A novel approach for the preparation of core-shell MOF/polymer composites as mixed-mode stationary phase

The nickel organic framework capped with polyvinylpyrrolidone was prepared and synergistically immobilized onto porous silica surface as the mixed-mode stationary phase for high-performance liquid chromatography. Here, polyvinylpyrrolidone firstly was chosen as functional molecules to change morphology and size of the metal organic framework. The silica microspheres were then modified by them via a simple bonding method rather than in-situ growth method with the aid of electrostatic interaction commonly used before. The stationary phase showed flexible selectivity for separation of both hydrophilic and hydrophobic compounds, especially for hydrophilic compounds such as carbohydrates, alkaloids and sulfonamides etc. The chromatographic behaviors were evaluated by investigating various factors, and a typical mixed-mode retention feature of the column was observed. The composites could be prepared repetitively, and relative standard deviations of retention time of objective compounds among different batches were less than 1.75%. It also showed excellent chromatographic reproducibility, stability and potentiality for application in real samples. In short, the composites can be used for a feasible option for analysis of multiple compounds as the mixed-mode stationary phase and it provides a general approach for preparing MOFs-based composites by changing morphology and size of MOFs.

Application of Metal-Organic Framework-Based Composites for Gas Sensing and Effects of Synthesis Strategies on Gas-Sensitive Performance

Gas sensing materials, such as semiconducting metal oxides (SMOx), carbon-based materials, and polymers have been studied in recent years. Among of them, SMOx-based gas sensors have higher operating temperatures; sensors crafted from carbon-based materials have poor selectivity for gases and longer response times; and polymer gas sensors have poor stability and selectivity, so it is necessary to develop high-performance gas sensors. As a porous material constructed from inorganic nodes and multidentate organic bridging linkers, the metal-organic framework (MOF) shows viable applications in gas sensors due to its inherent large specific surface area and high porosity. Thus, compounding sensor materials with MOFs can create a synergistic effect. Many studies have been conducted on composite MOFs with three materials to control the synergistic effects to improve gas sensing performance. Therefore, this review summarizes the application of MOFs in sensor materials and emphasizes the synthesis progress of MOF composites. The challenges and development prospects of MOF-based composites are also discussed.

Synthesis, characterization, and CO2 adsorption properties of metal organic framework Fe-BDC

The iron-containing Metal-Organic Frameworks (MOFs) have attracted a great deal of attention in the areas of gas separation, catalytic conversion, and drug delivery, due to their high surface area and activity, as well as the non-toxicity of iron. In this study, Fe-based MOFs using BDC ligands, MIL-101(Fe), MIL-53(Fe) and Amino-MIL-101(Fe) are synthesized by a solvothermal method and characterized by conventional methods such as BET, SEM, and TGA. Afterwards, the synthesized MOFs are investigated from the point of view of the adsorbing capability of carbon dioxide at different pressures and temperatures, and also their resistance to water and solvent. The results showed that Amino-MIL-101(Fe) achieved more CO2 adsorption than MIL-101(Fe) and MIL-53(Fe), equal to 13 mmol g-1 at 4 MP. Although MIL-53(Fe) has the best temperature resistance, around 350 °C, Amino-MIL-101(Fe) is more stable against water and ethanol and its surface area is increased from 670 to 915 m2 g-1 after washing with ethanol. The adsorption study reveals that CO2 is adsorbed not only by a physical adsorption mechanism, but also by chemisorption of acidic carbon dioxide by basic NH2 agent in the structure of Amino-MIL-101(Fe).

Encapsulation of a Porous Organic Cage into the Pores of a Metal-Organic Framework for Enhanced CO2 Separation

We present a facile approach to encapsulate functional porous organic cages (POCs) into a robust MOF by an incipient-wetness impregnation method. Porous cucurbit[6]uril (CB6) cages with high CO2 affinity were successfully encapsulated into the nanospace of Cr-based MIL-101 while retaining the crystal framework, morphology, and high stability of MIL-101. The encapsulated CB6 amount is controllable. Importantly, as the CB6 molecule with intrinsic micropores is smaller than the inner mesopores of MIL-101, more affinity sites for CO2 are created in the resulting CB6@MIL-101 composites, leading to enhanced CO2 uptake capacity and CO2 /N2 , CO2 /CH4 separation performance at low pressures. This POC@MOF encapsulation strategy provides a facile route to introduce functional POCs into stable MOFs for various potential applications.

