Fluolab【JACS】南京工业大学潘宜昌|勇夺567倍取向度提升！步进式配位编辑解锁高通量烷烃同分异构体分离膜
通讯作者： 潘宜昌（南京工业大学/苏州实验室）
文章概要
南京工业大学潘宜昌教授团队近期在《美国化学会志》（JACS）上发表了关于金属有机框架（MOF）分离膜的最新研究成果。研究提出了一种步进式配位编辑（SCES）策略，成功解决了铝基MOF（Al-MOFs）因各向异性生长导致的成膜难、取向差等瓶颈问题。通过调控铝中心与有机配体之间的配位动力学，团队制备出了具有高度垂直取向一维通道的Al-bttotb膜。该膜在轻石脑油中重要的C6烷烃同分异构体分离中展现出极高的渗透通量和选择性，证明了通道几何结构对分子输运的决定性作用。
Figure 1. Geometry-governed channel transport and stepwise coordination-editing synthesis of an oriented Al-bttotb membrane. (a–c) Comparison of transmembrane transport in MOF membranes with different pore dimensionalities: 3D cage-type and 2D nanosheet membranes exhibit tortuous pathways (Leff > Lmembr), whereas the vertically aligned 1D channel membrane achieved in this work enables near-direct transport across the membrane (Leff ≈ Lmembr). (d) Crystal structure of the Al-bttotb framework, highlighting two types of rhombic 1D channels with aperture sizes of 5 × 6 and 4.2 × 5 Å and their cross-sectional matching with C6 alkane isomers. (e) Schematic of the SCES process, showing the formation of an Al-carboxylate PSC scaffold that subsequently undergoes thermally induced terminal-to-bridging ligand exchange, enabling endogenous growth into a compact Al-bttotb membrane with vertically aligned 1D channels on a porous support.
引言
烷烃同分异构体的分离是炼油和精细化工中的关键步骤，直接影响燃油的辛烷值和能源利用效率。由于这些分子之间的尺寸差异仅为亚埃级，且物理性质极其相近，传统的分离技术面临巨大挑战。铝基MOF材料因其独特的一维直通道结构和优异的稳定性，被认为是实现高效分子筛分的理想载体。然而，Al-MOFs在生长过程中倾向于沿着轴向快速拉伸，导致横向融合困难，难以在支撑体表面形成连续且取向一致的薄膜。以往的研究往往通过牺牲孔道通达性来换取成膜完整性，这掩盖了一维通道本身具备的输运优势。为了打破这一僵局，开发一种能够兼顾膜连续性与通道取向度的合成新路径显得尤为迫切。
Figure 2. Morphology, structure, and transport properties of the Al-bttotb-7:3 membrane. (a) Top-view SEM image and (b) AFM phase image of the continuous PSC layer formed after SCES, step 1. (c) Cross-sectional SEM image of the PSC layer with a thickness of 500 nm. (d) Top-view SEM image and (e) AFM phase image of the crystalline Al-bttotb-7:3 membrane obtained after SCES, step 2. (f) Cross-sectional SEM image of the Al-bttotb-7:3 membrane. (g, h) 2D-XRD patterns of the Al-bttotb-7:3 membrane and randomly oriented Al-bttotb-0:1 membrane. (i) HR-TEM image and FFT pattern (inset) of the Al-bttotb-7:3 membrane. (j) N2 sorption isotherms at 77 K for the PSC scaffold and crystalline Al-bttotb samples obtained under progressively increasing crystallization temperatures (adsorption, closed; desorption, open). (k) Single-gas permeance of the PSC scaffold layer and the Al-bttotb-7:3 membrane as a function of kinetic diameter. (l) Comparison of H2 and N2 permeances for the fresh Al-bttotb-7:3 membrane and the membrane after C2H2 blocking with the inset exhibiting the relative contributions of MOF channels and non-MOF transport pathways.
主要实验及结论
研究人员创新性地引入了分阶段升温程序。在第一阶段的温和溶剂热条件下，利用单齿羧酸调节剂对铝离子进行动力学俘获，在多孔铝支撑体上预先构建出一层连续且饱和配位的非晶态预结构化配位（PSC）支架。这一阶段巧妙地避开了晶体的快速形核与各向异性生长，为后续的膜连续性奠定了基础。进入第二阶段后，通过升高温度触发配体交换，使多齿配体逐步取代单齿调节剂，驱动支架内部发生从终端配位到桥联配位的转变，实现了内源性结晶。这种生长模式不再依赖外部形核，确保了晶体能够以PSC支架为养分，在保持横向融合的同时实现沿一维通道方向的高度垂直生长。
Figure 3. Endogenous growth enabled by coordination-editing for improved membrane formation. (a) Relative crystallinity (%) as a function of reaction time for direct crystallization synthesis (DCS) conducted at different temperatures. The inset compares representative XRD patterns collected at 80 and 150 °C. t0 denotes the crystallization induction period. (52) (b) Morphological evolution of the crystal aspect ratio with reaction time for DCS and SCES-derived MOF samples; the inset exhibits that longitudinal growth dominates over lateral growth, dictated by the intrinsic coordination mode. (c) Time-resolved SEM images and corresponding schematic illustrations showing morphological evolution during the SCES process. (d) Top-view SEM images of the membranes at different stages: (d1) step 2, SCES after 1 h (inset: 2 h) and (d2) after 5 h (inset: 6 h), with identical scale bars for main images and insets. (e) XRD patterns corresponding to the membrane samples in (d), with the PSC layer shown for comparison.
