Chemistry, Ames Laboratory, Chemical and Biological Engineering
Journal or Book Title
The Journal of Physical Chemistry
High-resolution magic angle spinning (HRMAS) NMR spectroscopy was used to study the eﬀect of mixed solvent systems on the acidity at the solid−liquid interface of solid acid catalysts. A method was developed that can exploit beneﬁts of both solution and solid-state NMR (SSNMR) by wetting porous solids with small volumes of liquids (<2 >μL/mg) to create an interfacial liquid that exhibits unique motional dynamics intermediate to an isotropic liquid and a rigid solid. Results from these experiments provide information about the inﬂuence of the solvent mixtures on the acidic properties at a solid−liquid interface. Importantly, use of MAS led to spectra with full resolution between water in an acidic environment and that of bulk water. Using mixed solvent systems, the chemical shift of water was used to compare the relative acidity as a function of the hydration level of the DMSO-d6 solvent. Nonlinear increasing acidity was observed as the DMSO-d6 became more anhydrous. 1H HR-MAS NMR experiments on a variety of supported sulfonic acid functionalized materials, suggest that the acid strength and number of acid sites correlates to the degree of broadening of the peaks in the 1H NMR spectra. When the amount of liquid added to the solid is increased (corresponding to a thicker liquid layer), fully resolved water phases were observed. This suggests that the acidic proton was localized predominantly within a 2 nm distance from the solid. EXSY 1H−1H 2D experiments of the thin layers were used to determine the rate of proton exchange for diﬀerent catalytic materials. These results demonstrated the utility of using (SSNMR) on solid−liquid mixtures to selectively probe catalyst surfaces under realistic reaction conditions for condensed phase systems.
American Chemical Society
Johnson, Robert L.; Hanrahan, Michael P.; Mellmer, Max; Dumesic, James A.; Rossini, Aaron J.; and Shanks, Brent H., "The Solvent- Solid Interface of Acid Catalysts Studied by High Resolution MAS NMR" (2017). Chemistry Publications. 993.