Can I convert 72V to 12V using potential divider?
In general voltage dividers are a poor choice for power supplies because to acheive a stable output voltage over varying load the current wasted in the divider must be much greater than the current delievered to the load.

I would generally recommend an isolated converter unless you know exactly what you are doing, sharing return paths between high power high voltage systems and low voltage systems can be a recipie for trouble. 

Unfortunately 72V DC puts you in an awkward place, it's too high for most common DC-DC modules, but too low for mains power supplies. So converters are going to be specialised and hence expensive.