Figure 3 CH4, C2H6 and C3H8 adsorption isotherms of Ni(TMBDC)(DABCO)0.5 at 298 K in the pressure region of (a) 0 ~100 kPa and (b) 0 ~ 20 kPa
Figure 3 presents the CH4, C2H6 and C3H8 adsorption isotherms of Ni(TMBDC)(DABCO)0.5 at 298 K. The C3H8 and C2H6 uptakes on Ni(TMBDC)(DABCO)0.5 are much higher than CH4, reaching as high as 5.54 mmol/g and 5.81 mmol/g at 100 kPa, respectively. As shown in Figure 3(b), at low pressure region of 0 ~ 15 kPa, the C3H8isotherm exhibits a steeply increasing trend in C3H8 adsorption capacity and the uptake of each gas in Ni(TMBDC)(DABCO)0.5 decreased in the order of C3H8 > C2H6 > CH4. It implies that the interaction between C3H8 and Ni(TMBDC)(DABCO)0.5 is the strongest, that of C2H6 is in the second place, and that of CH4 is the weakest, which is mainly determined by the property of CH4, C2H6and C3H8, especially molecule polarizabilities. The polarizability is considered as an important intrinsic property of a molecule that reflects the ability to generate instantaneous dipole related to Van der Waals interactions within the molecule, which dominate the interactions between the molecule and an adsorbent.[32, 40] The polarizabilities of C3H8, C2H6 and CH4 are 62.9–63.7×1025, 44.3–44.7×1025, 25.93×1025 cm-3, respectively.[16] Therefore, C3H8 and C2H6 would likely exhibited stronger interaction towards the surface of Ni(TMBDC)(DABCO)0.5. The strong interaction is also evidenced by the isosteric heat (Qst). As presented in Figure S5, the Qst values of C3H8 and C2H6 reached 59 kJ/mol and 36 kJ/mol at 0.5 kPa, respectively, while that of CH4 is 14 kJ/mol, demonstrating strong interaction of C3H8and C2H6 within Ni(TMBDC)(DABCO)0.5. It is noticed that, at the pressure region of 15 ~ 100 kPa, the uptake of C3H8 is lower than C2H6. This should be attributed that the molecular kinetic diameter of C3H8 (4.3 ~ 5.1 Å) is larger than that of C2H6 (4.4 Å), thus less C3H8 molecules could be accommodated in the limited pore volume of Ni(TMBDC)(DABCO)0.5 compared to C2H6.
It is worth to mention that the Ni(TMBDC)(DABCO)0.5adsorbed C3H8 with a gate opening behavior at three temperatures as shown in Figure S6. The gate-opening pressure (Pgo ) for C3H8 decreased from 68 kPa at 308 K to 23 kPa at 288 K. Such breathing behavior on hydrocarbon adsorption were observed on ELM-11,[33]Cu(dhbc)2(4,4’-bipy),[34]USTA-300,[23] ZIF-7[35], etc. The gate-opening behavior of Ni(TMBDC)(DABCO)0.5 is considered to be induced by the adsorption of C3H8 molecule.[34]The framework of the material had strong interaction with the adsorbed C3H8 molecule due to the methyl group and methylene group in the channel, leading to a structural transition after the first saturated adsorption capacity of C3H8 was obtained. As the temperature decreased, the thermal motion of C3H8molecule slowed down so that the C3H8molecule could be adsorbed more easily on the Ni(TMBDC)(DABCO)0.5, thereby leading to lower gate-opening pressure (Pgo ).[34]
Particularly, since the concentrations of ethane and propane are relatively low in natural gas, the adsorption ability of ethane and propane at low pressure region (0 ~ 20 kPa) is basically important for the separation performance of C1/C2/C3. For comparison, Table 1 summarizes the low-pressure adsorption capacity of some materials for C2H6 and C3H8. It is clearly visible that the C3 and C2 uptakes in Ni(TMBDC)(DABCO)0.5 at low pressure region are comparable with those of MgMOF-74 and 0.3Gly@HKUST-1, and higher than other reported materials, such as MOFs and porous carbon materials, indicating a great potential of Ni(TMBDC)(DABCO)0.5 for separating C1/C2/C3.
Table 1 Comparison of C2H6/C3H8adsorption capacity and C2H6/CH4 and C3H8/CH4 selectivities of some reported materials