Figure 4. Mechanism for stepwise coordination-editing-regulated endogenous crystallization. (a) In situ XRD intensity map (2θ = 8.6–9.2°) of the PSC scaffold during heating (25–200 °C). (b) In situ DRIFTS contour map of the PSC scaffold recorded during heating (30–150 °C). (c) Product mass evolution as a function of reaction time under different synthesis protocols. (d) Thermogravimetric analysis (TGA) coupled to mass spectrometry (MS) for the powder samples obtained at different SCES stages and DCS. (e) High-resolution O 1s XPS spectra of the samples collected at different crystallization stages in the SCES process. (f) 27Al SSNMR spectra of the samples obtained at different SCES stages and by DCS (6 h). (g) Al K-edge XANES and (h) Fourier transform Al K-edge EXAFS spectra of the samples obtained via SCES and DCS (6 h). (i) Gibbs free energy (eV) as a function of temperature (K) for three models: M-L (red circles), M-FA (green diamonds), and M-AA (blue squares). (J) Mean square displacement (MSD) of H3bttotb ligands in the simulation system at different temperatures (25, 80, and 150 °C). (k) Snapshot of H3bttotb ligand distribution near the Al-COOH precursor after 30 ns of MD simulation at (k1) 80 and (k2) 150 °C. (l) Adsorption energy (eV) of different coordination motifs on different Al-bttotb crystal planes.
实验表征结果显示，通过SCES策略制备的Al-bttotb膜展现出了惊人的取向性能，其晶体优先取向（CPO）指数高达204，是随机取向膜的567倍。在分子探针实验中，该取向膜表现出了超高的氢气渗透通量（达2600 GPU），远超大多数已报道的MOF膜，充分证实了低曲折度一维通道对分子传输的强化作用。在关键的液相全蒸发分离实验中，该膜对正己烷/2,2-二甲基丁烷混合物的分离因子达到28，且渗透通量显著优于具有三维笼状结构的UiO-66膜。分子动力学模拟进一步揭示，分子在垂直取向的一维通道内的扩散速率是三维笼状结构的数倍，这证明了几何结构控制的通道取向能有效转化结构各向异性为输运优势。此外，该膜在面对包含C5-C7烷烃的十组分复杂体系时，仍能将线性异构体的浓度从30%大幅提升至81.5%，展现出极佳的工业应用潜力。
- Figure 5. Isomer separation performance of Al-bttotb-x:y membranes. (a) Dependence of n-Hex permeation flux and SF (n-Hex/2,2-DMB) on membrane thickness and orientation, quantified by the CPO index. (b) Separation of n-Hex from other C6 isomers using the Al-bttotb-7:3 membrane. (c) Linear/branched selectivity and n-Hex permeance of the Al-bttotb-7:3 membrane compared to those of reported MMMs, MOF membranes, and zeolite membranes. (d) Comparison of n-Hex permeation flux and n-Hex/2,2-DMB SF for the Al-bttotb-7:3, Al-bttotb-0:1, and UiO-66 membranes; here, ε describes the surface-accessible porosity of the membrane, while τ describes the transport tortuosity of diffusion channels. (e) Self-diffusion coefficients (D) of n-Hex in Al-bttotb and UiO-66 frameworks derived from molecular dynamics simulations. (f) Long-term operational stability of the Al-bttotb-7:3 membrane during continuous separation with feed switching. (g) Multicomponent pervaporation of a 10-component (C5–C7) alkane mixture that mimics light naphtha, with each component accounting for 10 wt % in the feed.
总结及展望
这项研究通过步进式配位编辑策略，成功开辟了各向异性Al-MOFs向功能化取向膜转化的新途径。该方法不仅实现了对膜微观形貌的精准调控，更深刻揭示了表面可达孔隙率与输运曲折度比值（ε/τ） 是决定膜分离效率的关键描述符。这种通过调节配位动力学来诱导内源性结晶的思路，为设计和制备其他各向异性多孔材料膜提供了通用性的指导方案。未来，这种高性能的取向MOF膜有望在石脑油升级、精细化工分离等领域发挥重要作用，为节能减排和流程工业的绿色化转型提供关键技术支持。