Figure 6. a) PXRD patterns of Mn-bpdc MOF after different
treatments by immersing in water (RT) for 60 days, boiled water (100 ℃)
for 24 hours and air for 60 days, respectively; b) Adsorption
isotherms of single component C4H6 at
298 K for Mn-bpdc samples after different treatments;c) Adsorption isotherms
of single component C4H6 for 10 cycles
at 298 K. Adsorption and desorption profiles are shown in closed and
open symbols, respectively; d) Cycling breakthrough tests for
C4H6/n-C4H8(1/1 (v/v)) separation with Mn-bpdc MOF at 298 K for five times. The
column was reactivated with a helium flow at 100 ℃ after each
breakthrough test.
The
excellent stability and regeneration of absorbents are prerequisites for
the practical applications in the adsorptive separation of
C4H6 from C4hydrocarbons.52 In order to examine the water
stability of Mn-bpdc MOF, we carried out stability tests of Mn-bpdc at
water (RT) for 60 days, air for 60 days and boiled water (100 ℃) for 24
h. As shown in Figure 6a , the PXRD patterns verifies that the
framework of Mn-bpdc MOF can well maintain even in those harsh
conditions. Furtherly, the C4H6 sorption
isotherms of Mn-bpdc MOF after different stability tests are shown inFigure 6b .
The
almost identical adsorption isotherms with original simple demonstrate
that it can still preserve its intact structure without phase transition
and framework collapse after stability treatments.53In addition, thermogravimetric analysis (TGA) was performed on the
Mn-bpdc MOF to survey its thermal stability. As shown in Figure
S3 , Mn-bpdc MOF can undergo a sustaining 4 % loss of weight up to 360
°C, which dues to the loss of all free molecules. Then, the weight
remains constant until the framework begins to collapse at 480 °C. The
excellent water and thermal stability of Mn-bpdc MOF can be attributed
to the strong coordinate bonds with high coordination numbers formed
between central hard acids (Mn2+) and connected four
hard bases (CH3COO-) and two
hard-medial bases
(C5H5N).54 The
regeneration ability of Mn-bpdc MOF was firstly evaluated by cycling
single component adsorption of C4H6 at
298 K for ten times. As shown in Figure 6c , all the adsorption
curves at each cycle are almost consistent and the uptake capacities do
not have any loss. Notably, the sample only needs regenerate by vacuum
treatment at room temperature instead of degassing by high temperature
before each cycle. Furtherly, the PXRD pattern of Mn-bpdc MOF after ten
times cycles was measured (Figure S7) , which is consistent with
activated MOF. In addition, the recycling breakthrough test of Mn-bpdc
for
C4H6/n-C4H8separation is shown in Figure 6d and the breakthrough times for
C4H6 remain nearly unchanged after five
cycles. Those results comprehensively reveal that the integrality of
frameworks can be retained after repeated uses and the
Mn-bpdc MOF possesses outstanding
recyclability for C4H6 adsorption and
separation. Importantly, it is a scarce report that confirms Mn-bpdc can
be well recycled many times by easily regenerating
under vacuum with room temperature
and the cost of regeneration will be relatively low. Hence, the
excellent water-stability and cycling tests furtherly manifest that it
is a great valuable adsorbent for industrial
C4H6 purification.