Suppressing Cracking Side Reactions to Enhance the Hydroisomerization of Polycyclic Aromatic Hydrocarbons into Alkyl Adamantanes

The hydroisomerization of bulky polycyclic aromatic hydrocarbons (PAHs) to adamantane-type high energy density fuels (HEDFs) using zeolite catalysts has excessive cracking of carbocations in ring-isomerization reactions, showing a low yield of alkyl adamantanes (AlAs). In the current study, the two-step approach, combining initial hydrogenation saturation followed by isomerization, not only avoids the cracking side reactions during hydrogenation but also exhibits broad applicability across different substrate systems. Due to the limitations of PAHs molecular size on mass transfer efficiency, the isomerization reactions occur at outer surface defects, within mesopores, and at accessible microporous mouths. High-temperature steam treatment was conducted to selectively modify the strong and medium acid sites of USY, effectively suppressing cracking side reactions, and to enlarge the volume of the micropores and mesopores, enhancing mass transfer efficiency, thereby achieving an improved one-step hydroisomerization yield from phenanthrene to AlAs (from 19.5% to 29.5%). The substrate scope study revealed that the yield of AlAs decreased from 65.3% to 2.1% as the number of benzene rings and molecular rigidity increases. The critical intermediate methyl-perhydrophenalene (MPHP) with a bridgehead carbon structure was detected, which will further be converted into various AlA compounds.