Multinuclear Solid-state NMR Research Articles

Evaluation of the Stability of Trimethyl Phosphate as a Li–O2 Battery Electrolyte via Multinuclear Solid-State NMR

Solid-state 17O NMR was used to compare the stability of two potential Li–O2 electrolytes—tetraethylene glycol dimethyl ether (TEGDME) and trimethyl phosphate (TMP). The TEGDME electrolyte demonstrated superior stability to the TMP electrolyte. Li2O2 and evidence of electrolyte breakdown was observed in the TEGDME cell, whereas only electrolyte breakdown products were discovered within the TMP cell. Potential decomposition pathways of TMP are proposed here that account for the formation of the discharge species observed in the 17O, 7Li, 1H, and 31P solid-state NMR of the cycled cathodes.

Monitoring and understanding the paraelectric-ferroelectric phase transition in the metal-organic framework [NH4 ][M(HCOO)3 ] by solid-state NMR spectroscopy.

The paraelectric-ferroelectric phase transition in two isostructural metal-organic frameworks (MOFs) [NH4 ][M(HCOO)3 ] (M=Mg, Zn) was investigated by in situ variable-temperature (25) Mg, (67) Zn, (14) N, and (13) C solid-state NMR (SSNMR) spectroscopy. With decreasing temperature, a disorder-order transition of NH4 (+) cations causes a change in dielectric properties. It is thought that [NH4 ][Mg(HCOO)3 ] exhibits a higher transition temperature than [NH4 ][Zn(HCOO)3 ] due to stronger hydrogen-bonding interactions between NH4 (+) ions and framework oxygen atoms. (25) Mg and (67) Zn NMR parameters are very sensitive to temperature-induced changes in structure, dynamics, and dielectric behavior; stark spectral differences across the paraelectric-ferroelectric phase transition are intimately related to subtle changes in the local environment of the metal center. Although (25) Mg and (67) Zn are challenging nuclei for SSNMR experiments, the highly spherically symmetric metal-atom environments in [NH4 ][M(HCOO)3 ] give rise to relatively narrow spectra that can be acquired in 30-60 min at a low magnetic field of 9.4 T. Complementary (14) N and (13) C SSNMR experiments were performed to probe the role of NH4 (+) -framework hydrogen bonding in the paraelectric-ferroelectric phase transition. This multinuclear SSNMR approach yields new physical insights into the [NH4 ][M(HCOO)3 ] system and shows great potential for molecular-level studies on electric phenomena in a wide variety of MOFs.

The Development of Organic Ionic Plastic Crystals As Solid State Electrolytes for Energy Storage

Organic ionic plastic crystals (OIPCs) are a class of solid-state electrolyte material quite unique in their mechanical and transport properties; they combine the advantages of fast ion conduction with good stability and non-volatility even above their melting point. Significant progress has been made recently in the development of OIPCs for electrochemical devices such as lithium batteries and dye-sensitised solar cells.[1] The negligible volatility of OIPCs renders them more suitable than molecular plastic crystals for long-term device use, while the high thermal and electrochemical stability of many OIPCs fulfils an essential requirement for solid state electrolytes for many device applications. Understanding the ion transport behaviour of OIPCs, both in their pure and doped state (i.e. in the presence of a small amount of a lithium salt, to enable their use in lithium batteries), is crucial for their development as solid electrolytes. Here we present our investigations into the ion transport mechanism in these materials using both experimental data (single crystal XRD, multinuclear solid-state NMR, DSC, ionic conductivity and SEM) and theoretical techniques (molecular dynamics and second moment-based solid static NMR line width simulations).[2][3] We also discuss the physical properties and application of the OIPC triisobutyl(methyl)phosphonium bis(fluorosulfonyl)imide (P1444FSI) doped with 4 mol% LiFSI. The good conductivity of this material (0.26 mS cm-1 at 22 °C) enables cycling of lithium cells (Li|LiFePO4) at room temperature, achieving about 160 mA h g-1 discharge capacity at 20 oC, which is the highest for OIPC based cells to date.[4] References [1] J. M. Pringle, Phys. Chem. Chem. Phys., 2013, 15, 1339 [2] L. Jin, K. M. Nairn, C. M. Forsyth, A. J. Seeber, D. R. MacFarlane, P. C. Howlett, M. Forsyth and J. M. Pringle, J. Am. Chem. Soc., 2012, 134, 9688-9697. [3] L. Jin, S. de Leeuw, M. V. Koudriachova, J. M. Pringle, P. C. Howlett, F. Chen, M. Forsyth, Phys. Chem. Chem. Phys., 2013, 15, 19570-19574. [4] L. Jin, P. C. Howlett, J. M. Pringle, J. Janikowski, M. Armand, D. R. MacFarlane and M. Forsyth, Energy and Environ. Sci., 2014, 7, 3352-3361.

