I would like to know if it is possible to characterize the property "quasi-compact" in the category of schemes by means of a pure categorical language. 

For example the property of being empty is categorical, because it just says that the scheme is initial. The terminal scheme is $\text{Spec}(\mathbb{Z})$, so this is also categorical. Further examples of categorical properties or schemes: Spectra of fields, the underlying set of a scheme (in particular surjective morphisms), connected schemes, $\text{Spec}(\mathbb{Z}_p)$, and much more, see [here][1]. The usual definition of quasi-compact involves open immersions, which are, a priori, not categorical.

[1]: http://mathoverflow.net/questions/56887/rigidity-of-the-category-of-schemes