Elucidation of the role of metals in the adsorption and photodegradation of herbicides by metal-organic frameworks.

Here, four MOFs, namely Sc-TBAPy, Al-TBAPy, Y-TBAPy, and Fe-TBAPy (TBAPy: 1,3,6,8-tetrakis(p-benzoic acid)pyrene), were characterized and evaluated for their ability to remediate glyphosate (GP) from water. Among these materials, Sc-TBAPy demonstrates superior performance in both the adsorption and degradation of GP. Upon light irradiation for 5 min, Sc-TBAPy completely degrades 100% of GP in a 1.5 mM aqueous solution. Femtosecond transient absorption spectroscopy reveals that Sc-TBAPy exhibits enhanced charge transfer character compared to the other MOFs, as well as suppressed formation of emissive excimers that could impede photocatalysis. This finding was further supported by hydrogen evolution half-reaction (HER) experiments, which demonstrated Sc-TBAPy's superior catalytic activity for water splitting. In addition to its faster adsorption and more efficient photodegradation of GP, Sc-TBAPy also followed a selective pathway towards the oxidation of GP, avoiding the formation of toxic aminomethylphosphonic acid observed with the other M3+-TBAPy MOFs. To investigate the selectivity observed with Sc-TBAPy, electron spin resonance, depleted oxygen conditions, and solvent exchange with D2O were employed to elucidate the role of different reactive oxygen species on GP photodegradation. The findings indicate that singlet oxygen (1O2) plays a critical role in the selective photodegradation pathway achieved by Sc-TBAPy.

Hydrogen Adsorption in Ultramicroporous Metal-Organic Frameworks Featuring Silent Open Metal Sites.

In this study, we utilized an ultramicroporous metal-organic framework (MOF) named [Ni3(pzdc)2(ade)2(H2O)4]·2.18H2O (where H3pzdc represents pyrazole-3,5-dicarboxylic acid and ade represents adenine) for hydrogen (H2) adsorption. Upon activation, [Ni3(pzdc)2(ade)2] was obtained, and in situ carbon monoxide loading by transmission infrared spectroscopy revealed the generation of open Ni(II) sites. The MOF displayed a Brunauer-Emmett-Teller (BET) surface area of 160 m2/g and a pore size of 0.67 nm. Hydrogen adsorption measurements conducted on this MOF at 77 K showed a steep increase in uptake (up to 1.93 mmol/g at 0.04 bar) at low pressure, reaching a H2 uptake saturation at 2.11 mmol/g at ∼0.15 bar. The affinity of this MOF for H2 was determined to be 9.7 ± 1.0 kJ/mol. In situ H2 loading experiments supported by molecular simulations confirmed that H2 does not bind to the open Ni(II) sites of [Ni3(pzdc)2(ade)2], and the high affinity of the MOF for H2 is attributed to the interplay of pore size, shape, and functionality.

Strengthening Aqueous Electrolytes without Strengthening Water.

Aqueous electrolytes typically suffer from poor electrochemical stability; however, eutectic aqueous solutions-25 wt.% LiCl and 62 wt.% H3 PO4 -cooled to -78 °C exhibit a significantly widened stability window. Integrated experimental and simulation results reveal that, upon cooling, Li+ ions become less hydrated and pair up with Cl- , ice-like water clusters form, and H⋅⋅⋅Cl- bonding strengthens. Surprisingly, this low-temperature solvation structure does not strengthen water molecules' O-H bond, bucking the conventional wisdom that increasing water's stability requires stiffening the O-H covalent bond. We propose a more general mechanism for water's low temperature inertness in the electrolyte: less favorable solvation of OH- and H+ , the byproducts of hydrogen and oxygen evolution reactions. To showcase this stability, we demonstrate an aqueous Li-ion battery using LiMn2 O4 cathode and CuSe anode with a high energy density of 109 Wh/kg. These results highlight the potential of aqueous batteries for polar and extraterrestrial missions.

Hypsochromically-shifted Emission of Metal-organic Frameworks Generated through Post-synthetic Ligand Reduction.

Luminescent materials with tunable emission are becoming increasingly desirable as we move towards needing efficient Light Emitting Diodes (LEDs) for displays. Key to developing better displays is the advancement of strategies for rationally designing emissive materials that are tunable and efficient. We report a series of emissive metal-organic frameworks (MOFs) generated using BUT-10 (BUT: Beijing University of Technology) that emits green light with λmax at 525 nm. Post-synthetic reduction of the ketone on the fluorenone ligand in BUT-10 generates new materials, BUT-10-M and BUT-10-R. The emission for BUT-10-R is hypsochromically-shifted by 113 nm. Multivariate BUT-10-M structures demonstrate emission with two maxima corresponding to the emission of both fluorenol and fluorenone moieties present in their structures. Our study represents a novel post-synthetic ligand reduction strategy for producing emissive MOFs with tunable emission ranging from green, white-blue to deep blue.

