Development of textured and high-flux gas-transporting membranes with high chemical stability

Abstract

The scope of the thesis is preparation of gas separation disk membranes with enhanced selectivity, permeability and operational stability. Membranes investigated rely on size-selective pores or solid-state ambipolar diffusion influenced by material composition and particle orientation. After a general fundamental overview on the gas separation process and relevant materials, six original research articles are presented into two parts. In the first part, porous materials zeolitic imidazolate frameworks (ZIFs) are grown on porous alumina supports for helium separation. During the solvothermal synthesis, metal ions react with imidazolate linkers and produce ZIF particles both in solvent and on supports. However, ZIFs prefer homogeneous nucleation, hindering the formation of continuous membranes. This problem was successfully overcome by filtrating a thin layer of two-dimensional materials on the support. By immersing the layer in a solution of metallic ions, it can host a high amount of metal ions and then acts as a buffer layer and provides anchor sites for heterogeneous nucleation to enable well-intergrown ZIF-67 and ZIF-62 membranes. For the latter case, the selectivity can be further improved by eliminating grain boundary of polycrystalline membranes through a thermal treatment, since ZIF-62 melts before decomposition and forms glass with preserved porosity as proved by gas adsorption and positron annihilation lifetime spectroscopy analysis. In the second part, dense ceramic membranes are developed for oxygen separation. The chemical stability against CO2 has been improved by reducing the content of alkaline-earth elements in perovskite oxides, specifically the membrane materials are high-entropy perovskites Pr1–xSrx(Cr,Mn,Fe,Co,Ni)O3–δ (x = 0 – 0.5) and dual-phase materials Ce0.9Pr0.1O2–δ – NdxSr1–xFe0.9Cu0.1O3–δ (x = 0.2, 0.5, 0.8) as well as Ce0.8M0.2O2–δ – La0.5Sr0.5Fe0.8Cu0.2O3–δ (M = La, Pr, Nd, Sm, or Gd). All membranes demonstrated considerable operational stability under pure CO2 sweep for more than 100 h, although the oxygen fluxes were lower than 1.0 mL min−1 cm−2. To obtain higher flux, the alkaline-earth-metal-free material Nd2NiO4+δ was employed, which exhibits faster oxygen migration in the crystallographic (a,b)-plane than along its c-axis. By controlling the particle orientation via a magnetic field generated by permanent magnets, a stable and high oxygen flux of 1.4 mL min−1 cm−2 was achieved for at least 120 h at 1223 K by a textured asymmetric disk membrane with 1.0 mm thickness under pure CO2 sweep.