Direct X‐ray Observation of Trapped CO2 in a Predesigned Porphyrinic Metal–Organic Framework

Metal–organic frameworks (MOFs) are emerging microporous materials that are promising for capture and sequestration of CO₂ due to their tailorable binding properties. However, it remains a grand challenge to pre‐design a MOF with a precise, multivalent binding environment at the molecular level to enhance CO₂ capture. Here, we report the design, synthesis, and direct X‐ray crystallographic observation of a porphyrinic MOF, UNLPF‐2, that contains CO₂‐specific single molecular traps. Assembled from an octatopic porphyrin ligand with [Co₂(COO)₄] paddlewheel clusters, UNLPF‐2 provides an appropriate distance between the coordinatively unsaturated metal centers, which serve as the ideal binding sites for in situ generated CO₂. The coordination of Co^II in the porphyrin macrocycle is crucial and responsible for the formation of the required topology to trap CO₂. By repeatedly releasing and recapturing CO₂, UNLPL‐2 also exhibits recyclability.     

Hierarchical Mesoporous Metal–Organic Frameworks for Enhanced CO2 Capture

Hierarchical porous materials are promising for catalyst, separation and sorption applications. A ligand‐assisted etching process is developed for template‐free synthesis of hierarchical mesoporous MOFs as single crystals and well‐intergrown membranes at 40 °C. At 223 K, the hierarchical porous structures significantly improve the CO₂ capture capacity of HKUST‐1 by more than 44 % at pressures up to 20 kPa and 13 % at 100 kPa. Even at 323 K, the enhancement of CO₂ uptake is above 25 % at pressures up to 20 kPa and 7 % at 100 kPa. The mesoporous structures not only enhance the CO₂ uptake capacity but also improve the diffusion and mass transportation of CO₂. Similarly, well‐intergrown mesoporous HKUST‐1 membranes are synthesized, which hold the potential for film‐like porous devices. Mesoporous MOF‐5 crystals are also obtained by a similar ligand‐assisted etching process. This may provide a facile way to prepare hierarchical porous MOF single crystals and membranes for wide‐ranging applications.     

A Porphyrin‐Based Porous rtl Metal–Organic Framework as an Efficient Catalyst for the Cycloaddition of CO2 to Epoxides

A porous rtl metal–organic framework (MOF) [Mn₅L(H₂O)₆?(DMA)₂]?5DMA?4C₂H₅OH ( 1? Mn) (H₁₀L=5,10,15,20‐tetra(4‐(3,5‐dicarboxylphenoxy)phenyl)porphyrin; DMA=N,N′‐dimethylacetamide) was synthesized by employing a new porphyrin‐based octacarboxylic acid ligand. 1? Mn exhibits high Mn^II density in the porous framework, providing it great Lewis‐acid heterogeneous catalytic capability for the cycloaddition of CO₂ with epoxides. Strikingly, 1? Mn features excellent catalytic activity to the cycloaddition of CO₂ to epoxides, with a remarkable initial turnover frequency 400 per mole of catalyst per hour at 20 atm. As‐synthesized 1? Mn also exhibits size selectivity to different epoxide substrates on account of their steric hindrance. The high catalytic activity, size selectivity, and stability toward the epoxides on catalytic cycloaddition of CO₂ make 1? Mn a promising heterogeneous catalyst for fixation and utilization of CO₂.     

Electrically Conductive Porous Metal–Organic Frameworks

Owing to their outstanding structural, chemical, and functional diversity, metal–organic frameworks (MOFs) have attracted considerable attention over the last two decades in a variety of energy‐related applications. Notably missing among these, until recently, were applications that required good charge transport coexisting with porosity and high surface area. Although most MOFs are electrical insulators, several materials in this class have recently demonstrated excellent electrical conductivity and high charge mobility. Herein we review the synthetic and electronic design strategies that have been employed thus far for producing frameworks with permanent porosity and long‐range charge transport properties. In addition, key experiments that have been employed to demonstrate electrical transport, as well as selected applications for this subclass of MOFs, will be discussed.     

Photoswitching CO2 Capture and Release in a Photochromic Diarylethene Metal–Organic Framework

We demonstrate herein a promising pathway towards low‐energy CO₂ capture and release triggered by UV and visible light. A photosensitive diarylethene ligand was used to construct a photochromic diarylethene metal–organic framework (DMOF). A local photochromic reaction originating from the framework movement induced by the photoswitchable diarylethene unit resulted in record CO₂‐desorption capacity of 75 % under static irradiation and 76 % under dynamic irradiation.     

