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 13C-labeled benzyl phenyl ether (BPE, a model compound with the same C-O bond energy as 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 operando 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 k1 = 0.17 s-1 gcat-1 and an apparent activation barrier of (80 ± 8) kJ mol-1. Subsequent phenol hydrogenation occurs much more slowly (k2 = 0.0052 s-1 gcat-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 (k3 = 0.046 s-1 gcat-1 at 175 °C). While the source of the reducing equivalents for both hydrogenolysis and hydrogenation is exclusively H2/D2(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. Finally ,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.
- Research Organization:
- Ames Lab., Ames, IA (United States); Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- National Science Foundation (NSF); USDOE Office of Science (SC), Biological and Environmental Research (BER)
- Grant/Contract Number:
- AC05-76RL01830; CBET-1512228; CBET-1604095; AC02-07CH11358; DMR-1121053
- OSTI ID:
- 1608537
- Alternate ID(s):
- OSTI ID: 1709115
- Report Number(s):
- PNNL-SA-148402; IS-J-10,099
- Journal Information:
- Journal of the American Chemical Society, Vol. 141, Issue 43; ISSN 0002-7863
- Publisher:
- American Chemical Society (ACS)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
Web of Science
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