Mechanistic Understanding of Anthracene Hydrocracking over HY Zeolite Encapsulated Single-Atom Pt Catalysts

Abstract

The hydrocracking of polycyclic aromatic hydrocarbons (PAHs) leading to the production of benzene, toluene, and xylene (BTX) is one of most important industrial petrochemical processes. Although experiencing extensive experimental effort and industrial practice, the underlying hydrocracking reaction mechanisms of PAHs over zeolite-supported metal catalysts are still elusive. In particular, when, where, and how the aromatic rings of PAHs are open via C–C bond breaking is not clear. In the present work, the hydrocracking reaction pathways of anthracene over the HY zeolite encapsulated single-atom Pt catalyst (Pt1/HY) as a demonstration case for the hydrocracking of PAHs have been investigated using density functional theory (DFT) calculations. The ring-opening processes of the terminal and the central rings of anthracene with respect to the saturated hydrogenation degrees of aromatic rings and the hydrogen sources, which result in different BTX products, have been systematically examined. The hydride transfer over the same aromatic ring is facile, while it is kinetically hindered at the connecting C atoms (Ca) between the neighboring aromatic rings. The hydride transfer between the terminal and the central rings, in the case of the isomerization of 2,3-dihydroanthracene to 9,10-dihydroanthracene, can be achieved through the intermolecular hydride transfer mechanism with the assistance of anthracene. Compared to the hydrogenation from the Pt1 site, the addition of a proton from the Brønsted acidic sites (BAS) on the aromatic ring of partially hydrogenated anthracene would pronouncedly weaken the C–C bond, resulting in the central ring-opening process. DFT calculation results show the central ring opening is kinetically favorable in the anthracene hydrocracking over the Pt1/HY catalyst, generating BTX as the major product rather than butylbenzene and n-butane. The first protonation step by BAS on both rings is the most kinetically relevant step. In addition, the hydrocracking reaction of branched PAHs, using octylanthracene as the probe molecule, has also been investigated. It has been found that the dealkylation of octylanthracene at the ring-branched chain connection position is kinetically more feasible than the central ring opening and the cleavage of the octyl chain. The present work provides a comprehensive and insightful mechanistic understanding of the hydrocracking reaction pathways of PAHs over bifunctional HY zeolite supported metal catalysts.