Production of 5,5′(oxy-bis(methylene)) bis-2-furfural from 5-hydroxymethylfurfural through the hydrogen-bonding catalysis of ionic liquids

5,5′(oxy-bis(methylene)) bis-2-furfural (OBMF) is an important biobased chemical that can be synthesized through the self-etherification of 5-hydroxymethylfurfural (HMF) in the presence of either homogeneous acid or solid acid catalysts, such as p-toluenesulfonic acid (PTSA) and molecular sieves. In previous studies, all authors believed that this process follows an acid-catalyzed mechanism, even though some argued that acid strength is not the key factor in the formation of OBMF. In this work, we present a new route for the synthesis of OBMF from HMF, utilizing hydrogen-bonding catalysis with protonated ionic liquids under metal-free conditions. [NMPH][OTf] featuring a cationic hydrogen bond donor and an anionic hydrogen bond acceptor, is effective for the self-etherification of HMF. Under the optimal conditions, the yield of OBMF reached approximately 92.4 %. Based on the results of Attenuated Total Reflectance Infrared Spectroscopy (ATR-IR), Nuclear Magnetic Resonance (NMR) and Density Functional Theory (DFT) calculation, it is determined that the cations and anions of ILs could activate the C O and O H bonds of HMF via hydrogen bonds respectively. This synergistically facilitates the self-etherification of two HMF molecules to form OBMF. After recycling the catalyst four times, the yield of OBMF did not decrease significantly. According to the Arrhenius law, the apparent activation energy and reaction order for [NMPH][OTf] in the self-etherification of HMF to OBMF are determined 40.7 kJ/mol and 1.0, respectively. The results suggested that [NMPH][OTf] is an excellent catalyst for the generation of OBMF from HMF. Therefore, this study offers a simple and environmentally friendly method for the production of OBMF, which holds promising potential for broad application prospect in the future.