Multinuclear NMR studies on the sol-gel preparation of sodium aluminophosphate glasses

Sodium aluminophosphate gels and glasses in the system NaPO₃–Al₂O₃ with P/Al ratios ranging from 9:1 to 1:1 have been synthesized by a novel sol–gel route based on the reaction of aluminum lactate with sodium polyphosphate in aqueous solution. The route from the solution to the gel and the final glass was monitored in situ by liquid and solid state NMR techniques, characterizing the influence of composition and pH on the hydrolysis, polymerization, and vitrification processes. The site distribution in the xerogels is strongly influenced by the gel-processing temperature. At temperatures near 150°C ligation with lactate groups can be nearly suppressed, resulting in maximum Al/P connectivity in the gel. Annealing the gels at temperatures near 400°C produces significant structural rearrangements, resulting in a glassy network that has close structural similarity to the glasses derived from the usual melt-cooling procedure at 1100–1450°C. This has been confirmed by extensive ²⁷Al, ³¹P and ²³Na MAS NMR as well as ²⁷Al{³¹P} and ²⁷Al {¹H} double resonance experiments. Compared to melt-cooling, the sol–gel process permits a significant extension of the glass-forming region towards higher alumina contents.     

Cd MAS and static NMR study of halogenocadmate crystals

¹¹³Cd magic-angle spinning (MAS) and static NMR spectra are measured using 23 kinds of halogenocadmate crystals with known structures to determine the isotropic chemical shifts (δ_iso), chemical shift anisotropies (Δδ), and asymmetry parameters η and then to discuss the relationship among these NMR parameters and the halide-anion co-ordination environments around Cd²⁺. The δ_iso(MAS) values of halide-anion co-ordination polyhedra of Cd²⁺ largely change with the kind of halide-anion and the halide-anion co-ordination number. The |Δδ| and η in halogenocadmate crystals is largely dependent on the structure type of crystal, but is independent of the halide-anion co-ordination number of Cd²⁺.     

On the synthesis of AlPO4-21 molecular sieve by vapor phase transport method and its phase transformation to AlPO4-15 molecular sieve

An experimental design was applied to the synthesis of AlPO₄-21 molecular sieve (AWO structure) by vapor phase transport (VPT) method, using tetramethylguanidine (TMG) as the template. In this study, the effects of crystallization time, crystallization temperature, phosphor content, template content and water content in the synthesis gel were investigated. The materials obtained were characterized by X-ray diffraction, scanning electron microscopy and fourier transform infrared spectroscopy (FT-IR). Microstructural analysis of the crystal growth in vapor synthetic conditions revealed a revised crystal growth route from zeolite AlPO₄-21 to AlPO₄-15 in the presence of the TMG. Homogenous hexagonal prism AlPO₄-21 crystals with size of 7 × 3 μm were synthesized at a lower temperature (120 °C), which were completely different from the typical tabular parallelogram crystallization microstructure of AlPO₄-21 phase. The crystals were transformed into AlPO₄-21 phase with higher crystallization temperature, longer crystallization time, higher P₂O₅/Al₂O₃ ratio and higher TMG/Al₂O₃ ratio.     

Molecular mobility of confined phases in model mesoporous (MCM-41) and microporous (AlPO4-5 zeolite) host materials

This paper discusses the self-assembly of block copolymers into vesicular morphology. After a brief state of art of the field, a system based on an amphiphilic poly(butadiene)-b-poly(-L-glutamic acid) (PB-b-PGA) diblock copolymer in aqueous solution is discussed in detail. The aggregation behavior of this block copolymer has been investigated by means of fluorescence spectroscopy, dynamic (DLS) and static (SLS) light scattering as well as transmission electron microscopy (TEM). The diblock copolymer was found to form well-defined vesicles in water. The size of these so-called polymersomes or peptosomes could be reversibly manipulated as a function of both pH and ion strength. Depending on the pH of the aqueous solution, the hydrodynamic radii of these vesicles were found to vary from 100 nm to 150 nm. By cross-linking the 1,2-vinyl double bonds present in the polybutadiene block, the ability to transform a transient supramolecular self-organized aggregate into a permanent “shape-persistent stimuli-responsive nanoparticle” has been demonstrated. Received 25 June 2002 and Received in final form 22 October 2002 Published online: 11 March 2003     

A solid-state NMR study of the formation of molecular sieve SAPO-34

This work examined the formation of a catalytically important microporous material, SAPO-34, in the presence of HF under hydrothermal synthesis conditions. The local environments of P, Al, F and Si atoms in several solid phases obtained at different stages of crystallization were characterized by several solid-state NMR techniques including ³¹P, ²⁷Al, ¹⁹F and ²⁹Si MAS, ²⁷Al triple-quantum MAS, ³¹P{²⁷Al} transfer of populations in double-resonance, ²⁷Al{³¹P} rotational-echo double-resonance (REDOR), ²⁷Al→³¹P heteronuclear correlation spectroscopy, ³¹P{¹⁹F} and ²⁷Al{¹⁹F} REDOR as well as ¹H→³¹P cross polarization. The NMR results provide the new insights into the formation of SAPO-34.     

