Keto–Enol Tautomerism-Driven Efficient Gold Recovery by Azo Covalent Organic Frameworks

Abstract Recovering gold from electronic waste represents a critical, yet challenging, field with substantial economic implications. Covalent organic frameworks (COFs), owing to their unique structural tunability and large specific surface area, provide a fundamental platform for the rational design of functionalities to enhance gold adsorption capacity. Among the various functional groups available for COF modification, the hydroxyl group is considered crucial because of its ability to form hydrogen bonds with gold species. Beyond the established mechanism of hydrogen bonds, this study innovatively investigates the influence of varying hydroxyl group quantities on the gold adsorption performance of COFs from the perspective of keto–enol tautomerism. The results demonstrate that keto–enol tautomerism not only influences electron transport efficiency but also creates synergistic sites between carbonyl oxygen and imine nitrogen that facilitate gold coordination. Consequently, AZO–COF-2OH demonstrated a superior adsorption capacity. This work indicates that the introduction of hydroxyl groups is not limited to the formation of hydrogen bonds, offering further insights into the application of hydroxyl-rich imine-based COFs in gold recovery from the perspective of keto–enol tautomerism.