Efficient Adsorption of Acetylene over CO₂ in Bioinspired Covalent Organic Frameworks

Rational design of covalent organic frameworks (COFs) to broaden their diversity is highly desirable but challenging due to the limited, expensive, and complex building blocks, especially compared with other easily available porous materials. In this work, we fabricated two novel bioinspired COFs, namely, NUS-71 and NUS-72, using reticular chemistry with ellagic acid and triboronic acid-based building blocks. Both COFs with AB stacking mode exhibit high acetylene (C2H2) adsorption capacity and excellent separation performance for C2H2/CO2 mixtures, which is significant but rarely explored using COFs. The impressive affinities for C2H2 appear to be related to the sandwich structure formed by C2H2 and the host framework via multiple host–guest interactions. This work not only represents a new avenue for the construction of low-cost COFs but also expands the variety of the COF family using natural biochemicals as building blocks for broad application

In Situ Formation of Micropore-Rich Titanium Dioxide from Metal–Organic Framework Templates

Phase and porosity control in titanium dioxide (TiO2) is essential for the optimization of its photocatalytic activity. However, concurrent control over these two parameters remains challenging. Here, a novel metal–organic framework templating strategy is demonstrated for the preparation of highly microporous anatase TiO2. In situ encapsulation of Ti precursor in ZIF-8 cavities, followed by hydrolysis and etching, produces anatase TiO2 with a high Brunauer–Emmett–Teller surface area of 335 m2·g–1 and a micropore surface area ratio of 48%. Photocatalytic hydrogen generation catalyzed by the porous TiO2 can reach a rate of 2459 μmol·g–1·h–1. The measured photocatalytic activity is found to be positively correlated to the surface area, highlighting the importance of porosity control in heterogeneous photocatalysts

Dimensional Impact of Metal–Organic Frameworks in Catalyzing Photoinduced Hydrogen Evolution and Cyanosilylation Reactions

Metal–organic frameworks (MOFs) have been widely studied as heterogeneous catalysts. Compared to the MOFs with three-dimensional (3D) topologies, two-dimensional (2D) MOF nanosheets can allow facile access to the active sites on their external surface, thus having huge potentials in catalysis. Herein, we fabricate 2D MOF nanosheets, UiO-67-NS, as photocatalyst for H2 evolution reaction, and study their photocatalytic performance in relation to their 3D bulk counterparts (UiO-67). The UiO-67-NS exhibit an 84-fold increase in photocatalytic efficiency compared to UiO-67. Postsynthetic cation grafting of the UiO-67-NS with titanium leads to further enhancement in photocatalytic efficiency, giving a hydrogen evolution rate of up to 393 μmol g–1 h–1, which is 13-times higher than that of the nongrafting nanosheets under the same condition. Our results indicate that the 3D-to-2D dimensionality reduction can be a viable strategy for the development of MOFs as efficient photocatalysts. In addition, we have demonstrated that the improvement of catalytic performance based on the strategy of 3D-to-2D framework dimensionality reduction can be easily extended to heterogeneous cyanosilylation reaction

Multivariate Polycrystalline Metal–Organic Framework Membranes for CO₂/CH₄ Separation

Membrane technology is attractive for natural gas separation (removing CO2, H2O, and hydrocarbons from CH4) because of membranes’ low energy consumption and small environmental footprint. Compared to polymeric membranes, microporous inorganic membranes such as silicoaluminophosphate-34 (SAPO-34) membrane can retain their separation performance under conditions close to industrial requirements. However, moisture and hydrocarbons in natural gas can be strongly adsorbed in the pores of those membranes, thereby reducing the membrane separation performance. Herein, we report the fabrication of a polycrystalline MIL-160 membrane on an Al2O3 substrate by in situ hydrothermal synthesis. The MIL-160 membrane with a thickness of ca. 3 μm shows a remarkable molecular sieving effect in gas separation. Besides, the pore size and environment of the MIL-160 membrane can be precisely controlled using reticular chemistry by regulating the size and functionality of the ligand. Interestingly, the more polar fluorine-functionalized multivariate MIL-160/CAU-10-F membrane exhibits a 10.7% increase in selectivity for CO2/CH4 separation and a 31.2% increase in CO2 permeance compared to those of the MIL-160 membrane. In addition, hydrophobic MIL-160 membranes and MIL-160/CAU-10-F membranes are more resistant to water vapor and hydrocarbons than the hydrophilic SAPO-34 membranes