Bismuth-Polyoxocation Coordination Networks: Controlling Nuclearity and Dimension-Dependent Photocatalysis.

Bismuth-oxocluster nodes for metal-organic frameworks (MOFs) and coordination networks/polymers are less prolific than other families featuring zinc, zirconium, titanium, lanthanides, etc. However, Bi3+ is non-toxic, it readily forms polyoxocations, and its oxides are exploited in photocatalysis. This family of compounds provides opportunity in medicinal and energy applications. Here, we show that Bi node nuclearity depends on solvent polarity, leading to a family of Bix-sulfonate/carboxylate coordination networks with x = 1-38. Larger nuclearity-node networks were obtained from polar and strongly coordinating solvents, and we attribute the solvent's ability to stabilize larger species in solution. The strong role of the solvent and the lesser role of the linker in defining node topologies differ from other MOF syntheses, and this is due to the Bi3+ intrinsic lone pair that leads to weak node-linker interactions. We describe this family by single-crystal X-ray diffraction (eleven structures), obtained in pure forms and high yields. Ditopic linkers include NDS (1,5-naphthalenedisulfonate), DDBS (2,2'-[biphenyl-4,4'-diylchethane-2,1-diyl] dibenzenesulphonate), and NH2-benzendicarboxylate (BDC). While the BDC and NDS linkers yield more open-framework topologies that resemble those obtained by carboxylate linkers, topologies with DDBS linkers appear to be in part driven by association between DDBS molecules. An in situ small-angle X-ray scattering study of Bi38-DDBS reveals stepwise formation, including Bi38-assembly, pre-organization in solution, followed by crystallization, confirming the less important role of the linker. We demonstrate photocatalytic hydrogen (H2) generation with select members of the synthesized materials without the benefit of a co-catalyst. Band gap determination from X-ray photoelectron spectroscopy (XPS) and UV-vis data suggest the DDBS linker effectively absorbs in the visible range with ligand-to-Bi-node charge transfer. In addition, materials containing more Bi (larger Bi38-nodes or Bi6 inorganic chains) exhibit strong UV absorption, also contributing to effective photocatalysis by a different mechanism. All tested materials became black with extensive UV-vis exposure, and XPS, transmission electron microscopy, and X-ray scattering of the black Bi38-framework suggest that Bi0 is formed in situ, without phase segregation. This evolution leads to enhanced photocatalytic performance, perhaps due to increased light absorption.

Photoactive Organo-Sulfur Polymers for Hydrogen Generation.

Herein, we report the synthesis of photoactive polymeric organo-sulfur (POS) materials. These polymers absorb light in the ultraviolet/visible and near-infrared region of the solar spectrum, and upon irradiation, they reduce water to hydrogen (H2 ). The decoration of POS materials with nitrile (-CN) groups is found to be the critical factor for enhanced interactions with the co-catalyst, Ni2 P, leading to greater H2 evolution rates compared to the nitrile-free POS material.

Wet flue gas CO2 capture and utilization using one-dimensional metal-organic chains.

Herein, we describe the use of an ultramicroporous metal-organic framework (MOF) with a composition of [Ni3(pzdc)2(ade)2(H2O)1.5]·(H2O)1.3 (pzdc: 3,5-pyrazole dicarboxylic acid; ade: adenine), for the selective capture of carbon dioxide (CO2) from wet flue gas followed by its conversion to value-added products. This MOF is comprised of one-dimensional Ni(II)-pyrazole dicarboxylate-adenine chains; through pi-pi stacking and H-bonding interactions, these one-dimensional chains stack into a three-dimensional supramolecular structure with a one-dimensional pore network. Upon heating, our MOF undergoes a color change from light blue to lavender, indicating a change in the coordination geometry of Ni(II). Variable temperature ultraviolet-visible (UV/vis) spectroscopy data revealed a blue shift of the d-d transitions, suggesting a change in the Ni-coordination geometry from octahedral to a mixture of square planar and square pyramidal. The removal of the water molecules coordinated to Ni(II) leads to the generation of a MOF with open Ni(II) sites. Nitrogen isotherms collected at 77 K and 1 bar revealed that this MOF is microporous with a pore volume of 0.130 cm3 g-1. Carbon dioxide isotherms show a step in the uptake at low pressure, after which the CO2 uptake is saturated. The step in the CO2 uptake is likely attributable to the rearrangement of the three-dimensional supramolecular structure to accommodate CO2 within its pores. The affinity of this MOF for CO2 is 35.5 kJ mol-1 at low loadings, and it increases to 41.9 kJ mol-1 at high loadings. While our MOF is porous to CO2 and water (H2O) at 298 K, it is not porous to N2, and the CO2/N2 selectivity increases from 28.5 to 31.5 as a function of pressure. Breakthrough experiments reveal that this MOF can capture CO2 from dry and wet flue gas with uptake capacities of 1.48 ± 0.01 and 1.14 ± 0.06 mmol g-1, respectively. The MOF can be regenerated and reused at least three times, demonstrating consistent CO2 uptake capacities. Upon understanding the sorption behavior of this MOF, catalysis experiments show that the MOF is catalytically active in the fixation of CO2 into an epoxide ring for the formation of a cyclic carbonate. The turnover frequency for this reaction is 21.95 ± 0.03 h-1. The MOF showed no catalytic deterioration after two cycles and maintained comparable catalytic activity when dry and wet CO2/N2 mixtures were used. This highlights that both N2 and H2O do not dramatically affect the catalytic activity of our MOF toward CO2 fixation.

