Self-Cooling Beverage Can  

Apart from...
being very expensive to make a tube that can hold gas under pressure during all the stages of filling, storagem, transportation, distribution etc. it would be quite a challenge to let the gas escape in a way that the cooling effect coincides with the cooling capacity of the can and it's content. What sounds simple on a patent description is not simple physics. One would also have to include an insulating membrane on the outside because the cold transfer is most likely faster on the outside than on the inside.
Nice idea, but there are very good reasons why it does not exist.