Geopolymer setting is seen to be substantially accelerated by addition of calcium and the objective of this study was to determine the mechanism for this effect by examining metakaolin geopolymers ...with and without calcium. Solid‐state 27Al NMR tests were used to examine the dissolution extent both qualitatively and quantitatively. Solid‐state 29Si NMR tests were conducted to determine the amount and structure of each phase. Prior to the quantitative tests, chemical extractions were used to facilitate assignment of peaks in each spectrum. On addition of calcium, it was found that both the rate and the extent of metakaolin dissolution were enhanced. Accelerating dissolution increases the Al concentration in solution, thus reducing Si/Al available for geopolymer gel formation and further accelerating the gel formation to cause faster setting. Although C‐A‐S‐H was observed in the calcium mix, no evidence indicated that it is directly involved in setting.
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Metal–organic frameworks (MOFs) are a new type of porous materials with numerous current and potential applications in many areas including ion-exchange, catalysis, sensing, separation, molecular ...recognition, drug delivery and, in particular, gas storage. Solid-state NMR (SSNMR) has played a pivotal role in structural characterization and understanding of host–guest interactions in MOFs. This article provides an overview on application of SSNMR to MOF systems.
► We reviewed the applications of solid-state NMR to metal–organic frameworks (MOFs). ► NMR of metal ions: directly probing local metal environments in MOFs. ► 129Xe NMR: establishing porosity and identifying adsorption sites in MOFs. ► 2H NMR: providing information on the dynamics of guest molecules and organic linkers. ► 1H and 13C SSNMR: characterizing organic linkers of the framework and guest species.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Structural characterization of metal–organic frameworks (MOFs) is crucial, since an understanding of the relationship between the macroscopic properties of these industrially relevant materials and ...their molecular-level structures allows for the development of new applications and improvements in current performance. In many MOFs, the incorporated metal centers dictate the short- and long-range structure and porosity of the material. Here we demonstrate that solid-state NMR (SSNMR) spectroscopy targeting NMR-active metal centers at natural abundance, in concert with ab initio density functional theory (DFT) calculations and X-ray diffraction (XRD), is a powerful tool for elucidating the molecular-level structure of MOFs. 91Zr SSNMR experiments on MIL-140A are paired with DFT calculations and geometry optimizations in order to detect inaccuracies in the reported powder XRD crystal structure. 115In and 139La SSNMR experiments on sets of related MOFs at two different magnetic fields illustrate the sensitivity of the 115In/139La electric field gradient tensors to subtle differences in coordination, bond length distribution, and ligand geometry about the metal center. 47/49Ti SSNMR experiments reflect the presence or absence of guest solvent in MIL-125(Ti), and when combined with DFT calculations, these SSNMR experiments permit the study of local hydroxyl group configurations within the MOF channels. 67Zn SSNMR experiments and DFT calculations are also used to explore the geometry near Zn within a set of four MOFs as well as local disordering caused by distributions of different linkers around the metal. SSNMR spectroscopy of metal centers offers an impressive addition to the arsenal of techniques for MOF characterization and is particularly useful in cases where XRD information may be ambiguous, incomplete, or unavailable.
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Metal–organic frameworks (MOFs) are an extremely important class of porous materials with many applications. The metal centers in many important MOFs are zinc cations. However, their Zn environments ...have not been characterized directly by 67Zn solid‐state NMR (SSNMR) spectroscopy. This is because 67Zn (I=5/2) is unreceptive with many unfavorable NMR characteristics, leading to very low sensitivity. In this work, we report, for the first time, a 67Zn natural abundance SSNMR spectroscopic study of several representative zeolitic imidazolate frameworks (ZIFs) and MOFs at an ultrahigh magnetic field of 21.1 T. Our work demonstrates that 67Zn magic‐angle spinning (MAS) NMR spectra are highly sensitive to the local Zn environment and can differentiate non‐equivalent Zn sites. The 67Zn NMR parameters can be predicted by theoretical calculations. Through the study of MOF‐5 desolvation, we show that with the aid of computational modeling, 67Zn NMR spectroscopy can provide valuable structural information on the MOF systems with structures that are not well described. Using ZIF‐8 as an example, we further demonstrate that 67Zn NMR spectroscopy is highly sensitive to the guest molecules present inside the cavities. Our work also shows that a combination of 67Zn NMR data and molecular dynamics simulation can reveal detailed information on the distribution and the dynamics of the guest species. The present work establishes 67Zn SSNMR spectroscopy as a new tool complementary to X‐ray diffraction for solving outstanding structural problems and for determining the structures of many new MOFs yet to come.
Natural‐abundance 67Zn solid‐state magic‐angle spinning (MAS) and static NMR spectra of several representative metal–organic frameworks were acquired at an ultrahigh magnetic field strength of 21.1 T. The 67Zn spectra are sensitive to the local environment of the Zn atom and can differentiate non‐equivalent Zn sites (see figure). A combination of 67Zn NMR data and molecular dynamics simulation provides information on the distribution and dynamics of guest species in the framework.
