At what temperature does simple syrup freeze?
It is pure water and pure sugar (glucose+fuctose) then it will freeze at or about 22F or -5.5C.  At 2:1 it will freeze at 12.5F or -11C.  Use a deep freeze as a normal freezer cannot get it that cold.   This is why slush is made from sugar water as at the freezing point of water the water will freeze while the sugar will not.