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.

Herein, we report that a new flexible coordination network, NiL (L=4-(4-pyridyl)-biphenyl-4-carboxylic acid), with diamondoid topology switches between non-porous (closed) and several porous (open) phases at specific CO and CH pressures. These phases are manifested by multi-step low-pressure isotherms for CO or a single-step high-pressure isotherm for CH . The potential methane working capacity of NiL approaches that of compressed natural gas but at much lower pressures. The guest-induced phase transitions of NiL were studied by single-crystal XRD, in situ variable pressure powder XRD, synchrotron powder XRD, pressure-gradient differential scanning calorimetry (P-DSC), and molecular modeling. The detailed structural information provides insight into the extreme flexibility of NiL . Specifically, the extended linker ligand, L, undergoes ligand contortion and interactions between interpenetrated networks or sorbate-sorbent interactions enable the observed switching.

Morphology influences the functionality of covalent organic networks and determines potential applications. Here, we report for the first time the use of Zincke reaction to fabricate, under either solvothermal or microwave conditions, a viologen-linked covalent organic network in the form of hollow particles or nanosheets. The synthesized materials are stable in acidic, neutral, and basic aqueous solutions. Under basic conditions, the neutral network assumes radical cationic character without decomposing or changing structure. Solvent polarity and heating method determine product morphology. Depending upon solvent polarity, the resulting polymeric network forms either uniform self-templated hollow spheres (HS) or hollow tubes (HT). The spheres develop via an inside-out Ostwald ripening mechanism. Interestingly, microwave conditions and certain solvent polarities result in the formation of a robust covalent organic gel framework (COGF) that is organized in nanosheets stacked several layers thick. In the gel phase, the nanosheets are crystalline and form honeycomb lattices. The use of the Zincke reaction has previously been limited to the synthesis of small viologen molecules and conjugated viologen oligomers. Its application here expands the repertoire of tools for the fabrication of covalent organic networks (which are usually prepared by dynamic covalent chemistry) and for the synthesis of viologen-based materials. All three materials-HT, HS, and COGF-serve as efficient adsorbents of iodine due to the presence of the cationic viologen linker and, in the cases of HT and HS, permanent porosity.

A Werner complex is highly selective for o-xylene in a vapor mixture containing all three isomers. However, in the absence of o-xylene, the substrate shows similar selectivity for m-xylene over p-xylene. Kinetic studies show a different trend whereby m-xylene is absorbed most rapidly, implying that thermodynamic factors must be responsible for the selectivity.

The control of structures and properties in crystalline materials has many returns that justify the increasing efforts in this direction. Traditionally, crystal engineering focused on the rational design of single component molecular crystals or supramolecular compounds (i.e., cocrystals). More recently, reports on crystalline solid solutions have become common in crystal engineering research. Crystalline solid solutions are characterized by a structural disorder that enables the variation of stoichiometry in continuum. Often such variation corresponds to a variation of structural and physicochemical properties, and offers an opportunity for the materials’ fine-tuning. In some cases, though, new and unexpected properties emerge. As illustrated here, both behaviors make solid solutions particularly relevant to the scope of crystal engineering.

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'.