Mimicking Enzymatic Non-Covalent Interactions with Functionalized Covalent Organic Frameworks for Improved Adsorption and Hydrolysis of Cellobiose.

Tuning catalytic centers in heterogeneous catalyst, both in a chemical and a spatial manner, is a powerful approach to improve the stability and the efficiency of catalysts. While the chemical aspects are largely understood, the spatial interactions around active sites, comprised of non-covalent interactions, are difficult to maintain and challenging to study. Herein, the unique properties of covalent organic frameworks (COFs) are utilized to establish an ideal reaction environment for the hydrolysis of cellobiose and other common disaccharides in mild, metal-free, and neutral aqueous conditions. The chosen COF, HCl-PSA-IM-COF-OMe ("HCl" for hydrochloric acid, "PSA" for propyl sulfonic acid, "IM" for imidazole, and "OMe" for methoxy), is modified to be ultra-stable in aqueous conditions and possesses sulfonic acid groups for general acid catalysis and for enhanced hydrogen bonding with reactants as well as intraporous chloride anions for oxocarbenium intermediate stabilization. In addition, the system also relies on the differences in adsorptive binding behavior, Kads , of the reactants and the products to the functionalized framework and benefits from a separate physical, kinetic process to boost the catalytic cycle. Due to its stability in aqueous conditions, HCl-PSA-IM-COF-OMe can be recycled and maintains its hydrolytic properties for five cycles before regeneration is needed.

Metal-Organic Framework Based Hydrogen-Bonding Nanotrap for Efficient Acetylene Storage and Separation.

The removal of carbon dioxide (CO2) from acetylene (C2H2) is a critical industrial process for manufacturing high-purity C2H2. However, it remains challenging to address the tradeoff between adsorption capacity and selectivity, on account of their similar physical properties and molecular sizes. To overcome this difficulty, here we report a novel strategy involving the regulation of a hydrogen-bonding nanotrap on the pore surface to promote the separation of C2H2/CO2 mixtures in three isostructural metal-organic frameworks (MOFs, named MIL-160, CAU-10H, and CAU-23, respectively). Among them, MIL-160, which has abundant hydrogen-bonding acceptors as nanotraps, can selectively capture acetylene molecules and demonstrates an ultrahigh C2H2 storage capacity (191 cm3 g-1, or 213 cm3 cm-3) but much less CO2 uptake (90 cm3 g-1) under ambient conditions. The C2H2 adsorption amount of MIL-160 is remarkably higher than those for the other two isostructural MOFs (86 and 119 cm3 g-1 for CAU-10H and CAU-23, respectively) under the same conditions. More importantly, both simulation and experimental breakthrough results show that MIL-160 sets a new benchmark for equimolar C2H2/CO2 separation in terms of the separation potential (Δqbreak = 5.02 mol/kg) and C2H2 productivity (6.8 mol/kg). In addition, in situ FT-IR experiments and computational modeling further reveal that the unique host-guest multiple hydrogen-bonding interaction between the nanotrap and C2H2 is the key factor for achieving the extraordinary acetylene storage capacity and superior C2H2/CO2 selectivity. This work provides a novel and powerful approach to address the tradeoff of this extremely challenging gas separation.

Enhancing Photocatalytic Hydrogen Production via the Construction of Robust Multivariate Ti-MOF/COF Composites.

Titanium metal-organic frameworks (Ti-MOFs), as an appealing type of artificial photocatalyst, have shown great potential in the field of solar energy conversion due to their well-studied photoredox activity (similar to TiO2 ) and good optical responsiveness of linkers, which serve as the antenna to absorb visible-light. Although much effort has been dedicated to developing Ti-MOFs with high photocatalytic activity, their solar energy conversion performances are still poor. Herein, we have implemented a covalent-integration strategy to construct a series of multivariate Ti-MOF/COF hybrid materials PdTCPP⊂PCN-415(NH2 )/TpPa (composites 1, 2, and 3), featuring excellent visible-light utilization, a suitable band gap, and high surface area for photocatalytic H2 production. Notably, the resulting composites demonstrated remarkably enhanced visible-light-driven photocatalytic H2 evolution performance, especially for the composite 2 with a maximum H2 evolution rate of 13.98 mmol g-1 h-1 (turnover frequency (TOF)=227 h-1 ), which is much higher than that of PdTCPP⊂PCN-415(NH2 ) (0.21 mmol g-1 h-1 ) and TpPa (6.51 mmol g-1 h-1 ). Our work thereby suggests a new approach to highly efficient photocatalysts for H2 evolution and beyond.

