Characterization and catalytic performance of [Cr]MCM-41 and [Cr]MCM-48 prepared by either classical or microwave heating

The chromium-substituted mesoporous materials [Cr]MCM-41 and [Cr]MCM-48 were prepared by microwave-assisted and conventional hydrothermal synthesis. A systematic comparison of the materials obtained by the two different procedures was carried out based on their characterization by XRD, chemical analysis (ICP-AES), DR–UV–Vis spectroscopy, and nitrogen adsorption. The microwave-assisted syntheses of [Cr]MCM-41 and [Cr]MCM-48 were completed within 1–2 h, whereas the conventional hydrothermal method took at least 12–24 h for completion. The yields and quality of the chromium-substituted mesoporous materials obtained from hydrothermal and microwave-assisted synthesis are comparable. The stability of chromium substitution in the synthesized materials was studied during the liquid-phase oxidation of cyclohexanol or cyclohexane using molecular oxygen as a “green” oxidant. In the cyclohexanol oxidation, the leaching of catalytically active chromium species was stronger for the mesoporous materials than for the microporous aluminophosphate [Cr]APO-5, but less than for bulk K2Cr2O7. In the oxidation of cyclohexane, the conversion over [Cr]APO-5 was higher than over the ordered mesoporous catalysts.