Facile synthesis of Zr-based metal-organic gel (Zr-MOG) using “green” sol-gel approach

Abstract

Zirconium-based metal–organic gels (Zr-MOG) was introduced to overcome the drawbacks such as macro-shaping and poor hydrostability of as-synthesized metal–organic frameworks in microcrystalline powder in industrial applications. Zirconium nitrate oxyhydrate and dicarboxylic acid were initially used as organic ligands to form a soft supramolecular structure in a sol–gel like processing method. Amorphous Zr-MOG were successfully synthesized without chloride ion and toxic modulators for the first time in an attempt to a sustainable and green approach. The estimated as well as observed pore size was at ca. 5 nm with a disordered wormhole-like structure analogous to mesoporous silica materials. The agitation method, aging duration and temperature were highly dependent factors for the formation of amorphous Zr-MOG xerogel in the absence of chloride ions and toxic organic solvents. The evidences suggested that simple and quick washing with ethanol of the obtained gel with aging in between as the key factor to successful formation at room temperature analogous to that of siliceous inorganic materials. X-ray photoelectron spectroscopy (XPS) spectra indicated that the Zr atoms and organic linker bonding were intact, forming an amorphous metal–organic gel network. The deconvolution of O1s spectra resulted in three peaks at 531.9 eV, 530 eV and 529 eV attributable to Zr-O-C, Zr-O-Zr and Zr-O bonds energy, respectively. The presence of Zr-O-C peak suggested successful formation of metal and organic moieties in Zr-MOG. In addition, the amorphous gel network was intact as evidenced in the presence of Zr-O-C peak in as-synthesized, dried gel.