Effect of HZSM-5 Coating Thickness upon Catalytic Cracking of n-Dodecane under Supercritical Condition

Supercritical catalytic cracking of hydrocarbon fuels over zeolite coatings is an important technique to improve cooling capacity of hydrocarbon fuels for the advanced aircraft. A series of HZSM-5 zeolite coatings with different thickness (6.13−18.27 μm) was prepared in the stainless steel tubes by washcoating methods to study the effect of the coating thickness on the catalytic activity, deactivation of catalyst, and adhesion strength. Scanning electron microscopy (SEM) characterization of prepared coatings indicated that the uniformity of the coating surface as well as the thickness was enhanced by repeated washcoating. The catalytic activity and deactivation of the coatings was investigated using catalytic cracking of n-dodecane under supercritical phase (550 °C, 4.0 MPa). It was found that the catalytic activity during 30 min raises from 12.5 to 19.2% (about 53.2% improvements) when the thickness of the HZSM-5 coating increases from 6.13 to 18.27 μm, but the significant deactivation was also observed due to more coke being generated (from 2.4 mg/cm2 to 10.7 mg/cm2). According to the temperature-programmed oxidation and SEM characterizations on deactivated HZSM-5 coatings, a hypothesis of thickness effect on coke distribution and formation in HZSM-5 coating was proposed and further confirmed by EDS anaylsis on cross-sections of deactivated HZSM-5 coatings.