Unlocking High Porosity: Post-Synthetic Solvothermal Treatment of Cu-Paddlewheel Based Metal-Organic Cages.

Metal-organic cages (MOCs) have garnered significant attention due to their unique discrete structures, intrinsic porosity, designability, and tailorability. However, weak inter-cage interactions, such as van der Waals forces and hydrogen bonding can cause solid-state MOCs to lose structural integrity during desolvation, leading to the loss of porosity. In this work, a novel strategy to retain the permanent porosity of Cu-paddlewheel-based MOCs, enabling their use as heterogeneous catalysts is presented. Post-synthetic solvothermal treatments in non-coordinating solvents, mesitylene, and p-xylene, effectively preserve the packing structures of solvent-evacuated MOCs while preventing cage agglomeration. The resulting MOCs exhibit an exceptional N2 sorption capacity, with a high surface area (SBET = 1934 m2 g-1 for MOP-23), which is among the highest reported for porous MOCs. Intriguingly, while the solvothermal treatment reduced Cu(II) to Cu(I) in the Cu-paddlewheel clusters, the MOCs with mixed-valenced Cu(I)/Cu(II) maintained their crystallinity and permanent porosity. The catalytic activities of these MOCs are successfully examined in copper(I)-catalyzed hydrative amide synthesis, highlighting the prospect of MOCs as versatile reaction platforms.

Crystalline Porous Organic Frameworks Based on Multiple Dynamic Linkages.

A novel class of crystalline porous materials has been developed utilizing multilevel dynamic linkages, including covalent B-O, dative B←N and hydrogen bonds. Typically, boronic acids undergo in situ condensation to afford B3O3-based units, which further extend to molecular complexes or chains via B←N bonds. The obtained superstructures are subsequently interconnected via hydrogen bonds and π-π interactions, producing crystalline porous organic frameworks (CPOFs). The CPOFs display excellent solution processability, allowing dissolution and subsequent crystallization to their original structures, independent of recrystallization conditions, possibly due to the diverse bond energies of the involved interactions. Significantly, the CPOFs can be synthesized on a gram-scale using cost-effective monomers. In addition, the numerous acidic sites endow the CPOFs with high NH3 capacity, surpassing most porous organic materials and commercial materials.

Double-Walled Covalent Organic Frameworks with High Stability.

Double-walled covalent organic frameworks, consisting of two same building blocks parallel to each other forming ladder-shape linkers, could enhance the stability of the frameworks and increase the density of functional sites, thus making them suitable for various applications. In this study, two double-walled covalent organic frameworks, namely DW-COF-1 and DW-COF-2, were successfully synthesized via imine condensation. The resulting DW-COFs exhibited a honeycomb topology, high crystallinity and stability. Particularly, DW-COF-2 showed excellent resistance toward boiling water, strong acid, and strong base, due to its double-walled structure, which limits the exposure of labile imine bonds to external chemical environments. The DW-COFs showed high porosity near 900 m2 /g, making them suitable for gas storage/separation. The selective gas adsorption experiments showed that at 273 K and 1 atm pressure, DW-COF-1 and DW-COF-2 exhibited a good IAST selectivity towards CO2 /N2 (15/85) adsorption, with selectivity values of 121.3 and 56.4 for CO2 over N2 , respectively.

Single Crystals of Insoluble Porous Salicylimine Cages.

Porous organic cages (POCs) are meanwhile an established class of porous materials. Most of them are soluble to a certain extend and thus processable in or from solution. However, a few of larger salicylimine cages were reported to be insoluble in any organic solvents and thus characterized as amorphous materials. These cages were now synthesized as single-crystalline materials to get insight into packing motifs and preferred intermolecular interactions. Furthermore, the pairs of crystalline and amorphous materials for each cage allowed to compare their gas-sorption properties in both morphological states.

Deinterpenetration of IRMOF-9.

