MM/P PhD defense Machiel van Essen

Last Friday, May 28, 2021, Machiel van Essen successfully defended his PhD thesis entitled ‘Design Aspects of Zeolitic Imidazolate Framework based Mixed Matrix Membranes for Gas Separation Applications’. In this work Machiel studied how design aspects zeolitic imidazolate frameworks (ZIFs), such as distribution, morphology and additive‑matrix interface, affected the gas separation performance of mixed matrix membranes (MMMs) with the goal to provide a knowledge framework for a sophisticated design of these materials.

In detail, Machiels’ thesis focused on structure‑property relationships of ZIF‑based MMM design aspects and the corresponding gas separation performance. Properties investigated were (1) the ZIF particle distribution, (2) the ZIF particle morphology, (3) the ZIF pore dimensions and functionality and (4) the ZIF‑matrix interfacial compatibility. Typically, the membranes have a thickness of a human hair (50 µm) and the ZIF additives in the membranes have submicron sized dimensions. Investigation of the first design aspect showed that controlling the dispersion of ZIFs in MMMs positively enhanced the membrane permeability by approximately 20%, as alignment of the ZIF particles in the direction of gas permeation enhanced the gas diffusivity. Regarding the particle morphology, ZIFs were either fabricated as rods or as sheets. For rod‑like particles the length of the permeation pathway through the ZIF increases with increasing the rod length and for sheets the tortuosity through the MMM is increased for gases that cannot enter the sheet‑like particles. MMMs containing the rod‑like particles showed that the length of the rods did not influence the separation performance. On the contrary, the separation performance of the MMMs containing the ZIF sheets resulted in the following two conclusions: the sheet‑like ZIF morphology can effectively enhance the MMM separation performance, but the ZIF should solely be accessible for one gas species (increasing the tortuosity of the retained species) and the ZIF‑matrix interface has to be defect free, such that the gas to be retained does not experience an enhanced diffusivity along the interfaces. The influence of the ZIF pore dimensions and functionality on the MMM separation performance was assessed by incorporating ZIFs that have the same network structures but have different pore dimensions and functionalities, where increasing the ZIF pore size (making them more accessible for gases) goes simultaneously with lowering the affinity of the ZIFs with gases. For two ZIF series, each consisting of three different ZIFs types, it was consistently found that bigger ZIF pore dimensions are preferred over pore interiors that have a high affinity with specific gases. This was the case because the biggest pore dimensions facilitated gas transport the most as they effectively increased the MMM solubility and diffusivity. Last, the ZIF‑matrix interfacial compatibility was tuned by adding a polymer to the matrix that contained moieties that were chemically similar to the organic moieties of ZIFs. Incorporation of this polymer improved the ZIF‑matrix interfacial compatibility and resulted in increased separation performances of the MMM, by means of increased selectivities. This enhanced interfacial compatibility was especially expressed by the difference in diffusivities between MMMs that either contained or did not contain the compatibilizing polymer: compatibilization resulted in lowered diffusivities because of lower amounts of interfacial defects.

For more information please contact Prof. dr. ir. Kitty Nijmeijer (d.c.nijmeijer@tue.nl) or Dr. ing. Zandrie Borneman (z.borneman@tue.nl).