I. Perdana, L. Lenny, and I. M. Bendiyasa, “Regeneration of geothermal-sludge based sodium silicate catalyst for transesterification of palm oil with methanol”, J. Mech. Eng. Sci., vol. 12, no. 4, pp. 4030–4043, Dec. 2018, doi: 10.15282/jmes.12.4.2018.04.0350.