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Sodium silicate‐activated slag‐fly ash binders (SFB) and slag‐metakaolin binders (SMKB) are room‐temperature hardening binders that have excellent mechanical properties and a significantly lower ...carbon footprint than ordinary Portland cement (OPC). The aim of this study was to use nuclear magnetic resonance (NMR) spectroscopy to study the nanostructure of poorly ordered phases in SFB by varying slag/fly ash ratio, curing time, and curing temperature. Fly ash was completely substituted with metakaolin and the effect of this substitution on the poorly ordered phases was studied. It was observed that the proportion of geopolymer was generally higher in SMKB when compared to SFB. Although C–N–A–S–H and geopolymer coexisted in SFB and SMKB, C–N–A–S–H was the major product phase formed. The mean chain length (MCL) and the structure of C–N–A–S–H gel were estimated as a function of time, temperature, and slag/fly ash ratio. The MCL was found to have a negative correlation with slag/fly ash ratio and Ca/(Si+Al) ratio, but positive correlation with curing temperature. The average Si/Al atom ratios for geopolymers were also estimated. Lastly, the increased proportion of five‐coordinated aluminum (Al(V)) in metakaolin resulted in the decreased unreacted metakaolin in the hardened binder but did not increase the geopolymer content.
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The nanostructural evolution during formation of geopolymers and its correlation with setting have not been well understood. In this study, penetration resistance and ultrasonic wave reflection tests ...were conducted to measure setting, and solid‐state 27Al NMR and liquid‐state 29Si NMR were used to examine nanostructural changes in a metakaolin geopolymer as a function of time. Aluminum was released rapidly during the first 10 hour after mixing and immediately condensed with silicate species in solution to form larger sized aluminosilicate oligomers, which then condensed to form large structural units. Our evidence suggests these units form near metakaolin particle surface. Smaller sized silicate ions in the sol phase then attach to these units to form a gel with a more interconnected network structure. The initial stage of this attaching process was seen to be associated with set, which in this mixture occurred at 15 hour.
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Heteroatom framework-substituted zeolites are important materials that enable shape- and size-selective catalysis. The efficacy of these materials for desired catalytic reactions depends critically ...on dispersive interactions between the microporous void of the zeolite and the reactant molecules stabilized within it. Here, we develop a post-synthetic method to synthesize base and transition metal-substituted (Ti, Nb, Ta, and Sn) FAU with ultralow Al contents (Si : Al > 900), which is confirmed using X-ray diffraction, elemental analysis, and N 2 volumetric adsorption and 29 Si MAS-NMR, DRUV-vis, and IR spectroscopic characterization. Turnover rates for styrene (C 8 H 8 ) epoxidation within Ti-FAU are 2- and 7-fold greater than in analogous Ti-BEA and Ti–SiO 2 , respectively; yet, turnover rates of H 2 O 2 decomposition are similar for all three materials. Consequently, Ti-FAU gives greater rates and selectivities for this reaction than common Ti-bearing silicates. The mechanism for epoxidation remains constant for all Ti-silicates examined ( i.e. , Ti-FAU, Ti-BEA, and Ti–SiO 2 ). Therefore, the improved performance of Ti-FAU reflects differences in activation free energies for epoxidation that show an enthalpic preference in Ti-FAU relative to Ti–SiO 2 and an entropic gain relative to Ti-BEA. These results demonstrate the synthesis of M-FAU with ultralow Al contents are useful for catalytic reactions involving bulky reactants that can not occur in smaller pore zeotype materials (Ti-MFI), that exhibit deactivation due to changes in Ti-atom coordination ( e.g. , Ti–SiO 2 ), and that are prone to losses catalyzed by residual Brønsted acid sites ( e.g. , epoxidations, oxidations, and isomerization reactions).
Hydroxyapatite (HAP) is a cost-effective material to remove excess levels of fluoride from water. Historically, HAP has been considered a fluoride adsorbent in the environmental engineering ...community. This paper substantiates an uptake paradigm that has recently gained disparate support: assimilation of fluoride to bulk apatite lattice sites in addition to surface lattice sites. Pellets of HAP nanoparticles (NPs) were packed into a fixed-bed media filter to treat solutions containing 30 mg-F/L (1.58 mM) at pH 8, yielding an uptake of 15.97 ± 0.03 mg-F/g-HAP after 864 h. Solid-state 19F and 13C magic-angle spinning nuclear magnetic resonance spectroscopy demonstrated that all removed fluoride is apatitic. A transmission electron microscopy analysis of particle size distribution, morphology, and crystal habit resulted in the development of a model to quantify adsorption and total fluoride capacity. Low- and high-estimate median adsorption capacities were 2.40 and 6.90 mg-F/g-HAP, respectively. Discrepancies between experimental uptake and adsorption capacity indicate the range of F– that internalizes to satisfy conservation of mass. The model was developed to demonstrate that F– internalization in HAP NPs occurs under environmentally relevant conditions and as a tool to understand the extent of F– internalization in HAP NPs of any kind.
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