Pore space partitioning (PSP) is methodically suited for dramatically increasing the density of guest binding sites, leading to the partitioned acs (pacs) platform capable of record-high uptake for CO2 and small hydrocarbons such as C2Hx. For gas separation, achieving high selectivity amid PSP-enabled high uptake offers an enticing prospect. Here we aim for high selectivity by introducing the bioisosteric (BIS) concept, a widely used drug design strategy, into the realm of pore-space-partitioned MOFs. New pacs materials have high C2H2/CO2 selectivity of up to 29, high C2H2 uptake of up to 144 cm3/g (298 K, 1 atm), and high separation potential of up to 5.3 mmol/g, leading to excellent experimental breakthrough performance. These metrics, coupled with exceptional tunability, high stability, and low regeneration energy, demonstrate the broad potential of the BIS-PSP strategy.
Metal-Organic Frameworks , Carbon Dioxide , Deuterium
An ideal material for C2H6/C2H4 separation would simultaneously have the highest C2H6 uptake capacity and the highest C2H6/C2H4 selectivity. But such material is elusive. A benchmark material for ethane-selective C2H6/C2H4 separation is peroxo-functionalized MOF-74-Fe that exhibits the best known separation performance due to its high C2H6/C2H4 selectivity (4.4), although its C2H6 uptake capacity is moderate (74.3 cm3/g). Here, we report a family of pore-space-partitioned crystalline porous materials (CPMs) with exceptional C2H6 uptake capacity and C2H6/C2H4 separation potential (i.e., C2H4 recovered from the mixture) despite their moderate C2H6/C2H4 selectivity (up to 1.75). The ethane uptake capacity as high as 166.8 cm3/g at 1 atm and 298 K, more than twice that of peroxo-MOF-74-Fe, has been achieved even though the isosteric heat of adsorption (21.9-30.4 kJ/mol) for these CPMs is as low as about one-third of that for peroxo-MOF-74-Fe (66.8 kJ/mol). While the overall C2H6/C2H4 separation potentials have not yet surpassed peroxo-MOF-74-Fe, these robust CPMs exhibit outstanding properties including high thermal stability (up to 450 °C) and aqueous stability, low regeneration energy, and a high degree of chemical and geometrical tunability within the same isoreticular framework.
Built from an unusual high-charge-density ligand 2,5-dioxido-1,4-benzenedicarboxylate (dobdc4-), MOF-74 M (M2dobdc) have unsurpassed gas uptake and separation properties. It is thus intriguing to mimic or replicate such ligand properties in other chemical systems. Here, we show a ligand charge separation (LCS) model that could offer one pathway toward this goal. Two new materials (CPM-74 and -75, corresponding to MOF-74 and IRMOF-74-II, respectively) are presented here to illustrate this concept and its feasibility. Specifically, the dobdc4- ligand in MOF-74 can be conceptually broken down into OH- and obdc3- (H3obdc = 2-hydroxyterephthalic acid), which leads to CPM-74, Zn2(OH)(obdc), that is nearly isomeric with MOF-74-Zn. Different from MOF-74, CPM-74 is made from homohelical rod packing. Moreover, CPM-74 has high hydrothermal and thermal stability uncommon for Zn-MOFs. It contains open Zn sites on 4-coordinated Zn2+ and its isosteric heat of adsorption for CO2 is 22% higher than that of MOF-74-Zn at low pressures.
