31P Solid-state NMR Research Articles

Fire and salvage logging increased recalcitrant soil organic matter and reduced soil functionality in Mediterranean pine forests.

Postfire management actions are used to mitigate damage caused by wildfires. Salvage logging, often employed to restore ecosystem functions in burnt stands, plays an essential role in reducing economic losses and the burn severity of future wildfires. However, its ecological implications for soil functionality still need to be understood, especially in the Mediterranean basin, which is prone to erosion and desertification. This study aimed to investigate the effects of fire on (i) soil organic matter (SOM) quality and composition using differential scanning calorimetry-thermogravimetry (DSC-TG) and solid-state nuclear magnetic resonance (13C CPMAS NMR) and (ii) phosphorus (P) forms using solid-state 31P NMR spectroscopy in a wildfire that affected 3200 ha in southeastern Spain in July 2017. One year after the fire, we monitored four Pinus halepensis Mill. stand categories based on soil burn severity (SBS): unburnt, low SBS, high SBS and high SBS areas with salvage logging (n=36, nine plots per SBS level). We collected soil samples and analysed soil pH, SOM content and SOM quality, along with biological activity indicators (carbon biomass, basal respiration, β-glucosidase, phosphatase activities) and P forms. We ran ANOVA statistical tests to identify significant differences in soil properties among SBS levels. We also established general linear regressions of thermo-recalcitrance values and aromaticity with biological soil quality indices to compare both techniques for detecting changes in SOM quality and composition. The results indicated that fire increased soil pH (up to 0.3), particularly in the plots with higher SBS levels. SOM decreased significantly with increasing SBS level (down to < 5 % at the high SBS level), with a shift from labile compounds (carbohydrates) to more recalcitrant ones (aromatics). Organic P forms were depleted, while orthophosphate levels rose, increasing the risk of irreversible fixation. This study also highlights that DSC-TG is a cost-effective technique for assessing SOM quality changes. Understanding these effects is essential for developing policies to conserve and restore fire-affected areas and to promote practices that enhance soil functionality and resilience.

Mechanochemical Polyoxometalate Super-Reduction with Lithium Metal.

In this first systematic investigation of mechanochemical polyoxometalate (POM) reduction, (TBA)3[PMo12O40] was reacted with n equiv of lithium metal (n = 1-24) to generate PMo12/n products which were shown to be mixtures of electron-rich PMo12Lix species. FTIR analysis revealed the lengthening/weakening of terminal Mo═O bonds with increasing levels of reduction, while EXAFS spectra indicated the onset of Mo-Mo bond formation at n ∼ 8 and a significant structural change at n > 12. Successive MoVI reductions were monitored by XANES and XPS, and at n = 24, results were consistent with the formation of at least one MoIV-MoIV bonded {MoIV3} triad together with MoV. Upon dissolution, the PMo12Lix species present in the solid PMo12/n products undergo electron exchange and single-peak 31P NMR spectra were observed for n = 1-12. For n ≥ 16, changes in solid state and solution 31P NMR spectra coincided with the emergence of features in the UV-vis spectra associated with MoV-MoV and {MoIV3} bonding in an ε-Keggin structure. Bonding between {Li(NCMe)}+ and 2-electron-reduced PMo12 in (TBA)4[PMo12O40{Li(NCMe)}] suggests that super-reduction gives rise to more extensive Li-O bonding that ultimately causes lithium-oxide-promoted TBA cation decomposition and POM degradation, which might explain the appearance of XPS peaks for Mo2C at n ≥ 16. This work has revealed some of the complex, unexplored chemistry of super-reduced POMs and establishes a new, solvent-free approach in the search for a better fundamental understanding of the electronic properties and reactivity of electron-rich nanoscale metal oxides.

On the Structure and Role of Avian Eggshells: A 31P, 1H, and 13C Solid-State NMR Study.

