Semi-continuous flow mechanochemistry as a tool for the valorization of spent coffee grounds for biorefinery

Spent coffee grounds (SCG) are a by-product of the coffee industry, with its production steadily increasing. In this study, a sequential biorefinery process for the valorization of SCG was investigated, aiming to produce biodiesel, 5-hydroxymethylfurfural (5-HMF) and activated carbon as products of significant industrial interest. In all these processes, conventional and mechanochemical-assisted methodologies were evaluated aiming at process intensification. Firstly, lipids were extracted from SCG and subsequently transformed into biodiesel using calcium diglyceroxide (CaDG) as heterogeneous catalyst, achieving a yield of 98.4 %. Afterwards, autohydrolysis of extracted SCG has allowed the recovery of hemicellulosic liquors, containing 22 % sugars in solution in the form of monomers (mainly C6) and oligomers. These sugar solutions were subjected to dehydration in a water, using CaCl₂ and Al₂O₃, resulting in a maximum 5-HMF yield of 65 %. Finally, the residue obtained after these sequential processes, primarily composed of cellulose and lignin, underwent pyrolysis with KOH as an activating agent, producing activated carbon with a maximum surface area of 1359 m²/g. This cascade biorefinery approach demonstrates a sustainable pathway for SCG utilisation by minimizing waste and maximizing the production of valuable products.