A Density Functional Theory Study On The Structures And Adsorption Properties Of Cu-SSZ-39 Zeolite

Cu-SSZ-39 zeolite with AEI framework structure shows excellent activity for the selective catalytic reduction with ammonia（NH₃-SCR）to reduce NOx emissions from diesel engine exhaust.The hydrothermal stability of Cu-SSZ-39 zeolite can be enhanced significantly by rare-earth Y modification.In this thesis,the relationship between Al distribution and the corresponding Br?nsted acidity of Si-rich and Al-rich SSZ-39 zeolites was investigated via density functional theory（DFT）calculations.Al distribution and Na⁺location in Na-SSZ-39zeolites with different Si/Al ratios were studied by comparing the substitution energies.The stable structures of active Cu species and Y species in SSZ-39 zeolites and the effects of these metal cations on the zeolite framework were confirmed.Finally,the adsorption properties of different NH₃-SCR reaction gases on Cu species were calculated.The main findings are as follows:（1）By comparing the substitution energies of framework Al in Si-rich SSZ-39,it is found that T3 site is the easiest to be substituted by the isolated Al and the H⁺prefers to be formed at O7 site near the isolated Al atom.In Al-rich SSZ-39 zeolite,the framework Al tends to form2Al pairings in Al-Si-Al（NNN）and Al-Si-Si-Al（NNNN）sequences.The most favorable distribution for NNN sequence involves in two Al atoms located in four-membered ring unit.While for NNNN sequence,two Al atoms occupying the diagonal of the same six-membered ring exhibit the lowest substitution energy.The Br?nsted acid strength of the SSZ-39 zeolite with different Si/Al ratios was characterized by the proton affinities（PA）,NH₃ desorption energies and ¹H NMR chemical shifts after d3-acetonitrile adsorption.It is found that the acid strength tends to decrease with the increase of Al content.These computing results agree well with the experimental results of NH₃-TPD and ¹H MAS NMR with d3-acetonitrile as probe molecule.（2）Al distribution and Na⁺location in Na-SSZ-39 zeolites with different Si/Al ratios were studied by calculating the substitution energies.For 1Al substitution,Na⁺ions mainly locate at 8-membered ring（8MR）above the double 6-membered ring（D6R）building unit.For paired Al substitution,the stable Al-Si-Si-Al are located in 6-MR,and the corresponding Na⁺ions are located at the upper and lower two six-membered ring planes of the D6R building unit sites,respectively.The calculated ²³Na NMR chemical shifts（δ）,quadrupolar interaction constants（Cq）and asymmetry factors（η）are consistent with the experimental ²³Na MAS NMR results.（3）The substitution energies of different Cu and Y species in SSZ-39 zeolite were also calculated.The isolated Cu⁺and Cu²⁺ions at the six-membered ring（6MR）are more stable,and Y（OH）₂⁺in the eight-membered ring（8MR）is the most stable site.When Y（OH）₂⁺and Cu²⁺coexist in the same AEI cage,Y（OH）₂⁺migrates to the 8MR and Cu²⁺is located in the6MR of the zeolite.The metal cations could form multiple coordination bonds with the framework O of the SSZ-39 zeolite,which leads to the shortened bond length and small bond angle,thus enhancing the strength of the zeolite framework.It is found that NH₃ has the largest adsorption energy on different Cu species among different gases adsorption on Cu-SSZ-39.Y（OH）₂⁺has strong binding ability to SO₂ and SO₃,and protects Cu active species to some extent in Y-modified Cu-SSZ-39 zeolite.