(540d) Dynamic Nuclear Polarization NMR Enables the Analysis of Sn-Beta Zeolite Prepared with Natural Abundance 119Sn Precursors

Pure-silica zeolites containing a small amount of tetravalent heteroatoms with open coordination sites  have emerged as highly active, water-tolerant solid Lewis acids for many important reactions. The catalytic activity of tin-containing zeolites, such as Sn-Beta, is critically dependent on the successful incorporation of the tin metal center into the zeolite framework. However, synchrotron-based techniques or solid-state nuclear magnetic resonance (ssNMR) of samples enriched with ¹¹⁹Sn isotopes are the only reliable methods to verify framework incorporation. For ¹¹⁹Sn (I = 1/2, natural abundance = 8.6%) magic angle spinning (MAS) NMR, the coupled effects of low natural abundance of the ¹¹⁹Sn isotope, low intrinsic NMR sensitivity, and low Sn loadings in the sample make NMR analysis impractical without ¹¹⁹Sn isotopic enrichment. Unfortunately, the high cost of isotopic enrichment drastically hinders high throughput screening, routine analysis, or analysis of low-yield syntheses with NMR. Dynamic nuclear polarization (DNP) can greatly improve sensitivity by transferring the larger polarization of electron spins, such as those found in stable exogenous radical compounds, to nuclear spins through irradiation with high-frequency microwaves. The target nuclei then become dynamically polarized, and their NMR signals are enhanced by orders of magnitude. This work demonstrates, for the first time, the use of DNP NMR for characterizing zeolites containing ∼2 wt % of natural abundance Sn without the need for ¹¹⁹Sn isotopic enrichment. To determine and minimize potential undesirable interactions of the radical with the Lewis acid center, four biradical polarizing agents with varying kinetic diameters were impregnated on the Sn-Beta sample. The biradicals TOTAPOL, bTbK, bCTbK, and SPIROPOL functioned effectively as polarizing sources, and the solvent enabled proper transfer of spin polarization from the radical’s unpaired electrons to the target nuclei. The larger bCTbK and SPIROPOL radicals did not interact directly with the intrapore Sn sites and bCTbK led to an enhancement (ε) of 75, allowing the characterization of natural-abundance ¹¹⁹Sn-Beta with excellent signal-to-noise ratios in <24 h. Without DNP, no ¹¹⁹Sn resonances were detected after 10 days of continuous analysis.