“… 8,9 BHPF can be obtained through the condensation reaction of 9-fluorenone and phenol under acidic catalysis ( Scheme 1a ). The reported catalysts include inorganic acids, 10 organic acids, 11 heteropoly acids, 12 solid acids 13 and acidic ionic liquids (ILs). 14 To increase the reaction rate, it is necessary to add co-catalysts, such as thiol, thioglycolic acid 15 etc.…”