MOF based MMMs with enhanced selectivity due to hindered linker distortion

In this paper the separation of the binary mixture H2/CH4 on (i) neat supported ZIF (zeolitic imidazolate framework) membranes of types ZIF-8 and ZIF-90, (ii) a neat supported Matrimid polymer membrane, and (iii) Mixed Matrix Membranes (MMMs) with 10, 20 and 30 vol-% ZIF-8 and ZIF-90 nanocrystals in Matrimid is compared. A high separation performance was found surprisingly for the MMMs and it was concluded that the polymer matrix prevents the framework flexibility of the embedded Metal Organic Framework (MOF) particles, thus increasing the selectivity of MMMs with particle loadings ≤20 vol-%. Higher loadings, however, lead to agglomerations and segregations, which result in MMMs with lower selectivities. To approve the idea of the hindered linker distortion caused by the surrounding polymer, we coated the top of neat supported ZIF-8 and neat ZIF-90 membranes with an additional Matrimid polymer layer (dual-layer membrane). Indeed, the polymer coating also caused a suppression of the linker distortion of the ZIF layer in contact with the polymer which results in an enhanced H2/CH4 selectivity.