A zeolite crystallisation model confirmed by in situ observation.

Probing nucleation and growth of porous crystals at a molecular level remains a cumbersome experimental endeavour due to the complexity of the synthesis media involved. In particular, the study of zeolite formation is hindered as these typically form in multiphasic synthesis media, which restricts experimental access to crystallisation processes. Zeolite formation from single phasic hydrated silicate ionic liquids (HSiL) opens new possibilities. In this work, HSiL zeolite crystallisation is investigated in situ using a specifically designed conductivity measurement set-up yielding access to crystallisation kinetics. Based on the conductivity data and final yields, a crystallisation model explaining the results based on a surface growth mechanism was derived. The excellent agreement between experiment and theory indicates zeolite crystallisation from highly ionic media proceeds via a multi-step mechanism, involving an initial reversible surface condensation of a growth unit, followed by incorporation of that unit into the growing crystal. The first step is governed by the liquid phase concentration and surface energy, while the final step shows a correlation to the mobility of the cation involved.

NMR Crystallography Reveals Carbonate Induced Al-Ordering in ZnAl Layered Double Hydroxide.

Layered double hydroxides (LDHs) serve a score of applications in catalysis, drug delivery, and environmental remediation. Smarter crystallography, combining X-ray diffraction and NMR spectroscopy revealed how interplay between carbonate and pH determines the LDH structure and Al ordering in ZnAl LDH. Carbonate intercalated ZnAl LDHs were synthesized at different pH (pH 8.5, pH 10.0, pH 12.5) with a Zn/Al ratio of 2, without subsequent hydrothermal treatment to avoid extensive recrystallisation. In ideal configuration, all Al cations should be part of the LDH and be coordinated with 6 Zn atoms, but NMR revealed two different Al local environments were present in all samples in a ratio dependent on synthesis pH. NMR-crystallography, integrating NMR spectroscopy and X-ray diffraction, succeeded to identify them as Al residing in the highly ordered crystalline phase, next to Al in disordered material. With increasing synthesis pH, crystallinity increased, and the side phase fraction decreased. Using 1 H-13 C, 13 C-27 Al HETCOR NMR in combination with 27 Al MQMAS, 27 Al-DQ-SQ measurements and Rietveld refinement on high-resolution PXRD data, the extreme anion exchange selectivity of these LDHs for CO3 2- over HCO3 - was linked to strict Al and CO3 2- ordering in the crystalline LDH. Even upon equilibration of the LDH in pure NaHCO3 solutions, only CO3 2- was adsorbed by the LDH. This reveals the structure directing role of bivalent cations such as CO3 2- during crystallization of [M2+ 4 M3+ 2 (OH)2 ]2+ [A2- ]1 ⋅yH2 O LDH phases.

Long-Term Generation of Longitudinal Spin Order Controlled by Ammonia Ligation Enables Rapid SABRE Hyperpolarized 2D NMR.

Symmetry breaking of parahydrogen using iridium catalysts converts singlet spin order into observable hyperpolarization. In this contribution, iridium catalysts are designed to exhibit asymmetry in their hydrides, regulated by in situ generation of deuterated ammonia governed by ammonium buffers. The concentrations of ammonia (N) and pyridine (P) provide a handle to generate a variety of stereo-chemically asymmetric N-heterocyclic carbene iridium complexes, ligating either [3xP], [2xP;N], [P;2xN] or [3xN] in an octahedral SABRE type configuration. The non-equivalent hydride positions, in correspondence with the ammonium buffer solutions, enables to extend singlet-triplet or S⟩→T0⟩ mixing at high magnetic field and experimentally induce prolonged generation of non-equilibrium longitudinal two-spin order. This long-lasting magnetization can be exploited in hyperpolarized 2D-OPSY-COSY experiments providing direct structural information on the catalyst using a single contact with parahydrogen. Separately, field cycling revealed hyperpolarization properties in low-field conditions. Controlling catalyst stereochemistry by introducing small and deuterated ligands, such as deuterated ammonia, simplifies the spin-system. This is shown to unify experimental and theoretically derived field-sweep experiments for four-spin systems.

Long-Term Generation of Longitudinal Spin Order Controlled by Ammonia Ligation Enables Rapid SABRE Hyperpolarized 2D NMR.

