Research On Accurate Control Of Acidic Site Of HZSM-5 Based On Alkali Neutralization And Its Anti-coking In Supercritical Catalytic Cracking

As a high-tech to maintain national security,the practical significance and research value of hypersonic vehicle is self-evident.Nowadays,the main research focuses of hypersonic vehicle are engine,thermal management and guidance control.Among them,using an active cooling system to solve the problem of heat treatment is a new idea.Endothermic hydrocarbon fuel is widely concerned as a fuel coolant.The chemical heat sink provided by endothermic hydrocarbon fuel can absorb heat well.HZSM-5 zeolites is widely used in the catalytic cracking of endothermic fuels because of its strong acidity and unique two-dimensional pore structure.The main reason for catalyst deactivation is carbon deposition,which is the by-product of cracking reaction.Therefore,it is a great significance to improve the catalytic activity and stability of HZSM-5 zeolites and to inhibit the harm of carbon deposition on zeolites.In this paper,HZSM-5 zeolites suspension was directly neutralized by low concentration Na OH aqueous solution at room temperature.The acid of HZSM-5zeolites suspension was accurately controlled by acid-base neutralization titration and hydrothermal synthesis.The modified catalyst was obtained by filtration,washing,drying and calcination.Then the crystal structure of HZSM-5 zeolites was characterized by XRD,SEM and N₂ adsorption The morpHology and pore properties of the modified HZSM-5 zeolites were systematically evaluated,Then,the acid density and acid strength of Br?nsted acid site of HZSM-5 zeolite were characterized by n-propylamine TPD;The catalytic activity of the modified catalyst was evaluated in a fixed bed reactor with n-dodecane as model fuel under supercritical conditions of 460℃and 4.0 MPa;Finally,the carbon deposition on the catalyst was characterized by temperature programmed oxidation（TPO）and isothermal oxidation,respectively,and the residual B acid sites of the catalyst were characterized by n-propylamine TPD.XRD,SEM and N₂ adsorption results show that the alkali neutralization treatment under mild conditions（room temperature,low alkali concentration）used will not cause significant damage to the crystal structure,morpHology and pore structure of HZSM-5zeolites,and this method can effectively control the acidity of HZSM-5 zeolites.The results of pH transformation curve and Zeta potential test in the process of acid-base neutralization titration show that that the main mechanism of alkali neutralization titration are the neutralization reaction of H⁺and OH^-in liquid pHase,the ion exchange process between Na⁺and H⁺in solid-liquid pHase,and the rearrangement of Na⁺and H⁺in solid pHase.The acidity of a series of modified samples with different neutralization degree was characterized by n-propylamine TPD method.The results show that the acid density of modified HZSM-5 zeolites have a linear relationship with neutralization degree,and the acid strength decreases with the increase of neutralization degree.The evaluation results of supercritical catalytic cracking of n-dodecane show that HZSM-5zeolites with neutralization degree of D_n=0.5 has the highest catalytic activity in supercritical catalytic cracking of n-dodecane,and HZSM-5 zeolites with neutralization degree of D_n=0.7 has the best catalytic stability.TPO results show that the amount of coke deposited on the catalyst after reaction is positively correlated with the acid content of fresh catalyst,and the decrease of acid strength plays a more important role in the properties and spatial distribution of coke deposited.The actual catalytic activity of catalyst in supercritical catalytic cracking mainly depends on the amount of acid that can be preserved after coke deposition,the amount of acid that can be preserved is determined by the initial amount of acid and the loss rate of acid sites caused by carbon deposition.With the increase of neutralization degree,the initial acid content decreases,but the loss of acid content caused by coke deposition decreases,so the catalytic effect of medium neutralization degree is the best.Then,HZSM-5 zeolites with different Si O₂/Al₂O₃ ratio were synthesized by hydrothermal method.The results of XRD and SEM show that the crystal morpHology of HZSM-5 zeolites is positive cylinder,and the crystal size is nanometer,which has a negative correlation with Si O₂/Al₂O₃ ratio.The acidity characterization of n-propylamine TPD showed that the acid amount decreased with the increase of Si O₂/Al₂O₃ ratio,but the acid strength did not change.In the process of supercritical catalytic cracking of n-dodecane,HZ-110 showed the best catalytic activity.When Si O₂/Al₂O₃≥110,because of the smaller particle size and the smaller diffusion resistance,HZSM-5 zeolites showed more stable catalytic activity.The results of TPO and residual acid analysis showed that with the increase of Si O₂/Al₂O₃ ratio,the amount of carbon deposition decreased,the intrinsic harm of carbon deposition to zeolites weakened,the spatial distribution and properties of carbon deposition have not obvious change,and the catalytic activity of zeolites was positively related to the amount of acid retained after reaction.Compared with two modification methods for acid control of HZSM-5 zeolites,alkali neutralization titration method has the characteristics of simple operation process,single variable,linear and accurate control of acid amount,weakening acid strength and significantly optimizing the spatial distribution of carbon deposition,which provides important research value for the modification of other similar molecular sieves.