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Liquid organic hydrogen carrier (LOHC) systems are an excellent alternative to pressurized gas and liquid hydrogen storage technologies due to their high volumetric storage capacities and straightforward adaptation to existing infrastructure. Homogeneous catalysts are promising for the selective and reversible release of hydrogen from LOHC. However, separation from product mixtures and recycling inhibit their use, particularly when comprised of costly low-abundance elements, motivating the development of heterogeneous versions that are more easily recovered and reused. Here, we describe two methods for the heterogenization of molecular Ru catalysts that efficiently dehydrogenate the polyalcohol LOHCs ethylene glycol (EG) and 1,2-propanediol (1,2-PDO). The heterogeneous versions of these catalysts maintain catalytic activity for hydrogen production comparable to the homogeneous complexes, with up to 81% conversion and 99% selectivity. DFT modeling indicates mechanistic similarities for the dehydrogenations of EG and 1,2-PDO, with the rate-limiting steps associated with protonation of the Ru-H bond to form H2 and the alkoxide species coordinated to Ru(II), followed by β-hydride elimination to regenerate the Ru-H bond. Overall, the data suggest these heterogenized molecular catalysts have potential for practical use in polyalcohol-based LOHC systems.

We report that linker ligand substitution involving just one atom induces a shape‐memory effect in a flexible coordination network. Specifically, whereas SIFSIX‐23‐Cu, [Cu(SiF6)(L)2]n, (L=1,4‐bis(1‐imidazolyl)benzene, SiF62−=SIFSIX) has been previously reported to exhibit reversible switching between closed and open phases, the activated phase of SIFSIX‐23‐CuN, [Cu(SiF6)(LN)2]n (LN=2,5‐bis(1‐imidazolyl)pyridine), transformed to a kinetically stable porous phase with strong affinity for CO2. As‐synthesized SIFSIX‐23‐CuN, α, transformed to less open, γ, and closed, β, phases during activation. β did not adsorb N2 (77 K), rather it reverted to α induced by CO2 at 195, 273 and 298 K. CO2 desorption resulted in α′, a shape‐memory phase which subsequently exhibited type‐I isotherms for N2 (77 K) and CO2 as well as strong performance for separation of CO2/N2 (15/85) at 298 K and 1 bar driven by strong binding (Qst=45–51 kJ/mol) and excellent CO2/N2 selectivity (up to 700). Interestingly, α′ reverted to β after re‐solvation/desolvation. Molecular simulations and density functional theory (DFT) calculations provide insight into the properties of SIFSIX‐23‐CuN. [ABSTRACT FROM AUTHOR]

Organic co-crystals have emerged as a promising class of semiconductors for next-generation optoelectronic devices due to their unique photophysical properties. This paper presents a joint experimental-theoretical study comparing the crystal structure, spectroscopy, and electronic structure of two charge transfer co-crystals. Reported herein is a novel co-crystal Npe:TCNQ, formed from 4-(1-naphthylvinyl)pyridine (Npe) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) via molecular self-assembly. This work also presents a revised study of the co-crystal composed of Npe and 1,2,4,5-tetracyanobenzene (TCNB) molecules, Npe:TCNB, herein reported with a higher-symmetry (monoclinic) crystal structure than previously published. Npe:TCNB and Npe:TCNQ dimer clusters are used as theoretical model systems for the co-crystals; the geometries of the dimers are compared to geometries of the extended solids, which are computed with periodic boundary conditions density functional theory. UV-Vis absorption spectra of the dimers are computed with time-dependent density functional theory and compared to experimental UV-Vis diffuse reflectance spectra. Both Npe:TCNB and Npe:TCNQ are found to exhibit neutral character in the S0 state and ionic character in the S1 state. The high degree of charge transfer in the S1 state of both Npe:TCNB and Npe:TCNQ is rationalized by analyzing the changes in orbital localization associated with the S1 transitions. [ABSTRACT FROM AUTHOR]

Porous sorbents are materials that are used for various applications, including storage and separation. Typically, the uptake of a single gas by a sorbent decreases with temperature, but the relative affinity for two similar gases does not change. However, in this study, we report a rare example of "crossover sorption," in which the uptake capacity and apparent affinity for two similar gases reverse at different temperatures. We synthesized two soft porous coordination polymers (PCPs), [Zn2(L1)(L2)2]n (PCP‐1) and [Zn2(L1)(L3)2]n (PCP‐2) (L1= 1,4‐bis(4‐pyridyl)benzene, L2=5‐methyl‐1,3‐di(4‐carboxyphenyl)benzene, and L3=5‐methoxy‐1,3‐di(4‐carboxyphenyl)benzene). These PCPs exhibits structural changes upon gas sorption and show the crossover sorption for both C2H2/CO2 and C2H6/C2H4, in which the apparent affinity reverse with temperature. We used in situ gas‐loading single‐crystal X‐ray diffraction (SCXRD) analysis to reveal the guest inclusion structures of PCP‐1 for C2H2, CO2, C2H6, and C2H4 gases at various temperatures. Interestingly, we observed three‐step single‐crystal to single‐crystal (sc‐sc) transformations with the different loading phases under these gases, providing insight into guest binding positions, nature of host–guest or guest‐guest interactions, and their phase transformations upon exposure to these gases. Combining with theoretical investigation, we have fully elucidated the crossover sorption in the flexible coordination networks, which involves a reversal of apparent affinity and uptake of similar gases at different temperatures. We discovered that this behaviour can be explained by the delicate balance between guest binding and host–guest and guest‐guest interactions. [ABSTRACT FROM AUTHOR]

