27Al Solid-state NMR Research Articles

Kaolins of high iron-content as photocatalysts: Challenges of acidic surface modifications and mechanistic insights

Surface modifications of natural kaolins (Felsőpetény, Hungary) with iron-content up to 9% (m/m) were carried out by varying acid concentration (5, 8 and 11 M HCl) and treatment time (1, 3 and 6 h) in order to evaluate the influence of treatment conditions on photochemical activity, amount of low coordinate Al defect sites, porosity, and acid/base character of the surface. Nitrogen adsorption and temperature-programmed desorption measurements showed that the acid treatment slightly reduced the pore volume and the surface area, while the average pore diameter and the number of acidic centers were increased. Solid-state 27Al NMR spectroscopy demonstrated the presence of mechanistically relevant Al defect sites. The distribution, concentration, and particle size of the iron oxide co-minerals were found to be influential to photochemical activity. Greater than 90% photochemical degradation efficiency of oxalic acid can be achieved by acid-treated samples upon 3 h exposure to 8 M HCl solution with good reproducibility.

Rates of levoglucosanol hydrogenolysis over Brønsted and Lewis acid sites on platinum silica-alumina catalysts synthesized by atomic layer deposition

Nanoscale coatings of AlOx were deposited onto SiO2 using atomic layer deposition (ALD) to synthesize amorphous silica-alumina (SiAl) catalysts, and these catalysts were investigated for levoglucosanol (Lgol) hydrogenolysis. With decreasing Al2O3 loading, the ratio of Brønsted to Lewis acid sites, measured by NH3-TPD and pyridine-FTIR, systematically increases, while the Al coordination, measured by solid state 27Al NMR, decreases. These structural changes correspond to an increasing mass-normalized rate of Lgol hydrogenolysis. We model the mass-normalized reaction rate as the sum of independent contributions from Brønsted and Lewis sites, showing that Brønsted acid sites on ALD-AlOx/SiO2 catalysts have a 6-times higher turnover frequency (TOF) than Lewis acid sites on these catalysts. Additionally, Lewis acid sites on ALD-AlOx/SiO2 catalysts (potentially related to Al(V) species) have a 4-times higher TOF than Lewis acid sites on bulk γ-Al2O3. The overall mass-normalized reactivity of ALD-AlOx/SiO2 catalysts is due to Lewis acid sites at the highest Al2O3 loading, while it is predominantly due to Brønsted acid sites at the lowest Al2O3 loadings. This work provides a new approach to synthesize amorphous SiAls with tunable Brønsted/Lewis acid site ratio and reveals differences in the reactivity of Brønsted and Lewis acid sites on these materials.

Exploring Structural Disorders in Aluminum-Containing Metal–Organic Frameworks: Comparison of Solid-State 27Al NMR Powder Spectra to DFT Calculations on Bulk Periodic Structures

We use solid-state 27Al NMR spectroscopy to test four aluminum-based metal–organic framework (MOF) materials—ICR-2, ICR-4, ICR-6, and ICR-7—against their structural models obtained from electron and X-ray diffraction. The lineshape analysis of the 27Al NMR spectra reveals varying degrees of disorders in the Al coordination environment, depending on the sample, which we were able to parameterize in terms of the extended Czjzek model (ECM) of the electric field gradient (EFG) distribution. The model’s degree-of-disorder parameter ε was found to correlate inversely with the crystallite size of the MOFs. This suggests that the EFG distribution is related to a high surface-to-volume ratio of the particles and can be ascribed to the deviation of molecular arrangements near the surface from the bulk crystalline order. The ECM’s parameters for a fixed part of the EFG tensor, a quadrupolar coupling constant CQ,0 and asymmetry parameter η0, were compared to the EFG parameters calculated on fully periodic models using density functional theory (DFT). The DFT-calculated CQ was found to exceed CQ,0 by 25–60%, depending on the MOF. Although this result finds support in the literature, an explanation of such apparent spectral narrowing is still lacking.

Al/Ga-Doped Li7La3Zr2O12 Garnets as Li-Ion Solid-State Battery Electrolytes: Atomistic Insights into Local Coordination Environments and Their Influence on 17O, 27Al, and 71Ga NMR Spectra.

