Non-isothermal in-situ (trans)esterification of lipids in pre-functionalized and lipid-dense post-hydrolysis spent coffee grounds with subcritical methanol at low subcritical condition

Pre-functionalized lipid-dense post-hydrolyzed spent coffee grounds (PHSCG) were used as feedstock for fatty acid methyl ester (FAME) production. The lipids in PHSCG were converted to FAME via non-isothermal in-situ (trans)esterification (ISTE). The conversion was achieved with methanol under subcritical conditions. This study investigates the influence of reactor loading (RL), solvent-to-solid ratio (SSR), temperature, and holding time. High FAME yield of ∼21 g/100g PHSCG can be achieved in about 1 h. Both extractable and bound fatty acids (FA) were converted to FAME. The apparent conversion (∼107%) or yield may exceed the expected extractable FA when using pre-functionalized and lipid-dense PHSCG without needing an additional catalyst. Under favorable conditions of 85% RL, 6 mL/g SSR, and reacted non-isothermally to 125 °C (∼0.8 MPa), a minimum conversion of ∼98% could be achieved depending on the PHSCG's characteristics. The residual acid sites in the post-ISTE residues recovered after ISTE still exhibited catalytic activity when used as a catalyst during the esterification of oleic acid with methanol. The catalytic activity of the residues was evaluated by reusing them for up to 7 cycles. Elemental analysis and FT-IR peak identification supported the presence of sulfonic acid sites providing the catalytic activity of PHSCG and its residues.