Harnessing Nuclear Magnetic Resonance Spectroscopy to Decipher Structure and Dynamics of Clathrate Hydrates in Confinement: A Perspective.

This perspective outlines recent developments in the field of NMR spectroscopy, enabling new opportunities for in situ studies on bulk and confined clathrate hydrates. These hydrates are crystalline ice-like materials, built up from hydrogen-bonded water molecules, forming cages occluding non-polar gaseous guest molecules, including CH4, CO2 and even H2 and He gas. In nature, they are found in low-temperature and high-pressure conditions. Synthetic confined versions hold immense potential for energy storage and transportation, as well as for carbon capture and storage. Using previous studies, this report highlights static and magic angle spinning NMR hardware and strategies enabling the study of clathrate hydrate formation in situ, in bulk and in nano-confinement. The information obtained from such studies includes phase identification, dynamics, gas exchange processes, mechanistic studies and the molecular-level elucidation of the interactions between water, guest molecules and confining interfaces.

Performance of ferrite nanoparticles in inductive heating swing adsorption (IHSA): how tailoring material properties can circumvent the design limitations of a system.

Inductive heating swing adsorption (IHSA) using hybrid adsorbents incorporating a porous material and ferrite nanoparticles holds promise to be a performant, electrified alternative for conventional gas separation. Successful implementation of hybrid adsorbents in IHSA depends on achieving a maximal specific absorption rate (SAR) in the conditions and at the frequency of the induction setup. This paper outlines and demonstrates successful strategies for optimization of the particle composition, tailoring the coercivity and susceptibility of the ferrite particles to optimal performance in a given alternating magnetic field.

Surface modification of mesostructured cellular foam to enhance hydrogen storage in binary THF/H2 clathrate hydrate.

This study introduces solid-state tuning of a mesostructured cellular foam (MCF) to enhance hydrogen (H2) storage in clathrate hydrates. Grafting of promoter-like molecules (e.g., tetrahydrofuran) at the internal surface of the MCF resulted in a substantial improvement in the kinetics of formation of binary H2-THF clathrate hydrate. Identification of the confined hydrate as sII clathrate hydrate and enclathration of H2 in its small cages was performed using XRD and high-pressure 1H NMR spectroscopy respectively. Experimental findings show that modified MCF materials exhibit a ∼1.3 times higher H2 storage capacity as compared to non-modified MCF under the same conditions (7 MPa, 265 K, 100% pore volume saturation with a 5.56 mol% THF solution). The enhancement in H2 storage is attributed to the hydrophobicity originating from grafting organic molecules onto pristine MCF, thereby influencing water interactions and fostering an environment conducive to H2 enclathration. Gas uptake curves indicate an optimal tuning point for higher H2 storage, favoring a lower density of carbon per nm2. Furthermore, a direct correlation emerges between higher driving forces and increased H2 storage capacity, culminating at 0.52 wt% (46.77 mmoles of H2 per mole of H2O and 39.78% water-to-hydrate conversions) at 262 K for the modified MCF material with fewer carbons per nm2. Notably, the substantial H2 storage capacity achieved without energy-intensive processes underscores solid-state tuning's potential for H2 storage in the synthesized hydrates. This study evaluated two distinct kinetic models to describe hydrate growth in MCF. The multistage kinetic model showed better predictive capabilities for experimental data and maintained a low average absolute deviation. This research provides valuable insights into augmenting H2 storage capabilities and holds promising implications for future advancements.

Computational Protocol for the Spectral Assignment of NMR Resonances in Covalent Organic Frameworks.

Solid-state nuclear magnetic resonance spectroscopy is routinely used in the field of covalent organic frameworks to elucidate or confirm the structure of the synthesized samples and to understand dynamic phenomena. Typically this involves the interpretation and simulation of the spectra through the assumption of symmetry elements of the building units, hinging on the correct assignment of each line shape. To avoid misinterpretation resulting from library-based assignment without a theoretical basis incorporating the impact of the framework, this work proposes a first-principles computational protocol for the assignment of experimental spectra, which exploits the symmetry of the underlying building blocks for computational feasibility. In this way, this protocol accommodates the validation of previous experimental assignments and can serve to complement new NMR measurements.

