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.

Catalytic Aerobic Oxidation of Cyclohexane and Ethyl Benzene Over Chromium-Containing Mesoporous Organosilicas

“…to Cr 3? [22,23]. This result supports the observations about the presence of Cr(VI) species in the calcined chromium-silica samples.…”

“…H 2 -TPR profiles of the calcined Cr-PMO samples and Cr-MCM-41 sample in the 100-800°C temperature range show that the reduction temperature maxima (T max ) of the catalyst vary with respect to the nature of catalyst preparation methods and are tabulated in Table 1. Kevan et al pointed out that the reduction temperature value is dependent on the reduction conditions like H 2 partial pressure, heating rate and final reduction temperature [22]. The peak observed in the range 420-460°C is attributed to the reduction of Cr 6?…”

Structure Characterization and Catalytic Properties of Cr2O3Doped with MgO Supported on MgF2

“…This signal is composed of two narrow isotropic lines with g 0 = 1.983 and g 0 = 1.971. In agreement with previous paper [11,12] [11]. The signal due to Cr 5+ disappeared for the sample with the higher Cr content (8%) and evacuated at room temperature and at 250…”

Vanadium-containing ordered mesoporous silicas: Synthesis, characterization and catalytic activity in the hydroxylation of biphenyl

“…Furthermore, the average oxidation state values calculated for V-MCM-41 and V/MCM-41 are at $4.5 and at 3.5, respectively, showing that the reduction of V (V) is more on the grafted catalyst than on the vanadiumincorporated mesoporous catalysts. However, as pointed out by Zhu et al [35], the reduction temperature and the average oxidation state values depend on the reduction conditions, like the H 2 partial pressure, the heating rate and the final reduction temperature. Hence, a comparison of the reduction values with those in literature reports is inappropriate, as such values show variations in the experimental set-up procedures.…”