Study On Secondary Growth Of HZSM-5Membrane And Properties For Catalytic Cracking Of Supercritical Hydrocarbon

For future hypersonic aircraft, hydrocarbon fuels not only provide propulsionpower via combustion, but also play a crucial role as an ideal coolant to offer anecessary sufficient cooling capacity (heat sink) for the advanced coolingtechnologies of engines and combustors. Importantly, the removal of the waste heatcan be achieved through endothermic cracking reactions of hydrocarbon fuels whenthey flow inside the microchannels of the heat exchangers, which is operated atelevated temperature and pressure. Specially, owing to its low pressure drop andthermal resistance, the catalytic membrane reactor (CMR) with zeolite membrane onthe inner wall is an ideal candidate. In this paper, a series of cracking reactions ofmodel fuel were carried out in the CMR under the simulated conditions of thesupercritical state that hypersonic aircrafts actually worked in.The HZSM-5membranes were prepared on the inner surface of stainless steeltubes by the secondary-growth method. The membranes were testified by the catalyticcracking of n-dodecane under supercritical conditions and the product distribution anddeactivation mode were carefully investigated.The HZSM-5membranes with tunable thickness of5.7-13.8μm and massloading of0.68-2.05mg/cm~2were synthesized by the repeated growth. The effects ofthickness and texture of membranes on the catalytic activity were studied. Thickermembrane highlights higher conversion of n-dodecane for the larger amount ofcatalyst and active sites, but the very thick membrane possessed the dense layer,which reduced the diffusion rate and resistance for coke formation. So it is necessaryto prepare ZSM-5membrane with suitable thickness.By tuning the Al content in the HZSM-5membrane, we produced the membranewith different Si/Al ratios and investigated its effect on the surface morphology,crystal size and acidity. It was found that weakening the acidity of zeolite membranesignificantly improved the catalytic acidity and stability, but much lower Al amountcaused insufficient of acid sites and cracking activity of n-dodecane was suppressed.The alkaline treatment of HZSM-5membrane was conducted to study the effectof the micro-and meso-pores on the n-dodecane cracking. The strong alkali treatmentcreated the mesopores in HZSM-5membrane while the weak basic treatment leads tocleaning of microporous channels, both facilitating the diffusion of primary productsin the cracking process. In this work, we studied the effects of mass loading, surface acidity and alkalinetreatment on the textures of HZSM-5membrane. The conversion of n-dodecane,products distribution, CMR and RM were used to deeply investigate the catalyticacidity, stability and deactivation mode.