Here we describe the combination of fast magic angle (MAS) and high resolution MAS (HRMAS) solid-state NMR spectroscopy to probe the structure and dynamics of heterogeneous catalysts coated with thin liquid layers. We demonstrate that MAS/ HRMAS solid-state NMR spectroscopy experiments can be applied to directly observe surface acid protons in exchange with the solvent,[1] mobile reactant/product molecules dissolved in the solvent and adsorbed reactant/products that are immobilized onto solid heterogeneous catalysts.[2] We have applied this approach to study alumina impregnated with dioxane solutions of bicyclic lactones in order to determine how alumina catalyzes decarboxylation of the lactone to form dihydrobenzenes.[2] Direct excitation 1H HRMAS NMR spectra generally show the sharp and intense 1H NMR signals associated with mobile solvent or reactant molecules. Conventional solution NMR experiments that exploit scalar couplings, such as 1H{13C} J-HMQC, can be performed to confirm the identity and structure of the mobile reactant molecules dissolved in solution. Dipolar based proton detected 2D 1H{13C} CP-HETCOR solid-state NMR spectra select the 1H and 13C NMR signals from immobilized bicyclic lactone molecules, confirming that a large fraction of the lactones are bound to the alumina surface. Proton detected 2D 27Al→1H D-RINEPT dipolar HETCOR NMR spectra[3] enables the selective observation of the 27Al NMR signals of the surface aluminum atoms and confirms immobilization of the lactone on the surface. The experimental NMR results are consistent with DFT models that predict the lactone is adsorbed on the alumina surface. We anticipate that the methods and approach presented here are generally applicable to study the interaction of dissolved reactants and products with catalyst surfaces under realistic reaction conditions for condensed phase systems. [1] Johnson, R. L; et al. The Journal of Physical Chemistry C 2017, 121, 17226. 1. [2] Pfennig, T.; Chemburkar, A.; Johnson, R. L.; Ryan, M. J.; Rossini, A. J.; Neurock, M.; Shanks, B. H. Manuscript in Preparation 2017. [3] Venkatesh, A.; Hanrahan, M. P.; Rossini, A. J. Solid State Nucl. Magn. Reson. 2017, 84, 171-181.

Magic angle spinning solid-state NMR spectroscopy of impregnated heterogeneous catalysts

255th National Meeting (2018)

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