Noncontact In Situ Multidiagnostic NMR/Dielectric Spectroscopy.

Introduction of a dielectric material in a nuclear magnetic resonance (NMR) probe head modifies the frequency response of the probe circuit, a phenomenon revealed by detuning of the probe. For NMR spectroscopy, this detuning is corrected for by tuning and matching the probe head prior to the NMR measurement. The magnitude of the probe detuning, "the dielectric shift", provides direct access to the dielectric properties of the sample, enabling NMR spectrometers to simultaneously perform both dielectric and NMR spectroscopy. By measuring sample dielectric permittivity as a function of frequency, dielectric permittivity spectroscopy can be performed using the new methodology. As a proof of concept, this was evaluated on methanol, ethanol, 1-propanol, 1-pentanol, and 1-octanol using a commercial cross-polarization magic angle spinning (CPMAS) NMR probe head. The results accurately match the literature data collected by standard dielectric spectroscopy techniques. Subsequently, the method was also applied to investigate the solvent-surface interactions of water confined in the micropores of an MFI-type, hydrophilic zeolite with a Si/Al ratio of 11.5. In the micropores, water adsorbs to BroÌ·nsted acid sites and defect sites, resulting in a drastically decreased dielectric permittivity of the nanoconfined water. Theoretical background for the new methodology is provided using an effective electric circuit model of a CPMAS probe head with a solenoid coil, describing the detuning resulting from the insertion of dielectric samples in the probe head.

Building a Cost-Efficient High-Pressure Cell for Online High-Field NMR and MRI Using Standard Static Probe Heads: An In Situ Demonstration on Clathrate Hydrate Formation.

High-pressure nuclear magnetic resonance (NMR) spectroscopy finds remarkable applications in catalysis, protein biochemistry and biophysics, analytical chemistry, material science, energy, and environmental control but requires expensive probe heads and/or sample cells. This contribution describes the design, construction, and testing of a low-cost 5 mm NMR tube suitable for high-pressure NMR measurements of up to 30 MPa. The sample cell comprises a standard, 5 mm single-crystal sapphire tube that has been fitted to a section of a relatively inexpensive polyether ether ketone (PEEK) HPLC column. PEEK HPLC tubing and connectors enable integration with a gas rig or a standard HPLC pump located outside the stray field of the magnet. The cell is compatible with any 5 mm static NMR probe head, exhibits almost zero background in NMR experiments, and is compatible with any liquid, gas, temperature, or pressure range encountered in HPLC experimentation. A specifically designed transport case enables the safe handling of the pressurized tube outside the probe head. The performance of the setup was evaluated using in situ high-field NMR spectroscopy and MRI performed during the formation of bulk and nanoconfined clathrate hydrates occluding methane, ethane, and hydrogen.

Eu3+ doped ZnAl layered double hydroxides as calibrationless, fluorescent sensors for carbonate.

The photoluminescence properties (PL) of Eu3+ hosted in the hydroxide layers of layered double hydroxides (LDHs) enables calibrationless quantification of anions in the interlayers. The concept is demonstrated during the nitrate-to-carbonate ion exchange in Zn2+/Al3+/Eu3+ LDHs and can be implemented as a remote optical sensor to detect intrusion of anions such as Cl- or CO32-.

Prediction of Cu Zeolite NH3-SCR Activity from Variable Temperature 1H NMR Spectroscopy.

Selective catalytic reduction (SCR) of NOx by ammonia is one of the dominant pollution abatement technologies for near-zero NOx emission diesel engines. A crucial step in the reduction of NOx to N2 with Cu zeolite NH3-SCR catalysts is the generation of a multi-electron donating active site, implying the permanent or transient dimerization of Cu ions. Cu atom mobility has been implicated by computational chemistry as a key factor in this process. This report demonstrates how variable temperature 1H NMR reveals the Cu induced generation of sharp 1H resonances associated with a low concentration of sites on the zeolite. The onset temperature of the appearance of these signals was found to strongly correlate with the NH3-SCR activity and was observed for a range of catalysts covering multiple frameworks (CHA, AEI, AFX, ERI, ERI-CHA, ERI-OFF, *BEA), with different Si/Al ratios and different Cu contents. The results point towards universal applicability of variable temperature NMR to predict the activity of a Cu-zeolite SCR catalyst. The unique relationship of a spectroscopic feature with catalytic behavior for zeolites with different structures and chemical compositions is exceptional in heterogeneous catalysis.

