Researchers Develop Process for High-Yield Conversion of Lignin to Bio-Hydrocarbons and Methanol

18 July 2008

Proposed routes for the conversion of lignin into alkanes and methanol. Click to enlarge.

Researchers at Peking University (PKU) and the Institut des Sciences et Ingénierie Chimiques Ecole Polytechnique Fédérale de Lausanne (EPFL) have developed a two-step process for converting lignin—a key component of plant cell walls—to alkanes (hydrocarbons) and methanol that obtains about 42 wt% C₈–C₉ alkanes, 10 wt% C₁₄–C₁₈ alkanes, and 11 wt% methanol—close to the calculated maximum.

The researchers, led by Professor Yuan Kou at the PKU Green Chemistry Center, published a report on their work 9 July in the journal ChemSusChem.

Professor Kou’s team last year reported on the development of an aqueous-phase Fischer-Tropsch process (earlier post) and had earlier shown that cellobiose, a dimeric model of cellulose, can be degraded to polyols through the precise cleavage of C-O-C bonds (Kou 2006). The current research applies the cleavage strategy reflected in the latter work to the degradation of lignin.

Lignin is a mixture of oxygenated polyaromatics with repeatedly, but randomly, cross-linked C₉ phenol units connected through C-O-C bonds (around two-thirds to three-quarters of all the linkages in white birch, the lignin source in the experiment) and C-C bonds (around one-third to one-quarter of the linkages).

The researchers used near-critical water as a solvent and treated the white birch sawdust over a series of active carbon supported catalysts (Ru/C, Pd/C, Rh/C, and Pt/C) under modest pressures of H₂ under varying conditions. Treatment of the sawdust in a mixture of dioxane and near-critical water (364°C and 190 atm) in a ratio of 1:1 combined with 1 wt% H₃PO₄ over Pt/C and Rh/C catalysts yielded about 45 wt% monomers (very close to the calculated maximum) and 12 wt% dimers. The catalysts can be reused directly after reaction without any apparent deactivation.

Following extraction, the monomers and the dimers were transferred to the second step for conversion into alkanes and methanol.

In principle, the hydrogenation of the monomers and the dimers may involve the hydrogenation of the aromatic groups, the removal of hydroxy groups, and the transformation of the methoxy groups into methanol. Again, as in the first step, a catalyst that is active under relatively mild conditions is required to prevent over-hydrogenation.

...Under the selected conditions, the mixture of the dimers and the monomers obtained in the first step were converted into the corresponding alkanes and methanol in near-quantitative yields. Alkane products including C₈H₁₆, C₉H₁₈, C₁₄H₂₆, C₁₅H₂₈, C₁₆H₃₀, C₁₇H₃₂, and C₁₈H₃₄ were identified. The yields for C₈–C₉ hydrocarbons, C₁₄–C₁₈ hydrocarbons, and methanol were over 90, 80, and 95 mol%, respectively.

...The process is simple, relatively green as it uses water as a solvent, atom-efficient, and adaptable. The dimers are obtained in yields below the predicted value of 28 wt%, thus further optimization of the catalyst would be advantageous. Also, it would be useful to find an alternative to phosphoric acid even hough, in principle, it could be recycled. The transformation of alkali lignin, a waste product from the paper industry, and enzymed lignin (i.e. the lignin recovered after enzymatic hydrolysis of cellulose and hemicellulose) from bioethanol to hydrocarbons and methanol is in progress and will be reported in due course.

—Kou (2008)

Resources

• Ning Yan, Chen Zhao, Paul J. Dyson, Chen Wang, Ling-tao Liu, Yuan Kou (2008) Selective Degradation of Wood Lignin over Noble-Metal Catalysts in a Two-Step Process, ChemSusChem Volume 1, Issue 7, Pages 626-629 doi: 10.1002/cssc.200800080

• N. Yan, C. Zhao, C. Luo, P. J. Dyson, H. Liu, Y. Kou, (2006) One-Step Conversion of Cellobiose to C6-Alcohols Using a Ruthenium Nanocluster Catalyst , J. Am. Chem. Soc. 128 (27), 8714 -8715 doi: 10.1021/ja062468t