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 -H 3 PMo V 8 Mo VI 4 O 40 Zn 4 (BTC)(IM)]·IM ( 1, TBA + = tetrabutylammonium ion, H 3 BTC = 1,3,5-benzenetricarboxylic acid, IM = imidazole) and [TBA] 2 [ ε -H 4 PMo V 8 Mo VI 4 O 40 Zn 4 (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.