Solvent-controlled synthesis of ferrocene-based aluminum metal–organic frameworks for high-efficiency proton conduction

Developing metal-organic frameworks (MOFs) that combine high stability with efficient proton transport remains an important challenge. Here, two ferrocene-based Al-MOFs, Al-CAU-1-FcF ( 1 ) and Al-MIL-68-FcF ( 2 ), were synthesized solvothermally from Al 3+ and 2-ferrocenyl terephthalic acid (FcF). Their structures were assigned by comparison with related framework types, and their stability and proton-conduction properties were systematically evaluated. Both MOFs showed good acid-base stability and remained structurally intact below 200 °C, consistent with the strong Al-O bonds and the presence of ferrocene groups. Although the same metal salt and linker were used, 1 was assigned a cage-type structure containing [Al 8 (OH) 4 (OCH 3 ) 8 ] 12+ clusters, whereas 2 was assigned a MIL-68 -type structure constructed from one-dimensional chains of AlO 4 (OH) 2 octahedra. The different framework types were obtained by changing the reaction solvent from methanol for 1 to isopropanol for 2 . At 100 °C and 98% relative humidity (RH), 1 reached a proton conductivity of 5.94 × 10 −3 S cm −1 , compared with 1.09 × 10 −3 S cm −1 for 2 . The higher conductivity of 1 is associated with its larger water uptake and the dense hydroxyl sites of the Al 8 cluster, which favor the formation of interconnected H-bonding networks. Arrhenius analysis indicated temperature-dependent transport behavior in 1 and predominantly Grotthuss-type transport in 2 under high humidity. This study demonstrates that solvent-induced structural regulation and ferrocene group incorporation are effective strategies for designing high-performance proton-conductive MOFs, offering valuable insights for the development of crystalline proton-conducting materials.