Microwave heating for the catalytic conversion of melon rind waste into biofuel precursors

Biomass waste is increasingly recognised as a good feedstock and a carbon source with different components and applications. One example of this type of feedstock is the agro-food waste from the external part of melon rind. Indeed, this material is fairly rich in carbohydrates, phenolic compounds and fatty acids. The carbohydrate fraction of melon rind is mainly composed of cellulose, hemicelluloses and pectin, with glucose, xylose and galactose being the major monosaccharides present. Typically, 5-hydroxymethylfurfural (5-HMF) is obtained by the dehydration of monosaccharides (hexoses such as glucose or fructose) at high temperature using an acid catalyst in aqueous solution. In this process levulinic acid (LA) and formic acid (FA) are formed as by-products. In the work described here, the application of microwave radiation and conventional heating in this process was studied along with the use of sulfuric acid (H₂SO₄) and the clay Montmorillonite KSF (M-KSF) as catalysts. Improvements were observed when the reactions were carried out in a biphasic system (water/THF) as this provides a more effective product extraction than the monophasic systems studied. The highest 5-HMF and LA recoveries (6 and 2%, respectively) were obtained with a combination of the biphasic systems and microwave radiation with M-KSF as the catalyst. The results obtained highlight the opportunities for the transformation of the melon rind carbohydrate fraction into a source of biofuel precursors. This transformation can be achieved in a clean and efficient manner using environmentally friendly techniques.