Releasing the Guest Molecules in 2D Polyoxometalate-Based Metal-Organic Frameworks for Enhancing Proton Conductivity...

Crystalline 2D polyoxometalate-based metal-organic frameworks (PMOFs) provide platforms for discussing structure-property relationships. However, properties regulated by releasing guest molecules in 2D PMOFs have rarely been reported. Herein, we fabricated two 2D PMOFs, [TBA]₃[ε-H₃PMo^V₈Mo^VI₄O₄₀Zn₄(BTC)(IM)]·IM (1, TBA⁺ = tetrabutylammonium ion, H₃BTC = 1,3,5-benzenetricarboxylic acid, IM = imidazole) and [TBA]₂[ε-H₄PMo^V₈Mo^VI₄O₄₀Zn₄(BTC)(BPE)] (2, BPE = 1,2-bis(4-pyridyl) ethylene), where the interlayer regions were occupied by large-sized TBA⁺ guests and dangled IM/BPE ligands. Through ion exchange, the TBA⁺ cations in 1 and 2 were partly replaced by Li⁺ or Na⁺. Noticeably, Li⁺@1 and Li⁺@2 exhibit the proton conductivity of 4.19 × 10^-3 and 6.12 × 10^-4 S cm^-1 at 50 °C and 95% RH, which are significantly higher than those of 1 and 2 with 7 and 4 orders of magnitude, respectively. In addition, Li⁺@1 and Li⁺@2 display outstanding iodine adsorption performance with adsorption capacities of 0.677 and 0.637 g g^-1, respectively. Mechanism studies reveal that the high iodine adsorption ability of Li⁺@1 and Li⁺@2 stems from their multiple-site (O, N, and Zn atoms and benzene rings) interactions with iodine. Meanwhile, the interaction between the surface O atom of polyoxometalate (POM) and iodine was explored by density functional theory (DFT) calculations.