Generation and Properties of Brønsted Acid Sites in Bifunctional Rh-, Ir-, Pd-, and Pt-Containing Zeolites Y Investigated by Solid-State NMR Spectroscopy

In the present work, the generation and properties of Bronsted acidic bridging OH groups (Si(OH)Al) in Rh-, Ir-, Pd-, and Pt-containing zeolites Y were quantitatively investigated by multinuclear solid-state NMR spectroscopy. The number of Si(OH)Al groups formed per noble metal atom upon their reduction was found to be 0.3 to 1.3 instead of the theoretically expected values of 2 (Pd2+, Pt2+) or 3 (Rh3+, Ir3+). For zeolites Y with low noble metal loadings, a significantly lower acid strength of Si(OH)Al groups in comparison with those of H,Na–Y zeolites occurred. Interestingly, noble metal-containing zeolites Y with lower acid strength have a significant content of not completely reduced noble metal atoms with partially cationic character. The spatial distribution of Si(OH)Al groups in noble metal-containing zeolites Y was found to differ significantly from that in H,Na–Y zeolites. This effect is explained by the migration of a significant content of noble metal atoms into the sodalite cages of zeolites Y ...

Effect of Impregnation on the Structure of Niobium Oxide/Alumina Catalysts Studied by Multinuclear Solid-State NMR, FTIR, and Quantum Chemical Calculations

Multinuclear solid-state 1H, 27Al, and 93Nb NMR experiments and DFT calculations were carried out for structural characterization of alumina-supported niobium oxide catalysts with high niobium content following an every stage in the catalyst preparation. It was found that the first stage of the impregnation procedure plays a key role in determining the catalyst structure and acidity. In order to monitor the presence in catalysts of aluminum niobate phase, AlNbO4, a series of 27Al and 93Nb NMR experiments was performed for several different individual AlNbO4 samples. Aluminum and niobium NMR parameters were determined for AlNbO4, which crystal structure contains two different crystallographic sites for each element. The compound was investigated through a combination of experimental 93Nb and 27Al NMR spectroscopy methods at several magnetic field strengths (9.4, 11.7, 19.4, and 21.1 T) and complemented by ab initio quantum chemical calculations of NMR parameters for these nuclei. The chemical shielding and...

Multinuclear Solid-State NMR Studies of Ionic Conduction Mechanisms in Low-Cost and Rare-Earth-Free Superior Fast Oxide-Ion Conductor Sr3-3xNa3xSi3O9-1.5x

