Ramazani, Ali, et al. “Kaolinite Powder As an Efficient Catalyst for the Preparation of Coumarins from in Situ Generated Stabilized Phosphorus Ylides in Solvent-Free Conditions”. Asian Journal of Chemistry, vol. 18, no. 1, Mar. 2010, pp. 120-4, https://asianpubs.org/index.php/ajchem/article/view/21267.