Experimental
Materials
All chemical reagents were purchased from commercial sources (J&K Chemical, Aladdin, Macklin, et.al.) and used without further purifications. 2,3,5,6-Tetramethylterephthalic acid (TMBDC) was synthesized based on a reported procedure in the literature.[30,31]
Synthesis of Ni(TMBDC)(DABCO)0.5
The Ni(TMBDC)(DABCO)0.5 was synthesized according to the reported procedure[30]. Typically, in a 20 mL vial, Ni(NO3)2·6H2O (0.093g, 0.32 mmol), DABCO (0.018g, 0.16 mmol), TMBDC (0.067 g, 0.03 mmol) were mixed in 8 mL DMF and then 1 drop of HNO3 was added, followed by sonication for 5 min. The resulting green solution was heated at 120 ℃ for 48 h to afford the green crystals. The crystals were collected by filtration and washed in DMF to remove the excess reactants and then soaked into ethanol for 2 days. The obtained Ni(TMBDC)(DABCO)0.5 was kept in ethanol before further experiments.
Characterizations
The N2 adsorption-desorption isotherms were measured on Micrometrics ASAP 2460 at 77 K using a liquid N2 bath. The BET surface area was calculated using the Brunauer-Emmett-Teller equation in the range P/P0 = 0.05 – 0.35 and the pore size distribution was calculated by DFT method based on the N2 adsorption amount at the pressure of P/P0 = 0.95. The sample was filtered using a Buchner funnel and degassed at 120 ℃ for 6 h before each analysis.
Powder X-ray diffraction (PXRD) was performed on the Bruker AXS D8 Advance using Cu Kα (λ=1.5406 Å) radiation at 5° – 40°. TGA curve was measured on NETZSCH STA 449F3 Simultaneous Thermal Analyzer (NETZSCH, Germany) at temperature range of 30 °C to 700 °C with 10 °C/min heating rate under N2 atmosphere.
Adsorption isotherms measurement
The single component adsorption isotherms of CH4, C2H6 and C3H8 were collected on 3Flex Surface Characterization Analyzer (Micromeritics, USA) at 288 K, 298 K and 308 K. Prior to each measurement, the ethanol-soaked sample was filtered and then degassed at 120 ℃ under vacuum for 6 h.
Stability test
To investigate the stability of Ni(TMBDC)(DABCO)0.5 in varied solution or under humidity conditions, the ethanol soaked sample was filtered and dried in the air firstly and then immersed into varied solution (acetone, hexane, dichloromethane) or exposed to humid air with RH =100% and RH = 55%, respectively. After 4 days, the samples were collected by filtration and characterized by PXRD technique.
Breakthrough experiments
The breakthrough curves of gas mixture CH4/C2H6/C3H8(85:10:5, v/v/v) were measured on a self-assembly experimental apparatus (ESI). The carrier gas was N2 with flow rate of 45 mL/min and the flow rate of CH4/C2H6/C3H8mixture gas was controlled at 5 mL/min by a mass flow controller (FMA-A200, America). The composition of exit gas stream from the adsorption column was determined on-line on a gas chromatography apparatus (GC-9560, Shanghai Huaai), equipped with a 2 m long Al2O3-packed column with an FID detector. Typically, about 300 mg dry sample was packed into a stainless steel column with inner dimension of Φ3 × 275 mm. The six-way valve was used to collect and send the sample gas with constant amount into the GC automatically every 90 seconds. For the cycling tests, the sample packed column was regenerated by purging N2 flow (30 mL/min) at 100 °C for 30 min. After that the gas flow was switched to CH4/C2H6/C3H8mixture for the next cycle.
Simulation details
Adsorption properties of pure CH4, C2H6 and C3H8 on Ni(TMBDC)(DABCO)0.5 were simulated by Grand Canonical Monte Carlo (GCMC) method. All GCMC simulations were carried out by the Sorption modules in Materials Studio (Version 2017 R2). In this study, the universal force field (UFF) was adopted. The Ewald summation method was used to calculate the electrostatic energy and the atom based method was applied to describe the van der Waals interaction.