Selective Production of Light Aromatics from Hydrocracking of Pyrolytic Lignin over Ni/HZSM-5

Pyrolytic lignin (PL), namely the water-insoluble fraction of bio-oil, is mainly composed of phenolic oligomers, and therefore, it has the potential to be converted into light hydrocarbons and phenols. This work studied the hydrocracking of PL for the generation of light aromatics (including monoaromatic hydrocarbons, double-ring aromatic hydrocarbons, and monophenols). By comparing different PL conversions, the reaction mechanism of PL hydrocracking over Ni/HZSM-5 was proposed, and particularly, the synergistic catalytic effects of Ni and acid sites on hydrodepolymerizatrion and hydrodeoxygenation were explained. Then, the effects of Ni loading on Ni/HZSM-5, reaction temperature, and reaction pressure were investigated and discussed based on the proposed reaction mechanism. It was found that a good balance of Ni and acid sites under the Ni loading of 10% could achieve the best synergistic effect for hydrodepolymerization and hydrodeoxygenation. A suitable reaction temperature of 500 °C achieved the efficient hydrodepolymerization and hydrodeoxygenation, while the removal of alkyl side groups attached to the benzene ring was not intense. Continuously increasing the reaction pressure to 4 MPa also facilitated the hydrodepolymerization of PL as well as the hydrodeoxygenation of the monophenols. Under the optimum reaction parameters, the yields of monoaromatic hydrocarbons, double-ring aromatic hydrocarbons, and monophenols reached 16.61%, 7.81%, and 9.36%, respectively. The spent catalyst was deactivated because of the carbonaceous deposits on it, and it could be well regenerated by burning these carbonaceous deposits.