Hydrogen bonds and local symmetry in the crystal structure of gibbsite

First‐principles quantum mechanical calculations of NMR chemical shifts and quadrupolar parameters have been carried out to assign the ²⁷Al MAS NMR resonances in gibbsite. The ²⁷Al NMR spectrum shows two signals for octahedral aluminum revealing two aluminum sites coordinated by six hydroxyl groups each, although the crystallographic positions of the two Al sites show little difference. The presence of two distinguished ²⁷Al NMR resonances characterized by rather similar chemical shifts but quadrupolar coupling constants differing by roughly a factor of two is explained by different character of the hydrogen bonds, in which the hydroxyls forming the corresponding octahedron around each aluminum site, are involved. The Al‐I site characterized by a C_Q = 4.6 MHz is surrounded by OH? groups participating in four intralayer and two interlayer hydrogen bonds, while the Al‐II site with the smaller quadrupolar constant (2.2 MHz) is coordinated by hydroxides, of which two point toward the intralayer cavities and four OH‐bonds are aligned toward the interlayer gallery. In high‐resolution solid‐state ¹H CRAMPS (combination of rotation and multiple‐pulse spectroscopy) four signals with an intensity ratio of 1:2:2:1 are resolved which allow to distinguish six nonequivalent hydrogen sites reported in the gibbsite crystal structure and to ascribe them to two types of structural OH groups associated with intralayer and interlayer hydrogen bonds. This study can be applied to characterize the gibbsite‐like layer—intergallery interactions associated with hydrogen bonding in the more complex systems, such as synthetic aluminum layered double hydroxides. Copyright © 2010 John Wiley & Sons, Ltd.     

铝在MCM-22分子筛骨架上分布的27Al MQ MAS NMR研究

利用二维多量子魔角旋转（2D MQ MAS）技术并结合量子化学计算，研究了铝在MCM-22分子筛骨架上的分布，并对铝的不等价四面体位进行了归属.在27Al 2D 5Q MAS NMR谱中骨架四配位铝的范围内观察到3个信号，证明MCM-22分子筛有3种骨架铝.经计算这3种骨架四配位铝的各向同性化学位移和四极作用常数分别为：δ 50.5、δ 57.3、δ 62.4和1.74、1.68、1.92 MHz. MCM-22分子筛结构中有8种结晶学不等价四面体（T）位.我们通过模拟MCM-22分子筛的27Al 2D 5Q MAS NMR谱，认为8种不等价T位分为3组. T2、T6位上的铝分别与δ 61、δ 49处的信号相关， T1、T3、T4、T5、T7、T8位上的铝对δ 56处的共振峰有贡献.当硅铝原子数比(Si/Al)在10～15之间变化时，铝在MCM-22分子筛的骨架上是无规占据的.     

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.     

Multinuclear high-resolution NMR study of compounds from the ternary system NaF-CaF2-AlF3: from determination to modeling of NMR parameters

27Al and 23Na NMR satellite transition spectroscopy and 3Q magic-angle-spinning spectra are recorded for three compounds from the ternary NaF-CaF2-AlF3 system. The quadrupolar frequency nuQ, asymmetry parameter etaQ, and isotropic chemical shift deltaiso are extracted from the spectrum reconstructions for five aluminum and four sodium sites. The quadrupolar parameters are calculated using the LAPW-based ab initio code WIEN2k. It is necessary to perform a structure optimization of all compounds to ensure a fine agreement between experimental and calculated parameters. By a comparison of experimental and calculated values, an attribution of all of the 27Al and 23Na NMR lines to the crystallographic sites is achieved. High-speed 19F NMR MAS spectra are recorded and reconstructed for the same compounds, leading to the determination of 18 isotropic chemical shifts. The superposition model developed by Bureau et al. is used, allowing a bijective assignment of the 19F NMR lines to the crystallographic sites.     

Understanding Trends in 27Al Chemical Shifts and Quadrupolar Coupling Constants in Chloroalkyl Aluminum [AlClx(Me)3 − x]n = 1 or 2 Compounds

The usage of alkyl aluminum compounds and related structures as co‐catalyst finds a broad range of application in homogeneous and heterogeneous catalysis. While understanding the nature of the aluminum species in solution or in solids can be a challenge, ²⁷Al solid state NMR is a powerful tool to understand the structures of Al species, but their assignment remains mostly empirical, typically by comparing chemical shifts with known compounds. In this work, the observed trends in ²⁷Al‐NMR parameters – chemical shift and quadrupolar coupling constant – of chloroalkyl aluminum compounds, a prototypical class of important Lewis activators, are traced back to their frontier orbitals and electron polarization through a natural localized molecular orbital analysis. This study thus provides guidelines to understand the nature of chemical shift and thereby assignment of possible structure.     

