Study On Carbon Deposition In Catalytic Cracking And Acid Distribution Of HZSM-5 Zeolite Based On Thermal Analysis

Hypersonic vehicle is becoming an important high-tech equipment to maintain national security.Thermal protection has become one of the key technologies restricting its development because of the large amount of aerodynamic friction heat generated by hypersonic flight.Active cooling using the supercritical catalytic cracking endothermic reaction of endothermic hydrocarbon fuels is a very promising solution.HZSM-5zeolite is a typical solid acid catalyst for catalytic cracking of hydrocarbon fuels.As a typical shape-selective catalyst,HZSM-5 zeolite has been widely used in several important process such as catalytic cracking to light olefins,methanol to olefins,methanol to aromatics and gasoline,etc.How to suppress catalyst deactivation caused by carbon deposition has always been a hot topic in related research.The Br?nsted acid site（B acid site）in the HZSM-5 zeolite is not only the active site for the main reaction of catalytic cracking,but also the active site for the side reaction that leads to the formation of carbon deposits.Among the multi-scale effects involved in heterogeneous catalytic reactions,the coupling effect of chemical reaction and product diffusion makes the distribution of acid sites at the grain scale have an important influence on the catalytic reaction.Therefore,characterizing the spatial distribution of B acid sites in HZSM-5 zeolite on the grain scale is of great significance for the design and synthesis of efficient HZSM-5 zeolite catalysts.It is a great challenge to quantitatively characterize the acid distribution of zeolite catalysts at the grain scale,up till the present moment,there is no mature,simple and reliable characterization method.The solution of this paper is as follows:Firstly,a method based on temperature-programmed decomposition of n-propylamine（n-propylamine TPD）was constructed to quantitatively characterize the average B acid site density and acid strength of HZSM-5 zeolite.Secondly,cover the B acid site of the HZSM-5 molecular sieve with the carbon deposit produced in the supercritical catalytic cracking of n-dodecane,and then the B acid sites are released layer by layer from the outer surface of the crystal to the center by using the strategy of shrinking nucleus and partial oxidation,and by modeling the oxidation process of carbon deposition,the spatial coordinates of released B acid layer on the grain were calculated quantitatively.Finally,the released B acid sites were quantitatively characterized by n-propylamine TPD method,and the differential distribution curves of B acid site density and acid strength on the grain were finally calculated.The n-propylamine TPD method for quantitative characterization of B acid sites of zeolite was established based on the principle of selective cracking reaction of organic amines on B acid sites to produce corresponding olefins and ammonia.Where the n-propylamine was used as probe molecule,and propylene produced by cracking was monitored on-line with FID detector of gas chromatography.And the peak area and peak temperature of the n-propylamine TPD profiles are the scales of acid density and acid strength respectively.By comparing the n-propylamine TPD profiles of HZSM-5zeolite and heteroatom isomorphously substituted ZSM-5 zeolite（such as H-[Fe]-MFI or H-[b]-MFI,which have lower intrinsic acid strength）,which is confirmed that the average peak temperature of n-propylamine TPD profile can be used as the apparent index of B acid strength.Finally,the influence of propylene reabsorption on n-propylamine TPD can be ignored by changing the sample loading,sample size and crystal size.Therefore,the surface reaction of n-propylamine is the controlling factor of n-propylamine TPD profile.The experimental results show that the isothermal oxidation of Coked-HZSM-5 at high temperature（≥640℃）and low oxygen concentration（2.1%）conforms shrinking core model.In order to obtain the spatial coordinates of released B-Acid sites on the grain,an enhanced nucleation model（ESCM）based on the traditional nucleation model is proposed in this paper to adapt to many challenges in the process of carbon deposition oxidation analysis,such as the non spherical and non-uniform distribution of grain size,and the non-uniform distribution of carbon deposition on the grain.Theoretically,if we can create two working conditions of"surface chemical reaction control"and"diffusion control in the product layer",and only rely on the analysis data of CO₂ in the gas phase obtained by constant temperature oxidation,we can realize the theoretical calculation method of spatial distribution of coke based on one-dimensional position coordinate.But the actual operation results show that the pure"diffusion control in the product layer"condition is not easy to obtain,so the effective application of this method needs further research.The micropore volume released by partial oxidation of carbon deposition was obtained by N₂ adsorption characterization.The intrinsic density of carbon deposition was correlated by the H/C ratio of carbon deposition.Combined with the CO₂ analysis data of constant temperature oxidation of carbon deposition,the one-dimensional spatial coordinateof oxidized carbon deposition layer in ESCM model was solved,and then the spatial distribution curve of carbon deposition on the grain scale of HZSM-5molecular sieve was obtained.Then,the released B acid was quantitatively characterized by n-propylamine TPD method,and combined with the coordinate,the differential spatial distribution curve of B-Acid sites on the grain scale was obtained by differential calculation.The results show that the acid density of HZSM-5 zeolite is roughly homogenous while the acid strength is heterogeneous within the crystal grains.From the outer surface to the center of the crystal grains,the strength of the Br?nsted acid sites increase gradually.