Catalytic hydroconversion of a wheat straw lignin: comprehensive analysis and reaction scheme
Résumé
Because of environmental concerns and needs for energetic independence, the transformation of lignocellulosic biomass is deeply investigated in order to provide biofuels and chemicals in a near future. Lignin, which represents almost 30% of this biomass, is a macromolecule containing mainly phenyl-propane units with ether linkages, its exact structure being strongly dependant on the vegetal source and on the extraction process. The total lignin availability exceeds 300 billion tons and continues to increase annually. Each year, more than 50 million tons of lignin are co-produced in pulp industry and burned as a low-value fuel. It has been established that a non-negligible part of this lignin can be used for other applications without weakening this industry. In addition, the improvement of the pulping processes, the production of cellulosic ethanol and new patented processes towards highly pure sulfur-free lignins should provide even greater quantities of high grade materials. The use of lignin as a precursor of hydrocarbons or phenolic compounds has then gained a strong interest due to its low cost and high availability. In order to get valuable bio-liquid from lignin, catalytic hydroliquefaction in hydrogen-donor solvent was proposed. High liquid yields were claimed but the mass balance and the identification of all the products and lignin residues were not clearly described. We report here the experimental results obtained in the hydroconversion of wheat straw soda lignin under hydrogen pressure in the presence of a supported catalyst, with tetralin as solvent in a batch reactor. The catalytic hydroconversion of the lignin reached 82 wt% with a high mass balance (>96 wt%) and with 71 wt% liquid yield. The lignin residue was converted until reaching 9 wt% of the starting lignin while the solids yield was decreasing until reaching initial ashes content. The size of the lignin, evaluated by GPC, diminished gradually from 26 units to 6 units after 28h, mainly by ether cleavage (B-O-4 linkages). By the same time, the aliphatic hydroxyl, carboxylic and methoxy groups were removed as observed by NMR. The GCxGC-MS analysis of the liquid fractions allowed the deep identification of the monomers families, including phenolic compounds, aromatics, naphthenes and alkanes. In each family the C1 to C6 alkyl-substituted derivatives were detected and quantified thanks to a FI detector. All those results allowed us to propose a reaction pathway for the overall transformation of the lignin on NiMo-based catalysts. It appeared that some specific chemical functions like aliphatic hydroxyl or carboxylic acids, exhibited a high reactivity during the reaction while syringyl units are progressively transformed in guaiacyl, catechol and phenolic units and then aromatics and naphthenic derivatives following the well-known reaction scheme described for guaiacol HDO. In addition, after 28h of use, the catalyst can be recycled.