Cascade Deoxygenation Process Integrating Acid and Base Catalysts for the Efficient Production of Second-Generation Biofuels

An innovative cascade deoxygenation concept coupling the thermal pyrolysis of lignocellulose with two consecutive (acidic and basic) catalytic beds in a single reactor has been developed. Thereby, thermal pyrolysis vapors produced by pyrolysis of acid-washed wheat straw have been upgraded by combining an acidic ZrO₂-supported ZSM-5 zeolite and a basic K-grafted USY zeolite, both being prepared in technical form by extrusion with attapulgite binder. Investigation of the different coupling alternatives has shown that the most efficient strategy relies on passing the pyrolysis vapors first by the acid catalyst followed by the basic one in a fixed-bed reactor arrangement. In this way, an enhanced degree of bio-oil deoxygenation is achieved since the K/USY catalyst promotes the occurrence of both aldol condensation and ketonization reactions, thus leading to further bio-oil upgrading by removal of oxygen in the form of CO₂ and H₂O while affording the formation of C–C bonds. Moreover, this second basic catalyst promotes the further conversion of the oligomers still present in the bio-oil. Accordingly, the sequential integration of the two catalytic systems leads to a clear synergetic effect, improving the efficiency of the bio-oil deoxygenation process and affording the production of liquid organic fractions with oxygen content as low as 12 wt % and energy yield of 32% regarding the raw lignocellulosic biomass.