Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech Catalytic hydrothermal liquefaction of lactuca scariola with a heterogeneous catalyst: The investigation of temperature, reaction time and synergistic efect of catalysts Halil Durak a, ⁎ , Salih Genel b a Van Yuzuncu Yil University, Vocational School of Health Services, 65080 Van, Turkey b Van Yuzuncu Yil University, Educational Faculty, 65080 Van, Turkey ARTICLE INFO Keywords: Lactuca scariola Hydrothermal liquefaction Heterogen catalyst Bio-oil ABSTRACT In this study, lignocellulosic biomass was converted into liquid products by catalytic hydrothermal liquefaction. Zn, Fe, and Zn + Fe were used to obtaining products with high energy value as heterogeneous catalyst systems in this study. The diferent experimental parameters were used to examine temperature (220, 240, 260, 280, 300 °C), reaction time (0, 5, 10, 15, 20, 30 min.), and the synergistic efect of catalysts (Zn + Fe) on conversion rate. The products obtained were examined by GC–MS, Elemental, FT-IR, 1 H NMR, SEM-EDX, and XRD analysis methods. According to the results of the experiment, it has been determined that Fe is the most efective catalyst for light bio-oil and heavy bio-oil yields and Zn + Fe is the most efective catalyst system for the gas + aqueous phase products. Fe catalyst in monoaromatics formation, Zn catalyst in polyaromatic and aliphatics compound formation, Zn + Fe catalyst system in oxygen compounds formation are efective. 1. Introduction The rapid increase in energy needs and environmental problems increase researchers interest in renewable energy sources rather than fossil fuels (Parsa et al., 2018). There are many resources that are considered as renewable energy sources, and one of the most important of these resources is biomass. Biomasses are living organism wastes that store solar energy and cheap, clean, and environmentally friendly po- tential energy sources that defned as biological material (Yucedag and Durak, 2019; and Kumar et al., 2018). Raw materials considered as biomass include plant and plant waste, industrial waste, animal waste, and household waste. Considering these resources, biomass resources can be recognized as renewable and sustainable resources. Biomass meets about 10–14% of the world's energy demand; this rate is expected to reach 25% in the future. Biomass, a heterogeneous mixture of or- ganic and small amounts of inorganic substances, contains 30–60% carbon, 30–40% oxygen, and 5–6% hydrogen in dry form. In addition, it consists of < 1% inorganic substances such as nitrogen, chlorine, and sulfur (Tekin et al., 2014; Briens et al., 2008). The polymers that make up the biomass generally consist of smaller units. Biomass consists of cellulose, lignin, hemicellulose, extract and ash. It consists of units of glucan (C 6 H 10 O 5 ), which is defned as an- hydrous glucose formed by the loss of 1 molecule of water from glucose molecule. Hemicellulose polymers are branched structure macro- molecules containing acetyl groups. Lignin polymers are formed by attaching phenyl propane subunits to C–C and C–O bonds at diferent points of the monomer. The molecular structure of lignin is more complex than cellulose and hemicellulose. In addition, in the biomass structure, lignocellulosic materials contain about 4% organic com- pounds consisting of protein, extractive and inorganic components (Colak et al., 2018). Thermochemical conversion methods (hydrothermal liquefaction, supercritical liquefaction, pyrolysis, and gasifcation) included in the chemical conversion method seem more advantageous than other methods in converting biomass into valuable chemicals with high pressure and temperature parameters (Chen, 2018). Organic solvents such as acetone and alcohols are mostly used in the supercritical liquefaction process. At the end of the process, these sol- vents cause pollution. In addition, these chemicals are expensive, thus increasing the cost. In this process, a high amount of liquid product is obtained, but the liquid product obtained may originate from the so- lution used. Pyrolysis is a thermochemical conversion process that aims to ob- tain new products by thermally breaking chemical bonds in the organic molecules of biomass by burning biomass in an oxygen-free environ- ment. As a result of the pyrolysis process, hydrocarbon-rich gas, oil-like https://doi.org/10.1016/j.biortech.2020.123375 Received 23 March 2020; Received in revised form 11 April 2020; Accepted 11 April 2020 ⁎ Corresponding author. E-mail address: halildurak@yyu.edu.tr (H. Durak). Bioresource Technology 309 (2020) 123375 Available online 15 April 2020 0960-8524/ © 2020 Elsevier Ltd. All rights reserved. T