Ultra‐wideline 27Al NMR Investigation of Three‐ and Five‐Coordinate Aluminum Environments

Ultra‐wideline ²⁷Al NMR experiments are conducted on coordination compounds with ²⁷Al nuclei possessing immense quadrupolar interactions that result from exceptionally nonspherical coordination environments. NMR spectra are acquired using a methodology involving frequency‐stepped, piecewise acquisition of NMR spectra with Hahn‐echo or quadrupolar Carr–Purcell Meiboom–Gill (QCPMG) pulse sequences, which is applicable to any half‐integer quadrupolar nucleus with extremely broad NMR powder patterns. Despite the large breadth of these central transition powder patterns, ranging from 250 to 700 kHz, the total experimental times are an order of magnitude less than previously reported experiments on analogous complexes with smaller quadrupolar interactions. The complexes examined feature three‐ or five‐coordinate aluminum sites: trismesitylaluminum (AlMes₃), tris(bis(trimethylsilyl)amino)aluminum (Al(NTMS₂)₃), bis[dimethyl tetrahydrofurfuryloxide aluminum] ([Me₂‐Al(μ‐OTHF)]₂), and bis[diethyl tetrahydrofurfuryloxide aluminum] ([Et₂‐Al(μ‐OTHF)]₂). We report some of the largest ²⁷Al quadrupolar coupling constants measured to date, with values of C_Q(²⁷Al) of 48.2(1), 36.3(1), 19.9(1), and 19.6(2) MHz for AlMes₃ , Al(NTMS₂)₃ , [Me₂‐Al(μ‐OTHF)]₂ , and [Et₂‐Al(μ‐OTHF)]₂ , respectively. X‐ray crystallographic data and theoretical (Hartree–Fock and DFT) calculations of ²⁷Al electric field gradient (EFG) tensors are utilized to examine the relationships between the quadrupolar interactions and molecular structure; in particular, the origin of the immense quadrupolar interaction in the three‐coordinate species is studied via analyses of molecular orbitals.     

Supported metallocene catalysts by surface organometallic chemistry. Synthesis, characterization, and reactivity in ethylene polymerization of oxide-supported mono- and biscyclopentadienyl zirconium alkyl complexes: establishment of structure/reactivity relationships

The reactions of CpZr(CH(3))(3), 1, and Cp(2)Zr(CH(3))(2), 2, with partially dehydroxylated silica, silica-alumina, and alumina surfaces have been carried out with careful identification of the resulting surface organometallic complexes in order to probe the relationship between catalyst structure and polymerization activity. The characterization of the supported complexes has been achieved in most cases by in situ infrared spectroscopy, surface microanalysis, qualitative and quantitative analysis of evolved gases during surface reactions with labeled surface, solid state (1)H and (13)C NMR using (13)C-enriched compounds, and EXAFS. 1 and 2 react with silica(500) and silica-alumina(500) by simple protonolysis of one Zr-Me bond by surface silanols with formation of a single well-defined neutral compound. In the case of silica-alumina, a fraction of the supported complexes exhibits some interactions with electronically unsaturated surface aluminum sites. 1 and 2 also react with the hydroxyl groups of gamma-alumina(500), leading to several surface structures. Correlation between EXAFS and (13)C NMR data suggests, in short, two main surface structures having different environments for the methyl group: [Al](3)-OZrCp(CH(3))(2) and [Al](2)-OZrCp(CH(3))(mu-CH(3))-[Al] for the monoCp series and [Al](2)-OZrCp(2)(CH(3)) and [Al]-OZrCp(2)(mu-CH(3))-[Al] for the bisCp series. Ethylene polymerization has been carried out with all the supported complexes under various reaction conditions. Silica-supported catalysts in the absence of any cocatalyst exhibited no activity whatsoever for ethylene polymerization. When the oxide contained Lewis acidic sites, the resulting surface species were active. The activity, although improved by the presence of additional cocatalysts, remained very low by comparison with that of the homogeneous metallocene systems. This trend has been interpreted on the basis of various possible parameters, including the (p-pi)-(d-pi) back-donation of surface oxygen atoms to the zirconium center.     

