A Radio Frequency Plasma-Modified SO42−-S2O82−/SnO2-Al2O3 Solid Superacid Catalyst Applied for the Esterification of Tyrosine

To overcome the limitations of conventional catalysts in sterically hindered esterification reactions, a radio frequency (RF) plasma-modified SO42−/S2O82−/SnO2-Al2O₃ solid superacid catalyst was synthesized via sol-gel and impregnation, followed by RF plasma treatment and calcination. Comprehensive characterization revealed that the RF plasma modification endowed the catalyst with a uniform particle distribution (4.32 nm average size), larger specific surface area (104.44 m2·g−1), elevated total acid content (142.86 μmol·g−1), and increased oxygen vacancy concentration (16.4%), compared to the conventional sol-gel–impregnation–calcination-prepared catalyst. The RF plasma-modified SO42−-S2O82−/SnO2-Al2O3 was subsequently applied to perform the esterification reaction of Tyr, with a higher steric hindrance. Mechanistic studies indicated that the plasma-induced surface etching and electronic redistribution/intensified electron-withdrawing capability of SO42−/S2O82− groups synergistically strengthened Brønsted/Lewis acidity. For the esterification of tyrosine—a sterically demanding substrate—the modified catalyst achieved a 92.1% methyl tyrosine yield under the optimized conditions (180 °C, 0.8 MPa N2, 6 h), where the catalyst exhibited a better mechanical strength and better lifetime with five cycles. This work not only provides a scalable plasma-assisted strategy for tuning solid superacids but also establishes an eco-friendly alternative to traditional catalysts, and was applied to the esterification reactions of some high-steric-hindrance substrates.