Insights into Paraben Adsorption by Metal–Organic Frameworks for Analytical Applications

Abstract

Metal-organic frameworks (MOFs) are attractive materials used as sorbents in analytical microextraction applications for contaminants of emerging concern (CECs) from environmental liquid matrices. The demanding specs for a sorbent in the analytical application can be comprehensively studied by considering the interactions of the target analytes with the frameworks by the use of single-crystal X-ray diffraction, computational analysis, and adsorption studies, including the kinetic ones. The current study intends a better understanding of the interactions of target CECs (particularly, propylparaben (PPB) as a model) and three Zn-based layered pillared MOFs: <b>CIM-81</b> [Zn₂(tz)₂(bdc)] (Htz = 1,2,4-triazole and H₂bdc = 1,4-benzenedicarboxylic acid) and their amino derivatives [Zn₂(NH₂-tz)₂(bdc)] <b>CIM-82</b> and [Zn₂(tz)₂(NH₂-bdc)] <b>CIM-83</b> (NH₂-Htz = 3-amino-1,2,4-triazole and NH₂-H₂bdc = 2-amino-1,4-benzenedicarboxylic acid). The crystal structures of the two solvate compounds (<b>dma@CIM-81</b> (dma = dimethylacetamide) and <b>acetone@CIM-81</b>) were solved by single-crystal X-ray diffraction to determine the points of interaction between the framework and the guest molecules. They also served as a starting point for the computational modeling of the <b>PPB@CIM-81</b> compound, showing that up to two PPB molecules can be hosted in one of the pores, while only one can be trapped in the second pore type, leading to a maximum theoretical capacity of 291.9 mg g^-1. This value is close to the value obtained by the adsorption isotherm experiment for <b>CIM-81</b> (283 mg g^-1). This value is, by far, higher than those previously reported for other materials for the removal of PPB from water, and also higher than the experimental values obtained for <b>CIM-82</b> (54 mg g^-1) and <b>CIM-83</b> (153 mg g^-1). The kinetics of adsorption is not very fast, with uptake of about 40% in 3 h, although a 70% release in methanol is achieved in 1 h. In addition, a further comparison of performance in analytical microextraction (requiring only 10 mg of <b>CIM-81</b>) was carried out together with chromatographic analysis to support all insights attained, with the method being able to monitor CECs as low as μg L^-1 levels in complex environmental water samples, thus performing successfully for water monitoring even in multicomponent scenarios.