(327d) Role of Lewis and Brønsted Acid Sites in the Dehydration of Glycerol over Niobia

Glycerol is a by-product of biodiesel production. It can be dehydrated to form acrolein, which is an important intermediate in the chemical industry. However, there is limited insight on the mechanism to optimize the reaction process. 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 acid sites. The proton of one of the primary alcohol groups dissociates to form a bridging alkoxy bond with two coordinatively unsaturated metal atoms, while the other primary OH group coordinates non-dissociatively to one of these metal atoms. The remaining secondary OH group forms a hydrogen bond to a surface oxygen atom. This multidentate surface species is irreversible in the presence of water vapor. 

When coordinated this way, the primary C-O bond involved in forming the bridging alkoxy bond is activated for dehydration to form 2-propene-1,2-diol. From DFT calculations, the adsorption of 2-propene-1,2-diol interacts with the surface via its primary OH group and alkene group, while its secondary OH group is tilted away from the surface, preventing its interaction with acid sites. In contrast, dehydration of the secondary alcohol group fn glycerol is favored over Brønsted acid sites in the absence of steric constraints. The primary product of this reaction, 1,3-propenediol, tautomerizes to form 3-hydroxypropionaldehyde. The latter is a very reactive and unstable molecule. It can further dehydrate to acrolein with the aid a Brønsted acid site. These pathways are supported by the linear correlation between the selectivity to hydroxyacetone and acrolein, and the acid ratio of Lewis and Brønsted acid sites, respectively. When more than a monolayer of glycerol is impregnated on niobia, monoaromatic compounds are also formed on the surface upon heating.