Pure Hydrogen

A family of poly(ethylene oxide) materials exhibits "outstanding" performance in removing acid gas impurities from hydrogen feed streams at high pressure, according to researchers at the University of Texas (UT), Austin, and Research Triangle Institute in Research Triangle Park, N.C., who prepared the materials (Science 2006, 311, 639).

Hydrogen produced industrially from hydrocarbons contains impurities such as carbon dioxide. Conventional materials for hydrogen purification allow H₂ molecules to diffuse through them in preference to larger molecules (C&EN, Oct. 3, 2005, page 49). The process lowers the pressure of H₂, which then has to be repressurized-at a cost-before use.

The novel materials allow H₂ purification to be carried out at high pressures. "Any improvement in purification efficiency could reduce the cost of H₂ and accelerate its use in energy applications in the future 'hydrogen economy,' " says UT chemical engineering professor Benny D. Freeman.

The polymers are "reverse selective" because acid gases rather than H₂ permeate through them. The cross-linked polymers exhibit increasing selectivity for acid gases with increasing acid gas partial pressure, Freeman points out.

The increasing selectivity is remarkable because polymer membranes swell in acid gases, observes Neil B. McKeown, a chemistry professor at Cardiff University, in Wales. "Swelling generally results in lower selectivities at higher pressures because all the gases diffuse faster through the extra space," he explains. "The advantage of the 'reverse-selective' type of membrane is that it can operate at pressures that are compatible with the conditions used in H₂ production via hydrocarbon reforming."