Valorization of cocoa's mucilage waste to ethanol and subsequent direct catalytic conversion into ethylene

BACKGROUND: The identification of new resources for producing biofuels and chemical-based products is crucial for processes sustainability. This study presents a valorization route to produce ethanol and ethylene using cocoa's mucilage juice (MJ) residue from cocoa farms of variety ‘Arriba’ (AC). The processing parameters to maximize the ethanol production and subsequent selective conversion into ethylene were determined. Ethanol production has been carried out by investigating the effect of three parameters: the temperature of fermentation, the initial fermentation pH and the addition of (NH4)2SO4 as an N source in the presence of free Saccharomyces cerevisiae NCYC 366. Consecutively, the selectivity of ethanol–ethylene conversion using a zeolite-based ZSM-5 catalyst was evaluated at different temperatures and ethanol concentrations.
RESULTS: During ethanol production, the best sugar conversion was reached at 30 °C, adjusting the initial pH to 5 and without nitrogen source, resulting in 86.83% sugar conversion, the maximum ethanol concentration of 68.65 g L-1 and maximum ethanol production rate of 2.03 g L-1 h-1 after 168 h of fermentation. On the other hand, ethylene was produced using ZSM-5-based zeolite catalyst with >99.9% of efficiency in the temperature range 240-300 °C. In addition, selective ethylene formation was found at 240 °C and 30 g L-1 ethanol.
CONCLUSION: The approach hereby presented shows the valorization of MJ waste of AC variety to produce ethanol and ethylene with minimum processing input costs, demonstrating a successful route to convert a farm residue into a bio-based product with enhanced marketability.