How does copper oxidise alcohols?
You likely had either or both of the following:

Copper had some oxide on it, and this oxidized the alcohol leaving the metal behind.

If there was air in your system, especially on heating copper could have reacted with it to generate the oxide (a relatively fast reaction with heating) and you're back to (1).  The metal would then, of course, be available to react with more air.  In effect this would be a catalytic oxidation.


Copper is actually known in negative oxidation states, but that does not happen or need to be invoked here.