Metal–Organic Frameworks with Target‐Specific Active Sites Switched by Photoresponsive Motifs: Efficient Adsorbents for Tailorable CO2 Capture

Photoresponsive metal–organic frameworks (PMOFs) are of interest for tailorable CO₂ adsorption. However, modulation of CO₂ adsorption on PMOFs is based on steric hindrance or structural change owing to weak interactions between CO₂ and active sites. It is challenging to fabricate PMOFs with strong but tailorable sites for CO₂ adsorption. Now, the construction of PMOFs with target‐specific (strong) active sites is achieved by introducing tetraethylenepentamine into azobenzene‐functionalized MOFs for tailorable CO₂ adsorption. Amines are specific active sites for CO₂, contributing to capture CO₂ selectively. Cis/trans isomerization of azobenzene motifs trigged by UV/Vis light adjusts the electrostatic potential of amines significantly, leading to exposure/shelter of amines and modulation of CO₂ adsorption on strong active sites. This system enables us to design adsorption processes for CO₂ capture from mixtures, which is impossible to realize by traditional PMOFs.     

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.     

Crystal Dynamics in Multi‐stimuli‐Responsive Entangled Metal–Organic Frameworks

An understanding of solid‐state crystal dynamics or flexibility in metal–organic frameworks (MOFs) showing multiple structural changes is highly demanding for the design of materials with potential applications in sensing and recognition. However, entangled MOFs showing such flexible behavior pose a great challenge in terms of extracting information on their dynamics because of their poor single‐crystallinity. In this article, detailed experimental studies on a twofold entangled MOF ( f‐MOF‐1) are reported, which unveil its structural response toward external stimuli such as temperature, pressure, and guest molecules. The crystallographic study shows multiple structural changes in f‐MOF‐1 , by which the 3 D net deforms and slides upon guest removal. Two distinct desolvated phases, that is, f‐MOF‐1 a and f‐MOF‐1 b , could be isolated; the former is a metastable one and transformable to the latter phase upon heating. The two phases show different gated CO₂ adsorption profiles. DFT‐based calculations provide an insight into the selective and gated adsorption behavior with CO₂ of f‐MOF‐1 b . The gate‐opening threshold pressure of CO₂ adsorption can be tuned strategically by changing the chemical functionality of the linker from ethanylene (?CH₂?CH₂?) in f‐MOF‐1 to an azo (?N=N?) functionality in an analogous MOF, f‐MOF‐2 . The modulation of functionality has an indirect influence on the gate‐opening pressure owing to the difference in inter‐net interaction. The framework of f‐MOF‐1 is highly responsive toward CO₂ gas molecules, and these results are supported by DFT calculations.     

Tuning the adsorption and fluorescence properties of aminal‐linked porous organic polymers through N‐heterocyclic group decoration

Aiming at tuning the adsorption and fluorescence properties of targeted porous organic polymer, four new aminal‐linked porous organic polymers (NAPOPs) were synthesized through the reaction of 1,4‐Bis(4,6‐diamino‐s‐triazin‐2‐yl) benzene (BATB) with four kinds of aldehydes substituted with different N‐heterocyclic groups. Among the polymers, NAPOP‐3 decorated with 5‐phenyl‐tetrazole group shows the largest CO₂ adsorption capacity (2.52 mmol g^?1 at 273 K and 100 kPa) because of its relative large surface area, while NAPOP‐1 decorated with piperazine groups shows relative large CO₂/N₂ adsorption selectivity (77 at 273 K and 100 kPa), attributable to its large CO₂ adsorption heats and cabined pore (<4 Å). Meanwhile, NAPOP‐1 and ?3 exhibit high adsorption rate toward iodine with a high capacity (>240 wt %). In addition, different luminescence emissions were also observed for NAPOPs, indicating different intramolecular charger transfer occurred inside polymer networks. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1724–1730     

Reversible Uptake of COS,CS2, and SO2: Ionic Liquids with O‐Alkylxanthate,O‐Alkylthiocarbonyl,and O‐Alkylsulfite Anions

CO₂‐binding organic liquids (CO₂BOLs) are mixtures of a base (typically an amidine or guanidine) and an alcohol, and have been shown to reversibly capture and release CO₂ with low reaction energies and high gravimetric CO₂ capacity. We now report the ability of such liquid blends to chemically bind and release other acid gases such as CS₂, COS, and SO₂ analogously to CO₂. These systems bind with sulfur‐containing acid gases to form colored ionic liquids with new O‐alkylxanthate, O‐alkylthiocarbonyl, and O‐alkylsulfite anions. The capture and thermal stripping of each acid gas from these systems and their applicability towards flue gas desulfurization is discussed.     

