Boc Protection for Diamine-Appended MOF Adsorbents to Enhance CO2 Recyclability under Realistic Humid Conditions.

Among the various metal-organic framework (MOF) adsorbents, diamine-functionalized Mg2(dobpdc) (dobpdc4- = 4,4-dioxidobiphenyl-3,3'-dicarboxylate) shows remarkable carbon dioxide removal performance. However, applying diamine-functionalized Mg2(dobpdc) in practical applications is premature because it shows persistent performance degradation under real flue gas conditions containing water vapor owing to diamine loss during wet cycles. To address this issue, we employed hydrophobic carbonate compounds to protect diamine groups in een-Mg2(dobpdc) (een-MOF, een = N-ethylethylenediamine). tert-Butyl dicarbonate (Boc) reacted rapidly with diamines at the pore openings of MOF particles to form dense secondary and tertiary hydrophobic amines, effectively preventing moisture ingress. The Boc-protected een-MOF-Boc1 maintained excellent CO2 adsorption even under simulated flue gas conditions containing 10% H2O. This observation indicates that Boc protection renders een groups intact during repeated wet cycles, suggesting that Boc-protected een groups are resistant to replacement by water molecules. To increase the practicability of the MOF adsorbent, we fabricated een-MOF/PAN-Boc1 composite beads by shaping MOF particles with polyacrylonitrile (PAN). Notably, the composite beads maintained their CO2 adsorption performance even after repeating the temperature swing adsorption process more than 150 times in 10% water vapor. Furthermore, breakthrough tests showed that the dynamic CO2 separation performance was retained under humid conditions. These results demonstrate that Boc protection provides an easy and effective way to develop promising adsorbents with high CO2 adsorption capacity, long-term durability, and the properties required for postcombustion applications.

Extended MOF-74-Type Variant with an Azine Linkage: Efficient Direct Air Capture and One-Pot Synthesis.

Direct air capture (DAC) shows considerable promise for the effective removal of CO2; however, materials applicable to DAC are lacking. Among metal-organic framework (MOF) adsorbents, diamine-Mg2(dobpdc) (dobpdc4- = 4,4-dioxidobiphenyl-3,3'-dicarboxylate) effectively removes low-pressure CO2, but the synthesis of the organic ligand requires high temperature, high pressure, and a toxic solvent. Besides, it is necessary to isolate the ligand for utilization in the synthesis of the framework. In this study, we synthesized a new variant of extended MOF-74-type frameworks, M2(hob) (M = Mg2+, Co2+, Ni2+, and Zn2+; hob4- = 5,5'-(hydrazine-1,2-diylidenebis(methanylylidene))bis(2-oxidobenzoate)), constructed from an azine-bonded organic ligand obtained through a facile condensation reaction at room temperature. Functionalization of Mg2(hob) with N-methylethylenediamine, N-ethylethylenediamine, and N,N'-dimethylethylenediamine (mmen) enables strong interactions with low-pressure CO2, resulting in top-tier adsorption capacities of 2.60, 2.49, and 2.91 mmol g-1 at 400 ppm of CO2, respectively. Under humid conditions, the CO2 capacity was higher than under dry conditions due to the presence of water molecules that aid in the formation of bicarbonate species. A composite material combining mmen-Mg2(hob) and polyvinylidene fluoride, a hydrophobic polymer, retained its excellent adsorption performance even after 7 days of exposure to 40% relative humidity. In addition, the one-pot synthesis of Mg2(hob) from a mixture of the corresponding monomers is achieved without separate ligand synthesis steps; thus, this framework is suitable for facile large-scale production. This work underscores that the newly synthesized Mg2(hob) and its composites demonstrate significant potential for DAC applications.

Postsynthetically Modified Alkoxide-Exchanged Ni2(OR)2BTDD: Synergistic Interactions of CO2 with Open Metal Sites and Functional Groups.

