Response surface optimization of ionic liquid pretreatments for maximizing cellulose nanofibril production

Pretreatments with aqueous protic ionic liquid (PIL)-ethanolamine bis(oxalate) ([MEA][(HOA)(H₂OA)]), combined with ultrasonic disintegration, were employed in cellulose nanofibril (CNF) production from pulp fibers. The optimization of pretreatment parameters is crucial for obtaining the maximum CNF yield. The response surface methodology was used to design the pretreatment conditions for preparing CNFs. This method consists of four factors: pretreatment time (A, 2-4 h), pretreatment temperature (B, 100-120 °C), liquid-to-solid ratio (C, 60-80 g g^-1), and PIL content (D, 20-40%). The predicted CNF yield (Y) followed a quadratic multinomial regression equation represented by Y = 84.43 + 3.59A + 8.22B + 2.22C - 2.13D - 0.85AB + 2.83AC + 5.95AD + 0.43BC - 2.98BD + 4.25CD - 6.04A² - 18.23B² - 4.98C² - 7.39D². The regression equation exhibited high model fit to the experimental CNF yields as evidenced by a determination coefficient of 0.9764. Results showed that a maximum CNF yield of 86.2% was obtained in the case with the following conditions: pretreatment temperature of 112 °C, pretreatment time of 3.2 h, liquid-to-solid ratio of 83 g g^-1, and PIL content of 29%. CNFs with high crystalline index (64.0%) and thermal stability (T_max = 348 °C) were prepared. This work favors the development of low cost PIL-based pretreatment systems for the clean production of CNFs.