Modulated assembly of multi-variate MOFs: The role of functional defects in high capacity Hg(II) and Pb(II) removal and asymmetric catalysis

Abstract

Mixed modulator is a novel approach for the one-pot insertion of various characteristics in the context of Metal-Organic Frameworks. In this work, a series of novel chiral defective MOFs (Zn.Mands) were synthesized using various ratios of terephthalic acid and L-Mandelic acid (L-Mand) as linker and modulator, respectively. The hydrogen-bond donation, bearing the Brønsted acid site as well as chirality induction are regarded as the main reasons for choosing L-mandelic acid. In the next step, we also applied a mixed-modulator approach, to prepare Zn.Mand family as multi-variate MOFs (MTV-MOFs) with broad applicability as well as chirality. We succeeded in showing, that the using of propionic acid, mercaptopropionic acid, or lauric acid in addition to L-mandelic acid, afforded a series of efficient frameworks Zn.Mand (PRA, MPA, LA) with high hydrophobicity, hierarchical porosity, hydrogen-bond donating along with chirality. Although utilization of a single chiral modulator (L-Mand) led to the appreciable result in promoting asymmetric ring-opening reactions with high enantiomeric excess, and also in the adsorption of heavy metal, mixed-modulators approach can boost these performances. The samples were synthesized with mixed modulators, especially Zn.Mand (MPA) showed excellent performance in Pb (II) and Hg (II) adsorption (qm = 1050 mg g−1, 900 mg g−1, respectively), owing to the simultaneous presence of thiol and hydroxyl functional groups. On the other hand, Lewis acid sites (defect centers), besides the high density of Brønsted acid sites (−SH and −OH), caused Zn.Mand (MPA) promotes the asymmetric ring-opening reactions (98 % conversion, 99 % ee) without significant loss of activity, to the fifth step of consecutive cycles. In similar conditions (12 h and 60 °C), this result is higher than Zn.Mand with 48 % conversion, revealing the significance of the mixed-modulator approach. This study provides insights into the benefits of modulator-based tuning of functionality in addition to exploring an avenue to broaden the industrial application scope of MOF. Furthermore, by employing this easy-to-operate, low-cost, and efficient synthesis route, multifunctional MOFs are produced which can't be achieved through conventional approaches.