Acid Strength–Dependent Oligomerization Pathways in Furan Acylation

The acid-catalyzed acylation of furan with acetic anhydride to 2-acetylfuran (AF) offers a promising route for biomass valorization, yet acid-induced oligomerization limits its practical scalability. Herein, we systematically investigate the effect of acid strength on AF selectivity and oligomer formation using five liquid Brønsted acids spanning a wide Hammett acidity range (H0 ≈ −4.66 to −14.6). The AF yield exhibits a non-monotonic dependence on acid strength, rising from 77.9% to 81.1% for moderately strong acids before dropping sharply to 52.5% under superacidic conditions. Gel-permeation chromatography (GPC) revealed that oligomer molecular weight increases monotonically with acid strength, while the oligomer fraction mirrors the non-monotonic AF yield trend. High-resolution LC–ESI–MS/MS identified diacetylfuran (DAF) as the dominant side product and distinguishes two mechanistically distinct oligomer families: those arising from nucleophilic substitution at the carbonyl group (NSCO) and those from ring substitution (NSRO), the latter emerging prominently only under strongly acidic conditions. Density functional theory (DFT) calculations reveal that activation free-energy barriers for AF formation, DAF formation, and oligomerization initiation scale linearly with catalyst deprotonation energy with statistically indistinguishable slopes, indicating that increasing acid strength uniformly accelerates both productive and unproductive pathways. These findings demonstrate that the observed selectivity behavior originates from a coupled interplay of kinetic and thermodynamic factors, and that acid-strength tuning alone is insufficient to suppress oligomerization.