Li7La3Zr2O12 (LLZO) garnets are among the most promising solid electrolytes for next-generation all-solid-state Li-ion battery applications due to their high stabilities and ionic conductivities. To help determine the influence of different supervalent dopants on the crystal structure and site preferences, we combine solid-state 17O, 27Al, and 71Ga magic angle spinning (MAS) NMR spectroscopy and density-functional theory (DFT) calculations. DFT-based defect configuration analysis for the undoped and Al and/or Ga-doped LLZO variants uncovers an interplay between the local network of atoms and the observed NMR signals. Specifically, the two characteristic features observed in both 27Al and 71Ga NMR spectra result from both the deviations in the polyhedral coordination/site-symmetry within the 4-fold coordinated Li1/24d sites (rather than the doping of the other Li2/96h or La sites) and with the number of occupied adjacent Li2 sites that share oxygen atoms with these dopant sites. The sharp 27Al and 71Ga resonances arise from dopants located at a highly symmetric tetrahedral 24d site with four corner-sharing LiO4 neighbors, whereas the broader features originate from highly distorted dopant sites with fewer or no immediate LiO4 neighbors. A correlation between the size of the 27Al/71Ga quadrupolar coupling and the distortion of the doping sites (viz. XO4/XO5/XO6 with X = {Al/Ga}) is established. 17O MAS NMR spectra for these systems provide insights into the oxygen connectivity network: 17O signals originating from the dopant-coordinating oxygens are resolved and used for further characterization of the microenvironments at the dopant and other sites.

The influence of low amounts on in situ-polymerized bisphenol-diamine net in cement slurries prepared in seawater – structural analysis after long-term contact with in situ-generated mud-acid fracturing fluid

ABSTRACT Inadequate zonal isolation in oil wells can provoke safety and environmental problems. New cement slurries can avoid such problems. In this work, novel cement slurries were prepared in the presence of seawater with in situ-formed epoxy resin–ethylenediamine. The new slurries were tested in relation to their mechanical properties and characterized by FTIR, XRD, TG/DTG, porosimetry by N2 adsorption-desorption and solid-state 29Si and 27Al NMR. Some kinetic features of long-term contact of the slurries with in situ-generated mud-acid were performed to evaluate the performances of the new slurries in routine acidizing procedures in oil well industry. The experimental kinetic data were well fitted to order-variable exponential kinetic model and a factorial design of four variables (24). In these studies, it was possible to evaluate in detail how some important experimental factors act on the quantities of acid that interact with the pastes, as well as to verify how these factors affect the speed with which the mud-acid interacts with the slurries. The results strongly suggest that the interactions occurred at epoxy/ethylenediamine cement slurry/mud-acid interfaces are due to surface reactions with preservation of the chemical structures of the cement slurries, even after long-term contact with mud-acid. The new cement slurry present good features to be used in environmental-friendly procedures in oil well field.

Synthesis of aggregation-resistant MFI nanoparticles

Abstract Zeolites with particles of nm-scale size and restricted aggregation propensity were designed by decoration of zeolite surface with specific functional groups to address the problem of irreversible aggregation of respective nanoparticles. Highly crystalline MFI nano zeolites synthesized using hydrothermal method were functionalized with silanes of different nature (monodentate vs. polydentate, hydrophobic vs. hydrophilic) to investigate the influence of the type of surface functional groups on the stability of zeolite nanoparticles against aggregation in solution, accessibility of acid sites located in zeolite framework, and catalytic properties of resulting hybrid materials. The as-prepared materials were characterized by XRD, DLS, nitrogen adsorption, and adsorption of acetonitrile followed by FTIR, solid state 29Si and 27Al NMR, ICP-OES, TEM. Finally, we investigated the catalytic performance of designed materials in acylation of p-xylene to demonstrate that surface functionalization does not lead to deterioration of catalytic activity of the zeolite nanoparticles. Characterization data and catalytic results confirmed the improved stability of prepared nanoparticles with reasonable accessibility of acid sites. This finding can be used for preparation of aggregation-resistant zeolites dispersible in liquid-phase reaction mixtures in order to mimic homogeneous catalytic systems.

