Stable Zr-Piperazine MOFs for Efficient Gold Recovery

Gold's importance in various industries, the ever-rising demand of electronics and shrinking gold reserves all underscore the pressing need to develop effective gold recovery methods from waste. In this study, we systematically evaluate several large-pore Zirconium-carboxylate Metal-Organic Frameworks (Zr-MOFs) having slightly varied internal surface functionality (-H, -CH3, -NH2 and -NO2) in gold recovery from acidic, simulated e-waste solutions. We compare two Zr-MOFs constructed from oligophenylene ligands with four MOFs built from structurally analogous piperazine-based ligands, including two unreported frameworks. The piperazine-based frameworks offer a critical sustainability advantage: by replacing the central benzene core with piperazine, they eliminate the need for expensive Pd-catalyzed coupling chemistry required for oligophenylene synthesis. In this work, we demonstrate for the first time that Zr-piperazine MOFs exhibit stability in acidic conditions, significantly surpassing their oligophenylene-based counterparts. One newly developed Zr-piperazine MOF, functionalized with both amine and methyl groups, substantially outperforms all analogs, displaying the highest stability alongside a gold adsorption capacity of up to 1826 mg/g and rapid adsorption kinetics in 40 ppm gold solution. X-Ray Absorption Near-Edge Structure (XANES) studies reveal distinct gold adsorption mechanisms for primary versus tertiary amines. These findings establish Zr-piperazine MOFs as robust, sustainable, and efficient materials for gold recovery from e-waste.