WE CLAIM:
1. Alumina exhibiting a structure with a porosity such that the volume of the pores having a diameter of between 70 and 2000 A is between 0.15 and 0.50 ml/g, and comprising at least one alkali metal (M), such that the content by weight of alkali metal, expressed as M20, is between 400 and 1500 ppm, with respect to the total weight of the alumina.
2. Alumina according to Claim 1, in which the volume of the pores having a diameter of between 70 and 2000 A is greater than or equal to 0.15 ml/g, preferably greater than or equal to 0.20 ml/g, and less than or equal to 0.50 ml/g, preferably less than or equal to 0.40 ml/g.
3. Alumine according to Claim 1 or 2, in which the content by weight of alkali metal, expressed as M20, is between 400 and 950 ppm, with respect to the total weight of the alumina.
4. Alumina according to one of the preceding claims, in which the total pore volume is greater than or equal to 0.50 ml/g.
5. Alumina according to one of the preceding claims, exhibiting a volume of the pores with a diameter of greater than or equal to 70 A, denoted V70A, of greater than or equal to 0.35 ml/g, preferentially of greater than or equal to 0.45 ml/g, in a preferred way of greater than or equal to 0.50 ml/g and more preferably still of greater than or equal to 0.52 ml/g.
6. Alumina according to one of the preceding claims, exhibiting a BET specific surface area of at least 65 m2/g, preferably of at least 150 m2/g, more preferably of at least 200 m2/g, and of at most 350 m2/g, preferably of at most 325 m2/g.
7. Alumina according to one of the preceding claims, containing at most 500 ppm by weight of silica, expressed as Si02, preferably at most 100 ppm by weight of silica, with respect to the total weight of said alumina, and more preferably being devoid of silica.
8. Alumina according to one of the preceding claims, having a structure comprising x-alumina and amorphous alumina.

9. Alumina according to one of the preceding claims, containing strictly less than 80% by weight of v-alumina, preferentially strictly less than 50% by weight of v-alumina and in a preferred way less than 35% by weight of v-alumina.
10. Alumina according to one of the preceding claims, containing strictly less than 35% by weight of n,-alumina, preferably strictly less than 12% by weight of n,-alumina, preferentially strictly less than 8% by weight of n,-alumina and more preferentially still strictly less than 5% by weight of n-alumina.
11. Alumina according to one of the preceding claims, obtained by rapid dehydration of an aluminium hydroxide or oxyhydroxide.
12. Process for the transformation of a feedstock comprising at least one alcohol into an olefinic effluent, said process comprising a stage of dehydration of said alcohol in the presence of the alumina according to one of Claims 1 to 11.
13. Process according to the preceding claim, in which the feedstock comprises at least 50% by weight of alcohol, preferably at least 70% by weight, preferentially at least 80% by weight, more preferably at least 85% by weight and more preferably still at least 90% by weight of said alcohol, with respect to the weight of said feedstock.
14. Process according to Claim 12 or 13, in which said alcohol comprises from 3 to 9 carbon atoms, preferably from 4 to 6 carbon atoms and more preferably still 4 carbon atoms.
15. Process according to one of Claims 12 to 14, in which said alcohol is a secondary and/or tertiary alcohol, preferably a tertiary alcohol.
16. Process according to one of Claims 12 to 15, in which the dehydration stage comprises a reaction section in which the dehydration reaction is carried out in the gas phase, in the liquid phase or in a mixed phase, at a temperature of between 200°C and 450°C, at a pressure of between 0.1 and 4 MPa and with an hourly space velocity (HSV) of between 0.5 h~1 and 10 h