Structural and Mechanistic Differences in Mixed-Linker Zeolitic Imidazolate Framework Synthesis by Solvent Assisted Linker Exchange and de Novo Routes

Mixed-linker zeolitic imidazolate frameworks (ZIFs) are a subclass of metal–organic frameworks (MOFs) amenable to significant property tuning by altering the functional groups on the imidazolate linkers. Solvent assisted linker exchange (SALE) and de novo synthesis of mixed-linker ZIFs have been demonstrated, but the differences in structural properties–most importantly the linker distributions–and synthesis mechanisms of these two different types of hybrid ZIFs are unknown. In this work, a combination of 1H NMR combined rotation and multiple pulse spectroscopy (CRAMPS), water adsorption, and nitrogen measurements reveal distinct differences in linker mixing between SALE and de novo ZIF-8–90 hybrids. Native-fluorescence confocal microscopy is shown to provide a direct means to visualize these differences. The effects of crystal size, temperature, and SALE duration were studied in detail, and a generalizable mechanism for SALE processes in ZIFs is proposed. The SALE process is found to follow a diffusion-limited behavior leading to core–shell morphologies. Under harsher SALE conditions, deviations from diffusion-limited behavior are found due to etching and partial dissolution of the initial ZIF-8 crystals. With the selection of appropriate reaction conditions, SALE processes appear to be capable of generating controlled core–shell ZIF structures of good morphological quality that complement the well-mixed structures obtained by de novo methods.

混合配体沸石咪唑酯骨架材料（mixed-linker zeolitic imidazolate frameworks, ZIFs）是金属有机骨架材料（metal–organic frameworks, MOFs）的一个子类，可通过调控咪唑酯配体上的官能团实现性能的显著调变。目前已证实可采用溶剂辅助配体交换（solvent assisted linker exchange, SALE）与从头合成两种方法制备混合配体ZIFs，但这两种不同杂化ZIF材料在结构性质——最核心的是配体分布——以及合成机制上的差异仍未明确。本研究结合氢核磁共振联合旋转多脉冲光谱技术（1H NMR combined rotation and multiple pulse spectroscopy, CRAMPS）、水汽吸附与氮气吸附测试，揭示了SALE法与从头合成法制备的ZIF-8-90杂化材料在配体混合模式上的显著差异。研究表明，原生荧光共聚焦显微镜可直接可视化呈现这类差异。本研究还系统考察了晶体尺寸、反应温度以及SALE反应时长对产物的影响，提出了ZIF材料中SALE过程的普适性机制。研究发现，SALE过程遵循扩散限制行为，最终生成核壳形貌结构；在更严苛的SALE反应条件下，由于初始ZIF-8晶体被刻蚀并发生部分溶解，产物会偏离扩散限制行为。通过合理选择反应条件，SALE法可制备得到形貌优良、结构可控的核壳型ZIF材料，可与从头合成法得到的均相混合配体结构形成互补。