Local structures of polar wurtzites Zn_{1-x}Mg_{x}O studied by Raman and {67}Zn/{25}Mg NMR spectroscopies and by total neutron scattering

Local compositions and structures of Zn_{1-x}Mg_{x}O alloys have been investigated by Raman and solid-state {67}Zn/{25}Mg nuclear magnetic resonance (NMR) spectroscopies, and by neutron pair-distribution-function (PDF) analyses. The E2(low) and E2(high) Raman modes of Zn_{1-x}Mg_{x}O display Gaussian- and Lorentzian-type profiles, respectively. At higher Mg substitutions, both modes become broader, while their peak positions shift in opposite directions. The evolution of Raman spectra from Zn_{1-x}Mg_{x}O solid solutions are discussed in terms of lattice deformation associated with the distinct coordination preferences of Zn and Mg. Solid-state magic-angle-spinning (MAS) NMR studies suggest that the local electronic environments of {67}Zn in ZnO are only weakly modified by the 15% substitution of Mg for Zn. {25}Mg MAS spectra of Zn_{0.85}Mg_{0.15}O show an unusual upfield shift, demonstrating the prominent shielding ability of Zn in the nearby oxidic coordination sphere. Neutron PDF analyses of Zn_{0.875}Mg_{0.125}O using a 2x2x1 supercell corresponding to Zn_{7}MgO_{8} suggest that the mean local geometry of MgO_{4} fragments concurs with previous density functional theory (DFT)-based structural relaxations of hexagonal wurtzite MgO. MgO_{4} tetrahedra are markedly compressed along their c-axes and are smaller in volume than ZnO_{4} units by ~6%. Mg atoms in Zn_{1-x}Mg_{x}O have a shorter bond to the $c$-axial oxygen atom than to the three lateral oxygen atoms, which is distinct from the coordination of Zn. The precise structure, both local and average, of Zn_{0.875}Mg_{0.125}O obtained from time-of-flight total neutron scattering supports the view that Mg-substitution in ZnO results in increased total spontaneous polarization.Comment: 12 pages, 14 figures, 2 table

Effect of Heteroatom Concentration in SSZ-13 on the Methanol-to-Olefins Reaction

SSZ-13 materials have been synthesized with varying amounts of Al to produce samples with different concentrations of Brønsted acid sites, and consequently, these SSZ-13 materials contain increasing numbers of paired Al heteroatoms with increasing Al content. These materials were then characterized and tested as catalysts for the methanol-to-olefins (MTO) reaction at 400 °C and 100% methanol conversion under atmospheric pressure. A SAPO-34 sample was also synthesized and tested for comparison. SSZ-13 materials exhibited significant differences in MTO reactivity as Si/Al ratios varied. Reduced Al content (higher Si/Al ratio) and, consequently, fewer paired Al sites led to more stable light olefin selectivities, with a reduced initial transient period, lower initial propane selectivities, and longer catalyst lifetime. To further support the importance of paired Al sites in the formation of propane during this initial transient period, a series of experiments was conducted wherein an H-SSZ-13 sample was exchanged with Cu²⁺, steamed, and then back-exchanged to the H form. The H-SSZ-13 sample exhibited high initial propane selectivity, while the steamed H-SSZ-13, the Cu²⁺-exchanged SSZ-13 sample, and the steamed Cu-SSZ-13 sample did not, as expected since steaming selectively removes paired Al sites and Cu²⁺ exchanges onto these sites. However, when it was back-exchanged to the proton form, the steamed Cu-SSZ-13 sample still exhibited the high initial alkane selectivity and transient period typical of the higher Al content materials. This is attributed to protection of paired Al sites during steaming via the Cu²⁺ cation. Post-reaction coke analyses reveal that the degree of methylation for each aromatic species increases with increasing Si/Al in SSZ-13. Further, SAPO-34 produces more polycyclic species than SSZ-13 samples. From these data, the paired Al site content appears to be correlated with both MTO reaction behavior and coke species formation in SSZ-13 samples

Structure-Directing Roles and Interactions of Fluoride and Organocations with Siliceous Zeolite Frameworks

Interactions of fluoride anions and organocations with crystalline silicate frameworks are shown to depend subtly on the architectures of the organic species, which significantly influence the crystalline structures that result. One- and two-dimensional (2D) ¹H, ¹⁹F, and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy measurements establish distinct intermolecular interactions among F^– anions, imidazolium structure-directing agents (SDA⁺), and crystalline silicate frameworks for as-synthesized siliceous zeolites ITW and MTT. Different types and positions of hydrophobic alkyl ligands on the imidazolium SDA⁺ species under otherwise identical zeolite synthesis compositions and conditions lead to significantly different interactions between the F^– and SDA⁺ ions and the respective silicate frameworks. For as-synthesized zeolite ITW, F^– anions are established to reside in the double-four-ring (D4R) cages and interact strongly and selectively with D4R silicate framework sites, as manifested by their strong ¹⁹F–²⁹Si dipolar couplings. By comparison, for as-synthesized zeolite MTT, F^– anions reside within the 10-ring channels and interact relatively weakly with the silicate framework as ion pairs with the SDA⁺ ions. Such differences manifest the importance of interactions between the imidazolium and F^– ions, which account for their structure-directing influences on the topologies of the resulting silicate frameworks. Furthermore, 2D ²⁹Si{²⁹Si} double-quantum NMR measurements establish ²⁹Si–O–²⁹Si site connectivities within the as-synthesized zeolites ITW and MTT that, in conjunction with synchrotron X-ray diffraction analyses, establish insights on complicated order and disorder within their framework structures

Long- and Short-Range Constraints for the Structure Determination of Layered Silicates with Stacking Disorder

Layered silicates have important applications as host materials, supports for catalysis, and zeolite precursors. However, their local structures are often challenging to establish due to disorder of the sheet assemblies. We present a new protocol that combines long- and short-range structural constraints from diffraction and solid-state NMR techniques, respectively, to determine the molecular structure of layered silicates in the presence of various extents of stacking disorder. Solid-state ²⁹Si NMR data are largely insensitive to the incomplete extent of three-dimensional (3D) crystallinity that limits the interpretation of diffraction data alone to the identification of possible unit cells and space groups. State-of-the-art NMR crystallography techniques consequently provide a simplified view of materials from which candidate framework structures can be built and evaluated based on local structural constraints, including interatomic distances, Si site numbers and multiplicities, and Si–O–Si connectivities, and refined using density functional theory. This protocol was applied to a new layered silicate material named CLS-1, of composition [Si₅O₁₁H]­[C₉N₂H₁₅]·1.9­(H₂O), synthesized by using a fluoride-based protocol and cationic alkylaminopyridinium as a structure-directing agent (SDA). Despite the intrinsic complexity and partial ordering of the intersheet arrangements and organic–inorganic interactions, this led to the identification of a single space group that is compatible with both NMR and diffraction data, from which the silicate framework structure could be established,. The remarkable similarities between the layered framework structures of CLS-1, HUS-2 (Tsunoji et al. <i>J. Mater. Chem.</i> <b>2012</b>, <i>22</i>, 13682), and another layered silicate material with a radically different morphology and extent of stacking order and interlayer dynamics, established by using a similar approach (Brouwer et al. <i>J. Am. Chem. Soc.</i> <b>2013</b>, <i>135</i>, 5641), point to the remarkable robustness of this previously unknown silicate framework type