(403b) Identifying and Controlling Defects in Siliceous Zeolites By Charge Density Matching in Presence of F-

Controlling defects in zeolites is crucial for tuning their adsorption and catalytic properties. We have performed an integrated zeolite synthesis, spectroscopy, and density functional theory study to test the limit of F^– as a charge-balancing agent that mitigates defects in siliceous zeolites. Siliceous zeolite LTA was synthesized with 1,2-dimethyl-3-(4-methylbenzyl) imidazolium (BULKY) as the primary organic structure-directing agent (OSDA) and tetramethyl ammonium (TMA⁺) as the secondary OSDA. Using the Raman spectroscopy, double four-membered rings (D4Rs) with a F^– residing inside (F^–/D4Rs) and empty D4Rs were identified. We also discovered that defect structures in D4R can be identified by Raman. The charge-balancing role and structure-directing role of F^– for D4R were confirmed. Based on these insights, we demonstrated that the amount of F^– required to synthesize defect-free, siliceous LTA could be substantially reduced by a factor of 6, which can benefit the industrial-scale synthesis of siliceous LTA, and also other pure- or high-silica D4R-containing zeolites.[1-3] The number of defects in the LTA synthesis can be also controlled by using TMA as the secondary OSDA. We will also discuss the process for removal of OSDA using a mild calcination method and its effect on the defect density.

References

1. Song Luo and Tongkun Wang et al. Chem. Mater. 2021, 33, 17, 6794-6803.
2. Tongkun Wang and Song Luo et al. J. Am. Chem. Soc. 2019, 141, 51, 20318-20324.
3. Song Luo and Tongkun Wang et al. Chem. Mater. 2022, 2022, 34, 4, 1789–1799.