Supported Pd Catalysts for the C—O Cleavage of the Lignin Derived Model Dimers through Intramolecular Hydrogenolysis Reaction

The selective depolymerization of the C—O ether bonds in lignin has attracted considerable attentions which would facilitate the valorization of lignin. The model compounds with β-O-4 type ether bonds were the most frequently stud- ied molecules. In this study, we used 2-(2-methoxyphenyl)oxy-1-phenethanol (1a) as model compound for the C—O cleav- age through intramolecular hydrogenolysis reaction. The hydroxymethyl group acted as the hydrogen source by dehydroge- nating over Pd catalysts. Then, the cleavage of C—O bond takes place on the supported Pd catalyst with Pd-H active species. All the catalysts were prepared and characterized by XRD, TEM and BET. Among different metal catalysts, Pd catalysts gave best results toward the C—O bond cleavage reaction. After that, the influences of solvent were studied and the best solvent was cyclohexane. Different additives such as acids and bases were also tested in the model reaction. The strong base (i.e. KOH) showed negative effect on the product yield, but a weak base (i.e. Na2HPO4) gave a promotion effect on the reaction. The result indicated that the modification of the Pd catalysts' supports would be a good option for the C—O bond cleavage reaction. Thus, different Pd catalysts with different supporting materials were prepared and tested, and found that MgO, a weak basic support, gave promoted performance on this reaction. Finally, to figure out the reaction pathway, the reaction mixtures with different reaction time were analyzed by GC-MS which showed that the ketone was the main intermediate of the reaction. The result indicated that the main route for the intramolecular hydrogenolysis reaction was (1) the dehydrogena- tion of -CCHOH to ketone to form Pd-H species and then (2) the C—O bond was cleavage by the Pd-H species to form the depolymerization products. Keywords lignin; depolymerization; Pd-catalyzed reaction; C—O bond cleavage; catalytic transfer hydrogenation