Improving Ethane/Ethylene Separation Performance under Humid Conditions by Spatially Modified Zeolitic Imidazolate Frameworks

Gas separation performances are usually degraded under humid conditions for many crystalline porous materials because of the lack of water stability and/or the competition of water vapor toward the interaction sites (e.g., open metal sites). Zeolitic imidazolate frameworks (ZIFs) are suitable candidates for practical applications in gas separation because of their excellent physical/chemical stabilities. However, the limitation of substituent positions in common ZIFs has prevented extensive pore engineering to improve their separation performance. In a type of gyroidal ZIFs with gie topology, the Schiff base moiety provides additional substituent positions, making it possible to modify the spatial arrangement of hydrophobic methyl groups. Herein, a new gyroidal ZIF, ZnBAIm (H2BAIm = 1,2-bis­(1-(1H-imidazol-4-yl)­ethylidene)­hydrazine), is designed, synthesized, and characterized. The spatially modified ZnBAIm exhibits improved thermal/chemical/mechanical stabilities compared to ZnBIm (H2BIm = 1,2-bis­((5H-imidazol-4-yl)­methylene)­hydrazine). ZnBAIm can remain intact up to about 480 °C in a N2 atmosphere and tolerate harsh treatments (e.g., 5 M NaOH aqueous solution at room temperature for 24 h and 190 MPa high pressure in the presence of water). Moreover, the modified pore and window sizes have improved significantly the ethane/ethylene selectivity and separation performance under humid conditions for ZnBAIm. Breakthrough experiments demonstrate efficient separation of a C2H6/C2H4 (50/50, v/v) binary gas mixture under ambient conditions; more importantly, the C2H6/C2H4 separation performance is unaffected under highly humid conditions (up to 80% RH). The separation performance is attributed to combined thermodynamic (stronger dispersion interaction with C2H6 than with C2H4) and kinetic factors (diffusion), determined by density functional theory calculations and kinetic adsorption study, respectively.诸多结晶多孔材料在潮湿环境下的气体分离性能通常会出现衰减，这源于其水稳定性不足，或是水蒸气与材料的作用位点（如开放金属位点（open metal sites））产生了竞争吸附。沸石咪唑酯骨架（Zeolitic imidazolate frameworks, ZIFs）凭借优异的物理与化学稳定性，成为气体分离实际应用的理想候选材料。然而，常规ZIFs的取代位点数量有限，阻碍了通过大规模孔道工程优化其分离性能。在一类具有gie拓扑的螺旋状ZIFs中，希夫碱（Schiff base）官能团提供了额外的取代位点，使得调控疏水甲基的空间排布成为可能。本研究设计、合成并表征了一种新型螺旋状ZIF材料ZnBAIm（配体H₂BAIm = 1,2-双(1-(1H-咪唑-4-基)亚乙基)肼）。经空间调控后的ZnBAIm，相较于ZnBIm（配体H₂BIm = 1,2-双((5H-咪唑-4-基)亚甲基)肼），展现出更优异的热稳定性、化学稳定性与机械稳定性。ZnBAIm在氮气氛围下可稳定至约480℃，且能够耐受严苛的处理条件：例如室温下于5 M氢氧化钠水溶液中浸泡24小时，以及含水环境下的190 MPa高压处理。此外，经调控的孔道与窗口尺寸，显著提升了ZnBAIm在潮湿环境下的乙烷/乙烯选择性与分离性能。穿透实验结果表明，ZnBAIm在常温环境下可高效分离50/50（体积比）的乙烷/乙烯二元混合气体；更关键的是，在高湿度环境（相对湿度（Relative Humidity, RH）最高达80%）下，其乙烷/乙烯分离性能未受影响。该分离性能源于热力学与动力学的协同作用：热力学上，ZnBAIm与乙烷的色散相互作用强于乙烯；动力学上则涉及扩散行为，上述结论分别通过密度泛函理论（density functional theory, DFT）计算与动力学吸附实验得以验证。