Flexible and Hierarchical Metal–Organic Framework Composites for High‐Performance Catalysis

The development of porous composite materials is of great significance for their potentially improved performance over those of individual components and extensive applications in separation, energy storage, and heterogeneous catalysis. Now mesoporous metal–organic frameworks (MOFs) with macroporous melamine foam (MF) have been integrated using a one‐pot process, generating a series of MOF/MF composite materials with preserved crystallinity, hierarchical porosity, and increased stability over that of melamine foam. The MOF nanocrystals were threaded by the melamine foam networks, resembling a ball‐and‐stick model overall. The resulting MOF/MF composite materials were employed as an effective heterogeneous catalyst for the epoxidation of cholesteryl esters. Combining the advantages of interpenetrative mesoporous and macroporous structures, the MOF/melamine foam composite has higher dispersibility and more accessibility of catalytic sites, exhibiting excellent catalytic performance.     

An Allosteric Metal–Organic Framework That Exhibits Multiple Pore Configurations for the Optimization of Hydrocarbon Separation

The function of allosteric enzymes can be activated or inhibited through binding of specific effector molecules. Herein, we describe how the skeletal deformation, pore configuration, and ultimately adsorptive behavior of a dynamic metal–organic framework (MOF), (Me₂NH₂)[In(atp)]₂ (in which atp=2‐aminoterephthalate), are controlled by the allocation and orientation of its counter ions triggered by the inclusion/removal of different guest molecules. The power of such allosteric control in MOFs is highlighted through the optimization of the hydrocarbon separation performance by achieving multiple pore configurations but without altering the chemical composition.     

Ligand Functionalization and Its Effect on CO2 Adsorption in Microporous Metal–Organic Frameworks

We report two new 3D structures, [Zn₃(bpdc)₃(2,2′‐dmbpy)] (DMF)_x(H₂O)_y ( 1 ) and [Zn₃(bpdc)₃(3,3′‐dmbpy)]?(DMF)₄(H₂O)_0.5 ( 2 ), by methyl functionalization of the pillar ligand in [Zn₃(bpdc)₃(bpy)] (DMF)₄?(H₂O) ( 3 ) (bpdc=biphenyl‐4,4′‐dicarboxylic acid; z,z′‐dmbpy=z,z′‐dimethyl‐4,4′‐bipyridine; bpy=4,4′‐bipyridine). Single‐crystal X‐ray diffraction analysis indicates that 2 is isostructural to 3 , and the power X‐ray diffraction (PXRD) study shows a very similar framework of 1 to 2 and 3 . Both 1 and 2 are 3D porous structures made of Zn₃(COO)₆ secondary building units (SBUs) and 2,2′‐ or 3,3′‐dmbpy as pillar ligand. Thermogravimetric analysis (TGA) and PXRD studies reveal high thermal and water stability for both compounds. Gas‐adsorption studies show that the reduction of surface area and pore volume by introducing a methyl group to the bpy ligand leads to a decrease in H₂ uptake for both compounds. However, CO₂ adsorption experiments with 1′ (guest‐free 1 ) indicate significant enhancement in CO₂ uptake, whereas for 2′ (guest‐free 2 ) the adsorbed amount is decreased. These results suggest that there are two opposing and competitive effects brought on by methyl functionalization: the enhancement due to increased isosteric heats of CO₂ adsorption (Q_st), and the detraction due to the reduction of surface area and pore volume. For 1′ , the enhancement effect dominates, which leads to a significantly higher uptake of CO₂ than its parent compound 3′ (guest‐free 3 ). For 2′ , the detraction effect predominates, thereby resulting in reduced CO₂ uptake relative to its parent structure 3′ . IR and Raman spectroscopic studies also present evidence for strong interaction between CO₂ and methyl‐functionalized π moieties. Furthermore, all compounds exhibit high separation capability for CO₂ over other small gases including CH₄, CO, N₂, and O₂.     

Chiral Metal–Organic Frameworks Bearing Free Carboxylic Acids for Organocatalyst Encapsulation

Two chiral carboxylic acid functionalized micro‐ and mesoporous metal–organic frameworks (MOFs) are constructed by the stepwise assembly of triple‐stranded heptametallic helicates with six carboxylic acid groups. The mesoporous MOF with permanent porosity functions as a host for encapsulation of an enantiopure organic amine catalyst by combining carboxylic acids and chiral amines in situ through acid–base interactions. The organocatalyst‐loaded framework is shown to be an efficient and recyclable heterogeneous catalyst for the asymmetric direct aldol reactions with significantly enhanced stereoselectivity in relative to the homogeneous organocatalyst.     

