Evaluation of 27Al and 51V electric field gradients and the crystal structure for aluminum orthovanadate (AlVO4) by density functional theory calculations

Three sets of crystal-structure data reported for AlVO(4) from two powder-XRD studies and a density functional theory (DFT) investigation, employing the Vienna ab initio simulation package (VASP), have been examined and refined using the DFT structure-optimization scheme implemented in the WIEN2k software. The crystal structures are evaluated on the basis of (27)Al and (51)V quadrupole coupling parameters recently reported for AlVO(4), employing the corresponding electric-field gradient (EFG) tensor elements obtained from the DFT calculations. The DFT calculations provide a reliable assignment of the (27)Al/(51)V resonances from three distinct Al and three V environments to the specific crystallographic sites in the asymmetric unit for AlVO(4). An improved agreement between experimental quadrupole tensor elements and calculated EFG tensors is achieved after the DFT structure optimizations and consistent results are obtained using the three different structures as starting points. The improvement of the structural data is also supported by an evaluation of the Al-O and V-O bond lengths before and after DFT structure optimization. The (51)V nuclear quadrupole moment, |Q((51)V)| = 4.8 +/- 0.1 fm(2), derived from the present analysis, represents a value of higher accuracy than earlier reported Q((51)V) values. The origin of the (27)Al and (51)V EFGs are investigated by an evaluation of the orientations of the EFG tensors in the crystal frame and by an examination of the individual contributions from the valence electrons and the surrounding lattice. The latter investigation shows that the magnitude and orientation of the tensors are largely determined by the p-p((27)Al) and p-p, d-d((51)V) orbital contributions to the valence electrons, while the lattice part only gives a minor contribution for both nuclei.     

Solid-state NMR detection, characterization, and quantification of the multiple aluminum environments in US-Y catalysts by (27)Al MAS and MQMAS experiments at very high field.

The detection of all of the aluminum present in steamed zeolite H-Y catalysts by (27)Al MAS NMR at 14.4 T (600 MHz for (1)H) and 18.8T (800 MHz for (1)H) is reported. Further, it is shown that it is possible by (27)Al MAS and MQMAS NMR measurements to clearly identify four separate aluminum environments which are characteristic of these materials and to unambiguously assign their coordinations. Average chemical shift and quadrupolar coupling parameters are used to accurately simulate the (27)Al MAS NMR spectra at 9.4 T (400 MHz for (1)H), 14.4 T (600 MHz for (1)H) and 18.8 T (800 MHz for (1)H) in terms of these four aluminum environments. In addition, these average chemical shift and quadrupolar coupling parameters are used to calculate peak positions in the (27)Al MQMAS isotropic dimension that are in good agreement with the experimental data acquired at 9.4 and 18.8 T.     

New synthesis of mullite. Structural evolution study by 17O, 27Al and 29Si MAS NMR spectroscopy

Mullite has been prepared from a new combination of precursors. An aluminum alkoxide, aluminium isopropoxide, and silicon tetrachloride, are hydrolysed in tetrahydrofuran solution by ¹⁷O enriched water. The resulting powder is chemically homogeneous, crystallizing into mullite at 980°C. The structural evolution has been studied by DTA, TGA, XRD and ¹⁷O, ²⁷Al and ²⁹Si MAS NMR spectroscopy.     

Dynamics on the microsecond timescale in microporous aluminophosphate AlPO-14 as evidenced by 27Al MQMAS and STMAS NMR spectroscopy

