Figure 4. (a) Adsorption isotherms of Mn-bpdc marked with four adsorption positions for C4H6 at 298 K;(b) In situ PXRD experiments collected under different gas pressures corresponding to the four adsorption positions as shown in(a) , point A (0 bar), point B (0.11 bar), point C (0.17 bar), point D (1 bar).
To confirm our hypothesis, firstly, the adsorption isotherms of C4H6 at different temperatures were collected. As shown in Figure 3a , the Mn-bpdc MOF exhibits abrupt adsorption phenomenon for C4H6 at all temperatures, including 273 K, 288 K, 298 K, 308 K,318 K and 323 K. As expected, the gate-opening pressure for C4H6is found to be as low as 0.13 bar at 298 K by virtue of the presence of two C=C double bonds. In addition, the C4H6uptake at 0.20 bar is about 30 cm3g-1, 93 % of that at 1 bar (32.12 cm3 g-1), which both demonstrates that C4H6with double C=C bonds has strong ability to open the doors of Mn-bpdc MOF.46 Besides, the gate-opening pressure is observed to be increased with the increasing of adsorption temperature. In detail, at 273 K, 288 K, 308 K,318 K and 323 K, the abrupt gate-opening pressures are 0.02 bar, 0.06 bar, 0.26 bar, 0.50 bar, 0.92 bar (Figure S5 ), corresponding to the uptake of 33.57 cm3g-1, 32.81 cm3g-1, 31.40 cm3g-1, 30.51 cm3 g-1and 26.04 cm3 g-1 at 1 bar, respectively. Consisting with the observations of C2H4 and C3H6 adsorptions, it can be found that lower temperatures are more conducive to opening the door of Mn-bpdc for C4H6, which dues to the gas molecules slow down at low temperatures and are more likely to be bound by MOF, thus providing adequate gate-opening capacity per unit time.47,48 In addition, compared with the gate-opening pressures of C2H4 (nearly 1 bar) and C3H6 (cannot open the gate even at 1 bar) at 298 K, C4H6 can open the flexible door of Mn-bpdc MOF at very low pressure (0.13 bar) (Figure S5 ), which is consistent with the previous inferences. It should be noticed that although the SD-65 MOF with gate-opening effect has been reported for C4H6separation from other C4 mono-olefines and paraffins, the gate-opening pressure of SD-65 reaches up to 0.6 bar at 298 K.41 Obviously, the very low gate-opening pressure of Mn-bpdc for C4H6is a huge advantageous to achieve efficient separation of C4H6from other C4 hydrocarbons at lower partial pressure of C4H6.
Subsequently, the adsorption isotherms for n-C4H8, iso-C4H8, n-C4H10 and iso-C4H10 were measured at 273 K (Figure 3b ) and 298 K (Figure 3c ), respectively. At 273 K and 1 bar, there are almost no uptakes for n-C4H8 (1.78 cm3g-1), iso-C4H8 (1.48 cm3 g-1), n-C4H10 (0.61 cm3g-1) and iso-C4H10(0.05 cm3 g-1), but high uptake for C4H6 (34.12 cm3g-1) is retained. Additionally, the uptake of C4H6 at the gate-opening pressure of 0.02 bar is closed to the maximum adsorption capacity. The distinct difference between adsorption isotherms of C4H6 and other C4hydrocarbons means great theoretical selectivity could be achieved and this MOF has huge potential for actual separation of C4H6 over other C4hydrocarbons. Importantly, the very low uptakes of n-C4H8 (0.80 cm3g-1), iso-C4H8 (0.74 cm3 g-1), n-C4H10 (0.29 cm3g-1) and iso-C4H10(0.02 cm3g-1) can be also observed at 298 K and 1 bar. The extreme low uptakes illustrate that the Mn-bpdc MOF is very negative to capture n-C4H8, iso-C4H8, n-C4H10 and iso-C4H10. On the contrary, by virtue of much stronger polarity of C4H6, it can actively push open the molecular door of Mn-bpdc MOF, which is definitely verified by the very low gate-opening pressure (0.13 bar) and close saturation adsorption capacity of C4H6 (32.2 cm3g-1) at the low pressure. As we known, the adsorption selectivity is a critical factor to evaluate the separation performance for adsorbent materials.49 Notably, the calculated uptake selectivities of Mn-bpdc MOF for C4H6/n-C4H8and C4H6/iso-C4H8are 40.0 and 45.0 at 298 K and 1 bar, respectively, both of which are much higher than that all previous porous materials. (Mg-gallate: 1.3, 15.1;50 Ni-gallate: 2.4, 15.4;50GeFSIX-14-Cu-i: 4.7, 6.4;37 NbFSIX-2-Cu-i: 1.2, 5.7;37 GeFSIX-2-Cu-i: 1.1, 2.9;37ZJNU-30a: 1.0, 1.2;36 and SD-65: 22.3, 25.4;41 Figure 3d and Table S2 ). Those results powerfully indicate the potential utility of Mn-bpdc MOF for the separation of C4H6 from other C4 hydrocarbons under ambient conditions.