The front cover artwork front cover artwork is provided by NMRCoRe, the Flemish NMR/X-Ray platform for Convergence Research and was designed by Ir. Ewoud Vaneeckhaute and Dr. Eric Breynaert. The image shows the reciprocity between parahydrogen, deuterated ammonia and iridium allowing for hyperpolarized 2D NMR via long-term availability of longitudinal spin order. Read the full text of the Article at 10.1002/cphc.202100079.

Water as a tuneable solvent: a perspective.

Water is the sustainable solvent of excellence, but its high polarity limits the solubility of non-polar compounds. Confinement of water in hydrophobic pores alters its hydrogen bonding structure and related properties such as dielectric constant and solvation power. Whether this special state of confined water can be rendered useful in chemical processes is hitherto underexplored. Confining water in hydrophobic nanopores could be a way to modulate water solvent properties, enabling the use of water as a tuneable solvent (WaTuSo). Applying pressure forces a heterogeneous mixture of poorly soluble molecules and water into hydrophobic nanopores of a host material where the lowered polarity of water enhances dissolution. Decompression after reaction causes expulsion of the solution from the pores and spontaneous demixing of reaction products because water returns to its normal polar state. Temporary dissolution enhancement during confinement is expected to be advantageous to chemical reaction and molecular storage. Nano-confined water offers a potential alternative to compression for storing CH4 and H2 gas, and opens new opportunities for green chemistry such as aqueous phase hydrogenation reactions which benefit from enhanced hydrogen solubility. Unprecedented control in time and space over H2O solvation properties in a WaTuSo system will enable new technologies with major scientific and societal impact.

Trace Level Detection and Quantification of Crystalline Silica in an Amorphous Silica Matrix with Natural Abundance 29Si NMR.

A protocol for the detection of trace amounts of quartz in amorphous silica gels by NMR spectroscopy was developed and tested on commercially available samples. Using natural abundance 29Si MAS NMR spectroscopy with CPMG acquisition and standard addition of crystalline quartz, quantitative detection of quartz concentrations down to 0.1% wt. was achieved. CPMG permitted to suppress the amorphous silica-derived signal, benefitting from the extremely long T2 relaxation time of quartz in 29Si and hence dramatically increasing the sensitivity. Dedicated post-processing exploiting the known CPMG spikelet frequencies allowed to probe the near-absence of quartz in commercial, 100% silica samples, enabling assessment of conformity of unknown samples to EU legislation (REACH).

Alumina: discriminative analysis using 3D correlation of solid-state NMR parameters.

Synthetic transition aluminas (χ, κ, θ, γ, δ, η, ρ) exhibit unique adsorptive and catalytic properties leading to numerous practical applications. Generated by thermal transformation of aluminium (oxy)hydroxides, structural differences between them arise from the variability of aluminium coordination numbers and degree of dehydroxylation. Unequivocal identification of these phases using X-ray diffraction has proven to be very difficult. Quadrupolar interactions of 27Al nuclei, highly sensitive to each site symmetry, render advanced 27Al solid-state NMR a unique spectroscopic tool to fingerprint and identify the different phases. In this paper, 27Al NMR spectroscopic data on alumina reported in literature are collected in a comprehensive library. Based on this dataset, a new 3D correlative method of NMR parameters is presented, enabling fingerprinting and identification of such phases. Providing a gold standard from crystalline samples, this approach demonstrates that any sort of crystalline, ill crystallized or amorphous, mixed periodic or aperiodically ordered transition alumina can now be assessed beyond the current limitations of characterisation. Adopting the presented approach as a standard characterisation of alumina samples will readily reveal NMR parameter-structure-property relations suitable to develop new or improved applications of alumina. Methodological guidance is provided to assist consistent implementation of this characterisation throughout the fields involved.

A Porous POSiSil Suited for Pressure-Driven Reversible Confinement of Solutions: PSS-2.

Polyoligosiloxysilicone (POSiSil; designated PSS-2) is a copolymer of double four-ring (D4R) cyclosilicate and dimethylsiloxane. It is synthesized by linking D4R units in tetrabutylammonium cyclosilicate crystals with dimethyldichlorosilane. The structure of PSS-2 was revealed using solid state NMR spectroscopy. In this 3D copolymer D4R units are connected systematically by short siloxane chains most likely composed of 2 to 3 dimethylsiloxane monomers. Controlling the conversion of the parent material allows for tuning the porosity of PSS-2. Residual parent material is embedded inside PSS-2 polymer and can be eliminated by calcination. This leaves nanovoids inside PSS-2, which is moderately hydrophobic. Pressure-driven intrusion-extrusion cycles of aqueous solution exhibit hysteresis, thus, PSS-2 can be used as reversible confinement for liquids with a capacity of around 1000 mm3 g-1 in porosity.

