A Density Functional Theory Study On The Structure And Adsorption Properties Of Al-rich Cu-Beta Zeolite

Beta zeolite consists of the distinctive three-dimensional intersecting 12-membered ring?12MR?channel systems and nine types of T-sites.At present,Al-rich Beta zeolite with Si/Al ratio less than 7 has been synthesized without organo-template,which include more framework Al atoms and corresponding counter ions.Al-rich Cu-Beta zeolite synthesized by ion-exchange process includes more Cu atoms and shows much higher activity in NH₃-SCR reaction.In this thesis,density functional theory has been performed to calculate Al distribution and Br???nsted acidity of Al-rich Beta and explore the relationship between the microscopic structures and Br???nsted acidity of zeolite.Subsequently,it is also investigated that the distribution of Cu active sites and the adsorption strength with the reactants of NH₃-SCR reaction in Al-rich Cu-Beta zeolite.The main findings are listed as follows:?1?The distinct Al locations in NNN and NNNN sequences were investigated for Al-rich Beta zeolite by density functional theory.Isolated Al atoms most likely locate in T1 and T9 sites,but T3,T4 and T8 sites are the most difficult to be substituted by Al atoms.For NNN sequences,5MR1-T92,5MR2-T15 sites are the most preferable Al locations of Al-rich H-Beta zeolite,while 5MR1-T17 sites represent the most possible Al distribution of Al-rich Na-Beta zeolite,which means 5MR1-T17 sites may be dealuminated to become extra-framework Al species during the ion-exchanging process of Na-Beta to H-Beta.The preferable Al locations in NNN sequence consist of two AlO4-tetrahedral in one 5MR but facing different channels.6MR1-T66 sites are the most easily substituted by Al atoms for NNNN sites of Al-rich Beta zeolites.?2?Density functional theory was applied to calculate the proton affinities,NH₃ adsorption energies and 1H chemical shifts of d5-pyridine adsorption on 2Al and 1Al sites of Al-rich Beta zeolites.The results reveal that the average Br?nsted acid strength of Al-rich sites is weaker than Al-isolated sites.However,NNNN sites of Al-rich sites own the similar Br???nsted acidity to Al-isolated sites.Al-rich Beta zeolite owing more Al-rich sites shows much weaker Br???nsted acidity than Si-rich Beta zeolite.The respective Br???nsted acid site in NNN sequence shows the obvious B-acidity difference.However,the two Br???nsted acid sites of 6MR1-T66 are relatively close to show the less acidity discrepancy,which leads to two NH₃ molecules only physically adsorbed on 6MR1-T66 sites.The linear relationship of proton affinities and 1H chemical shifts after adsorbing d5-pyridine is also satisfied for H-Beta zeolites.?3?Density functional theory was applied to study the stability of Cu⁺,Cu²⁺ and [CuOH]+ active species locating on different Al sites of Al-rich Cu-Beta zeolite,as well as the adsorption strength of Cu ions with NO,NO₂,NH₃,H2 O,SO₂ and SO3 molecules.The sequences of Cu active species exchanged in the zeolite framework are Cu²⁺ > [Cu OH]+ > Cu⁺,which shows that Cu²⁺ ions are the most possible active species of Al-rich Cu-Beta zeolite.Cu⁺ and Cu²⁺ ions are prone to locate in 6MR1-T66 sites,and [CuOH]+ ions most likely locate in Al-isolated sites.The strong adsorption strength of H2 O,SO₂ and SO3 molecules inhibit greatly the adsorption of NO and NO₂ on Cu active species,but have little effect on NH₃ adsorption.[CuOH]+ and Cu²⁺ are characteristic of selective adsorption of NO and NO₂ molecules...