1H MAS NMR Research Articles

Aqueous and non-aqueous Li+/H+ ion exchange in Li0.44La0.52TiO3 perovskite

The topotactic Li+/H+ exchange in Li0.44La0.52TiO3 has been studied in different aqueous and non-aqueous media with different acidity. With this purpose, aqueous hydrochloric and nitric acid solutions and benzoic acid/ethanol solution were employed. The pristine and exchanged samples have been characterized by 1H and 7Li MAS-NMR, TGA and XRD techniques. Aqueous hydrochloric and nitric acid solutions produce the powders degradation and the subsequent formation of Li2TiO3 and La2Ti2O7 phases. A detailed analysis of the 1H MAS NMR spectra of exchanged samples indicate that Li+/H+ exchange reaction in pure water produced formation of LiOH at the particles surface, band at 0 ppm, that could subsequently carbonated after exposition in air. Furthermore the presence of other OH signals at 8, 6 and 2ppm has been related to differences on octahedral Ti-O distances, produced by La/vacancy ordering in alternating planes of perovskites. In samples immersed into benzoic acid/ethanol solution results are similar, however the amount of LiOH species in particle surface is considerable lower. The exchange degree improved when increasing exchange temperature. The mechanical grinding of powders decreases the particle size improving exchange reactions; however, grinding treatments eliminated specific NMR bands of perovskite. In ground materials new 1H NMR bands at 6 and 4ppm were ascribed to an amorphous phase.

NMR and FTIR studies of clustering of water molecules: From low-temperature matrices to nano-structured materials used in innovative medicine

H-bond clustering of water molecules confined in an argon matrix and in nano-structured calcium hydroxyapatites (Ca10(PO4)6(OH)2, CaHA) was studied by means of FTIR and 1H MAS NMR spectroscopy. FTIR spectra of water in matrix were measured from T=9K to 40K stepping by 2K with sequent 2D correlation analysis (2DCOR) carried out in the range of O–H stretching vibrations 3000–3800cm−1. The peaks of monomer, dimer and the H-bond clusters from trimer to hexamer as well as the surface (dangling) modes were resolved. Some peaks in 2DCOR spectra drop out from the water clustering scheme. They were related to the enhancement of vibration–rotation motion due to increasing temperature and volume of the cavities in argon matrix. Water clusters were revealed in nano-CaHAs in FTIR spectra for the first time. Beside the sharp peak at 3570cm−1 of the stretching vibration of O–H− ion in the bulk, the bands of water dimer, tetramer, hexamer and several surface O–H and P–O–H modes were resolved at 2900–3900cm−1. They were assigned comparing with the spectra of water in argon matrix and in hydrophobic solvent (CCl4). The cluster structure and the size depends on the hydration level. Hexamer- or even higher structures were observed only in the samples containing significant amount of adsorbed water. The amount of adsorbed water in the studied nano-CaHAs was probed using the intensity of the broad 1H signal at ca 5.1ppm normalized respect to OH– peak at 0ppm. Two narrow signals observed in 1H MAS NMR spectra at 0.8 and 1.3ppm are originated from the surface structured H2O molecules. The NMR data perfectly correlate with those obtained by FTIR spectroscopy.

Differentiation of the solid-state NMR signals of gel, zeolite phases and water species in geopolymer-zeolite composites

Geopolymer-zeolite composites were synthesized using a silica-rich industrial byproduct from chlorosilane production and sodium aluminate. Pastes were cured at 80°C and 80% RH, and subsequently dried in two different climates: at 23°C and 50% RH, and under vacuum. 1H MAS, 23Na MAS and 29Si MAS NMR and XRD measurements were performed after the drying procedures as well as after subsequent aging. Zeolite Na-A was found beside traces of faujasite-type zeolite and zeolite EMT as major crystalline phases in the cured composites; the fraction of geopolymeric gel in the reaction products was determined to be ~18% on a molar basis. Various water species could be distinguished using 1H MAS and 1H-29Si CP MAS NMR, applying rotor-synchronized echo experiments. The largest fraction of the pore water resides in the α-cages of the zeolite Na-A and in the geopolymeric gel; in addition, water exists in the β-cages of the zeolites and adsorbed at sodium ions. The water species in α-cages and in the pores of the geopolymeric gel exhibit slightly different chemical shifts of 4.7ppm and 4.9ppm, respectively, in the 1H MAS NMR spectra. Changes of the water content in the geopolymer pores of differently dried samples were observed and led to slightly varied chemical shifts in the 29Si MAS NMR spectra too. Measurements after more than 500 days revealed no significant aging effects of the composites, which confirm their chemical stability.

