Niknam, Khodabakhsh, Sajadi, Seyed Ahmad, Hosseini, Roghayeh, Baghernejad, Mojtaba. Silica-bonded n-propyldiethylenetriamine sulfamic acid as a recyclable solid acid catalyst for the synthesis of coumarin and biscoumarin derivatives. Iranian Journal of Catalysis. 2014;4(3):163-173. doi: