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 Q(3) 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.