Niknam, Khodabakhsh, et al. “Silica-Bonded N-Propyldiethylenetriamine Sulfamic Acid As a Recyclable Solid Acid Catalyst for the Synthesis of Coumarin and Biscoumarin Derivatives”. Iranian Journal of Catalysis, vol. 4, no. 3, Feb. 2024, https://oiccpress.com/ijc/article/view/3806.