The separation of molecules with similar size and shape is an important technological challenge. For example, rare gases can pose either an economic opportunity or an environmental hazard and there is a need to separate these spherical molecules selectively at low concentrations in air. Likewise, chiral molecules are important building blocks for pharmaceuticals, but chiral enantiomers, by definition, have identical size and shape, and their separation can be challenging. Here we show that a porous organic cage molecule has unprecedented performance in the solid state for the separation of rare gases, such as krypton and xenon. The selectivity arises from a precise size match between the rare gas and the organic cage cavity, as predicted by molecular simulations. Breakthrough experiments demonstrate real practical potential for the separation of krypton, xenon and radon from air at concentrations of only a few parts per million. We also demonstrate selective binding of chiral organic molecules such as 1-phenylethanol, suggesting applications in enantioselective separation.

Metal−organic frameworks (MOFs) are known to facilitate energy-efficient separations of important industrial chemical feedstocks. Here, we report how a class of green MOFsnamely CD-MOFsexhibits high shape selectivity toward aromatic hydrocarbons. CD-MOFs, which consist of an extended porous network of γ-cyclodextrins (γ-CDs) and alkali metal cations, can separate a wide range of benzenoid compounds as a result of their relative orientation and packing within the transverse channels formed from linking (γ-CD) 6 body-centered cuboids in three dimensions. Adsorption isotherms and liquid-phase chromatographic measurements indicate a retention order of ortho-> meta-> para-xylene. The persistence of this regioselectivity is also observed during the liquid-phase chromatography of the ethyltoluene and cymene regioisomers. In addition, molecular shape-sorting within CDMOFs facilitates the separation of the industrially relevant BTEX (benzene, toluene, ethylbenzene, and xylene isomers) mixture. The high resolution and large separation factors exhibited by CD-MOFs for benzene and these alkylaromatics provide an efficient, reliable, and green alternative to current isolation protocols. Furthermore, the isolation of the regioisomers of (i) ethyltoluene and (ii) cymene, together with the purification of (iii) cumene from its major impurities (benzene, n-propylbenzene, and diisopropylbenzene) highlight the specificity of the shape selectivity exhibited by CD-MOFs. Grand canonical Monte Carlo simulations and single component static vapor adsorption isotherms and kinetics reveal the origin of the shape selectivity and provide insight into the capability of CD-MOFs to serve as versatile separation platforms derived from renewable sources. ■ INTRODUCTIONWith the expanding global demand for petrochemical feedstocks, the development of novel, low-cost materials that reduce the impact of chemical processing on the environment is critically important. Improving the efficiency of the refinement and separation of aromatic hydrocarbons is of particular importance, given the large volumes on which these compounds are produced. The sustained interest in metal− organic frameworks 1 (MOFs) as adsorbents and sequestering agents for industrially important gases, 2−4 e.g., H 2 , CH 4 , CO 2 and N 2 , as well as for the liquid-phase separation of larger molecular compounds, which include (1) constitutional isomers, 5 (2) chiral compounds, 6 (3) aliphatic hydrocarbons, 3b,5b,7 and (4) pharmaceuticals, 8 is leading to MOFs being investigated as alternatives to zeolites 9 and activated carbon 10 as separation media. The improvements 5−7 in separation efficiencies using MOFs over traditional size-and shape-selective materials can be attributed primarily to (i) the physiochemical properties imbedded in their diverse building blocks, (ii) their higher surface areas, and (iii) their larger adsorption capacities, which reduce the amount of adsorbent required for industrial processes. 7a,11 Consequently, MOFs represent emergent materials f...

