Synthesis of Furandicarboxylic Acid Esters From Nonfood Feedstocks Without Concomitant Levulinic Acid Formation

5-Hydroxymethylfurfural (HMF) is a versatile intermediate in biomass conversion pathways. However, the notoriously unstable nature of HMF imposes challenges to design selective routes to chemicals such as furan-2,5-dicarboxylic acid (FDCA). Here, a new strategy for obtaining furans is presented, bypassing the formation of the unstable HMF. Instead of starting with glucose/fructose and thus forming HMF as an intermediate, the new route starts from uronic acids, which are abundantly present in many agro residues such as sugar beet pulp, potato pulp, and citrus peels. Conversion of uronic acids, via ketoaldonic acids, to the intermediate formylfuroic acid (FFA) esters, and subsequently to FDCA esters, proceeds without formation of levulinic acid or insoluble humins. This new route provides an attractive strategy to valorize agricultural waste streams and a route to furanic building blocks without the co-production of levulinic acid or humins.

Selectivity Control in the Tandem Aromatization of Bio-Based Furanics Catalyzed by Solid Acids and Palladium

Bio-based furanics can be aromatized efficiently by sequential Diels-Alder (DA) addition and hydrogenation steps followed by tandem catalytic aromatization. With a combination of zeolite H-Y and Pd/C, the hydrogenated DA adduct of 2-methylfuran and maleic anhydride can thus be aromatized in the liquid phase and, to a certain extent, decarboxylated to give high yields of the aromatic products 3-methylphthalic anhydride and o- and m-toluic acid. Here, it is shown that a variation in the acidity and textural properties of the solid acid as well as bifunctionality offers a handle on selectivity toward aromatic products. The zeolite component was found to dominate selectivity. Indeed, a linear correlation is found between 3-methylphthalic anhydride yield and the product of (strong acid/total acidity) and mesopore volume of H-Y, highlighting the need for balanced catalyst acidity and porosity. The efficient coupling of the dehydration and dehydrogenation steps by varying the zeolite-to-Pd/C ratio allowed the competitive decarboxylation reaction to be effectively suppressed, which led to an improved 3-methylphthalic anhydride/total aromatics selectivity ratio of 80 % (89 % total aromatics yield). The incorporation of Pd nanoparticles in close proximity to the acid sites in bifunctional Pd/H-Y catalysts also afforded a flexible means to control aromatic products selectivity, as further demonstrated in the aromatization of hydrogenated DA adducts from other diene/dienophile combinations.