Thermal Decomposition of Acetic and Formic Acid Catalyzed by Red Mud—Implications for the Potential Use of Red Mud as a Pyrolysis Bio-Oil Upgrading Catalyst§Dedicated to Prof. Ulf Schuchardt on the occasion of his retirement.

Acetic and formic acid impart a high acidity on pyrolysis bio-oil (obtained by fast pyrolysis of ligno-cellulosic biomass), which is one of the factors preventing its direct use as a fuel. At temperatures ≥ 330 °C, Red Mud, a waste byproduct of the aluminum industry produced at >70 million tons p.a., is a good catalyst for thermal decomposition of these acids. Formic acid can serve as an internal source of hydrogen through the formation of synthesis gas and the water gas shift reaction. The formation of C6−C10 hydrocarbons in the nonpolar phase of the resulting product mixture and the identification of C3 and C4 hydrocarbons and CO2 in the gas phase and acetone in the polar liquid phases can be rationalized through mechanisms involving ketene as the intermediate formed by acetic acid dehydration, with subsequent formation of acetone. Higher hydrocarbons, mostly alkanes and alkenes, are then formed through iterative aldol condensation, hydrogenation, hydrogenolysis, and deoxygenation reactions of the primary products. During the reaction, the Red Mud used in these reactions undergoes a distinct color change to gray, yielding a nonalkaline magnetic material containing Fe3O4 and metallic iron rather than Fe2O3.