One of the iconic characteristics of metal-organic frameworks (MOFs) is the possesssion of guest-accessible pores. Increasing pore size has a direct and often beneficial impact on a MOF's adsorption and separation properties. However, as pore size increases, the resulting void spaces are often filled by interpenetrated frameworks, where one or more networks crystallize within the pore system of another identical network, reducing the MOF's free volume and pore size. Furthermore, due to the thermodynamic favorability of interpenetration during solvothermal synthesis, techniques to synthetically differentiate interpenetrated from non-interpenetrated MOFs are paramount. This study reports the synthesis of deinterpenetrated IRMOF-9 via halide mediated deinterpenetrative conversion of Zn4 O-derived IRMOF-9. IRMOF-9, when treated with ethylammonium bromide, is quasi-selectively etched, revealing the non-interpenetrated analogue, IRMOF-10 (deinterpenetrated IRMOF-9), which can be isolated prior to complete dissolution by the bromide solution. Dye adsorption, surface area and pore size distribution analysis, and powder X-ray diffraction are consistent with successful deinterpenetration.

Tailored Porous Transport Layers for Optimal Oxygen Transport in Water Electrolyzers: Combined Stochastic Reconstruction and Lattice Boltzmann Method.

The porous transport layer (PTL) plays an integral role for the mass transport in polymer electrolyte membrane (PEM) electrolyzers. In this work, a stochastic reconstruction method of titanium felt-based PTLs is applied and combined with the Lattice Boltzmann method (LBM). The aim is to parametrically investigate the impact of different PTL structures on the transport of oxygen. The structural characteristics of a reconstructed PTL agree well with experimental investigations. Moreover, the impact of PTL porosity, fiber radius, and anisotropy parameter on the structural characteristics of PTLs are analyzed, and their impact on oxygen transport are elucidated by LBM. Eventually, a customized graded PTL is reconstructed, exhibiting almost optimal mass transport performance for the removal of oxygen. The results show that a higher porosity, larger fiber radius, and smaller anisotropy parameter facilitate the formation of oxygen propagation pathways. By tailoring the fiber characteristics and thus optimizing the PTLs, guidelines for the optimal design and manufacturing can be obtained for large-scale PTLs for electrolyzers.

Turning on Asymmetric Catalysis of Achiral Metal-Organic Frameworks by Imparting Chiral Microenvironment.

The development of heterogeneous asymmetric catalysts has attracted increasing interest in synthetic chemistry but mostly relies on the immobilization of homogeneous chiral catalysts. Herein, a series of chiral metal-organic frameworks (MOFs) have been fabricated by anchoring similar chiral hydroxylated molecules (catalytically inactive) with different lengths onto Zr-oxo clusters in achiral PCN-222(Cu). The resulting chiral MOFs exhibit regulated enantioselectivity up to 83 % ee in the asymmetric ring-opening of cyclohexene oxide. The chiral molecules furnished onto the catalytic Lewis sites in the MOF create multilevel microenvironment, including the hydrogen interaction between the substrate and the chiral -OH group, the steric hindrance endowed by the benzene ring on the chiral molecules, and the proximity between the catalytic sites and chiral molecules confined in the MOF pores, which play crucial roles and synergistically promote chiral catalysis. This work nicely achieves heterogeneous enantioselective catalysis by chiral microenvironment modulation around Lewis acid sites.

Active Sites Decorated Nonpolar Pore-Based MOF for One-step Acquisition of C2 H4 and Recovery of C3 H6.

Herein, a 2-fold interpenetrated metal-organic framework (MOF) Zn-BPZ-TATB with accessible N/O active sites in nonpolar pore surfaces was reported for one-step C2 H4 purification from C2 H6 or C3 H6 mixtures as well as recovery of C3 H6 from C2 H6 /C3 H6 /C2 H4 mixtures. The MOF exhibits the favorable C2 H6 and C3 H6 uptakes (>100 cm3 g-1 at 298 K under 100 kPa) as well as selective adsorption of C2 H6 and C3 H6 over C2 H4 . The C3 H6 - and C2 H6 -selective feature were investigated detailedly by experimental tests as well as sorption kinetic studyies. Molecular modelling revealed the multiple interactions between C3 H6 or C2 H6 molecules and methyl groups as well as triazine rings in pores. Zn-BPZ-TATB not only can directly generate 323.4 L kg-1 and 15.4 L kg-1 of high-purity (≥99.9 %) C2 H4 from C3 H6 /C2 H4 and C2 H6 /C2 H4 mixtures, but also provide a large high-purity (≥99.5 %) C3 H6 recovery capacity of 60.1 L kg-1 from C3 H6 /C2 H4 mixtures. More importantly, the high-purity C3 H6 (≥99.5 %) and C2 H4 (≥99.9 %) with the productivities of 38.2 and 12.7 L kg-1 can be simultaneously obtained from C2 H6 /C3 H6 /C2 H4 mixtures through a single adsorption/desorption cycle.

