N. N. Saimon, S. Thavil, M. Jusoh, N. Ngadi, and Z. Y. Zakaria, “Preparation of Ti-SO₃H/ICG Using Microwave-Assisted for Esterification of Palm Fatty Acid Distillate to Methyl Ester”, Chemical Engineering Transactions, vol. 63, pp. 463-468, May 2018.