Highly Dispersed Sn-beta Zeolites as Active Catalysts for Baeyer–Villiger Oxidation: The Role of Mobile, In Situ 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 solidstate incorporation of active Sn in zeolites. Via an array of characterization tools including N 2 -physisorption, X-ray diffraction, diffuse reflectance UV−vis spectrocopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and 119 Sn Mossbauer spectroscopy, it is shown that preheating under an inert atmosphere (pre-pyrolysis) prior to air-calcination affords Snβ catalysts with the highest Sn dispersion and significantly less extra-framework SnO 2 compared to the classic calcination. In situ characterization during pre-pyrolysis by temperature-programed decomposition−mass spectrometry, thermogravimetric analysis, and 119 Sn Mossbauer spectroscopy reveals the in situ generation of Sn(II)O species that are more mobile than Sn(IV)O 2 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.

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

“…However, in literature it is reported that SnO2 as precursor gives inactive Snβ catalysts containing high amount of SnO2 clusters. 8,37 Figure 3 shows that untreated 5SnO2 deAlβ indeed did not possess any catalytic activity, which is in agreement with literature. After the reduction-reoxidation cycle, the 5SnO2 deAlβ samples reaches 63 mMCL• h -1 , exceeding the 5Snβ-air catalyst made with Sn(II) acetate and supporting our hypothesis.…”

“…The Ea values of the MPV and BVO reaction are comparable to the one reported for HT Snβ (63 kJ•mol -1 ), 33 and BVO PS SS Snβ (70 kJ•mol -1 ) 8 , respectively. Moreover, these Ea-values indicate that all reactions were performed in absence of mass transfer limitations.…”

“…Such low-dispersed Snβ starting samples were made by solid-state (SS) stannation of dealuminated beta zeolites with Sn(II) acetate via air-calcination without a pre-pyrolysis step. 8 These Snβ samples were reduced and the resulting Sn species were characterized by powder X-ray diffraction (PXRD) and spectroscopy techniques including temperature-programmed reduction mass-spectroscopy (TPR-MS), 119 Sn Mössbauer, diffuse reflectance UV-vis (DRUV-vis) and Fourier Transform infrared spectroscopy (FTIR).…”

“…To investigate the influence of extra-framework Sn oxide clusters on the formation of metallic Sn, we compared the 119 Sn Mössbauer data of 2Snβ-air (Figure S2A and B) with 2Snβ-N2/air (Figure S2C and D). This latter catalyst, made via two-step pre-pyrolysis/air-calcination, was previously shown to possess higher Sn dispersion relative to the corresponding one-step air calcination catalyst (58% vs. 20%) 8 , and hence less extra-framework Sn oxide clusters because of an improved incorporation of Sn into the framework. Under identical reduction at 700 °C, it was found that for the sample with lower Sn dispersion (single-step calcination), not only more Sn(IV) is reduced (83% vs. 75%), but also more metallic Sn is formed (31% vs. 14%).…”

“…Lewis acidic Sn incorporated in a beta (β) zeolite is highly active in industrially relevant reactions such as the reduction of ketones/aldehydes to alcohols by Meerwein-Ponndorf-Verley reaction (MPV), [1][2][3] the oxidation of ketones to esters with H2O2 (Baeyer-Villiger oxidation, BVO), [4][5][6][7][8] the isomerization and epimerization of sugars, [9][10][11][12][13][14][15][16][17][18] the conversion of carbohydrates into lactic acid, [19][20][21] and aldol condensations. 22,23 There is general consensus that Snβ catalysts contain Lewis acidity (LA), but the exact structural properties of the active Sn site is still debated.…”