Valorization of biomass derivatives: Keggin heteropolyacids supported on titania as catalysts in the suitable synthesis of 2-phenoxyethyl-2-furoate

Titania modified by tungstophosphoric (TPA) and tungstosilicic (TSA) acid (30% w/w) were synthesized by the sol-gel method, using urea as low cost pore-forming agent and annealing at 500°C for 2h (TiO2/TPA and TiO2/TSA respectively). The obtained materials were characterized by 31P, 29Si, 1H nuclear magnetic resonance (31P, 29Si, 1H MAS-NMR), X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS), Raman spectroscopy (FT-Raman), acid strength by potentiometric titration with n-butylamine. Mesoporous materials were obtained, without important microporosity, as determined from N2 adsorption-desorption isotherms by the Brunauer-Emmett-Teller (BET) method. The XRD patterns of the modified samples exhibited only peaks of anatase phase. According 31P/29Si MAS-NMR and XPS studies, the main species present in the TiO2/TPA and TiO2/TSA samples are the Keggin anions by forming surface acid species, and probably surface complex between Keggin anion and titania as well. Solids were evaluated in the synthesis of 2-phenoxyethyl-2-furoate, by esterification of 2-furoic acid, a valuable product which can be obtained from biomass, with 2-phenoxyethanol, where TiO2/TPA sample showed the highest catalytic activity. The reaction temperature, molar ratio acid:alcohol and catalyst amount were studied as variables using TiO2/TPA sample. Results of catalytic activity and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT-FT-IR) measurements suggested that the reaction mechanism may involve a protonated intermediate of 2-furoic acid polarizing the CO bond of the acid, and leaving that can be easily attacked by 2-phenoxyethanol.

Role of 12-Ring Channels of Mordenite in DME Carbonylation Investigated by Solid-State NMR

In this work, we have studied the role of the 12-ring channel of mordenite in DME carbonylation. Upon modification with pyridine, the catalytic lifetime of mordenite in DME carbonylation was prolonged dramatically although its activity decreased slightly and the induction period increased. 1H MAS NMR and PFG-NMR results demonstrated that the acid quantity decreased after pyridine was adsorbed, and the self-diffusion coefficient of CH4 decreased with the increase in pyridine quantity in the 12-ring channel. 129Xe NMR results reveal that Xe atoms are preferentially adsorbed in an 8-ring side pocket at low pressure; xenon can enter the 8-ring side pocket at high temperature through the 12-ring channels only. From these results, the function of the 12-ring with respect to the reaction rate is explored and a transfer path of reactants/products is proposed.

Non-exchanging hydroxyl groups on the surface of cellulose fibrils: The role of interaction with water

The interaction of water with cellulose stages many unresolved questions. Here 2H MAS NMR and IR spectra recorded under carefully selected conditions in 1H-2H exchanged, and re-exchanged, cellulose samples are presented. It is shown here, by a quantitative and robust approach, that only two of the three available hydroxyl groups on the surface of cellulose fibrils are exchanging their hydrogen with the surrounding water molecules. This finding is additionally verified and explained by MD simulations which demonstrate that the 1HO(2) and 1HO(6) hydroxyl groups of the constituting glucose units act as hydrogen-bond donors to water, while the 1HO(3) groups behave exclusively as hydrogen-bond acceptors from water and donate hydrogen to their intra-chain neighbors O(5). We conclude that such a behavior makes the latter hydroxyl group unreactive to hydrogen exchange with water.

