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The recalcitrance of lignin with non-hydrolyzable C–O–C bonds, as well as the tendency for its phenolic fragments to polymerize into more recalcitrant polymers, leads to low-efficiency lignin deconstruction processes, such as low lignin concentrations during treatment. Herein, we developed a tandem catalytic approach for the conversion of highly concentrated lignin to stable cyclic alkanes by introducing Pt/HAP into a Ni/ASA catalyst in dodecane media. The incorporated Pt/HAP changes the simultaneous hydrogenation and hydrogenolysis of lignin on Ni/ASA into tandem reactions on a physically mixed Pt/HAP and Ni/ASA catalyst, involving the initial selective lignin depolymerization over Pt/HAP and the following hydrodeoxygenation of phenols over Ni/ASA, as evidenced by kinetic studies. In this tandem approach, the preferred initial lignin depolymerization, instead of lignin hydrogenation, originates from the strong adsorption and high C–O cleavage selectivity of lignin on the introduced Pt/HAP, as confirmed by UV-Vis, gel permeation chromatography (GPC), 2D-heteronuclear single quantum coherence nuclear magnetic resonance (2D-HSQC NMR), and catalytic tests. The rates of lignin depolymerization and the hydrodeoxygenation of depolymerized phenols are matched when the mass ratio of Pt/HAP and Ni/ASA is set at 1 : 1, allowing this approach to proceed with the highest lignin concentrations (150 g L−1) so far reported, with conversion to cyclic alkanes with 42 wt% yield without coke formation in dodecane at 300 °C in the presence of 6 MPa H2. Promisingly, the developed approach may lay a solid foundation for future industrial applications relating to lignin valorization. [ABSTRACT FROM AUTHOR]

Hydrothermal reduction under aqueous conditions is widely used to convert biomass into more valuable products. However, the harsh conditions inherent in the process can irreversibly alter the intrinsic structure of the support, as well as dissolve the metal ions into the aqueous solution. In this work, for the first time we have synthesized a new highly hydrothermally stable Ru/LaCO3OH catalyst mostly consisting of Ru nanoparticles partially encapsulated by the LaCO3OH support with a strong metal–support interaction (SMSI), which confers high stability and activity to the catalyst under hydrothermal reduction conditions in the hydrogenolysis of the biomass model molecules guaiacol and glycerol. During impregnation, the RuCl3·3H2O precursor initially reacts with LaCO3OH to form a LaRu(CO3)2Cl2 complex and LaOCl. XPS demonstrated that Ru was present in the oxidized state, TEM and XRD showed the absence of Ru0, and the XRD pattern showed the presence of the characteristic lattice fringe of LaOCl. While the LaRu(CO3)2Cl2 complex was resistant to H2 reduction at 350 °C, the complex underwent facile reduction to Ru0 under hydrothermal conditions at 240 °C. In the subsequent process, LaRu(CO3)2Cl2 and LaOCl underwent hydrolysis, forming crystalline LaCO3OH (confirmed by Ag+ titration and XRD patterns), Ru(OH)3, and HCl. The Ru(OH)3 was reduced in situ to Ru0 nanoparticles, as revealed by XPS and TEM analysis. The simultaneous hydrothermal reduction of Run+ species and the formation of crystalline LaCO3OH result in the formation of Ru nanoparticles encapsulated by a protective LaCO3OH layer, as evidenced by HRTEM and DRIFTS CO adsorption measurements. The preparation of catalysts with this unique structure comprising metal nanoparticles protected by the support itself, which confers additional stability, is a novel strategy to prepare hydrothermally stable catalysts. [ABSTRACT FROM AUTHOR]