The strong acidity of Brønsted acid sites in zeolites often leads to uncontrollable reactions and rapid deactivation. Therefore, strategically weakening zeolite acidity is essential for balancing reactivity with stability. The local coordination environment of framework aluminum usually governs its acidity. Hydroxylation of framework aluminum is often inevitable under high-temperature and humid "working" conditions; however, the impact of hydroxyl coordination on the relevant acidity and catalytic behavior remains unclear. Here, we demonstrate how hydroxylation of framework aluminum strategically moderates acidity and enhances catalytic stability. Density functional theory (DFT) calculations predict that an increase in the hydroxyl groups coordinated to framework aluminum leads to a progressive weakening of Brønsted acidity. We experimentally validate this prediction by precisely manipulating hydroxylation in ZSM-5 zeolite via steaming treatment and analyzing the effects using 2D NMR spectroscopy with 2-13C-acetone as a probe molecule. The hydroxylated aluminum sites exhibit reduced adsorption and activation of methanol in the dehydration reaction, consistent with their weaker acidity. Catalytic tests reveal that the hydroxylated samples significantly enhance catalyst lifespan in the MTO process, while preserving stable reactivity. These findings provide key insights into how coordination perturbations influence zeolite acidity and catalytic performance, offering valuable guidance for designing zeolites with targeted catalytic functions.
Xing et al. (Tue,) studied this question.