Oxidative conversion of lignin and lignin model compounds catalyzed by CeO₂-supported Pd nanoparticles

Abstract

† Electronic Supplementary Information (ESI) available: [XPS spectra, H2chemsorption results, HPLC spectra of products from Organosolv lignin conversion, effects of O2 pressure for the conversion of PP-ol, conversion of PP-one under N2 and O2, conversion of methanol under N2, and the catalytic performances of CeO2 supported Au and Pt catalysts]. SeeThe oxidative transformation of lignin into aromatic compounds is an attractive route for chemical utilization of lignocellulosic biomass. Unlike hydrogenolysis, no consumption of expensive hydrogen is required for the oxidative transformation. However, only limited successes have been achieved for the oxidative conversion of lignin. Here, we report that cerium oxide-supported palladium nanoparticles (Pd/CeO2) can efficiently catalyze the one-pot oxidative conversion of 2-phenoxy-1-phenylethanol, a lignin model compound containing β-O-4 bond and Cα-hydroxyl group, in methanol in the presence of O2, producing phenol, acetophenone and methyl benzoate as major products. Pd nanoparticles played a pivotal role in the oxidation of Cα-hydroxyl group into Cα-ketonic group, which was crucial for the transformation of the model compound. The presence of Cα-ketonic group activated the β-O-4 bond, which was subsequently cleaved over the Pd/CeO2 catalyst, affording phenol and acetophenone. At the same time, the Cα−Cβ bond also underwent oxidative cleavage catalyzed by CeO2, producing benzoic acid and further methyl benzoate. The Pd/CeO2 catalyst could also catalyze the oxidative conversion of Organosolv lignin under mild conditions (458 K), producing vanillin, guaiacol and 4-hydroxylbenzaldehyde.Scheme 1 Structures of a representative lignin fragment and lignin model compound.

Heterogeneously catalyzed lignin depolymerization

“…This approach has been applied to wood pulp delignification, in which oxidation of the C a alcohol to a ketone favours the breakdown of b-O-4 linkages. Wang et al [85] also reported a bifunctional Pd/CeO 2 catalyst able to efficiently cleave b-O-4 bonds in the presence of C a -hydroxyl group, in which synergy between palladium activation of the oxidant and the redox properties of ceria facilitates b-O-4 cleavage; Pd is widely used in heterogeneous catalysis for aerobic alcohol oxidation [86][87][88][89][90]. This bifunctional catalyst was able to transform a range of Organosolv lignin in ethanol under oxygen to vanillin, guaiacol, and 4-hydroxybenzaldehyde.…”

Visible‐Light‐Driven Cleavage of C−O Linkage for Lignin Valorization to Functionalized Aromatics

“…[6] Cleavage of the b-O-4 linkager equires ab ond dissociation energy (BDE) of 54-72 kcal mol À1 . [3,6,[9][10][11] The two-step strategy of preoxidation and cleavage of the b-O-4 bond for lignin depolymerization can be performed under milderc onditions butu sually requires additional reductants/oxidants or acids/bases and generates large amountso fb yproducts. [3,6,[9][10][11] The two-step strategy of preoxidation and cleavage of the b-O-4 bond for lignin depolymerization can be performed under milderc onditions butu sually requires additional reductants/oxidants or acids/bases and generates large amountso fb yproducts.…”

Full Utilization of Lignocellulose with Ionic Liquid Polyoxometalates in a One‐Pot Three‐Step Conversion

“…Based on these challenges, the biomass pretreatment methodology or lignin-first concept was developed to provide an opportunity to realizet he full utilization of the three componentsinl ignocellulosic biomass. Supported metal catalysts including Pd, [12] Co, [13] Cu, [14] and V [15] gave For soft lignocellulose (pine), lignin was first decomposed into guaiacol and phenol with yields of 15.3 and 12.9 %a t9 8.6 % delignificatione fficiency at 130 8Cf or 14 h. Meanwhile, hemicellulose andc ellulose were intact during the delignifying process and were subsequently hydrolyzed to 3.5 %x ylose at 100 %h emicellulosec onversion efficiency at 150 8Cf or 14 h and 36.4 %g lucose at 100 %c ellulose conversion efficiency at 170 8Cf or 12 h, respectively.F or hard lignocellulose (poplar), the yields of guaiacol and phenol were 10.1 and 8.7 %a t 91.9 %d elignification efficiency at 130 8Cf or 14 h, whereas 12.9 %x ylose at 90.4 %h emicellulose conversion efficiency at 150 8Cf or 12 ha nd 32.9 %g lucose at 100 %c ellulose conversion efficiency at 170 8Cf or 12 hw ere obtained. Amongt he linkages, the b-O-4 motif is considered as the typical unit and the most abundant linkage, which composes up to 70 %o fl ignin.…”