(274b) Influence of Solid Acid Structure and Composition on Hydrocarbon Cracking Activity Under Supercritical Conditions

Optimization of hydrocarbon cracking catalysts under supercritical conditions is needed to develop endothermic fuel cooling systems for use on hypersonic aircraft. In this study, cracking of several different hydrocarbons over solid acid catalysts with various pore sizes and compositions was studied under supercritical conditions. The specific activity of porous aluminosilicate catalysts was substantially higher than that of non-porous tungstated zirconia (WO_x/ZrO₂) for cracking of n-dodecane, a surrogate molecule for jet fuel. Microporous H-ZSM-5 zeolite exhibited the highest turnover frequency (TOF), normalized by H+ sites titrated by n-propylamine decomposition, for cracking of linear hydrocarbons hexane and dodecane. However, the relatively small pore size of H-ZSM-5 negatively impacted the cracking of branched hydrocarbons (isooctane and isododecane) as well as military jet fuels (JP-8 and JP-10). Utilization of H-Y as an acid catalyst, which had a micropore size large enough to accommodate the kinetic diameter of JP-10 (exo-tetrahydrobicyclopentadiene), revealed excellent conversion of the hydrocarbon relative to H-ZSM-5, thus emphasizing the importance of proper catalyst/fuel pairing. The influence of temperature on the conversion of n-hexane and n-dodecane over H-ZSM-5, H-Y, and a mesoporous aluminosilicate revealed that hydrocarbon chain length and catalyst pore size significantly affected the apparent activation energy and observed product distribution. Mechanistic implications of the results will be discussed.