Mechanochemical Decomposition of Crystalline Cellulose in the Presence of Protonated Layered Niobium Molybdate Solid Acid Catalyst

Abstract

Direct depolymerization of crystalline cellulose into water-soluble sugars by solvent-free ball milling was examined in the presence of a strongly acidic layered metal oxide, HNbMoO , resulting in full conversion with 72 % yield of water-soluble sugars. Measurements by C cross-polarization magic angle spinning NMR spectroscopy and X-ray diffraction revealed that amorphization of cellulose occurred rapidly within 10 min. Scanning electron microscopy equipped with an energy dispersive X-ray indicated that the substrate and the catalyst were well mixed during milling. The time course of the product distribution showed that most of the resultant water-soluble sugars were produced not by successive degradation of oligosaccharides but by direct depolymerization of cellulose chains. The products included glucose, mannose, and cello-oligomers, as well as anhydrosugars. Addition of small amounts of polar solvents increased the sugar yield, whereas further addition of water decreased the selectivity to anhydrosugars. Calculations of the mechanical energy required for the ball-milling process showed that 0.02 % was utilized for the chemical transformation under the conditions examined in this study.

Solvent-Free Approaches in Carbohydrate Synthetic Chemistry: Role of Catalysis in Reactivity and Selectivity

“…More recently, another layered solid, protonated niobium molybdate catalyst (HNbMoO. ), was found to be especially performing in cellulose depolymerization [97].…”

Rational Design of Metal Oxide Solid Acids for Sugar Conversion

“…Protonated layered niobium molybdate and tantalum molybdate, HNbMoO 6 and HTaMoO 6 , function as solid acid catalysts, whereas other layered metal oxides such as HTiNbO 5 and HNbWO 6 do not work because reactants cannot enter the interlayers [12][13][14][15][16][17][18][19][20]. The characteristics of layered HNbMoO 6 and HTaMoO 6 as solid acid catalysts are their unique ability of intercalation of reactants within the interlayers with strong Brønsted acid sites and high tolerance against water.…”

“…In other words, cellulose depolymerization over HNbMoO 6 was difficult because cellulose is a big molecule and insoluble in water [13]. In these regards, the mechanochemical reaction as a different methodology was adopted for cellulose depolymerization [20]. It has been reported that ball-milling of crystalline cellulose in the presence of acidic kaolinite, a layered clay mineral could depolymerize it and the solid acidity of additives is important to accelerate the reaction [21].…”

“…It has been reported that ball-milling of crystalline cellulose in the presence of acidic kaolinite, a layered clay mineral could depolymerize it and the solid acidity of additives is important to accelerate the reaction [21]. The ball-milling of cellulose with layered HNbMoO 6 could produce a high yield of water-soluble sugars (72%) at full cellulose conversion without adding external heat, whereas the ball-milling of cellulose in the absence of catalyst afforded negligible yield of sugars (<1%) [20]. Not only cello-oligomers such as cellobiose and cellotriose but also corresponding anhydro-sugars were produced by the mechanochemical reaction under dry conditions.…”

“…G1: Glucose: G1 : Mannose; G2: Cellobiose; G3: Cellotriose; G4: Cellotetraose; G5: Cellopentaose; G6: Cellohexaose; A1: Anhydroglucose; A2: Anhydrocellobiose. Reproduced with permission from[20]; copyright (2018), John Wiley & Sons, Inc.…”