Tuning the acidity and porosity of zeolites through in-situ activated post-modification approaches

Abstract

Zeolites, the crystalline microporous aluminosilicates, have emerged as versatile catalysts in numerous industrial chemical processes like petroleum refineries, methanol conversion to olefins or gasoline, and biomass conversion. The catalytic performance is primarily dominated by their acidity and porosity, which can be easily adjusted. Post-modification is an attractive method for adjusting the zeolite properties due to its environmental friendliness and scalability. However, achieving precise atomic-level control of the acidity and homogeneous porosity via the conventional post-modification methods remains a frontier challenge in zeolite chemistry and science. The thesis aims to develop innovative in-situ activated post-modification methods to tackle this challenge and enable rational tuning of the acidity and porosity. The following are the designed strategies: 1. Introducing an innovative persulfate-based dealumination strategy, we unlock the potential for precise and selective removal of aluminum (Al) atoms/Brønsted acid sites (BASs) in zeolites by leveraging its in-situ activation and size-selective features. Our approach involves targeted delivery of persulfate molecules into specific zeolite channels, followed by thermal activation to in-situ release etching species and extract Al atoms from these channels. Through advanced techniques including synchrotron X-ray diffraction, high-energy X-ray total scattering, solid-state nuclear magnetic resonance (SSNMR) and electron paramagnetic resonance, we confirm the successful introduction and activation of persulfate, as well as the structural changes and selective dealumination it induces in the widely used mordenite zeolite. 2. Introducing a fluoride (F)-induced dehydroxylation method to modulate framework Lewis acid sites (LASs). Our approach involves delivering NH4F dissolved in methanol into the zeolite, followed by thermal treatment to partially break framework Al-O bonds by a dehydroxylation process. We validate the controlled modulation of framework LASs in our F-treated ZSM-5 zeolites through advanced SSNMR assisted with trimethylphosphine (TMP), trimethylphosphine oxide (TMPO) and 13C-2-acetone probes. Furthermore, we unravel the role of framework LASs in enhancing adsorption and catalytic conversion for propane. 3. Based on the understanding of the influence of F species on zeolite atomic structure from the above work, we further develop a novel hydrolysis-triggered in situ pore engineering strategy with methanol-mediated NH4F etching to fabricate hierarchical porosity in small-pore zeolites. Our method allows the in situ release of active F species for uniform and controlled removal of both framework Al and silicon (Si) atoms. SSNMR spectroscopy provides in-depth insight into the etchant interactions with the framework defect sites and the evolution of Al and Si species from framework to extra-framework, elucidating the structural and chemical changes occurring during the process. Overall, this thesis presents innovative post-modification methods that can address the challenge of achieving precise atomic-level control of acidity and homogeneous porosity. The conducted research in this thesis marks the beginning of a new era for rational design of zeolite materials via post-modification, offering versatility and significant potential for future advancements in zeolite research and development.