Contrasting Roles of Maleic Acid in Controlling Kinetics and Selectivity of Sn(IV)- and Cr(III)-Catalyzed Hydroxymethylfurfural Synthesis

Platform chemicals such as hydroxymethylfurfural (HMF) can be obtained through the conversion of biomass with the aid of metal catalyst (i.e., Cr(III) or Sn(IV)) systems. In this study, we examined the use of an environmentally compatible cocatalyst, namely, maleic acid (MA), to regulate the reaction kinetics and product selectivity in the metal-catalyzed conversion of rice waste to HMF. The maximum yields of HMF by Sn(IV) and Cr(III) systems were ∼36 and ∼26 mol %, respectively, in a medium of dimethyl sulfoxide (DMSO)/H₂O at 140 °C. The addition of MA to Sn(IV) system did not noticeably alter the conversion kinetics. In contrast, MA significantly changed the kinetics in the Cr(III) system, reducing the rate of glucose isomerization, while accelerating starch hydrolysis. The latter may be ascribed to the increased accessibility of starch, as malic acid (38.4%) emerging from hydration of MA in the Cr(III) system could have interrupted the hydrogen bond network of starch. HMF selectivity was also enhanced in Cr(III)/MA system, probably because of the favorable coordination of Cr(III) to MA and/or its derivatives, which could have moderated the Lewis acidity to suppress the side reactions that consume sugars and HMF. In this study, we elucidated the diverse roles of an organic acid in controlling the conversion selectivity and kinetics, and we highlighted the potential of applying a green cocatalyst to advance current catalytic systems for achieving efficient and sustainable production of HMF from biomass waste.