The eggshell is a composite and highly ordered structure formed by biomineralization. Besides other functions, it has a vital and intricate role in the protection of an embryo from various potentially harsh environmental conditions. Solid-state nuclear magnetic resonance (SSNMR) has been used for detailed structural investigations of the chicken, tinamou, and flamingo eggshell materials. 31P NMR spectra reveal that hydroxyapatite and β-tricalcium phosphate in the ratio 3:2 represent major constituents of phosphate species in the eggshells. All three eggshells exhibit similar spectra, except for the line widths, which implies different structural order of phosphate species in the chicken, tinamou, and flamingo eggshells. 1H NMR spectra for these materials are comparable, differentiating overlapped peaks in three spectral regions at around 7, 4-5, and 1-2 ppm. These spectral regions have been attributed to protons from NH or CaHCO3, water, and possibly isolated monomeric water molecules or hydroxyl groups in calcium-deficient hydroxyapatite. 1H-13C CP MAS NMR revealed the presence of organic matter in the form of lipids and proteins. Two overlapped resonances in the carbonyl region at around 173 and 169 ppm are assigned to the carbonyls of the peptide bonds and the bicarbonate unit in calcite, respectively. Fourier-transform infrared spectroscopy (FTIR) spectra confirmed the presence of structural units detected in the NMR spectra.

Structural evolution study by solid-state NMR of photopolymerizable aluminophosphosilicate hybrid materials

Aluminophosphosilicate materials of composition (mol.%) xSiO2[(Al2O3)50–(P2O5)50]100-x were prepared through sol-gel method and heat-treated yielding non-crystalline materials. The structural evolution from the hybrid to heat-treated material as a function of the SiO2 content was studied by 13C, 29Si, 31P and 27Al solid-state NMR, infrared spectroscopies, X-ray diffraction and differential scanning calorimetry (DSC). NMR analyses showed the AlPO4 formation in the matrix and heterocondensation, forming Al-O-P and P-O-Si linkages. The heat treatment promoted the formation of a more interconnected SiO2 tetrahedra network as the SiO2 content increased. Similar to the hybrid materials, the increase of SiO2 content induces the replacement of P-O-Al(IV) bonds by Si-O-Al(IV) bonds in the heat-treated materials as confirmed by 27Al NMR and 27Al{31P} REDOR data, which was also supported by Raman analysis. This sol-gel route provides a novel way to fabricate new homogeneous materials, which can be doped with rare earths and applied to new optical devices.

Probing Chemical Equilibrium in Frozen Sodium Phosphate Buffer Solution by 31P Solid-State NMR.

Phosphate buffers are crucial for cryopreservative stability in pharmaceuticals, food processing, biomedical sciences, and biology. However, their freeze concentrates lack quantitative characterization, especially regarding the physicochemical properties of phosphate salt species in equilibrium at subzero temperatures. This study employs 31P solid-state NMR (ssNMR) to analyze frozen sodium phosphate (NaP) solutions, providing insights into phase composition, ionic strength, and pH. For the first time, we have directly quantified phosphate species in frozen NaP buffer, including crystallized disodium phosphate dodecahydrate (Na2HPO4·12H2O) content and the concentrations of H2PO4- and HPO42- in the freeze concentrate. This enabled the calculation of the pH as well as the ionic strength in the freeze concentrate. Trehalose effectively mitigated pH shifts in buffer solutions by preventing the selective crystallization of salt, a spectroscopic phenomenon not previously observed experimentally.

Phosphaarsenes - Combining Phospha- and Arsa-Wittig-Reagents.

Dipnictenes of the type RPn=PnR (Pn=P, As, Sb, Bi) can be viewed as dimers of the corresponding pnictinidenes R-Pn. Phosphanylidene- and arsanylidenephosphoranes (R-Pn(PMe3); Pn=P, As) have been shown to be versatile synthetic surrogates for the delivery of pnictinidene fragments. We now report that thermal treatment of 1 : 1 mixtures of R-P(PMe3) and R'-As(PMe3) gives access to arsaphosphenes of the type RP=AsR'. Three examples are presented and the properties and reactivity of Mes*P=AsDipTer (1) (Mes*=2,4,6-tBu3-C6H2; DipTer=2,6-(2,6-iPr2C6H3)2-C6H3) were studied in detail. Solid state 31P NMR spectroscopy revealed a large 31P NMR chemical shift anisotropy with a span of ca. 920 ppm for 1 while computational methods were employed to investigate this pronounced magnetic deshielding of the P atom in 1. In the presence of the carbene IMe4 (IMe4=:C(MeNCMe)2) 1 is shown to be split into the corresponding NHC adducts Mes*P(IMe4) and DipTerAs(IMe4), which is additionally shown for diarsenes.

