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Synthesis of Zeolites with Controlled Defects by Understanding the Role of Charge Balance in Zeolite Crystallization
Author ORCID Identifier
Campus-Only Access for One (1) Year
Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
Prof. Wei Fan
Materials Science and Engineering
Zeolites are well-defined and ordered microporous crystalline materials constructed by the continuous linkage of corner sharing TO4 tetrahedra (taking T atoms (e.g., Si or Al) as the tetrahedral center). Due to the structural and compositional diversities and superior hydrothermal stability, zeolites have been broadly utilized to many industrial fields, including gas separation and catalysis heterogeneous catalysis. However, understanding zeolite crystallization mechanisms remains a tantalizing challenge, which causes challenges in tailoring this material for advanced applications. Raman spectroscopy has emerged as a powerful tool for probing medium-range (0.35 - 1 nm) structures. Since this scale is consistent with the micropore size of zeolites, using Raman can shed light on the developmental process of rings or more complex building units during zeolite crystallization. However, despite this importance, rigorous assignments of Raman spectra of zeolites are not completely understood. Therefore, herein, we reported a systematic zeolite synthesis, spectroscopy, and periodic DFT study of several all-silica zeolites. We discovered that zeolite Raman bands should be assigned to “tricyclic bridges” — three zeolite rings that share a common Si–O–Si bridge. Furthermore, we have found that the vibrational frequency of a given Raman band can be correlated to the smallest ring of its tricyclic bridge and
not to the ring that is actually vibrating. Finally, we have discovered a precise anti-correlation between Raman frequency and Si–O–Si angle. Based on the new discoveries regarding the assignments of Raman bands of zeolites, the crystallization process, from amorphous gel to final crystals, of siliceous LTA zeolite synthesized with HF was investigated by Raman spectroscopy complemented with XRD and NMR. By integrating experimental Raman with periodic density functional theory (DFT) calculations, it was discovered that the F--filled double four-membered ring (F-/D4R) and the empty D4R exhibit rather distinct Raman features, which opens a new window for studying defects in the D4Rs during zeolite formation. In particular, we have observed a 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. These defects appear to heal during further crystallization, leading to the formation of defect-free LTA zeolite crystals. These results for the LTA crystallization process provide deeper understanding on the roles of F- in charge balancing and stabilizing intact D4R units during zeolite formation. Controlling defects in zeolites is crucial for tuning their adsorption and catalytic properties. An integrated zeolite synthesis, spectroscopy, and density functional theory study was performed to investigate the minimum amount of F- as a charge-balancing agent that mitigates defects in siliceous zeolites. It was found that positive charges could be titrated into the resulting as-made siliceous LTA zeolite by varying the amount of tetramethylammonium cations (TMA+) introduced as the secondary organic structure-directing agent (OSDA). According to the results from 29Si solid-state MAS NMR, Raman spectroscopy, and density functional theory, those additional positive charges from TMA+ are balanced by framework defects of SiO- in the LTA crystals, rather than F-. In this way, the number of defects in the as-made LTA can be precisely controlled by the amount of TMA+ in the zeolite structures. A DFT thermodynamic analysis
explains that crowding of siliceous LTA pores by TMA+ impedes defect healing, leading to the formation of the controllable defects. Based on all these discoveries, siliceous isolated D4Rs (synthesized without F-; to help the formation of D4Rs in LTA product) and an appropriate amount of Al (to balance positivelycharged OSDAs) were added into the synthetic precursor, trying to increase the Si/Al ratio of LTA synthesized in a HF-free way. It was discovered that the Si/Al ratio could be increased to 7.55 (a progress of 37.2% compared to the previous highest one). 29Si NMR, 13C NMR, 27Al NMR, and TGA clearly shed light on the details of the picture of the charge balancing between OSDAs and tetrahedral Al in the LTA framework. The superior hydrothermal stability of this LTA sample was confirmed by an in-situ XRD experiment under temperatures as high as 1000 oC for 3 h. This research broadened the application scope for LTA zeolite in industry, and also provided inspirations in increasing the Si/Al ratios of other 37 zeolites featuring D4Rs synthesized in the absence of HF.
"Synthesis of Zeolites with Controlled Defects by Understanding the Role of Charge Balance in Zeolite Crystallization" (2023). Doctoral Dissertations. 2768.
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Available for download on Tuesday, August 01, 2023