Benzoic Acid/TEMPO as a Highly Efficient Metal‐Free Catalyst System for Selective Oxidation of 5‐hydroxymethylfurfural into 2, 5‐diformylfuran

Abstract

The combination of benzoic acid and 2,2,6,6‐teramethylpiperidin‐1‐oxyl (TEMPO) afforded an efficient catalytic system for the aerobic oxidation of 5‐hydroxymethylfurfural (HMF) into 2,5‐diformylfuran (DFF) under certain conditions. The maximum conversion of 86.7 % for HMF was obtained with a nearly 77.8 % DFF yield at 1 mmol benzoic acid and 0.4 MPa oxygen pressure in acetonitrile. The aerobic oxidation ability of HMF into DFF depended on the electron density of the organic acid skeletal structure with the carboxyl group. Meanwhile, the catalyst system of benzoic acid/TEMPO also exhibited good performance for aerobic oxidation of other alcohols. Based on experimental results, we propose that the benzoic acid acted as the primary catalyst and TEMPO was the key co‐catalyst, oxidized into oxoammonium cation through electron transport and then regenerated by a superoxide radical during the reaction.

Metal Nitrate Catalysis for Selective Oxidation of 5-Hydroxymethylfurfural into 2,5-Diformylfuran under Oxygen Atmosphere

“…32,33 However, there are only a few reported studies on applying N -oxyl radicals to catalyze the oxidation of HMF. 34,35 Yang et al 36 reported that benzoic acid/TEMPO obtained 86.7% conversion of HMF and 77.8% yield of DFF at 0.4 MPa O 2 in acetonitrile. Although interesting achievement was obtained by this method in the field of aerobic oxidation of HMF into DFF, it required relatively high operating pressures (0.4 MPa), high catalyst loading (50 mol %), special Teflon-lined autoclave reactors, high temperatures (up to 100 °C), and long reaction time (24 h).…”

Metal vs. Metal‐Free Catalysts for Oxidation of 5‐Hydroxymethylfurfural and Levoglucosenone to Biosourced Chemicals

“…A major reason that has led to this advance is the diversity of acid sites on the surface of solid acid catalysts and the ease of purification. [117][118][119] Although many protocols for the oxidation of furfural and HMF are described in the presence of acids, no systematic studies of the source of acid had been reported prior to the work of Choudhary et al then Lv et al [120][121][122] Before that, the oxidation of HMF in an oxidizing acid medium was reported by Descotes et al, who used 65 % nitric acid and water as solvent. The energy supply by heating or by sonication were compared, under these conditions.…”

“…This result should be compared with that described by Li et al, which also demonstrated the advantage of using aromatic acids as catalysts. [121] The beneficial effect can be attributed to a π-π stacking interaction between HMF and aromatic species. We will see a little further on how this feature will be exploited with the use of carbon catalysts with a polyaromatic structure related to graphite.…”

“…The presence of acetic acid as a cocatalyst was required under the optimized conditions. [134] The use of oxygen as a co-oxidant with TEMPO was studied by Li et al [121] This catalytic system is possible in the presence of acids and a screening was carried out by comparing mineral and organic acids. With inorganic acid, despite the high HMF conversion (95.6-98.2 %), DFF selectivity was very poor (less than 10 %) due to the oligomerization side reactions.…”

“…Although many protocols for the oxidation of furfural and HMF are described in the presence of acids, no systematic studies of the source of acid had been reported prior to the work of Choudhary et al. then Lv et al [120–122] …”