Nanospace Engineering of Metal-Organic Frameworks through Dynamic Spacer Installation of Multifunctionalities for Efficient Separation of Ethane from Ethane/Ethylene Mixtures.

Herein, a dynamic spacer installation (DSI) strategy has been implemented to construct a series of multifunctional metal-organic frameworks (MOFs), LIFM-61/31/62/63, with optimized pore space and pore environment for ethane/ethylene separation. In this respect, a series of linear dicarboxylic acids were deliberately installed in the prototype MOF, LIFM-28, leading to a dramatically increased pore volume (from 0.41 to 0.82 cm3 g-1 ) and reduced pore size (from 11.1×11.1 Å2 to 5.6×5.6 Å2 ). The increased pore volume endows the multifunctional MOFs with much higher ethane adsorption capacity, especially for LIFM-63 (4.8 mmol g-1 ), representing nearly three times as much ethane as the prototypical counterpart (1.7 mmol g-1 ) at 273 K and 1 bar. Meanwhile, the reduced pore size imparts enhanced ethane/ethylene selectivity of the multifunctional MOFs. Theoretical calculations and dynamic breakthrough experiments confirm that the DSI is a promising approach for the rational design of multifunctional MOFs for this challenging task.

A MOF-based Ultra-Strong Acetylene Nano-trap for Highly Efficient C2 H2 /CO2 Separation.

Porous materials with open metal sites have been investigated to separate various gas mixtures. However, open metal sites show the limitation in the separation of some challenging gas mixtures, such as C2 H2 /CO2 . Herein, we propose a new type of ultra-strong C2 H2 nano-trap based on multiple binding interactions to efficiently capture C2 H2 molecules and separate C2 H2 /CO2 mixture. The ultra-strong acetylene nano-trap shows a benchmark Qst of 79.1 kJ mol-1 for C2 H2 , a record high pure C2 H2 uptake of 2.54 mmol g-1 at 1×10-2  bar, and the highest C2 H2 /CO2 selectivity (53.6), making it as a new benchmark material for the capture of C2 H2 and the separation of C2 H2 /CO2 . The locations of C2 H2 molecules within the MOF-based nanotrap have been visualized by the in situ single-crystal X-ray diffraction studies, which also identify the multiple binding sites accountable for the strong interactions with C2 H2 .

Rational design of bifunctional conjugated microporous polymers.

Conjugated microporous polymers (CMPs) are an emerging class of porous organic polymers that combine π-conjugated skeletons with permanent micropores. Since their first report in 2007, the enormous exploration of linkage types, building units, and synthetic methods for CMPs have facilitated their potential applications in various areas, from gas separations to energy storage. Owning to their unique construction, CMPs offer the opportunity for the precise design of conjugated skeletons and pore environment engineering, which allow the construction of functional porous materials at the molecular level. The capability to chemically alter CMPs to targeted applications allows for the fine adaptation of functionalities for the ever-changing environments and necessities. Bifunctional CMPs are a branch of functionalized CMPs that have caught the interest of researchers because of their inherent synergistic systems that can expand their applications and optimize their performance. This review discusses the rational design and synthesis of bifunctional CMPs and summarizes their advanced applications. To conclude, our own perspective on the research prospects of these types of materials is outlined.

Metal-Organic Frameworks for Enzyme Immobilization: Beyond Host Matrix Materials.

