Elucidating transfer hydrogenation mechanisms in non-catalytic lignin depolymerization
[3];
[3];
[1];
[3]
- Joint BioEnergy Institute, Lawrence Berkeley National Laboratory, Berkeley, USA
- Total Raffinage Chimie, 92400 Courbevoie, France
- Joint BioEnergy Institute, Lawrence Berkeley National Laboratory, Berkeley, USA, Biological and Engineering Sciences Center
- Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, USA
- Total New Energies Inc., Emeryville, USA
Lignin undergoes catalytic depolymerization in the presence of a variety of transfer hydrogenation agents, however the mechanisms for non-catalytic depolymerization of lignin via transfer hydrogenation are not well understood; this makes process optimization difficult. Herein, for the first time a mechanism for this process is proposed. For the purposes of understanding the mechanisms involved in these non-catalytic lignin depolymerization processes, this study investigates the equilibrium system of formic acid, methyl formate and carbon monoxide, as agents for the depolymerization of lignin, in the presence of either water or methanol as solvents. In the methyl formate/water (at 300 °C) system, 73 wt%25 oil was produced which contained a significant amount of low molecular weight alkylphenols, with less than 1 wt%25 char produced. In aqueous media, the results showed that methyl formate maintains an equilibrium with formic acid which is itself in equilibrium with carbon monoxide. It was found that using either formic acid or methyl formate for non-catalytic transfer hydrogenation of lignin can produce high amounts of oil, and can be described as a two-stage mechanism. After 10 min of reaction at 300 °C, around a quarter of the formic acid is consumed via hydride transfer of the formate proton, preventing the condensation of lignin fragments. At the same time, approximately three quarters of the formic acid decomposes to carbon dioxide and carbon monoxide. Once the formic acid is consumed, the carbon monoxide was identified as the precursor to a reactive reductive reagent and was able to activate the proton of the water molecule preventing further condensation of the lignin fragments. It has been previously thought that transfer hydrogenation in lignin using formic acid occurs via the production of molecular hydrogen. Here it is demonstrated that formic acid reacts directly with the lignin, without this hydrogen formation. Therefore the key parameters for efficient transfer hydrogenation of the lignin to maximize bio-oil yield appear to involve controlling the reactions between lignin and formic acid, methyl formate or carbon monoxide under aqueous conditions, thereby reducing the reagent cost and loading while maintaining efficient lignin conversion.
- Research Organization:
- Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Biological and Environmental Research (BER)
- Grant/Contract Number:
- AC02-05CH11231
- OSTI ID:
- 1459223
- Alternate ID(s):
- OSTI ID: 1506292
- Journal Information:
- Green Chemistry, Journal Name: Green Chemistry Vol. 20 Journal Issue: 15; ISSN 1463-9262
- Publisher:
- Royal Society of ChemistryCopyright Statement
- Country of Publication:
- United Kingdom
- Language:
- English
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