Optimizing the Strategies for Tailoring Aluminum Lewis Acidity in Y Zeolites by Aluminum Exchange

High Resolution Image Download MS PowerPoint Slide This work investigates the formation of extra-framework aluminum Lewis acid sites (EFAl LAS) in Y zeolite via aluminum ion-exchange (Al-IE) as a function of parent zeolite Si/Al ratio, treatment conditions, i.e., with and without heating and stirring, and the presence of cocations, i.e., proton, ammonium, and sodium. High-silica Y zeolites (Si/Al ≥ 15) facilitate efficient EFAl incorporation via silanol-assisted anchoring while maintaining framework integrity and microporosity, as verified by 27 Al NMR, ICP-OES, XRD, and N 2 physisorption. The incorporated EFAl, octahedral under NMR conditions, served as LAS without affecting the Brønsted acidity, as demonstrated by pyridine-probed FTIR spectroscopy, and showed a strong correlation with enhanced catalytic activity in the Meerwein–Ponndorf–Verley (MPV) reduction of 4- tert -butylcyclohexanone. In very low silica Y zeolites, however, EFAl are incorporated at the expense of BAS, along with significant changes in framework integrity and microporosity. Under optimized Al-IE conditions, particularly ambient temperature Al-IE with moderate stirring, maximum generation of EFAl LAS and the respective MPV catalytic activity was achieved. Conversely, Al-IE under heating resulted in a lower number of LAS and reduced MPV rates, with LAS content and MPV activity being lowest due to Al-IE under combined heating and stirring despite maximum overall aluminum content. Minimal LAS formation and respective catalytic activity were observed in sodium-exchanged samples, underscoring the inhibitive effect of Na + on the EFAl formation. In contrast, zeolites in their protonic and ammonium forms equally favor the efficient formation of catalytically active EFAl LAS. These results outline approaches for modulating LAS in zeolites via nonframework aluminum modification without compromising Brønsted acidity and crystallinity.