Stabilizing defects in metal–organic frameworks: pendant Lewis basic sites as capping agents in UiO-66-type MOFs toward highly stable and defective porous materials

Abstract

An isostructural series of zirconium-based metal–organic frameworks (ZrMOFs) with UiO-66-type structure (including three previously unreported materials) have been successfully synthesized from combination of mono- and di-substituted terephthalic acids (H₂BDC-R, -R = -2OH: 2,5-dihydroxyterephthalic acid; -2SH: 2,5-dimercaptoterephthalic acid; -SMe: 2-methylsulfanyl-terephthalic acid; -2SEtSMe: 2,5-bis[[2-(methylthio)ethyl]thio]-terephthalic acid; -2OEtOMe: 2,5-bis[[2-(methyloxy)ethyl]oxy]terephthalic acid) and ZrCl₄ to produce the MOFs termed UiO-66-R. UiO-66-2SEtSMe is found to be highly defective, with approximately half the expected number of ligands missing (as compared to nondefective structure), and exhibiting mesoporosity. Thioether functionalities are shown to stabilize UiO-66-type MOFs in acidic (pH = 2) and basic (pH = 12) aqueous environments. In preparation of UiO-66-type MOFs with sterically demanding pendant groups (such as in -2SEtSMe and -2OEtOMe), pendant Lewis basic sites serve to stabilize the formation of highly defective MOFs, whereas removal of such pendant Lewis basic sites results in formation of amorphous material. UiO-66-2SH and -2SEtSMe both effectively adsorb Ag⁺ and Hg²⁺ from acidic aqueous solutions (up to >99% removal, K_d > 10⁴), whereas UiO-66-SMe shows potential to separate Ag⁺ from Hg²⁺ in acidic aqueous solutions.