One of the most promising next-generation energy conversion technologies is the solid oxide fuel cell (SOFC), which is more efficient and less polluting than conventional internal combustion engines (ICEs). However, in order to commercially compete with ICEs, SOFCs must be cost-competitive and operation-reliable. To achieve this goal, lowering the operating temperature of a SOFC from the current 700-1000oC to ≤ 600oC is vitally important. The realization of such intermediate-temperature SOFCs (IT-SOFCs) depends critically upon the availability of high-conductivity electrolytes and high-activity electrode catalysts. In recent years, significant progress has been made in the area of cathode catalysts through new material discovery and microstructural optimization, which has demonstrated a polarization resistance (area-specific resistance) as low as 0.1 W×cm2 even at 500oC. 1 2 In contrast, the development of high-conductivity oxide-ion conductors significantly lags behind that of IT-SOFC cathodes. Therefore, identification of new IT solid-state fast oxide-ion conductors is in high demand. A practical criterion for an electrolyte to be suitable for an IT-SOFC is its oxide-ion conductivity so ≥0.01 S/cm at an operating temperature < 600oC. None of the existing chemically stable oxide-ion conductors can satisfy this requirement at a temperature ≤500oC. Recently, the Goodenough group at the University of Texas at Austin discovered a new class of oxide-ion conductors that can meet this conductivity-temperature requirement. These new oxide-ion conductors have a generic formula Sr3-3xNa3xSi3O9-1.5x (or SNS hereinafter) and layered structure as shown in Fig.1 (a). 3 4 The oxide-ion conductivity of SNS is so»0.01 S/cm at 500oC, Fig.1 (b), the highest among all known chemically stable oxide-ion conductors. More importantly, it is stable over a broad pO2 range of approximately from 10-30 to 1 atm and an extended lifetime, making it an ideal electrolyte for IT-SOFCs. 5 Despite the exciting discovery of this superior ion conductor, so far very little is known about the transport mechanisms governing the high oxide-ion conductivity in the SNS. This study investigates the origin of high oxide-ion conductivity by probing the local structure and dynamics of ionic motion with high-temperature multinuclear (17O, 23Na, and 29Si) solid-state NMR techniques. It reveals the inhomegeneity of chemical phases in this electrolyte. The increased level of Na-doping has led to increased degree of amorphorization in structure, which may be correlated to the enhancement in ionic conductivity. The foundational knowledge gained from this research may have significant impact on understanding the critical structure-conductivity relationship and designing new solid-state fast ion conductors for advanced electrochemical energy conversion and storage systems. References (1) Liang, F. L.; Zhou, W.; Li, J.; Zhu, Z. H.: Microwave-plasma induced reconstruction of silver catalysts for highly efficient oxygen reduction. J Mater Chem A 2013, 1, 13746-13749. (2) Zhu, Y. L.; Chen, Z. G.; Zhou, W.; Jiang, S. S.; Zou, J.; Shao, Z. P.: An A-Site-Deficient Perovskite offers High Activity and Stability for Low-Temperature Solid-Oxide Fuel Cells. Chemsuschem 2013, 6, 2249-2254. (3) Singh, P.; Goodenough, J. B.: Monoclinic Sr1-xNaxSiO3-0.5x: New Superior Oxide Ion Electrolytes. J Am Chem Soc 2013, 135, 10149-10154. (4) Singh, P.; Goodenough, J. B.: Sr1-xKxSi1-yGeyO3-0.5x: a new family of superior oxide-ion conductors. Energ Environ Sci 2012, 5, 9626-9631. (5) Wei, T.; Singh, P.; Gong, Y. H.; Goodenough, J. B.; Huang, Y. H.; Huang, K.: Sr3-3xNa3xSi3O9-1.5x (x=0.45) as a superior solid oxide-ion electrolyte for intermediate temperature-solid oxide fuel cells. Energ Environ Sci 2014, 7, 1680-1684.