Illuminating the Black Box: A Perspective on Zeolite Crystallization in Inorganic Media.

ConspectusSince the discovery of synthetic zeolites in the 1940s and their implementation in major industrial processes involving adsorption, catalytic conversion, and ion exchange, material scientists have targeted the rational design of zeolites: controlling synthesis to crystallize zeolites with predetermined properties. Decades later, the fundamentals of zeolite synthesis remain largely obscured in a black box, rendering rational design elusive. A major prerequisite to rational zeolite design is to fully understand, and control, the elementary processes governing zeolite nucleation, growth, and stability. The molecular-level investigation of these processes has been severely hindered by the complex multiphasic media in which aluminosilicate zeolites are typically synthesized. This Account documents our recent progress in crystallizing zeolites from synthesis media based on hydrated silicate ionic liquids (HSIL), a synthesis approach facilitating the evaluation of the individual impacts of synthesis parameters such as cation type, water content, and alkalinity on zeolite nucleation, growth, and phase selection. HSIL-based synthesis allows straightforward elucidation of the relationship between the characteristics of the synthesis medium and the properties and structure of the crystalline product. This assists in deriving new insights in zeolite crystallization in an inorganic aluminosilicate system, thus improving the conceptual understanding of nucleation and growth in the context of inorganic zeolite synthesis in general. This Account describes in detail what hydrated silicate ionic liquids are, how they form, and how they assist in improving our understanding of zeolite genesis on a molecular level. It describes the development of ternary phase diagrams for inorganic aluminosilicate zeolites via a systematic screening of synthesis compositions. By evaluating obtained crystal structure properties such as framework composition, topology, and extraframework cation distributions, critical questions are dealt with: Which synthesis variables govern the aluminum content of crystallizing zeolites? How does the aluminum content in the framework determine the expression of different topologies? The crucial role of the alkali cation, taking center stage in all aspects of crystallization, phase selection, and, by extension, transformation is also discussed. New criteria and models for phase selection are proposed, assisting in overcoming the need for excessive trial and error in the development of future synthesis protocols.Recent progress in the development of a toolbox enabling liquid-state characterization of these precursor media has been outlined, setting the stage for the routine monitoring of zeolite crystallization in real time. Current endeavors on and future needs for the in situ investigation of zeolite crystallization are highlighted. Finally, experimentally accessible parameters providing opportunities for modeling zeolite nucleation and growth are identified. Overall, this work provides a perspective toward future developments, identifying research areas ripe for investigation and discovery.

Enabling hydrate-based methane storage under mild operating conditions by periodic mesoporous organosilica nanotubes.

Biomethane is a renewable natural gas substitute produced from biogas. Storage of this sustainable energy vector in confined clathrate hydrates, encapsulated in the pores of a host material, is a highly promising avenue to improve storage capacity and energy efficiency. Herein, a new type of periodic mesoporous organosilica (PMO) nanotubes, referred to as hollow ring PMO (HR-PMO), capable of promoting methane clathrate hydrate formation under mild working conditions (273 K, 3.5 MPa) and at high water loading (5.1 g water/g HR-PMO) is reported. Gravimetric uptake measurements reveal a steep single-stepped isotherm and a noticeably high methane storage capacity (0.55 g methane/g HR-PMO; 0.11 g methane/g water at 3.5 MPa). The large working capacity throughout consecutive pressure-induced clathrate hydrate formation-dissociation cycles demonstrates the material's excellent recyclability (97% preservation of capacity). Supported by ex situ cryo-electron tomography and x-ray diffraction, HR-PMO nanotubes are hypothesized to promote clathrate hydrate nucleation and growth by distribution and confinement of water in the mesopores of their outer wall, along the central channels of the nanotubes and on the external nanotube surface. These findings showcase the potential for application of organosilica materials with hierarchical and interconnected pore systems for pressure-based storage of biomethane in confined clathrate hydrates.

