Catalytic fast pyrolysis of biomass over Mg-Al mixed oxides derived from hydrotalcite-like precursors: Influence of Mg/Al ratio

In this work we examine a series of Mg-Al mixed oxides derived from hydrotalcite-like precursors prepared with different Mg/Al ratios i.e. Mg/Al = 2, 3, and 4, as catalysts for the catalytic fast pyrolysis (CFP) of wheat straw. Characterization of the Mg-Al mixed oxides by N-2 adsorption, X-ray diffraction and CO2 temperature programmed desorption revealed a decrease in the surface area and a higher number and proportion of basic sites corresponding to an increase in the Mg concentration in the hydrotalcite precursors. The Mg-Al mixed oxides derived from hydrotalcite precursors are active materials for the catalytic fast pyrolysis of biomass. The removal of oxygen during the catalytic fast pyrolysis of biomass over the Mg-Al-mixed oxides depends on their Mg/Al ratio, and an increase in gas production with a strong improvement in the decarboxylation capacity as the Mg concentration in the mixed oxides decreased, was observed. The Al-Mg mixed oxide catalyst with the lowest Al concentration (Mg/Al = 4.0) maintained the deoxygenation and bio-oil energy yield at levels comparable to those achieved over an acidic ZSM-5 zeolite widely used in the CFP of biomass. A moderate increment in ketone formation for the catalyst with Mg/Al = 4.0 suggests that these basic materials promote the aldol condensation and ketonization of pyrolytic compounds by removing the oxygen in the form of H2O and CO2.