Crystallization mechanisms and size control of metal-organic frameworks: Insights and applications

Metal-organic frameworks (MOFs) have gained significant scientific interest due to their wide-ranging applications across various disciplines. Precisely controlling the crystal size of MOFs plays a pivotal role in enhancing their performance for practical applications. Nevertheless, achieving the rational synthesis of MOFs with desired size distributions remains a formidable challenge. In this research, we elucidate the crystallization mechanism of MOFs and propose an enlightening approach to systematically synthesize MOFs with diverse size distributions. Using MIL-88A as a case study, we uncover its crystallization mechanism characterized as oriented assembling and Ostwald ripening. Our investigation reveals that the ratio of the assembling rate to the ripening rate, denoted as VA/VR, exerts a profound influence on the resulting size distribution of MOFs. Specifically, higher VA/VR values yield both larger average sizes and increased standard deviation in the size distribution. We introduce VA/VR as the size variation factor and demonstrate its efficacy as a guiding principle for tailoring the synthesis of MIL-88A with distinct size distributions. Furthermore, we showcase the versatility of the size variation factor VA/VR by applying it successfully to the controlled synthesis of other MOFs, including MOF-14 and HKUST-1. These findings offer profound insights into the fundamental principles governing MOFs crystallization, paving the way for more effective and precise synthesis methodologies.