Multinuclear Solid-state NMR Techniques Research Articles

Hydronium Ions on the Surface of a Partially Hydrolyzed α-Zirconium Phosphate: Solid-State 2H and 31P MAS NMR Evidence.

Characterization of the α-zirconium phosphates 1-D2Oh and 1-D2Oh/dr, partially hydrolyzed with D2O, by powder X-ray diffraction, scanning electron microscopy, and multinuclear solid-state NMR techniques led to an unprecedented observation of D3O+ ions located on the surface and stabilized by hydrogen bonds. These ions are formed after the surface phosphate groups have been lost.

Solid State NMR Techniques Study the Structural Characteristics of As-Synthesized ITQ-33

Multinuclear solid state NMR techniques have been applied to study the structural characteristics of extra-large-pore zeolite ITQ-33. Through analysis of 2D 29Si{19F} HETCOR NMR spectra, the configurations of Ge-D4R units in ITQ-33 can be confirmed to have the most separation between Si and Ge atoms. Because F anions are not in the center of D4R units for shorter Ge–F bond lengths and Ge-D4R units with Ge–F bonds in the crystals are related by mirror symmetries, 29Si NMR signals of D4R units are magnetically inequivalent, therefore two 29Si peaks are observed in 19F–29Si CP/MAS and 19F–29Si HETCOR spectra. The formation of specific D4R configurations proves that Ge atoms and F– anions play important structural directing roles in the formation process of zeolite ITQ-33. 27Al 5QMAS experimental results confirm that a major amount of Al atoms is incorporated into the 3-ring in the framework. 1D 13C–27Al S-RESPDOR experimental results show that Al atoms in the framework are spatially close to the methyl group...

Multinuclear Solid-State NMR Studies on the Formation Mechanism of Aluminophosphate Molecular Sieves in Ionic Liquids

In the present work, multinuclear solid-state NMR techniques together with power X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to monitor the crystallization process of aluminophosphate AlPO4-11 molecular sieves in 1-ethyl-3-methylimidazolium bromide ([emim]Br) ionic liquids (ILs). The local environments of the selected solid samples were probed by one-dimensional 27Al, 31P, and 19F MAS NMR experiments, and more information was obtained from two-dimensional 27Al→31P heteronuclear correlation (HETCOR) and 27Al triple-quantum MAS (3Q MAS) experiments. It is found that a large amount of amorphous aluminophosphates was formed with the F–Aloct–O–Ppar and Altet–O–Ppar structures in the initial stage of aging. With increasing crystallization time, the partially condensed framework phosphorus species disappeared gradually and became fully condensed. Meanwhile, the octahedral Al was transformed into the pentahedral and tetrahedral Al species with the structures of F–Alpenta–O–Pful and Altet–O–Pful in the AlPO4-11 frameworks. During the crystallization process, [emim] cations acting as the structure-directing agents were occluded into the channels, and F– anions were connected with pentahedral Al in the final AlPO4–11 phase. All of these findings can allow proposing a possible formation mechanism for the synthesis of AlPO4-11 in ionic liquids.

A Novel Phase Transformation Phenomenon in Mesostructured Aluminophosphate

A novel phase transformation phenomenon that involves two successive phase transformation events was found for the first time in the synthesis of mesostructured aluminophosphate and studied by XRD, TEM, and multinuclear solid-state NMR techniques. The results showed that a hexagonal phase Hex and two lamellar phases, L1 and L2, were formed after hydrothermal treatment for 1, 3, and 50 h, respectively. The status of the surfactant was found to be arrayed interdigitated in a bilayer with a tilt angle in L1 phase but upright in L2 phase. A mechanism that the exciting of the alkane tail of the surfactant together with the condensation of aluminophosphate cooperatively promoted the phase transformation was proposed for the observed phenomenon. Additionally, a ZON microporous structure was found for the first time existing in the framework of the mesostructured aluminophosphate.

Crystal Engineering of Pharmaceutical Co-crystals: Application of Methyl Paraben as Molecular Hook

Applicability of the O-H...N heterosynthon for synthesis of a pharmaceutical co-crystal comprised of a commonly used tablet excipient methyl paraben and quinidine, an anti-malarial constituent of Cinchona tree bark, has been successfully demonstrated. Insights into local conformation and hydrogen-bonding were derived from advanced multinuclear solid-state NMR techniques, where interpretation of the obtained NMR data was supported by DFT quantum-chemical computations. Furthermore, an approach for selective separation of quinidine from its stereoisomer quinine based on the molecular specificity of methyl paraben is presented. It was found that methyl paraben picked its target via hydrogen-bond-mediated molecular recognition, thereby acting as "molecular hook".

Multinuclear solid-state NMR studies on phase transition of mesostructured aluminophosphate

The phase transition process from a hexagonal to a lamellar mesostructured aluminophosphate was studied by multinuclear solid-state NMR techniques in combination with low-angle XRD and elemental analysis measurements. The XRD result showed that after hydrothermal treatment for 9 h, the initial hexagonal phase began to transform to a lamellar phase. It was also found by NMR that some initially formed aluminophosphate was hydrolyzed during the earliest 2 h of the hydrothermal treatment, and the inorganic aluminophosphate framework became more and more condensed during the phase transition. The organic surfactant mainly has a gauche conformation in the hexagonal phase, whereas it mainly has a trans conformation in the lamellar phase and is packed more compactly. The Br− anion is found to be involved in the phase transition and likely enters into the interface to compensate the lost of negative charges that generated by the condensation of the aluminophosphate. Finally, a mechanism for the phase transition is proposed.