Exploring Deactivation Reasons of Biomass-Based Phosphorus-Doped Carbon as a Metal-Free Catalyst in the Catalytic Dehydroaromatization of n-Heptane.

Catalytic dehydroaromatization of n-alkanes into high-value aromatics has garnered extensive interest from both academia and industry. Our group has previously reported that phosphorus-doped carbon materials exhibit high selectivity for C-H bond activation in the dehydroaromatization of n-hexane. In this study, using n-heptane as a probe, we synthesized biomass-based phosphorus-doped carbon catalysts to investigate the impact of hydrogen heat treatment and carbon deposition on catalyst structure. Despite achieving an initial conversion of n-heptane at approximately 99.6%, with a toluene selectivity of 87.9%, the catalyst activity fell quickly. Moreover, longer hydrogen treatment time and higher hydrogen concentrations were found to accelerate catalyst deactivation. Thermogravimetric analysis (TGA) and N2 adsorption measurements (BET) indicated that a small amount of coke deposition was not the primary cause of catalyst deactivation. Temperature-programmed desorption of ammonia gas (NH3-TPD) revealed a significant decrease in acid-active functional groups. X-ray photoelectron spectroscopy (XPS) and solid-state 31P NMR spectroscopy confirmed the reduction of active central phosphorus species. These results suggest that catalyst deactivation primarily arises from the decrease in acidity and the partial reduction of phosphorus-containing groups, leading to a substantial loss of active sites. This work contributes new perspectives to understanding the properties and design improvements of metal-free carbon catalysts.

Surface immobilization mechanisms of cobalt ions on hydroxyapatite catalyst supports

Cobalt deposition in an excess of solution was used to design Co-modified hydroxyapatite materials with various Co loadings and controlled dispersion. It is rationalized how the properties of hydroxyapatite supports (more or less stoichiometric compositions, nanorod or platelet morphologies and crystalline (100) zig-zag termination or non-apatitic hydrated external layer), influence the immobilization processes of Co by operating either with slightly acidic (natural) or basic pH of the suspension media. Four cobalt immobilization mechanisms impacting the final dispersion of Co on hydroxyapatites were identified by combining structural (XRD, 1H and 31P solid-state NMR, UV-Vis, Raman and X-ray fluorescence spectroscopies), surface (XPS) characterizations of Co-modified hydroxyapatites after drying and thermal treatment at 500 °C, and monitoring of the pH and the composition of the supernatant solutions during the Co deposition step. On dried Co-modified crystalline stoichiometric hydroxyapatite nanorods, cobalt is highly dispersed through cationic exchange or strong electrostatic adsorption (SEA) at slightly acidic or basic pH, respectively. After thermal treatment at 500 °C, only cation exchange preserved atomic dispersion of Co(II) ions since Co3O4 nanoparticles were observed on samples for which Co deposition occurred via SEA. On defective hydroxyapatite platelets, cobalt deposited at acidic natural pH could diffuse in an external non-apatitic layer, whereas under basic pH media, this surface layer was hydrolysed, resulting in the formation of a cobalt-substituted hydroxyapatite layer in which only a limited fraction of the surface cobalt species could be probed by XPS.

