Amine-Functionalized Metal–Organic Frameworks\nand Covalent Organic Polymers as Potential Sorbents for Removal of\nFormaldehyde in Aqueous Phase: Experimental Versus Theoretical Study

Abstract

Porous\nmaterials have been identified as efficient sorbent media to remove\nvolatile organic compounds. To evaluate their potential as adsorbents,\nthe adsorptive removal of formaldehyde (FA) in aqueous environments\nwas investigated using four materials, two water-stable metal–organic\nframeworks (MOFs) of UiO-66 (U6) and U6-NH₂ (U6N) and two\ncovalent organic polymers (COPs) with amine-functionality, CBAP-1-EDA\n(CE) and CBAP-1-DETA (CD). U6N exhibited the highest removal capacity\nof 93% (0.56 mg g^–1) of the tested materials [e.g.,\nCE (81.1%, 0.53 mg g^–1) > CD (67.2%, 0.43 mg\ng^–1) > U6 (66.9%, 0.42 mg g^–1)], which was 2 times higher than that of the reference sorbent,\nactivated carbon (AC: 50%, 0.30 mg g^–1). The results\nof Fourier transform infrared and powder X-ray diffraction analyses\nconfirmed the interactions between FA molecules and the amine components\nof the materials (U6N, CD, and CE). According to density functional\ntheory calculations, the formation of hydrogen bonds between FA molecules\nand amine components was apparent and was further verified by FA/amine\ndistance (CD: 2.83, CE: 2.88, and U6N: 2.66 Å) along with enthalpy\nvalues (CD: −32.4, CE: −45.5, and U6N: −272 kJ\nmol^–1). In case of U6, the major interactions occurred\nin the metal–clusters (−19.3 kJ mol^–1) via electrostatic interactions (distance: 5.49 Å). Furthermore,\nthe sorption by amine-functionalized materials such as U6N is suggested\nto be dominated by hydrogen bonding which ultimately led to the formation\nof imine. If the performance of the tested materials is evaluated\nin terms of partition coefficient, U6N (1153 mg g^–1 mM^–1) is found as the outperformer in all tested\nsubjects. Regeneration of spent MOFs/COPs was also plausible in the\npresence of ethanol to maintain their structural integrity even after\n10 adsorption–desorption cycles. Overall, the selected MOFs/COPs\nwere seen to have very high removal capacity for hazardous FA molecules\nin aqueous phase.