In the world of adult websites, a premium porn site is a site that requires a subscription. Thus, there are no truly free premium porn sites.
That said, some premium porn content is available for free on porn tube sites, particularly if they belong to shared ownership. For instance, you can find full-length premium porn videos taken from Brazzers and uploaded on Pornhub, but only because the same company owns both sites.
Free Premium Porn Sites
Brazzers porn videos:
Via Pornhub.
Bangbros porn videos:
Via XVideos.
Reality Kings porn videos:
Via Pornhub.
Xvideos Red porn videos:
Via Spankbang.