Complete valorization of defatted Cynara cardunculus through a microwave-assisted biorefinery process

Renewable resources such as lignocellulosic biomass are gaining interest as a sustainable alternative to fossil fuels, helping to reduce emissions and environmental impact. Cynara cardunculus is an interesting feedstock, being a perennial herbaceous crop that can grow on marginal lands. Up to now, the oil-rich seeds of the cardoon flower have found applications for bio-diesel production, whilst the cardoon residues called defatted cardoon (DC) are typically discarded despite its high polysaccharide content (37 wt%). This work presents the development of an innovative microwave-assisted biorefinery process of DC adopting a one-pot fractionation to obtain high-value chemicals and biofuels. An acidic biphasic organosolv pretreatment (water/n-butanol) was performed to easily isolate each biomass component: a solid cellulose-rich residue, an aqueous phase rich in xylose deriving from hemicellulose hydrolysis, and an organic phase containing solubilized lignin. The optimized organosolv pretreatment allowed the extraction of 72 wt% of lignin and 97 wt% of hemicellulose, enriching the solid residue up to 74 wt% in cellulose. This latter was used as substrate in a microwave-assisted one-pot butanolysis process catalyzed by H2SO4 to obtain n-butyl levulinate (BL), a promising bio-blendstock for Diesel. Under optimized reaction conditions, a BL yield of 46 mol% was achieved, higher than that obtained from untreated biomass, demonstrating the benefits of the pretreatment in enhancing cellulose accessibility and reactivity. The aqueous phase rich in xylose were further processed by microwave heating to produce furfural, an important platform-chemical for the synthesis of a pletora of other added value products, via acid-catalyzed dehydration reaching the highest yield of 52 mol%. The use of a raw hydrolysate instead of a pure substrate makes this approach particularly attractive from an industrial perspective. Finally, lignin solubilized in the organic phase was separated by butanol, allowing its possible reuse in the organosolv pretreatment or as reagent in the butanolysis step. The recovered lignin was characterized by FT-IR and elemental analysis proving its high purity. In conclusion, the proposed valorization strategy allowed the effective exploitation of each DC fraction using microwaves as an efficient heating method according to a sustainable biorefinery process.