Molecular mobility of confined phases in model mesoporous (MCM-41) and microporous (AlPO(4)-5 zeolite) host materials

The considered host materials are well suited to confine quasi-(1d) molecular phases, seeing that their porosities are composed of parallel unconnected cylindrical pores. For such a simple geometry, confinement effects can be simply described by a single parameter, the pore diameter phi. Our study concerns medium and ultra confinement ranges ( 40 A >or= ? >or= 7.3 A). The primary effect of such confinements is the decrease of the molecular interactions within the confined phase. As a consequence, we have observed strong triple point depressure Delta T(3t) effects for hydrogen and water confined phases in MCM-41 samples. In the limit case of (1d) phase (the neopentane/AlPO(4)-5 system) it seems that a molecular mobility is observed even at very low temperature T=5 K. The secondary confinement effect is an increase of the interactions between the host inner surface and the confined molecular assembly induced by the pore diameter decreasing. Such host material influence gives rise, for medium range confinement to the physisorption of a curved solid film on the inner surface before the capillary phase condensation (hydrogen/MCM-41 (24 A)) and for ultra confinement to the solidification of the confined phase when the molecular species are commensurate with the inner surface sites (methane/AlPO(4)-5).     

高质量AlPO4-5分子筛大单晶的合成与表征

以拟薄水铝石作为铝源,正磷酸为磷源,正三丙胺(TPA)作模板剂,采用水热法合成了AlPO4 5分子筛.在不同的反应物配比、温度以及晶化时间条件下,得到了不同尺寸的AlPO4 5分子筛单晶.实验表明,反应物质的摩尔比为Al2O3∶P2O5∶TPA∶C2H5OH∶H2O∶HF=1∶1. 2∶2. 66∶80∶1000∶0. 7,反应温度为457K,晶化时间48h是生成六方AlPO4 5分子筛大单晶较适宜的条件;在该反应条件下合成出纵向尺度最大达到2. 0mm、具有完整六方棱柱形态的AlPO4 5分子筛单晶;部分晶体的横向尺度达到0. 31mm.用扫描电镜观察晶体的形貌;AlPO4 5分子筛晶体结构通过SMART1000单晶衍射仪鉴定.     

Effects of T2-relaxation in MAS NMR spectra of the satellite transitions for quadrupolar nuclei: a 27Al MAS and single-crystal NMR study of alum KAl(SO4)2.12H2O

Asymmetries in the manifold of spinning sidebands (ssbs) from the satellite transitions have been observed in variable-temperature 27Al MAS NMR spectra of alum (KAl(SO4)2.12H2O), recorded in the temperature range from -76 to 92 degrees C. The asymmetries decrease with increasing temperature and reflect the fact that the ssbs exhibit systematically different linewidths for different spectral regions of the manifold. From spin-echo 27Al NMR experiments on a single-crystal of alum, it is demonstrated that these variations in linewidth originate from differences in transverse (T2) relaxation times for the two inner (m=1/2<-->m=3/2 and m=-1/2<-->m=-3/2) and correspondingly for the two outer (m=3/2<-->m=5/2 and m=-3/2<-->m=-5/2) satellite transitions. T2 relaxation times in the range 0.5-3.5 ms are observed for the individual satellite transitions at -50 degrees C and 7.05 T, whereas the corresponding T1 relaxation times, determined from similar saturation-recovery 27Al NMR experiments, are almost constant (T1=0.07-0.10 s) for the individual satellite transitions. The variation in T2 values for the individual 27Al satellite transitions for alum is justified by a simple theoretical approach which considers the cross-correlation of the local fluctuating fields from the quadrupolar coupling and the heteronuclear (27Al-1H) dipolar interaction on the T2 relaxation times for the individual transitions. This approach and the observed differences in T2 values indicate that a single random motional process modulates both the quadrupolar and heteronuclear dipolar interactions for 27Al in alum at low temperatures.     

用多核固体核磁共振研究CsH2PO4 和CsH5(PO4)2的制备

探讨可能用作燃料电池固体酸电解质的化合物CsH₂PO₄ (CDP)和CsH₅(PO₄)₂(CPDP)的制备. CDP和CPDP混合物由摩尔比1∶4的Cs₂CO₃∶H₃PO₄水溶液结晶而成,而CDP和Cs₂HPO₄·1.5H₂O (H-DCHP)混合物依其摩尔比1∶2的水溶液制备. 甲醇清洗能最有效地将CDP从其混合物中分离出来. CDP、CPDP以及H-DCHP的¹³³Cs 和³¹P 魔角旋转谱以及CPDP和H-DCHP的¹H魔角旋转谱均为首次报道,对各化合物的谱峰做了指认. 说明,透过确认合成的电解质以及合成过程所产生的副产物,多核固体核磁共振对于控制固体酸电解质合成的品质是一个非常有用的工具.     

