Tandem Reduction–Reoxidation Augments the Catalytic Activity of Sn-Beta Zeolites by Redispersion and Respeciation of SnO₂ Clusters

High dispersion of Sn in beta (β) zeolites is pivotal to obtain highly active Snβ catalysts with high productivity. However, at higher Sn loadings, Sn dispersion and the activity per Sn decrease. The present work highlights the augmentation of catalytic activity in the Baeyer–Villiger oxidation and Meerwein–Ponndorf–Verley reactions of the as-synthesized Snβ catalysts with Sn loadings of up to 10 wt % by tandem reduction–reoxidation and discusses the effect in terms of Lewis acid (LA) density, Sn dispersion, Sn speciation, and catalytic performance. To do so, nontreated, reduced, and reduced–reoxidized Snβ catalysts are characterized by N2 physisorption, X-ray diffraction (XRD), and a multitude of spectroscopic techniques such as temperature-programmed reduction-mass spectrometry (TPR-MS), probe Fourier transform infrared (FTIR), diffuse reflectance ultraviolet–visible (DRUV–vis), 119Sn Mössbauer, 119Sn magic angle spinning nuclear magnetic resonance (MAS NMR), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), and X-ray photoelectron spectroscopy (XPS). Although not mutually exclusive, the higher catalytic activity seems to arise from different phenomena depending on the Sn loading. At a lower Sn content (<5 wt %), the higher activity arises from the redispersion of SnO2 clusters into smaller active Sn species, whereas at a higher Sn content (≥5 wt %), the catalytic activity seems to improve based on the respeciation of SnO2 clusters.