Ether-Substituted Group 4 Metallocene Complexes: Cytostatic Effects and Applications in Ethylene Polymerization

The synthesis of the ether-substituted group 4 metallocene complexes [Ti(η5-C5H5)(η5-C5H4{CMeRCH(OMe)2})Cl2] (R = Me (4), Ph (5)) and [M(η5-C5H4{CMeRCH(OMe)2})2Cl2] (M = Ti, R = Me (6), Ph (7); M = Zr, R = Me (8), Ph (9)) is reported. In addition, the titanocene derivatives 4–7 have been supported on the mesoporous silica-based material KIT-6, giving the functionalized materials K4–K7, respectively. All compounds and materials have been characterized by different analytical techniques such as solution NMR spectroscopy, mass spectrometry, elemental analysis, solid-state multinuclear NMR spectroscopy, IR spectroscopy, N2 adsorption–desorption isotherms, X-ray fluorescence and diffraction, SEM, and TEM. Additionally, the molecular structures of the zirconocene derivative 8 and the hydrolysis product [Ti(η5-C5H5)(η5-C5H4{CMe2COO})Cl] (4a) of titanocene complex 4 have been determined by X-ray diffraction studies. The structure of 4a reveals a five-membered metallacyclic arrangement. The ether-substituted titan...

Structure-property relations in new fluorophosphate glasses singly- and co-doped with Er3+ and Yb3+

Rare earth (RE3+)-doped fluorophosphate glasses are among the most promising candidates for high-efficiency laser generation in the near-infrared spectral region. By proper choice of composition, these materials can combine the advantages of fluorides (low phonon energies, low refractive indices, extensive optical window, low hygroscopicity) and of oxides (high chemical and mechanical stability and high dopant solubility), resulting in enhancement of the RE3+ emissive properties. In this work, we present the synthesis and structural/spectroscopic investigation of new glasses with composition 25BaF225SrF2(30-x)Al(PO3)3xAlF3(20-z)YF3:zREF3, where x = 20 or 15, RE = Er3+ and/or Yb3+, z = 0.25–5.0 mol%. Results indicate considerable improvement of the emissive properties of both ions when compared to phosphate or even other fluorophosphate host compositions. Long excited state lifetimes (τ = 10 ms for the Er3+ level 4I13/2, and τ = 1.3 ms for the Yb3+ level 2F5/2) imply high fluorescence quantum efficiencies η (up to 85% for both ions). Structural characterization by Raman and multinuclear solid state NMR spectroscopies indicate that the metaphosphate-type chain structure of the Al(PO3)3 vitreous framework is partially depolymerized and dominated by Q(0) and Q(1) units crosslinked by six-coordinate Al species. As revealed by 27Al{31P} rotational echo double resonance (REDOR) NMR results the average local aluminum environment of the x = 20 sample comprises 1.6 phosphate and 4.4 fluoride species. These results indicate a clear bonding preference between aluminum and phosphorus, which is consistent with the desired dominance of fluoride species in the local environment of the rare earth and alkaline earth atoms in these glasses.

Intercalation of Benzoxaborolate Anions in Layered Double Hydroxides: Toward Hybrid Formulations for Benzoxaborole Drugs

Benzoxaboroles are a family of cyclic derivatives of boronic acids, whose reactivity makes them interesting candidates for the development of novel drugs. In this study, we describe the preparation of the first hybrid organic–inorganic materials involving benzoxaboroles, as a first step toward their use as new formulations for such drugs. The materials were prepared by intercalation of the simplest benzoxaborole (C7H6BO(OH), BBzx) or of its fluorinated analogue (C7H5FBO(OH), AN2690, a recently developed antifungal drug), both taken under their anionic form (benzoxaborolate), into a biodegradable inorganic matrix (a Mg–Al layered double hydroxide). Extensive characterization was carried out on the materials, notably by powder X-ray diffraction and multinuclear (11B, 27Al, 13C, 19F, 25Mg, and 1H) solid state NMR, in order to describe their structure, particularly in the vicinity of the organoboron species. Three crystalline phases involving benzoxaborolate anions in association with Ca2+ or Mg2+ cations wer...

Action of the multifunctional peptide BP100 on native biomembranes examined by solid-state NMR.

