Cluster-Cluster Co-Nucleation Induced Defective Polyoxometalate-Based Metal-Organic Frameworks for Efficient Tandem Catalysis.

The construction of defective sites is one of the effective strategies to create high-activity Metal-Organic frameworks (MOFs) catalysts. However, traditional synthesis methods usually suffer from cumbersome synthesis steps and disordered defect structures. Herein, a cluster-cluster co-nucleation (CCCN) strategy is presented that involves the in situ introduction of size-matched functional polyoxometalates (H6P2W18O62, {P2W18}) to intervene the nucleation process of cluster-based MOFs (UiO-66), achieving one-step inducement of exposed defective sites without redundant post-processing. POM-induced UiO-66 ({P2W18}-0.1@UiO-66) exhibits a classical reo topology for well-defined cluster defects. Moreover, the defective sites and the interaction between POM and skeletal cluster nodes are directly observed by Integrated Differential Phase Contrast in Scanning Transmission Electron Microscopy (iDPC-STEM). Owing to the molecular-level proximity between defective sites and POM in the same nano-reaction space, {P2W18}-0.1@UiO-66 exhibits efficient tandem catalysis in the preparation of γ-valerolactone (γ-GVL) from laevulinic acid (LA) by the combination of Lewis and Brønsted acids with 11 times higher performance than defective UiO-66 formed by conventional coordination modulation strategy. The CCCN strategy is applicable to different POM and has the potential to be extended to other cluster-based MOFs, which will pave a new way for the construction of functional MOFs with multi-centered synergistic catalysis.

Heterovalent Metal Pair Sites on Metal-Organic Framework Ordered Macropores for Multimolecular Co-Activation.

The precise design of catalytic metal centers with multiple chemical states to facilitate sophisticated reactions involving multimolecular activation is highly desirable but challenging. Herein, we report an ordered macroporous catalyst with heterovalent metal pair (HMP) sites comprising CuII-CuI on the basis of a microporous metal-organic framework (MOF) system. This macroporous HMP catalyst with proximity heterovalent dual copper sites, whose distance is controlled to ∼2.6 Å, on macropore surface exhibits a co-activation behavior of ethanol at CuII and alkyne at CuI, and avoids microporous restriction, thereby promoting additive-free alkyne hydroboration reaction. The desired yield enhances dramatically compared with the pristine MOF and ordered macroporous MOF both with solely isovalent CuII-CuII sites. Density functional theory calculations reveal that the Cu-HMP sites can stabilize the Bpin-CuII-CuI-alkyne intermediate and facilitate C-B bond formation, resulting in a smooth alkyne hydroboration process. This work provides new perspectives to design multimolecular activation catalysts for sophisticated matter transformations.

Non-planar Nest-like [Fe2 S2 ] Cluster Sites for Efficient Oxygen Reduction Catalysis.

Metal-nitrogen-carbon catalysts, as promising alternative to platinum-based catalysts for oxygen reduction reaction (ORR), are still highly expected to achieve better performance by modulating the composition and spatial structure of active site. Herein, we constructed the non-planar nest-like [Fe2 S2 ] cluster sites in N-doped carbon plane. Adjacent double Fe atoms effectively weaken the O-O bond by forming a peroxide bridge-like adsorption configuration, and the introduction of S atoms breaks the planar coordination of Fe resulting in greater structural deformation tension, lower spin state, and downward shifted Fe d-band center, which together facilitate the release of OH* intermediate. Hence, the non-planar [Fe2 S2 ] cluster catalyst, with a half-wave potential of 0.92 V, displays superior ORR activity than that of planar [FeN4 ] or [Fe2 N6 ]. This work provides insights into the co-regulation of atomic composition and spatial configuration for efficient oxygen reduction catalysis.

Constructing Mechanical Shuttles in a Three-dimensional (3D) Porous Architecture for Selective Transport of Lithium Ions.

Lithium (Li) extraction from brines is a major barrier to the sustainable development of batteries and alloys; however, current separation technology suffers from a trade-off between ion selectivity and permeability. Herein, a crown ether mechanically interlocked 3D porous organic framework (Crown-POF) was prepared as the porous filler of thin-film nanocomposite membranes. Crown-POF with penta-coordinated (four Ocrown atoms and one Ntert-amine atom) adsorption sites enables a special recognition for Li+ ion. Moreover, the four Ntert-amine atoms on each POF branch facilitate the flipping motion of Li+ ion along the skeletal thread, while retaining the specified binding pattern. Accordingly, the crown ether interlocked POF network displays an ultrafast ion transfer rate, over 10 times that of the conventional porous materials. Notably, the nanocomposite membrane gives high speed and selectivity for Li+ ion transport as compared with other porous solid-based mixed-matrix membranes.

Mechanical Bond Approach to Introducing Self-Adaptive Active Sites in Covalent Organic Frameworks for Zinc-Catalyzed Organophosphorus Degradation.

Mechanically interlocked molecules (MIMs) with discrete molecular components linked through a mechanical bond in space can be harnessed for the operation of molecular switches and machines, which shows huge potential to imitate the dynamic response of natural enzymes. In this work, rotaxane compounds were adopted as building monomers for the synthesis of a crown-ether ring mechanically intercalated covalence organic framework (COF). This incorporation of MIMs into open architecture implemented large amplitude motions, whose wheel slid along the axle in response to external stimulation. After impregnation with Zn2+ ions, the relative locations of two zinc active sites (crown-ether coordinated Zn(II) and bipyridine coordinated Zn(II)) are endowed with great flexibility to fit the conformational transformation of an organophosphorus agent during the hydrolytic process. Notably, the resulting self-adaptive binuclear zinc center in a crown-ether-threaded COF network is endowed with a record catalytic ability, with a rate over 85.5 µM min-1 for organophosphorus degradation. The strategy of synthesis for porous artificial enzymes through the introduction of mechanically bound crown ether will enable significant breakthroughs and new synthetic concepts for the development of advanced biomimetic catalysts.

