NMR Signatures of the Active Sites in Sn‐β Zeolite

Abstract

Dynamic nuclear polarization surface enhanced NMR (DNP-SENS), Mçssbauer spectroscopy, and computational chemistry were combined to obtain structural information on the active-site speciation in Sn-b zeolite. This approach unambiguously shows the presence of framework Sn IV -active sites in an octahedral environment, which probably correspond to so-called open and closed sites, respectively (namely, tin bound to three or four siloxy groups of the zeolite framework).Heterogeneous catalysts with well-defined isolated active sites do not only facilitate mechanistic investigations, but can also show unparalleled activity in a variety of important reactions. One example is TS-1 for the epoxidation of propene with H 2 O 2 .[1] A more recent example is Sn-b, which consists of Sn IV -sites embedded in the zeolite-b framework. [2] The uniform distribution of isolated Lewis acid sites, in combination with the unique hydrophobic pore architecture of the material, results in an unrivalled catalytic performance. Of particular interest is the Lewis acid-catalyzed isomerization and epimerization of sugars, key transformations in the upgrading of cellulose-based renewable feedstocks. [3] Sn-b is also active for other reactions, including the Baeyer-Villiger oxidation of ketones and aldehydes, [4] the carbonylene cyclization of citronellal, [5] and the Meerwein-Ponndorf-Verley-Oppenauer reaction. [6] Today, Sn-b can be conveniently synthesized with Sn-loadings varying between 0 and 10 wt %. [7] [*] P. Wolf, [+] M. Valla, [+] Dr. A. Comas-Vives, [+] Dr.

Selective Chemical Conversion of Sugars in Aqueous Solutions without Alkali to Lactic Acid Over a Zn-Sn-Beta Lewis Acid-Base Catalyst

“…This result indicates that the Zn-Sn-Beta zeolite does not appear to have extra-framework ZnO and SnO 2 which is consistent to the XRD data. In addition, Hermans et al confirmed that the presence of framework Sn in the Sn-Beta zeolite prepared by SSIE using dynamic nuclear polarization surface enhanced nuclear magnetic resonance24.…”

Theoretical aspects of Magic Angle Spinning - Dynamic Nuclear Polarization

“…The advent of Dynamic Nuclear Polarization (DNP), using gyrotron microwave (MW) sources, combined with MAS has opened the door to many advanced studies of solids at high magnetic fields [1]. The development and implementation of high field gyrotrons up to the Terahertz range has enabled the construction of high field MAS-DNP instruments, and led to the commercialization of MAS-DNP spectrometers by Bruker Inc. [2] operating up to 18 T. Theses high field DNP instrument developments have triggered a vast interest of the solid-state NMR community in MAS-DNP experiments, both for biological [3][4][5][6][7][8][9][10][11] and material science [12][13][14][15][16][17][18][19][20][21][22] applications. Most of these experiments performed today are using nitroxide based bi-radicals such as TOTAPOL [23,24,2,8,25,26,9], and recently new nitroxide based biradicals have been introduced with varying longitudinal relaxation times, phase memory times (or transverse relaxation time), electron-electron dipolar couplings, and relative gÀtensor orientations such as AMUPOL, bCTbK, or TEKPOL [27,26,28].…”

Interrogating the Lewis Acidity of Metal Sites in Beta Zeolites with 15N Pyridine Adsorption Coupled with MAS NMR Spectroscopy

“…For example, in the isomerization of glucose to fructose with Sn-Beta, various authors have attributed the observed decrease in turnover frequency with increasing metal loading to the resence of framework Sn sites with different activities or the formation of condensed SnO x species. 31,32 …”

“…However, many of these methods are difficult to implement reliably or routinely. 6 For instance, diffuse reflectance ultraviolet spectroscopy (DRUV), 30,31,33 X-ray diffraction, 30,34 119 Sn Mössbauer spectroscopy, 31 X-ray photoelectron spectroscopy (XPS), 33 and Raman spectroscopy 31,33 provide only qualitative or semi-quantitative measurements. Synchrotron-based techniques are powerful but difficult to implement on a routine basis.…”

“…6,16 Roman-Leshkov et al and Hermans et al have recently shown that hyperpolarization techniques based on dynamic nuclear polarization (DNP) MAS NMR can be implemented to characterize nonenriched Sn-Beta samples, but specialized NMR equipment is required. 31,35 Recently, Ivanova et al showed that other methods could be used to obtain enhanced signals with unlabeled samples. 36 Other metals of interest, including Ti, Zr, and Hf, suffer from extreme challenges in NMR sensitivity that stem from low natural abundance, large quadrupolar moment, and/or low gyromagnetic ratios (as shown in Table S1).…”