Tungsten‐Based Bimetallic Catalysts for Selective Cleavage of Lignin C−O Bonds

Abstract

Tungsten‐based bimetallic catalysts M–W/AC (M=Ru, Pt, and Pd; AC=activated carbon) exhibited high activity and selectivity in the hydrogenolysis of typical α‐O‐4 model compounds and deconstruction of lignin feedstock for the production of aromatic chemicals. Synergy effect was observed between tungsten and noble‐metal species, which on the one hand modulated the hydrogenolysis activity of tungsten and improved the activity of catalyst, and on the other hand suppressed overhydrogenation that avoided the formation of cycloalkanes. The conversion of α‐O‐4 model compounds with different substituents showed that electronic effects play an important role in stabilizing the reaction intermediates, and the electron‐donating methoxy group on the aromatic ring accelerated the hydrogenolysis efficiency. Solvent effect was another factor that determined hydrogenolysis efficiency; dipolar aprotic solvents such as n‐hexane provided the high yields of target aromatics. In the conversion of realistic lignins, the bimetallic catalyst showed excellent activity not only in α‐O‐4 cleavage but also in the hydrogenolysis of other major linkages such as β‐O‐4 and β−β between aromatic units, so that a high yield of liquid oil was obtained from lignin.

Heterogeneous Catalyst Design Principles for the Conversion of Lignin into High‐Value Commodity Fuels and Chemicals

“…The Pd/C catalyst had a 36.5 % product yield of cyclohexanol, whereas PdW/C had a 0.4 % product yield, and Pd had a 0 % product yield of phenol, whereas PdW had a 29 % product yield. The bimetallic synergy of Pd and W shows increased conversion to the desired toluene product, while limiting hydrogenation . Pt−WO 3 /C showed very similar trends to PdW/C upon reacting with m ‐cresol.…”

“…Noble metals are the most commonly used heterogeneous catalysts for the depolymerization of lignin. Very active catalysts include palladium, ruthenium, and rhodium, whereas transition metals, such as nickel, iron, and copper, are frequently used as well. The combination of alloys of metals can promote activity through electronic, interfacial, and synergistic effects.…”

“…The catalysts commonly used in HDO are Ni, Ru, Pd, Fe, Co, and W . Mono‐ and bimetallic catalysts of transition and noble metals were used in the cleavage of the C−O bond in BPE .…”

“…In comparison, W/C converts 79.5 % BPE with product yields of 40.3 % toluene and 33.8 % phenol. The combination of Ru and W bimetallic catalyst synergistically increased the conversion to 99.7 % and product yields to 92.6 and 61.5 %, respectively, which were higher than those of either monometallic catalyst . Each of these reactions were performed under the same conditions, as observed in Table .…”

“…Cyclohexane had a conversion of 92 % and n ‐hexane had a conversion of 99.7 %; each with higher product yields to toluene and phenol than that of the other solvents. Neither solvent has Lewis basicity nor is a hydrogen donor, and thus, leads to the best results for BPE hydrogenolysis with RuW/C . Many of these solvents were used in BPE depolymerization by ReO x /AC.…”