Waste-Derived Catalyst for Biodiesel Manufacturing in CO2-Free Manner: Preparation, Catalytic Activity, and Reuse Studies

Abstract

Currently, CaO-based catalysts for biodiesel manufacturing are produced by calcination of available limestone ore consisting of CaCO₃. The production of 1 ton of CaO catalyst resulted in 0.89 tons of CO₂ emission, and every ton of the catalyst provided only 20 tons of biodiesel in nonreusable manner. In this study, a CaO-based catalyst was obtained by calcination of Ca(OH)₂, a large-tonnage waste from acetylene manufacturing via calcium carbide route (carbide slag), producing H₂O as a by-product instead of CO₂. Amazingly, carbide slag-a waste material-was active in catalytic transesterification of soybean oil skipping calcination or any pretreatment steps and provided biodiesel in 28% yield. Moreover, biodiesel was obtained in 98% yield after catalyst calcination at 600 °C for at least 4 times accompanying zero CO₂ emission. The obtained catalysts were characterized by XRD, XRF, FTIR, TGA, SEM-EDX, and BET surface area analysis. The best conversion of soybean oil was achieved using 1 wt% CS₆₀₀, MeOH:oil ratio of 12:1, by boiling at 65 °C for 2 h. The catalyst reuse was found out using two approaches: “catalyst isolation” (5 cycles with yield ≥ 80%) and “fresh start” (up to 7–10 cycles with yield ≥ 80%).