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 footprint 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).

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Six isostructural hybrid ultramicroporous materials are prepared and characterized

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Crystal engineering approach enabled fine-tuning of pore size and chemistry

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Weak CO₂/strong C₂H₂ affinity resulted in high C₂H₂/CO₂ separation selectivities

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SIFSIX-21-Ni: benchmark selectivity/uptake capacity for C₂H₂/CO₂ separation

It is generally recognized that porous solids (sorbents) with high selectivity and high adsorption capacity offer potential for energy-efficient gas separations. Unfortunately, there is generally a trade-off between capacity and selectivity, which represents a roadblock to the utility of sorbents in key industrial processes. For example, acetylene (C₂H₂), an important fuel and chemical intermediate, is produced with CO₂ as an impurity, and the similar physicochemical properties of C₂H₂ and CO₂ mean that most sorbents are poorly selective. Hybrid ultramicroporous materials (HUMs) are candidates for gas separations as they exhibit benchmark selectivity for several key gas pairs. Unfortunately, existing HUMs are handicapped by low capacity. We report a new HUM, SIFSIX-21-Ni, that addresses the trade-off between selectivity and capacity that has plagued sorbents, as its high uptake and high selectivity renders it the new benchmark for C₂H₂/CO₂ separation performance.

Benchmark Acetylene Binding Affinity and Separation through Induced Fit in a Flexible Hybrid Ultramicroporous Material

Structural changes at the active site of an enzyme induced by binding to a substrate molecule can result in enhanced activity in biological systems. Herein, we report that the new hybrid ultramicroporous material sql-SIFSIX-bpe-Zn exhibits an induced fit binding mechanism when exposed to acetylene, C2 H2 . The resulting phase change affords exceptionally strong C2 H2 binding that in turn enables highly selective C2 H2 /C2 H4 and C2 H2 /CO2 separation demonstrated by dynamic breakthrough experiments. sql-SIFSIX-bpe-Zn was observed to exhibit at least four phases: as-synthesised (α); activated (β); and C2 H2 induced phases (β' and γ). sql-SIFSIX-bpe-Zn-β exhibited strong affinity for C2 H2 at ambient conditions as demonstrated by benchmark isosteric heat of adsorption (Qst ) of 67.5 kJ mol-1 validated through in situ pressure gradient differential scanning calorimetry (PG-DSC). Further, in situ characterisation and DFT calculations provide insight into the mechanism of the C2 H2 induced fit transformation, binding positions and the nature of host-guest and guest-guest interactions.

Tuning extreme anisotropic thermal expansion in 1D coordination polymers through metal selection and solid solutions

The thermal expansion behaviour of a series of 1D coordination polymers has been investigated. Variation of the metal centre allows tuning of the thermal expansion behaviour from colossal positive volumetric to extreme anomalous thermal expansion. Preparation of solid solutions increased the magnitude of the anomalous thermal expansion further, producing two species displaying supercolossal anisotropic thermal expansion (ZnCoCPHTαY2 = -712 MK-1, αY3 = 1632 MK-1 and ZnCdCPHTαY2 = -711 MK-1, αY3 = 1216 MK-1).

Supramolecular solvatochromism: mechanistic insight from crystallography, spectroscopy and theory

A solvatochromic dinuclear copper(ii) metallocycle effectively traps tetrahydrofuran, diethyl ether and pentane significantly above their boiling points.