Enzyme immobilization in metal-organic frameworks (MOFs) as a promising strategy is attracting the interest of scientists from different disciplines with the expansion of MOFs' development. Different from other traditional host materials, their unique strengths of high surface areas, large yet adjustable pore sizes, functionalizable pore walls, and diverse architectures make MOFs an ideal platform to investigate hosted enzymes, which is critical to the industrial and commercial process. In addition to the protective function of MOFs, the extensive roles of MOFs in the enzyme immobilization are being well-explored by making full use of their remarkable properties like well-defined structure, high porosity, and tunable functionality. Such development shifts the focus from the exploration of immobilization strategies toward functionalization. Meanwhile, this would undoubtedly contribute to a better understanding of enzymes in regards to the structural transformation after being hosted in a confinement environment, particularly to the orientation and conformation change as well as the interplay between enzyme and matrix MOFs. In this Outlook, we target a comprehensive review of the role diversities of the host matrix MOF based on the current enzyme immobilization research, along with proposing an outlook toward the future development of this field, including the representatives of potential techniques and methodologies being capable of studying the hosted enzymes.

Efficient separation of xylene isomers by a guest-responsive metal-organic framework with rotational anionic sites.

The separation of xylene isomers (para-, meta-, orth-) remains a great challenge in the petrochemical industry due to their similar molecular structure and physical properties. Porous materials with sensitive nanospace and selective binding sites for discriminating the subtle structural difference of isomers are urgently needed. Here, we demonstrate the adaptively molecular discrimination of xylene isomers by employing a NbOF52--pillared metal-organic framework (NbOFFIVE-bpy-Ni, also referred to as ZU-61) with rotational anionic sites. Single crystal X-ray diffraction studies indicate that ZU-61 with guest-responsive nanospace/sites can adapt the shape of specific isomers through geometric deformation and/or the rotation of fluorine atoms in anionic sites, thereby enabling ZU-61 to effectively differentiate xylene isomers through multiple C-H···F interactions. ZU-61 exhibited both high meta-xylene uptake capacity (3.4 mmol g-1) and meta-xylene/para-xylene separation selectivity (2.9, obtained from breakthrough curves), as well as a favorable separation sequence as confirmed by breakthrough experiments: para-xylene elute first with high-purity (≥99.9%), then meta-xylene, and orth-xylene. Such a remarkable performance of ZU-61 can be attributed to the type anionic binding sites together with its guest-response properties.

Secondary Sphere Effects on Porous Polymeric Organocatalysts for CO2 Transformations: Subtle Modifications Resulting in Superior Performance.

Albeit harnessing secondary sphere interactions to exert control over the reaction outcomes has primarily been applied to enzymatic and organometallic catalysis, there are seldom any studies that introduce outer-sphere modifiers into organocatalysts. This is even less in the corresponding heterogeneous catalytic system. In this contribution, we experimentally and computationally investigate the role of secondary effects in the reactivity of bromide anions toward CO2 transformations. Six pyridinium cationic porous frameworks have been synthesized and fully characterized. Structure-activity relationships and kinetics show that the type and the location of the substituents on the cationic framework have a significant impact on the nucleophilic reactivity of their bromide counter anion. Specifically, the attachment of amine substituent to the ortho position relative to a pyridinium motif produces a remarkably efficient catalyst for CO2 transformation, by a factor of six times greater in comparison to the pristine pyridinium-based polymer. The hydrogen-bond-interaction-promoted reagent activation and enhanced delocalization ability of bromide counter anion are believed to be the key to driving the reaction toward CO2 utilization. These observations, therefore, champion the leverage of secondary interaction for optimizing the reactivity of organocatalysts.

Solvent-assisted coordination driven assembly of a supramolecular architecture featuring two types of connectivity from discrete nanocages.

The rapid development of supramolecular chemistry provides a powerful bottom-up approach to construct various well-defined nano-architectures with increasing complexity and functionality. Compared to that of small and simple nanometric objects, the self-assembly of larger and more complex nanometric objects, such as nanocages, remains a significant challenge. Herein, we used a discrete nanocage as the monomer to successfully construct a novel three-dimensional (3D) supramolecular architecture, which comprises two types of nanocage building units with different connectivity, using the solvent-assisted coordination-driven assembly approach. The mechanism of this supramolecular assembly process was investigated by electrospray ionization mass spectrometry (ESI-MS) studies, which identified for the first time the formation of a nanocage dimer intermediate during the assembly process. The assembly of discrete nanocages into a 3D supramolecular architecture led to remarkable enhancement of stability and gas adsorption properties.

A Metal-Organic Framework Based Methane Nano-trap for the Capture of Coal-Mine Methane.