Mechanism of Proton Conduction in Perfluorinated Sulfonic Acid Polymers with High Ionic Exchange Capacity at Water-Freezing Temperature Region

Molecular-level understanding of dynamic behaviors of complex mass transports, especially proton and water- in polymer electrolytes is essential to design better membrane materials used for polymer electrolyte fuel cells (PEFCs). NMR is one of the suitable experimental techniques for analyzing such molecular dynamics, since both complex static structures and dynamical motions of molecules can be tracked by high-resolution NMR spectra and a pulsed field gradient NMR (PFG-NMR). This technique is widely used to understand proton dynamics however, it cannot clearly distinguish vehicle and grotthuss mechanisms. Recently, we have developed a combination method of 2H, 17O MAS NMR and 1H PFG NMR measurement to overcome the drawback. We applied the technique to an organic-inorganic composite membrane, Zr-SPP-SPES, which forms controlled configuration of proton functional groups [1,2]. In the paper we proposed enhanced grottuss mechanism in a water-freezing temperature region called "packed acid mechanism”. [6]In the present study we prepared perfluorinated sulfonic acid membranes with different equivalent weight (EW) 500, 600, 700 and 900, to understand relationship between value of ion exchange capacity (IEC = 1000/EW) and proton conduction mechanism in water freezing region. 17O labeled water was used in order to distinguish signals from non-exchangeable protons in the membranes, thus natural abundance 17O (0.04%) NMR signals of water in membrane cannot be detected. 1H and 17O MAS NMR measurements were carried out on Agilent NMR systems 400WB at 9.4 T. A 4 mm diameter air tight rotor was used for sample spin at 12 kHz. 1H and 17O 1D-NMR spectra were obtained by using a single pulse sequence. Sample temperature was calibrated by measuring the 79Br signal of KBr powder under the same measurement condisions as described elsewhere. [3] 1H PFG NMR measurements were carried out on JNM LA-400 at 9.4 T. NMR spectrometer equipped with maximum gradient strength of 12 T/m. The diffusion coefficient (D) was measured by using a pulsed-field-gradient stimulated-echo pulse sequence. [4, 5] 17O NMR spectrum in all the membrane show a sharp peak around 0ppm, from temperature 6 to 42℃. With lowering temperature from -20 to -45 ℃, the width of the spectra of all the membranes is drastically increased then finally the spectrum is vanished at T = -45 ℃. The behavior of 17O NMR in all samples were almost similar. Since the width of the NMR spectrum is inversely proportional to the movement of the observed nucleus, it indicates that absorbed water molecules are completely freezing.In contrast to the 17O NMR, solid 1H NMR spectra show a sharp peak even at the low temperature range, from -25 to -45 ℃. This result indicates that proton conductivity is still remaining, even while absorbed water molecules are freezing. 1H PFG-NMR also revealed that proton conduction is present at low temperature as well as the results of 1H NMR. The sample with highest IEC value ( IEC ~ 2 ) shows highest proton conductivity D = 7 x 10-7 (m2/s) at T = -25 ℃ and D = 0.5 x 10-7 (m2/s) at T = -60 ℃.These three results indicate that protons in the membranes with high IEC move independently like a "packed acid mechanism” in freezing temperature region. This conduction mechanism would happen efficiently because sulfonic acid is tightly-packed, which is a noteworthy property of the high IEC membranes.

Anatase nanoparticles boundaries resulting from titanium tetrachloride hydrolysis

An important factor that governs solar energy transformation into electrical or chemical energy, when using nanoparticles-based devices, is the spatial location of traps limiting electron transport. Evidences have been presented indicating that the electron diffusion is strongly influenced by nanoparticle boundaries, whose characteristics depend on the particles preparation. In the present work we have studied the role of hydrated excess proton structures in the formation of anatase nanoparticles boundaries in samples prepared by TiCl4 hydrolysis at low temperature and strong acidic conditions. The samples, constituted by anatase nanocrystals but, mainly, by amorphous titania, are studied by 1H-MAS NMR, FT-IR and HRTEM. The results indicate that hydrated excess proton species, generated by solvation of bridging hydroxyls protons of highly defective anatase and/or amorphous titania chains, favor the chains bonding to anatase nanoparticles and/or their condensation at the nanoparticles surface. The chains’ bonding facilitates the anatase particles aggregation producing oriented self-assemblings. The preparation conditions determine a strong condensation of amorphous titania chains and it should favor anatase transformation into rutile.