Membrane composition is a key factor that regulates the destructive activity of antimicrobial peptides and the non-leaky permeation of cell penetrating peptides in vivo. Hence, the choice of model membrane is a crucial aspect in NMR studies and should reflect the biological situation as closely as possible. Here, we explore the structure and dynamics of the short multifunctional peptide BP100 using a multinuclear solid-state NMR approach. The membrane alignment and mobility of this 11 amino acid peptide was studied in various synthetic lipid bilayers with different net charge, fluidity, and thickness, as well as in native biomembranes harvested from prokaryotic and eukaryotic cells. (19)F-NMR provided the high sensitivity and lack of natural abundance background that are necessary to observe a labelled peptide even in protoplast membranes from Micrococcus luteus and in erythrocyte ghosts. Six selectively (19)F-labeled BP100 analogues gave remarkably similar spectra in all of the macroscopically oriented membrane systems, which were studied under quasi-native conditions of ambient temperature and full hydration. This similarity suggests that BP100 has the same surface-bound helical structure and high mobility in the different biomembranes and model membranes alike, independent of charge, thickness or cholesterol content of the system. (31)P-NMR spectra of the phospholipid components did not indicate any bilayer perturbation, so the formation of toroidal wormholes or micellarization can be excluded as a mechanism of its antimicrobial or cell penetrating action. However, (2)H-NMR analysis of the acyl chain order parameter profiles showed that BP100 leads to considerable membrane thinning and thereby local destabilization.

NMR crystallography for structural characterization of oxovanadium(V) complexes: deriving coordination geometry and detecting weakly coordinated ligands at atomic resolution in the solid state.

NMR crystallography is an emerging method for atomic-resolution structural analysis of ubiquitous vanadium(V) sites in inorganic and bioinorganic complexes as well as vanadium-containing proteins. NMR crystallography allows for characterization of vanadium(V) containing solids, based on the simultaneous measurement of (51)V-(15)N internuclear distances and anisotropic spin interactions, described by (13)C, (15)N, and (51)V chemical shift anisotropy and (51)V electric field gradient tensors. We show that the experimental (51)V, (13)C, and (15)N NMR parameters are essential for inferring correct coordination numbers and deriving correct geometries in density functional theory (DFT) calculations, particularly in the absence of single-crystal X-ray structures. We first validate this approach on a structurally known vanadium(V) complex, ((15)N-salicylideneglycinate)-(benzhydroxamate)oxovanadium(V), VO(15)NGlySalbz. We then apply this approach to derive the three-dimensional structure of (methoxo)((15)N-salicylidene-glycinato)oxovanadium(V) with solvated methanol, [VO((15)NGlySal)(OCH3)]·(CH3OH). This is a representative complex with potentially variable coordination geometry depending on the solvation level of the solid. The solid material containing molecules of CH3OH, formally expressed as [VO((15)NGlySal)(OCH3)]·(CH3OH), is found to have one molecule of CH3OH weakly coordinated to the vanadium. The material is therefore best described as [VO((15)NGlySal)(OCH3)(CH3OH)] as deduced by the combination of multinuclear solid-state NMR experiments and DFT calculations. The approach reported here can be used for structural analysis of systems that are not amenable to single-crystal X-ray diffraction characterization and which can contain weakly associated solvents.

Structural refinement of the RT LaOF phases by coupling powder X-Ray diffraction, (19)F and (139)La solid state NMR and DFT calculations of the NMR parameters.

The structures of the β- and t-LaOF phases have been refined from XRPD patterns. For both phases, (19)F and (139)La solid-state NMR spectra recorded at high magnetic fields show the presence of a single F and a single La local environment, indicating a full anionic ordering in these oxyfluoride compounds. DFT calculations of the (19)F and (139)La chemical shielding tensors and of the (139)La EFG tensor have been performed for the proposed structural models. The observed good agreement between experimental and calculated NMR parameters for both phases highlights the accuracy of the structural data.

A multinuclear solid state NMR spectroscopic study of the structural evolution of disordered calcium silicate sol-gel biomaterials.

