Investigation of binder modified zeolites for the catalytic cracking of endothermic fuels
Creators: name: Hubesch, Roxanne
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340601
subject: Catalysis and mechanisms of reactions

description: A major challenge for high-speed aircrafts is the development of an active cooling system that can efficiently reduce the heat loads experienced at the combustor walls. One potential approach is to use the on-board hydrocarbon fuel to serve as both a propellant and an efficient heat sink. In this approach, the heat sink capacity of the fuel can work as a cooling system by absorbing waste heat to undergo endothermic reactions as the fuel circulates around the combustor wall. In this regard, catalytic cracking can provide a substantial chemical heat sink for so-called endothermic fuels. The use of a catalyst can control the reaction rates and form desired products of short ignition delay, such as short chain hydrocarbons, which is essential for efficient combustion of fuel. During the circulation of fuel in the channels of the heat exchangers, the fuel usually reaches its supercritical state, which results in different catalytic behaviour and lower carbon deposition. Therefore, the development of a catalyst which can lead to a great heat sink through the formation of products of short ignition delay, and the minimised deposition of carbon on the catalyst is key to a sustainable thermal management system. In this context, binders are known materials used in catalysis for their properties to modify the acidity and selectivity of a catalyst, which can also lead to the decrease of coke deposition.

The aim of this thesis is to study the addition of binders to zeolites to understand their impact on the catalytic behaviour and carbon deposition in the context of catalytic cracking for heat management of high-speed flight vehicles. Two binders, alumina and aluminophosphate (AlPO), were added in situ to three HY zeolites of varying Si:Al ratios with a zeolite:binder mass ratio of 70:30. The characterization of the catalysts showed that the addition of binders did impact the textural properties and the acidity of the catalysts. The addition of alumina induced additional mesoporosity in the catalyst, was not thought to bind on specific sites of the zeolite and only partially covered strong acid sites on the zeolites. The addition of AlPO did not affect the morphology of the catalyst as only small lumps seemed to have been formed, but AlPO decreased the catalyst acidity considerably by specifically binding on the strong Brønsted acid sites.

Catalysts were studied in three operating conditions for the catalytic cracking of methylcyclohexane (MCH): at low flow rate and partial pressure of MCH for fundamental understanding of the modified catalysts, and at high flow rates and pressures (under both subcritical and supercritical conditions) to assess their catalytic behaviour in realistic conditions. The catalytic cracking at low pressure and a low flow rate showed steady state catalytic activity. The addition of alumina to zeolites only slightly decreased the catalytic activity whereas the addition of AlPO considerably decreased the catalytic activity. The catalytic activity at high flow rates and pressures increased with increasing pressure, but also showed significant deactivation rates. The addition of alumina to zeolites only slightly decreased the initial catalytic activity but did not improve the deactivation rate. On the other hand, the addition of AlPO kept the catalytic activity lower but steady, with a low deactivation rate under subcritical conditions and no deactivation under supercritical conditions. Product selectivity also varied considerably based on the catalysts and operating conditions. Where low flow rate and pressure showed a majority of MCH isomers formed, high flow rate and increasing pressure led to a higher product selectivity towards short chain alkanes, which are desired products in terms of efficient combustion in high-speed flight vehicles. The addition of alumina only slightly decreased the formation of short chain alkanes. The addition of AlPO decreased the formation of short chain alkanes more than alumina while keeping a considerable product selectivity towards short chain alkanes under supercritical conditions. 

The deposition of carbon on the catalysts was lowered by the addition of binders, with AlPO addition leading to the best hinderance of carbon deposition. Because AlPO specifically binds on the strong acid sites of the zeolite, the formation of hard coke present as aromatic carbon was avoided, while the addition of alumina on zeolites only slightly lowered the presence of hard coke. Carbon deposition also decreased under supercritical conditions when comparing the amount of MCH converted to the amount of carbon deposited on the catalysts, and this improvement was observed to be the highest on AlPO modified zeolites. For these reasons, AlPO seemed the best candidate to adequately modify zeolites for sustainable catalytic activity, product selectivity and coke deposition.

One zeolite, one alumina modified zeolite and one AlPO modified zeolite were also washcoated on a 3D printed Inconel metallic support. This was done to mimic the conditions of wall-coated catalysts, which are the conditions in which catalysts would be present in heat exchanger channels, and assess the potential of binder modified zeolites in these conditions. The unexpectedly high catalytic conversion at a low flow rate and partial pressure of MCH was due to the probable synergy between metals migrating from the 3D printed support into the zeolite particles. This led to the very high product selectivity towards hydrogen, and short chain alkanes and alkenes.