Elucidating water’s role in post-synthesis zeolite modifications to control rare-earth and phosphorus incorporation

Zeolite catalysts are crucial to an economically viable petrochemical industry, with framework structures and post-synthetic modifications optimized for gas-phase hydrocarbon feeds. With increasing demands for sustainability and control of the carbon-atom economy, zeolites have also emerged as promising catalysts for hydrocarbon production from bio-derived feedstocks, waste streams, and other oxygenates that can expose the zeolite to both liquid- and vapor-phase water. Many studies on post-synthesis modifications of zeolites are essentially qualitative and focus on improving the zeolite’s stability against high-temperature steaming in catalyst regeneration steps. However, liquid- and vapor-phase water attack the zeolite framework by different mechanisms. To better adapt zeolites to these new and varied processes, then, a clearer understanding of how water impacts common post-synthesis modifications is needed. One of the most common post-synthesis modifications is the introduction of stabilizing cations by ion-exchange; most notably, the introduction of either rare earths or phosphorus, but routine methods of introducing non-framework cations via aqueous exchange can degrade the integrity of the zeolite framework, making the cations’ true impact on zeolite reactivity and stability unclear, particularly in the case of zeolite Y. By adjusting the cation exchange process in order to preserve the zeolite framework, defect-free catalysts with varied Lanthanum loadings were synthesized. Quantitative solid-state NMR experiments coupled with micro-reactor experiments and computational studies revealed both the nature of rare earth incorporation and its impact on catalyst reactivity and stability, clearly identifying preferred rare-earth siting in a faujasite zeolite Y catalyst. Additional NMR experiments combined with computational studies revealed that while the use of water as a solvent has negligible impact on phosphorus incorporation in ZSM-5, the presence or absence of water can change phosphate structure distributions in phosphorus-modified MFI zeolites. While hydration-dehydration cycles in Lanthanum-modified zeolite Y causes irreversible loss of framework acid sites, similar hydration-dehydration cycles in phosphorus-modified ZSM-5 are fully reversible, forming polyphosphates upon dehydration which, upon rehydration, reform the first-formed framework-tethered Al-O-P species as well as hydrogen-bonded H₃PO₄/BAS complexes.