Probing Water Dynamics in Octahedral Molecular Sieves: High Speed 1 H MAS NMR Investigations
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Probing Water Dynamics in Octahedral Molecular Sieves: High Speed 1H MAS NMR Investigations Todd M. Alam1, Jason Pless2, and Tina M. Nenoff3 1 Nanostructured and Electronic Materials, Sandia National Laboratories, Albuquerque, NM, 87185-0886 2 Modelling and Simulation, Sandia National Laboratories, Albuquerque, NM, 87185 3 Surface and Interface Sciences, Sandia National Laboratories, Albuquerque, NM, 87185-1415
ABSTRACT The water dynamics in a series of Sandia octahedral molecular sieves (SOMS) were investigated using high speed 1H magic angle spinning (MAS) NMR spectroscopy. For these materials both the 20% Ti-substituted material, Na2Nb1.6Ti0.4(OH)0.4O5.6•H2O and the 0% exchanged end member, Na2Nb2O6•H2O were studied. By combining direct one dimensional (1D) MAS NMR experiments with double quantum (DQ) filtered MAS NMR experiments different water environments within the materials were identified based on differences in mobility. Two dimensional (2D) DQ correlation experiments were used to extract the DQ spinning sideband patterns allowing the residual 1H-1H homonuclear dipolar coupling to be measured. From these DQ experiments the effective order parameters for the different water environments were calculated. The water environments in the two different SOMS compositions investigated revealed very large differences in the water mobility. INTRODUCTION The Sandia octahedral molecular sieves (SOMS) are materials with the general formula Na2Nb2-xMIVx(OH)xO6-x•H2O (MIV = Ti, Zr) [1, 2]. This class of materials has proven to be extremely efficient ion exchange materials. For example, the 20% Ti exchanged SOMS, Na2Nb1.6Ti0.4(OH)0.4O5.6•H2O exhibits selectivity for divalent cations or monovalent cations, with distribution coefficients (Kd) > 99,800 for Sr2+ and Ba2+ [1]. The impact of heterovalent substitution on the final material properties has been studied, and has shown that increasing Ti stabilizes the resulting SOMS framework, while increasing levels of Zr destabilizes the SOMS framework. In addition, it has been reported that the degree of MIV substitution within the framework changes the divalent cation selectivity within these materials [2]. The selectivity and the stability of these SOMS have been partially related to the presence of framework hydroxyl species and the possibility of hydrogen bonding within the pore structure. In this paper, the role of water and hydrogen bonding within the pore structure of the SOMS will be further explored; specifically to determine if the dynamics of the waters within the pores are related to the observed selectivity. Water dynamics in materials are commonly investigated using 2H NMR [3-10] which requires the isotopic synthesis or exchange to incorporate D2O within the material. This process can be time consuming and in some instances difficult. To bypass the need for this 2H labeling, we report 1H MAS NMR methods that allow details about the water environments to be determined at natural abundance.
THEORETICAL AND EXPERIMENTAL DETAILS Synthesis The synthetic
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