Certain kinds are hard to find. The one I'm looking for specifically absorbs things around the 5 angstrom range.
There's another one called "13x" that would be good too. It absorbs things less than 10 angstroms.
What I'm doing is trying to make a Pressure Swing Absorption system. Essentially, you put this 13X stuff in a container and then run air through it at high pressure. It removes the nitrogen leaving you with about 70-90% oxygen. If you lower the pressure, the nitrogen is released and you can re-use the material.
It's nothing new or special, but I need a LOT of oxygen so I have to build my own.