Fluorinated ZIF-71 with adjusted pore size to enhance separation performance of mixed matrix membrane for recovering R32 from R410A
Due to the high Global Warming Potential (GWP) of hydrofluorocarbons (HFCs), the Kigali Amendment to the Montreal Protocol explicitly calls for the phase-out or limited use of HFCs, and the replacement of HFCs by hydrofluoroolefins (HFOs) is imminent. Therefore, it is particularly significant to develop new technology to separate valuable difluoromethane (R32, GWP = 675) from the near-azeotropic refrigerant blend R410A (GWP = 2088), because the traditional distillation is not applicable. In this work, aiming to enhance the separation performance of mixed matrix membrane (MMMs) for R410A, 5-fluorobenzimidazole was introduced into ZIF-71 by the solvent assisted linker exchange (SALE) method for the first time to reduce the pore size of fluorinated ZIF-71 by benzene rings and form hydrogen bonds between R32 with ZIF-71. A novel series of MMMs consisting of fluorinated ZIF-71 and Poly(ether-block-amide) ®1657 (Pebax®1657) polymer were prepared to separate the components of the R410A. The separation performance of the MMMs was investigated at different feed pressures and filler loading. The results showed that the introduction of 5-fluorobenzimidazole contributed to a significant increase in R32 permeability and R32/R125 selectivity of the MMMs compared to pure Pebax membrane. The 15 wt% ZIF-71-F/Pebax MMM showed the best separation performance with R32/R125 selectivity up to 26.6, four times that of pure Pebax membrane and twice that of ZIF-71/Pebax MMM. The ZIF-71-F/Pebax MMM have outstanding data on R32/R125 selectivity and no significant disadvantage in permeability, far exceeding the reported separation performance of membranes for R410A. This confirms that the application of fluorinated metal organic frameworks (MOFs) to the separation of refrigerant blends is a promising strategy.
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