Study On The Acidity Adjustment Of ZSM-5/MCM-41 Hierarchical Zeolite By Reduction Strategy For Supercritical Catalytic Cracking Application

Catalytic cracking of hydrocarbon fuels as a strongly endothermic reaction can effectively solve the aerodynamic heat problem of hypervelocity?HFV?devices and provide a feasible solution for the design of active cooling systems for aircrafts.HZSM-5 zeolite is a commonly used catalyst for supercritical catalytic cracking of endothermic hydrocarbon fuels.However,due to the special nature of supercritical reaction fluids,there are problems in the diffusion of reactants and their products in the pores of microporous molecular sieves and serious deposition of carbon deposits.As a result,its catalytic activity and service life in the supercritical state are limited,so it is necessary to regulate the pore structure and acidity.In this paper,we studied the acidity regulation of HZSM-5 by two methods of ion exchange and alkali neutralization.These two methods can effectively regulate the acidity of HZSM-5 and have no significant effect on the crystal structure and pore characteristics of HZSM-5.However,the alkali neutralization method has problems of poor operability and repeatability,and the ion exchange method evades these two problems well and can effectively regulate the Br?nsted acidity of HZSM-5.In the normal pressure cracking of n-hexane,the catalytic activity is negatively correlated with the exchange degree of HZSM-5 protons,so the more acid content of HZSM-5 at atmospheric pressure is more conducive to the increase of catalytic activity,but in supercritical catalytic cracking,it is too strong.The acid easily causes deposition of carbon deposits and deactivates the catalyst instantaneously.Therefore,only moderate acidity and acid strength can help increase the catalytic activity and service life of HZSM-5 in the supercritical state.HZSM-5 molecular sieve was selected as raw material,and the composite molecular sieve HZSM-5/MCM-41 with micro-mesoporous structure was synthesized under hydrothermal conditions by post-synthesis.The synthesis conditions were optimized by orthogonal test.Ion exchange-reduction method was used.Under the ultrasonic conditions,NaCl solution and HZSM-5/MCM-41 zeolite were ion-exchanged to control their acidity and optimize their catalytic activity in n-dodecane catalytic cracking.Gas chromatography was used to analyze the change of catalyst activity during pyrolysis.The catalysts before and after the reaction were characterized by XRD,nitrogen adsorption,NH₃-TPD,temperature programmed oxidation?TPO?and temperature programmed surface reaction?TPSR?.The XRD and nitrogen adsorption-desorption curves show that the crystal form and pore properties are not affected by ion exchange.While NH₃-TPD,n-hexane cleavage probe reaction and n-dodecane temperature-programmed surface reaction show that different degrees of reduction of strong acid sites by ion exchange with different concentrations of NaCl solution.Although the initial conversion of the ion-exchanged composite zeolite in supercritical catalytic cracking of n-dodecane is slightly reduced,but its stability is greatly enhanced.The composite zeolite obtained after ion exchange with 0.8 M NaCl solution is after 3 hours of reaction,there was no apparent inactivation.In a word,by adjusting the acidity of the composite molecular sieve HZSM-5/MCM-41 by the reduction method,not only the activity and service life of the supercritical catalytic cracking are improved,but also the crystal structure and the pore properties are not significant affected.It is an advantageous exploration to optimize the performance of the supercritical catalytic cracking of the composite molecular sieve HZSM-5/MCM-41.Finally,based on coking analysis of the coked catalyst,we proposed a mechanism for the adsorption of acidic sites on the microporous and mesoporous of the composite zeolite catalyst:the catalytic activity is mainly provided by the acid sites in the microporous,and the coverage of strong acid sites by coke in supercritical catalytic cracking is a rapid process.There is a simple linear relationship between the loss of microporous and the effective coke blocking the pores,and mesoporous mainly improve the diffusion of reactants and products in the supercritical catalytic cracking.