Can an ex claim future inheritance after legal separation?

Probably not. An inheritance is not considered a marital asset that can be distributed for equitable distribution.  It may, however, be utilized for your ability to pay support if you are going to generate interest income or have a significant amount of principle that you can draw down.

You must be careful how you utilize this money. I would recommend you speak with an attorney well-versed in matrimonial and estate planning prior to filing a divorce action.