Abstract
Chromium-incorporating mesoporous material denoted as Cr-TUD-1 was synthesized by using a surfactant-free one-pot synthesis procedure. Samples with different chromium loading were prepared and extensively characterized by means of X-ray diffraction (XRD), UV-vis and Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Si-29 MAS NMR, high-resolution transition electron microscopy (HR-TEM), nitrogen physisorption and elemental analysis. Characterization results show that as a function of Cr loading, Cr exists in different morphological structures such as isolated tetrahedrally coordinated Cr6+, nano-particulates, and finally bulky crystalline Cr2O3. The catalytic performance of the Cr-TUD-1 samples was investigated in the gas phase photo-catalytic oxidation of a mixture of light hydrocarbons i.e. methane, ethylene, ethane, propylene, and propane by molecular oxygen. Isolated Cr6+ species were found to be more active than Cr3+. The sample with 5 wt.% Cr loading which contains a mixture of Cr6+ and Cr3+ moieties was found to be the most active sample. A mechanism is proposed to show the synergy between Cr6+ and Cr3+. Cr-TUD-1 was found to be stable under dry conditions, the photocatalytic oxidation rates of the different hydrocarbons were almost the same up to six runs, however, the presence of water vapour in the reaction mixture causes serious deactivation to the catalyst. The activity of Cr-TUD-1 was superior to other metals-incorporated TUD-1 (e.g. Ti, V, and Mo) or other Cr-containing mesoporous materials (e.g. Cr-MCM-41 and Cr-SiO2) under the same reaction conditions. (C) 2015 Elsevier B.V. All rights reserved.