Solid-state NMR tools for the structural characterization of POSiSils: 29 Si sensitivity improvement with MC-CP and 2D 29 Si-29 Si DQ-SQ at natural abundance.

The 1 H-29 Si multiple-contact cross polarization (MC-CP) MAS NMR experiment is evaluated for the class of silicate-siloxane copolymers called POSiSils, that is, polyoligosiloxysilicones. It proves a reasonably good solution to tackle the challenge of recording quantitative 29 Si NMR data in experimental time much reduced compared with single pulse acquisition. In a second time, we report 29 Si-29 Si MC-CP double-quantum single-quantum (MC-CP-DQ-SQ) NMR experiment, which provides information about the through-space proximities between all silicon species despite the high degree of heterogeneity of this material. This work furthers the NMR tools for NMR crystallography for inorganic polymers, as it covers flexible polymers with different dimensionalities and long or heterogeneous relaxation characteristics at low 29 Si natural abundance.

Reversible room temperature ammonia gas absorption in pore water of microporous silica-alumina for sensing applications.

Microporous silica and silica-alumina powders exhibit a reversible uptake and release of ammonia gas from water vapor containing gas mixtures at ambient temperature, with capacities of 0.9 and 2.0 mmol g-1 in the presence of 100 ppm and 1000 ppm NH3, respectively. The ammonia trapping mechanism was revealed using a combination of direct excitation 1H MAS, 1H-1H EXSY and 1H DQ-SQ NMR spectroscopy, indicating that the major part of the captured ammonia is blended in the hydrogen bonded water network in the pores of the adsorbent. A small fraction is irreversibly bound as result of protonation and chemisorption. While common ammonia adsorbents need thermal regeneration, microporous silica-alumina can be regenerated by sweeping with dry gas at ambient temperature, desorbing the physisorbed fraction together with occluded water. As carbon dioxide does not interfere with the ammonia absorption process, this reversible absorption process of ammonia gas at ambient temperature is particularly attractive for sensor applications.

Absolute Quantification of Water in Microporous Solids with 1H Magic Angle Spinning NMR and Standard Addition.

Zeolites are microporous materials driving industrial scale adsorption, ion exchange, and catalytic processes. Their water content dramatically impacts their properties, but its quantification with Karl Fisher titration or thermal gravimetric analysis is problematic. When standard addition of water is combined with 1H magic angle spinning (MAS) NMR detection, absolute quantification of water in microporous materials becomes possible. The method was demonstrated on five different, commercially available zeolites.

Alternating Copolymer of Double Four Ring Silicate and Dimethyl Silicone Monomer-PSS-1.

A new copolymer consisting of double four ring (D4R) silicate units linked by dimethylsilicone monomer referred to as polyoligosiloxysilicone number one (PSS-1) was synthesized. The D4R building unit is provided by hexamethyleneimine cyclosilicate hydrate crystals, which were dehydrated and reacted with dichlorodimethylsilane. The local structure of D4R silicate units and dimethyl silicone monomers was revealed by multidimensional solid-state NMR, FTIR and modeling. On average, D4R silicate units have 6.8 silicone linkages. Evidence for preferential unidirectional growth and chain ordering within the PSS-1 copolymer was provided by STEM and TEM. The structure of PSS-1 copolymer consists of twisted columns of D4R silicate units with or without cross-linking. Both models are consistent with the spectroscopic, microscopic and physical properties. PSS-1 chains are predicted to be mechanically strong compared to silicones such as PDMS, yet more flexible than rigid silica materials such as zeolites.

Impact of Amino Acids on the Isomerization of the Aluminum Tridecamer Al13.

