Insights into the In-Situ Growth of ZIF-L Metal–Organic Framework on Ultrafiltration Membrane as Selective Layer

Metal–organic framework (MOF)-functionalized membranes have attracted significant attention for precise liquid separation by using MOF layers as the selective separation layer. The in-situ growth method is considered reliable, as it eliminates MOF aggregation and uneven distribution issues that lead to defective membranes. In this study, a zeolitic imidazolate framework (ZIF-L) MOF was in-situ synthesized using a solution-based room temperature (SBRT) method on a poly(ether sulfone) (PES) ultrafiltration (UF) membrane by leveraging the −OH and −NH2 groups of the polydopamine (PDA). This study systematically establishes the structure–property–performance relationships underlying the in-situ growth of a ZIF-L layer on UF membranes as a selective layer. The in-situ growth kinetics, MOF size, layer compactness, and thickness were systematically assessed, and their effects on the membrane performance were thoroughly investigated. The MOF growth kinetics (0 to 48 h) revealed that 24 h of growth provided a 14.70 ± 3.08 μm MOF layer with micrometer-sized defects, and 48 h of growth resulted in a ∼20 μm-thick and compact (micrometer-sized defect-free) ZIF-L MOF layer. The 24 and 48 h ZIF-L functionalized membranes exhibited water permeances of 24 and 11 LMH/Bar, respectively. Water permeance measurements at different time scales provided valuable insights into the in-situ growth characteristics of ZIF-L, revealing how adjustments in the PDA configuration influenced water transport through the membrane. The 24 and 48 h ZIF-L functionalized membranes exhibited improved rejection of methyl orange (31% and 42%) and reactive orange 16 (25% and 44%) compared to PES-Blank and PDA-coated membranes (10%). The ZIF-L functionalized membranes exhibited stable water permeance over 2 days, with signs of gradual leaching observed after 7 days of continuous filtration at 5.5 bar.