Hydrogen Assisted Lignin Depolymerization with Pulp Mill Catalysts

Abstract: As part of the development of a process for production of a lignin-rich oil that can serve as a renewable feedstock to existing petroleum refineries, hydrogen assisted depolymerization of lignin in filtered black liquor has been investigated. This process offers the possibility to reduce fossil emissions from transportation by utilization of the largest renewable carbon resource in Sweden, namely the forest. Compared to petroleum feedstock, lignin is richer in oxygen content and its molecular size is much larger. Challenges in the catalytic treatment is to reduce the oxygen content and shift the molecular weight distribution to lower molecular weights. Lignin is by nature resistant to degradation and hydrothermal treatment of lignin is often associated with coke formation. One approach to suppress coke formation is to use organic solvents that has high solubility for lignin fragments and hydrogen gas. However, the consumption of organic solvents in combination with expensive catalyst is not favorable from a techno-economic perspective. The approach in this work involves only the alkaline solution already present in the black liquor and catalysts that are available in the pulp and paper mill. It was found that the molecular weight distribution was shifted to molecular weights below 10 kDa, mainly around 1 kDa, during long residence time under hydrogen pressure, 20 h and 190 bar. This can be compared to the filtered black liquor, which contain compounds up to 100 kDa. More experiments with the material needs to be done in order to investigate the catalytic effect compared to blank experiments at 20 h residence time. For 15 min residence time, the catalysts did not show any visible effect on the molecular weight distribution. The presence of hydrogen gas was shown to reduce the sulfur content, which is an indication of reduction of oxygen content as well.