The energy costs associated with the separation and purification of industrial commodities, such as gases, fine chemicals and fresh water, currently represent around 15 per cent of global energy production, and the demand for such commodities is projected to triple by 2050 (ref. 1). The challenge of developing effective separation and purification technologies that have much smaller energy footprints is greater for carbon dioxide (CO2) than for other gases; in addition to its involvement in climate change, CO2 is an impurity in natural gas, biogas (natural gas produced from biomass), syngas (CO/H2, the main source of hydrogen in refineries) and many other gas streams. In the context of porous crystalline materials that can exploit both equilibrium and kinetic selectivity, size selectivity and targeted molecular recognition are attractive characteristics for CO2 separation and capture, as exemplified by zeolites 5A and 13X (ref. 2), as well as metal-organic materials (MOMs). Here we report that a crystal engineering or reticular chemistry strategy that controls pore functionality and size in a series of MOMs with coordinately saturated metal centres and periodically arrayed hexafluorosilicate (SiF(2-)(6)) anions enables a 'sweet spot' of kinetics and thermodynamics that offers high volumetric uptake at low CO2 partial pressure (less than 0.15 bar). Most importantly, such MOMs offer an unprecedented CO2 sorption selectivity over N2, H2 and CH4, even in the presence of moisture. These MOMs are therefore relevant to CO2 separation in the context of post-combustion (flue gas, CO2/N2), pre-combustion (shifted synthesis gas stream, CO2/H2) and natural gas upgrading (natural gas clean-up, CO2/CH4).

The removal of CO 2 impurities from C 2 H 2 -containing gas mixtures is an important step in purifying C 2 H 2 , a feedstock chemical used in the production of several commodity chemicals. However, that C 2 H 2 and CO 2 exhibit similar size and physicochemical properties makes their separation by physisorption extremely difficult. In this work, we detail how two hybrid ultramicroporous materials (HUMs)-known variant SIFSIX-3-Ni and variant TIFSIX-2-Cu-i-exhibit exceptional CO 2 /C 2 H 2 and C 2 H 2 /CO 2 selectivity, respectively. SIFSIX-3-Ni sets a benchmark for CO 2 /C 2 H 2 selectivity at low partial pressures, whereas TIFSIX-2-Cu-i ranks among the best porous materials in the context of C 2 H 2 / CO 2 selectivity. The performance of these HUMs was confirmed by real-time dynamic breakthrough experiments. To our knowledge, such yin-yang inversion of selectivity in closely related compounds is unprecedented. We attribute this to the distinct sorbate binding sites in SIFSIX-3-Ni and TIFSIX-2-Cu-i, as revealed by modeling studies.

Highly selective separation and/or purification of acetylene from various gas mixtures is a relevant and difficult challenge that currently requires costly and energy-intensive chemisorption processes. Two ultramicroporous metal-organic framework physisorbents, NKMOF-1-M (M=Cu or Ni), offer high hydrolytic stability and benchmark selectivity towards acetylene versus several gases at ambient temperature. The performance of NKMOF-1-M is attributed to their exceptional acetylene binding affinity as revealed by modelling and several experimental studies: in situ single-crystal X-ray diffraction, FTIR, and gas mixture breakthrough tests. NKMOF-1-M exhibit better low-pressure uptake than existing physisorbents and possesses the highest selectivities yet reported for C H /CO and C H /CH . The performance of NKMOF-1-M is not driven by the same mechanism as current benchmark physisorbents that rely on pore walls lined by inorganic anions.

Purification of ethylene (C2H4), the largest-volume product of the chemical industry, currently involves energy-intensive processes such as chemisorption (CO2 removal), catalytic hydrogenation (C2H2 conversion), and cryogenic distillation (C2H6 separation). Although advanced physisorbent or membrane separation could lower the energy input, one-step removal of multiple impurities, especially trace impurities, has not been feasible. We introduce a synergistic sorbent separation method for the one-step production of polymer-grade C2H4 from ternary (C2H2/C2H6/C2H4) or quaternary (CO2/C2H2/C2H6/C2H4) gas mixtures with a series of physisorbents in a packed-bed geometry. We synthesized ultraselective microporous metal-organic materials that were readily regenerated, including one that was selective for C2H6 over CO2, C2H2, and C2H4.

