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

Abstract

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 ¹³C-labeled benzyl phenyl ether (BPE, a model compound for the major β-O-4 linkage in lignin), catalyzed by Ni/γ-Al₂O₃, was observed directly at elevated temperatures (150-175 °C) and pressures (79-89 bar) using <i>operando</i> magic-angle spinning NMR spectroscopy. Liquid-vapor partitioning in the NMR rotor was quantified using the ¹³C NMR resonances for the 2-propanol solvent, whose chemical shifts report on the internal reactor temperature. At 170 °C, BPE is converted to toluene and phenol with <i>k</i>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 (<i>k</i>₂ = 0.0052 s^-1 g_cat^-1 at 170-175 °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 (<i>k</i>₃ = 0.046 s^-1 g_cat^-1 at 175 °C). While the source of the reducing equivalents for both hydrogenolysis and hydrogenation is exclusively H₂/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 H₂ proceeds with an apparent activation barrier of (72 ± 13) kJ mol^-1. The <i>operando</i> NMR spectra provide molecularly specific, time-resolved information about the multiple simultaneous and sequential processes as they occur at the solid-liquid interface.