Characterization of liquid products from hydrothermal liquefaction (HTL) of biomass via solid-phase microextraction (SPME)

Abstract

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.