Effect of carbon coating on the thermal stability of nanocrystalline χ-Al2O3

χ-Al2O3 is a rarely studied low-temperature alumina phase that can be prepared in a finely dispersed form and attracts interest as a potential catalyst or catalyst support. Phase transformations of nanocrystalline χ-Al2O3 and χ-Al2O3@C core-shell materials in the temperature range of 650–1400 °C were studied by XRD and 27Al solid-state NMR. Deposition of the carbon shell was demonstrated to stabilize the oxide core size and extend the stability range of the low-temperature χ-Al2O3 phase to 1100 °C. Relatively high surface area of the oxide core consisting of χ-and κ-Al2O3, which was close to 80 m2/g after burning of the carbon shell, was preserved after the heat treatment at 1100 °C. Meanwhile, calcination of pure χ-Al2O3 at the same temperature yielded a sample consisting of α-and κ-Al2O3 with the surface are as low as 15 m2/g. Solid-state 27Al NMR with magic-angle spinning proved to be an efficient tool for investigation of χ- → κ- → α-Al2O3 phase transformations that can be used to obtain quantitative information on the alumina phase composition.

Palladium/Zeolite Low Temperature Passive NOx Adsorbers (PNA): Structure-Adsorption Property Relationships for Hydrothermally Aged PNA Materials

Zeolites with different framework structures (SSZ-13, ZSM-5, BEA) but similar Si/Al ratios (~ 12–15) and Pd loading (~ 1 wt%) were synthesized and evaluated as low-temperature passive NOx adsorbers (PNA). These materials exhibit high NOx adsorption efficiency with atomically dispersed Pd being the active adsorption site. Hydrothermal aging at 750 °C for 16 h in the presence of 10% water vapor in air resulted in the formation of PdO nanoparticles in all three samples as evidenced by high-energy XRD. Hydrothermal aging of the small-pore 1–3 wt% Pd/SSZ-13 (Si/Al = 6) materials, which contain ~ 100–90% atomically dispersed palladium ions, decreases its PNA performance only by ~ 10–20%, indicating agglomeration of only ~ 10–20% of atomically dispersed Pd into PdO. High-field solid state 27Al NMR studies on the fresh and aged samples reveal dealumination and significant changes in the distribution of Al (and thus, Bronsted acid) sites after hydrothermal aging. FTIR measurements with NO probe molecule and titration of Bronsted acid sites with nitrosyl (NO+) ions further corroborate the 27Al NMR data. Because framework aluminum atoms are the anchoring sites for atomically dispersed Pd ions, their elution from the framework causes the loss of active atomically dispersed Pd species. With the aid of HAADF-STEM imaging and synchrotron XRD studies, we further confirm and visualize the fate of these Pd species: they agglomerate into PdO nanoparticles on the external surface of zeolite. Consequently, these changes lead to the decrease in PNA performance of these materials after hydrothermal aging. The thus formed PdO agglomerates cannot be re-dispersed back to their ionic state due to the loss of framework Al T-sites and/or inherent stability of such large PdO particles. Our study demonstrates that, unlike in previous studies that found increased PNA performance upon HTA, high temperatures hydrothermal aging of PNA materials that contain atomically dispersed Pd initially results in a decrease in NOx storage efficiency due to the formation of PdO agglomerates. However, we also highlight the high hydrothermal stability of predominantly atomically dispersed 1–3 wt% Pd/SSZ-13 (Si/Al = 6), whose performance decreases only marginally after prolonged hydrothermal aging at 750 °C. This study shows that hydrothermally stable passive NOx materials can be prepared using small-pore SSZ-13 zeolite.

SYSU-6, A New 2-D Aluminophosphate Zeolite Layer Precursor.

Two-dimensional aluminophosphate is an important precursor of phosphate-based zeolites; a new Sun Yat-sen University No. 6 (SYSU-6) with |Hada|2[Al2(HPO4)(PO4)2] has been synthesized in the hydrothermal synthesis with organic structure-directing agent (OSDA) of N,N,3,5-tetramethyladamantan-1-amine. In this paper, SYSU-6 is characterized by single-crystal/powder X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray analysis, transmission electron microscopy, infrared and UV Raman spectroscopy, solid-state 27Al, 31P and 13C magic angle spinning (MAS) NMR spectra, and elemental analysis. The single-crystal X-ray diffraction structure indicates that SYSU-6 crystallized in the space group P21/n, with a = 8.4119(3), b = 36.9876(12), c = 12.5674(3), α = 90°, β = 108.6770(10)°, γ = 90°, V = 3704.3(2) Å3, Z = 4, R = 5.12%, for 8515 observed data (I > 2σ(I)). The structure has a new 4,12-ring layer framework topology linked by alternating AlO4 and PO4 tetrahedra. The organic molecules reside between the layers and are hydrogen-bonded to the inorganic framework. The new type of layer provides a greater opportunity to construct zeolite with novel topology.