Engineering of Phenylpyridine- and Bipyridine-Based Covalent Organic Frameworks for Photocatalytic Tandem Aerobic Oxidation/Povarov Cyclization.

Covalent organic frameworks (COFs) are emerging as a new class of photoactive organic semiconductors, which possess crystalline ordered structures and high surface areas. COFs can be tailor-made toward specific (photocatalytic) applications, and the size and position of their band gaps can be tuned by the choice of building blocks and linkages. However, many types of building blocks are still unexplored as photocatalytic moieties and the scope of reactions photocatalyzed by COFs remains quite limited. In this work, we report the synthesis and application of two bipyridine- or phenylpyridine-based COFs: TpBpyCOF and TpPpyCOF. Due to their good photocatalytic properties, both materials were applied as metal-free photocatalysts for the tandem aerobic oxidation/Povarov cyclization and α-oxidation of N-aryl glycine derivatives, with the bipyridine-based TpBpyCOF exhibiting the highest activity. By expanding the range of reactions that can be photocatalyzed by COFs, this work paves the way toward the more widespread application of COFs as metal-free heterogeneous photocatalysts as a convenient alternative for commonly used homogeneous (metal-based) photocatalysts.

Hydrogen bonding to oxygen in siloxane bonds drives liquid phase adsorption of primary alcohols in high-silica zeolites.

Upon liquid phase adsorption of C1-C5 primary alcohols on high silica MFI zeolites (Si/Al = 11.5-140), the concentration of adsorbed molecules largely exceeds the concentration of traditional adsorption sites: Brønsted acid and defect sites. Combining quantitative in situ1H MAS NMR, qualitative multinuclear NMR and IR spectroscopy, hydrogen bonding of the alcohol function to oxygen atoms of the zeolite siloxane bridges (Si-O-Si) was shown to drive the additional adsorption. This mechanism co-exists with chemi- and physi-sorption on Brønsted acid and defect sites and does not exclude cooperative effects from dispersive interactions.

Reversible Parahydrogen Induced Hyperpolarization of 15 N in Unmodified Amino Acids Unraveled at High Magnetic Field.

Amino acids (AAs) and ammonia are metabolic markers essential for nitrogen metabolism and cell regulation in both plants and humans. NMR provides interesting opportunities to investigate these metabolic pathways, yet lacks sensitivity, especially in case of 15 N. In this study, spin order embedded in p-H2 is used to produce on-demand reversible hyperpolarization in 15 N of pristine alanine and ammonia under ambient protic conditions directly in the NMR spectrometer. This is made possible by designing a mixed-ligand Ir-catalyst, selectively ligating the amino group of AA by exploiting ammonia as a strongly competitive co-ligand and preventing deactivation of Ir by bidentate ligation of AA. The stereoisomerism of the catalyst complexes is determined by hydride fingerprinting using 1 H/D scrambling of the associated N-functional groups on the catalyst (i.e., isotopological fingerprinting), and unravelled by 2D-ZQ-NMR. Monitoring the transfer of spin order from p-H2 to 15 N nuclei of ligated and free alanine and ammonia targets using SABRE-INEPT with variable exchange delays pinpoints the monodentate elucidated catalyst complexes to be most SABRE active. Also RF-spin locking (SABRE-SLIC) enables transfer of hyperpolarization to 15 N. The presented high-field approach can be a valuable alternative to SABRE-SHEATH techniques since the obtained catalytic insights (stereochemistry and kinetics) will remain valid at ultra-low magnetic fields.

Does Water Enable Porosity in Aluminosilicate Zeolites? Porous Frameworks versus Dense Minerals.