Metal oxide-triazole hybrids as heterogeneous or reaction-induced self-separating catalysts

The hybrid metal oxide-triazole materials [MoO3(trz)0.5] (1) and [W2O6(trz)] (2) (trz=1,2,4-triazole) have been hydrothermally synthesized and characterized by different techniques (TGA, SEM, 1H and 13C MAS NMR, FT-IR spectroscopy, and structure determination by Rietveld analysis of high resolution synchrotron powder XRD data). Materials 1 and 2 display distinct behaviors when applied as catalysts for oxidation reactions with alcohol, aldehyde, olefin and sulfide substrates, and are more effective with hydrogen peroxide as the oxidant than with tert-butylhydroperoxide. The MoVI hybrid 1 transforms into soluble active species during cis-cyclooctene epoxidation with H2O2. When consumption of H2O2 reaches completion, spontaneous reassembly of the 2-dimensional molybdenum oxide network of 1 takes place and the hybrid precipitates as a microcrystalline solid that can be easily separated and recycled. Reaction-induced self-separation behavior occurs with 1, H2O2 and other substrates such as methyl oleate and methylphenylsulfide. The WVI hybrid 2 behaves differently, preserving its structural features throughout the heterogeneous catalytic process.

2H NMR study of phase transition and hydrogen dynamics in hydrogen bonded organic antiferroelectric 55DMBP-H2ca

Hydrogen dynamics in one-dimensional hydrogen bonded organic antiferroelectric, co-crystal of 5,5’-dimethyl-2,2’-bipyridine (55DMBP) and chloranilic acid (H2ca), was investigated by use of 2H high resolution solid-state NMR. The two types of hydrogen bonds O-H …N and N+-H …O − in the antiferroelectric phase were clearly observed as the splitting of the side band of the 2H MAS NMR spectra of the acid-proton deuterated compound 55DMBP-D 2ca. The temperature dependence of the spin-lattice relaxation time was measured of the N+-H and O-H deuterons, respectively. It was suggested that the motion of the O-H deuteron is already in the antiferroelectric phase in the fast-motion regime in the NMR time scale, while that of the N+-H deuteron is a slow motion. In the high-temperature paraelectric phase, the both deuterons become equivalent and the fast motion of the deuterons in the NMR time scale is taking place with the activation energy of 7.9 kJ mol−1.

Novel Approach for the Characterization of Lewis Acidic Solid Catalysts by Solid‐State NMR Spectroscopy

AbstractAcetone‐2‐13C, trimethylphosphine oxide (TMPO), and ammonia were applied as probe molecules for solid‐state NMR investigations of Lewis acid sites on γ‐Al2O3, TiO2/anatase, and lithium‐exchanged zeolite Na‐Y. An indication for Lewis acid sites are the 31P MAS NMR signals at 48–51 ppm for TMPO‐loaded catalysts. The determination of the Lewis site density through the evaluation of these 31P MAS NMR signals, however, is difficult owing to the demanding TMPO adsorption procedure. Upon ammonia adsorption, the formation of ammonium ions at Brønsted acid sites and the coordination of ammonia at Lewis acid sites cause 1H MAS NMR signals at 6–7 ppm and −0.5–3 ppm, respectively. The integration of these signals results in the densities of Brønsted and Lewis acid sites. Future studies have to clarify whether the different 1H chemical shifts in the range of −0.5–3 ppm for ammonia coordinated at Lewis acid sites on solid catalysts are a hint at the different strengths of these surface sites.