Disordered sol-gel prepared calcium silicate biomaterials show significant, composition dependent ability to bond with bone. Bone bonding is attributed to rapid hydroxycarbonate apatite (HCA) formation on the glass surface after immersion in body fluid (or implantation). Atomic scale details of the development of the structure of (CaO)x(SiO2)1-x (x = 0.2, 0.3 and 0.5) under heat treatment and subsequent dissolution in simulated body fluid (SBF) are revealed through a multinuclear solid state NMR approach using one-dimensional (17)O, (29)Si, (31)P and (1)H. Central to this study is the combination of conventional static and magic angle spinning (MAS) and two-dimensional (2D) triple quantum (3Q) (17)O NMR experiments that can readily distinguish and quantify the bridging (BOs) and non-bridging (NBOs) oxygens in the silicate network. Although soluble calcium is present in the sol, the (17)O NMR results reveal that the sol-gel produced network structure is initially dominated by BOs after gelation, aging and drying (e.g. at 120 °C), indicating a nanoscale mixture of the calcium salt and a predominantly silicate network. Only once the calcium salt is decomposed at elevated temperatures do the Ca(2+) ions become available to break BO. Apatite forming ability in SBF depends strongly on the surface OH and calcium content. The presence of calcium aids HCA formation via promotion of surface hydration and the ready availability of Ca(2+) ions. (17)O NMR shows the rapid loss of NBOs charge balanced by calcium as it is leached into the SBF. The formation of nanocrystalline, partially ordered HCA can be detected via(31)P NMR. This data indicates the importance of achieving the right balance of BO/NBO for optimal biochemical response and network properties.

The structure of trimethyltin fluoride.

The solid-state structure of (CH3)3SnF was reinvestigated by X-ray diffraction techniques as well as by multi-nuclear solid-state NMR spectroscopy. Trimethyltin fluoride crystallizes from hot ethanol in the orthorhombic space group Pnma at room temperature and changes to a low-temperature orthorhombic phase (space group: Cmcm) below -70 °C. In both modifications, trimethyltin fluoride adopts a linear chain structure with symmetric fluorine bridges, in contrast to previous reports. During its synthesis, (CH3)3SnF precipitates in another, poorly crystalline modification, as shown by powder X-ray diffraction. Solid-state MAS NMR experiments of both room-temperature phases of (CH3)3SnF (non-recrystallized and recrystallized) were carried out for the (1)H, (13)C, (19)F, and (119)Sn nuclei. The (119)Sn{(19)F, (1)H} and (19)F{(1)H} NMR spectra offer unambiguous determination for the (19)F and (119)Sn shielding tensors. The (119)Sn{(1)H} solid-state NMR spectra are in agreement with pentacoordination of Sn in this compound for the non-recrystallized and the recrystallized modifications. Based on the solid-state NMR results, the non-recrystallized modification of (CH3)3SnF also consists of linear, symmetrically fluorine-bridged chains, and differs from the recrystallized orthorhombic phase only in packing of the chains.

Nano ranged 1,3-dithia-2-stannacyclopentane(IV) alkyldithiocarbonates: solid-state NMR, powder X-ray diffraction and thermal decomposition studies

The research work, performed during the course of present investigations is concerned with the synthesis, physico-chemical [elemental analysis and melting points determinations], spectral [IR, Far-IR, multinuclear (13C and 119Sn) solid-state NMR], powder XRD and thermal studies on 1,3-dithia-2-stannacyclopentane(IV) derivatives with alkyldithiocarbonates. The derivatives, [(CH2S)2Sn(S2COR)Cl] and [(CH2S)2Sn(S2COR)2] have been synthesized by the reaction of 1,3-dithia-2-stannacyclopentane(IV) dichloride with potassium alkyldithiocarbonates in 1:1 and 1:2 stoichiometry, respectively. Thermal decomposition study provides significant information about the formation of tin sulfide (SnS2) which has a number of commercial applications in various fields. Crystallite size of the derivatives has been found in nano range (∼132nm) with the help of powder XRD studies. Sideband patterns obtained in our respective derivatives indicate distorted geometry and increased shielding of tin in solid state as compared to solution state 119Sn NMR, could be an indication of high electron density at tin atom.

