Engineering of Stable Anionic/Neutral MOFs with Zinc-Adeninate Building Units for Efficient C₂H₂/CO₂ Separation

Using adenine and metal ions to form secondary building units (SBUs), further connected by a highly symmetrical multicarboxylic linker to construct an amino-modified porous framework with high porosity, is an effective strategy. By regulating the deprotonation and hydrolysis capacity of the synthesized solvent, it is possible to obtain different charged frameworks. In this work, two stable anionic/neutral MOFs, (Et₂NH₂)[Zn₃(TCPE)(adenine)₂CH₃COO]·DEF·3H₂O (1) and [Zn_3.5(adenine)(TCPE)_1.5(DMA)(H₂O)_0.5]·2DMA·2H₂O (2), have been synthesized based on zinc-adeninate building units and symmetric tetrakis(4-carboxyphenyl)ethylene (H₄TCPE) in N,N-diethylformamide (DEF) and N,N-dimethylacetamide (DMA) reaction systems, respectively. 1 is an anionic framework based on 1D rod zinc-adeninate SBU, containing 1D rectangular (14.3 × 6.3 Ų) and square (14.3 × 14.3 Ų) channels. While 2 is a neutral framework built from isolated zinc-adeninate SBU, it contains hexagonal cages with a dimension of 5.5 Å in the structure. Both of them have high porosity (61.6% for 1 and 46.3% for 2) and high stability in a wide range of pH. 1 and 2 show high C₂H₂ adsorption capacity at 298 K (48.1 and 70.1 cm³ g^-1, respectively) and selective capacity for C₂H₂/CO₂ mixtures, which was confirmed by the breakthrough experiments. Furthermore, the interaction between the frameworks and gas molecules has also been explained by theoretical calculation. This work provides a good example of the design and regulation of porous structures for adsorption and separation functions.