Encapsulation, characterization and catalytic properties of uranyl ions in mesoporous molecular sieves

doi:10.1016/S1381-1169(01)00350-8

Journal of Molecular Catalysis A: Chemical

Volume 181, Issues 1–2, 25 March 2002, Pages 91-97

https://doi.org/10.1016/S1381-1169(01)00350-8 Get rights and content

Abstract

Occlusion of uranyl ions (UO₂²⁺) in the pore channels of mesoporous MCM-41 and MCM-48 molecular sieves was accomplished using direct template ion-exchange method, and the samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), diffuse reflectance ultraviolet–visible spectroscopy (DRUV–VIS), and fluorescence spectroscopy. A shift in UO stretching IR band (Δν=−34 cm⁻¹), and the appearance of broad and diffused bands in the fluorescence spectra (480–620 nm) of the UO₂²⁺-exchanged samples indicate a definite electronic interaction of UO₂²⁺ species with the silicate (SiO⁻) surface. This inference is corroborated by DRUV–VIS results. Calcination in air/N₂ at 823 K resulted in the formation of well-dispersed α-U₃O₈/α-U₃O₇ moieties, accompanied by a marginal decrease in the concentration of UO₂²⁺ groups. The binding of UO₂²⁺ species to mesoporous materials framework remained intact even after calcination. The molecular sieves loaded with uranium oxide species showed appreciable activity, both for the oxidation of CO and for adsorption/decomposition of CH₃OH.

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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Encapsulation, characterization and catalytic properties of uranyl ions in mesoporous molecular sieves

Abstract

Introduction

Section snippets

Synthesis of MCM-41 and MCM-48

XRD studies

Conclusions

Acknowledgements

Prog. Solid State Chem.

J. Catal.

Catal. Today

Micropor. Mater.

Appl. Catal.

Adv. Mater.

Chem. Mater.

Chem. Rev.

J. Chem. Soc., Faraday Trans.

Adv. Mater.