High-Pressure Methane Storage and Selective Gas Adsorption in a Cyclohexane-Functionalized Porous Organic Cage

ABSTRACT Porous organic cages are a class of adsorbents that have shown promise for a variety of applications. The octahedral cyclohexane-functionalised imine-based cage, commonly referred to as CC3, is perhaps most recognisable in this regard. However, most gas adsorption studies concerning this cage have focused on chromatographic or noble gas separations. Here, we examine this cage for the adsorptive separation of a wide variety of small molecules, including acetylene, ethylene, ethane, methane, and others. The internal surface of the cage is an optimal binding environment for gaseous substrates and as such the material displays adsorption enthalpies of −15 to −35 kJ/mol, depending on adsorbate. These values are particularly high for an adsorbent lacking coordinatively unsaturated metal sites and are likely a result of optimal cage geometry, which can be inferred from its isostructural nature to one of the pores in a well-known metal-organic framework, HKUST-1. Given this and the soluble nature of this cage, we additionally report optimisation of the high-pressure methane storage properties of this cage by tuning its density. Graphical abstract


Synthesis of CC3b
The cage was synthesized using a strategy adapted from previous reports. A 1L Schlenk flask equipped with N2 line and stir bar was charged with 2.5914 g (22.69 mmol) (1R,2R)-(−)-1,2diaminocyclohexane, 2.4706 g (15.24 mmol) triformylbenzene, and 750 mL freshly distilled DCM. The vessel was capped with a septum and the atmosphere was purged with N2 for 1 hour. The purge needle was removed and the clear pale-yellow solution was allowed to stir under an N2 atmosphere for 3 days. After synthesis, three methods were explored for storage. A portion of the solution was evacuated to dryness, yielding an off-white crumbly powder which was stored inside a glove box. A portion of the solution was transferred to a smaller vessel and purged with N2 to dryness to allow for an air-free slow evaporation, yielding a flaky off-white solid which was stored inside a glove box. The remainder was preserved in solution under N2 until needed.

High-Pressure Methane Procedure
High-pressure isotherms were collected on a Sieverts' apparatus (PCT-Pro-2000 from Hy-Energy Scientific Instruments) using ultrahigh purity gases. Total adsorption was calculated using NIST Thermochemical Properties of CH4 and the pore volume of each material, as determined via 77 K N2 adsorption experiments.

Density Measurement
High-pressure methane gravimetric uptake was converted to volumetric uptake using three densities: the crystallographic density of the CC3b cage, the tap density of the activated cage, and a solvent-compacted density. The tap density of the activated cage was obtained by filling a tared, graduated glass tube with cage and gently tapping it until it was sufficiently settled. Density was calculated using the mass of the cage in the tube and the volume measured by from the graduation marks. Solvent-compacted density was calculated by adding DCM to the top of the tap density sample and letting it evaporate under flowing nitrogen. After complete activation of the sample using a Micromeritics Smart VacPrep, the volume was again determined from the graduation marks, yielding the density.

Powder X-ray Diffraction Data
Powder X-ray diffraction patterns were taken at room temperature on a Rigaku MiniFlex powder diffractometer. Figure S1. Powder X-ray diffraction patterns for the as-synthesized and activated cage as compared to the simulated pattern of the CC3b polymorph.   Thermogravimetric Analysis Thermogravimetric analysis (TGA) and TGA-combustion were performed using a TA Q5000 SA. For the TGA, the sample was heated to 600 °C at a ramp rate of 2 °C/min under a flow of N2. For the TGA-combustion, the sample was heated to 150 °C with a ramp rate of 10 °C/min under a flow of N2 to remove all solvent from the sample, and was subsequently cooled to 100 °C. The gas was switched from N2 to O2 and the sample was heated to 600 °C with a ramp rate of 3 °C/min to combust the sample.

Gas Adsorption Procedure
Low-pressure 77 K N2 gas adsorption measurements were obtained with a Micromeritics Tristar gas adsorption analyzer. Both flow and vacuum degassing were investigated. For flow activated samples, the Tristar tubes were purged with N2 on a Micromeritics SmartPrep system after being heated in an oven to remove water. After cooling, the tubes were sealed without exposure to air, weighed, and brought into the glove box. The cage was loaded into the tube, sealed, and degassed at room temperature overnight on the SmartPrep before measuring. For the vacuum activated samples, the Tristar tubes were evacuated on a Smart VacPrep system after being heated in an oven to remove water. After cooling and evacuating the air, the sealed tube was backfilled with N2 and weighed. The loading and activation procedure was the same as for the flow activated samples, using the Smart VacPrep instead of the SmartPrep. Once fully activated, the tubes were weighed to determine an accurate mass of the samples. N2 adsorption isotherm measurements were surveyed from 25 °C to 300 °C in increments of 25 °C. Once maximum surface area was achieved, new samples were then loaded and heated to their optimal temperature. Surface areas were measured via N2 isotherm in a 77 K liquid nitrogen bath and calculated using the Micromeritics software. All other isotherms were obtained using a Micromeritics 3Flex, submerging the sample in a temperature controlled bath.

Isotherm Fitting
The adsorption isotherms were fit with single-or dual-site Langmuir-Freundlich model (Eq. 1), where n is the absolute amount adsorbed in mmol/g, P is the pressure in bar, qsat,1 is the saturation capacity in mmol/g, b1 is the Langmuir parameter in bar -1 , and v1 is the Freundlich parameter for two sites, 1 and 2. The parameters used to fit the adsorption isotherms can be found in Tables S1-S9. Plots of the adsorption isotherms along with their fits are presented in Figures S7-S15.
The isosteric heats of adsorption, -Qst, were calculated through the use of the Clausius-Clapeyron equation (Eq. 2) for each gas using the single-or dual-site Langmuir-Freundlich fits for each material at 298 K, 308 K, and 318 K. P represents pressure, n is the amount adsorbed, T is the temperature, R is the universal gas constant, and C is a constant. The slope of (ln P)n as a function of 1/T was used to obtain the isosteric heats of adsorption for each gas, shown in Figure S16 and Table S10.   Table S1.  Table S2.  Figure S9. Pure component adsorption isotherm data for Argon. The black symbols refer to 298 K, red for 308 K, and blue for 318 K. The black lines are the respective Langmuir-Freundlich fits using the parameters in Table S3.   Table S4.  Table S5.  Table S6.  Table S7.  Table S8.  Table S9.