Hydrothermal Reactions of Algae Model Compounds.

Abstract

Hydrothermal liquefaction (HTL) refers to the process of converting biomass to bio-oil by contacting the biomass with water at high temperatures (> 200 °C) and sufficient pressures to maintain water in the liquid state. HTL of algae producing bio-oil has the advantage of energy efficiency and capability of dealing with wet biomass. Very little is known about the mechanism and chemistry of biomacromolecules reactions in high temperature water (HTW). In this project, we overcome the existing gap by studying ethyl oleate, phenylalanine, phytol, and DOPC as representative model compounds for triglycerides, proteins, chlorophyll and phospholipid, respectively, in HTW.

We examined the behavior of model compounds from 175 – 350 °C. Hydrolysis of ethyl oleate (to oleic acid) was autocatalytic. A phenomenological and mechanistic kinetics model was developed that was consistent with the experimental data. We suggest that hydrolysis of ester is catalyzed by both H+ and fatty acid. Phenylalanine primarily formed phenylethylamine, while minor products included styrene and phenylethanol isomers. Major products from phytol included neophytadiene, isophytol, and phytone, while pristene, phytene, phytane, and dihydrophytol were the minor products. Phenylalanine and phytol disappearance followed first order kinetics. DOPC hydrolyzed to form 1-acyl and 2-acyl lyso-phosphatidylcholine (LPC) along with oleic acid as primary products. LPC subsequently formed other phosphorus-containing intermediates, which finally led to phosphoric acid as the ultimate P-containing product. Hydrolysis of DOPC was catalyzed by oleic and phosphoric acids. A kinetics model based on proposed pathways was consistent with experimental data for the model compound.

Lastly, we studied the effects of inorganic (salts and boric acid) and organic (ethyl oleate) additives on phenylalanine conversion and product distribution at 250 °C and 350 °C, respectively. Inorganic additives increased phenylalanine conversion favoring formation of high molecular weight compounds. Higher concentration of ethyl oleate lead to an increase in the rates of deamination of phenylethylamine and hydration of styrene. Amides are also formed due to the interaction of fatty acid/ethyl oleate and amines/ammonia.

Taken collectively, the results from this study provide new insights into the reactions of algae during its HTL to produce crude bio-oil.