Fluolab【Angew.Chem.】北京大学杨四海、西安交通大学杨庆远|313 K下选择性高达11.7：具备“分子门控”效应的MOF实现乙烷/乙烯高效分离
通讯作者： Qing-Yuan Yang, Sihai Yang
文章概要
引言
在高分子工业中，每年对聚合级乙烯（C₂H₄） 的需求量超过2.1亿吨。然而，传统的乙烯/乙烷分离高度依赖高耗能的深冷蒸馏技术，约占全球能源消耗的0.3%。尽管基于吸附的分离技术被视为极具潜力的替代方案，但大多数金属有机框架（MOF）材料由于乙烯更强的四极矩和π电子效应，往往优先吸附乙烯而非乙烷，这导致从混合气中直接制备高纯乙烯的过程异常复杂。为了实现乙烯的一步法提纯，开发具有乙烷选择性的吸附剂成为当前研究的难点。本文研究团队提出了一种创新的“分子门控”策略，通过在稳固的Zn基MOF孔道中精准引入功能基团，利用动力学势垒的差异实现了极高的乙烷反向选择性。
Thermally tuneable preferential binding of C2H6 over C2H4 in ZAI-3N. (a–d) Adsorption isotherms showing temperature-induced selectivity inversion from 273 to 313 K. (e, f) Schematic energy diagrams illustrating the mechanism: dual-molecule adsorption at 273 K via π-π stacking versus selective C2H6 adsorption at 313 K through disrupted π-π stacking and retained C-H⋯π interactions.
主要实验及结论
研究人员通过溶剂热法合成了一系列同构的Zn基MOF材料，统称为ZAI系列。其中，通过在异烟酸配体的3位引入氨基修饰而成的ZAI-3N表现出了最卓越的分离性能。单晶X射线衍射和Rietveld精修分析表明，该材料在活化过程中发生了明显的结构收缩，Zn-N键缩短且孔径减小，形成了独特的受限孔道环境。在吸附测试中，ZAI-3N展现出了显著的温度响应性门控行为：在273 K时，材料对两种气体的吸附量差异较小；但当温度升高至313 K时，乙烯的吸附受到严重抑制，而乙烷依然能够触发“门控开启”进入孔道。此时，ZAI-3N的乙烷/乙烯吸附量比值达到了惊人的10.6，IAST选择性高达11.7，这一数值刷新了目前已报道的吸附剂性能基准。
Structural characterization of ZAI-3N activation. (a, b) Coordination geometry before and after activation. (c) Structural overlay showing framework alteration. (d, g) Crystal structures along c-axis. e,h) Pore dimensions change from 4.88 × 3.40 Å2 (as-synthesized) to 3.47 × 2.75 Å2 (activated). f, i) Channel cross-sections showing gate-narrowing effect. Blue represents as-synthesized state; orange represents activated state.
Gas adsorption and separation performance of ZAI materials. (a–c) C2H6 and C2H4 sorption isotherms of ZAI-3N recorded between 273 and 313 K. (d) 3D visualization of adsorption isotherms across different ZAI samples and temperatures. (e) Comparison of C2H6/C2H4 uptake ratio and IAST selectivity, showing ZAI-3N’s superior performance (uptake ratio: 10.6). (f) Influence of temperature on individual gas capacities and overall selectivity of ZAI-3N.
ZAI-3N benchmarking against ethane-selective sorbents in literature: (a) IAST selectivity plotted against C2H6 capacity and (b) C2H6/C2H4 uptake ratio as a function of C2H6 loading, (c) Bar-chart comparison of uptake ratios for all ZAI variants and leading reference materials at 100 kPa. (d, e) Time-resolved kinetic profiles illustrating the faster sorption rate of C2H6 relative to C2H4.
通过计算模拟和同步辐射X射线粉末衍射分析，研究团队揭示了这一现象的微观机制。乙烷分子中的甲基能够触发氨基基团的旋转，其活化能仅为~5 kJ/mol，从而顺利开启分子门并在孔道内形成多重C-H···N氢键接触。相比之下，乙烯的π电子与孔道环境产生的斥力使得其开启门的障碍大幅提升至 ~11 kJ/mol。这种基于旋转障碍差异的动力学门控效应，使得乙烷在较高的温度下依然能被优先捕获。在实际的动态突破实验中，ZAI-3N能够从不同比例的混合气中一步回收纯度高于99.4% 的聚合级乙烯，且在多次循环实验中展现了极佳的稳定性和重现性。
Host–guest interactions governing C2H6/C2H4 discrimination in ZAI-3N. Optimized binding geometries with characteristic interatomic distances (Å) for (a) C2H6 and (b) C2H4. GCMC-derived probability maps for adsorbed C2H6 in ZAI-3N: (c) spatial occupancy and (d) number-density contour. (e) Hirshfeld surface mapped onto the adsorbed C2H6 molecule. (f) Two-dimensional fingerprint plot highlighting the principal contact types. (g) Observed, calculated, and difference profiles from Rietveld fitting of synchrotron PXRD data [λ = 0.824495(1) Å] for C2H6-loaded ZAI-3N. (h) Crystallographic positions of adsorbed C2H6 molecules in ZAI-3N derived from SPXRD refinements. (i) Calculated energy landscape along the C2H6 and C2H4 migration coordinate, (j) snapshots depicting amino-group rotation that enables C─H⋯NH2 contacts during C2H6 diffusion.
Fixed-bed breakthrough behavior of ZAI-3N. Breakthrough curves for C2H6/C2H4 binary mixtures with ratios of (a) 5:5 (6 mL min−1) and (b) 1:9 (8 mL min−1) at 298 K and 1 bar. (c) Overlay of individual-cycle breakthrough curves confirming reproducible C2H6/C2H4 fractionation across successive runs.
总结及展望
本研究成功证明了通过在稳固的MOF孔道特定位置引入“分子门控”基团，可以利用细微的动力学势垒差异来实现极难处理的工业气体分离。ZAI-3N不仅在选择性和吸附量上达到了行业领先水平，其优异的水稳定性和循环再生能力也为其工业化应用奠定了基础。这种精准调控分子门开启能垒的设计理念，未来有望扩展到诸如丙烷/丙烯分离、天然气脱碳以及其他具有挑战性的轻烃分离领域，为开发下一代高性能、低能耗的工业分离材料提供了全新的设计思路。