Pei-Qin Liao

Strong and Dynamic CO2 Sorption in a Flexible Porous Framework Possessing Guest Chelating Claws

Using a bis-triazolate ligand and tetrahedral Zn(II) ion, we synthesized a flexible porous coordination polymer functionalized with pairs of uncoordinated triazolate N-donors that can be used as guest chelating sites to give very high CO(2) adsorption enthalpy and CO(2)/N(2) selectivity. The dynamic CO(2) sorption behavior could be monitored well by single-crystal X-ray diffraction.

Bimetal‐Organic Framework Derived CoFe2O4/C Porous Hybrid Nanorod Arrays as High‐Performance Electrocatalysts for Oxygen Evolution Reaction

Porous CoFe2 O4 /C NRAs supported on nickel foam@NC (denoted as NF@NC-CoFe2 O4 /C NRAs) are directly fabricated by the carbonization of bimetal-organic framework NRAs grown on NF@poly-aniline(PANI), and they exhibit high electrocatalytic activity, low overpotential, and high stability for the oxygen evolution reaction in alkaline media.

Turning on the flexibility of isoreticular porous coordination frameworks for drastically tunable framework breathing and thermal expansion

To study the potential flexibility of the (3,9)-connected xmz frameworks, a series of isoreticular metal carboxylate frameworks [M3(μ3-OH)(L)3], namely MCF-18(L,M), were constructed by 3-connected, tripodal pyridyl-dicarboxylate ligands (H2L1 = pyridine-3,5-dicarboxylic acid; H2L2 = 4,4′-(pyridine-3,5-diyl)dibenzoic acid; H2L3 = 2,6-di-p-carboxyphenyl-4,4′-bipyridine) and 9-connected, tricapped trigonal-prismatic M3(μ3-OH)(O2CR)6(py)3 (M = Fe, Co, Ni; py = pyridyl group) clusters. Powder and single-crystal X-ray diffraction studies showed that, while other isoreticular analogs do not show framework flexibility, the newly designed material MCF-18(L3,Ni) can drastically swell 70–105% in volume and 75–121% in length upon inclusion of different guests, the latter of which is the highest reported value to date. Comparison study showed that the nearly uniaxial framework breathing is generated by the special regulation effect of the xmz topology, but can only be activated by a ligand with a suitable shape. Moreover, the thermal expansion profile of MCF-18(L3,Ni) can be drastically tuned by guest, showing extremely large thermal expansion coefficients up to 430 × 10−6 K−1. The guest-included crystals also show water-like thermal expansion behaviors depending on the cooling rate, which is unanticipated for intrinsic crystalline materials.

Exceptional Hydrophobicity of a Large-Pore Metal–Organic Zeolite

Porous materials combining high hydrophobicity, large surface area, as well as large and uniform pore size are very useful but rare. The nanoporous zeolitic metal azolate framework, RHO-[Zn(eim)2] (MAF-6, Heim = 2-ethylimidazole), is an attractive candidate but thought to be unobtainable/unstable. In this work, the supramolecular isomerism of [Zn(eim)2] is thoroughly studied using a rapid solution mixing reaction of [Zn(NH3)4](OH)2 and Heim, which enables MAF-6 with high crystallinity, purity, and thermal/chemical stabilities to be synthesized in large quantity. Gas and vapor adsorption isotherms, gas chromatography, and water contact angle measurements, as well as transient breakthrough and molecular dynamics simulations show that MAF-6 exhibits large surface area (langmuir surface area 1695 m(2) g(-1)), pore volume (0.61 cm(3) g(-1)), pore size (d = 18.4 Å), and aperture size (d = 7.6 Å) with high hydrophobicity on both the internal pore and external crystal surfaces. It can barely adsorb water or be wetted by water (contact angle 143°) but readily adsorb large amounts of organic molecules including methanol, ethanol, mesitylene, adamantane, C6-C10 hydrocarbons, xylene isomers, and saturated/unsaturated analogues such as benzene/cyclohexene/cyclohexane or styrene/ethylbenzene. It can also separate these organic molecules from each other as well as from water by preferential adsorption/retention of those having higher hydrophobicity, lipophilicity, or oil/water partition coefficient. These properties are very different with other porous materials such as SOD-[Zn(mim)2] (Hmim = 2-methylimidazole, MAF-4/ZIF-8) with a hydrophobic pore surface but a hydrophilic crystal surface and small aperture size.

Controlling guest conformation for efficient purification of butadiene

The weaker adsorption of 1,3-butadiene in a metal-organic framework enables its separation from other C4 hydrocarbons. Selecting against cis conformers Before 1,3-butadiene can be used to make polymers, it must be separated from similar hydrocarbons in an energy-intensive distillation process. Liao et al. show that a zinc metal—organic framework can accommodate the cis isomer of 1,3-butadiene. It binds less tightly than butane and butene because its π-bond conjugation is broken. They used this preferential desorption to separate 1,3-butadiene with ≥99.5% purity under ambient conditions. Science, this issue p. 1193 Conventional adsorbents preferentially adsorb the small, high-polarity, and unsaturated 1,3-butadiene molecule over the other C4 hydrocarbons from which it must be separated. We show from single-crystal x-ray diffraction and computational simulation that a hydrophilic metal-organic framework, [Zn2(btm)2], where H2btm is bis(5-methyl-1H-1,2,4-triazol-3-yl)methane, has quasi-discrete pores that can induce conformational changes in the flexible guest molecules, weakening 1,3-butadiene adsorption through a large bending energy penalty. In a breakthrough operation at ambient temperature and pressure, this guest conformation–controlling adsorbent eluted 1,3-butadiene first, then butane, butene, and isobutene. Thus, 1,3-butadiene can be efficiently purified (≥99.5%) while avoiding high-temperature conditions that can lead to its undesirable polymerization.