Postsynthetic modifications (PSMs) of metal-organic frameworks (MOFs) play a crucial role in enhancing material performance through open metal site (OMS) functionalization or ligand exchange. However, a significant challenge persists in preserving open metal sites during ligand exchange, as these sites are inherently bound by incoming ligands. In this study, for the first time, we introduced alkoxides by exchanging bridging chloride in Ni2Cl2BTDD (BTDD=bis (1H-1,2,3,-triazolo [4,5-b],-[4',5'-i]) dibenzo[1,4]dioxin) through PSM. Rietveld refinement of synchrotron X-ray diffraction data indicated that the alkoxide oxygen atom bridges Ni(II) centers while the OMSs of the MOF are preserved. Due to the synergy of the existing OMS and introduced functional group, the alkoxide-exchanged MOFs showed CO2 uptakes superior to the pristine MOF. Remarkably, the tert-butoxide-substituted Ni_T exhibited a nearly threefold and twofold increase in CO2 uptake compared to Ni2Cl2BTDD at 0.15 and 1 bar, respectively, as well as high water stability relative to the other exchanged frameworks. Furthermore, the Grand Canonical Monte Carlo simulations for Ni_T suggested that CO2 interacts with the OMS and the surrounding methyl groups of tert-butoxide groups, which is responsible for the enhanced CO2 capacity. This work provides a facile and unique synthetic strategy for realizing a desirable OMS-incorporating MOF platform through bridging ligand exchange.

Aminal-Linked Covalent Organic Frameworks with hxl-a and Quasi-hcb Topologies for Efficient C2 H6 /C2 H4 Separation.

In reticular chemistry, topology is a powerful concept for defining the structures of covalent organic frameworks (COFs). However, due to the lack of diversity in the symmetry and reaction stoichiometry of the monomers, only 5% of the two-dimensional topologies have been reported to be COFs. To overcome the limitations of COF connectivity and pursue novel topologies in COF structures, two aminal-linked COFs, KUF-2 and KUF-3, are prepared, with dumbbell-shaped secondary building units. Linear dialdehydes and piperazine are condensed at a ratio of 1:2 to construct an aminal linkage, leading to unreported hxl-a (KUF-2) and quasi-hcb (KUF-3) structures. Notably, KUF-3 displays top-tier C2 H6 /C2 H4 selectivity and C2 H6 uptake at 298 K, outperforming most porous organic materials. The intrinsic aromatic ring-rich and Lewis basic pore environments, and appropriate pore widths enable the selective adsorption of C2 H6 , as confirmed by Grand Canonical Monte Carlo simulations. Dynamic breakthrough curves revealed that C2 H6 can be selectively separated from a gas mixture of C2 H6 and C2 H4 . This study suggests that topology-based design of aminal-COFs is an effective strategy for expanding the field of reticular chemistry and provides the facile integration of strong Lewis basic sites for selective C2 H6 /C2 H4 separation.

Switchable Xe/Kr Selectivity in a Hofmann-Type Metal-Organic Framework via Temperature-Responsive Rotational Dynamics.

The development of adsorbents for Kr and Xe separation is essential to meet industrial demands and for energy conservation. Although a number of previous studies have focused on Xe-selective adsorbents, stimuli-responsive Xe/Kr-selective adsorbents still remain underdeveloped. Herein, a Hofmann-type framework Co(DABCO)[Ni(CN)4 ] (referred to as CoNi-DAB; DABCO = 1,4-diazabicyclo[2,2,2]octane) that provides a temperature-dependent switchable Xe/Kr separation performance is reported. CoNi-DAB showed high Kr/Xe (0.8/0.2) selectivity with significant Kr adsorption at 195 K as well as high Xe/Kr (0.2/0.8) selectivity with superior Xe adsorption at 298 K. Such adsorption features are associated with the temperature-dependent rotational configuration of the DABCO ligand, which affects the kinetic gate-opening temperature of Xe and Kr. The packing densities of Xe (2.886 g cm-3 at 298 K) and Kr (2.399 g  cm-3 at 195 K) inside the framework are remarkable and comparable with those of liquid Xe (3.057 g cm-3 ) and liquid Kr (2.413 g cm-3 ), respectively. Breakthrough experiments confirm the temperature-dependent reverse separation performance of CoNi-DAB at 298 K under dry and wet (88% relative humidity) conditions and at 195 K under dry conditions. The unique adsorption behavior is also verified through van der Waals (vdW)-corrected density functional theory (DFT) calculations and nudged elastic band (NEB) simulations.

Electronic Effect-Modulated Enhancements of Proton Conductivity in Porous Organic Polymers.

