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.