You've probably found your answer by now, but it's a mineral zalled zeolite. Sciendirect: "Due to their high cation-exchange ability as well as to the molecular sieve properties, natural zeolites have been widely used as adsorbents in separation and purification processes in the past decades." When the Zeolite of the active generation vessel is saturated with O2 it switches automatically to the other, so you can continue to generate while the first vessel is purged from O2. It's a fully automatic process.