Directed hydrodeoxygenation of lignin depolymerization product pools for green aromatic hydrocarbon production: methoxy-induced inhibition and competitive reaction-sequence control

Hydrogenative lignin depolymerization produces oxygenated aromatic monomers that can be further upgraded to aromatic hydrocarbons by hydrodeoxygenation (HDO), providing an important route for renewable aromatic hydrocarbon production. However, in real lignin depolymerization product pools, multiple aromatic monomers usually coexist and may compete for catalytic sites, leading to different conversion sequences and mutual inhibition. Here, 4-vinylphenol (V), guaiacol (G), and 4-ethyl-2,6-dimethoxyphenol (S) were selected as representative lignin-derived aromatic monomers to clarify structure-dependent competitive HDO over a Pt-loaded Ni–Co MOF-derived acid–metal bifunctional catalyst (Pt@NC). Single-, binary-, and ternary-substrate hydrogenation experiments were combined with lumped/co-lumped kinetic analysis, in situ infrared spectroscopy, and DFT calculations. Single-substrate kinetics showed that V was readily converted to ethylbenzene through side-chain C=C hydrogenation followed by deoxygenation, whereas G followed branched pathways through C7, C8, and C6 oxygenated intermediate pools to form toluene and xylene products. S exhibited the lowest HDO activity because its higher methoxy and aromatic C–O contents delayed demethoxylation and deep deoxygenation. Binary experiments revealed asymmetric inhibition: hydrogenated intermediates derived from V suppressed G conversion, while G inhibited S conversion by delaying its demethoxylation and deep HDO pathways. In situ infrared spectroscopy and DFT calculations showed that methoxy and aromatic C–O structures strengthened adsorption and retention on Pt sites; when subsequent C–O activation was slow, this retention led to non-productive site occupation and reduced effective hydrogenation-site availability. To quantify this behavior, a competitive HDO inhibition index (CHII) was developed by integrating functional-group burden, adsorption strength, effective HDO kinetics, and apparent deactivation. The CHII followed S > G > V, opposite to the HDO priority observed in ternary experiments. This work identifies methoxy-induced site occupation and kinetic retardation as key origins of competitive HDO inhibition in lignin-derived aromatic monomer pools.