Many environmental pollutants inherently exist in their anionic forms and are therefore highly mobile in natural water systems. Cationic framework materials that can capture those pollutants are highly desirable but scarcely reported. Here we present a mesoporous cationic thorium-based MOF (SCU-8) containing channels with a large inner diameter of 2.2 nm and possessing a high surface area of 1360 m2 g−1. The anion-exchange properties of SCU-8 were explored with many anions including small oxo anions like ReO4 − and Cr2O7 2− as well as anionic organic dyes like methyl blue and the persistent organic pollutant, perfluorooctane sulfonate (PFOS). Both fast uptake kinetics and great sorption selectivity toward PFOS are observed. The underlying sorption mechanism was probed using quantum mechanical and molecular dynamics simulations. These computational results reveal that PFOS anions are immobilized in SCU-8 by driving forces including electrostatic interactions, hydrogen bonds, hydrophobic interactions, and van der Waals interactions at different adsorption stages.

Metal-organic frameworks (MOFs) based on zirconium phosphonates exhibit superior chemical stability suitable for applications under harsh conditions. These compounds mostly exist as poorly crystallized precipitates, and precise structural information has therefore remained elusive. Furthermore, a zero-dimensional zirconium phosphonate cluster acting as secondary building unit has been lacking, leading to poor designability in this system. Herein, we overcome these challenges and obtain single crystals of three zirconium phosphonates that are suitable for structural analysis. These compounds are built by previously unknown isolated zirconium phosphonate clusters and exhibit combined high porosity and ultrastability even in fuming acids. SZ-2 possesses the largest void volume recorded in zirconium phosphonates and SZ-3 represents the most porous crystalline zirconium phosphonate and the only porous MOF material reported to survive in aqua regia. SZ-2 and SZ-3 can effectively remove uranyl ions from aqueous solutions over a wide pH range, and we have elucidated the removal mechanism.

Uranium is not only a strategic resource for the nuclear industry but also a global contaminant with high toxicity. Although several strategies have been established for detecting uranyl ions in water, searching for new uranium sensor material with great sensitivity, selectivity, and stability remains a challenge. We introduce here a hydrolytically stable mesoporous terbium(III)-based MOF material compound 1, whose channels are as large as 27 Å × 23 Å and are equipped with abundant exposed Lewis basic sites, the luminescence intensity of which can be efficiently and selectively quenched by uranyl ions. The detection limit in deionized water reaches 0.9 μg/L, far below the maximum contamination standard of 30 μg/L in drinking water defined by the United States Environmental Protection Agency, making compound 1 currently the only MOF material that can achieve this goal. More importantly, this material exhibits great capability in detecting uranyl ions in natural water systems such as lake water and seawater with pH being adjusted to 4, where huge excesses of competing ions are present. The uranyl detection limits in Dushu Lake water and in seawater were calculated to be 14.0 and 3.5 μg/L, respectively. This great detection capability originates from the selective binding of uranyl ions onto the Lewis basic sites of the MOF material, as demonstrated by synchrotron radiation extended X-ray adsorption fine structure, X-ray adsorption near edge structure, and first principles calculations, further leading to an effective energy transfer between the uranyl ions and the MOF skeleton.

