Design and synthesis of tetralactam macrocycle-based porous organic polymers (POPs): application in the recovery of gold from e-waste

Abstract

Two porous organic polymers Mac-DMP and Mac-TMP based on macrocyclic tetralactam receptors for gold ions, that is, Mac-DM and Mac-TM, were synthesized by Friedel-Craft polyalkylation reaction and characterized comprehensively. The polymer Mac-DMP was found to exhibit better porosity as revealed by the BET surface area of 130 m² g^-1 than Mac-TMP, which exhibited 26 m² g^-1. The metal ion binding studies based on ICP-MS analysis, XPS studies and PXRD reveal high selectivity for the capture of Au ions. While the metal ions such as Mg, Al, Ni, Cu and Sn, the predominant constituents of e-waste, were found to be untouched by the POPs, Pd ions were found to be captured as well, albeit with a lesser efficiency than gold ions. Quantitatively, the capture capabilities of Mac-DMP and Mac-TMP from the solution of gold at pH 2 were determined to1.27 and 0.72 g per gram. These quantities are significantly higher than the amount possible based on the binding of one gold ion in one tetralactam receptor in the polymer, attesting to the fact that the heteroatoms and aromatic surfaces extant to the polymer facilitate the binding of gold nanoparticles within the micropores created by virtue of inefficient organization of the polymers. The fact that polymers can be employed for the extraction of gold from e-waste (printed circuit boards) is compellingly demonstrated. It is further shown that both polymers can be used in a recyclable manner without significant loss of their adsorption efficiencies up to three adsorption-desorption cycles. The results thus constitute the first demonstration of macrocyclic tetralactam-based polymers for gold recovery from electronic waste and illustrate the potential for further headways in the capture of metal ions in general by receptor-based POPs.