The evolution of the active phase in a Cu-doped mixed-metal oxide catalyst obtained by calcining a hydrotalcite precursor was studied during reductive depolymerization of lignin. Reactions were conducted in supercritical methanol in a liquid-phase batch reactor to model those involved in the depolymerization of lignin and saturation of the resulting aromatic fragments. The induction period prior to the onset of hydrogenolysis activity, as well as pronounced deactivation upon consecutive catalyst recycles, coincided with profound changes in the metal sites as the reaction proceeds. Spectroscopic and microscopic characterization of the catalyst reveals that Cu2+, which is initially well dispersed in the oxide lattice, evolved to Cu(0) nanoparticles in the presence of H2, or alcohols as H2 sources. Flow reactions performed in a packed-bed micro-reactor revealed that methanol reforming occurs concurrently with the extrusion and formation of small (&lt; 5 nm) Cu nanoparticles. Aging experiments demonstrated that the degree of Cu sintering depends on the reaction medium, as well as the reactant. Powder X-ray diffraction of the bulk metal oxide structure displayed that when H2O is generated as a byproduct, during long reaction times, that the hydrotalcite precursor structure reassembles, resulting in the loss of accessible surface area. Exposure to supercritical methanol alone did not cause further aggregation of the Cu nanoparticles, even after 72 h. However, when the reactor also contained poplar lignin, extensive sintering occurred within 6 h, resulting in the appearance of 29 nm particles. Examining the supported Cu nanoparticle catalyst reactivity with phenolic intermediates further elucidated the deactivation mechanism. Specifically, we propose that particle sintering occurred through chelated, Cu-metal ions, which are generated in situ from depolymerized lignin fragments, that have increased surface mobility.

Evolution of a heterogeneous Cu-based lignin hydrogenolysis catalyst in supercritical methanol

253rd National Meeting (2017)

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