Scalable phosphorylated cellulose production with improved environmental sustainability, crosslinkability and processability using 3D bioprinting for dye remediation

In the present work, phosphorylated cellulose (PC) gel has been produced following an environmentally benign approach using agro-based chemicals with improved yield. The PC gels produced were transparent, negatively charged with high consistency, charge content (1133.33 mmol/kg), degree of substitution (DS) of 0.183 and increased yield (>87 %). The XPS and EDS analysis confirms the covalently bonded phosphate groups at weight percent of 9.42 % and 11.01 %, respectively. The life cycle assessment (LCA) shows that PC gel production via the phosphorylation route is an ecologically favourable strategy compared with traditional TEMPO oxidation, resulting in 1.67 times lower CO2 emission. The rheological studies of PC gels show shear-thinning behaviour with improved 3D printability followed by heat-induced crosslinking of phosphate groups. The mechanistic insights for the condensation of phosphate to form a phosphoric ester group during cross-linking were evaluated through 31P solid-state NMR and XPS studies. Interestingly, the 3D-printed structures showed high structural stability under both compression and tensile load in both dry and wet conditions, with high water absorption (5408.33 %) and swelling capacity of 700 %. The structures show improved methylene blue (MB) remediation capabilities with a maximum removal efficiency of 99 % for 10–200 mg/L and more than seven times reusability. This work provides a green, facile and energy-efficient strategy for fabricating PCs with easy processability through additive manufacturing techniques for producing value-added products, opening up new avenues for high-performance applications.

Nanotube Structure of AsPS4-xSex (x = 0, 1).

Single-wall nanotubes of isostructural AsPS4-xSex (x = 0, 1) are grown from solid-state reaction of stoichiometric amounts of the elements. The structure of AsPS4 was determined using single-crystal X-ray diffraction and refined in space group . The infinite, single-walled AsPS4 nanotubes have an outer diameter of ≈1.1 nm and are built of corner-sharing PS4 tetrahedra and AsS3 trigonal pyramids. Each nanotube is nearly hexagonal, but the ≈3.4 Å distance between S atoms on adjacent nanotubes allows them to easily slide past one another, resulting in the loss of long-range order. Substituting S with Se disrupted the crystallization of the nanotubes, resulting in amorphous products that precluded the determination of the structure for AsPS3Se. 31P solid-state NMR spectroscopy indicated a single unique tetrahedral P environment in AsPS4 and five different P environments all with different degrees of Se substitution in AsPS3Se. Optical absorption spectroscopy revealed an energy band gap of 2.7 to 2.4 eV for AsPS4 and AsPS3Se, respectively. Individual AsPS4 microfibers showed a bulk conductivity of 3.2 × 10-6 S/cm and a negative photoconductivity effect under the illumination of light (3.06 eV) in ambient conditions. Thus, intrinsic conductivity originates from hopping through empty trap states along the length of the AsPS4 nanotubes.

Synthesis and morphological control of Ca5(PO4)3Cl and Ca2PO4Cl via the phase transformation of amorphous calcium phosphate in molten chlorides

In the present work, the phase conversion of amorphous calcium phosphate (ACP) in different molten chlorides (LiCl, NaCl, KCl, CaCl2) was investigated in detail. The main synthesis parameters influencing the phase purity and morphological features of the products include the chemical composition of molten salts, the heat treatment temperature and the ACP-to-flux ratio. The selective synthesis of single-phase Ca5(PO4)3Cl or Ca2PO4Cl depends on the content of CaCl2 in the reaction medium. The morphology control of Ca5(PO4)3Cl powders was achieved by varying the KCl/CaCl2 ratio in the flux, resulting in the formation of the particles of different size and shape. The KCl-rich fluxes led to the formation of relatively small nearly spherical particles, whereas the CaCl2-rich fluxes promoted an anisotropic growth of the Ca5(PO4)3Cl crystals resulting in the formation of monodispersed hexagonally-shaped microrods. Whereas the anisotropic growth was observed at relatively low temperature (750 °C) the increase of the reaction temperature up to 1200 °C significantly reduced this effect leading to the formation of the particles with obviously low aspect ratio. The phase crystallinity and purity were analyzed using powder X-ray diffraction, FTIR spectroscopy as well as 31P, 35Cl and 1H solid-state NMR. The morphological features and chemical composition of the synthesized products were studied by SEM/EDX analysis.

Small organic molecules containing amorphous calcium phosphate: synthesis, characterization and transformation.

