Preparation and Characterization of Nanoporous Membranes with Optimized Pore Structure for the Separation of Practice-Relevant Binary Gas Mixtures

Abstract

The present thesis is dedicated to the preparation and characterization of a series of supported nanoporous membranes, featuring various materials (metal-organic frameworks (MOFs), zeolites and polymers) and designs (single-component, multilayer and mixed matrix membranes (MMMs)), as well as the subsequent evaluation of their gas separation capabilities (Wicke-Kallenbach), using the binary gas mixtures propylene/propane and H2/CO2 as model systems.

In the case of MOF-74, the right orientation of its 1D channels (perpendicular to the corundum support surface in the best case) inside the dense layers is of utter importance and was eventually achieved to some extent, for the first time ever, by further developing known synthesis procedures. Unfortunately, all resulting Mg-MOF-74 membranes were only able to separate H2 and CO2 due to a previously unknown adsorption-founded blockage/self-hindrance in the case of propylene/propane. Neat zeolite type X (NaX) membranes, on the other hand, oftentimes suffer from severe structural issues which could be avoided by pre-synthetically modifying the supports with either PDA (polydopamine) or APTES (3-amino-propyltriethoxysilane), thus simultaneously improving their separation efficiency for both gas mixtures.

In order to investigate the possible benefits of using multilayered instead of monolayered systems for separating hydrogen from carbon dioxide, some of the PDA/NaX membranes were subsequently coated with a Matrimid top layer to create a novel kind of sandwich membrane. The same polyimide was also used to fabricate MMMs with good H2/CO2 separation capabilities, where it serves as flexible and protective matrix for the embedded separation-active zeolite X particles (inorganic filler phase).

Furthermore, it could be shown that using post-synthetic pore optimizations like ion-exchanged faujasite particles (PbX, CuX, NiX and CoX) instead of NaX in the Matrimid-based MMMs as well as modifying the open Mg sites inside MOF-74’s 1D channels with amine groups results in remarkably enhanced hydrogen selectivity due to stronger internal CO2-ion/amine interactions. By studying large Co-MOF-74 crystals and their gas uptake behavior before and after the contact with humid air (via infrared microscopy) it was eventually found that even a very short exposure to humidity causes a surface pore blockage (phase transition), completely preventing the adsorption of even small guest molecules, which could be subsequently annealed in a methanol atmosphere.

The thesis includes six publications (three with me as lead author), published in subject-specific, nationally and internationally renowned journals, which are reprinted and arranged in logical, rather than a chronological order.