are dominated by dichromate
. The overall intensity of Cr(V) increases in the order AlMCM-41 < SiMCM-41
< TiMCM-41 < ZrMCM-41. The intensity of Cr2O3 increases in the order of Ti
MCM-41 < ZrMCM-41 < SiMCM-41 < AlMCM-41. These results show that Ti and Zr
stabilize Cr(V) and Cr(VI) compared to Al and Si.