Degradation of G-Type Nerve Agent Simulant with Phase-Inverted Spherical Polymeric-MOF Catalysts.

For the neutralization of chemical warfare agents (CWAs), the generation of an effective catalyst that can be handled safely and applied in personal protective equipment is required. Recently, zirconium-based metal-organic frameworks (Zr-MOFs: UiO-66 and UiO-67) have shown great promise in the degradation of CWAs, including nerve agents. Their catalytic activity is owed to the interplay of both Zr(IV) Lewis acids and Lewis basic groups in the MOF structure. The latter act as proximal bases that can interact with CWAs and improve the catalytic activity of Zr-MOFs. The powder form of MOFs, though, makes them impractical catalysts, as it is challenging to handle, regenerate, and reuse them. To address this challenge, we have synthesized three Zr-MOFs with Lewis basic amino and pyridine functionalities and shaped them in spherical polymeric beads using the phase inversion method. Using this method, we can generate beads with many polymer and MOF combinations (MOF@polymer). We controlled the MOF loading in these beads, and scanning electron microscopy images revealed that the MOF crystals are evenly distributed in the polymeric matrix, ensuring effective catalytic activity. We used these beads to degrade dimethyl p-nitrophenyl phosphate (DMNP), a simulant for the G-type nerve agent. Using 31P NMR, we showed that UiO-66-NH2@PES and UiO-67-(NH2)2@PES PES: poly(ether sulfone) beads destruct DMNP to dimethyl phosphate (DMP) with a half-life (t1/2) of 5.09 and 4.34 min, respectively. Beads made of hydrophobic polymers such as poly(vinylidene fluoride) (PVDF), polystyrene (PS), and Zr-MOFs with pyridine functionalities show that the quantitative hydrolysis of DMNP requires more time compared to that seen with the UiO-66-NH2@PES beads. Our work highlights the facile shaping of MOF powders into beads that can be easily regenerated with their catalytic activity to be maintained for at least three cycles of use.

Lead-Free Antimony-Based Light-Emitting Diodes through the Vapor-Anion-Exchange Method.

Hybrid lead halide perovskites continue to attract interest for use in optoelectronic devices such as solar cells and light-emitting diodes. Although challenging, the replacement of toxic lead in these systems is an active field of research. Recently, the use of trivalent metal cations (Bi3+ and Sb3+) that form defect perovskites A3B2X9 has received great attention for the development of solar cells, but their light-emissive properties have not previously been studied. Herein, an all-inorganic antimony-based two-dimensional perovskite, Cs3Sb2I9, was synthesized using the solution process. Vapor-anion-exchange method was employed to change the structural composition from Cs3Sb2I9 to Cs3Sb2Br9 or Cs3Sb2Cl9 by treating CsI/SbI3 spin-coated films with SbBr3 or SbCl3, respectively. This novel method facilitates the fabrication of Cs3Sb2Br9 or Cs3Sb2Cl9 through solution processing without the need of using poorly soluble precursors (e.g., CsCl and CsBr). We go on to demonstrate electroluminescence from a device employing Cs3Sb2I9 emitter sandwiched between ITO/PEDOT:PSS and TPBi/LiF/Al as the hole and electron injection electrodes, respectively. A visible-infrared radiance of 0.012 W·Sr-1·m-2 was measured at 6 V when Cs3Sb2I9 was the active emitter layer. These proof-of-principle devices suggest a viable path toward low-dimensional, lead-free A3B2X9 perovskite optoelectronics.