Multiple-quantum magic angle spinning (MQMAS) and satellite-transition magic angle spinning (STMAS) are two well-known techniques for obtaining high-resolution, or "isotropic", NMR spectra of quadrupolar nuclei. It has recently been shown that dynamics-driven modulation of the quadrupolar interaction on the microsecond timescale results in linewidths in isotropic STMAS spectra that are strongly broadened, while, in contrast, the isotropic MQMAS linewidths remain narrow. Here, we use this novel methodology in an 27Al (I = 5/2) NMR study of the calcined-dehydrated aluminophosphate AlPO-14 and two forms of as-synthesized AlPO-14, one prepared with isopropylamine (C3H7NH2) as the template molecule and one with piperidine (C5H10NH). For completeness, the 31P and 13C (both I = 1/2) MAS NMR spectra are also presented. A comparison of the 27Al MQMAS and STMAS NMR results show that, although calcined AlPO-14 appears to have a rigid framework structure, the extent of motion in the two as-synthesized forms is significant, with clear evidence for dynamics on the microsecond timescale in the immediate environments of all four Al sites in each material. Variable-temperature 27Al STMAS NMR studies of the two as-synthesized AlPO forms reveal the dynamics to be complex, with the motions of both the guest water molecules and organic template molecules shown to be contributing. The sensitivity of the STMAS NMR experiment to the presence of microsecond timescale dynamics is such that it seems likely that this methodology will prove useful in NMR studies of host-guest interactions in a wide variety of framework materials.     

Characterization of framework and extra-framework aluminum species in non-hydrated zeolites Y by 27Al spin-echo, high-speed MAS, and MQMAS NMR spectroscopy at B0 = 9.4 to 17.6 T

27Al spin-echo, high-speed MAS (nu(rot) = 30 kHz), and MQMAS NMR spectroscopy in magnetic fields of B0 = 9.4, 14.1, and 17.6 T were applied for the study of aluminum species at framework and extra-framework positions in non-hydrated zeolites Y. Non-hydrated gamma-Al2O3 and non-hydrated aluminum-exchanged zeolite Y (Al,Na-Y) and zeolite H,Na-Y were utilized as reference materials. The solid-state 27Al NMR spectra of steamed zeolite deH,Na-Y/81.5 were found to consist of four signals. The broad low-field signal is caused by a superposition of the signals of framework aluminum atoms in the vicinity of bridging hydroxyl protons and framework aluminum atoms compensated in their negative charge by aluminum cations (delta(iso) = 70 +/- 10 ppm, C(QCC) = 15.0 +/- 1.0 MHz). The second signal is due to a superposition of the signals of framework aluminum atoms compensated by sodium cations and tetrahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (delta(iso) = 65 +/- 5 ppm, C(QCC) = 8.0 +/- 0.5 MHz). The residual two signals were attributed to aluminum cations (delta(iso) = 35 +/- 5 ppm, C(QCC) = 7.5 +/- 0.5 MHz) and octahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (delta(iso) = 10 +/- 5 ppm, C(QCC) = 5.0 +/- 0.5 MHz). By chemical analysis and evaluating the relative solid-state 27Al NMR intensities of the different signals of aluminum species occurring in zeolite deH,Na-Y/81.5 in the non-hydrated state, the aluminum distribution in this material was determined.     

Direct Observation of Various Al Sites in SAPO-34by ^27Al Multiquantum(MQ) Magic Angle Spinning (MAS)NMR

The elegant 2D multiquantum (MQ) MAS NMR has been applied to investigate the coordination of Al atoms in SAPO-34.The results show that the effects of the template and /or water on the coordination of Al atoms are significant,which lead to the formation of various Al species.Up to four kinds of Al species are distinctly resolved.     

Coordination Structure of Aluminum in Magnesium Aluminum Hydroxide Studied by ^27Al NMR

The coordination structure of aluminum in magnesium aluminum hydroxide was studied by ^27Al NMR.The result showed that tetrahedral aluminum (Al^IV) existed in magnesium aluminum hydroxide,and the contents of Al^TV increased with the increase of the ratio of Al/Mg and with the peptizing temperature.Al^IV originated from the so-called Al13 polymer with the structure of one Al tetrahedron surrounded by twelve Al octahedrons.     

HZSM-5分子筛焙烧脱铝的27Al MQMAS NMR研究

用29Si、27Al魔角旋转固体核磁共振(MAS NMR)结合二维多量子魔角旋转(2D MQMAS)技术对焙烧脱铝的HZSM-5分子筛中铝的配位状态进行了研究.结果表明,HZSM-5分子筛经焙烧后,在化学位移(δ)45处出现一宽峰信号,其主要来自扭曲四配位铝.通过二维三量子铝谱计算出扭曲四配位铝的四极作用常数约为5.2 MHz.对700和750 ℃焙烧样品的铝谱进行分峰拟合,发现在δ 30处又出现一个小峰,归属为非骨架五配位铝.同时,在750 ℃焙烧样品的二维多量子铝谱中直接观察到非骨架五配位铝的信号.焙烧温度低于700 ℃,脱铝不明显；高于700 ℃,引起分子筛骨架的显著脱铝.焙烧还造成部分骨架铝的信号变得“不可观测”.     

