Engineering Brønsted Acidic Microenvironments via Strong Metal–Support Interaction in Single-Atom Pd₁/CeO₂ for Acid-Free Acetalization Catalysis

Conventional acid-catalyzed acetalization faces significant challenges in catalyst recovery and poses environmental concerns. Herein, we develop a CeO2-supported Pd single-atom catalyst (Pd1/CeO2) that eliminates the reliance on liquid acids by creating a localized Hδ+-rich microenvironment through heterolytic H2 activation. X-ray absorption near-edge structure and extended X-ray absorption fine structure analyses confirm the atomic dispersion of Pd via Pd-O-Ce coordination, while density functional theory (DFT) calculations reveal strong metal-support interactions (SMSI) that facilitate electron transfer from CeO2 oxygen to Pd, downshifting the Pd d-band center and optimizing H2 activation. Heterolytic H2 cleavage at Pd sites generates CeO2-anchored Hδ+ species, mimicking Brønsted acidity for selective acetalization. DFT analysis reveals significant O 2p-Pd 4d hybridization, confirming the formation of Pd-O coordination bonds and inducing charge transfer to modulate the electronic states of active sites. Under mild conditions (90 °C, 1 MPa H2), the catalyst achieves 96.6% selectivity to cinnamal diethyl acetal with 100% conversion. Stability tests show no Pd aggregation or performance decay after six cycles, attributed to robust Pd-O-Ce bonding and SMSI stabilization. This work establishes a sustainable acid-free acetalization strategy by atomically engineering reactive Hδ+ sites, decoupling the process from corrosive acids.