Recently identified zeolite precursors consisting of concentrated, hyposolvated homogeneous alkalisilicate liquids, hydrated silicate ionic liquids (HSIL), minimize correlation of synthesis variables and enable one to isolate and examine the impact of complex parameters such as water content on zeolite crystallization. HSIL are highly concentrated, homogeneous liquids containing water as a reactant rather than bulk solvent. This simplifies elucidation of the role of water during zeolite synthesis. Hydrothermal treatment at 170 °C of Al-doped potassium HSIL with chemical composition 0.5SiO2:1KOH:xH2O:0.013Al2O3 yields porous merlinoite (MER) zeolite when H2O/KOH exceeds 4 and dense, anhydrous megakalsilite when H2O/KOH is lower. Solid phase products and precursor liquids were fully characterized using XRD, SEM, NMR, TGA, and ICP analysis. Phase selectivity is discussed in terms of cation hydration as the mechanism, allowing a spatial cation arrangement enabling the formation of pores. Under water deficient conditions, the entropic penalty of cation hydration in the solid is large and cations need to be entirely coordinated by framework oxygens, leading to dense, anhydrous networks. Hence, the water activity in the synthesis medium and the affinity of a cation to either coordinate to water or to aluminosilicate decides whether a porous, hydrated, or a dense, anhydrous framework is formed.

Absolute Quantification of Residual Solvent in Mesoporous Silica Drug Formulations Using Magic-Angle Spinning NMR Spectroscopy.

Porous silica is used as a drug delivery agent to improve the bioavailability of sparsely soluble compounds. In this approach, the active pharmaceutical ingredient (API) is commonly loaded into the porous silica by incipient wetness impregnation using organic solvents. Subsequent solvent elimination is critical as the residual solvent concentration cannot exceed threshold values set by health and safety regulations (e.g., EMA/CHMP/ICH/82260/2006). For dichloromethane and methanol, for example, residual concentrations must be below 600 and 3000 ppm, respectively. Today, EU and USA Pharmacopoeias recommend tedious procedures for residual solvent quantification, requiring extraction of the solvent and subsequent quantification using capillary gas chromatography with static headspace sampling (sHS-GC). This work presents a new method based on the combination of standard addition and absolute quantification using magic-angle spinning nuclear magnetic resonance spectroscopy (MAS qNMR). The methodology was originally developed for absolute quantification of water in zeolites and has now been validated for quantification of residual solvent in drug formations using mesoporous silica loaded with ibuprofen dissolved in DCM and MeOH as test samples. Interestingly, formulations prepared using as-received or predried mesoporous silica contained 5465 versus 484.9 ppm DCM, respectively. This implies that the initial water content of the silica carrier can impact the residual solvent concentration in drug-loaded materials. This observation could provide new options to minimize the occurrence of these undesired solvents in the final formulation.

Hydrogen-Bonded Water-Aminium Assemblies for Synthesis of Zeotypes with Ordered Heteroatoms.

Water plays a central role in the crystallization of a variety of organic, inorganic, biological, and hybrid materials. This is also true for zeolites and zeolite-like materials, an important class of industrial catalysts and adsorbents. Water is always present during their hydrothermal synthesis, either with or without organic species as structure-directing agents. Apart from its role as a solvent or a catalyst, structure direction by water in zeolite synthesis has never been clearly elucidated. Here, we report the crystallization of phosphate-based molecular sieves using rationally designed, hydrogen-bonded water-aminium assemblies, resulting in molecular sieves exhibiting the crystallographic ordering of heteroatoms. We demonstrate that a 1:1 assembly of water and diprotonated N,N-dimethyl-1,2-ethanediamine acts as a structure-directing agent in the synthesis of a silicoaluminophosphate material with phillipsite (PHI) topology, using SMARTER crystallography, which combines single-crystal X-ray diffraction and nuclear magnetic resonance spectroscopy, as well as ab initio molecular dynamics simulations. The molecular arrangement of the hydrogen-bonded assembly matches well with the shape and size of subunits in the PHI structure, and their charge distributions result in the strict ordering of framework tetrahedral atoms. This concept of structure direction by water-containing supramolecular assemblies should be applicable to the synthesis of many classes of porous materials.

Hydration of Wheat Flour Water-Unextractable Cell Wall Material Enables Structural Analysis of Its Arabinoxylan by High-Resolution Solid-State 13C MAS NMR Spectroscopy.

