Water Vapor Sorption in Hybrid Pillared Square Grid Materials

We report water vapor sorption studies on four primitive cubic, <b>pcu</b>, pillared square grid materials: <b>SIFSIX-1-Cu</b>, <b>SIFSIX-2-Cu-i</b>, <b>SIFSIX-3-Ni</b>, and <b>SIFSIX-14-Cu-i</b>. <b>SIFSIX-1-Cu</b>, <b>SIFSIX-3-Ni</b>, and <b>SIFSIX-14-Cu-i</b> were observed to exhibit negative water vapor adsorption at ca. 40–50% relative humidity (RH). The negative adsorption is attributed to a water-induced phase transformation from a porous <b>pcu</b> topology to nonporous <b>sql</b> and <b>sql-c*</b> topologies. Whereas the phase transformation of <b>SIFSIX-1-Cu</b> was found to be irreversible, <b>SIFSIX-3-Ni</b> could be regenerated by heating and can therefore be recycled. In contrast, <b>SIFSIX-2-Cu-i</b>, which is isostructural with <b>SIFSIX-14-Cu-i</b>, exhibited a type V isotherm and no phase change. <b>SIFSIX-2-Cu-i</b> was observed to retain both structure and gas sorption properties after prolonged exposure to heat and humidity. The hydrolytic stability of <b>SIFSIX-2-Cu-i</b> in comparison to its structural counterparts is attributed to structural features and therefore offers insight into the design of hydrolytically stable porous materials

Theoretical Investigations of CO₂ and H₂ Sorption in Robust Molecular Porous Materials

Molecular simulations of CO₂ and H₂ sorption were performed in MPM-1-Cl and MPM-1-TIFSIX, two robust molecular porous materials (MPMs) with the empirical formula [Cu₂(adenine)₄Cl₂]­Cl₂ and [Cu₂(adenine)₄(TiF₆)₂], respectively. Recent experimental studies have shown that MPM-1-TIFSIX displayed higher CO₂ uptake and isosteric heat of adsorption (<i>Q</i>_st) than MPM-1-Cl [Nugent, P. S.; et al. <i>J. Am. Chem. Soc.</i> <b>2013</b>, <i>135</i>, 10950–10953]. This was verified through the simulations executed herein, as the presented simulated CO₂ sorption isotherms and <i>Q</i>_st values are in very good agreement with the corresponding experimental data for both MPMs. We also report experimental H₂ sorption data in both MPMs. Experimental studies revealed that MPM-1-TIFSIX exhibits high H₂ uptake at low loadings and an initial H₂ <i>Q</i>_st value of 9.1 kJ mol^–1. This H₂ <i>Q</i>_st value is greater than that for a number of existing metal–organic frameworks (MOFs) and represents the highest yet reported for a MPM. The remarkable H₂ sorption properties for MPM-1-TIFSIX have been confirmed through our simulations. The modeling studies revealed that only one principal sorption site is present for CO₂ and H₂ in MPM-1-Cl, which is sorption onto the Cl^– counterions within the large channels. In contrast, three different sorption sites were discovered for both CO₂ and H₂ in MPM-1-TIFSIX: (1) between two TIFSIX groups within a small passage connecting the large channels, (2) onto the TIFSIX ions lining the large channels, and (3) within the small channels. This study illustrates the detailed insights that molecular simulations can provide on the CO₂ and H₂ sorption mechanism in MPMs

Tuning the gate-opening pressure in a switching pcu coordination network, X-pcu-5-Zn, by pillar ligand substitution

Coordination networks that reversibly switch between closed and open phases are of topical interest since their stepped isotherms can offer higher working capacities for gas‐storage applications than the related rigid porous coordination networks. To be of practical utility, the pressures at which switching occurs, the gate‐opening and gate‐closing pressures, must lie between the storage and delivery pressures. Here we study the effect of linker substitution to fine‐tune gate‐opening and gate‐closing pressure. Specifically, three variants of a previously reported pcu‐topology MOF, X‐pcu‐5‐Zn, have been prepared: X‐pcu‐6‐Zn, 6=1,2‐bis(4‐pyridyl)ethane (bpe), X‐pcu‐7‐Zn, 7=1,2‐bis(4‐pyridyl)acetylene (bpa), and X‐pcu‐8‐Zn, 8=4,4′‐azopyridine (apy). Each exhibited switching isotherms but at different gate‐opening pressures. The N2, CO2, C2H2, and C2H4 adsorption isotherms consistently indicated that the most flexible dipyridyl organic linker, 6, afforded lower gate‐opening and gate‐closing pressures. This simple design principle enables a rational control of the switching behavior in adsorbent materials

Crystal engineering of a rectangular sql coordination network to enable xylenes selectivity over ethylbenzene†