Investigating the vanadium environments in hydroxylamido V(V) dipicolinate complexes using 51V NMR spectroscopy and density functional theory

Using (51)V magic angle spinning solid-state NMR, SSNMR, spectroscopy and quantum chemical DFT calculations we have characterized the chemical shift and quadrupolar coupling parameters of a series of eight hydroxylamido vanadium(V) dipicolinate complexes of the general formula VO(dipic)(ONR1R2)(H2O) where R1 and R2 can be H, CH3, or CH2CH3. This class of vanadium compounds was chosen for investigation because of their seven-coordinate vanadium atom, a geometry for which there is limited (51)V SSNMR data. Furthermore, a systematic series of compounds with different electronic properties are available and allows for the effects of ligand substitution on the NMR parameters to be studied. The quadrupolar coupling constants, C(Q), are small, 3.0-3.9 MHz, but exhibit variations as a function of the ligand substitution. The chemical shift tensors in the solid state are sensitive to changes in both the hydroxylamide substituent and the dipic ligand, a sensitivity which is not observed for isotropic chemical shifts in solution. The chemical shift tensors span approximately 1000 ppm and are nearly axially symmetric. On the basis of DFT calculations of the chemical shift tensors, one of the largest contributors to the magnetic shielding anisotropy is an occupied molecular orbital with significant vanadium d(z)2 character along the V=O bond.     

Subsolidus area of the AlVO4−Al2(MoO4)3 system

The behaviour of Al₂(MoO₄)₃ towards AIVO₄ in the subsolidus area, over the whole component concentration range, has been studied using the DTA and XRD methods. The experimental results have been presented in the form of a phase diagram. It has been found that components of the system of interest do not remain in equilibrium, and AlVO₄?Al₂(MoO₄)₃ system is not a real two-component system, even in the subsolidus area.     

29Si and 27Al MAS NMR spectra of mullites from different kaolinites

Mullites synthesized from four kaolinites with different random defect densities have been studied by 27Al and 29Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) and X-ray diffraction (XRD). All these mullites show the same XRD pattern. However, 29Si and 27Al MAS NMR spectra reveal that the mullites derived from kaolinites with high defect densities, have a sillimanite-type Al/Si ordering scheme and are low in silica, whereas those mullites derived from kaolinites with low defect densities, consist of both sillimanite- and mullite-type Al/Si ordering schemes and are rich in silica.     

Cation distribution in co-doped ZnAl2O4 nanoparticles studied by X-ray photoelectron spectroscopy and 27Al solid-state NMR spectroscopy

Co(x)Zn(1-x)Al(2)O(4) (x = 0.01-0.6) nanoparticles were synthesized by the citrate sol-gel method and were characterized by X-ray powder diffraction and transmission electron microscopy to identify the crystalline phase and determine the particle size. X-ray photoelectron spectroscopy and (27)Al solid-state NMR spectroscopy were used to study the distribution of the cations in the tetrahedral and octahedral sites in Co(x)Zn(1-x)Al(2)O(4) nanoparticles as a function of particle size and composition. The results show that all of the as-synthesized samples exhibit spinel-type single phase; the crystallite size of the samples is about 20-50 nm and increases with increasing annealing temperature and decreases with Co-enrichment. Zn(2+) ions are located in large proportions in the tetrahedral sites and in small proportions in the octahedral sites in Co(x)Zn(1-x)Al(2)O(4) nanoparticles. The fraction of octahedral Zn(2+) increases with increasing Co concentration and decreases with increasing particle size. Besides the tetrahedral and octahedral coordinations, the presence of the second octahedrally coordinated Al(3+) ions is observed in the nanoparticles. The change of the inversion parameter (2 times the fraction of Al(3+) ions in tetrahedral sites) with Co concentration and particle size is consistent with that of the Zn fraction in octahedral sites. Analysis of the absorption properties indicates that Co(2+) ions are located in the tetrahedral sites as well as in the octahedral sites in the nanoparticles. The inversion degree of Co(2+) decreases with increasing particle size.     

