Ziyin Li

High Anhydrous Proton Conductivity of Imidazole-Loaded Mesoporous Polyimides over a Wide Range from Subzero to Moderate Temperature

On-board fuel cell technology requires proton conducting materials with high conductivity not only at intermediate temperatures for work but also at room temperature and even at subzero temperature for startup when exposed to the colder climate. To develop such materials is still challenging because many promising candidates for the proton transport on the basis of extended microstructures of water molecules suffer from significant damage by heat at temperatures above 80 °C or by freeze below -5 °C. Here we show imidazole loaded tetrahedral polyimides with mesopores and good stability (Im@Td-PNDI 1 and Im@Td-PPI 2) exhibiting a high anhydrous proton conductivity over a wide temperature range from -40 to 90 °C. Among all anhydrous proton conductors, the conductivity of 2 is the highest at temperatures below 40 °C and comparable with the best materials, His@[Al(OH)(1,4-ndc)]n and [Zn3(H2PO4)6(H2O)3](Hbim), above 40 °C.

Cobalt–citrate framework armored with graphene oxide exhibiting improved thermal stability and selectivity for biogas decarburization

A series of metal–organic framework (UTSA-16)–graphene oxide composites was synthesized. These composites are the first reported examples of core–shell type metal–organic framework composites armored with graphene oxide film. The parent materials (UTSA-16 and graphene oxide) and the nanocomposites were characterized using XRD, SEM, TEM, TGA and gas adsorption. The composites showed a greatly improved thermal stability compared with their parent materials. The UTSA-16–GO19 composite has a CO2/CH4 selectivity of 114.4, which is three times greater than that of UTSA-16 alone; of the previously reported metal–organic frameworks, only the polyamine-incorporated amine-MIL-101(Cr) has a higher CO2/CH4 selectivity. These graphene oxide composites provide a new direction for practical high-performance metal–organic framework materials.

Straightforward Loading of Imidazole Molecules into Metal–Organic Framework for High Proton Conduction

A one-step straightforward strategy has been developed to incorporate free imidazole molecules into a highly stable metal-organic framework (NENU-3, ([Cu12(BTC)8(H2O)12][HPW12O40])·Guest). The resulting material Im@(NENU-3) exhibits a very high proton conductivity of 1.82 × 10-2 S cm-1 at 90% RH and 70 °C, which is significantly higher than 3.16 × 10-4 S cm-1 for Im-Cu@(NENU-3a) synthesized through a two-step approach with mainly terminal bound imidazole molecules inside pores. Single crystal structure reveals that imidazole molecules in Im-Cu@(NENU-3a) isolate lattice water molecules and then block proton transport pathway, whereas high concentration of free imidazole molecules within Im@(NENU-3) significantly facilitate successive proton-hopping pathways through formation of hydrogen bonded networks.

Rationally tuning host–guest interactions to free hydroxide ions within intertrimerically cuprophilic metal–organic frameworks for high OH− conductivity

Hydroxide-anion-exchange membrane fuel cells (HEMFCs) are now considered as one of the most promising green energy-conversion technologies for stationary and mobile applications, showing high fuel conversion efficiency at high pH and low cost due to their ability to operate under basic conditions using non-precious metal catalysts. But one key impediment to commercialization is insufficient hydroxide ion (OH−) conductivity of the central HEM component. Here, we report the development of free OH− anion-containing metal–organic frameworks (FOMOFs) with rationally tunable host–guest interactions for HEMs with high OH− conductivity. Among three solids obtained by post-synthesis treatment of ultrastable MOF FJU-66 with various bases, free OH− anions are observed in FJU-66·[EVIm]OH with strong host–guest interactions between the MOF backbone and guest cations. Despite the lowest OH− concentration, FJU-66·[EVIm]OH exhibits the highest OH− conductivity close to 0.1 S cm−1. The high OH− conductivity achieved suggests the potential application of the FOMOFs for practical HEMs of fuel cells.

Mixed-Valence Cobalt(II/III) Metal–Organic Framework for Ammonia Sensing with Naked-Eye Color Switching

The construction of colorimetric sensing materials with high selectivity, low detection limits, and great stability provides a significant way for facile device implementation of an ammonia (NH3) sensor. Herein, with excellent alkaline stability and exposed N sites in molecule as well as with naked-eye color switching nature generated from changeable cobalt (Co) valence, a three-dimensional mixed-valence cobalt(II/III) metal-organic framework (FJU-56) with tris-(4-tetrazolyl-phenyl)amine (H3L) ligand was synthesized for colorimetric sensing toward ammonia. The activated FJU-56 demonstrates a limit of detection of 1.38 ppm for ammonia sensing, with high selectivity in ammonia and water competitive adsorption, and shows outstanding stability and reversibility in the cyclic test. The NH3 or water molecules binding to the exposed N sites with the hydrogen-bond are observed by single-crystal X-ray diffraction, determining that the attachment of guest molecules to the FJU-56 framework changes the valence of Co ions with a naked-eye color switching response, which provides an ocular demonstration for ammonia capture and a valuable insight into ammonia sensing.

