A Density Functional Theory Study On The Structures And Adsorption Properties Of Cu-SSZ-13 Zeolite Modified By Rare-earth Ion

SSZ-13 zeolite with a chabazite?CHA?topology structure has important applications in selective catalytic reduction of nitrogen oxides?NO_x?by ammonia?NH₃-SCR?to reduce diesel engine exhaust emissions.The hydrothermal stability of Cu-SSZ-13 zeolite can be enhanced significantly after modified by rare-earth ions.In this thesis density functional theory?DFT?was performed to investigate the different Al locations and Na⁺sites in Na-SSZ-13 zeolite as well as the corresponding Br?nsted acidity of H-SSZ-13 zeolite with various Si/Al ratios.The structures of Cu species in SSZ-13 zeolites with different Si/Al ratios and the effects of Ca²⁺and Y³⁺on the zeolite framework were further studied.Finally,the adsorption properties of different gas molecules on Cu species in NH₃-SCR reaction atmosphere were calculated.The main findings are summarized as follows:?1?By calculating the substitution energies of Na-type SSZ-13 zeolite with different Si/Al ratios,the most easily substituted structural model was determined.For the 1Al model,the Na⁺ion locates at the six-membered ring is of energy preference.For the NNNN and NNN sequences,the Na⁺ion preferentially occupies the SII site within the six-membered ring,and the secondary site for Na⁺is at the SIII'position within the eight-membered ring which is not in the same supercage of the previous Na⁺ion.The calculated ²³Na NMR chemical shifts,quadrupolar interaction constants and asymmetry factors are in good agreement with the experimental ²³Na MAS NMR results.?2?The calculated substitution energies revealed that the most favorable acid site in the1Al model is O?1?-H in which a proton is bonded with the O?1?atom near the isolated Al atom of the zeolite framework.Nevertheless,energy differences are rather small when comparing the substitution energies of an Al atom replacing an Si atom in the zeolite framework with a proton located at different O sites.As the Si/Al ratio decreases,the Al-rich H-SSZ-13 zeolite contains more Al substitutions in its framework.This system exhibits the lowest substitution energy when two Al atoms are located at the diagonal of the same six-membered ring for the Al-Si-Si-Al?NNNN?sequence in the framework of the Al-rich SSZ-13 zeolite.However,for the Al-Si-Al?NNN?sequence,the most favorable distribution involves two Al atoms located in different six-membered rings of the double six-membered ring units?D6R?.The proton affinities?PA?,NH₃ desorption energies,and ¹H NMR chemical shifts after acetonitrile adsorption were calculated in the most stable models to characterize the Br?nsted acidity of the SSZ-13 zeolite with different Si/Al ratios.All computing results suggest that the Al-rich SSZ-13 zeolite exhibits weaker Br?nsted acid strength than that of the Si-rich counterpart due to the presence of Si?2Al?groupings with the NNN sequence in the framework.The ¹H MAS NMR experiments after d3-acetonitrile adsorption confirmed that the Al-rich SSZ-13 zeolite has a weaker Br?nsted acid strength than the Si-rich SSZ-13zeolite.?3?The substitution energies of different Cu species in SSZ-13 zeolite were also calculated.Cu⁺and Cu²⁺ions preferentially locate at the six-membered ring plane of SSZ-13,and Cu?OH?⁺easily sit at the eight-membered ring.Then rare-earth Y³⁺,alkaline-earth Ca²⁺,Cu-Y and Cu-Ca substitutions in SSZ-13 zeolite were investigated by DFT calculations.By comparing the substation energies,the system is more stable with Cu²⁺at the six-membered ring after adding Ca²⁺or Y³⁺in Cu-SSZ-13.The addition of different metal ions in SSZ-13zeolite lead to a decrease of bond lengths between the Al atoms and the surrounding O atoms in the framework.Moreover,coordination bonds between Cu and O atoms or Y and O atoms in the framework are formed,which stabilizes the Al in the framework and enhances the hydrothermal stability of Al-rich SSZ-13.After calculating the desorption energies of different gas molecules on Cu-SSZ-13,it is found that NH₃ has the largest desorption energy on different Cu species.The adsorption ability of H₂O,SO₂ or SO₃ on different Cu species is stronger than that of NO or NO₂,which act as the reactant molecules in NH₃-SCR.Cu²⁺in the six-membered ring of the Al-rich Cu-SSZ-13 zeolite has the weakest ability to adsorb SO₂ and SO₃.Y?OH?²⁺has strong binding ability to H₂O,SO₂ and SO₃,and protects Cu active species to some extent in Y-modified Cu-SSZ-13 zeolite.