Promoting Deoxygenation of Bio-Oil by Metal-Loaded Hierarchical ZSM-5 Zeolites

The catalytic upgrading of crude bio-oils obtained through the pyrolysis of lignocellulosic biomass remarkably improves the properties of the final bio-oil. Here, it is demonstrated that the impregnation of hierarchically structured ZSM-5 zeolites with metal cations (Sn, Cu, Ni, or Mg) promotes oxygen removal. Remarkably, the Mg-loaded hierarchical zeolite has led to the best fuel characteristics, achieving the greatest reduction in the oxygen-content and the lowest acidity. The promotion of ketonization reactions of acids with aldehydes to produce ketones seems to be favored over the Lewis acid sites created after incorporation of Mg cations at the ion exchange sites. A slightly lower deoxygenation rate is obtained for Cu-loaded hierarchical ZSM-5 zeolite. However, some subtle differences are identified, such as the remarkable amount of evolved CO observed in the gas fraction. Thus, decarbonylation of acids to aldehydes at Cu cations incorporated at ion exchange positions seems to be the prevalent deoxygenation reaction for this solid. Similarly, the preferential mechanism for O-removal using hierarchically structured Ni and Sn-ZSM-5 zeolite catalysts seems to proceed through decarbonylation and decarboxylation reactions at the Lewis acid sites evolved after metal incorporation. Although a prevalent reaction mechanism could not be identified, lower cation incorporation at ion exchange positions could explain the inferior deoxygenation rate. In all the cation-loaded hierarchical zeolites, the incorporation of metallic species at the ion exchange sites decreases the production of aromatics, and this is linked with a lower amount of Brønsted acid sites.