The Complexity of Comparative Adsorption of C6 Hydrocarbons (Benzene, Cyclohexane, n-Hexane) at Metal–Organic Frameworks

Abstract

The relatively stable MOFs Alfum, MIL-160, DUT-4, DUT-5, MIL-53-TDC, MIL-53, UiO-66, UiO-66-NH2, UiO-66(F)4, UiO-67, DUT-67, NH2-MIL-125, MIL-125, MIL-101(Cr), ZIF-8, ZIF-11 and ZIF-7 were studied for their C6 sorption properties. An understanding of the uptake of the larger C6 molecules cannot simply be achieved with surface area and pore volume (from N2 sorption) but involves the complex micropore structure of the MOF. The maximum adsorption capacity at p p0−1 = 0.9 was shown by DUT-4 for benzene, MIL-101(Cr) for cyclohexane and DUT-5 for n-hexane. In the low-pressure range from p p0−1 = 0.1 down to 0.05 the highest benzene uptake is given by DUT-5, DUT-67/UiO-67 and MIL-101(Cr), for cyclohexane and nhexane by DUT-5, UiO-67 and MIL-101(Cr). The highest uptake capacity at p p0−1 = 0.02 was seen with MIL-53 for benzene, MIL-125 for cyclohexane and DUT-5 for n-hexane. DUT-5 and MIL-101(Cr) are the MOFs with the widest pore window openings/cross sections but the low-pressure uptake seems to be controlled by a complex combination of ligand and pore-size effect. IAST selectivities between the three binary mixtures show a finely tuned and difficult to predict interplay of pore window size with (critical) adsorptive size and possibly a role of electrostatics through functional groups such as NH2.

Evaluation of HKUST-1 as Volatile Organic Compound Adsorbents for Respiratory Filters

“…37,38 In addition, the HKUST-1 studied could compete with other materials specifically selected for cyclohexane adsorption, such as the COFs presented by Moroni et al, 39 with registered capacities around 6.5 mmol/g for pure cyclohexane. At low partial pressures (0.51 kPa), HKUST-1 also outperforms many of the several MOFs presented by Jansen et al 40 Cyclohexane adsorption isotherms feature two steps for HKUST-1 (Figure 2). The first step takes place at low partial pressures, filling the microporous volume, which in this case remains conserved after pressurization, even for the rest of the particle sizes (Figure S2).…”

“…This fact is striking since other studies show large drops in the adsorption capacity after a mechanical pressurization of HKUST-1. , In addition, the HKUST-1 studied could compete with other materials specifically selected for cyclohexane adsorption, such as the COFs presented by Moroni et al, with registered capacities around 6.5 mmol/g for pure cyclohexane. At low partial pressures (0.51 kPa), HKUST-1 also outperforms many of the several MOFs presented by Jansen et al…”

Enhanced sorption in an indium-acetylenedicarboxylate metal–organic framework with unexpected chains of cis-μ-OH-connected {InO6} octahedra

“…70 Vapor sorption studies in HHUD-4 and In-fum Toxic gases and vapors of volatile organic compounds (VOCs) in the atmosphere are a major concern and their removal, degradation and the separation of mixtures with MOFs is of scientific interest. [71][72][73] Benzene, cyclohexane and n-hexane are among the VOCs that can be found indoors and as they pose health risks their removal is of importance. Benzene is commonly regarded as the most toxic indoor VOC, as it cannot be biodegraded and thus is potentially cancerogenic, 74,75 but also n-hexane, which metabolizes to the nerve-damaging and toxic 2,5-hexanedione, 76,77 is of significance for removal.…”

Linker Vacancy Engineering of a Robust ftw‐type Zr‐MOF for Hexane Isomers Separation

“…In this context, MOFs assembled through coordination of inorganic clusters and organic linkers are particularly promising for discriminating similar molecules, because of their structural diversity, high porosity, exceptional tunability with respect to their pore dimensions, and functionality. [5][6][7] MOFs have demonstrated great promise in efficient separation of industrially relevant hydrocarbon mixtures such as propane/propylene, [8][9][10][11] ethane/ethylene, [12,13] xylene isomers, [14] etc., thanks to the successful practice of reticular chemistry which allows one to precisely tailor their pore structure and functionality. Fine-tuning of MOF structures is typically accomplished by making use of three strategies: linker engineering, metal/SBU engineering, and linker conformation engineering.…”

“…Adsorptive separation of these hydrocarbons of high similarity has stringent requirements on the structure, pore dimension, and functionality of the adsorbent, which have not been fully fulfilled by traditional materials. In this context, MOFs assembled through coordination of inorganic clusters and organic linkers are particularly promising for discriminating similar molecules, because of their structural diversity, high porosity, exceptional tunability with respect to their pore dimensions, and functionality [5–7] …”