A Green Method For Biomass Solubilization: Subcritical Water Process Combined With Microwave Pretreatment

Lignocellulosic biomass materials are attracting increasing attention as renewable, economical, and abundant resources to reduce dependency on petroleum resources and minimize energy and material feedstock costs. Despite their potential, the complex and rigid structures of biomass materials limit their use in such applications.

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Biomass materials must first be broken down into soluble components (e.g., oligo- and monosaccharides) in order to be efficiently converted into a range of products. Biomass hydrolysates from biomass can be used for producing a wide range of value-added products, including biofuels (ethanol, hydrogen, etc.), industrially important chemicals (e.g., solvents), and food products (sugar and sugar alcohols, etc.).

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 lignocellulosic 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 necessary to obtain a reduced molecular weight of carbohydrates from biomass to produce various biofuels and bioproducts.

Microwave pretreatment is an energy-efficient and environmentally benign technology that can be used to reduce the recalcitrance of complex biomass structure. Microwave-based pretreatment causes both thermal and non-thermal effects generated by an extensive intermolecular collision as a result of a realignment of polar molecules with microwave oscillations.

Selectively heating polar parts of lignocellulosic biomass through microwave radiation can make biomass vulnerable to thermochemical solubilization. Using microwave pretreatment with subcritical water as a hydrolysis medium could be an effective method to solubilize biomass in an environmentally friendly way without the use of any acid, base, enzyme or chemical catalysts. Subcritical water process that combined with innovative microwave irradiation pretreatment can be defined as ‘green treatment’ since it utilizes renewable biomass materials as feeds, uses water as a solvent and eliminates the use and generation of hazardous substances in the treatment process.

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Switchgrass (Panicum virgatum L.) and miscanthus (Miscanthus x giganteus) have a tolerance for cold weather and are highly adaptable to varying soil conditions. These perennial grasses are promising biomass candidates for biofuels, bioproducts and green chemicals production.

The study showed that microwave treatment enhanced the breakdown processes of miscanthus and switchgrass biomass materials in subcritical water. Miscanthus was more sensitive to destruction by microwave treatment; therefore, its recalcitrance was easily reduced at lower microwave pretreatment temperatures (60-80 °C). Switchgrass had more rigid structure than miscanthus; therefore, effective solubilization for switchgrass in subcritical water was achieved after higher temperatures microwave pretreatment was applied (120-150 °C). Switchgrass biomass consumed more microwave energy during the pretreatment compared to miscanthus.

Microwave processing reduced the biomass recalcitrance resulting in microwave treated materials having 7-10% higher solubility in subcritical water than untreated materials.

These findings are described in the article entitled Microwave pretreatment effects on switchgrass and miscanthus solubilization in subcritical water and hydrolysate utilization for hydrogen production, recently published in the journal Biomass and Bioenergy. This work was conducted by Sibel Irmak, Anjali Sandip, Jeyamkondan Subbiah, Robert B. Mitchell, and Gautam Sarath from the University of Nebraska-Lincoln, and Bahar Meryemoglu from Çukurova University.

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