(668d) Hierarchical MOF Nanoparticles for Natural Gas Liquids Adsorption

Metal-organic frameworks (MOFs) are three-dimensionally ordered porous materials consisting of metal nodes and organic linkers. MOFs (such as MOF-5, and MOF-74) have been shown to adsorb gases in large amounts, many of which are of practical importance, such as H₂, CO₂, C₂ and C₃ hydrocarbons. This gives MOFs great potential in gas storage. In addition to gas adsorption and storage, MOF materials are also used for gas separation by adsorption. Controlled chemical structures usually give MOFs selectivities in gas separations (such as open metal sites in MOF-74). Although gas separation with MOFs with tunable chemical structures have been well studied, there has been no exploration of hierarchical porosity in MOFs for gas separation. Pore size control among MOFs have been shown in isoreticular synthesis. However, these syntheses involve custom-synthesized linkers, which usually require dedicated organic synthesis labs. There have been very few reports on expanding pore size distribution by crystal engineering only. In this work, we show a general 2-step procedure to synthesize hierarchical MOFs with mesoporosity by controlling synthesis conditions, without the use of linkers of various lengths. The method reported in this work maintained the structural integrity of the MOFs during intergrowth, where the seed particle structures are preserved intact. These hierarchical features were shown to increase the adsorption of hydrocarbons. Adsorption of methane, ethane, ethene, propane, and propene pure gases were performed on the hierarchical MOFs as well as MOF seed particles at pressures ranging from 10^-5 Pa – 1 MPa (absolute). Results from adsorption have shown that the hierarchical MOFs not only adsorbed large amount of gas than the seed particles, they have also shown selectivity of ~50 in propane/CH₄ adsorption (from 1 atm 20% CH4 + 80% propane mixture, according to IAST).