Selective Recovery of Palladium From Spent Automotive Exhaust Catalysts Using Amidoxime‐Functionalized Calix[4]arene and Subsequent Reduction to Palladium Nanoparticles

Spent automotive exhaust catalysts represent the largest secondary resource of palladium (Pd), and their efficient recovery is essential to support the sustainability of Pd‐related industries. Inspired by supramolecular chemistry, a novel calixarene derivative, 5,11,17,23‐tetra‐ tert ‐butyl‐25,27‐bis[(N′‐hydroxyacetimidamide)ethoxy]‐26,28‐dihydroxycalix[4]arene (HAEC), was developed for Pd extraction from those wastes. HAEC was functionalized with amidoxime groups that possess strong chelation ability, in which multiple electron‐donating atoms could synergistically interact with Pd(II). Moreover, the 1,3‐alternate introduction of these groups on the lower rim of the calix[4]arene provided a favorable spatial configuration for Pd(II) binding. As a result of these characteristics, HAEC exhibited outstanding extraction performance toward Pd(II), achieving an extraction efficiency exceeding 99.9%. High selectivity of HAEC toward Pd(II) was observed, with a separation coefficient of 713.47 from Pt(IV), whereas those over other coexisting base metal ions all exceeded 1400. In addition, more than 99.9% of Pd(II) was still recovered after 10 successive extraction and stripping operations, confirming the remarkable reusability of HAEC. Mechanistic investigations revealed that Pd(II) recovery from acidic chloride systems is achieved through the synergistic coordination of −NH 2 , oxime nitrogen, and oxime oxygen within the amidoxime moieties of HAEC. Furthermore, size‐uniform nanoscale Pd powder, serving as a critical material for catalytic applications, was obtained from the Pd stripping solution using ascorbic acid (AA) as the reductant. This study integrates the efficient Pd recovery with the synchronous preparation of nanoscale Pd powder, providing a high‐value utilization pathway for spent automotive exhaust catalysts.