Additive-assisted solvothermal synthesis and properties of porous coordination polymer MOF-14 crystals with controlled morphology and size

Abstract

Meta-organic frameworks (MOFs) or porous coordination polymers (PCPs), which can be constructed by assembly of organic linkers with metal ions or metal clusters, are an important type of crystalline microporous materials and have attracted considerable attention in the past two decades due to their tailored pore structures, high surface areas and interesting properties such as gas adsorption, storage and separation, catalysis, molecular or ionic sensing as well as drug release. Nowadays remarkable research efforts have focused on bulk MOFs, however, for some applications the crystal size and morphology are also very important. In comparison with bulk materials, nanostructured MOFs may exhibit unique shape- and size-dependent properties, and also have superior properties than the bulk MOFs. However, due to the variety of MOFs structures and the poor understanding of the growth processes of MOFs, the controllable synthesis of nanostructured MOFs still remains a challenge. Therefore, developing efficient synthetic strategies for the tailored fabrication of well-defined nanostructured MOFs materials is highly desirable. In this paper, MOF-14 [Cu3(BTB)2] (H3BTB=4,4′,4″-benzene-1,3,5-triyl-tribenzoic acid) micro/nanocrystals with controllable morphology and size were successfully obtained by additive-assisted solvothermal method. The as-obtained products were characterized by powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FESEM), and gas adsorption measurements. Acetic acid and sodium acetate were employed in the reaction system as additives, and found to play an important role in determining the crystal morphology and size. With the amount of acetic acid increased, an evolution of particle morphology from cube, truncated rhombic dodecahedron, to rhombic dodecahedron was observed. Meanwhile, the particle size increased gradually. With the amount of sodium acetate increased, the particle morphology of MOF-14 crystals changed from cube to sphere, and the particle size decreased gradually. Moreover, the role of acetic acid and sodium acetate were explored in detail, and a general mechanism for the formation of MOF-14 crystals was proposed. The results of gas sorption measurements reveal that all the MOF-14 crystals with different morphology and size have large Langmuir surface area, good CO2 capture capacity and adsorption selectivity for CO2 over N2. The sorption profiles of the MOF-14 crystals were found to be dependent on the crystal morphology and size. Furthermore, microporous MOF-14 crystals prepared with sodium acetate have additional mesopores, which may supply a new method to expand the porous MOFs.