(569l) Confined Base Catalysis on Amine-Functionalized Zeolites Prepared Via Direct Synthesis

Confinement effects within microporous materials such as MOFs and zeolites significantly influence catalytic reactivity across the reaction coordinate (e.g., selective adsorption of reactive species, transition-state stabilization). The exploitation of confinement has historically been skewed towards acid catalysis and more limited for base catalysis, owing to the inherent and tunable Bronsted and Lewis acidity afforded by zeolites. Yet, base catalysis is crucial for many important industrial reactions including dehydration, hydrogenation, amination, etc. Here, we functionalize the microporous framework of zeolite Beta with covalently tethered aminopropyl functional groups that provide confined basic sites within pores with tailorable polarity and steric environments. To achieve intrapore amines, we use a direct co-condensation synthesis approach that involves the hydrolysis of alkoxysilane precursors. We vary the amine functionality through selection of the aminosilane precursor in the synthesis gel. Alkaline activation with ammonium hydroxide in methanol as a post-treatment procedure generated a high density of silanol defects in the framework. Structural characterization including x-ray diffraction, N₂ physisorption, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) confirmed the targeted BEA crystalline phase and presence of intrapore amine groups. Amine loadings of 1 mol% to 5.5 mol% with respect to silica were quantified through thermogravimetric analysis and potentiometric titration with perchloric acid. Batch experiments of aldol reactions (4-nitrobenzaldehyde and acetone cross-condensation, 2-butenal self-condensation) show that amine functionalized beta can effectively be used as a base catalyst with a turnover number (TON) of 2.8 moles per mole of active site. Furthermore, we show the impact of acid-base bifunctionality (previously shown in mesoporous silica) between basic amines and weakly acidic silanol defects. In the aminopropyl BEA, introducing silanols increased the TON by 24% compared to the low defect version. Looking ahead, these amine-functionalized Betas have broad applications in base catalysis (e.g., Knoevenagel condensation, esterification) and potentially low-concentration CO₂ capture.