Ramazani, Ali, et al. “Bentonite Powder Catalyzed Synthesis of Coumarins from in Situ Generated Stabilized Phosphorus Ylides in Solvent-Free Conditions”. Asian Journal of Chemistry, vol. 18, no. 1, Mar. 2010, pp. 698-00, https://asianpubs.org/index.php/ajchem/article/view/21430.