Preparation And Performance Of Metal-organic Framework Membranes

Metal-organic frameworks (MOFs) take the advantages of both organic andinorganic porous materials, built by metal ions and organic linker often causingfascinating structural topologies. MOFs porous materials also have the charactersof well stability and ordered structures, high suface areas and tailored properties,which make MOFs porous materials distinguish for the synthesis of excellentmembranes. In the last decade, MOF membranes are widely used in various fields,such as gas separation, catalyst, optical device, sensor and so on.MOFs possess uniform pore size and adsorption ability of gases, whichmake MOF membranes have outstanding perm-selectivity with specific gases andhave attracted the application in gas separation.Cu3(BTC)2metal-organic frameworks with three-dimensional ‘paddle-wheel’structure is constructed by copper ion and1,3,5-benzene acid and with the poresize of0.9nm. We have successfully fabricated Cu3(BTC)2membranes using asecondary growth method on pre-seeded α-Al2O3hollow ceramic fibers (HCFs)which are already modified with chitosan. A new support named HCFs with highmechanical strength and specific surface area are introduced. The300nmCu3(BTC)2seeds have been prepared in a simple approach, which are suitable forseeding without blocking the pore of HCFs support. Chitosan is used to strengthenthe binding force between HCFs support and the seeds on account of its fertileamino and hydroxyl groups. The performance of as-synthesized Cu3(BTC)2membranes are investigated by binary gas separation of H2/N2，H2/CO2andH2/CH4. The Cu3(BTC)2membranes show high H2perm-selectivity with theseparation factors are8.66,13.56and6.19separately and the permeance of H2isin the range of3.23×10-8mol·m-2·s-1·Pa-1to4.1×10-8mol·m-2·s-1·Pa-1. Besides,thermal, pressure and testing time effect are studied on H2separation. When thetemperature arrived at313K with a pressure of1atm, the selectivity ofCu3(BTC)2towards H2reaches the highest value. Cu3(BTC)2membrane alsomaintain well stability of perm-selectivity over long time gas separation.[Al4(OH)2(OCH3)4(H2N-BDC)3]·xH2O（CAU-1）is formed by aluminiumion and2-amino-1,4-benzenedicarboxylic acid (2-NH2-H2BDC) with a three- dimensional framework. There are two cages in the structure of CAU-1, which aredistorted octahedron and distorted tetrahedron with accessible window size of0.38nm. Continuous and high quality CAU-1membranes with a thinkness of4μmhave been successfully prepared by ‘dual-Al-source’ approach on pre-seeded α-Al2O3hollow ceramic fibers (HCFs). Energy-dispersive X-ray spectroscopy showsthe roles of HCFs support are dual, both as a support and suppling part of Al metalfor the synthesis of CAU-1. The performance of CAU-1membrane is examinedby the separation of two binary mixtures H2/N2, H2/CO2and H2/CH4, and theseparation factors are10.33,12.34and10.42, respectively. Taking advantages ofthe diffusion abilities and the kinetic diameters of the gas, the adsorption abilitiestowards four gases and the window size of CAU-1crystal, the mechanism ofCAU-1for gas separation is molecular sieving. The CAU-1membranes alsopossess high stability and reproducibility at different temperatures, pressures andwith long time testing.N, P or S element generally exists in the organic linker of MOFs, which hascorrosion resistance in acid liquor. Thus, MOFs with another new application is intheir infancy. We explored a new MOF membrane material with well anti-corrosion protection ability even in the alkaline.Super-hydrophobic and stable zeolitic imidazolate framework-8(ZIF-8) iscomposed of Zn ion and2-methylimidazole organic as a linker with three-dimensional structure. Even in the boil water and boil alkaline, ZIF-8crystals stillobtain good stability. Green eutectic mixture formed with choline chloride andurea is used as solvent,‘dual-Zn-source’ method is employed to prepare ZIF-8membrane with anti-corrosion performance on pre-seeded Zn slice. Owing to novapour pressure exists in choline chloride and urea eutectic mixture, themembrane could be made in an open vessel, which is benefit the industrialproduction. What’s more, the eutectic mixture have two roles, firstly, ZIF-8channel is blocked by eutectic mixture to hamper the corrosive medium destroythe Zn support by accessing into the channel of ZIF-8. Scondly, choline chlorideas a quaternary ammonium salt has the ability of corrosion protection. Theperformance of ZIF-8membrane and Zn substrate is studied by polarizationtesting in3.5wt%NaCl solution. The corrosion current density of ZIF-8 membrane decreased and the corrosion potential shifted to the positive direction,indicating that both in dynamics and thermodynamics ZIF-8membrane owns goodati-corrosion performance. Furthermore, H2evolution testing is investigated in3M KOH at25℃and45℃, respectively. The H2evolution rate of blank Znsubstrate is nearly4fold compared with ZIF-8membrane, it is implied once moreZIF-8membrane has the ability of corrosion protection.