Liquefaction and Hydrodeoxygenation of Polymeric Lignin Using a Hierarchical Ni Microreactor Catalyst

The depolymerization of lignin into liquid products in high yields is particularly challenging because of the poor solid-solid contact during heterogeneous catalysis and the formation of benzylic carbocations (Ph-C_αH-OH) and unstable intermediates bearing aldehyde groups, leading to repolymerization. Herein, we report a hierarchical Ni/ASA microreactor catalyst for the hydrodeoxygenation of lignin comprising ca. 17 aromatic monomers into light cyclic hydrocarbons in a nearly theoretical yield at a high reactant/catalyst ratio. The matched sizes in the macropores of Ni/ASA (3-6 μm) and the particle size of polymeric lignin (4.4 μm) resulted in fast lignin particle entrapment and an increased adsorption rate for larger amounts of lignin into the Ni/ASA, with better solid-solid contact compared to nonhierarchical Ni/ASA, as evidenced by UV-vis spectra. The lignin aromatics trapped in the macropores are quickly and fully hydrogenated to poly-cyclitols (stable intermediates), which are then transferred to the mesopores for further depolymerization and hydrodeoxygenation to the final cyclic hydrocarbons, as monitored and tracked by two-dimensional heteronuclear single quantum correlation NMR, in situ Fourier transform infrared, gel permeation chromatography, and time-of-flight mass spectrometry. The designed microreactor with suitable hierarchical pores facilitated the entrapment, adsorption, and catalyst-lignin macromolecule contact. The fast elimination of unstable intermediates of aromatics and aldehydes by efficient hydrogenation suppresses repolymerization and increases yields. We suggest that this can be a useful strategy for the efficient direct liquefaction of a diverse set of lignin or lignite polymeric materials and for the hydrogenation of polyaromatics in coal tar.