Correlations of Crystal and Electronic Structure via NMR and X-ray Photoelectron Spectroscopies in the RETMAl2(RE = Sc, Y, La–Nd, Sm, Gd–Tm, Lu; TM = Ni, Pd, Pt) Series

A total of 35 intermetallic aluminum compounds have been synthesized from the elements via arc melting and characterized by powder X-ray diffraction. A total of 15 of them have been previously reported; however, detailed property investigations were missing. Compounds of the RETMAl2 (rare earth metal RE = Sc, Y, La-Nd, Sm, Gd-Tm, Lu) series with transition metal TM = Ni, Pd, and Pt crystallize isostructurally in the orthorhombic MgCuAl2 type structure ( Cmcm, oC16, fc2). Single-crystal X-ray diffraction investigations were conducted on YNiAl2, LaNiAl2, YPdAl2, ScPtAl2, and YPtAl2. The TM and Al atoms form a [TMAl2]δ- polyanion, the RE atoms reside in cavities within the framework. While the Sc, Y, La, and Lu compounds exhibit Pauli-paramagnetic behavior, consistent with all atoms being closed shell, the other RETMAl2 compounds show paramagnetism along with magnetic ordering at low temperatures, in line with an open-shell trivalent oxidation state for the RE atoms. Solid-state 27Al NMR investigations were carried out on the Pauli-paramagnetic samples, all showing only a single central transition, in line with one crystallographic site for the respective atoms. The observed quadrupolar coupling constants and electric-field-gradient asymmetry parameters were found to be in good agreement with the density-functional-theory-calculated values. Isotropic resonance shifts are dominated by the Fermi-contact interactions with s-conduction electron densities at the Fermi edge (Knight shifts). The bonding characteristics mirror the electronic density of states and crystal chemistry of the family of intermetallic compounds under consideration. Both the Knight shifts and quadrupolar coupling constants can be predicted based on element-specific increments.

Synthesis of NiAl layered double hydroxides intercalated with aliphatic dibasic anions and their exchange with heptamolybdate

Layered double hydroxides (LDH) of general formula Ni1-x Alx(OH)2(A)x/2. mH2O were obtained by a co-precipitation method at constant pH, using succinate (C4H4O42−), adipate (C6H8O42−) and sebacate (C10H16O42−) as compensation anions and two x values, 0.3 and 0.5. The formation of NiAl LDH was evidenced, although in the Al-richer samples carbon analysis and solid-state 27Al NMR spectroscopy indicated the segregation of a small amount of an aluminum-containing phase. To prepare NiAlMo LDH, the original compensation anions were replaced by heptamolybdate ion using an anion exchange method. The results showed almost complete anion exchange in most samples, with small amounts of residual organic material, evidencing easy removal of the aliphatic dibasic anions as compared with terephthalate. The samples were characterized by FTIR, XRD, ICP-OES, CHN and Raman and 27Al MAS solid-state NMR spectroscopies.

Highly efficient Cr/β zeolite catalyst for conversion of carbohydrates into 5‑hydroxymethylfurfural: Characterization and performance

In the presented work, a series of Cr/β zeolites were prepared and used as bifunctional catalysts with Brønsted and Lewis acidity in the conversion of biomass-derived carbohydrates to 5‑hydroxymethylfurfural (HMF). The physicochemical properties of all catalysts were characterized by ICP-AES, N2 physisorption, solid-state 27Al NMR, XRD, SEM-EDX, UV–Vis, XPS, Raman, NH3-TPD and Py-FTIR techniques. It was found that the catalysts with appropriate total acidity, ratio of Lewis to Brønsted acid sites and acid density appear to be beneficial for HMF formation. Under mild reaction conditions, a superior 72% HMF yield with 83% selectivity was achieved from glucose in the H2O/THF biphasic system by using the 0.4-Cr/β zeolite catalyst. Moreover, the results of the stability evaluation showed that the catalytic activity of the 0.4-Cr/β zeolite was not significantly lost after four consecutive cycles, and the framework remained intact. Therefore, the catalytic approach proposed in this study is believed to provide potential prospects for the production of HMF from biomass material refining in the future.

