Highly permeable and selective mixed-matrix membranes for hydrogen separation containing PAF-1†
As the potential for a hydrogen economy re-emerges globally, its distribution is a key enabling capability to master. One option is to utilise the existing reticulated natural gas infrastructure, mixing in a minor component of hydrogen to form a composition commonly known as Hythane. After distribution, should the hydrogen be utilised within a fuel cell, it must be extracted from the Hythane mixture. Membranes offer the most efficient means of achieving such a separation, due to the lack of need for a phase change to occur. Sufficient selectivity within the membrane is required due to the relatively low (10–15%) composition of hydrogen within Hythane, and the very high purities required for fuel cell operation. Polymers of Intrinsic Microporosity (PIMs) are of interest for this application due to their innately high selectivity, albeit presently short of that required. Previously, we have shown that the porous framework PAF-1 can control physical aging within related systems. Here, we reveal that the use of a highly rigid ladder polymer, TPIM-2, can deliver a H2/CH4 selectivity as high as 16 in PAF-1 nanocomposites, which could reach 62.6 after aging. Encouragingly, the selectivity increase was also accompanied by a permeability enhancement, with as much as 4886 barrer recorded for hydrogen permeability. This favourable combination of hydrogen permeability and selectivity results in the aged MMMs surpassing the 2015 upper bound for H2/CH4 separation. Mechanistic investigations revealed that PAF-1 not only provided more gas transport channels for higher membrane permeability, but also accelerated the aging process leading to significantly improved membrane selectivity.