Abstract
The pyrolysis of lignocellulosic biomass generates a liquid product, known as pyrolysis oil, that can be further processed into biofuels and value-added chemicals. During pyrolysis, cellulose and hemicelulose fractions are converted into a range of water-soluble sugar derivatives, for which several valorisation strategies exist. On the other hand, lignin is broken down into so-called pyrolytic lignin, a water-insoluble complex mixture of aromatic oligomers that requires different upgrading strategies than the sugar rich fraction. Here, we report an experimental study on the valorisation of pyrolytic lignin via catalytic hydrotreatment in the absence of an external solvent. A variety of carbon-supported noble-metal heterogeneous catalysts were tested (Ru/C, Pd/C, Pt/C and Rh/C), as well as conventional hydrotreatment catalysts (sulphided NiMo/Al2O3 and CoMo/Al2O3) to obtain valuable low molecular weight aromatic and phenolic compounds. Overall, depolymerized liquids were obtained with organic yields of up to 63 wt%. Pd/C was shown to be the best catalyst, yielding 59 wt% of monomers based on the organic product (i.e. 33 wt% based on pyrolytic lignin intake). While noble metal catalysts favored higher monomer yields, though with some over-reduction of aromatic rings to saturated hydrocarbons, sulphided catalysts were more efficient in deoxygenation and aromatization reactions, but yielded less monomers. The hydrocracking efficiency was shown to be strongly dependent on the reaction temperature. Based on the experimental data, a global reaction network is proposed. The high monomer yields reveal the potential of catalytic hydrotreatment for the upgrading of pyrolytic lignin into valuable phenolics and aromatics.
Highlights
Environmental concerns and the need for sustainable solutions for the production of bioenergy, biofuels, biobased chemicals and materials have intensified research related to the valorisation of renewable feedstocks
The catalytic hydrotreatment of a pine-derived pyrolytic lignin (PL) to low molecular weight value-added products was investigated with a range of heterogeneous sulphided and non-sulphided catalysts
The maximum yield of desired compounds was 19.1 wt% and 12.9 wt% for Pd/C and NiMo/Al2O3, respectively
Summary
Environmental concerns and the need for sustainable solutions for the production of bioenergy, biofuels, biobased chemicals and materials have intensified research related to the valorisation of renewable feedstocks. Lignocellulosic biomass is readily available and as such has been identified as an attractive feedstock for these products. It yields up to 70 wt% [2] of so-called pyrolysis liquid [3]. Pyrolysis liquids can be further processed either as a whole, or after separation into a sugar and lignin fraction through a simple water fractionation [4]. This fractionation leads to the precipitation of the lignin-derived insoluble fraction, i.e. pyrolytic lignin (PL). Since PL and the water-soluble carbohydrate fraction present remarkably distinct characteristics, it enables the separate upgrading of the fractions through specific and more efficient strategies
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