As the primary solid phase, amorphous calcium phosphate (ACP) is a pivotal precursor in cellular biomineralization. The intrinsic interplay between ACP and Howard factor underscores the significance of understanding their association for advancing biomimetic ACP development. While organic compounds play established roles in biomineralization, this study presents the synthesis of ACP with naturally occurring organic compounds (ascorbate, glutamate, and itaconate) ubiquitously found in mitochondria and vital for bone remodeling and healing. The developed ACP with organic compounds was meticulously characterized using XRD, FTIR, and solid-state 13C and 31P NMR. The morphological analysis revealed the characteristic spherical morphology with particle size close to 20nm of all synthesized ACP variants. Notably, the type of organic compound strongly influences true density, specific surface area, particle size, and transformation. The in vitro analysis was performed with MC3T3-E1 cells, indicating the highest cell viability with ACP_ASC (ascorbate), followed by ACP_ITA (itaconate). The lowest cell viability was observed with 10 %w/v of ACP_GLU (glutamate); however, 1 %w/v of ACP_GLU was cytocompatible. Further, the effect of small organic molecules on the transformation of ACP to low crystalline apatite (Ap) was examined in Milli-Q® water, PBS, and α-MEM.

A copper-containing analog of the biomineral whitlockite: dissolution-precipitation synthesis, structural and biological properties.

In the present work, copper whitlockite (Cu-WH, Ca18Cu2(HPO4)2(PO4)12) was successfully synthesized and comprehensively characterized, founding the base knowledge for its future studies in medicine, particularly for bone regeneration. This material is a copper-containing analog of the well-known biomineral magnesium whitlockite (Mg-WH, Ca18Mg2(HPO4)2(PO4)12). The synthesis of powders was performed by a dissolution-precipitation method in an aqueous medium under hydrothermal conditions. Phase conversion from brushite (CaHPO4·2H2O) to Cu-WH took place in an acidic medium in the presence of Cu2+ ions. Optimization of the synthesis conditions in terms of medium pH, temperature, time, Ca/Cu molar ratio and concentration of starting materials was performed. The crystal structure of the synthesized products was confirmed by XRD, FTIR and Raman spectroscopy, 1H and 31P solid-state NMR, and EPR. Morphological features and elemental distribution of the synthesized powders were studied by means of SEM/EDX analysis. The ion release in SBF solution was estimated using ICP-OES. Cytotoxicity experiments were performed with MC3T3-E1 cells. The study on thermal stability revealed that the synthesized material is thermally unstable and gradually decomposes upon annealing to Cu-substituted β-Ca3(PO4)2 and Ca2P2O7.

Triclinic La7Zn2P11 with P3-, P24-, and P35- units: a combined study by 31P solid-state NMR spectroscopy and single crystal X-ray diffraction.

The ternary polyphosphide La7Zn2P11 was synthesized from the elements by using a salt flux or via a ceramic method in sealed quartz ampoules. The obtained samples were investigated by X-ray powder and single crystal diffraction: own type, P1̄, a = 775.33(13), b = 827.45(13), c = 1502.8(3) pm, α = 82.111(3), β = 77.034(3), γ = 89.996(3)°, wR2 = 0.1553, 5852 F2 values and 183 variables. This peculiar structure is characterized by the simultaneous presence of three distinct anionic phosphide species, namely P3-, P24-, and P35- units. La7Zn2P11 is an electron precise Zintl phase: (7La3+)21+(2Zn2+)4+(4P3-)12-(2P24-)8-(P35-). The P-P single bond distances range from 219.2 to 223.0 pm. The zinc sites show tetrahedral phosphorus coordination by three P3- and one P24- species. The tetrahedra are condensed to chains via common corners. The P35- units with P-P-P angles of 113.7° have exclusively lanthanum coordination. 31P solid-state NMR was used to probe the phosphorus local environments, connectivities and spatial proximities. The eleven crystallographically distinct phosphorus atoms were assigned with the help of two-dimensional homonuclear dipolar correlation experiments. Even though the application of 2D measurements on such phosphorus-based polyanionic compounds is exceedingly challenging because of the wide dispersion of chemical shifts, the fast irreversible decay of the transverse magnetization, and slow spin-lattice relaxation, a complete assignment is possible using radiofrequency-driven dipolar recoupling (RFDR), J-RESOLVED and total-through-bond correlation with R-sequence (R-TOBSY) techniques.