Tailoring selectivity in the liquid-phase isomerization of α-pinene on dealuminated ferrierite-type zeolites

Abstract Ferrierite-type zeolite (Zeolyst Intern.) with Si/Al = 10 was dealuminated sequentially using solutions of hydrochloric acid of different concentrations. The course of dealumination was followed by XRD, XPS, sorption of nitrogen, scanning electron microscopy, FTIR, and solid-state 29Si, 27Al and 1H MAS NMR. The dealuminated samples retained high crystallinity and revealed good sorption properties. The tetrahedrally coordinated aluminium in ferrierite was distributed between TB sites located in the main channels and TA ones siting in hidden positions. It was shown that dealumination removed aluminium from ferrierite in a non-random manner; up to 60% of aluminium was removed from TB and up to 19-% from TA sites. The amount and strength of Bronsted acid sites decreased with dealumination. The samples were tested in the liquid-phase isomerization α-pinene at 348–363 K; camphene and limonene were the main products. The initial reaction rates and turnover frequencies have been estimated and compared to other zeolite catalysts. The selectivity to camphene and limonene was in the range of 77–98.9 %.

The Structure and Stability of the Mg-HZSM-5/Al2O3 Catalyst for Synthesis of Olefins from DME: The Influence of Thermal and Hydrothermal Treatment

The studies were focused on changes in the structure, texture and acid properties of the Mg-HZSM-5/Al2O3catalyst upon its treatment at different temperatures (400 and 500 °С) and times (6 and 12 h). Physicochemical techniques such as X-ray fluorescence analysis, XPS, solid-state27Al NMR, nitrogen adsorption, DRIFT spectroscopy, ammonia TPD were used for sample characterization. The catalytic activitites of the initial and treated samples were studied regarding conversion of dimethyl ether (DME) to lower olefins. It was shown that the initial activity of the Mg-containing zeolite catalyst was dependent of the total acidity of the surface and independent of the initial strength distribution of the acid sites, and the catalyst stability depended on the morphology.

Exploiting in-situ solid-state NMR spectroscopy to probe the early stages of hydration of calcium aluminate cement

We report a high-field in-situ solid-state NMR study of the hydration of CaAl2O4 (the most important hydraulic phase in calcium aluminate cement), based on time-resolved measurements of solid-state 27Al NMR spectra during the early stages of the reaction. A variant of the CLASSIC NMR methodology, involving alternate recording of direct-excitation and MQMAS 27Al NMR spectra, was used to monitor the 27Al species present in both the solid and liquid phases as a function of time. Our results provide quantitative information on the changes in the relative amounts of 27Al sites with tetrahedral coordination (the anhydrous reactant phase) and octahedral coordination (the hydrated product phases) as a function of time, and reveal significantly different kinetic and mechanistic behaviour of the hydration reaction at the different temperatures (20 °C and 60 °C) studied.

Layered copper thioaluminate K2Cu3AlS4: Synthesis, crystal structure, characterization and solid-state 27Al and 39K NMR studies

The first example of alkali/transition metal thioaluminates K2Cu3AlS4 was synthesized and characterized. The compound crystallizes in the orthorhombic space group Pnna (no. 52), with unit cell parameters a = 5.471 (6) Å, b = 5.468 (6) Å and c = 13.111 (1) Å, within a pseudo-tetragonal lattice. Its crystal structure consists of 2D [Cu3AlS4]2- polyanionic anti-PbO type layers, in which Al and Cu atoms share the atomic positions, separated by K+ cations. The (Cu/Al) atomic positions are tetrahedrally coordinated by S atoms, whereas K atoms are surrounded by eight S atoms in a cubic fashion. The coordination environments of the Al and K cations were confirmed by solid-state 27Al and 39K NMR, respectively. The title compound is a semiconductor with a direct bandgap of 2.96 eV, and thermally stable up to 550 °C.

Ultrastabilization of Zeolite Y Transforms Brønsted-Brønsted Acid Pairs into Brønsted-Lewis Acid Pairs.

