Concentration-Driven Evolution of Crystal Structure, Pore Characteristics, and Hydrogen Storage Capacity of Metal Organic Framework-5s: Experimental and Computational Studies

Despite that metal organic frameworks (MOFs) have been considered as an effective hydrogen adsorbent, there has been little systematic information available on the effect of synthetic parameters on the evolution of structural features such as crystal structure and pore characteristics and further on hydrogen storage capacity of MOFs. We, therefore, carried out a systematic study to find the effects of the synthetic conditions on the evolution of crystal structure, pore characteristics, and hydrogen storage capacities of MOF-5s through both experimental and computational studies. We found that the precursor concentration had a noticeable influence on the degree of interweaving in the MOF-5s: high concentrations favored interwoven crystal forms, whereas low concentrations favored noninterwoven crystal forms. This variation has led to a substantial difference in the pore characteristics. Since the interweaving reduced pore volume and constricted pore dimensions, the Langmuir specific surface areas of the MOF-5s decreased from 2696 to 1006 m<sup>2</sup>/g, with concurrent evolution of ultramicroporosity. Moreover, the changes in the pore characteristics have significantly affected the hydrogen storage capacities of the product. Cryogenic hydrogen storage capacity at 1 bar of the interwoven MOF-5s was enhanced from 1.03 wt % (noninterwoven MOF-5) to 1.76 wt % due to highly developed ultramicroporosity. Our results indicate that this simple but efficient concentration controlled synthesis method not only provides highly interwoven and/or noninterwoven MOF-5s but also allows a control of the pore characteristics and H<sub>2</sub> storage capacity of MOF-type materials. The understanding of this correlation is particularly useful to establish favorable synthetic criteria for the preparation of MOF-type materials with rational designs.