Unveiling electronic effects of Brønsted acid substituents in AlCl3-based superacids: Key role in pitch-based mesophase development

The Lewis acid-catalyzed pyrolysis is crucial for producing mesophase pitch with high aromaticity and superior molecular orientation. While AlCl₃ has long been employed as a catalyst in mesophase pitch synthesis, previous studies have largely overlooked the essential role of Brønsted acids in forming Lewis-Brønsted superacid systems that provide the active catalytic centers. To fully activate the catalytic potential of AlCl₃, this study focuses on the impact of the electronic effects of various Brønsted acid (alcohols, water, carboxylic acids, and amides) substituents on catalytic performance. The characterization results demonstrate that Brønsted acids with strong electron-withdrawing groups and conjugated structures significantly enhance the catalytic performance, with cyanoacetic acid identified as the optimal candidate. Kinetic analysis reveals that cyanoacetic acid reduces the reaction activation energy by up to 6 kJ/mol compared to other Brønsted acids. The observed trends in activation energy and mesophase sphere size evolution align well with the electronic nature of Brønsted acid substituents. Furthermore, the superacid ratio critically determines mesophase morphology and carbonized sample properties. Insufficient Brønsted acid reduces active centers and polymerization rates, while excess acid increases counterion concentration, accelerating termination and suppressing polymerization. The bulk-type texture mesophase with 96.91 % anisotropic content was prepared only at the optimal cyanoacetic acid addition of 40 mol%. This study establishes a clear structure-activity relationship between Brønsted acid substituents and superacid catalytic efficiency, providing a rational approach for catalyst selection and optimization in mesophase pitch preparation.