Minecraft wiki entry for Development_versions, associated text: All sword enchantments can be applied to axes in the enchanting table.
This includes the enchantments Looting, Sweeping Edge, Knockback and Fire Aspect. Axes could previously only receive Sharpness, Smite or Bane of Arthropods through an anvil. This has been changed so that axes can receive these enchantments naturally in the enchanting table, alongside their new exclusive enchantment, Cleaving.