We proposed a new strategy to maximize the density of acidic groups by modulating the electronic effects of the substituents for high-performance proton conductors. The conductivity of the sulfonated 1-MeL40-S with methyl group corresponds to 2.29×10-1  S cm-1 at 80 °C and 90 % relative humidity, remarkably an 22100-fold enhancement over the nonsulfonated 1-MeL40. 1-MeL40-S maintains long-term conductivity for one month. We confirm that this synthetic method is generalized to the extended version POPs, 2-MeL40-S and 3-MeL40-S. In particular, the conductivities of the POPs compete with those of top-level porous organic conductors. Moreover, the activation energy of the POPs is lower than that of the top-performing materials. This study demonstrates that systematic alteration of the electronic effects of substituents is a useful route to improve the conductivity and long-term durability of proton-conducting materials.

Functionalization of Diamine-Appended MOF-Based Adsorbents by Ring Opening of Epoxide: Long-Term Stability and CO2 Recyclability under Humid Conditions.

Although diamine-appended metal-organic framework (MOF) adsorbents exhibit excellent CO2 adsorption performance, a continuous decrease in long-term capacity during repeated wet cycles remains a formidable challenge for practical applications. Herein, we present the fabrication of diamine-appended Mg2(dobpdc)-alumina beads (een-MOF/Al-Si-Cx; een = N-ethylethylenediamine; x = number of carbon atoms attached to epoxide) coated with hydrophobic silanes and alkyl epoxides. The reaction of epoxides with diamines in the portal of the pore afforded sufficient hydrophobicity, hindered the penetration of water vapor into the pores, and rendered the modified diamines less volatile. een-MOF/Al-Si-C17-200 (een-MOF/Al-Si-C17-y; y = 50, 100, and 200, denoting wt % of C17 with respect to the bead, respectively), with substantial hydrophobicity, showed a significant uptake of 2.82 mmol g-1 at 40 °C and 15% CO2, relevant to flue gas concentration, and a reduced water adsorption. The modified beads maintained a high CO2 capacity for over 100 temperature-swing adsorption cycles in the presence of 5% H2O and retained CO2 separation performance in breakthrough tests under humid conditions. This result demonstrates that the epoxide coating provides a facile and effective method for developing promising adsorbents with high CO2 adsorption capacity and long-term durability, which is a required property for postcombustion applications.

High Ammonia Uptake of a Metal-Organic Framework Adsorbent in a Wide Pressure Range.

Although numerous porous adsorbents have been investigated for NH3 capture applications, these materials often exhibit insufficient NH3 uptake, low NH3 affinity at the ppm level, and poor chemical stability against wet NH3 conditions. The NH3 capture properties of M2 (dobpdc) complexes (M=Mg2+ , Mn2+ , Co2+ , Ni2+ , and Zn2+ ; dobpdc4- =4,4-dioxidobiphenyl-3,3-dicarboxylate) that contain open metal sites is presented. The NH3 uptake of Mg2 (dobpdc) at 298 K was 23.9 mmol g-1 at 1 bar and 8.25 mmol g-1 at 570 ppm, which are record high capacities at both pressures among existing porous adsorbents. The structural stability of Mg2 (dobpdc) upon exposure to wet NH3 was superior to that of the other M2 (dobpdc) and the frameworks tested. Overall, these results demonstrate that Mg2 (dobpdc) is a recyclable compound that exhibits significant NH3 affinity and capacity, making it a promising candidate for real-world NH3 -capture applications.

Control of the Metal Composition in Bimetallic Mg/Zn(dobpdc) Constructed from a One-Dimensional Zn-Based Template.

While mixed-metal ions into a single framework can be randomly arranged in most reported cases, it is synthetically challenging to control and organize the distribution of different metal ions over a three-dimensional structure. In this context, for the family of M2(dobpdc) with broad applications, we present the first case of a bimetallic Mg/Zn(dobpdc) framework with a 1:1 compositional ratio, based on a one-dimensional Zn(H2dobpdc) template, which would not be obtained by the conventional reaction of the corresponding metal salts. Moreover, we demonstrate that the resultant compositional ratios in the bimetallic M'/Zn(dobpdc) (M' = Mg, Mn, Co, Ni) are governed by the ionic radii of the metals and the affinity of the metal ions for the linker groups. Notably, the unexpected gradual reduction in the adsorption enthalpy and the mixed CO2 adsorption feature are revealed in Mg/Zn(dobpdc) and its diamine-grafted framework, respectively.

A Hydrogen-Bonded Organic Framework (HOF) with Type†IV NH3 Adsorption Behavior.