Temperature-induced mobility in octacalcium phosphate impacts crystal symmetry: water dynamics studied by NMR crystallography.

Octacalcium phosphate (OCP, Ca8(PO4)4(HPO4)2·5H2O) is a notable calcium phosphate due to its biocompatibility, making it a widely studied material for bone substitution. It is known to be a precursor of bone mineral, but its role in biomineralisation remains unclear. While the structure of OCP has been the subject of thorough investigations (including using Rietveld refinements of X-ray diffraction data, and NMR crystallography studies), important questions regarding the symmetry and H-bonding network in the material remain. In this study, it is shown that OCP undergoes a lowering of symmetry below 200 K, evidenced by 1H, 17O, 31P and 43Ca solid-state NMR experiments. Using ab initio molecular-dynamics (MD) simulations and gauge including projected augmented wave (GIPAW) DFT calculations of NMR parameters, the presence of rapid motions of the water molecules in the crystal cell at room temperature is proved. This information leads to an improved description of the OCP structure at both low and ambient temperatures, and helps explain long-standing issues of symmetry. Remaining challenges related to the understanding of the structure of OCP are then discussed.

Amphiphilic titania Janus nanoparticles containing ionic groups prepared in oil-water Pickering emulsion.

Titania nanoparticles with a diameter of 8 nm underwent an anisotropic modification using apolar 6-bromohexylphosphonic acid and cationic polar N,N,N-trimethyl-6-phosphonohexan-1-aminium bromide. The Janus modification was achieved through a straightforward one-step Pickering emulsion approach using toluene-water mixtures. The resulting Janus particles were compared with isotropically and statistically modified titania particles, where either a single coupling agent is attached to the surface or both coupling agents are assembled over the surface randomly, respectively. The covalent binding of the phosphonic acids to the titania surface was confirmed by FTIR and 31P solid-state CP-MAS NMR analyses. The grafting density was assessed using TGA, elemental analysis, and ICP-MS, revealing grafting densities of 0.1 mmol g-1 to 0.5 mmol g-1 for the cationic coupling agent and 1.2 mmol g-1 to 1.5 mmol g-1 for the apolar coupling agent, respectively. ζ-Potential titration measurements of both pristine and modified particles revealed isoelectric points at pH 4.5 to 9.3, depending on the type of modification. The ability of the particles to stabilize Pickering emulsions was tested under various conditions, with statistically and Janus-modified particles demonstrating a significant increase in stabilization compared to their isotropically modified counterparts. Furthermore, Janus particles were deposited onto glass substrates by a simple layer-by-layer approach. Through the self-assembly of these Janus particles, the glass substrate's properties could be tailored from hydrophilic to hydrophobic to hydrophilic, depending on the dipping cycle.

In-depth characterization of phosphate intercalated Mg Al Layered double hydroxides and study of the PO4 release properties.