Monodentate hydroxide as a super strong yet reversible active site for CO2 capture from high-humidity flue gas

We demonstrate here that porous coordination frameworks, functionalized with monodentate hydroxide on the pore surface, can achieve ultrahigh CO2 adsorption affinity (124 kJ mol−1), adsorption capacity (9.1 mmol cm−3 at 298 K and 1 bar), and CO2/N2 selectivity (262 at 298 K) by the reversible formation/decomposition of bicarbonate in the adsorption/desorption processes. More importantly, these materials can capture up to 4.1 mmol cm−3 or 13.4 wt% of CO2 from simulated flue gases (CO2 pressure 0.10–0.15 bar at 313 K) even at high relative humidity (82%) and quickly release it under mild regeneration conditions (N2 purge at 358 K), representing the best CO2 capture performances reported to date.

Efficient purification of ethene by an ethane-trapping metal-organic framework.

Separating ethene (C2H4) from ethane (C2H6) is of paramount importance and difficulty. Here we show that C2H4 can be efficiently purified by trapping the inert C2H6 in a judiciously designed metal-organic framework. Under ambient conditions, passing a typical cracked gas mixture (15:1 C2H4/C2H6) through 1 litre of this C2H6 selective adsorbent directly produces 56 litres of C2H4 with 99.95%+ purity (required by the C2H4 polymerization reactor) at the outlet, with a single breakthrough operation, while other C2H6 selective materials can only produce ca. ⩽ litre, and conventional C2H4 selective adsorbents require at least four adsorption–desorption cycles to achieve the same C2H4 purity. Single-crystal X-ray diffraction and computational simulation studies showed that the exceptional C2H6 selectivity arises from the proper positioning of multiple electronegative and electropositive functional groups on the ultramicroporous pore surface, which form multiple C–H···N hydrogen bonds with C2H6 instead of the more polar competitor C2H4.

Modular and Stepwise Synthesis of a Hybrid Metal–Organic Framework for Efficient Electrocatalytic Oxygen Evolution

The paddle-wheel type cluster Co2(RCOO)4(LT)2 (R = substituent group, LT = terminal ligand), possessing unusual metal coordination geometry compared with other cobalt compounds, may display high catalytic activity but is highly unstable especially in water. Here, we show that with judicious considerations of the host/guest geometries and modular synthetic strategies, the labile dicobalt clusters can be immobilized and stabilized in a metal-organic framework (MOF) as coordinative guests. The Fe(na)4(LT) fragment in the MOF [{Fe3(μ3-O)(bdc)3}4{Fe(na)4(LT)}3] (H2bdc = 1,4-benzenedicaboxylic acid, Hna = nicotinic acid) can be removed to give [{Fe3(μ3-O)(bdc)3}4] with a unique framework connectivity possessing suitable distribution of open metal sites for binding the dicobalt cluster in the form of Co2(na)4(LT)2. After two-step, single-crystal to single-crystal, postsynthetic modifications, a thermal-, water-, and alkaline-stable MOF [{Fe3(μ3-O)(bdc)3}4{Co2(na)4(LT)2}3] containing the desired dicobalt cluster was obtained, giving extraordinarily high electrocatalytic oxygen evolution activity in water at pH = 13 with overpotential as low as 225 mV at 10.0 mA cm-2.

A Metal–Organic Framework with a Pore Size/Shape Suitable for Strong Binding and Close Packing of Methane

Much effort has been devoted to develop new porous structures for methane storage. We report a new porous coordination framework showing exceptional methane uptakes (e.g. 263 v/v at 298 K and 65 bar) and adsorption enthalpies (21.6 kJ mol(-1)) as high as current record holders functionalized by open metal sites. Computational simulations demonstrated that the hierarchical pore structure consisting of single-wall nanocages has suitable sizes/shapes and organic binding sites to enforce not only strong host-methane and methane-methane interactions but also dense packing of methane molecules.

Self-catalysed aerobic oxidization of organic linker in porous crystal for on-demand regulation of sorption behaviours.

Control over the structure and property of synthetic materials is crucial for practical applications. Here we report a facile, green and controllable solid-gas reaction strategy for on-demand modification of porous coordination polymer. Copper(I) and a methylene-bridged bis-triazolate ligand are combined to construct a porous crystal consisting of both enzyme-like O2-activation site and oxidizable organic substrate. Thermogravimetry, single-crystal X-ray diffraction, electron paramagnetic resonance and infrared spectroscopy showed that the methylene groups can be oxidized by O2/air even at room temperature via formation of the highly active Cu(II)-O2(˙-) intermediate, to form carbonyl groups with enhance rigidity and polarity, without destroying the copper(I) triazolate framework. Since the oxidation degree or reaction progress can be easily monitored by the change of sample weight, gas sorption property of the crystal can be continuously and drastically (up to 4 orders of magnitude) tuned to give very high and even invertible selectivity for CO2, CH4 and C2H6.