Green aviation fuel hydrocarbons from lignocellulosic biomass via hydrothermal liquefaction

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Abstract

Hydrothermal liquefaction (HTL) of lignocellulosic biomass in primary alcohols is a novel thermochemical method to produce sustainable aviation fuel range cyclic hydrocarbons. In this method, particularly the underutilized wood fractions, hemicellulose and lignin components are dissolved to produce biocrude. The current planned process comprises of three phases, such as production of biocrude through HTL, extraction of bio-aromatics from biocrude using deep eutectic solvents (DES) and further transformation of bio-aromatics into cyclic hydrocarbons via hydrodeoxygenation (HDO). The main aim of this work wasto utilize the heterogeneous catalysts in HTL method to produce biocrude at fixed temperature and under low initial hydrogen pressure in supercritical ethanol. The selection of catalysts was based on the acidity of the support material and transition metal immobilized on the support. The following catalysts were applied: 5 wt. % Fe-Beta-150 exhibiting with weak and medium Brønsted-Lewis acidity or non-acidic 5 wt. % Fe-SiO2 and weakly Brønsted acidic NbOPO4 or Pd/NbOPO4with weak Lewis acidity.

In-house prepared and characterized Fe-based catalysts were used in birch fractionation. The influence of gaseous atmosphere and catalyst acidity as well as formation of phenolic products during liquefaction was studied over 5 wt. % Fe-H-Beta 150 or 5 wt. % Fe-SiO2. After the experiments, the liquid, solid and gaseous products were fully characterized using several analytical techniques. The obtained results show that Brønsted-Lewis acidic 5 wt. % Fe-H-Beta-150 catalyst, gave rise to around 25 wt. % of biocrude with 68 wt. % aromatic products, while non-acidic 5 wt. % Fe-SiO2 catalyst only gave around 18 wt. % of biocrude with 38 wt. % aromatic products. A Brønsted acidic catalyst enhances the dissolution efficiency, particularly hemicellulose and lignin degradation into biocrude with increased formation of acids, esters and aromatic compounds. Relatively low amounts of gaseous products were formed and catalyst leaching during liquefaction was low. Based on X-ray diffraction measurements of the solid wood residue, amorphous cellulose part transformed into crystalline state during liquefaction. From the liquid phase analysis results as a function of time, a reaction mechanism was proposed for the lignin degradation products over 5 wt. % Fe-H-Beta 150 or 5 wt. % Fe-SiO2 catalysts. The main lignin degradation product was isoeugenol, while some other intermediate products, i.e. coniferyl, and sinapyl alcohol, 4-propenyl syringol, syringaresinol, along with syringyldehyde rapidly reacted further. To support the proposed reaction mechanism, the thermodynamic analysis was also performed using the Joback approach and Gibbs-Helmholtz equation. In case of Nb-based xcatalysts, Pd/NbOPO4 catalyst was prepared through wet-impregnation method and fully characterized. Reaction conditions were optimized and catalytic liquefaction over Pd/NbOPO4 was compared with fresh and acetone extracted birch. In addition, the influence of support, and metal upon the liquefaction of birch were determined. From the liquid phase results, the main product in lignin degradation was homosyringaldehyde, opposite to 5 wt. % Fe-H-Beta-150, for which isoeugenol was the main product contrary to Pd/NbOPO4 catalyzed hydrogenation of sinapyl alcohol to dihydroconiferyl alcohol. The fresh treated wood over Pd/NbOPO4 resulted in the formation of 34 wt. % of phenolic monomers composed of 76.9 wt. % of dimethoxyphenols and 16.5 wt. % of guaiacol related monomers. The liquefaction of acetone extracted birch treated over Pd/NbOPO4 gave 35 wt. % of lignin monomers containing 93.2 wt. % of dimethoxyphenols and 6.8 wt. % of guaiacol based monomers. The delignification efficiency during liquefaction of fresh and acetone-extracted birch over Pd/NbOPO4 was also determined. Based on the liquid phase results the following reaction mechanism was proposed: Lignin degradation occurs via cleavage of ether bonds, which is catalyzed by Lewis acid sites in the solid acid catalyst. Thereafter, dehydration/dealkylation of monophenols occurred over Pd metal under low initial hydrogen pressure. Furthermore, a novel extraction method to extract the aromatic fraction from biocrude was developed. A deep eutectic solvent (DES) composed of choline chloride: ethylene glycol (1:4 mol) was used in a step-wise method with methyl isobutyl ketone (MIBK) and water.
Original languageEnglish
Publisher
Print ISBNs 978-952-12-4039-3
Electronic ISBNs978-952-12-4040-9
Publication statusPublished - 2021
MoE publication typeG5 Doctoral dissertation (article)

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