Aluminium Oxide as Dehydrating agent According to this paper on dehydration by $\ce{Al2O3}$,


  Dehydroxylation causes a surface defects as Lewis acidic centers (unsaturated $\ce{Al^3+}$ ions) and basic Lewis centers ($\ce{O^2-}$ ions) creation. Water molecules deactivate Lewis centers and create strongly acidic Brönsted’s centers. Alcohols dehydration reaction performs on Brönsted’s active centers where hydroxyl groups disconnect proton from alcohol molecule. During reaction alcohol molecule adsorbed on aluminium oxide surface reacts with proton. 


But I am not able to understand what they are trying to convey here.
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.