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

doi:10.3724/SP.J.1123.2024.05024

Abstract

Abstract:

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.

Key words: multivariate strategy, tricomponent covalent organic framework, magnetic solid-phase extraction, post synthesis modification, fluorobenzoic acids, high performance liquid chromatography

CLC Number:

O658

WANG Yuelin, LIN Qian, GAO Jingnan, SHEN Jiwei, WEI Yinmao, WANG Chaozhan. Preparation of an amino/pentafluorophenyl dual-functional magnetic tricomponent covalent organic framework and its application in fluorobenzoic-acids adsorption[J]. Chinese Journal of Chromatography, 2024, 42(12): 1105-1116.

Figures/Tables 11

Fig. 1 Schematic of synthesis of MTV strategy-derived COF-PFBx-NH2(1-x) MTV: multivariate; DHB: 3,3-dihydroxybenzidine; DNB: 3,3-dinitrobenzidine; Tp: 2,4,6-triformylphloroglucinol; PFB: pentafluorobenzoyl chloride; COF: covalent organic framework. x=0, 0.25, 0.5, 0.75, 1, mole fraction of DHB.

Table 1 Composition of simulated formation water

Ingredient	C/(mg/L)	Ingredient	C/(mg/L)
NaCl	36.85	SrCl₂·6H₂O	0.4
KCl	0.63	NaHCO₃	0.2
CaCl₂·2H₂O	3.8	Na₂SO₄	0.05
MgCl₂·6H₂O	2.55	Crude oil	10^*
BaCl₂·2H₂O	0.1

Fig. 2 Adsorption rates of the four FBAs by COF-PFBx-NH2(1-x) (n=3) FBAs: fluorinated benzoic acids; 4-FBA: 4-fluorobenzoic acid; 2,3,4-TFBA: 2,3,4-trifluorobenzoic acid; 2,3,4,5-Tetra-FBA: 2,3,4,5-tetrafluorobenzoic acid; 3,5-BTFMA: 3,5-bis (trifluoromethyl) benzoic acid.

Fig. 3 SEM and TEM images of Fe3O4@SiO2 and Fe3O4@COF-PFB0.5-NH2(0.5) a. SEM image of Fe3O4@SiO2; b. TEM image of Fe3O4@SiO2; c. SEM image of Fe3O4@COF-PFB0.5-NH2(0.5); d. TEM image of Fe3O4@COF-PFB0.5-NH2(0.5).

Fig. 4 (a) XPS full spectra of different nanoparticles; (b) N 1s spectrum and (c) F 1s spectrum of Fe3O4@COF-PFB0.5-NH2(0.5) 1. Fe3O4@SiO2; 2. Fe3O4@COF; 3. Fe3O4@COF-PFB0.5; 4. Fe3O4@COF-PFB0.5-NH2(0.5).

Table 2 Elemental contents of various magnetic nanoparticles

Material	Elemental contents/%
Material	C 1s	O 1s	Si 2p	Fe 2p	N 1s	F 1s
Fe₃O₄@SiO₂	59.7	25.3	13.5	1.41	-	-
Fe₃O₄@COF	87.4	8.06	2.47	0.37	1.68	-
Fe₃O₄@COF-PFB_0.5	86.0	7.43	2.89	0.28	2.05	1.33
Fe₃O₄@COF-PFB_0.5-NH_2(0.5)	86.8	7.63	2.40	0.19	2.51	0.47

Fig. 5 (a) FT-IR, (b)XRD and (c) N2 adsorption -desorption analysis of the materials 1. Fe3O4@SiO2; 2. Fe3O4@COF; 3. Fe3O4@COF-PFB0.5; 4. Fe3O4@COF-PFB0.5-NH2(0.5)

Fig. 6 Effects of (a) salt concentration, (b) adsorption time and (c) eluent type on adsorption of FBAs (n=3)

Fig. 7 (a) Synthesis reproducibility and (b) regenera-tion of the adsorbents (n=3)

Fig. 8 Comparison of adsorption efficiency of the adsorbents on four groups Group 1: 2,3,4-TFBA and 2,3,4-trifluorobenzaldehyde (2,3,4-TRA); Group 2: 2,3,4-TFBA and 2,3,4-trifluorobenzenamine (2,3,4-TRH); Group 3: 2,3,4-TFBA and p-toluic acid (p-TOA); Group 4: 2,3,4-TFBA and 4-ethylphenol (4-EPH).

Table 3 Adsorption effects of FBAs in pure water and simulated formation water on Fe3O4@COF-PFB0.5-NH2(0.5) (n=3)

Analyte	Pure water		Simulated formation water
Analyte	Adsorption rate/%	RSD/ %	Adsorption rate/%	RSD/ %
4-FBA	84.5	0.4	85.7	3.1
2,3,4-TFBA	94.2	4.9	86.5	2.1
2,3,4,5-Tetra-FBA	97.4	4.1	94.9	0.5
3,5-BTFMA	85.4	3.6	82.4	0.3

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