Valorization of Spent Tea Waste into Sulfonated Heterogeneous Catalyst for Sustainable Biodiesel Production

The escalating energy crisis and environmental concerns associated with fossil fuel consumption have intensified the search for sustainable alternatives. This study reports the synthesis of a sulfonated heterogeneous catalyst derived from spent tea waste for biodiesel production via transesterification of waste cooking oil. The catalyst was prepared through sulfonation of spent tea waste with concentrated sulfuric acid and characterized using FT-IR, TGA, and XRF techniques. FT-IR analysis confirmed the successful grafting of sulfonic acid (-SO3H) functional groups onto the sulfonated carbonaceous framework, as evidenced by diagnostic S=O stretching bands at 1170 cm−1 and 1030 cm−1. XRF analysis revealed a substantial sulfur loading of 8.79 wt% in the sulfonated catalyst compared to 2.12 wt% in raw tea waste, while TGA demonstrated enhanced thermal stability of the catalyst up to 300 °C. The catalytic activity was systematically optimized through single-factor screening of reaction parameters. Under optimized conditions, 40 °C reaction temperature, 60 min reaction time, 2 wt% catalyst loading, and 12:1 methanol-to-oil molar ratio, a maximum biodiesel yield of 95.65% was achieved. The produced biodiesel exhibited favorable fuel properties, including a calorific value of 36.9 MJ kg−1, cloud point of 11.8 °C, and pour point below 1.3 °C, all conforming to ASTM D6751 specifications. This study demonstrates that sulfonated spent tea waste is a promising, cost-effective, and environmentally benign catalyst for biodiesel production, offering a sustainable solution for valorizing tea waste while contributing to renewable energy development.