Two-dimensional metal−porphyrin framework membranes for efficient molecular sieving

Two-dimensional (2D) membranes were constructed using porphyrin-based metal organic frameworks (MOFs), i.e., copper-tetrakis(4-carboxyphenyl) porphyrin (Cu-TCPP), for efficient molecular sieving of dye/salt mixture solutions. These novel membranes contained a regular Cu-TCPP nanosheet filter layer fabricated by the vacuum filtration method to retain the oriented interlayer structure of the MOF material, resulting in a robust 2D membrane. The hydrated Cu-TCPP membrane with a thickness of 300 nm displayed high retention rates for dyes (90.1% for crystal violet (CV), 93.3% for Chrome black T (CB-T), 99.7% for Methyl blue (MB) and 99.8% for Congo red (CR); CV, Mw= 407.9 g/mol; CB-T, Mw=416.4 g/mol) with diameters approximately greater than 1.3 nm. A water permeance of 840.1 L·m−2·h−1·bar−1 and a CR (Mw= 696.7 g/mol) rejection rate of 99.9% were obtained with the optimal metal−porphyrin framework membrane (small lamellar dimension nanosheets) undergoing the cross-flow mode filtration of dye/salt mixture solutions for 24 h. More importantly, due to the abundant interlayers and regular surface nanochannels, the Cu-TCPP membrane exhibited a low salt rejection rate (0.4% for NaCl) and an outstanding CR/NaCl selectivity (as high as 249.5), which was more superior compared with that of other reported two-dimensional membranes. Additionally, the nanochannels of the membrane presented strong rigidity under pressure load as high as 0.4 MPa, endowing the membrane with high tolerance to shear stress and good antifouling performance after several fouling-cleaning cycles.