Enhancing mechanical stability and uniformity of 2-D continuous ZIF-8 membranes by Zn(II)-doped polydopamine modification

Zeolitic imidazolate framework-8 (ZIF-8) with regular pore size (~ 0.34   nm) is widely attempted for gas separation membranes based on size sieving. However, the prospect is subjected to fabricating the continuous ZIF-8 membranes with high mechanical stability and uniformity. In response, an adhesive layer of Zn(II)-doped polydopamine (Zn-PDA) is pre-coated on the stainless steel nets in this work to improve ZIF-8 nucleation and growth. Zn-PDA layer shows high tolerance to ultrasonic agitation and heated methanol (system for ZIF-8 synthesis), little dissolution within 24   h, which supplies a high stable surface for ZIF-8 growth. Furthermore, the Zn(II) doped in PDA layer is opportunely serving as the even-distributed ZIF-8 nucleation starting sites, verified with the EDX and FE-SEM images for the aggregated Zn(II) clusters and the nascent ZIF-8 crystals. Therefore, an indented and hinged boundary is constructed between ZIF-8 and substrate to enhance the mechanical stability. The FE-SEM images after ultrasonic agitation show that the continuous ZIF-8 membranes prepared on Zn-PDA layers can tolerate the deformation to a certain extent. Finally, the prepared ZIF-8 membranes behave excellently with α H2/CO2 = 9.9, α H2/CO = 17.2, α H2/CH4 = 26.0, and permeation rate J H2 = 3980 GPU. This work supplied a useful approach to enhance mechanical stability and uniformity of defect-free ZIF-8 membranes.