Engineering Metal–Organic Frameworks for the Electrochemical Reduction of CO2: A Minireview

The electrochemical reduction of CO₂ holds great promise for lowering the concentration of CO₂ in the Earth′s atmosphere. However, several challenges have hindered the commercialization of this technology, including energy efficiency, the solubility of CO₂ in the aqueous phase, and electrode stability. In this Minireview, we highlight and summarize the main advantages and limitations that metal–organic frameworks (MOFs) may offer in this field of research, either when used directly as electrocatalysts or when used as catalyst precursors.     

A Highly Stable Zeotype Mesoporous Zirconium Metal–Organic Framework with Ultralarge Pores

Through topological rationalization, a zeotype mesoporous Zr‐containing metal–organic framework (MOF), namely PCN‐777, has been designed and synthesized. PCN‐777 exhibits the largest cage size of 3.8 nm and the highest pore volume of 2.8 cm³ g^?1 among reported Zr‐MOFs. Moreover, PCN‐777 shows excellent stability in aqueous environments, which makes it an ideal candidate as a support to incorporate different functional moieties. Through facile internal surface modification, the interaction between PCN‐777 and different guests can be varied to realize efficient immobilization.     

Isostructural Metal–Organic Frameworks Assembled from Functionalized Diisophthalate Ligands through a Ligand‐Truncation Strategy

Four isostructural metal–organic frameworks (MOFs) with various functionalized pore surfaces were synthesized from a series of diisophthalate ligands. These MOFs exhibit a new network topology of {4.6⁴.8}₂{4².6⁴}{6⁴.8²}₂{6⁶}. Hydrogen uptake as high as 2.67 wt % at 77 K/1 bar and CO₂ uptake of 15.4 wt % at 297 K/1 bar have been observed for PCN‐308, which contains ? CF₃ groups. The isostructural series of MOFs also showed reasonable adsorption selectivity of CO₂ over CH₄ and N₂.     

Metal–Organic Framework‐Templated Porous Carbon for Highly Efficient Catalysis: The Critical Role of Pyrrolic Nitrogen Species

Metal‐free catalysts are of great importance and alternative candidates to conventional metal‐based catalysts for many reactions. Herein, several types of metal–organic frameworks have been exploited as templates/precursors to afford porous carbon materials with various nitrogen dopant forms and contents, degrees of graphitization, porosities, and surface areas. Amongst these materials, the PCN‐224‐templated porous carbon material optimized by pyrolysis at 700 °C (denoted as PCN‐224‐700) is composed of amorphous carbon coated with well‐defined graphene layers, offering a high surface area, hierarchical pores, and high nitrogen content (mainly, pyrrolic nitrogen species). Remarkably, as a metal‐free catalyst, PCN‐224‐700 exhibits a low activation energy and superior activity to most metallic catalysts in the catalytic reduction of 4‐nitrophenol to 4‐aminophenol. Theoretical investigations suggest that the content and type of the nitrogen dopant play crucial roles in determining the catalytic performance and that the pyrrolic nitrogen species makes the dominant contribution to this activity, which explains the excellent efficiency of the PCN‐224‐700 catalyst well.     

Crystal Engineering of an nbo Topology Metal–Organic Framework for Chemical Fixation of CO2 under Ambient Conditions

Crystal engineering of the nbo metal–organic framework (MOF) platform MOF‐505 with a custom‐designed azamacrocycle ligand (1,4,7,10‐tetrazazcyclododecane‐N,N′,N′′,N′′′‐tetra‐p‐methylbenzoic acid) leads to a high density of well‐oriented Lewis active sites within the cuboctahedral cage in MMCF‐2, [Cu₂(Cu‐tactmb)(H₂O)₃(NO₃)₂]. This MOF demonstrates high catalytic activity for the chemical fixation of CO₂ into cyclic carbonates at room temperature under 1 atm pressure.     

Encapsulation of an Interpenetrated Diamondoid Inorganic Building Block in a Metal–Organic Framework

The first example of an inorganic–organic composite framework with an interpenetrated diamondoid inorganic building block, featuring unique {InNa}_n helices and {In₁₂Na₁₆} nano‐rings, has been constructed and structurally characterized. This framework also represents a unique example of encapsulation of an interpenetrated diamondoid inorganic building block in a metal–organic framework.