Ionic liquid-mediated deoxydehydration reactions: Green synthetic process for bio-based adipic acid

A novel recyclable Re-catalyzed deoxydehydration (DODH) reaction was developed with an ionic liquid (IL) as a reaction medium for an efficient synthesis of adipic acid (1), one of the commercially important dicarboxylic acids, from biomass galactaric acid. The carefully designed solubility of ILs allowed a homogeneous DODH reaction to produce muconate 3 in excellent yields, a key intermediate for I. Use of the IL also enabled an efficient separation of the DODH product 3 from the reaction mixture by simple decantation. The recovered IL layer containing the expensive Re catalyst was reused up to four times, yielding 3 without much decrease in yields. The target compound 1 was also produced in an excellent yield with the catalytic hydrogenation of 3 followed by the acidic hydrolysis. Thus, the overall process for bio-based adipic acid would become much more cost-effective, eco-friendly, and industrially viable, which could be applied to various biomass conversions. (C) 2017 Elsevier Ltd. All rights reserved.