Additive-Induced Supramolecular Isomerism and Enhancement of Robustness in Co(II)-Based MOFs for Efficiently Trapping Acetylene from Acetylene-Containing Mixtures

Although supramolecular isomerism in metal-organic frameworks (MOFs) would offer a favorable platform for in-depth exploring their structure-property relationship, the design and synthesis of the isomers are still rather a challenging aspect of crystal engineering. Here, a pair of supramolecular isomers of Co(II)-based MOFs (FJU-88 and FJU-89) can be directionally fabricated by rational tuning the additives. In spite of the fact that the isomers have the similar Co3 secondary building units and organic linkers, they adopt distinct networks with acs and snw topologies, respectively, which derive from the conformational flexibility of the organic ligands. It is noteworthy that the porous structure of FJU-88 would be collapsed after removal of the solvent from the pores. But FJU-89a shows permanent porosity accompanied with unusual hierarchical micro- and mesopores and superior gas selective adsorption performance. In addition, FJU-89a can efficiently trap C2H2 from C2H2/CO2 and C2H2/CH4 mixture gases through fixed-bed dynamic breakthrough experiments.

Facile synthesis of oxidized activated carbons for high-selectivity and low-enthalpy CO2 capture from flue gas

The prospect of a worsening climatic situation prompts us to develop energy-saving and cost-effective CO2 capture technologies. In this work, three oxidized activated carbons (ACO-n, n is 1–3) were prepared through a facile synthesis approach via oxidation in the presence of KMnO4 and concentrated H2SO4 for 0.5, 1.0 or 2.0 hours. Interestingly, these carbon materials ACO-n can exhibit high CO2 capacity with low adsorption enthalpy and the selectivity toward flue gas can be adjusted by altering the period of oxidation. Among the pristine activated carbon and ACO-n materials, ACO-2 can exhibit the highest CO2 capacity of 3.01 mmol g−1 under ambient conditions with an adsorption enthalpy of only 23.1 kJ mol−1, slightly larger than the CO2 vaporization enthalpy. Its selectivity of 48.5 is double the value of the pristine activated carbon. A column breakthrough experiment was conducted to evaluate the CO2 separation capability on ACO-2 toward a CO2/N2 (15 : 85 v/v) mixture under kinetic flow conditions, which suggests that the oxidized activated carbon made from sustainable sources is promising for CO2 capture.

Microporous metal-organic framework with open metal sites and π-Lewis acidic pore surfaces for recovering ethylene from polyethylene off-gas

There is an urgent demand for recovering ethylene (C2H4) from polyethylene off-gas since it allows for a more efficient conversion of monomers with high economic value. Herein, we report a three-dimensional (3D) porous MOF [Cu2(L)(H2O)2]·5DMF·dioxane·3H2O (FJU-101, H4L = N,N′-bis(5-isophthalic acid)naphthalenediimide, DMF = N,N′-dimethylformamide) with π-Lewis acidic pore surfaces and superior gas storage/separation performance. Under ambient conditions, the activated FJU-101a exhibits a high C2H4 uptake of 142 cm3(STP) per g and more importantly, the dynamic fixed-bed breakthrough test indicates that the recovery of C2H4 from simulated polyethylene off-gas through a column packed with FJU-101a adsorbent can be efficiently achieved. Such a separation is essential to recycle ethylene for its commercial usage. In addition, FJU-101a also exhibits an extraordinarily high gravimetric uptake of CO2 (219.1 cm3 g−1) at 273 K and 1 bar, which is only slightly lower than that of the best CO2 adsorption material, namely, MgMOF-74 (230 cm3 g−1) under similar conditions.

An Antiferromagnetic Metalloring Pyrazolate (Pz) Framework with [Cu12(µ2-OH)12(Pz)12] Nodes for Separation of C2H2/CH4 Mixture

The search for functional metalloring organic frameworks (MROFs) has been of continuous interest in the fields of both molecular rings and metal–organic frameworks (MOFs) due to their importance in many multifunctional applications. Herein, a hierarchical pyrazolate framework MROF-12 with the topology of the Schlafli symbol {460.66} is reported, which is constructed by an elongated rigid ligand and attractive metalloring cluster nodes [Cu12(μ2-OH)12(Pz)12]. MROF-12 not only adopts the unique functionalities of the metalloring clusters but also inherits the porous properties of MOFs. The large [Cu12(μ2-OH)12(Pz)12] metalloring nodes with the diameter of 12.202 A endow MROF-12 with excellent stability and antiferromagnetism, and they are bridged by linear pyrazolate ligands (H2NDI) to form micro–mesoporous MROF with permanent porosity. The activated sample MROF-12a can exhibit a relatively high Brunauer–Emmett–Teller/Langmuir surface area (460.7/571.9 m2 g−1) as well as pore volume of 0.240 cm3 g−1. Dynamic fixed bed breakthrough experiments indicate that the separation of C2H2/CH4 or CO2/CH4 mixtures can be efficiently achieved through a column packed with MROF-12a solid.