Role of Lewis and Brønsted Acid Sites in the Dehydration of Glycerol over Niobia

Abstract

The role of Lewis and Brønsted sites in the dehydration of glycerol on niobium oxide and Na+-exchanged niobium oxide is investigated using FTIR spectroscopy supported by DFT calculations. Glycerol is impregnated on the catalysts at room temperature using an ex-situ method. Under high vacuum conditions, glycerol forms a stable multidentate alkoxy species through its primary hydroxyl groups with the Lewis sites. When coordinated this way, the primary C–O bonds are polarized, favoring dehydration in this position to form hydroxyacetone. In contrast, dehydration of the secondary alcohol group is kinetically favored over Brønsted acid sites in the absence of steric constraints. The primary product of this reaction, 1,3-propenediol, is further dehydrated to acrolein. When more than a monolayer of glycerol is impregnated on niobia, monoaromatic compounds are also formed on the surface upon heating.