Aluminium Oxide as Dehydrating agent
Alumina absorbs water to give aluminium oxide hydroxide, a solid which removes the water from the reaction environment.

According to the paper by Pethrick et al. [1]:


  Aluminium oxide exists in several forms which on addition of water give a complex range of oxide-hydroxide; boehmite, bayerite and gibbsite.


References


Pethrick, R. A.; Hayward, D.; Jeffrey, K.; Affrossman, S.; Wilford, P. Investigation of the Hydration and Dehydration of Aluminium Oxide-Hydroxide Using High Frequency Dielectric Measurements between 300 KHz–3 GHz. J Mater Sci 1996, 31 (10), 2623–2629. https://doi.org/10/chcqs8.