Identifying Order and Disorder in Double Four-Membered Rings via Raman Spectroscopy during Crystallization of LTA Zeolite

Fluoride (F–) has been essential for the synthesis of low-defect siliceous zeolites. It has been hypothesized that F– balances the positive charges from organic structure-directing agents (OSDAs) and stabilizes key building units during zeolite crystallization such as the double four-membered ring (D4R). However, due to the lack of characterization techniques for investigating medium-range structures, including rings and cages formed during zeolite crystallization, the roles of F– in stabilizing building units and maintaining local charge balance during zeolite assembly are not yet fully understood. Here, the crystallization of siliceous Linde type A (LTA) zeolite in the presence of F– was investigated using Raman spectroscopy and periodic density functional theory (DFT) calculations. We have discovered that the F–-filled double four-membered ring (F–/D4R) and the empty D4R exhibit rather distinct Raman features. Both the F–/D4R and empty D4R are formed in the LTA zeolite synthesized in the presence of F– using 1,2-dimethyl-3-(4-methylbenzyl) imidazolium as the OSDA. Observed Raman bands of the F–/D4R and empty D4R, along with predictive DFT calculations on LTA supercells, reveal an ordered distribution of these two D4R units in the final as-made LTA zeolite. The discovery of these distinct Raman signatures of F–/D4R and empty D4R units opens an interesting new window for studying defects in the D4R during zeolite formation. In particular, we have observed variation in Raman intensities of F–/D4R and empty D4R bands during LTA crystallization; periodic DFT calculations indicate that the observed Raman behavior is consistent with empty D4R units containing one or two Si vacancies surrounded by Q3 Si—consistent also with solid-state nuclear magnetic resonance measurement. These defects appear to heal during further crystallization, leading to the formation of defect-free LTA zeolite crystals. Overall, our results provide deeper understanding on the roles of F– in charge balancing and stabilizing intact D4R units during zeolite formation.