Consecutive Single-Crystal-to-Single-Crystal Isomerization of Novel Octamolybdate Anions within a Microporous Hybrid Framework with Robust Water Sorption Properties.

The 3D hybrid framework [{Cu(cyclam)}3 (κ-Mo8 O27 )] ⋅ 14H2 O (1) (cyclam=1,4,8,11-tetraazacyclotetradecane) undergoes sequential single-crystal-to-single-crystal transformations upon heating to afford two different anhydrous phases (2 a and 3 a). These transitions modify the framework dimensionality and enable the isomerization of κ-octamolybdate (κ-Mo8 ) anions into λ (2 a) and µ (3 a) forms through metal migration. Hydration of 3 a involves condensation of one water molecule to the cluster to afford the γ-Mo8 isomer in 4, which dehydrates back into 3 a through the 6 a intermediate. In contrast, 2 a reversibly hydrates to form 5, exhibiting the same Mo8 cluster as that of 1. It is remarkable that three of the Mo8 clusters (κ, λ and µ) are new and that up to three different microporous phases can be isolated from 1 (2 a, 3 a, and 6 a). Water vapor sorption analyses show high recyclability and the highest uptake values for POM-based systems. The isotherms display an abrupt step at low humidity level desirable for humidity control devices or water harvesting in drylands.

Porous and Water Stable 2D Hybrid Metal Halide with Broad Light Emission and Selective H2 O Vapor Sorption.

In this work we report a strategy for generating porosity in hybrid metal halide materials using molecular cages that serve as both structure-directing agents and counter-cations. Reaction of the [2.2.2] cryptand (DHS) linker with PbII in acidic media gave rise to the first porous and water-stable 2D metal halide semiconductor (DHS)2 Pb5 Br14 . The corresponding material is stable in water for a year, while gas and vapor-sorption studies revealed that it can selectively and reversibly adsorb H2 O and D2 O at room temperature (RT). Solid-state NMR measurements and DFT calculations verified the incorporation of H2 O and D2 O in the organic linker cavities and shed light on their molecular configuration. In addition to porosity, the material exhibits broad light emission centered at 617 nm with a full width at half-maximum (FWHM) of 284 nm (0.96 eV). The recorded water stability is unparalleled for hybrid metal halide and perovskite materials, while the generation of porosity opens new pathways towards unexplored applications (e.g. solid-state batteries) for this class of hybrid semiconductors.

A Coordination Network Featuring Two Distinct Copper(II) Coordination Environments for Highly Selective Acetylene Adsorption.

Single crystals of 2D coordination network {Cu2 L2 ⋅ (DMF)3 (H2 O)3 }n (1-DMF) were prepared by reaction of commercial reagents 3-formyl-4-hydroxybenzoic acid (H2 L) and Cu(NO3 )2 in dimethylformamide (DMF). The single-crystal structure shows two distinct Cu(II) coordination environments arising from the separate coordination of Cu(II) cations to the carboxylate and salicylaldehydato moieties on the linker, with 1D channels running through the structure. Flexibility is exhibited on solvent exchange with ethanol and tetrahydrofuran, while porosity and the unique overall connectivity of the structure are retained. The activated material exhibits type I gas sorption behaviour and a BET surface area of 950 m2 g-1 (N2 , 77 K). Notably, the framework adsorbs negligible quantities of CH4 compared with CO2 and the C2 Hn hydrocarbons. It exhibits exceptional selectivity for C2 H2 /CH4 and C2 H2 /C2 Hn , which has applicability in separation technologies for the isolation of C2 H2 .

An Adaptive Hydrogen-Bonded Organic Framework for the Exclusive Recognition of p-Xylene.

Separation of xylene isomers is one of the most important but most challenging and energy-intensive separation processes in the petrochemical industry. Here, we report an adaptive hydrogen-bonded organic framework (HOF-29) constructed from a porphyrin based organic building block 4,4',4'',4'''-(porphyrin-5,10,15,20-tetrayl) tetrabenzonitrile (PTTBN), exhibiting the exclusive molecular recognition of p-xylene (pX) over its isomers of o-xylene (oX) and m-xylene (mX), as clearly demonstrated in the single crystal structure transformation and 1 H NMR studies. Single crystal structure studies show that single-crystal-to-single-crystal transformation from the as-synthesized HOF-29 to the pX exclusively included HOF-29⊃pX is triggered by the encapsulation of pX molecules, accompanied by sliding of the 2D layers and local distortion of the ligand, which provides multiple C-H⋅⋅⋅π interactions.

