Abstract
Kraft lignin, a side-product of the paper industry, is considered an attractive feedstock for the production of biorenewable chemicals. However, its recalcitrant nature and sulfur content render catalytic conversions challenging. This study demonstrates the efficacy of noble metal-based catalysts for the production of a lignin oil enriched in alkylphenolic and aromatic compounds, by a catalytic hydrotreatment of Kraft lignin without the use of an external solvent. Eight commercially available catalysts were evaluated using four different metals (Ru, Pt, Pd, Rh) on two supports (activated carbon and Al2O3). The product oils were extensively analyzed by means of GPC, GCxGC-FID, GC–MS-FID, and elemental analysis. The catalysts were characterized by various techniques (N2 physisorption, NH3-TPD, XRD and TEM) before and after reaction, and their physico-chemical properties were correlated with catalytic performance. Al2O3 as support gave better results than carbon as support in terms of lignin oil yield and composition, due to a combination of higher total acidity, mildly acidic sites and a mesoporous structure. The metallic phase also significantly affected product distribution. The best results were obtained using a Rh/Al2O3 catalyst, resulting in a lignin oil yield of 36.3 wt% on a lignin intake and a total monomer yield of 30.0 wt% on lignin intake including 15.3 wt% of alkylphenolic and 7.9 wt% of aromatic compounds, and with a sulfur content <0.01 wt%.
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•A catalytic, solvent-free hydrotreatment procedure for Kraft lignin is reported.•Noble metal-based catalysts give lignin oils enriched in low molecular weight aromatics and alkylphenolics.•34 wt% of total monomers on lignin intake were obtained with a Rh/Al2O3 catalyst.•Al2O3 is preferred as a support mainly due to its mild acidity and mesoporosity.•Catalyst regenerability is possible though metal sintering and specific surface loss was observed.