Acid-mediated green synthesis and optimization of the calcium-terephthalate metal-organic framework derived from waste eggshells

Abstract

Global plastic and food waste generation has climbed to unprecedented levels, inherently linked to exponential population growth since the 1950s accompanied by industrialization, urbanization and modernization. The reduction of waste generation is essential for ecosystem health and longevity as encompassed by the United Nations’ (UN) Sustainable Development Goal (SDG) 12. To this aim, this study valorized waste eggshells and post-consumer polyethylene terephthalate (PET) bottles into the value-added four-component calcium-terephthalate (Ca-BDC) metal-organic framework (MOF), with formula Ca(BDC)(DMF)(H2O). Previous reports synthesized Ca(BDC)(DMF)(H2O) from commercial calcium salts and ligands. Calcium salts, in turn, are manufactured by energy and chemical-intensive processes from naturally occurring dolomite, aragonite and limestone minerals, which are finite resources. This study bypassed the use of these minerals by using eggshells directly as a calcium precursor in the synthesis of eggshell-derived Ca(BDC)(DMF)(H2O). In an effort to optimize atom economy, purity, yield, porosity and energy efficiency, modulator-free, acetic acid (AA), formic acid (FA) and benzoic acid (BA) modulated syntheses of eggshell-derived Ca(BDC)(DMF)(H2O) were conducted and compared. In each case, modulator and solvent volumes, as well as reaction time and temperature were optimized in accordance with green chemistry principles. Phase studies were conducted to determine green synthetic conditions for achieving porous triclinic (Ca-BDC-tric) and non-porous orthorhombic (Ca-BDC-orth) phases of Ca(BDC)(DMF)(H2O). Finally, PET bottles were depolymerized into 1,4-benzenedicarboxylic acid (BDC), which was combined with eggshells, using optimized synthetic methods, to generate eggshell- and PET-derived Ca(BDC)(DMF)(H2O), presenting the greenest synthesis of this specific MOF to date. Modulator-free eggshell-derived Ca(BDC)(DMF)(H2O) syntheses arrived at a minimum H2O volume threshold, below which Ca(BDC)(DMF)(H2O) synthesis failed, but above which further increases in H2O volume enhanced Ca(BDC)(DMF)(H2O) product yield. Ca(BDC)(DMF)(H2O) syntheses from eggshells in the absence of a modulator at long reaction times (24 and 48 h) proved that the 120-150C temperature range results in undesirable dual-formation of both Ca-BDC-tric and Ca-BDC-orth phases, informing the avoidance of this temperature range in subsequent modulated syntheses. Modulator-free and BA-modulated syntheses achieved incomplete acidification of eggshell CaCO3, resulting in a CaCO3 impurity among the final Ca(BDC)(DMF)(H2O) product. This was overcome by the incorporation of AA (1.0 mL AA; 7.6 mL H2O) and FA (0.35 mL FA; 12 mL H2O) modulators in subsequent syntheses, which successfully liberated all calcium cations (Ca2+) from eggshell CaCO3 and improved atom economy by complete conversion of eggshell reactant to pure Ca(BDC)(DMF)(H2O) product. In both AA and FA modulated syntheses, the effect of reaction time was found to be negligible for both phases, where pure Ca-BDC-tric and Ca-BDC-orth was synthesized at 80-120C and 150C respectively, regardless of reaction times investigated, namely 12, 16, 20, 24 and 48 h. Thus, incorporation of AA and FA modulators enabled a 12 h synthesis for both phases, thereby halving solvothermal literature reaction times (24 h). The AA modulator produced the highest Ca-BDC-tric porosity, while the FA modulator further lowered energy consumption by making a novel 12 h room temperature synthesis (25C) of pure and highly-crystalline Ca-BDC-tric possible. Phase studies proved that the greenest synthetic parameters for AA-modulated pure Ca-BDC-tric and Ca-BDC-orth were 12 h at 80 and 150C respectively. However, when adopting the FA modulator, pure Ca-BDC-orth required longer reaction times (48 h) at high reaction temperatures (150C), providing a means to eliminate the undesirable non-porous Ca-BDC-orth phase in line with green chemistry principles. Lastly, the substitution of commercial BDC with PET-derived BDC in eggshell-derived Ca(BDC)(DMF)(H2O) retained results from AA and FA optimized syntheses by producing no alteration to Ca(BDC)(DMF)(H2O) purity, structure, morphology and bonding. Despite eggshell- and PET-derived Ca-BDC-tric demonstrating a slightly lower porosity than commercially-derived Ca-BDC-tric, porosities remained comparable, providing a means to valorize waste materials into the value-added Ca(BDC)(DMF)(H2O) MOF.