Physicochemical Characterization of Chromium Oxides Immobilized in Mesoporous MeMCM-41 (Me = Al, Ti, and Zr) Molecular Sieves

Abstract

A series of mesoporous siliceous MCM-41 (SiMCM-41) molecular sieves with silicon partially substituted by Al, Ti, and Zr were synthesized hydrothermally. A combination of electron spin resonance (ESR), diffuse reflectance UV−vis spectroscopy (UV−vis), Raman spectroscopy, and temperature-programmed reduction (TPR) was used successfully to characterize the chemical environment 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-41. With increasing Al content in AlMCM-41, the amount of Cr2O3 increases, which 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 immobilizes the chromium species and reduces the formation of Cr2O3. UV−vis and 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 TiMCM-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.