Unlocking the pre-nucleation state as a route to materials discovery in MOFs

金属有机框架（MOFs）是具有丰富的组成和结构可调性的扩展多孔材料，这使得它们对于许多重要应用（例如水收集、碳捕获、能量存储和传感）非常有吸引力。它们的结构由沸石咪唑酯框架（ZIF）例示，其由通过官能化咪唑酯连接基连接的四面体金属节点组成以形成作为经典沸石的直接类似物的拓扑结构。尽管已知数千种M0 F的事实，但239种已知沸石拓扑结构中仅15种被再现为ZIF。这表明ZIF有一个巨大的相空间有待发现，这是目前合成方法无法达到的。开发新发现范式的挑战是了解材料形成过程中的原子相关性，从而能够预测和利用不同合成参数的影响。然而，尽管有很大的兴趣，我们对支持MOF结晶的机制知之甚少，因此，我们缺乏对自组装和相选择的控制。这项研究将建立在原位测量和对ZIF预成核状态的理解的最新进展之上-ZIF预成核状态是在结晶之前和结晶期间存在于溶液中的复合物的动态演变混合物-揭示关键的中间体物种和与ZIF产品的结构关系，决定相选择。具体而言，所选ZIF形成反应的预成核状态将通过仔细的分析化学进行稳定，并使用高分辨率非原位技术进行表征，以确定关键中间体物质。一系列的技术将被使用，跨越几个长度尺度的灵敏度，从X射线总散射和核磁共振光谱（本地秩序），以电喷雾电离质谱（ESI-MS;复合物和低聚物）和X射线衍射（长程秩序），以产生一个详细的图片预成核状态。然后，这些数据将被用作解释关键原位实验的基础：时间分辨ESI-MS和同步加速器X射线散射，这将揭示在正常反应条件下ZIF结晶过程中中间体的演变。计算将确定中间体中的关键相互作用和不同产物的相对稳定性，验证竞争途径的重要性。重要的是，原位实验将产生动力学数据，从中提取相互转化和结晶的速率和活化能，以完成ZIF形成的详细定量模型。该项目的主要成果将是对ZIF预成核状态及其对相选择的影响的新的机理理解。它将揭示两个重要ZIF子集中的关键中间物种，即具有普通方钠石（SOD）拓扑结构的那些和具有大通道钠长石（GME）拓扑结构的那些，它们在碳捕获，使用和储存方面具有潜在的用途。它将显示预成核物质的演变如何导致在不同的合成条件下形成特定的ZIF相。总的来说，这些结果将建立ZIF结晶能量景观的定量模型，通过该模型可以合理化它们的形成，并可以预测理想的合成条件。这项研究将展示如何靶向中间体，稳定和组装以指导选定的ZIF结构，从而开辟了通过合理设计发现大量新相的可能性。开发的方法将可转移到其他MOF系统和晶体材料，在太阳能电池，催化和储能方面具有深远的应用。因此，它将为新一代创新产品和技术铺平道路。

Metal-organic frameworks (MOFs) are extended porous materials with a wealth of compositional and structural tunability that makes them very attractive for numerous important applications, such as water harvesting, carbon capture, energy storage, and sensing. Their structures are exemplified by zeolitic imidazolate frameworks (ZIFs), which consist of tetrahedral metal nodes connected by functionalised imidazolate linkers to form topologies that are direct analogues of classical zeolites. Despite the fact that several thousand MOFs are known, only 15 out of 239 known zeolite topologies have been reproduced as ZIFs. This suggests that there is a vast phase space of ZIFs that remains to be discovered, which is inaccessible to current synthetic methods. The challenge in developing a new discovery paradigm is to understand the atomic correlations during the materials formation process, thereby enabling the effects of different synthetic parameters to be predicted and utilised. However, despite significant interest, we know very little about the mechanisms that underpin MOF crystallisation and, therefore, we lack control over self-assembly and phase selection.This research will build on recent advances in in situ measurements and understanding of the ZIF pre-nucleation state - the dynamically evolving mixture of complexes that exists in solution prior to and during crystallisation - to reveal the key intermediate species and structural relationships with ZIF products that determine phase selection. Specifically, the pre-nucleation state of selected ZIF formation reactions will be stabilised through careful analytical chemistry and characterised using high-resolution ex-situ techniques to identify the key intermediate species. A range of techniques will be used that spans several length scales of sensitivity, from X-ray total scattering and nuclear magnetic resonance spectroscopy (local order) to electrospray ionisation mass-spectrometry (ESI-MS; complexes and oligomers) and X-ray diffraction (long-range order), in order to generate a detailed picture of the pre-nucleation state. These data will then be used as a basis with which to interpret key in situ experiments: time-resolved ESI-MS, and synchrotron X-ray scattering, which will reveal how the intermediates evolve during ZIF crystallisation under normal reaction conditions. Computational calculations will identify the key interactions in the intermediates and the relative stabilities of different products, verifying the importance of competing pathways. Importantly, the in-situ experiments will generate kinetic data, from which the rates and activation energies of interconversion and crystallisation will be extracted to complete a detailed, quantitative model of ZIF formation.The main output of this project will be a new mechanistic understanding of the ZIF pre-nucleation state and its effect on phase selection. It will reveal key intermediate species in two subsets of important ZIFs, namely those with the common sodalite (SOD) topology and those with the large channel gmelinite (GME) topology, which have potential uses in carbon capture, usage and storage. It will show how the evolution of pre-nucleation species leads to the formation of particular ZIF phases under different synthetic conditions. Collectively, these results will build a quantitative model of the ZIF crystallisation energy landscape by which their formation can be rationalised and ideal synthesis conditions can be predicted.This research will show how intermediates can be targeted, stabilised and assembled to direct the structures of selected ZIFs, thereby opening up the possibility to discover a wealth of new phases via rational design. The methodology developed will be transferable to other MOF systems and crystalline materials, with far-reaching applications in solar cells, catalysis and energy storage. Thus, it will pave the way for a new generation of innovative products and technology.