Process optimization and reusability study of a mesoporous CNT–Al ₂ O ₃ nanocatalyst for cottonseed oil transesterification

The limited durability and mass-transfer constraints of conventional heterogeneous catalysts restrict their practical application in biodiesel transesterification. To address this challenge, an acid-functionalized mesoporous carbon nanotube–alumina (CNT–Al2O3) nanocatalyst was synthesized and evaluated for cottonseed oil transesterification. Structural characterization using SEM, EDX, XRD, FTIR, and BET–BJH analyses confirmed the formation of a well-integrated CNT–Al2O3 framework with slit-shaped mesopores and high surface accessibility. Process parameters, including catalyst concentration, methanol-to-oil molar ratio, reaction temperature, and reaction time, were optimized using response surface methodology to establish quantitative process–performance relationships. Statistical analysis identified reaction time and methanol-to-oil molar ratio as the most influential variables. Under the optimized conditions (1431 ppm catalyst concentration, methanol-to-oil molar ratio of 13.4:1, reaction temperature of 116.4 °C, and reaction time of 145.9 min), a maximum methyl ester yield of 96.9% was experimentally achieved. The produced biodiesel satisfied ASTM specifications for viscosity, density, acid value, oxidation stability, and corrosion behavior. Reusability assessment demonstrated stable catalytic performance over four consecutive cycles, with only a minor reduction in yield thereafter, indicating good structural stability and resistance to active-site deactivation. These findings highlight the potential of the acid-functionalized CNT–Al2O3 nanocatalyst as a reusable catalyst for efficient cottonseed oil biodiesel production.