A green and sustainable biodiesel production from Chlorella vulgaris microalgae using functionalized carbon nanotube heterogenous catalysts derived from rice husk and mixed fruit waste

The present study investigates rice husk and fruit waste derived carbon nanotubes as potential heterogeneous catalysts for microalgal biodiesel production. Carbon nanotubes were synthesized by a two-step process of initial slow pyrolysis at 500 °C, 3h and 5 °C/min, followed by microwave assisted synthesis at 2.45 GHz, 5 min and 120 °C. The synthesized carbon nanotubes from rice husk and fruit waste were further purified and functionalized with hydroxyl group. The experiments were designed according to Box–Behnken Design approach in Response Surface Methodology and total of 27 experiments were conducted to determine biodiesel yield. Under optimal reaction conditions (65 °C, 2h, methanol to oil molar ratio of 9 and 1 %wt. of catalyst), f-RHCNTs yielded 97.45% of microalgal biodiesel. Also, f-FWCNTs yielded 93.24% biodiesel under optimal conditions of 65 °C, 3h, methanol to oil molar ratio of 9 and 3 %wt. of catalysts. Both waste derived catalysts demonstrated excellent reusability, maintaining biodiesel yields of 59.68% (f-RHCNTs) and 62.12% (f-FWCNTs) even after five successive reaction cycles. Additionally, the produced microalgal biodiesel aligned with ASTM and EN fuel specifications, confirming its suitability and potential for practical fuel applications. Rice husk and fruit waste proved to be highly effective precursors for synthesizing carbon nanotubes and developing heterogeneous catalysts, exhibiting strong catalytic performance, and promising reusability. Further emphasis should be laid on developing heterogenous catalysts exhibiting catalytic capability and reusability for biodiesel production. • A green and sustainable method is used to produce biodiesel from Chlorella vulgaris . • Carbon nanotube catalysts successfully derived from rice husk and mixed fruit waste. • Rice husk and fruit waste are effective precursors for synthesizing carbon nanotube. • The produced microalgal biodiesel aligned with ASTM and EN fuel specifications.