Spectral and nonlinear optical studies of Propane-1, 3-diaminium nitrate

Propane-1, 3-diaminium nitrate [C3H12N2] (NO3)2 (PDAN), an hybrid organic–inorganic nonlinear optical material combining an acentric octupolar moiety (nitrate) with a centrosymmetric organic molecule (Propane-1, 3-diaminium) was prepared by slow evaporation technique at room temperature from its aqueous solution. Good quality and well-developed crystals of size 0.133mm×0.092mm×0.078mm were harvested from the mother solution. The grown single crystals were characterized for their spectral, thermal, linear and second order nonlinear optical properties. Solid-state 13C and 1H MAS-NMR spectroscopies are in agreement with the X-ray structure. The decomposition of the title compound is confirmed by the thermogravimetric analysis (TGA). The UV–visible absorption spectrum, show that PDAN is suitable for frequency doubling applications in a wide spectral range in the visible and near IR. The NLO response of the crystal was evaluated using a SHG powder technique, indicating an effective quadratic nonlinear coefficient two times higher than that of KDP in spite of the low hyperpolarizability of the nitrate ion and of the centrosymmetric character of the diaminium derivative.

HR MAS 1H NMR and chemometrics as useful tool to assess the geographical origin of cocoa beans – Comparison with HR 1H NMR

Chocolate and cocoa-based products are among the goods with higher added value. A current trend of the cocoa market is to offer to the consumers high quality cocoa products, namely mono-origin cocoa. However, a reliable analytical method able to trace the geographical origin of cocoa is lacking. In this work we tested the capability of HR MAS 1H NMR combined with chemometrics to assess the geographical origins of 60 fermented and dried cocoa beans of 23 different cocoa producing countries from the three major crop-growing areas (Africa, Central/South America, Asia/Oceania). Metabolic profiling was determined by HR MAS 1H NMR directly on cocoa powder after the method optimization. The same samples were also subjected to extraction and analysis with HR 1H NMR.HR MAS 1H NMR, as 1H NMR analysis, allowed the simultaneous detection of amino acids, polyalcohols, organic acids, sugars, methylxanthines, catechins. Moreover, HR MAS allows the detection of lipids, not present in the aqueous extract utilized for 1H NMR. The data set obtained is therefore representative of all classes of cocoa compounds. Untargeted HR MAS 1H NMR and 1H NMR datasets were utilized as fingerprint of the samples and elaborated with multivariate statistical methods. A targeted quantitative approach of selected metabolites was possible only with HR 1H NMR data, because HR MAS 1H NMR does not give reliable quantitative results. All the approaches adopted showed a discrimination of the cocoa origins. HR MAS presents the advantages to obtain a very rapid picture of the samples, comprising both lipophilic and hydrophilic components, avoiding any sample manipulation.

Surface Selective 1H and 27Al MAS NMR Observations of Strontium Oxide Doped γ-Alumina

High-surface area γ-alumina is industrially used as a catalyst support. Catalytically active elements are doped onto the support, where they can bind to AlO4, AlO5 or AlO6 sites on the surface. Pretreating the surface with alkaline earth oxides can alter the availability of these surface sites, hence affecting the catalytic activity. The surface binding sites of strontium oxide (SrO) on γ-alumina were previously unknown. Direct 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) could not detect AlO5 sites at 9.4 T, so 1H–27Al cross-polarisation (CP) MAS NMR was used to preferentially select the surface environment signals. We directly observed the three surface environments on dehydrated γ-alumina as a function of SrO impregnation up to 4 wt% SrO. We found that Sr2+ preferentially binds to AlO5 and AlO6 surface sites. 1H MAS NMR revealed SrO impregnation causes a reduction in the terminal (−0.3 ppm) and bridging (2.2 ppm) hydroxyl environments, as well as the promotion of a new peak in between these sites, at 0.5 ppm. By using 1H–27Al CP/MAS NMR the relative proportions of surface sites on γ-alumina can be determined, allowing an optimal level of SrO doping that can saturate all the AlO5 sites. Importantly, this provides a method of subsequently depositing catalytically active elements on just the AlO4 or AlO6 sites, which can provide a different catalytic activity or stability compared to the AlO5 binding site.

