Investigation Of Adsorption And Diffusion Performance Of Hierarchical Porous FAU Zeolites

Hierarchical porous zeolites combine both the acidic and pore advantages of two or moredifferent molecular sieve materials,thus they are beneficial to improve the accessibility of the acidic center and diffusion rate,to increase the desorption rate of product molecules from the active center during the catalytic reactions.Accordingly,hierarchical porous zeolites show good synergy and excellent catalytic performance in many reactions.Hierarchical porous Y zeolites have been proved to be very prospective catalyst on heavy oil cracking,catalytic and selective adsorption of macromolecules and separation of aromatics,and so on.The adsorption and intracrystalline diffusion behavior of sorbate molecules in zeolite are the most important factors in a practical reaction course.In this thesis,the adsorption and diffusion behavior of n-octane,thiophene,benzene and p-xylene on hierarchical porous Y zeolites were systematically investigated by using frequency response?FR?technique combined with Infrared Spectra?IR?,NH₃ temperature programmed desorption?NH₃-TPD?and intelligent gravimetric analyzer?IGA?methods.The prefer performance in industrial applications would be supported theoretically.Major observations of this work are summarized as follows.First,the adsorption and diffusion performance of benzene and p-xylene on traditional Na Y zeolite and NaY zeolite synthesized by in-situ crystallization from Kaolin?designated as Kaolin-NaY?were investigated.The research on the physical and chemical properties shows that nano NaY zeolites with small size and good dispersion can be obtained by using loading crystallization method.The prepared nano Kaolin-NaY zeolites indicate weaker Lewis?L?acidity than traditional NaY,and their accessibility of heavy oil macromolecules is superior to the traditional NaY zeolite.FR spectra indicate that the mass transfer is a quick step for benzene on Kaolin.There are two adsorption processes on traditional NaY zeolite and Kaolin-NaY at a low temperature.Low frequency adsorption process is attributed to the?electronic interactions,while high frequency adsorption process belongs to pore filling physical adsorption.However,a medium frequency adsorption process caused by benzene rearrangement can be detected on NaY zeolites,in which diffusion is the controlling step on Kaolin-NaY at a high temperature.The rearrangement process of benzene in the pore almost can't be detected.Temperature programmed desorption curves and diffusion kinetic curves show that the interaction between benzene and traditional Na Y zeolite is stronger than that of Kaolin-NaY zeolites,owing to their better diffusion performance.Hence,the lower aromatic index?BMCI?values in tail oil during hydro cracking process can be expected.The interaction type and strength is consistent with FR method.The results of temperature programmed desorption and diffusion kinetic curves of p-xylene on zeolites further indicate that Kaolin-NaY is more beneficial to influent hydrocarbon molecular diffusion and reaction performance under certain conditions.Under the same conditions,the adsorption amounts and diffusion coefficient of benzene are greater than those of p-xylene.Second,the adsorption,diffusion and desorption of benzene and thiophene on NaY-Beta,CeY-Beta and CuY-Beta composite zeolites were investigated.NaY-Beta was firstly modified by using two ion-exchanged and two calcinations method.Pyridine infrared spectra?Py-IR?show both strong Br?nsted?B?acid and strong L acid for the surface acidity of Ce Y-Beta zeolites,middle strong B acid and strong L acid for the surface acidity of Cu Y-Beta and weak B acid and weak L acid for NaY-Beta zeolite.The adsorption of thiophene and benzene on zeolites obeys Langmuir equation.The adsorption capacity of thiophene is higher than that of benzene.Adsorption mechanism of thiophene and benzene on three molecular sieves is different,according to FR and DTG curves.The interaction between benzene and zeolites is?complexation.Benzene can be entirely desorbed from zeolites,which suggests that the adsorption of benzene on NaY-Beta,CeY-Beta and Cu Y-Beta is mainly physical adsorbed,and a weak chemical adsorption can be also detected at high temperature.The acid amounts and acid strength has little effect on adsorption of benzene.The interaction between thiophene and NaY-Beta is weak and mainly controlled by?electronic interaction.While the interaction between thiophene and CeY-Beta/CuY-Beta composites zeolites are strong,owing to S-M adsorption and?complexation interaction,and complex catalytic reaction processes are also detected with strong B acidic center under the condition of high temperature.Chemical adsorption is the main adsorption process and temperature has little influence on the adsorption capacity.Isosteric heats of sorption in descending order is consistent with the temperature range of desorption peaks.Under the same conditions,the diffusion coefficient of bezene is higher than that of thiophene.Third,the adsorption,diffusion and desorption of n-octane on NaY,H-Beta,NaY-Betacomposites and the mechanical mixture of Na Y and H-Beta?relative mass ratio of 1:1,designated as M-NaY-HBeta?were determined.The adsorption of n-octane on the four kinds of zeolites obeys Langmuir equation,and isotherms obey I type.The adsorption capacity is in the following orders:NaY>M-NaY-HBeta>H-Beta?NaY-Beta.According to FR spectra,the adsorption of n-octane is found to be the rate-controlling step and two different adsorption processes are found at 79.99-133.32 Pa and 333 K.In addition,a medium frequency adsorption process caused by n-octane molecular rearrangement can be detected on NaY-Beta and M-NaY-HBeta zeolites.The response intensity in descending order accords with that of adsorption capacity.Diffusion of n-octane is the rate-controlling step when the pressure is higher than 399.97 Pa.In the temperature range of 303-573 K,n-octane can be entirely desorbed from four molecular sieves,suggesting that the adsorption is a relatively weak process.At low temperature,the diffusion rate in hierarchical porous zeolites is slower than that of single porous zeolites because of molecular rearrangement.However,when the temperature is higher than 333 K,hierarchical porous zeolites exhibits optimum diffusion performance.