Abstract
Aqueous-phase acid-catalyzed reactions are essential for the conversion of cellulose-based biomass into chemicals. Brønsted acid and Lewis acid play important roles for these reactions, including hydrolysis of saccharides, isomerization and epimerization of aldoses, conversion of d-glucose into 5-hydroxymethylfurfural, cyclodehydration of sugar alcohols and conversion of trioses into lactic acid. A variety of metal oxide solid acids has been developed and applied for the conversion of sugars so far. The catalytic activity is mainly dependent on the structures and types of solid acids. Amorphous metal oxides possess coordinatively unsaturated metal sites that function as Lewis acid sites while some crystal metal oxides have strong Brønsted acid sites. This review introduces several types of metal oxide solid acids, such as layered metal oxides, metal oxide nanosheet aggregates, mesoporous metal oxides, amorphous metal oxides and supported metal oxides for sugar conversions.
Highlights
The fundamental features of solid acid catalysts are the reusability of the catalysts, their easy separation from the products and solvents and unnecessary neutralization, which are beneficial to saving energy and cost
5·nH2O, which had 3 mmol of water adsorbed on 1 g of the metal oxide, still had Lewis acid most the NbO4 tetrahedra was converted into NbO4 ·H2 O adducts
Followed by byfructose fructose dehydration (ii) stepwise glucose dehydration catalyzed by acid-base a Lewis followed dehydration andand (ii) stepwise glucose dehydration catalyzed by a Lewis acid-base pair
Summary
The fundamental features of solid acid catalysts are the reusability of the catalysts, their easy separation from the products and solvents and unnecessary neutralization, which are beneficial to saving energy and cost. Reactants have less functional groups with non-polarity and are converted in gas-phase flow systems These reaction conditions are very different from those of sugar conversion in the Catalysts 2019, 9, 907; doi:10.3390/catal9110907 www.mdpi.com/journal/catalysts. As water-tolerant solid acid catalysts, hydrophobic zeolites [4,5], solid heteropolyacids such as Cs2.5 H0.5 PW12 O40 [6], metal oxides including amorphous niobium oxide (Nb2 O5 ·nH2 O) [7,8], MoO3 -ZrO2 [9] and metal phosphates including niobium phosphate [10] have been reported Some of these water-tolerant solid acids are applicable for aqueous-phase sugar conversion.
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