Confinement synthesis in porous molecule-based materials: a new opportunity for ultrafine nanostructures.

A balance between activity and stability is greatly challenging in designing efficient metal nanoparticles (MNPs) for heterogeneous catalysis. Generally, reducing the size of MNPs to the atomic scale can provide high atom utilization, abundant active sites, and special electronic/band structures, for vastly enhancing their catalytic activity. Nevertheless, due to the dramatically increased surface free energy, such ultrafine nanostructures often suffer from severe aggregation and/or structural degradation during synthesis and catalysis, greatly weakening their reactivities, selectivities and stabilities. Porous molecule-based materials (PMMs), mainly including metal–organic frameworks (MOFs), covalent organic frameworks (COFs) and porous organic polymers (POPs) or cages (POCs), exhibit high specific surface areas, high porosity, and tunable molecular confined space, being promising carriers or precursors to construct ultrafine nanostructures. The confinement effects of their nano/sub-nanopores or specific binding sites can not only effectively limit the agglomeration and growth of MNPs during reduction or pyrolysis processes, but also stabilize the resultant ultrafine nanostructures and modulate their electronic structures and stereochemistry in catalysis. In this review, we highlight the latest advancements in the confinement synthesis in PMMs for constructing atomic-scale nanostructures, such as ultrafine MNPs, nanoclusters, and single atoms. Firstly, we illustrated the typical confinement methods for synthesis. Secondly, we discussed different confinement strategies, including PMM-confinement strategy and PMM-confinement pyrolysis strategy, for synthesizing ultrafine nanostructures. Finally, we put forward the challenges and new opportunities for further applications of confinement synthesis in PMMs. The space-, coordination-, and/or ion-confinement in porous molecule-based materials (PMMs) endow the PMM-confinement (pyrolysis) synthesis to construct a variety of ultrafine nanostructures.

在设计用于多相催化的高效金属纳米粒子（MNPs）时，活性与稳定性之间的平衡极具挑战性。一般来说，将MNPs的尺寸减小到原子尺度可以提供高原子利用率、丰富的活性位点以及特殊的电子/能带结构，从而极大地提高其催化活性。然而，由于表面自由能急剧增加，这种超细纳米结构在合成和催化过程中经常遭受严重的团聚和/或结构降解，极大地削弱了它们的反应性、选择性和稳定性。多孔分子基材料（PMMs）主要包括金属 - 有机框架（MOFs）、共价有机框架（COFs）以及多孔有机聚合物（POPs）或笼状结构（POCs），它们具有高比表面积、高孔隙率和可调节的分子限域空间，是构建超细纳米结构的有前景的载体或前体。其纳米/亚纳米孔或特定结合位点的限域效应不仅可以在还原或热解过程中有效限制MNPs的团聚和生长，还能稳定所得到的超细纳米结构，并在催化过程中调节它们的电子结构和立体化学。在这篇综述中，我们重点介绍了在PMMs中限域合成用于构建原子尺度纳米结构（如超细MNPs、纳米团簇和单原子）的最新进展。首先，我们阐述了典型的合成限域方法。其次，我们讨论了用于合成超细纳米结构的不同限域策略，包括PMM限域策略和PMM限域热解策略。最后，我们提出了PMMs中限域合成进一步应用所面临的挑战和新机遇。 多孔分子基材料（PMMs）中的空间、配位和/或离子限域赋予了PMM限域（热解）合成构建多种超细纳米结构的能力。