Synthesis and evaluation of N,O‐doped hypercrosslinked polymers and their performance in CO2 capture

A series of N, O‐doped hypercrosslinked microporous polymers (HCPs) with high surface area and rich microporosity were constructed via facile Friedel‐Crafts alkylation promoted by anhydrous ferric chloride (FeCl₃), which used benzyl alcohol (BA) and 2‐phenylimidazole (PID) as the basic building blocks. The effects of structural composition on the pore properties and gas adsorption properties of prepared HCPs were systematically investigated. The results show that by adjusting the ratio of PID to BA, the Brunauer–Emmett–Teller (BET) specific surface area and pore volume of the prepared HCPs can be controlled, and the BET specific surface area of the polymers range from 732–992 m²/g. Gas uptake experiments indicate that the obtained HCPs exhibit very high CO₂ adsorption capacity up to 3.55 mmol/g at 273 K/1.0 bar. Simultaneously, these N, O‐doped HCPs also show quite high isosteric heat of CO₂ sorption at the low coverage (up to 33 kJ/mol). In addition, the prepared HCPs have excellent selective adsorption performance, and the optimal selective adsorption of CO₂/N₂ is 62. These results demonstrate that these prepared HCPs are potential candidates for applications in CO₂ capture.     

Enhanced CO2 Adsorption over Polymeric Amines Supported on Heteroatom‐Incorporated SBA‐15 Silica: Impact of Heteroatom Type and Loading on Sorbent Structure and Adsorption Performance

Silica supported amine materials are promising compositions that can be used to effectively remove CO₂ from large stationary sources, such as flue gas generated from coal‐fired power plants (ca. 10 % CO₂) and potentially from ambient air (ca. 400 ppm CO₂). The CO₂ adsorption characteristics of prototypical poly(ethyleneimine)–silica composite adsorbents can be significantly enhanced by altering the acid/base properties of the silica support by heteroatom incorporation into the silica matrix. In this study, an array of poly(ethyleneimine)‐impregnated mesoporous silica SBA‐15 materials containing heteroatoms (Al, Ti, Zr, and Ce) in their silica matrices are prepared and examined in adsorption experiments under conditions simulating flue gas (10 % CO₂ in Ar) and ambient air (400 ppm CO₂ in Ar) to assess the effects of heteroatom incorporation on the CO₂ adsorption properties. The structure of the composite adsorbents, including local information concerning the state of the incorporated heteroatoms and the overall surface properties of the silicate supports, are investigated in detail to draw a relationship between the adsorbent structure and CO₂ adsorption/desorption performance. The CO₂ adsorption/desorption kinetics are assessed by thermogravimetric analysis and in situ FT‐IR measurements. These combined results, coupled with data on adsorbent regenerability, demonstrate a stabilizing effect of the heteroatoms on the poly(ethyleneimine), enhancing adsorbent capacity, adsorption kinetics, regenerability, and stability of the supported aminopolymers over continued cycling. It is suggested that the CO₂ adsorption performance of silica–aminopolymer composites may be further enhanced in the future by more precisely tuning the acid/base properties of the support.     

Flexible Zirconium Metal‐Organic Frameworks as Bioinspired Switchable Catalysts

Flexible metal–organic frameworks (MOFs) are highly desirable in host–guest chemistry owing to their almost unlimited structural/functional diversities and stimuli‐responsive pore architectures. Herein, we designed a flexible Zr‐MOF system, namely PCN‐700 series, for the realization of switchable catalysis in cycloaddition reactions of CO₂ with epoxides. Their breathing behaviors were studied by successive single‐crystal X‐ray diffraction analyses. The breathing amplitudes of the PCN‐700 series were modulated through pre‐functionalization of organic linkers and post‐synthetic linker installation. Experiments and molecular simulations confirm that the catalytic activities of the PCN‐700 series can be switched on and off upon reversible structural transformation, which is reminiscent of sophisticated biological systems such as allosteric enzymes.     

