Contents lists available at ScienceDirect Biomass and Bioenergy journal homepage: www.elsevier.com/locate/biombioe Research paper Microwave pretreatment effects on switchgrass and miscanthus solubilization in subcritical water and hydrolysate utilization for hydrogen production Sibel Irmak a,* , Bahar Meryemoglu b , Anjali Sandip a , Jeyamkondan Subbiah a,c , Robert B. Mitchell d , Gautam Sarath d a Biological Systems Engineering, Industrial Agricultural Products Center, University of Nebraska, Lincoln, NE 68583-0726, USA b Department of Chemistry, Çukurova University, 01330 Adana, Turkey c Food Science & Technology, University of Nebraska, Lincoln, NE 68588-6205, USA d USDA-ARS, Grain, Forage, and Bioenergy Research Unit, University of Nebraska-Lincoln, 251 Food Industry, Lincoln, NE 68583-0937, USA ARTICLE INFO Keywords: Biomass Microwave pretreatment Subcritical water APR Hydrogen ABSTRACT Microwave pretreatment is an energy-efficient and environmentally benign technology that can be used to re- duce the recalcitrance of complex biomass structure. Switchgrass (Panicum virgatum L.) and miscanthus (Miscanthus x giganteus) are perennial C4 grasses that are being developed as bioenergy crops because they have high yield potential and desirable agronomic traits. These materials are promising candidates for biofuels, bioproducts and green chemicals production from biomass. In the present study, miscanthus and switchgrass biomass were solubilized in subcritical water after pre- treatment by microwave at different processing temperatures. The hydrolysates obtained were evaluated for hydrogen-rich gas production by aqueous-phase reforming (APR). Higher temperature microwave processing reduced the biomass recalcitrance resulting in microwave treated materials having 7–10% higher solubility in subcritical water than untreated materials. However, gasification of pretreated biomass hydrolysates produced less gaseous products compared to untreated biomass for both switchgrass and miscanthus. Miscanthus biomass was more vulnerable to destruction by microwave treatment and recalcitrance of this biomass was achieved at lower temperature compared to switchgrass. Miscanthus biomass that was not microwave treated produced the highest gas yield. Microwave pretreatment caused sig- nificant increases in the formation of ungasified solid carbon residue in the APR process. 1. Introduction Among alternative biomass resources, switchgrass (Panicum vir- agatum) and miscanthus (Miscanthus x giganteus) are promising poten- tial sources for production of value-added products. They are non-ed- ible biomass materials with high total carbohydrate contents. These sources are C4 warm-season perennial grasses and both crops have a tolerance for cold weather and are highly adaptable to varying soil conditions [1,2]. Pretreatment is an essential prerequisite to make biomass accessible to deconstruction by altering structural features such as removing lignin and reducing cellulose crystallinity, thereby increasing porosity. Successful production of biofuels and other bioproducts from lig- nocellulosic biomass depends on the pretreatment and deconstruction methods applied as well as the physical and chemical properties of the biomass. An efficient pretreatment method followed by solubilization in aqueous media without using toxic and hazardous chemicals is neces- sary to obtain reduced molecular weight of carbohydrates from biomass to produce various biofuels and bioproducts. Microwaves are in the region of the electromagnetic spectrum be- tween infrared radiation and radio frequencies, in the 1 mm to 1 m wavelength range (300 MHz–300 GHz). This electromagnetic radiation is used to generate heat by the oscillation of molecules upon radiation absorption. Microwave-based pretreatment causes both thermal and non-thermal effects generated by an extensive intermolecular collision as a result of realignment of polar molecules with microwave oscilla- tions [3]. Selectively heating polar parts of lignocellulosic biomass with microwave radiation can reduce the recalcitrance of complex biomass structure. On the other hand, disrupting the crystallinity and altering the compact and rigid biomass structure can be achieved in an https://doi.org/10.1016/j.biombioe.2017.10.039 Received 2 August 2016; Accepted 28 October 2017 * Corresponding author. E-mail address: sibel.irmak@unl.edu (S. Irmak). Biomass and Bioenergy 108 (2018) 48–54 0961-9534/ © 2017 Published by Elsevier Ltd. MARK