The stability of the Keggin polycation ε-Al13 is monitored by 27Al NMR and ferron colorimetric assay upon heating aluminum aqueous solutions containing different amino acids with overall positive, negative, or no charge at pH 4.2. A focus on the effect of the amino acids on the isomerization process from ε- to δ-Al13 is made, compared and discussed as a function of the type of organic additive. Amino acids such as glycine and ß-alanine, with only one functional group interacting relatively strongly with aluminum polycations, accelerate isomerization in a concentration-dependent manner. The effect of this class of amino acids is also found increasing with the pKa of their carboxylic acid moiety, from a low impact from proline up to more than a 15-fold increased rate from the stronger binders such as glycine or ß-alanine. Amino acids with relatively low C-terminal pKa, but bearing additional potential binding moieties such as free alcohol (hydroxyl group) moiety of serine or the amide of glutamine, speed the isomerization comparatively and even more than glycine or ß-alanine, glutamine leading to the fastest rates observed so far. With aspartic and glutamic acids, changes in aluminum speciation are faster and significant even at room temperature but rather related to the reorganization toward slow reacting complexed oligomers than to the Al13 isomerization process. The linear relation between the apparent rate constant of isomerization and the additive concentration points to a first-order process with respect to the additives. Most likely, the dominant process is an accelerated ε-Al13 dissociation, increasing the probability of δ isomer formation.

27Al-27Al double-quantum single-quantum MAS NMR: Applications to the structural characterization of microporous materials.

In this paper, we review and illustrate applications, reported in the literature or used in our group, of 27Al-27Al double-quantum single-quantum (DQ-SQ) MAS NMR experiments for the structural characterization of Al-containing microporous solids, namely zeolites, aluminophosphates and metal-organic frameworks. Information regarding the periodic frameworks or the localization of the various aluminum species in the materials are obtained from the analysis of the two-dimensional NMR spectra, which allows getting local structural details sometimes inaccessible from other characterization technique. An application of 27Al-27Al of the DQ-SQ experiment for the detection of aluminum pairing in zeolite is shown.

High-field 95 Mo and 183 W static and MAS NMR study of polyoxometalates.

The potential of high-field NMR to measure solid-state 95 Mo and 183 W NMR in polyoxometalates (POMs) is explored using some archetypical structures like Lindqvist, Keggin and Dawson as model compounds that are well characterized in solution. NMR spectra in static and under magic angle spinning (MAS) were obtained, and their analysis allowed extraction of the NMR parameters, including chemical shift anisotropy and quadrupolar coupling parameters. Despite the inherent difficulties of measurement in solid state of these low-gamma NMR nuclei, due mainly to the low spectral resolution and poor signal-to-noise ratio, the observed global trends compare well with the solution-state NMR data. This would open an avenue for application of solid-state NMR to POMs, especially when liquid-state NMR is not possible, e.g., for poorly soluble or unstable compounds in solution, and for giant molecules with slow tumbling motion. This is the case of Keplerate where we provide here the first NMR characterization of this class of POMs in the solid state. Copyright © 2017 John Wiley & Sons, Ltd.

In Situ Solid-State (13)C NMR Observation of Pore Mouth Catalysis in Etherification of ß-Citronellene with Ethanol on Zeolite Beta.

The reaction mechanism of etherification of ß-citronellene with ethanol in liquid phase over acid zeolite beta is revealed by in situ solid-state (13)C NMR spectroscopy. Comparison of (13)C Hahn-echo and (1)H-(13)C cross-polarization NMR characteristics is used to discriminate between molecules freely moving in liquid phase outside the zeolite and molecules adsorbed inside zeolite pores and in pore mouths. In the absence of ethanol, ß-citronellene molecules enter zeolite pores and react to isomers. In the presence of ethanol, the concentration of ß-citronellene inside zeolite pores is very low because of preferential adsorption of ethanol. The etherification reaction proceeds by adsorption of ß-citronellene molecule from the external liquid phase in a pore opening where it reacts with ethanol from inside the pore. By competitive adsorption, ethanol prevents the undesired side reaction of ß-citronellene isomerization inside zeolite pores. ß-citronellene etherification on zeolite beta is suppressed by bulky base molecules (2,4,6-collidine and 2,6-ditertiarybutylpyridine) that do not enter the zeolite pores confirming the involvement of easily accessible acid sites in pore openings. The use of in situ solid-state NMR to probe the transition from intracrystalline catalysis to pore mouth catalysis depending on reaction conditions is demonstrated for the first time. The study further highlights the potential of this NMR approach for investigations of adsorption of multicomponent mixtures in general.

Solving the Hydrogen and Lithium Substructure of Poly(triazine imide)/LiCl Using NMR Crystallography.

