Exploring Cation Siting in Zeolites by Solid-State NMR of Quadrupolar Nuclei

Generation of basic centers in high-silica zeolites and their application in gas-phase upgrading of bio-oil

High-silica zeolites have been reported recently as efficient catalysts for liquid- and gas-phase condensation reactions because of the presence of a complementary source of basicity compared to Al-rich basic zeolites. Herein, we describe the controlled generation of these active sites on silica-rich FAU, BEA, and MFI zeolites. Through the application of a mild base treatment in aqueous Na2CO3, alkali-metal-coordinating defects are generated within the zeolite whereas the porous properties are fully preserved. The resulting catalysts were applied in the gas-phase condensation of propanal at 673 K as a model reaction for the catalytic upgrading of pyrolysis oil, for which an up to 20-fold increased activity compared to the unmodified zeolites was attained. The moderate basicity of these new sites leads to a coke resistance superior to traditional base catalysts such as CsX and MgO, and comparable activity and excellent selectivity is achieved for the condensation pathways. Through strategic acid and base treatments and the use of magic-angle spinning NMR spectroscopy, the nature of the active sites was investigated, which supports the theory of siloxy sites as basic centers. This contribution represents a key step in the understanding and design of high-silica base catalysts for the intermediate deoxygenation of crude bio-oil prior to the hydrotreating step for the production of second-generation biofuels.

Combination of the Metropolis Monte Carlo and Lattice Statics method for geometry optimization of H-(Al)-ZSM-5

In this article, the combination of the Metropolis Monte Carlo and Lattice Statics (MMC-LS) method is applied to perform the geometry optimization of crystalline aluminosilicate zeolite system in the presence of cationic species (H(+)), i.e., H-(Al)-ZSM-5. It has been proved that the MMC-LS method is very useful to allow H(+) ions in (Al)-ZSM-5 extra-framework to approach the global minimum energy sites. The crucial advantage of the combination MMC-LS method is that, in stead of simulating over thousands random configurations via the only LS method, the only one configuration is needed for the MMC-LS simulation to achieve the lowest energy configuration. Therefore, the calculation time can be substantially reduced via the performance of the MMC-LS method with respect to the only LS method. The calculated results obtained from the MMC-LS and the only LS methods have been comparatively represented in terms of the thermodynamic and structural properties.

Multinuclear Solid-State NMR Spectroscopy of Doped Lanthanum Fluoride Nanoparticles

Multinuclear solid-state NMR spectroscopy and powder X-ray diffraction (XRD) experiments are applied to comprehensively characterize a series of pure and lanthanide-doped LaF3 nanoparticles (NPs) that are capped with di-n-octadectyldithiophosphate ligands (Ln3+ = diamagnetic Y3+ and Sc3+ and paramagnetic Yb3+ ions), as well as correlated bulk microcrystalline materials (LaF3, YF3, and ScF3). Solid-state 139La and 19F NMR spectroscopy of bulk LaF3 and the LaF3 NPs reveal that the inorganic core of the NP retains the LaF3 structure at the molecular level; however, inhomogeneous broadening of the NMR powder patterns arises from distributions of 139La and 19F NMR interactions, confirming a gradual change in the La and F site environments from the NP core to the surface. 139La and 19F NMR experiments also indicate that low levels (5 and 10 mol %) of Ln3+ doping do not significantly change the LaF3 structure in the NP core. Similar doping levels of paramagnetic Yb3+ ions severely broaden 19F resonances, but only marginally effect 139La powder patterns, suggesting that the dopant ions are uniformly distributed throughout the NP core and occupy vacant La sites. Measurements of 139La T1 and T2 relaxation constants are seen to vary between the bulk material and NPs and between samples with diamagnetic and paramagnetic dopants. 45Sc NMR experiments confirm that the dopants are integrated into the La sites of the LaF3 core. Solid-state 1H and 31P magic-angle spinning (MAS) NMR spectra aid in probing the nature of the capping ligands and their interactions at the NP surface. 31P cross-polarization (CP)/MAS NMR experiments identify not only the dithiophosphate head groups but also thiophosphate and phosphate species which may form during NP synthesis. Finally, 19F-31P CP/MAS and 1H MAS experiments confirm that ligands are coordinated to the NP surface.

