Synergistic Effect of Functionalization and Crystallinity in Nanoporous Organic Frameworks for Effective Removal of Metal Ions from Aqueous Solution

In order to search effective adsorbents for removal of metal ions in water treatments, three nanoporous organic frameworks, TPPA, TPPA-SO₃H, and TPBD-SO₃H, with or without −SO₃H functionalities were selected and synthesized by the Schiff base reactions of 1,3,5-triformylphloroglucinol (TP) and p-phenylenediamine (PA), p-phenylenediaminesulfonic acid (PA-SO₃H), and 2,2′-benzidinedisulfonic acid (BD-SO₃H), respectively. Characterizations revealed that both TPPA and TPPA-SO₃H possess high crystallinity and porosity, while only amorphous powder with irregular pores of TPBD-SO₃H was obtained. The Brunauer–Emmett–Teller surface areas of 425.3 and 297.5 m²/g for TPPA and TPPA-SO₃H, respectively, were calculated, while only 45.1 m²/g was detected for TPBD-SO₃H. Metal ion adsorption experiments revealed that among all three framework materials, TPPA-SO₃H shows the best performances in adsorption capacities, removal efficiency, and adsorption rate to all of the tested metal ions, especially Fe³⁺, Cr³⁺, and Cd²⁺ ions. Compared with the absence of −SO₃H functionalities in TPPA and the amorphous nature of the extended framework material of TPBD-SO₃H, the high performances of TPPA-SO₃H should be ascribed to its synergistic effect of functionalization and crystallinity. This study revealed that besides the functional groups, the crystallinity of nanoporous organic frameworks is also a crucial factor in their applications.