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