Grand canonical Monte Carlo (GCMC) simulations were performed to investigate hydrogen sorption in an rht-type metal−organic framework (MOF), PCN-61. The MOF was shown to have a large hydrogen uptake, and this was studied using three different hydrogen potentials, effective for bulk hydrogen, but of varying sophistication: a model that includes only repulsion/dispersion parameters, one augmented with charge-quadrupole interactions, and one supplemented with many-body polarization interactions. Calculated hydrogen uptake isotherms and isosteric heats of adsorption, Q st , were in quantitative agreement with experiment only for the model with explicit polarization. This success in reproducing empirical measurements suggests that modeling MOFs that have open metal sites is feasible, though it is often not considered to be well described via a classical potential function; here it is shown that such systems may be accurately described by explicitly including polarization effects in an otherwise traditional empirical potential. Decomposition of energy terms for the models revealed deviations between the electrostatic and polarizable results that are unexpected due to just the augmentation of the potential surface by the addition of induction. Charge-quadrupole and induction energetics were shown to have a synergistic interaction, with inclusion of the latter resulting in a significant increase in the former. Induction interactions strongly influence the structure of the sorbed hydrogen compared to the models lacking polarizability; sorbed hydrogen is a dipolar dense fluid in the MOF. This study demonstrates that many-body polarization makes a critical contribution to gas sorption structure and must be accounted for in modeling MOFs with polar interaction sites.

As am ajor greenhouse gas,m ethane,w hichi s directly vented from the coal-mine to the atmosphere,h as not yet drawn sufficient attention. To address this problem, we report am ethane nano-trap that features oppositely adjacent open metal sites and dense alkyl groups in am etal-organic framework (MOF). The alkyl MOF-based methane nano-trap exhibits ar ecord-high methane uptake and CH 4 /N 2 selectivity at 298 Kand 1bar.The methane molecules trapped within the alkylMOF were crystalographically identified by single-crystal X-ray diffraction experiments,w hich in combination with molecular simulation studies unveiled the methane adsorption mechanism within the MOF-based nano-trap.T he IAST calculations and the breakthrough experiments revealed that the alkylM OF-based methane nano-trap is anew benchmark for CH 4 /N 2 separation, therebyproviding anew perspective for capturing methane from coal-mine methane to recover fuel and reduce greenhouse gas emissions.

We report MPM-1-TIFSIX, a molecular porous material (MPM) based upon the neutral metal complex [Cu2(adenine)4(TiF6)2], that self-assembles through a hydrogen-bonding network. This MPM is amenable to room-temperature synthesis and activation. Gas adsorption measurements and ideal adsorbed solution theory selectivity predictions at 298 K revealed enhanced CO2 separation performance relative to a previously known variant as well as the highest CO2 uptake and isosteric heat of adsorption yet reported for an MPM. MPM-1-TIFSIX is thermally stable to 568 K and retains porosity and capacity even after immersion in water for 24 h.

Porous materials capable of selectively capturing CO 2 from flue-gases or natural gas are of interest in terms of rising atmospheric CO 2 levels and methane purification. Sizeexclusive sieving of CO 2 over CH 4 and N 2 has rarely been achieved. Herein we show that acrystal engineering approach to tuning of pore-size in ac oordination network, [Cu(quinoline-5-carboxyate) 2 ] n (Qc-5-Cu)ena+bles ultra-high selectivity for CO 2 over N 2 (S CN % 40 000) and CH 4 (S CM % 3300). Qc-5-Cu-sql-b,anarrowpore polymorph of the square lattice (sql) coordination network Qc-5-Cu-sql-a, adsorbs CO 2 while excluding both CH 4 and N 2 .E xperimental measurements and molecular modeling validate and explain the performance. Qc-5-Cu-sql-b is stable to moisture and its separation performance is unaffected by humidity.