New Insights into the Short-Range Structures of Microporous Titanosilicates As Revealed by 47/49Ti, 23Na, 39K, and 29Si Solid-State NMR Spectroscopy

Seven prototypical microporous titanosilicates have been studied by multinuclear solid-state NMR (SSNMR) spectroscopy, representing four typical Ti environments: square-pyramidal TiO5 units (natisite, AM-1, ETS-4), edge-shared brookite-type TiO6 chains (AM-4), cubane-type Ti4O16 clusters (sitinakite, GTS-1), and corner-shared TiO6 chains (ETS-10, ETS-4). 47/49Ti SSNMR spectra at 21.1 T are related to the coordination, crystal symmetry, and local environment of Ti. Distortions in Ti–O bond lengths and O–Ti–O coordination angles are reflected via CQ(47/49Ti) values that range from 8 to 16 MHz. Several titanosilicates feature axially symmetric 47/49Ti electric field gradient (EFG) tensors that permit facile spectral assignment and detection of deviations in local symmetry. This study uses 29Si NMR experiments to assess phase purity and crystallinity. 23Na NMR is used to probe the location and mobility of the sodium ions in the framework. The potential of 39K SSNMR for investigation of extra-framework counter cations is demonstrated by ETS-10, with increased spectral resolution and enhanced sensitivity to changes in local environment versus 23Na experiments. Plane-wave DFT calculations predicted 47/49Ti NMR parameters assisting in spectral assignments and help correlate 23Na and 29Si NMR resonances to crystallographic sites. The approach described in this work should promote further SSNMR investigations of microporous solids, such as titanosilicates, with unknown or poorly defined structures.

An NMR crystallographic approach to monitoring cation substitution in the aluminophosphate STA-2

The substitution of the divalent cations Mg2+ and Zn2+ into the aluminophosphate (AlPO) framework of STA-2 has been studied using an “NMR crystallographic” approach, combining multinuclear solid-state NMR spectroscopy, X-ray diffraction and first-principles calculations. Although the AlPO framework itself is inherently neutral, the positive charge of the organocation template in an as-made material is usually balanced either by the coordination to the framework of anions from the synthesis solution, such as OH− or F−, and/or by the substitution of aliovalent cations. However, the exact position and distribution of the substituted cations can be difficult to determine, but can have a significant impact upon the catalytic properties a material exhibits once calcined.For as-made Mg substituted STA-2, the positive charge of the organocation template is balanced by the substitution of Mg2+ for Al3+ and, where required, by hydroxide anions coordinated to the framework [27] Al MAS NMR spectra show that Al is present in both tetrahedral and five-fold coordination, with the latter dependent on the amount of substituted cations, and confirms the bridging nature of the hydroxyl groups, while high-resolution MQMAS spectra are able to show that Mg appears to preferentially substitute on the Al1 site. This conclusion is also supported by first-principles calculations. The calculations also show that 31P chemical shifts depend not only on the topologically-distinct site in the SAT framework, but also on the number of next-nearest-neighbour Mg species, and the exact nature of the coordinated hydroxyls (whether the P atom forms part of a six-membered ring, P(OAl)2OH, where OH bridges between two Al atoms). The calculations demonstrate a strong correlation between the 31P isotropic chemical shift and the average 〈P–O–M〉 bond angle. In contrast, for Zn substituted STA-2, both X-ray diffraction and NMR spectroscopy show less preference for substitution onto Al1 or Al2, with both appearing to be present, although that into Al1 appears slightly more favoured.

