Nanofiltration with polyamide thin film composite membrane with ZIF-93/SWCNT intermediate layers on polyimide support

Among current separation processes, nanofiltration (NF), besides its easy scalability, exhibits low energy consumption, environmental impact and footprint, being widely used for water treatment and solvent recovery. The development of membranes for NF involves perfecting the skin selective layer, the sublayers and the support. In this work, a multilayer structure membrane is presented where a selective layer of polyamide (PA, thickness 40–60 nm) was interfacially polymerized on a sub-layer of MOF ZIF-93 (thickness ∼ 50 nm) grown on another sub-layer of single-walled carbon nanotubes (SWCNT, thickness ∼ 400 nm) vacuum filtrated on an asymmetric polyimide P84® support (thickness ∼ 200 μm). The membrane with structure PA/ZIF-93/SWCNT/P84 showed excellent results in water NF and methanol organic solvent NF of different dyes (with the highest values of water and methanol permeances of up to 57.6 and 84.5 L·m−2·h−1·bar−1, respectively, with rejections usually greater than 99%). By means of a wide range of characterization techniques (contact angle, AFM, XRD, ATR-FTIR and FIB-SEM) the role of every component in the membrane was elucidated. In fact, the presence of the sublayer of ZIF-93 increased the roughness and hydrophilicity as well as decreased the thickness of the PA layer. These effects are related to the fact that the sublayers subordinate the interfacial polymerization as well as influence the properties of the PA film and therefore its NF performance, even showing chlorine resistance as well as ten-day cross-flow NF stability.