Influence of sterically non-hindering methyl groups on adsorption properties of two classical zinc and copper MOF types

摘要

The metal-organic frameworks (three-dimensional porous coordination polymers) [Zn40(Me4BPDC)3] × 9 DMF, 2 · 9 DMF and [Cu2(Me4BPDC)2] × 9 DMF, 3 · 9 DMF are representatives of the classical Zn-IRMOF series and Cu paddle-wheel complexes with H2Me4BPDC = 2,2',6,6'-tetrarnethyl-4,4'-biphenyldicarboxylic acid, 1. The dicarboxylate linker of 1 is a representative of the non-planar biphenyl ligand family, known as an efficient scaffold for chiral molecules. There is a 90° twist angle between the phenyl rings in 1, dictated by the methyl groups, which leads to assembly of doubly interpenetrated pcu-a (in 2) and nbo-a (in 3) nets under low temperature solvothermal conditions in dimethylformamide (DMF). Activation by degassing (to yield 2), exchange with methanol or tetrahydrofuran and subsequent evacuation at elevated temperatures (to yield 3~I) gave materials with BET surface areas of 1735 m~2/g (2) and 1041 m~2/g (3~I). Adsorbed quantities of H2 were 1.26 wt% (2) and 1.02 wt% (3~I) (77 K, 1 bar), CO2 30.8 cm~3/g (2) and 50 cm~3/g (3~I) (273 K, 1 bar) and CH4 12.9 cm~3/g (2) and 11.4 cm~3/g (3~I) (273 K, 1 bar). The H2 and CO2 sorption values for 2 are similar to those of MOF-5 (IRMOF-1) with its almost doubled BET surface area. An increase is found concerning the adsorbed amounts of N2, H2, and CO2 for 3~I compared to related doubly interpenetrated nbo-a-type MOF-601, MOF-602, MOF-603 ([Cu2L2] with L = 2,2'-R2-4,4'-biphenyldicarboxylate, R=CN, Me, I, respectively).