Effect of solvent and pyrolysis upgrade on the products of liquefaction of Saccharina latissima

In this work, we report results for thermal liquefaction of the brown macroalga S. latissima with HZSM-5 catalyst in three solvent schemes (water-only, water-methanol as co-solvents, and methanol-only). We investigate the effects of these schemes on product yields and biocrude composition. We conducted the experiments in a batch reactor at 330 °C (sub-critical water with supercritical methanol) and 1 hr residence time. The highest biocrude yield (28 wt%) was observed in the catalytic run with methanol-only solvent, but the absence of water in this run meant that no water-soluble products (WSP) were obtained. The impact of the HZSM-5 catalyst in hydrothermal liquefaction appears to be solvent-dependent, with significant impact in the presence of water, but marginal changes for methanol. Methanol as a solvent or co-solvent led to significant fractions of fatty acids and esters in the biocrude (43.9 % - 69.1 %), which suggests a path for the production of sustainable aviation fuels (SAF). Methanol decomposition under liquefaction conditions led to the production of aliphatic chain alcohols, with selectivities ranging from 1.0 % (20 % methanol) to 6.1 % (100 % methanol). The highest yield of hydrocarbons (0.8 wt%) was obtained in the presence of HZSM-5 with 20 % methanol as a co-solvent. We further subjected the biocrude produced to upgrading via catalytic fast pyrolysis in HZSM-5. The upgrade at the biocrude-to-catalyst ratio 1:5 produced mostly aromatic hydrocarbons, but at a low yield of 4.1 wt%. These findings are promising to support the utilization of S. latissima as a potential resource to produce transportation drop-in fuels.