Understanding Gas Storage in Cuboctahedral Porous Coordination Cages

Abstract

Porous molecular solids are promising materials for gas storage and gas separation applications. However, given the relative dearth of structural information concerning these materials, additional studies are vital for further understanding their properties and developing design parameters for their optimization. Here, we examine a series of isostructural cuboctahedral, paddlewheel-based coordination cages, M24(tBu-bdc)24 (M = Cr, Mo, Ru; tBu-bdc2− = 5-tert-butylisophthalate), for high-pressure methane storage. As the decrease in crystallinity upon activation of these porous molecular materials precludes diffraction studies, we turn to a related class of pillared coordination cage-based metal-organic frameworks, M24(Me-bdc)24(dabco)6 (M = Fe, Co; Me-bdc2− = 5-methylisophthalate; dabco = 1,4-diazabicyclo[2.2.2]octane) for neutron diffraction studies. The five porous materials display BET surface areas from 1,057 – 1,937 m2/g and total methane uptake capacities of up to 143 cm3(STP)/cm3. Both the porous cages and cage-based frameworks display methane adsorption enthalpies of −15 to −22 kJ/mol. Also supported by molecular modeling, neutron diffraction studies indicate that the triangular windows of the cage are favorable methane adsorption sites with CD4-arene interactions between 3.7 and 4.1 Å. At both low and high loadings, two additional methane adsorption sites on the exterior surface of the cage are apparent for a total of 56 adsorption sites per cage. These results show that M24L24 cages are competent gas storage materials and further adsorption sites may be optimized by judicious ligand functionalization to control extra-cage pore space.

Stabilizing Metal–Organic Polyhedra (MOP): Issues and Strategies

“…Cotton's dimetallic building block approach has been used extensively to construct a variety of neutral supramolecular architectures from a wide range of organic linkers and dimetallic motifs (Mo 2 4+ and Rh 2 4+ ) . Recently, highly porous MOPs were developed by using a robust dimetallic Cr 2 secondary building block . Zhou and co‐worker proposed a bridging‐ligand‐substitutions strategy for the synthesis of various carboxylate MOPs .…”

“…As mentioned earlier, there are two main challenges to the formation of carboxylate MOPs with the desired stability, that is, 1) poor hydrolytic stability and 2) aggregation of MOPs after solvent removal, either or both of which may restrict the full potential of these materials . During MOP synthesis, some guest molecules are entrapped inside the host MOP, so prior to use the MOP active sites must be activated, that is, guest molecules must be removed.…”

“…However, substantial catalytic degradation was noted for the bare‐cage molecules even after the first catalytic run. Li and co‐workers synthesized four distinctly functionalized but analogous MOPs inside mesoporous silica (SBA‐16) by a double‐solvent strategy . They selected SBA‐16 with a cavity size around 6 nm larger than the MOP molecules, whereas the pore entrances were about 2 nm smaller than the MOP molecules (Figure ).…”

Entropy and Random Walk Trails Water Confinement and Non-Thermal Equilibrium in Photon-Induced Nanocavities

“…Any 2D topological transform is accompanied by a diversion of surface characteristics, such as porosity, sensing efficiency, chemical stability and extensive nanocavitation [ 41 , 42 , 43 , 44 , 45 , 46 ]. The adsorption of various molecules on 2D nanostructured surfaces [ 47 , 48 , 49 ], might well boost a plethora of surfactant effects along with molecular sensing [ 43 , 50 ], gas separation and storage [ 51 , 52 , 53 , 54 ], and also applications with particular emphasis on nanomedicine [ 55 ], bio-engineering [ 56 , 57 ] and drug delivery systems [ 58 , 59 ]. Among other polymeric matrixes, polyacrylamide (PAM) is a hydrophilic low toxic, biocompatible, water-soluble, synthetic linear or cross-linked molecule, modified accordingly for a wide range of applications, including oil recuperation, wastewater treatment, soil conditioner, cosmetics food and biomedical industries [ 60 , 61 , 62 ].…”

Self-assembly of a cobalt(II)-based metal–organic framework as an effective water-splitting heterogeneous catalyst for light-driven hydrogen production

“…Metal-organic frameworks (MOFs), as a kind of porous materials, have been the subject of much research, not only owing to their high porosity and the high surface area of the structures which makes them useful for potential applications in gas storage and separation (Lorzing et al, 2019;Wang et al, 2019b), drug delivery (Orellana-Tavra et al, 2020) and molecular recognition (Zhu et al, 2017;Mizutani et al, 2020), but also for the tunability and tailorability of their coordination modes and the numerous possible modifications with functional groups making them useful in catalysis (Zhou et al, 2016;Ji et al, 2019;Song et al, 2019;Feng et al, 2019;Wu et al, 2019), fluorescent sensing (Mi et al, 2019;Yang et al, 2019c;Pratik & Cramer, 2019) and magnetic materials (Lan et al, 2019;Gu et al, 2018;Li et al, 2019). The self-assembly of MOFs with various organic linkers and metal ions or clusters endows these materials with active sites and designable backbones, displaying the combined advantages of homogeneous and heterogeneous properties as catalysts, such as high activity and selectivity on one hand, and recovery and high controllability on the other hand.…”