Selective carbon dioxide sorption and heterogeneous catalysis by a new 3D Zn-MOF with nitrogen-rich 1D channels

We prepared a new C _2h-symmetric bridging ligand, 3,3'-(pyrazine-2,5-diyl)dibenzoic acid (3,3'-PDBA), through a Suzuki coupling reaction. 3,3'-PDBA contains a central pyrazine ring instead of the phenyl ring of 3,3'-terphenyldicarboxylic acid (3,3'-TPDC). Despite the geometrical similarity of the two bridging ligands, the reaction between Zn(NO₃)₂·6H₂O and 3,3'-PDBA in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) yielded a structurally different Zn-based metal-organic framework (Zn-MOF). The Zn-MOF, [Zn₂(3,3'-PDBA)₂(DABCO)_1.5]·2DMF·H₂O, had two-dimensional (2D) layers, and the interlocked 2D layers formed a porous 3D framework. Interestingly, one of the two available N atoms of DABCO remained intact. The uncoordinated N atoms of the dangling DABCO ligand and the pyrazinyl N atoms of the 3,3'-PDBA bridging ligand were fully exposed toward the 1D channels. Therefore, the 1D channels represented a highly nitrogen-rich environment. Gas sorption analysis indicated that the Zn-MOF was selective for adsorption of CO₂ at 196 K over N₂ (77 K) and H₂ (77 K). The exceptionally high zero surface coverage heat of CO₂ adsorption (Q _st = 79.5 kJ mol^-1) was attributable to the openly accessible multiple Lewis basic sites in the nitrogen-rich 1D channels. Zn-MOF also showed good Lewis base catalytic activities in three model aldol-type reactions with good recyclability due to chemically accessible 3° amine sites.