Cyclodextrins: a new and effective class of co-modulators for aqueous zirconium-MOF syntheses

Abstract

Inspired by the effectiveness of cyclodextrin (CD) auxiliary agents in the design of nanocomposite materials, we hypothesized that the same approach can be employed in the controlled assembly of zirconium-based metal–organic frameworks (Zr-MOFs) of the UiO-66-X topology, in aqueous media. Native CDs (α-CD, β-CD and γ-CD) in different proportions were initially applied as co-modulators to the reaction mixture of the Zr₆O₄(OH)₄(CH₃COO)₁₂ cluster zirconium precursor and the 2-aminoterephtalic acid linker, to assess their influence on the porous properties of the Zr-MOFs. Crystallinity and textural characteristics of the so-obtained materials were determined by means of X-ray diffraction and N₂-sorption analysis. Even at low concentrations, γ-CD was shown to have a deleterious effect on the Zr-MOFs' specific surface area and their overall N₂ sorption properties. Conversely, α-CD and β-CD aided synthesis of Zr-MOFs resulted in superior textural properties outperforming those obtained with classical hydrothermal syntheses, yielding a record surface area of S_BET = 1451 m² g⁻¹, suggesting that these co-modulators efficiently participate in the growth of UiO-66-NH₂. This effect is hereby rationalized through the different roles played by the CDs towards the reagents in the UiO-66-NH₂ reaction mixture, and a reaction mechanism is proposed.