Valorisation of lignin-rich residue to produce biofuel and chemicals

Lignin plays a significant potential role as a sustainable feedstock for bio-based chemicals and fuels, but its intricate and complex structure hampers its valorisation due to its intrinsic difficulty to be depolymerised. Converting lignin-rich residue (LRR) obtained as by-product of the biorefinery chain of lignocellulosic biomass into high-value added compounds is a challenge even tougher than valorising pure lignin itself. In fact, the impurities (ash, cellulose) present in LRR and the structural modification occurred in lignin after the biorefinery procedure render LRR even more difficult to be valorised than fresh lignin. This work explored the catalytic valorisation of LRR without any further pre-treatment and without the preliminary extraction of the contained lignin. The LRR investigated in this study was recovered after steam explosion pretreatment of wheat straw and subsequent enzymatic hydrolysis. Raney Nickel was used as the catalyst and several reaction conditions were varied in an initial screening step, including temperature, hydrogen pressure, reaction time, catalyst loading and solvent type (ethanol or methanol). The key products obtained were mainly phenols, esters and alkanes. Following this initial investigation, a statistical analysis using a fractional factorial design was conducted to determine the critical impact of the aforementioned variables on product distribution, with a focus on phenol and hydrocarbons production. The results revealed that the amount of catalyst was a key factor in the production of phenols and hydrocarbons, while the type of solvent only affected hydrocarbon production and temperature only affected phenol production. To maximise production of both, it is recommended that a high amount of catalyst, a high temperature, and ethanol as a solvent are used.