Encapsulation, characterization and catalytic properties of uranyl ions in mesoporous molecular sieves
Introduction
Mesoporous molecular sieves have received wide attention because of their diversified applications as shape selective catalysts, adsorbents, ion-exchangers and also in removal of heavy metal ions, radionuclides and organics from effluents [1], [2], [3]. The well-defined (tunable) pore sizes and the large pore openings of these molecular sieves render them unique host materials for occlusion/anchoring of large molecules or that of reactive metal complexes in their channels. The incorporation of specific functional groups onto the walls of the channels in these molecular sieves thus serves as an important step in heterogenizing the homogeneous catalyst systems. In consideration that uranium (i.e. uranyl ions or uranium oxides) may serve as promising oxidizing catalyst owing to its variable valence states vis-à-vis vacant f-orbitals [4], [5], we have attempted the encapsulation of uranyl species (UO22+) in the mesopores of hexagonal MCM-41 and cubic MCM-48 molecular sieves. The emphasis was to achieve a well-dispersed catalyst system for oxidation reactions. Earlier studies in this direction concern dispersion of uranium oxides over dense oxide supports such as Al2O3, TiO2, SiO2, MgO, etc. [6].
In the present investigation, we adopted a direct template ion-exchange method [7] for the entrapment of uranyl species (UO22+) in the mesopores of MCM-41 and MCM-48 silicates. The samples in the as-synthesized, as-exchanged and corresponding calcined forms were characterized by various techniques such as X-ray diffraction (XRD), induced coupled plasma–atomic emission spectroscopy (ICP–AES), Fourier transform infrared spectroscopy (FT-IR), diffuse reflectance ultraviolet–visible spectroscopy (DRUV–VIS), and fluorescence spectroscopy. The catalytic performance of these materials was evaluated for model reactions, viz. oxidation of CO and adsorption/decomposition of CH3OH over a temperature range 373–773 K.
Section snippets
Synthesis of MCM-41 and MCM-48
The mesoporous MCM-41 and MCM-48 silicates were synthesized hydrothermally as per the procedure described elsewhere [8], [9]. The typical gel (molar) composition was 10SiO2:1.35(CTA)2O: 0.75(TMA)2O:1.3Na2O:680H2O for MCM-41, and 10SiO2:3(CTA)2O:2.5Na2O:600H2O for MCM-48. The gels were crystallized in Teflon-lined stainless steel autoclaves at 373 K for 1 and 3 days for MCM-41 and MCM-48, respectively. The solid products obtained were washed with distilled water several times, filtered, and dried
XRD studies
XRD patterns of both as-synthesized and UO22+-exchanged MCM-41 showed typical reflections 100, 110, 200, and 210 in the range 2–5°, characteristic of MCM-41 [8]. Similarly, the XRD pattern of as-synthesized MCM-48 and corresponding UO22+-exchanged sample showed two major reflections at 211 and 220, in addition to six minor reflections 321, 400, 420, 332, 422, and 431 in the range 3.5–5°, typical of MCM-48 [8]. However, a decrease in the intensity of the reflections was observed in case of UO22+
Conclusions
In the present investigation, the successful entrapment of UO22+-ions within the mesopores of MCM-41 and MCM-48 is demonstrated. MCM-48 was found to be trapping higher amount of UO22+-ions than MCM-41. Under the experimental conditions, it was observed that both MCM-41 and MCM-48 suffer a partial loss in crystallinity, even though the structure does not collapse. Both FT-IR and fluorescence studies indicated the binding of UO22+-ions onto the silicate surface, possibly via an interaction
Acknowledgements
Authors thank Mr. S. Varma, Mr. B. Dhavekar, Mrs. M. Anita and Mrs. M.R. Pai for their help in recording FT-IR and fluorescence spectra. This work is supported by the Board of Research in Nuclear Sciences, Department of Atomic Energy, Mumbai under a Contract No. 98/37/31/BRNS/1049
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