From surface-assisted synthesis of porous aromatic bulk sheets to readily assembled membranes for nanofiltration

To address the challenges of global water scarcity and high energy consumption for water purification, nanofiltration by porous membranes provides a promising solution. The demand for precise pore modulation and enhanced stability in membranes renders porous organic frameworks (POFs) promising alternatives to polymeric membranes, as their intrinsic porosity can help overcome the typical selectivity-permeability trade-off. A key current goal is to synthesise POF membranes at size scales suitable for practical applications. Herein, a cost-effective and facile synthesis method is proposed to synthesize membranes comprising porous aromatic frameworks (PAFs), which are a class of porous frameworks with carbon-carbon-linked aromatic building units. Specifically, bulk PAF sheets are prepared on glass substrates by establishing noncovalent interactions between substrate hydroxyl groups and functional PAFs. A large number of PAF bulk sheets are directly synthesized on common substrates such as glass slides and silica wafers. These bulk sheets are readily assembled into membranes by spin coating. The obtained PAF membranes exhibit unprecedented nanofiltration performance, with water permeability up to 700 L m^-2 h^-1 MPa^-1 and rejection above 99% for the organic dye Congo Red. This surface-assisted synthesis and assembly of PAF membranes represent a step forward in the fabrication of porous organic separating membranes.