Effects of Nanopore Size on the Adsorption of Sulfamerazine from Aqueous Solution by β-Ketoenamine Covalent Organic Frameworks

Covalent organic frameworks (COFs) featuring long-range ordered pore size and large specific surface area have been recognized as a fantastic platform for contaminant removal. However, the influence of nanopore size on their adsorption behavior has not been well understood. Herein, three stable β-ketoenamine COFs, i.e., TpPa-1, TpBD, and TpTph, with the same type of linkage but diverse nanopore size were successfully obtained by a salt-mediated crystallization strategy. Their adsorption behavior and interaction mechanisms toward sulfamerazine (SMR) in aqueous solution were studied by adsorption experiments and density functional theory (DFT) calculations. The adsorption kinetics of SMR over COFs followed the order TpPa-1 < TpBD < TpTph, which is consistent with the changing trend of nanopore size, suggesting that a large nanopore is beneficial in boosting mass transfer. However, the maximum adsorption capacities followed the order TpPa-1 (82.9 mg/g) < TpTph (150 mg/g) < TpBD (188 mg/g). Referring to the data of specific surface area and nanopore size of COFs, it can be inferred that the specific surface area of COFs plays a less important role, and pore characteristics might be the dominant factor affecting the adsorption behavior of SMR. Furthermore, DFT results confirmed that SMR can be loaded into the channels of TpPa-1, TpBD, and TpTph due to its small molecule size, and the adsorption interactions are dominated by C–H···π and H-bonding. The different adsorption performance of SMR on TpPa-1, TpBD, and TpTph stems from their different number of adsorption sites and binding energies. These findings demonstrated the feasibility of adjusting the adsorption capacity of COFs by regulating the nanopore sizes, providing possible solutions for tailor-made COFs with superior adsorption performance.