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.