Slow-release fertilizers (SRFs) form the core of innovative strategies in sustainable agriculture. Layered Double Hydroxides (LDH), known for their high capacity to sequester plant nutrients, especially phosphate, are emerging as promising candidates for SRF synthesis. The phosphate release properties of MgAl LDH (with a targeted Mg/Al ratio of 2.0) intercalated with HPO42- anions were assessed in various aqueous environments. A comprehensive analysis, including in-depth chemical and structural characterizations (ICP-OES, XRD, PDF, 27Al NMR, 31P NMR, FTIR, SEM) of the as-prepared phase unveiled a more intricate composition than anticipated for a pure or ideal Mg2Al-HPO4 LDH, encompassing an excess of intercalated phosphate in conjunction with K+. Beyond the intercalated phosphate, solid state 31P NMR speciation identified multiple HxPO4(-3+x) environments, indicating a portion of the phosphate reacting with intralayer Mg2+ to form K-struvite. Additionally, some phosphates were adsorbed onto the surface of amorphous aluminum hydroxide, a side phase formed during MgAl coprecipitation. The phosphate release demonstrated rapid kinetics, occurring within 6 days. Moreover, the released phosphate increased significantly when reducing the Solid/Liquid (S/L) ratio (58%) and further increasing in the presence of carbonate ions (90%). The released phosphate varied from 12% to 90% under different release conditions, transitioning from water to a 3.33 mM NaHCO3 aqueous solution at a low S/L ratio (from 20 mg LDH per mL to 0.02 mg LDH per mL). The simultaneous release of K+, Mg2+, Al3+ indicated the complete dissolution of the K-struvite and partial dissolution of phosphate intercalated MgAl LDH. These results enhanced our understanding of the mechanism governing phosphate release from MgAl LDH, paving the way for potential phosphate recovery by LDH or for the development of LDH-based SRFs.

Site-directed cation ordering in chabazite-type AlxGa1-xPO4-34 frameworks revealed by NMR crystallography.

We report the first synthesis of the mixed-metal chabazite-type AlxGa1-xPO4-34(mim) solid solution, containing 1-methylimidazolium, mim, as structure directing agent (SDA), from the parent mixed-metal oxide solid solution, γ-(AlxGa1-x)2O3. 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 MO4 and one octahedral MO4F2 (M = Al, Ga). A combination of solid-state NMR spectroscopy and periodic DFT calculations reveals that the octahedral site is preferentially occupied by Al and the tetrahedral sites by Ga, leading to a non-random distribution of cations within the framework. Upon calcination to the AlxGa1-xPO4-34 framework, all metal sites are tetrahedral and crystallographically equivalent in the average R3̄ symmetry. The cation distribution was explored by 31P solid-state NMR spectroscopy, and it is shown that the non-random distribution demonstrated to exist in the as-made materials would be expected to give remarkably similar patterns of peak intensities to a random distribution owing to the change in average symmetry in the calcined materials.

Deconvolution of Charge-Transfer, Mass Transfer, and Ohmic Resistances of Phosphonic Acid-Sulfonic Acid Ionomer Binders in Electrochemical Hydrogen Pumps

