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.