Sequential Co-immobilization of Enzymes in Metal-Organic Frameworks for Efficient Biocatalytic Conversion of Adsorbed CO2 to Formate

The main challenges in multienzymatic cascade reactions for CO₂ reduction are the low CO₂ solubility in water, the adjustment of substrate channeling, and the regeneration of co-factor. In this study, metal-organic frameworks (MOFs) were prepared as adsorbents for the storage of CO₂ and at the same time as solid supports for the sequential co-immobilization of multienzymes via a layer-by-layer self-assembly approach. Amine-functionalized MIL-101(Cr) was synthesized for the adsorption of CO₂. Using amine-MIL-101(Cr) as the core, two HKUST-1 layers were then fabricated for the immobilization of three enzymes chosen for the reduction of CO₂ to formate. Carbonic anhydrase was encapsulated in the inner HKUST-1 layer and hydrated the released CO₂ to HCO3-. Bicarbonate ions then migrated directly to the outer HKUST-1 shell containing formate dehydrogenase and were converted to formate. Glutamate dehydrogenase on the outer MOF layer achieved the regeneration of co-factor. Compared with free enzymes in solution using the bubbled CO₂ as substrate, the immobilized enzymes using stored CO₂ as substrate exhibited 13.1-times higher of formate production due to the enhanced substrate concentration. The sequential immobilization of enzymes also facilitated the channeling of substrate and eventually enabled higher catalytic efficiency with a co-factor-based formate yield of 179.8%. The immobilized enzymes showed good operational stability and reusability with a cofactor cumulative formate yield of 1077.7% after 10 cycles of reusing.

Polyethyleneimine-Modified UiO-66-NH2(Zr) Metal-Organic Frameworks: Preparation and Enhanced CO2 Selective Adsorption

UiO-66-NH₂, a zirconium-based functional metal-organic framework (MOF), was postsynthetically modified via Schiff base reaction between aldehyde groups in glutaraldehyde and amino groups in UiO-66-NH₂ and CO₂-preabsorbed polyethyleneimine (PEI). The resulting PEI-modified MOFs, abbreviated as PEI^C@UiO, were characterized with ¹H NMR, Fourier transform infrared, powder X-ray diffraction, Brunauer-Emmett-Teller, scanning electron microscopy, and thermogravimetric analysis and evaluated as CO₂ adsorbents. In comparison with pristine UiO-66-NH₂, the PEI^C@UiO adsorbents have reduced specific surface area (7-150 m²/g) but maintained the same crystal structure. Particularly, the PEI^C96@UiO adsorbent exhibited significantly improved CO₂/N₂ adsorption selectivity (48 vs 25) and higher CO₂ adsorption capacity (3.2 vs 2.7 mmol/g). The adsorbent also displayed moderate desorption energy (68 kJ/mol CO₂), superior moisture endurance, and recyclability, which are very desirable for practical applications.

Stability of amine-functionalized CO2 adsorbents: a multifaceted puzzle

This review focuses on important stability issues facing amine-functionalized CO2 adsorbents, including amine-grafted and amine-impregnated silicas, zeolites, metal-organic frameworks and carbons. During the past couple of decades, major advances were achieved in understanding and improving the performance of such materials, particularly in terms of CO2 adsorptive properties such as adsorption capacity, selectivity and kinetics. Nonetheless, to pave the way toward commercialization of adsorption-based CO2 capture technologies, in addition to other attributes, adsorbent materials should be stable over many thousands of adsorption-desorption cycles. Adsorbent stability, which is of utmost importance as it determines adsorbent lifetime and operational costs of CO2 capture, is a multifaceted issue involving thermal, hydrothermal, and chemical stability. Here we discuss the impact of the adsorbent physical and chemical properties, the feed gas composition and characteristics, and the adsorption-desorption operational parameters on the long-term stability of amine-functionalized CO2 adsorbents. We also review important insights associated with the underlying deactivation pathways of the adsorbents upon exposure to high temperature, oxygen, dry CO2, sulfur-containing compounds, nitrogen oxides, oxygen and steam. Finally, specific recommendations are provided to address outstanding stability issues.