Porphyrin‐based covalent triazine frameworks: Porosity,adsorption performance,and drug delivery

Two porous porphyrin‐based covalent triazine frameworks (PCTFs), in which porphyrin is incorporated as building block, have been synthesized by the Friedel–Crafts reaction. The copolymer PCTFs show large Brunauer–Emmett–Teller specific surface area of up to 1089 m² g^?1, high CO₂ uptake capacity reaching 139.9 mg g^?1 at 273 K/1.0 bar, and good selectivity for CO₂/CH₄ adsorption attaining 6.1 at 273 K/1.0 bar. The resulting porous solids also can be used as matrices for drug delivery of ibuprofen in vitro. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 2594–2600     

Gas adsorption in Mg‐porphyrin‐based porous organic frameworks: A computational simulation by first‐principles derived force field

A novel type of porous organic frameworks, based on Mg‐porphyrin, with diamond‐like topology, named POF‐Mgs is computationally designed, and the gas uptakes of CO₂, H₂, N₂, and H₂O in POF‐Mgs are investigated by Grand canonical Monte Carlo simulations based on first‐principles derived force fields (FF). The FF, which describes the interactions between POF‐Mgs and gases, are fitted by dispersion corrected double‐hybrid density functional theory, B2PLYP‐D3. The good agreement between the obtained FF and the first‐principle energies data confirms the reliability of the FF. Furthermore our simulation shows the presence of a small amount of H₂O (≤ 0.01 kPa) does not much affect the adsorption quantity of CO₂, but the presence of higher partial pressure of H₂O (≥ 0.1 kPa) results in the CO₂ adsorption decrease significantly. The good performance of POF‐Mgs in the simulation inspires us to design novel porous materials experimentally for gas adsorption and purification. © 2017 Wiley Periodicals, Inc.     

Enhancing CO2 Separation Ability of a Metal–Organic Framework by Post‐Synthetic Ligand Exchange with Flexible Aliphatic Carboxylates

A series of porous metal–organic frameworks having flexible carboxylic acid pendants in their pores (UiO‐66‐ADn: n=4, 6, 8, and 10, where n denotes the number of carbons in a pendant) has been synthesized by post‐synthetic ligand exchange of terephthalate in UiO‐66 with a series of alkanedioic acids (HO₂C(CH₂)_n?2CO₂H). NMR, IR, PXRD, TEM, and mass spectral data have suggested that a terephthalate linker in UiO‐66 was substituted by two alkanedioate moieties, resulting in free carboxyl pendants in the pores. When post‐synthetically modified UiO‐66 was partially digested by adjusting the amount of added HF/sample, NMR spectra indicated that the ratio of alkanedioic acid/terephthalic acid was increased with smaller amounts of acid, implying that the ligand substitution proceeded from the outer layer of the particles. Gas sorption studies indicated that the surface areas and the pore volumes of all UiO‐66‐ADns were decreased compared to those of UiO‐66, and that the CO₂ adsorption capacities of UiO‐66‐ADn (n=4, 8) were similar to that of UiO‐66. In the case of UiO‐66‐AD6, the CO₂ uptake capacity was 34 % higher at 298 K and 58 % higher at 323 K compared to those of UiO‐66. It was elucidated by thermodynamic calculations that the introduction of flexible carboxyl pendants of appropriate length has two effects: 1) it increases the interaction enthalpy between the host framework and CO₂ molecules, and 2) it mitigates the entropy loss upon CO₂ adsorption due to the formation of multiple configurations for the interactions between carboxyl groups and CO₂ molecules. The ideal adsorption solution theory (IAST) selectivity for CO₂ adsorption over that of CH₄ was enhanced for all of the UiO‐66‐ADns compared to that of UiO‐66 at 298 K. In particular, UiO‐66‐AD6 showed the most strongly enhanced CO₂ uptake capacity and significantly increased selectivity for CO₂ adsorption over that of CH₄ at ambient temperature, suggesting that it is a promising material for sequestering CO₂ from landfill gas.     

