• Amino-functionalization of TS-1 zeolite enables uniform and stable immobilization of phosphotungstic acid, creating a robust solid acid catalyst.
• The catalyst achieves a high tributyl citrate yield of 96.2% and citric acid conversion of 98.1% under optimized conditions (150 °C, 6 h).
• The immobilized HPW provides strong Brønsted acid sites, while framework titanium contributes weak Lewis acidity, enhancing catalytic performance.
• The catalyst demonstrates good reusability over four cycles, with minor deactivation attributed to leaching of weakly anchored species.