Catalytic upgrading of lignin to aromatics can be achieved selectively with metal-based catalysts in solvents. The reactions often take place in triphasic solid-liquid-gas mixtures at T &gt; 150 °C and P &gt; 10 bar, and as a result, experimental studies are often challenged by difficulties in accessing mechanistic and kinetic information. Customized magic-angle-spinning (MAS) solid-state NMR rotors have been successfully adopted to study the lignin conversion chemistry under high temperature/pressure. The rotor can withstand an internal pressure of 250 bar at 250 °C under MAS conditions, which allows acquiring operando MAS-NMR measurements for lignin conversion catalyzed with supported metal catalysts. The hydrogenolysis of lignin α-O-4 and β-O-4 ether linkages has been demonstrated with model compounds, benzyl phenyl ether (BPE) and 2-phenoxy-1-phenylethan-1-ol (PPE). Selective 13C labeling in both molecules permits quantitative measurements of kinetic profiles of the model compounds and their products. Various catalysts have been studied, including γ-Al2O3 supported Ni, Pd, etc., at 150-225 °C with 50-135 bar H2. Besides, MAS sharpens resonances with increased resolution, enabling identification of key reaction intermediates. The mechanistic investigation has been verified with selective isotope labeling of hydrogen gas and solvent 2-PrOH with 2H. The operando MAS-NMR makes it possible to probe different reactivities of the two C-O bonds in PPE.

Operando MAS-NMR spectroscopy in catalytic lignin hydrogenolysis

2019 Spring National Meeting

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