Solvent-free transformation of spray coated ZnO layers to ZIF-8 membranes

Abstract

ZIF-8 membranes were fabricated by a two-step procedure consisting of (1) a flexible and simple spray coating process to deposit uniform ZnO layers on different supports and (2) subsequent solvent-free transformation of the ZnO layers into dense ZIF-8 membranes by a vapor-solid reaction. The spray coating offered superior flexibility regarding the choice of support and yielded even and homogeneous ZnO layers with thicknesses of 5–10 μm on non-porous crude steel plates, porous α-Al2O3 and porous stainless steel discs. The texture of the ZnO layer did not depend on the texture of the underlying support. The conversion of the ZnO to ZIF-8 via vapor-solid reaction of gaseous 2-mIm and ZnO was performed without the use of a solvent and no further activation or seeding was applied. Except the spray coated ZnO, no further zinc species were added to synthesize the ZIF-8 layers. The particle size, morphology and the degree of intergrowth were tunable by adjusting the temperature of the vapor-solid reaction in a range of 25–130 °C. The variation of the reaction time for the fabrication of membranes on α-Al2O3 discs at 80 °C resulted in well-intergrown layers without visible defects. The ideal gas selectivities increased with increasing temperature and the best membrane showed a H2 permeance of 2.3·10−8 mol m−2 s−1 Pa−1 at 100 °C and ideal selectivities of H2/CO2, H2/N2 and H2/CH4 of 5.4, 7.1 and 9.5.