Crystallization of magnesium substituted aluminophosphate of type-36 as studied by solid-state NMR spectroscopy

The crystallization process of aluminophosphate molecular sieve MgAPO-36 is characterized by the evolution of intermediate gels. Multinuclear solid-state NMR techniques in combination with powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM) reveal in detail the nature of the intermediate gels and their evolution into crystalline structure. The long-range ordering of the aged as well as heated intermediate gels are probed by PXRD, showing that the crystallization of the framework begins after heating the gels at 423 K for 1.5 h. 31P NMR indicated that the structural Al–O–P and Mg–O–P units from the ATS framework are formed at the same time. More information about the local ordering of the gels is obtained from two-dimensional 27Al → 31P HETCOR and 1H → 31P CP/MAS experiments. Two types of microstructure regions could be differentiated by partially and fully condensed P species, in which five types of P( nAl) ( n = 2–4) units are identified by 31P/ 27Al double-resonance experiments. A possible evolution mechanism consisting of three stages is proposed for the crystallization process.

Crystallization of AlPO4-5 Aluminophosphate Molecular Sieve Prepared in Fluoride Medium: A Multinuclear Solid-State NMR Study

In the present work, multinuclear solid-state NMR techniques, combined with powder X-ray diffraction (PXRD) and infrared (IR) spectroscopy, are employed to monitor the crystallization of AlPO4-5 aluminophosphate prepared in the presence of HF under hydrothermal condition. The crystallization process is characterized by the evolution of intermediate gels, in which the long-rang ordering arrangement is probed by PXRD, revealing the threshold of the crystallization around 120 min. The appearance of 31P signals at ca. -22 and -29 ppm due to the structural P-O-Al unit and 19F signal at -120 ppm due to the structural F-Alpen-O-P unit in the NMR spectra of the series gels indicates that the crystalline framework is starting to form. The onset of the crystallization is also evidenced by the presence of the pentacoordinated Al in the structural F-Alpen-O-P unit which is considered to be associated with the ordered framework. More information about the local ordering of the gels is obtained from two-dimensional 27Al --> 31P heteronuclear correlation (HETCOR) and 31P/27Al double-resonance experiments. In combination with 1H --> 31P cross-polarization/magic-angle spinning (CP/MAS) experiments, two microdomains can be identified in the 120 min heated gel. A possible evolution mechanism of the gels consisting of three successive stages is proposed for the crystallization process.

Acidity of Mesoporous MoOx/ZrO2 and WOx/ZrO2 Materials: A Combined Solid-State NMR and Theoretical Calculation Study

The acidity of mesoporous MoO(x)/ZrO2 and WO(x)/ZrO2 materials was studied in detail by multinuclear solid-state NMR techniques as well as DFT quantum chemical calculations. The 1H MAS NMR experiments clearly revealed the presence of two different types of strong Brønsted acid sites on both MoO(x)/ZrO2 and WO(x)/ZrO2 mesoporous materials, which were able to prontonate adsorbed pyrine-d5 (resulting in 1H NMR signals at chemical shifts in the range 16-19 ppm) as well as adsorbed trimethylphosphine (giving rise to 31P NMR signal at ca. 0 ppm). The 13C NMR of adsorbed 2-(13)C-acetone indicated that the average Brønsted acid strength of the two mesoporous materials was stronger than that of zeolite HZSM-5 but still weaker than that of 100% H2SO4, which was in good agreement with theoretical predictions. The quantum chemical calculations revealed the detailed structures of the two distinct types of Brønsted acid sites formed on the mesoporous MoO(x)/ZrO2 and WO(x)/ZrO2. The existence of both monomer and oligomer Mo (or W) species containing a Mo-OH-Zr (or W-OH-Zr) bridging OH group was confirmed with the former having an acid strength close to zeolite HZSM-5, with the latter having an acid strength similar to sulfated zirconia. On the basis of our NMR experimental and theoretical calculation results, a possible mechanism was proposed for the formation of acid sites on these mesoporous materials.