Light Triggered Pore Size Tuning in Photoswitching Covalent Triazine Frameworks for Low Energy CO2 Capture

Recently, photo switching porous materials have been widely reported for low energy costed CO₂ capture and release via simply remoted light controlling method. However, most reported photo responsive CO₂ adsorbents relied on metal organic framework (MOFs) functionalisation with photochromic moieties, and MOF adsorbents still suffered from chemically and thermally unstable issues. Thus, further metal free and highly stable organic photoresponsive adsorbents are necessary to be developed. CTFs, because of their high porosity and stability, have attracted great attention for CO₂ capture. Considering the high CO₂ uptake capacity and structural tunability of CTFs, it suggests high potential to fabricate the photoswitching CTF materials by the same functionalisation method as MOFs. Herein, the first series of photo switching CTFs were developed for low energy CO₂ capture and release. Apart from that, the CO₂ switching efficiency could be doubled either through the azobenzene numbers adjusting method or through the previously reported structural alleviation strategy. Furthermore, the pore size distribution of azobenzene functionalised PCTFs also could be tuned under UV exposure, which may contribute to the UV light induced decrease of CO₂ uptake capacity. These photoswitching CTFs represented a new kind of porous polymers for low energy costed CO₂ capture.     

MIL‐53(Al) as a Versatile Platform for Ionic‐Liquid/MOF Composites to Enhance CO2 Selectivity over CH4 and N2

Five different imidazolium‐based ionic liquids (ILs) were incorporated into a metal–organic framework (MOF), MIL‐53(Al), to investigate the effect of IL incorporation on the CO₂ separation performance of MIL‐53(Al). CO₂, CH₄, and N₂ adsorption isotherms of the IL/MIL‐53(Al) composites and pristine MIL‐53(Al) were measured to evaluate the effect of the ILs on the CO₂/CH₄ and CO₂/N₂ selectivities of the MOF. Of the composite materials that were tested, [BMIM][PF₆]/MIL‐53(Al) exhibited the largest increase in CO₂/CH₄ selectivity, 2.8‐times higher than that of pristine MIL‐53(Al), whilst [BMIM][MeSO₄]/MIL‐53(Al) exhibited the largest increase in CO₂/N₂ selectivity, 3.3‐times higher than that of pristine MIL‐53(Al). A comparison of the CO₂ separation potentials of the IL/MOF composites showed that the [BMIM][BF₄]‐ and [BMIM][PF₆]‐incorporated MIL‐53(Al) composites both showed enhanced CO₂/N₂ and CO₂/CH₄ selectivities at pressures of 1–5 bar compared to composites of CuBTC and ZIF‐8 with the same ILs. These results demonstrate that MIL‐53(Al) is a versatile platform for IL/MOF composites and could help to guide the rational design of new composites for target gas‐separation applications.     

生物金属有机框架ZnBTCA对CO2的选择性吸附性能

采用溶剂热法制备的生物金属有机框架ZnBTCA,在CO_2的吸附过程中表现出有趣的主客体化学现象,通过计算发现随着CO_2吸附量的增加,吸附焓逐渐增大。采用4种模型分别对其进行拟合,并通过理想吸附溶液理论模拟预测ZnBTCA对二元混合气体的选择性,试图找到ZnBTCA反常吸附行为的合理解释:ZnBTCA带有氨基和N-活性位点,且孔道内含有大量的客体离子。虽然ZnBTCA孔径较大,但由于其本身的阴离子框架,孔道中的(CH3)2NH2+抗衡离子并未完全去除而降低了可接触的微孔体积,在吸附过程中存在分子筛效应,在二元混合气体中对CO_2具有很高的吸附选择性。     

UV‐triggered CO2‐responsive behavior of nanofibers and their controlled drug release properties

