(415c) Catalytic Depolymerization of Lignin Extracted From Kraft Black Liquor

Catalytic Depolymerization of Lignin Extracted from Kraft Black Liquor

Bethany D. Carter and David A. Bruce*

Department of Chemical and Biomolecular Engineering, Center for Advanced Engineering Fibers and Films, Clemson University, Clemson, SC, 29634-0909, USA.

* Corresponding author. E-mail: dbruce@clemson.edu

Lignin is the only renewable source of aromatic species, consisting of a network of guaiacyl, syringyl, and p-hydroxyphenyl units connected through various types of ether linkages forming an alkyl aryl ether backbone functionalized with methoxyl, benzyl alcohol, and phenolic hydroxyl species. Lignin conversion into lower molecular weight, monomeric type species with valuable functionality, such as phenols and aromatic acids, would shift dependence away from fossil fuels to a renewable source of aromatics. Lignin is a major component of woody biomass constituting 20 to 30 percent of woody biomass.

Lignin was extracted from Kraft black liquor via the continuous Sequential Liquid Lignin Recovery and Purification (SLRP) process whereby the pH of the incoming black liquor (pH ~ 13.5) was lowered to a pH ~ 9.5 by acidification with CO₂resulting in lignin precipitation. The precipitated, liquid lignin phase is then acidified with sulfuric acid until a final pH ~ 2.5 is achieved, whereby the lignin is fully acidified.   

Catalytic conversion of SLRP lignin, as well as small model compounds, was investigated using a porous, heterogeneous solid acid catalyst containing noble a metal sites. The bifunctional nature of the catalyst is such that cleavage of the ether linkages occurs at the acid site, while product stabilization occurs via hydrogenation at the noble metal site. Hydrogenation of reaction products helps prevent repolymerization, which leads to improved conversion. The heterogeneous nature of the catalyst allows for separation and reuse of the catalyst.

Lignin depolymerization experiments were carried out in small batch reactor in the presence of the catalyst and under hydrogen pressure. Characterization of depolymerized lignin fractions was done using gas chromatography-mass spectrometry to analyze lighter molecular weight fractions.  Higher molecular weight fractions, which are not easily volatilized, were analyzed using NMR spectroscopy.