Solid superacid SO42−-S2O82−/SnO2-Nd2O3-catalyzed esterification of α-aromatic amino acids

α -Aromatic amino acid esters are important platform compound for drugs, sweeteners and fluorescent sensors. Due to the amphoteric dissociation and steric effect, the esterification of α -aromatic amino acids is difficult. Here, a green and efficient solid superacid, SO 4 2− -S 2 O 8 2− /SnO 2 -Nd 2 O 3 (SSSN), was synthesized and firstly used for the methyl esterification of α -aromatic L-tyrosine and L-tryptophan. The methyl esters yield was optimized with the highest L-TyrOMe yield of 93.1% and L-TrpOMe yield of 92% over SSSN-650 (SO 4 2− -S 2 O 8 2− /SnO 2 -Nd 2 O 3 calcined under 650 °C). Furthermore, it was clarified that the doping of Nd enhanced the electron-withdrawing inductive effect of SO 4 2− /S 2 O 8 2− with increased Brønsted and Lewis acidity, which were beneficial to the esterification of L-tyrosine and L-tryptophan. Besides, Nd-doping reduced the activation energy during L-Tyr esterification by kinetics study. Finally, SSSN-650 could be recycled six times with > 70% of their initial ester yield. The decrease of catalyst activity was caused by carbon deposition and species leaching. This work launched a novelty, rapid and environmental-benign technology for the esterification of α -aromatic amino acid using solid superacid.