What is the effect of zeolite topology on the hydrocracking pathways of 9,10-dihydrophenanthrene (9,10-DHP)?

9,10-Dihydrophenanthrene (9,10-DHP) is a partially hydrogenated polycyclic aromatic hydrocarbon (PAH) that is relevant in the upgrading of heavy oil fractions. The study investigates how different zeolite topologies influence the hydrocracking pathways of 9,10-DHP. Zeolites are microporous aluminosilicate materials with different pore structures that can act as catalysts in hydrocracking processes. The study focuses on three industry-friendly zeolites: H-ZSM-5, H-USY, and H-Beta, which vary in their pore topology, acidity, and crystal size.

The hydrocracking of 9,10-DHP is initiated through two primary routes: bimolecular hydrogen transfer and monomolecular protolytic attack on the central rings. The key finding is that the relative influence of these two hydrocracking pathways is predominantly governed by the pore topological structure of the zeolites. Specifically:
H-ZSM-5 (10MR pores): Predominantly favors the monomolecular protolytic attack pathway, leading to extensive hydrogenation and cracking of 9,10-DHP into small alkanes (propane, butanes, pentanes, etc.). This is due to its small pore structure, which facilitates deep cracking.
H-USY (12MR pores): Favors the bimolecular hydrogen transfer pathway, resulting in the formation of phenanthrenes, tetrahydropenanthrene (THP), and other aromatic compounds. This is attributed to its larger pore size, which allows for bimolecular reactions.
H-Beta (12MR pores with a smaller sinusoidal channel): Exhibits a hybrid performance characteristic of both small-pore and large-pore zeolites. It shows a higher proportion of small molecular alkanes and benzene compared to H-USY, due to its unique pore structure that facilitates both protolytic cracking and dealkylation reactions.