Liquefaction and hydrodeoxygenation of polymeric lignin using a hier-archical Ni micro-reactor catalyst

Abstract

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-CαH-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.