Microwave-assisted catalytic upgrading of 2,5-hexanedione to 2,5-dimethyltetrahydrofuran over metal triflate under H2-free conditions

Biomass refinery is considered as an attractive alternative to achieve global sustainability. Among its derivatives, 2,5-dimethyltetrahydrofuran (DMTHF) is a key biochemical that serves as an eco-friendly solvent, a fuel additive and an intermediate for organic synthesis. In this study, various metal triflates were employed as catalysts in the microwave-assisted upgrading of 2,5-hexanedione (2,5-HD) to DMTHF, utilizing alcohol as the hydrogen donor under H 2 -free conditions. Experimental results demonstrate that the product distribution of this cascade process is strongly influenced by both the catalyst and the alcohol. The complex (OTf) 3 Sc-OR···H, formed between Sc(OTf) 3 and isopropanol, features appropriate Lewis and Brønsted acidic strengths. This complex showed the best catalytic activity, giving 100 % 2,5-HD conversion and 92.5 % DMTHF yield at 180 °C after 80 min. The main reaction pathway refers to the sequential reduction of the two carbonyl groups of 2,5-HD to produce 5-hydroxy-2-hexanone and 2,5-hexanediol via transfer hydrogenation. This process is facilitated by Lewis acidic sites based on the classic MPV reduction mechanism. Subsequently, DMTHF is obtained by intramolecular dehydration of the two hydroxyl groups of 2,5-hexanediol catalyzed by Brønsted acidic sites. Notably, the Sc(OTf) 3 catalyst can be effectively recovered through vacuum distillation and reused for five cycles without significant loss of activity. Overall, this work presents an attractive route for producing DMTHF from 2,5-HD upgrading under H 2 -free conditions.