Transformation of Sugars into Furfurals

1 Synthesis of 5-Hydroxymethylfurfural from Fructose and Glucose Using Heterogeneous Catalysts 

Fructose, one of the most common ketohexoses, readily dehydrates to afford HMF in the presence of Br0nsted acids in polar solvents. A variety of aprotic polar solvents, including DMSO, DMF, N,N-dimethylacetamide (DMA), and sulfolane, are used for these liquid-phase reaction because of the solubility of carbohydrates. A variety of solid acids, such as ion-exchange resins [156], zeolites [157, 158], metal oxides, and heteropoly acid salts, have been examined for HMF production from fructose [159,160]. Niobic acid, niobium phosphate, vanadium phosphate, sulfated zirconia, Amberlyst-15, and acid-functionalized mesoporous silicas are also found to exhibit high catalytic activity for fructose dehydration [161-167]. Moreover, soHd acid catalysts have also been examined in ionic liquids [168-175]. 

Unlike fructose, glucose, one of the most common aldohexoses, is difficult to transform directly to HMF. It is the most abundant monosaccharide obtained from the depolymerization of cellulose. To achieve successful HMF production from glucose, heterogeneous catalyzed isomerization to fructose therefore appears as a key reaction. There are two mechanisms for glucose isomerization via proton transfer and intramolecular hydride shift. These are generally catalyzed by a base and Lewis acid, respectively. 

The latter isomerization via an intramolecular hydride shift is catalyzed by Lewis acids. Tin-containing P-zeoHte exhibits remarkable activity for the glucose-fructose isomerization in water. and NMR studies have elucidated this Lewis acid-catalyzed isomerization mechanism, and show a clear difference between Sn-fl (Lewis acid catalysis) and NaOH (Bronsted base catalysis). 

In addition to group V elements (Nb and Ta), mesoporous titanium nanoparticles with Lewis add sites are elTective for HMF formation from glucose [180, 181]. 

---