Lignin, containing rich aromatic structures, is an attractive and renewable source to produce fine chemicals and fuels through depolymerization and upgrading. Hydrodeoxygenation (HDO) over metal supported catalysts is one of the most efficient upgrading strategies. Metals like Pt, Ru, Pd and Ni have been widely studied in hydrogenation of lignin model compounds, such as anisole, phenol and guaiacol. However, those metal catalysts prefer to saturate the aromatic rings. It remains challenging to selectively produce aromatic hydrocarbons (AHs) from lignin-derived compounds over metal supported catalysts, especially in liquid-phase conditions. Herein, we synthesized Mn-doped Cu/AlOx catalysts derived from layered double hydroxide (LDH) by the co-precipitation method, and employed them in liquid-phase HDO of anisole. The Mn addition significantly increased AHs selectivity. The highest AHs selectivity of 65.9% was achieved over 4Cu/8MnAlOx, which is 6 times of that on 4Cu/AlOx (~10%). Without Mn, the selectivity of saturated hydrogenation products over Cu/AlOx remained more than 70% no matter how the Cu/Al ratio changed. Clearly, the addition of Mn is the key in selective HDO of anisole to AHs. To further elucidate the role of Mn in HDO of anisole, we synthesized a series of Cu/MnAlOx with different Cu/Mn ratios and characterized the structure by XRD, TEM, XPS, Raman, and so on. It is found that Mn successfully inserted into the LDH structure and a Mn-containing spinel phases formed after calcination with the Cu/Mn ratio of 0.5 to 2. With the Cu/Mn ratio decreasing, the Cu particle size reduced and oxygen vacancies concentration increased, which might benefit from the spinel structure and interaction between Cu and MnOx. Combined with in situ FTIR experiments, oxygen vacancies in MnOx were proposed as the crucial sites for anisole adsorption and following CAr-OCH3 bond dissociation. In this way, instead of aromatic rings, the oxygen atom in anisole was activated, leading to a highly selective HDO to AHs. This insights to the role of MnOx in Cu/MnAlOx for anisole selective HDO may bring new possibilities in catalysts design for lignin conversion to AHs.

Effect of MnOx in Cu/MnAlOx for lignin-derived anisole selective hydrodeoxygenation to aromatic hydrocarbons

ACS Spring Meeting 2021

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