Modulator-Free Green Synthesis of the Calcium-Terephthalate Metal-Organic Framework Derived from Waste Eggshells

Abstract

Conventionally, the four-component calcium-terephthalate (Ca-BDC) metal–organic framework (MOF), Ca(BDC)(DMF)(H2O), has been synthesized from commercial calcium nitrate (Ca(NO3)2) and calcium carbonate (CaCO3), which in turn are manufactured from naturally-occurring minerals by energy-intensive processes. This study presents a primary investigation on the modulator-free solvothermal synthesis of Ca(BDC)(DMF)(H2O) derived from waste eggshells as green precursors. Ca(BDC)(DMF)(H2O) crystallizes as two possible phases, namely the triclinic (Ca-BDC-tric) and orthorhombic (Ca-BDC-orth) phase. This study presents an in-depth investigation on the influence of reaction temperature and solvent volumes on eggshell-derived Ca(BDC)(DMF)(H2O) product phase. Investigations were conducted at long reaction times to ensure complete reaction of green eggshell precursors. Low water (H2O) and high N,N-dimethylformamide (DMF) volumes were found to inhibit solvothermal synthesis. 7.6 mL H2O was determined to be the minimum H2O volume threshold for successful Ca-BDC-tric synthesis, above which further increasing the H2O volume produced porous Ca-BDC-tric at higher yield, which proved favorable due to reduction of toxic DMF. Ca-BDC-orth and Ca-BDC-tric were obtained at 150 °C and 110 °C respectively, whereas dual-formation of both phases prevailed at 130 °C, thereby informing the rough temperature range at which Ca-BDC-tric morphs into Ca-BDC-orth. Ultimately, this study introduces a green solvothermal method for valorizing eggshell waste into value-added Ca(BDC)(DMF)(H2O) for sustainable eggshell waste management and alleviation of environmental health concerns associated with eggshell disposal.