Characterization of the malate-aluminum(III) complex using 1H and 27Al NMR spectroscopy

Structural elucidation of a malate-aluminum(III) complex has been carried out using 1H and 27Al NMR spectroscopy. The 1H chemical shift perturbation clearly indicated the interaction between malate and Al(III) ion. The measurements of 27Al NMR and 1H-13C HSQC spectra demonstrated that the major form of a complex comprised two equivalent malate ions and two unequivalent Al(III) ions. With this constraint, an equilibrium geometry of the complex was proposed by a semi-empirical molecular orbital calculation.     

Structure of Framework Aluminum Lewis Sites and Perturbed Aluminum Atoms in Zeolites as Determined by 27Al{1H} REDOR (3Q) MAS NMR Spectroscopy and DFT/Molecular Mechanics

Zeolites are highly important heterogeneous catalysts. Besides Brønsted SiOHAl acid sites, also framework Al_FR Lewis acid sites are often found in their H‐forms. The formation of Al_FR Lewis sites in zeolites is a key issue regarding their selectivity in acid‐catalyzed reactions. The local structures of Al_FR Lewis sites in dehydrated zeolites and their precursors—“perturbed” Al_FR atoms in hydrated zeolites—were studied by high‐resolution MAS NMR and FTIR spectroscopy and DFT/MM calculations. Perturbed framework Al atoms correspond to (SiO)₃AlOH groups and are characterized by a broad ²⁷Al NMR resonance (δ_i=59–62 ppm, C_Q=5 MHz, and η=0.3–0.4) with a shoulder at 40 ppm in the ²⁷Al MAS NMR spectrum. Dehydroxylation of (SiO)₃AlOH occurs at mild temperatures and leads to the formation of Al_FR Lewis sites tricoordinated to the zeolite framework. Al atoms of these (SiO)₃Al Lewis sites exhibit an extremely broad ²⁷Al NMR resonance (δ_i≈67 ppm, C_Q≈20 MHz, and η≈0.1).     

27Al and 29Si solid-state NMR characterization of calcium-aluminosilicate-hydrate

Calcium silicate hydrate (C-S-H) is the main constituent of hydrated cement paste and determines its cohesive properties. Because of the environmental impact of cement industry, it is more and more common to replace a part of the clinker in cement by secondary cementitious materials (SCMs). These SCMs are generally alumina-rich and as a consequence some aluminum is incorporated into the C-S-H. This may have consequences on the cohesion and durability of the material, and it is thus of importance to know the amount and the location of Al in C-S-H and what the parameters are that control these features. The present paper reports the (29)Si and (27)Al MAS NMR analyses of well-characterized C-A-S-H samples (C-S-H containing Al). These samples were synthesized using an original procedure that successfully leads to pure C-A-S-H of controlled compositions in equilibrium with well-characterized solutions. The (27)Al MAS NMR spectra were quantitatively interpreted assuming a tobermorite-like structure for C-A-S-H to determine the aluminum location in this structure. For this purpose, an in-house written software was used which allows decomposing several spectra simultaneously using the same constrained spectral parameters for each resonance but with variable intensities. The hypothesis on the aluminum location in the C-A-S-H structure determines the proportion of each silicon site. Therefore, from the (27)Al NMR quantitative results and the chemical composition of each sample, the intensity of each resonance line in the (29)Si spectra was set. The agreement between the experimental and calculated (29)Si MAS NMR spectra corroborates the assumed C-A-S-H structure and the proposed Al incorporation mechanism. The consistency between the results obtained for all compositions provides another means to assess the assumptions on the C-A-S-H structure. It is found that Al substitutes Si mainly in bridging positions and moderately in pairing positions in some conditions. Al in pairing site is observed only for Ca/(Si+Al) ratios greater than 0.95 (equivalent to 4 mmol.L(-1) of calcium hydroxide). Finally, the results suggest that penta and hexa-coordinated aluminum are adsorbed on the sides of the C-A-S-H particles.