Ulla Gro Nielsen

Mg/Al Ordering in Layered Double Hydroxides Revealed by Multinuclear NMR Spectroscopy

The anion-exchange ability of layered double hydroxides (LDHs) has been exploited to create materials for use in catalysis, drug delivery, and environmental remediation. The specific cation arrangements in the hydroxide layers of hydrotalcite-like LDHs, of general formula Mg2+1–xAl3+xOH2(Anionn–x/n)·yH2O, have, however, remained elusive, and their elucidation could enhance the functional optimization of these materials. We applied rapid (60 kilohertz) magic angle spinning (MAS) to obtain high-resolution hydrogen-1 nuclear magnetic resonance (1H NMR) spectra and characterize the magnesium and aluminum distribution. These data, in combination with 1H-27Al double-resonance and 25Mg triple-quantum MAS NMR data, show that the cations are fully ordered for magnesium:aluminum ratios of 2:1 and that at lower aluminum content, a nonrandom distribution of cations persists, with no Al3+-Al3+ close contacts. The application of rapid MAS NMR methods to investigate proton distributions in a wide range of materials is readily envisaged.

Local Environments and Lithium Adsorption on the Iron Oxyhydroxides Lepidocrocite (γ-FeOOH) and Goethite (α-FeOOH): A 2H and 7Li Solid-State MAS NMR Study

2H and 7Li MAS NMR spectroscopy techniques were applied to study the local surface and bulk environments of iron oxyhydroxide lepidocrocite (gamma-FeOOH). 2H variable-temperature (VT) MAS NMR experiments were performed, showing the presence of short-range, strong antiferromagnetic correlations, even at temperatures above the Néel temperature, T(N), 77 K. The formation of a Li+ inner-sphere complex on the surface of lepidocrocite was confirmed by the observation of a signal with a large 7Li hyperfine shift in the 7Li MAS NMR spectrum. The effect of pH and relative humidity (RH) on the concentrations of Li+ inner- and outer-sphere complexes was then explored, the concentration of the inner sphere complex increasing rapidly above the point of zero charge and with decreasing RH. Possible local environments of the adsorbed Li+ were identified by comparison with other layer-structured iron oxides such as gamma-LiFeO2 and o-LiFeO2. Li+ positions of Li+-sorbed and exchanged goethite were reanalyzed on the basis of the correlations between Li hyperfine shifts and Li local structures, and two different binding sites were proposed, the second binding site only becoming available at higher pH.

Characterization of phosphate sequestration by a lanthanum modified bentonite clay: a solid-state NMR, EXAFS, and PXRD study.

Phosphate (Pi) sequestration by a lanthanum (La) exchanged clay mineral (La-Bentonite), which is extensively used in chemical lake restoration, was investigated on the molecular level using a combination of (31)P and (139)La solid state NMR spectroscopy (SSNMR), extended X-ray absorption spectroscopy (EXAFS), powder X-ray diffraction (PXRD) and sorption studies. (31)P SSNMR show that all Pi was immobilized as rhabdophane (LaPO4·n H2O, n ≤ 3), which was further supported by (139)La SSNMR and EXAFS. However, PXRD results were ambiguous with respect to rhabdophane and monazite (LaPO4). Adsorption studies showed that at dissolved organic carbon (DOC) concentration above ca. 250 μM the binding capacity was only 50% of the theoretical value or even less. No other La or Pi phases were detected by SSNMR and EXAFS indicating the effect of DOC is kinetic. Moreover, (31)P SSNMR showed that rhabdophane formed upon Pi sequestration is in close proximity to the clay matrix.

Characterization of defects and the local structure in natural and synthetic alunite (K, Na, H3O)Al3(SO4)2(OH)6 by multi-nuclear solid-state NMR spectroscopy

Abstract The local structural environments in a series of natural and synthetic alunite samples [ideally AAl3(SO4)2(OH)6, A = H3O+, D3O+, Na+, and K+] have been probed by solid-state 1H, 2H, 23Na, 27Al, and 39K NMR spectroscopy. The natural alunite [KAl3(SO4)2(OH)6] and synthetic hydronium alunite samples contain few structural defects, whereas the synthetic natroalunite and alunite samples have ca. 10% Al vacancies based on 27Al NMR. A new 27Al local environment (AlD) was observed and assigned to Al with one Al vacancy in the first cation sphere. Three different proton environments, Al2-OH, Al - OH2, and H3O+ are detected by 1H and 2H MAS NMR. The hydronium ion (H3O+) is only observed in hydronium alunite, and is associated with the stoichiometric regions of the sample. It was not detected in 1H and 2H NMR spectra of alunite and natroalunite despite K (Na) occupancies of significantly less than 100%, as determined from elemental analysis. Thus, our NMR results suggest that the common assumption, namely that an A vacancy and an Al3+ vacancy are compensated by adding an H3O+and 3H+ (creating 3 Al-OH2 groups), respectively, is too simplistic. Instead, a significant fraction of the Al3+ vacancies are compensated for by 4 H+ ions, resulting in 4 Al-OH2 groups per vacancy. This substitution is accompanied by the simultaneous deprotonation of a H3O+ ion present on the A site. The resultant H2O molecule is unnecessary for charge balance, accounting for the A-site deficiency often observed. The presence of Al3+ and A+ vacancies appears closely correlated based on NMR.

