A Density Functional Theory Modeling on the Framework Stability of Al-Rich Cu-SSZ-13 Zeolite Modified by Metal Ions

Abstract

The metal ion-modified aluminum-rich Cu-SSZ-13 zeolite possesses high hydrothermal stability (especially with modifications using yttrium). In this study, DFT was used to model the location of Cu2+, Cu+, Ca2+, and Y3+ on Al-rich SSZ-13. The Al distribution in the framework of SSZ-13 zeolite affects the locations of metal ions. The isolated Cu+, Cu2+, Ca2+, and Y­(OH)2+ ions are most stable in the six-membered ring, and the easiest site for Cu­(OH)+ or Y­(OH)2 + is in the eight-membered ring. When Cu2+ and Y­(OH)2+ or Cu2+ and Ca2+ are in the same cage, Cu2+ is located in the six-membered ring and Y­(OH)2+ or Ca2+ migrates to the eight-membered ring of the zeolite. The Y3+ exhibits the strongest effect on stabilizing the framework Al followed by Ca2+. In addition, metal ions could form multiple coordination bonds with the zeolite framework O, which further stabilizes the framework Al and enhances the hydrothermal stability of SSZ-13 zeolite.

Rare‐earth Yttrium Exchanged Cu‐SSZ‐39 Zeolite with Superior Hydrothermal Stability and SO2‐Tolerance in NH3‐SCR of NOx

“…DFT calculations were conducted to model the locations of the Cu and Y species in the zeolites, thus providing further insights into the source of the improved hydrothermal stability and SO 2 ‐tolerance in Y‐containing catalysts. Generally, it is considered that metal ions are located in sites with lower substitution energies [18] . Therefore, following the calculated substitution energies of the optimized geometries of the Cu species in the AEI zeolite (Figure S13 and Table S1), we predicted that the Cu 2+ /Cu + and Cu(OH) + species would be located in the 6MR and 8MR sites of the SSZ‐39 zeolite, respectively, which is similar to the Cu distribution in CHA zeolite [18–19] .…”

“…Generally, it is considered that metal ions are located in sites with lower substitution energies [18] . Therefore, following the calculated substitution energies of the optimized geometries of the Cu species in the AEI zeolite (Figure S13 and Table S1), we predicted that the Cu 2+ /Cu + and Cu(OH) + species would be located in the 6MR and 8MR sites of the SSZ‐39 zeolite, respectively, which is similar to the Cu distribution in CHA zeolite [18–19] . Comparing the substitution energies of the optimized geometries of the Y species (Figure S14 and Table S2), the more stable configurations occur when Y(OH) 2 + and Y(OH) 2+ are found in the 8MR and 6MR, respectively.…”

“…In accordance with our previous study, [18,33] the atomic coordinates of SSZ‐39 zeolite were obtained from the International Zeolite Association (IZA) database. The outermost dangling bonds in all of the cluster models were saturated with hydrogen atoms, and the Si−H bond length was set to 1.47 Å [34] .…”

“…A cluster model of 34 T atoms was constructed, which is large enough to study the distribution of the metal cations in the AEI exchange‐sites [18,33] . The details of DFT calculations were listed in the Supporting Information.…”

“…The DFT calculations show how the zeolite framework retains its integrity under otherwise unfavorable conditions, inhibiting dealumination during hydrothermal aging: Y(OH) 2 + in the 8MR form three coordination bonds with the three O atoms in the zeolite framework (Figure S15). The shortened AlÀ O bond length and narrower bond angle in the AEI zeolite framework (Table S3) suggest that the introduction of Y cation strengthens the AlÀ O bond, [18] thereby stabilizing the Al framework and improving the hydrothermal stability of the Cu(0.25)-Y(0.12)-AEI(10) catalyst.…”