Modulation of the Sorption Characteristics for an H-bonded porous Architecture by Varying the Chemical Functionalization of the Channel Walls.

Five isostructural microporous supramolecular architectures prepared by H-bonded assembly between the hexa-anionic complex [Zr2 (Ox)7 ]6- (Ox=oxalate, (C2 O4 )2- ) and tripodal cations (H3 -TripCH2 -R)3+ with R=H, CH3 , OH and OBn (Bn=CH2 Ph) are reported. The possibility to obtain the same structure using a mixture of tripodal cations with different R group (R=OH and R=CH3 ) has also been successfully explored, providing a unique example of three-component H-bonded porous framework. The resulting SPA-1(R) materials feature 1D pores decorated by R groups, with apparent pore diameters ranging from 3.0 to 8.5 Å. Influence of R groups on the sorption properties of these materials is evidenced through CO2 and H2 O vapor sorption/desorption experiments, as well as with I2 capture/release experiments in liquid media. This study is one of the first to demonstrate the possibility of tuning the porosity and exerting precise control over the chemical functionalization of the pores in a given H-bonded structure, without modifying the topology of the reference structure, and thus finely adjusting the sorption characteristics of the material.

What Is the Smallest Zeolite That Could Be Synthesized?

Zeolites with a few unit cells are promising as catalyst and adsorbents. The quest to synthesize the smallest zeolites has recently resulted in 4 to 8 nm nanozeolites, about 2 to 4 unit cells. These findings pose the question of what is the smallest zeolite that could be obtained by hydrothermal synthesis. Here we address this question using molecular simulations and thermodynamic analysis. The simulations predict that amorphous precursors as small as 4 nm can crystallize zeolites, in agreement with the experiments. We find that interfacial forces dominate the structure of smaller particles, resulting in size-dependent compact isomers that have ring and pore distributions different from open framework zeolites. The instability of zeolites smaller than 3±0.5 nm precludes a classical mechanism of nucleation from solution or through assembly of small nanoslabs.

Electron Diffraction Enables the Mapping of Coke in ZSM-5 Micropores Formed during Methanol-to-Hydrocarbons Conversion.

Unveiling the coke formation in zeolites is an essential prerequisite for tackling the deactivation of these catalysts in the transformations of hydrocarbons. Herein, we present the direct mapping of coke in the micropores of ZSM-5 catalysts used in methanol-to-hydrocarbons conversion by single-crystal electron diffraction analysis. The latter technique revealed a polycyclic aromatic structure along the straight channel, wherein the high-quality data permit refinement of its occupancy to about 40 %. These findings were exploited to analyze the evolution of micropore coke during the reaction. Herein, coke-associated signals, which correlate with the activity loss, indicate that the nucleation of coke commences in the intersections of sinusoidal and straight channels, while the formation of coke in the straight pores occurs in the late stages of deactivation. The findings uncover an attractive method for analyzing coke deposition in the micropore domain.

MOF Synthesis Prediction Enabled by Automatic Data Mining and Machine Learning.

Despite rapid progress in the field of metal-organic frameworks (MOFs), the potential of using machine learning (ML) methods to predict MOF synthesis parameters is still untapped. Here, we show how ML can be used for rationalization and acceleration of the MOF discovery process by directly predicting the synthesis conditions of a MOF based on its crystal structure. Our approach is based on: i) establishing the first MOF synthesis database via automatic extraction of synthesis parameters from the literature, ii) training and optimizing ML models by employing the MOF database, and iii) predicting the synthesis conditions for new MOF structures. The ML models, even at an initial stage, exhibit a good prediction performance, outperforming human expert predictions, obtained through a synthesis survey. The automated synthesis prediction is available via a web-tool on https://mof-synthesis.aimat.science.

Superior Metal-Organic Framework Activation with Dimethyl Ether.

