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

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-H3PMoV8MoVI4O40Zn4(BTC)(IM)]·IM (1, TBA+ = tetrabutylammonium ion, H3BTC = 1,3,5-benzenetricarboxylic acid, IM = imidazole) and [TBA]2[ε-H4PMoV8MoVI4O40Zn4(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.