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Depolymerisation of kraft lignin under hydrothermal conditions was investigated at short residence times (1–12 min) with glycerol being used as a capping agent. The weight average molecular weight (Mw) of the products decreased within the first minute of residence time, with the inter-unit ether linkages breaking accordingly. Furthermore, the Mw 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 Mw of the products, it seemed to increase the yield of char and therefore might not be a suitable capping agent for kraft lignin depolymerisation. [ABSTRACT FROM AUTHOR]

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 °C. 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. [ABSTRACT FROM AUTHOR]

In this study, Kraft lignin was depolymerised by hydrothermal liquefaction in near-critical water (290–335 °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. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Oxygenated water soluble aliphatics were the most common HTL products. • High-value carboxylic acids constituted the core resilient to process tailoring. • Tetralin promoted formation of aromatics through solubilization and scavenging. • Acetone promoted low molecular weight oxygenates through retro-aldol splitting. Abstract The influence of co-solvents on hydrothermal liquefaction (HTL) of Fallopia Japonica was studied as a function of temperature. Combination of low and high-resolution mass spectrometry with multi-layered data mining strategy resulted in a comprehensive characterization of the reaction products, mostly water-soluble organics (WSO) with a broad spectrum of chemical functionalities. The non-targeted analysis revealed the presence of a core composition in the samples independent of the process conditions and consisting of hydroxycarboxylic acids, imids, lactones, lactams, phenolics, various short-chain oxygenated aliphatics, and cyclohexane derivatives. Changes in process conditions did not affect those species showing that a part of the HTL product is not susceptible to process tailoring by addition of co-solvents. The findings indicated that the effect of tetralin is a combination of solubilization and scavenging resulting in an increased abundance of monomeric aromatics. For acetone, the results pointed to the promotion of retro-aldol splitting yielding low molecular weight oxygenates. [ABSTRACT FROM AUTHOR]

Although hydrothermal liquefaction of biomass (HTL) is considered one of the most promising techniques for production of drop-in biofuels, the challenges associated with its development and expansion are still significant. One of the issues is concerned with characterization of the liquid product (biocrude) and by-product (aqueous phase), which, due to their complexity and polarity, are considered an analytical challenge. In this study, solid-phase microextraction (SPME) combined with gas chromatography mass spectroscopy (GC–MS) were applied for a qualitative characterization of both the aqueous phase and the biocrude from HTL. Furthermore, a method for an optimal application of SPME on water soluble organics (WSO) was developed with regard to the fiber type and a number of extraction parameters. For the biocrude, the optimization was limited to the fiber type. Four different SPME fibers were used, namely 65 μm polydimethylsiloxane divinylbenzene (PDMS/DVB), 85 μm polyacrylate (PA), 7 μm polydimethylsiloxane (PDMS), and 100 μm polydimethylsiloxane (PDMS), covering a wide range of potential compounds. The results have shown that characterization of liquid products from HTL is significantly improved by application of SPME. Four groups of compounds were identified: I) Low molecular weight (MW) aliphatics; II) Cyclic compounds; III) Aromatics; and IV) High MW compounds. The oil phase consisted of deoxygenated species of chemicals present in the water phase, including complex polymerized cyclic structures and aromatic-aliphatic assemblies. 65 μm PDMS/DVB fiber was the most efficient one for adsorption of compounds from both the biocrude and the aqueous phase. [ABSTRACT FROM AUTHOR]