Metal-organic frameworks (MOFs) are demonstrated to be readily activated by treatment with the low surface tension, low boiling point solvent dimethyl ether (DME). The mildness of the method enables access to high surface areas by avoiding structural changes in the framework that often plague thermal activation methods. A distinction from previous methods is that DME activation succeeds for materials with coordinatively unsaturated sites (CUS) and non-CUS MOFs as well. DME displaces solvent molecules occupying the pores of the MOF as well as those coordinated to metal centers; reducing evacuation temperature by using a coordinating, yet highly volatile guest enables low temperature activation with structural retention as demonstrated surface area measurements that match or exceed existing activation protocols.

Shell-like Xenon Nano-Traps within Angular Anion-Pillared Layered Porous Materials for Boosting Xe/Kr Separation.

Adsorptive separation of xenon (Xe) and krypton (Kr) is a promising technique but remains a daunting challenge since they are atomic gases without dipole or quadruple moments. Herein we report a strategy for fabricating angular anion-pillared materials featuring shell-like Xe nano-traps, which provide a cooperative effect conferred by the pore confinement and multiple specific interactions. The perfect permanent pore channel (4-5 Å) of Ni(4-DPDS)2 MO4 (M=Cr, Mo, W) can host Xe atoms efficiently even at ultra-low concentration (400 ppm Xe), showing the second-highest selectivity of 30.2 in Ni(4-DPDS)2 WO4 and excellent Xe adsorption capacity in Ni(4-DPDS)2 CrO4 (15.0 mmol kg-1 ). Crystallography studies and DFT-D calculations revealed the energy favorable binding sites and angular anions enable the synergism between optimal pore size and polar porosity for boosting Xe affinity. Dynamic breakthrough experiments demonstrated three MOFs as efficient adsorbents for Xe/Kr separation.

Low Temperature Deposition of Highly Cyclable Porous Prussian Blue Cathode for Lithium-Ion Microbattery.

Small dimension Li-ion microbatteries are of great interest for embedded microsystems and on-chip electronics. However, the deposition of fully crystallized cathode thin film generally requires high temperature synthesis or annealing, incompatible with microfabrication processes of integrated Si devices. In this work, a low temperature deposition process of a porous Prussian blue-based cathode on Si wafers is reported. The active material is electrodeposited under aqueous conditions using a pulsed deposition protocol on a porous dendritic metallic current collector that ensures good electronic conductivity of the composite. The high voltage cathodes exhibit a huge areal capacity of ≈650 µAh cm-2 and are able to withstand more than 2000 cycles at 0.25 mA cm-2 rate. The application of these electrode composites with porous Sn based alloying anodes is also demonstrated for the first time in full cell configuration, with high areal energy of 3.1 J cm-2 and more than 95% reversible capacity. This outstanding performance can be attributed to uniform deposition of Prussian blue materials on conductive matrix, which maintains electronic conductivity while simultaneously providing mechanical integrity to the electrode. This finding opens new horizons in the monolithic integration of energy storage components compatible with the semiconductor industry for self-powered microsystems.

Effect of NaOH molarities to the microstructure and sodium storage performance of the Sn-MOF derived SnO2microporous rod.

In this study, we demonstrated a facile method to prepare a novel SnO2microporous rod with various microstructures by controlling NaOH molarities in precursor synthesis processes. Four different molarities of NaOH solution (0.005 M, 0.048 M, 0.12 M and 0.5 M) were used together with o-phthalic acid in Sn-MOF synthesis to determine the effect of ligand [o-C6H4CO222-] concentration on microstructure evolution. It was found that increasing NaOH molarity can effectively decrease the size of Sn-MOF rods. Then, the SnO2microporous rods were obtained by calcinating the as-prepared Sn-MOF as microstructures. Under an optimized experimental condition (NaOH molarity of 0.12 M), the SnO2rods shows a modest initial coulombic efficiency of 61.3% with a high reversible sodium storage capacity of 503 mAh g-1after 150 cycles at 50 mA g-1. Moreover, an impressive reversible sodium storage capacity of 206 mAh g-1can be obtained at long-term cycling performance (800 cycles at current density of 2 A g-1). Effects of morphologies to electrochemical performances have been further discussed in aspects of intrinsic resistance, pseudocapacitive contribution, surface area and porous structure and microstructural stability, and the enhanced electrochemical performance could be attributed to factors of enhanced pseudocapacitive charge contribution, optimized microstructures, and structural stability, which ensure the SnO2-0.12 M to have a good rate performance and cyclability. This nanoscale-engineering method adopted here could be a promising path to fabricate SnO2-based anodes with novel microstructures for sodium storage applications.