Comparison of hydrocracking and cracking of pyrolytic lignin over different Ni-based catalysts for light aromatics production

Abstract

This work compared hydrocracking and cracking of pyrolytic lignin (PL) for light (single-ring and double-ring) aromatics production over Ni-based catalysts. Due to the supports' original acidities and the interactions between metal and supports, these Ni-based catalysts had different amounts of available Ni and acid sites, which affected depolymerization and deoxygenation in both cracking and hydrocracking. During cracking, the PL depolymerization could be promoted by increasing Ni and acid sites but this promotion was limited by the hydrogen inside PL, whereas both Ni and acid sites exhibited poor performance in deoxygenation. The cooperation of Ni and acid sites achieved efficient hydrodepolymerization and hydrodeoxygenation in hydrocracking. Acid sites benefitted the cracking of linkages in PL and the resulting intermediates could be converted into monophenols through Ni-catalyzed hydrogenation. In the selective hydrodeoxygenation mechanism of monophenols to monoaromatic hydrocarbons, acid sites facilitated the initial tautomerization and the final dehydration reactions, while the middle hydrogenation step was catalyzed by Ni. Additionally the micropore structure of catalyst also favored the generation of light aromatics. Therefore, hydrocracking over Ni/HZSM-5 with sufficient available Ni and acid sites and the micropore structure, realized the most effective conversion of PL to light aromatics, especially aromatic hydrocarbons.