Microwave-assisted oxidative digestion of lignin with hydrogen peroxide for TOC and color removal

Microwave-Assisted Lignin Wet Peroxide Oxidation to C4 Dicarboxylic Acids

Innovative methodologies, such as microwave-assisted reaction, can help to valorize lignin with higher productivity and better energy efficiency. In this work, microwave heating was tested in the wet peroxide oxidation of three lignins (Indulin AT, Lignol, and Eucalyptus globulus lignins) as a novel methodology to obtain C₄ dicarboxylic acids. The effect of temperature, time, and catalyst type (TS-1 or Fe-TS1) was evaluated in the production of these acids. The TS-1 catalyst improved succinic acid yield, achieving up to 9.4 wt % for Lignol lignin. Moreover, the microwave heating specifically enhanced Lignol conversion to malic acid (34 wt %), even without catalyst, showing to be an attractive path for the future valorization of organosolv lignins. Overall, compared to conventional heating, microwave heating originated a rapid lignin conversion. Nevertheless, for prolonged times, conventional heating led to better results for some target products, e.g., malic and succinic acids.

On the use of microwave pretreatment to assist zero-waste chemical-free production process of nanofibrillated cellulose from lime residue

Microwave (MW) pretreatment as an energy-efficient method to enhance the production of nanofibrillated cellulose (NFC) from lime (Citrus aurantifolia Swingle) residue after juice extraction is proposed. NFC was prepared by subjecting lime residue to MW pretreatment for up to 3 rounds; this was followed by high-shear and high-pressure homogenization. Repeated application of MW pretreatment helped remove non-cellulosic components and resulted in an increased cellulose content and crystallinity index but a decrease in fiber diameter. Freshly prepared NFC sample exhibited gel-like behavior. G' and G″ of suspension prepared from dried NFC markedly decreased, indicating the loss of gel-like property upon drying. Proper pectin molecular weight as well as pectin content were noted to play an important role in controlling aggregation of NFC during drying and hence water redispersibility of dried NFC. Significant amounts of pectin and limonin could be recovered and utilized as co-products after the first round of MW pretreatment.