Production of biodiesel and hydrogen by using a double-function heterogeneous catalyst derived from spent coffee grounds and its thermodynamic analysis

Biodiesel and hydrogen are promising liquid and gas energy source alternatives to satisfy fossil fuel demand. In this study, a heterogeneous catalyst, spent coffee grounds derived activated carbon supported, was synthesized using KOH as an activation agent. The highest iodine number was obtained at 600 °C carbonization temperature and the synthesized catalyst was subjected to the characterization and evaluated in terms of biodiesel and hydrogen production efficiencies. The optimal transesterification reaction parameters were determined as 3 wt% catalyst loading, 9:1 methanol-to-waste cooking oil molar ratio, 90 °C reaction temperature, and 120 min reaction time. Under optimal reaction conditions, the biodiesel yield was 91.57%. For hydrogen production, 100% conversion was observed in all cases. Among the experimental conditions, the fastest reaction was obtained with the addition of 0.25 g superior catalyst with 2.5% of NaBH4 concentration at 30 °C reaction temperature. Moreover, the chosen superior catalyst was successfully reused for five cycles to test the reusability of the catalyst. The catalyst performance was almost the same as of the first cycle after regeneration for both transesterification and dehydrogenation reactions. Additionally, the activation energy of the transesterification and methanolysis reactions was calculated as 19.15 and 15.48 kJ/mol, respectively. In the thermodynamic aspect, both reactions were endergonic and unspontaneous. Additionally, it can be concluded that the reactants were converted to the products very well due to the indication of entropy change.