Concluding remarks: on NMR crystallography.

This explored the field of NMR crystallography, and considered recent developments in experimental and theoretical approaches, new advances in machine learning and in the generation and handling of large amounts of data. Applications to a wide range of disordered, amorphous and dynamic systems demonstrated the range and quality of information available from this approach and the challenges that are faced in exploiting automation and developing best practice. In these closing remarks I will reflect on the discussions on the current state of the art, questions about what we want from these studies, how accurate we need results to be, how we best generate models for complex materials and what machine learning approaches can offer.

Synthesis of the large pore aluminophosphate STA-1 and its application as a catalyst for the Beckmann rearrangement of cyclohexanone oxime.

The preparation of stable large pore aluminophosphate (AlPO) zeotypes offers materials for applications in adsorption and catalysis. Here we report the synthesis of the pure AlPO with the SAO topology type (AlPO STA-1) using ,'-diethylbicyclo[2.2.

Mechanistic Insights into Cs-Ion Exchange in the Zeolite Chabazite from Powder X-Ray Diffraction.

Zeolites contain extraframework cations that are exchangeable under favorable aqueous conditions; this is the fundamental feature for their application in water purification and necessary to produce cation forms for other applications such as catalysis. Optimization of the process is common, but there is little fundamental understanding based on real-time experiments of the mechanism of exchange for most zeolites. The sodium and potassium forms of zeolite chabazite selectively uptake Cs by ion exchange, leading to its application in removing radioactive Cs from industrial nuclear waste streams, as well as from contaminated environments in the aftermath of the Fukushima and Three Mile Island accidents.

Rapid preparation of binary mixtures of sodium carboxylates as anodes in sodium-ion batteries.

Sodium-ion batteries are emerging as a sustainable solution to tackle the growing global energy demands. In this context, organic electrode materials complement such technologies as they are composed of earth-abundant elements. As organic anodes, sodium carboxylates exhibit promising applicability in a wide range of molecules.

An NMR crystallographic characterisation of solid (+)-usnic acid.

We use a combination of one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) calculations to obtain a full assignment of the H and C signals for solid (+)-usnic acid, which contains two molecules in the asymmetric unit. By combining through-space H-H correlation data with computation it is possible to assign signals not just to the same molecules (relative assignment) but to assign the signals to specific crystallographic molecules (absolute assignment). Variable-temperature measurements reveal that there is some variation in many of the C chemical shifts with temperature, likely arising from varying populations of different tautomeric forms of the molecule.

Site-directed cation ordering in chabazite-type AlGaPO-34 frameworks revealed by NMR crystallography.

We report the first synthesis of the mixed-metal chabazite-type AlGaPO-34(mim) solid solution, containing 1-methylimidazolium, mim, as structure directing agent (SDA), from the parent mixed-metal oxide solid solution, γ-(AlGa)O. This hitherto unreported family of materials exhibits complex disorder, arising from the possible distributions of cations over available sites, the orientation of the SDA and the presence of variable amounts of water, which provides a prototype for understanding structural subtleties in nanoporous materials. In the as-made forms of the phosphate frameworks, there are three crystallographically distinct metal sites: two tetrahedral MO and one octahedral MOF (M = Al, Ga).

Single-crystal X-ray Diffraction Studies of Physisorption and Chemisorption of SO within a Metal-Organic Framework and Its Competitive Adsorption with Water.

Living on an increasingly polluted planet, the removal of toxic pollutants such as sulfur dioxide (SO) from the troposphere and power station flue gas is becoming more and more important. The CPO-27/MOF-74 family of metal-organic frameworks (MOFs) with their high densities of open metal sites is well suited for the selective adsorption of gases that, like SO, bind well to metals and have been extensively researched both practically and through computer simulations. However, until now, focus has centered upon the binding of SO to the open metal sites in this MOF (called chemisorption, where the adsorbent-adsorbate interaction is through a chemical bond).

Investigations of the ADOR Process Using Solid-State NMR Spectroscopy.