A novel nanofibrous mat featuring an ultraviolet (UV)‐induced CO₂‐responsive behavior was fabricated via electrospinning and used as a controlled drug release system. First, a random copolymer for electrospinning, poly(N,N‐diethylaminoethyl acrylamide‐co‐N‐benzylacrylamide‐co‐N,N‐dimethyl‐N‐(2‐nitrobenzyl)‐ethaneamine acrylamide‐co‐4‐acryloyloxy benzophenone) [P(DEEA‐co‐BA‐co‐DMNOBA‐co‐ABP)], was prepared based on pentafluorophenyl esters via an “active ester‐amine” chemistry reaction. Subsequently, doxorubicin hydrochloride (DOX)‐loaded P(DEEA‐co‐BA‐co‐DMNOBA‐co‐ABP) nanofibers were fabricated, yielding a new drug‐loaded nanofibrous mat as a potential wound dressing. These DOX‐loaded nanofibers can respond to UV irradiation and CO₂ stimulation. Interestingly, without UV irradiation, the fabricated nanofibers cannot exhibit any responsiveness. Therefore, the majority of the DOX was steadily stored in the nanofibers, even in the presence of CO₂. However, upon UV irradiation, the CO₂‐responsive behavior of the nanofibers was activated and the prepared nanofibers swelled slightly, resulting in the release of around 42% DOX from the nanofibers. Upon further purging with CO₂, the release amount of DOX from the nanofibers could reach up to approximately 85%, followed by the morphological transition from a nanofibrous mat to a porous hydrogel film. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1580–1586     

CO<Subscript>2</Subscript> Biofixation of <Emphasis Type="Italic">Actinobacillus succinogenes</Emphasis> Through Novel Amine-Functionalized Polystyrene Microsphere Materials

CO₂-derived succinate production was enhanced by Actinobacillus succinogenes through polystyrene (PSt) microsphere materials for CO₂ adsorption in bioreactor, and the adhesion forces between A. succinogenes bacteria and PSt materials were characterized. Synthesized uniformly sized and highly cross-linked PSt microspheres had high specific surface areas. After modification with amine functional groups, the novel amine-functionalized PSt microspheres exhibited a high adsorption capacity of 25.3 mg CO₂/g materials. After addition with the functionalized microspheres into the culture broth, CO₂ supply to the cells increased. Succinate production by A. succinogenes can be enhanced from 29.6 to 48.1 g L^?1. Moreover, the characterization of interaction forces between A. succinogenes cells and the microspheres indicated that the maximal adhesive force was about 250 pN. The amine-functionalized PSt microspheres can adsorb a large amount of CO₂ and be employed for A. succinogenes anaerobic cultivation in bioreactor for high-efficiency production of CO₂-derived succinate.     

Bimetallic Ni–Co‐based metal–organic framework: An open metal site adsorbent for enhancing CO2 capture

An open metal site framework named UTSA‐16 was synthesized and modified as a high‐capacity adsorbent for reversible CO₂ capture. Partial substitution of intrinsic Co²⁺ sites of UTSA‐16 with Ni²⁺ centres was realized in the molar composition range 0–75% Ni with the aim of increasing CO₂ uptake. Synthesized bimetallic Nix‐UTSA‐16 (x = 0, 20, 50, 75) materials were characterized using various techniques to assess the influence of chemical composition on CO₂ binding affinity and any subsequent physical change in morphology, crystal size and porosity on the total uptake. Experimental isotherm adsorption studies showed the following trend for CO₂ adsorption capacity employing the Nix‐UTSA‐16 series: Ni20‐UTSA‐16 > UTSA‐16 > Ni50‐UTSA‐16 > Ni75‐UTSA‐16. According to the dynamic breakthrough CO₂ profiles measured for a mixture of CO₂ and CH₄ (15/85 molar ratio), Ni20‐UTSA‐16 exhibited 2 times the breakthrough time with 1.5 times the loading capacity at 75 Nml min^?1 feed flow rate, compared to the parent UTSA‐16. In addition, the Ni20‐UTSA‐16 bimetallic metal–organic framework exhibited lower isosteric heat of adsorption compared to UTSA‐16 (ΔH_ave = 28.54 versus 46.85 kJ mol^?1). As a result, more than 95% of its capacity was restored by applying a partial vacuum for only 1 h at room temperature without involving any other time‐ and energy‐consuming regenerative step.     