To enable its structural characterization by nuclear magnetic resonance (NMR) spectroscopy, the native structure of cereal water-unextractable arabinoxylan (WU-AX) is typically disrupted by alkali or enzymatic treatments. Here, WU-AX in the wheat flour unextractable cell wall material (UCWM) containing 40.9% ± 1.5 arabinoxylan with an arabinose-to-xylose ratio of 0.62 ± 0.04 was characterized by high-resolution solid-state NMR without disrupting its native structure. Hydration of the UCWM (1.7 mg H2O/mg UCWM) in combination with specific optimizations in the NMR methodology enabled analysis by solid-state 13C NMR with magic angle spinning and 1H high-power decoupling (13C HPDEC MAS NMR) which provided sufficiently high resolution to allow for carbon atom assignments. Spectral resonances of C-1 from arabinose and xylose residues of WU-AX were here assigned to the solid state. The proportions of un-, mono-, and di-substituted xyloses were 59.2, 19.5, and 21.2%, respectively. 13C HPDEC MAS NMR showed the presence of solid-state fractions with different mobilities in the UCWM. This study presents the first solid-state NMR spectrum of wheat WU-AX with sufficient resolution to enable assignment without prior WU-AX solubilization.

HSIL-Based Synthesis of Ultracrystalline K,Na-JBW, a Zeolite Exhibiting Exceptional Framework Ordering and Flexibility.

A reproducible synthesis strategy for ultracrystalline K,Na-aluminosilicate JBW zeolite is reported. The synthesis uses a Na-based hydrated silicate ionic liquid (HSIL) as a silicon source and gibbsite as the aluminum source. 27Al and 23Na NMR spectra exhibit crystalline second-order quadrupole patterns in the hydrated as well as dehydrated states and distinct resonances for different T-sites demonstrating an exceptional degree of order of the elements of the JBW framework, observed for the first time in a zeolite. Detailed structural analysis via NMR crystallography, combining powder X-ray diffraction and solid-state NMR of all elements (27Al, 29Si, 23Na, 39K, and 1H), reveals remarkable de- and rehydration behavior of the JBW framework, transforming from its as-made hydrated structure via a modified anhydrous state into a different rehydrated symmetry while showing astonishing flexibility for a semicondensed aluminosilicate. Its crystallinity, exceptional degree of ordering of the T atoms and sodium cations, and the fully documented structure qualify this defect-free K,Na-aluminosilicate JBW zeolite as a suitable model system for developing NMR modeling methods.

Ion-Pairs in Aluminosilicate-Alkali Synthesis Liquids Determine the Aluminum Content and Topology of Crystallizing Zeolites.

Using hydrated silicate ionic liquids, phase selection and framework silicon-to-aluminum ratio during inorganic zeolite synthesis were studied as a function of batch composition. Consisting of homogeneous single phasic liquids, this synthesis concept allows careful control of crystallization parameters and evaluation of yield and sample homogeneity. Ternary phase diagrams were constructed for syntheses at 90 °C for 1 week. The results reveal a cation-dependent continuous relation between batch stoichiometry and framework aluminum content, valid across the phase boundaries of all different zeolites formed in the system. The framework aluminum content directly correlates to the type of alkali cation and gradually changes with batch alkalinity and dilution. This suggests that the observed zeolites form through a solution-mediated mechanism involving the concerted assembly of soluble cation-oligomer ion pairs. Phase selection is a consequence of the stability for a particular framework at the given aluminum content and alkali type.

Nucleation of Porous Crystals from Ion-Paired Prenucleation Clusters.

Current nucleation models propose manifold options for the formation of crystalline materials. Exploring and distinguishing between different crystallization pathways on the molecular level however remain a challenge, especially for complex porous materials. These usually consist of large unit cells with an ordered framework and pore components and often nucleate in complex, multiphasic synthesis media, restricting in-depth characterization. This work shows how aluminosilicate speciation during crystallization can be documented in detail in monophasic hydrated silicate ionic liquids (HSILs). The observations reveal that zeolites can form via supramolecular organization of ion-paired prenucleation clusters, consisting of aluminosilicate anions, ion-paired to alkali cations, and imply that zeolite crystallization from HSILs can be described within the spectrum of modern nucleation theory.