Advanced zeolite designs via structural transformation and their applications.

Abstract

Zeolites, aluminosilicate materials, are widely known for their large surface area, defined pore structures, diverse chemical properties, and high hydrothermal stability. However, the applications of synthetic zeolites in the industry are currently limited due to their high manufacturing costs from the synthesis conditions and initial gel precursors and a lack of suitable functions in practical uses. Therefore, the focuses of this thesis are to explore a recent zeolite synthesis technique using alternative silica and alumina sources and design novel zeolites for various applications, ranging from catalytic ethanol conversion to alkane and alkene adsorption. The details of all studies are described below.

First, the transformations of commercial beta (*BEA) and faujasite (FAU) to Mobil–5 (MFI) and analcime (ANA) zeolites have been conducted. The effects of synthesis compositions and conditions on the phase and crystallinity of zeolitic products and transformation pathways are examined. The results indicate the preferential conversion route of *BEA and FAU into ANA frameworks when high synthesis alkalinity is applied. The MFI formation, on the other hand, requires the use of a high Si/Al parent zeolite and TPAOH under lower synthesis alkalinity; thereby, in this study, only *BEA can convert into MFI structure. The *BEA-MFI synthesis is also suggested to involve interzeolite transformation due to the successful metal encapsulation in MFI

topology during the conversion of reduced metal/*BEA. The integration of zeolitic transformation and metal encapsulation techniques preserves metals in the zeolitic structure despite the high basic solution used. However, these methods lead to the inclusion of most metals in the inaccessible area of the zeolitic pore. This study successfully demonstrates another technique to encapsulate metals in zeolite via zeolitic structural transformation. Although this method is advantageous in reducing metal degradation and loss from zeolites, the problem of metal inaccessibility in zeolitic topology must be addressed before their utilization.

Second, one-pot syntheses of cancrinite (CAN) and ANA frameworks from mordenite (MOR) natural zeolite have been performed for the first time. The effects of the synthesis gel composition and alkalinity on the phase formation and crystallinity of the product are investigated. All studied zeolites are evaluated for their catalytic activities in ethanol conversion reactions from 473 K to 673 K and at 1 atm. The results suggest that the formation pathways of CAN and ANA are highly dependent on the alkalinity of the synthesis solution and the starting material regardless of the synthesis template. The use of natural MOR zeolite in the synthesis can contribute to highly crystalline ANA structures comparable to the ANA products obtained from fumed silica and NaAlO2. The quantity and locations of the acidic-cationic dual active sites of the proton-form product zeolites are suitable for synthesizing acetaldehyde from ethanol with over 90% product selectivity. However, further development of synthesis procedures to enhance the surface area is needed to improve the ethanol conversion of the resulting zeolites. This work provides a new perspective on highly crystalline and pure-phase zeolitic structural formations from natural zeolite and their potential application in ethanol conversion.

Third, the synthesis of robust Pd-based MFI composite adsorbents consisting of Pd-ZSM- 5 zeolite as an inner complex and S-1 zeolite as an outer complex with high ethylene adsorption capacity in the humid environment is reported. The influence of secondary-layer introduction and its thickness on the hydrophobicity of composite products is examined. The ethylene adsorption loadings in uncoated and coated zeolites are evaluated in dry and humid environments. The result reveals that the prepared composite zeolites (S-1/Pd-ZSM-5) demonstrate excellent ethylene adsorption ability due to the synergistic effect between the Si-rich layer and Pd nanoparticles. Compared with the conventional ZSM-5-supported Pd adsorbent, S-1/Pd-ZSM-5 exhibits a 2.1- fold enhancement in ethylene removal in the presence of moisture (0.195 mmol/g vs. 0.416 mmol/g). Nevertheless, the second crystallization of S-1 over Pd-ZSM-5 has to be performed for at least 72 h to achieve excellent confinement over the Pd nanoparticles and inner complex. This novel design of S-1/Pd-ZSM-5 provides the valuable applicability of the composite zeolite for actual ethylene regulation in post-harvest crops in the future.

Last, the investigations on the adsorption behaviors of light alkanes and alkenes, including ethane, ethylene, propane, and propylene, are conducted over H-ZSM-5 with Si/Al ratios of 15-140. The impacts of adsorption temperature and Si/Al ratio of adsorbents on the single-component adsorption kinetics are examined to evaluate the separation efficacy of adsorbents in alkane-alkene processes. The results demonstrate that the low Si/Al MFI zeolite can be a good candidate for ethane-ethylene separation by selectively chemisorbed with ethylene over ethane. Propane molecules preferentially bind with low Si/Al zeolite over propylene at a low temperature of 303 K where the physisorption is dominant for the propane-propylene process. The propane diffusion appears to be primarily controlled by the acid site density of samples instead of the adsorption temperature, indicating the influence of the molecular size of adsorbate and Si/Al ratio on adsorption kinetics. The study also reveals the possibility of using high Si/Al MFI for selective C2H6/C2H4 separation via the physisorbed adsorptive-adsorbent interaction. This work offers the potential uses of MFI zeolite in alkane-alkene separation, where the Si/Al ratio of the adsorbent plays a vital role in both chemisorption and adsorption selectivity.