Contents lists available at ScienceDirect Biomass and Bioenergy journal homepage: www.elsevier.com/locate/biombioe Short communication Microwave-assisted solubilization of microalgae in high-temperature ethylene glycol Shuntaro Tsubaki a,* , Kiriyo Oono b , Ayumu Onda b , Takashi Kadono c , Masao Adachi c , Tomohiko Mitani d a School of Materials and Chemical Technology, Tokyo Institute of Technology, Ookayama 2-12-1 E4-3, Meguro, Tokyo, 152-8550, Japan b Research Laboratory of Hydrothermal Chemistry, Faculty of Science and Technology, Kochi University, Akebono-cho 2-5-1, Kochi, 780-8520, Japan c Laboratory of Aquatic Environmental Science, Faculty of Agriculture and Marine Science, Kochi University, Otsu-200, Monobe, Nankoku, Kochi, 783-8502, Japan d Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, 611-0011, Japan ARTICLE INFO Keywords: Microwave heating Microalgae Ethylene glycol Dielectric property ABSTRACT Four types of microalgae (Chlorella vulgaris, Nannochloropsis oculata, Fistsulifera solaris (JPCC DA0580), and Phaeodactylum tricornutum Strain NRIA-065) were liquefied through noncatalytic solvothermal solubilization under microwave (MW) irradiation using ethylene glycol (EG) as a reaction media. C. vulgaris and N. occulata exhibited higher solubilization rates and bio-crude oil yields in a temperature range at 300 °C while diatoms (Fistsulifera. Solaris and Phaeodactylum tricornutum) were recalcitrant to MW treatment owing to its high ash content. Bio-crude oil yield increased depending on the reaction temperature and reaction time. In particular, nearly 38% bio-crude oil was obtained under a reaction at 300 °C for 40–60 min from C. vulgaris. Higher heating values of bio-crude oil of 39 and 28 MJ kg -1 were attained in the chloroform and ethyl acetate layers in EG soluble fraction, respectively. Dielectric measurement of the reaction system indicated the effectiveness of ethylene glycol as a medium for absorbing MW energy. EG was efficient reaction medium for rapid heating up by MWs to facilitate solubilization of microalgae at low pressure. Microalgae and macroalgae are regarded as the third-generation biomass feedstock after starchy and lignocellulosic biomass [1]. Mi- croalgae offers advantages of high growth rate and large production in a unit area with high oil content accumulated in algal bodies [2–4]. A large energy consumption for drying wet algae and environmental impact during oil recovery using organic solvents are critical techno- logical and economic issues for the efficient utilization of microalgae. Various pretreatment processes have been investigated to effectively utilize microalgae biomass such as enzymatic, acid and alkali, ultra- sound, pulse electric field, mechanical, and microwave (MW) pre- treatments [5,6]. Hydrothermal liquefaction of microalgae is another method of directly utilizing wet algae by converting them to bio-crude oils under sub- or supercritical water and affords microalgae conversion without drying and oil separation [7–12]. The sub- and supercritical water afford a high reactive environment for converting biomass owing to their unique properties arising from their dielectric constant, visc- osity, and dissociation constant [13]. During hydrothermal liquefac- tion, algae undergoes dissolution and degradation to produce bio-crude oil. The bio-crude oil is further upgraded to light hydrocarbons by removing oxygen and nitrogen through hydrotreatment. The high re- action temperature is, however, still a challenging step for hydro- thermal liquefaction processes of microalage. MW heating is a rapid and efficient heating method as the direct interaction of MWs and irradiated materials affords a rapid, direct, and efficient heating of high-dielectric-loss materials such as aqueous so- lutions [14]. MW heating has been applied to various industrial pro- cesses such as organic synthetic reactions [15], material processing [16], and drying processes [17]. Furthermore, MWs have been applied to biorefinery purposes, for instance, enhancement in pyrolysis of mi- croalgae [18–21], bio-diesel production through transesterification [22–27], and hydrothermal pretreatment of lignocellulose for im- proving their enzymatic susceptibility [28]. Two important parameters must be considered for designing effi- cient hydrothermal reaction by MWs: (1) transparent hydrothermal reactor with sufficient durability to withstand vapor pressure; (2) im- provement in dielectric property of reaction media at elevated tem- peratures [29]. Commercial MW reactors are made of Teflon or glass, both of which are not suitable for hydrothermal reactions above 250 °C. https://doi.org/10.1016/j.biombioe.2019.105360 Received 27 May 2019; Received in revised form 21 August 2019; Accepted 17 September 2019 * Corresponding author. E-mail addresses: tsubaki.s.aa@m.titech.ac.jp, shuntaro.tsubaki@gmail.com (S. Tsubaki), jm-kiriyo@kochi-u.ac.jp (K. Oono), aonda@kochi-u.ac.jp (A. Onda), kadono-takashi@kochi-u.ac.jp (T. Kadono), madachi@kochi-u.ac.jp (M. Adachi), mitani@rish.kyoto-u.ac.jp (T. Mitani). Biomass and Bioenergy 130 (2019) 105360 0961-9534/ © 2019 Published by Elsevier Ltd. T