A novel nanoporous metal−organic framework NPC-4 with excellent thermal stability was assembled from 2,3,5,6-tetramethylbenzene-1,4-diisophthalate (TMBDI) and the paddle-wheel secondary building unit (Cu 2 (COO) 4 ). The porous structure comprises a single type of nanoscale cage (16 Å diameter) interconnected by windows (5.2 × 6.3 Å), which give a high pore volume. CH 4 (195−290 K), CO 2 (198−303 K), N 2 (77 K), and H 2 (77 K) adsorption isotherms were studied for pressures up to 20 bar. NPC-4 exhibits excellent methane and carbon dioxide storage capacities on a volume basis with very high adsorbate densities, under ambient conditions. Isobars were investigated to establish the relationship for adsorption capacities over a range of storage temperatures. The isosteric enthalpies of adsorption for both CH 4 and CO 2 adsorption did not vary significantly with amount adsorbed and were ∼15 and ∼25 kJ mol −1 , respectively. The adsorption/desorption kinetics for CH 4 and CO 2 were investigated and activation energies, enthalpies of activation, and diffusion parameters determined using various kinetic models. The activation energies for adsorption obtained over a range of uptakes from the stretched exponential kinetic model were 5.1− 6.3 kJ mol −1 (2−13.5 mmol g −1 ) for CO 2 and 2.7−5.6 kJ mol −1 (2−9 mmol g −1 ) for CH 4 . The activation energies for surface barriers and diffusion along pores for both CH 4 and CO 2 adsorption obtained from a combined barrier resistance diffusion model did not vary markedly with amount adsorbed and were <9 kJ mol −1 . Comparison of kinetic and thermodynamic parameters for CH 4 and CO 2 indicates that a surface barrier is rate determining at high uptakes, while intraparticle diffusion involving diffusion through pores, consisting of narrow windows interconnecting with nanocages, being rate determining at very low uptakes. The faster CH 4 intraparticle adsorption kinetics compared with CO 2 for NPC-4 was attributed to faster surface diffusion due to the lower isosteric enthalpy of adsorption for CH 4 .

Direct synthesis of the ultrathin and discrete 2D MOF nanosheets is extremely challenging.Herein, we present the first facile, continuous bottom-up strategy for preparing ultrathin (~3 nm) 2D MOF nanosheets with high crystallinity comprising of assemblies of a few layers.Unlike conventional solvothermal synthetic methods for 2D MOFs, the weak interlayer interaction in the vertical direction of the 2D materials is restricted under microdroplet flow reaction conditions. 2D MOF nanosheets with a large lateral area and a few layers thick were directly synthesized by suppressing the lamellar stacking of the nanosheets under the dynamic growth conditions. The 2D MOF nanosheets were characterized by scanning and transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, powder Xray diffraction, infrared spectroscopy, thermogravimetric analysis, gas adsorption and light scattering techniques, which were supported by Density Functional Theory (DFT) calculations and molecular simulations. The properties of the 'as-prepared' 2D MOF nanosheets were compared with the corresponding pristine solvothermal MOF with extended structure perpendicular to the laminar assembly. The ultrathin 2D MOF nanosheets have greater external surface area, resulting in a far higher gas adsorption and colloidal suspensions

A series of flexible 3-fold interpenetrated lanthanide-based metal organic frameworks (MOFs) with formula [Ln(HL)(DMA) 2 ]•DMA•2H 2 O where Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy and Er; DMA = dimethylacetamide and H 4 L = 5,5'-(2,3,5,6-tetramethyl-1,4-phenylene)bis(methylene)bis(azanediyl)diisophthalic acid, have been prepared. [Sm(HL)(DMA) 2 ]•DMA•2H 2 O was studied as an exemplar of the series. The activated Sm(HL)(DMA) 2 framework exhibited reversible single-crystal to single-crystal (SCSC) structural transformations in response to adsorption and desorption of guest molecules. X-ray single crystal structural analysis showed that activation of [Sm(HL)(DMA) 2 ]•DMA•2H 2 O by heat treatment to form Sm(HL)(DMA) 2 , involves closing of 13.8 × 14.8 Å channels with coordinated DMA molecules rotating into the interior of the channels with a change from trans to cis Sm coordination and unit cell volume shrinkage of ~20%, to a void volume of 3.5%. Solvent exchange studies with CH 2 Cl 2 gave [Sm(HL)(DMA) 2 ]•2.8CH 2 Cl 2 which, at 173 K, had a structure similar to trans-[Sm(HL)(DMA) 2 ]•DMA•2H 2 O. CH 2 Cl 2 vapor sorption on activated cis-[Sm(HL)(DMA) 2 ] results in gate opening and the fully loaded structure has a similar pore volume to trans-[Sm(HL)(DMA) 2 ]•2.8CH 2 Cl 2 structure at 173 K. Solvent exchange and heat treatment studies also provided evidence for intermediate framework structural phases. Structural, thermodynamic and kinetic aspects of the molecular gating mechanism were studied. The dynamic and structural response of the endothermic gate opening process is driven by the

