Synthesis Of Orientation-tuned HZSM-5 Bi- Layers And Its Performance In Catalytic Cracking

Catalytic cracking of hydrocarbon fuel is an endothermic process,have potential to address thermal barriers in hypersonic flight.HZSM-5 membranes which have surprisingly high thermal and hydrothermal stabilities,low pressure drop,as well as thermal resistance,is an ideal structural catalyst for catalytic cracking of hydrocarbons under supercritical conditions.However,the synthesis of ZSM-5 zeolite membrane with high diffusion properties is still difficult.This paper mainly focused on the synthesis of HZSM-5 bi-layers with different structures by in situ growth method and studied their catalytic performance using a model reaction of catalytic cracking of n-dodecane.In this work,HZSM-5 bi-layers with b,b&a and b&a&h0h/c orientation on the inner surface of stainless steel tubes were synthesized by in situ crystallization.It was found that HZSM-5 bi-layers with b orientation exhibit better catalytic performance than the other oriented membranes.Compared to the b&a oriented membrane,the reaction rate was increased by 42%and the inactivation rate was reduced to 33%for catalytic cracking over b oriented membrane.Secondly,b-oriented HZSM-5 bi-layers were prepared on the inner surface of stainless steel tubes by in situ hydrothermal crystallization method with various aluminum sources.In addition,catalytic cracking of supercritical n-dodecane was carried out over the as-synthesized b-oriented HZSM-5 bi-layers at 500°C and 4 MPa in the zeolite-grown tubes as a reactor.The ZSM-5 layers synthesized with inorganic aluminum sources had high b-orientation and almost full coverage,whereas those obtained with organic aluminum sources gave the opposite results.It was found that the layers prepared with inorganic aluminum with highest b-orientation exhibited the cracking rate of n-dodecane higher than 3.5×10^-2 mol·g^-1·s^-1 and the deactivation rate lower than 3%.Finally,b-oriented HZSM-5 bi-layers were prepared on the inner surface of stainless steel tubes by in situ hydrothermal crystallization method with various aluminum contents,and their catalytic performance were evaluated by catalytic cracking of n-dodecane.It was found that HZSM-5 bi-layer could be synthesized with a low silica to aluminum value?37.4?,low b-orientation?0.86?,as well as low continuity.For the layer prepared with a higher Si/Al ratio?68.6?,the b-orientation is 0.94 and crystal coverage is 0.94.The layer prepared with the synthesis solution with a Si/Al ratio value equal to 100 showed the most rapid cracking rate of n-dodecane(3.7×10^-2mol·g^-1·s^-1)and the lowest deactivation rate.