Hydrothermal synthesis and characterization of uranium-containing MCM-48 samples

Abstract

MCM-48 samples containing uranium (U-MCM-48) to the extent of 1 to 6 wt % were synthesized hydrothermally and characterized using powder XRD, ²⁹Si MAS NMR, FTIR spectroscopy, DR UV-Vis spectroscopy, N₂ sorption, and TEM techniques. These analyses confirmed the formation of an ordered cubic-phase MCM-48 mesomaterial where the uranium existed in the hexavalent state forming a local uranate type structure. A part of the uranyl groups were found to be incorporated in the framework positions, while the rest were anchored on to the wall of the host silica matrix. Also, the U-MCM-48 samples exhibited an enlargement of the cubic cell unit as well as an increase in the degree of cross-linking, as revealed by XRD and MAS NMR results. The samples exhibited N₂ sorption isotherms of type IV and an increase was observed in the pore diameter as a result of uranium incorporation. TEM investigation revealed that although U-MCM-48 samples were of cubic symmetry similar to that of Si-MCM-48, some distortion in the pore structure was clearly noticeable. Furthermore, in case of the samples with U loading of 6 wt %, formation of nanocrystallites of α -U₃O₈ was also detected after calcination and such samples consisted of a multiple-size pore system.