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Polychlorinated dibenzo-p-dioxin and polychlorinated dibenzofuran species are classes of extremely toxic compounds generated in very low concentrations in postcombustion gases and these may be removed by adsorption on porous carbons. Their extreme toxicity and very low volatility prevent detailed study of their adsorption characteristics, and therefore, models for dioxins have been used in this study. Chlorobenzene, 2-chlorotoluene, 1,3-dichlorobenzene, and 2-chloroanisole were used as models to investigate factors influencing the adsorption characteristics of dioxins on porous carbons. Adsorption studies were carried out under conditions of very low concentration and temperatures up to 453 K, which simulate those found in dioxin abatement systems. Adsorption of 2-chloroanisole on three carbons with various micro/ mesoporous structures showed that microporous structure was a critical adsorbent characteristic under these conditions. A microporous activated carbon was selected for detailed thermodynamic and kinetic studies of adsorption of chloroaromatic species in relation to adsorbate structure and adsorbent surface functional groups. Virial equation analysis of adsorption isotherms was used to determine the Henry's Law constants and isosteric enthalpies of adsorption at zero surface coverage to compare adsorbate-adsorbent interactions. The van't Hoff equation was used to determine the enthalpy of adsorption as a function of surface coverage. The role of surface functional groups on adsorption thermodynamics was investigated by oxidizing and reducing the carbon in nitric acid and hydrogen, respectively. The important factor influencing adsorption at very low concentrations is the adsorbate adsorbent interaction. Oxidation of the carbon adsorbent only has a small effect on the isosteric enthalpy of adsorption. The adsorption kinetics for each isotherm pressure increment were described by the stretched exponential equation. The activation energies and enthalpies of activation were calculated as a function of surface coverage for adsorption kinetics of chloroaromatic species. The planar molecules studied had lower activation energies and enthalpies of activation than isosteric enthalpy of adsorption indicating that a site-to-site surface hopping mechanism is the main factor in determining the adsorption kinetics. In comparison, 2-chloroanisole is nonplanar with a methoxy group giving rise to a larger minimum cross-section size and higher barrier to diffusion than isosteric enthalpy of adsorption at low surface coverage leading to the adsorption kinetics being mainly determined by diffusion through constrictions in the porous structure under these conditions. The isosteric enthalpies of adsorption initially increase with increasing surface coverage and this is attributed to π-π interactions of planar aromatic molecules confined in microporosity. The trends in the kinetic barriers and isosteric enthalpies of adsorption with surface coverage for 2-chlorotoluene are similar irrespective of adsorbent oxidation/reduction,...

There is extensive interest in postcombustion flue gas treatment for mitigating CO2 emissions and removal of acid gases. In this study we investigate the adsorption of the main flue gas components (CO2, N2, SO2, and water vapor) on Filtrasorb 400 activated carbon to understand adsorption characteristics of the main components and competitive adsorption effects. The adsorption isotherms of the pure components of flue gas, CO2 (273.15–318.15 K and 0–50 bar), N2 (298.15–313.15 K and 0–150 bar), SO2 (273.15–303.15 K and 0–3.6 bar), and water vapor (293.15–303.15 K and 0–41 mbar), were investigated. The isosteric enthalpies of adsorption were determined to be a function of surface excess. The enthalpies at zero surface coverage have the order SO2 > H2O > CO2 > N2. However, the SO2 isosteric enthalpy decreases with increasing surface excess and is lower than that of water vapor at high surface excess uptake values. The temperature range for CO2 adsorption covers the subcritical to supercritical gas transition. There was no evidence for isosteric enthalpy differences over this temperature range. The adsorption kinetics for SO2 (290.65–303.15 K) and H2O (293.15–303.15 K) adsorption were measured for each isotherm pressure increment. In both cases the adsorption kinetics followed the linear driving force model. The adsorption mechanisms for both SO2 and H2O kinetic trends are discussed in terms of the adsorption mechanisms. The water vapor adsorption kinetics showed a minimum in the region where water molecules form clusters around functional groups, which merge in the pores. The SO2 adsorption kinetics also show a minimum with increasing surface coverage, and this is attributed to dipole–dipole interactions. The activation energies for diffusion of both SO2 and H2O into F400 were very low. Both the N2 and CO2 adsorption kinetics were too fast to be measured accurately by the gravimetric method used in this study. Ideal adsorbed solution theory (IAST) was used to calculate competitive adsorption of SO2/CO2 and CO2/N2 from the isotherms of the pure components. The competitive adsorption of CO2/N2 was investigated by using the integral mass balance (IMB) experimental method, and this was used for validation of the IAST. The results provide an insight into the role of competitive adsorption in the capture of CO2 and SO2 from flue gases by adsorption from both thermodynamic and kinetic perspectives.