Valorization of lignocellulosic wastes for low-cost and sustainable algal biodiesel production using biochar-based solid acid catalyst

Recently, carbon-based solid acid catalysts have come into prominence among different types of catalyst for production of biodiesel. Solid acid catalysts can be produced from carbon based materials such as biomass wastes by reducing the cost. Utilization of these wastes to prepare solid acid catalyst provides solid waste management and the potential greenhouse gas emissions decreases. Since algal biodiesel production is not so economical and needs to be improve, using these catalysts produced from waste biomass can be promising for making the process more cost effective and a low cost applicable zero-waste biorefinery approach can be put forth. In this study, walnut shell, olive stone and macroalgae were utilized to produce biochar based solid acid catalyst to be evaluated in microalgal biodiesel production. Walnut shell carbonized at 500 °C and then sulfonated by sulfuric acid was selected for biodiesel production due to having higher sulfonic acid density (0.35 mmol/g) and preferable structural properties. The statistical evaluation of transesterification parameters effect on microalgal biodiesel yield were evaluated and yield of 82% was found as the highest which was obtained by using 5 wt% catalyst, 12:1 methanol/oil molar ratio at the reaction time of 60 min. Stability test also showed the catalyst can be re-used up until the 5th cycle. Low cost feedstock and reusability of the catalyst presented a simple, cost-effective and environment-friendly process. At last, lignocellulosic wastes can be suggested for not only as biochar production also can be evaluated as solid acid catalyst and provide a cost-efficient biodiesel production.