Two pairs of Brønsted acid sites have been identified in H,Na-Y zeolite, located in the supercage and in the sodalite cage, which upon ultrastabilization (dealumination) are transformed into pairs of Brønsted and Lewis acid sites. This mild postsynthetic modification step is an important process for converting this material into an active catalyst for large-scale commercial reactions. Pairing structures and their transformations have been investigated using 1 H double-quantum NMR spectroscopy experiments for dehydrated zeolite, H,Na-Y, and its ultrastabilized form, H,Na-USY. This approach enables the detection of pairs of Brønsted and Lewis acid sites with unprecedented 1 H resolution and distinguishing them from isolated acid sites. The dealumination is also detected by static 27 Al solid-state NMR experiments.

CH4 oxidation to oxygenates with N2O over iron-containing Y zeolites: Effect of preparation

Abstract Developing effective iron-incorporated zeolites and determining their active centers for the direct oxidation of CH4 to oxygenates have remained challenging topics so far. In this paper, we successfully prepare the highly-dispersed iron supported Y zeolites by a facile solid-state ion-exchange method with ferrocene, which was conducted under water-free conditions followed by a series of calcination. Moreover, extra-framework dinuclear Fe2 + complexes are identified as so-called active α-iron sites on zeolites. ICP-OES, N2 adsorption–desorption test, X-ray diffraction, solid-state 27Al NMR, N2O titration, TEM, EPR and 57Fe Mossbauer spectra were carried out to characterize properties of sample structure, acid sites, as well as the supported iron species. Characterization results indicate that high-temperature treatments have no effect on the typical structure feature of zeolites. Compared with catalysts synthesized by conventional impregnation, the samples prepared by the facile approach possess abundant dinuclear Fe2 + complexes but no Fe2O3 bulks and show weak acidity. These lead to a higher oxygenate selectivity in CH4 oxidation to oxygenates. Remarkably, the oxygenate (HCHO and CH3OH) selectivity of 6.5% at 375°C can be eventually obtained.

Hierarchical MTW zeolites in tetrahydropyranylation of alcohols: Comparison of bottom-up and top-down methods

Despite a widespread application of zeolites in catalysis, these microporous materials still suffer from a low accessibility of active sites located deep in crystals due to limited size of channels (< 1 nm). Here, we prepared a series of micro-mesoporous zeolites with MTW topology and similar Si/Al molar ratio (50). Two topical approaches were applied here: i. bottom-up synthesis with a surfactant-SDA leading to so-called nanosponge material; ii. top-down desilication of bulk crystals by NaOH solution in the presence of tetraalkylammonium cations (TAA+). Prepared materials were characterized by powder X-ray diffraction, sorption of nitrogen, adsorption of pyridine and di-tert-butyl-pyridine followed by FTIR, solid state 27Al NMR, ICP-OES, SEM, TEM, and STEM-EDS. It was found that the desilication of MTW in the presence of TAA+ proceeds in different pathway compared to well-studied MFI. This variance was attributed to the different dimensionality of their channel systems (1D vs. 3D). Materials were tested in tetrahydropyranylation of alcohols differing in a chain length and degree of branching. The nanosponge MTW shows generally very good performance. Nevertheless, in cases of the branched alcohols MTW materials desilicated in the presence of TAA+ exhibited comparable or even higher activity than nanosponge based on TOF values. Therefore, desilicated MTW presents cheaper and equally or more effective catalyst compared to nanosponge material with the same topology.

Critical role of (100) facets on γ-Al2O3 for ethanol dehydration: Combined efforts of morphology-controlled synthesis and TEM study

In this work, the effect of crystal facets on the catalytic behavior of γ-Al2O3 was investigated by X-ray diffraction, transmission electron microscopy, temperature-programmed desorption of ethanol, solid-state 27Al NMR, infrared spectroscopy, and ethanol dehydration reaction. A series of platelet γ-Al2O3 were synthesized, in which the relative ratio of (100) facets had been systematically increased. Ethylene formation increased with increasing (100) facets, clearly demonstrating the critical role of these facets as active sites for ethanol dehydration on γ-Al2O3. This systematic approach is helpful for a better understanding of facet-dependent catalytic properties of γ-Al2O3 that arise from the interaction between the supported metal and the crystal facets.