An investigation of the roles of surface aluminum and acid sites in the zeolite MCM-22

Ammonia adsorption studies reveal that the observed Lewis acidity in the zeolite MCM-22 is derived from at least two types of framework aluminum sites (AlF), that is, octahedral AlF and three-coordinate AlF. Comparative ammonia or trimethylphosphine (TMP) adsorption experiments with MCM-22 confirm that octahedral Al species gives rise to the signal at delta(iso) approximately 0 in the 27Al NMR spectrum; this is a superposition of two NMR signals from the different Al species on the water-reconstructed zeolite surface. A sharp resonance assigned to framework Al reversibly transforms on ammonia adsorption to delta(iso)27Al approximately 55 from tetrahedral AlF, while the broad peak is assigned to nonframework aluminum which results from hydrothermal treatment. This study also demonstrates the effectiveness of 27Al magic angle spinning (MAS) and multiple quantum (MQ) MAS NMR spectroscopy as a technique for the study of zeolite reactions.     

Silica‐Grafted Tris(neopentyl)aluminum: A Monomeric Aluminum Solid Co‐catalyst for Efficient Nickel‐Catalyzed Ethene Dimerization

A silica‐supported monomeric alkylaluminum co‐catalyst was prepared via surface organometallic chemistry by contacting tris(neopentyl)aluminum and partially dehydroxylated silica. This system, fully characterized by solid‐state ²⁷Al NMR spectroscopy augmented by computational studies, efficiently activates (ⁿBu₃P)₂NiCl₂ towards dimerization of ethene, demonstrating comparable activity to previously reported dimeric diethylaluminum chloride supported on silica. Three types of aluminum surface species have been identified: monografted tetracoordinated Al species as well as two types of bisgrafted Al species—tetra‐ and pentacoordinated. Of them, only the monografted Al species is proposed to be able to activate the (ⁿBu₃P)₂NiCl₂ complex and generate the active cationic species.     

Cd-113 and Al-27 NMR relaxation study of aquoeus solutions adsorbed on silica gels and zeolite NaY

Cd-113 and Al-27 NMR parameters were used to investigate the Cd²⁺ and Al³⁺ hexaaquo metal ions adsorbed on some silica gels and zeolite NaY. No significant variations of the chemical shifts of cadmium and aluminum signals were observed for the aquated ions onto the pores with respect to those of the bulk solutions, thus indicating the absence of oservable specific interactions with the surface. On the contrary an increasing broadness of the signals as the pore size decreased as well as dramatic shortenings of Cd-113 and Al-27 NMR spin-lattice relaxation times were obtained. These variations were mainly ascribed to a reduced mobility of the aquo ions inside the pores. From the comparative analysis of Cd-113 and Al-27 relaxation data the correlation times of Cd(H₂O)₆²⁺ and Al(H₂O)₆³⁺ adsorbed species were estimated together with the quadrupolar coupling constants for Al(H₂O)₆³⁺.     

Ziegler–Natta catalysts on silica for ethylene polymerization

The amount of dibutylmagnesium (DBM) or triethylaluminum (TEAL) that reacted with silica at 55–60°C depended on the silica calcining temperature. Lower silica calcining temperatures resulted in more Mg or Al fixed to the silica surface, indicating greater amounts of DBM or TEAL reacting with the silica. The amount of the metal alkyls butyl(octyl) magnesium ethoxide, ethylaluminum dichloride, tri-n-hexylaluminum, and diethyl(ethyldimethylsilanolato) aluminum that reacted with 600°C calcined silica was also determined. The metal alkyl can react with the silica at two sites, a surface hydroxyl group and a siloxane group. The silica surface hydroxyl groups can be chemically converted to trimethylsilyl groups so that only the siloxane groups are available for attack. After the metal alkyl was reacted with silica, the resulting intermediate was treated with titanium tetrachloride to yield an ethylene polymerization catalyst in the presence of TEAL. When no metal alkyl was employed, titanium tetrachloride reacted only with the silica surface hydroxyl groups to yield a weakly active ethylene polymerization catalyst.     

Silica-Grafted Tris(neopentyl)aluminum: A Monomeric Aluminum Solid Co-catalyst for Efficient Nickel-Catalyzed Ethene Dimerization

A silica-supported monomeric alkylaluminum co-catalyst was prepared via surface organometallic chemistry by contacting tris(neopentyl)aluminum and partially dehydroxylated silica. This system, fully characterized by solid-state ²⁷Al NMR spectroscopy augmented by computational studies, efficiently activates (ⁿBu₃P)₂NiCl₂ towards dimerization of ethene, demonstrating comparable activity to previously reported dimeric diethylaluminum chloride supported on silica. Three types of aluminum surface species have been identified: monografted tetracoordinated Al species as well as two types of bisgrafted Al species—tetra- and pentacoordinated. Of them, only the monografted Al species is proposed to be able to activate the (ⁿBu₃P)₂NiCl₂ complex and generate the active cationic species.     

