Figure
2. a) VT- PXRD patterns of Mn-bpdc MOF at different
temperatures under atmospheric environment; b) PXRD patterns of
Mn-bpdc MOF after different treatments by immersing in acetone,
dichloromethane and water, respectively; c) Adsorption
isotherms of Mn-bpdc MOF for single componentC2H4 andC2H6 at different temperatures;d) Adsorption isotherms of Mn-bpdc MOF for single componentC3H6 and
C3H8 at different temperatures.
To investigate the flexible structure of Mn-bpdc, firstly, the VT-PXRD
measurements at different temperatures under atmospheric environment
were measured, as drawn in Figure 2a . At the temperatures of
303 K and 323 K, the PXRD patterns are consistent with the simulated
ones. When the temperature raises from 373 K to 473 K, the diffraction
peak at around 11.3° slightly shifts to the high angle, while new
diffraction peak appears at 26.3° and the pristine diffraction peaks at
24.1°of Mn-bpdc at 303 K and 323 K
completely disappears, which illustrate temperature responded structural
transformation of Mn-bpdc. It is worth noting that when the temperature
drops from 473 K to 303 K, the PXRD patterns were restored to the
original state at 303 K, which indicates that the reversible structure
transformation of Mn-bpdc upon thermal stimulation. To explore the guest
molecule induced structural transformation of Mn-bpdc, we measured the
PXRD of the Mn-bpdc MOF samples by immersing in water, dichloromethane
and acetone, respectively. The PXRD pattern of Mn-bpdc under water
environment is consistent with that simulated one (Figure 2b ).
However, the PXRD patterns change obviously when the MOF immersed in
acetone or dichloromethane, and the diffraction peaks at 24.1° disappear
while new diffraction peaks appear at 26.3°, which indicates that guest
molecules can also induce the structure transformation of Mn-bpdc. In
addition, when the dichloromethane treated
Mn-bpdc
MOF was exchanged by water, the consistent PXRD pattern of water
exchanged Mn-bpdc MOF (Figure S4) with simulated ones credibly
verifies the structural reversibility of Mn-bpdc MOF. In a word, it
illustrates that both thermal stimulation and guest molecule induction
can trigger the structural transformation of the Mn-bpdc MOF.
Taking into account the peculiarity of both temperature and guest
molecular can induce structural transformation, gas molecules with
different polarity might also be able to regulate the structural
transformation of the Mn-bpdc MOF at proper temperature. To study the
stimulus-response behavior of
Mn-bpdc towards gas molecules, the single-component gas adsorption
isotherms of
C2H4and
C3H6were collected at 273 K, 288 K,
298 K and 323 K, respectively, as shown in Figure 2c . For the
C2H4 adsorption at 323K, there is almost
no observed adsorption even up to 1 bar, indicating that
C2H4 cannot open the door of Mn-bpdc at
323 K. However, when the temperature reduces, obvious adsorption begins
to be observed at 273 K, which shows typical S-typed curves and implies
structural transformation in the adsorption
process.44,45 In particular, the lower adsorption
temperatures, the lower gate-opening pressures can be achieved,
illustrating that low temperature is more conducive for
C2H4 to open the door of Mn-bpdc.
Similar results also can be observed for
C3H6 adsorption (Figure 2d ).
We
can obviously find that it fails to open the door at both 298 K and 323
K for C3H6 adsorption, but the
gate-opening effects are appeared at 273 K and 288 K, corresponding to
the gate-opening pressure points
at 0.35 bar and 0.80 bar, respectively. These results mean that the
gate-opening pressure points of adsorption can be well regulated by
different temperatures and object gas molecule. In addition, the single
component gas sorption isotherms of Mn-bpdc MOF for
C2H6 (Figure 2c ) and
C3H8 (Figure 2d ) were measured
both at 273 K and 298 K. Obviously, no matter at 273 K or 298 K, they
have no ability to open the door of Mn-bpdc MOF, which suggests that the
occurrence of gate-opening can be induced by the
C=C double bond of olefins.
Furtherly, compared the adsorption curves of
C2H4 and
C3H6 at 273 K, it should be noticed that
the gate-opening points of C2H4 (0.2
bar) is lower than that of C3H6 (0.35
bar). Similar changes of gate-opening pressure can also be observed for
C2H4 and
C3H6 adsorption at 298 K. It verifies
that C2H4 has higher capability to
induce the gate-opening behavior of Mn-bpdc than that of
C3H6, and gate-opening pressure of
Mn-bpdc will increase when the number of carbon atoms increased in
mono-olefines.
Enlightened by the above experimental results, it implies that Mn-bpdc
depending on its structural response to different types olefins can
achieve the separation of C4H6 from
other C4 at room temperature. In detail, since
C4H6 has two C=C double bonds, it could
be more easily to open the door of Mn-bpdc MOF with lower gate-opening
pressure. On the contrary, for other C4 components, the
gate-opening ability will be weaker due to the increases of carbon atoms
numbers. Since C3H6 cannot open the door
of Mn-bpdc even up to 1 bar at 298 K, we can infer that
n-C4H8 and
iso-C4H8 will fail to open the door at
298 K although they have one C=C double bond. On the other hand,
n-C4H10 and
iso-C4H10 also certainly cannot open the
gate of Mn-bpdc at 298 K owning to the absence of C=C double bond.
Hence, theoretically, the Mn-MOF can efficiently separate
C4H6 from other C4hydrocarbons by recognizing C4H6 and
rejecting other C4 hydrocarbons to enter the framework
of Mn-bpdc MOF at room temperature.