Ion-pair high-temperature polymer electrolyte membranes (HT-PEMs) paired with phosphonic acid ionomer electrode binders have substantially improved the performance of HT-PEM electrochemical hydrogen pumps (EHPs)1, 2 and fuel cells3. Recently, blending poly(pentafluorstyrene-co-tetrafluorostyrene phosphonic acid) (PTFSPA) with NafionTM improved ionomer conductivity under anhydrous conditions in the temperature range of 100 °C to 250 °C. Using the said polymer blend as an electrode binder resulted in a 2 W.cm-2 peak power density of fuel cells4 at 240 °C (a HT-PEM fuel cell record). However, much is still unknown about how phosphonic acid ionomers blended with perfluorosulfonic acid (PFSA) materials affect electrode kinetics and gas transport in porous electrodes. In this work, we studied the ionic conductivity, electrode kinetics, and gas transport resistance of 3 types of phosphonic acid ionomers, poly(vinyl phosphonic acid), poly(vinyl benzyl phosphonic acid) by themselves and when blended with Aquivion®. These studies were performed in the context of an EHP platform – both membrane electrode assemblies (MEAs) and interdigitated electrode arrays (IDAs) decorated with nanoscale platinum electrocatalysts with thin film ionomer electrolytes. For all phosphoric acid ionomer types, the addition of Aquivion® promoted ionic conductivity, hydrogen oxidation/evolution reaction kinetics (HOR/HER), and hydrogen gas permeability. Solid-state 31P NMR revealed that the addition of Aquivion® significantly reduced phosphate ester formation in phosphoric acid ionomers and this played a vital role in enhancing ionomer blend conductivity. Using the best blend variant, PTFSPA-Aquivion®, a remarkable EHP performance of 5.1 A.cm-2 at 0.4 V at T = 200 °C was attained. Density functional theory (DFT) simulations identified that phosphonic acids with electron-withdrawing moieties reduced the propensity of the phosphonic acid to adsorb to platinum electrocatalyst surfaces. All phosphonic acid ionomers are anticipated to have a greater affinity for adsorption on platinum when compared to sulphonic acid ionomers (PFSAs). The relative adsorption affiliation of the various phosphoric acid ionomers from DFT is consistent with an explanation that stronger anion-specific adsorption has a detrimental impact and is commensurate with experimentally obtained MEA charge-transfer kinetics. A machine learning-aided compositional model5 revealed that the addition of Aquivion® reduced activation and concentration overpotentials in EHP MEAs and improved exchange current density and diffusivity in EHP IDAs. Future work involves employing the EHPs in a real-world application for separating and compressing hydrogen from the natural gas mixture.References Venugopalan, G.; Bhattacharya, D.; Andrews, E.; Briceno-Mena, L.; Romagnoli, J.; Flake, J.; Arges, C. G., Electrochemical Pumping for Challenging Hydrogen Separations. ACS Energy Letters 2022, 7 (4), 1322-1329.Venugopalan, G.; Bhattacharya, D.; Kole, S.; Ysidron, C.; Angelopoulou, P. P.; Sakellariou, G.; Arges, C. G., Correlating high temperature thin film ionomer electrode binder properties to hydrogen pump polarization. Materials Advances 2021, 2 (13), 4228-4234.Atanasov, V.; Lee, A. S.; Park, E. J.; Maurya, S.; Baca, E. D.; Fujimoto, C.; Hibbs, M.; Matanovic, I.; Kerres, J.; Kim, Y. S., Synergistically integrated phosphonated poly(pentafluorostyrene) for fuel cells. Nat Mater 2021, 20 (3), 370-377.Lim, K. H.; Lee, A. S.; Atanasov, V.; Kerres, J.; Park, E. J.; Adhikari, S.; Maurya, S.; Manriquez, L. D.; Jung, J.; Fujimoto, C.; Matanovic, I.; Jankovic, J.; Hu, Z.; Jia, H.; Kim, Y. S., Protonated phosphonic acid electrodes for high power heavy-duty vehicle fuel cells. Nature Energy 2022, 7 (3), 248-259.Briceno-Mena, L. A.; Venugopalan, G.; Romagnoli, J. A.; Arges, C. G., Machine learning for guiding high-temperature PEM fuel cells with greater power density. Patterns 2021, 2 (2), 100187. Figure 1

Instantaneous and efficient removal of glyphosate with low concentration by amine-functionalized SBA-15 chelated with Fe(III)

Glyphosate has drawn great attention due to its potentially detrimental effect to ecosystems, but the selective removal of low-concentration glyphosate from surface water remains challenging. Herein, a novel adsorbent (Fe(III)/NN-SBA-15) for the adsorption of low-concentration glyphosate from aqueous solution was prepared by using ethylenediamine (EDA) chelating with Fe(III) to modify SBA-15. Adsorption study showed that the Fe(III)/NN-SBA-15 has excellent adsorption capacity for glyphosate (71.4 mg/g). Most importantly, the Fe(III)/NN-SBA-15 exhibited fast adsorption kinetics for glyphosate, 99% of glyphosate could be quickly removed within 1 min when the glyphosate concentration from 5 mg/L to 21 mg/L, which was outperforms many other adsorbents previous reported. In addition, the results of the effect of competing ions on the adsorption process showed that Fe(III)/NN-SBA-15 had good selectivity for glyphosate removal from water. According to the FTIR and 31P solid-state NMR spectra, the direct Fe(III)-phosphate complexation played the dominant role in glyphosate removal on Fe(III)/NN-SBA-15. In summary, the high adsorption capacity, fast adsorption kinetics, and good selectivity make Fe(III)/NN-SBA-15 a promising sorbent for high-efficient removal of glyphosate from aqueous solutions with low initial concentration.