The assembly-disassembly-organization-reassembly (ADOR) process for the transformation of zeolite using water has been studied by using Si and O solid-state NMR spectroscopy. The results show that the intermediate materials that are formed during the reaction are extremely dynamic and that the process involves both irreversible changes in structure that define the overall pathway and reversible lability of the Si-O-Si linkages that have no effect on the overall structure. The combination of processes occurring during the ADOR reaction means that the mechanism is considerably more complex than initially proposed.

C pNMR shifts of MOFs based on Cu(II)-paddlewheel dimers - DFT predictions for spin-1/2 defects.

We present DFT predictions (CAM-B3LYP/II level) for the paramagnetic Nuclear Magnetic Resonance (pNMR) spectra of small molecular models based on the Cu(II)-paddlewheel dimer motif that is present in metal-organic frameworks (MOFs, notably the HKUST and STAM families). We explore potential point defects with spin-1/2 discovered through electron paramagnetic resonance (EPR) experiments. We consider defects through substitution of one Cu(II) centre in the dimer with protons, or through one-electron reduction, affording a mixed-valence dimer.

Organosolv Pretreatment of Cocoa Pod Husks: Isolation, Analysis, and Use of Lignin from an Abundant Waste Product.

Cocoa pod husks (CPHs) represent an underutilized component of the chocolate manufacturing process. While industry's current focus is understandably on the cocoa beans, the husks make up around 75 wt % of the fruit. Previous studies have been dominated by the carbohydrate polymers present in CPHs, but this work highlights the presence of the biopolymer lignin in this biomass.

Computational NMR investigation of mixed-metal (Al,Sc)-MIL-53 and its phase transitions.

Compositionally complex metal-organic frameworks (MOFs) have properties that depend on local structure that is often difficult to characterise. In this paper a density functional theory (DFT) computational study of mixed-metal (Al,Sc)-MIL-53, a flexible MOF with several different forms, was used to calculate the relative energetics of these forms and to predict NMR parameters that can be used to evaluate whether solid-state NMR spectroscopy can be used to differentiate, identify and characterise the forms adopted by mixed-metal MOFs of different composition. The NMR parameters can also be correlated with structural features in the different forms, giving fundamental insight into the nature and origin of the interactions that affect nuclear spins.

Exploring cation distribution in ion-exchanged Al,Ga-containing metal-organic frameworks using O NMR spectroscopy.

A mixed-metal metal-organic framework, (Al,Ga)-MIL-53, synthesised by post-synthetic ion exchange has been investigated using solid-state nuclear magnetic resonance (NMR) spectroscopy, microscopy and energy dispersive X-ray (EDX) spectroscopy. O enrichment during the ion-exchange process enables site specific information on the metal distribution to be obtained. Within this work two ion-exchange processes have been explored.

Pressure-induced postsynthetic cluster anion substitution in a MIL-53 topology scandium metal-organic framework.

Postsynthetic modification of metal-organic frameworks (MOFs) has proven to be a hugely powerful tool to tune physical properties and introduce functionality, by exploiting reactive sites on both the MOF linkers and their inorganic secondary building units (SBUs), and so has facilitated a wide range of applications. Studies into the reactivity of MOF SBUs have focussed solely on removal of neutral coordinating solvents, or direct exchange of linkers such as carboxylates, despite the prevalence of ancillary charge-balancing oxide and hydroxide ligands found in many SBUs. Herein, we show that the μ-OH ligands in the MIL-53 topology Sc MOF, GUF-1, are labile, and can be substituted for μ-OCH units through reaction with pore-bound methanol molecules in a very rare example of pressure-induced postsynthetic modification.

Synthesis of FeAPO-34 Molecular Sieve under Ionothermal Condition.

FeAPO-34 with a chabazite (CHA) topology structure was successfully synthesized under ionothermal conditions using 1-ethyl-3-methylimidazoliumchloride (EMIMCl) ionic liquid in the presence of ethylenediamine (EDA). The material was characterized using powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), elemental analyses, and solid-state NMR spectroscopy. Incorporation of iron within the covalent framework of the material was confirmed by the presence of broad signals between 1000 and 14,000 ppm in the P NMR spectrum, corresponding to the P(O)(OAl) species.

Origin of the temperature dependence of C pNMR shifts for copper paddlewheel MOFs.

