Study on the hydrocarbon-rich bio-oil from catalytic fast co-pyrolysis cotton stalk and polypropylene over alkali-modified HZSM-5

The quality of bio-oil produced from fast pyrolysis of lignocellulosic biomass needs urgent upgrading due to its undesirable characteristics. In this study, HZSM-5 modified with different alkali solutions is used to catalytic fast co-pyrolysis of cotton stalk and polypropylene, investigating pyrolysis temperatures, catalysts modified with different alkali solutions, the cotton stalk (CS) to polypropylene (PP) ratio, and the feedstock to catalyst ratio. Thermogravimetric experiments revealed that PP undergoes rapid decomposition between 408 °C and 465 °C, during which a substantial amount of hydrogen free radicals is released. These radicals subsequently react with oxygen-containing compounds emitted by carbonaceous substances CS, thereby facilitating weight loss and minimizing coke formation. The FTIR results also indicate that the presence of PP significantly increases the transmittance of O-H groups at 3600–3000 cm −1 and C-O groups at 1290–900 cm −1 compared to CS. The TPAOH-modified HZSM-5 possesses a mesoporous structure and porous channels, facilitating the completion of deoxygenation of pyrolysis vapors at the acid sites. CFP of CS over HZSM-5 modified TPAOH (1.5 mol/L) at 600 °C is carried out, resulting in the highest hydrocarbon content in the bio-oil reaching 63.3 %. Concurrently, the BTEX concentrations attain their maximum values of 6.5 %, 12.9 %, 3.4 %, and 12.6 %. In addition to the hydrocarbon pool mechanism, the Diels-Alder reaction between furan and olefin is a crucial synergistic effect for generating aromatics. Upon the addition of PP in a 1:1 ratio, the contents of BTEXs attain maximum values of 8.1 %, 15.5 %, 2.3 %, and 15.5 %. Response surface methodology (RSM) indicates that the organic alkali concentration used for catalyst modification is the most significant factor influencing aromatics generation. Overall, this study aims to improve bio-oil quality and detail the reaction mechanism. • Catalytic fast co-pyrolysis of CS and PP over HZSM-5 alkali modified was studied. • TPAOH modification was superior to NaOH modification in improving HZSM-5. • Diels-Alder reaction further promoted hydrocarbons production. • Hydrocarbons of 78.16 % in the optimal operating conditions were obtained. • The concentration of TPAOH solution used for modification was foremost factor.