Solid Acid Catalysts for Endothermic Fuels

Vehicles traveling at high supersonic and hypersonic flight speeds experience high thermal heat loads that exceed the heat sink capacity of existing thermal management systems. Conventional thermal management systems are limited by the physical heat sink capacity of hydrocarbon fuels. One method to increase the heat sink capacity of hydrocarbon fuels is to carry out endothermic reactions on the hydrocarbon fuel near the heat load. The breaking of C-C bonds over a catalyst, otherwise known as catalytic cracking, is an endothermic reaction that can provide a substantial heat sink for so called endothermic fuels. Understanding the effects of catalyst porosity and hydrocarbon molecular structure for catalytic cracking under supercritical conditions is necessary for the rational design of catalyst/fuel pairings for endothermic fuels.

In this study, reactions of linear, branched, and cyclic hydrocarbons were performed over micro- and mesoporous solid acid catalysts under supercritical conditions. Measurements of intrinsic rate parameters were used to develop a fundamental understanding of the reaction mechanism under supercritical conditions. The specific activity of porous aluminosilicate catalysts was substantially higher than that of non-porous tungstated zirconia (WOx/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. This work illustrates the importance of proper catalyst/fuel pairing for endothermic fuel cracking reactions.