Catalytic Cracking of Supercritical n-Dodecane over Wall-Coated HZSM-5 with Different Si/Al Ratios

A series of wall-coated catalysts were prepared in stainless-steel microchannels with HZSM-5 zeolites with a Si/Al molar ratio of 25-140 by washcoating methods. Catalytic cracking of supercritical n-dodecane (4 MPa and 550 degrees C) was used to examine the catalytic activity and stability of HZSM-5 coatings (ZC). It is found that catalytic cracking activities and stabilities of the coatings increase by the following order: ZC25 < ZC50 < ZC100 < ZC140 (with the numbers representing the Si/Al ratio), which is in well accordance with the increasing Lewis acid and decreasing Bronsted acid amount of parent HZSM-5. Temperature-programmed oxidation (TPO) characterization of cokes deposited on the HZSM-5 coatings indicates that the coke amount deposited over coatings also increases with Si/Al ratios of parent zeolites, owing to the interaction of the coke formation and its supercritical extraction. Rapid deactivation of ZC25 was possibly caused by pore-mouth plugging by a little amount of coke, while a large amount of cokes of ZC140 may be a result of the gradual and uniform buildup of cokes in the pore under kinetic control.