Understanding Hydrocarbon Adsorption in the UiO-66 Metal–Organic Framework: Separation of (Un)saturated Linear, Branched, Cyclic Adsorbates, Including Stereoisomers

Abstract

The low coverage adsorption properties of alkanes, alkenes, and aromatics of the linear, branched, and cyclic type (ca. 70 molecules) were studied using inverse pulse gas chromatography at zero coverage on the zirconium metal− organic framework UiO-66 and its functionalized analogues UiO-66-Me, UiO-66-NO 2 , UiO-66-Me 2 in the temperature range 433−573 K. In our study, we determined and analyzed the adsorption enthalpy, Henry constants, and entropic factors. Preferential adsorption of bulky molecules is observed with specific adsorbate and cage size effects, yielding very specific, preferential adsorption. Remarkably high adsorption selectivity factors (up to 14) for cyclo-compared to nalkanes were found. The presence of additional groups (methyl, nitro) on the linkers in the framework influences adsorption properties significantly, mainly by reducing the effective pore size. Whereas increased selectivity is observed for UiO-66-Me, this effect decreases again upon addition of a second methyl group, UiO-66-Me 2 . The latter allows for tuning confinement factors inside the pores, thus adsorption properties of the metal−organic framework. The selective adsorption results from the interaction in the smallest octahedral cage. The extreme confinement in the tetrahedral cage allows for stereoselective separation of disubstituted cycloalkanes and cis/trans alkenes. Monte Carlo simulations were performed for the unfunctionalized UiO-66 framework. First, a comparative study between the force fields Dreiding and UFF is performed with n-alkanes to obtain accurate and reproducible values. The simulations show adsorbate molecular size−adsorbent cage size effects similar to window/cage effects reported for zeolites (e.g., silicalite). Second, adsorption properties were simulated for selected cases, including stereoisomers. Careful analysis of the adsorbate's molecular positioning in the framework confirms the experimental data. The framework's selectivity results from adsorption in the tetrahedral cage at zero coverage. Furthermore, simulations show important contributions of entropic factors to the observed adsorption selectivity.

Origin of the structure-directing effect resulting in identical topological open-framework materials

“…The atomic charges were calculated using the charge equilibrium (QEq) method31, which adjusts atomic charges dynamically to represent the polarization effects. The combination of UFF and QEq has been used successfully to describe many different inorganic materials32. The long-range electrostatic energy was calculated using the Ewald summation, and the VDW interaction was calculated using a 12.5 Å cutoff with a tail correction.…”

Exploiting Large‐Pore Metal–Organic Frameworks for Separations through Entropic Molecular Mechanisms

“…This is exclusively attributed to entropy.I ns tructures like UiO-66, the molecules are well separated in small cavities, and intermolecularinteractions for hexane isomers are low.I ng eneral, because there are two types of cavities in UiO-66, molecules of ac ertain type can preferentially adsorb in one of the two types of cage (or transition between them), depending on the temperature and loading. [58] Entropy differences between components in am ixture can be substantial, even at low loading, and mixture separations can be driven by rotational entropy. [43,59] At the start of an adsorption process, DH < TDS and DG < 0, and the process transfers molecules in the direction of lower free energy (which results in adsorption).…”

Statistical mechanics of binary mixture adsorption in metal–organic frameworks in the osmotic ensemble

“…MOFs constitute a family of soft porous crystals, described by Kitagawa and co-workers [11] as 'porous crystals that possess both a highly ordered network and structural transformability'. MOFs have high potential for use in adsorptive separation processes [12][13][14][15][16][17][18][19][20][21] due to the unique characteristics of structural pores which may be adapted to permit adsorption to take place on the basis of molecular shape and size and in particular structural flexibility [22][23][24][25][26][27][28]. In MOFs, metal framework centres and organic units link thereby allowing structural transitions to occur upon gas adsorption [29][30][31][32][33][34].…”