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

Abstract

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.

Titrating Controlled Defects into Si-LTA Zeolite Crystals Using Multiple Organic Structure-Directing Agents

“…As displayed in Figure 3 and Table 2, for the sample without TMA, no Q 3 peak was found in the 29 Si MAS NMR spectrum, implying a defect-free LTA structure with all positive charge from BULKY balanced by F − , consistent with our previous study. 20 With increasing TMA, the ratio of Q 3 /(Q 3 + Q 4 ) grows accordingly, consistent with an increasing amount of anionic framework defects to balance charge. From the NMR data in Figure 3, we computed the Si−O − and SiOH distributions, which show a positive correlation between Si−O − and TMA + (Table S2).…”

“…Our previous computational and experimental study found negligible effects on the Raman spectra from these very low Al contents. 20 For this reason, we denote the resulting materials as Si-LTA. As reflected by XRD results (Figure 2a), all of the four siliceous LTA samples are fully crystalline with the characteristic diffraction peaks from the LTA framework.…”

“…The method of introducing defects into initial intact systems and calculating theoretical Raman spectra are the same with our previous work. 20 If more than one defect presents, we calculated energy differences of possible configurations to find thermodynamically favored ones. Theoretical chemical shifts of intact and defected systems were calculated using the PBE exchange-correlation functional at an energy cutoff value of 800 eV based on the linear response method.…”

“…Two of the three D4R units in the LTA unit cell are filled with F − , resulting in two negative charges per unit cell that balance the two positive charges from the BULKY dimer. 20 Because of the charge balance achieved by F − and BULKY OSDA, a defectfree Si-LTA zeolite can be made from this pathway. Interestingly, a range of different OSDAs 27 can be used for the synthesis of Si-LTA zeolite, thus allowing various amounts of OSDA-positive charge to be incorporated into the zeolite.…”

Tailored Synthesis of ZSM-5 Nanosheets with Controllable b-Axis Thickness and Aspect Ratio: Strategy and Growth Mechanism

“…Zeolites, which have uniform sub-nanometer-sized pore channels, tunable composition, acidic properties, and tailorable morphology, are widely used in the chemical industry as selective catalysts, adsorbents, and membranes. − Among them, ZSM-5 zeolite (MFI-type framework), which has two intersecting 10-membered ring channels has attracted much attention of researchers. Its micropores have the dimensions in the range of 0.5–0.6 nm and the interconnected cavities on the framework (Figure S1) are useful for the synthesis of valuable chemical intermediates. − However, a significant drawback in the zeolite catalyst is that the very presence of the micropores also brings diffusion limitation.…”