Abstract
Lewis acidic zeolites are rapidly emerging liquid-phase Lewis acid catalysts. Nevertheless, their inefficient synthesis procedure currently prohibits greater utilization and exploitation of these promising materials. Herein, we demonstrate that SnIV-containing zeolite beta can readily be prepared both selectively and extremely rapidly by solid-state incorporation (SSI) method. Through a combination of spectroscopic (XRD, UV/Vis, X-ray absorption, magic-angle spinning NMR, and diffuse reflectance infrared Fourier transform spectroscopy) studies, we unambiguously demonstrate that site-isolated, isomorphously substituted SnIV sites dominate the Sn population up to a loading of 5 wt % Sn. These sites are identical to those found in conventionally prepared Sn-beta, and result in our SSI material exhibiting identical levels of intrinsic activity (that is, turnover frequency) despite the threefold increase in Sn loading, and the extremely rapid and benign nature of our preparation methodology. We also identify the presence of spectator sites, in the form of SnIV oligomers, at higher levels of Sn loading. The consequences of this mixed population with regards to catalysis (Meerwein–Pondorf–Verley reaction and glucose isomerization) are also identified.
Highlights
Lewis acid catalysts have found widespread applicability for a number of liquid-phase oxidation and isomerization reactions.[1,2] Amongst this class of catalysts are Lewis acidic zeolites, which are important catalysts in the area of sustainable chemistry.[3,4,5] These crystalline, porous, inorganic Lewis acids possess several major advantages over conventional homogeneous analogues, such as AlCl3 and ZnCl2
B zeolite material (SiO2/Al2O3 > 1000) possessing vacant framework sites, within which SnIV can be incorporated through post-synthetic methodologies
Metal loadings were determined by inductively coupled plasma mass spectrometry (ICP–MS), and are consistent— within experimental error—to the nominal loadings used during preparation
Summary
Lewis acid catalysts have found widespread applicability for a number of liquid-phase oxidation and isomerization reactions.[1,2] Amongst this class of catalysts are Lewis acidic zeolites, which are important catalysts in the area of sustainable chemistry.[3,4,5] These crystalline, porous, inorganic Lewis acids possess several major advantages over conventional homogeneous analogues, such as AlCl3 and ZnCl2. Given the lack of a widely applicable and scalable preparation methodology, significant academic and industrial research has recently focused on the development of new methodologies for preparing Sn-b.[19,20,21,22,23,24] Recently, Hammond and co-workers[20] demonstrated how simple, post-synthetic modifications of commercially available zeolites could readily enhance the availability and practicality of Lewis acidic zeolites for sustainable catalytic transformations.[20] Dealumination of a commercially available b zeolite with HNO3 was found to lead to a highly siliceous framework possessing vacant tetrahedral (T)-sites, into which SnIV could be placed by means of solid-state incorporation (SSI) with tin(II) acetate Does this approach avoid the long synthesis timescales associated with conventional Sn-b synthesis procedures (the total time for mechanochemical and heat-treatment steps is only 8 h), but it allows for the synthesis of a material with significantly smaller. The identity of these spectator sites, and their consequences for Lewis acid catalysis, is rationalized with XAS, MAS NMR, UV/Vis, XRD and diffuse reflectance infrared Fourier transform spectroscopy studies (DRIFTS)
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