Nature, structure and strength of the acidic sites of amorphous silica alumina: an IR and NMR study

IR spectroscopy of probe molecules (pyridine, 2,6-dimethylpyridine, and CO) as well as high-resolution solid state NMR and especially double-resonance experiments give a new insight into the acidic sites of amorphous silica alumina (ASA). ASA samples are heterogeneous compounds that contain a silica alumina mixed phase as well as aluminum clusters and pure silica zones. The distribution of various forms depends both on the preparation method and on the Si/Al ratio. Formation of mixed phase leads to the creation of acidic hydroxyl groups of various strength, up to that present in dealuminated HY zeolite. Detailed spectroscopic analyses show that these acidic OH groups correspond to the silanol groups located in close vicinity to an Al atom in tetrahedral environment. The strength of the acidity of the OH species of ASA could be modified both by the location of the vicinal Al atom on the surface or in the bulk and by the number of aluminum atoms in the vicinity of silanol group. Cogelification of high silica-containing ASA appears as the best mean to prepare homogeneous amorphous aluminosilicate, which exhibits the strongest Br?nsted acidity.     

NMR heteronuclear correlation between quadrupolar nuclei in solids

We show for the first time that it is possible to acquire high-resolution heteronuclear NMR correlation experiments in solid state between second-order-broadened half integer quadrupolar nuclei (i.e., 27Al and 17O) using the scalar J-coupling. The sensitivity of the experiment is dramatically improved at high fields (gain proportional to the fourth power of the principal field) with a combination of signal enhancement techniques. This turns a challenging experiment into a real tool. We apply this experiment to characterize a calcium aluminate glass in which we prove the presence of tricluster mu3 oxygen sites and describe the signature of their directly bonded aluminum sites. Applications involve a large range of possible pairs of quadrupolar nuclei in different materials, such as glasses, porous or mesoporous framework materials, zeolites, hybrid organic-inorganic, and bioinvolved materials.     

Chemistry of the silica surface: liquid-solid reactions of silica gel with trimethylaluminum

The reaction of trimethylaluminum and dry, high-surface-area (500 m2/g) silica gel in a mixed slurry was studied using multinuclear, solid-state NMR spectroscopy. The products of the initial reaction were characterized, and their progress through subsequent washing with diethyl ether and reactions with measured amounts of water was followed. The quantitative distribution of different chemical forms of carbon deposited on the silica surface by the initial reaction was measured. The products of the initial reaction are dominated by methyl species of the types Al(CH3)n (with Si-O-Al linkages), Si-O-CH3, and (Si-O)4-nSi(CH3)n; aluminum is seen to exist predominantly as a five-coordinate species. Subsequent treatment with diethyl ether fails to remove any surface species, but instead the ether becomes strongly associated with the surface and highly resistant to removal. Stepwise additions of water hydrolyze the Al-CH3 and Si-O-CH3 moieties, leading to conversion of five-coordinate aluminum to four- and six-coordinate aluminum, and affect the partial release of the surface-associated diethyl ether; Si-CH3 moieties remain. The effect of aromatic and saturated solvents on the initial reaction was examined and found to cause a small but significant change in the distribution of products. Structures of aluminum-centered species on the silica surface consistent with the spectroscopic data are proposed.     

Advances in structural analysis of fluoroaluminates using DFT calculations of 27Al electric field gradients

Based on the analysis of 23 aluminum sites from 16 fluoroaluminates, the present work demonstrates the strong potential of combining accurate NMR quadrupolar parameter measurements, density functional theory (DFT)-based calculations of electric field gradients (EFG), and structure optimizations as implemented in the WIEN2k package for the structural and electronic characterizations of crystalline inorganic materials. Structure optimizations are essential for compounds whose structure was refined from usually less accurate powder diffraction data and provide a reliable assignment of the 27Al quadrupolar parameters to the aluminum sites in the studied compounds. The correlation between experimental and calculated EFG tensor elements leads to the proposition of a new value of the 27Al nuclear quadrupole moment Q(27Al) = 1.616 (+/-0.024) x 10(-29) m2. The DFT calculations provide the orientation of the 27Al EFG tensors in the crystal frame. Electron density maps support that the magnitude and orientation of the 27Al EFG tensors in fluoroaluminates mainly result from the asymmetric distribution of the Al 3p orbital valence electrons. In most cases, the definition of relevant radial and angular distortion indices, relying on EFG orientation, allows correlations between these distortions and magnitude and sign of the Vii.     