Compared to rigid physisorbents, switching coordination networks that reversibly transform between closed (non-porous) and open (porous) phases offer promise for gas/vapour storage and separation owing to their improved working capacity and desirable thermal management properties. We recently introduced a coordination network, X-dmp-1-Co, which exhibits switching enabled by transient porosity. The resulting "open" phases are generated at threshold pressures even though they are conventionally non-porous. Herein, we report that X-dmp-1-Co is the parent member of a family of transiently porous coordination networks [X-dmp-1-M] (M = Co, Zn and Cd) and that each exhibits transient porosity but switching events occur at different threshold pressures for CO2 (0.8, 2.1 and 15 mbar, for Co, Zn and Cd, respectively, at 195 K), H2O (10, 70 and 75% RH, for Co, Zn and Cd, respectively, at 300 K) and CH4 (<2, 10 and 25 bar, for Co, Zn and Cd, respectively, at 298 K). Insight into the phase changes is provided through in situ SCXRD and in situ PXRD. We attribute the tuning of gate-opening pressure to differences and changes in the metal coordination spheres and how they impact dpt ligand rotation. X-dmp-1-Zn and X-dmp-1-Cd join a small number of coordination networks (<10) that exhibit reversible switching for CH4 between 5 and 35 bar, a key requirement for adsorbed natural gas storage. [ABSTRACT FROM AUTHOR]

The enrichment and purification of coal‐bed methane provides a source of energy and helps offset global warming. In this work, we demonstrate a strategy involving the regulation of the pore size and pore chemistry to promote the separation of CH4/N2 mixtures in four nickel‐based coordination networks, named Ni(ina)2, Ni(3‐ain)2, Ni(2‐ain)2, and Ni(pba)2, (where ina=isonicotinic acid, 3‐ain=3‐aminoisonicotinic acid, 2‐ain=2‐aminoisonicotinic acid, and pba=4‐(4‐pyridyl)benzoic acid). Among them, Ni(ina)2 and Ni(3‐ain)2 can effectively separate CH4 from N2 with top‐performing performance because of the suitable pore size (≈0.6 and 0.5 nm) and pore environment. Explicitly, Ni(ina)2 exhibits the highest ever reported CH4/N2 selectivity of 15.8 and excellent CH4 uptake (40.8 cm3 g−1) at ambient conditions, thus setting new benchmarks for all reported MOFs and traditional adsorbents. The exceptional CH4/N2 separation performance of Ni(ina)2 is confirmed by dynamic breakthrough experiments. Under different CH4/N2 ratios, Ni(ina)2 selectively extracts methane from the gaseous blend and produces a high purity of CH4 (99 %). Theoretical calculations and CH4‐loading single‐crystal structure analysis provide critical insight into the adsorption/separation mechanism. Ni(ina)2 and Ni(3‐ain)2 can form rich intermolecular interactions with methane, indicating a strong adsorption affinity between pore walls and CH4 molecules. Importantly, Ni(ina)2 has good thermal and moisture stability and can easily be scaled up at a low cost ($25 per kilogram), which will be valuable for potential industrial applications. Overall, this work provides a powerful approach for the selective adsorption of CH4 from coal‐bed methane. [ABSTRACT FROM AUTHOR]

Controlling gas sorption by simple pore modification is important in molecular recognition and industrial separation processes. In particular, it is challenging to realize the inverse selectivity, which reduces the adsorption of a high‐affinity gas and increases the adsorption of a low‐affinity gas. Herein, an "opposite action" strategy is demonstrated for boosting CO2/C2H2 selectivity in porous coordination polymers (PCPs). A precise steric design of channel pores using an amino group as an additional interacting site enabled the synergetic increase in CO2 adsorption while suppressing the C2H2 adsorption. Based on this strategy, two new ultramicroporous PCP physisorbents that are isostructural were synthesised. They exhibited the highest CO2 uptake and CO2/C2H2 volume uptake ratio at 298 K. Origin of this specific selectivity was verified by detailed density functional theory calculations. The breakthrough separation performances with remarkable stability and recyclability of both the PCPs render them relevant materials for C2H2 purification from CO2/C2H2 mixtures. [ABSTRACT FROM AUTHOR]

C2H2/CO2 separation is an industrially important process that remains challenging because of the similar physicochemical properties of C2H2 and CO2. We herein report that the new square lattice (sql) coordination network [Cu(bipy‐xylene)2(NO3)2]n, sql‐16‐Cu‐NO3, 16 = bipy‐xylene = 4,4′‐(2,5‐dimethyl‐1,4‐phenylene)dipyridine, exists in at least three forms, as‐synthesised (α), activated (α′) and hydrated (β). The activated phase, sql‐16‐Cu‐NO3‐α′, is an ultramicroporous material that exhibits high selectivity towards C2H2 over CO2 as revealed by dynamic gas breakthrough experiments (1:1, C2H2/CO2) that afforded 99.87% pure CO2 in the effluent stream. The separation selectivity at 298 K and 1 bar, 78, is the third best value yet reported for C2H2 selective physisorbents whereas the mid‐loading performance sets a new benchmark. The performance of sql‐16‐Cu‐NO3‐α′ is attributed to a new type of C2H2 binding site in which CH···ONO2 interactions enable moderately strong sorbent‐sorbate binding (Qst (C2H2) = 38.6 kJ/mol) at low loading. Conversely, weak CO2 binding (Qst (CO2) = 25.6 kJ/mol) at low loading means that (ΔQst)AC [Qst (C2H2)–Qst (CO2)] is 13 kJ/mol at low coverage and 11.4 kJ/mol at mid‐loading. Analysis of in situ powder X‐ray diffraction and modelling experiments provide insight into the sorption properties and high C2H2/CO2 separation performance of sql‐16‐Cu‐NO3‐α′. [ABSTRACT FROM AUTHOR]