Surface and interlayer base-characters in lepidocrocite titanate: The adsorption and intercalation of fatty acid

While layered double hydroxides (LDHs) with positively-charged sheets are well known as basic materials, layered metal oxides having negatively-charged sheets are not generally recognized so. In this article, the surface and interlayer base-characters of O2− sites in layered metal oxides have been demonstrated, taking lepidocrocite titanate K0.8Zn0.4Ti1.6O4 as an example. The low basicity (0.04mmol CO2/g) and low desorption temperature (50–300°C) shown by CO2− TPD suggests that O2− sites at the external surfaces is weakly basic, while those at the interlayer space are mostly inaccessible to CO2. The liquid-phase adsorption study, however, revealed the uptake as much as 37% by mass of the bulky palmitic acid (C16 acid). The accompanying expansion of the interlayer space by ~0.1nm was detected by PXRD and TEM. In an opposite manner to the external surfaces, the interlayer O2− sites can deprotonate palmitic acid, forming the salt (i.e., potassium palmitate) occluded between the sheets. Two types of basic sites are proposed based on ultrafast 1H MAS NMR and FTIR results. The interlayer basic sites in lepidocrocite titanate leads to an application of this material as a selective and stable two-dimensional (2D) basic catalyst, as demonstrated by the ketonization of palmitic acid into palmitone (C31 ketone). Tuning of the catalytic activity by varying the type of metal (Zn, Mg, and Li) substituting at TiIV sites was also illustrated.

Ion-exchange synthesis and improved Li insertion property of lithiated H2Ti12O25 as a negative electrode material for lithium-ion batteries

We successfully prepared the lithiated H2Ti12O25 sample by the H+/Li+ ion exchange synthetic technique in the molten LiNO3 at 270 °C using H2Ti12O25 as a starting compound. Chemical composition of the obtained lithiated H2Ti12O25 sample was determined to be H1.05Li0.35Ti12O25-δ having δ = 0.3 by ICP-AES and DTA-TG analyses. The H+/Li+ ion exchange was also confirmed by powder XRD, 1H-MAS NMR, and 7Li-MAS NMR measurements. Electrochemical Li insertion and extraction measurements revealed that the initial coulombic efficiency was improved from 88% in H2Ti12O25 to 93% in the lithiated H2Ti12O25 sample. In addition, superior capacity retention properties for the charge and discharge cycling performance and good charge rate capability of the present lithiated H2Ti12O25 were confirmed in the electrochemical measurements. Accordingly, the lithiated H2Ti12O25 is suggested to be one of the promising high-voltage and high-capacity oxide negative electrodes in advanced lithium-ion batteries.

Structure of С@Al2O by multinuclear solid-state NMR spectroscopy

The 27Al, 1H MAS NMR method is used to study initial nanosized metastable aluminum oxides of a pseudoboehmite series (γ- and δ-Al2O3) after being coated with graphene (С@Al2O3) and annealed in the air (С@Al2O3-Т). It is demonstrated that aluminum nanoparticles coated with graphene and annealed at high temperatures (to 750°C for γ and 1180°C for δ) preserve their phase composition but differ from initial oxides by a very low concentration of defects (ОН groups). After the annealing of the graphene coating the hydroxyl cover of oxides is reduced, however, the set of ОН groups differs greatly from that of the initial oxides. Only one type of terminal ОН groups with a ~0.2 ppm shift and one type of bridging μ2-ОН groups with a 1.8 ppm shift for γ-Al2O3 from OH-μ2-AlVAln and 2.1 ppm for δ-Al2O3 OH-μ2-AlIVAln are observed. The data obtained make it possible to characterize in detail the δ-Al2O3 pure phase.