17O magic angle spinning NMR studies of Brønsted acid sites in zeolites HY and HZSM-5

High-resolution 17O/1H double resonance NMR spectra were obtained for two zeolites, one with a low Si/Al ratio (zeolite HY) and one with a high Si/Al ratio (HZSM-5), to investigate their local structure and Br?nsted acidity. Two different oxygen signals, corresponding to Br?nsted acid sites in supercages and sodalite cages of zeolite HY were readily resolved in the two-dimensional (2-D) 1H-17O heteronuclear correlation (HETCOR) NMR spectra allowing the 17O isotropic chemical shift (deltaCS) and quadrupolar coupling parameters (quadrupolar coupling constant, QCC, and asymmetry parameter, eta) for the two oxygen atoms to be extracted. Similar experiments for HZSM-5 showed that the sites in this system are associated with a much larger distribution in NMR parameters than found in HY. 17O-1H rotational echo double resonance (REDOR) NMR was applied to probe the O-H distances in zeolites HY and HZSM-5. Weaker 17O-1H dephasing was observed for zeolite HZSM-5 in comparison to that of HY, consistent with longer O-H bonds and/or increased proton mobility.     

Structural characterization of MAO and related aluminum complexes. 1. Solid-state (27)Al NMR with comparison to EFG tensors from ab initio molecular orbital calculations.

Experimental and ab initio molecular orbital techniques are developed for study of aluminum species with large quadrupole coupling constants to test structural models for methylaluminoxanes (MAO). The techniques are applied to nitrogen- and oxygen-containing complexes of aluminum and to solid MAO isolated from active commercial MAO preparations. (Aminato)- and (propanolato)aluminum clusters with 3-, 4-, and 6-coordinate aluminum sites are studied with three (27)Al NMR techniques optimized for large (27)Al quadrupole coupling constants: field-swept, frequency-stepped, and high-field MAS NMR. Four-membered (aminato)aluminum complexes with AlN(4) coordination yield slightly smaller C(q) values than similar AlN(2)C(2) sites: 12.2 vs 15.8 MHz. Planar 3-coordinate AlN(2)C sites have the largest C(q) values, 37 MHz. In all cases, molecular orbital calculations of the electric field gradient tensors yields C(q) and eta values that match with experiment, even for a large hexameric (aminato)aluminum cage. A D(3d) symmetry hexaaluminum oxane cluster, postulated as a model for MAO, yields a calculated C(q) of -23.7 MHz, eta = 0.7474, and predicts a spectrum that is too broad to match the field-swept NMR of methylaluminoxane, which shows at least three sites, all with C(q) values greater than 15 MHz but less than 21 MHz. Thus, the proposed hexaaluminum cluster, with its strained four-membered rings, is not a major component of MAO. However, calculations for dimers of the cage complex, either edge-bridged or face-bridged, show a much closer match to experiment. Also, MAO preparations differ, with a gel form of MAO having significantly larger (27)Al C(q) values than a nongel form, a conclusion reached on the basis of (27)Al NMR line widths in field-swept NMR spectra acquired from 13 to 24 T.     

Mutable Lewis and Brønsted acidity of aluminated SBA-15 as revealed by NMR of adsorbed pyridine-15N

(1)H and variable-temperature (15)N NMR techniques have been used to study the effect of the gradual alumination of SBA-15 on the structure and adsorption properties of this mesoporous material. The interpretation of experimental spectra suggests that aluminum chlorhydrol most effectively reacts with silica surfaces in the confinement of the cavities of rough mesopore walls, instead of forming a homogeneous aluminum film. This first leads to a gradual filling of the cavities and finally results in aluminum islands on the inner surfaces of mesopores. In the sample with a Si/Al atomic ratio of 4.1, up to half of the inner surface area of the mesopores is covered with aluminum. The alumination produces Br?nsted acid sites attributed to silanol groups interacting with aluminum but does not affect the proton-donating ability of isolated silanol groups. At high Si/Al ratios, the surface contains only one type of Lewis site attributed to tetracoordinated aluminum. At lower Si/Al ratios, Lewis acid sites with a lower electron-accepting ability appear, as attributed to pentacoordinated aluminum. The numerical values of the surface densities of all chemically active sites have been estimated after annealing at 420 and 700 K. We were surprised to observe that gaseous nitrogen can occupy Lewis acid sites and hinder the interaction of the aluminum with any other electron donor. As a result, aluminated surfaces saturated with nitrogen do not exhibit any Br?nsted or Lewis acidity. At room temperature, it takes days before pyridine replaces nitrogen at the Lewis acid sites.     

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.