Abstract
One way of valorizing the lignin waste stream from the pulp and paper industries is depolymerizing it into low-molecular-mass compounds (LMMC). However, a common problem in the depolymerization of Kraft lignin is the low yields of small aromatic molecules obtained. In the present work, the combination of the repeated depolymerization of lignin and the separation of LMMC from depolymerized lignin to upgrade them into value-added chemicals was studied. In so doing, we investigated the possibility of depolymerizing black liquor retentate (BLR). The base-catalyzed depolymerization of BLR was performed using a continuous flow reactor at 170–210 °C, with a 2 min residence time. The results obtained indicate that BLR can be depolymerized effectively under the experimental conditions. Depolymerized lignin LMMC can be successfully separated by a GR95PP membrane, and thus be protected from repolymerization. Through combining membrane filtration with base-catalyzed depolymerization, more than half of the lignin could be depolymerized into LMMC. Around 46 mg/g of lignin monomers (guaiacol, vanillin, acetovanillone, and acetosyringone), which can potentially be upgraded to high-valued chemicals, were produced. On the basis of our results, we suggest use of a recycling Kraft lignin depolymerization and filtration process for maximizing the production of LMMC under mild alkaline conditions.
Highlights
Increasing interest has been directed over recent decades at the use of lignin, the most abundant natural source of aromatic polymers, as a feedstock for the production of renewable chemicals and fuels [1]
In order to suppress repolymerization, we have developed a novel continuous flow reactor (CFR) to depolymerize ‘Indulin AT’ at 170–240 ◦ C for 1–4 min of residence time by using NaOH as a catalyst [18]
There was a total of 5.52 g/L of hemicellulose to be found in the raw material. 3 g/L of total solids was removed from the 170–190 ◦ C depolymerized black liquor retentate (BLR)
Summary
Increasing interest has been directed over recent decades at the use of lignin, the most abundant natural source of aromatic polymers, as a feedstock for the production of renewable chemicals and fuels [1]. Lignin is currently treated as a by-product in many different processes, such as the Kraft process, the soda process, the sulphite process, and the second-generation bioethanol bio-refinery process. Around 130 million tons of Kraft pulp are generated annually [2]. During the Kraft process, more than 90% of the lignin is degraded into fractions and is dissolved in alkali solutions. It represents one of the major constituent of Kraft black liquor
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