Directing the Structural Features of N2‐Phobic Nanoporous Covalent Organic Polymers for CO2 Capture and Separation

A family of azo‐bridged covalent organic polymers (azo‐COPs) was synthesized through a catalyst‐free direct coupling of aromatic nitro and amine compounds under basic conditions. The azo‐COPs formed 3D nanoporous networks and exhibited surface areas up to 729.6 m² g^?1, with a CO₂‐uptake capacity as high as 2.55 mmol g^?1 at 273 K and 1 bar. Azo‐COPs showed remarkable CO₂/N₂ selectivities (95.6–165.2) at 298 K and 1 bar. Unlike any other porous material, CO₂/N₂ selectivities of azo‐COPs increase with rising temperature. It was found that azo‐COPs show less than expected affinity towards N₂ gas, thus making the framework “N₂‐phobic”, in relative terms. Our theoretical simulations indicate that the origin of this unusual behavior is associated with the larger entropic loss of N₂ gas molecules upon their interaction with azo‐groups. The effect of fused aromatic rings on the CO₂/N₂ selectivity in azo‐COPs is also demonstrated. Increasing the π‐surface area resulted in an increase in the CO₂‐philic nature of the framework, thus allowing us to reach a CO₂/N₂ selectivity value of 307.7 at 323 K and 1 bar, which is the highest value reported to date. Hence, it is possible to combine the concepts of “CO₂‐philicity” and “N₂‐phobicity” for efficient CO₂ capture and separation. Isosteric heats of CO₂ adsorption for azo‐COPs range from 24.8–32.1 kJ mol^?1 at ambient pressure. Azo‐COPs are stable up to 350 °C in air and boiling water for a week. A promising cis/trans isomerization of azo‐COPs for switchable porosity is also demonstrated, making way for a gated CO₂ uptake.     

High‐Performance K–CO2 Batteries Based on Metal‐Free Carbon Electrocatalysts

Metal–CO₂ batteries have attracted much attention owing to their high energy density and use of greenhouse CO₂ waste as the energy source. However, the increasing cost of lithium and the low discharge potential of Na–CO₂ batteries create obstacles for practical applications of Li/Na–CO₂ batteries. Recently, earth‐abundant potassium ions have attracted considerable interest as fast ionic charge carriers for electrochemical energy storage. Herein, we report the first K–CO₂ battery with a carbon‐based metal‐free electrocatalyst. The battery shows a higher theoretical discharge potential (E^?=2.48 V) than that of Na–CO₂ batteries (E^?=2.35 V) and can operate for more than 250 cycles (1500 h) with a cutoff capacity of 300 mA h g^?1. Combined DFT calculations and experimental observations revealed a reaction mechanism involving the reversible formation and decomposition of P12₁/c1‐type K₂CO₃ at the efficient carbon‐based catalyst.     

Enhanced CO2 Adsorption Affinity in a NbO‐type MOF Constructed from a Low‐Cost Diisophthalate Ligand with a Piperazine‐Ring Bridge

A metal–organic framework (NPC‐6) with an NbO topology based on a piperazine ring‐bridged diisophthalate ligand was synthesized and characterized. The incorporated piperazine group leads to an enhanced adsorption affinity for CO₂ in NPC‐6, in which the CO₂ uptake is 4.83 mmol g^?1 at 293 K and 1 bar, ranking among the top values of CO₂ uptake on MOF materials. At 0.15 bar and 293 K, the NPC‐6 adsorbs 1.07 mmol g^?1 of CO₂, which is about 55.1 % higher than that of the analogue MOF NOTT‐101 under the same conditions. The enhanced CO₂ uptake combined with comparable uptakes for CH₄ and N₂ leads to much higher selectivities for CO₂/CH₄ and CO₂/N₂ gas mixtures on NPC‐6 than on NOTT‐101. Furthermore, an N‐alkylation is used in the synthesis of the PDIA ligand, leading to a much lower cost compared with that in the synthesis of ligands in the NOTT series, as the former does not require a palladium‐based catalyst and borate esters. Thus, we conclude that NPC‐6 is a promising candidate for CO₂ capture applications.