Metal–Organic Framework (MOF)-based CO2 Adsorbents

Rising CO2 levels in the atmosphere resulting from fossil fuel combustion is one of the most significant global environmental concerns. Carbon capture and sequestration (CCS), primarily post-combustion CO2 capture, is an essential research area to reduce CO2 levels and avoid environmental destabilization. Recently, metal–organic frameworks (MOFs) have been attracting attention in the scientific community for potential applications in gas storage and separation, including CCS, owing to their novel properties, such as a large surface area, tunable pore shape and size, and tailored chemical functionality. This chapter starts with a brief introduction about the significance of CO2 adsorption and separation, followed by how MOF-based research endeavors were initiated and explored, and why MOFs are unique for gas adsorption. Secondly, we reviewed the relationship between CO2 adsorption and MOF properties including surface area, pore size and volume, amine functionality, nature of linkers, and structural flexibility, and analyzed the reported data based on the possible adsorption mechanism. The humidity effects on CO2 capture over MOFs and implementation of MOF composites were considered as well. Finally, some conclusions on the status of the developed MOFs and perspectives for future research on MOFs for the practical application of CO2 adsorption and separation were mentioned.

Host-guest selectivity in a series of isoreticular metal-organic frameworks: observation of acetylene-to-alkyne and carbon dioxide-to-amide interactions

We report a series of six isoreticular metal–organic frameworks (MOFs) for selective gas adsorption, specifically for selective adsorption of CO₂ and C₂H₂.

In order to develop new porous materials for applications in gas separations such as natural gas upgrading, landfill gas processing and acetylene purification it is vital to gain understanding of host–substrate interactions at a molecular level. Herein we report a series of six isoreticular metal–organic frameworks (MOFs) for selective gas adsorption. These materials do not incorporate open metal sites and thus provide an excellent platform to investigate the effect of the incorporation of ligand functionality via amide and alkyne groups on substrate binding. By reducing the length of the linker in our previously reported MFM-136, we report much improved CO₂/CH₄ (50 : 50) and CO₂/N₂ (15 : 85) selectivity values of 20.2 and 65.4, respectively (1 bar and 273 K), in the new amide-decorated MOF, MFM-126. The CO₂ separation performance of MFM-126 has been confirmed by dynamic breakthrough experiments. In situ inelastic neutron scattering and synchrotron FT-IR microspectroscopy were employed to elucidate dynamic interactions of adsorbed CO₂ molecules within MFM-126. Upon changing the functionality to an alkyne group in MFM-127, the CO₂ uptake decreases but the C₂H₂ uptake increases by 68%, leading to excellent C₂H₂/CO₂ and C₂H₂/CH₄ selectivities of 3.7 and 21.2, respectively. Neutron powder diffraction enabled the direct observation of the preferred binding domains in MFM-126 and MFM-127, and, to the best of our knowledge, we report the first example of acetylene binding to an alkyne moiety in a porous material, with over 50% of the acetylene observed within MFM-127 displaying interactions (<4 Å) with the alkyne functionality of the framework.

Facile synthesis of MOFs with uncoordinated carboxyl groups for selective CO2 capture via postsynthetic covalent modification

A facile strategy involving dual-acyl chloride has been developed to introduce uncoordinated carboxyl groups into amine containing metal–organic frameworks.

Imine-Linked Polymer Based Nitrogen-Doped Porous Activated Carbon for Efficient and Selective CO2 Capture

The preparation of nitrogen-doped activated carbon (NACs) has received significant attention because of their applications in CO2 capture and sequestration (CCS) owing to abundant nitrogen atoms on their surface and controllable pore structures by carefully controlled carbonization. We report high-surface-area porous N-doped activated carbons (NAC) by using soft-template-assisted self-assembly followed by thermal decomposition and KOH activation. The activation process was carried out under different temperature conditions (600-800 °C) using polyimine as precursor. The NAC-800 was found to have a high specific surface area (1900 m2 g-1), a desirable micropore size below 1 nm and, more importantly, a large micropore volume (0.98 cm3 g-1). NAC-800 also exhibits a significant capacity of CO2 capture i.e., over 6. 25 and 4.87 mmol g-1 at 273 K and 298 K respectively at 1.13 bar, which is one of among the highest values reported for porous carbons so far. Moreover, NAC also shows an excellent separation selectivity for CO2 over N2.

