Efficient depolymerization of lignins to alkylphenols using phosphided NiMo catalysts

Abstract

Greening up the chemical industry by using waste biomass streams as feed is a topic of high relevance. Residual lignins from for example the pulp and paper industry and second-generation bioethanol plants are interesting resources for the synthesis of biobased aromatics and alkylphenols. We here report experimental studies on the catalytic hydrotreatment of Kraft lignin to alkylphenols using non-precious metal, sulfur tolerant catalysts in the form of phosphided NiMo catalysts on different supports (AC, SiO₂Al₂O₃, SiO₂, MgO–Al₂O_3, and TiO₂) in the absence of an external solvent. The catalysts were prepared by an incipient wetness impregnation method and characterized in detail (BET surface area, SEM, TEM, X-ray diffraction, and temperature-programmed desorption of NH₃/CO₂). Hydrotreatment experiments were carried out in a batch autoclave at a temperature of 400 °C, for 2 h and 100 bar initial H₂ pressure. The lignin oils were analyzed extensively by GPC, GC-MS, GC×GC-FID, and elemental analysis. The highest monomer yield (51.8 wt% on lignin intake) was obtained with the NiMoP catalyst on SiO₂ (5.6 wt% Ni, 9.1 wt% Mo and 5.9 wt% P), which is among the best reported in the literature so far. Of the monomers, alkylphenols are the dominant component group (30.6%), followed by aliphatics (8.1%) and aromatics (5.7%). Clear relations between support characteristics and performance were absent. The only exception is the support acidity, and apparently, intermediate acidity is required for best performance. The SiO₂-supported NiMoP catalyst was also applied for the hydrotreatment of Lignoboost and Alcell lignin under the same reaction conditions. Whereas Lignoboost gave highly comparable results to Kraft lignin in terms of oil and monomer yield, Alcell lignin gave a considerably lower monomer yield (34.4 wt% on lignin intake). These results are rationalized by considering P/S exchange in the catalyst formulation during the reaction.