Solid-state 27Al NMR studies of aluminophosphate molecular sieves. Enhanced resolution by quadrupole nutation and double-rotation.

Solid-state 27Al NMR spectra of several aluminophosphate molecular sieves have been recorded with conventional magic-angle spinning (MAS), double-rotation (DOR) and quadrupole nutation with fast MAS. Enhanced resolution was obtained in the quadrupole nutation experiment at certain radiofrequency pulse strengths. This extra resolution can be comparable to that attainable using DOR, and does not introduce spinning sidebands.     

In situ high temperature MAS NMR study of the mechanisms of catalysis. Ethane aromatization on Zn-modified zeolite BEA

Ethane conversion into aromatic hydrocarbons over Zn-modified zeolite BEA has been analyzed by high-temperature MAS NMR spectroscopy. Information about intermediates (Zn-ethyl species) and reaction products (mainly toluene and methane), which were formed under the conditions of a batch reactor, was obtained by ¹³C MAS NMR. Kinetics of the reaction, which was monitored by ¹H MAS NMR in situ at the temperature of 573 K, provided information about the reaction mechanism. Simulation of the experimental kinetics within the frames of the possible kinetic schemes of the reaction demonstrates that a large amount of methane evolved under ethane aromatization arises from the stage of direct ethane hydrogenolysis.     

Multinuclear MAS NMR Investigation of Sol-Gel and Ball-Milled Nanocrystalline Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>

⁷¹Ga magic-angle spinning (MAS) nuclear magnetic resonance (NMR) has been used to characterize the structural evolution of nanocrystalline Ga₂O₃ samples prepared by sol-gel and ball-milling techniques. ²⁹Si and ²⁷Al MAS NMR have also been used to characterize silica and alumina Zener pinning phases. ⁷¹Ga NMR parameters are reported for the α- and β-Ga₂O₃ phases, and more tentatively for the δ-Ga₂O₃ phase. By simulating the octahedrally coordinated gallium NMR line of β-Ga₂O₃ using Gaussian distributions in χ_Q, the extent of disorder in the Ga₂O₃ crystallites has been quantified. The ball-milled samples contain much more inherent disorder than the sol-gel samples in the nano-phase, which was observed from simulations of the ⁷¹Ga MAS NMR spectra. The silica pinning phase produced highly crystalline and densely aggregated nanocrystalline Ga₂O₃, as well as the smallest nanocrystal sizes. Authors' address: Mark E. Smith, Department of Physics, University of Warwick, Coventry CV4 7AL, UK     

Strategies for extracting NMR parameters from 23Na MAS, DOR and MQMAS spectra. A case study for Na4P2O7

The 23Na magic-angle spinning (MAS), double rotation (DOR) and multiple-quantum magic-angle spinning (MQMAS) NMR spectra of anhydrous sodium pyrophosphate, Na4P2O7, measured at five different Larmor frequencies (nuL) ranging from 105.8 MHz (corresponding to 400 MHz 1H frequency) to 211.6 MHz (800 MHz) are analysed and the complete set of NMR parameters (C(qcc), etaQ and delta(iso)) of the four crystallographically inequivalent sodium sites were determined with high accuracy. Different approaches of spectra evaluation are discussed and their results are compared. The most reliable results are obtained from a combined evaluation of five DOR and three MQMAS spectra but also from two DOR and one MAS spectra or even from a single MQMAS spectrum all data can be derived. It is shown that Na4P2O7 may serve as a useful reference material for experimental set-up and reliability tests of the various NMR experiments.     

The determination of the average 27Al-31P distance in aluminophosphate molecular sieves with SEDOR NMR.

The average 27Al-31P distances in an aluminophosphate, AlPO4-5, and in a silicoaluminophosphate, SAPO-11, were determined by using Spin Echo DOuble Resonance (SEDOR). A calculated SEDOR curve was fitted to the data in order to obtain the dipolar coupling constant. The tetrahedral surrounding of 27Al by four 31P atoms in the second coordination sphere was taken into account in the calculation of the theoretical SEDOR curve. The average 27Al-31P distances obtained by using this technique proved to be in good agreement with X-ray diffraction data.     