An S-shaped gas isotherm pattern displays high working capacity in pressure-swing adsorption cycle, as established for CO2 , CH4 , acetylene, and CO. However, to our knowledge, this type of adsorption behavior has not been revealed for NH3 gas. Herein, we design and characterize a hydrogen-bonded organic framework (HOF) that can adsorb NH3 uniquely in an S-shape (type IV) fashion. While conventional porous materials, mostly with type I NH3 adsorption behavior, require relatively high regeneration temperature, this platform which has significant working capacity is easily regenerated and recyclable at room temperature.

Diaminopropane-Functionalized Metal-Organic Frameworks with Controllable Diamine Loss and One-Channel Flipped CO2 Adsorption Mode.

Diamine-functionalized metal-organic frameworks (MOFs) based on Mg2(dobpdc) (dobpdc4- = 4,4'-dioxidobihyenyl-3,3'-dicarboxylate) have been frequently reported as promising CO2 adsorbents due to their characteristic step-shaped adsorption behavior. However, high CO2 desorption temperatures for diamine-Mg2(dobpdc)-based adsorbents led to gradual diamine loss while the existence of an exotic CO2 adsorption mode remains experimentally unanswered. Herein, we present CO2 adsorbents obtained by functionalizing Mn2(dobpdc) with a diaminopropane series. These adsorbents offer low regeneration energies, allowing CO2 desorption at lower temperatures than the reported Mg-based analogs. Our first-principles density functional theory calculations showed that the binding strength between the diamine and Mn ions in Mn2(dobpdc) was stronger than that between the diamine and Mg ions in Mg2(dobpdc), preventing diamine loss even at high temperatures and enabling efficient regeneration. Additionally, the computational and experimental data demonstrated that primary-tertiary diamine-functionalized MOFs exhibit one-channel flipped adsorption structures that have not been experimentally revealed. Our findings provide insights into the role of metal ions in diamine loss for the future development of efficient amine-based CO2 adsorbents.

Protocol for the fabrication of porous organic polymer-based composites for photocatalytic degradation of a sulfur mustard simulant.

Although porous organic polymer (POP) has been explored as a promising photosensitizer, its powdered form makes it unfavorable for practical applications. Here, we demonstrate a protocol for fabricating imidazoline-based POP composites using fabric and sponge as substrates. This fabrication is limited to POPs with aldehyde containing organic building blocks. For complete details on the use and execution of this protocol, please refer to Kim et al. (2022).

Understanding Correlation Between CO2 Insertion Mechanism and Chain Length of Diamine in Metal-Organic Framework Adsorbents.

Although CO2 insertion is a predominant phenomenon in diamine-functionalized Mg2 (dobpdc) (dobpdc4- =4,4-dioxidobiphenyl-3,3'-dicarboxylate) adsorbents, a high-performance metal-organic framework for capturing CO2 , the fundamental function of the diamine carbon chain length in the mechanism remains unclear. Here, Mg2 (dobpdc) systems with open metal sites grafted by primary diamines NH2 -(CH2 )n -NH2 were developed, with en (n=2), pn (n=3), bn (n=4), pen (n=5), hn (n=6), and on (n=8). Based on CO2 adsorption and IR results, CO2 insertion is involved in frameworks with n=2 and 3 but not in systems with n≥5. According to NMR data, bn-appended Mg2 (dobpdc) exhibited three different chemical environments of carbamate units, attributed to different relative conformations of carbon chains upon CO2 insertion, as validated by first-principles density functional theory (DFT) calculations. For 1-hn and 1-on, DFT calculations indicated that diamine inter-coordinated open metal sites in adjacent chains bridged by carboxylates and phenoxides of dobpdc4- . Computed CO2 binding enthalpies for CO2 insertion (-27.8 kJ mol-1 for 1-hn and -20.2 kJ mol-1 for 1-on) were comparable to those for CO2 physisorption (-19.3 kJ mol-1 for 1-hn and -20.8 kJ mol-1 for 1-on). This suggests that CO2 insertion is likely to compete with CO2 physisorption on diamines of the framework when n≥5.

Shaping of a Metal-Organic Framework-Polymer Composite and Its CO2 Adsorption Performances from Humid Indoor Air.