Brönsted and Lewis Acidity of the BF3/γ-Al2O3 Alkylation Catalyst as Revealed by Solid-State NMR Spectroscopy and DFT Quantum Chemical Calculations

Multinuclear solid-state NMR techniques and DFT quantum chemical calculations were employed to investigate the detailed structure of acid sites on the BF3/gamma-Al2O3 alkylation catalyst. The NMR experiment results indicate that gaseous BF3 is able to react with the hydroxyl groups present on the surface of gamma-Al2O3, leading to the formation of new Brönsted and Lewis acid sites. The 1H/11B and 1H/27Al TRAPDOR (TRAnsfer of Population in DOuble Resonance) experiments suggest that the 3.7 ppm signal in 1H NMR spectra of the BF3/gamma-Al2O3 catalyst is due to a bridging B-OH-Al group that acts as a Brönsted acid site of the catalyst. On the other hand, a Lewis acid site on the surface of the catalysts, as revealed by 31P MAS and 31P/27Al TRAPDOR NMR of adsorbed trimethylphosphine, is associated with three-coordinate -OBF2 species. 13C NMR of adsorbed 2-13C-acetone indicates that the Brönsted acid strength of the catalyst is slightly stronger than that of zeolite HZSM-5 but still weaker than that of 100% H2SO4, which is in good agreement with theoretical prediction. In addition, DFT calculations also reveal the detailed structure of various acid sites formed on the BF3/gamma-Al2O3 catalyst and the interaction of probe molecules with these sites.

Solid state NMR and X-ray diffraction study of three forms of the aluminophosphate AlPO 4-ZON

As-synthesized, calcined and calcined-rehydrated AlPO 4- ZON materials have been studied using powder XRD and multinuclear solid state NMR techniques. A new assignment for the four 31P signals of the as-synthesized sample is presented. A Rietveld refinement on synchrotron data allows to elucidate the structure of the calcined material. Although the space group (Pbca) and the framework topology are similar to those of the previously studied as-synthesized and calcined-rehydrated materials, all four Al sites now exhibit a fourfold coordination. This is in agreement with the result of an 27Al 5QMAS NMR experiment. Furthermore, the bond distances deduced from this structural refinement, combined with a 31P homonuclear correlation experiment, allow assignment of the 31P resonances. Only a partial assignment of the 27Al signals could be achieved for the rehydrated material.

W/HZSM-5 catalyst for methane dehydroaromatization: a multinuclear MAS NMR study

Multinuclear solid-state NMR techniques were employed to investigate the interaction between tungsten species and zeolite in W/HZSM-5 catalysts prepared by controlled impregnation. Al-27, Si-29 NMR experiments suggest that introduction of W species leads to the formation of distorted four-coordinate framework and six-coordinate extra-framework aluminum species due to the interaction of the W species and framework aluminum, and the former can be further transformed into the latter, especially at high W loadings. H-1 MAS NMR indicates that some of the W species disperse on the external surface and some of them diffuse into the internal channels of the zeolite, which causes a decrease of the Bronsted acid sites and silanols as well as an increase of the extra-framework AI-OH species in the W/HZSM-5 catalysts. Cooperation of the W species and the remaining Bronsted acid sites is responsible for the activity of the W/HZAM-5 catalysts for methane dehydroaromatization. As revealed by C-13 NMR experiments, an induction period exists for methane dehydroaromatization reaction and W6+ oxide on/in the zeolite is likely to be converted into W4+ oxide during the reaction. It is W4+ oxide rather than carbide tungsten that acts as active metal sites for initial activation of methane on the W/HZSM-5 catalysts. (C) 2003 Elsevier Science B.V. All rights reserved.

Methane Dehydro-aromatization over Mo/HZSM-5 in the Absence of Oxygen: A Multinuclear Solid-State NMR Study of the Interaction between Supported Mo Species and HZSM-5 Zeolite with Different Crystal Sizes

The interaction between Mo species and a conventionally microsized and particularly nanosized HZSM-5 support was studied by high-resolution multinuclear solid-state NMR techniques. As proved by 27Al and 29Si MAS as well as CP/MAS NMR investigations, this interaction was so strong that the framework aluminum of both microsized and nanosized HZSM-5 zeolites could be extracted. With increasing Mo loading, more nonframework aluminum, resonanced at ca. 30 ppm, appeared in the 27Al MAS NMR spectrum of the Mo-loaded nanosized HZSM-5 catalyst. Meanwhile, this strong interaction led to the formation of more new Al2(MoO4)3 crystallines on the nanosized HZSM-5 support than on the microsized HZSM-5 support. The appearance of Al2(MoO4)3 crystallines resulted in fewer active catalysts for the methane dehyro-aromatization. The results of 1H MAS NMR using perfluorotributyl amine as a probe molecule demonstrated that Mo species preferentially reacted with the silanols and nonframework AlOH on the external surface of microsized and nanosized HZSM-5 zeolites. In addition, impregnated Mo species remained predominantly on the external surface of the nanosized HZSM-5 zeolite, although there was a possibility that they might migrate into the lattice channels of the microsized HZSM-5 zeolite. The migration of some Mo species into the zeolite channels might be beneficial for the conversion of methane to aromatics in the absence of oxygen.

Recent advances in applications of multinuclear solid-state NMR to heterogeneous catalysis and inorganic materials

The interface between heterogeneous catalysis and the science of inorganic materials and high-resolution solid-state NMR is recently an area of much activity. A wide variety of novel techniques in multinuclear solid-state NMR have provided attractive possibilities in the mechanistic studies in catalysis as well as the design of catalysts and functional materials. Current state-of-the-art topics, by means of significant multinuclear solid-state NMR techniques, were selected to illustrate the scope of applications to the studies in the field of heterogeneous catalysis and inorganic materials.