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.