Diamine-functionalized metal-organic frameworks (MOFs) are known as desirable adsorbents that can capture CO2 even at low pressures, but the humidity instability of bare MOF powders as well as their shaping have not yet adequately addressed for practical applications. Herein, we report an effective synthetic strategy for fabricating millimeter-sized MOF/poly(vinylidene fluoride) (PVDF) composite beads with different amounts of PVDF binders (30, 40, and 50 wt %) via a phase inversion method, followed by the postfunctionalization of 1-ethylpropane-1,3-diamine (epn). Compared with the pristine MOF powder, the diamine-grafted bead, epn-MOF/PVDF40, upon mixing with 40% binder polymers, exhibited a superior long-term performance without structural collapse for up to 1 month. The existence of the hydrophobic PVDF polymer in the composite material is responsible for such durability. This work provides a promising preparative route toward developing stable and shaped MOFs for the removal of indoor CO2.

High-Throughput Discovery of Ni(IN)2 for Ethane/Ethylene Separation.

Although ethylene (C2 H4 ) is one of the most critical chemicals used as a feedstock in artificial plastic chemistry fields, it is challenging to obtain high-purity C2 H4 gas without any trace ethane (C2 H6 ) by the oil cracking process. Adsorptive separation using C2 H6 -selective adsorbents is beneficial because it directly produces high-purity C2 H4 in a single step. Herein, Ni(IN)2 (HIN = isonicotinic acid) is computationally discovered as a promising adsorbent with the assistance of the multiscale high-throughput computational screening workflow and Computation-Ready, Experimental (CoRE) metal-organic framework (MOF) 2019 database. Ni(IN)2 is subsequently synthesized and tested to show the ideal adsorbed solution theory (IAST) selectivity of 2.45 at 1 bar for a C2 H6 /C2 H4 mixture (1:15), which is one of the top-performing selectivity values reported for C2 H6 -selective MOFs as well as excellent recyclability, suggesting that this material is a promising C2 H6 -selective adsorbent. Process-level simulation results based on experimental isotherms demonstrate that the material is one of the top materials reported to date for ethane/ethylene separation under the conditions considered in this work.

Metal-Organic Framework Adsorbent for Practical Capture of Trace Carbon Dioxide.

Control of indoor CO2 concentration to a safe level is important to human health. Metal-organic-framework-based adsorbents show superior adsorption performance at moderate CO2 concentration compared to other solid adsorbents but suffer from low capacities and high regeneration temperature at indoor CO2 concentrations and poor humidity stability. Herein, we report epn-grafted Mg2(dobpdc) (epn = 1-ethylpropane-1,3-diamine) showing a CO2 capacity of 12.2 wt % at an acceptable concentration of 1000 ppm and a practically low desorption temperature of 70 °C, which surpasses the performance of conventional solid adsorbents under the given conditions. After poly(dimethylsiloxane) coating, this material reveals a significant adsorption amount (∼10 wt %) in humid conditions (up to 98% relative humidity) with structural durability.

Fine-tuning of wettability in a single metal-organic framework via postcoordination modification and its reduced graphene oxide aerogel for oil-water separation.

Elaborate control of wettability in a single platform is essential for materials' applications towards oil-water separation, but it still remains challenging. Herein, we performed postcoordination modification of Mg2(dobpdc) with monoamines of various alkyl chain lengths, enabling both long-term hydrolytic stability and facile fine-tuning of wettability. An efficient separation of oil-water mixtures was achieved by using the octylamine-appended framework (OctA). We also prepared an OctA/reduced graphene oxide aerogel that showed exceptional absorption capacities towards organic solvents and oil as well as superb recyclability with maintained absorbency.

PDMS-coated hypercrosslinked porous organic polymers modified via double postsynthetic acidifications for ammonia capture.

A hypercrosslinked porous organic polymer was modified by post-oxidation and post-sulfonation to obtain a porous platform with a high density of acidic groups. Such an acidified material exhibits record high NH3 adsorption capacity per surface area, fast adsorption rate, and recyclability at low desorption temperature. Noticeably, the coating of the polymer with PDMS represents a facile and efficient route to enable both a significant improvement of low-pressure NH3 adsorption capacity (∼40-fold enhancement; from 0.04 to 1.41 mmol g-1) with respect to the non-modified polymer at 500 ppm and hydrophobicity associated with the selective sorption of NH3 over water vapor (hydrophilic for the non-coated material). This material is easy to prepare, cost-effective, and scalable to mass production.