Sorbents with high efficiency for CO2 capture based on amines-supported carbon for biogas upgrading

Sorbents for CO₂ capture have been prepared by wet impregnation of a commercial active carbon (Ketjen-black, Akzo Nobel) with two CO₂-philic compounds, polyethylenimine (PEI) and tetraethylenepentamine (TEPA), respectively. The effects of amine amount (from 10 to 70wt.%), CO₂ concentration in the feed, sorption temperature and gas hourly space velocity on the CO₂ capture performance have been investigated. The sorption capacity has been evaluated using the breakthrough method, with a fixed bed reactor equipped with on line gas chromatograph. The samples have been characterized by N₂ adsorption-desorption, scanning electron microscopy and energy dispersive X-ray (SEM/EDX). A promising CO₂ sorption capacity of 6.90 mmol/g_sorbent has been obtained with 70wt.% of supported TEPA at 70°C under a stream containing 80vol% of CO₂. Sorption tests, carried out with simulated biogas compositions (CH₄/CO₂ mixtures), have revealed an appreciable CO₂ separation selectivity; stable performance was maintained for 20 adsorption-desorption cycles.

Improved Interfacial Affinity and CO2 Separation Performance of Asymmetric Mixed Matrix Membranes by Incorporating Postmodified MIL-53(Al)

Asymmetric mixed matrix membranes(MMMs) with MOFs hold great application potential for energy-efficient gas separations. However, the particle aggregation and nonselective interfacial microvoids restrict the gas separation performance of asymmetric MMMs. Herein, nanoporous metal-organic framework (MOF) of MIL-53(Al) was modified with aminosilane after solvothermal synthesis. The postfunctionalization by grafting alkyl chains can form hydrogen bonds with polymer chains to enhance the affinity with polymer matrix and facilitate the preferential adsorption of CO2 by dipole-quadrupole interaction with the functional group. Then the postmodified MIL-53(Al) was incorporated as filler into poly(ether imide) Ultem1000 to fabricate high-quality asymmetric MMMs with well dispersed particles in polymer matrix and good adhesion at the MOFs-polymer interface. The Ultem/S-MIL-53(Al) asymmetric MMMs exhibited remarkable combinations of gas permeance and ideal selectivity for CO2/N2 separation at 10 wt % filler loading. The CO2 permeance achieved 24.1 GPU, an increase of 165% compared with pure Ultem membrane. Meanwhile, the ideal CO2/N2 selectivity also increased from 31.0 up to 41.1. The strategy of post covalent modification for MOFs provides an effective way to improve the interfacial affinity and gas separation performance.

Covalent Triazine-Based Frameworks with Ultramicropores and High Nitrogen Contents for Highly Selective CO2 Capture

Porous organic frameworks (POFs) are a class of porous materials composed of organic precursors linked by covalent bonds. The objective of this work is to develop POFs with both ultramicropores and high nitrogen contents for CO2 capture. Specifically, two covalent triazine-based frameworks (CTFs) with ultramicropores (pores of width <7 Å) based on short (fumaronitrile, FUM) and wide monomers (1,4-dicyanonaphthalene, DCN) were synthesized. The obtained CTF-FUM and CTF-DCN possess excellent chemical and thermal stability with ultramicropores of 5.2 and 5.4 Å, respectively. In addition, they exhibit excellent ability to selectively capture CO2 due to ultramicroporous nature. Especially, CTF-FUM-350 has the highest nitrogen content (27.64%) and thus the highest CO2 adsorption capacity (57.2 cc/g at 298 K) and selectivities for CO2 over N2 and CH4 (102.4 and 20.5 at 298 K, respectively) among all CTF-FUM and CTF-DCN. More impressively, as far as we know, the CO2/CH4 selectivity is larger than that of all reported CTFs and ranks in top 10 among all reported POFs. Dynamic breakthrough curves indicate that both CTFs could indeed separate gas mixtures of CO2/N2 and CO2/CH4 completely.

Amine post-functionalized POSS-based porous polymers exhibiting simultaneously enhanced porosity and carbon dioxide adsorption properties

We provide a possibility for post-synthetic amine functionalization of porous polymers exhibiting enhanced CO₂ capacity and selectivity without compromising the porosity.

Polyaniline-intercalated MIL-101: selective CO2sorption and supercapacitor properties

A novel hybrid material polyaniline@MIL-101 was synthesized and characterized. The obtained compound shows excellent selectivity towards CO₂over nitrogen.