Site-specific engineering of nitrogen centers in porous organic polymers for enhanced trace SO2 uptake

Sulfur dioxide (SO2) emissions from human activities significantly impact the ecological environment and human health. Consequently, developing effective adsorbents for SO2 removal, particularly for trace amounts, holds substantial industrial and societal importance. Herein, three types of porous organic polymers (POP-Pyr, POP-Bce, and POP-Phe) with different nitrogen adsorption sites were prepared via radical polymerization using the corresponding vinyl-functional monomers: pyridazine (v-Pyr), benzo[c]cinnoline (v-Bce), and phenanthroline (v-Phe). The precise chemical compositions and porous structures of the resulting materials were verified through multiple characterization methods, including solid-state NMR, N2 sorption measurements, FT-IR, and TEM. The SO2 adsorption capacities of these materials were investigated, revealing that POP-Pyr, POP-Bce, and POP-Phe exhibit moderate to excellent capacities of 11.7, 13.0, and 15.4 mmol g−1 at 273 K, respectively. These values are among the highest ever reported for this class of materials. At 298 K, the capacities decrease to 6.8, 7.7, and 9.7 mmol g−1, while the calculated adsorption heats (17–21 kJ mol−1) indicate that the adsorption process is primarily physical in nature. The IAST selectivities for SO2/CO2 were determined to be 153, 115, and 450 for POP-Pyr, POP-Bce, and POP-Phe, respectively, with corresponding SO2/N2 selectivities of 1278, 1466, and 2014. These results were further validated by dynamic breakthrough experiments using a SO2/CO2/N2 gas mixture (0.17/15/84.83 v%). Specifically, POP-Phe demonstrated an exceptionally long breakthrough time of 969.3 min g−1, accompanied by a saturated breakthrough capacity of 1.47 mmol g−1. Notably, the SO2/CO2 selectivity and breakthrough time of POP-Phe are comparable to those of reported materials such as MOFs, COFs, and other related sorbents, demonstrating its outstanding promise for dynamic SO2 separation from simulated flue gas.