Unraveling the Dynamic Network in the Reactions of an Alkyl Aryl Ether Catalyzed by Ni/γ-Al2O3 in 2-Propanol

The reductive cleavage of aryl ether linkages is a key step in the disassembly of lignin to its monolignol components, where selectivity is determined by the kinetics of multiple parallel and consecutive liquid-phase reactions. Triphasic hydrogenolysis of C-13-labeled benzyl phenyl ether (BPE, a model compound for the major beta-O-4 linkage in lignin), catalyzed by Ni/gamma-Al2O3, was observed directly at elevated temperatures (150-175 degrees C) and pressures (79-89 bar) using operando magic-angle spinning NMR spectroscopy. Liquid-vapor partitioning in the NMR rotor was quantified using the C-13 NMR resonances for the 2-propanol solvent, whose chemical shifts report on the internal reactor temperature. At 170 degrees C, BPE is converted to toluene and phenol with k(1) = 0.17 s(-1) g(cat)(-1) and an apparent activation barrier of (80 +/- 8) kJ mol(-1). Subsequent phenol hydrogenation occurs much more slowly (k(2) = 0.0052 s(-1) g(cat)(-1) at 170-175 degrees C), such that cyclohexanol formation is significant only at higher temperatures. Toluene is stable under these reaction conditions, but its methyl group undergoes facile H/D exchange (k(3) = 0.046 s(-1) g(cat)(-1) at 175 degrees C). While the source of the reducing equivalents for both hydrogenolysis and hydrogenation is exclusively H-2/D-2(g) rather than the alcohol solvent at these temperatures, the initial isotopic composition of adsorbed H/D on the catalyst surface is principally determined by the solvent isotopic composition (2-PrOH/D). All reactions are preceded by a pronounced induction period associated with catalyst activation. In air, Ni nanoparticles are passivated by a surface oxide monolayer, whose removal under H-2 proceeds with an apparent activation barrier of (72 +/- 13) kJ mol(-1). The operando NMR spectra provide molecularly specific, time-resolved information about the multiple simultaneous and sequential processes as they occur at the solid-liquid interface.