Nature and catalytic activity of Lewis acid extra-framework species in zeolites
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Author
Date
2023Type
- Doctoral Thesis
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Abstract
Zeolites are aluminosilicate materials possessing well-defined pore sizes, hydrothermal stability, and structural tunability. Due to the combination of unique characteristics, they hold a central place in heterogeneous catalysis and continue to find an impressive number of potential applications. The role of zeolites as acid catalysts originates from the co-presence of Lewis acid sites (LAS) and Brønsted acid sites (BAS). The structure of BAS is well defined as the hydroxyls bridging the framework of silicon and aluminum. In contrast, LAS is less defined owing to the multiplicity of their structure and origin. Understanding the nature, origin, and activity of acid sites is crucial in tuning the structure of zeolites for end applications. The Lewis acidity of zeolites remains an extensive area of research and application. In comparison, the Lewis acidity of aluminum in zeolites is ambiguous and demands thorough investigation. In particular, many essential questions about the nature and the quantitative structure-performance relations of extra-framework aluminum (EFAl) motifs must be carefully addressed.
The present work, thus, explores the Lewis acidity of aluminum in zeolites while emphasizing the dedicated design of EFAl LAS and understanding their structure, acidity, and catalytic performance. Chapters 1 and 2 discuss the existent literature about the structure and applications of Lewis acidic zeolites, different ways of incorporating Lewis acidity, and open questions related to the Lewis acidity of EFAl species. Chapter 3 explains the required characterization techniques and synthesis protocols employed in the present work. These chapters determine the scope of the thesis.
The ultra-stable zeolite Y (USY), prepared by post-synthetic steaming, shows much higher hydrothermal stability and catalytic activity than non-steamed Y. In Chapter 4, Lewis acidity was introduced into zeolite Y by facile ion-exchange of aluminum cations. FTIR and NMR spectroscopies were employed to evaluate the Lewis acidity and coordination of introduced aluminum. A quantitative agreement was observed between the concentration of these introduced EFAl species with the total Lewis acid content from pyridine-FTIR and with the catalytic activity in Meerwein–Ponndorf–Verley (MPV) reduction of 4-tert butyl cyclohexanone. This illustrates that the newly introduced aluminum LAS adopt octahedral coordination under the conditions of NMR measurement and are responsible for enhanced Lewis acid catalytic activity of the aluminum exchanged zeolites. The preservation of Brønsted acid sites after treatments further endorses the charge neutrality of these extra-framework aluminum complexes.
The efficiency of the aluminum-exchange route to introduce the EFAl LAS while retaining the zeolite’s framework and inherent porous characteristics depends on many factors. The work presented in Chapter 5 explores the generation of aluminum-exchanged EFAl LAS in zeolite Y employing different Si/Al ratios and cationic forms of parent zeolite and different aluminum-exchange conditions. A constant stirring of the zeolite in an aluminum-exchange solution and higher Si/Al ratios of parent zeolite favor the maximum incorporation of catalytically active EFAl LAS, followed by maximum zeolite structure retention. The presence of sodium co-cation not only hampers the incorporation of acid sites but also negatively affects the crystallinity and pore structure. The catalytic performance depends equally on the number of EFAl LAS and the retention of the inherent framework of zeolite.
These fundamental insights about the rational design of EFAl LAS were employed in Chapter 6 to introduce Lewis acidity into zeolites of different morphologies. This work aimed to evaluate the factors affecting the generation and activity EFAl LAS in zeolites. The increase in EFAl LAS in BEA and Y was appreciable, whereas MOR and MFI showed minimal uptake of aluminum. This quantitatively follows the catalytic activity for MPV reduction of 4-tert butyl cyclohexanone. The incorporation of EFAl LAS and their catalytic activity depend on the zeolite framework type, pore size, and aluminum location within the framework. Likewise, the selectivity towards cis and trans 4-tert butyl cyclohexanols was affected by the zeolite's pore size and framework type and not by the number or structure of LAS. Irrespective of zeolite morphology, aluminum-exchange incorporates neutral LAS.
The MPV reduction of ketones occurs under mild conditions as it uses secondary alcohols for the hydride transfer reaction; thus, it can also be catalyzed by weak and medium-strength LAS (Chapters 4-6). Therefore, Chapter 7 explores the strength and hydrothermal stability of aluminum-exchanged EFAl LAS in activating the C-H bond during n-butane dehydrogenation. Aluminum-exchange significantly increases the conversion of n-butane with enhanced selectivity to dehydrogenation products. No significant change in selectivity by lowering the BAS content of aluminum-exchanged samples by Na-IE proposes that dehydrogenation occurs on EFAl LAS. The preservation of structure and Lewis acidity of EFAl species in the regenerated catalysts confirms that the thermal stability and strength of neutral EFAl LAS, produced by Al-IE, are capable of cleaving the C-H bonds of alkanes.
Zeolite BEA is an efficient catalyst for MPV reaction, and the aluminum species partly connected to the framework (FA-Al) (and not the EFAl) are usually reported as active sites. The aluminum-exchange produces EFAl LAS without affecting the content of FA-Al. In Chapter 6, an increase in EFAl LAS in BEA follows an increase in MPV activity. Therefore, Chapter 8 explores the distribution of aluminum LAS and the associated activity of BEA after alumination (by aluminum-exchange), dealumination (by acid treatment), and realumination by aluminum-exchange). The aluminated and dealuminated-realuminated BEA significantly increase the activity during MPV reduction. The correlations of catalytic activity with the total content of LAS from pyridine- and CO-FTIR and the content of EFAl and FA-Al species from 27Al NMR suggest that both EFAl and FA-Al can serve as MPV active sites. The Lewis acidity and catalytic activity of parent BEA are mainly due to FA-Al species. In contrast, the alumination and realumination by aluminum-exchange increase the catalytic activity and the number of LAS due to the introduction of EFAl. The EF-Al were systematically distinguished from EFAl by characterizing the zeolites under different cations forms.
The insights about the structure of the MPV active site were employed to revisit the so-called ‘very high frequency’ (VHF) hydroxyls at ⁓3780 cm-1 of zeolites and the structure of aluminum species attached (Chapter 9). The literature describes These hydroxyls in many ways, but the exact assignment is unclear. In BEA, these hydroxyls are considered highly acidic as they are proposed to be connected to the MPV active site. Therefore, this chapter aims to systematically explore the evolution of VHF in hydroxyl and base stretching regions under different conditions. The FTIR spectroscopy of adsorbed carbon monoxide and pyridine on dehydrated zeolites, in combination with 27Al-1H HETCOR NMR spectroscopy on evacuated zeolites, describes that the VHF-OH band correlates neither to the strongly acidic FA-Al species nor to the EFAl species generated by aluminum-exchange.
In conclusion, this work provides significant insights into the rational design of EFAl LAS in zeolites without Brønsted acidity of zeolite. Evaluating the factors affecting the generation of aluminum-exchanged LAS guides the pathways towards maximizing the Lewis acidity yet preserving the intrinsic properties of zeolite. A combination of different spectroscopic techniques, catalytic evaluation, and diverse treatment conditions can be used to quantitatively discern the different Lewis acidic aluminum species and address the open questions related to the Lewis acidity of extra-framework aluminum in zeolites. Show more
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https://doi.org/10.3929/ethz-b-000648031Publication status
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ETH ZurichSubject
chemistry; heterogeneous catalysis; zeolitesOrganisational unit
03746 - Van Bokhoven, Jeroen A. / Van Bokhoven, Jeroen A.
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