Microwave-assisted alkali pre-treatment medium for fractionation of rice straw and catalytic conversion to value-added 5-hydroxymethyl furfural and lignin production

Abstract

In the current study, the use of microwave-assisted sodium hydroxide medium (MWSH) for pre-treatment and saccharification of rice straw to obtain sugar syrup for the production of 5-hydroxymethyl furfural (5-HMF) was investigated. The optimization of the MWSH pre-treatment was carried out using central composite methodology, resulting in a maximum reducing sugar yield of 350 mg/g of treated rice straw (TRS) and a glucose yield of 255 mg/g of TRS under the conditions of a microwave power of 681 W, a NaOH concentration of 0.54 M, and a pre-treatment time of 3 min. Additionally, the microwave assisted transformation of sugar syrup with titanium magnetic silica nanoparticle as catalyst, producing 41.1 % yield of 5-HMF from the sugar syrup after 30 min microwave irradiation at 120 °C with catalyst loading of 2.0:200 (w/v)). The structural characterization of the lignin was analysed using 1H NMR techniques, and the surface carbon (C1s spectra) and oxygen (O1s spectra) composition changes of the rice straw during pre-treatment were analysed using X-ray photoelectron spectroscopy. The rice straw based bio-refinery process which contains MWSH pretreatment followed by dehydration of sugars achieved high efficiency of 5-HMF production.