An efficient protocol for the calculation of C pNMR shifts in metal-organic frameworks based on Cu(ii) paddlewheel dimers is proposed, which involves simplified structural models, optimised using GFN2-xTB for the high-spin state, and CAM-B3LYP-computed NMR and EPR parameters. Models for hydrated and activated HKUST-1 and hydrated STAM MOFs with one, two and three Cu dimers have been used. The electronic ground states are low-spin and diamagnetic, with pNMR shifts arising from thermal population of intermediate- and high-spin excited states.

Solid-state NMR spectroscopy.

Solid-state nuclear magnetic resonance (NMR) spectroscopy is an atomic-level method used to determine the chemical structure, three-dimensional structure, and dynamics of solids and semi-solids. This Primer summarizes the basic principles of NMR as applied to the wide range of solid systems. The fundamental nuclear spin interactions and the effects of magnetic fields and radiofrequency pulses on nuclear spins are the same as in liquid-state NMR.

O NMR spectroscopy of crystalline microporous materials.

Microporous materials, containing pores and channels of similar dimensions to small molecules have a range of applications in catalysis, gas storage and separation and in drug delivery. Their complex structure, often containing different types and levels of positional, compositional and temporal disorder, makes structural characterisation challenging, with information on both long-range order and the local environment required to understand the structure-property relationships and improve the future design of functional materials. In principle, O NMR spectroscopy should offer an ideal tool, with oxygen atoms lining the pores of many zeolites and phosphate frameworks, playing a vital role in host-guest chemistry and reactivity, and linking the organic and inorganic components of metal-organic frameworks (MOFs).

Single-step synthesis and interface tuning of core-shell metal-organic framework nanoparticles.

Control over the spatial distribution of components in metal-organic frameworks has potential to unlock improved performance and new behaviour in separations, sensing and catalysis. We report an unprecedented single-step synthesis of multi-component metal-organic framework (MOF) nanoparticles based on the canonical ZIF-8 (Zn) system and its Cd analogue, which form with a core-shell structure whose internal interface can be systematically tuned. We use scanning transmission electron microscopy, X-ray energy dispersive spectroscopy and a new composition gradient model to fit high-resolution X-ray diffraction data to show how core-shell composition and interface characteristics are intricately controlled by synthesis temperature and reaction composition.

Exploring cation disorder in mixed-metal pyrochlore ceramics using O NMR spectroscopy and first-principles calculations.

Characterising the local structures (e.g., the cation distribution) of mixed-metal ceramics by NMR spectroscopy is often challenging owing to the unfavourable properties (low γ, large quadrupole moment and/or low abundance) of many metal nuclei.

Mechanochemically assisted hydrolysis in the ADOR process.

The ADOR (Assembly-Disassembly-Organisation-Reassembly) process for zeolites has been shown to produce a number of previously unknown frameworks inaccessible through conventional synthesis methods. Here, we present successful mechanochemically assisted hydrolysis of germanosilicate zeolite leading to ADOR products under mild conditions, low amounts of solvent and in short reaction times. The expansion of zeolite synthesis into the realm of mechanochemistry opens up feasible pathways regarding the production of these materials, especially for industrial purposes, as well as an exciting application for economical enrichment of materials with the low natural abundance NMR-active isotope of oxygen, O.

Application of NMR Crystallography to Highly Disordered Templated Materials: Extensive Local Structural Disorder in the Gallophosphate GaPO-34A.

We present an NMR crystallographic investigation of two as-made forms of the recently characterized gallophosphate GaPO-34A, which has an unusual framework composition with a Ga:P ratio of 7:6 and contains both hydroxide and fluoride anions and either 1-methylimidazolium or pyridinium as the structure-directing agent. We combine previously reported X-ray crystallographic data with solid-state NMR spectroscopy and periodic density functional theory (DFT) calculations to show that the structure contains at least three distinct types of disorder (occupational, compositional, and dynamic). The occupational disorder arises from the presence of six anion sites per unit cell, but a total occupancy of five of these, leading to full occupancy of four sites and partial occupancy of the fifth and sixth (which are related by symmetry).