Plasma-Activated Solid Superacid Catalysts: Boosting Phenylalanine Esterification on SO42−/TiO2-HZSM-5

To address the challenges of zwitterionic dissociation and steric hindrance in the esterification of α-aromatic amino acids, this study prepared the solid superacid catalyst SO42−/TiO2/HZSM-5 (STH) and its plasma-modified derivative SO42−/TiO2/HZSM-5 (STH-RF) via an aging-impregnation method. Systematic characterization revealed that plasma modification optimizes the crystal morphology and particle dispersion of the catalyst, while also achieving pore clearance and an increase in the specific surface area. Furthermore, it gradationally enhances acidic properties by increasing the abundance of strong acid and Lewis acid sites, and promotes uniform loading and stable bonding of the SO42− active component. Performance evaluation using the synthesis of L-phenylalanine methyl ester as a model reaction demonstrated that STH-RF exhibits optimal catalytic activity, affording a product yield of 85.7%, which is significantly higher than that of unmodified STH (19%) and the homogeneous catalyst H2SO4 (63%). This superior performance originates from a “structure–acidity” synergistic effect, combining the thermodynamic advantage of a lower energy barrier for the rate-determining step (12.6 Kcal·mol−1) with efficient kinetics under optimal process conditions (1.0 MPa, 2000 rpm, 170 °C). Moreover, STH-RF maintained a yield above 80% after four consecutive reaction cycles, indicating excellent stability. This work provides a novel catalytic system for the green and efficient synthesis of highly hindered α-amino acid derivatives, holding significant theoretical and practical implications.