Haiqing presents his paper entitled, "Reverse-selective, highly branched polymers for purification of hydrogen and other light gases" at the 2004 North American Membrane Society (NAMS) meeting in Honolulu, Hawaii. Co-authors Haiqing Lin and Benny D. Freeman (UT Austin); L. Toy, V. Bondar, and R. Gupta (Research Triangle Institute); S. Pas and Anita Hill (CSIRO Manufacturing Science and Technology); and D. Dworak and M. Soucek (The University of Akron).
Abstract
Polymer membranes are used in many applications, including gas separations, due to their inherently low energy requirements for molecular scale separations. Hydrogen, a potential energy source for fuel cells, is produced by steam reforming of hydrocarbons and requires removal of byproducts such as CO2 and H2S. Highly efficient membrane materials will be more permeable to large impurity molecules (e.g., CO2) than to H2 to produce purified H2 at high pressure; such behavior is opposite to that exhibited by most polymers, which sieve penetrants mainly based on relative molecular size, and are, therefore, more permeable to H2 than to CO2. We present results of studies aimed at separating molecules based on the relative affinity of the penetrants for the membrane. Based on an extensive survey of interactions between polar moieties in polymers and CO2, the polar ether units in ethylene oxide are promising candidates for preparing materials with high acid gas permeability and high selectivity for larger CO2 and H2S over smaller H2.
We have prepared and characterized a systematic series of polar, rubbery, branched hydrogels based on poly (ethylene glycol diacrylate) (PEGDA, which is a crosslinker) and the monomers poly(ethylene glycol methyl ether acrylate) (PEGMEA, which has a methyl ether end group) and poly (ethylene glycol acrylate) (PEGA, which has a hydroxyl end group). Crystallization of poly(ethylene oxide) can be completely suppressed by the choice of branch length (i.e., monomer molecular weight) and composition. Introducing PEGMEA into PEGDA improves CO2 permeability by 400% (from 110 to 570 Barrers) and CO2/H2 selectivity by 65% (from 7 to 12) at infinite dilution and 35C, as PEGMEA content increases from zero to 99 wt.%. However, copolymers of PEGDA and PEGA do not show any improvement in permeation properties relative to those of PEGDA alone. A copolymer containing 91 wt.% PEGMEA and the balance of PEGDA exhibits a mixed gas H2S permeability of 2,500 Barrers and H2S/H2 selectivity of 50 at 22C. Temperature could be manipulated to achieve better separation properties. For example, a copolymer containing 70 wt.% PEGMEA and the balance of PEGDA exhibits CO2 permeability of 52 Barrers and CO2/H2 selectivity of 40 at -20C, based on pure gas measurements at an upstream pressure of 4.4 atm. These materials exhibit the best CO2/H2 separation performance reported to date for solid non- facilitated transport membranes. Examples of the separation properties of these materials for CO2/CH4 and CO2/N2 separations will also be shown. The results are interpreted in terms of the effects of polymer chemical structure on free volume, as probed by density and positron annihilation lifetime spectroscopy (PALS), and glass transition temperature.