Preparation of an amino/pentafluorophenyl dual-functional magnetic tricomponent covalent organic framework and its application in fluorobenzoic-acids adsorption

Fluorobenzoic acids (FBAs) are important intermediates in the synthesis of many drugs, agrochemicals, and other organic compounds, and are also used as water tracers. FBAs are environmentally friendly, unnatural, have stable properties, are poorly adsorbed in aquifers, and are used in low dosages as groundwater tracers, which are their main advantages along with non-interfering diversity; as a result, their applications scope continues to expand. While FBAs are used to trace the movement of groundwater, direct analysis is often challenging owing to their low concentrations and the complexity of the groundwater matrix. Therefore, sample pretreatment for trace and ultra-trace FBAs in complex matrices is important prior to instrumental analysis. Covalent organic frameworks (COFs) are porous crystalline polymers constructed from organic units through strong covalent bonds and are characterized by low densities, large specific surface areas, permanent porosities, excellent chemical/thermal stabilities, and ease of functionalization for required purposes.

In this study, we used a multivariate (MTV) strategy to construct a series of tricomponent COFs with different hydroxy/nitro ratios from 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde (Tp), 3,3'-dihydroxybenzidine (DHB), and 3,3'-dinitrobenzidine (DNB) as building blocks using Schiff-base reactions; these COFs are referred to as Tp-DHB_xDNB_(1-x) (x=0, 0.25, 0.5, 0.75, and 1). We then anchored pentafluorophenyl and amino groups to these COFs via post-synthetic modification; the adsorption performance of the COF-PFB_x-NH_2(1-x) samples prepared in this manner led to the selection of COF-PFB_0.5-NH_2(0.5) for its FBAs-adsorbing ability. A magnetic fluorophilic/anion-exchange/reversed-phase mixed-mode COF adsorbent (Fe₃O₄@COF-PFB_0.5-NH_2(0.5)) was prepared via an in-situ growth method using Fe₃O₄ nanoparticles as cores to facilitate separation of the material from solution. Notably, the Fe₃O₄@COF-PFB_0.5-NH_2(0.5) particles were very dispersible in water owing to their abundant amino groups and facilitated magnetic solid-liquid separation. In addition, Fe₃O₄@COF-PFB_0.5-NH_2(0.5) exhibited excellent operational stability for the magnetic solid-phase extraction (MSPE) of FBAs in aqueous samples. The structural properties of Fe₃O₄@COF-PFB_0.5-NH_2(0.5) were characterized using various techniques, including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), powder X-ray diffractometry (PXRD), Fourier-transform infrared spectroscopy, and the Brunauer-Emmett-Teller method, all of which revealed that Fe₃O₄@COF-PFB_0.5-NH_2(0.5) had been successfully prepared; it also has uniformly distributed mesopores and a specific surface area of 107 m²/g. Adsorption-mechanism studies revealed that Fe₃O₄@COF-PFB_0.5-NH_2(0.5) is highly adsorptive toward all four FBAs (4-fluorobenzoic acid (4-FBA), 2,3,4-trifluorobenzoic acid (2,3,4-TFBA), 2,3,4,5-tetrafluorobenzoic acid (2,3,4,5-tetra-FBA) and 3,5-bis(trifluoromethyl)benzoic acid (3,5-BTFMA)) through electrostatic, hydrogen bonding, F-F affinity, and π-π and hydrophobic interactions through synergy between the dual-functional fluorine and carboxyl binding sites, as well as the benzene rings in their skeletons. Isothermal FBAs-adsorption experiments involving Fe₃O₄@COF-PFB_0.5-NH_2(0.5) revealed maximum adsorption capacities of 73.5, 64.9, 38.4, and 253 mg/g for 4-FBA, 2,3,4-TFBA, 2,3,4,5-tetra-FBA and 3,5-BTFMA, respectively. Adsorption kinetics were also investigated, which revealed that FBAs-adsorption by Fe₃O₄@COF-PFB_0.5-NH_2(0.5) follows pseudo-second-order kinetics. Remarkably, no significant decrease in adsorption efficiency was observed after five adsorption-desorption cycles, highlighting good material reusability. We next optimized the factors that affect MSPE efficiency. Fe₃O₄@COF-PFB_0.5-NH_2(0.5) exhibited adsorption efficiencies of 85.7%, 86.5%, 94.9%, and 82.4% for 4-FBA, 2,3,4-TFBA, 2,3,4,5-tetra-FBA and 3,5-BTFMA from simulated underground water, respectively, under the optimal extraction conditions, which indicates that Fe₃O₄@COF-PFB_0.5-NH_2(0.5) holds significant potential for the separation and enrichment of FBAs in real samples.