Hydrothermal Liquefaction of Kraft Lignin - The influence of capping agents and residence time

In the context of exploring alternatives to replace fossil resources, lignin has been acknowledged as a renewable source of various aromatic compounds that have the potential of being precursors to chemicals as well as fuel additives. Originating from lignocellulosic biomass such as wood, lignin is an amorphous polymer with a high content of aromatic units and, in order to harness these units, it must be depolymerised. A major problem with current depolymerisation techniques, however, is that lignin repolymerises after being depolymerised, and forms an undesirable char fraction. The addition of capping agents and fine-tuning the reaction conditions can be used to mitigate such formation of char.This work has investigated the depolymerisation of kraft lignin in hydrothermal conditions under varying temperatures (290-335 \ub0C), residence times (1-12 min) and charges of isopropanol (IPA/dry lignin, 0-4.9) which, aside from being a co-solvent, was hypothesised as acting as a capping agent. The influence of these reaction parameters on the molecular weights, yields and elemental compositions of the products was studied, along with changes in the molecular structure compared to the starting lignin. The product is a suspension of solid material, i.e. char, in an aqueous phase and thus any desired organic liquid phase requires extraction from the aqueous product. While the yield of char increased with temperature and residence time, it decreased with increasing isopropanol loading, suggesting that the isopropanol does in fact act as a capping agent.Most of the lignin forms a water-soluble fraction that precipitates when the aqueous product phase is acidified, thereby forming the precipitated solids fraction (PS). The components remaining dissolved after acidification of the product phase are known as acid soluble organics (ASO). A portion of the ASO fraction was aromatic monomers, with guaiacol dominating: this result was expected since the lignin was sourced from softwood. The amount of such monomers increased with residence time in the reactor.Molecular weight analyses showed a rapid depolymerisation of the lignin within 1 min of hydrothermal liquefaction (HTL) treatment via a significant decrease in the molecular weight of all product fractions: char, PS and ASO. Moreover, the carbon-oxygen inter-unit linkages were found to break in this timeframe as well. The repolymerisation reactions started to exceed depolymerisation between residence times of 4 and 12 min, causing the weight average molecular weight (Mw) to increase again. Although minimising the residence time allows the char yield and Mw to be kept low, more monomers were formed at longer residence times. This calls for careful tuning of the residence time in the HTL of kraft lignin

On the hydrothermal depolymerisation of kraft lignin using glycerol as a capping agent

Depolymerisation of kraft lignin under hydrothermal conditions was investigated at short residence times (1–12\ua0min) with glycerol being used as a capping agent. The weight average molecular weight (M w) of the products decreased within the first minute of residence time, with the inter-unit ether linkages breaking accordingly. Furthermore, the M w of the product fractions decreased at increasing residence times, while the char yield increased. Short residence times thus appear to be beneficial for mitigating the formation of char. Also, addition of NaOH reduced the yield of char. Although the addition of glycerol caused a decrease in the M w of the products, it seemed to increase the yield of char and therefore might not be a suitable capping agent for kraft lignin depolymerisation

Using guaiacol as a capping agent in the hydrothermal depolymerisation of kraft lignin

The depolymerisation of softwood kraft lignin was investigated, under hydrothermal conditions at 290 \ub0C and 250 bar, with guaiacol in the reactor feed to evaluate its impact on the formation of char and on the molecular weights of the products. The effect of residence time was investigated in the time span 1-12 min. Lignin is depolymerised during the process and guaiacol is both formed and consumed during the reaction, with clearly noticeable changes as early as in the first minute of reaction. Although the addition of guaiacol in the reactor feed causes a reduction in the weight average molecular weight of the products, the yield of char increases. Longer residence times result in repolymerisation of the reaction products as well as a further increase in the yield of monoaromatic components and char

Towards understanding kraft lignin depolymerisation under hydrothermal conditions

Kraft lignin depolymerisation using hydrothermal liquefaction suffers from the formation of char, resulting in a decreased product yield as well as causing operational problems. While this may be mitigated by the addition of capping agents such as phenol and isopropanol, other reaction parameters, for example reaction time and temperature, are also important for the product yields. In this work, the effect of short reaction times on the hydrothermal liquefaction of kraft lignin in an alkaline water and isopropanol mixture was investigated at 1-12 min and 290 \ub0C. The results show that there were swift initial reactions: the major ether bonds in the lignin were broken within the first minute of reaction, and the molecular weight of all product fractions was halved at the very least. Longer reaction times, however, do not cause as pronounced structural changes as the initial reaction, indicating that a recalcitrant carbon-carbon skeleton remained in the products. Nevertheless, the yields of both char and monomers increased slowly with increasing reaction time. The swift initial depolymerising reactions were therefore followed by slower repolymerisation as well as a slow formation of monomers and dimers, which calls for careful tuning of the reaction time

Using Isopropanol as a Capping Agent in the Hydrothermal Liquefaction of Kraft Lignin in Near-Critical Water

In this study, Kraft lignin was depolymerised by hydrothermal liquefaction in near-critical water (290-335 degrees C, 250 bar) using Na2CO3 as an alkaline catalyst. Isopropanol was used as a co-solvent with the objective of investigating its capping effect and capability of reducing char formation. The resulting product, which was a mixture of an aqueous liquid, containing water-soluble organic compounds, and char, had a lower sulphur content than the Kraft lignin. Two-dimensional nuclear magnetic resonance studies of the organic precipitates of the aqueous phase and the char indicated that the major lignin bonds were broken. The high molar masses of the char and the water-soluble organics, nevertheless, indicate extensive repolymerisation of the organic constituents once they have been depolymerised from the lignin. With increasing temperature, the yield of char increased, although its molar mass decreased. The addition of isopropanol increased the yield of the water-soluble organic products and decreased the yield of the char as well as the molar masses of the products, which is indicative of a capping effect