Poly(triazine imide) with incorporated lithium chloride has recently attracted substantial attention due to its photocatalytic activity for water splitting. However, an apparent H/Li disorder prevents the delineation of structure-property relationships, for example, with respect to band-gap tuning. Herein, we show that through a combination of one- and two-dimensional, multinuclear solid-state NMR spectroscopy, chemical modelling, automated electron diffraction tomography, and an analysis based on X-ray pair distribution functions, it is finally possible to resolve the H/Li substructure. In each cavity, one hydrogen atom is bound to a bridging nitrogen atom, while a second one protonates a triazine ring. The two lithium ions within each cavity are positioned between two nitrogen atoms of neighbouring triazine rings. The thereby induced local dipole moments cause slight buckling of the framework and lateral displacements of the Cl- ions at a coherence length below 2 nm. Nevertheless, the average structure conforms to space group P21 21 21 . In this way, we demonstrate that, in particular, the above-mentioned techniques allow for smart interplay in delineating the real structure of PTI/LiCl.

Zeolite Beta Formation from Clear Sols: Silicate Speciation, Particle Formation and Crystallization Monitored by Complementary Analysis Methods.

The formation of silicate nanoaggregates (NAs) at the very early stages of precursor sols and zeolite beta crystallization from silicate nanoparticles (NPs) are investigated in detail using a combination of different analysis methods, including liquid-state 29 Si, 27 Al, 14 N, and 1 H NMR spectroscopy, mass spectrometry (MS), small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), and transmission electron microscopy at cryogenic temperatures (cryo-TEM). Prior to hydrothermal treatment, silicate NAs are observed if the Si/OH ratio in the reaction mixture is greater than 1. Condensation of oligomers within the NAs then generates NPs. Aluminum doped into the synthesis mixtures is located exclusively in the NPs, and is found exclusively in a state that is fourfold connected to silicate, favoring their condensation and aggregation. These results are in agreement with general trends observed for other systems. Silicate NAs are essential intermediates for zeolite formation and are generated by the aggregation of hydrated oligomers, aluminate, and templating cations. Subsequent further intra-nanoaggregate silicate condensation results in the formation of NPs. 1 H and 14 N liquid NMR as well as diffusion ordered spectroscopy (DOSY) experiments provide evidence for weakly restricted rotational and translational mobility of the organic template within NAs as a consequence of specific silicate-template interactions. NAs thus appear as key species in clear sols, and their presence in the precursor sol favors silicate condensation and further crystallization, promoted either by increasing the Si/OH ratio or by heating.

A Breathing Zirconium Metal-Organic Framework with Reversible Loss of Crystallinity by Correlated Nanodomain Formation.

The isoreticular analogue of the metal-organic framework UiO-66(Zr), synthesized with the flexible trans-1,4-cyclohexanedicarboxylic acid as linker, shows a peculiar breathing behavior by reversibly losing long-range crystalline order upon evacuation. The underlying flexibility is attributed to a concerted conformational contraction of up to two thirds of the linkers, which breaks the local lattice symmetry. X-ray scattering data are described well by a nanodomain model in which differently oriented tetragonal-type distortions propagate over about 7-10 unit cells.

Two Compartmentalized Inner Receptors for the Tetramethylammonium Guest within a Keplerate-Type Capsule.

The host-guest interactions between the spherical porous Keplerate anion, [Mo132O372(CH3CO2)30(H2O)72](42-) (abbreviated {Mo132}) and the tetramethylammonium cation have been investigated extensively by one- and two-dimensional (EXSY, ROESY, and DOSY) and variable-temperature NMR. Evidence of two inner receptor sites specific for a NMe4(+) guest appears consistent with a quite striking compartmentalization phenomenon. ROESY NMR analyses showed that both sites exhibit a close spatial proximity with the hanging inner acetate groups, while a quantitative EXSY study revealed that these two sites are differentiated by their exchange rates. These NMR data support the hypothesis that these two inner sites could be delimited by the hanging inner acetate groups forming triangular (S1) or pentagonal (S2) hydrophobic pockets on the inner side of the capsule wall. Furthermore, the stability constants associated with the trapping process of the NMe4(+) guest on both the S1 and S2 sites have been determined, showing that the stability constant of the S1 sites decreases significantly as the concentration of the capsule increases gradually, while that of the S2 sites remains nearly unaffected. Such an observation has been interpreted as a result of the plugging process of the {Mo9O9} pores by the counterions NH4(+), which causes unfavorable electrostatic interactions for the NMe4(+) coordination on the proximal S1 site. Finally, the thermodynamic parameters of the NMe4(+) transfer from the solvated situation to the interior of the capsule were estimated from variable-temperature NMR experiments that provide the split of the global process into two successive events corresponding to the plugging and transfer across the inorganic shell.