Improving the Antioxidant Activity, Yield, and Hydrocarbon Content of Bio-Oil from the Pyrolysis of Açaí Seeds by Chemical Activation: Effect of Temperature and Molarity

Abstract

Biomass-derived products are a promising way to substitute the necessity for petroleum-derived products, since lignocellulosic material is widely available in our atmosphere and contributes to the reduction of greenhouse gases (GHGs), due to zero net emissions of CO2. This study explores the impact of temperature and molarity on the pyrolysis of açaí seeds (Euterpe oleracea, Mart.) activated with KOH and subsequently on the yield of bio-oil, hydrocarbon content of bio-oil, antioxidant activity of bio-oil, and chemical composition of the aqueous phase. The experiments were carried out at 350, 400, and 450 °C and 1.0 atmosphere, with 2.0 M KOH, and at 450 °C and 1.0 atmosphere, with 0.5 M, 1.0 M, and 2.0 M KOH, at laboratory scale. The composition of bio-oils and the aqueous phase were determined by GC-MS, while the acid value, a physicochemical property of fundamental importance in biofuels, was determined by AOCS methods. The antioxidant activity of bio-oils was determined by the TEAC method. The solid phase (biochar) was characterized by X-ray diffraction (XRD). The diffractograms identified the presence of Kalicinite (KHCO3) in biochar, and those higher temperatures favor the formation peaks of Kalicinite (KHCO3). The pyrolysis of açaí seeds activated with KOH show bio-oil yields from 3.19 to 6.79 (wt.%), aqueous phase yields between 20.34 and 25.57 (wt.%), solid phase yields (coke) between 33.40 and 43.37 (wt.%), and gas yields from 31.85 to 34.45 (wt.%). The yield of bio-oil shows a smooth exponential increase with temperature. The acidity of bio-oil varied between 12.3 and 257.6 mg KOH/g, decreasing exponentially with temperature, while that of the aqueous phase varied between 17.9 and 118.9 mg KOH/g, showing an exponential decay behavior with temperature and demonstrating that higher temperatures favor not only the yield of bio-oil but also bio-oils with lower acidity. For the experiments with KOH activation, the GC-MS of bio-oil identified the presence of hydrocarbons (alkanes, alkenes, cycloalkanes, cycloalkenes, and aromatics) and oxygenates (carboxylic acids, phenols, ketones, and esters). The concentration of hydrocarbons varied between 10.19 and 25.71 (area.%), increasing with temperature, while that of oxygenates varied between 52.69 and 72.15 (area.%), decreasing with temperature. For the experiments with constant temperature, the concentrations of hydrocarbons in bio-oil increased exponentially with molarity, while those of oxygenates decreased exponentially, showing that higher molarities favor the formation of hydrocarbons in bio-oil. The antioxidant activity of bio-oils decreases with increasing temperature, as the content of phenolic compounds decreases, and it decreases with increasing KOH molarity, as higher molarities favor the formation of hydrocarbons. Finally, it can be concluded that chemical activation of açaí seeds with KOH favors not only the yield of bio-oil but also the content of hydrocarbons. The study of process variables is of utmost importance in order to clearly assess reaction mechanisms, economic viability, and design goals that could be derived from chemically activated biomass pyrolysis processes. The study of the antioxidant properties of pyrolysis oils provides insight into new products derived from biomass pyrolysis.