Hierarchically Ordered Macro-Microporous Polyoxometalate-Based Metal-Organic Framework Single Crystals.

Facile construction of ordered macroporous polyoxometalate-based metal-organic frameworks (POM@MOFs) to break the intrinsic microporous restriction is significant but remains challenging. On one hand, the POMs introduced improve the structural stability and modify the pores of MOFs, e.g., introducing functional catalytic and adsorptive units. Meanwhile, the acidic POMs severely affect the nucleation and growth of the POM@MOFs, resulting in complicated synthesis and difficult assembly control. Herein, a general approach has been developed to fabricate ordered macroporous POM@MOF single crystals, involving close-packed polystyrene (PS) nanosphere templates. The artificially selected polar solvents exerting strong solvent effect with POMs weaken the affinity between POMs and metal ions, thereby effectively stabilizing the precursors from assembly before filling into the PS template interstices. The weak alkaline carboxylate used regulates the in situ nucleation and growth of POM@MOFs through deprotonation of the ligands as well as coordinating modulation, affording a series of hierarchically cuboctahedral POM@MOF single crystals with ordered macropores (ca. 180 nm) and intrinsic micropores after template removal. The ordered macroporous structure and thinned microporous skeleton markedly improve mass diffusion properties, while the integral single-crystal lattice retains superior stability.

Constructing Uranyl-Specific Nanofluidic Channels for Unipolar Ionic Transport to Realize Ultrafast Uranium Extraction.

High-speed capturing of uranyl (UO22+) ions from seawater elicits unprecedented interest for the sustainable development of the nuclear energy industry. However, the ultralow concentration (∼3.3 µg L-1) of uranium element leads to the slow ion diffusion inside the adsorbent particle, especially after the transfer paths are occupied by the coexisted interfering ions. Considering the geometric dimension of UO22+ ion (a maximum length of 6.04-6.84 Å), the interlayer spacing of graphene sheets was covalently pillared with phenyl-based units into twice the ionic length (13 Å) to obtain uranyl-specific nanofluidic channels. Applying a negative potential (-1.3 V), such a charge-governed region facilitates a unipolar ionic transport, where cations are greatly accelerated and co-ions are repelled. Notably, the resulting adsorbent gives the highest adsorption velocity among all reported materials. The adsorption capacity measured after 56 days of exposure in natural seawater is evaluated to be ∼16 mg g-1. This novel concept with rapid adsorption, high capacity, and facile operating process shows great promise to implement in real-world uranium extraction.

Inorganic nanocrystal-dynamic porous polymer assemblies with effective energy transfer for sensitive diagnosis of urine copper.

Despite their remarkable mechanical, optical, and electrical properties, inorganic particles and dynamic polymer assemblies encounter difficulties in their compatibility with regards to structural order and complexity. Here, covalent organic frameworks (COFs) constructed through reversible coupling reactions were exploited as dynamic porous polymers to prepare inorganic nanocrystal-polymer assemblies. Under an in situ growth process, carbon quantum dots (CDs) were gradually prepared in the COF cavity, with a narrow size distribution (2 ± 0.5 nm). The well-established assemblies achieve effective energy transfer from the inorganic to the organic part (efficiency > 80%), thus rendering a ∼130% increase in quantum yield compared with the pristine COF network. Notably, the hybrid material realizes a simple, selective, and sensitive diagnostic tool for urine copper, surpassing the detection limit of COF solid by 150 times. Beyond the scientific and fundamental interests, such hybrid assemblies are attractive from technological perspectives as well, for example, in energy storage, electronics, catalysis, and optics.

Molecularly Imprinted Porous Aromatic Frameworks Serving as Porous Artificial Enzymes.

Artificially designed enzymes are in demand as ideal catalysts for industrial production but their dense structure conceals most of their functional fragments, thus detracting from performance. Here, molecularly imprinted porous aromatic frameworks (MIPAFs) which are exploited to incorporate full host-guest interactions of porous materials within the artificial enzymes are presented. By decorating a porous skeleton with molecularly imprinted complexes, it is demonstrated that MIPAFs are porous artificial enzymes possessing excellent kinetics for guest molecules. In addition, due to the abundance of accessible sites, MIPAFs can perform a wide range of sequential processes such as substrate hydrolysis and product transport. Through its two functional sites in tandem, the MIPAF subsequently manifests both hydrolysis and transport behaviors. Advantageously, the obtained catalytic rate is ≈58 times higher than that of all other conventional artificial enzymes and even surpasses by 14 times the rate for natural organophosphorus hydrolase (Flavobacterium sp. strain ATCC 27551).

Molecularly Imprinted Porous Aromatic Frameworks and Their Composite Components for Selective Extraction of Uranium Ions.

Selective extraction of uranium from water has attracted worldwide attention because the largest source of uranium is seawater with various interference ions (Na+ , K+ , Mg2+ , Ca2+ , etc.). However, traditional adsorbents encapsulate most of their functional sites in their dense structure, leading to problems with low selectivity and adsorption capacities. In this work, the tailor-made binding sites are first decorated into porous skeletons, and a series of molecularly imprinted porous aromatic frameworks are prepared for uranium extraction. Because the porous architecture provides numerous accessible sites, the resultant material has a fourfold increased ion capacity compared with traditional molecularly imprinted polymers and presents the highest selectivity among all reported uranium adsorbents. Moreover, the porous framework can be dispersed into commercial polymers to form composite components for the practical extraction of uranium ions from simulated seawater.