S. Gowri Krishnan, F.- ling Pua, K. Palanisamy, S. Nabihah Syed Jaafar, and K. Subramaniam, “Oil Palm Empty Fruit Bunches Fiber-Supported Heterogeneous Acid Catalyst for Esterification of Oleic Acid: Effect of Different Transition Metal Sulfate”, IJET, vol. 7, no. 4.35, pp. 870–875, Nov. 2018, doi: 10.14419/ijet.v7i4.35.26272.