Highly Dispersed Sn-beta Zeolites as Active Catalysts for Baeyer–Villiger Oxidation: The Role of Mobile, <i>In Situ</i> Sn(II)O Species in Solid-State Stannation

Abstract

Solid-state incorporation of Sn into beta (β) zeolites is a fast and efficient method to obtain Lewis acidic Snβ catalysts with high activity. The present work emphasizes the fundamental role of the heat-treatment atmosphere in the solid-state incorporation of active Sn in zeolites. Via an array of characterization tools including N2-physisorption, X-ray diffraction, diffuse reflectance UV–vis spectrocopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and 119Sn Mössbauer spectroscopy, it is shown that preheating under an inert atmosphere (pre-pyrolysis) prior to air-calcination affords Sn-β catalysts with the highest Sn dispersion and significantly less extra-framework SnO2 compared to the classic calcination. In situ characterization during pre-pyrolysis by temperature-programed decomposition–mass spectrometry, thermogravimetric analysis, and 119Sn Mössbauer spectroscopy reveals the in situ generation of Sn(II)O species that are more mobile than Sn(IV)O2 species generated during calcination. This mobility property essentially enables the high Sn dispersion in Snβ. Based on this knowledge, active sites per catalyst weight are maximized while retaining high turn-over frequencies for the Baeyer–Villiger oxidation reaction (300 h–1 at 80 °C). For Lewis acid densities above 200 μmol·g–1, the catalytic activity unexpectedly leveled off to 93 mM·h–1, even under kinetic control. We tentatively ascribe the activity plateau to the incorporation of Sn in less favorable T-sites at high Sn-loadings.