Temperature-induced amorphization of Na-zeolite A: A view from multi-nuclear high-resolution solid-state NMR

Despite efforts to understand the amorphization mechanisms of zeolites upon heating and subsequent dehydration, little is known about the extent of Si-Al disorder and topological variations in both crystalline and amorphous phases during amorphization. In this study, we investigated the atomic structure and the extent of configurational disorder (e.g., Si-Al ordering) in Na-zeolite A and other dehydrated phases during their temperature-induced amorphization using multi-nuclear solid-state NMR. We also report the first multi-nuclear (17O, 29Si, and 27Al) NMR spectra of the intermediate amorphous phases. 29Si MAS NMR results confirm the prevalence of amorphous phases up to ~1073 K and variation in Q-species for the crystalline phases. The 27Al quadrupolar coupling constant of the [4]Al peak in Na-zeolite A and the intermediate amorphous phases increase with increasing temperature, which suggests an increase in the topological disorder associated with the structural distortion around [4]Al. 2D 17O 3QMAS NMR spectra resolve the crystallographically distinct Si-O-Al sites in Na-zeolite A and three types of oxygen linkages namely, Si-O-Al, Si-O-Si, and Al-O-Al in the intermediate amorphous phases, which provides an unambiguous experimental evidence for an increase in the Si-Al disorder during the amorphization of zeolite. The detailed structural changes in Na-zeolite A and other dehydrated phases at various temperatures provide insights into the structural changes of other aluminosilicates during amorphization, thereby highlighting the changes in Si-Al ordering.

Multinuclear Solid-State NMR Studies of Polymer-Supported Scandium Triflate Catalysts

Scandium(III) trifluoromethanesulfonate [Sc(OTf)3] is extensively used in organic synthesis to catalyze a wide variety of carbon–carbon bond-forming reactions in aqueous media. It has previously been demonstrated that it is possible to immobilize Sc(OTf)3 in polystyrene (PS) to form the heterogeneous catalysts, microencapsulated (ME) Sc(OTf)3. The ME catalysts are recoverable, reusable, often reduce metal leaching, and have activity similar to that of their homogeneous counterparts. Aside from preliminary scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) imaging studies and solution 45Sc NMR studies, there is little information available about the molecular and bulk structure of the ME catalyst. In this regard, we have conducted a 45Sc solid-state NMR investigation of the Sc environments in the crystalline and ME forms of anhydrous Sc(OTf)3 and hydrated Sc(OTf)3·8H2O. Additional solid-state 1H, 19F, and 13C NMR and powder X-ray diffraction experiments provide information that is complementary to that obtained from the 45Sc NMR spectra, allowing for structural models of ME complexes to be proposed. The principal findings are that Sc(OTf)3 is hydrated upon microencapsulation to form Sc(OTf)3·8H2O. 1H–45Sc TRAPDOR and 1H–19F CP SSNMR experiments and powder X-ray diffraction experiments suggest that Sc is dispersed throughout the polymer within nanocrystalline domains of Sc(OTf)3·8H2O. The approach outlined here should be applicable for the characterization of many other polymer-supported metal-based catalysts.

Coordination Polymers Based on Alkylboronate Ligands: Synthesis, Characterization, and Computational Modelling

AbstractBoronate ligands [R–B(OH)3–] have recently started to attract attention for the elaboration of coordination polymer networks. Here, three new crystalline structures involving butyl‐ and octylboronate ligands are described: Sr[Bu–B(OH)3]2, Ca[Oct–B(OH)3]2 and Sr[Oct–B(OH)3]2 (Bu = C4H9, Oct = C8H17). All were obtained as microcrystalline powders, and their structures were solved by synchrotron powder X‐ray diffraction. IR and multinuclear (13C, 11B, 43Ca, 87Sr and 1H) solid‐state NMR characterizations were performed on the materials. Computational models of the new Sr[Bu–B(OH)3]2 phase and the previously reported Sr[Ph–B(OH)3]2·H2O structure were then developed. The IR O–H stretching modes and NMR parameters were calculated for these models and are discussed in view of the experimental spectra. This work confirms the importance of computational studies on boronate phases to determine the nature of the H‐bond network within the materials and to better understand their spectroscopic signatures.