A combined 17O RAPT and MQ-MAS NMR study of L-leucine

We report the application of rotor-assisted population transfer (RAPT) to measure the quadrupolar coupling constant (C(q)) for spin 5/2 nuclei. Results from numerical simulations are presented on the magnitude of enhancement factor as a function of frequency offsets, i.e. the RAPT profile. Experimental O17 RAPT profile is traced for the amino acid L-leucine. In addition, results from MQ-MAS experiments are incorporated to determine the quadrupolar asymmetry parameter (eta(q)). Unlike previous reports, the O17 NMR parameters for an amino acid, L-leucine, is reported at a relatively low field of 9.4 T.     

A Al MAS, MQMAS and off-resonance nutation NMR study of aluminium containing silica-based sol-gel materials

Aluminium containing hybrid materials were prepared via the sol-gel method using aluminium sec-butoxide complexed with ethylacetoacetate (Al(OBu^s)₂EAA or Al(OBu^s)₃/EAA mixtures). As silanes, phenyltrimethoxysilane (PhTMS) or phenyltriethoxysilane (PhTES), 3-glycidoxypropyl trimethoxysilane (Glymo) and tetraethylorthosilicate (TEOS) were used. After room temperature drying of the samples the ²⁷Al single pulse excitation (SPE) magic angle spinning (MAS) NMR shows that octahedral (5 ppm) and tetrahedral (55 ppm) coordinated aluminium species are present in the materials. The relative amount of these two species depends on the preparation method. However, the Al(IV)/Al(VI) ratio is lower than 3 (typically 2.3) in all materials, indicating the presence of a small amount of an aluminate phase. Annealing of the samples at 100, 150 and 200 °C results in the formation of an extra signal at 30 ppm (peak maximum measured at 11.7 T). Based on the resonance frequency this signal is generally assigned to a pentahedrally coordinated aluminium species. Hydration/dehydration processes of annealed samples were studied with ²⁷Al SPE MAS NMR, multiple-quantum MAS NMR (MQMAS) and off-resonance nutation NMR. Upon hydration of the annealed sample the signal intensity around 30 ppm decreases in intensity and at the same time the intensity of the signal around 55 ppm increases by the same amount (tetrahedrally coordinated aluminium). The MQMAS spectra reveal that the signal around 30 ppm is not caused by a fivefold-coordinated aluminium species but mainly by tetrahedrally coordinated aluminium species in a distorted environment, experiencing large quadrupole induced shifts and small chemical shifts due to conformational changes in the polymeric network. From the MQMAS NMR spectra it can be concluded that the linebroadening observed in the ²⁷A1 MAS NMR spectra is due to both a distribution in isotropic chemical shifts and a distribution in quadrupole coupling constants (C_qcc = e²qQ/h). Hydration of the sample results in a decrease of the average C_qcc for the tetrahedrally coordinated aluminium from 6 to 4 MHz, whereas the average C_qcc of the octahedrally coordinated aluminium is hardly influenced (4 MHz). These MQMAS results are confirmed by off-resonance nutation experiments.     

Brillouin scattering study of transverse elastic waves in the α, incommensurate and β phases of AlPO4

Transverse and pseudo-transverse elastic waves have been studied in several scattering geometries in order to investigate the temperature dependences of C E ₆₆ and C E ₁₄ over the range 300-1100 K, including the transitions near 860 K. These results complete those on C E ₄₄ we have obtained in a previous work. All these constants display discontinuities at the lock-in transition. In the phase, the results are analysed in term of lowest order couplings between strains (e) and the order parameter (Q). The main features are described by the lowest order biquadratic e2Q2 coupling, in particular for C E ₄₄ in a large temperature range. However, it appears that a contribution of the next coupling term arises for C E ₆₆ below K and that the first two lowest order terms have to be taken into account even just below the lock-in transition in the case of C E ₁₄ . The temperature dependence of Q has been deduced and it can be well described in the framework of Landau's theory. Received: 2 October 1997 / Received in final form: 3 December 1997 / Accepted: 29 January 1998     

Critical factors in obtaining meaningful fast MAS NMR spectra of non-integral spin quadrupolar nuclei. A review with particular emphasis on27Al MAS NMR of catalysts and minerals

In the last decade, magic angle spinning (MAS) NMR has become an extremely important method for studying the structure of inorganic solids. Advances in NMR technology have greatly aided in understanding the structure of catalysts, minerals, clays, ceramics, glasses, etc. Obtaining meaningful MAS spectra of spin-1/2 nuclei such as²⁹Si and³¹P is relatively straightforward and well understood. In contrast, obtaining meaningful MAS spectra is far from simple with non-integral spin quadrupolar nuclei such as¹¹B (I=3/2),¹⁷O (I=5/2),²³Na (I=3/2),²⁷Al (I=5/2),⁶⁹Ga (I=3/2), and⁷¹Ga (I=3/2)?to name some of the most commonly studied nuclei. Many additional factors have to be considered. This paper will deal with these factors and the utility of very fast MAS for studying non-integral spin quadrupolar nuclei in inorganic solids.