As a major greenhouse gas, methane, which is directly vented from the coal-mine to the atmosphere, has not yet drawn sufficient attention. To address this problem, we report a methane nano-trap that features oppositely adjacent open metal sites and dense alkyl groups in a metal-organic framework (MOF). The alkyl MOF-based methane nano-trap exhibits a record-high methane uptake and CH4 /N2 selectivity at 298 K and 1 bar. The methane molecules trapped within the alkyl MOF were crystalographically identified by single-crystal X-ray diffraction experiments, which in combination with molecular simulation studies unveiled the methane adsorption mechanism within the MOF-based nano-trap. The IAST calculations and the breakthrough experiments revealed that the alkyl MOF-based methane nano-trap is a new benchmark for CH4 /N2 separation, thereby providing a new perspective for capturing methane from coal-mine methane to recover fuel and reduce greenhouse gas emissions.

Promoting Frustrated Lewis Pairs for Heterogeneous Chemoselective Hydrogenation via the Tailored Pore Environment within Metal-Organic Frameworks.

Frustrated Lewis pairs (FLPs) have recently been advanced as efficient metal-free catalysts for catalytic hydrogenation, but their performance in chemoselective hydrogenation, particularly in heterogeneous systems, has not yet been achieved. Herein, we demonstrate that, via tailoring the pore environment within metal-organic frameworks (MOFs), FLPs not only can be stabilized but also can develop interesting performance in the chemoselective hydrogenation of α,ß-unsaturated organic compounds, which cannot be achieved with FLPs in a homogeneous system. Using hydrogen gas under moderate pressure, the FLP anchored within a MOF that features open metal sites and hydroxy groups on the pore walls can serve as a highly efficient heterogeneous catalyst to selectively reduce the imine bond in α,ß-unsaturated imine substrates to afford unsaturated amine compounds.

Tuning Pore Heterogeneity in Covalent Organic Frameworks for Enhanced Enzyme Accessibility and Resistance against Denaturants.

Achieving high-performance biocomposites requires knowledge of the compatability between the immobilized enzyme and its host material. The modular nature of covalent organic frameworks (COFs), as a host, allows their pore geometries and chemical functionalities to be fine-tuned independently, permitting comparative studies between the individual parameters and the performances of the resultant biocomposites. This research demonstrates that dual pores in COFs have profound consequences on the catalytic activity and denaturation of infiltrated enzymes. This approach enforces a constant pore environment by rational building-block design, which enables it to be unequivocally determined that pore heterogeneity is responsible for rate enhancements of up to threefold per enzyme molecule. More so, the enzyme is more tolerant to detrimental by-products when occupying the larger pore in a dual-pore COF compared to a corresponding uniform porous COF. Kinetic studies highlight that pore heterogeneity facilitates mass transfer of both reagents and products. This unparalleled versatility of these materials allows many different aspects to be designed on demand, lending credence to their prospect as next-generation host materials for various enzyme biocomposites catalysts.

Ultralight Conductive Silver Nanowire Aerogels.

Low-density metal foams have many potential applications in electronics, energy storage, catalytic supports, fuel cells, sensors, and medical devices. Here, we report a new method for fabricating ultralight, conductive silver aerogel monoliths with predictable densities using silver nanowires. Silver nanowire building blocks were prepared by polyol synthesis and purified by selective precipitation. Silver aerogels were produced by freeze-casting nanowire aqueous suspensions followed by thermal sintering to weld the nanowire junctions. As-prepared silver aerogels have unique anisotropic microporous structures, with density precisely controlled by the nanowire concentration, down to 4.8 mg/cm3 and an electrical conductivity up to 51 000 S/m. Mechanical studies show that silver nanowire aerogels exhibit "elastic stiffening" behavior with a Young's modulus up to 16 800 Pa.

Multiphase separation of copper nanowires.

This communication reports a new method to purify copper nanowires with nearly 100% yield from undesired copper nanoparticle side-products formed during batch processes of copper nanowire synthesis. This simple separation method can yield large quantities of long, uniform, high-purity copper nanowires to meet the requirements of nanoelectronics applications as well as provide an avenue for purifying copper nanowires in the industrial scale synthesis of copper nanowires, a key step for commercialization and application of nanowires.