Reverse Hierarchy of Alkane Adsorption in Metal–Organic Frameworks (MOFs) Revealed by Immersion Calorimetry

Abstract

Immersion calorimetry into liquids of different dimensions is a powerful tool to learn about the pore size and shape in nanoporous solids. In general, in the absence of specific interactions with the solid surface, the accessibility of the liquid probe molecule to the inner porosity, and the associated enthalpy value decreases with an increase in its kinetic diameter (bulkier molecules have lower accessibility and packing density). Although this is true for majority of solids (e.g. activated carbons and zeolites), this study anticipates that this is not straightforward in the specific case of MOFs. The evaluation of different hydrocarbons and their derivatives reveals the presence of reverse selectivity for C 6 isomers (2,2-dimethylbutane>2-methylpentane>n-hexane) in UiO-66 and HKUST-1, while size exclusion effects take place in ZIF-8. The immersion calorimetric findings have been compared with vapor adsorption isotherms and computational studies. Monte Carlo simulations suggest that the reverse selectivity in UiO-66 are attributed to the strong confinement of the di-branched hydrocarbons in the small tetragonal cages, while the presence of strong interactions with the open metal sites account for the preferential adsorption in HKUST-1. These results open the gate towards the application of immersion calorimetry for the pre-

Molecular sieving of linear and branched C6 alkanes by tannin-derived carbons

“…Figures 4a and 4b show that nHEX is able to access the inner porous structure of both DMC and OMC. The uptakes at 45 °C, 3.8 and 3.3 mmol g -1 for the DMC and OMC, respectively, are comparable to those reported recently for nHEX adsorption in MOFs [58].…”

“…However, the high adsorption uptake, even at low relative pressures, and the high selectivity observed over the entire pressure range evaluated for the OMC, constitute one of the best results reported so far in the literature for this specific application [63][64][65]. to present a "reverse" hierarchy of adsorption, meaning that these MOFs have preferential adsorption of C 6 isomers in the order: di-branched > mono-branched > linear [58,65].…”

“…gathers some data on the adsorption performance of different materials such as carbons, zeolites, MOFs, and zeolitic imidazolate frameworks (ZIFs) tested for the separation of C 6 isomers compared to those of the tannin-derived MMCs of this study[4,[20][21][22]58,65,66]. Even more, to directly compare the C 6 separation performance of the present MMCs derived from MT extract with that of MOFs used for gas separation, IAST calculations were performed using the single-component adsorption isotherms of HKUST-1, UiO-66, and ZIF-8, previously reported by Cuadrado-Collados et al[58].…”

“…gathers some data on the adsorption performance of different materials such as carbons, zeolites, MOFs, and zeolitic imidazolate frameworks (ZIFs) tested for the separation of C 6 isomers compared to those of the tannin-derived MMCs of this study[4,[20][21][22]58,65,66]. Even more, to directly compare the C 6 separation performance of the present MMCs derived from MT extract with that of MOFs used for gas separation, IAST calculations were performed using the single-component adsorption isotherms of HKUST-1, UiO-66, and ZIF-8, previously reported by Cuadrado-Collados et al[58]. Given the available data for the MOFs in the aforementioned study, in order to compare the performance consistently, the singlecomponent adsorption isotherms of DMC and OMC were acquired at 25 °C (not shown) and IAST calculations were carried out using the adsorption branch instead of the desorption branch.…”

Kerogen Swelling in Light Hydrocarbon Gases and Liquids and Validity of Schroeder’s Paradox

“…Different bias schemes have been proposed to increase the insertion acceptance rate in GCMC simulations of complex adsorbate molecules. − The configurational-bias GCMC (CB-GCMC) method based on the Rosenbluth sampling scheme has been widely applied to simulate adsorption of long-chain hydrocarbons in metal–organic frameworks (MOFs), − zeolite, − and confined slits. − Instead of inserting the molecule as a whole, CB-GCMC starts with one atom of the chain and inserts the molecule part by part biasing the growth process toward energetically favorable configurations in the void space of porous materials . The process is effective in reducing the overlap between adsorbents and adsorbates and accelerating the convergence.…”