Although comprehensive progress has been made in the area of coordination polymer (CP)/metal-organic framework (MOF)-based proton-conducting materials over the past decade, searching for a CP/MOF with stable, intrinsic, high anhydrous proton conductivity that can be directly used as a practical electrolyte in an intermediate-temperature proton-exchange membrane fuel cell assembly for durable power generation remains a substantial challenge. Here, we introduce a new proton-conducting CP, (NH)[Zr(HPO)] (ZrP), which consists of one-dimensional zirconium phosphate anionic chains and fully ordered charge-balancing NH cations. X-ray crystallography, neutron powder diffraction, and variable-temperature solid-state NMR spectroscopy suggest that protons are disordered within an inherent hydrogen-bonded infinite chain of acid-base pairs (N-H···O-P), leading to a stable anhydrous proton conductivity of 1.45 × 10 S·cm at 180 °C, one of the highest values among reported intermediate-temperature proton-conducting materials. First-principles and quantum molecular dynamics simulations were used to directly visualize the unique proton transport pathway involving very efficient proton exchange between NH and phosphate pairs, which is distinct from the common guest encapsulation/dehydration/superprotonic transition mechanisms. ZrP as the electrolyte was further assembled into a H/O fuel cell, which showed a record-high electrical power density of 12 mW·cm at 180 °C among reported cells assembled from crystalline solid electrolytes, as well as a direct methanol fuel cell for the first time to demonstrate real applications. These cells were tested for over 15 h without notable power loss.

We report one of the most efficient scavenger materials, a cationic crystalline coordination polymer SBN for trapping ReO 4 − , a surrogate for 99 TcO 4 − , as an anionic radioactive contaminant of great concern. The uptake capacity for ReO 4 − reaches 786 mg/g, a value noticeably higher than the state of art anion-exchange resins and other inorganic or hybrid anion sorbents. Once being captured, ReO 4− is greatly immobilized, as almost no ReO 4 − can be eluted using large excess of nitrate, carbonate, and phosphate anions. The processes are featured by a complete and irreversible single-crystal to single-crystal structural transformation from SBN to the ReO 4 − -incorporated phase (SBR). The coordination environments of NO 3 − and ReO 4 − probed by single-crystal structures clearly unravel the underlying mechanism, where each ReO 4 − in SBR binds to multiple Ag + sites forming strong Ag−O−Re bonds, and to 4,4′-bipyridine through a dense hydrogen bond network. These structural insights lead to a significant difference in solubility product constants between SBN and SBR, which is further confirmed by first principle calculations showing a large binding energy difference of 35.61 kcal/mol. To the best of our knowledge, SBR is the least soluble perrhenate/pertechnetate salt reported, which may be considered as a potential waste form for direct immobilization of TcO 4 − .

Real-time and accurate detection of pH in aqueous solution is of great significance in chemical, environmental, and engineering-related fields. We report here the use of 8-hydroxyquinoline-functionalized covalent organic framework (COF-HQ) for dual-mode pH sensing. In the fluorescent mode, the emission intensity of COF-HQ weakened as the pH decreased, and also displayed a good linear relationship against pH in the range from 1 to 5. In addition, COF-HQ showed discernible color changes from yellow to black as the acidity increased and can be therefore used as a colorimetric pH sensor. All these changes are reversible and COF-HQ can be recycled for multiple detection runs owing to its high hydrolytical stability. It can be further assembled into a mixed matrix membrane for practical applications.

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Despite tremendous efforts having been made in the exploration of new high-performance proton-conducting materials, systems with superprotonic conductivity higher than 10−1 S·cm−1 are scarcely reported. We show here the utilization of bridging uranyl oxo atoms, traditionally termed cation–cation interaction (CCI), as the hydrogen bond acceptor to build a dense and ordered hydrogen bond network, affording a unique uranyl-based proton-conducting coordination polymer (H3O)4UO2(PO4)2 (HUP-1). This compound contains a densely connected hydronium network that is substantially stabilized by uranyl oxo atoms and exhibits high proton conductivities over a wide temperature range. At 98 °C, 98% relative humidity, a superprotonic conductivity of 1.02 × 10−1 S·cm−1 is observed for the system, one of the highest values reported for a solid-state proton-conducting material. This property originates from the thermally induced phase transformation from HUP-1 to another uranyl compound also with a CCI bond, (H3O)UO2PO4·(H2O)3 (HUP-2), accompanied by the partial generation of phosphorus acid that is further trapped in the structure of HUP-2, demonstrated by solid-state NMR analysis. The superprotonic conductivity of H3PO4@HUP-2 is persistent under the testing condition.