| The catalytic cracking of supercritical hydrocarbons provides a solution to the aerodynamic thermal problems generated in the flight of ultra-high-speed vehicles.Using hydrocarbon fuel as a propellant and coolant during UHP flight,the fuel absorbs heat and provides positive thermal protection.ZSM-5 zeolites are widely used in alkane cracking reactions because of their high stability,high skeletal density distribution,and special shape selectivity.However,the diffusion limitation and lower acidic site accessibility of microporous molecular sieves in the catalytic cracking of supercritical hydrocarbons greatly affect the reactivity,Therefore,the introduction of mesopores in microporous zeolite is an effective solution.In this paper,hierarchical ZSM-5 zeolite catalysts were prepared by post-processing and in situ synthesis,and the catalytic cracking of n-dodecane under supercritical conditions（500°C,4 MPa）was used as a model reaction to evaluate the catalytic performance of ZSM-5 zeolite coating and to analyze the relationship between the chemical properties of ZSM-5 zeolite and the catalytic cracking performance of n-dodecane.Etching ZSM-5 zeolite by a mixture of HF and NH4F solution can avoid the formation of extra-framework aluminum while constructing mesopore channels,which is an effective method for constructing hierarchical ZSM-5 zeolite.However,the formation of mesopores is accompanied by a large loss of active components.Using oxygen phosphorus components,and quaternary ammonium salts as the protective agents can effectively slow down the etching process.It was found that tetramethylammonium bromide（TMABr）and tetrapropylammonium bromide（TPABr）tend to protect the silica component of the zeolite in an acidic environment,while trimethylphosphine oxide（TMPO）and tributylphosphine oxide（TBPO）can adsorb on the B-acid sites of the zeolite and provide preferential protection to aluminum,thus improving the skeletal active species concentration and acidic site accessibility of the ZSM-5 zeolite.The zeolite was subsequently coated on the inner wall of stainless steel to prepare a hierarchical ZSM-5 zeolite coating.The initial conversion of n-dodecane was increased by 51%and the deactivation rate was reduced by 56%for the catalyst prepared by TBPO-assistance compared to the non-fluorinated ZSM-5 zeolite coating,and the improved catalytic performance could be attributed to the improved diffusion performance and the increased amount of accessible acid.High-strength hierarchical ZSM-5 zeolite coatings were prepared on stainless steel carriers by in situ dry glue transformation method using silanized starch as a binder and mesoporous templating agent.The silanized starch is hydrolyzed to produce a large amount of silica-methoxy,which is similar to a"gripper"and makes the silica-aluminum component crystallize and nucleate around it,avoiding the isolated generation of zeolite particles and the formation of cracked gaps in the coating.The coating showed stronger mechanical stability,compared with the coatings prepared with starch as well as those without silylated starch or starch.Meanwhile,silylated starch as a polymeric macromolecule also acts as a templating agent to construct mesoporous pores and improve the diffusion performance of the ZSM-5 zeolite-coated catalyst.Compared with the ZSM-5 zeolite catalyst without starch as a templating agent,the initial conversion of n-dodecane over the coating prepared with the assistance by moderate amount of silylated starch increased by 46%and the deactivation rate decreased by 56%,which can be attributed to its excellent diffusion performance. |