Solid-State 23Na and 7Li NMR Investigations of Sodium- and Lithium-Reduced Mesoporous Titanium Oxides

Mesoporous titanium oxide synthesized using a dodecylamine template was treated with 0.2, 0.6, and 1.0 equiv of Li- or Na-naphthalene. The composite materials were characterized by nitrogen adsorption, powder X-ray diffraction, X-ray photoelectron spectroscopy, elemental analysis, thermogravimetric analysis, and solid-state 23Na and 7Li NMR spectroscopy. In all cases the wormhole mesoporosity was retained as evidenced by BET surface areas from 400 to 700 m(2)/g, Horvath-Kawazoe pore sizes in the 20 Angstroms range, and a lack of hysteresis in the nitrogen adsorption isotherms. Variable-temperature conductivity studies show that the Li-reduced materials are semiconductors, with conductivity values 3 orders of magnitude higher than those of the Na-reduced materials. Electrochemical measurements demonstrate reversible intercalation/deintercalation of Li+ ions into pristine mesoporous Ti oxides with good cycling capacity. Solid-state 23Na NMR reveals two distinct Na environments: one corresponding to sodium ions in the mesoporous channels and the other corresponding to sodium ions intercalated into the metal framework. 23Na NMR spectra also indicate that the relative population of the framework site increases with increased reduction levels. Solid-state 7Li NMR spectra display a single broad resonance, which increases in breadth with increased reduction levels, though individual resonances inferring the presence of channel and framework Li species are not resolved. Comparisons of the lithium chemical shifts with published values suggests an "anatase-like structure" with no long-range order in the least-reduced samples but a "lithium titanate-like structure" with no long-range order in the higher reduced materials.

Anion-Promoted Cation Motion and Conduction in Zeolites

The motion of sodium cations in sodalite and cancrinite has been investigated by force field calculations, solid-state NMR, and impedance spectroscopy. Special emphasis is dedicated to the influence of anions on sodium mobilities. Local cation motion is promoted when they interact with anions. However, not all systems with high local mobilities exhibit good ion conductivities, as cooperativity of the motion appears to be an important factor, as well. The activation barrier for local sodium motion (calculations) and long-range transport (dc conductivities) is lowered in sodalite when halogenide anions, Cl(-), Br(-), or I(-), are present. The activation barriers increase with increasing size of the anion and decreasing coordination in the transition state. On the basis of (23)Na solid-state NMR data, all the sodium ions in the dense sodalite structure are rather rigid up to 470 K. All the cations in chromate sodalite, and Na(+) in the small cancrinite epsilon-cages without anion interactions, show a restricted local motion at higher temperatures. There is a selective high local motion of Na(+) in the neighborhood of chromate anions in the more open channel system of cancrinite. These results suggest that sodium migration can be enhanced, at least locally, in open channel systems by anion interactions. A dynamics coupling between anion reorientation and cation mobility was not observed.

23Na solid state MAS NMR of sodium halides occluded in zeolites

Occlusion of sodium chloride and sodium bromide in zeolitic pores was performed by heating mixtures of the salts with zeolites NaY and NaA under high vacuum conditions. The obtained samples were subjected to various further pretreatments like washing with water and zinc-exchange, and were investigated spectroscopically with the 23Na MAS NMR technique at various Zeeman field strengths. In the case of NaY, the halides are occluded in both types of cages of the faujasite structure. About 90% of the sodalite cages are shown to have incorporated salt which is concluded to be part of [Na4Hal]3+ clusters as in the case of sodalite type materials.