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 recovery of gold ions, were synthesized by the Friedel–Crafts polyalkylation reaction and characterized comprehensively. The polymer Mac-DMP was found to exhibit better porosity, as revealed by its 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 metal ions such as Mg, Al, Ni, Cu and Sn, the predominant constituents of e-waste, were found to be untouched by these POPs, Pd ions were found to be captured as well, albeit with less efficiency than gold ions. Quantitatively, the capture capabilities of Mac-DMP and Mac-TMP from a solution of gold at pH = 2 were determined to be 1.27 and 0.72 g g⁻¹, respectively. 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 polymers facilitate the binding of gold nanoparticles within the pores 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 headway in the capture of metal ions in general by receptor-based POPs.