Natural Abundance ¹⁷O DNP NMR Provides Precise O−H Distances and Insights into the Brønsted Acidity of Heterogeneous Catalysts

Abstract

Heterogeneous Brønsted acid catalysts are tremendously important in industry, particularly in catalytic cracking processes. Here we show that these Brønsted acid sites can be directly observed at natural abundance by O DNP surface-enhanced NMR spectroscopy (SENS). We additionally show that the O-H bond length in these catalysts can be measured with sub-picometer precision, to enable a direct structural gauge of the lability of protons in a given material, which is correlated with the pH of the zero point of charge of the material. Experiments performed on materials impregnated with pyridine also allow for the direct detection of intermolecular hydrogen bonding interactions through the lengthening of O-H bonds.

The Physico-Chemical Characterization of Mesoporous Sulfated Zirconia Nanoparticles Prepared under Structure-Directing-Free for <i>n</i>-Hexane Isomerization

“…Therefore, based on the clue given by the EDXS analysis, we have quantitatively determined the metal and non-metals by ICP-AES and by elemental analysis and X-ray fluorescence spectroscopy, respectively, yielding (wt%) Zr (51.85), S (2.388), H (3.15), N (0), and Cl (0). Combined with the data derived from the thermal gravimetric and elemental analyses, the pristine complex can be represented by an empirical formula: Zr 4 (S 2 O 7 ) 0.25 O 2 (OH) 5 (CH 3 COO) 6 •H 2 O. Figure 10 shows the ion current curve for evolved gases in the TG-MS measurement of the catalyst.…”

Enhanced 1H-X D-HMQC performance through improved 1H homonuclear decoupling

“…[28][29][30] To address the drawbacks of the CP-HETCOR approaches, HETCOR methods utilizing γencoded 31 and non-γ-encoded dipolar recoupling have been developed. The techniques making use of γ-encoded recoupling make use of the PRESTO [32][33][34][35] (phase-shifted recoupling effects a smooth transfer of order) and D-HUQC 36 (dipolar-heteronuclear universal quantum correlation) pulse sequences, which yield a direct polarization transfer and the indirect detection of heteronuclear n-tuple-quantum coherences (with n = 0, 1 and 2), respectively. While these techniques can provide high sensitivity as well as robustness to spinning instabilities, γ-encoded heteronuclear dipolar recoupling is severely dipolar-truncated, 33 which prevents them from being used to detect long-range correlations.…”

Improved strategies for DNP-enhanced 2D 1H-X heteronuclear correlation spectroscopy of surfaces

“…In particular, the DNP-enabled, multidimensional correlation NMR experiments can yield unprecedented structural and conformational details about the surfacebound species. [23][24][25][26][27][28][29] In modern high-field DNP SSNMR, solvents are used to disperse the biradical polarization agents within the sample. The solvent can also facilitate the transport of hyperpolarized magnetization from the radical source to the bulk of the observed nuclei by 1 H-1 H spin diffusion.…”