Figure 5 Simulated adsorption density distribution colored by adsorption potential of (a) CH4, (b) C2H6 and (c) C3H8 in Ni(TMBDC)(DABCO)0.5 at 1 kPa
To further understand the adsorption mechanism of CH4, C2H6 and C3H8 in Ni(TMBDC)(DABCO)0.5, the GCMC was applied to examine the adsorption density distribution of the three gases in the adsorbent. As illustrated in Figure 5, the CH4, C2H6 and C3H8 were adsorbed on two sites in the framework of Ni(TMBDC)(DABCO)0.5, which are termed as Site I and Site II. The gases adsorbed on Site I was mainly influenced by the methylene group from DABCO pillar, while those adsorbed on Site II was affected synergistically by the methyl group from TMBDC ligand and the methylene group from the adjacent DABCO pillar. That is to say, strong interaction was formed on CH4, C2H6 and C3H8 molecule owing to the presence of methyl and methylene group, and thus these hydrocarbon molecules were more preferentially adsorbed on the Site II and Site I.
In addition, it shows that the adsorption potentials of these alkanes in the Ni(TMBDC)(DABCO)0.5 follow the order: C3H8 > C2H6 > CH4, which is consistent with the order of the adsorption capacity at low pressure region. This phenomenon further confirmed that the framework of Ni(TMBDC)(DABCO)0.5 showed stronger affinity toward C3H8 than C2H6 and CH4, which was attributed to the existence of both methyl groups and methylene groups in the channel.
Figure 6 Breakthrough curves of CH4/C2H6/C3H8(85:10:5, v/v/v) ternary mixtures through Ni(TMBDC)(DABCO)0.5 packed column
To evaluate the dynamic separation performance of the sample for CH4/C2H6/C3H8mixture, breakthrough experiments at 298 K were performed, in which the stimulated industrial gas mixture of CH4/C2H6/C3H8(85:10:5, v/v/v) was applied. Figure 6 presents the breakthrough curves of ternary mixtures CH4/C2H6/C3H8(85:10:5, v/v/v) through the packed column with Ni(TMBDC)(DABCO)0.5. It demonstrates that these three gases were completely separated with the breakthrough time of 80 min for C3H8, 24 min for C2H6, and 4 min for CH4. In addition, the recyclability of Ni(TMBDC)(DABCO)0.5was also examined. Figure S7 presents the breakthrough curves of three adsorption-desorption cycles on Ni(TMBDC)(DABCO)0.5. It shows that the breakthrough curves of these ternary mixtures almost overlap, indicating excellent recycling performance of Ni(TMBDC)(DABCO)0.5. These great separation property and recyclability of the Ni(TMBDC)(DABCO)0.5 would make it a great candidate for challenging separation of CH4/C2H6/C3H8mixture or recovering low content of C2H6 and C3H8 from natural gas.