Encapsulation and Protection of Ultrathin Two-Dimensional Porous Organic Nanosheets within Biocompatible Metal–Organic Frameworks for Live-Cell Imaging

Despite the rapid development of ultrathin two-dimensional (2D) organic nanosheets, it still remains a challenge to stabilize them and prevent restacking so that they could be used in aqueous environments for biological applications such as live-cell bioimaging. Herein, we report an effective approach to stabilize and protect ultrathin 2D porous organic nanosheets (PONs) by encapsulating them with biocompatible zeolitic imidazolate framework-8 (ZIF-8) for in vitro live-cell imaging. We rationally design and synthesize few-layered 2D PONs named NUS 27–29 containing flexible tetraphenylethylene units as aggregation-induced emission (AIE) molecular rotors. The micrometer-sized freestanding 2D nanosheets of NUS 27–29 with thicknesses of 2–5 nm can be easily obtained by exfoliation from their bulk powders. We demonstrate that these 2D nanosheets can be armored by ZIF-8 crystals grown in situ for inhibition of restacking. Importantly, we find that the dynamics of the AIE molecular rotors of NUS 27–29 can be restricted by noncovalent interactions between the 2D nanosheets and ZIF-8 armor, as proved through experimental studies and theoretical simulations. As a result, the integration of these 2D nanosheets in ZIF-8 leads to highly stable, porous, and fluorescent composites. We further demonstrate that these composites can be employed as biological fluorescent probes for in vitro live-cell imaging. Our strategy shows the first example of transporting hydrophobic 2D organic nanosheets into live cancer cells by encapsulating within biocompatible MOFs, which should facilitate the further development of ultrathin 2D nanomaterials for various biological applications

Process-Tracing Study on the Postassembly Modification of Highly Stable Zirconium Metal–Organic Cages

Metal–organic cages (MOCs) are discrete molecular assemblies formed by coordination bonds between metal nodes and organic ligands. The application of MOCs has been greatly limited due to their poor stability, especially in aqueous solutions. In this work, we thoroughly investigate the stability of several Zr-MOCs and reveal their excellent stability in aqueous solutions with acidic, neutral, and weak basic conditions. In addition, we present for the first time a process-tracing study on the postassembly modification of one MOC, ZrT-1-NH₂, highlighting the excellent stability and versatility of Zr-MOCs as a new type of molecular platform for various applications

Restriction of Molecular Rotors in Ultrathin Two-Dimensional Covalent Organic Framework Nanosheets for Sensing Signal Amplification

Covalent organic frameworks (COFs) have emerged as promising crystalline porous materials with well-defined structures, high porosity, tunable topology, and functionalities suitable for various applications. However, studies of few-layered ultrathin two-dimensional (2D) COF nanosheets, which may lead to unprecedented properties and applications, are still limited. Herein, we report the targeted synthesis of three azine-linked and imine-linked 2D COFs named NUS 30–32 using monomers containing aggregation-induced emission (AIE) rotor-active tetraphenylethylene (TPE) moieties, affording micro- and meso-dual pores in NUS-30 and NUS-32 and triple pores in NUS-31. For the first time, we demonstrate that these isostructural bulk COF powders can be exfoliated into ultrathin 2D nanosheets (2–4 nm thickness) by a temperature-swing gas exfoliation approach. Compared with TPE monomers and COF model compounds, the AIE characteristic of NUS 30–32 nanosheets is distinctly suppressed because of the covalent restriction of the AIE molecular rotors in the confined 2D frameworks. As a result, the enhancement of conjugated conformations of NUS 30–32 nanosheets with unusual structure relaxation shows signal amplification effect in biomolecular recognition of amino acids and small pharmaceutical molecules (l-dopa), exhibiting much higher sensitivity than their stacked bulk powders, TPE monomer, and COF model compound. Moreover, the binding affinity of the COF nanosheets toward amino acids can be controlled by increasing the number of azine moieties in the structure. Density functional theory calculations reveal that binding affinity control results from the crucial geometric roles and stronger host–guest binding between azine moieties and amino acids. In addition, we demonstrate that minimal loading of the NUS-30 nanosheets in composite membranes can afford excellent performance for biomolecule detection. Our findings pave a way for the development of functional ultrathin 2D COF nanosheets with precise control over the nature, density, and arrangement of the binding active sites involved in enhanced molecule recognition