I’ve been googling the question regarding koshering silicone mats and pot holders, etc. for Pesach. They can withstand the heat of boiling but I cannot find anything in writing…
Silicone mats and pot holders should not be kashered for Pesach, and separate Pesach ones should be used. The reason for this is because silicone is a new material and it is not known if it will indeed expel the absorbed taste that is contained inside it, therefore for Pesach we don’t kasher them.
Igros Moshe O:CH 2-92, The Kosher Kitchen pg. 344, The Halachos Of Pesach ( R’ Eider) pg. 139, Chut Shani (Pesach) 10-11(1).