Solid-state 51V MAS NMR spectroscopy determines component concentration and crystal phase in co-crystallised mixtures of vanadium complexes

The oxo-peroxo-vanadium(V) and dioxo-vanadium(V) complexes of N,N-bis(2-pyridylmethyl)glycinate (bpg−), VO(O2)(bpg) (1) and V(O)2(bpg) (2) co-crystallize in variable ratios in an anhydrous and a dihydrate phase with the overall formulation 1x21−x·nH2O, n = 0 or 2, where 0 ≤ x ≤ 1 (alternatively V(O)2−x(O2)x(bpg)·nH2O, n = 0 or 2, where 0 ≤ x ≤ 1). The seven-coordinated complex 1 contains a η2-peroxide ligand (O22−) whereas the six coordinated 2 contains a monoatomic oxide ligand (O2−) in the same position. Characteristic chemical shift differences for the vanadium atoms of the two complexes mean that 51V solution state and MAS NMR spectroscopy can be used to determine the concentration of 1 and 2 in bulk samples. Significantly, however, 51V MAS NMR spectroscopy also reports on the identity of the crystal phase. This is possible because the isotropic 51V resonances are sensitive to intermolecular interactions specific to each crystal phase. The solid-state 51V MAS NMR spectroscopic data show that the different phases do not co-precipitate but the concentration of the solute (which can be either 1 or 2) can vary. Thus co-crystallised mixtures of 1 and 2 can be classed as a molecular mixture capable of forming continuous solid solutions.

Solid State 13C and 2H NMR Investigations of Paramagnetic [Ni(II)(acac)2L2] Complexes

Nine structurally related paramagnetic acetylacetonato nickel(II) complexes: [Ni(acac)2] and trans-[Ni(acac)2(X)2]nH/D2O, X = H2O, D2O, NH3, MeOH, PMePh2, PMe2Ph, or [dppe]1/2, n = 0 or 1, dppe = 1,2-bis(diphenylphosphino)ethane, as well as cis-[Ni(F6-acac)2(D2O)2], F6-acac = hexafluoroacetylonato, have been characterized by solid state (13)C MAS NMR spectroscopy. (2)H MAS NMR was used to probe the local hydrogen bonding network in [Ni(acac)2(D2O)2]D2O and cis-[Ni(F6-acac)2(D2O)2]. The complexes serve to benchmark the paramagnetic shift, which can be associated with the resonances of atoms of the coordinated ligands. The methine (CH) and methyl (CH3) have characteristic combinations of the isotropic shift (δ) and anisotropy parameters (d, η). The size of the anisotropy (d), which is the sum of the chemical shift anisotropy (CSA) and the paramagnetic electron-nuclei dipolar coupling, is much more descriptive than the isotropic shift. Moreover, the CSA is found to constitute up to one-third of the total anisotropy and should be taken into consideration when (13)C anisotropies are used for structure determination of paramagnetic materials. The (13)C MAS NMR spectra of trans-[Ni(acac)2(PMe2Ph)2], trans-[Ni(acac)2(PMePh2)2], and the noncrystallographically characterized trans-[Ni(acac)2(dppe)]n were assigned using these correlations. The complexes with L = H2O, D2O, NH3, and MeOH can be prepared by a series of solid state desorption and sorption reactions. Crystal structures for trans-[Ni(acac)2(NH3)2] and trans-[Ni(acac)2(PMePh2)2] are reported.

A solid state NMR study of Layered double hydroxides intercalated with para-amino salicylate, a tuberculosis drug

Para-amino salicylate (PAS), a tuberculosis drug, was intercalated in three different layered double hydroxides (MgAl, ZnAl, and CaAl-LDH) and the samples were studied by multi-nuclear ((1)H, (13)C, and (27)Al) solid state NMR (SSNMR) spectroscopy in combination with powder X-ray diffraction (PXRD), elemental analysis and IR-spectroscopy to gain insight into the bulk and atomic level structure of these LDHs especially with a view to the purity of the LDH-PAS materials and the concentration of impurities. The intercalations of PAS in MgAl-, ZnAl-, and CaAl-LDHs were confirmed by (13)C SSNMR and PXRD. Moreover, (13)C MAS NMR and infrared spectroscopy show that PAS did not decompose during synthesis. Large amounts (20-41%) of amorphous aluminum impurities were detected in the structure using (27)Al single pulse and 3QMAS NMR spectra, which in combination with (1)H single and double quantum experiments also showed that the M(II):Al ratio was higher than predicted from the bulk metal composition of MgAl-PAS and ZnAl-PAS. Moreover, the first high-resolution (1)H SSNMR spectra of a CaAl LDH is reported and assigned using (1)H single and double quantum experiments in combination with (27)Al{(1)H} HETCOR.

