Metal-free catalytic systems based on imidazolium chloride and strong bases for selective oxidative esterification of furfural to methyl furoate

Homogeneous and heterogeneous catalytic systems based on 1-benzyl-3-methylimidazolium chloride (BzmimCl) assisted by strong bases (Cs2CO3 or 1, 8-diazabicyclo [5.4.0] under-7-ene (DBU)) were applied in the oxidative esterification of furfural, to obtain 2-methyl-furoate (MF). The reaction was carried out in one step, using oxygen as a unique source of oxidation and methanol as an esterification reagent. Commercial BzmimCl was used as a homogeneous ionic catalyst whereas the heterogeneous ionic one (HCP-BzmimCl) was prepared through a Friedel-Crafts reaction between biphenyl and BzmimCl. The catalytic systems here reported yield MF with a selectivity > 92% and furfural conversions > 82% after 24 h. The homogeneous catalyst systems have better catalytic activity than the similar ionic liquid systems assisted by bases published so far. Besides, HCP-BzmimCl is the first heterogeneous metal-free catalyst containing imidazole chloride groups for this reaction. HCP-BzmimCl can be recycled up to 5 times using both bases, obtaining quantitatively methyl furoate. © 2023 The Authors