Furtherly, to explore the structure variations upon the adsorption of C4H6 and directly observe the guest-induced structural transformations, we measured the in situPXRD of Mn-bpdc under C4H6 environment with different pressures (0 bar, 0.11 bar, 0.17 bar and 1 bar) at 298 K (Figure 4b ). The four different pressures points at in situ PXRD data are corresponded to the detailed adsorption points in the adsorption curves of C4H6 at 298 K (Figure 4a). As shown in Figure 4b , compared with the PXRD pattern of Mn-bpdc under vacuum environment (point A), obvious new diffraction peak at 25.4° appears and slight peak at 24.4° begins to appear when the pressure increases to 0.11 bar (point B), which means small amount adsorption of C4H6 before isotherm obviously jumping can lead to the initial transformation of the structure of Mn-bpdc MOF and proves the sensitivity of the structure for C4H6. Subsequently, the diffraction peak at 26.5° completely disappears and the peak at 24.4° gradually increases when the pressure raises to 0.17 bar (point C), which corresponds to a large amount of C4H6 (28.4 cm3 g-1) enter the pore structure and furtherly facilitate the structural changes. Then, when the pressure reaches 1 bar and the amount adsorption of C4H6 reaches the saturated state, the diffraction peak at 25.4° disappears and the intensity of diffraction peak at 24.4° reaches its maximum value (point D). These results together reveal gradual transformation of the structure of Mn-bpdc in the adsorption process of C4H6. In addition, it is found that the strong peaks at around 11.6° under vacuum transforms into several unidentifiable weaker diffraction peaks when the pressures increased to 0.11 bar and 0.17 bar, then, furtherly splits into two strong peaks at around 11° at 1 bar, which due to the formation of metastable distorted structure of Mn-bpdc at the adsorption process of C4H6 and also verifies the huge structure transformation of Mn-bpdc induced by C4H6.41,51 Furtherly, the PXRD pattern of Mn-bpdc after C4H6desorption is highly consistent with that of initial vacuum state, which means the well reversibility of structure transformation for C4H6 adsorption (Figure S6 ).
Figure 5. Column breakthrough experiment for C4H6/n-C4H8(1/1 (v/v)) (a) ; C4H6/iso-C4H8(1/1 (v/v)) (b) , C4H6/n-C4H10(1/1 (v/v)) (c) , C4H6/iso-C4H10(1/1 (v/v)) (d) at 298 K, respectively.
To explore the actual separation performance, dynamic fixed-bed column breakthrough experiments were conducted by using binary gas mixtures of C4H6/n-C4H8(1:1 v/v), C4H6/iso-C4H8(1:1 v/v), C4H6/n-C4H10(1:1 v/v) and C4H6/iso-C4H10(1:1 v/v) at 298 K, respectively. As shown in Figure 5(a-d) , the n-C4H8, iso-C4H8, n-C4H10 and iso-C4H10 all begin to break through the bed gradually as soon as the feeding of target gas mixture, meanwhile, C4H6 keeps continuous adsorption with breakthrough retention times of 167.1 s, 171.8 s, 175.7 s, 177.4 s, respectively, which attributes to its super sensitive gate-opening effect on C4H6and unrecognized responses for n-C4H8, iso-C4H8, n-C4H10 and iso-C4H10. In consequence, the excellent dynamic separation performance toward C4H6 for all the binary C4H6/n-C4H8, C4H6/iso-C4H8, C4H6/n-C4H10and C4H6/iso-C4H10gas mixtures furtherly verifies that this MOF is a very promising soft MOF adsorbent for C4H6 purification.