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.

Sequestration of CO2, either from gas mixtures or directly from air (direct air capture, DAC), could mitigate carbon emissions. Here five materials are investigated for their ability to adsorb CO2 directly from air and other gas mixtures. The sorbents studied are benchmark materials that encompass four types of porous material, one chemisorbent, TEPA-SBA-15 (amine-modified mesoporous silica) and four physisorbents: Zeolite 13X (inorganic); HKUST-1 and Mg-MOF-74/Mg-dobdc (metal-organic frameworks, MOFs); SIFSIX-3-Ni, (hybrid ultramicroporous material). Temperature-programmed desorption (TPD) experiments afforded information about the contents of each sorbent under equilibrium conditions and their ease of recycling. Accelerated stability tests addressed projected shelf-life of the five sorbents. The four physisorbents were found to be capable of carbon capture from CO2-rich gas mixtures, but competition and reaction with atmospheric moisture significantly reduced their DAC performance.

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.

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.

The first sorbent with high CO2 selectivity and poor water affinity addresses need for trace CO2 remediation in confined spaces.

Fine-tuning of HUMs through pillar substitution can significantly enhance trace CO sorption performance and stability. The resulting materials, exemplified by the new material TIFSIX-3-Ni, [Ni(pyrazine)(TiF)], are shown through temperature programmed desorption experiments to remove trace quantities of CO from moist gas mixtures.

Capture of CO2 from flue gas is considered to be a feasible approach to mitigate the effects of anthropogenic emission of CO2. Herein we report that an isostructural family of metal organic materials (MOMs) of general formula [M(linker)2(pillar)], linker = pyrazine, pillar = hexaflourosilicate and M = Zn, Cu, Ni and Co exhibits highly selective removal of CO2 from dry and wet simulated flue gas. Two members of the family, M = Ni and Co, SIFSIX-3-Ni and SIFSIX-3-Co, respectively, are reported for the first time and compared with the previously reported Zn and Cu analogs.

Sequestration of CO, either from gas mixtures or directly from air (direct air capture), is a technological goal important to large-scale industrial processes such as gas purification and the mitigation of carbon emissions. Previously, we investigated five porous materials, three porous metal-organic materials (MOMs), a benchmark inorganic material, ZEOLITE 13X: and a chemisorbent, TEPA-SBA-15: , for their ability to adsorb CO directly from air and from simulated flue-gas. In this contribution, a further 10 physisorbent materials that exhibit strong interactions with CO have been evaluated by temperature-programmed desorption for their potential utility in carbon capture applications: four hybrid ultramicroporous materials, SIFSIX-3-CU: , DICRO-3-NI-I: , SIFSIX-2-CU-I: and MOOFOUR-1-NI: ; five microporous MOMs, DMOF-1: , ZIF-8: , MIL-101: , UIO-66: and UIO-66-NH2: ; an ultramicroporous MOM, NI-4-PYC: The performance of these MOMs was found to be negatively impacted by moisture. Overall, we demonstrate that the incorporation of strong electrostatics from inorganic moieties combined with ultramicropores offers improved CO capture performance from even moist gas mixtures but not enough to compete with chemisorbents.This article is part of the themed issue 'Coordination polymers and metal-organic frameworks: materials by design'.