Monte Carlo Simulations to Examine the Role of Pore Structure on Ambient Air Separation in Metal–Organic Frameworks

Abstract

MOFs can separate O 2 /N 2 mixtures via various mechanisms, making them a great candidate for many air separation applications. We show that the adsorptive behavior of MOFs can be classified based on whether they contain open metal sites (OMSs) and if those sites form a coordination bond with oxygen molecules. Existing data suggests that OMS MOFs that can bind O 2 have high O 2 /N 2 selectivity, while OMS MOFs that cannot form such a bond have increased selectivity toward N 2 . Monte Carlo simulations show that saturated metal site (SMS) MOFs selectively adsorb O 2 based on dispersion forces. Inaccuracies in simulations on nonbinding OMS MOFs are suggested as evidence that there are other nonbinding interactions impacting O 2 /N 2 adsorption in OMS MOFs. Monte Carlo simulations on SMS MOFs are used to define a volume-weighted average pore diameter to establish clear correlations between complex pore size distributions and adsorptive properties.

Biocompatible MOFs for Storage and Separation of O2: A Molecular Simulation Study

“…We would like to add that the top-performing MOFs identified for adsorption-based O 2 /N 2 separation (given in Table ) are not the same as those identified for O 2 storage as we expected. The pore geometry and/or pore sizes have a significant effect on O 2 /N 2 adsorption selectivity as discussed in the literature . Since pore volume has a more pronounced effect on O 2 storage, the best MOFs which have the highest O 2 working capacities are not the best candidates for adsorbent-based O 2 /N 2 separation.…”

“…It is also required for the enrichment of air during catalyst regeneration in industrial catalytic units . In order to produce pure O 2 , air separation is widely used which purifies O 2 and nitrogen (N 2 ) . Cryogenic distillation, which requires high pressure (from 1320 to 650 kPa) and ultralow temperature (−185 °C), is a traditional method to separate air mixtures based on the boiling point differences of components.…”

“…In a recent study, Moghadam et al computationally screened 2932 existing MOFs for O 2 uptake and reported that the maximum volumetric O 2 working capacity (249 cm 3 (STP) cm –3 ) and the maximum gravimetric O 2 working capacity (20.4 mol O 2 /kg) are achieved by UMCM-152 (University of Michigan Crystalline Material) and DIDDOK, respectively. McIntyre et al also performed GCMC simulations of IRMOFs- n ( n = 1–16), MOF-177, and UiO-66 and showed that IAST (ideal adsorption solution theory) selectivities of these MOFs for O 2 /N 2 mixture are between 1 and 1.3 at 298 K. Wang et al examined the effect of metal ions on the O 2 adsorption performance of M 3 (BTC) 2 -type materials (M = Cr, Mn, Fe, Co, Ni, and Cu; BTC = 1,3,5-benzenetricarboxylate acid) and found out that Ni 3 (BTC) 2 shows 11 kJ mol –1 higher interaction energy toward O 2 than that for N 2 , indicating that Ni 3 (BTC) 2 can be a promising material for O 2 /N 2 separation. Similarly, Parkes et al computationally investigated M 2 (dobdc) (M = Cr, Mn, Fe) MOF series for pure gas and competitive gas adsorption of O 2 and N 2 and showed that unsaturated metal sites enhance O 2 selectivity due to strong interactions between O 2 and the metal centers of MOFs.…”

“…3 In order to produce pure O 2 , air separation is widely used which purifies O 2 and nitrogen (N 2 ). 4 Cryogenic distillation, which requires high pressure (from 1320 to 650 kPa) and ultralow temperature (−185 °C), is a traditional method to separate air mixtures based on the boiling point differences of components. However, this technology is complex, expensive, and energy intensive.…”

Core–shell adsorbents by electrospun MOF‐polymer composites with improved adsorption properties: Theory and experiments

“…Since 1989, metal–organic‐frameworks (MOFs) have been touted as a promising material that may be tailor‐made for a variety of chemical selective applications including separation and catalysis . The theoretically infinite combination of metal nodes and ligands allows for the rational design of active sites, pore networks, and surface area . While some industrial applications have emerged as we see in the ION‐X gas storage cylinder developed by NuMat Technologies, the extensive use of MOFs is still limited.…”