Competitive reactions during synthesis of zinc aluminum layered double hydroxides by thermal hydrolysis of urea

Homogeneous precipitation by thermal hydrolysis of urea (“The urea method”) is preferred for the preparation of pure and highly crystalline layered double hydroxides (LDHs). However, our recent study revealed large concentrations of amorphous aluminum hydroxide (AOH) in several zinc(II) aluminum(III) LDHs (ZnAl-LDHs) prepared by this method. The origin of this AOH, the nature of an elusive zinc-rich phase, and whether phase pure LDHs can be obtained by the urea method remained unanswered [J. Phys. Chem. C., 2015, 119, 27695–27707]. Therefore, a series of ZnAl-LDHs were prepared by the urea method at four different reaction times (7, 12, 16, and 24 h) and characterized by bulk (PXRD, TEM, and elemental analysis) and local techniques (27Al SSNMR, FT-IR, and Raman spectroscopies) in combination with a time-resolved synchrotron PXRD study of the reaction mixture. The products obtained are a mixture of hydroxylated phases: an ill-defined aluminum hydroxide with similarities to gibbsite precipitates at 7 h, which is followed by a competitive formation of a crystalline ZnAl-LDH (≈8 h) and a poorly crystalline hydrozincite during the later stages of the reaction. Their relative concentrations vary depending on the synthesis conditions, e.g., reaction time, urea/metal ratio, metal source, and hydrothermal treatment post synthesis, with less than half of the total Al3+ content incorporated in ZnAl-LDHs according to solid state 27Al NMR. Thus, the preparation of highly crystalline and pure LDHs remains a challenge. Furthermore, these impurities will have a significant impact on application studies using LDHs prepared by the urea method.

Structural Investigation of Zn(II) Insertion in Bayerite, an Aluminum Hydroxide

Bayerite was treated under hydrothermal conditions (120, 130, 140, and 150 °C) to prepare a series of layered double hydroxides (LDHs) with an ideal composition of ZnAl4(OH)12(SO4)0.5·nH2O (ZnAl4-LDHs). These products were investigated by both bulk techniques (powder X-ray diffraction (PXRD), transmission electron microscopy, and elemental analysis) and atomic-level techniques ((1)H and (27)Al solid-state NMR, IR, and Raman spectroscopy) to gain a detailed insight into the structure of ZnAl4-LDHs and sample composition. Four structural models (one stoichiometric and three different defect models) were investigated by Rietveld refinement of the PXRD data. These were assessed using the information obtained from other characterization techniques, which favored the ideal (nondefect) structural model for ZnAl4-LDH, as, for example, (27)Al magic-angle spinning NMR showed that excess Al was present as amorphous bayerite (Al(OH)3) and pseudoboehmite (AlOOH). Moreover, no evidence of cation mixing, that is, partial substitution of Zn(II) onto any of four Al sites, was observed. Altogether this study highlights the challenges involved to synthesize pure ZnAl4-LDHs and the necessity to use complementary techniques such as PXRD, elemental analysis, and solid-state NMR for the characterization of the local and extended structure of ZnAl4-LDHs.

Thermodynamics and crystal chemistry of rhomboclase, (H5O2)Fe(SO4)2·2H2O, and the phase (H3O)Fe(SO4)2 and implications for acid mine drainage

Abstract The system Fe2O3-SO3-H2O contains the most important minerals of acid mine drainage (AMD), iron oxides, and iron sulfates. For geochemical modeling of the AMD systems, reliable thermodynamic data for these phases are needed. In this work, we have determined thermodynamic data for the most acidic sulfates rhomboclase [(H5O2)Fe(SO4)2⋅2H2O or (H3O)Fe(SO4)2⋅3H2O] and the phase (H3O)Fe(SO4)2. The actual compositions of the studied phases are (H3O)1.34Fe(SO4)2.17(H2O)3.06 (molecular mass of 344.919 g/mol) and (H3O)1.34Fe(SO4)2.17 (289.792 g/mol). Structural details for both phases were refined from synchrotron powder X-ray diffraction data. Enthalpies of formation were determined by acid-solution calorimetry. Low-temperature heat capacity was measured for rhomboclase by relaxation calorimetry but a critical analysis of entropies for several oxysalts showed that these data are too high. Entropies for both phases were estimated from a Kopp-rule algorithm. The enthalpies of formation and entropies were combined with previously published temperature-relative humidity brackets to generate an internally consistent thermodynamic data set for rhomboclase: ΔfH° = –3202.03 kJ/mol, S° = 378.7 J/(mol⋅K); and for (H3O)1.34Fe(SO4)2.17 : ΔfH° = –2276.25 kJ/mol, S° = 253.2 J/(mol⋅K). Solubility experiments at room temperature and at T = 4 °C agree well with previously reported data in the system Fe2O3-SO3-H2O. An inspection of the extended Pitzer model for Fe3+-SO4 solutions shows that this model reproduces the general topology of the phase diagram, but the position of the calculated solubility curves deviates substantially from the experimental data. Solid-state 2H MAS NMR spectra on deuterated rhomboclase show two isotropic chemical shifts δiso(2H) = of 8 ± 1 and 228 ± 1 ppm, assigned to D5O2+