Defects in structure as the sources of the surface charges of kaolinite

The sign and magnitude of the surface charge govern the particle interactions and hence determine the physical properties of dispersions. The present work is to evaluate the distortions, isomorphic and broken surface bond as the sources of the permanent or variable surface charges of kaolinite. The theoretical permanent charge density (σcc) was calculated by XRF. The experimental permanent charge density (σp) and variable charge density (σv) were obtained by the method of potentiometric titration. The degree of broken bonds (Db, b) of Al–O–Al and Si–O–Si at the side of kaolinite structure were characterized by the numbers and strength of surface hydrogen in 1H MAS NMR, absorbance of Al–OH or Si–OH in FTIR and edge fractures in FESEM. The surface area was measured by N2 adsorption–desorption. The defects such as basal z corrugation (∆z), tetrahedral rotation angles (θ) and octahedral flattening angles (ψ) were obtained by the XRD rietveld refinement. The negative σcc or σp of three samples KA, KB and KC are 0.012, 0.007 and 0.009C·m−2 or 0.24, 0.19 and 0.22C·m−2, respectively. Db, b of three samples are Db, b-KA>Db, b-KC>Db, b-KB. The σv depend on the degree of the broken bonds at edges of kaolinite and can be affected by pH significantly. The negative variable charge density at alkaline condition decreased with the increase of the degree of broken bonds at the side of kaolinite structures. Surface charges of kaolinite come from the defects such as isomorphic substitutions and broken bonds Al–O–Al and Si–O–Si. The distortions in structure e.g. ∆z, θ and ψ may not directly create the surface charges.

Hydroxylation and dealumination of a metakaolinite-rich brick under acid conditions, and their influences on metal adsorption: One- and two-dimensional (1H, 27Al, 23Na, 29Si) MAS NMR, and FTIR studies

Brick was treated at 90 °C with a 6 M solution of HCl at various reaction times (1–26 h), leading to hydroxylation and dealumination of brick metakaolinite. Alkali treatment of HCl-treated brick improved the adsorption performance of this material in the removal of divalent metals (Fe2+ and Pb2+). The effects of chemical and thermal treatments on the structure of the brick were examined using one- (1D) and two-dimensional (2D) NMR and FTIR spectroscopies. The applied techniques included: (1D) 1H, 23Na, 29Si and 27Al MAS NMR; and 27Al/1H D-HMQC and 1H/29Si CP-HETCOR NMR. Correlation NMR methods provided new insights into the complicated issue relative to the nature of generated 1H atoms and their connectivity/proximity with neighboring 27Al and 29Si atoms in hydroxylated metakaolinite. The contribution of Brönsted acid sites before and after their neutralization by NaOH on the metal-adsorption process was interpreted in terms of “active” negatively charged sites at the material surface. The metal-adsorption capacity of neutralized brick was found to be intimately related to its Na amount and/or number of ΞSiO−Na+ and ΞAlO−Na+ functions.

Si-deficient, OH-substituted, boron-bearing vesuvianite from Sakha-Yakutia, Russia: a combined single-crystal, 1H MAS-NMR and IR spectroscopic study

Single crystals of Si-deficient vesuvianite with significant degree of hydrogarnet-type (SiO 4 ) 4− –(O 4 H 4 ) 4− substitution occur as epitactic overgrowth on the surface of large wiluite crystals from the Wiluy River, Sakha–Yakutia, Russia. Electron-microprobe analysis revealed considerable Si-deficiency, Si ranging from 16.30 to 17.50 apfu . The crystal structure of the mineral has been refined in the P 4/ nnc space group, a = 15.5876(4), b = 11.8021(5) A to R 1 = 0.028 for 1533 unique observed reflections. The refinement of the site-occupancy factors confirmed significant vacancy at the Z (1) and Z (2) sites (27 % and 10 %, respectively), with associated increase of the Z (1)–O and Z (2)–O bond lengths to 1.697 and 1.655 A, respectively. The increased size of the Z (1) tetrahedra results in the compression of the X (1) polyhedra, while the expansion of the Z (2) tetrahedra is compensated by the compression of the X (4) polyhedra. As a result, the significant degree of the hydrogarnet-type substitutions does not have an essential influence upon the unit-cell parameters compared to the usual defect-free vesuvianite. Thermogravimetric (TGA) and differential scanning calorimetry (DSC) curves indicated two steps of the weight loss in the temperature ranges of 600–900°C (2.08 wt%) and 1014°C (2.22 wt%). The total weight loss is estimated as 4.30 %, which is in good agreement with the total content of OH estimated as 13.72 apfu . The 1 H solid state MAS-NMR demonstrates the presence of a strong additional line at 3.38 ppm compared to the usual vesuvianite, which is explained by the existence of the additional H(3) site. The infrared spectrum in the OH-stretching vibration region shows the additional (B′) absorption band at 3618 cm −1 typical for a hydrogarnet-type substitution.