Zeolitic Imidazolate Framework-67 Based Membranes For Gas Separation

As emerging nanocrystalline porous materials,zeolite imidazole frameworks（ZIFs）have been widely used in storage,separation and purification of various chemical gases and medicines,gas sensing,catalysis and many other fields due to their advantages including high specific surface area,adjustable pore structure and easy modification.ZIF-67,a typical Co-based ZIF material,have exhibited promising application potentials in membrane separation,especially in gas separation fields because of its high chemical and good thermal stability,suitable pore size and functionalized imidazole ligands.However,it is difficult to prepare a continuous,dense,and high-quality ZIF-67 crystalline membrane for ZIF-67 due to its low heterogeneous nucleation density and poor binding force with inorganic substrates.Meanwhile,the easy agglomeration between crystals and the trade-off effect between permeability and selectivity also make it face great challenges in preparation of mixed matrix membranes（MMMs）.In this paper,continuous and dense ZIF-67 crystalline membranes were prepared by using“Inorganic Surface Modification（ISM）”strategy,and the separation performance for H₂/CO₂ was improved by ammoniation modification,Pebax/ZIF-L@67 MMMs were further prepared using ZIF-L@67 composite nanosheets synthesized by heteroepitaxial growth strategy as fillers,and the gas separation performance for CO₂/CH₄ was studied as follows:（1）Dense and well-intergrown ZIF-67 membranes with a thickness of ca.3μm was synthesized using α-Al₂O₃ supported cobalt carbonate hydroxide nano-layer as the inorganic surface modifier.The formation and conversion process of cobalt nanolayers（Co-NL）were studied in a time-dependent experiment system,finally,the cobalt nanosheets layer（Co-NSL）with smooth,uniform and high porosity were prepared without any modification of the substrate.In the process of self-conversion to ZIF-67membrane,Co-NSL provides the nucleation center and active site for the formation of ZIF-67 crystal,and also provides the metal cobalt sources for the subsequent membrane formation.In addition,the good binding force between the inorganic cobalt nanolayer and the substrate greatly improves the stability of the ZIF-67 membrane.More importantly,the Co-NSL can be completely dissolved after the reaction,which reduces the overall membrane thickness and is conducive to the improvement of permeability.（2）Due to the strong adsorption between amine groups and CO₂ gas molecules,the ZIF-67 membrane synthesized in the previous chapter was modified with ethylenediamine（EDA）and the separation performance of H₂/CO₂ was tested.Amine modification greatly enhanced the adsorption capacity of the membrane for CO₂.The single and mixed gas permeability tests showed that the modified ZIF-67 membrane improved the separation efficiency of H₂/CO₂ by 80.3%and 70.1%,respectively.（3）Using ZIF-L as a precursor,ZIF-L@67 composite nanosheet with a two-dimensional structure was synthesized by a heterogeneous epitaxial growth strategy,which was used as a filler to blended with Pebax,and enventually prepared a Pebax/ZIF-L@67 mixed matrix membrane.ZIF-L@67 with a core-shell structure can perfectly combine the tortuous and complex interlayer gap structure formed by large-size ZIF-L@67 in Pebax with the molecular sieve channel of the inner core structure（ZIF-L）and the CO₂ adsorption effect of the outer shell structure（ZIF-67）.It greatly promotes the transmission of CO₂,and at the same time improves the separation effect of CO₂/CH₄.Finally,the permeability of CO₂ and the separation factor of CO₂/CH₄ are increased by 48.1%and 33.6%,respectively.