Separation of the C8 aromatic isomers, p-xylene (PX), m-xylene (MX), o-xylene (OX) and ethylbenzene (EB), is relevant thanks to their widespread application as chemical feedstocks but challenging because of their similar boiling points and close molecular dimensions. Physisorptive separation could offer an energy-efficient solution to this challenge but sorbents which exhibit strong selectivity for one of the isomers remain a largely unmet challenge despite recent reports of OX or PX selective sorbents with high uptake capacity. For example, the square lattice, sql, topology coordination network [Co(bipy)2(NCS)2]n (sql-1-Co-NCS) exhibits the rare combination of high OX selectivity and high uptake capacity. Herein we report that a crystal engineering approach enabled isolation of the mixed-linker sql coordination network [Co(bipy)(bptz) (NCS)2]n (sql-1,3-Co-NCS, bipy = 4,4′-bipyridine, bptz = 4,4′-bis(4-pyridyl)tetrazine) and study of its C8 vapour and liquid sorption properties. sql-1,3-Co-NCS was found to exhibit high adsorption capacity from liquid xylenes (∼37 wt%) and is to our knowledge the first sorbent to exhibit high selectivity for each of xylene isomer over EB (SOX/EB, SMX/EB, SPX/EB > 5). Insights into the performance of sql-1,3-Co-NCS are gained from structural studies which reveal stacking interactions between electron-deficient bptz linkers and the respective xylenes. sql-1,3-Co-NCS is the first N-donor mixed-linker sql coordination network studied for its gas/vapour sorption properties and represents a large and diverse class of understudied coordination networks

supporting information from Flue-gas and direct-air capture of CO₂ by porous metal-organic materials

Sequestration of CO₂, 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, <b>Zeolite 13X</b> and a chemisorbent, <b>TEPA-SBA-15</b>, for their ability to adsorb CO₂ directly from air and from simulated flue gas. In this contribution, a further 10 physisorbent materials that exhibit strong interactions with CO₂ have been evaluated by temperature-programmed desorption for their potential utility in carbon capture applications: four hybrid ultramicroporous materials, <b>SIFSIX-3-Cu</b>, <b>DICRO-3-Ni-i</b>, <b>SIFSIX-2-Cu-i</b> and <b>MOOFOUR-1-Ni</b>; five microporous MOMs, <b>DMOF-1</b>, <b>ZIF-8</b>, <b>MIL-101</b>, <b>UiO-66</b> and <b>UiO-66-NH₂</b>; an ultramicroporous MOM, <b>Ni-4-PyC</b>. 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 CO₂ capture performance from even moist gas mixtures but not enough to compete with chemisorbents

Efficient CO2 removal for ultra-pure CO production by two hybrid ultramicroporous materials

Removal of CO2 from CO gas mixtures is a necessary but challenging step during production of ultra‐pure CO as processed from either steam reforming of hydrocarbons or CO2 reduction. Herein, two hybrid ultramicroporous materials (HUMs), SIFSIX‐3‐Ni and TIFSIX‐2‐Cu‐i, which are known to exhibit strong affinity for CO2, were examined with respect to their performance for this separation. The single‐gas CO sorption isotherms of these HUMs were measured for the first time and are indicative of weak affinity for CO and benchmark CO2/CO selectivity (>4000 for SIFSIX‐3‐Ni). This prompted us to conduct dynamic breakthrough experiments and compare performance with other porous materials. Ultra‐pure CO (99.99 %) was thereby obtained from CO gas mixtures containing both trace (1 %) and bulk (50 %) levels of CO2 in a one‐step physisorption‐based separation process

Impact of partial interpenetration in a hybrid ultramicroporous material on C2H2/C2H4 separation performance

Phases of a 2‐fold pcu Hybrid Ultramicroporous Material (HUM), SIFSIX‐14‐Cu‐i, exhibiting 99%, 93%, 89%, and 70% partial interpenetration have been obtained. 1:99 C2H2/C2H4 gas separation studies reveal that as the proportion of interpenetrated component decreases, so does the separation performance

Breaking the trade-off between selectivity and adsorption capacity for gas separation

The trade-off between selectivity and adsorption capacity with porous materials is a major roadblock to reducing the energy foot-print of gas separation technologies. To address this matter, we report herein a systematic crystal engineering study of C2H2 removal from CO2 in a family of hybrid ultramicroporous materials (HUMs). The HUMs are composed of the same organic linker ligand, 4-(3,5-dimethyl-1H-pyrazol-4-yl)pyridine, pypz, three inorganic pillar ligands, and two metal cations, thereby affording six isostructural pcu topology HUMs. All six HUMs exhibited strong binding sites for C2H2 and weaker affinity for CO2. The tuning of pore size and chemistry enabled by crystal engineering resulted in benchmark C2H2/CO2 separation performance. Fixed-bed dynamic column breakthrough experiments for an equimolar (v/v = 1:1) C2H2/CO2 binary gas mixture revealed that one sorbent, SIFSIX-21-Ni, was the first C2H2 selective sorbent that combines exceptional separation selectivity (27.7) with high adsorption capacity (4 mmol.g-1